2 Be ™) NOZIO
Bulletin of the
British Museum (Natural History)
A Global Analysis of the Ordovician—
Silurian boundary
Edited by L. R. M. Cocks & R. B. Rickards
Geology Vol 43 28 April 1988
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World List abbreviation: Bull. Br. Mus. nat. Hist. (Geol.)
© British Museum (Natural History), 1988
The Geology Series is edited in the Museum’s Department of Palaeontology
Keeper of Palaeontology: DrL.R.M. Cocks
Editor of the Bulletin: Dr M. K. Howarth.
Assistant Editor: Mr D. L. F: Sealy
ISBN 0 565 07020 7
ISSN 0007-1471
M3 Ta wef y Geology series
British Museum (Natural History): * Vol 43 complete
Cromwell Road ‘ we oe
London SW7 5BD ser ana Issued 28 April 1988
A Global Analysis of the Ordovician—
ae Silurian boundary
Edited by L. R. M. Cocks
Department of Palaeontology, British Museum (Natural History),
Cromwell Road, London SW7 5BD
and R. B. Rickards
Sedgwick Museum, Downing Street, Cambridge CB2 3EQ
Bulletin British Museum (Natural History)
Geology Series
Vol. 43
International Union of Geological Sciences
Sponsored Publication
The papers incorporated in this volume represent contributions from the International Work-
ing Group on the Ordovician-Silurian Boundary, a constituent body of the International
Commission on Stratigraphy within the International Union of Geological Sciences.
Contents
GRO CECH OTM years, dorsie aise nierere sersneie nierate oie eters ets L. R. M. Cocks & R. B. Rickards
The Ordovician-Silurian Boundary and its Working Group ............ L. R. M. Cocks
Ordovician—Silurian Boundary Sections
EUROPE:
Dob’s Linn — the Ordovician-Silurian Boundary Stratotype .......... S. H. Williams
Conodonts from the Ordovician-Silurian Boundary Stratotype, Dob’s Linn, Scotland
C. R. Barnes & S. H. Williams
Preliminary arcritarch and chitinozoan distributions across the Ordovician—Silurian
boundary stratotype at Dob’s Linn, Scotland ........................... G. M. Whelan
Ordovician—Silurian junctions in the Girvan District, S.W. Scotland ...D. A. T. Harper
Base of the Silurian in the Lake District and Howsgill Fells, Northern England
R. B. Rickards
The Ordovician-Silurian boundary at Keisley, Cumbria ................... A. D. Wright
Ordovician-Silurian boundary strata in Wales ..................22..eeeeee eee J. T. Temple
La Limite Ordovicien-Silurien en France .... C. Babin, R. Feist, M. Melou & F. Paris
The Ordovician-Silurian boundary in the Oslo region, Norway ........ L. R. M. Cocks
ast baltiCMRESIOMM ns sise.s0 sect seeneeecicees cee e natets D. Kaljo, H. Nestor & L. Pélma
The Ordovician-Silurian boundary in Poland ...................0.0....00eeeeeee L. Teller
The Ordovician-Silurian boundary in the Prague Basin, Bohemia .......... P. Storch
The Ordovician-—Silurian boundary in the Saxothuringian Zone of the Variscan Orogen
H. Jaeger
The Ordovician-Silurian boundary in the Carnic Alps of Austria ...... H. P. Schonlaub
ASIA:
The Ordovician-Silurian boundary in China ....................0000eeeeee ees Mu En-zhi
The Ordovician-Silurian boundary beds of the north-east U.S.S.R.
T. N. Koren, M. M. Oradovskaya & R. F. Sobolevskaya
The Ordovician-Silurian boundary in the Altai Mountains, U.S.S.R.
E. A. Yolkin, A. M. Obut & N. V. Sennikov
Nature of the Ordovician—Silurian boundary in south Kazakhstan, U.S.S.R.
M. K. Apollonov, T. N. Koren, I. F. Nikitin, L. M. Paletz & D. T. Tsai
The Ordovician-Silurian boundary in Saudi Arabia ...................... H. A. McClure
AFRICA AND AUSTRALASIA:
The Ordovician-Silurian boundary in Morocco............ J. Destombes & S. Willefert
The Ordovician-Silurian boundary in the Algerian Sahara................... P. Legrand
The Ordovician-Silurian boundary in Mauritania .........................05- S. Willefert
Ordovician-Silurian boundary in Victoria and New South Wales, Australia
A. H. M. Vandenberg & B. D. Webby
The base of the Silurian System in Tasmania ..................220e0eeeee eee M. R. Banks
AMERICA:
Stratigraphy and Palaeontology of the Ordovician-Silurian boundary interval,
Anticostiolslands @uebecs @amaday ernest sseleretele-leleielele eteleter= C. R. Barnes
Graptolites at and below the Ordovician-Silurian boundary on Anticosti Island,
(CANAD 354550000 s0008de neneaadoOdetbasbeccutanon seme HasepoeeaoonunEUunoDTnaapcmrrane J. Riva
ered, Oiieloe, (CAME .osoccenss vg duoc nb ocne coocospsaedaas coop padEboooE P. J. Lespérance
The Ordovician-Silurian boundary on Manitoulin Island, Ontario, Canada
C. R. Barnes & T. E. Bolton
Preliminary report on Ordovician-Silurian boundary rocks in the Interlake area,
Mianittobam Canad ammeeprrerceereree cere ec Gernecenim cc romances H. R. McCabe
nN
145
165
7/7)
183
191
195
211
239
247
255
CONTENTS
The Ordovician-Silurian boundary in the Rocky Mountains, Arctic Islands and
HudsonuPlatfonmsCanadauyeeereseseraseeo tea eee eee a eee cere B. S. Norford
Ordovician-Silurian boundary, northern Yukon, Canada
A. C. Lenz & A. D. McCracken
The Ordovician-Silurian boundary in the United States
S. M. Bergstrom & A. J. Boucot
The Ordovician-Silurian boundary in South America ....................... A. J. Boucot
The Ordovician-Silurian boundary in Bolivia and Argentina
A. Cuerda, R. B. Rickards & C. Cingolani
The Ordovician-Silurian boundary in the Sierra de Villicum, Argentine Precordillera
B. A. Baldis & E. D. Pothe de Baldis
Palaeobiology and Environmental changes
Late Ordovician and Early Silurian Acritarchs ...................6...0eeeeeeeeee F. Martin
Brachiopods across the Ordovician-Silurian boundary .................. L. R. M. Cocks
Chitinozoan stratigraphy in the Ashgill and Llandovery ....................... Y. Grahn
Conodont biostratigraphy of the Uppermost Ordovician and Lowermost Silurian
C. R. Barnes & S. M. Bergstrom
Graptolite faunas at the base of the Silurian ..........................000- R. B. Rickards
Land plant spores and the Ordovician-Silurian boundary ....................... J. Gray
TODOS Wes 8, aie. tenes Aen eee Ee OE CACM ARSE oN oOnIROE SS P. J. Lespérance
Environmental changes close to the Ordovician-Silurian boundary ..... P. J. Brenchley
Introduction
L. R. M. Cocks & R. B. Rickards
The base of the Silurian System was agreed by the I.U.G.S. Executive Committee in May 1985
(published June 1985 in Bassett 1985), and was taken at the base of the acuminatus Zone at
Dob’s Linn, Scotland (Cocks 1985).
This volume closely reflects the achievements of the Ordovician—-Silurian Boundary Working
Group from its formation in 1974 to its disbandment in 1985. A detailed account of the
activities of the Group is given in the next chapter, including the procedures followed which led
to the decision on the definition of the boundary. We have taken the opportunity to gather in
this book a global review of the Ordovician—Silurian boundary. These contributions are partly
based on submissions on places and fossil groups made during the lifetime of the Working
Groups and circulated by the Secretary, but these, if used in this volume, have been thoroughly
updated by the respective authors and their colleagues. In addition we have commissioned a
number of papers to give an overview of the many places where the boundary is exposed, as
well as others on the global analysis of sedimentary events, and the evolutionary progress of the
most important biological groups across the boundary.
It has always been clear from discussions that unanimous agreement would never be poss-
ible. Different countries have different traditions and philosophies, for example with respect to
stratigraphical principles. This is especially true of the concepts of zones, and of the utility of
zones for correlative purposes. For example, Mu (this volume) attempts a very detailed correla-
tion of what are regarded elsewhere as potential subdivisions of the acuminatus Zone, claiming
that an ascensus fauna underlies the acuminatus Zone (as it is, indeed, seen in China). But in
some of the most precisely and exhaustively collected sections, such as at Dob’s Linn, Scotland,
it seems clear that the two species appear more or less simultaneously, albeit with ascensus
more abundant low in the zone, and acuminatus more common in the upper part of the zone
and outlasting ascensus. Thus, whilst there is a case for locally subdividing an acuminatus Zone,
as Teller (1969) and others have sensibly done, it should be made clear that on current
information these subdivisions correlate in total with the acuminatus Zone at Dob’s Linn. In
sections where the record is perhaps not very complete, or the fauna not abundant, it may
appear that acuminatus follows ascensus.
Barnes (this volume) considers that, although the systemic boundary has now been fixed, its
‘reconsideration may be necessary’ (Lespérance et al. 1987). The main grounds for this opinion
are that the Anticosti sequence has a future potential for further studies; has all the attributes
for a boundary stratotype; and that ‘important stratigraphic principles have been disregarded
or overruled in making the final stratotype decision’. It cannot be overemphasized that the
procedures adopted by the Working Party Group throughout its life were correct, proper,
democratic, and always in accord with I.U.G:S. guidelines and with specific guidance from
LU.GS.
If some stratigraphical ideas have been disregarded or overruled, then a substantial majority
of the Working Group took the decisions to do so: the voting which took place is recorded in
the next section. ‘Potential’ is always a difficult commodity to evaluate: and the judged poten-
tial of a section cannot delay for ever what will always be arbitrary decisions in the end. By the
time a reconsideration was worked through (? ten years) another section would no doubt be
vying with Anticosti in terms of its potential. Where then?
That Anticosti has most of the attributes necessary for a boundary stratotype is beyond
question. That is why it was on a short list of two, voted upon by the Working Group. Other
sections were of an almost equally high standard, for example, in China and the Lake District
of England. But Anticosti does have one very serious drawback in any current discussions on
Bull. Br. Mus. nat. Hist. (Geol) 43: 5—7
6 COCKS & RICKARDS
correlations about the boundary, and that is its seemingly poor record of graptolites. It may be
that at some future time graptolites may be relatively demoted in value for correlative purposes,
but that time is still far away on present information. Dob’s Linn also has most of the attributes
of a boundary stratotype, and the Working Group, after eleven years of study, considered it
better than Anticosti. In fact, the boundary has now been certainly put at the correct level,
using the best group for correlation, the graptolites. Despite the fact that the Hirnantia brachio-
pod fauna is very often overlain by persculptus Zone graptolites, unequivocal evidence from
both Kazakhstan (Koren et al. this volume) and the Lake District of England (Cocks this
volume) shows that it also occurs rarely within the persculptus Zone. There is a strong feeling
amongst most biostratigraphers that they prefer to regard the Hirnantia fauna as Ordovician
rather than Silurian in age and not straddling the systematic boundary, and this assignment to
the Ordovician can be achieved only by a sub-acuminatus Zone boundary, as was eventually
decided.
A more interesting question is the precise age, in terms of graptolite zones, of the maximum
glacio-eustatic drop in sea level, and this is still not yet definitively answered although it was
probably about half way through the persculptus Zone—there are some well-dated persculptus
bearing post-glacial transgressive beds in parts of North Africa. On the other hand, the precise
duration and extent of the glacial episode (Fig. 1) certainly varied from place to place—
commencing even in late Caradoc and early Ashgill times in some parts of Gondwana, and
certainly continuing into post-Hirnantia fauna times, perhaps into the Rhuddanian, in others,
e.g. South Africa. It is also important to note that detailed investigation indicates that the ‘end
Hirnantia fauna
Mucronaspis
tillites etc.
Glacial directions
Fig. 1 Distribution of the latest Ordovician glacial deposits in Gondwana and adjacent areas (after
Cocks & Fortey 1988).
INTRODUCTION 7
Ordovician’ faunal extinctions were by no means synchronous. No other faunal or floral group
than graptolites yet approaches the sensitivity and exactness of the graptolites during the
period in question—for example from the mid-Ashgill (base of the Rawtheyan) to the end of the
early Llandovery (Rhuddanian) there are no fewer than eight graptolite zones, as compared
with three or four conodont zones, and four successive brachiopod faunas, three or four
ostacod faunas, three or four trilobite faunas etc. This, from a period of only perhaps 7 or 8
million years (McKerrow et al. 1985), makes the graptolites compare well with Mesozoic
ammonites or Tertiary foraminifera as a precise dating tool.
The coverage in this volume of the Ordovician-—Silurian sections themselves cannot be total
partly because several regions are little known. However, it is worth drawing attention here to
probable additional Ordovician—Silurian boundary sections in Libya (Klitzsch 1981), Burma
(Mitchell et al. 1977; Wolfart et al. 1984) and Greenland (e.g. Hurst & Kerr 1982; Surlyk &
Hurst 1984). In addition we are aware of preliminary work on strata about the boundary in
Vietnam, Thailand, Malaysia and other parts of SE Asia. In the instance of central Nevada,
U.S.A., we have not republished a revised preliminary submission because there is nothing yet
new to add to the work by Berry (1986). There is also further work in preparation on Scandina-
via.
We would like to end this introduction with a tribute to the many people involved, both as
members of the Working Group and as contributors to the present volume, who patiently took
part in the meetings, newsletter, activities and final decision-making, and thank them all for
their patience, support, good humour and international friendship; despite the controversy of
the eventual scientific conclusion.
References
Bassett, M. G. 1985. Towards a ‘Common Language’ in Stratigraphy. Episodes, Ottawa, 8: 87-92.
Berry, W. B. N. 1986. Stratigraphic significance of Glyptograptus persculptus group graptolites in central
Nevada, U.S.A. Spec. Publs geol. Soc. Lond. 20: 135-143.
Cocks, L. R. M. 1985. The Ordovician-Silurian boundary. Episodes, Ottawa, 8: 98-100.
—— & Fortey, R. A. 1988. Lower Palaeozoic facies and faunas round Gondwana. Geol. Soc. Lond. Spec.
Publ. (in press).
Hurst, J. M. & Kerr, J. W. 1982. Upper Ordovician to Silurian facies patterns in eastern Ellesmere Island
and western North Greenland and their bearing on the Nares Strait lineament. Meddel. om Grgn.
Geosci. 8: 137-145.
Klitzsch, E. 1981. Lower Palaeozoic rocks of Libya, Egypt, and Sudan. In Holland, C. H. (ed.), Lower
Palaeozoic of the Middle East, Eastern and Southern Africa, and Antarctica: 131-163. London.
Lespérance, P. J., Barnes, C. R., Berry, W. B. N., Boucot, A. J. & Mu En-zhi 1987. The Ordovician—
Silurian boundary stratotype: consequences of its approval by I.U.GS. Lethaia, Oslo, 20: 217-222.
McKerrow, W. S., Lambert, R. St.J. & Cocks, L. R. M. 1985. The Ordovician, Silurian and Devonian
periods. Mem. geol. Soc. Lond. 10: 73-80.
Mitchell, A. H. G., Marshall, T. R., Skinner, A. C., Baker, M. D., Amos, B. J. & Bateman, J. H. 1977.
Geology and exploration geochemistry of the Yadanatheingi and Kyaukme-Longtawkno areas. North-
ern Shan States, Burma. Overseas Geol. Miner. Resour., London, 51: 1-35, pls 1, 2.
Surlyk, F. & Hurst, J. M. 1984. The evolution of the early Palaeozoic deep-water basin of North
Greenland. Bull. geol. Soc. Am., New York, 95: 131-154.
Teller, L. 1969. The Silurian biostratigraphy of Poland based on graptolites. Acta geol. Pol., Warsaw, 19:
393-501.
Wolfart, R. et al. 1984. Stratigraphy of the Western Shan Massif, Burma. Geol. Jb., Hannover, (B) 57:
3-92.
December 1986
L. R. M. Cocks, Department of Palaeontology, British Museum (Natural History), Cromwell
Road, London SW7 SBD.
R. B. Rickards, Sedgwick Museum, Downing Street, Cambridge CB2 3EQ.
The Ordovician-Silurian Boundary and its
Working Group
L. R. M. Cocks
Department of Palaeontology, British Museum (Natural History), Cromwell Road,
London SW7 5BD
Synopsis
After a brief history of the study and definition of the Ordovician—Silurian boundary in the nineteenth and
early twentieth centuries, the process of setting up the Ordovician-Silurian Boundary Working Group is
described, together with its progress, publications and final decision-making during the period 1974-1985.
Both the Cambrian and the Silurian Systems were established as formal system names by
Sedgwick and Murchison respectively amicably enough in 1835, but during the next thirty
years it become clear that the upper part of Sedgwick’s Cambrian occupied the same time and
space as the lower part of Murchison’s Silurian. It was not until after the deaths of both men
that Charles Lapworth in 1879 established the Ordovician System to occupy the chief overlap-
ping ground between the older part of the Silurian and the younger part of the Cambrian. In
contrast to the rather generalized earlier definitions of the boundaries of the Cambrian and
Silurian, Lapworth’s definition of the limits of the Ordovician was admirably precise: he
defined the new Ordovician System as the ‘strata included between the base of the Lower
Llandovery formation and that of the Lower Arenig’ (Lapworth 1879: 14). There were subse-
quently problems (which are still not entirely resolved today) about the position and interna-
tional correlation of the ‘base of the Lower Arenig’, but these are the province of the
Cambro—Ordovician Boundary Working Group and will not be further discussed here. “The
base of the Lower Llandovery’ has been much less ambiguous, and thus in general any dispute
surrounding the definition of the Ordovician—Silurian boundary has always been of a much
lesser magnitude than the problems of the Cambro—Ordovician and the Siluro—Devonian
boundaries.
From the time of Murchison onward, ‘the base of the Lower Llandovery was defined
primarily in terms of shelly facies and without much precision, and usually recognized by the
incoming of various pentameride brachiopods such as Stricklandia. However, following Charles
Lapworth’s classic work on the Ordovician and Silurian rocks of Scotland in the period 1870
to 1880, it became clear that the best national and international correlation tool in rocks of
those ages was the sequence of graptolite zones, and these zones were subsequently used in
practice, with Lapworth himself, and subsequently the great graptolite monograph of Elles &
Wood (1901-1918), using the acuminatus Zone (type locality Dob’s Linn, Scotland) as the de
facto base of the Silurian. The acuminatus Zone was poorly developed as such in Wales, and so
Jones (1909) erected the persculptus Zone (type locality Pont Erwyd, Wales), which was subse-
quently realised to be of the same age as the lower part of Lapworth’s broad acuminatus Zone
in Scotland. Thereafter most stratigraphers treated the persculptus Zone as the base of the
Silurian, e.g. in the Lexique stratigraphique international (Whittard 1961), and this horizon was
also taken as the base of the Silurian by Cocks et al. (1970) when they erected stages for the
Llandovery Series, with a basal boundary defined at Dob’s Linn.
It was probably the identification of the problems surrounding the Silurian—Devonian
boundary and their subsequent illumination and solution that gave impetus to the internation-
al effort and will to define properly the exact horizon and identify a type locality for the various
systemic divisions of the Phanerozoic. The Siluro-Devonian Boundary Working Group
worked formally between 1960 and 1972 (Martinsson 1977), but that work was preceded by a
period of uncertainty, during which some of the procedures within the International Geological
Congresses and the International Union of Geological Sciences were being developed.
Bull. Br. Mus. nat. Hist. (Geol) 43: 9-15 Issued 28 April 1988
10 L. R. M. COCKS
And so it was during the Ordovician—Silurian symposium at Brest, France, in 1971 that
Claude Babin was the first to identify vocally the need for a group to be formally established to
investigate and stabilize the Ordovician—Silurian boundary. This was put to the nascent Com-
mission on Stratigraphy at the International Geological Congress in Montreal, Canada, in
1972, who felt that such a boundary working group should be established not by that commis-
sion directly, but at a suitable international meeting and through the joint coordination of the
then proposed Ordovician and Silurian Subcommissions. These last two bodies were finally
established at the Ordovician Symposium at Birmingham, England, in September 1974, and
one of their first acts was to arrange the initial meeting of the Ordovician—Silurian Boundary
Working Group, which first met at Birmingham on 19th September 1974. Those present at that
meeting were C. Babin (France), C. R. Barnes (Canada), S. M. Bergstrom (USA), A. J. Boucot
(USA), L. R. M. Cocks (UK), J. Destombes (Morocco), J. K. Ingham (UK), V. Jaanusson
(Sweden), P. J. Lespérance (Canada), D. J. McLaren (Canada), L. Marek (Czechoslovakia),
F. Martin (Belgium), R. B. Rickards (UK), P. Sartenaer (Belgium), N. Spjeldnaes (Denmark),
L. Teller (Poland), J. T. Temple (UK) and E. A. Yolkin (USSR). It was decided that 6 voting
members of the Working Group should be nominated by both the Ordovician and the Silurian
Subcommissions, plus their two chairmen ex officio, and that 3 voting members from the USSR
and | from Czechoslovakia should be nominated by their respective academies of science. Thus
the Ordovician Subcommission nominated Barnes, Bergstrom, W. B. N. Berry (USA), Des-
tombes, Ingham and Jaanusson, with A. Williams (UK) ex officio as their Chairman, and the
Silurian Subcommission nominated Boucot, Cocks, S. Laufeld (Sweden), Lesperance, Rickards
and Temple, with Spjeldnaes ex officio as their Chairman. Any interested and active worker on
Ordovician-Silurian boundary problems could be accepted as a Corresponding Member. At
that first meeting R. B. Rickards was elected by those present as the Chairman of the Working
Group, and L. R. M. Cocks as the Secretary. It was also decided that most of the Group’s
activities and communication would take the form of circulars to be issued by the Secretary,
and this is what subsequently happened, although field and discussion meetings also took
place, and that the circulars should include reports on various Ordovician-—Silurian sections or
countries and also on the different fossil groups. The first circular was issued in October 1974:
it reported the formation of the Working Group, and listed which members had promised to
prepare reports.
In the next few years many circulars were issued, which included reports on boundary
sections in Australia, Austria, Belgium, Canada (many areas), China, Czechoslovakia, England,
France, Italy, Morocco, Poland, Scotland, Sweden, Wales, USA and USSR (Altai Mountains,
East Baltic, Kazakhstan and NE Siberia), and also on acritarchs, chitinozoa, conodonts, grap-
tolites and physical changes near the boundary. Many people became Corresponding
Members, and the Voting Members were increased by D. L. Kaljo, T. N. Koren and I. F.
Nikitin from the USSR, L. Marek from Czechoslovakia and Mu En-zhi from China, all of these
nominations being accepted and ratified at the appropriate times by the I.U.G.S. Commission
on Stratigraphy, the parent body of the Working Group. Meetings were held at the Interna-
tional Geological Congress at Sydney, Australia, in August 1976 and informal meetings at
Alma-Ata, USSR, in May 1977 and at the Ordovician Symposium at Columbus, USA, in
August 1977, and it became clear that a more substantial meeting of the Working Group would
be valuable so that future plans of action could be formulated. This coincided with an
expressed wish by various geologists to see the classic sections of Great Britain, and accord-
ingly a meeting was arranged from 30th March to 11th April 1979, jointly with the Silurian
Subcommission. By that time R. J. Ross jr and C. H. Holland had taken over the chairman-
ships of the Ordovician and Silurian Subcommissions respectively.
Those attending the British meeting in 1979 were (Voting Members of the Ordovician—
Silurian Boundary Working Group with an asterisk): *C. R. Barnes (Canada), M. G. Bassett
(UK), *L. R. M. Cocks (UK), *C. H. Holland (Ireland), *J. K. Ingham (UK), J. S. Jell
(Australia), Jin Chun-tai (China), *D. L. Kaljo (USSR), P. Legrand (France), *P. J. Lespérance
(Canada), Lin Bao-yu (China), F. Martin (Belgium), A. Martinsson (Sweden), *Mu En-zhi
(China), *R. B. Rickards (UK), H.-P. Schénlaub (Austria), B. S. Sokolov (USSR), L. Teller
THE ORDOVICIAN-SILURIAN BOUNDARY WORKING GROUP 11
Fig. 1 The British field meeting, April 1979, outside Ludlow Castle, Shropshire. From left to right
L. R. M. Cocks, Jin Chun-tai (obscured), B. D. Webby, C. R. Barnes, J. S. Jell, Wang Wei, Lin
Bao-yu (obscured), D. Kaljo, Mu En-zhi, D. J. Siveter, F. Martin (obscured), L. Teller, P. J.
Lespérance (obscured), D. E. White, A. Martinsson, B. S. Sokolov, P. Legrand, J. T. Temple, H. P.
Schonlaub, M. G. Bassett, R. B. Rickards. (Photo C. H. Holland).
(Poland), *J. T. Temple (UK), G. B. Vai (Italy) and B. D. Webby (Australia). Thus more than
half the Voting Members and a considerable breadth of both stratigraphical and palaeontol-
ogical expertise were represented (Fig. 1). Sections were examined in Wales (Llandovery,
Meifod, Hirnant and Pont Erwyd), the Lake District of England (Yewdale, Skelgill and
Spengill), and Scotland (Dob’s Linn), but, more importantly, business meetings were held in the
evenings. Following a long-standing tradition of the Commission on Stratigraphy (whose then
Chairman, Martinsson, and Secretary, Bassett, were present) all of the people present were
allowed to participate freely in the discussions and also to take part in the informal voting
which took place.
The various animal and plant groups were discussed and reviewed in turn, and it was agreed
that only graptolites, brachiopods, conodonts, and to a lesser extent trilobites, were important
in the Ordovician-Silurian boundary discussions in the present state of knowledge. Localities
were then considered. Having inspected the type Llandovery area, all members present were
unanimous in rejecting that area as the boundary stratotype, large due to the unfossiliferous
nature of the A, Sandstone of Jones (1928) at the base of the succession, the sporadic exposure
near the base, and the lack of stratigraphically critical fossils, particularly graptolites and
conodonts, then known from beds near the boundary (although this situation has been much
improved by subsequent work, Cocks et al. 1984). Other localities were graded in turn, with the
following scheme: A, a possible section for placing the boundary; B, an important section
which may be considered further in discussing the boundary, and C, a section or area unlikely
to prove important in boundary definition. The grading was as follows:
A Anticosti Island (Canada), Dob’s Linn (Scotland). . tag’
B_ Carnic Alps (Austria), Cornwallis Island (Canada); Hupei (China), Mirnyi Creek (Siberia,
2 L. R. M. COCKS
USSR), Missouri (USA), Nevada (USA), Pont Erwyd (Wales), Szechuan (China) and Yewdale
Beck (Lake District, England).
C Australia, Bala district (including Hirnant area, Wales), Belgium, Bohemia
(Czechoslovakia), France, Garth (Wales), Hudson Platform (Canada), Kazakhstan (USSR),
Kweichow (China), Lake District (apart from Yewdale Beck, England), Manitoba (Canada),
Manitoulin Island (Canada), Morocco, Newfoundland (Canada), North American mid-
continent (except Missouri and Nevada), Pembrokeshire (Wales), Perce (Canada), Poland,
Scania (Sweden), Shensi (China) and Yukon (Canada).
In addition the Working Group then felt that more reports were needed from Algeria,
Bornholm (Sweden), Burma, Dalarna and Vasterg6étland (Sweden), Estonia (USSR), India and
the Himalayas, Norway, Rae Grain (Scotland), Portugal, South America, Spain and West
Nevada: however, although more data on some of these areas were subsequently gathered,
none proved to have much extra to offer in the main definition of a stratotype. Because
Anticosti Island, Canada, was one of the leading contenders for the definitive boundary section,
it was agreed that a further field meeting should be held there. Other briefer meetings were also
held in Paris, France, during the 1980 International Geological Congress, and in the Carnic
Alps of Austria in late July and early August 1980. Meanwhile the debate persisted as to the
best method of correlation across the boundary interval, and whether the actual boundary
should be defined by the use of conodonts or graptolites. It was generally agreed that brachio-
pods and trilobites should not be used in the definition, except that there was a strong feeling
that the widespread Hirnantia brachiopod fauna should be included within the Ordovician
rather than the Silurian.
The Working Group circulars also contained various discussion and position papers between
1978 and 1982 on the theory and practice of defining the boundary both geographically and
biostatigraphically. Opinions differed as to whether or not the stratotype could be satisfactorily
placed within a nearly exclusively graptolite sequence such as Dob’s Linn, and, if the boundary
was to be defined on graptolites, whether it was to be at the base of the extraordinarius, the
persculptus or the acuminatus Zone. There was no real consensus on the answers to these
questions.
The field meeting to Quebec, which was partly in Anticosti Island and partly in the Gaspé
Peninsula, was held in July 1981, again jointly with the Silurian Subcommission. Those attend-
ing (apart from various other Canadian hosts) were T. W. Amsden (USA), *C. R. Barnes
(Canada), *A. J. Boucot (USA), *L. R. M. Cocks (UK), *C. H. Holland (Ireland), P. Legrand
(France), *P. J. Lespérance (Canada), F. Martin (Belgium), G. M. Philip (Australia), *R. J. Ross
jr (USA), H.-P. Schonlaub (Austria) and L. Teller (Poland). This was a rather disappointing
attendance, particularly of Voting Members, and hence the evening discussion meetings were
not as representative of the differing positions of the complete group as they might have been if
the attendance had been better. A review was given of each of the relevant biological groups,
and general discussions ensued, with the following points noted. There were very favourable
general impressions of the simplicity of structure and good exposure at Anticosti, but reser-
vations on the lack of graptolites there near the Ordovician—Silurian boundary and the relative
lack of work done on groups other than conodonts on the beds near the boundary. Opinions
differed about the accessibility of Anticosti Island and also about the importance of the struc-
tural complexity of the Dob’s Linn area. At the end of the meeting, two straw votes indicated
that those present thought that Anticosti was the best available section across the Ordovician—
Silurian boundary in the shelly facies, and that, other things being equal, it would be preferable
to have the Ordovocian-Silurian boundary stratotype in the same area as the stratotype area
for the lowest series of the Silurian System. The latter point was relevant since at that time
Anticosti was one of the three candidates under consideration by the Silurian Subcommission
(the other two being Llandovery itself and the Oslo Region, Norway) for the stratotype for the
lowest Silurian series. Shortly after this meeting, R. B. Rickards resigned as Chairman of the
Working Group, and, because it was clear that the decisions on the boundary were close to
being taken, the Commission on Stratigraphy subsequently appointed the Chairmen of the
THE ORDOVICIAN-SILURIAN BOUNDARY WORKING GROUP 13
Ordovician and Silurian Subcommissions, R. J. Ross jr and C. H. Holland, as Co-Chairmen of
the Group; which they remained until its closure.
After the formal circulation of a number of further views on the position and correlation of
the future boundary stratotype through the Circular, and informal discussion between inter-
ested people, it was agreed that maximum publicity and attendance should be sought for a
meeting of the Working Group at the Ordovician Symposium at Oslo, Norway, so that
progress would be made on the boundary decision. At that symposium, two meetings of the
boundary Working Group were held, as well as seven papers on the boundary being presented
within the normal symposium sessions. The meetings, on 20th and 23rd August 1982, attracted
53 and 76 people respectively, including the following Voting Members: Barnes, Bergstrom,
Berry, Cocks, Destombes, Holland, Jaanusson, Kaljo, Lespérance, Rickards and Ross. After
lengthy discussion, the first decision taken was whether or not the time was yet ripe for a
formal vote on deciding the boundary stratotype and horizons, and, despite strong pleas for
delays to enable more research to be done from several speakers, it was decided by 47 votes to
14 that the time had now come. The choice of stratotype boundary had been narrowed to
three:
(i) the first appearance of the conodont Ozarkodina oldhamensis at 50cms above the Oncolitic
Platform Bed at Ellis Bay, Anticosti Island, Canada.
(11) the base of the persculptus graptolite Zone at Dob’s Linn, near Moffat, Scotland.
(iii) the base of the acuminatus graptolite Zone at Dob’s Linn.
At the Oslo meeting two informal votes were then taken: (i) Anticosti was preferred to the
persculptus Zone at Dob’s Linn by 34 votes to 13, with 25 abstentions; (ii) Anticosti was
preferred to the acuminatus Zone at Dob’s Linn by 35 votes to 13, with 26 abstentions. The
same questions were also informally voted upon by the 30 Voting and Corresponding Members
of the Working Group who were present, and 17 preferred Anticosti against 7 for the per-
sculptus Zone (6 abstentions); and 19 preferred Anticosti against 5 for the acuminatus Zone (6
abstentions). Therefore, it was clear that a substantial majority of those at the meeting then
preferred to place the base of the Silurian at Anticosti Island using conodonts, and that the
Voting Members of the Working Group should take part in a formal postal ballot in the light
of this knowledge. Thus Circular No 17 was distributed to the members in October 1982 with a
ballot paper to be returned by the end of January 1983. There followed a period during which
various letters were informally circulated and lobbying took place, although none formally
through the Secretary apart from a paper by P. Legrand which was very critical of the Oslo
decision and which was distributed with Circular 17.
At the end of the formal voting period, the votes returned stood as follows:
(i) Which do you prefer—Anticosti or the persculptus Zone at Dob’s Linn?
For Anticosti: Barnes, Bergstrom, Boucot, Holland, Lespérance, Ross: total 6.
For persculptus Zone: Berry, Cocks, Destombes, Ingham, Kaljo, Koren, Laufeld, Marek,
Nikitin, Rickards, Temple: total 11.
No vote received: Jaanusson, Mu: total 2.
(iu) Which do you prefer—Anticosti or the acuminatus Zone at Dob’s Linn?
The votes received were identical to the persculptus Zone vote.
These results were distributed to all members of the Working Group in Circular 18 in March
1983. Since there had been an outright majority on the selection of Dob’s Linn rather than
Anticosti, this was accepted by the officers as a decision, and a second formal postal vote was
called for, firstly to give Voting Members an opportunity to change their minds, and secondly
to decide between the persculptus and the acuminatus Zones at Dob’s Linn for the stratotype
horizon. Opportunity was also given to the Corresponding Members to formally express their
opinions. The results of this second ballot was announced in Circular No. 19 in August 1983,
and were as follows:
(i) the place of the stratotype.
Voting Members. Dob’s Linn: Berry, Cocks, Destombes, Holland, Ingham, Kaljo, Koren,
14 L. R. M. COCKS
Laufeld, Marek, Nikitin, Rickards, Temple: total 12. Anticosti: Barnes, Bergstrom, Boucot,
Lespérance, Ross: total 5. Abstain: Jaanusson, Mu: total 2. In addition 14 Corresponding
Members voted for Dob’s Linn, 8 for Anticosti, and 4 abstained.
(ii) the horizon of the stratotype.
Voting Members. Base of acuminatus Zone: Cocks, Holland, Ingham, Jaanusson, Kaljo,
Koren, Marek, Nikitin, Rickards, Temple: total 10. Base of persculptus Zone: Berry, Des-
tombes, Laufeld, Mu, Ross: total 5. Abstain: Barnes, Bergstrom, Boucot, Lespérance: total 4.
13 Corresponding Members voted for the base of the acuminatus Zone, 9 for the base of the
persculptus Zone, and 5 abstained.
In addition the question of possible parastratotypes was also voted upon, with the possibility
of erecting one parastratotype on Anticosti Island and the other in China, but on this question
only 8 Voting Members voted for the erection of these, with 3 against and 8 abstentions, and so
the officers decided not to proceed further on that topic, and they were assisted in that decision
by informal advice against parastratotypes from the Commission on Stratigraphy.
Thus since there was a clear majority for placing the Ordovician-Silurian stratotype bound-
ary at the base of the acuminatus graptolite Zone at Dob’s Linn, Scotland, this decision was
formally forwarded to the Commission on Stratigraphy for consideration with various other
matters at their meeting at the International Geological Congress at Moscow, USSR in August
1984. The decision was endorsed by a postal vote of that committee, who subsequently for-
warded it to the I.U.G:S. for ratification. The proposals were reported to a meeting of the full
I.U.G.S. Executive Committee in Rabat, Morocco, in February 1985 and submitted to the
I.U.G.S. Executive for a postal ballot, whose result was declared in May 1985, and published in
June 1985 (Bassett 1985), together with an article describing the Ordovician—Silurian boundary
at Dob’s Linn (Cocks 1985). The Ordovician—Silurian Boundary Working Group was finally
dissolved in its Circular No. 20, distributed in June 1985.
The life of the Ordovician—Silurian Boundary Working Group was therefore somewhat over
ten years long, but it was useful not only in determining the position and horizon of the
boundary itself, but also in stimulating a great deal of research in various parts of the world,
and in encouraging international understanding and cooperation.
References
Bassett, M. G. 1985. Towards a ‘Common Language’ in Stratigraphy. Episodes, Ottawa, 8: 87-92.
Cocks, L. R. M. 1985. The Ordovician-Silurian Boundary. Episodes, Ottawa, 8: 98-100.
, Toghill, P. & Ziegler, A. M. 1970. Stage names within the Llandovery Series. Geol. Mag., Cam-
bridge, 107: 79-87.
, Woodcock, N. H., Rickards, R. B., Temple, J. T. & Lane, P. D. 1984. The Llandovery Series of the
type area. Bull. Br. Mus. nat. Hist., London, (Geol.) 38 (3): 131-182.
Elles, G. L. & Wood, E. M. R. 1901-18. A monograph of British Graptolites. Palaeontogr. Soc. (Monogr.),
London. m + clxxi + 539 pp., 52 pls.
Jones, O. T. 1909. The Hartfell-Valentian succession in the district around Plynlimon and Pont Erwyd
(North Cardiganshire). Q. Jl geol. Soc. Lond. 65: 463-537, pls 1, 2.
Lapworth, C. 1879. On the tripartite classification of the Lower Palaeozoic Rocks. Geol. Mag., London,
(dec. 2) 6: 1-15.
Martinsson, A. (ed.) 1977. The Silurian—Devonian Boundary. Int. Un. geol. Sci. (A) 5: 1-349.
Whittard, W. F. (ed.) 1961. Lexique Stratigraphique International 1 Europe. (3aV: Angleterre, Pays de
Galles, Ecosse; Silurien.) 273 pp. Paris, C.N.R.S.
THE ORDOVICIAN-SILURIAN BOUNDARY WORKING GROUP 15
Appendix
MEMBERSHIP OF THE ORDOVICIAN-SILURIAN
BOUNDARY WORKING GROUP
Those names with an asterisk* were Voting Members, the remainder were Corresponding
Members.
Amsden, T. W. USA Lin Bao-yu China
Apollonov, M. K. USSR *Marek, L. Czechoslovakia
Babin, C. France Martin, F. Belgium
*Barnes, C. R. Canada Martinsson, A. Sweden
Bassett, M. G. UK McLaren, D. J. Canada
Bergstrom, J. Sweden *Mu En-zhi China
*Bergstrom, S. USA *Nikitin, I. F. USSR
*Berry, W. B. N. USA Norford, B. S. Canada
Bolton, T. E. Canada Nowlan, G. S. Canada
*Boucot, A. J. USA Oradovskaya, M. M. USSR
Brenchley, P. J. UK Poulsen, V. Denmark
Bruton, D. L. Norway *Rickards, R. B. UK
*Cocks, L. R. M. UK Rong Jia-yu China
Cramer, F. H. Spain *Ross, R. J. jr USA
*Destombes, J. Morocco Sartenaer, P. J. M. J. Belgium
Hamada, T. Japan Schonlaub, H. P. Austria
*Holland, C. H. Ireland Sheehan, P. M. USA
*Ingham, J. K. UK Sokolov, B. S. USSR
*Jaanusson, V. Sweden Spjeldnaes, N. Denmark
Jackson, D. E. UK Teller, L. Poland
Jaeger, H. East Germany *Temple, J. T. UK
Jin Chun-tai China Toghill, P. UK
*Kaljo, D. USSR Wang Xiao-feng China
Kobayashi, T. Japan Webby, B. D. Australia
*Koren, T. N. USSR Williams, A. UK
*Laufeld, S. Sweden Williams, S. H. UK
Legrand, P. France Wright, A. D. UK
Lenz, A. C. Canada Yolkin, E. A. USSR
*Lespérance, P. J. Canada
Dob’s Linn — the Ordovician-Silurian Boundary
Stratotype
S. H. Williams
Department of Earth Sciences, Memorial University of Newfoundland, St John’s,
Newfoundland A1B 3X5, Canada
Synopsis
Dob’s Linn, north-east of Moffat, southern Scotland, has been designated the Ordovician-Silurian bound-
ary stratotype by the Ordovician-Silurian Boundary Working Group of the I.U.G.S. Commission on
Stratigraphy. The boundary is placed at the base of the P. acuminatus Zone, marked by the first
occurrence of Akidograptus ascensus and Parakidograptus acuminatus, s.l., 1-6m above the base of the
Birkhill Shale in the Linn Branch section.
The stratigraphical interval covering this boundary consists of richly graptolitic black shale. Occasional
metabentonites are also present. The underlying Upper Hartfell Shale is composed predominantly of pale
grey-green, non-graptolitic shale and mudstone, with several black graptolitic bands referred to the
Complanatus, Anceps and Extraordinarius Bands. The rich faunal assemblage of the Anceps Band reduces
to only three diplograptid taxa in the Extraordinarius Band. This major extinction is recorded at an
equivalent horizon by all other graptolitic sequences throughout the globe.
Sediments of the Upper Ordovician to Lower Silurian Moffat Shale Group were probably deposited
entirely by distal turbidites in the abyssal depths of the Iapetus Ocean. Northerly-directed subduction
subsequently transported the site of shale deposition into a proximal turbidite environment, resulting in a
diachronous transition into coarse clastics of the overlying Gala Greywacke Group. Deformation related
to subduction also produced imbricate thrusting and raised the area to a prehnite-pumpellyite facies
metamorphic grade. Geophysical evidence indicates that the region is underlain by continental basement;
this suggests that the Southern Uplands are allochthonous.
Historical introduction
Graptolites were first recorded from Dob’s Linn, southern Scotland over one hundred years
ago. The earliest publications to include descriptions of the fauna from the Moffat Shales (e.g.
Carruthers 1858; Nichoison 1867; Dairon 1869; Hopkinson 1871) paid little or no attention to
their stratigraphical importance. Elsewhere during this period, an ever increasing volume of
articles on graptolites was being published, including a number which recognized their great
potential for both regional and global correlation (Nicholson 1876). These included studies on
the Lower Ordovician of northern England (e.g. Nicholson 1870, 1875), South Wales
(Hopkinson & Lapworth 1875) and eastern Canada (Hall 1858, 1865; Billings 1865).
In 1864 Charles Lapworth obtained a teaching post connected with the Episcopal Church at
Galashiels some 30km north-east of Dob’s Linn (Gibson 1921). He had no previous geological
training or experience, but soon developed an interest in the local geology of the Southern
Uplands. Harkness (1851) had described the repeated, faulted nature of this area composed of
thick greywacke and containing a shale sequence termed the “Moffat Series’. Otherwise this
structurally complex region still defied satisfactory interpretation despite attempts by several
other eminent geologists (e.g. Sedgwick 1850).
Lapworth’s first publication on the Lower Palaeozoic (1870) concerned the geology of the
Galashiels area. During these early years in his geological career, he recorded graptolites both
from within the thick greywacke sequence of the Southern Uplands and from the underlying
black shales. A summary of Lapworth’s early lithostratigraphical division was published in
1872. During the next five years he completed an exercise of detailed geological mapping,
logging of sections and bed-by-bed faunal collecting throughout the Moffat area. A selection of
new graptolite taxa were figured and discussed briefly in 1876, while similar faunas were also
illustrated from equivalent strata in northern Ireland (Lapworth 1877).
Bull. Br. Mus. nat. Hist. (Geol) 43: 17-30 Issued 28 April 1988
18 S. H. WILLIAMS
Lapworth’s major stratigraphical synthesis ‘On the Moffat Series’ was published in 1878,
where he established beyond doubt the precise, ordered stratigraphical change in graptolite
assemblages through the sequence of black and grey shales. With the exception of the Glenkiln
Shale (best developed at Glenkiln Burn, south-east of Moffat) and the lowermost portion of the
Lower Hartfell Shale (best exposed at Hartfell Spa, north of Moffat), Lapworth used the Main
Cliff and Linn Branch sections of Dob’s Linn as the standard reference for the Moffat Shale.
While working at Dob’s Linn, Lapworth stayed at Birkhill Cottage only a few hundred metres
above the locality. The uppermost black shale division of the group was named after this
cottage.
Lithological sections measured at Dob’s Linn, together with graptolite assemblages and
biostratigraphical divisions, were figured by Lapworth (1878: figs 27-30) in his major work,
where the lithostratigraphical division of the Moffat Shale was also clearly defined. An earlier,
less detailed log of the Moffat Shale from Lapworth’s notes, covering the Glenkiln Shale to
basal Birkhill Shale, is still preserved in Birmingham University, and is here illustrated for
comparison (Fig. 1). Note that Lapworth’s assignment of the lower part of the Upper Hartfell
Shale to the “Belcraig Shale’ (after Beldcraig Burn near Moffat) was apparently never published.
During this research, Lapworth was appointed in 1875 to an Assistant Mastership at Madras
College, St Andrews. In 1881 he was elected to the Chair of Geology at the recently established
Mason College, Birmingham, which subsequently became the University of Birmingham. In
addition to elucidating the structure of the Southern Uplands, Lapworth established the Ordo-
vician System in 1879, solving the embittered feud between the schools of Murchison and
Sedgwick (see Bassett 1985). He also made an equally painstaking, detailed stratigraphical
study at Girvan on the south-west coast of Scotland (Lapworth 1882). His conclusions regard-
Adbef,
wh
Cie (tore
« pe, Ak
a thierry thal, WSS a niarey blk bat beck eth
Ce sadted fase waned foes pe fv C1 1645 Co hone
: a Vg. exraest, pha
Puccll sreeps
P. Mack auksy,
2 bcheedgta ree ie gaye Meee.
Graptolite zone
C. vesiculosus
6 Marte that 01 yeler
P. linearis
ftfb0 Rh Fliaye 3 Alles: hard Mean by Mali
LOWER
SAL O- clingani
Sadao: 3 torcharpiricre”
a ON
ln lellid Muu retbet Bach Hhrle
Diesyan: rarriasis
i
E Extraordinarius Band
A Anceps Bands
C Complanatus Bands
Fig. 1 Reproduction of an unpublished section through Moffat Shale from Lapworth’s notebook
(preserved at Birmingham University), predating his modified version published in 1878. Modern
measured section with graptolite zones is included for comparison.
THE ORDOVICIAN-SILURIAN BOUNDARY STRATOTY PE 19
ing the relationship between the strata at Girvan and those of the Southern Uplands were
published in 1889.
The work of Lapworth was drawn upon heavily by Peach & Horne (1899), who also
described many confirmatory sections through the Moffat Shale in the Southern Uplands. With
the exception of one taxonomic paper published by Lapworth in 1880, most of his new
graptolite taxa were first described fully by Elles & Wood (1901-18), whose work he supervised
throughout its production. Following this major publication, little taxonomic or stratigraphical
work was attempted at Dob’s Linn for half a century. One notable exception was an article by
Davies (1929), who included Dob’s Linn in his detailed study of late Ordovician and early
Silurian graptolites.
A series of recent biostratigraphical and taxonomic papers was initiated by Packham (1962),
who described the evolution of Glyptograptus tamariscus and related diplograptids from the
Birkhill Shale of Dob’s Linn and from the Lower Silurian of the Rheidol Gorge, mid Wales.
Toghill (1968) discussed the evolution of the earliest monograptids and formally established the
presence of the G. persculptus Zone at Dob’s Linn. He also gave a biostratigraphical summary
of the entire Birkhill Shale with listings of the zonal assemblages (1968a), but none of the fauna
was described or illustrated.
Toghill (1970) subsequently published a revision of graptolites from the Upper Hartfell Shale
and top Lower Hartfell Shale. Brief taxonomic descriptions and illustrations were included; this
paper added little in terms of refinement to Lapworth’s biostratigraphical divisions, but was
important in demonstrating the presence of previously unrecorded graptolite species. Cocks et
al. (1970) used this data to make a premature proposal of Dob’s Linn as the Ordovician—
Silurian boundary stratotype, where they placed the boundary at the base of the Birkhill Shale
(= ‘base’ of G. persculptus Zone). Rickards (1979) added the record of the C.? extraordinarius
Zone, based on the discovery by Ingham (1979) of a black, graptolitic shale band midway
between the top Anceps Band of the Upper Hartfell Shale and the basal Birkhill Shale.
A geological locality map of Dob’s Linn was given by Toghill (19684), but most geologists
visiting the locality were still guided by the remarkably detailed geological map published by
Lapworth in 1878. Following several years’ critical work using aerial and ground photographic
overlays and modern structural synthesis, Ingham (1979) published a totally revised geological
map of Dob’s Linn. During the course of this research, Ingham found that several of Toghill’s
measured sections through the Upper Hartfell and lowermost Birkhill Shales were disrupted
structurally and measurements were consequently revised. Ingham’s recognition of unbroken
sections and several new graptolitic bands in the Upper Hartfell Shale (upper Complanatus
Band, Anceps Band A and the Extraordinarius Band), permitted critical faunal recollection of
the Moffat Shale. This task was begun by the present author in 1978, leading to a series of
taxonomic and biostratigraphical papers covering the top 8m of the Lower Hartfell Shale
(Williams 1982a), the Complanatus Bands (Williams & Ingham, in prep.), the Anceps Bands
(Williams 1982), the Extraordinarius Band and the basal 2 m of Birkhill Shale (Williams 1983).
These papers confirmed Lapworth’s original faith in graptolites as a critical bio-
stratigraphical tool and gave more precise definitions of the zonal boundaries. Of particular
importance is the revised, unambiguous definition of the boundary between the G. persculptus
and P. acuminatus Zones, the horizon now defined as the Ordovician—Silurian boundary.
Regional setting, stratigraphy and depositional environment
The geology of the Southern Uplands is dominated by a thick package of monotonous, sparse-
ly graptolitic greywackes, belonging to the Gala Greywacke Group. This is of unequivocal
turbidite origin. The underlying Moffat Shale Group is exposed as a series of elongate, narrow,
east-west inliers which Lapworth (1878) and Peach & Horne (1899) considered to represent
tight, isoclinal anticlines. It is now considered (Webb 1983) that these structures were formed
through progressive shearing of early folds. The appearance of simple, reverse faulting postu-
lated by Craig & Walton (1959), Leggett et al. (1979), Eales (1979) and other recent workers is
due to almost complete removal of the shorter, south-eastern limbs.
20 S. H. WILLIAMS
An overall younging and progressive lateral change in lithologies from north to south was
recognized in the Southern Uplands by Peach & Horne (1899). They divided the regions into
three tracts, namely the Northern, Central and Southern Belts. The rock types and age ranges
of strata characterizing each belt have since been summarized in detail by Leggett et al. (1979),
who considered division into ten discrete sequences to be more appropriate. In the most
northerly sequences red cherts, siliceous mudstones and pillow basalts of Arenig to Llandeilo
age are overlain by Llandeilo-Caradoc greywackes. This succession passes southwards to
Llandeilo—Llandovery cherts and black shales overlain by Llandovery greywackes. The diach-
ronous base of the greywackes youngs progressively to the south, with consequently extended
black shale deposition. The most southerly sequences of the Southern Uplands are composed
entirely of Wenlock greywackes.
Both the structural pattern of the Moffat Shale outcrops and the diachronous base of the
greywackes were explained in a model proposed by Mitchell & McKerrow (1975) and expand-
ed by McKerrow et al. (1977) and Leggett et al. (1979). These authors considered the Southern
Uplands to have formed as an accretionary prism over a northerly dipping subduction zone on
the northern margin of the Iapetus Ocean. The prehnite-pumpellyite metamorphic facies could
have resulted from burial and tectonic processes during such accretion (Oliver et al. 1984).
Geophysical studies (Powell 1971; Hall et al. 1983), however, indicate crystalline, continental
material underlying the area, rather than the oceanic basement required for this model. Bluck
(1984) discussed this apparently contradictory evidence; he concluded that the Southern
Uplands are probably allochthonous. More recently, Needham & Knipe (1986) reiterated the
accretionary prism model, but this was considered inadequate by Murphy & Hutton (1986),
who concluded that subduction at both Iapetus margins was complete by late Ordovician times
and that the Silurian turbidites were deposited in a successor basin.
The Moffat Shale Group is divided into four formations: the Glenkiln Shale, Lower Hartfell
Shale, Upper Hartfell Shale and Birkhill Shale (Lapworth 1878). The Glenkiln Shale is com-
posed of an unknown thickness of pale grey and black, heavily silicified argillites. At Dob’s
Linn the formation is poorly exposed as a series of disconnected, fault-bounded slivers. It is
generally unfossiliferous and due to heavy shattering of the competent, siliceous component,
even black lithologies rarely yield identifiable graptolites. Useful comparative sections are
exposed at the type section of Glenkiln Burn and at several other inliers in the Moffat area
(Lapworth 1878; Peach & Horne 1899).
The Glenkiln Shale apparently passes gradationally into the almost continuously black
Lower Hartfell Shale, which yields a more abundant graptolite fauna and is over 20m thick.
The lower half of the formation remains highly siliceous; the proportion of chert to black shale
decreases upwards throughout the unit, black shale becoming predominant in the upper 5m.
The overlying Upper Hartfell Shale is composed mostly of monotonous, non-graptolitic, pale
grey/green shales and mudstones 28m thick (Figs 1, 2). Its lower boundary is marked by a
transitional 3cm interval of alternating pale grey and black laminae. Three groups of graptoli-
tic, black shale bands occur within the formation, named the Complanatus, Anceps and Extraor-
dinarius Bands (Ingham 1974, 1979) after their diagnostic zonal assemblages (Fig. 1). Other
atypical lithologies include nodular limestones and one detrital limestone. The latter horizon is
a very pale grey, coarse-grained limestone 6:5cm thick, lying 1-5m below the lower Com-
planatus Band in the banks of the Linn Branch stream. Unfortunately it has been totally
recrystallized and affected by strain-induced pressure solution, but it was presumably of detrital
origin.
One medium grey nodular limestone, 4cm thick and lying 2m above the base of the Upper
Hartfell on the North Cliff section, displays uncompacted bioturbation with horizontal to
subvertical simple burrows 1-2mm in diameter. Other nodular horizons present in the Linn
Branch section include that known to yield a blind, dalmanitid trilobite 0-1m below the
Extraordinarius Band (Ingham 1979) and a second, apparently unfossiliferous bed 0:25m below
the base of the Birkhill Shale. Three of these four limestones were not known prior to recent
recollecting for conodont samples (Barnes & Williams, this volume) and other similar horizons
in the Upper Hartfell Shale probably still await discovery.
THE ORDOVICIAN-SILURIAN BOUNDARY STRATOTYPE 21
beg DEL
Lee Pas . = y . E>: = a Se 7 1 y V 2 " a F
Fig. 2 Sectioned slabs and bedding surfaces from the Moffat Shale at Dob’s Linn (all x 2). A.
Lower boundary of Anceps Band C. B. Typical uniformly laminated black shale, lower Birkhill
Shale. C. Black shale, thin metabentonite and micaceous horizons from Anceps Band D (note
grading shown by metabentonite). D. Uniform pale grey Upper Hartfell Shale lithology, Anceps
Bands. E. Irregular laminae and compacted bioturbation, base of lower Complanatus Band. F.
Bioturbation above upper Complanatus Band. G. Irregular laminae, compacted bioturbation and
low-angle synsedimentary faulting from reversal in Birkhill Shale 0-5m above base. H. Bedding
plane section from base of slab shown in Fig. 2G. I. Black shale injection through pale mudstone,
Anceps Bands. J. High-angle, post-compactional microfaulting, Anceps Band D. K, L. Bedding
surfaces with coarse mica flakes, Anceps Band D.
ee
2D, S. H. WILLIAMS
The 43m of Birkhill Shale is composed of black, continuously graptolitic shale in the lower
part, with the exception of a temporary reversal to an ‘Upper Hartfell’ type lithology 0-46—
0-56 m above its base. The shales become progressively siltier, less fissile and paler towards the
top of the formation, culminating in a transition to coarse turbidites of the overlying Gala
Greywacke Group.
The precise depositional environment of the Moffat Shale Group is still uncertain. Lapworth
(1897) envisaged black shale formation in a partially restricted ‘Sargasso Sea’ setting. Later
Walton (1963) considered the Moffat Shale to have been deposited on a regional high in a deep
ocean environment, explaining the lack of turbidites which are found elsewhere as lateral
equivalents and overlying the group.
With the recognition of the Lower Palaeozoic Iapetus Ocean in recent years, it has become
evident that the Moffat Shale was deposited within a wide, open ocean of complex history. This
suffered continued narrowing throughout the Upper Ordovician and Silurian due apparently to
subduction on both northern and southern margins (Moseley 1978, Bluck 1984). The signifi-
cance of sedimentary features such as postulated winnowing of graptolities, lithological colour
alternation, soft-sediment deformation and presence of limited bioturbation (Fig. 2) was dis-
cussed by Williams & Rickards (1984). Further observations have emphasized the variation in
contacts between pale and black lithologies, from sharp and laminar (Fig. 2A) to gradational
and irregular (Figs 2E—G). They have also confirmed the presence of coarse, silty laminae with
biotite flakes up to 1mm diameter, particularly within the Anceps Bands of the Upper Hartfell
Shale (Figs 2K—L). These strongly suggest a hemipelagic, distal turbidite origin for the sedi-
ments, in contradiction to Dewey (1971), Leggett (1980) and Leggett et al. (1979), who con-
sidered the shale to be of oceanic, truly pelagic origin formed during periods of high eustatic
sea level stands. Several black shale sequences elsewhere are known to have been deposited as
distal turbidites, including beds of the Burgess Shale of British Columbia (Piper 1972) and of
the Cow Head Group, western Newfoundland (Coniglio 1985). It was therefore evident that
during Lower Palaeozoic times black shales could form within an unrestricted oceanic setting
lacking any degree of restriction, unlike those deposited during Mesozoic and Recent times (e.g.
Jenkyns 1978; Stow & Piper 1984).
The presence of metabentonites throughout much of the Moffat Shale indicates sporadic
acidic volcanism. Most of these are only laminae or thin beds (Fig. 2C), but they occasionally
reach over 5cm thick. Their lateral impersistence was noted by Williams & Rickards (1984),
who suggested variable deposition due to a gently undulating sea floor. It seems likely that the
metabentonites were transported by a turbidite mechanism in a similar fashion to the remain-
ing lithologies; they would not, therefore, have significance in terms of proximity to volcanic
activity. The single coarse-grained limestone below the lower Complanatus Band was probably
also deposited by a powerful, carbonate-rich, turbidite flow. Such carbonate detritus was prob-
ably derived from a northerly source, such as the sites of fore-arc, shelf and slope deposition at
Girvan (Bluck 1984).
No critical sedimentological studies have been carried out on the Moffat Shale at Dob’s
Linn. With recent advances in both understanding of depositional mechanisms in deep-water,
hemipelagic sedimentation (Stow & Piper 1984; Coniglio 1985) and development of new tech-
niques to assist the study of fine-grained sediments, a detailed review of argillites at Dob’s Linn
and at comparitive sections is now warranted.
Late Ordovician and Early Silurian graptolite biostratigraphy
The following account is based on detailed logging through a trench constructed on the north
valley side of the Linn Branch (Figs 3, 4), excepting that of the Lower Hartfell Shale (from the
North Cliff trench, Fig. 3) and lower part of the Upper Hartfell Shale, including the Com-
planatus Bands (Linn Branch stream bed).
The uppermost 5m of the continuously black Lower Hartfell Shale is encompassed within
the Pleurograptus linearis Zone (Williams 1982a). Following this level, 9m of unfossiliferous
grey shale and mudstone belonging to the Upper Hartfell Shale is present before the black,
(9.2)
N
THE ORDOVICIAN-SILURIAN BOUNDARY STRATOTYPE
<=
1)
i=
o
=
=
—
pas
oO
<£
~
= /,
°
2:
i
Fig. 3 Photograph showing northern side of Linn Branch gorge, indicating key collecting localities.
Interpretation of geology and structure adapted after Ingham (1974: fig. 25).
24 S. H. WILLIAMS
Cel ad :
“(UPPER HARTFELL SHALE 7“,
Mp aap eta
alee ate MH Let C6
ASS (ae UK,
ec
ag
Goi srt Si
pacificus ;~ f
ae SUlzOMe 2-7
1
4
“ D. anceps /i<-
/'= Zone
ee
AN-°®
,o
> C? extraord-*"'
\ v, Marius Zone)
iy
RUN, ce
\
.
Fig. 4 Photograph of Linn Branch trench with interpretation, photographed from same position as
Fig. 2. Notebook lies on position of Ordovician-Silurian boundary.
THE ORDOVICIAN-SILURIAN BOUNDARY STRATOTYPE 25
graptolitic Complanatus Bands are reached. Lapworth (1878: 316; fig. 28) originally recorded
‘Dicellograptus forchhammeri, Climacograptus scalaris? and Diplograptus truncatus’. These speci-
mens were subsequently recognized as new taxa, described by Lapworth (1880) and Elles &
Wood (1901-18) as Dicellograptus complanatus, Climacograptus scalaris miserabilis and
Orthograptus truncatus socialis. Davies (1929: 18) relocated the graptolite horizon, as did
Toghill (1970). Ingham (1974) proved the existence of a second narrow, black seam about 0:-4m
above the 4cm thick, previously recorded band. Williams (1987) records D. complanatus Lap-
worth, D. minor Toghill, C. miserabilis Elles & Wood, C. tubuliferus Lapworth and O. socialis
(Lapworth) from the lower band. The upper band yields D. complanatus and rare specimens of
Orthoretiograptus pulcherrimus (Keble & Harris). Williams & Lockley (1983) described well
preserved specimens of an inarticulate brachiopod, both from within and directly above the
upper band, which were assigned to a new genus and species Barbatulella lacunosa. Rare,
usually fragmented specimens of this brachiopod also occur at several grey mudstone horizons
within the following Anceps Bands.
The Anceps Bands are separated from the Complanatus Bands by 13m of grey barren shale
and mudstone. They comprise a series of alternating black and grey shales with common
metabentonites, covering an interval which ranges in thickness from 1:6m on the Main Cliff, to
2:0 m in the Linn Branch trench and 4-5 m in the Long Burn section. The last of these localities
is separated from the former two by the Main Fault, and may have been deposited at some
distance apart. Other lateral variation in thickness was probably due to deposition on an
irregular sea floor and synsedimentary erosion as discussed by Williams & Rickards (1984).
Lapworth (1878: 253, 317) erected the Dicellograptus anceps Zone in his major publication
on the Moffat Shale, owing to the distinctive nature of the faunal assemblage in the black
Anceps Bands. Toghill (1970: 6; fig. 1) recorded four black shales; Ingham (1974) however
established the presence of five bands or groups of bands, now referred to Bands A to E. The
rich, diverse fauna contained within these black shales (Fig. 5) allowed Williams (1982) to divide
the zone into the Dicellograptus complexus and Paraorthograptus pacificus Subzones. In addi-
tion to those species’ ranges shown on the range chart, rare specimens of Climacograptus
hastatus Hall and Glyptograptus posterus Koren & Tsai have been found in the D. complexus
and P. pacificus Subzones respectively. These taxa confirm correlation with the Australian and
Chinese graptolite zonal schemes.
Ingham (1979) was first to discover the Extraordinarius Band 0-:96m above Anceps Band E.
This narrow, dark brown shale contains a sparse graptolite assemblage, identified by Rickards
(1979) and Williams (1983) as Climacograptus? extraordinarius (Sobolevskaya), Climacograptus
sp. indet. and Glyptograptus? sp. indet. The grey strata separating the Extraordinarius Band
from Anceps Band E is unfossiliferous, with the exception of a nodular limestone 0-1 m below
the Extraordinarius Bands which yields rare fragmentary specimens of a blind dalmanitid tri-
lobite (Ingham 1979).
The lower boundary of the Birkhill Shale lies 1:17 m above the Extraordinarius Band.
Following a basal, unfossiliferous black shale interval 0:15m thick, an abundant but poorly
diverse graptolite fauna is present, including Climacograptus normalis Lapworth, C. miserabilis
Elles & Wood and Glyptograptus? ‘venustus cf. venustus’ (Legrand). A temporary reversal to
alternating grey/green and black shales occurs at 0-46 to 0-56m above the base. This is
followed by black shales yielding a better preserved, more diverse assemblage with the addition
of Glyptograptus cf. persculptus (Salter) and Glyptograptus? avitus Davies. Lapworth (1878)
referred the basal Birkhill Shale to the P. acuminatus Zone. The G. persculptus Zone was first
separated as a biostratigraphical unit underlying the P. acuminatus Zone in central Wales by
Jones (1909, 1921), where he also considered it to be lithologically different. Davies (1929)
ratified the presence of two distinct zones and recognized the interval equivalent to the G.
persculptus Zone in both northern England and southern Scotland. It appears that he referred
three ‘horizons’ below the first occurrence of Parakidograptus acuminatus (Nicholson) and
Akidograptus ascensus Davies to the G. persculptus Zone at Dob’s Linn (1929: 22; fig. 32), but
this is not stated unequivocally in the text. Adoption of the G. persculptus Zone as a formally
defined, distinct biostratigraphic unit at Dob’s Linn was not realized prior to Toghill’s revision
26 S. H. WILLIAMS
BIRKHILL SHALE
Me - j \
D, ornatus” Sey
- =
q s
o > s
0 2 H
28 i
, s
o oO s
iJ
ail aT T
> wo oO oO m NN
i \
metres .
pee \ oF (e)
— & - « , ° ° 2
D, anceps , AN ow ros
| os
——— O90
f a _ | a S >
D,. complaxus | o A= o «
o o >
{ ° ° 2 D z =o
}/ 2) ce ry)
| \ ® ! © re
o o ~ <
-- — i | oO o oo cu (=
D, aff. complexus ; = » oC re) joo
/ \ o o @ . o >
i u I c a a = >
/ “sd 3 ° BD '
ae “4G 0c = a 3 3
D, minor ‘ 2 5 c
nN 2 a
o
fe}
=
o
KO
C. trifills
ener
KLADAAR MM.
C, medius
C. normalis
aot
miserabilis
C
iF
O, abbreviatus G. sp
O. fastigatus , G? ‘venustus cf, venustus
P. paciticus
CT ' es
—
O, denticulatus G? avitus
ees
ON
“SSOcc> |
N. velatus d A. ascensus
<=
=)
| | 1
Prrervecoueye : \
Srnecerssaay
P? lautus P. acuminatus
P? craticulus
\
P. pacificus C? extraordinarius |
Subzone \ Zone ,
D. complexus
Subzone
G persculptus
Zone
Fig. 5 Detail of sediments and graptolite ranges for the top Upper Hartfell Shale and basal Birkhill
Shale.
THE ORDOVICIAN-SILURIAN BOUNDARY STRATOTYPE 27
of the Birkhill Shale in 1968. Rickards (1970) and Hutt (1974) also used this zone as the basal
biostratigraphic division of the Lower Silurian Skelgill Formation in northern England.
The base of the P. acuminatus Zone is marked by the first appearance of Akidograptus
ascensus Davies and Paraorthograptus acuminatus (Nicholson) s.l. at 1-6m above the base of the
Birkhill Shale (Fig. 5). It is this level which has now been adopted as the defined Ordovician—
Silurian boundary (Cocks 1985), marked by the first occurrence of A. ascensus. Most previous
publications (e.g. Toghill 1968a; Cocks et al. 1970) have taken the base of the Birkhill Shale as
marking the Ordovician-Silurian boundary. This interval covers a change from grey to black
shale, is unfossiliferous and clearly unsuited as a zonal boundary, let alone for an international
system boundary stratotype. Similar barren intervals seem to occur at this level in every other
graptolitic succession in the world; they are probably related to eustatic sea level changes
induced by late Ordovician glaciation in the southern hemisphere (see Rong 1984).
In addition to the problem of barren intervals, faunal changes accompanying the transition
between the G. persculptus and C.? extraordinarius Zones are poorly understood. Few graptol-
ite taxa are present, following the mass extinction at the D. anceps—C.? extraordinarius zonal
boundary. Elles (1922, 1925) referred the basal interval of the Birkhill Shale to the ‘Zone of
Glyptograptus persculptus and Cephalograptus acuminatus’. In the earlier of these publications
(1922: 195) she suggested that this lowest Llandovery zone should perhaps be assigned to the
Ordovician owing to the lack of monograptids. It is interesting to note that this proposal has
now been partially adopted.
Atavograptus ceryx (Rickards & Hutt) occurs 1:9 to 2:3m above the base of the Birkhill
Shale. Recent recollecting indicates that it is probably restricted to such a level low in the P.
acuminatus Zone at Dob’s Linn. A. ceryx was first recorded from strata referred to the G.
persculptus Zone in the English Lake District (Rickards & Hutt 1970), but was later found in
the basal P. acuminatus Zone of that area (Hutt 1975), in association with A. ascensus.
Monograptus cyphus praematurus Toghill and Atavograptus atavus (Jones) are the next mono-
graptids found at Dob’s Linn, marking the boundary between the P. acuminatus and Cysto-
graptus vesiculosus Zones (Toghill 1968a). Lapworth (1882: 624) recorded an assemblage of
‘Climacograptus scalaris, Dimorphograptus acuminatus and ?Monograptus tenuis’ from a section
through the Lower Silurian at Girvan, south-west Scotland. Jones (1921: 155) remarked that
such an assemblage seemed anomalous for the P. acuminatus Zone; it may, however, prove that
the monograptid was A. ceryx and that the interval was equivalent to the early P. acuminatus
Zone of Dob’s Linn and northern England. Relocation and recollection of Lapworth’s horizon
could clearly prove significant as a comparative basal Silurian section.
Future research
Dob’s Linn has now been adopted as Ordovician-Silurian boundary stratotype, the boundary
being set at the base of the P. acuminatus Zone 1:6m above the base of the Birkhill Shale in the
Linn Branch trench (Figs 4, 5). This renders necessary ratification and expansion of Williams’
(1983) study of the interval. Other outstanding research still required includes:
1. Detailed study of the basal Birkhill Shale at remaining sections of Dob’s Linn, and at
other comparative localities in the Central Belt of the Southern Uplands.
2. Biostratigraphical and taxonomic revision of the Glenkiln Shale, the lower part of the
Lower Hartfell Shale and remainder of the Birkhill Shale, employing continuous, bed-by-bed
collecting techniques.
3. Critical sedimentological logging and study of the Moffat Shale at Dob’s Linn, with
subsequent integration of faunal data, in order to provide a clearer understanding of original
depositional setting.
Acknowledgements
I thank J. K. Ingham for his invaluable supervision of my original work at Glasgow University, which was
funded by a NERC postgraduate fellowship. I. Strachan provided efficient guidance through Lapworth’s
28 S. H. WILLIAMS
original material stored at Birmingham University and critically read the manuscript. Several colleagues
at Memorial University gave fruitful, often lively discussion on biostratigraphical and seimentological
problems, particularly C. R. Barnes and M. Coniglio.
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Palaeontogr. Soc. (Monogr.), London. 108 pp., 8 pls.
1979. [New information on some Ordovician-Silurian boundary sections in Great Britain.] Izv.
Akad. Nauk kazakh. SSR, Alma-Ata, (Geol.) 4: 103-107 [In Russian].
— & Hutt, J. E. 1970. The earliest monograptid. Proc. geol. Soc., London, 1663: 115-119.
Rong Jia-yu 1984. Distribution of the Hirnantia fauna and its meaning. In D. L. Bruton (ed.), Aspects of
the Ordovician System: 101-112. Universitetsforlaget, Oslo.
30 S. H. WILLIAMS
Sedgwick, A. 1850. On the Geological Structure and Relations of the Frontier Chain of Scotland. Edinb.
new phil. J. 51: 250-258.
Stow, D. A. V. & Piper, D. J. W. 1984. Deep water fine-grained sediments: facies models. In D. A. V. Stow
& D. J. W. Piper (eds), Fine-grained sediments: 611-646. Boston.
Toghill, P. 1968. The stratigraphical relationships of the earliest Monograptidae and the Dimorphograp-
tidae. Geol. Mag., Hertford, 105: 46-51.
1968a. The graptolite assemblages and zones of the Birkhill Shales (Lower Silurian) at Dobb’s Linn.
Palaeontology, London, 11: 654-668.
1970. Highest Ordovician (Hartfell Shales) graptolite faunas from the Moffat area, South Scotland.
Bull. Br. Mus. nat. Hist., London, (Geol.) 19: 1-26, pls 1-16.
Walton, E. K. 1963. Sedimentation and structure in the Southern Uplands. In M. R. W. Johnson & F. H.
Stewart (eds), The British Caledonides: 71-97. Edinburgh.
Webb, B. 1983. Imbricate structure in the Ettrick area, Southern Uplands. Scott. J. Geol., Edinburgh, 19:
387-400.
Williams, S. H. 1982. The Late Ordovician graptolite fauna of the Anceps Bands at Dob’s Linn, southern
Scotland. Geologica Palaeont., Marburg, 16: 29-56, 4 pls.
1982a. Upper Ordovician graptolites from the top Lower Hartfell Shale (D. clingani and P. linearis
zones) near Moffat, southern Scotland. Trans. R. Soc. Edinb. (Earth Sci.) 72: 229-255.
1983. The Ordovician-Silurian boundary graptolite fauna of Dob’s Linn, southern Scotland. Palae-
ontology, London, 26: 605-639.
1987. Upper Ordovician graptolites from the D. complanatus Zone of the Moffat and Girvan districts
and their significance for correlation. Scott. J. Geol., Edinburgh, 23: 65-92.
—— & Lockley, M. G. 1983. Ordovician inarticulate brachiopods from graptolitic shales at Dob’s Linn,
Scotland; their morphology and significance. J. Paleont., Tulsa, 57: 391400.
—— & Rickards, R. B. 1984. Palaeoecology of graptolitic black shales. In D. L. Bruton (ed.), Aspects of
the Ordovician System: 159-166. Universitetsforlaget, Oslo.
Conodonts from the Ordovician—Silurian Boundary
Stratotype, Dob’s Linn, Scotland
C. R. Barnes’ and S. H. Williams?
‘Geological Survey of Canada, 601 Booth St, Ottawa, Ontario K1A OE8, Canada
?Department of Earth Sciences, Memorial University of Newfoundland, St John’s,
Newfoundland A1B 3X5, Canada
Synopsis
About one hundred poorly preserved conodonts have been collected from surfaces of shale from seven
graptolite zones of the Dob’s Linn boundary stratotype section, mainly from the D. anceps Zone.
Attempts to recover conodonts by dissolving siltstones and cherts from the section were unsuccessful.
When preserved, the conodont phosphatic material provides Colour Alteration Index values of CAI 5-7,
indicating burial temperatures in excess of 300°C. The sparse, low diversity faunas assist in correlating
conodont and graptolite zones. Amorphognathus sp. and Scabbardella sp. cf. S. altipes were found in the G.
persculptus Zone, suggesting that the conodont turnover must lie at least high within this zone. Lowest
Silurian strata yielded rare, undiagnostic coniform taxa and an element referred tentatively to Oulodus?
kentuckyensis. The results encourage further efforts in retrieving conodonts from graptolitic shale
sequences, but the precise correlation of the conodont turnover with respect to the defined base of the
Silurian remains in question.
Introduction
The Ordovician—Silurian boundary was finally designated in 1985 at 1:-6m above the base of
the Birkhill Shale in the Linn Branch section of Dob’s Linn, southern Scotland, at the base of
the Parakidograptus acuminatus Zone (Williams 1983 and this volume; Cocks 1985). Detailed
work on the rich graptolite faunas has been carried out by a number of previous researchers,
especially Lapworth, Elles & Wood, Toghill and Williams (see Williams 1983, this volume). The
section, however, has yielded no other biostratigraphically useful fossils in abundance; there are
rare inarticulate brachiopods (Williams & Lockley 1983) and a species of a blind dalmanitid
trilobite. Lamont & Lindstr6m (1957) reported conodonts from cherts in the Southern Uplands
of Scotland, including Dob’s Linn, but only gave identifications and details of the Arenig and
Llandeilo faunas.
One critical problem in the debate concerning the definition of the Ordovician—Silurian
boundary and subsequent selection of a stratotype was that few candidate sections contained
both graptolites and conodonts. At the level of the G. persculptus and P. acuminatus Zones
(Fig. 1) in particular, there are difficulties in correlating the graptolite and conodont zones and
the two respective extinction events (e.g. Barnes & Bergstr6m, this volume). It is, therefore, both
encouraging and important to report in this paper the discovery of conodonts at several levels
in the Dob’s Linn boundary stratotype section.
While scanning shale surfaces under the microscope during the investigation of graptolites,
Williams observed a number of microfossils which have since been identified by Barnes.
Further collections were made by Williams in 1985; this time, in addition to the scanning of
shale surfaces, samples of shales, siltstones and cherts were processed through a variety of
standard chemical rock digestion techniques employed for conodonts (e.g. acetic and hydro-
fluoric acids; bleach). The latter results were disappointing in that most lithologies appeared to
be barren of conodonts, although this may have been due to inadequate preservation (see
below). The remaining new collections revealed many additional conodont horizons, but
yielded few diagnostic elements from new stratigraphical levels. This project however demon-
strates that conodonts are present, and moderately abundant at some horizons, in graptolitic
shales deposited in a deep oceanic environment which has been interpreted as an accretionary
prism (McKerrow et al. 1979; see other recent interpretations by Needham & Knipe 1986 and
Bull. Br. Mus. nat. Hist. (Geol) 43: 31-39 Issued 28 April 1988
32 BARNES & WILLIAMS
STRATOT YPE
TRENCH
0?
waterfall
key section
sheep track
AUT rocky bluff
uy
Wi
\
Ys
scree
~----- boundary
fault
Gala
Greywacke
5 Sees Birkhill
Shale
acuminatus
persculptus
extraorainarnss
anceps Upper
Hartfell
SSeS ae Shale
Lower
Bs tee See Hartfell
Shale
wilsoni___ .
peltifer se NS seman
e conodonts
e Extraordinarius Band
a Anceps Bands
Cc Complanatus Bands
Fig. 1 Simplified geological map and stratigraphical section of Dob’s Linn, showing position of
conodont localities and horizons (after Williams 1980).
CONODONTS FROM THE ORDOVICIAN-SILURIAN BOUNDARY STRATOTYPE 33
Murphy & Hutton 1986). Careful microscopical examination of similar shales in other
sequences should reveal many new conodont faunas and assist integration of graptolite and
conodont biostratigraphic zonation schemes.
Results
Following the discovery of the microfossils, re-examination of earlier material, together with
the new shale collections, has involved the study of several hundred surfaces for conodonts.
Conodonts and rare scolecodonts are present. The conodonts always occur as isolated ele-
ments; no fused clusters or natural assemblages were discovered. The elements are poorly
preserved, typically being fractured by tectonic stretching and commonly with only part of the
phosphatic skeletal material preserved. This may, in part, explain the difficulty in obtaining
identifiable conodonts from dissolved samples. For some, only an external mould remains, but
latex casts have been successfully made which permit specific identifications (e.g. Pl. 1, fig. 10;
Pl. 2, fig. 12). The conodonts provide Colour Alteration Index values of CAI 5-7. This is in
agreement with the general high thermal values reported elsewhere in the Southern Uplands of
Scotland by Bergstrom (1980), indicating burial temperatures exceeding 300°C.
About one hundred conodont elements have been recognized, the majority of which are
identifiable only to generic level. The diversity of the fauna is low, but zonal species are present.
Nearly all the conodonts come from Ordovician strata, in particular the D. anceps Zone;
unfortunately, conodonts are especially rare near the Ordovician-Silurian boundary.
Hartfell Shale conodonts
Most of the conodonts from the Dob’s Linn section come from the Hartfell Shale. They were
recovered at various levels within the Dicranograptus clingani, Pleurograptus linearis, Dicello-
graptus complanatus and D. anceps Zones, but principally from the latter zone. Details of
stratigraphy, together with a revision of the graptolite faunas from the D. clingani, P. linearis,
and D. anceps Zones, have been published by Williams (1982a, b). Conodonts from the D.
clingani Zone were not identifiable; those from the P. linearis Zone included two specimens of
Amorphognathus superbus (Rhodes) from 1-1—1:2 m and 0-3—0-45 m below the top of the Lower
Hartfell Shale, several specimens of Amorphognathus sp. and a specimen of Walliserodus
unidentifiable to species level.
The precise level at which A. superbus evolved into A. ordovicicus (i.e. base of the A. ordovi-
cicus Zone) in terms of graptolite zones remains to be established. This zonal boundary appears
to lie within the upper Pusgillian Stage or lower Cautleyan Stage (Bergstr6m 1971, 1983;
Orchard 1890; Bergstrom & Orchard 1985), although Savage & Bassett (1985) tentatively
suggest a late Caradoc age. In North America, this boundary occurs in the lower Maysvillian
Stage (Sweet & Bergstrom 1971). The D. clingani—P. linearis zonal boundary is approximately
equivalent to, or slightly predates, the base of the earliest Ashgill Pusgillian Stage (Williams &
Bruton 1983). The samples yielding A. superbus are from the top of the Lower Hartfell Shale
(mid P. linearis Zone; Williams 1982a: fig. 3) which probably falls within the Pusgillian Stage.
A single identifiable conodont was recovered from the D. complanatus Zone of the Upper
Hartfell Shale, namely Amorphognathus ordovicicus Branson & Mehl from the lower Com-
planatus Band. At Myoch Bay in the Girvan area, southern Scotland, the D. complanatus Zone
of the Upper Whitehouse Group also yields shelly fossils of Pusgillian age (Ingham 1978;
Harper 1979). Conodonts from these strata (Sweet & Bergstrom 1976: 135-136; Bergstrom &
Orchard 1980) do not allow a zonal assignment. It must be emphasized that the material at
hand comprises only a single, poorly preserved amorphognathodontiform element; this limited
evidence suggests that the A. ordovicicus Zone boundary lies within the Pusgillian rather than
the Cautleyan.
The D. anceps Zone is recognized in the Upper Hartfell Shale by a series of thin black shales
assigned to Anceps Bands A-E (e.g. Williams 1982b). These contain the most abundant cono-
dont fauna from the Dob’s Linn section. No significant difference was observed in the conodont
fauna of the various bands except in terms of relative abundance. Band A yielded rare speci-
34 BARNES & WILLIAMS
CONODONTS FROM THE ORDOVICIAN-SILURIAN BOUNDARY STRATOTYPE 35
mens assignable to only two species: Amorphognathus ordovicicus and Protopanderodus liripipus
Kennedy, Barnes & Uyeno. Band B produced conodonts referred to A. sp. cf. A. ordovicicus,
Scabbardella altipes (Henningsmoen) and an oistodontiform element that probably belonged to
Hamarodus europaeus (Serpagli). Band C contained only P. liripipus, and Band D yielded A. sp.
cf. A. ordovicicus and S. altipes; both had only rare fragmentary conodonts. Band E contained
slightly more specimens including Amorphognathus sp., P. liripipus and S. altipes. The D. anceps
Zone therefore yields conodonts belonging to the A. ordovicicus Zone. Orchard (1980) reco-
vered H. europaeus from only Rawthyan and Hirnantian strata although the range of this
species has now been extended into the Cautleyan by Barnes & Bergstrém (this volume).
No conodonts were recovered from the l-cm black shale Extraordinarius Band of the top
Upper Hartfell Shale, which yields C.? extraordinarius Zone graptolites of probable mid-
Hirnantian age (Williams 1983).
Birkhill Shale conodonts
The Birkhill Shale includes the upper part of the top Ordovician G. persculptus Zone, the basal
Silurian Parakidograptus acuminatus Zone and subsequent Llandovery graptolite zones (Toghill
1968; Williams 1983). The lower few metres of the Birkhill Shale is the critical interval from
which turnover conodonts need to be recovered, but unfortunately no especially diagnostic
taxa were found.
In strata of the G. persculptus Zone, the few specimens observed were all coniform except for
one slightly crushed and distorted specimen of Amorphognathus sp. from 0-12—-0-2m above the
base of the Birkhill Shale. The coniform taxa include Dapsilodus obliquicostatus (Branson &
Mehl) and Scabbardella sp. cf. S. altipes. The latter occurs at 0-95m above the base of the
Birkhill Shale. Neither Amorphognathus nor Scabbardella are known with certainty from Silu-
rian strata. This limited evidence, based on rare, poorly preserved specimens, suggests that most
of the G. persculptus Zone may lie below the main Ordovician—Silurian conodont turnover (see
discussion by Barnes & Bergstrom, this volume).
The P. acuminatus Zone, beginning at 1-6m above the base of the Birkhill Shale, and the
overlying Cystograptus vesiculosus Zone contained a few conodonts assigned to Dapsilodus
obliquicostatus and Decoriconus sp. In addition a single, small, poorly preserved ligonodiniform
element was found at 1:75m above the base of the Birkhill Shale. The form of the lateral
PLATE 1 Conodonts from the Lower and Upper Hartfell Shale, Dob’s Linn, Scotland.
Figs 1,2 Amorphognathus superbus (Rhodes) x 35. 1, dextral blade element, upper view. HM Y155.
1-1m below top of Lower Hartfell Shale, P. linearis Zone. North Cliff. 2, dextral blade element,
upper view of mould. HM Y157. 0-3—0-45 m below top of Lower Hartfell Shale, North Cliff.
Fig.3 Walliserodus sp. x 70. Lateral view. HM Y201. Top of Lower Hartfell Shale, North Cliff.
Figs 4, 5,13 Amorphognathus ordovicicus Branson & Mehl. x 35. Upper Hartfell Shale. 4, sinistral
blade element, upper view of mould. HM Y159. Lower Complanatus Band. 5, dextral blade
element, upper view of mould. HM Y107. Anceps Band A. Long Burn. 13, dextral blade element,
upper view of mould. HM Y129. Anceps Band D. Main Cliff.
Figs 6, 12, 16 Protopanderodus liripipus Kennedy, Barnes & Uyeno. x 55. Upper Hartfell Shale. 6,
scandodontiform element. HM Y109a. Anceps Band A, Long Burn. 12, symmetrical element. HM
Y121. Anceps Band C. Main Cliff. 16, scolopodontiform element. HM Y135. Anceps Band E. Linn
Branch.
Figs 7, 11, 14, 15,18 Scabbardella altipes Henningsmoen. Lateral views. x 55. Upper Hartfell Shale.
7, 2acodontiform element. HM Y203. Anceps Band B. Linn Branch. 11, distacodontiform element.
HM Y112. Anceps Band B. Main Cliff. 14, acodontiform element. HM Y202. Anceps Band D. Linn
Branch. 15, distacodontiform element. HM Y126. Anceps Band D. Long Burn. 18, dis-
tacodontiform element. HM Y204. 40cm above Anceps Band E, Linn Branch.
Fig. 8 Hamerodus europaeus (Serpagli). x 55. Oistodontiform element. HM Y205. Anceps Band B.
Linn Branch.
Figs 9, 10 Amorphognathus sp. cf. A. ordovicicus Branson & Mehl. x 35. 9, dextral blade. Upper
view of mould. HM Y114b. Anceps Band B. Main Cliff. 10, latex cast of HM Y114b (Fig. 9).
Fig. 17 Amorphognathus sp. x 35. Dextral blade element, upper view of mould. HM Y136. Anceps
Band E. Linn Branch.
BARNES & WILLIAMS
36
CONODONTS FROM THE ORDOVICIAN-SILURIAN BOUNDARY STRATOTYPE Si
process extends into the shale but its shape is revealed by a latex cast. The element is assigned
tentatively to Oulodus? kentuckyensis (Branson & Mehl). The latter species is known only from
Silurian strata elsewhere (e.g. Anticosti Island, McCracken & Barnes 1981).
Summary
About 100 conodont elements have been observed on shale surfaces from the Dob’s Linn
boundary stratotype section. Most are from black shales, but occasional specimens also occur
within paler grey shales and siltstones. The elements are poorly preserved, fractured and
commonly occur as moulds; the Colour Alteration Index values are in the range of CAI 5-7
indicating burial temperatures exceeding 300°C. Identification of most elements can be made
only to generic level; a selection of the better specimens are here illustrated (Figs 2, 3) but the
photography for many proved difficult and not all details of micromorphology could be repro-
duced. The diversity of the faunas is low, typically 3—5 species per graptolite zone interval. This
may be expected in the deep oceanic environment of the Hartfell Shale and Birkhill Shale, but
is probably also related to the limited material discovered. Siltstone, shale and chert samples
were also processed chemically but yielded no identifiable conodonts. Although the sparse
fauna and poor preservation must be taken into account, the following biostratigraphic conclu-
sions may be drawn from this study.
1. Amorphognathus superbus is present in the Pleurograptus linearis Zone near the top of the
Lower Hartfell Shale (based only on amorphognathodontiform, not holodontiform elements).
Amorphognathus ordovicicus occurs in the Dicellograptus complanatus Zone of the Upper Hart-
fell Shale. This suggests that the A. superbus—A. ordovicicus zonal boundary is not far removed
from that of the P. linearis and D. complanatus Zones and lies within the Pusgillian Stage.
2. Most of the conodonts come from the Dicellograptus anceps Zone; all the Anceps Bands
A-E of the Upper Hartfell Shale yielded specimens, which are indicative of the A. ordovicicus
Zone. Conodonts also occur at several grey, silty, non-graptolitic horizons during this interval.
3. No conodonts were recovered from the 1-cm black shale of the Climacograptus? extraordi-
narius Zone.
4. The lower 1-6m of the Birkhill Shale, belonging to the Glyptograptus persculptus Zone,
contained two poor specimens of Amorphognathus sp. and Scabbardella sp. cf. S. altipes, known
only from Ordovician strata. This suggests that the major conodont turnover (Barnes &
Bergstrom, this volume) occurred at a level equivalent to at least high in the G. persculptus
Zone.
PLATE 2 Conodonts from the Birkhill Shale, Dob’s Linn, Scotland. Figs 1-16 arranged in order of
stratigraphical occurrence of specimens. G. persculptus Zone (Figs 1-9); P. acuminatus Zone (Figs
10-14); C. vesiculosus Zone (Figs 15, 16).
Fig. 1 Amorphognathus sp. x 35. Upper view, distorted specimen. HM Y142. 0:12-0:2m above base
of Birkhill Shale.
Figs 2, 3, 5,9 Dapsilodus sp. Lateral views. 2, HM Y206. x 90. 0-55 m above base of Birkhill Shale.
3, HM Y207. x 55. 0:95m above base of Birkhill Shale. 5, HM Y208. x 80. 0-95m above base of
Birkhill Shale. 9, HM Y209. x 55. 1:5m above base of Birkhill Shale.
Figs 4,6 Scabbardella altipes Henningsmoen. Lateral views. x 55. 4, HM Y210. 0-95m above base
of Birkhill Shale. 6, HM Y211. 0-95 m above base of Birkhill Shale.
Figs 7, 15, 16 Dapsilodus obliquicostatus (Branson & Mehl). Lateral views. 7, HM Y213. x 70. 1m
above base of Birkhill Shale. 15, HM Y214. x 55. 5m above base of Birkhill Shale. 16, HM Y215.
x 55. 5-5m above base of Birkhill Shale.
Fig. 8 Drepanoistodus sp. x 70. Lateral view. Drepanodontiform element. HM Y212. 1m above
base of Birkhill Shale.
Figs 10, 12, 13 Decoriconus sp. x 55. Lateral views. 10, HM Y216. 1:75m above base of Birkhill
Shale. 12, HM Y217. 1-75m above base of Birkhill Shale. 13, latex cast of HM Y217 (Fig. 12). re
Figs 11, 14 cf. Oulodus? kentuckyensis (Branson & Branson). x 105. Lateral views. 11, ligonodini-
form element. HM Y218. 1-75m above base of Birkhill Shale. 14, latex cast of HM Y218 (Fig. 11).
38 BARNES & WILLIAMS
5. Silurian conodonts from the Parakidograptus acuminatus and Cystograptus vesiculosus
Zones include mostly coniform taxa (Dapsilodus, Decoriconus) which cross the systemic bound-
ary at other localities. A poor single element assigned tentatively to Oulodus? kentuckyensis,
which elsewhere is known only from Silurian strata, was found in the P. acuminatus Zone.
These results suggest that more attention should be made to recover conodonts from shales,
particularly in graptolitic shale sequences. The above data must be used with caution until
more material is discovered. However, the situation is perhaps analogous to the presence of
poorly preserved, rare graptolites within the conodont-rich Anticosti Island carbonate bound-
ary sequence (McCracken & Barnes 1981; Riva, this volume). It remains one of the future
challenges to find a boundary sequence that yields both well preserved and abundant, bio-
stratigraphically diagnostic conodonts and graptolites across the systemic boundary.
Acknowledgements
Ms Felicity H. C. O’Brien provided invaluable research assistance aspects and C.R.B. acknowledges
financial support from the Natural Sciences and Engineering Research Council of Canada.
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CONODONTS FROM THE ORDOVICIAN-SILURIAN BOUNDARY STRATOTYPE 39
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Preliminary acritarch and chitinozoan distributions
across the Ordovician-Silurian boundary stratotype at
Dob’s Linn, Scotland
G. M. Whelan
Department of Geology, Glasgow University, Glasgow G12 8QQ.
Synopsis
Palynomorph distribution has been investigated across the Ordovician-Silurian boundary at Dob’s Linn,
where the Hartfell Shale and Birkhill Shale are well exposed. Samples were taken from the anceps,
extraordinarius, persculptus, and acuminatus graptolite Biozones at the stratotype Linn Branch section and
also the Main Cliff. Graptolite debris is the dominant component of the organic fraction, but acritarchs,
chitinozoans and scolecodonts also occur in small numbers. Although it has not been possible to define
the position of the Ordovician-Silurian boundary by microflora, the presence of palynomorphs indicates
that detailed sampling might provide the stratigraphical resolution necessary to do this.
At Dob’s Linn in the Southern Uplands of Scotland, continuous sections through the Hartfell
and Birkhill Shales (Caradoc to Llandovery) bracket the Ordovician—-Silurian boundary. These
shales are replaced vertically by greywackes (the Gala Greywackes) in the maximus Zone. Fault
bounded tracts showing similar transitions are common in the Southern Uplands. Systematic
variations in the regional timing of this transition, and the complex younging relationships
between and within tracts, are thought to reflect the progressive growth of an accretionary
prism (McKerrow et al. 1977) during closure of the Iapetus Ocean. The 90m of Hartfell and
Birkhill Shales exposed here (Williams 1981) represent a substantially condensed sequence, as
an equivalent sequence a hundred kilometres to the west, at Girvan, is over 3000 m thick.
This is a preliminary report of the distribution of acritarchs and chitinozoans across the
newly formalized Ordovician—Silurian boundary at Dob’s Linn. Data from the anceps, extraor-
dinarius, persculptus (all Ordovician) and acuminatus (Silurian) graptolite Biozones are present-
ed. Palynomorphs were recovered from hydrofluoric and hydrochloric acid-etched residues and
studied using the scanning electron microscope, or transmitted light microscope. Whilst grap-
tolites are common at Dob’s Linn, and other fossils, such as scolecodonts, have been found
sporadically, this is the first major palynological survey that has been undertaken on the
Ordovician and the basal Silurian there.
The older Upper Hartfell Shale is a sequence (28 m thick) of finely bioturbated massive grey
mudstones (Williams & Rickards 1984), with subordinate thin black shale bands (two com-
planatus bands, five anceps bands and one extraordinarius band), and metabentonite horizons.
The Birkhill Shale (48 m) comprises a laminated, pyritous, black shale with abundant graptol-
ites, and representing the persculptus to maximus Zones. The systemic boundary of the
Ordovician-Silurian has been fixed at the base of the acuminatus graptolite Biozone, 1:6m
above the base of the Birkhill Shale (Cocks 1985).
Samples have been collected from two localities spanning the boundary, the Main Cliff and
the Linn Branch section (the world stratotype of the Ordovician—Silurian Boundary). At Main
Cliff the wilsoni to acuminatus graptolite Zones are exposed, and although some strike slip
faulting has caused repetition of the upper anceps and extraordinarius black shale bands, the
beds are consistently the right way up (Williams 1980). At the Linn Branch, the anceps to
maximus Zones are present, and although the beds are overturned, the stratigraphy is not
complicated by repetition. To date sampling has concentrated on the extraordinarius and
anceps Zones. However, work in progress aims to characterize the distribution of palyno-
morphs across the boundary.
Bull. Br. Mus. nat. Hist. (Geol) 43: 41-44 Issued 28 April 1988
42 G. M. WHELAN
3 4
Figs 1-4 Chitinozoans and acritarchs from Dob’s Linn. 1, Ancyrochitina ancyrea (Eisenack 1931)
Eisenack 1955. SU/DL/41, acumunatus Zone, Main Cliff, x 250. 2, Cyathochitina kukersiana
(Eisenack 1934) Eisenack 1965. SU/DL/9, anceps Zone, Main Cliff, x 250. 3, Solisphaeridium
nanum (Deflandre 1945) Turner 1984. SU/DL/12, anceps Zone, Main Cliff, x 530. 4, Diexallophasis
sp. 1. SU/DL/10, anceps Zone, Main Cliff, x 470.
Both groups of palynomorphs are unevenly distributed throughout the two sections although
they are generally more abundant at Main Cliff. Acritarchs appear to be more important and
better preserved in the grey mudstones, while chitinozoans appear to be more common in the
black shales, although this is not always the case. Palynomorph colour varies from grey to
black within a single sample, and probably reflects differences in wall thickness.
Acritarchs can be divided into several groups (Downie et al. 1963): (a) Sphaeromorphs which
are spherical. These are of limited biostratigraphical use as can be seen in Figs | and 2, and will
not be mentioned further; (b) Acanthomorphs which have spines or processes; (c) Herkomorphs
which have crested ridges forming polygonal fields; (d) Polygonomorphs which have a limited
number of processes, usually between three and five; and (e) Netromorphs which are generally
fusiform in shape. The Dob’s Linn samples are noticeably dominated by acanthomorph acri-
tarchs and only a few samples contain representatives of the other groups.
Anceps Zone
Six samples have been studied from Main Cliff (only one of which is a grey mudstone) and
sixteen acritarch and chitinozoa taxa have been found (Fig. 5). The chitinozoans Cyathochitina
campanulaeformis (Eisenack), C. kukersiana (Eisenack) and Rhabdochitina gallica Taugourdeau
all suggest a Caradoc to Ashill age. Hercochitina cf. turnbulli Jenkins has previously been
described from the Caradoc of Oklahoma (Jenkins 1969), but only one poorly preserved speci-
men was found at Dob’s Linn. The acritarch Solisphaeridium nanum (Deflandre) Turner ranges
from Arenig to Devonian and is therefore a poor biostratigraphical indicator. Of the other
acritarchs recovered Stellechinatum brachyscolum Turner has been described only from the
Caradoc of Shropshire (Turner 1984), and Veryhachium reductum (Deunff) Jekhowsky from the
Tremadoc to the Silurian. Diexallophasis sp. 1 has also been found from the Silurian sedgwickii
Zone and is probably a new species (pers. comms Molyneux 1986). Thus palynomorphs indi-
cate an Upper Ordovician age for the anceps Zone, primarily on the evidence of chitinozoan
distribution. Samples from the anceps Zone at the Linn Branch section have yielded no palyno-
morphs and this is attributed to the extreme weathering of this part of the section.
Extraordinarius Zone
The chitinozoans and the acritarch Veryhachium corpulentum Colbath found in this zone (Figs
5, 6) suggest a Caradoc to Ashgill age, although the acritarchs Veryhachium lairdii and V.
reductum both range from Lower Ordovician to Silurian in age. The Linn Branch section has
only yielded two non-sphaeromorph acritarchs: the acanthomorphs Baltisphaeridium sp. 1 and
Armoricanium sp. 2 (Fig. 6).
PRELIMINARY ACRITARCH AND CHITINOZOAN DISTRIBUTIONS, DOB’S LINN 43
GRAPTOLITE SAMPLE LITH al CHITINOZOANS
ZONE NUMBER
=
ACANTHOMORPH
ACRITARCHS
SPHAEROMORPH OTHER
ACRITARCHS ACRITARCHS
ANCYROCHITINA ANCYREA
ACUMINATUS SUDL 41 ANCYROCHITINA SP 1
KALOCHITINA SP1
LEIOSPHAERIDIA SP 1
PERSCULPTUS SU DL —| RHABDOCHITINA MAGNA
DICTYOTIDIUM SP 1
VERYHACHIUM LAIRDII
SUDL 38
MULTIPLICISPHAERIDIUM SP.1 LP 2 V. CORPULENTUM
; _ SYNSPHAERIDIUM SP 1 Vv. REDUCTUM
= esl MICRHYSTRIDIUM SP 1 VSP
SUDL 17 CYATHOCHITINA HYMENOPHORA Lo sp 4
aes MICRHYSTRIDIUM SP.2 ACTINOTODISSUS SP 1
EXT RAORDINARIUS|_ |
Se ==
| ;
+—
| MULTIPLICISPHAERIDIUM SP.1
MULTIPLICISPHAERIDIUM SP 1
——
Ll. sp 1
L spe2
ee
SOLISPHAERIDIUM NANUM Lead
STELLECHINATUM BRACHYSCOLUM
MICRHYSTRIDIUM SP 1 ESESPa2:
ANCEPS a a ee a= Sew noel ea Se a |
L sp 1
L. SP. 2 |
GONIOSPHAERIDIUM SP.1 L sp. VERYHACHIUM REDUCTUM
DIEXALLOPHASIS SP1
MULTIPLICISPHAERIDIUM SP. 2
MICRHYSTRIDIUM SP1; M.SP3 L Se ECA UMESE A
CYATHGCHITINA KUKERSIANA 7] Spin
©. CAMPANULAEFORMIS
RHABDOCHITINA GALLICA eee
HERCOCHITINA CF TURBULLI J
Fig. 5 Distribution of acritarchs and chitinozoans at Main Cliff, Dob’s Linn. In column 3
(lithology), horizontal lines indicate a black shale sample, and the dots represent a grey mudstone.
Persculptus Zone
At Main Cliff the chitinozoan Rhabdochitina magna Eisenack and the herkomorph acritarch
Dictyotidium sp. 1 have been found, while two samples from the Linn Branch section have
yielded Kalochitina sp. 1 and Conochitina tormentosa Taugourdeau. This assemblage suggests a
Caradoc to Ashgill age, although Rhabdochitina magna is known to range into the Llandovery.
Acuminatus Zone
One sample from Main Cliff has yielded 24 specimens of the important Lower Silurian form
Ancyrochitina ancyrea (Eisenack) Eisenack and a single specimen of Kalochitina sp. 1. At the
Linn Branch Rhabdochitina magna is found, and both this species and Kalochitina sp. 1 extend
across the boundary, and are thus of little biostratigraphical use as boundary markers.
Because of the long range of most species the distributions of acritarchs and chitinozoans are
less refined biostratigraphical indicators than those of graptolites. The sample from the acumin-
atus Zone can be dated accurately as Lower Silurian, while all other samples which yielded an
unequivocal age determination are of Upper Ordovician age. It is important to note that the
chitinozoans have proved most useful in this survey and that they are often very abundant in
the black shales. As the boundary is within the Birkhill Shale, it is possible that bed by bed
processing will yield sufficient chitinozoan taxa to determine the position of the Ordovician—
Silurian boundary accurately in terms of the microflora. As palynomorphs often occur in rocks
which lack datable macrofossils, even a crude biostratigraphical zonation based on chitin-
ozoans would have considerable use in word-wide correlation.
44 G. M. WHELAN
a eee = on] PRE - sos
GRAPTOLITE SAMPLE LITH CHITINOZOANS ACANTHOMORPH SPHAEROMORPH
ZONE NUMBER ACRITARCHS ACRITARCHS
——. + — —
SU DL38 —— LEIOSPHAERIDIA SP 1
———-
t—
SU DL 37 RHABDOCHITINA MAGNA
| |
— + +
SU DL 36 (L, SR. 2
|
= eee == = a a
\
SU DL35 KALOCHITINA SP 1 7 os SP. 2
PERSCULPTUS
ACUMINATUS
‘sale ]
SU DL34 CONOCHITINA TORMENTOSA | L. SP. 2
|
= +
|, Us SP
| &. Sp 2
Jt JL —_- St
| aaa as BALTISPHAERIDIUM SP 1 | es SPiat
SU DL32| © |
AREMORICANIUM SP 2 | L. sp.2
EXTRAORDINARIUS |
|} L. SP. 14
SU DL 31 |
| & SR 2
ANCEPS SU DL 43
1 = : a
Fig. 6 Distribution of acritarchs and chitinozoa at the Linn Branch section, Dob’s Linn. Lithology
symbols as in Fig. 5.
Acknowledgements
I am grateful to C. J. Burton, G. B. Curry, K. J. Dorning, P. D. W. Haughton and S. G. Molyneux for
their help, and I should also like to thank D. Maclean for printing the diagrams. A N.E.R.C. grant is
gratefully acknowledged.
References
Cocks, L. R. M. 1985. The Ordovician-Silurian boundary. Episodes, Ottawa, 8: 98-100.
Downie, C., Evitt, W. R. & Sarjeant, W. A. S. 1963. Dinoflagellates, hystrichospheres and the classification
of the acritarchs. Stanf. Univ. Publs, Palo Alto, 17(3): 3-16.
Jenkins, W. A. M. 1969. Chitinozoa from the Ordovician Viola and Fernvale Limestones of the Arbuckle
Mountains, Oklahoma. Spec. Pap. Palaeont., London, 5. 44 pp., 9 pls.
McKerrow, W. S., Leggett, J. K. & Eales, M. H. 1977. Imbricate thrust model of the Southern Uplands of
Scotland. Nature, Lond. 267: 237-239.
Turner, R. E. 1984. Acritarchs from the type area of the Ordovician Caradoc Series, Shropshire, England.
Palaeontographica, Stuttgart, (B) 190: 87-157.
Williams, S. H. 1980. An excursion guide to Dob’s Linn. Proc. geol. Soc. Glasgow 121/122: 13-18.
(1981). The Ordovician and Lowest Silurian Graptolite Biostratigraphy in Southern Scotland.
Unpublished Ph.D. Thesis, University of Glasgow.
—— & Rickards, R. B. 1984. Palaeoecology of graptolitic black shales. In D. L. Bruton (ed.), Aspects of
the Ordovician System: 159-166. Universitetsforlaget, Oslo.
Ordovician-Silurian junctions in the Girvan district,
S.W. Scotland
D. A. T. Harper
Department of Geology, University College, Galway, Ireland
Synopsis
The Ordovician—Silurian boundary at Girvan is represented by a variety of unconformable contacts; the
basal Silurian rocks both overstep and overlap the upper Ordovician strata south and southwestwards.
The most complete section across the junction is in a regressive shelly facies located north of the Girvan
valley in the Craighead inlier. The Hirnantian High Mains Formation contains a moderately diverse
Hirnantia fauna within channel fill sandstones. The overlying basal Silurian unit, the middle Rhuddanian
Mulloch Hill Conglomerate, was deposited in submarine canyons at a variety of depths and contains an
entrained Cryptothyrella fauna. The continuing regression evident across the junction and the facies
patterns in the lowermost Silurian are related to the local emergence of fault-bounded blocks.
Introduction
The Ordovician and Silurian rocks of the Girvan district, S.W. Scotland contain a wide variety
of siliciclastic sediments, together with locally diverse shelly and graptolite faunas; deposition
occurred in a proximal fore-arc environment (Bluck 1983). In contrast to the graptolite facies of
the Ordovician-Silurian boundary sections in the shale inliers of the Southern Uplands, the
most stratigraphically complete junction section at Girvan is in a shelly facies.
Lapworth’s detailed study of the Girvan succession (1882) was largely confirmed by the
similarly substantial researches of Peach & Horne (1899). But neither study was aware of the
terminal Ordovician unit, the High Mains Formation; thus the marked contrast between the
faunas of the Ladyburn Mudstones of the Upper Drummuck Group and those of the Mulloch
Hill Group led Lapworth (1882: 622) to consider the apparent hiatus between the top of his
Ardmillan Series and the base of his Newlands Series to represent ‘the grandest palaeonto-
logical break in the entire Girvan succession’.
In a detailed appraisal of the Drummuck Group, Lamont (1935: 294) noted the presence of a
hitherto unrecognized unit of buff-weathering sandstone overlying the Drummuck Group and
containing a distinctive shelly fauna. He considered the unit, the High Mains Sandstone, to
represent the base of the Mulloch Hill Group and moreover (Lamont 1935: 289) suggested a
correlation with the lower Llandovery. From this unit he briefly described and figured speci-
mens of his new genus Hirnantia, which he based on material of Orthis sagittifera M‘Coy from
both the High Mains Sandstone and the Hirnant beds of Bala, north Wales, and noted the
presence of Meristella sp. (Hindella crassa incipiens). Subsequently, Lamont (1949) described the
trilobite Flexicalymene scotica from the High Mains Sandstone and modified his views on the
correlation of the unit to include the possibility of a Hirnantian age. Ingham & Wright (1970)
subsequently emphasized the presence of key elements of the terminal Ordovician Hirnantia
fauna and concluded a correlation with the Hirnantian Stage.
Harper (1979b) noted the presence of two distinct associations of the Hirnantia fauna within
the High Mains Sandstone and suggested the inapplicability of the term ‘community’ to
contain the marked diversity of associations within the Hirnantia fauna. The formation has
been described and mapped in detail and bulk samples of the two shelly associations investi-
gated (Harper 1981). The thirteen taxa of brachiopod are currently being described (Harper
1984 and in preparation), whilst Owen (1986) has completed a monographic study of the five
taxa of trilobites.
Bull. Br. Mus. nat. Hist. (Geol) 43: 45-52 Issued 28 April 1988
46 D. A. T. HARPER
The junction sections
The basal Silurian strata both overstep and overlap the upper Ordovician rocks of the district
south and southwestwards (Cocks & Toghill 1973). The most stratigraphically complete bound-
ary section is thus north of the Girvan valley in the Craighead inlier (Fig. 1) whilst the largest
hiatus is developed in the coastal exposures south of the Girvan Valley and southwest of the
main outcrop (Fig. 2).
(1) Craighead inlier. The terminal Ordovician unit, the High Mains Formation, crops out in the
vicinity of High Mains farmhouse (Fig. 1). The unit is poorly exposed, and the detailed outcrop
pattern (Harper 1981) was investigated by trenching and mechanical digging. The formation
contains two associations of the Hirnantia fauna and a Hirnantian age is indicated. The High
Mains Formation is overlain by the Mulloch Hill Conglomerate (the Lady Burn Conglomerate
of Cocks & Toghill, 1973) but although the junction is not exposed it is assumed to be fairly
sharp with a slight angular discordance.
(ti) Main Outcrop. The main outcrop of Silurian rocks in the Girvan district extends from
Saugh Hill approximately northeast to Straiton (Cocks & Toghill 1973: fig. 1). The presence of
major bedding-parallel structures have locally tectonized the shale units and may be
responsible for the variation of thicknesses, along strike, of several of the formations. The
junction of the Silurian with the underlying Ordovician is exposed on the west bank of Pen-
whapple Burn (National Grid ref. NX 23279769) some 500 metres downstream from Penwhap-
ple Bridge (Cocks & Toghill 1973: fig. 4). Here, the local base of the Silurian is represented by
the Tralorg Formation. At the junction the succession is inverted; however, the Tralorg Forma-
tion appears to overlie conformably grey micaceous sandstones and shales of the Shalloch
Formation; the junction is apparently tectonized as are the shales within the underlying Shal-
loch Formation. In an adjacent quarry, graptolites of the anceps Zone indicate a middle Ashgill
age for this part of the Shalloch Formation. Both units dip steeply south.
(111) Coastal Exposures. The two main coastal exposures of the Ordovician—Silurian junction
clearly demonstrate the southward overstep and overlap of the basal Silurian units. At the
northernmost of the two exposures, the Haven (Cocks & Toghill 1973: fig. 3), the Craigskelly
Conglomerate overlies the Shalloch Formation unconformably. However, farther south on
Woodland Point the Woodland Formation unconformably overlies lower horizons of the
Shalloch Formation, although pockets of Craigskelly Conglomerate lie between the two.
Faunal and facies changes at the Ordovician—Silurian junction
As noted above, the most complete boundary section is near High Mains farmhouse in the
central part of the Craighead inlier (Fig. 1). The highest Ordovician strata in the Girvan
district, in ascending order the Shalloch Formation, the Drummuck Group and the High
Mains Formation, are sporadically exposed and the latter two units are locally highly fossil-
iferous (Figs 3—22). Within the Drummuck Group a variety of shelly associations dominated by
brachiopods have been noted (Harper 1979b), and are currently under detailed description,
together with the continuing documentation of the brachiopod taxa (Harper 1984 and in
preparation). The associations are thought to have inhabited a spectrum of environments
upslope and adjacent to the proximal parts of a submarine fan system. The highest strata of the
group, the upper Rawtheyan South Threave Formation (Harper 1982), contain highly fossil-
iferous sandstones (the Ladyburn Starfish Beds of the Farden Member) and probable mudflow
units (the Cliff Member); nevertheless background sedimentation is represented by bedded
green mudstones and occasional siltstones containing low diversity faunas of minute inarticu-
late, enteletacean and plectambonitacean brachiopods. The boundary with the overlying Hir-
nantian High Mains Formation, although not exposed, is assumed to be fairly sharp. The High
Mains Formation consists of fine-medium and medium grained quartz sandstones. The beds
are massive with an apparent lack of internal sedimentary structures; horizons of shelly debris
47
ORDOVICIAN-SILURIAN JUNCTIONS IN S.W. SCOTLAND
‘passnosip do19jno UBLINIIS JO svore UIUT 9014} JO suOI}Isod ayeuNIxOI1dde oy) sMoOyYs ‘1J9] WOI10q ‘JasuT
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48 D. A. T. HARPER
i = = — : = = = =r
5 7 GRAPT E
! STAGES CRAIGHEAD INLIER MAIN OUTCROP CORSTAL OUIEE
EXPOSURES BIOZONES
— 1
= Glenwells Shale Tralorg Formation | Woodland Formation
ey — SS a cyphus 2
> | <x
2 RHUDDANIAN Mulloch Hill | Craigskelly =
ZS Gandstone Conglomerate =)
xt vesiculosus jl
=] —
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Conglomerate
S| SN NON ae Ne es}
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RAWTHEYAN =<
4 Drummuck Group ©
| —
= anceps
(@) >
I} CAUTLEYAN O
op) Qa
Shalloch Formation See] fe)
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PUSGILLIAN Shalloch Formation complanatus
mi : IL
Fig. 2 Correlation of Ordovician—Silurian junction sections across the Girvan district with each
other and the established shelly stages and graptolite biozones.
are developed at various levels in the formation. The lower 2m of the formation exposed in the
High Mains trench (Harper 1981: 250) comprises grey-green fine-medium grained, thinly
bedded sandstones, whilst the upper 5-5 m is a hard medium-grained, thickly-bedded sandstone.
Changes in grain size, bedding characteristics and faunal composition indicate a minor regres-
sion within the sequence. In view of the incomplete exposure and an apparent absence of
Figs 3-22 Brachiopods and trilobites from the High Mains Formation (Hirnantian), High Mains
trench, Girvan. Repository: Hunterian Museum, Glasgow. Fig. 3, fossiliferous block of the High
Mains sandstone dominated by internal moulds of both pedicle and brachial valves of Hindella
crassa (J. de C. Sowerby) incipiens (Williams) and crinoid ossicles, x 1. Figs 4, 8, Plaesiomys aff.
porcata (M‘Coy). 4, HM L12238, latex cast of internal mould of brachial valve, x 2; 8, HM
L12239, latex cast of external mould of pedicle valve, x 2. Figs 5, 9, 13, Eochonetes cf. advena
Reed. 5, HM L12115, internal mould of pedicle valve, x 4; 9, HM L12117, latex cast of internal
mould of brachial valve, x 4; 13, HM L12118, latex cast of external mould of pedicle valve, x 3.
Figs 6, 7, 10, 11, Hindella crassa (J. de C. Sowerby) incipiens (Williams). 6, 10, HM L12242, latex
cast and internal mould of pedicle valve, both x 2; 7, HM L12244a, external mould of brachial
valve, x 3; 11, HM L12244b, latex cast of internal mould of brachial valve, x 3. Figs 12, 14-16,
Eostropheodonta aff. hirnantensis (M‘Coy). 12, HM L12105, latex cast of internal mould of brachial
valve, x 2; 14, HM L12104, internal mould of pedicle valve, x 1; 15, HM L12103, latex cast of
external mould of pedicle valve, x 2; 16, HM L12653, latex cast of internal mould of brachial
valve, x 2. Figs 17, 18, Hemiarges extremus Owen, HM A16153, external mould and latex cast of
cranidium, both x 2. Figs 19-21, Hirnantia sagittifera (M‘Coy). 19, HM L12654, latex cast of
brachial valve exterior, x 2; 20, 21, HM L1986, latex cast and internal mould of brachial valve,
both x 2. Fig. 22, Achatella cf. truncatocaudata (Portlock), HM A16152, internal mould of
cephalon, x 2.
ORDOVICIAN-SILURIAN JUNCTIONS IN S.W. SCOTLAND 49
sedimentary structures, palaeoenvironmental analysis of the High Mains Formation is equivo-
cal. Nevertheless the thickness, geometry and lithology of the unit are compatible with deposi-
tion within channels which developed on the deeper parts of the shelf and the upper parts of the
slope. Such environments (Dott & Bird 1979) may be characterized by apparently massive and
structureless sandstones comprising channel fills in the order of 25m thick. Elsewhere, various
modes of channelling characterize predominantly argillaceous upper Ashgill sequences; these
developed during the time of regression in response to the end Ordovician glacio-eustatic event
(Brenchley & Newall 1980). At Girvan, however, a fall in sea level in excess of the 50-100m
estimated (Brenchley & Newall 1980: 34) is required and thus additional tectonic controls must
be invoked.
Liiqoo|i2
4 Hunt Mus
50 D. A. T. HARPER
To date, the High Mains Formation contains a fauna of thirteen brachiopod (Harper 1981)
and five trilobite (Owen 1986) taxa. The brachiopods are characterized by a relative abundance
of Hirnantia sagittifera (M‘Coy), Eostropheodonta aff. hirnantensis (M‘Coy) and Hindella crassa
(J. de C. Sowerby) incipiens (Williams), important elements of the terminal Ordovician Hirnan-
tia fauna, and less common Glyptorthis, Plaesiomys, Platystrophia, Eochonetes, Eopholidostro-
phia, Fardenia, Rostricellula, Hypsiptycha and Eospirigerina and an indeterminate enteletacean.
With the exception of Hypsiptycha, all these forms have congeners in the underlying Drum-
muck Group. Moreover small individuals of H. crassa incipiens have been described previously
from the Ladyburn Starfish Beds within the upper Rawtheyan South Threave Formation near
the summit of the Drummuck Group (Reed 1917: 955; pl. 24, fig. 55) whilst Mitchell (1977: 54)
has described and figured a species of Hirnantia from the Cautleyan Killey Bridge Formation,
which is along strike in the Pomeroy inlier of the north of Ireland.
The Girvan fauna is quite distinct from other Hirnantia faunas (cf. Rong 1984a); whilst the
fauna is dominated by key members of the Hirnantia fauna, it is of moderate diversity and
supplemented by essentially relict North American forms. It is nevertheless different from other
coeval assemblages, for example the Holorhynchus and Older Edgewood faunas (Rong 1984b:
117). Similarly, the trilobite fauna is dominated by North American relicts (Ingham in Harper
1981; Owen 1986).
The succeeding Mulloch Hill Conglomerate unconformably overlies the Drummuck Group.
This formation is dominated by units of polymict, poorly sorted, of either clast- or matrix-
supported conglomerate. The clasts range in diameter from a few centimeters up to 15cm; a
variety of lithologies is represented as is a range of shapes from near rounded to angular. The
conglomerate units are separated by thinner beds of coarse impure quartz sandstone which are
locally fossiliferous. Cocks & Toghill (1973) considered a shallow water environment of deposi-
tion likely for the unit whilst more recently Walton (1983: 133) indicated the sedimentology
and fauna of the formation to be suggestive of shallow, shelf conditions. The available data
however suggest an equally feasible alternative. The nature and thickness of the formation, in
excess of 100m, together with an ability to cut through some 350m of strata over a distance of
about five miles, suggest the Mulloch Hill Conglomerate was deposited in a channel across a
gradient of depths. Clearly in the vicinity of Girvan the unconformity was not subaerial but
rather resulted from downslope channelling during the earliest Silurian (see also Ingham 1978).
The fauna of the Mulloch Hill Conglomerate, although locally abundant within the sand-
stone units, is of low diversity. It is dominated by crinoid ossicles and the brachiopods Crypto-
thyrella angustifrons (Salter) and a species of Rhynchotreta (Cocks & Togill 1973). Both species
have near identical relatives in the fauna of the upper Rawtheyan Ladyburn Starfish Beds
(Harper 1979a). Such associations characterize shallow water environments created during the
early Llandovery global transgression (Sheehan 1977).
Discussion
The faunal succession across the Ordovician-Silurian junctions indicates three phases of devel-
opment: (a) above the Rawtheyan—Hirnantian transition a marked decrease in diversity con-
comitant with the development of a fauna comprising relict middle Ashgill elements of the
North American province together with more abundant key taxa of the Hirnantia fauna, (b)
during the early and middle Rhuddanian very low diversity faunas characteristic of the, then,
recently colonized shallow water environments in the North American province, and (c) the
arrival during the middle and late Rhuddanian of diverse, more typically Llandovery, shelly
faunas. The former two events are accompanied by channel development during the regression
whilst the latter is concomitant with net transgression. Similarly in the more complete and
stable boundary section of the Oslo Basin relict Ordovician forms are not displaced by more
typical Silurian elements until at least the middle Rhuddanian (Baarli & Harper 1986).
The mutual relationships of the basal Silurian facies and their southwestward overlap and
overstep have been rationalized recently by Bluck (1983: fig. 6). Such features are considered to
be the result of deposition on blocks of Ordovician strata separated by high-angle listric faults
ORDOVICIAN-SILURIAN JUNCTIONS IN S.W. SCOTLAND 51
with approximately east to west trends. Evidence of fault-controlled sedimentation has been
documented within the middle Ordovician succession of the Girvan district in the classic study
by Williams (1962), more recently refined by Ince (1984). Whilst the disposition and relative
movement of such blocks can at least partly explain lower Silurian facies patterns in the Girvan
district, a mechanism is available also to provide substantial and continued local regressions
during the late Ordovician and early Silurian. The relative downfaulting of sequential blocks to
the south, during extensional phases, may have resulted in the rotation of each block about an
axis parallel to the trend of the listric faults; consequently the leading apex of each block may
have become emergent. The overall effect locally is one of regression and channel development
across relatively steep slopes. Both faunal and facies development thus occurred in a tectoni-
cally active environment at Girvan, against a background of global regression and transgres-
sion during the late Ashgill and early Llandovery respectively.
Acknowledgements
I thank Dr D. M. Williams for advice regarding sedimentology and for his comments on the manuscript.
Dr A. W. Owen kindly provided unpublished data on the High Mains trilobites and valuable discussion.
Much of the fieldwork was carried out during tenure of a N.E.R.C. research studentship at Queen’s
University, Belfast.
References
Baarli, B. G. & Harper, D. A. T. 1986. Relict Ordovician brachiopod faunas in the Lower Silurian of
Asker, Oslo Region, Norway. Norsk geol. Tidsskr., Oslo, 66: 87-98.
Bluck, B. J. 1983. Role of the Midland Valley of Scotland in the Caledonian orogeny. Trans. R. Soc.
Edinb. (Earth Sci.) 74: 119-136.
Brenchley, P. J. & Newall, G. 1980. A facies analysis of Upper Ordovician regressive sequences in the
Oslo region, Norway: a record of glacio-eustatic changes. Palaeogeogr. Palaeoclimat. Palaeoecol.,
Amsterdam, 31: 1-38.
Cocks, L. R. M. & Toghill, P. 1973. The biostratigraphy of the Silurian rocks of the Girvan District,
Scotland. Q. JI geol. Soc. Lond. 129: 209-243, pls 1-3.
Dott, R. H. jr & Bird, K. J. 1979. Sand transport through channels across an Eocene shelf and slope in
southwestern Oregon, U.S.A. Spec. Publs Soc. econ. Paleont. Miner., Tulsa, 27: 327-342.
Harper, D. A. T. (1979a). The brachiopod faunas of the Upper Ardmillan succession (upper Ordovician),
Girvan, S.W. Scotland. Unpublished Ph.D. thesis, Queen’s University, Belfast.
1979b. The environmental significance of some faunal changes in the Upper Ardmillan succession
(upper Ordovician), Girvan, Scotland. Spec. Publs geol. Soc. Lond., 8: 439-445.
—— 1981. The stratigraphy and faunas of the Upper Ordovician High Mains Formation of the Girvan
district. Scott. J. Geol., Edinburgh, 17: 247-255.
—— 1982. The stratigraphy of the Drummuck Group (Ashgill), Girvan. Geol. J., Liverpool, 17: 251-277.
—— 1984. Brachiopods from the Upper Ardmillan succession (Ordovician) of the Girvan district, Scot-
land. Part 1. Palaeontogr. Soc. (Monogr.), London. 78 pp., 11 pls.
Ince, D. 1984. Sedimentation and tectonism in the Middle Ordovician of the Girvan district, S.W.
Scotland. Trans. R. Soc. Edinb. (Earth Sci.) 75: 225—237.
Ingham, J. K. 1978. Geology of a continental margin. 2: Middle and Late Ordovician transgression,
Girvan. Geol. J., Liverpool, (Spec. Iss.) 10: 163-176.
& Wright, A. D. 1970. A revised classification of the Ashgill Series. Lethaia, Oslo, 3: 233-242.
Lamont, A. 1935. The Drummuck Group, Girvan: a stratigraphical revision with descriptions of fossils
from the lower part of the group. Trans. geol. Soc. Glasg., 19: 288-334, pls 7-9.
—— 1949. New species of Calymenidae from Scotland and Ireland. Geol. Mag., Hertford, 86: 313-323.
Lapworth, C. 1882. The Girvan succession. Part 1. Stratigraphy. Q. JI geol. Soc. Lond. 38: 537-666, pls
24-25.
Mitchell, W. I. 1977. The Ordovician Brachiopoda from Pomeroy, Co. Tyrone. 138 pp., 28 pls. Palaeon-
togr. Soc. (Monogr.), London.
Peach, B. N. & Horne, J. 1899. The Silurian rocks of Britain. I, Scotland. Mem. geol. Surv. U.K., London:
1-749.
Owen, A. W. 1986. The uppermost Ordovician (Hirnantian) trilobites of Girvan, S.W. Scotland with a
review of coeval trilobite faunas. Trans. R. Soc. Edinb. (Earth Sci.) 77 (3): 231-239.
oe D. A. T. HARPER
Reed, F. R. C. 1917. The Ordovician and Silurian Brachiopoda of the Girvan District. Trans. R. Soc.
Edinb. 51: 795-998, pls 1-24.
Rong Jia-Yu 1984a. Distribution of the Hirnantia fauna and its meaning. In D. L. Bruton (ed.), Aspects of
the Ordovician System: 101-112. Universitetsforlaget, Oslo.
—— 1984b. Brachiopods of latest Ordovician in the Yichang district, western Hubei, central China. In
Nanjing Institute of Geology and Palaeontology, Academia Sinica, Stratigraphy and Palaeontology of
Systemic Boundaries in China: Ordovician—Silurian boundary 1: 111-190, pls 1-14. Anhui Sci. Tech. publ.
House.
Sheehan, P. M. 1977. Late Ordovician and earliest Silurian meristellid brachiopods in Scandinavia. J.
Paleont., Tulsa, 51: 23-43, pls 1-3.
Walton, E. K. 1983. Lower Palaeozoic—Stratigraphy. In G. Y. Craig (ed.), The Geology of Scotland:
105-137. Edinburgh.
Williams, A. 1962. The Barr and Lower Ardmillan Series (Caradoc) of the Girvan district, south-west
Ayrshire, with descriptions of the Brachiopoda. Mem. geol. Soc. Lond. 3: 1-267, pls 1-25.
Base of the Silurian in the Lake District and Howsgill
Fells, Northern England
R. B. Rickards
Department of Earth Sciences, Downing St, Cambridge CB2 3EQ
Synopsis
The basal Silurian in the Lake District and Howsgill Fells is divided into four slightly different deposi-
tional zones, only one of which shows a provable base to the acuminatus Zone, being underlain by a
persculptus fauna and overlain by an atavus fauna. Other sections have ‘Basal Beds’ which certainly
represent very condensed deposition of carbonates, perhaps involving non-sequences. The varied
environments are interpreted as part of an offshore fault-scarp-cum-ridge-and-hollow system paralleling
the Iapetus Suture and situated upon the southern (northward-dipping) plate.
There are essentially four rather different depositional environments at the Ordovician—Silurian
boundary in the Lake District proper and in the Howgill Fells; and these are each different
again from the facies and faunal development at Cross Fell, dealt with by Wright elsewhere in
this volume. The four types are shown in Figs 1—4: although drawn diagrammatically it is
important to realize that there are no exposure gaps in the region of the boundary, and that the
sections in the Howsgill Fells and western Lake District (Figs 1, 4) can be confirmed in many
other nearby sections.
The acuminatus Zone fauna, the new basal Silurian zone, is well represented except in one
small region only, namely the classic Skelgill section (Fig. 3), the type section of the Skelgill
Beds black shale formation. On this section there is a thin, hard, partly calcareous and partly
siliceous shelly mudstone (usually referred to in the literature as the Basal Beds). A similar bed
occurs in the Howgill Fells, but the age on Skelgill could range from the persculptus Zone to
the lower atavus Zone inclusive, for it is underlain by Ashgill Shales (Hirnantian; and probably
of anceps Zone age) and overlain by upper atavus Zone black shales. The Basal Beds certainly
represent a period of condensed deposition and possibly of non-sequence. There is no direct
evidence of hardground criteria. The shelly fossils include Atrypa flexuosa and may represent
relatively deep water community life with low diversity.
In the Howsgill Fells and the eastern Lake District (Figs 1, 2) the acuminatus Zone is well
established but its base, and hence the base of the Silurian, cannot be proved: the Basal Beds in
the Howgill Fells might be of persculptus Zone age, but a possible bentonite separates those
beds from the thin acuminatus Zone black shale; and at Browgill a 0:08 m rottenstone, possibly
the lithological and stratigraphical equivalent of the Basal Beds, separates Hirnantian Ashgill
shales from black, acuminatus Zone shales.
Only in the western Lake District (Fig. 4) can the base of the Silurian be unequivocally
placed, albeit on numerous sections in the region. The Yewdale Beck section is well and
continuously exposed, and above 0:3m of beds with a good persculptus Zone fauna are 11m of
black shales with a very rich assemblage of acuminatus Zone graptolites (Hutt 1974). The
persculptus Zone also contains numerous shelly fossils of most groups, but they have not been
extensively studied. The Ashgill Shales below them yield numerous brachiopods and rarer
trilobites giving a Hirnantian age to the Ashgill Shales, but graptolites in these beds are rare.
The acuminatus Zone black shales yield shelly fossils only very infrequently and none to date
have proved to be of diagnostic value. In every other respect, however, the Yewdale Beck
section provides a good confirmatory section for the base of the Silurian, especially as an
almost infinite number of both natural and artificial sections are available in the general region
of Coniston and on the fells and streams to the southwest of that town. Graptolites from these
sections can be collected by the hundred and, as with all other acuminatus Zone faunas
mentioned above, almost all the typical species of the zonal assemblage occur.
Bull. Br. Mus. nat. Hist. (Geol) 43: 53-57 Issued 28 April 1988
54
Fig. 1
R. B. RICKARDS
continuous exposure
into Wenlock
Cc
‘)
5 black shales
ao] atavus Zone
3 7
= mM.
ag
O-Im.
acuminatus Zone
<— ?P bentonite
black shales
O:lm
Waar av aaa a
fe)
Skelgill Beds
Paleestis: |
Lia
P persculptus Zone
in part
banded grey
: calcareous nodules
shales & silts
anceps Zone
in part
Hirnantian
Ashgill Shales
~~
tm
Cc
@
E
a
©
@
>
®
a2)
@
Qa.
>
=_—
~—
wharfe
conglomerate
Howsgill Fells: beds about the Ordovician—Silurian boundary on Spengill, Grid Reference
SD 698998.
BASE OF THE SILURIAN IN THE LAKE DISTRICT 5)5)
I continuous exposure
into Browgill Beds
(in type development)
w
S| ,
5 a black shales
3 = atavus Zone
2
ao 7p)
brown-weathering :
acuminatus Zone
008m! black shales a
ol “(7171 =P persculptus Zone
S 3 rey shales
re
= (dp) ae,
Cc
So =
=e! £
ae (7p)
= § not to scale:
thickness of
units given
in metres
Fig. 2 Eastern Lake District: beds about the Ordovician—Silurian boundary on Browgill, NY 4974
0587.
Rickards (1978) attempted a general interpretation of the environment of deposition of the
basal Silurian strata, envisaging a west- or northwest-facing fault scarp, according to Hutt
(1974) active during deposition of the early Llandovery, against which were deposited deeper
offshore, black shales and upon which and behind which were deposited the Basal Beds and
their equivalents. By upper atavus Zone times the scarp feature was further submerged and
covered in black shale deposition. Associated with these features were a series of ridges and
hollows striking ENE/WSW, that is roughly the same as the fault scarp strike. The hollows
received a greater thickness of black shale in a more highly anaerobic environment (Rickards
1964). The ridge and hollow system persisted in the Howsgill Fells region, and possibly in the
main Lake District outcrop, until late in the Llandovery.
Thus the onset of the Silurian in the Lake District is marked by condensed deposition of
shelly limestone, and possible non-sequences, in the eastern, presumed shoreward or shallower
region; and by relatively thick, black shale deposition in the western Lake District. The post-
glacial marine transgression is recorded in the gradual spread of black shale deposition over the
whole region, the last area to succumb being the eastern Lake District area of Skelgill which is
interpreted as being on the crest of an old scarp structure, itself certainly operative as far back
as the Caradoc. It seems likely that the region was situated atop the northward-dipping plate,
south of the Iapetus Suture. The scarp and ridge/hollow systems may be a result of the
northwards subduction process, to which they are parallel, and which resulted in a combination
of compressional and extensional features.
t continuous exposure
to crispus Zone
= oo
=
= Cc
(Ss (2)
Oo] v0 Ee black shales
3 m upper atavus Zone
ox — ®
©l=5
Zo
=< @
n>
—_—
? persculptus —
lower atavus Zones
grey shales
P anceps Zone
and silts
Hirnantian
w
2
tS)
or
op)
oO
‘iS
D
4
Coniston
limestone
not to scale:
thickness of
units given
in metres
Fig. 3 Eastern Lake District: beds about the Ordovician—Silurian boundary on Skelgill, NY 3964
0320.
BASE OF THE SILURIAN IN THE LAKE DISTRICT 57
t continuous exposure
into convolutus Zone
black shales atavus Zone
black shales acuminatus Zone
Rhuddanian
Skelgill Beds
blue/grey shales persculptus Zone
0O-3m
grey shales
and siltstones
+
calcareous
nodules
Hirnantian
Ashgill Shales
exposure failure
not to scale:
thickness of
units given
in metres
Fig. 4 Western Lake District: beds about the Ordovician-Silurian boundary at Yewdale Beck,
SD 3073 9858.
References
Hutt, J. E. 1974. The Llandovery graptolites of the English Lake District. Part 1. 56 pp., 10 pls. Palaeon-
togr. Soc. (Monogr.), London.
Rickards, R. B. 1964. The graptolitic mudstone and associated facies in the Silurian strata of the Howgill
Fells. Geol. Mag., Hertford, 101: 435-451.
—— 1978. In J. K. Ingham et al., The Upper Ordovician and Silurian Rocks. In F. Moseley (ed.), The
Geology of the Lake District. Occ. publ. Yorks. geol. Soc. 3: 121-245.
The Ordovician—Silurian boundary at Keisley,
Cumbria
A. D. Wright
Department of Geology, The Queen’s University of Belfast, Belfast BT7 1NN, Northern Ireland
Synopsis
At Keisley, in the Cross Fell Inlier of Cumbria, the lowest Silurian graptolite biozone recorded until
recently was that of A. atavus, with the topmost of the underlying carbonates regarded as being of either
Lower Llandovery or Hirnantian age. A temporary excavation has confirmed the Hirnantian age of the
latter, and with the discovery in the overlying clastic sediments of the biozones of both G. persculptus and
P. acuminatus, the Ordovician—Silurian boundary is now accurately located.
Although Upper Ordovician and Lower Silurian rocks crop out in the Cross Fell Inlier of
northern England, the area is much faulted (Shotton 1935). Moreover, where reasonably con-
tinuous graptolite sequences of the Lower Silurian are exposed in Swindale Beck (Knock) and
in Great Rundale Beck (Marr & Nicholson 1888: 699; Burgess & Rushton 1979: 23), the lowest
biozones (below Coronograptus cyphus) are missing. Until recently, the earliest Silurian graptol-
ite biozone was that recorded from the road cutting to Keisley Quarry by Marr (1906: 485) and
reported by him as indicating the Dimorphograptus confertus Zone of Marr & Nicholson (1888).
The lowest part of that zone has been shown by Rickards (1970) to equate with the Ata-
vograptus atavus Biozone, and the presence of beds of this age was confirmed by Rickards from
graptolite material excavated in 1965 from this locality by Temple (1968: 2).
On the Upper Ordovician side of the boundary the stratigraphical relationships and precise
age of the main unit, the Keisley Limestone, have been debated for many years. The limestone
has been a source of geological interest since the last century as it contains a prolific shelly
fauna, is of distinctive lithology, and has a peculiar morphological form referred to as a ‘knoll’
by Marr (1906: 485). The views on various aspects of this mudmound have been discussed by
Wright (1985); only the relationships of the carbonate mudmound to the atavus Biozone
graptolite shales are relevant in the present context.
Marr (1906: 485) noted that the Ashgill Shales, which do occur in Swindale Beck, were not
present at Keisley; and as there was insufficient room for these beds between the Silurian
graptolite shales and the nearest outcrops of Keisley Limestone, he interpreted the junction as a
faulted one. Burgess (1968: 343) noted that along the track leading to the quarry, the massive
limestone was succeeded by calcareous mudstones and limestone nodules which were in turn
overlain by the graptolite shales ‘in apparently conformable sequence’, and the presence of this
apparently unfaulted and conformable relationship was subsequently reiterated by Burgess et
al. (1970: 170), despite the discontinuous nature of the outcrops. An extensive brachiopod and
trilobite fauna was collected by Temple (1968, 1969) from weathered limestone bands associated
with unfossiliferous shales at the bend in the quarry track; this outcrop was separated by a few
metres from those of both the underlying massive limestone and the overlying atavus Biozone
shales, and the extensive fauna interpreted by Temple as being of Lower Llandovery age, a view
supported by Burgess & Rushton (1979: 23) but not by Ingham & Wright (1972: 47), who
regarded it as being of Hirnantian age.
The difficulty with the Keisley locality is that the beds immediately below the established
atavus Biozone graptolite shales are concealed beneath the trackway to the quarry. To over-
come this a temporary trench was dug with the aid of a mechanical digger and the complete
sequence exposed (Wright 1985). Fig. 1 shows the position of the trench across the trackway
and Fig. 2 the lithological log obtained.
Bull. Br. Mus. nat. Hist. (Geol) 43: 59-63 Issued 28 April 1988
60 A. D. WRIGHT
aC
Keisley a
—_ Keisley New Quarry
Ss ob
q vd
as
5 metres
Fig. 1 Plan showing the position of the temporary trench excavated across the trackway at the
eastern end of Keisley New Quarry to reveal the Ordovician—Silurian boundary (National Grid
Ref. NY 7137 2379). The strikes in the trench were taken on the trench floor except for the two
strikes at the southern end which are in the trench walls and thus well up in the atavus Biozone.
Stippling in the bank to the east of the trench indicates outcrops of fossiliferous weathered lime-
stones, the fauna of which was described by Temple (1968, 1969). Stylized trees (not to scale)
represent two ash (light outlines), two sycamores (dark outlines) and a hawthorn (small figure). The
inset figure shows the position of Keisley in the Cross Fell Inlier (shaded) in relation to north-west
England (C—Carlisle; K—Kendal).
The lower part of the sequence up to and including unit 8 (numbering as in Wright 1985)
consists of alternations of bedded limestones or calcareous nodules with calcareous siltstones.
The bedded bioclastic limestones are fresh and although pelmatozoan debris, bryozoan frag-
ments and the occasional brachiopod (including Hirnantia sagittifera) are to be seen on the bed
surfaces, faunal lists are scant compared with those of Temple (1968) obtained from the well
weathered material above the trackway. Gastropods, ostracodes and a few trilobites have been
observed in thin sections of the trench limestones in which abundant Girvanella is probably the
most revealing element palaeoenvironmentally.
The unit 7 siltstone, while by no means abundantly fossiliferous, does have a shelly fauna in
the form of moulds, albeit in a broken and fragmented state. The diverse fauna includes the
brachiopods Dolerorthis praeclara, Hindella sp., Hirnantia sagittifera, ? Oxoplecia, Paracraniops
sp., Reuschella inexpectata, Skenidioides scoliodus, Sphenotreta sp. and Toxorthis proteus
ORDOVICIAN-SILURIAN BOUNDARY IN CUMBRIA
20 cm
— oo |
Volcanic clay
Black shale
Blotchy siltstone
Green siltstone
Calcareous siltstone
Rottenstone breccia
Calcareous nodules
Limestone
HEIL IMU
61
SILURIAN
ORDOVICIAN
Hirnantia
shelly
fauna
Fig. 2 The lithological log obtained for the trench cutting across the quarry track of Fig. 1, showing
the position of the Ordovician—Silurian boundary. Numbers of lithological units discussed in the
text are as in Wright (1985). The black spots and bars to the right of the log respectively indicate
specific horizons or bulk samples yielding graptolite assemblages.
62 A. D. WRIGHT
together with dalmanellid, lingulide, orthid, sowerbyellid, strophomenide and triplesiid frag-
ments. In addition to pelmatozoan and bryozoan debris, trilobite, bivalve and hyolith fragments
are also recorded. This is a Hirnantia shelly fauna, and differs principally from Temple’s fauna
in the apparent complete absence of craniids which accounted for more than two-fifths of the
entire brachiopod assemblages from the weathered limestones (Temple 1968: 9).
Overlying these beds is a thin (7cm) rottenstone breccia (units 9 and 10). This is the only
indication of a break in the sequence and is interpreted as the result of minor tectonic move-
ment along the surface of lithological change from the underlying carbonate dominated
sequence to the overlying fine-grained and non-carbonate clastics. Angular clasts of both
fossiliferous shelly Hirnantian and unfossiliferous greenish siltstone (matching the unit 11
sediment) occur in the breccia. No diagnostic shelly fossils have been located in the sequence
above unit 10. The first graptolites recovered by Rickards are from a horizon 2cm below the
top of unit 11 and indicate the Glyptograptus persculptus Biozone. This fauna comprises Cli-
macograptus cf. miserabilis, Climacograptus ? medius, Glyptograptus sp. and Glyptograptus ex gr.
persculptus.
Unit 12 is an 8cm unit of silt with a blotchy and mottled appearance produced by an
increase in the proportion of dark muddy silt that first appears in the greenish siltstones of unit
11 (Wright 1985: 269). Despite clear evidence of bioturbation, a small graptolite fauna from a
bulk sample of the unit contained specimens of Climacograptus normalis and cf. Parakido-
graptus acuminatus, and indicates the presence of the Parakidograptus acuminatus Biozone. The
Ordovician-Silurian boundary at Keisley is accordingly placed at the base of lithological unit
12. This seems to be the most logical horizon although, as noted previously (Wright 1985),
there is clearly a little uncertainty regarding the precise appearance of the acuminatus fauna
within a bulk sample taken from the 8 cm unit.
Unit 13 lithologically shows a further stage in the transition from the greenish siltstones at
the base of unit 11 towards the micaceous black silty shales of the overlying sequence. In this
unit the dark material is dominant, although some horizons and patches of the greenish-grey
siltstones still occur; concomitantly with the overall colour change, bioturbation disappears. At
2:5cm above the base of this unit, the first of a series of bentonite clays occurs. A fauna
collected from a bulk sample above this clay (Fig. 2) yielded Climacograptus medius, Cli-
macograptus cf. normalis and Dimorphograptus sp. This assemblage is identified by Rickards as
a post-acuminatus one, 1.e. from the base of the atavus Zone. Accordingly the acuminatus—atavus
boundary is placed at the thin bentonite band, which is a useful marker that may assist with
correlation elsewhere, although the appearance of atavus Biozone bentonites in the Keisley
trench is a major surprise in the northern England context (Wright 1985). The increasingly rich
graptolite faunas from the overlying sequence in the trench all belong to the atavus Biozone.
Thus although the persculptus and acuminatus Biozones occur in thin lithological units at
Keisley, both do occur and accordingly enable the Ordovician—Silurian boundary to be pre-
cisely located.
References
Burgess, I. C. 1968. p. 343 in F. W. Shotton, A. J. Wadge & I. C. Burgess, Cross Fell Area (Field Meeting).
Proc. Yorks. geol. Soc., Leeds, 36: 340-344.
, Rickards, R. B. & Strachan, I. 1970. The Silurian strata of the Cross Fell area. Bull. geol. Surv. Gt
Br., London, 32: 167-182.
& Rushton, A. W. A. 1979. Skelgill Shales. In I. C. Burgess & D. W. Holliday, Geology of the country
around Brough-under-Stainmore. Mem. geol. Surv. Gt Br., London, Sheet 31. 131 pp.
Ingham, J. K. & Wright, A. D. 1972. The North of England. In A. Williams et al., A correlation of
Ordovician rocks in the British Isles. Spec. Rep. geol. Soc. Lond. 3: 43-49.
Marr, J. E. 1906. On the stratigraphical relations of the Dufton Shales and Keisley Limestone of the
Cross Fell Inlier. Geol. Mag., London, (dec. 5) 3: 481-487.
& Nicholson, H. A. 1888. The Stockdale Shales. Q. JI geol. Soc. Lond. 44: 654-732.
Rickards, R. B. 1970. The Llandovery (Silurian) graptolites of the Howgill Fells, Northern England.
Palaeontogr. Soc. (Monogr.), London. 108 pp., 8 pls.
ORDOVICIAN-SILURIAN BOUNDARY IN CUMBRIA 63
Shotton, F. W. 1935. The stratigraphy and tectonics of the Cross Fell Inlier. Q. Jl geol. Soc. Lond. 91:
639-701.
Temple, J. T. 1968. The Lower Llandovery (Silurian) brachiopods from Keisley, Westmorland. Palaeon-
togr. Soc. (Monogr.), London. 58 pp., 10 pls.
— 1969. Lower Llandovery (Silurian) trilobites from Keisley, Westmorland. Bull. Br. Mus. nat. Hist.,
London, (Geol.) 18: 197—230.
Wright, A. D. 1985. The Ordovician-Silurian boundary at Keisley, northern England. Geol. Mag., Cam-
bridge, 122: 261-273.
Ordovician-Silurian boundary strata in Wales
J. T. Temple
Department of Geology, Birkbeck College, Gresse Street, London W1P 1PA
Synopsis
Ordovician-Silurian boundary strata in Wales belong to the shelly facies in the south and east, and to the
graptolitic facies in the north and west. In the graptolitic facies the zones of Dicellograptus anceps,
Glyptograptus persculptus and Parakidograptus acuminatus occur, but the Climacograptus? extraordinarius
Zone is not known. The anceps Zone is restricted to central and west Wales; the persculptus Zone is
widespread and is preceded by a sudden lithological change; the acuminatus Zone is preceded by a more
gradual lithological change. Graptolites occur sporadically in boundary strata of the shelly facies but are
not abundant enough for the base of the acuminatus Zone to be recognized in this facies. Records of the
Hirnantia fauna in Wales are summarized.
Introduction
As a result of Caledonian and Hercynian folding the Ordovician-Silurian boundary strata in
Wales form a complex arcuate pattern striking approximately NE-SW through much of central
Wales but becoming east-west in south-west Wales and SE-NW in north-east Wales. The
length of outcrop is approximately 750 km. The outcrop is shown in Fig. 1, together with index
numbers by which individual areas and the references relating to them are cited in the text.
In places on the outward (S, SE or E) side of the Caledonian fold belt in Wales, as in the
adjoining parts of England, the local base of the Silurian is formed by late Llandovery (post-
convolutus or post-sedgwickii) or Wenlock strata transgressive onto pre-Ashgill strata. This
relation is found in the southernmost outcrop (but not in the main northern outcrop) at
Haverfordwest (la), near Llandeilo (2), from north of Llandovery (4) to Garth (5a, b), near
Builth Wells (6), east of Abbey-Cwmhir (7), and east of Welshpool (25, 26). Flanking this
marginal area of late Llandovery transgression there is an unconformity of lesser magnitude
between the early Llandovery and the Ashgill (and Caradoc) near Welshpool (27) and Llan-
santffraid ym Mechain (31), and although the gap continues to diminish northwards and
westwards it is recorded as still present in the Meifod and Vyrnwy areas (28, 29). Elsewhere in
Wales the early Llandovery is believed to follow the topmost Ordovician with no sedimentary
gap.
Boundary strata
Ordovician-Silurian boundary strata in Wales show two facies, shelly and graptolitic. The
shelly facies consists of detrital sediments, mainly of the silt and sand grades, with a fauna
predominantly of brachiopods. The graptolitic facies consists of fine detrital sediments
(mudstones and shales) with some coarser horizons interpreted as turbidites, and with a fauna
almost exclusively of graptolites.
In pre-persculptus Zone strata the dichotomy into shelly and graptolitic facies is not as
clearly defined as later. The strictly graptolitic facies, as defined by the recorded presence of the
Dicellograptus anceps Zone, is much more restricted in occurrence (to central and west Wales—
16, 18, 19, 20) than the persculptus Zone, and even where both zones occur in the same area the
intervening strata are either unfossiliferous (16, 18, 19) or include shelly fossils (20). Along the
outcrop north-west of the Towy anticline (8-14), for instance, where the persculptus Zone is
graptolitic, the very thick underlying strata yield only sporadic graptolites (not diagnostic of
the anceps Zone), being otherwise unfossiliferous or with a few shelly fossils. The restriction of
the demonstrable anceps Zone to central and west Wales and the wider extent eastwards of the
persculptus and acuminatus Zones are consistent with regression during anceps Zone time
followed by transgression during the persculptus Zone. The extraordinarius Zone has not been
Bull. Br. Mus. nat. Hist. (Geol) 43: 65-71 Issued 28 April 1988
66 J. T. TEMPLE
36H
oH
y 33 32a,b
He 34,35° Llangollen
Tp
HBerwyn Hills
39 30a,b
cs a
te Welshpool
25
(>-- shelly facies
er : Early
C= graptolitic facies Mandowen,
absent by overlap
Numbers refer toindex of areas & references
H Hirnantia fauna recorded
of
4? Ulandovery
S/3a 4
Haverfordwest meee
< see 2
Se
Swansea
e
QO 5 10 15 2O 25 3)
feo,
kilometres
Fig. 1 Ordovician—Silurian boundary outcrop areas in Wales and the Welsh Borderland. 1, Haver-
fordwest: la, Strahan et al. 1914; 1b, Cocks & Price 1975. 2, Llandeilo, Williams 1953. 3, 4,
Llandovery: 3a, Jones 1925; 3b, Jones 1949; 4, Cocks et al. 1984. 5, Garth: Sa, Andrew 1925; 5b,
Williams & Wright 1981. 6, Builth Wells, Jones 1947. 7, Abbey-Cwmhir, Roberts 1929. 8, 9,
Rhayader: 8, Lapworth 1900; 9, Kelling & Woollands 1969. 10, Rhayader to Abergwesyn, Davies
1928. 11, Abergwesyn to Drygarn, Davies 1926. 12, Pumpsaint, Davies 1933. 13, Llansawel, Drew
& Slater 1910. 14, Llangranog, Hendricks 1926. 15, Llanidloes, Jones 1945. 16, Plynlimon, Jones
1909. 17, Machynlleth, Jones & Pugh 1916. 18, Towyn and Abergynolwyn, Jehu 1926. 19, Corris,
Pugh 1923. 20, Dinas Mawddwy, Pugh 1928. 21, Llanuwchllyn-Llanymawddwy, Pugh 1929. 22,
Bala: 22a, Elles 1922; 22b, Bassett et al. 1966. 23, Cerrigydrudion, Marr 1880. 24, Conwy, Elles
1909. 25, Shelve area, Whittard 1932. 26, 27, Welshpool: 26, Wade 1911; 27, Cave 1965. 28, Meifod,
King 1928. 29, Lake Vyrnwy, King 1923. 30, Berwyns: 30a, Wedd et al. 1929; 30b, Brenchley &
Cullen 1984. 31, Llansantffraid ym Mechain, Whittington 1938. 32, Llangollen: 32a, Groom &
Lake 1908; 32b, Hiller 1980. 33, Corwen, Lake & Groom 1893. 34, Llangollen, Wills & Smith 1922.
35, Llangollen, Wedd et al. 1927. 36, Mynydd Cricor, Smith 1935. 37, Criccieth, Roberts 1967. 38,
Anglesey, Greenly 1919. 39, W. Berwyn, A. W. A. Rushton & J. T. Temple (unpublished). 40,
Aberystwyth and Machynlleth, Cave & Hains 1986.
ORDOVICIAN-SILURIAN BOUNDARY STRATA IN WALES 67
recognized in Wales, but there is ample room for it: the barren strata between the anceps and
persculptus Zones in areas 16, 18, 19, 20 are respectively 730m, 1000 m, 690 m, and 180m thick.
In the persculptus Zone and the succeeding early Llandovery the dichotomy into shelly and
graptolitic facies is well shown. The shelly facies forms a narrow belt running through Haver-
fordwest (1a, b), the Llandovery (3a, b, 4) and Garth (5a, b) areas (which form north-westward
salients from the adjacent line of outcrop along which the strata are missing), and the eastern
end of the Berwyn dome (27-32, 34-36). The transition from shelly to graptolitic facies of the
persculptus Zone and early Llandovery takes place in south and central Wales across the Towy
anticline (between for instance Llandovery [3a, b, 4] and Pumpsaint [12]), and in north-east
Wales probably north-westwards across the Berwyn dome. The persculptus and acuminatus
Zones are widespread, having been recorded from north-west of the Towy anticline (10-12) as
well as through most of central and west Wales (14-21, 40). G. persculptus occurs on the
western outcrop around the Berwyn Hills at Nant Pant-y-llidiart, north of Lake Vyrnwy (39),
and there is an informal record of the species at Bwlch yr Hwch, 5km SE of Bala (Jones in
Pugh, 1929: 274-S). The persculptus Zone (but not the acuminatus Zone) has also been recorded
from the north end of the Towy anticline (7, 9), and G. cf. persculptus occurs at Garth (5a). The
early Llandovery graptolite succession between Bala (22a, b) and Conwy (24) is still in need of
reinvestigation. In the two small isolated outliers near Criccieth (37) and in Anglesey (38) the
early Llandovery is in graptolite facies, but in both cases the relationship to the Ordovician is
obscure and neither the persculptus nor the acuminatus Zones are recorded.
A sudden and striking lithological change heralds the incoming of persculptus Zone graptol-
ites in west and central Wales (14—20, 40): the underlying strata are very thick, usually
unfossiliferous, often unbedded, well cleaved or doubly cleaved, and with many ‘grit’ bands; the
persculptus Zone strata (the ‘Mottled Beds’) are mudstones 5—25m thick, well-bedded, often
with mottled pale bands (interpreted as bioturbated—Cave & Hains 1986) and with a thin
band crowded with the zone fossil about 1m above the base. The suddenness of the lithologi-
cal change preceding the appearance of G. persculptus in this part of Wales betokens some
physical change in the conditions of deposition, and this evidence also is consistent with a
persculptus transgression following regression. A similar lithological contrast at this horizon is
also found north of the Towy anticline (9-11), although not strongly marked in the south of the
outcrop (12).
There is also a lithological change below the acuminatus Zone in west and central Wales
(15-20, 40), but it is more gradual than that below the persculptus Zone, the hard resistant
mottled mudstones of the latter zone being gradually replaced by rusty red- and yellow-
weathering mudstones without bioturbation (40). A similar change occurs at this horizon north
of the Towy anticline (10-12). In both areas the change probably precedes the end of the
persculptus Zone (40, 12).
Hirnantia fauna in Wales
Around the Berwyn dome and near Llangollen there are developed ‘grits’ which have been
taken as either topmost Ordovician (35) or basal Silurian (28, 29): Craig-wen Sandstone (28),
Meristina crassa Sandstone (29), Allt-g6ch Grit (30), Corwen Grit (33), Glyn Grit (32), Plas
uchaf Grit (35). These grits have been interpreted as channel-fill deposits formed during the
Hirnantian regression (30b). ‘Grits’, possibly of the same age as those around the Berwyns, also
occur in the north and east of the Bala area (Calettwr Quartzite—22b) and along the little-
known outcrops north of Bala, i.e. at Cerrigydrudion (23) and Conwy (Conwy Castle Grit—24).
South of the Berwyns there are ‘grit’ bands near Abbey-Cwmhir (7) which are mapped as
topmost Ordovician but whose relationship to the persculptus Zone strata occurring about
3km to the west needs reinvestigation.
Many of the ‘grits’ in these different areas include elements of the Hirnantia fauna (Fig. 1), for
which Brenchley & Cullen (1984: 122) give faunal lists at various Welsh localities. To these
68 J. T. TEMPLE
authors’ list for ‘Meifod’ (i.e. Craig-wen quarry, near Meifod) may be added the record of the
tretaspid indet. discovered on the Silurian Subcommission excursion in 1979, although the
presence of pebbles of underlying strata in the Craig-wen Sandstone suggests the possibility of
this being a derived fossil. The Hirnantia fauna also occurs in Afon Tanat on the western
outcrop of the Berwyn Hills (39). The Hirnantia fauna at its type area south of Bala (22a) was
considered by Pugh (1929: 273) to be pre-persculptus in age although no single section (except
Jones’ record at Bwlch yr Hwch—see above—which awaits confirmation) shows the one fauna
succeeding the other. Further southwestwards along the outcrop (beyond 20) in west and
west-central Wales the Hirnantia fauna dies out while the persculptus Zone fauna becomes
more clearly developed. South of the Towy anticline the Hirnantia fauna has been recorded
from Garth (5a, b) apparently in association with G. cf. persculptus (Williams & Wright 1981:
38), and from Haverfordwest (1b) in the St Martin’s Cemetery Beds (Cocks & Price 1975: 710)
whose relations to the persculptus Zone are unknown. The Hirnantia fauna has also recently
been found in the Llandovery area (4) where it is considered (Cocks et al. 1984: 144) to underlie
strata probably representing the persculptus Zone.
At Conwy (24) the Hirnantia fauna is underlain, as in the English Lake District, by strata
containing abundant Dalmanitina [Mucronaspis auctt.], and this relationship is found also at
Bala (22a) and in the Llanuwchllyn-Llanymawddwy area to the south (21). The trilobite persists
southwestwards along the outcrop, as the facies change and the rocks thicken, even further
(20, ?19) than the Hirnantia fauna. On the other hand the Hirnantia fauna around the Berwyns
(28-30), at Abbey-Cwmhir (7) and at Garth (5) is not accompanied or preceded by Dalmanitina
(except for a possible record in area 28—King 1928: 687), and although the absence of the
latter trilobite in the Berwyns may be due to a stratigraphical gap below the Hirnantia ‘grits’,
there is no evidence for such a gap at Abbey-Cwmhir or Garth, nor indeed at Llandovery
where Dalmanitina is also absent. At Haverfordwest (1b) Dalmanitina occurs as part of an
unusually rich Hirnantia fauna but is not found in underlying strata.
Descriptions of sections
Boundary strata of four areas merit description: Plynlimon-Machynlleth (16, 17, 40), where the
sequence is graptolitic throughout and where the persculptus and acuminatus Zones are well
developed; Llandovery (3a, b, 4) where the base of the Llandovery was originally defined;
Haverfordwest (1a, b) and Garth (5a, b), in both of which there are apparently continuous
successions in strata of predominantly shelly facies.
Plynlimon—Machynlleth (16, 17, 40). The succession in this area, which is wholly in the graptoli-
tic facies, has recently been described in detail (Cave & Hains 1986). The best sections of the
Mottled Mudstone Member are at the Cardiganshire Slate Quarry (National Grid ref.
SN 6991 9595) and in a stream near Eisteddfa-Gurig (SN 7951 8409), but the faunal transition
between the persculptus and acuminatus Zones has not been investigated in detail.
Strata above Mottled Mudstone Member.
Dark grey rusty-weathering mudstones: in middle, sand-
stones and siltstones near top of acuminatus Zone.
70-145 m Cwmere 5—25m Mottled Mudstone Member.
Formation Banded mudstone with pale bioturbated layers and
phosphatic concretions (both disappearing in topmost
3m). The lowest beds are unfossiliferous but about 1m
above the base is a thin layer (15—-30cm) with abundant
G. persculptus. Pyritized G. persculptus also occur above
this layer.
Bryn-glas Massive mudstone with splintery,
Formation phacoidal cleavage.
ORDOVICIAN-SILURIAN BOUNDARY STRATA IN WALES 69
Garth (5b). The following section is obtained by mapping in strata of predominantly shelly
facies near Garth, 32km NE of Llandovery, Powys (Williams & Wright 1981).
250m + Sandstones & mudstones Rhuddanian shelly fossils
77m+ Garth Bank Formation
11-S5im Cwm Clyd Formation Eostropheodonta hirnantensis
0-30m Speckly Sandstone Hirnantia fauna. (Andrew [5a] records G. cf.
Member persculptus and Mesograptus cf. modestus
parvulus probably from this Member)
Wenallt
0-20m Ooid Member Hirnantia fauna
Formation
0-65m Siltstones Brongniartella cf. robusta
(high Rawtheyan)
Llandovery (4). The following section (transect i, of Cocks et al. 1984) is exposed almost
continuously along a forestry road in the north Llandovery area (base of section at Grid ref.
SN 8467 3962). The Hirnantia fauna, however, is extrapolated from 1-3 km further west.
120m Bronydd Formation Rhuddanian shelly fossils and
graptolites suggesting atavus
and acinaces Zones. Near base
Climacograptus normalis
70m Scrach Formation (Hirnantia fauna in west)
— Tridwr Formation Rawtheyan shelly fossils and
‘uppermost Ordovician’
graptolites
Haverfordwest (1b). The following section (Cocks & Price 1975) is obtained by mapping in
strata of predominantly shelly facies at Haverfordwest, Dyfed, but there are continuous expo-
sures in road and railway sections upwards from about the middle of the Haverford Mudstone
Formation (base of road section at Grid ref. SM 9573 1547).
85m Gasworks Sandstone Formation Graptolites indicating acinaces or atavus Zones
at top. Rhuddanian shelly fossils throughout
370m Haverford Mudstone Formation Rhuddanian shelly fossils. Climacograptus cf. normalis
near middle. ?C. normalis at 9m above base
65m Portfield Formation Hirnantia fauna at top, including Diplograptid
undescr. sp.
— Slade & Redhill Mudstone Formation Rawtheyan shelly fossils
Conclusions
On the assumption (cf. Temple 1978) that graptolite zones are definable and recognizable
entities, then because of the wide extent of the persculptus and acuminatus faunas in central
Wales, the Ordovician-Silurian boundary defined beneath the acuminatus Zone is in principle
widely applicable in Wales. It is not however directly applicable in the marginal belt character-
ized by boundary strata of shelly facies. Even in the recently reinvestigated Llandovery area (4),
where there is an intermingling of shelly fossils and graptolites, the persculptus and acuminatus
Zones are not firmly enough identified for the boundary to be recognized accurately.
70 J. T. TEMPLE
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La Limite Ordovicien—Silurien en France
C. Babin,! R. Feist,? M. Mélou! et F. Paris*
1 Université Claude Bernard-Lyon 1—Département des Sciences de la Terre—27—43 Boulevard
du 11 Novembre—69622 VILLEURBANNE Cédex, France
2 Centre d’Etudes et de Recherches Géologiques et Hydrogéologiques— Place Eugene
Bataillon—34060 MONTPELLIER Cédex, France
3 Institut de Géologie—Faculté des Sciences—Avenue du Général Leclerc—35042 RENNES
Cédex—et GRECO 130007 du C.N.R.S., France
Synopsis
The Ordovician and Silurian systems are well represented in France, but the boundary between them
remains imprecise because there is generally a gap in the lower part of the Llandovery and/or the upper
part of the Ordovician. The zctual documentation for the Armorican Massif and the south-west of France
is briefly revised.
Les systémes ordovicien et silurien sont largement représentes dans les massifs paléozoiques
francais (notamment dans le Massif armoricain et en Montagne Noire au Sud du Massif
central). Pourtant, la limite entre les deux systémes n’est nulle part reconnue avec precision
dans l’état actuel des investigations. Une lacune sedimentaire semble, en réalité, étre assez
généralisée, au moins pour la partie inférieure du Llandovery. Elle peut résulter de l’interférence
d’un ensemble de causes, climatiques et variations eustatiques induites, €pirogéniques et tecton-
iques distensives (échos taconiques) et manifestations volcaniques subordonnéees.
Nous préciserons briévement ces propos pat l’examen de quelques successions.
Le Massif Armoricain
Différents domaines peuvent y étre considérés.
En Normandie, la présence d’Ashgill est attestée par des Conodontes (zone a Amorpho-
gnathus ordovicicus) pour le Calcaire de Vaux (Weyant et al. 1977). Des fragments de ces
calcaires sont repris dans la formation glacio-marine dite des ‘pélites a fragments’ ou Tillite de
Feuguerolles qui est également rapportee a l’Ashgill supérieur grace aux Chitinozoaires qu'elle
renferme (F. Paris inédit). Dans les formations sus-jacentes l’absence apparente des Graptolites
du Llandovery inférieur suggére une lacune correspondant au moins a celui-ci et débutant
peut-étre dans l’Hirnantien.
Dans les parties centrales et orientales du Synclinorium median armoricain, la limite
Ordovicien-Silurien se place entre les Formations de Saint-Germain-sur-Ille et de la Lande
Murée (Fig. 1). Le passage entre ces deux formations est expose dans diverses coupes des
synclinoria du Ménez-Belair et de Laval.
La Formation de Saint-Germain-sur-Ille, dans sa totalité, appartient a l’'Ordovicien supé-
rieur. Elle est habituellement subdivisée en deux unités lithologiques: un Membre inferieur a
dominante arénacée, puissant de 200m environ, et un Membre supérieur, argileux, et nettement
moins développé (quelques dizaines de métres d’épaisseur).
Des interlits argileux noirs s’intercalent dans l'ensemble grésoquartziteux constituant le
Membre inférieur. Déposé dans un environnement littoral, voire tidal, ces grés livrent locale-
ment une abondante faune, généralement rassemblée dans des lits d’accumulation. On y
reconnait notamment des Brachiopodes (Drabovinella erratica), des Trilobites (Calymenella
bayani, Homalonotidae), des Bivalves, et surtout des Graptolites qui ont permis de dater une
partie de ce Membre inférieur (Skevington & Paris 1975). Ces Graptolites, limités a quelques
niveaux gréso-micacés, sont exclusivement représentés par des Diplograptidae (Orthograptus
truncatus truncatus, O. truncatus abbreviatus, O. truncatus pauperatus ainsi que de rares spéeci-
Bull. Br. Mus. nat. Hist. (Geol) 43: 73-79 Issued 28 April 1988
74 BABIN, FEIST, MELOU & PARIS
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= 7
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ra Fig. 1 Colonne stratigraphique a la limite
S
ri
Ordovicien-Silurien dans le Synclinortum du
Menez-Belair. 1. Quartzites et mudstones.
2. Grés et quartzites. 3. Mudstones et silt-
stones. 4. Ampeélites.
=I]
iB
i
hr]
ly
Ww
i
mens de ? Climacograptus miserabilis et de ? Diplograptus fastigatus). S'appuyant sur la fre-
quence relative des diverses sous-espéces de O. truncatus, Skevington & Paris (1975) admettent
que les plus anciens niveaux a Graptolites de la Formation de Saint-Germain-sur-Ille appar-
tiendraient a la partie supérieure de la Zone a D. complanatus, tandis que les niveaux les plus
réecents représenteraient la Zone a D. anceps. La partie supérieure du Membre inférieur de la
formation a donc été attribuée a l’Ashgill. Les Trilobites étudiés par Henry (1980) et les
Brachiopodes, revisés recemment par Mélou (1985), n’apportent pas de précisions strati-
graphiques complémentaires. Quant aux Chitinozoaires, ils n’ont pas été observés dans les
termes les plus élevés de ce Membre inférieur (Paris 1981).
Le Membre supérieur marque un net changement dans la lithologie. Sa base ravine le toit du
Membre inférieur et ses caractéres sedimentologiques (mudstones et siltstones noirs a ‘ball and
pillow structures’) rappellent certains faciés des formations glacio-marines décrites dans
Y’Ordovicien terminal armoricain (Paris 1986). Aucune macrofaune n’y est connue. En revanche
les Acritarches et les Chitinozoaires y sont relativement abondants. En dépit d’un état de
conservation trés médiocre, ces microfossiles évoquent des formes de I’Ashgill supérieur. Si l’on
accepte un parallélisme entre ce Membre supérieur et des formations glacio-marines finiordovi-
ciennes telles que la Formation des ‘Pélites a fragments’ de Normandie ou les argiles micro-
conglomératiques du Nord de l’Afrique, le sommet de la Formation de Saint-Germain-sur-Ille
appartiendrait a l’Ashgill supérieur et vraisemblablement a |’Hirnantien.
La Formation de la Lande Murée débute par un Membre inférieur constitué de quelques
metres de quartzites noirs, pyriteux, admettant des intercalations de mudstones a Graptolites,
trés riches en matiére organique (ampélites) et montrant des teneurs anormalement élevées en
éléments-traces (Dabard & Paris 1986). Le contact avec le Membre supérieur de la Formation
LA LIMITE ORDOVICIEN—SILURIEN EN FRANCE 75
de Saint-Germain-sur-Ille, correspondant a un brusque changement lithologique (Paris 1977),
est plus ou moins bien exposé dans divers affleurements des synclinoria du Ménez-Bélair et de
Laval (ex. carriére des ‘Planches’, en Guitté; carriére de ‘Pont-Douve’, en Médréac; carriére
‘Pioc’, en Vieux-Vy-sur-Couesnon; carriére du Rocher a Andouille-Neuville:; tranchée de
Pautoroute Laval—Le Mans, a Ouest de Saint-Jean-sur-Erve; le ‘Moulin du Few’ en Balazé).
Dans le Synclinorium du Menez-Belair, les premiers niveaux a Graptolites, parfois situés a
moins d’un metre au-dessus du contact entre les deux formations, appartiennent déja au Tely-
chien (sommet de la Zone a turriculatus ou Zone a crispus, selon les localités) (cf. Paris et al.
1980). Dans le Synclinorium de Laval, les premiers Graptolites récoltés dans la partie inférieure
de la Formation de la Lande Murée appartiennent au Wenlock (Paris & Robardet, inédit). De
toute évidence, il existe une lacune sédimentaire séparant les derniers dépéts ordoviciens
(sommet du Membre supérieur de la Formation de Saint-Germain-sur-Ille) des premiers sédi-
ments siluriens (base de la Formation de la Lande Murée). Cette lacune est d’ampleur variable
selon les localités. Dans le Synclinorium du Ménez-Bélair, elle correspond au moins au Rhud-
danien et a I’Aeronien (et peut-étre au sommet de l’Ashgill). Dans le Synclinorium de Laval
cette lacune parait plus importante puisqu elle implique l'ensemble du Llandovery et une partie
du Wenlock.
Au Sud de Rennes, dans le Synclinorium de Martigne-Ferchaud, des travaux cartographiques
(Herrouin, sous presse) ont recemment permis de préciser la succession lithologique locale, au
voisinage de la limite Ordovicien—Silurien.
Succedant aux siltstones micacés, a lits gréseux, de la Formation de Riadan (tradit-
ionnellement rapportée au Caradoc et a lAshgill pro parte), on trouve la Formation de la
Chesnaie (60 a 80m de puissance). Cette unite comprend un ensemble inférieur gréso-
quartziteux et une partie supérieure a4 dominante argileuse. Pour l’instant, la Formation de la
Chesnaie n’a livré aucune faune exploitable. Au-dessus se placent les grés et quartzites blancs de
la Formation de Poligné (60 a 70 m d’épaisseur). Le plus souvent azoique, cet ensemble arénacé
contient localement quelques Graptolites (Philippot 1950) de conservation trop médiocre pour
fournir une attribution stratigraphique réellement fiable. Les premiéres faunes siluriennes sig-
nificatives (Philippot 1950) apparaissent dans les mudstones noirs susjacents (ampélites). I]
s’agit de riches assemblages de Graptolites de la base du Telychien (Zone a turriculatus).
Dans le Synclinorium de Martigné-Ferchaud, la limite Ordovicien—Silurien se place donc
entre le toit de la Formation de Riadan et les ampelites de la base du Telychien. En absence de
tout controle paléontologique rigoureux, la position de cette limite reste donc trés approx-
imative. Une lacune d’une partie du Llandovery, quoique vraisemblable, ne peut pour l’instant
étre demontrée.
Dans la partie occidentale du Synclinorium median, la presquile de Crozon permet
d’approcher la limite Ordovicien—Silurien dans deux contextes différents et tous deux incom-
plets.
La succession observée dans lunité nord de la presquwile (plage du Veryarc’h en Camaret)
demeure d’interprétation difficile (Fig. 2). En concordance sur la Formation des Grés de
Kermeur, datée du Caradoc dans sa partie moyenne (biozone 14 a Jenkochitina tanvillensis;
Paris 1981), la Formation du Cosquer (Hamoumi 1981; Guillocheau 1983) débute par des
shales noirs a lamines gréseuses, bien stratifieés, puis se caracterise par un ensemble a blocs
glissés qui passent progressivement a des boules (‘ball and pillow structures’). Les quartz de
cette formation ont une origine glaciaire (Hamoumi et al. 1981). Vers le sommet, glissements et
déformations s’atténuent, ce qui assure le passage a4 une stratification normale de grés a minces
interlits de schistes noirs (Grés de Lamm-Saoz puissants de 6 métres environ). Ces grés sont
surmonteés par les ampélites de la base du Groupe de Kerguillé qui livrent des Graptolites du
Wenlock (Philippot 1950). La Formation du Cosquer n’a fourni aucun fossile dans les sédiments
autochtones et son 4ge demeure imprécis. Un age ashgillien a cependant été proposé (Paris et
al. 1981) par comparaison notamment avec celui attribué a la formation glacio-marine de la
Tillite de Feuguerolles de Normandie.
Les Grés de Lamm-Saoz furent, pour des raisons de géométrie, rapportés au Valentien
(Silurien inférieur) par Philippot (1950). La recente découverte par l'un de nous (F.P.) de Armor-
| SILURIEN
WENLOCK
ASHGILL
GROUPE DE
COSQUER
KERGUILLE
(2)
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3 og
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ud
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ac
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Zz
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SILURIEN
LUDLOW
GROUPE DE
KERGUILLE
2
DE
LLANDOVERY
CALCAIRES
?
HIRNANTIEN
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TUFS
HIRNANTIEN
DES
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ud
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(0)
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Fig. 2 Contact Ordovicien-Silurien dans la
coupe du Veryarc’>h (Camaret, presquile de
Crozon). 1. Grés et quartzites. 2. Shales.
3. Shales a ‘ball and _ pillow structures’.
10m 4. Megaslumps. 5. Ampelites.
Fig. 3 Contact Ordovicien-Silurien le long de
YAulne, a Est de Tregarvan (presqu’ile de
Crozon). 1. Schistes. 2.Grés et quartzites.
3. Hyaloclastites. 4. Calcaires. # niveau a Hir-
(6+ Ts nantia.
LA LIMITE ORDOVICIEN—SILURIEN EN FRANCE 77
ichitina nigerica dans le dernier interlit noir de ces grés, situeé 4 30cm sous les ampélites
wenlockiennes, permet désormais de proposer, par comparaison avec les pélites a fragments du
Sahara, un 4ge ashgillien supérieur pour la partie sommitale des Grés de Lamm-Saoz. Ainsi se
trouve confirmée l’importance de la lacune qui correspond, dans cette unité nord, a la totalité
du Llandovery.
Dans l’unité sud de la presqu’ile de Crozon, la Formation des Grés de Kermeur est surmon-
tée par un ensemble volcano-sédimentaire désigné Formation des Tufs et calcaires de Rosan.
Aucun affleurement ne permet l’observation continue de la colonne correspondante. La base est
concordante sur la Formation de Kermeur (falaise de Raguenez). Les coupes de la carriére du
four a chaux de Rosan et de la route contigué livrent en abondance Nicolella actoniae. La
récente révision de cette espéce (Harper 1984) permet de considérer que nous sommes ici en
présence de N. actoniae ramosa, sous-espéce de |’Ashgill. Ailleurs, a Lostmarc’h, les calcaires de
Rosan sont également attribuables a l’Ashgill d’aprés les assemblages de Conodontes (zone a
Amorphognathus ordovicicus) selon Paris et al. (1981). Un affleurement isolé le long de I’Aulne, a
Coat-Garrec, a livre des Echinodermes (Chauvel & Le Menn 1972) qui ont confirmé l’age
ashgillien proposé pour cet affleurement par Melou (1971) d’aprés la faune de Leptestiina.
Enfin, il semble que la partie la plus élevée de cette formation soit représentée a l'Est de
Trégarvan, le long de la riviére Aulne. La sedimentation carbonatée y régresse au profit des
dépots arénacés (Fig. 3). L’un de nous a réecemment découvert dans cette coupe (Mélou 1987)
un niveau a Hirnantia sagittifera au-dessus duquel 90 métres de grés et de hyaloclastites avec
quelques bancs carbonatés n’ont jusqu’a present fourni aucun fossile. Cette partie sommitale de
la formation peut donc encore correspondre a l’Hirnantien ou représenter déja la base du
Llandovery. La pile est tronquée par une faille importante qui la met en contact avec une partie
élevée (Ludlow probablement) du Groupe de Kerguillé. Notons que ces observations nouvelles
en presquile de Crozon tendent a réhabiliter un certain synchronisme des Formations du
Cosquer et de Rosan qui avait été mis en doute recemment dans divers schémas (Paris et al.
1981; Guillocheau 1983).
Dans le Sud-Ouest du Massif armoricain, les données relatives a unite vendéenne demeurent
fragmentaires (Ters 1979). Les schistes et grés schisteux rapportés a l’Ordovicien supérieur
comme les schistes et phtanites a Radiolaires attribués au Llandovery n’ont pas livré de fossiles
déterminants.
En conclusion, la présence de l’Ashgill, longtemps méconnue dans le Massif armoricain, y est
désormais attestée dans plusieurs domaines et son extension inclut l’Hirnantien. Le Silurien,
par contre, parait en general amputé de sa partie basale au niveau d’une lacune qui peut,
suivant les régions, intéresser Rhuddanien et Aeronien (Synclinorium de Martigné-Ferchaud,
Synclinorium du Menez Belair) ou affecter ensemble du Llandovery (presquile de Crozon). Le
Massif armoricain ne permet donc aucune observation de la limite Ordovicien-Silurien.
Le Sud-ouest de la France
En Aquitaine, ’étude récente de sondages dans le socle paléozoique sous la couverture
mé€socénozoique, a permis a l’un de nous (F.P.) de constater, d’aprés les Chitinozoaires, la
presence d’Ashgill terminal directement surmonté par des niveaux assez élevés du Llandovery.
Une lacune du Silurien basal parait donc également reconnaissable dans cette région.
En Montagne Noire, la succession de l’Ordovicien et du Silurien est observable en deux
endroits connus depuis Chaubet (1937): au-dessus de la “‘Tranchée noire’ pres de la Grange du
Pin et au Petit Glauzy. De fagon générale, la succession ordovicienne se termine par des
alternances calcaréo-argileuses dites ‘calcaires 4 Cystoides’ et reputées d’age ashgillien depuis
Dreyfus (1948). Dans une récente révision des Brachiopodes de ces niveaux, Havli¢ek (1981)
remet en cause cet age et estime que les associations décrites indiqueraient plutdot le Caradoc
supérieur. L’Age ashgillien demeure néanmoins plausible et si la faune a Hirnantia n’a pas été
reconnue, les calcaires, quoique trés pauvres en Conodontes, ont livré a l'un de nous (R.F.)
quelques restes d’Amorphognathus ordovicicus. Ces niveaux terminaux, assez détritiques, n’ont
fourni aucun Graptolite. Ceux-ci n’apparaissent que quelques métres plus haut dans les argilites
78 BABIN, FEIST, MELOU & PARIS
carburées. Dans la partie basale de ces schistes noirs, a la Grange du Pin, des Conodontes,
extraits des nodules calcaires, indiquent selon Centene & Sentou (1975), la zone a celloni
(€quivalente des zones 20 a 23 des Graptolites, Llandovery moyen). Les mémes niveaux livrent
au Petit-Glauzy, selon ces auteurs, Monograptus sedgwickii, M. uncinatus, M. nudus du Llando-
very moyen également (zone 21). On constate ainsi que la limite Ordovicien—Silurien ne peut
étre reconnue avec précision en Montagne Noire dans I’état actuel de la documentation. Faute
de fossiles dans les niveaux qui assurent le passage entre les derniers carbonates a Cystoides et
les premiéres ampélites a septaria, il demeure impossible de conclure a la continuité ou a
l'existence de lacunes.
References
Centene, A. & Sentou, G. (1975). Graptolites et Conodontes du Silurien des Massifs du Midi mediterranéen.
Thése 3e cycle. Université des Sciences et Techniques du Languedoc, Montpellier (inédit.). 176 pp.,
13 pls.
Chaubet, M. C. 1937. Contribution a étude du Gothlandien du versant méridional de la Montagne
Noire. Mem. Trav. Lab. Geol. Univ. Montpellier 1: 1-223, pls 1-7.
Chauvel, J. & Le Menn, J. 1973. Echinodermes de lOrdovicien supérieur de Coat-Carrec, Argol
(Finistére). Bull. Soc. géol. miner. Bretagne, Rennes, 4 (1): 39-61, pls 1—3.
Dabard, M.-P. & Paris, F. 1986. Palaeontological and geochemical characteristics of Silurian black shale
formations from the Central Brittany Domain of the Armorican Massif (Northwest France). Chem.
geol., Amsterdam, 55 (1/2): 17-29.
Dreyfus, M. 1948. Contribution a etude geologique et paléontologique de ’Ordovicien supérieur de la
Montagne Noire. Mem. Soc. geol. Fr., Paris, 58: 1-63.
Guillocheau, F. 1983. La sedimentation paleozoique ouest-armoricaine. Histoire sédimentaire; relations
tectonique-sédimentation. Bull. Soc. geol. miner. Bretagne, Rennes, (C) 14 (2): 45-62.
Hamoumi, N. 1981. Comparaison des coupes du Veryarc’h et de l’Aber-Kerglintin. In: Analyse sedimentol-
ogique des formations de |’Ordovicien superieur en presqu ile de Crozon (Massif Amoricain).
, Le Ribault, L. & Pelhate, A. 1981. Les Schistes du Cosquer (Ordovicien supérieur, Massif Armor-
icain occidental): une formation glacio-marine a la peripherie d’un islandsis ordovicien. Bull. Soc. géol.
France, Paris, (7) 23: 279-286.
Harper, D. A. T. 1984. Brachiopods from the Upper Ardmillan succession (Ordovician) of the Girvan
district, Scotland. Part 1. Palaeontogr. Soc. (Monogr.), London. 78 pp., 11 pls.
Havlicek, V. 1981. Upper Ordovician Brachiopods from the Montagne Noire. Palaeontographica, Stutt-
gart, (A) 176: 1—34, pls 1-9.
Henry, J. L. 1980. Trilobites ordoviciens du Massif Armoricain. Mem. Soc. géol. miner. Bretagne, Rennes,
22: 1-250, pls 1-48.
Herrouin, Y. (1987). Carte geologique de la France 1/50000e. Feuille de Bain-de-Bretagne 388.
B.R.G.M. éd.
Melou, M. 1971. Nouvelle espéce de Leptestiina dans ’Ordovicien supérieur de l’Aulne (Finistére). Mem.
Bur. Rech. geol. minier., Paris, 73: 93-105, pls 1, 2.
1985. Révision d*Orthis’ berthoisi ROUAULT, 1849, Orthida (Brachiopoda) de l’Ordovicien du
Massif Armoricain. Geobios, Lyon, 18: 595-603, pls 1, 2.
—— 1987. Découverte de Hirnantia sagittifera (M’Coy 1851) (Orthida Brachiopoda) dans l’Ordovicien
supérieur (Ashgillien) de l’extrémité occidentale du Massif Armoricain. Géobios, Lyon, 20: 679-686,
pl. 1.
Paris, F. 1977. Les formations siluriennes du Synclinorium du Ménez-Bélair; comparaisons avec d’autres
formations siluriennes du Massif armoricain. Bull. Bur. Rech. geol. Min., Paris, (2e sér., 1) 2: 75-87.
1981. Les Chitinozoaires dans le Paléozoique du Sud-Ouest de ’Europe. Mem. Soc. géol. miner.
Bretagne, Rennes, 26: 1-412, pls 1-41.
1986. Les formations paléozoiques et leur structuration. In: Notice géologique de la feuille Combourg
a 1/50 000eme. B.R.G.M. (doct provisoire).
——, Pelhate, A. & Weyant, M. 1981. Conodontes ashgilliens dans la Formation de Rosan, coupe de
Lostmarc’h (Finistére, Massif Armoricain); conséquences paléogéographiques. Bull. Soc. geol. miner.
Bretagne, Rennes, (C) 13 (2): 15-35.
, Rickards, R. B. & Skevington, D. 1980. Les assemblages de Graptolites du Llandovery dans le
Synclinorium du Ménez-Bélair (Massif Armoricain). Geobios, Lyon, 13: 153-171, pls 1, 2.
LA LIMITE ORDOVICIEN—SILURIEN EN FRANCE 1S)
Philippot, A. 1950. Les Graptolites du Massif Armoricain, étude stratigraphique et paléontologique. Mem.
Soc. geol. miner. Bretagne, Rennes, 8: 1—295.
Skevington, D. & Paris, F. 1975. Les Graptolites de la Formation de Saint-Germain-sur-Ille (Ordovicien
supérieur du Massif Armoricain). Bull. Soc. geol. Fr., Paris, (7) 17: 260-265.
Ters, M. 1979. Les synclinoriums paléozoiques et le Précambrien sur la fagade occidentale du Massif
vendéen. Stratigraphie et structure. Bull. Bur. Rech. geol. Min., Paris, (2e sér., 1) 4: 293-302.
Weyant, M., Dore, F., Le Gall, J. & Poncet, J. 1977. Un episode calcaire ashgillien dans Est du Massif
Armoricain, incidence sur l’age des depots glacio-marins finiordoviciens. C.r. hebd. Seanc. Acad. Sci.,
Paris, (D) 284: 1147-1149.
The Ordovician—Silurian boundary in the Oslo
region, Norway
L. R. M. Cocks
Department of Palaeontology, British Museum (Natural History), Cromwell Road, London
SW7 SBD
Synopsis
The Ordovician-Silurian boundary is exposed sporadically throughout the southern and central parts of
the Oslo region; to the north there is an unconformity. In the central Oslo—Asker districts a well-
developed Hirnantia fauna underlies beds with acuminatus Zone graptolites; other beds yield Holo-
rhynchus faunas in the late Ordovician and early members of the Stricklandia lineage in the overlying
Llandovery. Some early Silurian conodonts and acritarchs are recorded.
Lower Palaeozoic rocks outcrop in the Oslo region within a 230km by 50km area which is
separated from the Precambrian to the east by the faults of a Permian graben. Within this
broad region, most recent work in the late Ordovician and early Silurian has been achieved in
the Oslo—Asker district, which lies in the approximate centre of the region, and also in the
Hadeland district, some 50 km to the north of Oslo. These and other districts will be reviewed
in turn. The Ordovician and Silurian beds in the area have been known since the early work of
Murchison, Kjerulf, Broegger and others, and were the subject of a monumental study near the
turn of the century by Kjaer (e.g. 1908). During the past fifteen years much new work has been
done, for example Worsley et al. (1983) proposed a modern system of stratigraphical nomencla-
ture for the Silurian rocks of the region.
Oslo—Asker District. The formation names for the late Ordovician stratigraphy (Fig. 1) were
erected by Brenchley & Newall (1975), and its biostratigraphy and ecology elucidated by
Brenchley & Cocks (1982), its trilobites described by Owen (1980, 1981) and its brachiopods by
Cocks (1982). The topmost few metres of the Husberg¢ya Shale carries the trilobite Tretaspis
sortita broeggeri, which Owen (1980) regarded as indicative of the uppermost Rawtheyan Stage.
A Hirnantia fauna is known from horizons near the base of the Langgyene Sandstone Forma-
tion and within the Langara Limestone-Shale Formation (Brenchley & Cocks 1982: 796), and
includes common Dalmanella testudinaria, Hirnantia sagittifera, Cliftonia aff. psittacina, Hindella
cassidea, Eostropheodonta hirnantensis, Mucronaspis mucronata kjaeri, bryozoans and cricco-
conariids, and less common Acanthocrania, Glyptorthis, Lingula, Leptaena, Orbiculoidea, Oxo-
plecia, Philhedra, Calyptaulax, Illaenus, Platycoryphe, Toxochasmops, molluscs, crinoids and
carpoids. Elements of the Hirnantia fauna persist above this horizon in Hindella—Cliftonia and
Dalmanella associations higher in the Lang¢yene Sandstone and there are also other faunas
there such as one dominated by Trematis norvegica and modiolopsid bivalves. Above these, in
the west of the area in Asker there occur thick beds largely composed of Holorhynchus gigan-
teus, but with 13 other brachiopods and 17 other animals also recorded from them (Brenchley
& Cocks 1982: 802), whilst in the east of the area, in the Oslo District, only trace fossils occur
in rocks believed to represent a shore-face environment. At the top of the Langgyene Sandstone
there occur shallow-water channel sequences which in some cases bear high abundance, low
diversity faunas dominated by brachiopods such as Brevilamnulella kjerulfi and Thebesia scopu-
losa. This total sequence represents a regression since at least mid-Rawtheyan times, but above
the channel-fill beds there occurs a metre-thick couplet of sandstone and limestone over the
whole district which carries faunas, which are not age-diagnostic, of small shells such as
Onniella, Eoplectodonta, Leangella, Paucicrura and Dolerorthis, as well as crinoids and bryozoa
(and 16 other rarer forms). This couplet is lithologically included within the Langgyene Forma-
tion, but in fact marks the start of the ‘early Silurian’ transgression in the area. It is conform-
Bull. Br. Mus. nat. Hist. (Geol) 43: 81-84 Issued 28 April 1988
82 L. R. M. COCKS
ASKER OSLO RINGERIKE HADELAND
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Fig. 1 Latest Ordovician and early Silurian stratigraphy in the Asker, Oslo, Ringerike and
Hadeland districts of the Oslo Region.
ably followed by the basal organic-rich shales of the Solvik Formation in the Oslo District,
which carries no shelly fauna, but from which Howe (1982) has identified Climacograptus
transgrediens Waern from an horizon 11m above the base of the formation at Ormd¢ya, which
he attributes to an horizon low in the acuminatus Zone (or perhaps high in the persculptus
Zone). In the west of the area, in the Asker District, there was no break in the deposition of
shelly faunas, and brachiopods are recorded from all three members of the Solvik Formation
there, in a similar way to the higher parts of the formation in the Oslo District (Baarli 1985;
Baarli & Harper 1986). The first occurrence of Stricklandia lens prima is at 95m above the base
of the Solvik Formation (Myren Member) and the transition from S. lens prima to S. lens lens
occurs between 122 and 130m above the base (Baarli 1986). Conodonts of the Icriodella
discreta—I. deflecta Zone are known from 8m above the base of the Solvik Formation at
Konglungen, Asker (Aldridge & Mohamed 1982). Above the three members of the Solvik
Formation, the Rytteraker Formation yields pentamerides and conodonts of Aeronian age:
Nakrem (1986) has identified the Distomodus kentuckyensis—D. staurognathoides conodont zonal
boundary as occurring at about the boundary of the Solvik and Rytteraker Formations.
The Ringerike area. The latest Ordovician of the Ringerike area remains unrevised, and thus
the old stage terminology of Kiaer (1897, 1908) is employed—tt carries a rich brachiopod fauna,
but one not identical to that from the Oslo—Asker region and no Hirnantia fauna is known
from the area; it also differs in the presence of bioherms and patch reefs within Stage 5b, the
most notable of which is at Ullerntangen. The relationships between the Ordovician and
Silurian beds are obscure and a local unconformity is postulated here (Fig. 1). The overlying
beds of the Saelabonn Formation are shallow-water storm deposits with lenses of displaced
ORDOVICIAN-SILURIAN BOUNDARY IN NORWAY 83
shelly faunas (Thomsen 1982); their detailed age is indeterminate, but probably includes the
Lower Llandovery. The overlying Rytteraker Formation includes the Borealis-Pentamerus
transition near its base (Mérk 1981), and that horizon is certainly now in the Aeronian.
Smelrgr (1987) has identified the acritarch zones 1 and 2 of Hill (1974) as occurring in the
Saelabonn Formation.
The Hadeland area. Owen (1978) has revised the late Ordovician and early Silurian of this area
and established a Rawtheyan age for the Kjérrven Formation which underlies the Kalvsj¢
Formation, which carries a sparse trilobite fauna, some brachiopods and the cystoid Hemi-
cosmites and the calcareous alga Palaeoporella which indicate an Ordovician rather than a
Silurian age. Above this the 120m thick Sk¢yen Sandstone Formation appears to straddle the
Ordovician-Silurian boundary, since beds with Zygospiraella and other typical early Silurian
brachiopods occur from about the middle of the formation. The Skdyen Sandstone is succeeded
conformably by the Rytteraker Formation which yields Borealis borealis near its base.
Other areas. From the Skien and Porsgrunn district near the south of the Oslo Region, for
example in a section at Hergyavegen, Porsgrunn, Holorhynchus beds followed by early Silurian
beds yielding Zygospiraella duboisi (Verneuil) and Eostropheodonta mullochensis (Reed) are
known, but the stratigraphy is unrevised. In the Oslo region north of Hadeland there is an
unconformity between the late Caradoc and early Ashgill Mjgesa Limestone and the early
Silurian, for example Mller (1986) has described the succession at Brummunddal, where the
Helggya Quartzite of probable Aeronian age bearing Borealis borealis rests on the Mjgesa
Limestone.
References
Aldridge, R. J. & Mohamed, i. 1982. Conodont biostratigraphy of the early Silurian of the Oslo Region. In
D. Worsley (ed.), Field meeting, Oslo region, 1982. I.U.G.S. Subcommission on Silurian Stratigraphy:
109-120, 2 pls. Universitetsforlaget, Oslo (Pal. Contr. Univ. Oslo 278).
Baarli, G. 1985. The stratigraphy and sedimentology of the early Llandovery Solvik Formation, central
Oslo Region, Norway. Norsk geol. Tiddsskr., Oslo, 65: 229-249.
—— 1986. A biometric re-evaluation of the Silurian brachiopod lineage Stricklandia lens/S. laevis. Palae-
ontology, London, 29: 187—205, pl. 21.
& Harper, D. A. T. 1986. Relict Ordovician brachiopod faunas in the Lower Silurian of Asker, Oslo
Region, Norway. Norsk geol. Tidsskr., Oslo, 66: 87-98.
Brenchley, P. J. & Cocks, L. R. M. 1982. Ecological associations in a regressive sequence: the latest
Ordovician of the Oslo—Asker district, Norway. Palaeontology, London, 25: 783-815, pls 85-86.
—— & Newall, G. 1975. The stratigraphy of the upper Ordovician Stage 5 in the Oslo—Asker district,
Norway. Norsk geol. Tidsskr., Oslo, 55: 243-275.
Cocks, L. R. M. 1982. The commoner brachiopods of the latest Ordovician of the Oslo—Asker District,
Norway. Palaeontology, London, 25: 755-781, pls 78-84.
Hill, P. J. 1974. Stratigraphic palynology of acritarchs from the type area of the Llandovery and the
Welsh Borderland. Rev. Palaeobot. Palynol., Amsterdam, 18: 11-23.
Howe, M. P. A. 1982. The Lower Silurian graptolites of the Oslo Region. In D. Worsley (ed.), Field
meeting, Oslo region, 1982. I.U.G.S. Subcommission on Silurian Stratigraphy: 21-32, 2 pls. Uni-
versitetsforlaget, Oslo (Pal. Contr. Univ. Oslo 278).
Kiaer, J. 1897. Faunistische Uebersicht der Etage 5 des norwegischen Silursystems. Skr. VidenskSelsk.
Christiania (Math.-nat.) 1897 (3): 1-76.
—— 1908. Das Obersilur im Kristianiagebiete. Skr. VidenskSelsk. Christiania (Math.-nat.) 19€6 II: 1-595,
pls 1-24.
Mller, N. K. 1986. Evidence of synsedimentary tectonics in the Lower Silurian (Llandovery) strata of
Brumunddalen, Ringsaker, Norway. Norsk geol. Tidsskr., Oslo, 66: 1-15.
Mérk, A. 1981. A reappraisal of the Lower Silurian brachiopods Borealis and Pentamerus. Palaeontology,
London, 24: 537-553, pls 83-85.
Nakrem, H.-A. 1986. Llandovery conodonts from the Oslo Region, Norway. Norsk geol. Tidsskr., Oslo,
66: 121-133.
84 L. R. M. COCKS
Owen, A. 1978. The Ordovician and Silurian stratigraphy of Central Hadeland, south Norway. Norg. geol.
Unders., Oslo, 338: 1—23, pl. 1.
1980. The trilobite Tretaspis from the upper Ordovician of the Oslo region, Norway. Palaeontology,
London, 23: 715—747, pls 89-93.
—— 1981. The Ashgill trilobites of the Oslo Region, Norway. Palaeontographica, Stuttgart, (A) 175: 1-88,
pls 1-17.
Smelrgr, M. 1987. Early Silurian acritarchs and prasinophycean algae from the Ringerike District, Oslo
Region (Norway). Rev. Palaeobot. Palynol., Amsterdam, 52: 137-159, pls 1-5.
Thomsen, E. 1982. Saelabonn Formationen (nedre Silur) i Ringerike, Norge. Arsskr. Dansk geol. Foren.
1981: 1-11.
Worsley, D., Aarhus, N., Bassett, M. G., Howe, M. P. A., M¢@rk, A. & Olaussen, S. 1983. The Silurian
succession of the Oslo Region. Norg. geol. Unders., Oslo, 384: 1-57.
East Baltic Region
D. Kaljo, H. Nestor and L. Polmat+
Institute of Geology, Estonian Academy of Sciences, Estonia Puistee 7, Tallinn 200101, USSR
+ L. Polma died in January 1988.
Synopsis
Five confacies belts from north to south, from Estonia through Latvia to Lithuania, are described briefly
through the late Ordovician and early Silurian, with their varied facies and faunas. Despite clear breaks
corresponding to the Ordovician-Silurian boundary at the edges of the depositional basin, rocks of
Hirnantian age are identified from the centre of the basin, including Hirnantia and Dalmanitina faunas in
the Porkuni Regional Stage and basal Silurian faunas, including some graptolites, chitinozoans, brachio-
pods and conodonts, from the overlying Juuru Regional Stage. Any stratigraphical break at the boundary
appears to be represented by no more than a facies change.
Introduction
The East Baltic area is a part of the extensive gulf-like Baltic sedimentary basin (Mannil 1966;
Kaljo & Jiirgenson 1977). The uppermost Ordovician and the lowermost Silurian are mostly
represented by carbonate or terrigenous-carbonate rocks with an exceptionally rich benthic
shelly fauna; however, pelagic groups of fossils, especially graptolites, are of a more restricted
distribution. The rocks are tectonically undisturbed, and unmetamorphozed (CAI 1-1-5), with
only a little dolomitization in places, and the fossils are well preserved. The bedding is almost
horizontal and dips slightly to the centre of the basin. The distribution of the Ashgill—
Llandovery rocks in the East Baltic is shown in Fig. 1. The outer margin of the area is
erosional and corresponds to the base of the Ashgill (Vormsi Regional Stage). The axial part of
the basin with the most deep-water rocks corresponds to the Baltic Syneclise (IV belt), and
along its margins there occur shallower-water sediments.
Most of the area is covered by younger rocks. The outcrops of the Ordovician—Silurian
boundary strata are confined to North Estonia (Belt I in Fig. 1), where only comparatively
shallow-water deposits are exposed. A more complete succession of facies in the basin can be
seen in borehole sections. Fig. 2 presents a cross section of Ashgill and Lower and Middle
Llandovery strata along the Orjaku-Remte—Ukmerge line, which is shown in Fig. 1. The
section goes across the main facies belts of the basin and shows the relations between local
lithostratigraphical units and their general lithology. In the figure stratigraphical units are
marked with letter-indexes: their full nomenclature is given in Fig. 3.
Confacies belts
In the East Baltic five confacies belts can be distinguished in Ordovician—Silurian boundary
beds. Their distribution is shown in Fig. 1 and their lithological composition in Fig. 2.
Type 1—the most shallow-water sections in North Estonia and Hiiumaa Island represented
by aphanitic, bioclastic and biohermal limestones. In the Raikkiila Formation there occur
primary argillaceous dolomites of lagoonal origin in places. Some considerable stratigraphical
gaps have been established (Fig. 3). The Ordovician ends with Early Porkuni bioclastic, bio-
hermal and arenaceous limestones (Arina Formation), which carry a Streptis brachiopod com-
munity (Hints 1986), disconformably overlain by Juuru aphanitic (Koigi Member) and
biomicritic limestones (Varbola Formation) with a Stricklandia community (Rubel 1970).
Type II—sections in central Estonia and Saaremaa Island. Represented by marls, aphanitic
and biomicritic nodular limestones. The sections are more complete than in Type I. A distinct
hiatus has been established only in the upper part of the Porkuni Regional Stage and in the
west at the top of the Raikkiila Stage. The Ordovician-Silurian boundary interval is similar to
the sections of Type I, but southwards the Arina Formation and the Koigi Member thin out
Bull. Br. Mus. nat. Hist. (Geol) 43: 85-91 Issued 28 April 1988
86
KALJO, NESTOR & POLMA
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Fig. 1 Distribution of Ordovician—Silurian boundary rocks in the East Baltic area. 1—boreholes,
2—administrative boundaries, 3—outer margin of the distribution of Ashgill and Llandovery
rocks, 4—boundaries of main types of sections, marked with Roman numbers.
and the boundary of the systems continues in a comparatively monotonous complex of nodular
limestones and marls. In places the Porkuni Regional Stage may be missing.
Type III—sections in south Estonia and north-west Latvia. Marls and argillaceous lime-
stones, including their red-coloured varieties, are significant lithologies. In the Llandovery,
aphanitic limestones alternate with marls. A considerable erosional gap corresponds to the
upper part of the Pirgu Regional Stage, and this gap increases westwards. The uppermost
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Ordovician is represented by marls and argillaceous limestones with the Dalmanitina Fauna
(Kuldiga Formation). Above this occur biosparitic, oolitic and arenaceous limestones of the
Saldus Formation. The Silurian begins with marls and argillaceous limestones of the Ohne
Formation with the Clorinda community (Rubel 1970).
Type IV—sections in southeast Estonia, considerable part of Latvia, west Lithuania and the
Kaliningrad Region. The studied stratigraphical interval begins and ends with dark graptolitic
mudstones with the assemblage of the Pleurograptus linearis Zone in the Ordovician part
(Fjacka Formation) and of the Coronograptus cyphus—Monograptus sedgwickii Zones in the
Silurian (Dobele Formation). Between these key beds there occur red and grey calcareous
mudstones, marls and aphanitic limestones. The uppermost Ordovician is analogous to the
sections of Type III. The Silurian begins with marls and aphanitic limestones of the Staciunai
Formation which have yielded few fossils good for correlation.
Type V—sections in east Lithuania and southeast Latvia with an extensive hiatus at the
boundary interval. More or less continuous Upper Ordovician deposits are represented by
marls and various limestones which end at the top of the Pirgu Regional Stage with the
aphanitic limestones of the Taucionys Formation which yield a Holorhynchus fauna. There is a
hiatus at the level of the Porkuni, Juuru and Raikkiila Regional Stages, or in places there occur
thin residual tongues and lenses of the Kuldiga, Saldus and Apascia Formations, which are
transgressively overlain by mudstones and marls of the late Llandovery Adavere Regional
Stage.
In the westernmost part of Lithuania and in the Kaliningrad District the rocks of the
Ordovician-Silurian boundary interval become still more argillaceous and graptolites occur
throughout the whole section, with the exception of the uppermost Ordovician which yields a
shelly Hirnantia fauna. This is a transition to a different type of facies belt which is distributed
in north Poland and the southern part of the present Baltic Sea.
Thus analysis of the lithologies and fossils of the various sections shows that by the end of
the Ordovician the Baltic basin had experienced a considerable regression which reached its
maximum in the second half of the Porkuni. This is indicated both by hiatuses in the sections
(Fig. 3) and by the presence of calcareous oolites and early diagenetic (or sedimentary) dolomi-
REGIONAL
STAGE
NORTH
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WEST LATVIA,
W. LITHUANIA
FAST LATVIA,
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Fig. 3. Stratigraphical scheme of the late Ordovician and early Silurian boundary rocks in the East
Baltic area.
= nooner HES rna =
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SECTIONS
EAST BALTIC REGION 89
tes in the Saldus Formation in the axial part of the basin. The character of the transition from
the Porkuni to the Juuru Regional Stage and the lithology of the sequences indicate a rapid
deepening of the basin, obviously of glacial eustatic origin (Kaljo et al., in press).
Local stratigraphy
Knowledge of the local stratigraphy of rocks near the Ordovician—Silurian boundary has
considerably improved in the past few decades. The correlation chart presented in Fig. 3 is
based on the decisions of the regional stratigraphical conferences in Vilnius in 1976 and in
Tallinn in 1984 (Grigelis 1978). The chart was compiled from material in many publications (see
further references in the papers by Mannil 1966, Kaljo 1970, Kaljo & Klaaman 1982, PaSkevié-
ius 1979, Grigelis 1982, Ulst et al. 1982).
Dynamics of the faunas
From the five regional stages from Vormsi to Raikkiila which correspond to the Ashgill and
lower and middle Llandovery, extremely rich fossil faunas have been collected. The present
paper uses the data obtained through the study of eight groups of fossils: stromatoporoids,
tabulate corals, brachiopods, trilobites, ostracodes, chitinozoans, conodonts and graptolites. In
total 734 species from 313 genera and 105 families have been identified. Table 1, which is based
on data by Nestor et al. (in press), shows the distribution of species and genera by stages. It
shows that the associations of the Porkuni and Juuru Regional Stages are the least diverse; and
also that they have almost no common species, whereas about one third of the genera occur in
both stages. At the Ordovician—Silurian boundary, besides intensive extinction of the Ordovi-
cian fauna, the rate of the appearance of new fauna also rose. In Porkuni times extinction
prevailed and Juuru times were characterized by the appearance of new faunas.
Table 1 Numbers of species and genera of eight fossil groups recorded from the
Vormsi to Raikila Regional Stages.
Regional Stage Vormsi Pirgu Porkuni Juuru Raikkula
Species, total number 195 252 154 a 221
transitional from
the underlying 43 38 17 4 22
stage, %
Genera, total number 150 175 125 109 130
transitional from
the underlying 57 69 49 32 57
stage, %
The dynamics of the fossil groups varied according to their ecology. For example, the
shallowing of the basin in the Late Ordovician led to the radiation of the shallow-water
stromatoporoids and corals, whereas the graptolites emigrated completely from the East Baltic
area at the same time as the general crisis of graptolites noted by Rickards (1978) became
apparent. Shallowing was also of great influence on the benthic trilobites and ostracodes, which
usually inhabited deeper shelf areas and a remarkable decrease in their diversity took place in
Pirgu and Porkuni times. The reverse tendency can be seen during the rapid deepening of the
basin at the beginning of Juuru times; however, at that time shallow-water groups, particularly
stromatoporoids and corals, were chiefly affected.
Biostratigraphy and correlation
Space does not allow a more detailed analysis here of the diverse biota from the boundary
beds, and so only selected lists of species for each stage are presented, those which are most
90 KALJO, NESTOR & POLMA
valuable for correlation (in brackets the index of the formation is shown where the species has
been found).
Vormsi Regional Stage
Catenipora wrighti Klaamann (Kr), Plaesiomys solaris Buch (Kr), Kullervo complectens (Wiman)
(Td), Acanthochitina barbata Eisenack (Td, Fj, Ml), Tretaspis seticornis (Hisinger) (Fj),
Orthograptus quadrimucronatus (Hall) (Fj), Climacograptus styloides Lapworth (Fj), Hamarodus
estonicus Viira (Fj), Belodina compressa (Branson & Mehl) (M1).
The above species enable a clear determination of the position of the Stage at the level of the
graptolite Pleurograptus linearis Zone.
Pirgu Regional Stage
In the lower part: Eospirigerina sulevi (Alichova) (Mo, Jn, Sv), Foramenella parkis (Neckaja)
(Mo, Jn, Sv, Ad, Uk), Amorphognathus ordovicicus Branson & Mehl (Mo, Jl), Dicellograptus cf.
complanatus Lapworth (Mo), Rectograptus gracilis (Roemer) (HI, Jn), Panderia megalophthalma
(Linnarsson) (Jn), Tretaspis latilimba (Linnarsson) (Jn, Jl, K1).
In the middle part: Clathrodictyon microundulatum Nestor (Ad), Catenipora tapaensis
(Sokolov) (Ad), Esthonia asterisca Roemer (Ad, Uk), Maclurites neritoides (Eichwald) (Ad),
Belodina compressa (Branson & Mehl) (Ad).
In the topmost part: Conochitina taugourdeaui Eisenack (Kb), Climacograptus supernus Elles
& Wood (Kb), Holorhynchus giganteus Kiaer (TC).
The graptolites shown above enable a correlation of the stage with the zones of Dicello-
graptus complanatus and D. anceps.
Porkuni Regional Stage
Paleofavosites rugosus Sokolov (Ar), Rhabdotetradium frutex Klaamann (Ar), Streptis undifera
(Schmidt) (Ar), I/laenus angustifrons depressa Holm (Ar), Apatochilina falocata Sarv (Ar), Dalma-
nella testudinaria (Dalman) (Kl), Hirnantia sagittifera (M‘Coy) (Kl), Eostropheodonta hirnan-
tensis (M‘Coy) (KI), Dalmanitina (Mucronaspis) mucronata (Brongniart) (KI, Sl), Brongniartella
platynota (Dalman) (K1), Pseudulrichia norvegica Henningsmoen (K1), Conochitina postrobusta
subsp. A (Nolvak, Ms).
The representatives of the Hirnantia and Dalmanitina communities enable correlation with
the Hirnantian Stage at the level of the zones of Climacograptus extraordinarius and Glyp-
tograptus persculptus.
Juuru Regional Stage
Clathrodictyon boreale Riabinin (Vr, Tm), Paleofavosites paulus Sokolov (Vr, Tm, Oh), Strick-
landia lens prima Williams (Vr, lower pt), S. lens lens Williams (Vr, upper pt), Borealis borealis
(Eichwald) (Tm), Calymene ansensis Mannil (Vr, Tm), Acernaspis estonica Mannil (Oh), Aitilia
senecta Sarv (Vr), Steusloffina eris Neckaja (Vr, Tm, Oh), Ozarkodina ex gr. oldhamensis
(Rexroad) (Oh, lower pt), Distomodus cf. kentuckyensis Branson & Branson (Oh), Ancyrochitina
laevaensis Nestor (Oh, lower pt), Conochitina postrobusta Nestor (Oh), Dimorphograptus con-
fertus (Nicholson) on upper pt), Pribylograptus incommodus Te etch (on top).
The top of the Juuru Regional Stage is well defined by graptolites, suggesting that this level
approximately coincides with the boundary of the Dimorphograptus confertus (equivalent to the
Orthograptus vesiculosus) and Coronograptus cyphus Zones (Kaljo et al. 1984). The age of the
lower limit of the stage can be established by Stricklandia lens prima (according to Cocks, 1971,
it equates to the level of the Parakidograptus acuminatus Zone) and by the listed chitinozoans
and conodonts, indicating that there was no substantial regional hiatus at the base of the
Silurian in the East Baltic. However, distinct breaks occur at the margins of the basin, particu-
larly to the southeast.
The correlation of the Raikkiila Regional Stage is clearly defined by graptolites within the
Coronograptus cyphus and Demirastrites convolutus Zones (Kaljo 1967; Kaljo 1970; Kaljo et al.
1984). Detailed correlations in Estonia were considerably improved by the study of chitin-
ozoans (Nestor 1976).
EAST BALTIC REGION 91
The present data from graptolites and other evidence permit only general correlation of the
East Baltic section with the Dob’s Linn section, but finds of Climacograptus supernus at the top
of the Pirgu and D. confertus at the top of the Juuru Regional Stage do not contradict the
placing of the Ordovician-Silurian boundary (the base of the P. acuminatus Zone) at the top of
the Porkuni Regional Stage.
Correlation with the Anticosti section is possible by means of chitinozoans and conodonts.
In this section (Achab 1981; McCracken & Barnes 1981) Member 5 of the Ellis Bay Formation
is characterized by the presence of Conochitina taugourdeaui, C. micracantha and C. gama-
chiana. J. Nolvak has found the first two and a form similar to the third species at the top of
the Pirgu Regional Stage. At the base of Member 6 in Anticosti Ozarkodina oldhamensis
appears, and somewhat higher Distomodus kentuckyensis and above bioherms Ancyrochitina
spongiosa are recorded. P. Mannik, V. Nestor and V. Viira have found all these species or
closely related forms in the lower part of the Juuru Regional Stage. Thus, in the Anticosti
section we do not see equivalents of the Porkuni Regional Stage (at least of its upper part)
which is characterized by Conochitina postrobusta subsp. A.
References
Achab, A. 1981. Biostratigraphie par les Chitinozoaires de POrdovicien Supérieur—Silurien Inférieur
de I’Ile d’Anticosti. Résultats préliminaires. In P. J. Lespérance, (ed.), Field Meeting, Anticosti-Gaspe,
Quebec, 1981 2 (Stratigraphy and paleontology): 143-157. Montreal (I{UGS Subcommission on Silurian
Stratigraphy Ordovician-Silurian Boundary Working Group).
Cocks, L. R. M. 1971. Facies relationships in the European Lower Silurian. Mem. Bur. Rech. geol. minier.,
Paris, 73: 223-227.
Grigelis, A. A. (ed.) 1978. Decisions of the East Baltic regional stratigraphical conference (1976). 88 pp. and
correlation charts. Leningrad, Interdep. Strat. Comm. USSR. [In Russian].
—— (ed.) 1982. Geology of the Soviet Baltic republics. 304 pp. Leningrad, Nedra [In Russian].
Hints, L. 1986. Genus Streptis (Triplesiidae, Brachiopoda) from the Ordovician and Silurian of Estonia.
Proc. Acad. Sci. Estonian SSR, Tallinn, (Geology) 35: 20-26 [Eng]. summ. ].
Kaljo, D. 1967. On the age of lowermost Silurian of Estonia. Eesti NSV Tead. Akad. Toim., Yallinn,
(Keem. Geol.) 16: 62-68 [Eng]. summ. ].
— (ed.) 1970. Silurian of Estonia. 343 pp. Tallinn, Valgus. [Engl. summ. ].
—— & Jiirgenson, E. 1977. Sedimentary facies of the East Baltic Silurian. In: Facies and fauna of the
Baltic Silurian: 122-148. Tallinn, Acad. Sci. [Eng]. summ. ].
& Klaamann, E. (eds) 1982. Ecostratigraphy of the East Baltic Silurian. 112 pp. Tallinn, Valgus.
——, Nestor, H., Polma, L. & Einasto, R. 1988 (in press). Late Ordovician glaciation and its influence on
the ecology in the Baltic cratonic basin. In: Essential biotic events in the earth history. Tallinn, Acad. Sci.
[In Russian].
——, Paskevitius, I. & Ulst, R. 1984. Graptolite zones of the East Baltic Silurian. In: Stratigraphy of the
East Baltic Early Palaeozoic: 94-118. Tallinn, Acad. Sci. [Eng]. summ.].
McCracken, A. D. & Barnes, C. R. 1981. Conodont biostratigraphy across the Ordovician—Silurian
boundary, Ellis Bay Formation, Anticosti Island, Québec. In P. J. Lespérance (ed.), Field Meeting,
Anticosti-Gaspe, Quebec, 1981 2 (Stratigraphy and paleontology): 61-69. Montréal (IUGS Subcommis-
sion on Silurian Stratigraphy Ordovician—Silurian Boundary Working Group).
Mannil, R. 1966. Evolution of the Baltic basin during the Ordovician. 200 pp. Tallinn, Valgus. [Engl.
summ. ].
Nestor, H., Klaamann, E., Meidla, T., Mannik, P., Mannil, R., Nestor, V., Nolvak, J., Rubel, M., Sarv, L.
& Hints, L. 1988 (in press). Faunal dynamics in the East Baltic basin at the Ordovician and Silurian
boundary. In: Essential biotic events in the earth history. Tallinn, Acad. Sci. [In Russian].
Nestor, V. 1976. A microplankton correlation of boring sections of the Raikkiila Stage, Estonia. Eesti
NSV Tead. Akad. Toim., Tallinn, (Keem. Geol.) 25: 319-324 [In Russian with Eng]. summ. ].
Paskevitius, J. 1979. Biostratigraphy and graptolites of the Lithuanian Silurian. 268 pp. Vilnius, Mokslas.
(Engl. summ. ].
Rickards, R. B. 1978. Major aspects of evolution in the graptolites. Acta palaeont. pol., Warsaw, 23:
585-594.
Rubel, M. 1970. On the distribution of brachiopods in the lowermost Llandovery of Estonia. Eesti NSV
Tead. Akad. Toim., Tallinn, (Keem. Geol.) 19: 69-79.
Ulst, R., Gailite, L. & Jakoyleva, V. 1982. Ordovician of Latvia. 294 pp. Riga [In Russian].
The Ordovician—Silurian boundary in Poland
L. Teller
Zaklad Paleobiologii PAN, Newelska 6, Warsaw 01-447, Poland
Synopsis
Outcrops in the Holy Cross Mountains and Sudetes, as well as boreholes in the Polish lowlands, show
Ordovician-Silurian boundary sediments to be variably developed or sometimes absent. The Hirnantia
fauna is developed, but most other rocks are in graptolitic facies.
Ordovician-Silurian boundary beds have been recognized in Poland both in outcrops and in
boreholes. However, despite abundant documentation obtained from both types of sections as
well as intensive investigations carried out, the boundary in Poland is still inadequately known.
This is mainly because of the presence of many sedimentary gaps in the known sections, which
are a result of the Taconic orogenic phase, and also because of the lack of good index fossils. In
consequence, this boundary is not sharply defined in the Polish profiles, which makes good
correlation with the adjacent regions difficult (Teller 1969).
The Ordovician-Silurian boundary beds outcrop in Poland only in the Holy Cross Moun-
tains and in the Sudetes. In the Bardo Range of the Sudetes (Teller 1962) there are no fossils
known from near the junction, so the boundary has been arbitrarily designated by the presence
of Lower Llandovery graptolites in black siliceous shale among the liddites The upper Ordovi-
cian sediments appear to be represented in this area by alternating beds of sandstone and shale
without fossils which underlie the Silurian liddites. The Ordovician—Silurian boundary has been
put at the contact of these two formations, but it is not known for certain whether or not the
clastic Ordovician corresponds to the uppermost Ashgill.
In the Holy Cross Mountains, the boundary beds are known to occur in the Zalesie profile
(Kielan 1956, 1957; Temple 1965), in the southern limb of the Bardo syncline in the Kielce
region. The uppermost Ashgill silty beds contain a Hirnantia fauna with Mucronaspis mucro-
nata Brongniart), M. olini Temple, Dalmanella testudinaria (Dalman), Hirnantia sagittifera
(M‘Coy) and Eostropheodonta hirnantensis (M‘Coy) amongst others, and are covered by black
shales with Akidograptus acuminatus at their base, accompanied by Climacograptus scalaris
normalis and A. ascensus, indicating the acuminatus Zone.
Thus the boundary separates the Upper Ashgill siltstone formation, containing a Hirnantia
fauna, from the Lower Llandovery black shale formation with graptolites. This rapid change in
facies suggests a lack of sedimentary continuity particularly since there are no graptolites in the
uppermost Ashgill. In profiles in other parts of the world, the Hirnautia fauna (Cocks 1985) is
generally older, or is to be found below the Ordovician Glyptograptus persculptus Biozone, the
top of which is now taken as the boundary between the Ordovician and the Silurian.
In many other sections in the Holy Cross Mountains (Tomezyk 1962; Bednarczyk 1973) a
sedimentary gap is noted at this boundary. This gap embraces the entire Upper and partly the
top of the Lower Ashgill as well as the lowermost Llandovery, and appears to be a result of the
Taconic phase of orogeny.
In the Polish Lowlands, the Ordovician-Silurian boundary beds show great facies variability
(Modlinski 1973). In many boreholes, sedimentary gaps embrace various time spans and a
change of facies toward a marly-arenaceous one is noted, which appears to indicate a gradual
regression. Graptolites have only been found in the deeper parts of the platform slope clayey
facies, including the Upper Ashgill Biozone of Glyptograptus persculptus and the Lower Llan-
dovery A. acuminatus Zone, for example in the Lebork borehole (Tomezyk 1965).
Bull. Br. Mus. nat. Hist. (Geol) 43: 93-94 Issued 28 April 1988
94 L. TELLER
References
Bednarezyk, W. 1971. Stratigraphy and paleogeography of the Ordovician in the Holy Cross Mountains.
Acta geol. Pol., Warsaw, 21 (4): 573-616, pls 1—4.
Cocks, L. R. M. 1985. The Ordovician—Silurian boundary. Episodes, Ottawa, 8: 98-100.
Kielan, Z. 1956. Stratygrafia gornego ordowiku w Gorach Swieto krzyskich. Acta geol. Pol., Warsaw, 6:
253-272, pls 1-4. [Engl. summ. ].
— 1960. Upper Ordovician trilobites from Poland and some related forms from Bohemia and Scandin-
avia. Palaeont. Pol., Warsaw, (for 1959) 11. 198 pp., 36 pls.
Modlinski, Z. 1973. Stratigraphy and development of the Ordovician in North-Eastern Poland. Pr. Inst.
geol., Warsaw, 72: 1—74, pls 1-5.
Teller, L. 1962. Zagadnienie granicy Ordowik-Sylur w Gorach Bardzkich. In E. Passendorfer (ed.), Ksiega
pamiatkowa ku czci prof. Jana Samsonowicza: 171-186. Warsaw. Akademia Nauk. [Engl. summ.].
1969. The Silurian biostratigraphy of Poland based on graptolites. Acta geol. Pol., Warsaw, 19:
393-501.
Temple, J. T. 1965. Upper Ordovician brachiopods from Poland and Britain. Acta palaeont. Pol., Warsaw,
10: 379-427, pls 1-21.
Tomezyk, H. 1962. Problem stratygrafii ordowiku 1 syluru w Polsce w Swietle ostatnich badan. Pr. Inst.
geol., Warsaw, 35: 1-134, pls 1-4. [Eng]. summ. ].
The Ordovician-Silurian boundary in the Prague
Basin, Bohemia
P. Storch
Geological Survey, P.O. Box 85, Prague 011, 118 21 Czechoslovakia
Synopsis
The Ordovician-Silurian boundary in the Prague Basin is marked by an abrupt change in facies develop-
ment and faunal assemblages, without significant breaks in purely marine sedimentation. Shallow marine
sandstones and petromict conglomerates of the upper Kosov (Hirnantian) are followed by bioturbated
mudstones due to the initial phase of a new transgression, with an abundant Hirnantia fauna in the
uppermost Kosov. The mudstones are followed by dark graptolitic shales at the base of the Silurian (in
the Prague Basin at the base of the Akidograptus ascensus Subzone). During the Parakidograptus acumin-
atus Subzone another change of sedimentation appeared as a transition from silty-clay shales to sandy-
micaceous laminites. This change corresponds to a local break in sedimentation in the north limb of the
Prague Basin and in the Pankrac area, where the break continued to the Monoclimacis griestoniensis
Zone. The sequence and the succession of faunal assemblages indicate an accelerated rate of transgression
just below and above the Ordovician-Silurian boundary. Analysis of the faunal assemblages allows a
detailed stratigraphical subdivision of the boundary beds in the Prague Basin and wide international
correlation.
Introduction
In Bohemia, the Ordovician-Silurian boundary is well developed in the Prague Basin
(Barrandian area). The Prague Basin is a tectonically predisposed linear sedimentary depression
in which the sedimentation continued from the lowermost Ordovician up to the Middle Devo-
nian without substantial interruptions (Havlicek 1981, 1982). In the Prague Basin, the
Ordovician-Silurian boundary coincides with the boundary between the Kosov and Zelkovice
Formations. Perner & Kodym (1919) supposed that there was a stratigraphical gap at the base
of the Silurian in Bohemia caused by the emersion phase of the Taconic orogeny. Later, the
lowermost Silurian graptolite zones, including the Parakidograptus acuminatus Zone, were
documented in the Barrandian area by Marek (1951) and by Bouéek (1953) in an isolated
outcrop near Béchovice. These authors denied the existance of the boundary gap east of Prague
at Béchovice, but they admitted its presence in the rest of the Prague Basin. Horny (1956, 1960)
found the earlier A. ascensus Zone along the whole southern limb of the Basin. He recorded
that the rocks of the basal Silurian graptolite zones were only absent locally due to minor
erosion caused by epeirogenetic movements that represented the aftermath of tectonic activity
during the deposition of the Kosov Formation. Havliéek (1981, 1982) explained both the
flysch-like Kosov Formation and the change in lithologic development at the Ordovician—
Silurian boundary by invoking synsedimentary tectonic movements in the basin.
More recently, basal Silurian graptolite zones have also been discovered in the northern limb
of the Prague Basin and the boundary hiatus was verified only in a restricted part of the basin
(Storch 1982, 1986). Investigation of the early Kosov (Storch & Mergl, in press) has shown the
sequence in Bohemia to be very similar to that explained by glacio-eustatic environmental
changes (Brenchley & Cocks 1982; Brenchley & Cullen 1984; Brenchley & Newall 1984). The
glacio-eustatic conception of the late Ordovician to early Silurian facies and faunal changes
(Brenchley 1984; Brenchley & Newall 1984) also appears to explain the Ordovician-Silurian
boundary sequence in the Prague Basin.
Sequence of the latest Ordovician
Considerable changes preceding the Ordovician—Silurian boundary event were recorded at the
top of the Kraliv Dviur Series in the Prague Basin (Storch & Mergl, in press). The deep water
Bull. Br. Mus. nat. Hist. (Geol) 43: 95-100 Issued 28 April 1988
96 p. STORCH
mudstones of the Kraluv Dvur Formation, with deep water faunal assemblages, were followed
by coarse grained subgraywackes and silty shales at the base of the Kosov Formation. The
high-diversity Proboscisambon Community of the uppermost Kraliv Dvur Formation was
replaced by the low-diversity and short-lived Mucronaspis Community (Storch & Mergl, in
press), the last record of which (bivalves and trilobite fragments) occurs in the shale of the
lowermost Kosov Formation.
The basal Kosov subgraywackes and shales were succeeded by flysch-like sediments which
form most of the thickness of the Kosov Formation. This regressive sequence culminated in the
deposition of shallow-water sandstones and petromict conglomerates in the upper part of the
formation. In the uppermost sandstone layers a monotonous assemblage of infaunal bivalves
provides evidence of intertidal environments (Havlicek 1982). In the uppermost part of the
Kosov Formation, the quartz sandstones with shaly intercalations are replaced by siltstones
and mudstones. Pale grey, often bioturbated calcareous mudstones and claystones containing a
rich Hirnantia sagittifera Community occur near the top of the formation. The Hirnantia fauna,
interpreted by Havlicek (1982) as representing a subtidal environment, has been found only in
the eastern part of the Prague Basin. A gradual deepening of the sea seems likely in the
uppermost Kosov (Hirnantian) of the Prague Basin.
The cosmopolitan Hirnantia fauna found in the uppermost part of the Kosov Formation
permits a broad international correlation. In the Prague Basin it was first recorded at Bécho-
vice near Prague (Marek 1963; Marek & Havlicek 1967). Later, it was found at Nova Ves,
Pankrac, Repy and Reporyje (all within the Prague area) and near Tachlovice. All the fossil-
iferous localities yielded faunal associations of similar taxonomic composition, but without the
depth-controlled variations of the associations reported by Brenchley & Cocks (1982) and
Brenchley & Cullen (1984) from the Oslo region, Norway. Lists of the Hirnantia faunas from
Bohemia were published by Havliéek (1982) and Storch (1986). The graptolite Glyptograptus
bohemicus (Marek) accompanies the Hirnantia sagittifera Community in Bohemia and supports
the international biostratigraphic correlation of the sequence. The layer containing the Hirnan-
tia fauna is separated from the first graptolitic shales by at least 0-3m thickness of mudstone,
often heavily bioturbated, with frequent limonite impregnations originating from pyrite
weathering (Storch 1986).
Ordovician-Silurian boundary and lowermost Silurian sequence
In general, sedimentation is continuous through the Ordovician—Silurian boundary in the
Prague Basin, in spite of some differences between the separate sections. By using distinctive
features of the boundary sequence and also the basal Silurian lithologies, the Prague Basin may
be formally subdivided into five areas (Storch 1986).
The quietest sedimentation, in probably the deepest parts of the basin, is limited to the
sections along the whole south limb of the basin (South limb area—Zelkovice, Vseradice, Béleé,
Votkov, Zadni Treban, Hlasna Tiebani, Karlik, Cernosice and Velka Chuchle).
A complete succession starting with the Akidograptus ascensus Subzone has been preserved
in all the localities (exemplified by the Karlik section, Fig. 1). Clayey shales with climaco-
graptids and rare glyptograptids were recorded even below the first occurrences of A. ascensus
at Zelkovice and Votkov and could represent the upper part of the Glyptograptus persculptus
Zone. The ascensus Subzone is represented by clayey shales with subsidiary variable siltstones.
Sandy-micaceous laminites start within the Parakidograptus acuminatus Subzone. The laminites
disappear in the western part of the south limb at Zelkovice and VSeradice in the Cystograptus
vesiculosus Zone, and towards the east in the Coronograptus cyphus Zone, and sometimes they
even reach up to the Demirastrites triangulatus Zone (Storch 1986). In the same way, the onset
horizon of siliceous shales migrates in the south limb on the vesiculosus Zone at Zelkovice to
the Demirastrites pribyli Zone at Cernogice and Velka Chuchle. The Repy and Béchovice
sections differ in having more rapid sedimentation, giving the greatest thicknesses of graptolite
zones (Repy section, Fig. 1) in this part of the Prague Basin. The layer referred to the per-
|
SILURIAN
ORDOVICIAN
Fig. 1
ORDOVICIAN-SILURIAN BOUNDARY IN BOHEMIA
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KARLIK
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ZELKOVICE
Lithology, stratigraphy and faunal distribution in selected Ordovician—Silurian boundary
sections of the Prague Basin; location of the sections.
97
98 Pp. STORCH
sculptus Zone is also developed there. Laminites appear in the acuminatus Zone and pass up
into the vesiculosus Zone.
Detailed studies of both biostratigraphy and lithostratigraphy (Storch 1986) revealed that the
laminites represent more condensed sedimentation than the clayey and silty shales. The onset of
laminite deposition in the southern limb of the basin appears to have been synchronous with
the start of the break in sedimentation in the acuminatus Zone in the north limb of the basin at
Sedlec and Lodénice.
The longest break in sedimentation is known from the Mala Chuchle, Pankrac, Nova Ves
and Tachlovice sections (Pankrac area). The topmost Ordovician mudstones are followed there
by graptolitic shales of the Litohlavy Formation, with upper Llandovery graptolites of the
Monoclimacis griestoniensis Zone. In this case, reworking possibly took place of previously
deposited, incoherent, clayey and muddy sediments of the basal Silurian (ascensus Subzone, the
lower part of the acuminatus Subzone), and perhaps also of the topmost Ordovician (several
tens of centimetres in thickness). Near Stodtlky and Reporyje (Reporyje section, Fig. 1), this
break in sedimentation splits into two shorter gaps. The earlier of them starts above the
ascensus Subzone and thus supports the explanation of the break presented in different parts of
the Prague Basin.
Sedimentation and assumed bathymetric changes
The Kosov Formation, which is about 100m thick, shows sedimentation which was presum-
ably controlled by glacio-eustatic regression. The subsequent transgression started in the
uppermost Kosov and strongly accelerated at the base of the Silurian (Brenchley & Newall
1984). Considerable transgression is also documented by a decrease of the rate of sedimentation
at the Ordovician—Silurian boundary. In the Prague Basin, the rate of sedimentation in the
lowermost Silurian was approximately calculated (Storch 1986) to range between 1 m and 7-5m
per 10° years in contrast to nearly 100m per 10° years during the Kosov Series (Hirnantian).
During the acuminatus Zone the transgression caused a further deepening of the Prague Basin
and was probably the origin of a fairly intensive bottom current in the deeper central part of
the linear depression of the Prague Basin. This current is considered to have caused local
breaks in sedimentation, in places perhaps accompanied by mild subaquatic erosion (Storch
1986). In the sites where this current had less erosive power, condensed sedimentation of
laminites occurred, and in the quietest parts of the basin floor there were deposited siliceous
shales and silty silicites ((phtanites’) which first appeared in the vesiculosus Zone.
Stratigraphy
The Hirnantia fauna occurs in the upper part of the Kosov Series well above the disappearance
of the Mucronaspis Community in the basal part of the Series. The Hirnantia fauna, which is
accompanied by Glyptograptus bohemicus, can be referred to the upper Hirnantian, namely to
the upper part of the Climacograptus extraordinarius Zone or the lower part of the persculptus
Zone.
In Bohemia, the base of the Silurian System coincides with the base of the ascensus Subzone,
which is defined by the first appearance of Akidograptus ascensus Davies (usually accompanied
by Diplograptus modestus Lapworth). When compared with the British Isles, the base of the
subzone in Bohemia is comparable to the base of the acuminatus Zone at the type section Dob’s
Linn (Williams 1983). In the Prague Basin, the base of the ascensus Subzone mostly corre-
sponds to a sudden change in both the colour and the composition of the sediments, in which
the pale grey bioturbated mudstones are replaced by dark grey clayey graptolitic shales.
However, a low-diversity climacograptid—glyptograptid assemblage has been recorded from
several localities at the base of the graptolitic shales just below the ascensus Subzone, which is
separated by an unfossiliferous bioturbated mudstone from the bohemicus Zone beneath. The
first assemblage of graptolitic shales below the ascensus Subzone is referred to the upper part of
SILURIAN
CHRONO-
STRATIGRAPHY
LITHOSTRATIGRAPHY
BIOSTRATIGRAPHY
Llandovery
Rhuddanian Aeronian
Zelkovice Fm.
?
persculptus —
ascensus
acuminatus
vesiculosus
triangulatus
Ww)
>}
S
E
o
fe
fo}
a
cyphus
Glyptograptus bohemicus Marek
Climacograptus aff. miserabilis Elles & Wood
Climacograptus normalis Lapworth
Glyptograptus sp. (ex gr. persculptus)
Glyptograptus sp. (aff. avitus)
Diplograptus modestus Lapworth
Akidograptus ascensus Davies
Diplograptus elongatus Churkin & Carter
Diplograptus aff. parvulus (Lapworth)
Diplograptus parajanus Storch
Cystograptus ancestralis Storch
Climacograptus aff. premedius Waern
Climacograptus trifilis Manck
Parakidograptus acuminatus (Nicholson)
Climacograptus longifilis Manck
Diplograptus diminutus apographon Storch
Cystograptus vesiculosus (Nicholson)
Climacograptus aff. rectangularis McCoy
Glyptograptus ex gr. tamariscus
Atavograptus atavus (Jones)
Dimorphograptus confertus (Nicholson)
Orthograptus obuti Rickards & Koren
Rhaphidograptus toernquisti (Elles s Wood)
Lagarograptus aff. acinaces (Térnquist)
Pribylograptus argutus (Lapworth)
Monograptus austerus austerus Tornquist
Monograptus cf. sudburiae Hutt
Limpidograptus cf. posohovae Chaletzkaja
Diplograptus cf. thuringiacus Eisel
Diplograptus fezzanensis Desio
Coronograptus cyphus cyphus (Lapworth)
Orthograptus cyperoides (Tornquist)
Monograptus austerus vulgaris Hutt
Monograptus difformis Tornquist
Petalograptus ovatoelongatus (Kurck)
Rastrites longispinus Perner
Demirastrites triangulatus (Harkness)
Coronograptus gregarius gregarius (Lapworth)
Fig. 2. Chronostratigraphy, lithostratigraphy, biostratigraphy and graptolite species ranges through
the Ordovician-Silurian boundary interval in the Prague Basin.
100 p. STORCH
the persculptus Zone, in spite of the fact that true Glyptograptus persculptus has not yet been
found there.
The ranges of graptolites up to the base of the triangulatus Zone are shown in Fig. 2. The
rich graptolite assemblages of the Prague Basin were briefly described by Boucek (1953), and
more recently they have been described by Storch (1986).
Acknowledgements
I would like to thank V. Havli¢éek and J. Kfiz for critically reading the manuscript.
References
Boucéek, B. 1953. Biostratigrafie, vyvoj a korrelace zelkovickych a motolskych vrstev ¢eského siluru.
(Biostratigraphy, Development and Correlation of the Zelkovice and Motol Beds of the Silurian of
Bohemia). Sb. ustred. Ust. geol., Prague, 20: 421-484.
Brenchley, P. J. 1984. Late Ordovician Extinctions and their Relationship to the Gondwana Glaciation.
In P. J. Brenchley (ed.), Fossils and Climate: 291-315. London.
& Cocks, L. R. M. 1982. Ecological associations in a regressive sequence: the latest Ordovician of
the Oslo—Asker District, Norway. Palaeontology, London, 25: 783-815, pls 85-86.
—— & Cullen, B. 1984. The environmental distribution of associations belonging to the Hirnantia
fauna—evidence from North Wales and Norway. In D. L. Bruton (ed.), Aspects of the Ordovician
System: 113-125. Universitetsforlaget, Oslo. (Pal. Contr. Univ. Oslo 295).
—— & Newall, G. 1984. Late Ordovician environmental changes and their effect on faunas. In D. L.
Bruton (ed.), Aspects of the Ordovician System: 65—79. Universitatsforlaget, Oslo. (Pal. Contr. Univ.
Oslo 295).
Havlicek, V. 1981. Development of a linear sedimentary depression exemplified by the Prague Basin
(Ordovician—Middle Devonian; Barrandian area—central Bohemia). Sb. geol. Véd., Prague, (Geol.) 35:
7-48.
—— 1982. Ordovician in Bohemia: development of the Prague Basin and its benthic communities. Sb.
geol. Véd., Prague, (Geol.) 37: 103-136.
Horny, R. 1956. Zona Akidograptus ascensus v jiznim kfidle barrandienského siluru. Vést. ustred. Ust.
geol., Prague, 31: 62-69.
1960. Stratigrafie a taktonika zapadnich uzavéru silurodevonskeho synklinoria v Barrandienu. Sb.
ustred. Ust. geol., Prague, 26: 495-530.
Marek, L. 1951. The find of Akidograptus acuminatus (Nicholson) in the Silurian of Bohemia. Vést. ustred.
Ust. geol., Prague, 24: 382-384.
1963. Zprava o vyzkumu fauny vrstev kosovskych ¢eskeho ordoviku. Zpravy o geol. vyzkumech 1962:
103-104.
—— & Havli¢ek, V. 1967. The articulate brachiopods of the Kosov Formation (Upper Ashgillian). Vést.
ustred. Ust. geol., Prague, 42 (4): 275-284, pls 1-4.
Perner, J. & Kodym, O. 1919. O rozéleneni svrchniho siluru v Cechach. Cas. Mus. Kral. éesk., Prague, 93:
6-24.
Storch, P. 1982. Ordovician-Silurian boundary in the northernmost part of the Prague Basin (Barrandian,
Bohemia). Vést. ustred. Ust. geol., Prague, 57 (4): 231-236.
— 1986. Ordovician-Silurian boundary in the Prague Basin (Barrandian area, Bohemia). Sb. geol. Véd.,
Prague, (Geol.) 41: 69-99, 8 pls.
—— & Mergl, M. (in press). Kralovdvor-—Kosov boundary and the late Ordovician environmental
changes in the Prague Basin {Barrandian area, Bohemia). Sb. geol. Véd., Prague, (Geol.) 44.
Williams, S. H. 1983. The Ordovician—Silurian boundary graptolite fauna of Dob’s Linn, southern Scot-
land. Palaeontology, London, 26: 605-639.
The Ordovician—Silurian boundary in the
Saxothuringian Zone of the Variscan Orogen
H. Jaeger
Museum fur Naturkunde, Palaontologisches Museum, Invalidenstrasse 43, 104 Berlin, DDR.
Synopsis
In the Saxothuringian Zone of the Variscan Orogen in Thuringia, Saxonia and north Bavaria the poorly
fossiliferous, thick arenaceous-argillaceous Ordovician rocks are abruptly but conformably succeeded by
the very condensed sequence of Silurian—Early Devonian graptolitic alum shales and lydites beginning in
both major facies with the Zone of Akidograptus ascensus. Below it, shaly interbeds in the uppermost
Ordovician Dobra Sandstone yielded chiefly non-zonal graptolites, and in one section Diplograptus bohe-
micus about | m below the lithological boundary.
Introduction
The Saxothuringian and Lugian (= West Sudetic) Zones form the middle of the three major
depositional and tectonic belts of the Variscan Orogen in central Europe. They constitute the
metamorphic zones that are situated between the internal Moldanubian Zone (internids) and
the external Rhenohercynian Zone (externids). The latter is exemplified by the Rheinisches
Schiefergebirge and the Harz Mountains, in both of which the nature of the Ordovician—
Silurian junction is unknown. In this paper only the type area of the Saxothuringian Zone is
considered; it lies west of the River Elbe in Saxonia, Thuringia, north Bavaria and north
Bohemia. Together with the Lugian Zone (situated east of the Elbe), it forms the northern part
of the Bohemian Massif and is the largest outcropping fragment of the broken Variscan orogen
in central Europe. In a wider palaeogeographical and geotectonical context, the
Saxothuringian—Lugian Zones are part of the Mediterranean province, and of the Palaeotethys
geosyncline and sea, that is the Tethys of the early and middle Palaeozoic.
In the whole of the Palaeotethys area, the Ordovician-Silurian transition is marked by a
drastic change in the depositional regime. In the Saxothuringian Zone the typically 2000m
thick Ordovician, consisting of poorly fossiliferous, arenaceous-argillaceous rocks with some
sedimentary iron ore bodies, is rapidly replaced by 50m thick Silurian, which is made up
almost entirely of interbedded euxinic lydites and alum shales rich in graptolites. From the
middle Ludlow to the Pridoli, the graptolitic shales are interrupted by a peculiar limestone
(Ockerkalk) or grey-green clay shales, both of which are poorly aerated deposits. Sedimentation
of the alum shales, and regionally also of the lydites, recurred in the uppermost Silurian, and
lasted well into the Lower Devonian (Lochkov).
The Silurian (and Devonian) graptolitic shales of the Thuringian type, that is alum shales
and black lydites, contain large quantities of pyrite, phosphorite (in nodules and layers) and
carbon (in beds, laminae and lenses). These rocks cover vast areas in the deeper parts of the
Palaeotethys sea between Thuringia and north Africa. They are the result of one of the largest
oceanic anoxic events in the history of the earth, both areally and temporally.
Thuringian and Bavarian Facies
In the geosynclinal Palaeozoic of the Saxothuringian Zone two major facies (or rather series of
facies—Faziesreihen) are distinguished, at least in the rock-sequences from the Ordovician to
the Lower Carboniferous. These are known as the ‘Thuringian’ and ‘Bavarian Facies’, but it is
beyond the scope of this paper to outline their features in detail. The following points may
however be made.
Bull. Br. Mus. nat. Hist. (Geol) 43: 101-106 Issued 28 April 1988
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ORDOVICIAN-SILURIAN BOUNDARY IN VARISCAN OROGEN 103
The Thuringian Facies represents a monotonous basin facies that exhibits only moderate
lateral changes, if any. By contrast, the Bavarian Facies is complex. In the simplest model
(Jaeger 1977: text-fig. 3) its site is depicted as a swell flanked on either side by deep furrows
(deeper than the Thuringian basin). The central swell of the Bavarian Facies region is charac-
terized by intermittent carbonate sedimentation that lasted demonstrably from the Silurian into
the Carboniferous. On the swell the nature of the Ordovician-Silurian boundary is unknown.
In most or all of the Saxothuringian Zone the swell-limestones are known from allochthonous
blocks (olistholites) or even only from boulders, for example, the Middle to Upper Devonian
stromatoporoid-coral reef-limestones at Frankenberg. The flanking depressions received non-
carbonate sediments throughout their history. Typical of this Bavarian basin facies is the
continuous sequence of cherts, siliceous shales and clay shales (Kieselschiefer-Fazies) spanning
the long interval from the base of the Silurian to the top of the Devonian. In the Silurian
interbedded graptolitic black lydites and alum shales are the typical rocks, as in the Thuringian
Facies, whereas throughout most of the Devonian conodont-bearing brighter grey-green and
even red cherts, siliceous shales and clay shales occur.
The region of the Bavarian Facies was, at least in its Bavarian type area, the site of large-
scale basic vulcanism which lasted intermittently from the earliest Ordovician to the Carbon-
iferous, whereas in the Thuringian Facies the geosynclinal basic vulcanism was virtually
confined to a brief phase of violent eruptions and intrusions at the beginning of the Upper
Devonian.
Rocks of the Thuringian Facies cover large areas in the Saxothuringian Zone. Minor
occurrences are known from the southern margin of the Lugian Zone in Czechoslovakia. The
Bavarian Facies rocks form a discontinuous belt that runs along the strike near the middle of
the Saxothuringian Zone. They are confined to narrow strips (at the most several kilometres
broad) on either side of the so-called Zwischengebirge (Betwixt Mountains) of Miinchberg,
Wildenfels and Frankenberg. East of the Elbe, the Bavarian Facies reappears at the Eichberg
near Weissig immediately north of the plutons that build up the area between Dresden and
Gorlitz. From the Eichberg the Bavarian Facies can be traced through all of the Lugian Zone
as far as the southern end of the Sowie Gory (Eulen-Gneis), where it is particularly well
developed. Outside its main belt, the Bavarian Facies is typified by the Palaeozoic of the
Elbtalschiefergebirge southeast of Dresden. The palaeogeography of the area of the Bavarian
Facies may be envisaged as an island arc (the use of which term does not necessarily denote the
implications of the theory of plate tectonics).
Ordovician-Silurian Boundary
At the Ordovician-Silurian boundary the distinctness of the two contrasting regional facies is
particularly pronounced. In the Thuringian Facies the uppermost Ordovician is represented by
the peculiar Lederschiefer, a monotonous, almost black, buff-weathering, non-bedded silty shale
with high content of mica. Predominantly arenaceous rock-detritus and isolated sandstone
boulders up to 30cm across (some attaining even several metres) occur in varying quantities
throughout the 250m thick formation, for which it is noted. Whether the boulders represent
glacial drop-stones or whether they originated from slumping are much debated questions.
While the matrix of the Lederschiefer is barren, many boulders contain brachiopods, bryo-
zoans, various trilobites and echinoderms, particularly loose cystoids. Most of these exotic
fossils await modern expert study. Strata that compare closely lithologically with the Leder-
schiefer are of wide distribution in the Mediterranean province, for example in the Orea Shale
in Spain.
In the uppermost two to three metres of many Thuringian sections it can be seen that the
sand grains and mica flakes disappear, while many pyrite nodules appear in the shales, herald-
ing the change to the otherwise abrupt transition to the Silurian euxinic graptolitic rocks. By
contrast, the occurrence of sandstone beds in the uppermost Lederschiefer has been reported
(Troeger 1959, 1960; Freyer 1959) from eastern sections (near Oelsnitz) that lie near the Bavar-
ian Facies belt.
104 H. JAEGER
In view of the intense folding, sections that exhibit a tectonically undisturbed transition from
the Ordovician to the Silurian are hardly to be expected between rocks with such different
mechanical properties as the Lederschiefer (below) and the lydites/alum shales (above). Never-
theless, a century ago Akidograptus acuminatus was recovered from the basal graptolite shales
at Ronneburg and Oelsnitz by Eisel. Recently Alder (1963) and Schauer (1971) found 4A.
ascensus in the basal 4m of interbedded alum shales and lydites below the acuminatus fauna at
the Weinberg near Hohenleuben in what would appear to be the most intact boundary sec-
tions. The zone fossil is associated with Diplograptus modestus and several forms of Cli-
macograptus (C. medius, C. rectangularis, C. scalaris normalis and C. miserabilis); there also
occur unnamed climacograptids that have branched virgellae or virgellae with a distal vesicular
appendage (Schauer 1971).
In the succeeding half metre, Akidograptus acuminatus occurs together with all the species
that are already present in the ascensus Zone, but in addition, the highly characteristic Cli-
macograptus trifilis Manck and C. longifilis Manck make their first appearance.
In the Bavarian basin facies the uppermost Ordovician is represented by the Dobra Sand-
stone. This is an almost black, fine-grained, often quartzitic sandstone with subordinate shaly
interbeds, with a maximum thickness in excess of 40m. Some sandstone beds exhibit
magnificently-developed sole markings (load casts), others roll- and ball-structures. Greiling
(1966: 12) interprets the Dobra Sandstone essentially as a turbidite. This peculiar rock is a
characteristic formation of the Bavarian Facies, and is of wide distribution. It can be traced
intermittently throughout the Saxothuringian and Lugian Zones for a total length of 400km
and it has a far greater linear extent in central Europe than the coeval Lederschiefer.
Lithologically virtually identical (Carnic Alps) or dissimilar (Kosov Quartzite in the
Barrandian) sandstones occur in the same or analogous stratigraphical position in many areas
of the Mediterranean province. In some regions they may range considerably higher, through
much of the Llandovery, and not start until the base of the Silurian.
The Dobra Sandstone is practically unfossiliferous, except for the uppermost two metres
which yielded graptolites in shaly interbeds. Stein (1965: 119; text-figs 5, 20 and others)
described Climacograptus medius, C. scalaris normalis, Diplograptus modestus, and a single
thabdosome of D. cf. persculptus (Salter) from 1:90 m below its top at Dobra.
At the Silurberg locality in Obermthlbach near Frankenberg Diplograptus bohemicus
(Marek) was described by Jaeger (1977) from the uppermost Dobra Sandstone. This species
occurs there abundantly, but to the exclusion of other graptolites, in a layer just a few mm
thick in the middle of a 0-70-0-75m thick bed of homogeneous grey-black clay shale that
underlies a prominent 30cm thick quartzite. The latter is overlain by 4m of platy sandstone
and shale showing slickensiding, which is succeeded by 1m of broken and mylonitized alum
shales and lydites indicating a major fault that throws Ludlovian (colonus and chimaera Zone)
graptolite shales against the Ordovician Dobra Sandstone. The same sequence, particularly the
0:70-0:75 m thick bed of shale and the overlying compact 30cm sandstone bed, have been
traced to the northeast as far as Starbach. This sequence is therefore shown as the typical one
in Fig. 2 (right column). In the apparently undisturbed boundary section at Starbach the 30cm
thick compact sandstone bed is immediately overlain by 40cm of weathered clay shales and
siliceous shales, which in turn are succeeded by typical alum shales and lydites. Graptolites
were not found in the Dobra Sandstone at other localities, nor was the occurrence of the basal
Silurian graptolite zones established in this northeastern part of the Saxothuringian Zone.
The basal Silurian graptolite zones were recovered in the lowermost alum shales and lydites
of the type area of the Bavarian Facies along the northwest side of the Mtinchberg gneis at
Dobra, Fortschenbach, Ober-Brumberg and Rauhenberg (Greiling 1957, 1966; Stein 1965).
Though these workers did not formally distinguish between the Zones of A. ascensus and
acuminatus 1t would appear evident from Stein’s precise documentation that the two can be
differentiated. The thicknesses are approximately the same as in the Thuringian Facies, or
slightly less. The associations are also the same, though the number of listed forms is somewhat
smaller. Climacograptus trifilis and C. longifilis occur as frequently as in the Thuringian Facies.
ORDOVICIAN-SILURIAN BOUNDARY IN VARISCAN OROGEN 105
Thuringian Basin Facies Bavarian Basin Facies
Fs Upper Graptolitic === Upper Graptolitic
Shales 15m —— Shales 10m
= M.hercynicus - =e | Mieneymicus—
_—— M.transgrediens |a=s—ae M.uniformis
alee] SS
Ockerkalk == /Grey-Green Shales
10 -20m — mn
M.chimaera F M.chimaera 35/34
— Zam Solee ==
— —— Lower
== Lower == Graptolitic Shales
ame Craptolitic Shales ——= 35-40m
M.gregarius 19 mee 8=M.gregarius
s=S== Mees 18 =—= M.cyphus
== peeom === S©vesiculosus
== C.vesiculosus os 4
——— 04-13m 17 —— nd)
Pee A.acuminatus See § A.acuminatus
et O04 = 05 m ; 6 a 02 = 05 mM
——— A.ascensus == A.ascensus 16
a 04-0,5m Sateen 0Q2-05m
.=.=-. | sandstone & shale
: 5m
Lederschiefer pce | quartzite bed Q3m
250m = grey shale 07m
Uppermost =s2a02 || Dipl. bohemicus
Ordovician =~ | Débra Sandstone
>40m
et =? (SSI
Fig. 2 Composite sections across the Ordovician-Silurian boundary in the Thuringian (left) and
Bavarian basin facies (right). Legend: 1. Lydites (black layered cherts). 2. Alum shales. 3. Grey to
green argillaceous shales. 4. Homogeneous non-bedded silty shales. 5. Arenaceous rocks. 6. Lime-
stones. 7. Phosphoritic nodules.
106 H. JAEGER
Two points of general interest may be made. Firstly, in the Saxothuringian Zone, the change
from the Ordovician Lederschiefer and Dobra Sandstone, respectively, to the Silurian grapto-
litic rocks takes place at the base of the Zone of A. ascensus and above beds with D. bohemicus
which have only been found in one section of the Bavarian Facies. Secondly, in the Saxo-
thuringian region, A. ascensus and A. acuminatus indicate two successive graptolite zones, as in
the Barrandian area and southern Spain (Jaeger & Robardet 1979: 693, section 4), although 4.
ascensus ranges into the acuminatus Zone, and in Sardinia even into the next higher Zone of
Cystograptus vesiculosus (Jaeger 1976: pl. 3, fig. 7).
References
Alder, F. (1963). Biostratigraphie und Taxionomie der Graptolithen des Weinberges bei Hohenleuben. 95 pp.,
pls 1-47, text-figs 1-19. Diplomarbeit, Bergakademie Freiberg (unpublished).
Freyer, G. 1959. Die Ausbildung der Grenze Ordovicium/Silur im Bereich der Vogtlandischen Haupt-
mulde. Beitr. Geol., Berlin, 1: S—12, 2 text-figs.
Greiling, L. 1957. Das Gotlandium des Frankenwaldes (Bayerische Entwicklung). Geol. Jb., Hannover, 73:
301-356.
—— 1966. Sedimentation und Tektonik im Palaozoikum des Frankenwaldes. Erlanger geol. Abh., 63:
1-60, pls 1-2.
Jaeger, H. 1976. Das Silur und Unterdevon vom thtringischen Typ in Sardinien und seine regional-
geologische Bedeutung. Nova Acta Leopoldina, Halle a.S., 45 (224): 263-299, pls 1-3.
— 1977. Das Silur/Lochkovy-Profil im Frankenberger Zwischengebirge (Sachsen). Freiberger ForschHft.,
Berlin, (C) 326: 45—S9, pl. 1.
— & Robardet, M. 1979. Le Silurien et le Dévonien basal dans le Nord de la Province de Seville
(Espagne). Géobios, Lyon, 12: 687-714, pls 1-2.
Schauer, M. 1971. Biostratigraphie und Taxionomie der Graptolithen des tieferen Silurs unter besonderer
Berticksichtigung der tektonischen Deformation. Freiberger ForschHft., Berlin, (C) 273: 1-185, pls 1-45.
Stein, V. 1965. Stratigraphische und palaontologische Untersuchungen im Silur des Frankenwaldes. N. Jb.
Geol. Palaont. Abh., Stuttgart, 121: 111—200, pls 1-2.
Troeger, K. A. 1959. Kaledonische und frtihvariscische Phasen im Vogtland und den angrenzenden
Gebieten. Freiberger ForschHft., Berlin, (C) 73: 1-152.
1960. Das untere Silur im Vogtland. In J. Svoboda (ed.), Prager Arbeitstagung uber die Stratigraphie
des Silurs und des Devons (1958): 315-325, text-figs 1-2. Prague.
Wiefel, H. 1974. Ordovizium. In W. Hoppe & G. Seidel (eds), Geologie von Thiiringen: 165-194. Gotha/
Leipzig.
Zitzmann, A. 1966. Neue Conodontenfunde in der devonischen Kieselschiefer-Serie der bayerischen Fazies
des Frankenwaldes. Geol. Bl. Nordost-Bayern 16 (1): 1-39.
—— 1968. Das Palaozoikum im Grenzbereich zwischen Bayerischer und Thtringischer Faziesreihe des
Frankenwaldes. Geol. Jb., Hannover, 86: 579-654, pls 1-3.
The Ordovician—Silurian boundary in the Carnic Alps
of Austria
H. P. Schonlaub
Geologische Bundesanstalt, PO Box 154, Rasumofskygasse 23, A 1031 Vienna, Austria
Synopsis
Although the Ordovician-Silurian boundary is represented in some places by a considerable uncon-
formity in the Carnic Alps, in other sections a Hirnantia fauna in the Plocken Formation and possibly
persculptus Zone graptolites are succeeded by the Bischofalm facies which in places has yielded graptolites
of the acuminatus Zone. The shallow-water facies and unconformities at and near the boundary were
partly caused by the global eustatic fall and rise in sea level and partly by Caledonian tectonic activity.
Introduction
The long geological history of the Carnic Alps of Austria and northern Italy lasts from the late
Ordovician to middle Triassic times. For many years in this region several sections which cross
the Ordovician-Silurian boundary and represent different environmental settings have been
well known. Based on earlier studies by Gaertner (1931), Walliser (1964), Fliigel (1965), Serpagli
(1967), Schonlaub (1969, 1971), Vai (1971), and Jaeger et al. (1975), a brief summary of knowl-
edge of this interval up to the year 1975 was submitted and published in an earlier circular of
the Ordovician-Silurian Boundary Working Group.
Based on the final decision of the Commission on Stratigraphy that the base of the Silurian
System shall be at the base of the A. acuminatus Biozone, the present paper revises the strati-
graphy of the boundary beds in the Carnic Alps. In addition new field data are presented and
summarized in this updated version of previous reports. I acknowledge the help of H. Jaeger,
Berlin, and R. Schallreuter, Hamburg, who kindly provided unpublished data on graptolites
and ostracods.
Upper Ordovician sediments and stratigraphy
All known late Ordovician and early Silurian boundary sequences show clear evidence of a
regressive-transgressive relationship. Except for one section representing the deep water ‘Bi-
schofalm graptolite facies’, for which, however, biostratigraphical data are missing for the late
Ordovician, the lithology and faunal composition in the upper Ordovician reflect a stable
environment of shallow to moderate depths with a considerable clastic influx in the Caradoc
Stage. During this time the fossiliferous Uggwa Shales, up to 100m thick, were deposited. They
comprise sandy shales and pass laterally into greenish and brownish mudstones and siltstones,
the latter being widely distributed in the Central Carnic Alps in the surroundings of Plock-
enpass and Lake Wolayer. In contrast to the typical Uggwa Shales, in these beds only very few
fossils occur. This shale and siltstone sequence grades laterally and in part also vertically into
40-60 m of thick well-bedded and locally cross-bedded sandstones also known as the Himmel-
berg Sandstone. Fossils, if any, are extremely rare except for the under- and overlying strata
which suggest a late Caradoc age for this unit. Hence, this sandstone is in part equivalent to
the Uggwa Shale, which is also supported by field observations. The fauna of the clastic upper
Ordovician sequence is dominated by bryozoans and less frequently brachiopods, trilobites,
gastropods and cystoids occur. According to Vai (1971) and Havlicek et al. (1987), this fauna
suggests a close relationship to middle Caradoc sequences of Sardinia and other regions of
southern Europe as well as to Bohemia.
The Caradoc Uggwa Shale and its equivalent, the Himmelberg Sandstone, are overlain by
distinctive limestones of Ashgill age. Two lithological types are developed in the Carnic Alps,
Bull. Br. Mus. nat. Hist. (Geol) 43: 107-115 Issued 28 April 1988
H. P. SCHONLAUB
108
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ORDOVICIAN-SILURIAN BOUNDARY IN AUSTRIA 109
the first being the nodular Uggwa Limestone and the other its lateral equivalent, the coarse-
grained biodetrital Wolayer Limestone. The Uggwa Limestone represents a quiet water shelf
environment and contains relatively abundant microfossils, for example conodonts, ostracods
and foraminiferans, but also a few trilobites, bryozoans, brachiopods and cephalopods. Yet age
assignments within the Ashgill are not precisely known except for its upper part, in which the
Hirnantia fauna is found.
The second type, the Wolayer Limestone, comprises biodetrital cystoid-bearing light grey
limestones which may be up to 18m thick, three times as much as the Uggwa Limestone. Its
palaeogeographic setting suggests carbonate mud mounds on the outer shelf surrounded by
rather uniform and more widely distributed shelf carbonates of the Uggwa Limestone. There is
no indication of close proximity to a land area for either type. In the Carnic Alps lateral
changes between the two limestone types can occur over a few km in the same tectonic unit. In
other places they are tectonically separated. As shown in the diagrams (Figs 1, 2) the individual
boundary sections exhibit significant differences in thickness and lithology, as far as the latest
Ordovician is concerned.
The Boundary Beds
At the top of the Ordovician sequence in the Carnic Alps a widespread sandy facies occurs, the
so-called Plocken Formation. In the old literature this horizon was termed ‘Untere Schichten’.
It succeeds the Uggwa Limestone but is missing at the top of the coeval Wolayer Limestone
(see below). Reinvestigation of the Plocken Formation indicates that it represents a regressive
sequence starting with offshore shaly mud intercalations in the uppermost Uggwa Limestone
and above, and developing into shoreface calcareous sands. In these beds contorted deforma-
tion structures are very common. In the lower parts they are associated with channel fillings of
coarse bioclastic material.
The Hirnantian fauna which first occurs in laminated greenish-greyish mudstones overlying
the Uggwa Limestone at Cellon shows evidence of transportation. The same is true for the
Hoher Trieb section east of Cellon (Figs 4E, 4F). The poorly sorted, mostly disarticulated fossil
debris occurs in several layers. They are characterized by internal erosional surfaces, small-scale
channelling, reworking of sediment, bioturbation with subsequent infilling of fossils, and pro-
nounced load deformation structures. Higher up in the sequence channelling and reworking of
the sediment increase, although laminated mudstones are here less abundant. Usually channels
are connected with contorted beds the thickness of which is usually between 10 and 20cm but
which may reach 60cm.
The channel filling consists of coarse-grained bioclastic limestones which cut into the under-
lying mudstones and shales. Fossils include representatives of the Hirnantia fauna (mainly
brachiopods and trilobites), pyritized ostracods and spicules. According to Jaeger et al. (1975)
and Schonlaub (1980: fig. 27 and 1985: fig. 25a) the following taxa have been found in the latest
Ordovician Hirnantian Stage:
Kinnella kielanae (Temple) Dalmanitina mucronata (Brongniart)
Rafinesquina sp. Icriodella sp.
Clarkeia sp. Quadrijugator harparum (Troedsson)
Hirnantia sagittifera (M‘Coy) Scanipisthia rectangularis (Troedsson)
Dalmanella testudinaria (Dalman) Eocytherella troedssonia Bonnema
Cryptothyrella sp. Dornbuschia ostseensis Schallreuter
At Cellon (Fig. 3) and Hoher Trieb (Figs 4E, 4F) the channels are connected or grade into
contorted beds composed of less pure limestones. They are irregularly coloured brownish and
greyish marls with floating brachiopod valves and loosely packed matrix-supported subangular
clasts of different rock types including carbonates of different size up to 20-30cm in diameter,
sandstone pebbles, shales or small black phosphorite nodules. At the Nolblinggraben section at
the base of the Plécken Formation there is even a layer with clasts of granitic composition
(Schonlaub & Daurer 1977).
110 H. P. SCHONLAUB
ORDOVICIAN/ZSILURIAN BOUNDARY SOU
:
|
1 2 3
FEISTRITZ— STEINWEN NOLBLING—
GRABEN DER - HUTTE GRABEN
WASSERFALL |
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crenulatus oO |
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ORDOVICIAN
Fig. 2 Comparative sections through the Carnic Alps near the Ordovician-Silurian boundary.
The Plécken Formation has a thickness of between 1-5 and 9m, the latter occurring on the
southern slope of Mount Rauchkofel. Based on the occurrences of the Hirnantia faunal
assemblage at the Cellon, Hoher Trieb and Uggwa sections, a late Ashgill age, i.e. the Hirnan-
tian Stage, is deduced for the Plécken Formation. This is in agreement with earlier reports of
Glyptograptus cf. persculptus (Salter) from the ‘Feistritzgraben’ section in the Western Kara- |
wanken Alps (Jaeger et al. 1975). We correlate this level with the basal Plocken Formation in |
the Carnic Alps, although the lithologies are slightly different.
t
The Base of the Silurian
On the carbonate shelf which was already shallow in pre-Hirnantian times the shallow water
carbonate facies was re-established in the Silurian. However, in this facies disconformities with
distinct karst surfaces are widely developed and depositional hiatuses are well known. The relief
may be several cm or more. In particular this phenomenon can be seen on top of the carbonate
mounds of the Wolayer Limestone which apparently became subaerially exposed from the
ORDOVICIAN-SILURIAN BOUNDARY IN AUSTRIA HU
siaimN ALPS
4 5 6 7
HOHER TRIEB CELLON RAUCHKOFEL RAUCHKOFEL
SUD BODEN
Kok-Fm
Kok - Fm ao S0gitta - Zone
Kok- Fm conodonts
2 77 Telychian (Sheinwoodian )
conodonts
Fm.
en
Hirnantia Fauna
MivAreitcic
Uggwa
Wolayer
latest Ordovician to the middle or even upper Silurian (see Fig. 2, section no. 7). In other
sequences stratigraphical gaps are of shorter duration. In any case there is an abrupt upward
transition from the Hirnantian Plocken Formation to either cephalopod limestones of the Kok
Formation or to the uniform dark grey graptolitic shales of the basal Silurian Bischofalm facies.
According to unpublished new data of H. Jaeger (cited by Schonlaub, 1985: 78) in the Carnic
Alps the graptolite facies starts in the A. acuminatus Biozone. At the ‘Steinwenderhiitte-
Wasserfall’ locality the graptolitic shales succeed the greyish Bischofalm Quartzite. At other
places, for example at Nolblinggraben, D. vesiculosus, the index graptolite of the lower Silurian
graptolite zone 17, has been reported overlying an almost 2m thick quartzitic rock. Due to the
lack of fossils the stratigraphical relationship between the two quarzitic members is yet poorly
understood. They may represent fan deposits of different ages, the lower one being deposited in
basin areas of the Hirnantian low sea level stand and the latter at or near the beginning of the
transgressive graptolite sequence at the presumed base of the Rhuddanian Stage. In either case,
in this part of the Carnic Alps an almost complete succession of strata across the Ordovician—
Silurian boundary can be assumed.
112 H. P. SCHONLAUB
ORDOVICIAN-SILURIAN BOUNDARY IN AUSTRIA 1113}
Conclusion
The Ordovician-Silurian boundary beds in the Carnic Alps reflect a regressive-transgressive
cycle. Alongside probably continuous sedimentation across the systemic boundary in sections
representing deeper environments, in the shallow carbonate shelf areas stratigraphical gaps are
very common. This relation is in accordance with data from other regions in the world.
However, this event was not solely caused by worldwide eustatic changes of sea level attributed
to the famous glacial event in the southern hemisphere. Vertical block movements of Caledon-
lan age also affected the Carnic Alps in the late Ordovician and, consequently, were also
responsible for differences in thickness of closely-related sections as well as for greatly differing
facies that developed in the Silurian after a less pronounced facies pattern in the Ordovician.
Fig. 3 Ordovician—Silurian boundary beds at the Cellon section in the central Carnic Alps of
Austria. A: Cellon section, lower part showing Uggwa Limestone in the lower portion and Plocken
Formation above. Indicated is a coarse grained channel filling limestone bed at the base of the
Plocken Formation. B: Detail from A in the upper portion of the Plocken Formation showing a
multilayered fold. C: Detail from A. Coarse-grained limestone bed at the base of the Plocken
Formation (no. 6 is a reference point of O. H. Walliser’s conodont-based collection). D: Internal
erosional surface in the uppermost Uggwa Limestone Formation at level no. 5 of Walliser (1964).
Length of the cut approx. 4cm. E: Reworked limestone clast at the same horizon as Fig. D. Long
axis approx. 3-5cm. F: Fossil debris representing components of the Hirnantia fauna in the
uppermost Uggwa Limestone Formation at horizon no. 5 of Walliser (1964). Width of the brachio-
pod valve is 3cm. G: Same horizon as Figs D—F showing bioturbation and infilling at an internal
erosional surface in mudstones. Length of the cut approx. 4cm.
114 H. P. SCHONLAUB
ORDOVICIAN-SILURIAN BOUNDARY IN AUSTRIA 115
References
Fligel, H. 1965. Vorbericht tiber mikrofazielle Untersuchung des Silurs des Cellon-Lawinenrisses
(Karnische Alpen). Anz. 6st. Akad. Wiss. mat.-nat. Kl., Wien, 1965: 289-297.
Gaertner, H. R. von 1931. Geologie der Zentralkarnischen Alpen. Denkschr. Akad. Wiss. Wien 102:
113-199.
Havlicek, V., Kriz, J. & Serpagli, E. 1987. Upper Ordovician Brachiopod assemblages of the Carnic Alps,
Middle Carinthia and Sardinia. Boll. Soc. paleont. ital., Modena, 25: 277-311, 9 pls.
Jaeger, H., Havlicek, V. & Schonlaub, H. P. 1975. Biostratigraphie der Ordovizium/Silur-Grenze in den
Stidalpen—Ein Beitrag zur Diskussion um die Hirnantia-Fauna. Verh. geol. Bundesanst., Wien 1975:
271-289.
Schonlaub, H. P. 1969. Das Palaozoikum zwischen Bischofalm und Hohem Trieb (Zentrale Karnische
Alpen). Jb. geol. Bundesanst. Wien 112: 265-320.
—— 1971. Palaeo-environmental studies at the Ordovician/Silurian boundary in the Carnic Alps. Mem.
Bur. Rech. géol. minier., Paris, 73: 367-376.
—— 1980. Field Trip A: Carnic Alps. In H. P. Schonlaub (ed.), Guidebook, Abstracts. Second European
conodont symposium. Abh. geol. Bundesanst., Wien, 35: 5—60, 10 pls.
—— 1985. Das Palaozoikum der Karnischen Alpen. Exkursion Wolayersee. Arbeitstag. geol. Bundesanst.,
Wien, 1985: 34-69.
— & Daurer, A. 1977. Ein auffallender Gerollhorizont an der Basis des Silures im Nolblinggraben
(Karnische Alpen). Verh. geol. Bundesanst., Wien 1970: 361-365.
Serpagli, E. 1967. I conodonti dell’Ordoviciano Superiore (Ashgilliano) delle Alpi Carniche. Boll. Soc.
paleont. ital., Modena, 6: 30-111, 25 pls.
Vai, G. B. 1971. Ordovicien des Alpes Carniques. Mem. Bur. Rech. geol. minier., Paris, 73: 437-450, 4 pls.
Walliser, O. H. 1964. Conodonten des Silurs. Abh. hess. Landesamt. Bodenforsch., Wiesbaden, 41: 1-106,
32 pls.
Fig. 4 Ordovician-Silurian boundary sections at Rauchkofel-Boden, Rauchkofel-Sud and Hoher
Trieb in the central Carnic Alps. A: Rauchkofel-Boden section, disconformity between the Ashgill
Wolayer Limestone (left) and the darker cephalopod-bearing Kok Formation (right). At the base of
the latter sagitta-Zone conodonts of middle Wenlock age occur. B: Rauchkofel-Siid section
showing contact between the nodular Uggwa Limestone (left) and the overlying Plocken Forma-
tion (right). C, D: Reworked limestone clasts containing an Amorphognathus ordovicicus conodont
fauna in the lower part of the Plocken Formation at the Rauchkofel-Stid section. E, F: Hoher
Trieb section. Uggwa Limestone (left) and basal part of the Plocken Formation (right). Note
channel filling coarse-grained bioclastic bed near the base of the Plécken Formation. This bed
contains representatives of the Hirnantia fauna (Hirnantia sagittifera, Dalmanella testudinaria, Kin-
nella kielanae, Cryptothyrella sp. and also Clarkeia sp.).
The Ordovician—Silurian boundary in China
Mu En-zhit
Nanjing Institute of Geology and Palaeontology, Academia Sinica, Chi-Ming-Ssu, Nanjing,
China.
+ Professor Mu died in April 1987.
Synopsis
After a general account of the Chinese graptolite zones about the boundary, a précis is given of the
Chinese type section for the boundary, at Wangjiawan, which includes the faunal characteristics. It is
followed by similar details for nine other major Chinese sections and a synthesis of the biofacial types.
After a discussion of correlation problems about the boundary, it is concluded that the ascensus Zone of
some European sections is equivalent to the Chinese persculptus Zone, and that the base of the Silurian is
best taken above the bohemicus Zone and its correlatives, the Hirnantia—Dalmanitina fauna.
Introduction
Ordovician and Silurian strata are well developed in China. Many Ordovician-Silurian bound-
ary sections have been defined in the Yangtze Region (or the Central China region) where the
Ordovician and Silurian consist of platform deposits. These sections are small in thickness and
rich in fossils, mainly graptolites, known as the Ashgill Wufeng Formation and the early
Llandovery Lungmachi Formation. Between these two formations there is usually a thin bed of
shelly facies, namely the Hirnantia—Dalmanitina bed (HD) or the Kuanyinchiao bed. The grap-
tolite sequences of the Wufeng Formation and the Lungmachi Formation are quite complete,
and thirteen graptolite zones have been established in descending order as follows:
Lungmachian: L, Monograptus sedgwickii Zone
L, Demirastrites convolutus Zone
L,; Demirastrites triangulatus Zone
L, Pristiograptus cyphus Zone
L, Orthograptus vesiculosus Zone
L, Parakidograptus acuminatus Zone
L, Glyptograptus persculptus Zone
Wufengian: W, Diplograptus bohemicus Zone
s Paraorthograptus uniformis Zone
W,, Diceratograptus mirus Zone
W, Tangyagraptus typicus Zone
W,. Dicellograptus szechuanensis Zone
W, Amplexograptus disjunctus yangtzeensis
Zone or Pleurograptus lui Zone
The establishment of the Wufengian and Lungmachian graptolite zones is of great impor-
tance in stratigraphical correlation and in the determination of the exact position of the
Hirnantia—Dalmanitina bed (HD). The HD bed is underlain by beds of varying age from the
Tangyagraptus typicus Zone (W3) to the lower part of the Diplograptus bohemicus Zone (W6) in
different localities. By comparison, the earliest Silurian shelly facies, known as the ‘Eospiri-
gerina bed or the Wulipo bed, has a less wide distribution and its upper limit varies in different
places and may reach as high as the Pristiograptus cyphus Zone (L4). The relationship between
the Ordovician-Silurian boundary graptolite zones and the shelly beds may be shown in
Table 1.
As shown in the table, the Ordovician-Silurian boundary should be drawn between the
Diplograptus bohemicus Zone (W,)/Hirnantia—Dalmanitina bed and the Glyptograptus per-
sculptus Zone (L,)/‘Eospirigerina’ bed. The striking faunal changes from the topmost Ordovi-
cian (W,) and the lowermost of the Silurian (L,) support this assertion. Therefore, nearly all
Bull. Br. Mus. nat. Hist. (Geol) 43: 117-131 Issued 28 April 1988
118 MU EN-ZHI
Table 1 A correlation between the graptolite and shelly sequences across the Ordovician—Silurian
boundary.
L, Pristiograptus cyphus
L, Orthograptus vesiculosus 3
Lis Parakidograptus acuminatus | a
s
EF Glyptograptus persculptus ‘Eospirigerina fauna =
Hirnantia—Dalmanitina
Ws Diplograptus bohemicus (Wo) fauna (HD)
lower
bed
Paraorthograptus uniformis
Kuanyinchiao
Diceratograptus mirus
Ww, Tangyagraptus typicus
geologists and palaeontologists in China agree that the Ordovician—Silurian boundary should
be placed between the D. bohemicus Zone (W,) (or the Hirnantia—Dalmanitina bed (HD)) and
the G. persculptus Zone (L,).
Description of the Ordovician—Silurian boundary sections
In 1983 the writer reviewed sixteen Ordovician—Silurian boundary sections distributed in four
stratigraphical regions and described nine sections in the Yangtze Region in detail. In recent
years, some sections have been revised and some new sections recognized. There are 33 well
defined Ordovician—Silurian boundary sections distributed in four regions of China. Among
them, 26 are in the Yangtze Region, three in the Xizang (Tibet}}W. Yunnan Region, two in the
Zhujiang Region (S. China Region) and one in the Northwest Region, as shown in the map
(Fig. 1). In the northernmost region, the Ordovician—Silurian strata are very thick, complicated
in structure and fossils are rare, and thus no ideal Ordovician—Silurian boundary section has
been found in this region. There are no Silurian deposits in the Huanghe Region (N. China
Region).
In the present paper, the type section, the Wangjiawan section of Yichang, W. Hubei, and
nine selected sections are described as follows.
1. The Wangjiawan Ordovician-Silurian Boundary section is the type section in China. In
1982, this section was restudied by Mu En-zhi, Zhu Zhao-ling, Lin Yao-kun, Zou Xi-ping, Wu
Hong-ji, Chen Ting-en, Geng Liang-yu and Dong Xi-ping. The section is as follows (after Mu
et al. 1984).
Lower Silurian Lungmachi Formation (basal part):
15. Black argillaceous shale weathered greyish black, yielding (ACC768) Orthograptus vesiculosus
(Nicholson), Climacograptus normalis Lapworth and C. cf. medius Tornquist more than 1:0m
14. Brownish-grey siliceous shale intercalated with black shale, with 7 siliceous beds in a distance of
20cm, yielding (ACC767) Parakidograptus acuminatus (Nicholson), Climacograptus normalis Lapworth, C.
sinitzini (Chaletzkaya), Glyptograptus tamariscus magnus Churkin & Carter and Paraorthograptus sp.
0:60 m
13. Black shale with (ACC766) Parakidograptus acuminatus (Nicholson), Climacograptus bicaudatus Chen
& Lin, C. normalis Lapworth, C. angustus Perner and C. sinitzini (Chaletzkaya). 0:35m
12. Black shale with sandy shale (0:15m thick) in the upper part, weathered greyish black, containing
(ACC765) Akidograptus ascensus Davies, Glyptograptus sinuatus (Nicholson), G. tamariscus magnus
119
ORDOVICIAN-SILURIAN BOUNDARY IN CHINA
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Churkin & Carter, G. tamariscus linearis Perner, G. ex gr. tamariscus Nicholson, Climacograptus angustus
Perner, C. bicaudatus Chen & Lin and C. normalis Lapworth 0:20m
(ACC764a) Glyptograptus sinuatus (Nicholson), G. tamariscus linearis Perner, Climacograptus angustus
Perner, C. wangjiawanensis Mu & Lin, Diplograptus modestus Lapworth and Rhaphidograptus minutus
Chen & Lin 0:04m
11. Black argillaceous shale weathered brownish grey in colour, rich in graptolites including (ACC763d)
Glyptograptus persculptus (Salter), G. sinuatus (Nicholson), G. ex gr. tamariscus Nicholson, G. tamariscus
linearis Perner, Diplograptus modestus Lapworth, Orthograptus guizhouensis Chen & Lin, Paraorthog-
raptus innotatus (Nicholson), Climacograptus angustus Perner, C. normalis Lapworth, C. wangjiawanensis
Mu & Lin and Rhaphidograptus minutus Chen & Lin 0-:16m
(ACC763c) Glyptograptus sinuatus (Nicholson), G. lunmaensis Sun, G. tamariscus linearis Perner, G.
tamariscus magnus Churkin & Carter, Diplograptus cf. coremus Chen & Lin, Orthograptus angustifolius
Chen & Lin, O. guizhouensis Chen & Lin, O. bellulus Tornquist, Climacograptus angustus Perner and C.
wangjiawanensis Mu & Lin 0-08 m
(ACC763b) Glyptograptus sinuatus (Nicholson), G. lunmaensis Sun, G. ex gr. tamariscus Nicholson, G.
tamariscus linearis Perner, G. tamariscus magnus Churkin & Carter, Diplograptus modestus Lapworth,
Orthograptus angustifolius Chen & Lin, Paraorthograptus innotatus (Nicholson), P. sp., Climacograptus
angustus Perner and C. normalis Lapworth 0:06 m
(ACC763a) Glyptograptus persculptus (Salter), G. sinuatus (Nicholson), G. lungmaensis Sun, G. tamariscus
linearis Perner, G. tamariscus magnus Churkin & Carter, Diplograptus modestus Lapworth, Climacograptus
angustus Perner and C. normalis Lapworth 0:06 m
Upper Ordovician Wufeng Formation:
10. Bluish grey argillaceous calcareous silicolites weathered whitish-yellow and greyish-yellow, yielding
abundant brachiopods and trilobites: (ACC762) Leptaenopoma trifidum Marek & Havliéek, Kinnella
kielanae (Temple), Dalmanella testudinaria (Dalman), “Paracraniops’ patillis Rong, Cliftonia cf. oxople-
cioides Wright, Hirnantia sagittifera (M‘Coy), Draborthis cf. caelebs Marek & Havli¢ek, Aphanomena ultrix
(Marek & Havlicek), Aegiromena cf. ultima Marek & Havlicek and Dalmanitina yichangensis Lin, D. sp.
0-33m
9. Black argillaceous shale and mudstone, yielding (ACC761) Diplograptus bohemicus (Marek) and
Paraothograptus typicus Mu with a few brachiopods and cephalopods 0:26m
8. Black shale intercalated with a few siliceous shale beds of the same colour, yielding: (ACC760) Diplo-
graptus bohemicus (Marek), D. sp., Glyptograptus sp., Climacograptus supernus Elles & Wood and
Paraorthograptus sp. 0:23 m
7. Black argillaceous shale with siliceous shale intercalation, yielding in the upper part (ACC759) Dicel-
lograptus ornatus Elles & Wood, Climacograptus supernus Elles & Wood, C. longicaudatus Geh, C. sp.,
Glyptograptus sp., Orthograptus truncatus Lapworth and Paraorthograptus uniformis Mu & Li 0:-42m
Middle part (ACC758) Tangyagraptus typicus Mu, Climacograptus supernus Elles & Wood, C. venustus
Hsu, Amplexograptus suni (Mu) and Paraplegmatograptus sp. 0:70m
Lower part (ACC758a) Dicellograptus szechuanensis Mu, D. ornatus Elles & Wood, Climacograptus
supernus Elles & Wood, C. sp., Orthograptus truncatus Lapworth, Orthograptus maximus Mu and Amplex-
ograptus suni (Mu) 1:73m
6. Black carbonaceous siliceous shale, yielding (ACC757) Dicellograptus szechuanensis Mu, Amplexo-
graptus disjunctus yangtzensis Mu & Lin, Pseudoclimacograptus sp., Orthograptus abbreviatus Elles &
Wood and Parareteograptus sinensis Mu 0-40 m
5. Black carbonaceous shale, yielding abundant graptolites: (ACC756) Amplexograptus disjunctus yang-
zeensis Mu & Lin, A. suni (Mu), Orthograptus cf. pauperatus Elles & Wood and Parareteograptus sp.
0-43 m
4. Black carbonaceous shale intercalated with a few siliceous beds, yielding abundant graptolites
(ACC755) Leptograptus extremus modestus Chen, Dicellograptus sp., Climacograptus chiai Mu, Pseudocli-
macograptus spp., Amplexograptus disjunctus yangtzeensis Mu & Lin, Orthograptus cf. maximus Mu, O.
truncatus Lapworth, O. cf. pauperatus Elles & Wood and O. sp. and inarticulate brachiopods 0-20m
3. Dark grey to greyish green mudstone 0:12m
Linhsiang Formation:
2. Dark yellow mudstone 0-:05m
1. Yellowish green to green argillaceous nodular limestone, yielding the trilobites (ACC754) Ham-
matocnemis sp. and Microparia sp. about 2:00m
2. ‘Baoshan’ (the ‘Treasure Hill’) section, Huanghuachang, Yichang, W. Hubei (after Mu et al.
1984).
ORDOVICIAN-SILURIAN BOUNDARY IN CHINA 121
Lower Silurian Lungmachi Formation (basal part):
9. Black siliceous rock weathered greyish-yellow, yielding: (ACC744) Parakidograptus acuminatus
(Nicholson), Climacograptus normalis Lapworth, C. sinitzini (Chaletzkaya) 0-:10m
8. Black carbonaceous shale, black siliceous shale weathered blackish grey, containing: (ACC743) Glyp-
tograptus persculptus (Salter), G. sinuatus (Nicholson), Climacograptus sp. (cf. normalis Lapworth) 0-45m
Upper Ordovician Wufeng Formation:
7. Black calcareous argillaceous siliceous mudstone weathered greyish-white to greyish-yellow, yielding
abundant brachiopods, trilobites and other fossils, including (ACC742) Hirnantia sagittifera (M‘Coy),
Kinnella kielanae (Temple), Aphanomena ultrix (Marek & Havlicek), Cliftonia cf. psittacina (Wahlenberg),
Triplesia sp., Dalmanella testudinaria (Dalman), Aegiromena cf. ultima (Marek & Havliéek), Meristina
crassa incipiens (Williams) and Dalmanitina yichangensis Lin 0:10m
5—6. Black argillaceous siliceous shale, weathered dark grey, yielding (ACC741) Diplograptus bohemicus
(Marek) and a few brachiopods in the upper part 0:45 m
3-4. Black siliceous shale intercalated with argillaceous shale, containing (ACC740) Dicellograptus ornatus
Elles & Wood, D. sp., Glyptograptus sp., Climacograptus supernus Elles & Wood, C. hastatus Hall, C. sp.
and Paraorthograptus uniformis Mu & Li 0:-51m
2. Black shale intercalated with black siliceous shale, yielding (ACC739) Diceratograptus mirus Mu, D.
ornatus brevispinus Chen, Glyptograptus sp., Climacograptus hastatus Hall 0:20m
1. Black shale with a few siliceous shale intercalations, rich in graptolites including (ACC737) Tangya-
graptus uniformis Mu, Dicellograptus ornatus Elles & Wood, D. ornatus brevispinus Chen, Glyptograptus
sp., Climacograptus supernus Elles & Wood, C. supernus longus Geh, C. tumidus Geh, Amplexograptus suni
(Mu), Orthograptus abbreviatus Elles & Wood, Yinograptus disjunctus (Yin & Mu), Y. brevispinus Mu,
Paraplegmatograptus connectus Mu 0-15m
Black shale with siliceous shale intercalation, yielding abundant graptolites, including (ACC737a) Tangy-
agraptus typicus Mu, T. uniformis Mu, T. sp., Climacograptus supernus Elles & Wood, C. supernus longus
Geh, Orthograptus truncatus Lapworth, Glyptograptus sp., Amplexograptus suni (Mu), Yinograptus dis-
Junctus (Yin & Mu), Y. grandis Mu, Paraplegmatograptus sp. Ld
3. Renhuai section (after Geng Liang-yu et al. 1984).
Lower Silurian Lungmachi Formation (basal part):
Greyish-black silty, carbonaceous shale (0-05 m thick in single bed), cream-coloured sandy shale (in basal
part), yielding an abundant graptolite fauna of Glyptograptus kaochiapienensis Hsu, G. cf. lungmaensis Sun
and Orthograptus sp. etc. associated with some brachiopods 1-8m
Upper Ordovician Wufeng Formation:
2. Kuanginchiao bed, including the following units:
c. dark grey thick-bedded bioclastic limestone in upper part (ADR557-3) with numerous solitary corals
such as Brachylasma sp., Crassilasma sp. and Dansiphyllum? sp. 1:14m
b. Dark greyish thin-bedded bioclastic limestone in the middle part (ADR557-2) including Hirnantia
sagittifera (M‘Coy), Dalmanella testudinaria (Dalman), Aphanomena ultrix Marek & Havli¢ek, Dalmanitina
sp., Modiolopsis sp., rugose corals, and the chitinozoan Conochitina cf. sp. A of Achab 0:29m
a. Dark greyish medium-bedded limestone in lower part (ADR557-1) with the monotomous chitin-
ozoan Conochitina cf. sp. A of Achab 0-67 m
1. Greyish-black carbonaceous shale with a minor quantity of clayey shale in the upper part, dark greyish
dolomitic limestone in the lower part and 4cm greyish black carbonaceous shale in basal part, yielding
abundant graptolites such as Climacograptus hastatus Hall, C. sp., Paraorthograptus typicus Mu, P. sp.,
Dicellograptus ornatus Elles & Wood, D. tenuisculus Mu et al., D. szechuanensis Mu and Pleurograptus lui
Mu 41m
4. The Nanzheng Formation of Liangshan, Nanzheng county, S. Shaanxi, was considered to be
basal Silurian for a long time. However Zhu et al. (1986) have revised this to a late Ordovician
age. According to their detailed work, the Nanzheng Formation is the equivalent of the Wufeng
Formation and indicates a mixed biofacies. The Liangshan Ordovician—Silurian boundary
section, Nanzheng, measured by them may be summarized as follows:
Lower Silurian Lungmachi Formation (basal part):
11. Brownish grey shales with Climacograptus angustus (Perner), Diplograptus uniformis Li, Glyptograptus
lungmaensis Sun, G. tamariscus distans Packham, G. tamariscus linearis Perner 0-Sm
1 MU EN-ZHI
10. Brownish grey and pinkish shale with a few cephalopods and brachiopods (NZ10) and Climacograptus
normalis Lapworth, C. miserabilis Elles & Wood, C. angustus (Perner), Diplograptus ex gr. modestus
Lapworth, D. uniformis Li, Glyptograptus lungmaensis Sun 0:27-0:32 m
Upper Ordovician Nanzheng Formation:
9. Brownish-yellow calcareous shale rich in (NZ9) Climacograptus angustus (Perner), Orthograptus sp.,
Glyptograptus sp., Platycoryphe sinensis (Lu), Dalmanitina sp.; the bivalve Deceptrix sp. and some com-
pressed cephalopods 0:17-0:22m
8. Brownish-grey medium-bedded argillaceous limestone with (NZ8) Diplograptus cf. bohemicus (Marek),
Orthograptus sp., Climacograptus sp., Pleurorthoceras shanchongense Zou, P. jingxianense Zou, P. slender-
tubulatum Zou, P. cf. clarksvillense (Foerste), Michelinoceras sp., Aegiria? sp., Platycoryphe sinensis (Lu)
and Dalmanitina nanchengensis Lu 0:74m
7. Brownish argillaceous limestone, containing (NZ7) Dalmanitina nanchenensis Lu, Platycoryphe sinensis
(Lu), the gastropod Rhaphistomina? sp., and brachiopod fragments 0:-46m
6. Brownish to light grey, coarse quartzitic sandstone 0:83 m
5. Light brown shale intercalated with sandstone containing bivalve fragments in the top part (NZ6)
2:30m
4. Greyish shale containing a few graptolites (NZ5) including Climacograptus sp. 0:25m
3. Grey clayey and aluminal shale rich in fossils (NZ4) with Orthograptus maximus Mu, O. cf. abbreviatus
Elles & Wood, Climacograptus normalis Lapworth, Diplograptus sp., Parareteograptus sp., Dictyonema sp.,
Orbiculoidea, Euklesdenella, the bryozoans Stictopora, Hallopora and Escharopora; Conularia and Meto-
conularia (?) proteica (Barrande) 0:28 m
2. Light grey siliceous shale containing (NZ2) Orthograptus maximus Mu, Climacograptus angustus
(Perner) in the lower part 0:15m
1. Light grey and brownish siltstone and shale 0:-5m
Linhsiang Formation:
Light green and brownish argillaceous limestone, with Nankinolithus sp. and Protopanderodus insculptus
(Branson & Mehl) in the upper (NZ2) and Paraceraurus cf. longisulcatus Lu in the lower (NZ1) 1:10m
5. Gaojiawan section, Xixiang, S. Shaanxi. A most detailed Ordovician-Silurian section was
measured by Yu et al. (1986) as follows:
Lower Silurian Lungmachi Formation:
10. Black siliceous and carbonaceous shale containing (XF 162-155) Orthograptus vesiculosus (Nicholson),
Climacograptus transgrediens Waern and C. medius Tornquist. 2:77m
9. Black siliceous shale interbedded with carbonaceous shale rich in graptolites (KF 154-135) with Paraki-
dograptus acuminatus (Nicholson), Akidograptus ascensus Davies, A. xixiangensis Yu, Fang & Zhang, A.
parallelus Li & Jiao, Climacograptus sinitzini (Chaletzkaya) and Orthograptus lonchoformis Chen & Lin
4-63m
8. Black siliceous shale intercalated with black carbonaceous shale rich in graptolites (KF134—125) with
Glyptograptus persculptus Salter, G. persculptus—sinuatus transient, G. tamariscus (Nicholson), G. lung-
maensis Sun, G. zhui Yang, Climacograptus normalis Lapworth, Orthograptus lonchoformis Chen & Lin,
Akidograptus ascensus Davies and A. xixiangensis Yu, Fang & Zhang 0-89 m
Upper Ordovician Wufeng Formation:
7. Black siliceous shale weathered purplish brown in colour, containing (XF124—-118) Diplograptus bohe-
micus (Marek), D. orientalis Mu, Climacograptus normalis Lapworth, Glyptograptus sp. 0:64m
6. Greyish to pale siltstone and quartzitic sandstone containing (XF117-115) Dalmanitina wuningensis
Liu, Leonaspis (Eoleonaspis) olinini (Troedsson), Hirnantia sagittifera (M‘Coy), Kinnella kielanae (Temple)
0:22m
5. Black siliceous and carbonaceous shale rich in graptolites (XF 114-112) with Paraorthograptus uniformis
Mu & Li, Orthograptus truncatus Lapworth, Climacograptus hastatus Hall, Paraplegmatograptus sp. and
Dicellograptus sp. 0:26m
4. Black carbonaceous shale and siliceous shale containing graptolites (KF111—110) Paraorthograptus
typicus Mu, Climacograptus supernus Elles & Wood, C. hastatus Hall, Paraplegmatograptus sp., Dicello-
graptus graciliramosus Yin & Mu 0:17m
3. Black shale weathered brown, containing (KXF109-107) Tangyagraptus typicus Mu, Paraorthograptus
typicus Mu, Climacograptus hastatus Hall, C. venustus Hsu, Amplexograptus suni (Mu), Dicellograptus
ornatus Elles & Wood, Yinograptus disjunctus (Yin & Mu), Parareteograptus sp. 0-33m
ORDOVICIAN-SILURIAN BOUNDARY IN CHINA 123
2. Dark grey shale with (KF 106-104) Dicellograptus szechuanensis Mu, D. excavatus Mu, Pleurograptus lui
Mu, Climacograptus supernus Elles & Wood, Parareteograptus sinensis Mu, Orthoreteograptus denticulatus
Mu 0:-42m
1. Dark grey to black shale, containing (XF 103-101) Pleurograptus lui Mu, Dicellograptus elegans Carru-
thers, Climacograptus supernus Elles & Wood, Pseudoclimacograptus sp., Glyptograptus sp., Parareteo-
graptus sinensis Mu, Orthoreteograptus denticulatus Mu 0:-44m
Jiancaogou Formation:
Grey and yellowish green mudstone with Nankinolithus, etc.
In the section listed above, unit 1 is the Pleurograptus lui Zone which is equivalent to the
Amplexograptus disjunctus yangtzensis Zone (W,). Unit 2 is the Dicellograptus szechuangensis
Zone (W,) and unit 3 is the Tangyagraptus typicus Zone (W;). Unit 4 is the equivalent of the
Diceratograptus mirus Zone (W,) but D. mirus itself has not been found. Unit 5 is the Paraor-
thograptus uniformis Zone (W.), unit 6 is the Hirnantia—Dalmanitina bed (HD) and unit 7 is the
Diplograptus bohemicus Zone (W,). Unit 8 is the Glyptograptus persculptus Zone (L,) character-
ized by the occurrence of G. persculptus, G. persculptus—sinuatus transient, G. zhui and G.
lungmaensis. It is noteworthy that Akidograptus ascensus first appears in the lower part of this
zone and A. xixiangensis appears in the upper part. Unit 9 is the Parakidograptus acuminatus
Zone (L,) characterized by the incoming of P. acuminatus and Climacograptus sinitzini in
association with A. ascensus and A. xixiangensis. Unit 10 is the Orthograptus vesiculosus Zone
(L;) characterized by the incoming of O. vesiculosus.
6. Bajaokou Ordovician-Silurian boundary section, Ziyang county, S. Shaanxi. The Lower
Silurian Banjuguan Formation and the Upper Ordovician Bajaokou Formation are all in
graptolite facies, without shelly beds. They are composed of dark grey to black carbonaceous
and siliceous slate and rich in graptolites, which were deposited in deep water on the south
slope of the East Qinling trough and on the north margin of the Yangtze platform. The
thickness of the basal Silurian is much greater than that of the uppermost Ordovician. The
section measured by Fu and others may be outlined as follows.
Lower Silurian Banjiuguan Formation (basal part). Black carbonaceous and siliceous slate:
L, Orthograptus vesiculosus Zone with O. vesiculosus, Neodicellograptus, Rhaphidograptus, and Atavo-
graptus 274m
L, Parakidograptus acuminatus Zone with P. acuminatus and Climacograptus sinitzini (F 14) 20:8 m
L, Glyptograptus persculptus—sinuatus transient zone 10:-5m
4. G. persculptus—sinuatus transient, and G. tamariscus (F 13)
3. Akidograptus ascensus, Climacograptus miserabilis, Orthograptus, and Atavograptus (F 12)
2. Glyptograptus cf. persculptus, Orthograptus lonchoformis and Diplograptus cf. modestus (F 11)
1. G. cf. persculptus, G. sinuatus, G. gracilis, Diplograptus modestus, Climacograptus normalis, and C.
miserabilis (F10)
Upper Ordovician Bajaokou Formation (upper part). Dark grey to black carbonaceous and siliceous slate:
W2 Diplograptus spp., Climacograptus sp., Orthograptus sp. (F9, F8) 2m
Wi Climacograptus extraordinarius, Diplograptus spp. (F7, F6) 1-Sm
W, Paraorthograptus uniformis (F 4) 1:-2m
W, Diceratograptus mirus (F3) 0-6m
7. Tangshan Ordovician—Silurian boundary section near Nanjing (Jiao & Zhang 1984).
Lower Silurian Kaochiapien Formation (basal part):
10. Greyish and yellowish shale with chert (ND8), containing Glyptograptus caudatus Ge, Climacograptus
normalis Lapworth, and Orthograptus sp. 0.30m
9. Variegated siliceous shale with (ND7) Glyptograptus lungmaensis Sun, Orthograptus sp. and Akido-
graptus? sp. 0-40 m
8. Purple siliceous shale rich in graptolites (ND6) with Diplograptus sp., Glyptograptus sp. and Cli-
macograptus sp. 0:02 m
124 MU EN-ZHI
Upper Ordovician Wufeng Formation:
7. Kuanyinchiao bed: greyish siliceous mudstone rich in shelly fossils (ND5) with Dalmanitina
yichangensis Lin, Leonaspis sinensis Chang, Platycoryphe sp., Paromalomena polonica (Temple), Aegiro-
mena ultima Marek & Havli¢ek, Triplesia? sp., Holopea? sp., Loxonema sp., Nuculoidea sp. and Hyolithes?
0:19m
6. Black sandy shale (ND4), containing Diplograptus cf. bohemicus (Marek) and Climacograptus extraordi-
narius (So6) 0-28 m
5. Variegated calcareous mudstone 0:09 m
4. Purple greyish siliceous shale with graptolites (ND3) Diplograptus sp. and Climacograptus sp. 0:09 m
3. Brownish yellow shale (ND2) with the brachiopod Manosia sp., the gastropod Planetochidea and
trilobite and crinoid fragments. 0:30m
2. Grey siliceous pale-weathered shale 0:-45m
1. Black siliceous shale with (ND1) Dicellograptus sp. and Climacograptus supernus Elles & Wood 0-83m
8. Xainze area, Northern Xizang (Tibet) (after Mu & Ni, 1983).
Lower Silurian Dewukaxia Formation (basal part):
Black graptolitic shale with Climacograptus normalis Lapworth, C. miserabilis Elles & Wood, C. xain-
zaensis Mu & Ni, Glyptograptus elegantulus Mu & Ni, G. nanus Mu & Ni, G. asthenus Mu & Ni,
Diplograptus lacertosus Mu & Ni, D. spanis Mu & Ni and D. temalaensis (Jones).
Upper Ordovician Xainza Formation:
Grey argillaceous limestone with Hirnantia, Kinnella, Cliftonia, Paromalomena, Hindella, Aphanomena and
dalmanitid trilobite 8-82m
Greyish-yellow shale with Glyptograptus asthenus Mu & Ni, G. daedalus Mu & Ni, G. elegantulus Mu &
Ni, G. nanus Mu & Ni, Diplograptus bohemicus (Marek), D. charis Mu & Ni, D. flustrianus Mu & Ni, D.
maturatus Mu & Ni, D. ojsuensis (Koren & Mikhaylova), D. orientalis Mu et al., D. spanis Mu & Ni, D.
viriosus Mu & Ni, Climacograptus cf. extraordinarius (Sobolevskaya), C. miserabilis Elles & Wood, C.
normalis Lapworth, C. xainzaensis Mu & Ni, C. xizangensis Mu & Ni and Orthograptus sp. 5:27m
Upper Ordovician Gangmusang Formation:
Limestone with shelly fauna.
9. Mangjiu section of Luxi (after Ni et al., 1983).
Lower Silurian Lower Jenhochiao Formation (basal part):
4. Black shale with Climacograptus normalis Lapworth, C. miserabilis Elles & Wood, C. trifilis lubricus
Chen & Lin, Akidograptus ascensus Davies, Orthograptus guizhouensis Chen & Lin, Diplograptus bifurcus
Mv et al., etc. 41m
3. Sandy mudstone with Climacograptus normalis Lapworth and C. sp. c.0-Sm
Upper Ordovician Wanyaoshu Formation (top part):
2. Greyish-white mudstone with Hirnantia sagittifera (M‘Coy), Hindella crassa incipiens (Williams), Cool-
inia cf. dalmani Bergstrom, Plectothyrella cf. crassicosta (Dalman), Paromalomena polonica (Temple), Aph-
anomena ultrix Marek & Havli¢éek and Dalmanitina sp. c.2m
1. Black shale, containing Climacograptus latus Elles & Wood, C. angustus Perner and Orthograptus
maximus Mu.
10. The Ordovician—Silurian boundary strata are well developed at the locality of Shahechang,
about 15km NW of Baoshan, Yunnan, where a number of graptolites were collected from the
uppermost Ordovician by Ni Yu-nan, Cai Cong-yang, Chen Ting-en, Li Guo-hua, and Wang
Ju-de. The stratigraphical sequence is as follows (in descending order):
Lower Silurian Lower Jenhochiao Formation (basal part):
3. Upper part: Black siliceous shale with Pristiograptus sp. and Climacograptus sp.
Lower part: Greyish white sandy shale with Climacograptus normalis Lapworth, C. xainzaensis Mu & Ni
and Glyptograptus sp. (ex gr. persculptus) in the basal 2m.
Upper Ordovician:
2. Greyish black sandy shale, rich in graptolites, the top part with Diplograptus bohemicus (Marek),
Diplograptus ojsuensis (Koren & Mikhaylova), Climacograptus normalis Lapworth (ACJ196), Cli-
ORDOVICIAN-SILURIAN BOUNDARY IN CHINA 125
macograptus cf. normalis Lapworth, C. xainzaensis Mu & Ni, C. extraordinarius (Sobolevskaya), Diplo-
graptus cf. orientalis Mu et al., D. yunnanensis Ni (ACJ195). The middle part yields Glyptograptus daedalus
Mu & Ni and Climacograptus extraordinaris (Sobolevskaya) (ACJ194); and the basal part Glyptograptus
cf. elegantulus Mu & Ni, G. daedalus Mu & Ni, Diplograptus maturatus Mu & Ni, D. ojsuensis (Koren &
Mikhailova) and D. temalaensis (Jones) (ACJ193).
1. Yellow argillaceous limestone with Nankinolithus? sp., Cyclopyge sp., etc.
Analysis of the boundary sections
The strata across the Ordovician-Silurian boundary in China fall into different biofacies types
as follows.
1. Where the graptolitic Glyptograptus persculptus Zone (L,) lies upon the graptolitic Diplo-
graptus bohemicus Zone (W,) without intervening shelly beds, as in the Bajaokou section,
Ziyang, S. Shaanxi.
2. Where the graptolitic Glyptograptus persculptus Zone or its equivalents (L,) lies upon the
graptolitic Diplograptus bohemicus Zone (W,) with a shelly bed below, as in the Xixiang section,
Xixiang, S Shaanxi; the Ganxi section, Yanhe, NE Guizhou; and the Shahechang section,
Baoshan, W Yunnan.
3. Where the graptolitic facies with the Glyptograptus persculptus Zone or its equivalents
(L,) lies upon shelly Hirnantia—Dalmanitina beds (HD) with a graptolitic facies below, as at the
Wangjiawan, Huanghuachang, Fenxiang and Tangya Sections, all in Yichang, W Hubei; the
Sintan section, Zigui, W Hubei; the Shuanghezhen section, Changning, SW Sichuan; the
Guanyiqiao section, Quyiang, S Sichuan; the Xiushan section, SE Sichuan; the Songtao section,
NE Guizhou; the Hanjiadian and Liangfengya sections, Tongzi, N Guizhou; the Renhuai and
Bijie sections, NW Guizhou; the Yanjin and Daguan sections, NE Yunnan; the Luxi section,
W Yunnan; and the Xainza sections of Xizang (Tibet).
4. Where the graptolitic facies with Glyptograptus persculptus or its equivalents (L,) lies upon
a mixed facies with graptolitic facies below, such as in the Honghuayuan section, Tongsi, N
Guizhou; the Liangshan section, Nanzheng, S Shaanxi; the Xinkailing section, Wuning,
NW Jiangxi; the Shanchong section, Jingxian, S Anhui; and the Tangjia section, Yuqiau, W
Zhejiang.
5. Where the shelly Wulipo bed with an ‘Eospirigerina fauna lies upon the shelly Hirnantia—
Dalmanitina bed with graptolitic facies below, as at Donggongsi, Zunyi, in N Guizhou.
Strata of the first type are only known in the transitional belt between the Yangtze basin and
the East Qinling trough to the north, whereas the last type is only known in the southern
marginal belt of the Yangtze basin. The Ordovician—Silurian boundary sections of the second
and fourth types are important for the correlation of the Diplograptus bohemicus Zone (W.,) and
the Hirnantia—Dalmanitina fauna (HD). The Ordovician-Silurian boundary sections of the third
type are most common and widespread in the Yangtze region. The Wufengian (Ashgill)
Yangtze sea was bounded by surrounding lands and swells and became a semi-enclosed sea
under aerobic conditions, but the surface water above the anoxic layer was oxygenated. The
strata of the third type are rich in organic matter and graptolites flourished.
The diversity of the Wufeng graptolitic fauna increases upwards stratigraphically from the
Amplexograptus disjunctus yangtzeensis Zone (W,) to the Tangyagraptus typicus Zone (W3).
More than twenty genera occur in the Dicellograptus szechuanensis Zone (W), apart from the
dendroids. The decline of graptolite diversity took place from the Diceratograptus mirus Zone
(W,) to the Diplograptus bohemicus Zone (W,) (Table 2). At the end of the Ordovician, all the
axonolipous graptoloids were nearly extinct except for a few Dicellograptus which remained in
China. In contrast, the Wufengian benthic shelly fauna increased in diversity. The well-known,
cosmopolitan Hirnantia fauna first appeared in the equivalents of the Diceratograptus mirus
Zone (W,) with 7 genera, and increased gradually to 23 genera in the uppermost Ordovician
Hirnantia—Dalmanitina bed (Table 3). The sea level was lowered in late Ordovician due to the
formation of the ice cap in North Africa. In the late Wufengian W,—W,g, a shallow and better
aerated environment occurred due to ventilation of sea waters. The maximum glaciation was
126 MU EN-ZHI
Table 2 Stratigraphical range of graptolite genera in the Wufeng
Formation
=
fe
We ic
Leptograptus
Pleurograptus
Dicellograptus
Diceratograptus ar = +
Dicranograptus
Tang yagraptus
Glyptograptus
Amplexograptus
Climacograptus
Pseudoclimacograptus
Diplograptus
Orthograptus
Paraorthograptus
Parareteograptus
Orthoreteograptus
Sinoreteograptus
Neurograptus
Nymphograptus
Arachniograptus
Phormograptus —
Plegmatograptus +
Paraplegmatograptus = +
Yinograptus = af =F
Y angzigraptus = +
a ar ar ||
JP ap ar
ap ar ar || =
++++
+++
+++
+++
qe fe te ob te de apap || te
++++4
+++
+++
}++++4+]4+)++++4 |
++4++
++
+ + +
+ — —
Table 3 Stratigraphical range of brachiopod
genera in the Upper Wufeng Formation
Wie Wow we
Paracraniops
Dalmanella
Paromalomena
Leptaena
Aphanomena
Coolinia
Hindella
Trematis
Hirnantia —
Cliftonia ~-
Plectothyrella --
Dorytreta =
Philhedra —
Philhedrella —_—
Acanthocrania — —
Kinnella — —
Draborthis — —
Mirorthis — a=
Aegiromena — —
Leptaenopoma = =
Toxorthis
Dysprosorthis
Trucizetina
Onychoplecia
iste etrect eet cect
| Sb GP Ab sip ae ae Ge ie ae ab IP ae
++etettetet+e | ++tetest
++tetettet+ettete | t+tett+
ORDOVICIAN-SILURIAN BOUNDARY IN CHINA 127
reached at the end of the Ordovician (W,) and the whole Yangtze basin became a nearly
normal shallow sea in which the Hirnantia—Dalmanitina fauna flourished.
At the beginning of the Silurian a new graptolite fauna occurred, notably with monograptids
and typical Silurian diplograptids such as the Diplograptus cf. modestus and Glyptograptus cf.
tamariscus groups during the Glyptograptus persculptus Zone (L,) time interval. A new brachio-
pod fauna, known as the ‘Eospirigerina fauna, appeared above the Hirnantia fauna in the
nearshore region. The rapid change in biofacies and faunal composition is due to the rising of
sea level caused by rapid melting of the ice cap.
Correlation of the Ordovician—Silurian boundary sections
All the Ordovician—Silurian boundary sections may be easily correlated in China by the stan-
dard of the Wufengian—Lungmachian graptolite zones and the Hirnantia—Dalmanitina bed. In
order to define the Ordovician-Silurian boundary throughout the world, a precise correlation
of the Diplograptus bohemicus, Glyptograptus persculptus and Parakidograptus acuminatus
Zones with shelly faunas is necessary. Thus, the subdivision and correlation of the Diplograptus
bohemicus Zone with the Hirnantia—Dalmanitina bed is of great importance.
In the Yichang sections, Western Hubei, the uppermost Hirnantia—Dalmanitina bed is under-
lain by the Diplograptus bohemicus Zone (W,) and overlain by the Glyptograptus persculptus
Zone (L,), whereas in the Xixiang section, S. Shaanxi, the Hirnantia—Dalmanitina bed is under-
lain by the Paraorthograptus uniformis Zone (W) and overlain by the Diplograptus bohemicus
Zone (W,), which is succeeded by the Glyptograptus persculptus Zone (L,). Therefore, the D.
bohemicus Zone of Yichang is equivalent to the lower part of the D. bohemicus Zone (Wé), and
the D. bohemicus Zone of Xixiang is equivalent to the upper part of the D. bohemicus Zone
(W2). Thus the Hirnantia—Dalmanitina bed of Yichang is the equivalent of the upper part of the
D. bohemicus Zone (W2), and that of Xixiang is the equivalent of the lower part of the D.
bohemicus Zone (W¢). Climacograptus extraordinarius and Diplograptus orientalis usually occur
in the lower part of the D. bohemicus Zone (W6).
The Glyptograptus persculptus Zone (L,) is marked by the incoming of Glyptograptus per-
sculptus, G. sinuatus, G. lungmaensis, G. gracilis, Diplograptus modestus, Akidograptus ascensus
and monograptids. It represents the beginning of a new developmental stage of graptolite
faunas, the fifth (or monograptid) fauna as defined by the writer (Mu 1984). Thus the base of
the G. persculptus Zone should be considered an important stratigraphical boundary, that
between the Ordovician and Silurian.
It is noteworthy that the Akidograptus ascensus Zone, directly overlying the Hirnantia—
Dalmanitina beds of Europe, is usually regarded as the equivalent of Parakidograptus acumin-
atus by some foreign colleagues. For defining the Ordovician—Silurian boundary the correlation
of the Akidograptus ascensus Zone with the Glyptograptus persculptus Zone and the boundary
between the Glyptograptus persculptus Zone and the Parakidograptus acuminatus Zone must be
clarified.
The Parakidograptus acuminatus Zone (L,) is marked by the incoming of P. acuminatus in
association with Climocograptus sinitzini which also characterizes the P. acuminatus Zone.
Akidograptus ascensus itself first appeared in the persculptus Zone (L,), much earlier than P.
acuminatus, although the two forms may be present together in the P. acuminatus Zone (L,),
whereas P. acuminatus is confined to the P. acuminatus Zone. Yu and his colleagues are of the
opinion that Parakidograptus acuminatus is directly derived from Akidograptus ascensus and a
transitional form Akidograptus xixiangensis Yu et al. was described and illustrated from the
basal Lungmachi formation of Xixiang, S. Shaanxi. A. xixiangensis appears higher than A.
ascensus and lower than P. acuminatus. It posseses akidograptid thecae in the proximal portion
of the rhabdosome and parakidograptid thecae in the distal portion. A similar form Akido-
graptus giganteus was described by Yang (1964) from the basal Silurian of W. Zhejiang. Li &
Ge (1981) and Fu (1983) tried to propose a new genus for these transitional forms between
Akidograptus and Parakidograptus.
128 MU EN-ZHI
It is clear that the Akidograptus ascensus Zone of Europe may be correlated with the
Glyptograptus persculptus Zone in China. This view was confirmed by the works of Nilsson
(1984) in Sweden, and Storch (1982) in Bohemia. The same is true, in my view, for the Mirny
Creek section, northeast USSR, studied by Koren et al. (1983). The Mirny Creek Ordovician—
Silurian boundary section of mixed biofacies measured by Koren and her colleagues may be
outlined mainly by graptolites as follows:
Members 65 and 66 Paraorthograptus pacificus Zone
Members 67 and 68 Climacograptus extraordinarius Zone with Hirnantia—Dalmanitina fauna
Members 69 to 72 Diplograptus bohemicus Zone (=‘persculptus’ Zone) with Hirnantia—Dalmanitina fauna
Members 73 and 74 Akidograptus ascensus Zone, incoming of Diplograptus of modestus group, Glyp-
tograptus of the tamariscus group and Akidograptus ascensus.
Members 75 to basal part of member 78 Parakidograptus acuminatus Zone, incoming of P. acuminatus
and Climacograptus sinitzini..
Member 78 Orthograptus vesiculosus Zone, incoming of Orthograptus vesiculosus.
It is obvious that the Paraorthograptus pacificus Zone (65—66) corresponds to the Paraortho-
graptus uniformis Zone (W;), that the Climacograptus extraordinarius Zone (67-68) corresponds
to the lower part of the Diplograptus bohemicus Zone (W3), and the Diplograptus bohemicus
Zone (=‘persculptus’ Zone, 69-72) corresponds to the upper part of the Diplograptus bohemicus
Zone (W2). The lower part of the ‘acuminatus—ascensus Zone’ (members 73-74) of Koren and
others is equivalent to the Akidograptus ascensus Zone of Europe, and corresponds to the
Glyptograptus persculptus Zone (L,) of China, whereas the upper part of the ‘acuminatus—
ascensus Zone’ (75—basal 78) is the Parakidograptus acuminatus Zone, corresponding to the
Parakidograptus acuminatus Zone (L,) of China and Europe.
I am convinced that the Akidograptus ascensus Zone of the European continent is equivalent
to the Glyptograptus persculptus Zone of Britain and Denmark. The Parakidograptus acumin-
atus Zone and the Glyptograptus persculptus Zone of the Dob’s Linn section of Britain corre-
spond to the P. acuminatus Zone (L,) and G. persculptus Zone (L,) of China respectively. The
C. extraordinarius band of the Dob’s Linn section falls within the lower part of the Diplograptus
bohemicus Zone (Wé), and the blind dalmanitid band of Dob’s Linn possibly falls within the
upper part of the D. bohemicus Zone (W2). It seems to me that the G. persculptus Zone of Dob’s
Linn as well as elsewhere represents the beginning of the Silurian transgression due to the rapid
melting of the ice-cap in North Africa.
Conclusions
1. The Ordovician—-Silurian boundary sections are widely distributed in China. Many
Ordovician-Silurian boundary sections have been defined in the Yangtze platform of the
Central China Region.
2. The graptolite sequence of the upper Ordovician (Wufengian W,—W,) and the Lower
Silurian (Lungmachian L,—L,) affords a valuable standard for correlation. The position of the
Hirnantia—Dalmanitina bed is confined to W,-W,. The Diplograptus bohemicus Zone (W,) is the
highest level reached by the well-known and cosmopolitan Hirnantia fauna.
3. By this standard all the Ordovician—Silurian boundary sections may be easily correlated
in China and even outside China.
4. The acuminatus Zone is marked by the incoming of Parakidograptus acuminatus. The
underlying Akidograptus ascensus Zone of Europe is equivalent to the Glyptograptus persculptus
Zone, which is the beginning of the Silurian transgression due to the rapid melting of the
ice-cap in north Africa. The G. persculptus Zone was also the beginning of the monograptid
fauna stage in the history of the development of the graptolite faunas. It is reasonable to place
the Ordovician—Silurian boundary between the G. persculptus Zone (L,) or ‘“Eospirigerina’ bed
and the D. bohemicus Zone (W,) or the Hirnantia—Dalmanitina bed (HD).
5. The C. extraordinarius Zone of the north-east USSR or the C. extraordinarius band of
Dob’s Linn, Scotland, correspond to the lower part of the D. bohemicus Zone (W}). The ‘G.
ORDOVICIAN-SILURIAN BOUNDARY IN CHINA 129
persculptus (= D. bohemicus) Zone of the north-east USSR corresponds to the upper part of the
D. bohemicus Zone (W2) of China.
6. Many kinds of fossils have been found in the Ordovician-Silurian boundary sections such
as graptolites, brachiopods, trilobites, ostracods, corals, bivalves, cephalopods, gastropods,
bryozoa, crinoids, conularia, conodonts, chitinozoa, and so on. The increasing number of finds
of conodonts is of great importance for correlation with the Anticosti section of Canada. At
present, the correlation with Anticosti is difficult. Unfortunately there are many weak points in
the Dob’s Linn section, and it is difficult to use as an international Ordovician—Silurian
boundary stratotype.
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The Ordovician—Silurian boundary beds of the
north-east USSR
T. N. Koren’, M. M. Oradovskaya’ and R. F. Sobolevskaya°*
'VSEGEI, Srednii prospekt 74, 199026 Leningrad, USSR
2PGO ‘Sevvostokgeologia’, 44 Proletarskaya, 685000 Magadan, USSR
3VNIIOkeangeologia, 120 Moika, 190121 Leningrad, USSR
Synopsis
Graptolites of the supernus, extraordinarius, persculptus, acuminatus and ascensus Zones are present in
sections in the north-east USSR, with the best section at Mirny Creek. Brachiopod and coral faunas also
occur with the Tcherskidium and Holorhynchus beds in the supernus Zone and the Hirnantia? beds present
in the persculptus Zone, both within the Tirekhtyakh Horizon. The succeeding acuminatus and ascensus
Zone graptolites are developed in the Chalmak Horizon, which also bears a sparse shelly fauna.
Introduction
The late Ordovician and early Silurian boundary beds in the north-east USSR crop out on the
Omulev Uplift in the upper Kolyma Basin. They are built up by terrigenous-carbonate and
terrigenous deposits which are variable in composition and contain a mixed shelly-graptolite
fauna. The rocks are exposed on limbs of extensive anticlines and show either a monoclinal
succession, such as at Mirny Creek, Neznakomka River and Drevnyaya River, or represent
large fragments of sections among complex faulted sequences, such as at the Ina River. The
Upper Ashgill and Lower Llandovery deposits include the supernus, extraordinarius, per-
sculptus, acuminatus and ascensus graptolite Zones and have a total thickness of about 300m
(Fig. 1). This part of the section is designated the Tirekhtyakh and Chalmak horizons. The
lower part of the Tirekhtyakh horizon (the supernus Zone) (Fig. 2) shows a diversity of facies
from deep water shales yielding graptolites, for example at Khekandya River and Lukavy
Creek, to biohermal and biogenic—detrital carbonates with mixed brachiopod—coral-graptolite
faunas as at Mirny Creek and the Ina and Neznakomka rivers. The upper part of the
Tirekhtyakh horizon (the extraordinarius and persculptus Zones) and the lower part of the
Chalmak horizon (the acuminatus and ascensus Zones) are represented by sequences more
Fig. 1 Distribution of Ordovician—Silurian
boundary beds on the Omulev Uplift. I, Mirny
Creek; II, Ina River; III, Neznakomka River
Basin; IV, Tirekhtyakh River Basin; V, Mount
Kharkindzha; VI, Levaya Khekandya River;
VII, Drevnyaya River; VIII, Lukavy Creek.
Seimchan ©
Bull. Br. Mus. nat. Hist. (Geol) 43: 133-138 Issued 28 April 1988
134
KOREN, ORADOVSKAYA & SOBOLEVSKAYA
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ORDOVICIAN-SILURIAN BOUNDARY IN NORTH-EAST USSR 135
diverse in composition. The Upper Tirekhyakh deposits consist mainly of dolomites, marls, and
siltstones representing the termination of the late Ordovician regressive cycle and the Chalmak
dark carbonate clay sequences mark the beginning of the Llandovery transgression.
Of greatest interest is the key section at Mirny Creek, which has the best exposed
Ordovician-Silurian boundary deposits. This has been studied in detail, and forms a type
section for such regional units as formations and horizons.
The Tirekhtyakh horizon
At the Mirny Creek and Ina River sections the horizon is 250m thick and represented by a
formation of the same name (upper unit M to unit Q) which is composed of bedded and
massive limestones with tabulate corals, brachiopods, ostracodes and gastropods. The lime-
stones are interbedded with siltstones yielding graptolites. In the Neznakomka River the forma-
tion is 315m thick and represented mainly by biohermal and biogenic-clastic limestones
interbedded with siltstones. The rocks contain chiefly brachiopods but the siltstones yield rare
graptolites.
In the south-eastern Omulev Mountains (Khekandya River, Yasachnaya Basin, Lukavy
Creek and Drevnyaya River) the Tirekhtykh horizon exhibits changes in composition. Its lower
part consists of the Iryudi Formation (500-600 m) and Lukavaya sequence (100m). The Iryudi
Formation is composed of clay and pelitomorphic, unevenly bedded limestones with abundant
corals and brachiopods. The Lukavaya unit is represented by dark platey limestones inter-
calated with calcareous shales containing abundant graptolites and rare brachiopods. As at
Mirny Creek, the upper part of the horizon includes siltstones.
The Terekhtyakh horizon has been subdivided by means of graptolites in sections at Mirny
and Lukavy creeks, the Khekandya and Drevnyaya rivers, and at Mount Kharkindza, and by
means of brachiopods mainly in Mirny Creek and the Neznakomka River (Fig. 3). The lower
part of the horizon is equated with the Climacograptus longispinus supernus Zone and the
Tcherskidium unicum beds. The supernus Zone is subdivided into two subzones, the lower
Climacograptus longispinus longispinus Subzone and the upper Paraorthograptus pacificus
Subzone. The lower subzone contains Climacograptus longispinus longispinus Hall, C. |. supernus
Elles & Wood, C. hastatus Hall, C. trifidus spectabilis Koren & Sobolevskaya, and Dicello-
graptus complanatus Lapworth, whose appearance marks its lower boundary. The pacificus
Subzone is recognized as a taxon biozone and, along with Dicellograptus ornatus ornatus Elles
& Wood and subspecies of Climacograptus longispinus, contains Climacograptus latus hekan-
daensis Koren & Sobolevskaya and C. pogrebovi Koren & Sobolevskaya, while the upper part
yields Glyptograptus? ojsuensis Koren & Mikhailova, Climacograptus angustus (Perner), C.
normalis Lapworth and others.
The supernus Zone is equated with the Tcherskidium unicum beds which also contain
Ptychoglyptus bellarugosus Cooper, Holorhynchus ex gr. giganteus Kiaer and Eostropheodonta
hirnantensis lucavica Oradovskaya. There are also abundant corals of the genera Agetolites,
Heliolites, Propora, Calapoecia, Coxia and others (Preobrazhensky 1966). The brachiopod-coral
assemblage allows the lower Tirekhtyakh horizon to be correlated with the Sb beds of Norway.
On Mirny Creek the deposits also contain trilobites, gastropods, ostracodes and other fossils
(Sokolov et al. 1983).
The upper Tirekhtyakh horizon corresponds to the Climacograptus? extraordinarius and
Glyptograptus? persculptus Zones. The extraordinarius Zone, which was first established on
Mirny Creek (Koren & Sobolevskaya 1979), corresponds to the index-species range. Apart
from the latter, it contains Climacograptus? ex gr. extraordinarius (Sobolevskaya), C. angustus
(Perner), C. normalis Lapworth and C. mirnyensis (Obut & Sobolevskaya). Climacograptus aff.
medius Tornquist and scarce Glyptograptus sp. appear in the upper part of the zone.
The persculptus Zone was recognized as equal to the full range of the index-species and the
zonal assemblage also contains Climacograptus angustus (Perner), C. normalis Lapworth, C.
mirnyensis (Obut & Sobolevskaya) and C. torosus Koren & Sobolevskaya.
4
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Fig. 3. Correlation chart of the
Ordovician-Silurian boundary
beds of Mirny Creek, Ina River
and the Neznakomka River
Basin. For legend see Fig. 2.
ORDOVICIAN-SILURIAN BOUNDARY IN NORTH-EAST USSR 37)
At Mirny Creek, the Khekanda and Neznakomka rivers and Mount Kharkindzha, this zone
is equated with the Hirnantia? beds (Oradovskaya 1977). Amongst the brachiopods the most
common are Dolerorthis? savagei Amsden, Brevilamunella thebesensis (Savage), Rafinesquina?
latisculptilis (Dalman) and Giraldibella bella (Bergstrom), and the trilobites Bumastus (Bumastus)
commodus Apollonov and Mucronaspis kolymica Chugaeva. Dalmanitina olini Temple occurs
near the top of the zone.
The Chalmak horizon
In the Omulev Mountains the lower Chalmak horizon includes the Maut Formation, and the
main Chalmak Formation corresponds to the horizon in the Yasachnaya Basin. On the
Omulev Uplift, the Maut Formation consists of dark calcareous shales, shales and cherts
containing graptolites which are interbedded with detrital and conglomerate-like limestones
with a scarce neritic fauna. Coarse clastic rocks dominate the coeval deposits further south-east.
The lower part of the horizon corresponds to the Parakidograptus acuminatus and Akido-
graptus ascensus Zones recognized in Mirny Creek, the Ina and Khekanda rivers, and Mount
Kharkindzha. The most complete graptolite assemblage was reported from Mirny Creek (Obut
et al. 1967). As well as P. acuminatus and A. ascensus, the assemblage includes Climacograptus
rectangularis (M‘Coy), C. transgrediens Waern, Paraclimacograptus sinitzini Chalatskaya, Diplo-
graptus ex gr. modestus Lapworth and Glyptograptus ex gr. tamariscus (Nicholson). The bound-
ary of the zone is drawn by the appearance and disappearance of the diagnostic species.
The acuminatus and ascensus Zone corresponds to the lower Skenidioides beds containing
Skenidioides cf. scolioides Temple, Leptaena aff. aequalis Amsden, Eospirigerina putilla
Oradovskaya, Zygospiraella sp. and Protatrypa sp. The assemblage is similar to the brachiopod
fauna from the lower Llandovery of the Northern Appalachians (Ayrton et al. 1969). The beds
also contain trilobites such as Acernaspis sp., Tropidocoryphinae gen. et sp. indet. and the
corals Palaeofavosites balticus Rukhin, and Propora conferta Edwards & Haime, among others.
The systemic boundary
The most complete and well known section of the Tirekhtyakh and Chalmak horizons is
exposed along the Mirny Creek. A point 2.5km from its mouth was chosen as a regional type
section for the Ordovician—Silurian boundary in the north-east USSR. The systemic boundary
is drawn at the base of unit 73 which is 1-5m thick and coincides with the base of the Maut
Formation (Figs 2, 3). This level corresponds to the base of the acuminatus and ascensus Zone
which in the section studied is substantiated by the appearance of representatives of such
typically Silurian groups as Diplograptus modestus Lapworth and Glyptograptus tamariscus
(Nicholson) (unit 73). The index-species Akidograptus ascensus Davies is known from the base
of unit 74, 1-5m above the boundary, and Parakidograptus acuminatus (Nicholson) occurs in
the lower part of unit 75, 11m above the boundary. Their absence from the basal layer can be
attributed to the difficulty in searching for graptolites in the beds. In the section at Mount
Kharkindzha, akidograptids are known from the basal beds of the Maut Formation associated
with other typical diplograptids.
The principal criteria for establishing the boundary on a regional scale are distinct changes in
the lithological composition of the deposits as well as the change in the assemblages of graptol-
ites (persculptus/acuminatus and ascensus), brachiopods (Hirnantia?/Skenidioides) and trilobites.
Graptolites allow interregional and global correlations of the level.
The section at Mirny Creek is well exposed and shows a continuous succession of uniformly
dipping deposits containing diverse fossils. Its major advantage is bed-by-bed graptolite control
within the range of the Dalmanitina—Hirnantia assemblage and the presence of shelly fauna (the
Skenidiodes beds) from the base of the acuminatus and ascensus Zones.
Abundant graptolites, brachiopods and corals and rare trilobites, ostracodes and conodonts
are known from the Ordovician-Silurian boundary beds in the north-east USSR. All faunal
groups except ostracodes and conodonts have been monographically described in different
138 KOREN, ORADOVSKAYA & SOBOLEVSKAYA
publications (Sokolov et al., 1983; Nikolaev et al., 1977; Nikolaev & Sapelnikov 1969; Obut et
al. 1967; Opornii razrez (Anon.) 1974; Oradovskaya 1963; Polevoi atlas (Anon.) 1968; Polevoi
atlas (Anon.) 1975; Preobrazhensky 1966 and Sobolevskaya 1970, 1974).
References
(Anon.) 1974. Opornii razrez verkhnego ordovika na Severo-Vostoke SSSR [The Upper Ordovician key
section in the north-east USSR]. In: Opornye razrezy paleozoya Severo-Vostoka SSSR: 3-136.
Magadan.
— 1968. Polevoi atlas ordovikskoi fauny Severo-Vostoka SSSR [Field atlas of the Ordovician fauna in the
north-east USSR]. 286 pp. Magadan.
1975. Polevoi atlas siluriiskoi fauny Severo-Vostoka SSSR [Field atlas of the Silurian fauna in the
North-East USSR]. 382 pp. Magadan.
Ayrton, W. G., Berry, W. B. N., Boucot, A. J., Lajoie, J., Lesperance, P. J., Pavlides, L. & Skidmore, W. B.
1969. Lower Llandovery of the northern Appalachians and adjacent regions. Bull. geol. Soc. Am., New
York, 80: 459-484.
Koren, T. N. & Sobolevskaya, R. F. 1979. A graptolite zonation of the Ordovician-Silurian boundary
deposits of the Omulev Mountains. In M. M. Oradovskaya & R. F. Sobolevskaya (eds), Guidebook to
field excursion to the Omulev Mountains. Pacific Sci. Ass. 14 Pacific Sci. Cong., Magadan: 91—92.
Nikolaev, A. A., Oradovskaya, M. M. & Sapelnikov, V. P. 1977. [Biostratigraphical review of the Ordovi-
cian and Silurian pentamerids of the north-east USSR]. Trudy Inst. Geol. Geokhim. Akad. Nauk SSSR
ural. nauch. Tsentr, Sverdlovsk, 126: 32-67, 11 pls.
Obut, A. M., Sobolevskaya, R. F. & Nikolaev, A. A. 1967. Graptolity i stratigrafia nizhnego silura
okrainnykh podnyatii Kolymskogo massiva (Severo-V ostok SSSR) [Graptolites and the stratigraphy of
the lower Silurian of marginal uplifts of the Kolyma Massif, the north-east USSR]. 162 pp. Moscow.
Oradoyskaya, M. M. 1963. Ordovikskie otlozhenia khrebta Cherskogo [The Ordovician deposits of the
Chersky Ridge]. In: Materialy po geologii i poleznym iskopaemym Severo-Vostoka SSSR 16: 140-162.
Magadan.
— 1977. Verkhnyaya granitsa ordovika na Severo-Vostoke SSSR [The upper boundary of the Ordovi-
cian in the north-east USSR]. Dokl. Akad. Nauk SSSR, Leningrad, 236 (4): 954-956.
Preobrazhensky, B. V. (1966). Biostratigraficheskoe obosnovanie granitsy mezhdu ordovikom i silurom
Severo-Vostoka SSSR po tabulyatomorfnym korallam [Biostratigraphic substantiation of the
Ordovician-Silurian in the north-east USSR based on tabulate corals]. Avtoref. dis. kand. geol. min.
nauk., Novosibirsk. 20 pp.
Sobolevskaya, R. F. (1970). Biostratigrafia srednego i verknego ordovika okrainnykh podnyatii Kolyma
Massif po graptolitam [Middle and Upper Ordovician biostratigraphy of the Kolyma uplifts based on
graptolites]. Avtoref. dis. kand. geol. min. nauk., Novosibirsk. 26 pp.
1974. Novye Ashgillskie graptolity v basseine srednego techenia r. Kolymy [ New Ashgillian graptol-
ites from the middle Kolyma River basin]. In A. M. Obut (ed.), Graptolity SSSR, Trudy I Vses.
Kollokviuma: 63-71. Novosibirsk.
Sokolov, B. S., Koren, T. N. & Nikitin I. F. (eds) 1983. Granitsa Ordovika i Silura na Severo-Vostoke SSSR
[The Ordovician-—Silurian boundary in the north-east USSR]. 205 pp. Leningrad.
The Ordovician—Silurian boundary in the Altai
Mountains, USSR
E. A. Yolkin, A. M. Obut and N. V. Sennikov
Institute of Geology and Geophysics, Siberian Branch, Academy of Science, 630090
Novosibirsk, USSR
Synopsis
The Ordovician-Silurian boundary is repeatedly exposed in the Altai-Sayan fold belt, with the best-
studied outcrops in the Charysh—Inya structural zone near Ust’-Chagyrka and Chineta, where the per-
sculptus and acuminatus zones are both known in association with shelly faunas.
Ordovician and Silurian deposits in the western part of the Altai-Sayan fold-belt are not only
widely distributed in the Altai, but in the Kuznetsk Alatau, Salair and Shoria Mountains as
well. The boundary interval, however, is known only from the Altai Mountains in two
structural-formational zones, the Anui-Chuya and Charysh—Inya zones. In the first zone there
are several sections where it is possible to see a normal stratigraphical succession from Ordovi-
cian to Silurian. However, most of them are not well characterized palaeontologically, espe-
cially the boundary beds (Yolkin et al. 1978; Sennikov & Sennikov 1982). Because of this, the
boundary interval is shown as a biostratigraphical break in the stratigraphical correlation
charts for this zone (Khomentovskiy & Tesakov 1983).
The Ordovician-Silurian boundary interval is better known in the Charysh—Inya Zone. Here,
in different areas, there are now more than ten known sections. In each such area there are
usually several sections with transitional continuity between the two systems, though there are
some differences in the faunas from area to area. The best of these sections occur near Ust’-
Chagyrka and Chineta villages (Yolkin & Zheltonogova 1974; Sennikov et al. 1979, 1982,
1984). The faunal assemblages in these sections in the two areas include graptolites, conodonts,
trilobites, gastropods, orthoconic cephalopods, brachiopods, ostracodes, corals, chitinozoans
and polychaetes, part of which have been monographed (Sennikov 1976, 1978; Moskalenko
1977; Severgina 1978, 1984; Yolkin 1983). The most important fossils for the subdivision and
correlation of these sections are the graptolites. They are the predominant group numerically
and have by far the best international distribution stratigraphically.
It is important to draw attention to the association, in the boundary beds, of graptolites,
conodonts and trilobites, especially Dalmanitina. This indicates the possibility for future work
directed towards clarifying and refining the correlation of Ordovician—Silurian boundary beds
in the Altai, but perhaps also globally. The best boundary in the Altai, as in China (Chen Xu
1984) would be somewhat below the acuminatus Zone decided by the Ordovician-Silurian
Boundary Working Group (Holland 1985). The beds with persculptus correspond to the onset
of a wide transgression.
Bull. Br. Mus. nat. Hist. (Geol) 43: 139-143 Issued 28 April 1988
OBUT & SENNIKOV
YOLKIN,
140
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Legend for Figs 2-7. 1—conglomerates, 2—sandstones, 3—silty sandstones, 4—siltstones, 5—cherty
rocks, 6—limestones, 7—sandy limestones, 8—argillaceous limestones, 9—boundaries, 10—faults,
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YOLKIN, OBUT & SENNIKOV
142
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ORDOVICIAN-SILURIAN BOUNDARY IN THE ALTAI MOUNTAINS 143
References
Chen Xu 1984. The Silurian graptolite zonation of China. Can. J. Earth Sci., Ottawa, 21: 241-257.
Holland, C. H. 1985. Series and Stages of the Silurian System. Episodes, Ottawa, 8: 101—103.
Khomentovskiy, V. V. & Tesakov, Y. I. (eds) 1983. Resheniya Vsesoyuznogo stratigraficheskogo sovesh-
chaniya po dokembriyu, paleozoyu i chetvertichnoy sisteme Sredney Sibiri, Novosibirsk, 1979. Ch. 1:
Verkhniy proterozoi i nizhniy paleozoi. In: Mezhvedomstvenniy Stratigraficheskiy Komitet USSR.
215 pp. Novosibirsk.
Moskalenko, T. A. 1977. Ashgill’skiye konodonty na Gornom Altaye. In A. B. Kanygin, et al., Problemy
stratigrafii ordovika i silura Sibiri. Trudy Inst. Geol. Geofiz. Sib. Otdel., Novosibirsk, 372: 74-83.
Sennikoy, N. V. 1976. Graptolity i stratigrafiya nizhnego silura Gornogo Altaya. Trudy Inst. Geol. Geofiz.
Sib. Otdel., Moscow, 304. 270 pp., 17 pls.
1978. O nakhodke graptolitov zony persculptus na Gornom Altaye. In Novoe v stratigrafii i paleon-
tologii nizhnego paleozoya Sredney Sibiri. Trudy Inst. Geol. Geofiz. Sib. Otdel., Novosibirsk, 141-144.
——,, Petrunina, Z. E., Gladkikh, L. A., Ermikoy, V. D., Zinov’eva, T. V., Mamlin, A. N. & Shokal’sky,
S. P. 1984. Novye pogranichnye Ordoviksko-Siluriyskie razrezy na Gornom Altaye. Geol. Geofiz. 1984
(7): 23-27.
——, Puzyrev, A. A. & Russkikh, V. G. 1979. Ordovik i nizhniy silur rayona s.Ust’-Chagyrka (Gorniy
Altai). In P. P. Kuznetsov (ed.), Problemy stratigrafti i tektoniki Sibiri: 30-45. Novosibirsk (Akad. Nauk
SSSR, Sib. Otdel. Inst. Geol. Geofiz.).
—— & Russkikh, V. G. 1982. Etalon llandoveriyskikh graptolitovykh zon na Gornom Altaye. Geol.
Geofiz. 1982 (2): 28-35.
Sennikoy, V. M. & Sennikoy, N. V. 1982. Stratigrafiya ordovika Anuysko-Chuyskogo sinklinoriya (Gorniy
Altai). Geol. Geofiz. 1982 (6): 17-25.
Severgina, L. G. 1978. Brakhiopody i stratigrafiya verkhnego ordovika Gornogo Altaya, Salaira 1 Gornoy
Short. In J. I. Tesakov & N. P. Kulkov (eds), Fauna i biostratigrafiya verkhnego ordovika i silura
Altaye-Sayanskoy oblasti. Trudy Inst. Geol. Geofiz. Sib. Otdel., Moscow, 495: 3-41, pls 1-6.
—— 1984. Nekotoriye verkhneordovikskiye (Ashgillskiye) brakhiopody Gornogo Altaya. In Paleon-
tologiya 1 biostratigrafiya paleozoya Sibiri. Trudy Inst. Geol. Geofiz. Sib. Otdel., Novosibirsk, 584:
39-48, pls 3, 4.
Yolkin, E. A. 1983. Zakonomernosti evolutsii dekhenellid 1 biokhronologiya silura i devona. Trudy Inst.
Geol. Geofiz. Sib. Otdel., Moscow, 571. 116 pp., 16 pls.
——, Obut, A. M. & Sennikov, N. V. 1978. O granitse ordovika i silura v Gornom Altaye. In B. S.
Sokolov & E. A. Yolkin (eds), Pogranichniye slo ordovika 1 silura Altaye-Sayanskoy oblasti 1 Tyen-
Shanya. Trudy Inst. Geol. Goefiz. Sib. Otdel., Moscow, 397: 5-14.
—— & Zheltonogova, V. A. 1974. Drevneyshiye dekhenellidy (trilobity) 1 stratigrafiya silura Gornogo
Altaya. Trudy Inst. Geofiz. Sib. Otdel., Novosibirsk, 130. 96 pp., 13 pls.
it
s) fe
rh
Nature of the Ordovician—Silurian boundary in south
Kazakhstan, USSR
M. K. Apollonov', T. N. Koren’, I. F. Nikitin’, L. M. Paletz! and D. T. Tzai‘
1 Institute of Geology, Academy of Sciences of Kazakhstan SSR, Kalinina 69A, Alma-Ata
480100, USSR
? All-Union Geological Research Institute (VSEGEI), Sredni Prospekt 74, Leningrad
199026, USSR
Synopsis
Kazakhstan was the region where the coeval nature of the Dalmanitina mucronata—Hirnantia faunas with
the persculptus Zone faunas was first established. The best sections are in the Chu-Ili Mountains of South
Kazakhstan, the Ashchisu River and the Zhideli and Karasay sequences. A summary is given of the upper
Ashgill and lower Llandovery biostratigraphy and the position of the systemic boundary. The litho-
stratigraphy is also outlined.
To have the Ordovician-Silurian boundary at the base of the acuminatus Zone was first
advanced by Kazakhstan geologists (Rukavishnikova et al. 1968; Mikhailova 1970; Nikitin
1972; Apollonov et al. 1973; Apollonov 1974; Poltavtseva & Rukavishnikova 1972) after the
discovery of Glyptograptus persculptus in association with Dalmanitina mucronata and Hirnantia
in the Chu-Ili Mountains. This showed that the persculptus Zone did not succeed the Dalmani-
tina beds, as was previously thought in western Europe, and that, on the contrary, it was partly
coeval with the Dalmanitina mucronata—Hirnantia beds which have always been assigned to the
Ordovician. Thus it became clear that tracing the persculptus boundary in the neritic facies was
impossible. This new evidence has been widely discussed in the literature (Williams et al. 1972;
Bergstrom et al. 1973; Lespérance 1974; Rozman 1976; Rickards 1976).
The Kazakhstan Ordovician—Silurian boundary deposits are best studied in the Chu-Ili
Mountains in south Kazakhstan, in the upper reaches of the Ashchisu River (Durben and Ojsu
wells), as well as along the Zhideli and Karasay dry channels (Apollonov et al. 1980; Nikitin et
al. 1980: textfigs 1-6). This paper is a summary of the upper Ashgill and lower Llandovery
biostratigraphy and describes the position of the system boundary established in Kazakhstan
on the basis of continuous sections.
The succession is divided into three conformable lithostratigraphic units: the Chokpar,
Zhalair and Salamat Formations. The latter is overlain by the Betkainar Formation (Figs 1—6).
The Chokpar Formation consists of dark-grey and greenish-grey regularly bedded mudstones
and siltstones yielding abundant graptolites characteristic of the supernus Zone (Apollonov et
al. 1980). A more detailed zonation can now be suggested. The lowermost part of the Chokpar
Formation contains Dicellograptus ornatus minor Toghill, Climacograptus longispinus supernus
Elles & Wood, Amplexograptus inuiti (Cox) and Orthograptus amplexicaulis (Hall) and com-
prises the inuiti Zone. The graptolites present above this, and in most of the Chokpar Forma-
tion, are characteristic of the pacificus Zone and include Dicellograptus ornatus Elles & Wood,
Climacograptus manitoulinensis Caley, Orthograptus socialis (Lapworth), Paraorthograptus paci-
ficus (Ruedemann) (rare) and Nymphograptus velatus Elles & Wood. The uppermost Chokpar
Formation locally contains limestone beds which are best developed in the Osju section where
they are placed in a local stratigraphic unit—the Osju Limestones. The unit consists of dark-
grey argillaceous limestones interbedded with aphanitic sandy limestones in which terrigenous
clastics account for 15 to 20%. The Osju Limestones yields abundant brachiopods and tri-
lobites including Giraldiella bella Bergstrom, Streptis altosinuata (Holtedahl), Leptaena rugosa
Dalman, Cryptothyrella sp., Tscherskidium cf. ulkuntasensis Sapelnikov & Rukavishnikova,
Prostricklandia prisca Rukavishnikova & Sapelnikov, Platycoryphe sinensis sinensis (Lu),
Bull. Br. Mus. nat. Hist. (Geol) 43: 145-154 Issued 28 April 1988
146 APOLLONOV, KOREN, NIKITIN, PALETZ & TZAI
e2
e3 Semipalatinsk
Karaganda
CENTRAL °6
KAZAKHSTAN Fig. 1 Localities of the Ordovician—Silurian
boundary deposits in Central and South
Kazakhstan. 1, Sarysu-Teniz watershed and
Zhaksykon River; 2, Northeast of Central
Kazakhstan—Kombabasor lake; 3, Akjar—
Zhartas watershed; 4-6, Chingiz Range and
Pre-Chingiz Range: 4, Mount Otyzbes; 5,
Mount Mizek: 6, Mount Akdombak; 7-12,
Chu-Ili Mountains: 7, Karasay River; 8,
Zhideli River; 9, Anzhar River; 10, Ojsu well;
11, Durben well; 12, Mount Dulankara.
@ALMA~ ATA
Bumastus commodus Apollonov, Decoroproetus artus Apollonov, D. cf. evexus Owens, Otarion
curvulum Apollonoyv, O. gibberum Apollonov, Dicranogmus confinis Apollonov, and Leonaspis
sp. There also occur conodonts, bivalves, gastropods and cepalopods, among them Acodus
similaris Rhodes, Eobelodina fornicala Stauffer, Icriodella sp., Tshuiliceras lobatum Barskov,
Michelinoceras procurens Barskov and Geisonoceras fustis Barskov.
The numerous graptolites that are characteristic of the pacificus Zone occur in argilliceous
limestone layers. Present are Climacograptus longispinus supernus Elles & Wood, C. cf. normalis
Lapworth, C. tatianae Keller, Glyptograptus posterus Koren & Tzai, G.? ojsuensis Koren &
Mikhailova, Paraorthograptus pacificus (Ruedemann), Orthograptus amplexicaulis (Hall) and
Plegmatograptus nebula lautus Koren & Tzai. Rare tabulate corals, radiolarians and algae are
also known (Apollonov et al. 1980).
The uppermost Chokpar Formation in other sections (as at the Anzhar River) is represented
by massive biogenic-detrital limestones (the so-called Ulkuntas Limestones) overcrowded with
tabulate corals and heliolitids. The assemblage includes Agetolites cf. mirabilis Sokolov, Hemi-
agetolites insignis Poltavceva, Catenipora inordinata Kovalevsky, Plasmoporella papillatiformis
Kovalevsky, Propora cancellatiformis Sokolov and Heliolites parvulus Kovalevsky. Some penta-
merids such as Holorhynchus giganteus Kiaer, Proconchidium tchuilensis Rukavishnikova &
Sapelnikov and Tcherskidium? ulkuntasense Rukavishnikova & Sapelnikov have been found.
There occur the trilobites Holotrachellus punctillosus Tornquist, Amphylicas sp. and Sphaerex-
ochus sp., which are characteristic of biohermal environments. The thickness of the Ojsu and
Ulkuntas Limestones varies from 14 to 55m and the Chokpar Formation totals 140 to 180m.
The Zhalair Formation rests conformably on the Chokpar deposits and is exposed in all
sections studied. The section at Durben may serve as a stratotype (Figs 2, 3). The formation is
composed of tobacco-green and greenish-grey siltstones interbedded locally with grey and
reddish-brown fine-grained poorly sorted sandstones, the latter being of carbonate and quartz-
feldspathic composition. Locally, sandstones form a separate unit more than 80m thick, for
example at the Ojsu section. The lowermost Zhalair Formation includes the Durben Limestone
which is 9 to 40m thick, and is easily discernible in many of the sections studied (Fig. 4). It
consists of well-bedded dark grey pelitomorphic limestones. The upper part of the Zhalair
Formation contains local beds of dark grey and green silty tuffites.
The lower Zhalair Formation (the Durban horizon) contains graptolites of the extraordi-
narius and persculptus Zones (Koren & Nikitin 1983). The former zone yields Climacograptus
angustus (Perner), C. normalis Lapworth, C.? extraordinarius (Sobolevskaya) (=Glyptograptus?
persculptus forma A and G. aff. persculptus of Apollonov et al. 1980) and Pseudoclimacograptus
ORDOVICIAN-SILURIAN BOUNDARY IN SOUTH KAZAKHSTAN 147
Fig. 2 Schematic geological map of the Durben well area. 1, 2, Kysylsai Formation (?): 1, black
siltstones and sandstones; 2, yellow sandstones; 3, Chokpar Formation black mudstones and
siltstones; 4, 5, Zhalair Formation: 4, dark fine-crystalline and fine-clastic limestones; 5, green
siltstones and fine-grained sandstones; 6, 7, Betkainar Formation: 6, basal conglomerate and
sandstones; 7, grey sandstones; 8, red sandstones; 9, Koichin Formation: red sandstones and
siltstones; 10, faults; 11, localities of fauna; 12, strike and dip.
sp. The latter zone may be distinguished by the occurences of Glyptograptus persculptus (Salter)
(=G. persculptus forma B of Apollonov et al. 1980), Glyptograptus sp. and Climacograptus
angustus (Perner). A shelly fauna was found in limestones and siltstones within both graptolite
zones, namely a typical Dalmanitina—Hirnantia assemblage including Platycoryphe sinensis (Lu),
Dalmanitina mucronata (Brongniart), Dalmanitina olini Temple, Leonaspis olini Troedsson, Dic-
ranopeltis sp., Dalmanella testudinaria (Dalman), Hirnantia sagittifera (M‘Coy), Anisopleurella
novemcostata Nikitin, Aegiromena durbenensis Nikitin, Aphanomena ultrix (Marek & Havlicek),
A. aff. urbicola (Marek & Havli¢ek), Bracteoleptaena polonica Temple, Eostropheodonta bublits-
chenki Nikitin and Coolinia iliensis Nikitin.
Fig. 3 A—Section on the north-east limb of the Ashchysu anticline near the Durben well. (a) the
Chokpar Formation, (b—d) the Zhalair Formation: (b) limestones, (c) carbonaceous sandstone, (d)
limestones, (e) siltstones, (f) Betkainar Formation; 354, f-287—localities of fauna. In the back-
ground to the right are hills composed of coarse-grained sandstones of Betkainar Formation on
the south-western limb of the anticline.
B—enlarged part of the same section.
C—section near the Ojsu well. (a) Ojsu Limestones of the uppermost part of the Chokpar Forma-
tion; (b) limestones with Dalmanitina assemblage; (c) siltstone of the basal Silurian. In the fore-
ground an exposure of the Ojsu Limestones is seen.
D—transgressive onlapping of the basal conglomerate of the Betkainar Formation (b) on siltstones
of the middle Zhalair Formation (a) in the Durben well area. Photographs I. F. Nikitin.
ORDOVICIAN-SILURIAN BOUNDARY IN SOUTH KAZAKHSTAN 149
The thickness of the lower Zhalair Formation (the extraordinarius and persculptus Zones)
varies from 122 to 127m in the southeastern Chu-Ili Mountains (the Durben and Osju wells),
to 55m in the Zhideli River and to half a metre in the Karasay River in the northwestern
Chu-Ili Mountains.
The upper Zhalair Formation (the Alpeis horizon) yields early Silurian graptolites. The
acuminatus Zone is well defined in the strata overlying the persculptus Zone in sections in the
Karasay, Zhideli, and Ashchysu Rivers. The zonal assemblage includes abundant graptolites,
namely Climacograptus acceptus Koren & Mikhailova, C.? jidelensis Koren & Mikhailova, C.
mirnyensis (Obut & Sobolevskaya), C. ex gr. normalis Lapworth, Pseudoclimacograptus
(Metaclimacograptus) fidus Koren & Mikhailova, P. (M.) pictus Koren & Mikhailova, Diplo-
graptus modestus primus Mikhailova, G. madernii Koren & Tzai, Akidograptus cf. ascensus
Davies, A. ascensus cultus Mikhailova, Parakidograptus cf. acuminatus (Nicholson) and Ortho-
graptus illustris Koren & Mikhailova.
The younger beds of the Zhalair Formation are eroded over most of the area studied (Fig. 4)
and they are exposed only in the lower Karasay River. There, in beds overlying the acuminatus
Zone, the graptolites Climacograptus miserabilis Elles & Wood, Glyptograptus sp. and abundant
Priblylograptus sp. and Atavograptus sp., characteristic of the vesiculosus Zone, were found. The
section is capped by strata yielding Climacograptus angustus (Perner), C. mirnyensis (Obut &
Sobolevskaya), C. normalis Lapworth, Pseudoclimacograptus (Metaclimacograptus) hughesi
(Nicholson), Coronograptus cyphus (Lapworth), C. gregarius (Lapworth), Monograptus revolutus
praecursor Elles & Wood, Atavograptus sp. and Dimorphograptus dessicatus Elles & Wood.
Shelly fauna is scarce in the Silurian part of the Zhalair Formation. In the acuminatus Zone
only a single trilobite of the family Odontopleuridae occurs (exposure 280). The Zhalair Forma-
tion is 51 to 133 m thick.
The Salamat Formation consists of green sandstones and siltstones with abundant graptolites
of the gregarius Zone. The overlying Betkainar Formation, with basal conglomerate beds,
transgresses deposits of different ages, including in places the Dalmanitina mucronata beds of the
Durben horizon (Figs 2, 4).
The Ordovician-Silurian boundary in the Chu-Ili Mountains is drawn at the base of the
acuminatus Zone, which is marked by the appearance of Akidograptus ascensus Davies, Glyp-
tograptus madernii Koren & Tzai, Orthograptus illustris Koren & Mikhailova and Diplograptus
modestus primus Mikhailova.
The Chokpar and Zhalair Formations reflect a distinct regressive-transgressive cycle (Fig. 5).
Dark pelitomorphic deposits of the Chokpar Formation (the supernus Zone) are comparatively
deep-water and might have accumulated in an extensive, open, flat-bottomed sea with a remote
source of terrigenous sediments. That sea was inhabited by diverse graptolites (more than 15
species). Towards the end of Chokpar time, the sea bed was elevated and a number of bio-
hermal chains were developed. Each bioherm had a trail of clastic carbonate material (the
Ulkuntas Limestones).
In early Durben time (the extraordinarius Zone), the areas of continuously growing elevation
were surrounded by thick beds chiefly consisting of limey coarse-grained sands (Fig. 6), and a
broad band of the fine dark Durben Limestones accumulated which were 40m thick near the
elevations and 0-5m thick further away. The areas of limestone sedimentation were inhabited
by a trilobite assemblage including Dalmanitina mucronata, D. olini and Platycoryphe sinensis.
In the deep-water limestones near the village of Karasay a single species of blind Dalmanitina
was found. Brachiopods are commonly represented by the single species Bracteoleptaena polon-
ica. The graptolite assemblage consists of 2 to 4 species, and all the fossils are large-sized,
numerous but taxonomically restricted. Late Durben time (the persculptus Zone) saw the depo-
sition of green fine-grained sandstones and cross-bedded siltstones with traces of turbidity and
slumping. The benthic fauna shows a greater diversity (the Hirnantia—Dalmanitina assemblage)
but the graptolites are limited to two to three species.
An abrupt increase in the supply of tuffaceous material coincided with the beginning of the
acuminatus Zone. A new and diverse (up to 15 species) graptolite assemblage appeared;
however, benthic faunas are almost unknown from this level. The cosmopolitan distribution of
APOLLONOV, KOREN, NIKITIN, PALETZ & TZAI
150
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Fig. 4 Chart showing a correlation of the Ordovician—Silurian boundary deposits in the Chu-Ili
Mountains. 1, black mudstones, siltstone and silty mudstones; 2, dark grey to black mudstone and
siltstone; 3, grey tuffaceous pelite, tuffaceous mudstone; 4, grey siltstone, mudstone, fine-grained
detrital
6,
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151
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Fig. 5 Chart showing the lateral variation of different lithogenetic types within the Ordovician—
Silurian boundary interval in South Kazakhstan. D, Durben Limestones; O, Ojsu Limestones; U,
Ulkuntas Limestones. 1, black mudstones; 2, green sandstone; 3, detrital thin-bedded micro-
crystalline limestone; 6, sandstone; 7, conglomerate and gritstone; 8, tuffite; 9, unconformity; 10,
non-deposition; 11, fossils: (a) trilobites, (b) graptolites, (c) brachiopods, (d) corals, (e) other fauna
groups.
the acuminatus graptolite assemblage may be due to the widespread early Llandovery transgres-
sion. A great crisis in graptolite evolution within the extraordinarius and persculptus Zones
took place at the end of the Ordovician regressive cycle.
The basal lower Silurian deposits (the acuminatus Zone) outside the Chu-Ili Mountains are
established in eastern Central Kazakhstan in the Otyzbes Mountains, near the Kombabasor
Lake east of the town of Bajanaul and at the watershed of the Akzhar—Zhartas Rivers north-
east of Karaganda (Bandaletov 1969; Apollonov et al. 1980; Fig. 1 herein).
The uppermost Ashgill deposits (the Dalmanitina mucronata beds of the Durben horizon) are
known from the Zhaksykon River basin at the Sarysu-Teniz watershed in the Chingiz Range
(near the town of Akdombak) and south-western Chingiz area (Nikitin 1972; Nikitin et al.
1980). The systemic boundary in the regions within the neritic development is defined by the
appearance of the diagnostic brachiopods Eospirifer cinghizicus and Holorhynchus cinghizicus
and tabulate corals (Borisyak et al. 1969; Nikitin 1972).
However, direct correlation between the graptolite and shelly sequences within the Silurian
basal beds is still not fully established, and the problem of the identification of shelly faunas
diagnostic of the acuminatus Zone remains open in Kazakhstan as elsewhere.
ORDOVICIAN-SILURIAN BOUNDARY IN SOUTH KAZAKHSTAN 153
Fig. 6 Schematic depositional patterns in the South Kazakhstan Palaeo-basin in the uppermost
Ordovician. A (supernus Zone): a, coarse sandstones; b, biohermal (Ulkuntas) limestones; c, detrital
(Ojsu) limestone; d, microitic (Ojsu) limestone; e, black (Chokpar) mudstones.
B (extraordinarius and persculptus Zones): a, coarse sands; b, biohermal limestones; c, thin-bedded
micritic (Durben) limestones with Dalmanitina association; d, fine sandstones with Dalmanitina—
Hirnantia association.
Arrows indicate the direction of transport of the clastic material.
154 APOLLONOV, KOREN, NIKITIN, PALETZ & TZAI
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Rukavishnikova, T. B., Tokmacheva, S. G. & Salin, B. A. 1968. Novye dannye po stratigrafii otlozhenii
verkhnego ordovika i nizhnego silura Chu-Ilijskikh gor [New evidence on the upper Ordovician—lower
Silurian stratigraphy in Chu-Ili Mountains]. Dokl. Akad. Nauk SSSR, Leningrad, 183: 420-423.
Williams, A., Strachan, I., Bassett, D. A., Dean, W. T., Ingham, J. K., Wright, A. D. & Whittington, H. B.
1972. A correlation of Ordovician rocks in the British Isles. Spec. Rep. geol. Soc. Lond. 3. 74 pp.
The Ordovician—Silurian boundary in Saudi Arabia
H. A. McClure
Arabian American Oil Company, Box 2376, Dhahran, Saudi Arabia
Synopsis
An account is given of the environments of deposition across the Ordovician—Silurian boundary which
occurs within the Tabuk Formation, Saudi Arabia. The results of much recent work are appraised and
earlier conclusions are reassessed with respect to it. The late Ordovician glaciation is considered to have
been a prime factor influencing sedimentation, for example by restricting land derived clastic input. There
appears to be no regionally significant depositional hiatus, and the beds about the boundary are best
regarded as conformable. The general environment of deposition was of prograding sandstones, tidal flats,
delta cycles and intermittent marine incursions on a tectonically stable structural platform. A basically
normal graptolite sequence is deduced across the boundary region, and a précis is given of the relative
dating achieved by these and other fossil groups.
Introduction
Early Palaeozoic rocks of the Arabian Peninsula are almost exclusively siliciclastics whose
primary source was erosion from the western part of the Precambrian Arabian Shield. These
rocks were successively deposited to the east along a regressive shoreline in fluvio-deltaic and
shallow water marine environments. The Ordovician—Silurian boundary in Saudi Arabia
occurs within the Tabuk Formation of this suite of rocks.
The Tabuk Formation was originally designated by R. A. Bramkamp in 1954 in an unpub-
lished report of the Arabian American Oil Company. His definition in amended form was
presented on U. S. Geological Survey Miscellaneous Geologic Investigations Map I-270A
(1963). The formation was formally defined by Powers et al. (1966). A summary of details of the
formation is given by Powers (1968).
The type section, in the Tabuk area of northwest Saudi Arabia (Fig. 1), consists of 1071 m of
shale, siltstone and sandstone, deposited in shallow water in a complex of fluviatile, littoral
beach, deltaic, and tidal flat sediments. Holomarine shale members, recording marine transgres-
sive phases, occur at the base, near the middle and near the top. These are designated, respec-
tively, the Hanadir, the Ra‘an, and the Qusaiba shales. However, in the vicinity of the type
section, only the basal member, the Hanadir, shows easily mappable lateral continuity.
Powers (1966) designated a reference section of 677-2m thickness in the Qasim (Qusaiba)
area (Fig. 1) which is a composite section from several excellent exposures in the vicinities of
Jebal Hanadir, Khashm Ra‘an, and in the Qusaiba depression. For the local definition of the
Ordovician-Silurian boundary this section is best, with the advantages that (1) all three holo-
marine shale members are well developed and well exposed, (2) all three shale members are
graptolite-bearing, (3) additional fossil collections, including graptolites and trilobites, have
been made in more recent years and serve to refine previous age assignments and strati-
graphical relationships within the formation on outcrop as well as in subsurface areas several
hundred kilometres to the east, and (4) glacial beds recording an ‘end-of-the-Ordovician’ glaci-
ation event and stratigraphical and sedimentary details have been recently studied in the area.
Fig. 2 shows a generalized stratigraphical section in the Qasim area.
Stratigraphy and sedimentation
Rocks of the Tabuk Formation were deposited in shallow water on a very broad and extensive,
gently sloping epicontinental shelf, reflecting the underlying basement structure of a nearly flat,
gently dipping, stable homoclinal platform (Powers et al. 1966). Present dips on outcrop
average less than 2° eastward and have been little disturbed since deposition. Graptolitic shales
Bull. Br. Mus. nat. Hist. (Geol) 43: 155-163 Issued 28 April 1988
156 H. A. MCCLURE
Tabuk Fm. Outcrop
SAUDI ARABIA
4
Arabian Sea
Fig. 1 Outcrop map of the Tabuk Formation in Saudi Arabia. Equivalent rocks on the surface and
in the subsurface have been found as far east as Oman.
and sands with other macrofossils and trace fossils as well as a palynomorph suite of chitin-
ozoans, acritarchs and plant spores occur on surface outcrop as well as in the deeper subsurface
section of the eastern part of Saudi Arabia and Oman. The Tabuk Formation gradually
thickens basinward to the east, where it is extensive in the subsurface, but, except that the three
marine shales tend to be less distinct as discrete units, no gross changes in facies or depositional
environment are apparent. Carbonate beds are known only as rare thin lenses at outcrop.
Lithologies of the Tabuk Formation comprise three basic types: (1) medium-grained, partly
cross-bedded, partly massive-bedded, channelled sandstone, (2) fine-grained, laminated and
ripple-marked, micaceous sandstones and siltstones, and (3) laminated and micaceous, fossil-
iferous shales, the Hanadir, the Ra‘an and the Qusaiba. The shales grade upwards through
siltstone interbeds at the top into type 2 lithology. Tabuk lithologies are thus arranged in three
generally coarsening-upward cycles that, together with the regional sedimentological and struc-
tural framework, suggest deposition in a prograding deltaic environment dominated by fluvial
sediment input. Lithology type 1 probably represents material derived from a fluvially-fed delta
plain and deposited in the distributary system of a delta front; type 2, fine sandstone and
siltstone, may have been deposited in intermediate mouth bars; and type 3 is considered to
represent a mud-dominated platform in the pro-delta, off-shore area, where holomarine condi-
tions prevailed. Each of the three cycles from bottom to top probably represents sand and silt
ORDOVICIAN-SILURIAN BOUNDARY IN SAUDI ARABIA Sy)
facies of a delta plain and delta front prograding during periods of eustatic stand-stills over
pro-delta muds, which were the product of intermittent, possibly eustatically controlled, marine
incursions.
Intertidal deposition as part of the delta plain appears to have been widespread, Skolithos
beds and tidalite sands being prominent towards the top of the Ordovician part of the Tabuk
Formation on outcrop as well as in the subsurface. A non-barred, tidally dominated, sandy
coastline was probably present, where extensive fluvially-dumped sediments were contempora-
neously reworked and redistributed during periods of active progradation.
Graptolite zonations, documentation of the glacial event, and sedimentary observations are
the principal aids available in the area to define the nature of the Ordovician-Silurian bound-
ary. Analysis of the Ra‘an and Qusaiba shales and the intervening sandstone is particularly
informative, since these units bracket the boundary. (The Hanadir, the basal shale member of
the Tabuk, of Llanvirn age, is not discussed here, except briefly in the section on bio-
stratigraphy below.)
The Ra‘an is the least distinctive and persistent of the three shale members. At the type
locality at Khashm Ra‘an (latitude 26° 52’ N, longitude 43° 23’ E), the lithology consists of 67m
of green-grey, silty, micaceous shale and fine-grained, red-brown, ripple marked, micaceous
sandstone and siltstone with trace fossils towards the top. Glacial beds are erratically associ-
ated with the top of the Ra‘an in many places at outcrop. Very rare graptolites, trilobites,
brachiopods and molluscs are concentrated in several thin zones at the bottom and top.
The range of the graptolite Orthograptus amplexicaulis, which occurs in the lower part of the
Ra‘an, is from the clingani Zone to the anceps Zone. Glyptograptus persculptus occurs at the top
of the Ra‘an and, although formerly considered to represent the lowest Silurian, is now taken as
uppermost Ordovician. The trilobites indicate a less precise age ranging from about the middle
Caradoc to about the late Ashgill. Overall considerations indicate the Ra‘an member at
outcrop is probably late Caradoc to the latest Ashgill, persculptus Zone, in age.
The sandstone overlying the Ra‘an, which is similar to the sandstone underlying it, is partly
cross-bedded, partly massive-bedded, medium-grained and occasionally channelled. This unit,
about 240m thick in the Qasim area, is probably lower Rhuddanian in age because of its
conformable position below the well-dated Aeronian Qusaiba shale and above the persculptus
Zone. It is generally barren of body fossils, but poorly preserved moulds of molluscs and
brachiopods (mostly lingulids) are sometimes present. Trace fossils such as Cruziana are
frequent.
The Qusaiba shale, like the Ra‘an, is erratically distributed along the length of the outcrop.
At its best exposure and type locality in the Qusaiba Depression (26° 53’N, 43° 34’E), it
consists of 57m of varicoloured, red and grey-green laminated shale with thin interbeds of
yellow shale, and red, hematitic, ripple-marked, micaceous and fine-grained sandstone with
trace fossils towards the top. A medium-grained, cross-bedded sandstone overlies the shaly-silty
interbedded unit. The Qusaiba is especially rich in graptolites, but also contains rare trilobites,
brachiopods and molluscs. Graptolites serve to date the Qusaiba as Aeronian, convolutus Zone.
Nature of the Ordovician—Silurian boundary
In the Arabian section, both on outcrop and in the subsurface to the east, the persculptus Zone
is present near the top of the Ra‘an shale. On the surface, persculptus occurs just below the
glacial beds. While this zone has not been documented above the glacial horizon on outcrop, in
the subsurface it occurs just above a diamictite suspected of being of glacial origin (Fig. 2).
The contact between the Ordovician and the Silurian, both at outcrop and in the subsurface
further to the east, is apparently conformable. Nothing appears to have happened across the
boundary that drastically altered the depositional mode characteristic throughout the Tabuk
Formation of prograding sands, delta cycles, and intermittent marine incursions on a tectoni-
cally stable structural platform. Within the Ra‘an, however, extreme cold water conditions were
apparently manifested in an impoverished fauna, and local glacial activity took place within the
top part of the Ra‘an. Fluvioglacial channels, tillite deposits, striated and faceted megaclasts,
158 H. A. MCCLURE
exotic igneous boulders, pro-glacial sandstone, and other evidence of glaciation occur in this
part of the section (McClure 1978; Young 1981). This event is assumed to be approximately
coeval with glaciation at this time in north Africa (Beuf et al. 1969; Hambrey & Harland 1981).
The glaciation in Saudi Arabia is confined within the top part of the Ra‘an, apparently
within the persculptus Zone, but is ice-marginal and ice-contact and not glacio-marine. Sub-
aerial exposure due to sea level drop at the maximum of glaciation during the later phase of the
Ashgill probably occurred. Thus, super-imposed upon the Ra‘an is a subsidiary sequence of
events composed of (1) glacio-eustatic sea level regression, during which glaciation took place,
(2) deglaciation during which glaciofluvial and fluvial sands were deposited, finally followed by
(3) glacio-eustatic sea level rise, during which the upper part of the persculptus Zone shale was
deposited. Sea level dropped again toward the beginning of the Silurian and regressive sands
were deposited in Rhuddanian time. In later Llandovery (Aeronian) time, a marine transgres-
sion apparently unrelated to glacial events deposited the Qusaiba shale. The glacio-eustatically
controlled regressive—transgressive sequence at the top of the Ra‘an may be synchrononous
with similar world-wide events such as those documented by Brenchley & Newall (1980) at the
end of the Ordovician in the Oslo region, Norway, and those proposed by Berry & Boucot
(1973). The Ordovician—Silurian boundary in Saudi Arabia may thus be taken at the base of the
sandstone unit between the Ra‘an and Qusaiba shales, or above the persculptus Zone.
Lithofacies to the east in the deep subsurface vary little from the outcrop sequence, except
that the Ra‘an as a discrete shale unit with easily determined top and base is not always present
and the sandstone of presumed Rhuddanian age between the Ra‘an and the Qusaiba at outcrop
is poorly developed. The contact between the Ra‘an and the Qusaiba is consequently indistinct,
and the Qusaiba sequence is considerably thicker. A distinctive feature of the subsurface is a
regionally persistent and prominent, thin, highly organic, pyritic euxinic black shale, often
bearing common or abundant Glyptograptus persculptus with no benthic fossils and overlying a
sandstone with diamictite suspected of being equivalent to the glacial tillite of outcrop. This
shale may be equivalent to the post-glaciation upper part of the persculptus Zone of outcrop
mentioned above and helps place the glaciation event as within the persculptus Zone.
The graptolite succession of the deep subsurface requires further study, but appears similar to
that of the outcrop. Several differences are that graptolites assignable to the clingani to anceps
Zones as found at the base of the Ra‘an on outcrop have not been documented in the sub-
surface, and a graptolite suite assignable approximately to the boundary between the magnus
and leptotheca Subzones of the gregarius Zone has been recovered in one drill hole. The most
perplexing anomaly, however, is that, in another representative drill hole with continuous core
sequence, a convolutus Zone graptolite suite occurs within about 6m of the euxinic persculptus
Zone shale. Intervening graptolite zones of the lower Llandovery therefore appear to be largely
missing or drastically telescoped. (See Note, p. 163).
The ‘missing’ graptolite zones are assumed to be represented on outcrop by the non-
fossiliferous Rhuddanian sandstone and their apparent absence in the deeper section where this
sandstone is not present and shales were continuously deposited is puzzling. However, these
zones are also ‘compressed’ in some standard British successions (R. B. Rickards, personal
communication) and lower Llandovery marine fossils are rare on a worldwide basis (Berry &
Boucot 1973). The apparent gap in the graptolite succession of Saudi Arabia is probably not
due. to events peculiar to the Arabian platform. It is tempting to consider the euxinic black
shale as well as the condensing or absence of the early Llandovery graptolite zones as in some
way related to the glaciation event. Cessation or drastic restriction of fluvial flow regimes and
consequent constriction of clastic input due to tie-up of water in glacial ice may have resulted
in stagnant, euxinic conditions in more distal intra-platform areas on what was already a cold
water, carbonate-starved platform. Fig. 2 presents outcrop and subsurface correlations within
the Tabuk Formation.
Thus, a firmer calibration of a time scale with depositional and climatic events and faunal
occurrences is essential to define more precisely the nature of the Ordovician-Silurian bound-
ary on the Arabian platform and correlate it with sequences elsewhere. The evidence accumu-
lated to date, however, is informative, and the following conclusions can be tentatively made.
ORDOVICIAN-SILURIAN BOUNDARY IN SAUDI ARABIA 159
W E
Outcrop 600km +» Subsurface
Graptolite Zones
, Graptolite Zones
convolutus
Llandovery
SILURIAN
— convolutus >
euxinic sh. persculptus
el
a a ees = aoe ee aed cf, )| COCO aCO CON |
glacial sediments CO=GIEGIENIEN CamEEone * fp .€ Os glacial sediments
persculptus
Caradoc~—Ashgill
clingani-anceps
Llandeilo
murchisoni
2
<
O
>
ie)
fa)
c
e)
murchisoni
Llanvirn
Shale Sandstone
Fig. 2 Section comparing the Tabuk Formation in the outcrop of NE Saudi Arabia (left) with that
in the subsurface to the east (right). The three shale horizons at outcrop are termed Hanadir
(Llanvirn), Ra‘an (Caradoc to basal Llandovery) and Qusaiba (Middle Llandovery). The Idwian
Stage shown is now regarded as the lower part of the Aeronian Stage. The base of the Tabuk
Formation is at the base of the Llanvirn.
1. Rates of sediment deposition may have varied on the Arabian platform across the
Ordovician-Silurian boundary and can probably be attributed to the effects of glaciation.
Land-derived clastic input may have been drastically restricted, resulting in euxinic, starved and
stagnant areas, but:
2. No regionally significant depositional hiatus is evident and the contact between the Ordo-
vician and Silurian may be considered conformable.
3. Nothing except glaciation happened at the boundary to upset significantly the mode of
deposition characteristic throughout the Tabuk Formation of prograding sandstone, tidal flats,
delta cycles and intermittent marine incursions on a tectonically stable structural platform.
4. The graptolite succession across the boundary appears normal, the apparent gap of early
Llandovery graptolite zones being probably attributable to world-wide events and not intra-
platform activity.
5. The significant boundary event on the Arabian platform appears to have been the glaci-
ation at the end of the Ordovician that affected sedimentation rates and faunal suites.
160 H. A. MCCLURE
Biostratigraphy
Early fossil collections listed by Powers et al. (1966) and Powers (1968) were sparse. Additional
surface collecting in more recent years in the Qasim (Qusaiba) area has provided fossils that
serve to refine previous age assignments as well as to reveal more about the palaeobiology of
the Tabuk Formation and faunal events across the Ordovician-Silurian boundary. Drill hole
cores available in recent years from the deep subsurface of the eastern part of Saudi Arabia,
where rocks across the boundary are extensively distributed, also provide useful information.
All three shale members of the Tabuk Formation, the Hanadir, the Ra‘an, and the Qusaiba,
are fossiliferous holomarine shales deposited as pro-delta muds. Intervening sands are largely of
tidalite and shoreface origin and are mostly unfossiliferous of body fossils, but frequently
contain trace fossils including Skolithos and Cruziana. Thin siltstone beds near the tops of the
shales rarely contain poorly preserved moulds of bivalves, brachiopods (lingulids) and tri-
lobites. All three shale units contain graptolites, trilobites, and an assortment of benthic fauna
in addition to palynomorphs (chitinozoans, acritarchs, and spores).
Except for graptolites, trilobites, and palynomorphs, the fossil suite has been little studied.
R. B. Rickards has been working with the graptolites in recent years; Thomas (1977), Fortey &
Morris (1982) and El-Khayal & Romano (1985) have studied some of the trilobites, H. A.
McClure is working on chitinozoan and acritarch suites and J. Gray, A. J. Boucot and H. A.
McClure are currently investigating spores of possible land plant affinity. The following
analysis should be considered preliminary. The following lists are comprehensive compilations
from both outcrop sequences (Qasim area) and cored holes of the subsurface to the east.
Though not strictly pertinent to the boundary problem, the fossils of the Hanadir shale at the
base of the Tabuk Formation are also listed. The Hanadir at its type section (26° 27’N, 43°
27’ E) consists of 60m of varicoloured, laminated, micaceous shale, with thin, red-brown, ripple
marked siltstone and fine grained sandstone at the top. Fossils of the Hanadir include:
Graptolites: Didymograptus murchisoni murchisoni (Beck), D. murchisoni geminus (Hisinger),
D. pakrianus Jaanusson, D. aff. D. acutus Ekstrom, Amplexograptus cf. A. coelatus (Lapworth),
A. sp. Trilobites: Neseuretus tristani (Desmarest), Plaesiacomia vacuvertis Thomas, ?Marrolithus
sp. Cephalopod: Orthoceras sp. Brachiopods: ?Monobolina sp. and other articulate species and
unidentified lingulids. Molluscs: ?Glyptarca sp., unidentified bivalves, unidentified gastropods.
Beyrichids and other unidentified ostracodes; unidentified conodonts and palynomorphs
(chitinozoans, acritarchs, spores, and scolecodonts). Based mainly on the graptolites, the
Hanadir is Llanvirn in age, murchisoni Zone.
The Ra‘an shale contains the following fossils, derived mainly from several thin zones at the
base and toward the top from outcrop and from cores of the subsurface: Graptolites: Glyp-
tograptus persculptus (Salter) s.s., Orthograptus amplexicaulis Hall s.s., Orthograptus sp. nov.,
Diplograptus sp., Climacograptus angustus/normalis, ?Dictyonema sp., ?Climacograptus misera-
bilis and ?Diplograptus modestus. Trilobites: Kloucekia sp. and Onnia sp. Brachiopods: Com-
atopoma sp. or Hirnantia sp., others (mostly lingulids). Molluscs: unidentified gastropods and
bivalves and the cephalopod Orthoceras sp.; unidentified conodonts and palynomorphs
(chitinozoans, acritarchs, spores, and scolecodonts).
The range of Orthograptus amplexicaulis is from the clingani to the anceps Zones. Glyp-
tograptus persculptus places the top part of the Ra‘an in the persculptus Zone.
The Qusaiba shale contains the following: Graptolites, Suite 1: Climacograptus scalaris
(Hisinger), C. aff. C. rectangularis Tornquist, Glyptograptus aff. G. incertus Elles & Wood, G.
tamariscus tamariscus (Nicholson), G. (Pseudoglyptograptus) sp., Lagarograptus sp., Mono-
graptus capis Hutt, M. communis Lapworth, M. convolutus (Hisinger), M. decipiens Tornquist,
M. gregarius gregarius Lapworth, M. lobiferus (M‘Coy), M. cf. M. delicatulus (Elles & Wood),
M. ex gr. tenuis (Portlock), Orthograptus cyperoides Tornquist, Petalograptus ovatoelongatus
(Kurk), P. sp., Pristiograptus regularis (T6rnquist), Pseudoclimacograptus (Clinoclimacograptus)
retroversus Bulman & Rickards, P. (Pseudoclimacograptus) sp. nov., Rastrites spina Richter,
Retiolites perlatus (Nicholson), Rhaphidograptus tornquisti Elles & Wood. Graptolites, Suite 2:
Climacograptus tamariscus s.l., Coronograptus gregarius cf. C. minisculus Obut & Sobolovskaya,
ORDOVICIAN-SILURIAN BOUNDARY IN SAUDI ARABIA 161
Climacograptus cf. C. rectangularis, Diplograptus cf. D. magnus, Monograptus lobiferus (M‘Coy),
Pristiograptus ?concinnus. Trilobite: Platycoryphe dyaulax Thomas. Bivalves: Nuculites, among
others. Bellerophontids, unidentified gastropods; the cephalopod Orthoceras sp.; brachiopods:
‘Camarotoechia’ and other unidentified articulates. Unidentified conodonts and palynomorphs
(chitinozoans, acritarchs, spores, and scolecodonts); ostracodes, Tentaculites, ophiuroids and
fish remains.
On graptolite evidence of Suite 1, the outcrop of the Qusaiba is Llandovery, Aeronian Stage,
convolutus Zone. A slightly older zone in the subsurface is represented by Suite 2, from the
gregarius Zone, approximately on the boundary between the magnus and leptotheca Subzones,
still within the Aeronian.
Palaeoecology and Palaeobiogeography
The fossil content of the Tabuk Formation was the product of a remarkably stable
environment and static physical conditions for a considerable period of time. Two kinds of
faunal association are represented in the Tabuk suite: (1) planktic, with graptolites, chitin-
ozoans and acritarchs, and (2) level-bottom benthic, with an epifauna of what were probably
mostly vagrant shelly benthos such as brachiopods, molluscs, trilobites and ostracodes. Other
taxa such as conodonts, scolecodonts and ophiuroids are also represented. Fine layering and
lamination and lack of bioturbation of the shales indicates that a significant infauna was
probably not present. In general terms, population densities were high for the planktic level and
low for the benthic. Diversity was moderately high for the graptolites and very high for the
chitinozoans and acritarchs. Shelly benthics identified to date indicate a low diversity.
Continuity of the Tabuk suite extends for hundreds of kilometres, the fossils known from
cored sequences in deep drill holes in the subsurface further to the east do not differ signifi-
cantly from those of outcrop. There are no obvious indications that any element of the Tabuk
biota is allochthonous.
In the shales of the Tabuk, graptolites are common but of low diversity in the Hanadir, rare
and of low to moderate diversity in the Ra‘an, and abundant and of high diversity in the
Qusaiba. Molluscs (especially bivalves), brachiopods, trilobites and ostracodes are the next
most common taxa, occurring in about equal abundance and approximately equal diversities.
The shelly benthos is certainly not brachiopod-dominated as in more northern biogeographic
realms. Conodonts occur in all three shale members, but are very rare and to date very little is
known about them. Scolecodonts occur as a minor part of the palynomorph suites. Chitin-
ozoans and acritarchs are common to abundant and of high diversity in the Hanadir, compara-
tively rare and of comparatively low diversity in the Ra‘an and abundant and of high diversity
in the Qusaiba. Spores, including tetrahedral tetrads that possibly represent early vascular land
plants, are rare to common in the palynomorph suites. Ophiuroids are very rare; only several
specimens of less than 0:5cm size are recorded from the Qusaiba. Tentaculites occurs rarely in
the Qusaiba. Orthoceras is rare but ubiquitous in all three shales, being more common and
robust in the Qusaiba. In one limited locality, near the base of the Ra‘an, it is concentrated in
small planoconvex lenses of calcareous debris associated with algal nodules. (This is the closest
resemblance to the Orthoceras limestone lenses recorded as widespread in the Silurian of north
Africa by Berry & Boucot, 1973. The Arabian occurrence possibly represents a storm event.)
All the shelly benthic species are small, brachiopods and molluscs being rarely more than one
centimetre in maximum dimension. Shelly specimens appear to have been weakly calcified or
subjected to early dissolution. Most of the material is composed of moulds of the composite
type on poorly defined bedding planes and laminae. As in the case of composite-type moulds
(McAlister 1962), fine interior and external morphological features are often well preserved.
Although taxonomic diversity is generally maximised in shallow marine environments (Boucot
1981), this is not the case with the Tabuk fauna. The condition of the shelly benthics of the
Tabuk shales may indicate the influence of low salinity, but cold-water conditions (especially
marked during the glaciation at the end of the Ordovician) was most likely the over-riding
control. Fortey & Morris (1982) regard the trilobite genus Neseuretus, present in the Llanvirn
(Hanadir) of the Tabuk, to be a reliable and sensitive indicator of inshore facies in cold water.
162 H. A. MCCLURE
Planktics do not appear to have been affected by some cold, but were clearly affected by the
excessive glacial cold conditions at the end of the Ordovician. An extensive platform covered
with hyposaline water can exist if an adjacent continent has a river system adequate to provide
a steady influx of fresh water. In such environs today, there is a low taxic diversity, and there is
no reason to think that extensive river regimes of the past flowing off large land masses may
not have had the potential for producing similar hyposaline environments with appropriate
faunal consequences (Boucot 1981). This condition may have prevailed on the Arabian plat-
form during Ordovician and Silurian times. Outcrop sequences of the Tabuk sands, silts, and
shales are oxidized to shades of red, pink, yellow and green. However, subsurface equivalents
invariably range from light grey to dark grey and black. Tidalite sands are especially rich in
carbonaceous laminae, each lamina possibly representing nutrient material transported by a
single tidal event. Tabuk shales in the subsurface are usually medium grey to dark grey and
black, the extreme case being the black, highly radioactive, ‘sooty’ shale of the subsurface
persculptus Zone.
Some of the palynological samples yield a distinct tetrahedral tetrad type of suspected land
origin. This kind of evidence for vascular land plants may be recorded as early as the Llanvirn
(Hanadir shale) in the Arabian Tabuk section. Berry & Boucot (1973) suggest that a black,
radioactive shale at the ‘base of the Silurian’ in north Africa could be due to blooms of plants in
mud flats and lagoons at this time. An apparently synchronous event occurs across much of
north Africa and Arabia. A readily accessible and presumably useable supply of nutrients might
therefore be assumed for both planktic and bottom benthics. Nutrient kind and availability
may have been a significant factor in the palaeoecology of chitinozoans and acritarchs espe-
cially, and perhaps also graptolites.
Temperature is probably the most important variable affecting both plant and animal dis-
tribution of the present and continental glaciation episodes of the end of the Ordovician and
Permian—Carboniferous are associated with global diversity gradients (Boucot 1981). The
change in faunal composition associated with the Ordovician glaciation is now well docu-
mented (Berry 1973; Berry & Boucot 1973). A Silurian warming followed the Ordovician cold
in the area and this may be reflected in taxa of the Qusaiba fossil suite being relatively more
diverse and populations denser, especially planktic ones.
In summary, the Tabuk palaeogeography and palaeoenvironment was probably that of a
broad pro-delta mud substratum on a shallow-water, clastic-fed, carbonate-starved, tectonically
stable platform area, with sediment and high nutrient input derived from a low, rapidly eroding
landmass, possibly with primitive plant cover, and transported via extensive fluvial, tidal and
deltaic systems. Conditions of low salinity and cold-water temperatures probably controlled
diversity and density of parts of the faunal community. Conditions may be considered to have
been optimum for planktic taxa such as chitinozoans and acritarchs, favourable for graptolites,
and generally unfavourable for benthics.
Lovelock et al. (1981) record chitinozoans and acritarchs, trace fossils, ?dalmanellid brachio-
pods, and the trilobite ?Neseuretus from Early and Middle Ordovician rocks of the Amdeh
Formation of Oman. Rocks of southern Jordan of age equivalent to the Tabuk Formation are
sandstones, shales and siltstones bearing graptolites, brachiopods, bivalves and gastropods,
nautiloids, Conularia and trace fossils such as Cruziana and Skolithos (Bender 1975). Exact
equivalents in these two areas to individual units of the Tabuk Formation, the precise defini-
tion of the Ordovician—Silurian boundary, and the comparison with the Tabuk fauna remain
yet to be worked out.
Conclusions
Pending further study and documentation of the palaeobiology of Arabian Ordovician-Silurian
fossil suites, the following conclusions are presented:
1. The fossils of all three Tabuk shales are similar in composition, diversity, population
density and abundance levels and may be taken to represent one community.
2. Two trophic levels are readily identifiable: (a) planktic—consisting of graptolites, chitin-
ozoans and acritarchs, and (b) benthic—consisting largely of vagrants on a flat-bottom mud
ORDOVICIAN-SILURIAN BOUNDARY IN SAUDI ARABIA 163
substratum.
3. Water temperature was probably the main environmental control on the community.
4. Salinity was possibly a secondary control on the community.
5. Nutrient material was readily available and may have played a significant role in the
palaeocology.
6. An extensive pro-delta mud platform provided the main palaeogeographical control for
the bulk of the Tabuk fauna; inshore sandy facies and tidal flats were less important features.
7. The main event that affected the biological community across the Ordovician—Silurian
boundary was stress imposed by glaciation at the end of the Ordovician. Otherwise, the
conditions that affected the community throughout deposition of the Tabuk were also oper-
ative in boundary times.
8. Similarities in the palaeobiology, palaeogeography and palaeoecology occur in the plat-
form rocks of the north African Silurian sections.
References
Bender, F. 1975. Geology of the Arabian Peninsula—Jordan. Prof. pap. U.S. geol. Surv., Washington,
560-1: 1-36.
Berry, W. B. N. 1973. Silurian—Early Devonian graptolites. In A. Hallam (ed.), Atlas of Palaeobiogeog-
raphy: 81-87. Elsevier Sci. Publ. Co.
& Boucot, A. J. 1973. Glacio-eustatic control of Late Ordovician—Early Silurian platform sedimen-
tation and faunal changes. Bull. geol. Soc. Am., New York, 84: 275-284.
Beuf, S., Biju-Duval, B., Stevaux, J. & Kulbicki, G. 1969. Extent of ‘Silurian’ glaciation in the Sahara: its
influences and consequences upon sedimentation. In W. H. Kanes (ed.), Geology, Archaeology and
Prehistory of the southwestern Fezzan, Libya. Ann. Field Conf., Pet. Explor. Soc. Libya, 11th: 103-116.
Boucot, A. J. 1981. Principles of Benthic Marine Paleocology. 463 pp. New York, Academic Press.
Brenchley, P. J. & Newall, G. 1980. A facies analysis of Upper Ordovician regressive sequences in the
Oslo Region, Norway: a record of glacio-eustatic changes. Palaeogeogr. Palaeoclimat. Palaeoecol.,
Amsterdam, 31: 1—38.
Carney, R. S. 1981. Nutrients. In A. J. Boucot (ed.), Principles of Benthic Marine Paleoecology: 136-142.
New York.
El-Khayal, A. A. & Romano, M. 1985. Lower Ordovician trilobites from the Hanadir shale of Saudi
Arabia. Palaeontology, London, 28: 401-412, pl. 47.
Fortey, R. A. & Morris, S. F. 1982. The Ordovician trilobite Neseuretus from Saudi Arabia, and the
palaeogeography of the Neseuretus fauna related to Gondwanaland in the earlier Ordovician. Bull. Br.
Mus. nat. Hist., London, (Geol.) 36 (2): 63-75.
Hambrey, M. J. & Harland, W. B. (eds) 1981. Earth’s pre-Pleistocene Glacial Record. 1004 pp. Cambridge.
Lovelock, P. E. R., Potter, T. L., Walsworth-Bell, E. B. & Wiemer, W. M. 1981. Ordovician rocks in the
Oman Mountains: the Amdeh Formation. Geologie Mijnb., Den Haag, 60: 487—495.
McAlister, A. L. 1962. Mode of preservation in early Paleozoic pelecypods and its morphologic and
ecologic significance. J. Paleont., Tulsa, Ok., 36: 69-73, pl. 16.
McClure, H. A. 1978. Early Paleozoic glaciation in Arabia. Palaeogeogr. Palaeoclimat. Palaeoecol.,
Amsterdam, 25: 315-326.
Powers, R. W. 1968. Lexique Stratigraphique International 3 Asie (10b 1: Arabie Saoudite). 177 pp. Paris,
C.R.N.S.
—, Ramirez, L. F., Redmond, C. D. & Elberg E. L. jr 1966. Geology of the Arabian Peninsula:
Sedimentary Geology of Saudi Arabia. Prof. Pap. U.S. geol. Surv., Washington, 560-D: i—vi, DI-D147.
Thomas, A. T. 1977. Classification and phylogeny of homalonotid trilobites. Palaeontology, London, 20:
159-178.
Young, G. M. 1981. Early Palaeozoic tillites of the northern Arabian Peninsula. In M. J. Hambrey & W.
B. Harland (eds), Earth’s pre-Pleistocene Glacial Record. 338 pp. Cambridge.
Note added in page proof. The atavus Zone (Rhuddanian) has recently been documented in the
Arabian Silurian section. Atavograptus atavus (Jones) is present in both the Tabuk area of
outcrop and the deep subsurface of eastern Saudi Arabia. In the outcrop section, it occurs with
Climacograptus normalis Lapworth.
The Ordovician—Silurian boundary in Morocco
J. Destombes and S. Willefert
Direction de la Géologie, Ministére de l’Energie et des Mines, B.P. 6208, Rabat-Instituts,
Morocco
Synopsis
At only one locality, Moulay bou Anane, in Jbilet, the persculptus and acuminatus Zones are both found,
although the acuminatus Zone is known from many localities throughout Morocco. The early Llandovery
usually consists of transgressive shales, ranging from acuminatus Zone to cyphus Zone and above in age,
overlying usually unfossiliferous glacial sandstones and microconglomerates of the latest Ordovician, from
one of which a Hirnantia fauna is recorded.
General survey
The Ordovician-Silurian boundary in Morocco is always marked by a very acute change of
facies between the two systems. The glacial episode which concludes the Ordovician deposited
relatively coarse material, such as sandstones, quartzites and microconglomeratic clays, which
strongly contrast with the fine argillaceous or siliceous deposits which characterize the begin-
ning of the Silurian. Consequently, the scenario is one of more or less important interruption in
sedimentation, the development of glaciogenic sediments, and the transgressive development of
a Silurian sea after the melting of the continental ice sheet. Under these conditions, the faunas
of the two systems are naturally different, apart from the single exception of Jbilet, at Moulay
bou Anane (Locality 1, of Fig. 1), where selected graptolites for the official boundary (Cocks
1985), Glyptograptus persculptus Salter and Akidograptus acuminatus (Nicholson), succeed each
other in the same section. Elsewhere, only A. acuminatus dates the beginning of the Silurian
above more or less terminal beds of the Hirnantian:
(1) in the western Anti-Atlas, at Ain Oui n’Deliouine (Locality 2);
(2) in the eastern Anti-Atlas, at Tizi ou Mekhazni (Tizi Ambed) (Locality 3) and at Oued
Bou-Leggou (Oued bou Oubagou) (Locality 3’);
(3) on the northern slope of the central High Atlas, at Ghogoult (Locality 4) and west of
Tiwghaza (Locality 4’);
(4) in the substratum of the Plateau des Phosphates (Locality 5);
(5) in the Moroccan central massif in the Azrou area, at Bou Ourarh (Locality 6);
(6) in the Palaeozoic inliers of the north of the middle Atlas at Tazekka (Locality 7) and
Immouzer du Khandar (Locality 8).
Some other outcrops of the transgressive Silurian are still later Rhuddanian:
(a) in the central Anti-Atlas, at Rich Mel’Alg, where graptolitic beds with Cystograptus
vesiculosus (Nicholson), Dimorphograptus, and Coronograptus cyphus (Lapworth) are separated
by a red layer from sandstones and clays of the Deuxieme Bani (Upper Ashgill);
(b) in the coastal Meseta, at Oulad Said, south of Casablanca, where Atavograptus atavus
(Jones) occurs in a boring;
(c) in the Qasbat-Tadla-Azrou area, at Jbel Eguer-Iguiguena, where the same association as
in (a) occurs.
For (b) and (c) it is not possible to determine with precision the age of the underlying beds.
The very widespread Silurian in Morocco more generally begins either with Aeronian
beneath a siliceous facies alternating with phthanitic ribbons, more sandy in the Anti-Atlas at
the east of the meridian of Icht, or sometimes with argillaceous—siliceous Telychian, or, in rare
cases, with the upper Wenlock and/or Ludlow.
Bull. Br. Mus. nat. Hist. (Geol) 43: 165-170 Issued 28 April 1988
J. DESTOMBES & S. WILLEFERT
166
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Description of partial sections
(1) Eastern Jbilet, Moulay bou Anane, Locality 1 (Topographical Sheet Attaouia ech Chaibia,
1:50000, at x = 322, y = 157-2) (Fig. 1). Roch (1939) described this area as forming part of the
‘Mountains to the East of Marrakech’. Huvelin (1977) emphasized the peculiar features of the
Hercynian massif of Jbilet. Huvelin and others refined the section near the boundary in 1980.
Roch only pointed out that “Miss G. Elles and G. Waterlot have recognised: Monograptus (sic)
modestus, M. sandersoni, M. cyphus, M. revolutus, Glyptograptus incertus, G. persculptus, Cli-
macograptus Tornquisti and Cl. normalis from the base of Llandovery’ (p. 113). Specimens of
Glyptograptus persculptus, determined by G. Elles, were obtained from a siliceous sandstone,
weathered pink-beige, but more greyish on fresh fracture, in beds on which they are nearly
orientated. They are of great size, the septum always starting at the fourth or fifth theca, and
they are preserved as internal moulds, in whole or half relief.
Vertical section 1 summarizes more recent collections. The usual suite of terminal Hirnantian
occurs over 20m and consists of microconglomeratic clays, argillites, and sandstones with
orientated sedimentary features. This is followed by a layer of quartzose sandstone not much
different from those of Roch, but coarser, which yields dispersed G. persculptus with a few more
irregularly orientated and smaller forms with a septum beginning at a lower level (in the third
theca when visible). They are always internal moulds and are apparently narrower than those
identified by G. Elles, but they show more relief. The thickness of the layer is 30cm and it can
be presumed that the Roch assemblage is rather nearer the top than the base.
Above this coarse facies, and without transitional beds, pink and pink-beige shales with a
little mica and with a very fine cleavage, contain at their base: Climacograptus normalis
(Lapworth), C. miserabilis Elles & Wood, C. rectangularis (M‘Coy), Diplograptus modestus
Lapworth, Akidograptus ascensus Davies and A. acuminatus (Nicholson). The thickness of this
argillaceous level is 30cm and occurs below alternations between more phthanitic beds and
more or less siliceous clays which terminate the Rhuddanian. The boundary is therefore very
sudden and with a sharp change of facies.
(2) Western Anti-Atlas, Ain oui n’Deliouine, Locality 2 (Topographical Sheet Tiglit, 1:50000, at
x = 1076-4, y = 764-2). The boundary was figured in some detail in Destombes et al. (1985: 242,
fig. 46). Above green microconglomeratic strata representing the glacial upper Ordovician, a
red bed makes a clear transition with argillites which are very similar in colour, although a few
are greener, and shows the same alteration and preservation for fossils as at Moulay bou
Anane, although the cleavage is coarser. At the contact there is C. normalis and D. modestus
and two metres above a single, small, aseptate specimen of G. persculptus, together with C.
normalis, C. transgrediens Waern, D. modestus, and A. acuminatus.
The similarities between the two areas are striking for the early Silurian beds and, from the
palaeontological point of view, the abundant D. modestus shows some intraspecific variations
which recall Davies’s (1929) considerations on the similarities of G. persculptus and D. modestus,
and whether it is a case of convergence or of real relationship. Internal moulds in iron-oxides
only emphasize, once again, all the pitfalls in determining deformed graptolites by comparison
with material which has preserved its proteic skeleton. Finally, from these two localities, which
appear to be the most characteristic of those actually known from Morocco, it is difficult to
imagine any Ordovician-—Silurian boundary without a break.
(3) Eastern Anti-Atlas, Tizi ou Mekhazni (Tizi Ambed), Locality 3 and Bou Leggou (Oued Bou
Oubagou), Locality 3’. A peculiar feature of the sections near the boundary is the presence,
above conglomeratic sandstones and quartzites and lenses with very coarse green and pink
sandstones, of a green siltstone with a very probable hard ground between the two deposits.
(a) At Tizi ou Mekhazni (Topographic Sheet Erfoud, 1:100000, at about x = 588-8,
y = 73-8), Destombes et al. (1985: 257-258, figs 54 and 55) report 10m of greenish silts followed
by a black marker bed about 10 m thick of very fine silicified slates with tuff layers, followed by
75m of fine silicified white, pink and reddish violet slates, the base of which includes C.
normalis, C. transgrediens, D. modestus, A. ascensus?, and A. acuminatus in the first 5m. The
168 J. DESTOMBES & S. WILLEFERT
Rhuddanian and the Aeronian continue up to the M. sedgwickii Zone within 125m of siliceous
sandstones, sometimes in plaquettes which weather to a very dark ferrugineous colour, but
lighter on splitting.
(b) At Bou-Leggou (Topographical Sheet Erfoud, 1:100000, at about x = 589-2, y = 56-6),
the Rhuddanian includes about 60m of green silts which contain nine levels with classic
climacograptids (Cl. normalis, transgrediens, praemedius Waern, medius and rectangularis),
which are sometimes crossed by small sandy nodular structures. Towards the top, at the
transition with siliceous shales, Dimorphograptus confertus Lapworth and D. confertus cf. swan-
stoni Elles & Wood are found, showing a difference in thickness for the first part of the Silurian
between the two localities. No trace of the black marker bed can be seen at Bou Leggou.
These sections give rise to a problem in the appreciation of the precise age for the base of the
silts. However, given the usual conditions of sedimentation between the end of the Ordovician
and the first Silurian and the fact that there is no proof of A. acuminatus at the beginning of its
biozone, one can, for cartographical purposes, take the Silurian as beginning with the silts. It
remains to analyze the mineralogy of the black marker beds, and perhaps also the siliceous
shales, to see whether they reflect volcanic activity, even if only very distant from this district of
the eastern Anti-Atlas.
(4) On the northern slope of the central High Atlas, at Ghogoult (Locality 4) and east of Tiwghaza
(Locality 4’). The important Hercynian tectonics which are manifest in the central High Atlas,
formerly known as the ‘Mountains to the East of Marrakech’ (Roch 1939) or ‘Demnate Atlas’
(Lévéque 1961), do not enable us to establish a sure succession for the boundary in this part of
Morocco. The Silurian with A. acuminatus is present in the allochthonous inliers of Ait Mallah
and Ait Mdioual (geological map Azilal 1:100000, 1985) and in the autochtonous deposits to
the west of Tiwghaza (boundary of topographical sheets Telouat and Skoura 1:100000).
In Ait Mallah, C. normalis, D. modestus, A. acuminatus, C. vesiculosus, Monograptus revolutus
s.l. (Kurck), Pribylograptus incommodus (Tornquist) and A. cf. atavus have been identified; in Ait
Mdioual, only the lower third in argillaceous or argillaceous-siliceous shales, with a very thin
cleavage (overlain by drier, resonant shales, sometimes with drifted micas), and higher coarser
beds with C. cyphus. The relations with the Ordovician cannot be defined since the earlier
Silurian ‘constitutes a level of preferential disharmony’ (Jenny & Le Marrec 1980).
West of Tiwghaza, D. modestus, A. acuminatus and C. vesiculosus are recognized from the
base of the first 5m of sandy, coarse, micaceous shales underlying siliceous and phthanitic ones
of the Llandovery succession. Jenny & Le Marrec (1980) described the last three metres of the
upper Ordovician as composed of classic ‘massive or irregular decimetrical sandstones-
quartzites, sometimes with oscillation-ripples, whitish colour with dark patina and black micro-
brechic or microconglomeratic sandstones or clays with round and matt quartz’.
(5) In the substratum of the Plateau des Phosphates (Locality 5). An oil-boring—BJ 105—on the
geological map Qasbat Tadla (1:100000, 1985, at x = 417-7, y = 216-8) terminated at a depth
of 1017m in the upper Ashgill. In a fragment of core between 963 to 988-5 m, in an argillaceous,
graphitic, more or less siliceous facies, the lowest associations contain: (a) more argillaceous
than siliceous beds with many slip planes with C. normalis, C. rectangularis, D. modestus, A.
acuminatus, followed by (b) a more siliceous layer with the same association underlying the C.
vesiculosus, Dimorphograptus and C. cyphus Zones. Although information is insufficient to
define the boundary, a sudden change in facies (here between 988:5 and 1017 m) is found, with
the same pattern as in other areas.
(6) In the Moroccan central massif, Azrou area, at Bou-Ourarh (Locality 6) (Topographical Sheet
Ain Leuh, 1:50000, at about x = 503-5, y = 302-5). The Silurian here occurs as a siliceous facies
alternating with real phthanites weathering light grey. It is the “Formation dite de Mokattam’
of Choubert (1956). It always lies upon ridges of sandy or even quartzitic material, which are
more resistant in the landscape, and which can be assigned to the upper Ordovician without
more precision in dating. Graptolites are found more or less at the contact. At one locality,
there is C. normalis, C. medius, C. rectangularis, C. vesiculosus, A. acuminatus, Glyptograptus sp.
ORDOVICIAN-SILURIAN BOUNDARY IN MOROCCO 169
or Orthograptus sp., P. incommodus, A. ex gr. atavus and Raphidograptus toernquisti (Elles &
Wood). The beds with A. acuminatus are less compact than those with C. vesiculosus. Rhudda-
nian and Aeronian rocks with the Coronograptus gregarius Zone are found down a small valley.
Sandy layers occur at several levels in the Mokattam Formation and the sequence is repetitive.
It is now known that this area has suffered greatly through Hercynian tectonism, so it seems
that Bou-Ourarh is constructed of a number of tectonic slices in which the Silurian has often
played the role of soapstones, and so it is not possible to find any Silurian beds conformably
against the Ordovician sandstones. Although this district is not important for the boundary
definition, it is a supplementary paleogeographical marker for the distribution of the A. acumin-
atus Zone.
(7) In the Palaeozoic inliers of the north middle Atlas.
(a) Tazekka (Eastern Morocco) (Locality 7). The same tectonics as at Bou-Ourarh cause
repetition of the upper Ordovician and lower Silurian. At Souk et Tleta des Zerarda
(Topographical Sheet Ribat el Kheir, 1:50000, at x = 594-5, y = 373-7) at the top of the usual
quartzites, almost vertical upper Ashgill black argillaceous-siliceous and siliceous beds contain
C. normalis, C. medius, C. rectangularis, C. probably longifilis Manck, C. probably trifilis
Manck, D. modestus and A. acuminatus. Silurian beds follow, but not quite in the same section.
(b) Immouzer du Khandar (Locality 8). The same situation exists at the NW end of the
Immouzer du Khandar inlier (Topographical Sheet Sefrou, 1:100000, at about x = 540-1,
y = 353-7), where A. acuminatus, C. normalis, C. miserabilis, C. rectangularis and D. modestus are
found in the argillaceous facies of the Mokattam Formation, but the locality is altered and
schistosed, with bedding plane thrusts. This has contact with sandy pelites and big well-
rounded quartzites of the upper glacial Ordovician, which are equivalent to the Upper Deux-
i¢me Bani Formation (Upper Ashgill) of the Anti-Atlas.
Conclusions
The base of the Silurian is seen in many areas of Morocco, and invariably in argillaceous facies,
underlying sandstone levels and never in true phthanites. It is remarkable that in these sections
no Ordovician faunas have been found, except at Moulay bou Anane. However, in the central
Anti-Atlas, Tagounite area, at Jbel Larjame and at Oued Mooulili, some badly preserved bra-
chiopods are known from the upper part of the Upper Deuxi¢me Bani Formation; these are
from a more western region (south flank of Jbel Addana, south of Akka) and consist of
Hirnantia sagittifera (M‘Coy), Eostropheodonta squamosa Havli¢ek, and Plectothyrella chauveli
Havlicek, from grits above microconglomeratic clays. These faunas are very important in
dating the intra-Hirnantian tillite, and in these areas of the central Anti-Atlas the Silurian
begins with a hardground followed by graptolites of later Llandovery age. We do not expect to
find more significant faunas in the Ordovician rocks and future studies must turn to the
sedimentology of the glacial phenomena and the volcanic influences in the eastern Anti-Atlas;
and also to the description and illustration of the graptolites themselves.
References
Choubert, G. (ed.) 1956. Lexique Stratigraphique International 4 Afrique (1a: Maroc). 165 pp. Paris,
C.N.R.S.
Cocks, L. R. M. 1985. The Ordovician-Silurian boundary. Episodes, Ottawa, 8: 98-100.
Davies, K. A. 1929. Notes on the graptolite faunas of the Upper Ordovician and Lower Silurian. Geol.
Mag., London, 66: 1—27.
Destombes, J. 1981. Hirnantian (Upper Ordovician) tillites on the north flank of the Tindouf basin,
Anti-Atlas, Morocco. In J. Hambrey & W. B. Harland (eds), Earth’s pre-Pleistocene glacial record:
84-88. Cambridge.
, Hollard, H. & Willefert, S. 1985. Lower Palaeozoic rocks of Morocco. In C. H. Holland (ed.), Lower
Palaeozoic of north-western and west central Africa: 91-336. London.
170 J. DESTOMBES & S. WILLEFERT
Graf, C. (1976). Synthése géologique du bassin de Kasba-Tadla, Beni-Mellal, Tanhasset (d’aprés les
données géophysiques et de forages). Rapp. BRPM/DEP. 89 pp., 15 pls (unpublished).
Huvelin, P. 1977. Etude géologique et gitologique du Massif hercynien des Jebilet (Maroc occidental).
Notes Mem. Serv. géol. Maroc, Rabat, 232 bis: 1-307, 12 pls, 3 maps.
Jenny, J. & Couyreur, G. 1985. Carte geologique du Maroc au 100000e, feuille Azilal. Notes Mem. Serv.
geol. Maroc, No. 339.
—— & Le Marrec, A. 1980. Mise en evidence d’une nappe a la limite méridionale du domaine hercynien
dans la boutonniére d’Ait-Tamlil (Haut Atlas central, Maroc). Eclog. geol. Helv., Basel, 73: 681-696.
Levéque, P. (1961). Contribution a l’etude geologique et hydrologique de |’Atlas de Demnate (Maroc). These
Sci., Paris. 242 + 161 + 42 pp. (unpublished).
Roch, E. 1939. Description géologique des montagnes a Est de Marrakech. Notes Mem. Serv. Mines
Carte geol. Maroc, Paris, 51: 1-438, 7 pls.
Verset, Y. 1985. Carte géologique du Maroc au 100 000e, feuille Qasbat-Tadla. Notes Mem. Serv. geéol.
Maroc, No. 340.
The Ordovician—Silurian boundary in the Algerian
Sahara
P. Legrand
Directeur Laboratoires Exploration (Groupe) TOTAL, 218-228 Ave du Haut-Lévéque, 33605
PESSAC Cédex, France.
Synopsis
Two sections, at eastern Tassili-n-Ajjer and at El] Kseib, demonstrate the Ordovician—Silurian boundary,
with graptolites at intervals and rare shells, however the acuminatus Zone itself is not recorded. The
sections are internationally important firstly in demonstrating excellent glacial and periglacial sediments
during the late Ashgill, and secondly in showing that this continental ice-mass melted and was succeeded
by, but was not the origin of, the transgression during the latest Ordovician, in persculptus Zone times.
Introduction
Because of the uplift that probably affected most of the Algerian Sahara near the end of the
Ordovician, and the circumpolar conditions which caused the development of a continental ice
sheet (Debyser et al. 1965), the Algerian Sahara seemed originally an unlikely country for
biostratigraphical study of the Ordovician—Silurian boundary. However, detailed observations
from the boundary beds enable us to show clearly an almost continuous succession from the
Ordovician to the Silurian in the eastern Tassili-n-Ajjer, whereas to the west, in the Ougarta
range, there is a probable hiatus. Moreover, these observations suggest some interesting conclu-
sions about the palaeogeography because this is a country where the glacial events are particu-
larly striking (Fig. 1).
Eastern Tassili-n-Ajjer sections of the Djanet—In Djerane Oued
tray and of the In Djerane Oued
Kilian (1928) drew attention to this area by pointing out the presence of a fauna of lowermost
Llandovery age. Unhappily, this discovery was forgotten and it was many years later when
interest was aroused again following a preliminary collection by the ‘Mission sédimentologique
sur la couverture sédimentaire du Boudin sahariem’ in 1965. Two further studies were carried
out in the field (1978, 1982) despite substantial logistical difficulties; but only some of the
successive results have been published, others are in press.
The stratigraphical succession is as follows (Fig. 2):
Above the Gara Tembi sandstones with a glacial relief:
(a) the Arrkine argillaceous sandy formation (about 90m) in which a new fauna with Cli-
macograptus (Climacograptus) gelidus nov. sp., C. (Climacograptus) arrikini nov. sp. and C.
(Climacograptus) normalis ajjeri Legrand occurs near the base.
(b) The shaley formation of Oued In Djerane in which the following distinctions can be made:
Lower member (80m) of silty claystones and siltstones with a few carbonate levels; the fauna
is as follows: C. (Climacograptus) normalis ajjeri Legrand, C. (Climacograptus) pseudo-
venustus Legrand, C. (Climacograptus) pretilokensis nov. sp., C. (Climacograptus) tilokensis
Legrand and Zygospiraella sp.
Middle member (about 110m) with: a lower submember of shales with C. (Climacograptus)
normalis ajjeri Legrand, Diplograptus (?) kiliani Legrand; an upper submember of siltstones
and silty shales with C. (Climacograptus) freuloni nov. sp., C. (Climacograptus?) incommodus
nov. sp., and Glyptograptus (Glyptograptus) sahariensis nov. sp. and near the top C.
(Climacograptus) imperfectus Legrand, and ?G. (Glyptograptus) aff. persculptus (Salter).
Bull. Br. Mus. nat. Hist. (Geol) 43: 171-176 Issued 28 April 1988
172 P. LEGRAND
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Fig. 1 Outcrops of lower Palaeozoic in Algeria apart from the Intermediate Series (1); Intermediate
Series and Cambro-Ordovician of the syneclise of Taoudeni (2); and Precambrian and Interme-
diate Series (3).
Upper member of sandstones with argillaceous silty intercalations. Fossils are only found
near the base and include Diplograptus africanus Legrand, and G. (Glyptograptus) tariti
Legrand and then, above, Diplograptus fezzanensis Desio.
A lower Llandovery age was originally suggested for the whole Oued In Djerane Formation
(Legrand 1976, 1981, 1985a); then an Ordovician-Silurian boundary level at the top of the
Diplograptus (?) kiliani Zone was proposed (Legrand 1985b, 1986), but a further possibility, of a
boundary at the top of the Middle member, must be considered. The arguments in favour of
this last possibility are as follows:
(i) A new subspecies very near to Diplograptus (?) kiliani is known in the Kurama Range,
Usbekistan (but not in Kazakhstan) and it occurs, according to T. N. Koren, not below the
Parakidograptus acuminatus Zone, as formerly believed, but below some beds where C.
(Climacograptus?) extraordinarius or G. (Glyptograptus) persculptus was collected.
(ii) On the other hand, C. (Climacograptus) incommodus has some affinities with C.
(Climacograptus) extraordinarius and in this respect the position of Zygospiraella, a genus
ORDOVICIAN-SILURIAN BOUNDARY IN THE ALGERIAN SAHARA 173
3° section
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Fig. 2 Distribution of the principal faunas in the sections of the Djanet-In Djerane Oued tray and
the In Djerane Oued, Algeria.
174 P. LEGRAND
only so far definitely recorded from the Silurian, would be the same as that in Kazakhstan
(Oysu River section).
(iii) Finally, rare specimens of ?G. (Glyptograptus) aff. persculptus have been gathered just below
the top of the middle member of the Oued In Djerane Formation.
The objections to the hypothesis are the following:
(i) G. (Glyptograptus) sahariensis is very close to G. (Glyptograptus) tariti and has the aspect of a
Silurian Glyptograptus.
(ii) Diplograptus africanus seems to belong to the Coronograptus cyphus Zone (Legrand 1976),
and consequently there is a very small thickness for the Parakidograptus acuminatus Zone
and the Cystograptus vesiculosus Zone. The sandstones that form the top of the middle
member may be thought to be the equivalent of the zone.
(ii) Parakidograptus acuminatus has not yet been found; one can think of the sandstones that
form the top of the middle member as the equivalent of the biozone characterized by this
species. However, nor has it been found near the Libyan boundary, where the shales take the
place of the sandstones owing to the later transgression there, and where the sedimentation
seems to have been more continuous.
(iv) Perhaps in this apparently very confined area the vertical range of species many not have
been absolutely the same as in less restricted regions.
To conclude, two hypotheses can be proposed for the position of the Ordovician—Silurian
boundary, but the highest seems the most likely. Moreover, there is no characteristic fauna of
the Ordovician in the lower part of the section and this sets problems of correlation with the
standard sections (Dob’s Linn, Kolyma River, Yangtse Valley), and consequently this section in
Algeria can only be a local reference. On the other hand, it has important palaeogeographical
significance since it shows the beginning of the transgression onto the southeastern part of the
Saharan shield before the end of the Ordovician, which must have involved the melting of the
continental ice sheet, at least locally, before the beginning of the Silurian (Legrand 1985).
Ougarta Range—El Kseib section
In the Ougarta Range, the stratigraphical succession of the upper part of the Ordovician
includes the argillaceous sandy Bou M‘haoud Formation, which is overlain by the argillaceous
sandy Jebel Serraf Formation. A mappable unconformity separates these two formations
(Arbey 1962; Gomes Silva et al. 1963; BRP et al. 1964; Legrand 1974). In the eponymous
locality, where that formation seems the most complete, the upper part of the Bou M‘haoud
N.W. S.E.
Surface a modele glaciaire Faune a Airnantia
Graptolites
du Llandoverien moyen
Membre EBACE
greso-conglomeratique
“SS Membre supérieur | Formation
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Membre moyen argileux d’E! Kseib
Formation gréso-conglomératique du Djebel Serraf
(0) is) 10 15 20 km. Ph. LEGRAND . 1967-1981
Fig.3 Section in the vicinity of the Ordovician—Silurian boundary at El Kseib, Ougarta range, Algeria.
ORDOVICIAN-SILURIAN BOUNDARY IN THE ALGERIAN SAHARA 175
Formation is apparently of Lower Caradoc age, with Kloucekia (Kloucekia?) nov. sp.,
Calymenella sp., Drabovinella grandis Mergl, and Drabovia sp.
At first this fauna was attributed to the Upper Caradoc and the beds from which it was
collected were considered to belong to the lower member of the formation subjacent to the
Jebel Serraf Formation. Going to the north west (in the Daoura), the succession is apparently
complete up to the lower Ashgill. Above this the Jebel Serraf Formation appears to be absent
or very thin in Bou M‘haoud village, with siltstones and sandstones (channel deposits), but no
fossils have been found. The quality of the outcrops does not allow us to see the contact with
the lowest Silurian shales. Thus, it is near Ougarta that the Ordovician—Silurian boundary
must be investigated.
In the classical El Kseib section discovered by Menchikoff (1930), the Bou M‘haoud Forma-
tion is reduced to its lower member. Above, the Jebel Serraf Formation consists of a well-
developed sandy, conglomeratic lower member, then the microconglomeratic shales of El Kseib
that prove a periglacial environment; and above these, the sandstones of the “Ksar d’Ougarta’,
It is at Ougarta that some brachiopods were gathered from this member by Poueyto (1950).
Unhappily this fauna (which has been recollected since 1961) is poorly diversified and consists
of Plectotyrella chauveli Havlitek, Hirnantia aff. sagitiffera (M‘Coy), Lingulella sp., Pseudobolus
sp., Conchilolites sp. and a homalonotid pygidium. The age of this member is uppermost Ashgill
(Destombes 1971; Legrand 1974, 1985a, b). Above this the Oued Ali formation is found, whose
base is characterized by a ferrugineous sandstone with ferrugineous nodules and then a bed of
sandstone; there follows some varicoloured shales and coarse shaly sandstones with C.
(Climacograptus) sp., and the member ends with black shales with C. (Climacograptus) aff.
rectangularis M‘Coy, Orthograptus aff. mutabilis Elles & Wood, ?P. (Metaclimacograptus) phry-
gonius Tornquist, and Rastrites sp., indicating a Middle Llandovery age.
Although this section is only interesting from a local point of view for the definition of the
Ordovician—Silurian boundary, it has the wider advantage of showing that the glacial or
periglacial environment ended just before the end of the Ashgill.
Conclusions
The Algerian Sahara is surprisingly important in increasing our knowledge of the Ordovician—
Silurian boundary period. Studies in eastern Tassili-n-Ajjer show, in an almost continuous
section through coastal sediments, the nature of the endemic faunal succession, which, however,
has some affinities with southern Siberia. A palaeogeography can be drawn showing the area
more or less neighbouring the South Pole, and the observations in the Ougarta Range strongly
suggest the almost complete melting of the Upper Ordovician continental ice sheet before the
Silurian transgression. This leads us to reconsider the importance of the melting in the mecha-
nism of the transgression (Legrand 1985).
References
Arbey, F. 1962. Données nouvelles sur la sedimentation au Cambro—Ordovicien dans les monts d’Ougarta
(Saoura). C.r. hebd. Seanc. Acad. Sci., Paris, 254: 3726-3728.
Bureau de recherches de pétrole et al. (compagnies pétroliéres) 1964. Essai de nomenclature litho-
stratigraphique du Cambro-Ordovicien Saharien (colloque). Mem. Soc. geol. Fr., Paris (h.s.) 2. 55 pp., 11
pls.
Destombes, J. 1968. Sur la présence d’une discordance générale de ravinement d’age Ashgill supérieur
dans l’Ordovicien terminal de l’Anti-Atlas (Maroc). C.r. hebd. Seanc. Acad. Sci., Paris, (D) 267: 565—567.
Debyser, J., Charpal, de O. & Merabet, O. 1965. Sur le caractére glaciaire de la sedimentation de Unité
IV au Sahara Central. C.r. hebd. Seanc. Acad. Sci., Paris, 261: 5575.
Gomes Silva, M., Pacaud, M. & Wiel, F. 1963. Contribution a l’etude du Cambro-Ordovicien des Chaines
d’Ougarta (Sahara algérien). Bull. Soc. géol. Fr., Paris, (7) 5: 134-141.
Kilian, C. 1928. Sur la présence du Silurien a l’Est et au Sud de l’Ahaggar. C.r. hebd. Seanc. Acad. Sci.,
Paris, 186 (8): 508-509.
176 P. LEGRAND
Legrand, P. 1970, Les couches a Diplograptus du Tassili de Tarit (Ahnet, Sahara algérien). Bull. Soc. Hist.
nat. Afr. N., Algiers, 60 (3—4): 3-58.
1974. Essai sur la paleogeographie de lOrdovicien au Sahara algerien. Notes Mem. Comp. Franc.
Petrol., Paris, 11: 121-138, 8 pl.
1981. Contribution a étude des graptolites du Llandovérien inférieur de POued In Djerane Tassili
N’Ajjer Oriental (Sahara algerien). Bull. Soc. Hist. nat. Afr. N., Algiers, 67 (1-2): 141-196.
—— 198la. Essai sur la paléogéographie du Silurien au Sahara algérien. Notes Mem. Comp. Franc.
Petrol., Paris, 16: 9-24, 9 pls.
1985. Lower Palaeozoic Rocks of Algeria. In C. H. Holland (ed.), Lower Palaeozoic of North Western
and West Central Africa: 6-29. London.
1985a. Réflexions sur la transgression silurienne au Sahara algérien. Act. Cong. Nat. Soc. Sav. Sect.,
6: 233-244.
— 1986. The lower Silurian graptolites of Oued In Djerane: a study of populations at the Ordovician—
Silurian Boundary. Spec. Publs geol. Soc. Lond. 20: 145-153.
Menchikoff, N. 1930. Recherches géologiques et morphologiques dans le Nord du Sahara occidental. Rev.
geogr. phys. et geol. dyn. 3 (2): 103-247.
Poueyto, A. 1950. Coupe stratigraphique des terrains gothlandiens a Graptolites au N d’Ougarta (Sahara
occidental). C.r. somm. Seanc. Soc. geol. Fr., Paris, 1950: 44-46.
The Ordovician—Silurian boundary in Mauritania
S. Willefert
Direction de la Géologie, Ministére de l’Energie et des Mines, B.P. 6208, RABAT-Instituts,
Morocco
Synopsis
Three sections are described across the Ordovician-Silurian boundary in Mauritania, each bearing well-
developed glacial deposits succeeded by graptolitic shales. In general, fossils of the latest Ordovician and
earliest Silurian are absent, apart from the southeastern section between Aratane and Oualata, at a cliff in
Hodh, where the persculptus and atavus Zones are recorded.
Introduction
Three areas in Mauritania (Fig. 1) shed some light on the question of the Ordovician—Silurian
boundary; however, the pioneer stage of work in these large areas encourages caution. The
areas are:
1 Zemmour Noir (northern Mauritania), known from the masterly contribution of Sougy
(1964) and included in the northern flank of the Reguibat uplift in Deynoux et al. (1985).
2 The Mauritanian Adrar, monographed by Trompette (1973), in the western part of the
Taoudeni Basin (Deynoux et al. 1985).
3 Hodh, whose Precambrian and Ordovician glacial deposits were studied by Deynoux (1980);
this is in the eastern extension of Tagant, which reaches the Adrar towards the S and SE. The
Hodh escarpment frames a Cambro—Ordovician-Silurian ribbon to the N of the southern
margin of the Taoudeni Basin before the post-Palaeozoic oversteps it (Deynoux et al. 1985).
In each area, the glacial upper Ordovician has been carefully studied and these deposits are
more remarkable than those of Morocco, since they were nearer to the Lower Palaeozoic pole,
and so record even more glacial activity, and, moreover, the glacial episode lasted for a longer
time. The Ordovician-—Silurian relationships are very gradual at Hodh and marked by an acute
change of facies at Adrar and Zemmoutr.
Regional descriptions
1 Zemmour Noir (Fig. 2A, but chiefly Deynoux et al. 1985: 347, fig. 4; 354, fig. 6; and 369,
fig. 7). The upper Ordovician consists of the Garat el Hamoueid Group and overlies rocks of
Precambrian to Llanvirn age. Its upper boundary is correlated with the upper Ashgill by
analogy with comparable deposits in Morocco and Algeria and its thickness varies between 0
and 200m. The rocks are typical glacial deposits but these characteristics become less clear to
the NW in the Dhlou Chain because of tectonic complications. Some sedimentological features
suggest a more periglacial regime near the top. Faunas are very rare and consist only of
‘indeterminable Camarotoechia compared by Havlicék (1971) with other brachiopods of the
upper sandstones of the Deuxiéme Bani of Morocco; and of Cornulites.
The base of the Silurian is marked by a very sharp discontinuity, and the system is well
developed on the eastern margin of Zemmour, striking SSW—NNE. It always starts with
Demirastrites triangulatus (Harkness) (determined by A. Philippot) in a facies of black, argilla-
ceous, and some micaceous, shales. Its thickness seems to decrease evenly from 30m in the
north to 6m in the south.
Among the detailed sections of Sougy (1964), the more northern, west of Gara Bouya Ali, has
its base concealed by about 27m of sandy ‘oued’: in the 3m of overlying shales there are
specimens of Monograptus sedgwickii (Portlock) (determined by A. Philippot), while a 30cm
bed of sandstones separates the top of the Garat el Hamoueid Group from the hidden part.
Bull. Br. Mus. nat. Hist. (Geol) 43: 177-182 Issued 28 April 1988
178
Carboniferous
Folded chains
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Siluro-Devonian
Terminal Precambrian and
Cambro-Ordovician
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(1980).
Elsewhere, the surface of the sandstones at the contact with the shales is sometimes covered by
a yellow coating. At Gara Foug Gara there is 2m between ‘Camarotoechia’ and Demirastrites
triangulatus. There is therefore not much hope of defining the boundary exactly in Zemmour
Noir, unless new discoveries are made in the western tectonized part. The Silurian has been
noted in the Dhlou Chain but has not been systematically studied.
2 The Mauritanian Adrar (Fig. 2B, but chiefly Deynoux et al. 1985: 371, fig. 11; 374, fig. 12;
378, table 3). This area geomorphologically consists of (roughly from NNE to SSW), the Atar
plain, the cliff, the plateaus (tabular zone) and the SW margin (folded zone), overlapped by the
Mauritanides chain. The Ordovician-Silurian boundary is exposed in the two last units, but the
area can be treated as a whole, whilst noting that the Silurian becomes more sandy to the
179
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WSW. The glacial formation and the Silurian have been called ‘Supergroup 3’ by Trompette
(1973), subdivided into the Abteilli Group and the Oued Chig Group.
(a) The Abteilli Group represents the glacial upper Ordovician whose lower boundary is
difficult to establish because the glacial deposits occur in a landscape long exposed to continen-
tal deposition and weathering. The only earlier marine palaeontological horizon consists of
lingulids of probable Cambro-Ordovician boundary age (determined by P. Legrand). The top
of the group is marked by sandy eskers which reflect the withdrawal of the land ice to the
south-east. At the time of Monod’s survey (1952) in this district, some brachiopods in a
sandstone from the folded zone at Ayoun Lebgar were determined by D. Le Maitre, who
recognized the genera Camarotoechia, Rhynchonella (especially R. ex gr. borealis), Orthis, Dal-
manella etc., but frequently with nomenclatural doubt. Monod thought that these sandstones
were of Silurian age and that influenced the palaeontologist in her attribution to a high level in
the Wenlock. However, these brachiopods may perhaps better be compared with those from
Gara Foug Gara. J. Drot considers that in Zemmour as well as in Adrar all these fossils are
indeterminable, but it is tempting to compare the total fauna directly. In the section, collected
again by Trompette (1973), the usual graptolitic shales are immediately above the brachiopod-
bearing lenticular sandstones, which indicate a marine incursion which might have been con-
temporaneous with those of Zemmour or the upper sandstones of the Deuxiéme Bani, and so
Trompette has suggested that they belong to the lower Silurian. However, prudence is neces-
sary with such weak data and both possibilities remain hypotheses.
(b) The base of the Oued Chig Group. In the fifteen sections and complementary support
sections, Trompette (1973) was able to verify the concordance between the Abteilli Group and
the Oued Chig Group and also the striking difference in sedimentation between the two groups.
Their contact is rarely clear: there is often 1m or more of sandy debris masking the extreme
base of the Silurian. The oldest graptolites are: Climacograptus normalis Lapworth, C. cf.
rectangularis (M‘Coy), C. cf. scalaris (Hisinger), ?C. sp. or Pseudoglyptograptus sp., cf. Pseudo-
climacograptus (Metaclimacograptus) hughesi (Nicholson), Diplograptus magnus Lapworth,
D. modestus Lapworth or D. magnus, Pristiograptus regularis (TOrnquist), Lagarograptus tenuis
(Portlock), M. sedgwickii and ?Cyclograptus sp. or Calyptograptus sp. There is no Akidograptus
acuminatus (Nicholson) but a part of the Rhuddanian may be present when the lowest associ-
ation contains only the first Climacograptus and Diplograptus either modestus or magnus. In
Adrar it appears that the Llandovery Series begins earlier than in Zemmour because of the
scarcity of monograptids at the base.
3 The Hodh (Fig. 2c, but chiefly Deynoux et al. 1985: 389, fig. 16 and unpublished
determinations). The subdivisions adopted here are Tichit Sandstones for the glacial formation
and Aratane Group for the sandstones and shales with graptolites. The definition of the
Ordovician-Silurian boundary (Cocks 1985) may modify somewhat the Silurian attribution of
some of the basal graptolitic sediments.
The glacial complex rests on any formation among those defined as Cambro-Ordovician.
The major erosional disconformity which opens the glacial cycle is perhaps also in places an
angular unconformity, for example in Tagant (Dia et al. 1969). Deynoux (1980) has recognized
a lower and an upper part in a total thickness of the order of 100-150 m. The upper part, with
several members, includes sandstones and microconglomeratic clays underlying a landmark
sandstone R,, followed by sandy clays (still with microconglomeratic layers) under a second
sandy landmark R,, above which are the clays with graptolites of the Aratane Group. To the
east there are further sandstones termed R; and R,. This group ranges from 100-130m in
thickness.
In the more southeastern section, about halfway between Aratane and Oualata, a bed with
graptolites between R, and R, contains some diplograptids identified as amplexograptids of
Ashgill type. Following the escarpment to the north and west, the sandy landmarks become less
easy to correlate but the zone of Glyptograptus persculptus is well represented:
(a) The more western layer, a portion of the Aratane cliff, appears to be deposited in a glacial
gully under R, and contains only Climacograptus normalis and C. transgrediens Waern.
ORDOVICIAN-SILURIAN BOUNDARY IN MAURITANIA 181
(b) The persculptus Zone contains: Glyptograptus persculptus (Salter), ?Acanthograptus sp. or
?Koremagraptus sp., C. normalis, C. miserabilis Elles & Wood, C. transgrediens, C. cf. praeme-
dius Waern, C. medius (Tornquist), C. cf. rectangularis, C. cf. indivisus Davies, C. minutus? Elles
& Wood, a more amplexograptid than climacograptid new form which recalls some figures of
Comatograptus Obut & Sobolevskaya or Hedrograptus Obut, although more oval; rare frag-
ments of Orthograptus ex gr. truncatus Lapworth, and ?Akidograptus sp. Some climacograptids
show basal spines (Elles & Wood 1906; series of species of Manck 1924 (see Miinch 1952);
reminiscent of more ancient species such as those described by Ross & Berry, 1963). The septa
of G. persculptus begins at the 4th theca.
These beds, except one, are in the portion of the Oualata-cliff, therefore to the NW-SE and
above R, (but Deynoux cannot always decide between R, and R, towards the NW) in a facies
of argillaceous shales and sandy layers and lenses, and some more micaceous beds.
(c) Above in the same member and in the portion of Oualata-cliff:
(i) A layer in a more sandy facies: C. normalis, C. transgrediens, C. medius, C. probably
praemedius, the amplexograptid form, a proximal part of Rhaphidograptus?, a proximal part
of Akidograptus? and some monograptid thecae.
(ii) In the same facies as (b): C. normalis, C. miserabilis, C. minutus, amplexograptid form
narrower than those above, Orthograptus truncatus abbreviatus Elles & Wood, Dimorpho-
graptus sp., Pribylograptus incommodus (Tornquist) and Atavograptus ex gr. atavus (Jones).
(iii) C. normalis, C. miserabilis, Pseudoclimacograptus (Metaclimacograptus) hughesi or
undulatus (Kurck), Diplograptus modestus, D. diminutus Elles & Wood, and a single Peira-
graptus or pathological specimen of Diplograptus sp.?
(d) The landmark bed R, is above these layers, except in one section where it has not been
recognized (C. normalis, P. (M.) hughesi, Dimorphograptus cf. confertus Lapworth), and the same
facies as (b) begins again with C. normalis, C. rectangularis, P. (M.) hughesi or undulatus, D.
modestus, Glyptograptus ex gr. tamariscus (Nicholson), G. tamariscus linearis? Perner, G. either
angulatus Packham or distans Packham, ?Raphidograptus sp., A. atavus, A. strachani Hutt &
Rickards, Lagarograptus acinaces? (Tornquist), and Coronograptus cyphus? (Lapworth).
To the north of Aratane, beyond the post-Palaeozoic cover, towards Mejahouda and in the
vicinity of Tinioulig, Sougy & Trompette (1976) have sampled the usual climacograptids, D.
modestus, Cystograptus vesiculosus (Nicholson) and 4A. ex gr. atavus. All these graptolites are
often irregularly flattened, preserved in iron oxides or with a fragile black pellicule. There is
never an impression of fusellar tissue. Their deposit is rarely homogeneous along the rhabdo-
some. Some layers contain brachiopods and numbers of other organic fragments.
The Ordovician-Silurian boundary is therefore situated between the sandy landmarks R,
and R, in the east of the Hodh. G. persculptus terminates the Ordovician, A. acuminatus is only
suspected, and the remaining Rhuddanian is well represented. One should not forget that these
collections are the first made systematically from this adverse environment, and reflect limited
field-work, which was part of a large programme executed in a short time and with no
possibility of immediate revision. The cliff at Hodh, in the Oualata area, if it were more
accessible, would nevertheless be a first-rate place for a parastratotype, since it records the end
of the African glacial phenomenon and has a good Ordovician-—Silurian transition.
Recently, Legrand (1986) has described in detail (before the choice of the boundary) the lower
Silurian at Oued in Djerane, Algeria, and has recognized new taxa. There is certainly some
correlation between the Hoggar margin and the west of the Taoudeni Basin. However, before
defining an ‘African’ fauna, it would be very useful to demonstrate with more certainty the
effects of diagenesis on the preservation of graptolites, the more so because sections in proteic
tissues have revealed the ability of the cortical layers to trap exogeneous particles. These
extraneous particles could, of course, modify considerably any part of a rhabdosome.
Conclusions
From the Hodh to the Adrar, the post-glacial transgression would seem to have begun in the
Ordovician and extended towards the west in the earliest Silurian, arriving later in the
182 S. WILLEFERT
Zemmour. The cliff to the north-west of Oualata is the best exposure of the local Ordovician—
Silurian boundary, though it is still necessary to fully describe and figure the graptolites and
complementary faunas from there.
References
Cocks, L. R. M. 1985. The Ordovician—-Silurian boundary. Episodes, Ottawa, 8: 98-100.
Deynoux, M. 1980. Les formations glaciaires du Précambrien terminal et de la fin de l’Ordovicien en
Afrique de l'Ouest. Deux exemples de glaciation d’inlandsis sur une plate-forme stable. Trav. Lab. Sci.
Terre St Jerome, Marseille, (B) 17: 1-315.
——., Sougy, J. & Trompette, R. 1985. Lower Palaeozoic Rocks of West Africa and the western part of
Central Africa. In C. H. Holland (ed.), Lower Palaeozic of north-western and west central Africa:
337-495. London.
Dia, O., Sougy, J. & Trompette, R. 1969. Discordances de ravinement et discordance angulaire dans le
Cambro-Ordovicien de la region de Mejeria (Tagant occidental, Mauritanie). Bull. Soc. géol. Fr., Paris,
(7) 11: 207-221.
Elles, G. L. & Wood, E. M. R. 1901-18. A monograph of British Graptolites. Palaeontogr. Soc. (Monogr.),
London. m + clxxi + 539 pp., 52 pls.
Haylicék, V. 1971. Brachiopodes de lOrdovicien du Maroc. Notes Mém. Serv. géol. Maroc, Rabat, 230:
1-135, pls 1-26.
Legrand, P. 1986. The lower Silurian graptolites of Oued In Djerane: a study of populations at the
Ordovician-Silurian boundary. Spec. Publs geol. Soc. Lond. 20: 145-153.
Manck, E. 1924. Grosskolonien von Climacograptus, Abdriicke von Zelltieren von Graptolithen. Natur,
Leipzig, 16.
Monod, T. 1952. L’Adrar mauritanien (Sahara occidental). Esquisse géologique. Bull. Dir. Mines Afr. occ.
fr., Dakar, 15.
Munch, A. 1952. Die graptolithen aus dem Anstehenden Gotlandium Deutschlands und der Tschechoslo-
wakei. Geologica, Berl. 7: 1-157, pls 1-62.
Ross, R. J. & Berry, W. B. N. 1963. Ordovician Graptolites of the Basin Ranges in California, Nevada,
Utah and Idaho. Bull. U.S. geol. Surv., Washington, 1134: 1-177.
Sougy, J. 1964. Les formations paléozoiques du Zemmour noir (Mauritanie septentrionale); étude strati-
graphique, pétrographique et paléontologique. Annls Fac. Sci. Univ. Dakar 15: 1-695.
Trompette, R. 1973. Le Précambrien supérieur et le Paléozoique inferieur de !Adrar de Mauritanie
(bordure occidentale du bassin de Taoudeni, Afrique de l'Ouest). Un exemple de sédimentation de
craton, étude stratigraphique et sédimentologique. Trav. Lab. Sci. Terre St Jerome, Marseille, (B) 7:
1-702.
Ordovician—Silurian boundary in Victoria and New
South Wales, Australia
A. H. M. VandenBerg! and B. D. Webby?
‘Geological Survey Division, Department of Industry, Technology & Resources, P.O. Box 173,
East Melbourne, Victoria, 3002, Australia
*Department of Geology & Geophysics, University of Sydney, New South Wales, 2006,
Australia
Synopsis
The late Ordovician and early Silurian is often represented by an unconformity or otherwise by beds
bearing graptolites: no significant shelly faunas are known. In Darraweit Guim, Victoria, and in the
Forbes—Parkes area of New South Wales, there may be beds spanning the Ordovician-Silurian boundary
without a break, but nowhere have both the persculptus and acuminatus Zones been found in a single,
structurally uncomplicated, succession.
Introduction
Ordovician and Silurian rocks crop out extensively in the Lachlan Fold Belt of southeastern
Australia (Figs 1 and 3). A variety of facies is represented, from deep marine chert, black shale
and turbidites, to shallow marine mudstone and sandstone. Carbonates and volcaniclastics
occur, associated with island arc-type andesites in central New South Wales. The turbidite—
black shale—chert association often contains rich and diverse graptolite assemblages and cono-
donts, but virtually no shelly fossils. Mixed graptolite-shelly fossil assemblages occur in some
of the volcaniclastic deposits, but the shallow marine carbonates only contain shelly fossils.
Sections in central and eastern Victoria
No single section spanning the Ordovician—Silurian boundary has yet been located in
Victoria, although there is reasonably convincing evidence of a complete but fault-disrupted
succession at Darraweit Guim, near Melbourne (Fig. 1). Poor exposure and deep weathering,
and the scarcity of fossils in the Silurian rocks, are the main difficulties in locating further
sections. Another limiting factor is due to the effects of the Benambran Orogeny, a major
accretionary event which took place at about the Ordovician—Silurian boundary and produced
the Wagga Metamorphic Belt in eastern Victoria (Cooper & Grindley 1982). The orogeny is
marked by a prominent facies change, from black shale with or without turbidites, to massive
mudstone or quartzite. East of the metamorphic belt, the facies change follows a break in
sedimentation, which in some places was accompanied by folding.
No such break in sedimentation occurs in the Melborne Trough in central Victoria, but here
the lithological contrast produced by the Benambran Orogeny is such that the boundary
interval became the preferred site for strike faulting during the Middle Devonian Tabberab-
beran orogeny, thus causing considerable complexity in the boundary sections.
Darraweit Guim
The only apparently complete succession spanning the Ordovician-Silurian boundary in Victo-
ria occurs at Darraweit Guim, a hamlet 46km NNW of Melbourne (Fig. 1). It is situated near
the western margin of the Melbourne Trough, a basin in which there is record of continuous
marine sedimentation from early Ordovician to late Early Devonian time (VandenBerg &
Wilkinson, in Cooper & Grindley 1982). The boundary sequence recognized by VandenBerg et
al. (1984) consists of three units, the Bolinda Shale, Darraweit Guim Mudstone and Deep Creek
Siltstone (Fig. 2).
Bull. Br. Mus. nat. Hist. (Geol) 43: 183-190 Issued 28 April 1988
184 A. H. M. VANDENBERG & B. D. WEBBY
Ns Ww
VICTORIA
ORRYONG
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MELBOURNE s PROVINCE
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@ MORWELL ae SILURIAN
[ised ORDOVICIAN
: REGIONAL METAMORPHICS
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_ SS ee |
BASS STRAIT
Fig. 1 Distribution of Ordovician and Silurian rocks in central and eastern Victoria. Localities
mentioned in text and Fig. 2 are: 1, Darraweit Guim; 2, Mount Easton region; 3, Yalmy River; 4,
Delegate (southeast N.S.W.).
The Bolinda Shale is composed of 800m or more of thin-bedded coarse-grained black shale
and fine sandstone with a rich Bolindian graptolite fauna, comprising mostly cosmopolitan
species. The assemblage consists of very abundant Climacograptus latus, C. longispinus supernus
and Orthograptus amplexicaulis (sensu lato), somewhat less abundant C. hastatus, C. cf. tubuli-
ferus, Paraorthograptus pacificus pacificus and Dicellograptus ornatus, and rare specimens of
Orthograptus fastigatus, Orthoretiograptus denticulatus and Pleurograptus linearis (sensu lato).
This assemblage constitutes the Zone of D. ornatus and C. latus of VandenBerg (in Webby et al.
1981) and is virtually identical to that of the Paraorthograptus pacificus Subzone at Dob’s Linn
(Williams 1982).
The overlying Darraweit Guim Mudstone consists of. 20 to 45m of sparsely fossiliferous
black calcareous mudstone and slump-folded mudstone of partly evaporitic origin, and may be
the only unit in Australia to show the effects of the late Ordovician glaciation (VandenBerg, in
prep.). The impoverished shelly fauna consists of small bivalves, hyolithids, straight nautiloids,
and. a single trilobite, Songxites darraweitensis. More important, however, is the occurrence of
Climacograptus? extraordinarius which is associated with C. angustus and C. cf. acceptus
(VandenBerg et al. 1984). This assemblage represents the upper Bolindian Zone of C.? extraor-
dinarius and is considered to correlate with the C.? extraordinarius Zone at Dob’s Linn
(Williams 1983).
Contacts between the Darraweit Guim Mudstone and the overlying Deep Creek Siltstone
are usually poorly exposed and marked by bedding-parallel faults. The Deep Creek Siltstone is
very thick (800-1000 m) and consists of poorly bedded, massive and bioturbated siltstone and
thin rippled sandstone. Fossils are very rare. The lowest graptolite horizon occurs about 75m
above the base of the formation (and about 90m above C.? extraordinarius) and contains
Glyptograptus sp. (VandenBerg et al. 1984: fig. 11). A somewhat richer assemblage occurs 85m
ORDOVICIAN-SILURIAN BOUNDARY IN AUSTRALIA 185
GLOBAL| GRAPTOLITE ZONAL MELBOURNE TROUGH | YALMY
SERIES @| BIOSTRATIGRAPHY eects RIVER — DELEGATE
STAGES DARRAWEIT | MT EASTON | MOUNT (SE NSW)
| BRITISH |AUSTRALIAN| GUIM PROVINCE | TINGARINGY
PZ
ya 7
|S Mc ADAM a Sere elasyay 8
te th |_ magnus | SANDSTONE 5 |
<x DEEP |
S| CREEK 1275275775] SC sanast.| ||| (1/1
4/0 SILTSTONE < |
= atavus | TH
=r | OT
(© | acuminatus | acuminatus * / fault contact | P| | | | || |
2 || |
persculptus | persculptus * LJ J |
if Zz
extraordinarius ——— < at Saad Uy), ] AKUNA MST
a 7) ornatus- S AKUNA MST eee
ro} | pacificus i 5 BOLINDA *
= Q SEES fe) SHALE * *
< & |complexus MOUNT
complanatus uncles s * *
gravis Zz * EASTON x Wansiccot WARBISCO
; A : |= RIDDELL * SHALE
ts * * SHALE
linearis hians kirki 5 SANDSTONE SMAUE
4 3 baragwanathi 5 * * *
Sel spiniferus n.ssp. a * x *
Fig. 2 Correlation chart of Ordovician—Silurian boundary sections in Victoria. For location of
columns, see Fig. 1. Graptolite horizons are shown by asterisks.
and 95m higher in the same section (VandenBerg et al. 1984: fig. 3), and contains Cli-
macograptus normalis, C. angustus, and Glyptograptus? persculptus or a species very close to it.
This assemblage is considered to correlate with the British G.? persculptus Zone at Dob’s Linn
(Williams 1983).
The next graptolite zone, the Zone of Parakidograptus acuminatus, is based on a single
described specimen of P. acuminatus cf. acuminatus (VandenBerg et al. 1984) which came from
the core of an anticline north of Darraweit Guim, low in the Deep Creek Siltstone, but
unfortunately structurally isolated from the more complete sections west of Darraweit Guim.
Its precise stratigraphical relationship with the G.? persculptus Zone is therefore not known.
The same applies to an assemblage from PL665, low in the Deep Creek Siltstone NW of
Darraweit Guim, consisting entirely of Glyptograptus? venustus (Legrand non Mu) (figured as C.
normalis in VandenBerg et al. 1984: fig. 10A).
Little work has been done on the sparse graptolite fauna higher in the Deep Creek Siltstone
(Harris & Thomas 1937, 1949), and much of it is in need of revision. Sufficient material has
been collected, however, to indicate that the graptolite record is far from complete and can only
be correlated with reference to the standard British sequence.
Mount Easton
In the Mount Easton Province, farther east in the Melborne Trough (Fig. 1), VandenBerg (in
Webby et al. 1981) has recognized a nearly complete Upper Ordovician sequence of graptolite
faunas in the Mount Easton Shale (Fig. 2). Faunas range from the Darriwilian Zone of Pseudo-
climacograptus? decoratus to the Bolindian Zone of Dicellograptus ornatus and Climacograptus
latus. VandenBerg (1975) has recorded a possibly conformable relationship with overlying
siltstone near Eildon, but elsewhere the shale is in fault contact with the 500m thick McAdam
186 A. H. M. VANDENBERG & B. D. WEBBY
Sandstone (VandenBerg 1975). The latter contains a small late Llandovery graptolite
assemblage including Retiolites geinitzianus (recorded as Stomatograptus australis), Mono-
graptus exiguus, M. turriculatus, M. spiralis permensus, M. priodon and M. pandus (Keble &
Harris 1934; Harris & Thomas 1947). There is a single record of Silurian graptolites, listed as
Glyptograptus tamariscus, Climacograptus sp. and Monograptus spp. (Harris & Thomas 1954)
from an outcrop adjacent to Mount Easton Shale in the structurally complex Mount Welling-
ton Belt.
Eastern Victoria and the borderland with New South Wales
In the Yalmy River-Mount Tingaringy district in eastern Victoria (Fig. 1), the Warbisco Shale
comprises about 500m of black shale. This contains a graptolite sequence which is recorded by
VandenBerg (1981) as complete from the Gisbornian Zone of Nemagraptus gracilis, to the
Bolindian D. ornatus—C. latus Zone (Fig. 2). Locally, the black shale is overlain by a thin unit of
sandstone and siltstone, the Akuna Mudstone, still with a full D. ornatus—C. latus zonal
assemblage comprising Dicellograptus ornatus, Climacograptus latus, C. longispinus supernus, C.
hastatus, Paraorthograptus pacificus and Orthoretiograptus denticulatus. This unit was formerly
placed in the Yalmy Group (VandenBerg, in Webby et al. 1981: 33) but its relationship is not
completely clear. In most places, the contact between Warbisco Shale and undoubted Yalmy
Group is faulted, and the entire Akuna Mudstone is absent.
The 3700m thick Yalmy Group consists of about 2700m of siltstone containing very large
lenses of deltaic? sandstone, overlain by about 1000m of orthoquartzite turbidites (Fig. 2).
Several small graptolite assemblages occur high in the siltstone unit, but only one has been
studied sufficiently to permit correlation and it comprises Petalograptus sp., Glyptograptus sp..,
Retiolites cf. perlatus, and a variety of monograptids including M. convolutus which correlate
with the mid-Llandovery M. convolutus Zone of Britain.
At Delegate in southeastern New South Wales, to the northeast of the Yalmy River-Mount
Tingaringy district (Fig. 1), the 200-300 m thick Akuna Mudstone (R. A. Glen, in prep.) overlies
the entire Warbisco Shale (Fig. 2). Most of the latter formation consists of black shale, ranging
in age from Gisbornian (with Climacograptus bicornis bicornis) to Bolindian (with C. latus and
Orthograptus fastigatus). A prominent facies change from black shale to grey-green siltstone
occurs at the boundary with the Akuna Mudstone and may correlate with the transition from
Warbisco Shale to Akuna Mudstone farther west. No fossils have been collected from the
upper part of the Akuna Mudstone, but there is a good possibility that the unit extends into the
Silurian.
The contact between the Akuna Mudstone and the overlying Tombong Beds is a low-angle
unconformity, attributable to the Benambran Orogeny which, elsewhere in the same district,
marks a period of strong folding (Glen & VandenBerg 1985, 1987). The Tombong Beds are
thick and unfossiliferous, but a small graptolite assemblage has been recorded from the overly-
ing Meriangaah Siltstone by Crook et al. (1973). They suggest a broad late Llandovery—early
Wenlock age, based on the occurrence of Retiolites geinitzianus angustidens, ‘Monograptus cf.
auduncus’ (presumably Monoclimacis adunca), and M. ex gr. priodon.
Sections in central New South Wales
Similarly, in New South Wales no section has yet been demonstrated to exhibit a complete
record of beds across the Ordovician-Silurian boundary. The main limiting factors are the poor
exposure, the structural complexity and the lack of continuity of richly fossiliferous successions.
Even in the tableland areas the topography is generally subdued, and the sequences are often
deeply weathered. The effects of the latest Ordovician—early Silurian Benambran Orogeny are
noticeable in many areas of New South Wales, as in eastern Victoria. This major event resulted
in the closing of the Wagga Marginal Sea, and then of its deformation, metamorphism and
plutonism to produce the upraised Wagga Metamorphic Belt (Fig. 3). No proven Silurian
deposits are known to occur to the west of the Wagga Metamorphic Belt, and many areas to
the east appear to have a less than complete record of deposition through the Ordovician—
ORDOVICIAN-SILURIAN BOUNDARY IN AUSTRALIA 187
QP
Y
YY
Forbes B OY
Canowindra
SYDNEY
34°
Griffith »
S>
SS \
> we <¢
SS
BS
SAX
SILURIAN
LO ORDOVICIAN
{¢)
100km
laa pa
Fig.3 Map showing the distribution of Ordovician and Silurian rocks in central and southern New
South Wales, and the location of Ordovician-Silurian boundary sections represented in Fig. 4.
188 A. H. M. VANDENBERG & B. D. WEBBY
Silurian boundary interval. The latest Ordovician deposits east of the Wagga Metamorphic
Belt accumulated with associated graptolites in deeper waters as did much of the overlying
Early Silurian, but many sections show physical breaks (unconformities, disconformities with
associated facies changes or faults) reflecting the Benambran orogenesis or subsequent events.
The few sections which appear to show conformity unfortunately have an incomplete record
of Late Ordovician to Early Silurian graptolite assemblages—late Bolindian occurrences fol-
lowed by a significant barren interval to the succeeding mid-Llandovery assemblages, making it
impossible to position the boundary closely (Figs 3—4). In addition to the rarity of proven early
Llandovery deposits, there is an even greater paucity of established late Bolindian to early
Llandovery shelly faunas. Indeed the graptolites are the only group to be adequately represent-
ed in the New South Wales successions. The sections with the best potential for establishing the
Ordovician—Silurian boundary in New South Wales are in the Forbes area and east of Cano-
windra. Two less important sections occur in the Angullong—Four Mile Creek area and east of
Goulburn.
1. Forbes—Parkes. The Cotton Siltstone of the Forbes area comprises separate exposures of a
lower unit of late Ordovician age and an upper unit of Early Silurian age (Sherwin 1970, 1973)
with an extensive strip of ground in between, representing unexposed intervening beds. Sherwin
identified two graptolite assemblages from the lower unit, fauna A characterized by Cli-
macograptus supernus, C. hastatus, C. latus, Dicellograptus cf. elegans and Orthograptus trun-
catus subsp., and assigned a Bolindian age; and fauna B typified by C. normalis and placed by
Sherwin at or just above the Ordovician—Silurian boundary. The upper unit contains faunas C
and D which are correlated with the late Llandovery (sedgwickii and turriculatus Zones); see
also Sherwin (1974). C. normalis is the only determinable graptolite in fauna B and is a
long-ranging species, and consequently can be of little use in establishing the position of the
GLOBAL
SERIES &
STAGES
GRAPTOLITE ZONAL
BIOSTRATIGRAPHY
FORBES- EAST OF (3) ANGULLONG- (4) EAST OF
PARKES CANOWINDRA |'°’FOUR MILE CK.| ~ GOULBURN
S
>|< Ww
m|=a
Oa BEDS
Lu a =
=> || ae
O (shales)
ZZ,
= Ss COTTON
<u MILLAMBRI
tias
al = °
+|z< FORMATION
(sandstones
& siltstones)
ornatus/
/otus
hians kirki
AUSTRALIAN
ZONES & STAGES
Fig. 4 Correlation chart of Ordovician—Silurian boundary sections in central New South Wales.
ANGULLONG UNNAMED *
ROCKDALE x TUe) EAS
GOONUMBLA |FM. (siltstones) ORDOVICIAN
VOLCANICS 0 | CANOMODINE MALONGULLI * SHALES *
%* | LIMESTONE 0 | FORMATION * *
* Graptolite horizon () Shelly faunal horizon
BOLINDIAN
ASHGILL
/inearis
)
EASTONIAN
ORDOVICIAN-SILURIAN BOUNDARY IN AUSTRALIA 189
boundary. Sherwin (in Pickett 1982) estimated the Cotton Siltstone of the Forbes area to be a
total of 1500m thick, and a large part of this is unexposed. For instance, only 100m of the
upper unit is well exposed in the road cutting and quarry near Cotton Trig north-west of
Forbes (Sherwin 1973: fig. 4).
At ‘The Secrets’ north of Parkes, a 90m thick sequence of the Cotton Siltstone includes
several graptolite assemblages (Sherwin 1976) which do not occur near Forbes. These probably
come from stratigraphical levels equivalent to the unexposed gap (between faunas B and C) of
the Forbes section. The assemblages range in age from late lower to early middle Llandovery
(M. cyphus to M. triangulatus Zones). The earliest assemblages, represented through the interval
from 60-70m on Sherwin’s (1976: fig. 3) measured column, include elements such as Cli-
macograptus normalis, Pseudoclimacograptus sp., Glyptograptus sp. and Monograptus? strachani.
Unfortunately, however, there is as yet no evidence in the sections of the Cotton Siltstone near
Forbes and Parkes of the presence of either the latest Ordovician graptolite zones of C.?
extraordinarius and G. persculptus, or the earliest Llandovery zones of P. acuminatus or C.
vesiculosus. Attempts are to be made to arrange the drilling of the unexposed part of the Forbes
section, as it promises to provide the most complete, well preserved and structurally most
uncomplicated record of graptolite assemblages through the Ordovician—Silurian boundary
interval in Australia.
2. East of Canowindra. It is also possible that the Millambri Formation, as redefined by Ryall
(1965), contains a continuous sequence of beds across the Ordovician—Silurian boundary but
this 1240 m thick siliciclastic (poorly bedded arenite and well bedded siltstone) succession needs
to be studied in much more detail. In its type area, in the core of the Cranky Rock Anticline
east of Canowindra, Ryall (1965) has recognized the Millambri Formation as resting conform-
ably on the Rockdale Formation. This siltstone unit has a Late Ordovician graptolite
assemblage identified by Ryall (1965) as Climacograptus bicornis (probably erroneously), C. sp.,
Dicellograptus sp. and Glyptograptus sp. Judging from its stratigraphical relationships with the
underlying Canomodine Limestone, the Rockdale Formation is unlikely to be older than early
Bolindian (Webby et al. 1981). In a separate faulted sliver at Lidcombe Pools, to the east of the
type area, the top of the Millambri Formation has produced a graptolite fauna of middle
Llandovery age, that is about the level of the M. gregarius Zone. Elements of this fauna
have been recorded by Percival (1976) as including Glyptograptus tamariscus, Monograptus
jonesi, Pseudoclimacograptus (Metaclimacograptus) hughesi, P. (M.) andulatus and P.
(Clinoclimacograptus) retroversus.
3. Angullong—Four Mile Creek. In the Angullong—Four Mile Creek area, Jenkins (1978) has
found a late Bolindian assemblage in the uppermost part of the Angullong Tuff and referred the
fauna of Climacograptus supernus, C. latus, C. normalis and Dicellograptus ornatus ornatus to the
D. anceps Zone. Jenkins (1978) has also noted that the horizon lies beneath the top of the
Angullong Tuff, so that volcanic activity may have continued somewhat beyond the end of
anceps Zone time. These tuffs are succeeded disconformably by clastics and limestones of the
Cadia Group, the basal part being judged by Jenkins to be about the level of the C. vesiculosus
Zone. This implies a break of possibly two graptolite zones of the latest Ordovician and one of
the earliest Silurian.
4. East of Goulburn. Sherwin (in Pickett 1982) has noted that while the Early Silurian shales of
the Jerrara Beds east of Goulburn ‘are closely associated with a great thickness of Late
Ordovician strata of similar rock kinds, and because of structural uncertainties and known
faults in this belt it is not known if sedimentation was continuous from Late Ordovician to
Silurian times or not’. Graptolite assemblages of Bolindian and middle—late Llandovery ages
have been recorded from many localities, and in one road section on the Hume Highway, a
tightly folded succession of shales exhibits both Bolindian assemblages and Llandovery
assemblages ranging from the M. cyphus to M. convolutus Zones (Creaser 1973). However,
again there appears to be a significant break (or barren interval) representing the latest Ordovi-
cian (two zones) and the earliest Silurian (two zones).
190 A. H. M. VANDENBERG & B. D. WEBBY
Acknowledgement
The first author publishes with the permission of P. R. Kenley, Acting Director of the Geological Survey
Division of the Victorian Department of Industry, Technology & Resources.
References
Cooper, R. A. & Grindley, G. W. (eds) 1982. Late Proterozoic to Devonian sequences of southeastern
Australia, Antarctica and New Zealand and their correlation. Spec. Publs geol. Soc. Aust., Sydney, 9.
103 pp.
Creaser, P. H. (1973). The geology of the Goulburn—Brayton—Bungonia area. B.Sc. Hons. Thesis, Aust.
Nat. Univ. (Canberra) (unpublished).
Crook, K. A. W., Bein, J. A., Hughes, R. J. & Scott, P. A. 1973. Ordovician and Silurian history of the
southeastern part of the Lachlan Geosyncline. J. geol. Soc. Aust., Sydney, 20: 113-138.
Glen, R. A. & VandenBerg, A. H. M. 1985. Evaluation of the I-S line in the Delegate area, southeastern
Australia, as a possible terrane boundary. Abstr. geol. Soc. Aust., Sydney, 14: 9195.
1987. Thin-skinned tectonics in part of the Lachlan Fold Belt near Delegate, southeastern
Australia. Geology, Boulder, Colo. 15: 1070-1073.
Harris, W. J. & Thomas, D. E. 1937. Victorian Graptolites (New Series), Part IV. Min. geol. J., Mel-
bourne, 1 (1): 68-79.
1947. Notes on the geology of the Yarra Track area near Mount Matlock. Min. geol. J.,
Melbourne, 3 (1): 44-49.
—— —— 1949. Victorian graptolites, Part XI. Silurian graptolites from Jackson’s Creek, near Sydenham,
Victoria. Min. geol. J., Melbourne, 3 (5): 52-55.
—— —— 1954. Notes on the geology of the Wellington—Macalister area. Min. geol. J., Melbourne, 5 (3):
34-49.
Jenkins, C. J. 1978. Llandovery and Wenlock stratigraphy of the Panuara area, central New South Wales.
Proc. Linn. Soc. N.S.W., Sydney, 102: 109-130.
Keble, R. A. & Harris, W. J. 1934. Graptolites of Victoria; new species and additional records. Mem. natn
Mus. Melb. 8: 166-183.
Percival, I. G. 1976. The geology of the Licking Hole Creek area, near Walli, central western New South
Wales. J. Proc. R. Soc. N.S.W., Sydney, 109: 7-23.
Pickett, J. 1982. The Silurian System in New South Wales. Bull. geol. Surv. N.S.W., Sydney, 29. 264 pp.,
5 pls.
Ryall, W. R. 1965. The geology of the Canowindra East area, N.S.W. J. Proc. R. Soc. N.S.W., Sydney, 98:
169-179.
Sherwin, L. 1970. Preliminary results on studies of graptolites from the Forbes district, New South Wales.
Rec. geol. Surv. N.S.W., Sydney, 12: 75—76.
1973. Stratigraphy of the Forbes-Bogan Gate district. Rec. geol. Surv. N.S.W., Sydney, 15: 47-101.
—— 1974. Llandovery graptolites from the Forbes district, New South Wales. Spec. Pap. Palaeont.,
London, 13: 149-175.
—— 1976. The Secrets section through the Cotton Beds north of Parkes. Q. Notes geol. Surv. N.S.W.,
Sydney, 24: 6—10.
VandenBerg, A. H. M. 1975. Definitions and descriptions of Middle Ordovician to Middle Devonian rock
units of the Warburton District, East Central Victoria. Geol. Surv. Rep. 1975/6. 66 pp. Mines Dept.,
Melbourne, Victoria.
—— (1981). A complete Late Ordovician graptolitic sequence at Mountain Creek, near Deddick, eastern
Victoria. Unpubl. Rep. geol. Surv. Victoria 1981/81, Open file. Dept. Industry, Technology and
Resources, Melbourne, Victoria.
—— (in prep.). Explanatory Notes to the Kilmore 1:500000 geological map. Geol. Surv. Rep. 83. Dept.
Industry, Technology and Resources, Melbourne, Victoria.
, Rickards, R. B. & Holloway, D. J. 1984. The Ordovician—Silurian Boundary at Darraweit Guim,
central Victoria. Alcheringa, Sydney, 8: 1—22.
Williams, S. H. 1982. The Late Ordovician graptolite fauna of the Anceps Bands at Dob’s Linn, southern
Scotland. Geologica Palaeont., Marburg, 16: 29-56, 4 pls.
—— 1983. The Ordovician-Silurian boundary graptolite fauna of Dob’s Linn, southern Scotland. Palae-
ontology, London, 26: 605-639.
Webby, B. D., VandenBerg, A. H. M., Cooper, R. A., Banks, M. R., Burrett, C. F., Henderson, R. A.,
Clarkson, P. D., Hughes, C. P., Laurie, J., Stait, B., Thomson, M. R. A. & Webers, G. F. 1981. The
Ordovician System in Australia, New Zealand and Antarctica. Correlation chart and explanatory notes.
64 pp., 4 figs., 2 charts. Paris & Ottawa (Int. Union Geol. Sci. Publ. 6).
The base of the Silurian System in Tasmania
M. R. Banks
Department of Geology, University of Tasmania, Box 252C GPO, Sandy Bay, Hobart,
Tasmania, Australia
Synopsis
The base of the Silurian System in Tasmania lies within the Westfield Sandstone, probably just below an
horizon exposed in the road cutting immediately east of Westfield Quarry and containing a rich fauna
including ?Akidograptus, Atavograptus, Climacograptus normalis and Glyptograptus persculptus.
Introduction
The base of the Silurian System in Tasmania lies within the uppermost formation of the
Gordon Group, the Westfield Sandstone (this includes the Westfield Beds of Corbett & Banks
1974 and equals the Arndell Sandstone of Baillie 1979). The Gordon Group is a predominantly
shallow water sequence, deposition of which began in the Canadian and continued apparently
without interruption into the early Silurian. Within this group in the Florentine Valley (lat. 42°
37’ S, long. 146° 22’ E) the uppermost carbonate formation, the Benjamin Limestone, is over-
lain by the Westfield Sandstone. Stratigraphically equivalent limestones are overlain by silt-
stones and/or sandstones in the Linda Valley in western Tasmania and Mole Creek in northern
Tasmania, but only in the Florentine Valley are the sequences sufficiently exposed, structurally
simple enough and known well enough for consideration in the context of this volume.
The relevant sections in the Florentine Valley lie within the Westfield Syncline and the Tiger
Syncline of the Florentine Synclinorium (Corbett & Banks 1974). These structures in the
relevant areas appear to be simple and most of the dips lie between 30° and 50° (Fig. 1). The
two areas of particular importance are the Westfield Syncline and the eastern flank of the Tiger
Syncline.
Biostratigraphy
In the Westfield Syncline the top of the Benjamin Limestone, e.g. at Corbett & Banks (1974)
locality 13, contains stromatoporoids (Webby & Banks 1976), rugose corals including Foer-
stephyllum sp., Palaeophyllum spp., Favistina sp., Cyathophylloides sp., favositids including
Palaeofavosites sp., auloporids including Eofletcheria sp., heliolitids including Calapoecia sp.
and Coccoseris, halysitids including Catenipora sp. and Falsicatenipora cf. chillagoensis
(Etheridge), ?Beloitoceras sp., Dinorthis sp. (Laurie 1982) and the conodonts Belodina compressa
and Phragmodus undatus (Banks & Burrett 1980). The assemblage suggests correlation with the
P. linearis Zone (Webby et al. 1981) and is clearly Ordovician.
No contact between the Benjamin Limestone and the Westfield Sandstone is exposed. Local-
ities F1 of Baillie & Clarke (1976) and C.&B.15 of Corbett & Banks (1974) are clearly close to
the base of the Sandstone. F1 and F9 of Baillie & Clarke (1976) are closely similar faunally (see
Table 1) as are GB15 and GB16 of Corbett & Banks (1974), and differences between F1 and F9
on the one hand and C.&B.15 and 16 on the other may be ecological rather than temporal
since F1 and F9 are in sandstone and the other two in siltstone. The fauna from F3 of Baillie &
Clarke (1976) is similar to that of C.&B.15 and 16 and is also in siltstone. All five localities can
conveniently be grouped together as different from other and higher horizons. Glossograptus sp.
and a trinucleid related to Guandacolithus suggest that these horizons are late Ordovician. A
few metres stratigraphically above F1 is an horizon, L6 of Laurie (1982), containing Hirnantia
sp. and Isorthis (Ovalella) n. sp. (Laurie 1982). A further 40m stratigraphically higher is a richly
fossiliferous horizon (C.&B.18, B.&C.F2, L11) with Onniella sp., Eospirifer sp., and other bra-
chiopods, Pterinea sp., Orthodesma sp., Encrinuraspis sp., Brongniartella sp., Eokosovopeltis sp.,
Bull. Br. Mus. nat. Hist. (Geol) 43: 191-194 Issued 28 April 1988
192
M. R. BANKS
My SSS)
NW a 262A
ie
Q Quaternary
Bee Formations in the Tiger Range Group
Sw Westfield Sandstone
Ogb Ordovician limestone
F3 etc Fossil localities of Baillie & Clarke (1976)
C&B15 etc. 5 » Corbett & Banks (1974)
L2 etc » Laurie (1982)
SS5So= contours (metres)
r-) limestone outcrops
EA Late Carboniferous and younger
fee] Westfield Sandstone & Tiger Range Group
(ca Gordon Group (excluding Westfield Sst.)
Denison Group
He Cambrian
Map of Tasmania showing localities
mentioned.
Numbers in margins of figures 1a,b,c refer to
grid co-ordinates.
Fig. 1 Ordovician-Silurian Boundary outcrops in Tasmania. la, The Tiger Syncline; 1b, The West-
field Syncline; 1c, The Florentine Valley, also showing the positions of Figs la and 1b; 1d, The
Florentine and Linda Valleys and Mole Creek within Tasmania.
BASE OF THE SILURIAN SYSTEM IN TASMANIA 193
Table 1 Biostratigraphical range chart of fossils from the Westfield Sandstone, Tasmania.
CB18
CB15 F3 F9 Fil L2 L3 L6 F4 CBI6 F2 F8 F5
Taxon L11
|
|
Lepidocyclus x — x x
**Pterinea sp. A P.&G.-T.
Onniella x — Xx Xx — ~ ~ Xk —
*20nniella n. sp. L.
cf. Calymene birmanicus x x
cf. Guandacolithus x x
cf. Heterorthis = x
Byssoconchia —- x
x
x
x
|
|
Bumastus =
Flexicalymene —
2Dalmanophyllum —- _
?Holophragma = —
Dolerorthis — —
Kjerulfina
*Hirnantia n. sp. L. “x « x -
*Tsorthis (Ovalella) n. sp. L. x
*Kinnella cf. kielanae T.S. x
Bekkeromena x
Hedstroemina x —
Orthodesma
Pterinea
**T asmanoconularia sp. Parfrey
Glossograptus
retiolitid
favositids
** Eospirifer sp. S.&B.
Brongniartella
Bumastoides
Encrinuraspis
Encrinurus
Eokosovopeltis
Gravicalymene
**? Akidograptus B.B.&R.
** Atavograptus B.B.&R.
**Climacograptus normalis Lapworth
**Glyptograptus persculptus
**Glyptograptus cf. persculptus
** Fospirifer tasmaniensis S.&B. x
x
x KX X X
x XK X X
XS OS OS OK
|
KK OS, OK OK KK KEKE: OK OX), OX
|
|
**Indicates published description and/or figure.
*Indicates figured and described in a Ph.D. thesis (Laurie 1982).
Other taxa names based on preliminary to somewhat detailed examination.
Records from Baillie (1979); Baillie, Banks & Rickards (1978); Baillie & Clarke (1976); Banks & Burrett (1980);
Corbett & Banks (1974); Laurie (1982); Parfrey (1982); Pojeta & Gilbert-Tomlinson (1977); Sheehan & Baillie (1981);
Webby & Banks (1976).
Bumastoides sp., Gravicalymene sp., ?Akidograptus sp., Atavograptus sp., Climacograptus normal-
is Lapworth, Glyptograptus persculptus (Salter) and G. cf. persculptus. The graptolites suggest
either the persculptus Zone or an horizon low in the acuminatus Zone (Baillie et al. 1978). In
view of the recent decision to place the base of the Silurian System at the base of the acuminatus
Zone (Cocks 1985), this horizon must lie close to the base of the System.
Horizons (L2, L3 of Laurie) contain Hirnantia sp. and one of these also contains Kinnella
cf. kielanae (Laurie 1982). The stratigraphical positions of these horizons are not clear and one
194 M. R. BANKS
or both could be stratigraphically below F2 (both are some tens of metres topographically
lower).
The brachiopods Bekkeromena sp., Hedstroemina sp. and Onniella sp. have been collected
from an horizon (F4 of Baillie & Clarke 1976) on the eastern flank of the Tiger Syncline. A
slightly higher horizon (F5 of Baillie & Clarke) on the flank of the Tiger Range contains
Eospirifer tasmaniensis Sheehan & Baillie (1981) in abundance. This occurs 65m below the top
of the Westfield Sandstone which is overlain by the Gell Quartzite and then the Richea
Siltstone of the Tiger Range Group (Baillie 1979). The Richea Siltstone contains graptolites in
an horizon 300m above that with E. tasmaniensis and the graptolites indicate a very late
Llandovery age (Baillie 1979).
References
Baillie, P. W. 1979. Stratigraphic relationships of Late Ordovician to Early Devonian rocks in the
Huntley Quadrangle, south-western Tasmania. Pap. Proc. R. Soc. Tasm., Hobart, 113: 5—13.
——, Banks, M. R. & Rickards, R. B. 1978. Early Silurian graptolites from Tasmania and their signifi-
cance. Search, Sydney, 9 (1-2): 46-47.
—— & Clarke, M. J. (1976). Preliminary comments on Early Palaeozoic (Late Ordovician—Early Silurian)
rocks and fossils in the Huntley Quadrangle. Tasmania Dept Mines Unpub. Rept. 1976/41.
Banks, M. R. & Burrett, C. F. 1980. A preliminary Ordovician biostratigraphy of Tasmania. J. geol. Soc.
Aust., Adelaide, 26: 363-376.
Cocks, L. R. M. 1985. The Ordovician—Silurian Boundary. Episodes, Ottawa, 8: 98—100.
Corbett, K. D. & Banks, M. R. 1974. Ordovician stratigraphy of the Florentine Synclinorium, southwest
Tasmania. Pap. Proc. R. Soc. Tasm., Hobart, 107: 207-238.
Laurie, J. R. (1982). The taxonomy and biostratigraphy of the Ordovician and Early Silurian articulate
brachiopods of Tasmania. Ph.D. thesis, Univ. Tasmania (unpublished).
Parfrey, S. M. 1982. Palaeozoic conulariids from Tasmania. Alcheringa, Adelaide, 6: 69-77.
Pojeta, J. & Gilbert-Tomlinson, J. 1977. Australian Ordovician pelecypod molluscs. Bull. Bur. Miner.
Resour. Geol. Geophys. Aust., Melbourne, 174: 1—64.
Sheehan, P. M. & Baillie, P. W. 1981. A new species of Eospirifer from Tasmania. J. Paleont., Tulsa, 55:
248-256, pl. 1.
Webby, B. D. & Banks, M. R. 1976. Clathrodictyon and Ecclimadictyon (Stromatoporoidea) from the
Ordovician of Tasmania. Pap. Proc. R. Soc. Tasm., Hobart, 110: 129-137.
——, VandenBerg, A. H. M., Cooper, R. A., Banks, M. R., Burrett, C. F., Henderson, R. A., Clarkson,
P. D., Hughes, C. P., Laurie, J., Stait, B., Thomson, M. R. A. & Webers, G. F. 1981. The Ordovician
System in Australia, New Zealand and Antarctica. Correlation chart and explanatory notes. 64 pp., 4 figs,
2 charts. Paris & Ottawa (Int. Union Geol. Sci. Publ. 6).
Stratigraphy and Palaeontology of the
Ordovician—Silurian boundary interval, Anticosti
Island, Quebec, Canada
C. R. Barnes
Geological Survey of Canada, 601 Booth St, Ottawa, Ontario KIA OE8, Canada
Synopsis
Anticosti Island provided the principal alternative boundary stratotype to Dob’s Linn, Scotland, for the
base of the Silurian System. It represents the best exposed, most fossiliferous, continuous section across
the systemic boundary and has virtually all the attributes required of a stratotype. The 1100m Upper
Ordovician—Lower Silurian (Richmondian to Jumpersian stages) sequence of limestone with minor shale
represents deposition in a marginal carbonate basin. The latest Ordovician Ellis Bay and earliest Silurian
lower Becscie formations contain a record of eustatic sea level change and profound faunal changes. The
seven members in the Ellis Bay Formation appear to reflect eustatic changes associated with the Saharan
glaciation. The Ellis Bay—lower Becscie interval has yielded some 300 species of most invertebrate phyla.
Correlation of this interval is best achieved through conodonts, ostracodes and palynomorphs, together
with brachiopods and trilobites. There is a profound faunal change in conodonts and palynomorphs at
90cm above the base of member 7, Ellis Bay Formation which is taken as the systemic boundary. Precise
correlation of this level to the P. acuminatus graptolite Zone is difficult, but it probably lies at or just
below this zonal level, somewhere within the upper G. persculptus Zone. The Anticosti sequence represents
a standard reference for carbonate platform successions across the boundary and it also holds much
information in regard to the processes and timing of the various faunal/floral extinctions which together
form a Phanerozoic extinction event second in significance only to the terminal Permian event.
Introduction
The best exposed, most fossiliferous and complete section through the Ordovician—Silurian
boundary interval occurs on Anticosti Island, Quebec. In these qualities as well as the lack of
deformation, excellent preservation and diversity of faunas, Anticosti is comparable to other
outstanding stratigraphical sections of Ordovician and Silurian strata such as the type Cincin-
natian Series, the type Wenlock Series, the Silurian of Gotland and the type Pridoli Series.
Dob’s Linn and Anticosti-Gaspé were the only boundary sections formally visited by the
Ordovician-Silurian Boundary Working Group, in 1979 and 1981 respectively. Arguments
supporting Anticosti as a boundary stratotype were advanced by Barnes et al. (1981), Barnes &
McCracken (1981a, b) and McCracken & Barnes (1981). The I.U.G.S., however, has ratified the
decision of the Ordovician—Silurian Boundary Working Group to choose Dob’s Linn, Scot-
land, as the boundary stratotype (Cocks 1985) and this issue is considered elsewhere in this
volume. However, it is the view of this author, and others, that a serious error of judgement has
been made in this decision and that reconsideration should occur in the near future (Lespérance
et al. 1987). In this paper, a general review is presented of the stratigraphy and palaeontology of
the boundary interval on Anticosti. Many data were presented by workers in the volumes
prepared for the Anticosti field excursion edited by Lespérance (1981). Some additional data
have been published in the intervening period and some new conodont data are presented
herein.
Anticosti Island lies in the Gulf of St Lawrence and is approximately 200km long and up to
50 km wide (Fig. 1). The only town is Port Menier on the western end which can be reached by
plane (Québecair) from Sept Iles on the north shore, or by ferry from Rimouski on the south
shore of the Gulf. The island has a network of logging roads, reflecting the main economic
activity of the past fifty years. In 1975, the island was expropriated by the Province of Quebec
and converted to a hunting and fishing reserve: it has over 70000 deer and some of North
Bull. Br. Mus. nat. Hist. (Geol) 43: 195-219 Issued 28 April 1988
([96]) ‘Jp 12 soureg UT [IeJaP UI PaquOsap sUON|IaS Koy JO UONRIO] pur SUONRULIOJ JO UONNG!]SIP SuIMoYs purys] Hsoonuy jodeyw | “By
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196
ORDOVICIAN-SILURIAN BOUNDARY IN ANTICOSTI 197
America’s best salmon rivers. Port Menier has a hotel; cabins and camping facilities have been
developed; vehicles may be rented, or ferried from Rimouski; travel to the eastern and central
parts of the island requires a permit.
Stratigraphy
The island exposes an Upper Ordovician—Lower Silurian (Richmondian, Gamachian,
Menierian, Jumpersian stages) succession, approximately 1100 m thick, comprising the Vauréal,
Ellis Bay, Becscie, Gun River, Jupiter and Chicotte formations (Figs 1, 2). These limestones and
minor shales and sandstones were deposited in the Anticosti Basin. Older parts of the suc-
cession are exposed as a discontinuous, narrow belt on the north shore of the Gulf, and in
western Newfoundland. Offshore basinal equivalent strata are exposed to the south of the
Logan’s Line structural front in the Gaspé Peninsula. Oil exploration wells on Anticosti and
seismic work south of the island have provided additional information on the regional strati-
graphy (Roliff 1968; Petryk 1981d; Roksandic & Granger 1981). The strata dip at less than two
degrees to the southwest and conodont colour alteration indices (CAI 1) indicate that burial
temperatures did not exceed 80°C (Nowlan & Barnes 1987). Excellent exposure is present as
cliff sections around the coast and as a wide wave-cut platform in some places; inland, expo-
sures occur mainly along rivers and roadcuts. Key boundary sections are described by Barnes
et al. (1981) and McCracken & Barnes (1981a); section numbers referred to in the former paper
are also shown in Fig. 1. Space limitations do not permit a full review of previous studies (see
references in McCracken & Barnes 1981a; Lespérance 1981). Key contributions on stratigraphy
include those of Schuchert & Twenhofel (1910), Twenhofel (1914, 1928), Bolton (1961, 1972),
Copeland & Bolton (1975) and Petryk (1979, 19814).
During the Early and Middle Ordovician, the Anticosti Basin acted as a stable platform
receiving shallow water carbonates. In response to tectonic activity of the Taconic Orogeny, the
area was converted into a foreland basin first receiving the black shales of the Macasty Forma-
tion (Maysvillian), followed by 1100m of shale and limestone of the Vauréal Formation. Only
the upper third of the Vauréal outcrops at the surface on Anticosti and it forms most of the
northern and western coastal outcrops. Bolton (1972) recognized that the units referred to the
English Head and Vauréal Formations by Twenhofel (1921, 1928) belonged to the same forma-
tion; he proposed a lower shale and an upper limestone member. Petryk (1981a, c) recognized
five informal members in the Vauréal Formation. Bolton’s upper member, 150m thick, consists
of thin- to medium-bedded, grey, lime mudstone to skeletal wackestone with rare skeletal
packstone, and interbedded grey shale. Intraformational limestone conglomerate and ball and
pillow slump structures are common. Trace fossils are abundant; small coral-stromatoporoid
bioherms occur near the top; some beds have concentrations of the stromatoporoid Aulacera
(Beatricea) up to 3m in length. Sedimentological data (Petryk 1981a) and conodont palaeoecol-
ogy (Nowlan & Barnes 1981) indicate a general upward shallowing sequence. The numerous
minor cycles in the relative abundance of the conodont genera Drepanoistodus and Panderodus
(Nowlan & Barnes 1981: fig. 4) may represent climatic Milankovitch cycles which produced
repetitive oceanic water mass interactions. The faunas of the Vauréal Formation suggest a
Richmondian age (Fig. 2); the main study by Twenhofel (1928) was followed by others on
graptolites (Riva 1969; Riva & Petryk 1981), ostracodes (Copeland 1970), chitinozoans (Achab
1977a, b), and conodonts (Nowlan & Barnes 1981).
The upper Vauréal and Ellis Bay Formations represent the final phase of infilling of the
foreland basin and a return to a pattern of stable, outer carbonate platform sedimentation that
persisted through the Llandovery (Anticostian). The Ellis Bay Formation, however, comprises
an alternation of lithologies permitting recognition of seven members. Six of these were long
recognized (Twenhofel 1928; Bolton 1972) and minor stratigraphical revision by Petryk (1979,
1981a) modified these to seven. This alternation has been interpreted as caused by eustatic
sea-level changes associated with the Late Ordovician north African glaciation (McCracken &
Barnes 198la; Petryk 1981b; Johnson et al. 1981; Barnes 1986). The Ellis Bay Formation,
redefined by Petryk (1979, 1981a) to extend only up to the level of the bioherms, is about 75m
C. R. BARNES
198
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ORDOVICIAN-SILURIAN BOUNDARY IN ANTICOSTI 199
thick. Members 1, 3 and 5 are more argillaceous than members 2, 4 and 6 and are more
recessive; they consist dominantly of nodular, argillaceous limestone, mainly skeletal wackes-
tone to packstone, with lenses of packstone to grainstone; interbeds and films of green and grey
shale are common. These members are particularly fossiliferous with abundant brachiopods
and common cephalopods, gastropods, trilobites, bivalves, aulacerid stromatoporoids, ostra-
codes, conodonts, and palynomorphs. Members 2 and 4 consist dominantly of thin- to
medium-bedded limestone, mainly lime mudstone, with minor regular interbeds of grey shale;
member 6 is a higher energy, cross laminated wackestone to packstone. Members 2, 4 and 6
are less fossiliferous than the other interbedded members, yielding sparse brachiopods, corals,
aulacerids and microfossils. Member 7 consists of a basal oncolitic platform bed, 40cm thick at
Ellis Bay, which extends over most of the island and on which are developed small bioherms,
typically 2m high and 4-8 m wide (Figs 3, 4). These can be studied in vertical profile in the cliffs
and in sequential horizontal profiles in the wave platform. Detailed stratigraphical descriptions
of Ellis Bay Formation sections, particularly across the boundary interval, are given by Barnes
et al. (1981) and McCracken & Barnes (1981a). The faunas of the Ellis Bay are abundant and
diverse. In his pioneer study, Twenhofel (1928) described 172 species; later studies, particularly
on microfossils not considered by Twenhofel, have probably doubled this figure. Twenhofel
(1928) recognized that the Ellis Bay was of post-Richmondian age and proposed the term
Gamachian for this latest Ordovician interval. This stage (Fig. 2) was largely ignored for half a
century, but the recent Anticosti conodont work has demonstrated its validity as a North
American regional stage (McCracken & Barnes 1981a; Barnes et al. 1981; Barnes, in press;
McCracken & Nowlan, in press). Member 7 of the Ellis Bay Formation includes the
Ordovician—Silurian boundary as defined on conodonts (McCracken & Barnes 1981a); the
correlation of this level with the base of the A. acuminatus Zone at the Dob’s Linn stratotype is
discussed below.
The Becscie Formation was initially estimated at about 80m thick by Twenhofel (1928) and
Bolton (1972). Petryk (1979, 1981a) included most of Bolton’s member 6 of the Ellis Bay
Formation in the lower Becscie and his enlarged Becscie measures 131—-173m thick, with four
informal members. The formation consists primarily of thin to thick bedded lime mudstone to
bioturbated skeletal wackestone with brachiopod packstone and grainstone, intrarudstone, and
some ball and pillow slump structures. In the upper third, packstone and grainstone are more
prominent together with green shale. Much of the formation is extremely fossiliferous with
concentrations of Virgiana barrandei (Billings) as well as corals, bryozoans and algae. Cono-
donts (McCracken & Barnes 1981a; Fahraeus & Barnes 1981) and ostracodes (Copeland 1974)
indicate an early Llandovery age (Rhuddanian; Menierian).
Above the Becscie lie the Gun River, Jupiter and Chicotte formations. These cover the
middle to late Llandovery interval and are not part of this present paper. P. Copper has been
studying the brachiopods of Anticosti (e.g. Copper 1977, 1981) and preliminary results of
acritarch and chitinozoan studies have been published (Duffield & Legault 1981; Achab 1981).
There have been few detailed studies of the sedimentology of the Anticosti litho-
stratigraphical units. General reviews and interpretations have been given by Petryk (1981a)
and in the several papers dealing with conodont faunas referred to above. Near the boundary,
the sedimentology and palaeoecology of the bioherms, mainly from the eastern part of the
island, was undertaken by Lake (1981). Orth et al. (1986) failed to detect any iridium anomaly
across the Anticosti boundary interval that may have explained the systemic boundary extinc-
tions through a bolide impact. Seguin & Petryk (1984) have produced some preliminary results
of palaeomagnetic studies and J. Kirschvink and colleagues have recently begun a project to
determine a possible magnetostratigraphic record in the sequence.
Palaeontology
Within the overall stratigraphy of the Anticosti sequence described above, consideration of the
faunas and floras will be restricted here largely to the boundary interval.
200 Cc. R. BARNES
Macropalaeontology
Graptolites. A separate paper by Riva (this volume, p. 221) reviews the Anticosti graptolite
faunas.
Trilobites. Bolton (1981) reported and illustrated the most abundant and diverse of the Anti-
costi trilobite faunas which occurs in the upper member of the Vauréal Formation as the
Ceraurinus icarus (Billings) Richmondian fauna. A less diverse fauna occurs in the Ellis Bay
Formation and includes Isotelus, Toxochasmops anticostiensis (Twenhofel), Otarion anti-
costiensis (Twenhofel), with a member 7 interbiohermal association of Primaspis n. sp.,
Cyphoproteus(?) sp., Calymene sp. and Amphilichas sp. The boundary interval fauna is currently
under study and preliminary results have been presented by Chatterton et al. (1983) and
Lésperance (1985). They report that trilobite genera typical of the Ordovician disappear at the
oncolitic platform bed, member 7 of the Ellis Bay Formation including Celtencrinurus, Isotelus,
Nahannia, Platycorphe and Toxochasmops. The overlying 45m of the lower Becscie Formation
(of Petryk) does not contain diagnostic trilobites until the appearance of Acernaspis. Lespérance
(1985) emphasizes the significance of this occurrence and infers a correlation with the A.
acuminatus Zone. Barnes & Bergstrom (this volume), however, caution that its first appearance
in Norway is higher, as could be its appearance on Anticosti.
Brachiopods. Lespérance (1985) has reviewed the boundary interval brachiopod data. Vellamo,
a typical Ordovician genus, ranges up to 30cm above the oncolitic platform bed, member 7,
Ellis Bay Formation. As with the trilobites, the next 40 m of the lower Becscie contains few
diagnostic brachiopods (e.g., Parastrophinella reversa in growth position; Stricklandia sp.). At
about 100m above the base of the Becscie is the first appearance of Virgiana sp., a level which
Lespérance considers may be as low as the A. acuminatus Zone or Cystograptus vesiculosus
Zone.
The distribution of the atrypoid brachiopods was reviewed by Copper (1981). Three species
of Spirigerina occur in the Ellis Bay Formation and this genus is only known elsewhere in
North America Ordovician strata from the Edgewood Group, Missouri (?Gamachian). Differ-
ent forms of this genus, together with Atrypina gamachiana (Twenhofel), occur above the
oncolitic platform bed which Copper (1981) considered as a suitable level for the systemic
boundary. Zygospiraella planoconvexa, a typical Rhuddanian index fossil, occurs higher in the
lower Becscie, below or at a level where Virgiana and the trilobite Acernaspis occur (e.g.
Lespeérance 1985: figs 3, 4).
Cocks & Copper (1981) reported a Hirnantia fauna from a thin interval, 4.5m below the
oncolitic platform bed, in eastern Anticosti. This level is about 5m below the occurrence of
Silurian conodonts at this locality (Nowlan 1982). Since no internal moulds were illustrated,
Lespérance (1985) has queried the assignment of these brachiopods to the Hirnantia fauna, but
recognized that this fauna does appear at an equivalent level to the south in the Gaspé region.
Other macrofossils. Although commonly abundant in the Anticosti sequence, insufficient work
has been completed or published on other groups of macrofossils to add much resolution to
defining the systemic boundary in this region. Aulacerid stromatoporoids range only into
member 7, Ellis Bay Formation and are present in the oncolitic platform bed (Bolton 1981;
Cocks & Copper 1981; Petryk 1982c). The global change from a labechiid to a clathrodictyid
assemblage near the systemic boundary was documented by Webby (1980). The coral genus
Calapoecia, typically regarded as Ordovician, occurs in the bioherms and up to 20m above the
base of the Becscie Formation of Petryk (Bolton 1981). Another such Ordovician genus, Acid-
olites, is also known to extend into the upper Becscie Formation (Bolton 1981) and the
distribution of species on Anticosti, especially in the member 7 bioherms, has been documented
by Dixon (1986). Some preliminary work on algae, including those in the bioherms, have been
published by Copper (1977), Bolton (1981), and Gauthier-Coulloudon & Mamet (1981). Bolton
(1981) reviewed the occurrence of echinoderms, molluscs, and bryozoans but none of these
groups is sufficiently well documented to be of biostratigraphical value for the boundary
interval.
ORDOVICIAN-SILURIAN BOUNDARY IN ANTICOSTI 201
Micropalaeontology
Microfossils have been systematically collected from all of the Anticosti succession and provide
the most precise biostratigraphic control. Ostracodes were investigated initially, followed by
extensive conodont work, and acritarch—chitinozoan studies are now in progress with much of
this collecting being tied to the conodont samples.
Ostracodes. The Anticosti ostracode faunas have been documented by Copeland (1970, 1973,
1974, 1981, 1983) for the Anticosti sequence and a series of zones and subzones established
(Fig. 2). Increasing faunal provincialism occurs with the Silurian faunas (Copeland & Berdan
1977). In broadest terms, two distinct faunas occur. An older, predominantly Ordovician,
hollinacean fauna is developed through the Vauréal, Ellis Bay and the lower 35m of the Becscie
formations and is assigned to the Jonesites semilunatus Zone with ten subzones. Much of this
fauna is replaced (e.g., extinction of the Tetradellidae and Eurychilinidae) abruptly by an
endemic beyrichiacean zygobolbid fauna. However, this turnover is not precisely defined since
there is a 10m interval in the lower Becscie which yields only sparse undiagnostic ostracodes.
The Euprimitia gamachei Subzone, the highest in the Jonesites semilunatus Zone, occurs in the
lower 35m of the Becscie Formation of Petryk. Copeland (1983) reported the distinctive Baltic
species Steusloffina cuneata, considered to be of Ordovician age, from 6m above the base of the
Becscie Formation. The earliest Silurian zygobolbinid ostracodes occur about 40-50m above
the first occurrence of Virgiana and Acernaspis and 70m above the first appearance of Silurian
conodonts. Most of the ostracode distributions are plotted by member and/or formation by
Copeland (1970, 1973, 1974) which limits the degree of resolution of ostracode biostratigraphy.
Palynomorphs. The chitinozoan faunas from the Vaureal and Ellis Bay formations have been
described by Achab (1977a, b, 1981). A doctoral study of the latest Ordovician and the Silurian
acritarchs was undertaken by Duffield (1982) and the preliminary results published (Duffield &
Legault 1981). In both groups, significant turnovers occur at the level of the bioherms similar to
that of the conodonts (see below).
For the chitinozoans, members 5 and 6 contain Conochitina gamachiana Achab, C. micra-
cantha Eisenack and C. taugourdeaui Eisenack, which range up to the base of the bioherms.
Above the bioherms, the fauna consists only of Cyathochitina kuckersiana Eisenack and Ancy-
rochitina spongiosa Achab with Conochitina sp. 1 of Achab higher in the Becscie.
The acritarch floral assemblage of the upper Ellis Bay Formation is of low diversity and
abundance. Dominant taxa are Baltisphaeridium plicatispinae Gorka and Multiplicisphaeridium
sp. 1 of Duffield & Legault. These taxa dominate up to the bioherms but the 2m biohermal
interval is generally barren of acritarchs. Some taxa range into the overlying Becscie but above
the bioherms several new distinctive taxa appear including Goniosphaeridium oligospinosum,
Multiplicisphaeridium birminghamensis and members of the M. denticulatum group. This diverse
upper assemblage contains forms described elsewhere from Silurian strata in North America
and Belgium.
Conodonts (Plates 1—3). The entire Anticosti outcrop was sampled at 2m intervals for cono-
donts by Barnes and later expeditions have provided more intensive collections, particularly in
the boundary interval. In all, some 700 samples have yielded over 150000 conodonts. Most of
the basic taxonomic and biostratigraphical results have now been published (McCracken &
Barnes 1981a; Nowlan & Barnes 1981; Uyeno & Barnes 1983); for the upper Becscie—Gun
River interval only preliminary results have appeared (Fahraeus & Barnes 1981). These data
have been important in a revision of North American chronostratigraphy using the Anticosti
sequence as a reference section (Barnes & McCracken 1981; Barnes, in press) for the Gama-
chian, Menierian and Jumpersian stages (Fig. 2).
The Vauréal Formation yielded a diverse and particularly abundant conodont fauna of
Richmondian age (Nowlan & Barnes 1981). The pattern of conodont communities reflects the
gradually upward-shallowing sequences with Phragmodus and Amorphognathus—Plectodina
dominated assemblages eventually being replaced by an Oulodus—Aphelognathis assemblage
(Nowlan & Barnes 1981: figs 2, 3).
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ORDOVICIAN-SILURIAN BOUNDARY IN ANTICOSTI 203
In the upper Vauréal a new distinctive genus, Gamachignathus, appears (McCracken et al.
1980) and then dominates the fauna of the entire Ellis Bay Formation, particularly the western
sections. The Ellis Bay fauna contains many taxa ranging up from the Vauréal Formation but
also new taxa such as Aphelognathus sp. aff. A. grandis and Staufferella inaligera as well as an
absence of Plectodina. McCracken & Barnes (1981a) established conodont Fauna 13 for this
Ellis Bay interval (following Faunas 1-12 of Sweet et al. (1971); see also Sweet (1984) for new
conodont chronozones). This Gamachignathus fauna has since been recognized in other latest
Ordovician marginal basins in North America, including the Matepedia Group, Gaspé
(Nowlan 1981) and the Grog Brook Group, New Brunswick (Nowlan 1983), the Hanson Creek
Formation, Ely Springs Dolomite, and Unnamed Limestone at Ikes Canyon, Toquima Range,
Nevada and California (Ross et al. 1982: C11), the Fish Haven Dolomite of Utah (Leatham
1985), the Road River Formation of the Yukon (McCracken & Nowlan in press; McCracken &
Lenz in press) and the Cape Phillips Group, Cornwallis Island, Canadian Arctic Archipelago
(McCracken & Nowlan in press). This distinctive genus appears to have evolved in the latest
Richmondian from Birksfeldia (Barnes & Bergstrom, this volume, p. 325).
McCracken & Barnes (198la: fig. 12) have shown the distribution of nearly 40 form and
multielement conodont species through the members of the Ellis Bay Formation. A remarkable
turnover in the fauna occurs at the level of the bioherms. The Ordovician taxa range up to a
level 50cm above the oncolitic platform bed, that is in the lower 50 cm of the interbiohermal
strata. At this level, taxa typical of the Silurian first appear (e.g. Ozarkodina oldhamensis). These
intermingle with only a few taxa extending from underlying strata: Gamachignathus ensifer, G.
hastatus, Oulodus robustus and the coniform taxa of Panderodus, Pseudooneotodus, Decoriconus,
Walliserodus and Staufferella. Of these, Gamachignathus and Staufferella become extinct 1-5—
2-0 m higher in the section, at the base of the Becscie Formation of Petryk. Within a few metres
of the first appearance of Silurian conodonts, several other distinctive Silurian taxa appear
including Distomodus sp. aff. D. kentuckyensis, Icriodella discreta, I. deflecta, Oulodus? ken-
tuckyensis, O.? nathani and Spathognathodus manitoulinensis. The base of the Silurian on Anti-
costi was defined using conodonts as the first appearance of Ozarkodina (O. hassi and/or O.
oldhamensis) (McCracken & Barnes 198la; Barnes & McCracken 1981). These authors also
PLATE 1 All figures x 70 except fig. 2 x 100, fig. 11 x 85 and figs 12, 13, and 17 x 35. Type
specimens deposited in the Geological Survey of Canada, Ottawa; sample number given in parenth-
eses after GSC type number.
Figs 1-8 Gamachignathus hastatus McCracken, Nowlan & Barnes. (1, 6) Posterior and inner
lateral views of keislognathiform elements; GSC 84971, GSC 84976 (S-1). (2, 5) Inner lateral views
of cyrtoniodiform elements; GSC 84972, GSC 84975 (S-1). (3) Posterior view of hibbardelliform
element; GSC 84973 (S-1). (4, 7) Outer lateral and inner lateral views of modified prioniodiform
elements; GSC 84974 (S-1), GSC 84977 (2B-2). (8) Outer lateral view of cordylodiform element;
GSC 84978 (2B-3).
Figs 9-19 Gamachignathus ensifer McCracken, Nowlan & Barnes. (9) Inner lateral view of cyrtonio-
diform element; GSC 84979. (10) Posterior view of keislognathiform element; GSC 84980. (11)
Posterior view of hibbardelliform element; GSC 84981. (12, 13) Inner lateral and outer lateral
views of modified prioniodiform elements; GSC 84982, GSC 84983. (14, 16, 17) Inner lateral, inner
lateral and outer lateral views of prioniodiform elements; GSC 84984, GSC 84986, GSC 84987.
(15, 18) Inner lateral and outer lateral views of cordylodiform elements; GSC 84985, GSC 84988.
(19) Inner lateral view of falodiform element; GSC 84989. All specimens from sample S-1.
Figs 20, 24 Pseudobelodina dispansa (Glenister). (20) Lateral view of furrowed element; GSC 84990.
(24) Lateral view of non-furrowed element; GSC 84994. Both specimens from sample S-1.
Figs 21-23 Phragmodus undatus Branson & Mehl. (21) Inner lateral view of trichonodelliform
element; GSC 84991. (22) Outer lateral view of oistodiform element; GSC 84992. (23) Inner lateral
view of cordylodiform—cladognathiform element; GSC 84993. All specimens from sample S-1.
Figs 25, 26 Plegagnathus dartoni (Stone & Furnish). (25) Outer lateral view of recurved element;
GSC 84995 (S-145). (26) Inner lateral view of reclined element; GSC 84996 (S-1).
Figs 27, 28 Pseudobelodina vulgaris vulgaris Sweet. (27) Inner lateral view of broadly curved
element; GSC 84997 (S-1). (28) Outer lateral view of tightly curved element; GSC 84998 (S-1).
C. R. BARNES
ORDOVICIAN-SILURIAN BOUNDARY IN ANTICOSTI 205
established the Oulodus? nathani Zone for the earliest Silurian strata, lying below the Dis-
tomodus kentuckyensis Zone known elsewhere in North America (Fig. 2). In all the Anticosti
conodont studies this conodont faunal turnover is by far the most profound and it is also a
global event (Barnes & Bergstrom, this volume). In other carbonate sequences this same sharp
boundary level can also be recognized. The O.? nathani Zone has been recognized elsewhere, for
example in Gaspé, Quebec (Nowlan 1983) and the Oslo region of Norway (Aldridge &
Mohamed 1982) based on the presence there of O.? cf. O. nathani.
The precise conodont faunal changes across the systemic boundary at the Ellis Bay and
Salmon River sections, western and east-central Anticosti, were documented by McCracken &
Barnes (1981a: figs 12, 14, tables 1-7). Cluster analysis was used to determine the changing
community patterns with time, particularly with respect to east-west facies change. Additional
collecting across the boundary interval was made by Duffield & Barnes in 1979 and the author
in 1982 at Pointe Laframboise (Petryk 1981a: fig. 11), and west and east sides of Ellis Bay
(Petryk 1981a: figs 12, 14) and at Salmon River (Petryk 1981a: figs 22, 23). These sections are
described in both McCracken & Barnes (1981a) and Barnes et al. (1981).
The new conodont data are shown in Fig. 3 and Table 1. These three sections were closely
sampled in each of these three sets of collections, resulting in sampling across the boundary
interval at 10-20cm intervals with each sampled interval being about 10cm in thickness. In all,
over 250 samples were taken through the 4-5 m interval at these three sections. The number of
specimens per species per sample were tabulated by McCracken & Barnes (1981a) and Table 1
herein records similar data for the 1979 and 1982 collections. The latter two collections were
taken close to the bioherms and produced much lower yields. Conodonts in general are rare in
biohermal facies and to test this in the Anticosti sequence several samples (e.g. 2A.13—2A.15;
2B.14—2B.15) were taken from within the bioherms (Figs 3, 4B). All but one were barren and the
exception contained only one specimen.
The faunal change occurring in this boundary interval described by McCracken & Barnes
(1981a) and further by McCracken & Nowlan (in press), is substantiated in the new collections
at each of the three sections. Some slight adjustments to the ranges of certain species can be
noted. The general pattern is of an assemblage of Ordovician taxa up to the level of, and
including, the oncolitic platform bed, member 7, Ellis Bay Formation, dominated by Gamachig-
nathus ensifer and G. hastatus. At both the Pointe Laframboise and west side of Ellis Bay
PLATE 2 All figures x 70 except figs 3, 6, 11, 17, 24, and 26 x 85, figs 7, 16, 18-23 and 27 x 35
and fig. 10 x 60. Sample numbers are as shown in Fig. 3, p. 211, except for S-143, 2m below S-144;
C-24, 1m below oncolitic platform bed, east side Ellis Bay (Loc. 2C; Fig. 1).
Figs 1-3, 6-8 Oulodus robustus (Branson, Mehl & Branson). (1) Posterior view of zygognathiform
element; GSC 85032 (2B-3). (2) Inner lateral view of cordylodiform element; GSC 85033 (2B-3).
(3, 6). Inner lateral views of eoligonodiniform elements; GSC 85034, GSC 85037 (2B-3). (7) Outer
lateral view of prioniodiniform element; GSC 85038 (C-24). (8) Posterior view of oulodiform
element; GSC 85039 (2B-3).
Figs 4, 5, 9, 10, 12 Oulodus ulrichi (Stone & Furnish). (4) Inner lateral view of eoligonodiniform
element; GSC 85035 (2B-3). (5, 9) Posterior views of zygognathiform elements; GSC 85036, GSC
85040 (2B-3). (10) Posterior view of trichonodelliform element; GSC 85041 (2B-3). (12). Posterior
view of oulodiform element; GSC 85043 (S-143).
Figs 11, 13-19 Oulodus rohneri Ethington & Furnish. (11, 13) Posterior views of trichonodelliform
elements; GSC 85042, GSC 85044 (2B-3). (14, 16) Posterior views of zygognathiform elements;
GSC 85045, GSC 85047 (2B-3). (15) Inner lateral view of eoligonodiniform element; GSC 85046
(2B-3). (17) Inner lateral and posterior views of prioniodiniform element; GSC 85048 (S-143).
(18, 19) Posterior view of oulodiform elements; GSC 85049, GSC 85050 (S-143).
Figs 20-27 Aphelognathus sp. aff. A. grandis Branson, Mehl & Branson. (20) Posterior view of
trichonodelliform element; GSC 85051. (21, 26) Posterior views of zygognathiform elements; GSC
85052, GSC 85057. (22) Inner lateral view of cyrtoniodiform element; GSC 85053. (23, 27) Lateral
views of aphelognathiform elements; GSC 85054, GSC 85058. (24) Inner lateral view of eoligono-
diniform element; GSC 85055. (25) Inner lateral view of prioniodiniform element; GSC 85056. All
specimens from sample S-143.
206 C. R. BARNES
Table 1 Distribution of conodont species in the Ordovician—Silurian boundary interval, Anticosti Island,
Québec. A: Pointe Laframboise (Locality 2A; Fig. 1). B: West side of Ellis Bay (Locality 2B; Fig. 1). C: 9 mile
pool, Salmon River (Locality 5B; Fig. 1). Stratigraphical position of samples shown in Fig. 3 from collections by
Table 1A: Pointe Laframboise (Loc. 2A)
Species/Sample number F3) P47) R6) =E7 F859) E10 Et E12) ls ETS
Amor phognathus ordovicicus
Aphelognathus aff. A. grandis
Decoriconus costulatus 1 —
Drepanoistodus suberectus
Gamachignathus ensifer — 23 3 9 1 7 6 3
G. hastatus — 22
Oulodus robustus — 25
O. rohneri
O. ulrichi
Panderodus spp. —- — 1 1 2 186 76
Phragmodus undatus —
Plegagnathus dartoni
Pseudobelodina dispansa
P. v. vulgaris
Pseudooneotodus beckmanni — 3 1 2 _
Staufferella inaligera
Walliserodus cf. W. curvatus a 2 2 1
Distomodus aff. D. kentuckyensis 1 2 — —
Icriodella discreta 5
Oulodus? kentuckyensis 2 —
O.? nathani
zarkodina hassi 4 13 22 2
O. oldhamensis
(+ramiforms of O. hassi) 6 3 5 5) —_- — 72 56 2
Spathognathodus manitoulinensis 1 2 a — —_
Walliserodus curvatus 38 31 =
oo
Total specimens/sample OS Yiu 7 14 #11 8 2 6 316 188 24
ORDOVICIAN-SILURIAN BOUNDARY IN ANTICOSTI 207
Duffield & Barnes, and Barnes; distribution data for other samples given in McCracken & Barnes (1981a).
Average sample weight is 2 kg.
2A-1 2A-2 2A-3 2A-4 2A-5 2A-6 2A-7 2A-8 2A-9 2A-10 2A-11 2A-12 2A-13 2A-14 2A-15
_
Nn
N
—
BS
—
208
Table 1B: West side, Ellis Bay (Loc. 2B)
Species/Sample number
Amorphognathus ordovicicus
Aphelognathus aff. A. grandis
Decoriconus costulatus
Drepanoistodus suberectus
Gamachignathus ensifer
G. hastatus
Oulodus robustus
O. rohneri
O. ulrichi
Panderodus spp.
Phragmodus undatus
Plegagnathus dartoni
Pseudobelodina dispansa
P. v. vulgaris
Pseudooneutodus beckmanni
Staufferella inaligera
Walliserodus cf. W. curvatus
Distomodus aff. D. kentuckyensis
Icriodella discreta
Oulodus? kentuckyensis
O.? nathani
Ozarkodina hassi
O. oldhamensis
(+ramiforms of O. hassi)
Spathognathodus manitoulinensis
Walliserodus curvatus
Total specimens/sample
C. R. BARNES
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ORDOVICIAN-SILURIAN BOUNDARY IN ANTICOSTI 209
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ORDOVICIAN SILURIAN
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ORDOVICIAN-SILURIAN BOUNDARY IN ANTICOSTI 213
sections (Localities 2A, 2B, on Fig. 1), the first Silurian taxa (Ozarkodina oldhamensis, O. hassi,
Spathognathodus manitoulinensis and Oulodus? nathani) appear about 90cm above the base of
the oncolitic platform bed, that is 50cm above the top of this bed within the interbiohermal
strata (Fig. 4D). At the Salmon River section (Locality 5B on Fig. 1; Fig. 4F), the later
collections show the first occurrence to be still 90cm above the base of the oncolitic platform
bed but since this bed has thickened to 90cm, from 40cm in the western sections, the top 10cm
of this bed have now yielded Silurian taxa (Table 1). This is about 50 cm lower than the level
reported by McCracken & Barnes (1981a) and perhaps the level reported by Nowlan (1982)
from a coastal section further to the east. In the three sections, Gamachignathus, Oulodus
robustus and Staufferella inaligera range through the next two metres, mixed with the early
Silurian forms. At a level approximating to the base of Petryk’s Becscie Formation (typically
2m above the base of the oncolitic platform bed, and equivalent to a level within a metre of the
top of the bioherms) these residual Ordovician taxa disappear and the earlier Silurian taxa are
joined by other Silurian forms such as Icriodella discreta, Icriodella deflecta, Distomodus sp. aff.
D. kentuckyensis and Oulodus? kentuckyensis.
Biostratigraphical correlations
This paper has reviewed the sequence of faunas through the systemic boundary interval on
Anticosti and added new conodont data. Many of the references noted above include sections
on the regional biostratigraphical correlations. Space limitations prevent a comprehensive dis-
PLATE 3 All figures x 70 except figs 4-8, 18, 20, 21 x 85 and fig. 31 x 35. Sample numbers are as
shown in Fig. 3, p. 211, except for S-154, S-155, 2 and 1:5 m above S-153; C-38 in Lower Becscie, 1-2
km east of Cap a l’Aigle (Loc. 3B; Fig. 1); F-16 is 2 m above F-15.
Figs 1-8 Oulodus? nathani McCracken & Barnes. (1, 3) Inner lateral views of modified oulodiform
elements; GSC 84999, GSC 85001. (2) Posterior view of trichonodelliform element; GSC 85000.
(4, 8) Posterior view of zygognathiform elements; GSC 85002, GSC 85006. (5, 6) Inner lateral views
of lonchodiniform elements; GSC 85003, GSC 85004. (7) Inner lateral view of ligonodiniform
element; GSC 85005. All specimens from sample S-154 except (1) which is from sample C-38.
Figs 9-12 Oulodus? kentuckyensis (Branson & Branson). (9) Lateral view of modified oulodiform
element; GSC 85007 (F-15). (10) Lateral view of eupriodiodiniform element; GSC 85008 (S-153).
(11) Posterior view of zygognathiform element; GSC 85009 (S-154). (12) Inner lateral view of
ligonodiniform element; GSC 85010 (S-154).
Figs 13, 14 Ozarkodina oldhamensis (Rexroad). (13) Lateral view of spathognathodiform element;
GSC 85011 (S-155). (14) Inner lateral view of ozarkodiniform element; GSC 85012 (S-155).
Figs 15-19 Ramiform elements of O. oldhamensis and O. hassi complex. (15) Lateral view of syn-
prioniodiniform element; GSC 85013. (16) Posterior view of zygognathiform element; GSC 85014.
(17, 19) Inner lateral views of ligonodiniform elements; GSC 85015, GSC 85017. (18) Posterior view
of trichonodelliform element; GSC 85016. All specimens from sample S-155.
Figs 20, 21 Ozarkodina hassi (Pollock, Rexroad & Nicholl). (20) Inner lateral view of ozarkodini-
form element; GSC 85018 (S-153). (21) Lateral view of spathognathodiform element; GSC 85019
(2A-10).
Fig. 22 Spathognathodus manitoulinensis Pollock, Rexroad & Nicholl. Inner lateral view of spathog-
nathodiform element; GSC 85020 (S-8).
Figs 23-28 Distomodus sp. aff. D. kentuckyensis Branson & Branson. (23, 24) Upper view of platform
elements; GSC 85021, GSC 85022. (25) Inner lateral view of distomodiform element; GSC 85023.
(26) Inner lateral view of modified ambalodiform element; GSC 85024. (27) Inner lateral view of
eoligonodiniform element; GSC 85025. (28) Posterior view of zygognathiform element; GSC 85026.
All specimens from sample F-16.
Figs 29-31, 33 Icriodella discreta Pollock, Rexroad & Nicholl. (29) Outer lateral view of sagit-
todontiform element; GSC 85027 (2B-13). (30) Inner lateral view of ambalodiform element; GSC
85028 (2B-13). (31) Upper view of icriodelliform element; GSC 85029 (2B-12). (33) Posterior view of
trichonodelliform element; GSC 85031 (2B-13).
Fig. 32 Icriodella deflecta Aldridge. Upper view of icriodelliform element; GSC 85030 (C-55: base of
Gun River Formation, Locality SC on Fig. 1).
214 C. R. BARNES
ORDOVICIAN-SILURIAN BOUNDARY IN ANTICOSTI 215
cussion here of the correlations suggested by all the different fossil groups. Fairly precise
correlations can be made from Anticosti to the various sections in Gaspe and New Brunswick,
to sections in central and western North America, and to Norway (e.g. Lespérance 1985;
Barnes & Bergstrom, this volume, p. 325). These correlations can be effected best through use of
conodonts, ostracodes, shelly fossils and palynomorphs (Fig. 2).
The more difficult correlation is with oceanic graptolitic sequences, for example with the
Dob’s Linn stratotype. This problem has been addressed from different viewpoints by Barnes &
Bergstrom; Barnes & Williams; and Riva (all in this volume). There is no precise correlation
since Dob’s Linn contains few fossils other than graptolites and these are rare in the Anticosti
boundary interval. Barnes & Bergstrom (this volume) conclude that the conodont faunal turn-
over, so dramatically seen on Anticosti and elsewhere, must occur at a level within the upper
Glyptograptus persculptus Zone up to, but no higher than, the base of the Akidograptus acumin-
atus Zone (the formally defined base of the Silurian). The major extinction event in conodonts
and graptolites thus occurs within latest Ordovician time and not at the new systemic bound-
ary. The earliest Silurian conodonts on Anticosti referred to in this paper may therefore be of
latest Ordovician age (e.g. latest G. persculptus Zone) but at this point it is both impossible to
be so precise and impractical not to refer them to the Silurian, since they are so distinctively
different from Ordovician forms and form the basis for correlation in the extensive Lower
Silurian carbonate platform sequences.
The conodonts, palynomorphs, aulacerid stromatoporoids, and, to a lesser extent, the bra-
chiopods and trilobites show distinct faunal changes at essentially the same level in member 7
of the Ellis Bay Formation. Some other groups, however, seem to show a significant change
within 20-SOm higher in the sequence (e.g. ostracodes, corals). Assuming that the extinctions
are induced directly or indirectly by the glacial climatic events (e.g. Brenchley 1984; Barnes
1986) it is to be expected that different fossil groups would respond to such environmental
pressures in different ways and at slightly different times.
Future studies and potential
The beauty of Anticosti Island is not only in its modern fauna, flora and scenery but in the
magnificent quality and potential of the stratigraphical sections. The Ellis Bay section has
virtually all the attributes for a boundary stratotype: well exposed, continuous sedimentation,
variable lithology, abundant and diverse faunas and floras, no structural deformation, little
thermal alteration, geographically accessible, a reasonably sound knowledge base and long-
term protection. In comparison to Dob’s Linn, it lacks abundant graptolites and _ historical
precedence. However, in most of the other criteria, the Dob’s Linn section has serious weak-
nesses to the point at which important stratigraphical principles have been disregarded or
overruled in making the final stratotype decision (Lespérance et al. 1987). Whereas there may
be little more significant faunal data to be extracted from the well-collected Dob’s Linn section,
Fig. 4 Ordovician-Silurian boundary interval, Anticosti Island, Québec. A—C, Point Laframboise
(Locality 2A, fig. 1); A: 2m tape is at level of bioherms, member 7, Ellis Bay Formation overlain by
lower Becscie Formations; B: detail of biohermal—interbiohermal relationships, grainstones well
developed against upper quarter of bioherm; C: detail of upper bioherm surface with crinoidal
grainstones abutting and overlapping coral heads. D-G, West side of Ellis Bay, north of Cap Henri
(Locality 2B, Fig. 1); D: view of cliff exposures of member 5, Ellis Bay Formation (background)
and member 6 (foreground), wave platform covered by high tide; E: members 6 and 7, Ellis Bay
Formation, hammer (40cm) rests on top of oncolitic platform bed which forms base of member 7,
overlain by this recessive shale and interbiohermal strata; F: similar view to E but showing
bioherm developed on oncolitic platform bed above hammer; systemic boundary drawn SOcm
above top of oncolitic platform bed; G: lower Becscie Formation, hammer is 40cm. H, Salmon
River, 9 mile pool (Section 8B, Fig. 1); massive bed in centre is 90cm oncolitic platform bed,
member 7, Ellis Bay Formation, overlain by interbiohermal strata; hammer is 40 cm, earliest
Silurian conodonts occur in top 10 cm of massive bed.
216 C. R. BARNES
the Anticosti sequence will clearly continue to yield a wealth of new data and its future
potential in studies through the Ordovician—Silurian boundary is probably unsurpassable.
Although the systemic boundary has been decided, its reconsideration may be necessary
(Lespérance et al. 1987). Future work will also concentrate on unravelling the type and timing
of processes that caused such major extinctions. Sepkoski (1982) has calculated that 22 per cent
of all families became extinct at this boundary, making it second only to the terminal Permian
extinction in severity among Phanerozoic biotic crises.
Acknowledgements
The author acknowledges financial research support from the Natural Sciences and Engineering Council
of Canada. Research assistance from F. H. C. O’Brien is greatly appreciated. This paper summarizes the
recent work of many specialists working on the Anticosti Island sequence and the author’s own work has
benefited considerably from the logistic help and scientific discussions of, in particular, T. E. Bolton, M. J.
Copeland, S. L. Duffield, P. J. Lespérance, A. D. McCracken, G. S. Nowlan, A. A. Petryk and T. T.
Uyeno. G. S. Nowlan kindly criticized an early draft of this paper.
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1981a. Stratigraphy, Sedimentology, and Paleogeography of the Upper Ordovician—Lower Silurian
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Amsterdam, 32: 1-19.
Graptolites at and below the Ordovician—Silurian
boundary on Anticosti Island, Canada
J. Riva
Department of Geology, Laval University, Ste Foy, Québec, Canada, G1K 7P4
Synopsis
Graptolites in the lower and middle Vauréal Formation of Anticosti Island, Canada, form a discrete
assemblage renamed the Amplexograptus prominens Zone, characterized by Amplexograptus latus, Recto-
graptus abbreviatus, Amplexograptus prominens and Paraclimacograptus decipiens sp. nov.: these suggest
correlation with the Dicellograptus anceps Zone of Scotland, the Climacograptus pacificus Zone of north-
eastern Siberia and Kazakhstan, and the Wufeng Shale of Central china. Graptolites are rare in the upper
Vaureal Formation. A few have been collected from the upper members of the Ellis Bay Formation and
the lower members of the Becscie Formation, but not in sufficient numbers to form a zonal assemblage.
Most of them belong to the normalis group for which the new genus Scalarigraptus is proposed. The most
common graptolite is Scalarigraptus angustus, which is known to range through the upper Ashgill and the
lower Llandovery Series. Two fragmentary specimens identified as Rectograptus abbreviatus have been
collected from the top (Member 6) of the Ellis Bay Formation. This species is only known from the Upper
Ordovician and may be taken to indicate that the top members (6 and 7) of the formation belong to the
Ordovician System.
Introduction
In an earlier paper Riva (Riva & Petryk 1981) reviewed and updated the work done by
previous workers on graptolites from the Island of Anticosti, either as part of a general
palaeontological study (Twenhofel 1928) or as detailed morphological studies of isolated grap-
tolites (Barrass 1953; Strachan 1954). It also updated the study of subsurface collections which
had been extracted by Riva (1969) from three drill cores during the summers of 1964 and 1965,
and presented an evaluation of 33 new collections made by A. A. Petryk from 1975 to 1979
from the upper Vauréal to the Jupiter Formations. An accompanying range chart showed the
stratigraphical position of all graptolites hitherto identified from surface collections. This chart
will undergo further revisions and refinements as new morphological studies and revisions of
type collections are made known. Part of this work is incorporated into this paper together
with data on new collections made by Petryk from 1981 to 1983.
This paper is primarily concerned with the graptolites collected at or just above or below the
Ordovician-Silurian boundary now located at the Ellis Bay—Becscie formational contact (Fig.
1) (Lespérance 1985). It also re-evaluates the fauna of the Amplexograptus prominens Zone of
the lower and mid-Vauréal Formation and correlates it with the zonal successions of Scotland,
the U.S.S.R., China and Australia. Figure 1 shows the range of all graptolites hitherto identified
from the mid-Vauréal to the lower Becscie Formations plotted against the revised surface
stratigraphy and nomenclature of Petryk (1979). The graptolites from below the mid-Vauréal
Formation, which are known only from drill-cores, have been treated separately (Riva 1969).
A graptolite zone and other graptolites
The Amplexograptus prominens Zone. This is the youngest of the zones proposed by Riva (1969)
from his study of drill cores and the only one recognized from surface exposures of the Vaureal
Formation. In the N.A.C.P. well (Riva 1969: fig. 12) it spans much of the lower Vauréal
between the 2047-1734 ft level (614-412 m), for a thickness of 202m. In both the N.A.C.P. and
the L.G.P.L. wells (Riva 1969: figs 11 and 12) it follows on the Dicellograptus complanatus Zone
which spans most of the underlying ‘English Head’ Formation (to be renamed the Princeton
Lake Formation) for a thickness of 193m. Originally, Riva (1969) named the A. prominens Zone
Bull. Br. Mus. nat. Hist. (Geol) 43: 221-237 Issued 28 April 1988
Vy) J. RIVA
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Fig. 1 Graptolite ranges in the upper Vauréal, Ellis Bay and lower Becscie Formations of Anticosti
Island.
the Climacograptus prominens—elongatus Zone and interpreted its fauna (constituted primarily
of biserial graptolites not easily related to those of other successions) as representing a level
‘intermediate between ... the youngest Ordovician and the oldest Silurian’ (1969: 551). He also
referred the species used to name the zone to Climacograptus rather than Amplexograptus, as
Barrass (1953) had done, because most specimens recovered from the core possessed clima-
cograptid thecae with everted apertures rather than amplexograptid thecae. In 1981 he re-
named the zone the Amplexograptus inuiti Zone on the recognition that A. elongatus Barrass
was identical to Amplexograptus inuiti described by Cox (1933) and also its junior synonym. He
also re-interpreted Amplexograptus prominens Barrass as a subspecies of A. inuiti.
In 1985, I studied and sorted out the type material of Climacograptus latus Elles & Wood
and came to the conclusion that this species belongs to Amplexograptus rather than Cli-
macograptus, s.l., and is also identical to, and the senior synonym of, A. inuiti. A. latus was
erected on flattened, fragmentary material and A. inuiti (Figs 4b—c) on excellent, isolated speci-
mens from Akpatok Island in northeastern Canada. Cox (1933: 2) pointed out the similarity of
A. inuiti to A. latus, but refrained from considering the two species identical because the thecal
apertures of A. latus were ‘more even’ and lacked genicular flanges. In reviewing the type
specimens of A. latus, | recognized apertural lappets in all specimens retained in the species and
also residual genicular flanges (Figs 2a—h), but not in the specimens that I have excluded from it
(Figs 2i-)), which belong to Climacograptus tubuliferus Lapworth. These features are even more
pronounced in the topotype material recently identified as C. latus by Williams (1982: pl. 3, figs
12-18). The occurrence of A. latus in the A. prominens Zone of Anticosti is critical, for it allows
us to correlate this zone with the Dicellograptus anceps Zone of Scotland and the Cli-
GRAPTOLITES FROM ANTICOSTI ISLAND 223
macograptus pacificus Zone of the U.S.S.R. and their equivalents in China, the Yukon, and
elsewhere, something which had not hitherto been possible. I have, however, refrained from
naming this zone after either D. anceps or C. pacificus because neither graptolite has been
recovered from Anticosti.
Amplexograptus prominens itself is morphologically quite distinct from Amplexograptus latus
and cannot be regarded as a mere subspecies or a morphological variant of it. The study of an
original collection of A. prominens (made by Col. C. C. Grant) from the type strata at Observa-
tion Cliff on the north shore of Anticosti Island fully confirms Barrass’ (1953) original diagnosis
of this species. A. prominens is characterized by broad, short rhabdosomes which expand
rapidly from a narrow proximal end (first pair of thecae), by prominent genicular flanges and
the absence of a mesial spine on th 11. The long genicular flanges and the lack of a mesial spine
on th 1’ set A. prominens well apart from all other species of Amplexograptus, although it shares
with them a similar type of proximal-end development (early prosoblastic) and thecal style
(amplexograptid with well-developed lappets) (Riva 1987). A. prominens is a unique species,
known up to now only from the upper Vauréal Formation of Anticosti. It is the last Amplexo-
graptus. It could well be the immediate ancestor to Paraclimacograptus decipiens sp. nov. which
has a long range through the upper Vauréal and with which it has been confused in the past. P.
decipens differs from A. prominens both in thecal form and the nature of genicular ornaments
(Fig. 2s) but otherwise it shares with it the same type of proximal development and general
distal rhabdosome structure (Figs 20-r). On the other hand, the isolated specimens from
Manitoba referred to A. prominens by Jackson (1973: 2-4; text-figs 2B, E and F) are close to the
topotypes of the older Paraclimacogratus manitoulinensis (Caley) shown here as Figs 5g, h and 1.
Occasional low or incipient lappets are present both on the everted thecal apertures of the
Manitoba specimens and the topotype specimens of P. manitoulinensis, and the Manitoba
specimens have also a keel-like appression on outer margin of th 1’. One specimen referred to
Amplexograptus inuiti by Jackson (1973: text-fig 2D) has also a mesial spine on th 1! in
addition to the keel-like structure. This sort of structure has not been observed in topotype
specimens of P. manitoulinensis, but a mesial spine has been reported and figured by Walters
(1977) in specimens from the Lorraine Group of the St Lawrence Lowlands.
The name Paraclimacograptus decipiens is proposed below for the short, stubby biserial
graptolites which stratigraphically follow on A. /atus in the upper Vauréal Formation (Fig. 1).
P. decipiens is morphologically close to A. prominens for which it may be easily mistaken (hence
its specific name), but its thecae are of the paraclimacograptid type with clearly everted thecal
apertures and reduced genicular flanges supported by two short genicular spines (Fig. 2s). The
development of the proximal end is of the prosoblastic type and th 1! lacks a mesial spine,
much as in A. prominens. The problem now arises as to the proper generic affiliation of the new
species, which could be either in the genus Paraclimacograptus Pribyl, 1948 or Paraortho-
graptus Mu, 1974. Paraclimacograptus has P. innotatus (Nicholson) as type species. P. innotatus
(Figs 5l-n) is a thin, short graptolite, restricted to the lower Llandovery, with an advanced
prosoblastic type of proximal-end development, thecae slightly inclined to the axis of the
thabdosome with wide apertural excavations, everted thecal apertures and short genicular
processes which turned out to be flanges in isolated Siberian specimens (Crowther 1981: pl. 13,
fig. 4). It lacks a mesial spine on th 1'. Rickards (1970: 32) has also noted a complete median
septum on deformed specimens identified as C. innotatus, but it is probably the trace of the
virgula. Paraorthograptus has P. typicus Mu from the Upper Ordovician Paraorthograptus
uniformis Zone of the Wufeng Shale of central China as type species. This species was described
as having *... thecae of the orthograptid type with paired ventral spines ... pointed obliquely
downward at the proximal end, horizontal at the distal end ... Interthecal septa straight,
slightly inclined, not curved; apertural margins everted, not horizontal .... (Mu et al. 1974: 161;
translated). No mention was made of the proximal end, which is not preserved in the holotype
specimen (Fig. Sa); it is preserved, however, on a complete specimen on the type slab (Fig. 5b)
and shows an apparently advanced type of proximal-end development, much as in Paraortho-
graptus pacificus (Ruedemann) (Figs Sc-f). The type species of Paraclimacograptus and Para-
orthogratus share the same basic rhabdosome morphology, i.e. a prosoblastic type of
224 J. RIVA
proximal-end development, thecae inclined to the rhabdosome axis and wide thecal excavations
with everted apertural margins. They differ, however, in the type and size of genicular processes
which are flanges in species assigned to Paraclimacograptus (Fig. 5j) and genicular spines of
various length in species included into Paraorthograptus. The latter also have a mesial spine on
th 1’, a virgella and antivirgellar spines, whereas the former generally lack a mesial spine on th
1* (except in some specimens of P. manitoulinensis figured by Walters 1977) and also, appar-
ently, antivirgellar spines in the younger species such as P. innotatus (see Crowther 1981: pl. 13,
fig. 4). The problem is whether two genera are needed to group species on the basis of external
morphology, conspicuous as it may be. Lin & Chen (1984: 216), for instance, have tried to solve
this problem by simply assigning Climacograptus innotatus Nicholson to Paraorthograptus in
describing Chinese specimens identified and figured as Paraorthograptus innotatus (Nicholson).
However, a study of the Chinese specimens has revealed that they are fragmentary growth or
juvenile stages of P. typicus. One of them, complete with mesial spine on th 1’ and long, paired
genicular spines, is shown here as Fig. 5k. This deviation notwithstanding, I feel that the genus
Paraclimacograptus should group species characterized by a prosoblastic proximal develop-
ment (advanced as in the type species or more primitive as in P. manitoulinensis), thecae
inclined to the rhabdosome axis, wide thecal excavations, everted apertures and genicular
flanges. The genus Paraorthograptus should group all species which, in addition to the basic
morphology of the paraclimacograptids, have genicular spines rather than flanges, a mesial
spine on th 1' and antivirgellar spines. Paraclimatograptus decipiens has genicular processes
consisting of reduced flanges supported by short, lateral spines (Fig. 2s). It may be regarded as
a transitional form between species assigned to Paraclimacograptus and Paraorthograptus, but
the fact that flanges are still present, genicular spines poorly developed and the rhabdosome
lacks a mesial spine on th 1' support its inclusion in Paraclimacograptus, and it will be so
described below. |
The following graptolites have been identified from the P. prominens Zone from surface
outcrops and the N.A.C.P. drill core (Fig. 1): Amplexograptus latus (Elles & Wood), Amplexo-
graptus prominens Barrass, Paraclimacograptus decipiens n.sp., Glyptograptus cf. G. hudsoni
Jackson, Peiragraptus fallax Strachan, Rectograptus abbreviatus (Elles & Wood), Orthograptus?
and Desmograptus sp. In the N.A.C.P. well (Riva 1969), Amplexograptus latus has a short, 34m
long range at the base of the P. prominens Zone, from the 2047 to the 1933 ft level (614-579 m),
whereas P. decipiens ranges through the middle and upper part of the zone, from the 1647 to
the 1376 ft level (493-412 m), for a total of at least 80m. Glyptograptus cf. G. hudsoni (Figs 2k—n)
was described by Jackson (1971) from the Upper Ordovician of Southampton Island, north of
Labrador and Akpatok Island; in the N.A.C.P. well it has a long range extending through both
the D. complanatus and the A. prominens Zones to terminate somewhere in the upper Vauréal
Formation (Fig. 1), for a total of at least 650m; P. fallax is a rare graptolite and has been
recognized in only one collection from the mouth of the Patate River in association with A.
latus, R. abbreviatus and G. cf. G. hudsoni (Riva & Petryk 1981); R. abbreviatus occurs sporadi-
cally through both the D. complanatus and A. prominens Zones and two specimens were also
collected by A. A. Petryk from member 6 of the Ellis Bay Formation, just below the
Ordovician-Silurian boundary (Fig. 31).
Correlation of the A. prominens Zone. A. latus is a cosmopolitan graptolite long recorded from
the D. anceps Zone of southern Scotland and, especially, the D. complexus and P. pacificus
Subzones (Williams 1982). This allows us definitely to correlate the A. prominens Zone of
Anticosti Island with the uppermost British Ordovician. A. latus also occurs in the C. supernus
Zone of Kazakhstan (Koren et al. 1980), where it has been described as Amplexograptus
stukalinae, the C. pacificus Subzone of the Omulev Mountains of Siberia (Koren et al. 1983),
where it is represented by A. latus hekandaensis, the Amplexograptus yangtzensis to the Diplo-
graptus bohemicus Zones of the Wufeng Shale of central China (Mu & Lin 1984), where A. latus
has been called A. suni and A. yangtzensis (Fig. 4a), and from the Bolindian D. ornatus and C.
latus Zones of Victoria, Australia (VandenBerg 1981a). The A. prominens Zone of Anticosti is
correlated with all the above-mentioned zonal levels (Fig. 1).
GRAPTOLITES FROM ANTICOSTI ISLAND 225
Graptolites from the Ellis Bay and lower Becscie Formations. Graptolites are scarce above the
A. prominens Zone. Few graptolites have been collected above member 2 of the Vauréal
Formation besides a few specimens of G. cf. hudsoni (Figs 2l-n). Graptolites are also scarce in
the Ellis Bay and Becscie Formations: the few collected are either indicative of the uppermost
Ordovician or are long-ranging species that straddle the Ordovician—Silurian boundary.
Members 4 and 7 of the Ellis Bay Formation have yielded fragmental climacograptids which I
have assigned to Scalarigraptus angustus (Elles & Wood); one of them is shown as Fig. 31. In
Scotland this graptolite ranges through the D. anceps Zone (Williams 1983: fig. 3) and may be
taken to indicate that member 6 of the Ellis Bay is of uppermost Ordovician age. At Salmon
River, the Becscie Formation has yielded fragments of S. angustus from its contact with the top
of reef structures of the Ellis Bay upwards (Fig. 1). An excellent three-dimensional specimen of
S. angustus was collected by A. A. Petryk a few metres above the base of the Becscie (Figs 3t, u);
two small collections of this species were made 7 and 30m above the base of the formation at
pool 9 on Salmon River (Figs 3j—s) and one specimen (Fig 3v) was collected from the Gun River
Formation, well above the Ordovician—Silurian boundary. This is the longest specimen of S.
angustus collected on Anticosti Island. S. angustus ranges from the Ashgill to the lower Llando-
very, and it is common in the D. anceps, G. persculptus, A. acuminatus and other Zones at or
above the Ordovician-—Silurian boundary and cannot be regarded as a good zonal indicator. In
closing, it will be noted that a large climacograptid approaching Scalarigraptus normalis
(Lapworth) in size (Fig. 3w) was collected by T. E. Bolton from the basal Becscie Formation on
the east side of Ellis Bay at Cap-a-l’Aigle. S. normalis is only known from the G. persculptus
Zone to the lower Llandovery.
The new genus Scalarigraptus. The occurrence of graptolites of the normalis (or scalaris) group
in the Ellis Bay and Becscie Formations brings again to the fore the problem of their generic
affiliation which cannot any longer be the traditional polyphyletic genus Climacograptus
Hall. Climacograptus was created by Hall (1865: 111-112) for ‘simple stipes with sub-parallel
margins having a range of cellules (thecae) on each side’, which were to be ‘short and square’.
Graptolithus bicornis was designated as the type species and the members of the G. scalaris
group of Linné were ‘conceived’ as the ‘veritable species of this genus’. (The generic name
Climacograptus was obtained by adopting the Greek noun klimax, equivalent to the Latin
scala, ladder, of which scalaris is the adjective). Since its creation, this genus has known
enormous popularity, having been used as a generic umbrella for all sorts of biserial graptolites
characterized by square or climacograptid thecae, at least in the mature or distal part of the
thabdosome. Elles & Wood (1906) attempted to deal with the large number of British grapto-
lites assigned to Climacograptus by dividing them into five groups on the basic of thecal outline,
type of apertural excavation or thecal ornaments such as spines, but did not propose new
genera or subgenera. Pribyl (1947, 1948), on the other hand, went a step further and proposed
the genus Pseudoclimacograptus for climacograptids characterized by a zig-zag median septum
connected by transverse rods to the thecal septa and the genus Paraclimacograptus for cli-
macograptids with genicular spines. The genus Pseudoclimacograptus has since been widely
accepted by graptolite specialists, but the genus Paraclimacograptus has been overshadowed by
the genus Paraorthograptus Mu, 1974. Riva (1974b, 1976) showed, on the basis of three-
dimensional topotype material, that C. bicornis had a primitive diplograptid, or streptoblastic,
type of proximal-end development and thus differs significantly from other climacograptids
with a prosoblastic type of proximal-end development. The graptolites of the scalaris group,
considered by Hall (1865: 112) as the ‘veritable species’ of Climacograptus, have an advanced
prosoblastic type of proximal-end development and cannot be regarded as true cli-
macograptids, although they share with C. bicornis a similar distal development. For this
reason, I am proposing the new genus Scalarigraptus for all graptolites of the ‘scalaris’ or
normalis group and for all Ordovician climacograptids with an advanced prosoblastic type of
proximal-end development and with a septate or partly septate rhabdosome. C. normalis will be
designated as the type species of the new genus.
In 1949 Obut erected the genus Hedrograptus for early Silurian climacograptids with insig-
nificant or incomplete apertural excavations on one side of the rhabdosome and complete on
226 J. RIVA
the other. The figures of the type species, H. janischewskyi Obut, show that it is also character-
ized by an advanced prosoblastic type of proximal-end development, just like C. normalis. In
1975, Obut extended Hedrograptus to include all climacograptids of the scalaris group. This
would mean that Hedrograptus rather than the proposed Scalarigraptus is actually the genus
intended for climacograptids of the scalaris group. I have, however, thanks to the cooperation
of A. M. Obut, been able to examine a latex cast of the holotype of H. janischewskyi (Fig. 6a)
and conclude that Hedrograptus is based on an incomplete and distorted specimen preserved in
+-face view which does not allow us to ascertain whether the thecae are climacograptid or
glyptograptid. Another specimen from the type locality, also preserved in }-face view (Fig. 6b),
is much larger than the holotype of H. janischewskyi and probably not conspecific with it. For
these reasons, I have been reluctant to adopt Hedrogratus and propose instead the genus
Scalarigraptus.
Systematic palaeontology
Family DIPLOGRAPTIDAE Lapworth, 1873
Genus AMPLEXOGRAPTUS Elles & Wood, 1907
Amplexograptus latus (Elles & Wood)
Figs 2a—h, 4
1906 Climacograptus latus Elles & Wood: 204-205; pl. 27, figs 3a—e and g, non figs 3f-h; text-figs
135a—d.
1933 Climacograptus inuiti Cox: 1-19, pls 1, 2.
1953 Amplexograptus elongatus Barrass: 62-66; figs 6-8.
non 1970 Climacograptus latus Elles & Wood; Toghill: 22; pl. 15, figs 1, 2.
1974 Amplexograptus disjunctus yangtzensis Mu & Lin; Mu et al.: 162; pl. 70, fig. 6.
1980 Amplexograptus stukalinae Mikhailova; Koren et al.: 125-126; pl. 4, figs 1, 2.
1982 Climacograptus latus Elles & Wood; Williams: 39-40; pl. 3, figs 12-18. [See also for other
synonyms. |
1983 Climacograptus latus hekandaensis subsp. nov.; Koren & Sobolevskaya: 116-117; pl. 30, figs
2-6; pl. 31, figs 1-3.
1983 Climacograptus latus Elles & Wood; Wang et al.: pl. 3, fig. 1.
1984 Amplexograptus suni (Mu); Mu & Lin: 56; pl. 5, figs 4-6.
Fig. 2 Type specimens of Amplexograptus latus (Elles & Wood, 1906) and graptolites from the
Vauréal Formation. ah, Type specimens of A. /atus from the upper Hartfell Shale, Main Cliff,
Dob’s Linn; a, SM 19683b (Elles & Wood 1906: text-fig. 135a), paralectotype, x 5; b, SM A19680
(Elles & Wood 1906: pl. 27, fig. 3a), proposed lectotype, x 5; c, BU 1195 (Elles & Wood 1906: pl.
27, fig. 36), paralectotype, x 5; d, SM 19682a (Elles & Wood 1906: pl. 27, fig. 3g, text-fig. 135c),
paralectotype, x 5; e, BU 1412b, unfigured paralectotype (on the same slab as BU 1412a of
Fig. 2j), x 5; f, SM A19683c, unfigured growth stage, x 10; g, BU 1411a (Elles & Wood: pl. 27, fig.
3e), paralectotype, x 5; h, BU 1411b, unfigured paralectotype, x 5; 1-j, Scalarigraptus tubuliferus
(Lapworth) originally included in the type material of A. latus; 1, BU 1413 (Elles & Wood: pl. 27,
fig. 3h) doubtfully included, x 5; j, BU 1412a (Elles & Wood 1906: pl. 27, fig. 3f), x 5; k—n,
Glyptograptus cf. G. hudsoni Jackson; k, G.S.C. 82880, from the 2739 ft (822 m) level in the N.A.C.P.
core, x 5; 1, m, G.S.C. 82881, from member 2 of the Vauréal Formation at Cap Crotté, Anticosti
Island (A. A. Petryk’s coll. 76 AP29-1), respectively x 10 and x 5; n, G.S.C. 82882, same locality
and collection, x 5; o-s, Paraclimacograptus decipiens sp. nov.; 0, G.S.C. 82883, holotype, longest
specimen recovered from the 1376ft (413m) level in the N.A.C.P. core x 5; p, GS.C. 82884,
paratype, a large macerated specimen (A. A. Petryk’s coll. 83 AP6-5), from 90m above the mouth
of Patate River, member 2, Vauréal Formation, x 5; q-s, G.S.C. 82885, 82886, paratypes, isolated
specimens from the 1381 ft (414m) level in the N.A.C.P. core showing the development of the
proximal-end thecal structure, x 15. Note the development of vertical cortex filaments in the
apertural excavations of th 2? and 37.
MDT
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Lectotype. SM A19680 (Fig. 2b) (Elles & Wood 1906: pl. 27, fig. 3a) from the upper Hartfell
Shale, D. anceps Zone, Main Cliff, Dob’s Linn, Scotland. Herein selected.
PARALECTOTYPES. SM A19683b and A19682a (Figs 2a, b), BU 1195 and 1411a (Figs 2c, g), BU
1414 and 1196 (not figured because of poor preservation) and the following specimens from the
type collection, not previously figured: BU 1412b (Fig. 2e), 1411b (Fig. 2h) and a growth stage,
SM A19683c (Fig. 2f). BU 1413 and 1412 (Figs 21, j) have been excluded from A. latus and
assigned to C. tubuliferus.
OTHER MATERIAL EXAMINED. Several topotype specimens of A. inuiti from Akpatok Island, the
N.A.C.P. drill core and surface collections made by A. A. Petryk from member 2 of the Vauréal
Formation, Anticosti Island. The type and topotype material of Amplexograptus stukalinae
Mikhailova and of Climacograptus latus hekandaensis Koren & Sobolevskaya stored either at
the VSEGEI in Leningrad or at the Institute of Geology and Palaeontology of the Akademya
Nauk, Moscow, U.S.S.R.; the type or topotype material of Amplexograptus suni (Mu) and
Amplexograptus disjunctus yangtzensis Mu & Lin at the Institute of Geology and Palaeontol-
ogy, Academia Sinica, Nanjing, and at the Institute of Geology and Mineral Resources,
Academy of Geological Sciences, Yichang, China.
DESCRIPTION. Rhabdosome up to 5 to 6cm in length, gradually widening from 0-8—1-1 mm at
the level of th 17 aperture to a maximum of 2:2-2:-4 (exceptionally 2:6) mm distally, attained
within 2 or 3 cm. The average width, however, is less than 2mm, generally 1-6-1:83mm. A
waist-like constriction may also be noted in some specimens above the the first pair of thecae.
Thecae 14-12 in 10mm proximally, decreasing to 11-12 distally. Development of proximal end
of prosoblastic type (Cox 1933: 6, 7; figs 1-21). The sicula is 1-5 mm long; it secretes a virgella
and two antivirgellar spines. Th 1' originates low in the metasicula, grows down along the
virgellar side to the sicular aperture, then turns out and upwards, secreting a mesial spine at the
point of upward growth; th 1? buds off from the downward-growing portion of th 1', grows
around the reverse side of the sicula to turn up at the point of issuance of the antivirgellar
spines (Fig. 4). Th 2’ buds off th 1’ and th 2? from th 1? and so on alternately to the distal end
of the aseptate rhabdosome. Thecae are of the amplexogratid type with apertural lappets and
thecal excavations occupying about 4 of the rhabdosome width. A selvage runs around the
thecal apertures and the infragenicular walls to form a short genicular flange.
REMARKS. The type material of A. latus was mixed, containing two specimens herein assigned to
C. tubuliferus (Figs 2i, j). Because of its world-wide distribution, this species has been identified
and described as C. latus and also under a number of names such as C. inuiti and A. stukalinae
Mikhailova, Climacograptus latus hekandaensis Koren & Sobolevskaya for specimens from
Kazakhstan and NE Siberia, and as Amplexograptus disjunctus Mu & Zhang, Climacograptus
suni (Mu) and Amplexograptus disjunctus yangtzensis Mu & Lin for specimens from the Upper
Ordovician Wufeng Shale of central China. A. yangtzensis is a species in its own right and not a
subspecies of A. disjunctus, a nomen nudum, the type of which could not be located in a recent
study visit to Nanjing. It is based on a single three-dimensional specimen (Mu et al. 1974: pl.
70, fig. 4), here refigured as Fig. 4a, from a zone of the same name in the lower Wufeng Shale,
where graptolites are generally preserved in relief in a black shale. Farther up in the Wufeng
Shale, A. yangtzensis is replaced by A. suni, which differs from A. yangtzensis only in being
preserved as flattened, brown, flaky films.
The specimens from Dob’s Linn, Scotland, identified as C. latus by Toghill (1970) belong to
either S. normalis or S. tubuliferus.
STRATIGRAPHICAL AND GEOGRAPHICAL OCCURRENCE. A. latus is restricted to the D. anceps Zone
of Scotland (Williams 1982) and may be considered as one of its diagnostic fossils. It is a widely
distributed, cosmopolitan species, known from the C. supernus Zone of Kazakhstan and NE
Siberia, the Upper Ordovician of China and correlative strata elsewhere. In SE Australia it
helps name the upper Bolindian D. ornatus—C. latus Zone (VandenBerg 198 1a).
GRAPTOLITES FROM ANTICOSTI ISLAND 229
Genus PARACLIMACOGRAPTUS Pribyl, 1948
TYPE SPECIES (by original designation). Climacograptus innotatus Nicholson (Nicholson 1869:
238; pl. 11, figs 16, 17).
DIAGNOSIS (amended from Pribyl 1948: 40-41, 47-48, fig. 6). Rhabdsome aseptate, apparently
ovoid in cross-section; thecae of the paraclimacograptid type, inclined to the axis of the
thabdosome; apertural excavations wide and deep with everted thecal apertures and genicular
flanges, strengthened by a selvage (list) split into two short spines at the geniculum in some
species. Proximal end characterized by a prosoblastic type of development, and provided with a
virgella, antivirgellar spines and, exceptionally, a mesial spine on th 1' (in older species).
INCLUDED SPECIES. The following species may be included in Paraclimacograptus: Paraclimaco-
graptus innotatus (Nicholson), Paraclimacograptus manitoulinensis (Caley), Paraclimacograptus
decipiens sp. nov., Paraclimacograptus sp., an undescribed species from the Climacograptus
wilsoni Zone of Gaspé, Canada.
Climacograptus innotatus nevadensis Carter (Riva 1974a: figs 2k—m) from the late mid-
Ordovician of Nevada, Texas (Marathon region), Oklahoma (unpubl. data) and Australia
(VandenBerg 1981b) is close to Scalarigraptus. This species has an advanced prosoblastic
proximal-end development, thecae of the climacograptid type with stiff genicular spines in the
first six to twelve pairs, a long virgella accompanied by a sicular downgrowth, a long inflated
virgula and a sicula lacking the prosicula. These characteristics brings it closer to the
scalarigraptids of the tubuliferus group of the Upper Ordovician rather than to the para-
climacograptids.
Paraclimacograptus decipiens sp. nov.
Figs 20-s
Ho.otyPe. G.S.C. 82883 (Fig. 20), from the 1376 ft (413m) level in the N.A.C.P. core, upper
Vauréal Formation, Anticosti Island.
PARATYPES. G.S.C. 82884 (Fig. 2p), from 90m above the mouth of Patate River, Anticosti
Island, member 2 of the Vauréal Formation; G.S.C. 82885 and 82886 (Figs 2q-s), isolated
growth stages from the 1381 ft (414 m) level of the N.A.C.P. core, upper Vauréal Formation.
Name. Latin decipiens, deceiving.
DESCRIPTION. Rhabdosome of moderate length, usually not exceeding 2 to 3cm, maximum
observed 4cm (Fig. 20), widening rapidly from 0-8-1-0mm at the level of the aperture of th 1?
to 1-6—2-0mm (maximum observed 2:-4mm) at the level of the 4th to Sth pair of thecae. Thecae
numbering 8 in 5mm, or 15 in 10mm, proximally, decreasing to 12-13 in 10mm distally, of the
paraclimacograptid type with everted thecal apertures, except for the first two which have low
lappets (faintly visible also on the second pair of thecae in Fig. 2s). Interthecal septa inclined at
20° to 40° to the rhabdosome axis; supragenicular walls parallel or slightly inclined to it.
Thecal excavations wide, occupying + of the rhabdosome width, reinforced by a selvage
running around the thecal aperture and the infragenicular wall and terminating as two short,
stiff genicular spines supporting a reduced hood (Fig. 2s). Development of the proximal end of
the prosoblastic type. Sicula about 1-Smm long, partly exposed on the obverse side of the
thabdosome (Figs 20, s). Th 1! originates low in the metasicula, grows down the virgellar side
to the sicular aperture before turning out and upwards to terminate about level with its point
of origin. Th 1? buds off the downward-growing portion of th 1', grows diagonally around and
up on the obverse side of the rhabodosome (Fig. 2r); th 2! buds off from th 17 and th 2? from
th 1? and so on alternately to the distal end of the rhabdosome. A thin nema passes through
the rhabdosome and extends a short distance beyond it. The rhabdosome is aseptate.
REMARKS. The development of the proximal end of P. decipiens is identical to that of A. latus
and A. prominens, suggesting a close genetic relationship between the three species. P. decipiens
is much larger than P. innotatus which has a proximal development of the advanced prosoblas-
230 J. RIVA
tic type. P. decipiens is much closer to P. manitoulinensis from the lower Upper Ordovician of
NE North America (Riva 1969) (Figs 5g, h and 1), but this species is thinner, of uniform width
and has genicular flanges strengthened by a thickened selvage (Fig. 5j). A mesial spine on th 1!
may occur sporadically in some rhabdosomes (Walters 1977).
STRATIGRAPHICAL AND GEOGRAPHICAL OCCURRENCE. P. decipiens is only known from the 4A.
prominens Zone of Anticosti, where it has a stratigraphical range of at least 80m in the upper
Vauréal Formation (Riva 1969). It has been found also sporadically in recent surface collections
made by A. A. Petryk and in an older collection (Y.P.M. 3036/4) made by W. H. Twenhofel and
stored at the Peabody Museum of Yale University (Riva & Petryk 1981: 160).
Genus SCALARIGRAPTUS nov.
TYPE SPECIES. Climacograptus normalis Lapworth (Lapworth 1877: 138; pl. 6, fig. 31; Elles &
Wood 1906: pl. 26, fig. 2a; Williams 1983: text-fig. 4a).
NAME. From the Latin scalaris, ladder-like.
DIAGNOsIS. Rhabdosome septate or partly septate, ovoid to subrectangular in cross-section;
thecae of the climacograptid type with definite genicula, deep horizontal apertural excavations
and straight supragenicular walls, usually parallel to the axis of the rhabdosome. Proximal-end
development of the advanced prosoblastic type with only th 1’ initially growing down along
the sicula. The virgella is the only proximal spine.
INCLUDED SPECIES. The following species, among others, fall within the limits of the diagnosis of
Scalarigraptus: C. normalis, C. angustus (Perner), C. transgrediens Waern, C. medius Tornquist,
C. praemedius Waern, C. rectangularis M‘Coy, C. brevis Elles & Wood, C. putillus (Hall), C.
tubuliferus Lapworth, C. nevadensis Carter, C. yumenensis Mu and C. biformis (Mu & Lee).
Fig. 3 Syntypes of Climacograptus miserabilis Elles & Wood, 1906 and graptolites from the Ellis
Bay and the lower Becscie Formations. a—c, Syntypes of C. miserabilis; a, BU 1148b (Elles & Wood
1906: text-fig. 120b), proximal end with long virgella (freed from matrix), x 5; b, BU 1150 (Elles &
Wood 1906: text-fig. 120a), typical specimen with long virgella (freed from matrix), x 5; c, BU
1146a (Elles & Wood 1906: pl. 26, fig. 3b and text-fig. 120c), distal fragment showing thread-like
virgula passing through the thin rhabdosome, x 5. d-h, Scalarigraptus angustus (Perner) from the
Ellis Bay Formation; d, G.S.C. 82887, obverse view of growth stage preserved in relief, showing
climacograptid thecae and wavy median septum, from the oncolite platform bed, basal member 7
(A. A. Petryk’s collection 84AP8-2-1F), Pointe Laframboise, Cape Henry, x 10; e-h, G.S.C. 82888—
82891, large distorted or fragmentary rhabdosomes from upper member 4 (A. A. Petryk’s collection
81AP3-2), Baie des Navots, Ellis Bay, x 5. i, G.S.C. 82892, Rectograptus abbreviatus (Elles &
Wood), macerated specimen from member 5, Ellis Bay Formation, immediately below reef bio-
herms, 7 km upriver from mouth of Salmon River, right bank (A. A. Petryk’s collection 75APt3-3),
x 5. jm, S. angustus (Perner) from the basal beds of the Becscie Formation; j, k, G.S.C. 82893,
82894, a growth stage and an adult individual showing a thin virgella distally prolonged (A. A.
Petryk’s collection 81AP13-1-1F), from pool 9, Salmon River, 13m above the base of the forma-
tion, x 5; l-m, G.S.C. 82895, 82896, from the basal Becscie at pool 9 on Salmon River (collected by
J. Riva 1981), x 5. n-s, G.S.C. 82897-82902, growth series of S. angustus (A. A. Petryk’s collection
79AP48-4), 7m above base of the Becscie, base of pool 9 on Salmon River, x 5. t, u, G.S.C. 82903,
observe view of S. angustus preserved in excellent relief, showing wavy median septum in proximal
part of rhabdosome (A. A. Petryk’s collection 76AP22-30-6’), 2-3m above base of Becscie Forma-
tion on Salmon River, respectively x 10 and x 5. v, G.S.C. 82904, longest specimen of S. angustus
recovered from the mid-part of the Gun River Formation, 3-5km from mouth of Chute Creek,
eastern Anticosti (A. A. Petryk’s collection 75MPt18-L8C-1F), x 5. w, G.S.C. 69157, Scalarigraptus
normalis (Lapworth), collected by T. E. Bolton in 1981 from the basal Becscie Formation on the
east shore of Ellis Bay near Cap-a-l’Aigle, Anticosti Island, x 5.
231
GRAPTOLITES FROM ANTICOSTI ISLAND
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Scalarigraptus angustus (Perner, 1895)
Figs 3a—u
1895 Diplograptus (Glyptograptus) euglyphus Lapworth var. angustus Perner: 48; pl. 8, figs 14a, b.
1906 Climacograptus scalaris (Hisinger) var. miserabilis Elles & Wood: 186; pl. 26, figs 3a, b, d, e, g, h,
non figs 3c, f; text-figs a—c.
1951 Climacograptus angustus (Perner) Pribyl: 7; pl. 2, figs 2-9.
1975 Climacograptus angustus (Perner); Bjerreskov: 23; fig. 9A.
1980 Climacograptus angustus (Perner); Koren et al.: 131; pl. 37, figs 2-7; text-figs 34a-e.
1983 Climacograptus angustus (Perner); Koren & Sobolevskaya: 106-108; pl. 27, figs 1—5; text-fig. 34.
21983 Climacograptus mirnyensis (Obut & Sobolevskaya); Koren & Sobolevskaya: 132-133; pl. 37, figs
2-5; text-figs 47K—H.
1983 Climacograptus miserabilis Elles & Wood; Williams: 615-616; text-figs 3fH, ?j, 4f, Sa—b. [See
also for a more extended pre-1983 synonymy. |
Ho.otyPe. National Museum of Prague CD 1835, partly figured by Perner (1895: pl. 8, figs
14a—b) and refigured in full by Pribyl (1951: pl. 2, fig. 8).
MATERIAL STUDIED. The type collection of C. miserabilis in the Lapworth collection of Birming-
ham; part of the collections made by P. Toghill at Dob’s Linn; the type and topotype material
of C. angustus in Prague; the collections of C. angustus and C. mirnyensis at VSEGEI, Lenin-
grad, several collections made by A. A. Petryk from the Ellis Bay, Becscie and Gun River
Formations of Anticosti Island.
DESCRIPTION. Rhabdosome up to 2cm in length, widening imperceptibly from 0-8—0-9mm at
the level of th 1? aperture to a maximum of 1-0-1-1 mm (exceptionally 1-2 mm) within one pair
of thecae. Thecae of the climacograptid type, numbering 11-12 in the first 10mm, decreasing to
10-11 distally, with sharp genicula and supragenicular walls parallel to slightly inclined to the
rhabdosome axis. Thecal apertures horizontal to slightly everted; thecal excavations wide and
semicircular, occupying about 4 of the rhabdosome width and reinforced by a thin selvage
around the aperture and the infragenicular walls, terminating as a slight genicular flange
(Figs 3d, t). Development of the proximal end of the advanced prosoblastic type. Sicula from
1-2 to 1-6mm long, secreting a long virgella; it is mostly exposed on the obverse side of the
rhabdosome (Williams 1983: text-fig. 3h). Th 1’ first grows down along the sicula and then
turns out and upwards at the sicular aperture (Figs 3d, t); th 1? grows up from th 1! and th 2?
from th 17. Th 2! is also the dycalical thecae which gives rise to two independent linear series
separated by a median septum. The median septum begins on the obverse side of the rhabdo-
some at about the level of th 1* aperture (its point of origin is marked by a notch in some
specimens, Fig. 3t) and follows a wavy pattern through the first 5 or 6 pairs of thecae before
straightening out (Figs 3d and t). A thin, thread-like nema passes through the rhabdosome and
extends for some distance beyond.
REMARKS. In 1951 Pribyl pointed out that C. miserabilis Elles & Wood 1906 was identical to,
and the junior synonym of, C. angustus (Perner 1895). This synonymy was accepted by some
workers (for instance Bjerreskov 1975: 23) but not by British workers for a number of reasons
best summarized by Williams (1983: 616). Recently, I have been able to study the type material
of both C. miserabilis and of S. angustus. C. miserabilis is based on seven specimens from the D.
complanatus Zone and two from the D. anceps Zone of Dob’s Linn, Scotland. The two speci-
mens from the D. anceps Zone do not belong to C. miserabilis: one, BU 1145b (Elles & Wood
1906: pl. 26, fig. 3c), is a distal fragment of tubuliferus, and the other, BU 1149 (Elles & Wood:
pl. 26, fig. 3f), is of uncertain affiliation. The specimens from the D. complanatus Zone (three of
which are shown here as Figs 3a—c) are preserved as thin, flaky, abraded films. They all belong
to C. miserabilis. They are from 0-8 to 1-1mm wide and have 12-11 thecae per 10mm proxi-
mally and 11 distally. The proximal end bears a long virgella, and a thin nema passes through
the rhabdosome. This is all that can be learned from the type material of C. miserabilis. The
type and topotype material of S. angustus is more diversified and contains several specimens in
partial relief. (I was unable to draw any specimens, but was assisted in my work by Dr A.
GRAPTOLITES FROM ANTICOSTI ISLAND 233
Fig. 4 a, N.I.G.P. Catalogue Number 21410, holotype of Amplexograptus yangtzensis Mu & Lin
(=A. latus), x 20; b and c, SEM montages of Amplexograptus inuiti (Cox) (=A. latus) from
Akpatok Island, Canada: b, SM A102524, obverse view; c, SM A102521, reverse view, both x 20
(courtesy of Peter Crowther).
Pribyl). The specimens attain a width of 1:0-1-:1 mm, have 12-11 thecae per 10mm proximally
and 10 distally. The thecae are all of the climacograptid type with strong genicula. The proxi-
mal end bears a long virgella and a thin virgula passes through the rhabdosome. The holotype
is a complete, not partial, specimen as claimed by Strachan (1971: 34); it has been refigured in
full by Pribyl (1951: pl. 2, fig. 8). With the aforesaid in mind, I do not see any morphological
differences between the types of C. miserabilis and S. angustus and do not hesitate to place the
former in synonymy with the latter.
The specimens from the basal Becscie Formation (Figs 3j—u) are all practically identical to
the type of S. angustus and so are those from the Gun River Formation. The specimens from
member 4 of the Ellis Bay Formation (Figs 3e—-h) are wider (from 1-1 to 1-3mm) because of
poor preservation and distortion; that from the base of member 7 (Fig. 3d) has the same
dimensions as the holotype in Prague.
STRATIGRAPHICAL AND GEOGRAPHICAL OCCURRENCE. S. angustus is a cosmopolitan graptolite
ranging through the Upper Ordovician and part of the Lower Silurian. In NE Siberia (Omulev
Mountains) it is common from the base of the C. extraordinarius Zone to the top of the A.
acuminatus Zone (Koren et al. 1983: figs 62, 64). On Anticosti Island it first occurs at the top of
the P. manitoulinensis Zone (Riva 1969: figs 11, 13), below the base of the D. complanatus Zone,
and extends all the way up into the Gun River Formation of mid-Llandovery age.
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Fig. 5 a, b, Paraorthograptus typicus Mu; a, N.I.G.P. Cat. No. 21418a, counterpart of the holotype
(better preserved than the part) from the Wufeng Shale north of Yichang, central China, showing
the characteristic long, paired genicular spines of the species but with the proximal end missing (a
thabdosome of Climacograptus longispinus supernus Elles & Wood lies diagonally across its proxi-
mal end), x 5; b, unfigured specimen of P. typicus, with a complete proximal end, occurring on the
same slab as the holotype, x 5. c-f, U.S.N.M. 415038415401, rhabdosomes of Paraorthograptus
pacificus (Ruedemann) from the Phi Kappa Formation at Trail Creek, Idaho, U.S.A., near the type
locality of the species, showing their characteristic short genicular spines, both paired and triple,
and stubby form; note the tectonic deformation undergone by specimens of Figs Sc and d lying
normal to each other, x 5. gj, G.S.C. 56899, 56895, 56900 and 56901, respectively, topotypes of
Pseudoclimacograptus manitoulinensis (Caley) from the upper Whitby Formation, 5 km south of
Little Current west side of Rt 68, Manitoulin Island, Ontario, Canada; g—1, growth series showing
distinct fusellar rings, x 10; j, detail of thecal excavations showing everted thecal apertures and
well-developed genicular lappets strengthened by a selvage, x 20. k, N.ILG.P. Cat. No. 82816,
proximal end of P. typicus figured as Paraorthograptus innotatus (Nicholson) by Lin & Chen (1984:
pl. 4, fig. 7), showing the spinose processes typical of the species: virgella, antivirgellar spines,
mesial spine on th 1! and genicular spines, x 10. l-n, Paraclimacograptus innotatus (Nicholson),
topotypes from the lower Birkhill Shale (Lower Silurian) at Dob’s Linn, southern Scotland; 1, SM
A20222, specimen figured by Elles & Wood (1906: pl. 27, fig. 10a) as a ‘typical specimen’ (but not
the ‘type’ of Nicholson), x 5; m, n, SM A20232 (op. cit.: pl. 27, fig. 106), specimen showing
advanced prosoblastic development of proximal end and a partly uncovered sicula below th 17,
x Sand x 10, respectively.
GRAPTOLITES FROM ANTICOSTI ISLAND DBS
a
Dh
a
{7
a aN
=le raul
a }
elas
ley [
|
ais P 4
a4 tH
ac
WW
all iad
wae
|
i : \
Ly } ) Sp
LB
iss t
| 5 J
( rs fia
LS | 2 Fig. 6 a, I.G:G—COAH-SSSP No. 278/5, 1945,
te 2 a camera lucida drawing of latex cast of the
: i 2 holotype of Hedrograptus janischewskyi Obut
from the Lower Silurian (Llandovery) of the
southern Ural Mountains, U.S.S.R., preserved
as a 3-face view impression, x 4; b, IL.G.G—
COAH-SSSP No. 278/6, 1945, a ‘topotypic
specimen’ of H. janischewskyi ‘from the same
locality as the holotype and the closest to the
type’ (Obut, in litt. 1984), preserved as a 3-face
impression in a light-grey aphanitic limestone
b with most of the periderm missing, x 4.
)
=e
toa =
Wah
a
——
Das ==
SS Re Neen
Acknowledgements
I thank Dr Barrie Rickards for hospitality and facilities during several visits to the Sedgwick Museum, Mr
P. J. Osborne for the loan of specimens from the Lapworth Collection, Birmingham, Dr Tatyana N.
Koren, VSEGEI, Leningrad, for permission to study the collections from the Omuley Mountains and
Kazakhstan; Dr A. Pribyl for his help in studying graptolites at the National Museum of Prague, Dr
A. A. Petryk for permission to study his Anticosti collections, Miss Claire Carter, U.S.G.S., for the loan of
236 J. RIVA
topotype specimens of P. pacificus, Li Ji-jin for his assistance at the Academia Sinica in Nanjing, Wang
Xiao-feng for organizing a field excursion near Yichang, Mme Aicha Achab for the use of the INRS-
Géoressources photolaboratory and my daughter Patricia for translations of Russian papers. This work
was supported in part by a research grant from the N.R.C. of Canada and by a sabbatical travel grant
from Université Laval.
References
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Bjerreskov, M. 1975. Llandoverian and Wenlockian graptolites from Bornholm. Fossils Strata, Oslo, 8:
1-94, pls 1-13.
Cox, I. 1933. On Climacograptus inuiti sp. nov. and its development. Geol. Mag., London, 70: 1-19.
Crowther, P. R. 1981. The fine structure of the graptolite periderm. Spec. Pap. Palaeont., London, 26:
1-119.
Elles, G. L. & Wood, E. M. R. 1901-18. A monograph of British Graptolites. Palaeontogr. Soc. (Monogr.),
London. m + clxxi + 539 pp., 52 pls.
Hall, J. 1865. Graptolites of the Quebec Group. Figures and Descriptions of Canadian organic-remains,
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—— 1948. Some new subgenera of graptolites from the Families Dimorphograptidae and Diplograptidae.
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— 1974a. Graptolites with multiple genicular spines from the Upper Ordovician of Western North
America. Can. J. Earth Sci., Ottawa, 11: 1455-1460.
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— 1974b. A revision of some Ordovician graptolites of eastern North America. Palaeontology, London,
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—— 1971. A synoptic supplement to ‘A Monograph of British Graptolites by Miss G. L. Elles and Miss
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System in Australia, New Zealand and Antarctica: 2-6. 1.U.G.S. Publication 6.
—— (1981b). A complete Late Ordovician graptolite sequence at Mountain Creek near Deddick, eastern
Victoria. Unpubl. report, geol. Surv. Victoria 1981/81.
Walters, M. 1977. Middle and Upper Ordovician graptolites from the St Lawrence Lowlands, Québec,
Canada. Can. J. Earth Sci., Ottawa, 14: 932-952.
Wang Xiao-feng 1983. Latest Ordovician and earliest Silurian faunas from the eastern Yangtze Gorges,
China, with comments on Ordovician-Silurian boundary. Bull. Yichang Inst. Geol. Min. Res. 6: 129-
163.
Williams, S. H. 1982. The Late Ordovician graptolite fauna of the Anceps Bands at Dob’s Linn, southern
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— 1983. The Ordovician-Silurian boundary graptolite fauna at Dob’s Linn, southern Scotland. Palae-
ontology, London, 26: 605-639.
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P. J. Lespérance
Département de Geologie, Université de Montréal, Casier Postal 6128, Montreal,
Canada H3C 3J7
Synopsis
The Ordovician-Silurian boundary in the Perce area occurs within the Matapédia Group. This boundary
has not been identified within the Grande Coupe beds, which yield a brachiopod and trilobite fauna with
pronounced northwestern European affinities. The Ordovician—Silurian boundary can, however, be recog-
nized within the White Head Formation. The Cote de la Surprise Member is Hirnantian and yields both
Hirnantia and Mucronaspis Communities. The overlying L’Irlande Member is presumed to be totally
Silurian, but its basal part has not been positively dated.
Introduction
Southeastern Québec is unique within the North American continent in that it contains two
complete sequences near and at the Ordovician—Silurian boundary. A flat-lying sequence of
diverse limestones occurs on Anticosti Island (Barnes, this volume), which was originally depos-
ited in a shallow open-marine platform. The Percé sequence is also predominantly limestones,
but is decidedly a deeper-platform deposit. This Percé sequence lies within the Appalachian
folded belt, at the eastern end of the Aroostook—Percé Anticlinorium, which can be followed
from central Maine (USA) to Percé (Ayrton et al. 1969), a distance of approximately 500 km.
The Aroostook—Percé Anticlinorium in Québec, that is, in Gaspé, lies between the Siluro—
Devonian Gaspé—Connecticut River Synclinorium to the north and the Baie des Chaleurs
Synclinorium to the south. The Percé area is the most fossiliferous area within the Aroostook—
Perce Anticlinorium and, furthermore, the lithostratigraphy there outlined is useful throughout
Québec. Thus Percé stands as a local standard for the afore-mentioned anticlinorium.
The Anticosti platform, or the lateral equivalents of it, was probably the source of the
carbonates for the Percé sequence. Brachiopods and trilobites are predominantly endemic to
each sequence, although corals, conodonts, and ostracodes share some species. Ecological
control of these faunas thus appears evident. The Ordovician faunas of the Anticosti and Perce
sequences have different faunal affinities: the Anticosti sequence is related to the North Amer-
ican faunas, whereas the Perce faunas have a distinct northwestern European affinity, first
recognized by Schuchert & Cooper (1930).
The recognition of the Ordovician—Silurian boundary on Anticosti and around Percé has
been treated in detail by Lespérance (1985). A lithostratigraphical and palaeoecological revision
of the Early Ashgill to Late Llandovery strata of the Matapédia Group of the Percé area is to
be found in Lespérance et al. (1987). The lithostratigraphical revision follows the outlines given
by Skidmore & Lespérance (1981), while the palaeoecological treatment, relying on the com-
munity framework of Boucot (1975), is entirely new. The present contribution will summarize
data from Lespérance et al. (1981), Lespérance (1985), and Lespérance et al. (1987), but will also
draw from other sources and unpublished data.
Lithostratigraphical framework
The Aroostook—Perce Anticlinorium in Québec is composed of two main lithostratigraphical
sequences: a predominantly carbonate suite termed the Matapédia Group, and a deeper-water,
largely turbiditic suite termed the Honorat Group. The Taconic orogeny affected this part of
the Appalachians, apparently culminating in the early Caradoc; both the Honorat and Mata-
pedia Groups are younger than early Caradoc. The Honorat does not range into the Silurian
(although about a dozen Hirnantian brachiopod localities are known), but the Matapédia
Bull. Br. Mus. nat. Hist. (Geol) 43: 239-245 Issued 28 April 1988
240 P. J. LESPERANCE
Group is as young as upper Telychian, on the basis of the conodont Aulacognathus bullatus
(Nicoll & Rexroad 1969) (as reported by Nowlan 1983), present in the Des Jean Member of the
White Head Formation in the Percé area.
Within the immediate vicinity of Perce (Skidmore & Lespérance 1981; Lespérance et al.
1987) strata of the Matapédia Group occur in two distinct structural bands. The northeast
band is structurally complex, enough so that its total thickness is unknown. It is composed of
locally varying proportions of calcilutites and shales, with rare calcarenites, predominantly
pelmatozoan-bearing. This northeast band is in fault contact with Cambrian strata to the
southwest. The exact lithostratigraphical correlation of these beds with the southwest band (the
White Head Formation) is uncertain, which is the main reason why the northeast band of
strata has been termed the Grande Coupe beds. Some non-limey shales occur along the sea at
Grande Coupe (stream); these have been assigned to the (undivided) Honorat Group, but
otherwise, all the Ordovician-Silurian strata of the Percé area are assigned to the Matapédia
Group. =
The southwest structural band of the Percé area lies with angular unconformity on Cam-
brian strata. This band is a monoclinal sequence of Ashgill to Llandovery strata which, in turn,
are unconformably overlain by the Carboniferous Bonaventure Formation. The lower part of
this band is composed of calcareous terrigenous strata and is assigned to the Rouge Member of
the Pabos Formation. Above these are limestones, with minor intercalations of fine-grained
terrigenous strata, which terminate along the sea at White Head (Cap Blanc); these strata are
properly named the White Head Formation. Usage of the term White Head Formation before
Skidmore & Lespérance (1981) included the Grande Coupe beds and the Rouge Member of the
Pabos Formation, so that care in interpreting previous faunal lists must be exercised.
The stratotypes of the Rouge Member, as well as the four members of the White Head
Formation, are all within 6km of Percé, so that Fig. 1 is representative of the overall strati-
graphy. The Rouge Member of the Pabos Formation consists of basal conglomeratic strata and
coarse-grained sandstones, followed upward by mud-shales, sandstones, calcarenites, sandy
limestones and calcilutites. Terrigenous content decreases upward, and when it reaches less
than 50%, this signals the beginning of the White Head Formation.
The basal member of the White Head Formation consists of interbedded thinly bedded
calcilutites with thinner interbeds of mudstones, with some calcarenites; these strata form the
Burmingham Member. The next member, the Cote de la Surprise, is very predominantly dark
green readily-weathering calcareous mudstone. The L’Irlande Member, composed of thin to
medium bedded calcilutites and common very thinly bedded mud-shales, as well as rare thin-
bedded calcarenites, overlies the Cote de la Surprise Member. Within the middle part of this
member are significant clay-shale horizons. The youngest member of the White Head Forma-
tion, the Des Jean Member, does not crop out along the type section of the White Head
Formation along the sea, and is composed of argillaceous calcilutites, with minor silty and
sandy limestones, calcarenites and limestone conglomerates, in fine to very thick beds. The
Grande Coupe beds are Ashgill, the Cote de la Surprise Member Hirnantian, and the L’Irlande
Member Llandovery. A geological map of the Percé area will be found in Lespérance et al.
(1987).
Biostraiigraphy
Brachiopod-dominated communities, assigned to Benthic Assemblage 4 or 5 (Boucot 1975),
dominate the Rouge Member of the Pabos Formation. Extensive brachiopod and trilobite
faunas are known from this member (Sheehan & Lespérance 1979), but it is notable that
cyclopygid trilobites, as well as the trilobites Calyptaulax and Lonchodomas, are absent from
this member, while Stenopareia and Tretaspis, on the other hand, are rare; this is in striking
contrast with the partly coeval Grande Coupe beds. From a study of encrinurid trilobites,
Lespérance & Tripp (1985) suggested that the age of this member was probably Cautleyan.
The Burmingham Member of the White Head Formation is also dominated by brachiopods,
which are locally abundant, but their study is difficult because of their preservation in calcilu-
PERCE, QUEBEC, CANADA 241
CARBONIFEROUS
Seip
Formations Members
300
TH
ota
i
GaGRO
nha
Des Jean
=a
o
o
o
TELYCHIAN
Te
Hult
HH Q
=
(oc
Ww
=>
O
O
ZZ
<L
Ll
zal
White Head
L'Irlande
y
y
Matapédia Group
Wy y
Cote de la
Surprise
Burmingham
ASHGILL
PUSGILLIAN
CAMBRIAN
Fig. 1 Columnar section of the Pabos and White Head Formations in the Percé area, as taken from
the type sections of the various members (covered intervals within type sections filled in by data
from adjacent sections). The fossil localities shown within the L’Irlande Member occur along the
sea at White Head, where its thickness below the central clay-shale unit is 22m greater than
the one shown for its type section along the Deuxiéme Rang section. Compiled from data in
Lespérance et al. (1987). Symbols as in Fig. 2.
tites. Only four trilobite species are known from this member, but corals are present (Bolton
1980). The base of the Gamachian Stage (from Anticosti) is drawn 34m above the base of this
130m thick member along the shore at White Head, its stratotype (Lesperance 1985: 841). A
Benthic Assemblage 4 has been assigned to this member.
The Des Jean Member fauna is sparsely distributed and dominated by trilobites, notably
Acernaspis (Acernaspis) primaeva (Clarke 1908) and Stenopareia sp., with infrequent brachio-
pods. Study of the Des Jean Member and the underlying L’Irlande Member brachiopods is
hampered by the preservation in calcilutites and/or calcarenites, and thus most identifications
are only precise at the familial level. Nonetheless, these two members have in common Eospiri-
fer, a new atrypacid genus and a new athyridacid genus, as well as Eoplectodonta cf. stri-
atacostatus (Twenhofel 1928); all but the first of these taxa are illustrated in Sheehan &
Lespérance (1981: pl. 1). Oxoplecia sp. and Atrypa sp. are present, but restricted to the Des Jean
Member (Lespérance & Sheehan 1981).
Grande Coupe beds
The fauna from the Grande Coupe beds is the best-known fauna from the Percé area, and is the
242 P. J. LESPERANCE
one with the striking northwestern European faunal affinity. No less than 45 different trilobites,
20 brachiopods and 22 cephalopods, to name but these, are known from these beds. The
Priest's Road, Grande Coupe and southern fiank of Mont Joli (Cooper & Kindle 1936) are its
most fossiliferous localities. Stenopareia perceensis (Cooper in Schuchert & Cooper 1930)
[ =CSC] and cyclopygid trilobites are abundant, as are locally Tretaspis clarkei CSC, Loncho-
domas longirostris CSC, and the brachiopods Glyptorthis sublamellosa CSC, Sowerbyella
gigantea CSC, Holtedahlina parva CSC and Christiania dubia CSC. A Benthic Assemblage 6
position is indicated, but with local accumulations of pelagic taxa (cyclopygid trilobites and
cephalopods), the Foliomena Community (Sheehan & Lespérance 1978), or Benthic Assemblage
4 storm deposits (yielding, notably, colonial corals with encrusted algae).
Hirnantian faunas, or for that matter Silurian faunas, have not been recognized within the
Grande Coupe beds.
Cote de la Surprise Member e
The stratotype of this member is along the sea at White Head. From a talus slope, approx-
imately in the middle of the member, Lespérance & Sheehan (1976) described the brachiopods
and listed other elements present in this fauna: Dalmanella? sp., Eostropheodonta siluriana
(Davidson 1871), Hirnantia sagittifera (M‘Coy 1851), Kinnella kielanae (Temple 1965), Plec-
tothyrella crassicosta (Dalman 1828), rare Phillipsinella parabola s.l. (Barrande 1846), one
pygidium of Mucronaspis mucronata (Brongniart 1822), and favositid, cornulitid, conulariid and
pelmatozoan taxa. This fauna is a typical Hirnantia Community fauna, and assigned a Benthic
Assemblage 4 position.
The contact between the Cote de la Surprise Member and the L’Irlande Member is faulted
along the sea, and a boundary stratotype has been suggested along the adjacent Deuxiéme
Rang [=Flynn road, Irishtown road] section, where the contact is undisturbed. Here, the
uppermost 3m of the 44m thick Cote de la Surprise Member is composed of quartz arenites,
and these have yielded (Lesperance & Sheehan 1981; Sheehan & Lespérance 1981) abundant
brachiopods: an inarticulate, Dalmanella testudinaria (Dalman 1828), Hirnantia sagittifera, Kin-
nella kielanae, Eostropheodonta siluriana, Plectothyrella crassicosta, P. n. sp., and Hindella? sp.
(Hindella, however, is locally abundant in the Honorat Group west of Percé). This has been
assigned a Benthic Assemblage possibly transitional between 3 and 4.
The Cote de la Surprise Member also crops out 17km west-northwest of Percé (Lespérance
1974; Skidmore & Lesperance 1981) (Fig. 2). The fauna there consists almost entirely of
trilobites, with some graptolites, and is a typical Benthic Assemblage 6 fauna. The horizon with
the most fossils is between the two covered intervals of Fig. 2; fossils have not been recovered
above the uppermost covered interval, nor in the overlying L’Irlande Member. Revision of all
previous faunal lists now indicates the presence of: Brongniartella robusta (Lespérance 1968),
Cryptolithus portageensis sp. nov. Lespérance (this volume, p. 370), Mucronaspis mucronata, M.
olini (Temple, 1956), the sponge Astylospongia praemorsa (Goldfuss, 1826), a lingulid and a
pholidostrophid brachiopod, a bivalve, and the graptolites Climacograptus normalis s.s. Lap-
worth (1877) (J. Riva, personal communication, 1984), and Orthograptus sp. This is considered a
Mucronaspis Community; the presence of graptolites suggests nearness to pelagic (graptolite
and other) communities.
L’Irlande Member
Sparsely distributed, often isolated, trilobites and brachiopods occur in the upper three-
quarters of the L’*Irlande Member, but they are abundant only in infrequent calcarenite beds,
often associated with ostracodes. Trilobites are the most abundant taxa in the member, and
more specifically Acernaspis (A.) primaeva. The L’Irlande Member has been assigned a Benthic
Assemblage 6 position, and named the Acernaspis Community (which also includes the over-
lying Des Jean Member). Although the fauna is sparsely distributed, the total fauna includes
species of Acernaspis (Murphycops), Bolbineossia, Monograptus, as well as brachiopods (those
previously cited as also occurring in the Des Jean Member, as well as Homoeospira?, Streptis
and Triplesia), conodonts and trilobites, and is distinctly Llandovery in age. Fossiliferous
horizons within and above the clay-shales in the middle of the member are Telychian.
PERCE, QUEBEC, CANADA 243
70 65
: 505 —150
L'Irlande 20 =
lee
Member i
10 @ Rimouski Perce
48° LQ 48
Opis
que. 3
Peary g
/ \
: N.B.
m
46 Fredericton 46
Cote de la
Surprise
CALCILUTITES
nodules
Member ARGILLACEOUS CALCILUTITES
SILTY AND/OR SANDY
CALCILUTITES
i CALCARENITES AND
1 CALCIRUDITES
SHALES, MUDSHALES,
MUDSTONES
: SANDSTONES
minor
faults
FOSSILS (SEE TEXT)
Fig. 2 Columnar section of the Cote de la Surprise Member in the Portage river area (modified
from Skidmore & Lespérance 1981). Fossil localities shown by arrowheads are those discussed in
the text; numerous others are known. Insert shows location of Percé and the Portage river area
(starred); Me.: Maine; N.B.: New Brunswick; Qué.: Québec.
Extensive and closely spaced sampling through the lowest 10m of the L’Irlande Member
along the Deuxi¢me Rang section stratotype has proven fruitless for conodonts (Nowlan 1983:
102).
The L’Irlande Member along the sea at White Head is locally faulted, but, nonetheless, 466 m
are present (Lespérance et al. 1987). Strata below the middle clay-shale unit (faulted out along
the sea) are less fossiliferous than those above, but an extensive trilobite fauna is known 35m
below the clay-shale (62-L31 or locality E of Lesperance in Ayrton et al. 1969: 479), with
Eoplectodonta cf. striatacostatus, and the new atrypacid and athyridacid genera less than a
metre above the trilobites (62-L32). A cephalon of Acernaspis sp. occurs 80m (62-L41; erron-
eously referred to as a pygidium by Skidmore & Lespérance 1981: 37) below the clay-shales
and a pygidium of Acernaspis? sp., with Triplesia sp., E. cf. striatacostatus and the two new
genera previously quoted (62-L43 of Sheehan & Lespérance 1981: 255) 148m below the clay-
shales. Uncollectable pygidia of Acernaspis sp. occur below this last level, some 20-40 m above
the base of the member. These are the lowest occurrences of Silurian fossils in the L’Irlande
Member in the Percé area.
Lespérance (1985) has attempted to relate the acuminatus Zone, the base of the Silurian, to
shelly sequences, and has concluded that Acernaspis is apparently the only taxon of Silurian
244 P. J. LESPERANCE
aspect, or previously known Silurian distribution, to originate at the acuminatus boundary. In
view of the presence of the Hirnantian in the topmost Codte de la Surprise Member, the
monotonous nature of the L’Irlande Member, and the absence of Ordovician fossils, it appears
logical to assign the base of the L’Irlande Member to the Silurian.
Conclusions
Although typical Hirnantian faunas are present in the Percé area, the base of the Silurian
cannot be accurately positioned because of the lack of diagnostic graptolites, or, for that
matter, other diagnostic taxa. It is surmised that the base of L’*Irlande Member is of acuminatus
Zone age, because Acernaspis occurs low in this member.
The Matapédia Group in the immediate Percé area thus consists, in the Ordovician, of
deep-water communities (Grande Coupe beds) and shallower communities (Rouge, Burmin-
gham and Hirnantia Community of the Cote de la Surprise Member), while the Silurian part
reverts to deep-water communities, intermediate between the Clorinda and pelagic graptolite
communities. The widely accepted glaciation at the end of the Ordovician, although of prob-
lematical length (Hambrey 1985), could explain, by rapid eustatic sea-level rise following
melting of the ice-caps, the abrupt change from the Céte de Surprise mudstones to the thin-
bedded calcilutites of the L’Irlande.
Acknowledgements
Most of the data on the Percé area were gathered under the auspices of the Ministére de Energie et des
Ressources du Québec, to which the writer is grateful for continual help. Grants from the Natural
Sciences and Engineering Council of Canada were essential to the pursuit of the Perce investigations
throughout, and it is with pleasure that the writer expresses his best thanks.
References
Ayrton, W. G., Berry, W. B. N., Boucot, A. J., Lajoie, J., Lesperance, P. J., Pavlides, L. & Skidmore, W. B.
1969. Lower Llandovery of the Northern Appalachians and adjacent regions. Bull. geol. Soc. Am., New
York, 80: 459-484.
Bolton, T. E. 1980. Colonial coral assemblages and associated fossils from the Late Ordovician Honorat
Group and White Head Formation, Gaspé Peninsula, Québec. In Current Research. Geol. Surv. Pap.
Can., Ottawa, 80-1C: 1-12.
Boucot, A. J. 1975. Evolution and extinction rate controls. Developments in Palaeontology and Strati-
graphy, 1. 428 pp. Elsevier.
Cooper, G. A. & Kindle, C. H. 1936. New brachiopods and trilobites from the Upper Ordovician of Perce,
Quebec. J. Paleont., Menasha, Wis., 10: 348-372.
Hambrey, M. J. 1985. The Late Ordovician—Early Silurian glacial period. Palaeogeogr. Palaeoclimat.
Palaeoecol., Amsterdam, 51: 273-289.
Lesperance, P. J. 1974. The Hirnantian fauna of the Percé area (Québec) and the Ordovician—Silurian
boundary. Am. J. Sci., New Haven, 274: 10-30.
—— 1985. Faunal distributions across the Ordovician-Silurian boundary, Anticosti Island and Percé,
Québec, Canada. Can. J. Earth Sci., Ottawa, 22: 838-849.
——, Malo, M., Sheehan, P. M. & Skidmore, W. B. 1987. A stratigraphical and faunal revision of the
Ordovician-Silurian strata of the Percé area, Québec. Can. J. Earth Sci., Ottawa, 24 (1): 117-134.
& Sheehan, P. M. 1976. Brachiopods from the Hirnantian stage (Ordovician—Silurian) at Perce,
Québec. Palaeontology, London, 19: 719-731, pls 109-110.
—— 1981. Hirnantian fauna in and around Percé, Québec. In P. J. Lespérance (ed.), Field Meeting,
Anticosti—Gaspe, Quebec, 1981 2 (Stratigraphy and paleontology): 231-245. Montréal (I.U.G.S. Sub-
commission on Silurian Stratigraphy Ordovician—Silurian Boundary Working Group).
—, & Skidmore, W. B. 1981. Correlation of the White Head and related strata of the Percé region.
In P. J. Lespérance (ed.), Field Meeting, Anticosti—Gaspe, Quebec, 1981 2 (Stratigraphy and
paleontology): 223-229. Montréal (1.U.G.S. Subcommission on Silurian Stratigraphy Ordovician—
Silurian Boundary Working Group).
PERCE, QUEBEC, CANADA 245
—— & Tripp, R. P. 1985. Encrinurids (Trilobita) from the Matapédia Group (Ordovician), Percé, Québec.
Can. J. Earth Sci., Ottawa, 22: 205-213.
Nowlan, G. S. 1983. Early Silurian conodonts of eastern Canada. Fossils Strata, Oslo, 15: 95-110, 2 pls.
Schuchert, C. & Cooper, G. A. 1930. Upper Ordovician and Lower Devonian stratigraphy and paleon-
tology of Percé, Quebec. Part I. Stratigraphy and fauna (C. Schuchert). Am. J. Sci., New Haven, 20:
161-176. Part II. New species from the Upper Ordovician of Percé (G. A. Cooper). Loc. cit.: 265-288,
365-392.
Sheehan, P. M. & Lesperance, P. J. 1978. The occurrence of the Ordovician brachiopod Foliomena at
Perce, Québec. Can. J. Earth Sci., Ottawa, 15: 454-458.
1979. Late Ordovician (Ashgillian) brachiopods from the Percé region of Québec. J. Paleont.,
Tulsa, 53: 950-967.
1981. Brachiopods from the White Head Formation (Late Ordovician—Early Silurian) of the
Percé region, Québec, Canada. In P. J. Lesperance (ed.), Field Meeting, Anticosti—Gaspe, Quebec, 1981
2 (Stratigraphy and paleontology): 247-256. Montréal (I.U.G.S. Subcommission on Silurian Strati-
graphy Ordovician-Silurian Boundary Working Group).
Skidmore, W. B. & Lesperance, P. J. 1981. Percé Area. The White Head Formation, Percé. In P. J.
Lespérance (ed.), Field Meeting, Anticosti—Gaspe, Quebec, 1981 1 (Guidebook): 31-40. Montréal
(I.U.G.S. Subcommission on Silurian Stratigraphy Ordovician—Silurian Boundary Working Group).
ae
ib.
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NR ine
The Ordovician—Silurian boundary on Manitoulin
Island, Ontario, Canada
C. R. Barnes and T. E. Bolton
Geological Survey of Canada, 601 Booth St, Ottawa, Ontario, K1A OE8, Canada
Synopsis
The Ordovician-Silurian boundary in southern Ontario is reviewed. Sections on Manitoulin Island have
been regarded by earlier workers as representing continuous sedimentation in a shallow carbonate plat-
form environment on the north-east flank of the Michigan Basin. The best section across the boundary,
exposed in the Kagawong West Quarry, is described and illustrated. Lithological studies have demon-
strated a minor karst development near the systemic boundary. Conodont and macrofossil data demon-
strate that the Kagawong Member, Georgian Bay Formation and the lower 15cm of the overlying
Manitoulin Formation are of Richmondian age (Ordovician, Cincinnatian Series). The remainder of the
Manitoulin Formation is of Rhuddanian age (Silurian, Llandovery (Anticostian) Series). A hiatus is shown
to occur 15cm above the base of the Manitoulin Formation that represents the Gamachian Stage,
Cincinnatian Series and possibly also the latest Richmondian Stage and earliest Rhuddanian Stage.
Although the section on Manitoulin Island possesses many of the prerequisites of a boundary stratotype,
the hiatus at the systemic boundary ruled it out of consideration as the formal stratotype. It 1s, however,
one of many similar sections in the North American Midcontinent with a hiatus of this proportion at this
level which is interpreted as reflecting the eustatic sea level drop in the latest Ordovician related to the
north African continental glaciation.
Regional setting
In southern Ontario, undeformed, gently-dipping Ordovician and Silurian carbonates form the
eastern margin of the Michigan Basin, affected slightly by the Algonquin Arch (Fig. 1). Over
much of this area, the boundary between Ordovician and Silurian strata is a disconformity, but
to the north, on Manitoulin Island (Fig. 1), several previous workers have considered it to be
conformable with continuous sedimentation. More recent palaeontological and sedimentologi-
cal work has revealed a paraconformable relationship.
South of the Algonquin Arch (Fig. 1) exposures of the systemic boundary near the base of the
Niagara Escarpment reveal a sharp disconformable contact between the Queenston and
Whirlpool formations. The Queenston red shales have been generally regarded as continental
deposits of the “Queenston Delta complex’ with their widespread distribution being attributed
to lowered sea-level caused by the Late Ordovician glaciation (Dennison 1976). A few limestone
interbeds low in the Queenston Formation have yielded a marine fauna, including conodonts,
brachiopods, and bryozoans with at least the former indicating a littoral community (Barnes et
al. 1978) and suggesting a Richmondian (Late Ordovician) age. The overlying Whirlpool For-
mation is a white, cross-bedded sandstone barren of diagnostic fossils, but overlying shales
within the Medina Group yield Llandovery fossils. The classic reference section for this area is
that of the Niagara Falls gorge.
North of the Algonquin Arch (Fig. 1), the red shales are replaced progressively by shallow
water limestone with minor grey shale of the Kagawong Member (30m) of the Georgian Bay
Formation (130m). On Manitoulin Island the red shales are absent and these Late Ordovician
carbonates are overlain by carbonates of the Manitoulin Formation (20m), regarded as
approximately equivalent to the sandstone of the Whirlpool Formation of the Niagara region.
These regional stratigraphical relationships are illustrated in Fig. 1.
Bull. Br. Mus. nat. Hist. (Geol) 43: 247-253 Issued 28 April 1988
248 C. R. BARNES & T. E. BOLTON
MANITOULIN ALGONQUIN NIAGARA
ISLAND ARCH GORGE
CABOT
HEAD SBINSB YY.
z CABOT
=a
< HEAD
=
2) CABOT
= |
@ | MANITOULIN ~ HEAD
MANITOULIN ~
_——————————
WHIRLPOOL WHIREROOL
Oe OD gO ee ee
z > ZZ
SS <a
(3) a c
2 S
> Ze ~
5 < 3 QUEENSTON QUEENSTON
a 6 «
Cs ares bd 10
o [e)
Ww
Lo Joel m
Collingwood
LAKE ONTARIO
Toronto
Hamilton
Niagara Gorge
Kilometres
Fig. 1 Map of southern Ontario showing Manitoulin Island, main tectonic elements, and generalized
stratigraphical successions across the Ordovician—Silurian boundary for Manitoulin Island, Algonquin
Arch, and Niagara gorge.
Detailed stratigraphy
On Manitoulin Island, the systemic boundary is best exposed and most accessible at the small,
disused Kagawong West Quarry (Figs 2, 3) on Highway 540, 3km west of Kagawong (Alguire
& Liberty 1968, Stop 2; Sanford & Mosher 1978, Stop 10; Telford et al. 1981, Stop 14; Kobluk
& Brookfield 1982, Stop 6.3). The adjacent roadcut exposes additional strata of the Kagawong
Member and the Manitoulin Formation. The following sequence is exposed:
Manitoulin Formation:
6:5m Dolostone, massive to thick bedded at base, weathering into thin beds separated by
irregular shale partings; medium to light brown with grey patches, weathering to a buff
colour; medium crystalline; minor vugs in basal 15cm; abundant fossil debris, especially
brachiopods and rugose corals; minor silicification.
0-15m Dolostone, thin bedded to laminated, argillaceous; medium brown, weathering to a
very light brown colour; finely crystalline; beds separated by even shale partings; sharp
upper and lower contacts; recessive unit.
ORDOVICIAN-SILURIAN BOUNDARY ON MANITOULIN ISLAND 249
me
Fig. 2 Kagawong West Quarry showing Kagawong Member, Georgian Bay Formation and Manitoulin
Formation. Ordovician-Silurian boundary is drawn (black arrow) at top of 15cm recessive argillaceous
dolostone unit.
Georgian Bay Formation, Kagawong Member:
1-7m Dolomitic limestone, medium bedded weathering to thin beds; medium grey brown,
weathering to blue grey; finely crystalline; poorly fossiliferous, bryozoans and stromatopo-
roids.
Liberty (1954: 13) and Bolton & Liberty (1954: 28) placed the systemic boundary at the top of
the shaly recessive unit, including it within the Kagawong Member. Later Alguire & Liberty
(1968: 8) included it in the Manitoulin Formation and considered this sequence to represent
continuous sedimentation with no disconformity. Sanford & Mosher (1978: 13) and Sanford et
al. (1978: 99) from lithological and geochemical evidence placed the systemic boundary 11cm
above the top of the shaly recessive unit, the unconformity probably developing under sub-
marine rather than subaerial conditions. Kobluk (1984) defined two paleokarst surfaces—
erosional disconformities below the base and 10cm above the top of the recessive shaly unit.
The lower paleokarst was regarded as at, or very close to, the systemic boundary. Johnson &
Telford (1985), however, noted that the disconformable contact between the Manitoulin and
Georgian Bay Formations is devoid of scour, rill or other features indicative of extended
periods of erosion.
Palaeontology
Conodonts. Eight samples from this section, with particular emphasis on the Georgian Bay—
Manitoulin formational contact, yielded nearly 1000 conodonts (Fig. 3). This fauna formed part
of earlier studies by Tarrant (1977) and Barnes et al. (1978). The fauna of the Kagawong
Member of the Georgian Bay Formation was listed by Barnes et al. (1978: fig. 3) and includes
Aphelognathus grandis (Kohut & Sweet), A. pyramidalis (Branson, Mehl & Branson), Oulodus
ulrichi (Stone & Furnish), Panderodus staufferi (Branson & Mehl), Pseudobelodina vulgaris
Sweet, Rhipidognathus symmetricus Branson, Mehl & Branson. The last species dominates the
fauna in the uppermost bed, indicating a littoral environment (e.g. Rhipiodognathus community
250 C. R. BARNES & T. E. BOLTON
of Barnes & Fahraeus 1975). The progressive decrease in diversity upwards in the member also
suggests upward shallowing. Most taxa are of late Maysvillian to Richmondian age. In the
Composite Standard Section for the Middle and Upper Ordovician rocks of the Midcontinent
Province, Sweet (1984, Appendix) reports A. pyramidalis and P. staufferi as restricted to the
Richmondian interval. The Kagawong West fauna is herein assigned to the Richmondian
Aphelognathus divergens Zone. Although several of the taxa range into Gamachian strata on
Anticosti Island (McCracken & Barnes 1981: fig. 12), the presence on Manitoulin of Plectodina
tenuis, A. grandis rather than A. sp. cf. A. grandis, P. staufferi rather than P. sp. cf. P. staufferi,
and the absence of Gamachignathus spp., suggests a Richmondian rather than a Gamachian
age. The fauna may be generally correlative with other Richmondian units such as the Bull
Fork and Drakes formations, Cincinnati area (Sweet 1979a), the Noix Limestone, Edgewood
Group of Missouri (McCracken & Barnes 1982) and the Vauréal Formation of Anticosti Island
(Nowlan & Barnes 1981), but biofacies differences between these faunas make precise correla-
tion difficult.
The thin shaly recessive bed, at the base of the Manitoulin Formation, contains a similar
fauna with Rhipidognathus (Fig. 3). Only P. gracilis and possibly O. sp. are known to range into
Silurian strata elsewhere; no characteristic early Silurian taxa are present. The shaly recessive
unit is therefore considered to be of Ordovician (Richmondian) age.
The dolostones of the Manitoulin Formation yielded a conodont fauna (Fig. 3) that includes
Icriodella discreta Pollock, Rexroad & Nicoll, Spathognathodus comptus Pollock, Rexroad &
Nicoll s.f., and Ozarkodina hassi Pollock, Rexroad & Nicoll. The conodont fauna from the
Lower Silurian of southern Ontario, including Manitoulin Island, and northern Michigan was
described by Pollock et al. (1970), with other documentation by Barnes et al. (1978). The lower,
but not lowest, part of the Manitoulin Formation thus includes forms indicative of the Icrio-
dina irregularis Zone of Pollock et al. (1970), who also noted (p. 746) that in some sections ‘the
oldest parts of the Manitoulin ... seems to correspond with the pre-/criodina irregularis Zone in
the Midwest ... and with the lower part of Walliser’s (1964) Bereich I.’ I. discreta and O. hassi
are known from earliest Silurian strata, Menierian Stage of Barnes (in press), in the Anticosti
Island sections that are continuous across the Ordovician—Silurian boundary although S.
comptus is absent (McCracken & Barnes 1981: fig. 12; Barnes, this volume). Herein, the Mani-
toulin Formation is assigned to the Icriodella discreta—I. deflecta Zone of Aldridge (1972). In
the Manitoulin section, there is therefore no evidence of the latest Ordovician conodont Fauna
13 characteristic of the Gamachian Stage as described by McCracken & Barnes (1981) from
Anticosti Island. Other sections in the Midcontinent in North America also lack this interval,
e.g. the Cincinnati area (Sweet 1979a; Sweet et al. 1971; Sweet 1984), the Noix Limestone and
Bowling Green Dolomite of the Edgewood Group, Missouri (McCracken & Barnes 1982), and
elsewhere in the western Midcontinent (Sweet 1979b), and the Hudson Bay region (LeFevre et
al. 1975). McCracken & Barnes (1981) attributed this pattern to the latest Ordovician
(Gamachian) regression, induced by the north African glaciation, which restricted areas of
continuous sedimentation to subsiding marginal cratonic basins or non-eroding oceanic basins.
Macrofossils. The general fauna of the Kagawong Member, Georgian Bay Formation, as
detailed by Caley (1936), suggests the inclusion of these carbonates within the standard North
American Richmondian Stage. Within the upper 5m, only Stromatocerium, Tetradium and
poorly preserved undiagnostic bryozoans, bivalves and gastropods have been identified.
According to Copper (1982: 680), ‘the post-Richmondian Ellis Bay Spirigerina—Hindella faunas
of Anticosti Island are absent, suggesting an interval of erosion or non-deposition’.
The fauna of the overlying Lower Silurian Manitoulin Formation is scattered throughout
with concentrations confined to the uppermost beds (Bolton 1966, 1968). Characteristic forms
include the corals Paleofavosites asper (d’Orbigny), Palaeophyllum williamsi Chadwick, cystoid
Brockocystis tecumseth (Billings), brachiopods Resserella eugeniensis (Williams), Mendacella sp.,
‘Orthorhynchula bidwellensis Bolton, Zygospiraella planoconvexa (Hall), Sypharatrypa (?) lati-
corrugata (Foerste), Eospirigerina parksi (Williams), and Dolerorthis sp. An early Llandovery
(Anticostian) pre-C, age, within the “Coelospira’ planoconvexa—Atrypa laticorrugata Zone of
ORDOVICIAN-SILURIAN BOUNDARY ON MANITOULIN ISLAND 251
Ss (ep) = [= <x = a & (2
< io ~E | KAGAWONG |Z \ x , S > + Zz
=, <x Oo ct ~ ie = ='©@
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D 2) Se) || ROMDOUY |S=\ §S a = Roa > B aS
re SECON JOB \ 6 4 @ = & e229 O
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Zz S35 2 8 oe SEO! ts
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< =| a > Eo oot .2= a ©
— no 5 © 8S SESS aAOVWVSELNQ
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4 = = Q OSs = @ Os
a SS <K KG OM MAAR
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ee edie © 5 6 43 55
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lw = ze)
aqi|>o S86 8s 2 8
Ze = tis COVERED Sees es
2 |<] mG | INTERVAL Sess ue ee
5 Pez > qa 260 & ®
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ae ae
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COVERED
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6 58 2 3230043 140251 1013 14 10 2123244
Fig. 3 Section at Kagawong West Quarry showing distribution of conodont species in upper Kagawong
Member, Georgian Bay Formation and in lower Manitoulin Formation, across the Ordovician—
Silurian boundary.
Ehlers & Kesling (1962: 7), is assigned to the Manitoulin Formation. In the Kagawong West
Quarry, proper Brockocystis tecumseth was discovered near the base of the Manitoulin Forma-
tion and the first brachiopod concentration was located 1m above the base. Copper (1978: 51)
reported ‘the atrypoid Zygospiraella, an index genus from earliest Llandoverian (A) strata on
the Siberian platform and in the Baltic area is common’ from the basal few centimetres of the
Manitoulin Formation (above the recessive shaly dolostone bed). A Llandovery A age is also
assigned to the Manitoulin and overlying Cabot Head formations by Johnson (1981).
Summary
In the classic Niagara gorge section of southern Ontario there is an undisputed disconformity
between late Ordovician and early Silurian strata. To the north, on Manitoulin Island, several
252 C. R. BARNES & T. E. BOLTON
previous workers have argued for continuous sedimentation within a carbonate sequence
across the systemic boundary. Recent studies of the last decade on both conodonts and macro-
fossils now indicate a paraconformable relationship with the systemic boundary lying 15cm
above the base of the Manitoulin Formation and associated with subtle paleokarst develop-
ment. The Kagawong Member of the upper Georgian Bay Formation and the basal 15cm of
the Manitoulin Formation are assigned to the Aphelognathus divergens Zone of the Richmon-
dian Stage, Cincinnatian Series. The Manitoulin Formation is assigned to the Icriodella
discreta—Icriodella deflecta Zone and the Llandovery A, i.e. Rhuddanian Stage (Menierian
Stage), Llandovery (Anticostian) Series. The hiatus within the lower Manitoulin Formation
therefore represents the Late Ordovician Gamachian Stage and possibly the latest Richmon-
dian and earliest Rhuddanian (Menierian) as well. This hiatus is regionally extensive across the
Midcontinent (Barnes et al. 1981; Ross et al. 1982) and is interpreted as a result of eustatic
sea-level drop related to the Late Ordovician continental glaciation in north Africa.
The Kagawong West Quarry section is well exposed, undeformed with low burial tem-
peratures of CAI 1-5 (Legall et al. 1982) and with strata dipping at less than five degrees,
moderately fossiliferous, readily accessible, and has other qualities expected of a boundary
stratotype. However, even as the best potential section in southern Ontario, the recent demon-
stration through detailed faunal and lithologic studies of a hiatus at the systemic boundary
ruled out this section as the boundary stratotype.
Acknowledgements
Glen Tarrant completed a M.Sc. study on some of the samples noted in this paper under the supervision
of C.R.B. at the University of Waterloo and the financial support for this and the present study by the
Natural Sciences and Engineering Research Council of Canada is acknowledged. L. Nowlan drafted the
figures and A. Reid typed the manuscript.
References
Alguire, S. L. & Liberty, B. A. 1968. Itinerary. In Geology of Manitoulin Island. A. Fld Excurs. Michigan
Basin geol. Soc., Lansing, 1968: 6-17.
Barnes, C. R. (in press). Lower Silurian chronostratigraphy of Anticosti Island, Québec. In C. H. Holland
(ed.), A global standard for the Silurian System. National Museum of Wales, Cardiff.
—— & Fahraeus, L. E. 1975. Provinces, communities, and the proposed nektobenthic habit of Ordovician
conodontophorids. Lethaia, Oslo, 8: 133-149.
——, Norford, B. S. & Skevington, D. 1981. The Ordovician System in Canada, correlation chart and
explanatory notes. Int. Un. geol. Sci., Stuttgart, 8: 1-27.
——, Telford, P. G. & Tarrant, G. A. 1978. Ordovician and Silurian conodont biostratigraphy, Manitou-
lin Island and Bruce Peninsula, Ontario. Spec. Pap. Michigan Basin geol. Soc., 3: 63-71.
Bolton, T. E. 1966. Illustrations of Canadian fossils. Silurian faunas of Ontario. Geol. Surv. Pap. Can.,
Ottawa, 66-5: 1—46, 19 pls.
—— 1968. Silurian faunal assemblages, Manitoulin Island, Ontario. In The Geology of Manitoulin
Island. A. Fld Excurs. Michigan Basin geol. Soc., Lansing, 1968: 38—49.
& Liberty, B. A. 1954. Description of stops. In The stratigraphy of Manitoulin Island, Ontario,
Canada. A. Fld Trip Michigan geol. Soc. 1954: 27-30.
Caley, J. F. 1936. The Ordovician of Manitoulin Island, Ontario. Mem. geol. Surv. Brch Canada, Ottawa,
202: 21-91.
Copper, P. 1978. Paleoenvironments and paleocommunities in the Ordovician—Silurian sequence of Mani-
toulin Island. Spec. Pap. Michigan Basin geol. Soc. 3: 47-61.
—— 1982. Early Silurian atrypoids from Manitoulin Island and Bruce Peninsula, Ontario. J. Paleont.,
Tulsa, 56: 680-702.
Dennison, J. M. 1976. Appalachian Queenston delta related to eustatic sea-level drop accompanying Late
Ordovician glaciation centred in Africa. In M. G. Bassett (ed.), The Ordovician System: 107-120.
University of Wales Press.
Ehlers, G. M. & Kesling, R. V. 1962. Silurian rocks of Michigan and their correlation. Jn Silurian rocks of
the southern Lake Michigan area. A. Fld Conf. Michigan Basin geol. Soc., 1962: 1—20.
ORDOVICIAN-SILURIAN BOUNDARY ON MANITOULIN ISLAND 253
Johnson, M.D. & Telford, P. G. 1985. Paleozoic geology of the Kagawong area, District of Manitoulin.
Ontario Geol. Surv., Engineering and Terrain Publication, Prelim. Map P.2669.
Johnson, M. E. 1981. Correlation of Lower Silurian strata from the Michigan Upper Peninsula to
Manitoulin Island. Can. J. Earth Sci., Ottawa, 18: 869-883.
Kobluk, D. R. 1984. Coastal paleokarst near the Ordovician-Silurian boundary, Manitoulin Island. Bull.
Can. Petrol. Geol., Calgary, 32 (4): 398—407.
—— & Brookfield, M. E. 1982. Excursion 12A: Lower Paleozoic carbonate rocks and paleoenvironments
in southern Ontario. Intern. Assoc. Sedimentologists, Field excursion Guide Book. 62 pp.
LeFevre, J., Barnes, C. R. & Tixier, M. 1976. Paleoecology of Late Ordovician and Early Silurian
conodontophorids, Hudson Bay basin. In C. R. Barnes (ed.), Conodont Paleoecology. Spec. Pap. geol.
Ass. Can., Toronto, 15: 69-89.
Legall, F. D., Barnes, C. R. & Macqueen, R. W. 1982. Thermal maturation, burial history, and hotspot
development, Paleozoic strata from southern Ontario—Québec, from conodont and acritarch colour
alteration studies. Bull. Can. Petrol. Geol., Calgary, 29: 492-539.
Liberty, B. A. 1954. Ordovician of Manitoulin Island. In The Stratigraphy of Manitoulin Island, Ontario,
Canada. A. Fld Trip Michigan geol. Soc. 1954: 7-11.
—— 1968. Ordovician and Silurian stratigraphy of Manitoulin Island, Ontario. In Geology of Manitoulin
Island. A. Fld Excurs. Michigan Basin geol. Soc., Lansing, 1968: 25-37.
McCracken, A. D. & Barnes, C. R. 1981. Conodont biostratigraphy and paleoecology of the Ellis Bay
Formation, Anticosti Island, Québec, with special reference to Late Ordovician—Early Silurian chrono-
stratigraphy and the systemic boundary. Bull. geol. Surv. Can., Ottawa, 329 (2): 51-134, 7 pls.
—— —— 1982. Restudy of conodonts (Late Ordovician—Early Silurian) from the Edgewood Group,
Clarkesville, Missouri. Can. J. Earth Sci., Ottawa, 19: 1474-1485, 2 pls.
Nowlan, G. S. & Barnes, C. R. 1981. Late Ordovician conodonts from the Vauréal Formation, Anticosti
Island, Québec. Bull. geol. Surv. Can., Ottawa, 329 (1): 1-49, 8 pls.
Pollock, C. A., Rexroad, C. B. & Nicoll, R. W. 1970. Lower Silurian conodonts from northern Michigan
and Ontario. J. Paleont., Tulsa, 44: 743-764, 4 pls.
Ross, R. J. & 28 co-authors 1982. The Ordovician System in the United States. Correlation chart and
explanatory notes. Int. Un. geol. Sci., (A) 12. 73 pp.
Sanford, J. T. & Mosher, R. E. 1978. Road logs. Spec. Pap. Michigan Basin geol. Soc., 3: 1-28.
& Kennedy, J. W. 1978. The Ordovician—Silurian boundary. Spec. Pap. Michigan Basin geol.
Soc., 3: 95-99.
Sweet, W. C. 1979a. Conodonts and conodont biostratigraphy of post-Tyrone Ordovician rocks of the
Cincinnati region. Prof. Pap. U.S. geol. Surv., Washington, 1066-G: G1—G26.
—— 1979b. Late Ordovician conodonts and biostratigraphy of the western Midcontinent Province.
Geology Stud. Brigham Young Univ., Provo, 26 (3): 45-85, 5 pls.
— 1984. Graphic correlation of upper Middle and upper Ordovician rocks, North American Mid-
continent Province, U.S.A. In D. L. Bruton (ed.), Aspects of the Ordovician System: 23-35. Uni-
versitetsforlaget, Oslo.
, Ethington, R. L. & Barnes, C. R. 1971. North American Middle and Upper Ordovician Conodont
Faunas. In W. C. Sweet & S. M. Bergstrom (eds), Symposium on Conodont Stratigraphy. Mem. geol.
Soc. Am., Boulder, Col., 127: 163-193, 2 pls.
Tarrant, G. A. (1977). Taxonomy, biostratigraphy, and paleoecology of Late Ordovician conodonts from
southern Ontario. Unpubl. M.Sc. thesis, Univ. Waterloo, Ontario. 240 pp.
Telford, P. G., Johnson, M. & Verma, H. 1981. Field Trip Guidebook, Canadian Paleontology and Bio-
stratigraphy Seminar, Manitoulin Island September 26-29, 1981. 32 pp. Ontario geol. Survey.
Walliser, O. H. 1964. Conodonten des Silurs. Abh. hess. Landesamt. Bodenforsch., Wiesbaden, 41: 1—106.
——— A
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Preliminary report on Ordovician—Silurian boundary
rocks in the Interlake area, Manitoba, Canada
H. R. McCabe
Manitoba Mineral Resources Geological Services, 535-330 Graham Avenue, Winnipeg,
Manitoba R3C 43, Canada
Synopsis
Both Ashgill and early Llandovery rocks are represented in both surface outcrop (Stonewall Quarry) and
the subsurface of Manitoba, but there is no definite evidence of continuous sedimentation through the
boundary period.
The Interlake area of central Manitoba and its northwestward extension to eastern Saskatche-
wan (Fig. 1) provides the only outcrop area for the Lower Palaeozoic strata of the Williston
Basin, and the only Lower Palaeozoic outcrops between Hudson Bay and the western Cor-
dillera. Unfortunately, outcrops are sparse and expose only limited stratigraphical intervals, so
that it is not possible at present to propose a definitive locality for the Ordovician—Silurian
boundary there. No single outcrop area is at present known which exposes completely the
required stratigraphical interval. Nevertheless, because of the critical location of the Manitoba
outcrop belt, the following will present a brief summary of data relevant to the delineation of
the boundary.
Stearn (1953, 1956), on the basis of detailed faunal studies, placed the Stonewall Formation
in the Ordovician and placed the Ordovician—Silurian boundary at the contact between the
Stonewall Formation and the overlying Fisher Branch dolomite of the Silurian Interlake
Group. However, because of erosion of the uppermost beds, the type section of the Stonewall
Formation at the Stonewall Quarry is incomplete. At the time of Stearn’s studies, firm correla-
tion with the complete subsurface sequence had not been established. Subsequently, Porter &
Fuller (1959) established a subsurface reference section for the Stonewall Formation, based on
correlation of regional marker horizons (B. A. Morriseau, 3-20-90-6W; 875’-920’). Detailed
correlations between the Morriseau well and the Stonewall Quarry (about 72km to the east)
indicate that, at the Stonewall Quarry, the uppermost 4 to 6m of the Stonewall beds, including
a prominent medial arenaceous-argillaceous marker bed (t-horizon) has been eroded. Brindle
(1960), from subsurface faunal studies, suggested that the Ordovician—Silurian boundary falls
within the Stonewall Formation, rather than at the top, and may be marked by the medial
arenaceous bed. It must be noted that marker beds at the top, middle and bottom of the
Stonewall Formation can be correlated through almost the entire Williston Basin, indicating
little or no stratigraphical discontinuity at the Ordovician—Silurian boundary.
Preliminary results of conodont studies (C. R. Barnes, personal communication) indicate an
Ordovician—Richmondian (Ashgill) age for the Stonewall Quarry beds. Also, a possible late
Lower Llandovery fauna was obtained from a core hole drilled near the outcrop belt north of
Grand Rapids. Exact correlation of this core hole with the surface section is uncertain, but it
appears that the sampled interval may be upper Stonewall, and the upper Stonewall beds may,
at least in part, fill the apparent gap between the lower Stonewall beds of Ashgill age and the
Middle Llandovery Fisher Branch beds.
Recent stratigraphical core hole drilling in the Interlake outcrop belt, and mineral explora-
tion drilling in the area north and west of Grand Rapids, have obtained a number of cores for
the Fisher Branch—Stonewall-Stony Mountain succession, so that the complete lithological
sequence through the Ordovician—Silurian boundary interval is now available. Also, recent
geological mapping has outlined several new outcrops that may expose this interval. Although
no systemic boundary outcrop can be defined with certainty, two newly accessible occurrences
may possibly include the boundary zone, but precise faunal data for these outcrops are not yet
Bull. Br. Mus. nat. Hist. (Geol) 43: 255-257 Issued 28 April 1988
256 H. R. MCCABE
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Fig. 1 Correlation of the Stonewall Formation and adjacent rocks in the Interlake area, Manitoba,
Canada (in part after Porter & Fuller 1959). Correlation with the subsurface is also shown.
available. A thin sequence of dolomites, including an argillaceous marker bed believed to be the
mid-Stonewall (t-horizon) marker, is exposed at the parking lot for the Manitoba Hydro
powerhouse at Grand Rapids, but the remaining stratigraphical exposure is minimal.
A large bedrock hill south of the village of Cormorant (approx. Sec. 14, Tp. 60, Rge. 22
WPM), on the south shore of Cormorant Lake, is traversed by a recent extension of Provincial
Road 287. This hill is believed to comprise an outlier of the Stonewall Formation, although
exposure is by no means complete (Fig. 1). Good exposures occur in a roadcut at the top of the
hill, in a small quarry near the top, and in a number of scattered natural outcrops on the slopes
of the hill. Total topographic relief (partially exposed stratigraphical section) is 33m, and the
estimated Stonewall thickness is only about 10-6m. A preliminary examination shows, at the
top of the hill, a 2-3m cap of massive to nodular bedded, buff mottled, variably fossiliferous
dolomite with numerous corals and minor brachiopods and gastropods, but no recognizable
ORDOVICIAN-SILURIAN BOUNDARY ROCKS IN MANITOBA AST
Virgiana decussata (the diagnostic fossil of the Fisher Branch Formation). These beds have not
yet been identified palaeontologically, but on the basis of lithology are believed to be Fisher
Branch Formation (Middle Llandovery). These beds overlie sharply, and with apparent slight
unconformity, a pebbly argillaceous marker bed (0:9m), which in turn is underlain by fine-
grained dense conglomeratic dolomite (2:87m). This in turn overlies a 0:64m reddish grey
dolomitic shale and argillaceous dolomite (possible t-marker?) which passes downward to
microcrystalline dense dolomites. The conglomeratic beds are believed to be stratigraphically
equivalent to similar dolomites described by Stearn for an outcrop on P.T.H. 10 near Rocky
Lake, 26-7 miles (42:-6km) north of The Pas (Stearn 1956: 13). Stearn reported an Ordovician
fauna from these strata, suggesting that, at this locality and at Cormorant, a portion of the
Upper Stonewall may be missing because of non-deposition or pre-Fisher Branch (Middle
Llandovery) erosion.
Core-hole drilling and microfossil studies for the Cormorant section and for the Stonewall
area, planned for 1986-87, may permit more precise determination of the Ordovician—Silurian
boundary in Manitoba. It should be noted that the conglomeratic beds occurring in the
Stonewall Formation in central Manitoba (e.g. the Cormorant area) are not known in southern
Manitoba, where the Stonewall beds are slightly thicker and possibly comprise a more com-
plete, but not completely exposed, Ordovician—Silurian boundary sequence.
The summary faunal list for the Stonewall Formation is as follows:
Upper Stonewall fauna (after Brindle 1960 for Saskatchewan subsurface):
Above t-marker: streptelasmid, Favosites cf. favosus Goldfuss, Syringopora sp., bryozoan.
Below t-marker: Halysites (Catenipora) gracilis Hall, ?Oepikina stonewallensis Stearn.
Spathognathus manitoulinensis (Pollock, Rexroad & Nicoll)—C. R. Barnes (pers. comm.
1975).
Lower Stonewall fauna (Stonewall Quarry section—after Stearn 1956): Kochoceras cf. pro-
ductum, Antiplectoceras shammattawaense, Paleofavosites capax, P. okulitchi, Tryplasma
gracilis, Angopora manitobensis, Beatricea regularis, Megamyonia nitens, ?Oepikina stone-
wallensis, Ephippiorthoceras minutum, Metaspyroceras meridionale, Bickmorites insignis.
(after C. R. Barnes 1975, pers. comm.): Belodina profunda (Branson & Mehl), Rhipidognathus
symmetrica discreta Bergstrom & Sweet, Panderodus staufferi (Branson, Mehl & Branson).
References
Brindle, J. E. 1960. The faunas of the lower Palaeozoic carbonate rocks in the subsurface of Saskatche-
wan. Res. Rep. Saskatchewan Dept. Min. 52: 1—45, pls 1-8.
Porter, J. W. & Fuller, J. G. C. M. 1959. Lower Paleozoic rocks of the northern Williston Basin and
adjacent areas. Bull. Am. Ass. Petrol. Geol., Tulsa, Ok., 43: 124-189.
Stearn, C. W. 1953. Ordovician—Silurian boundary in Manitoba. Bull. geol. Soc. Am., New York, 64:
1477-1478.
—— 1956. Stratigraphy and palaeontology of the Interlake Group and Stonewall Formation of southern
Manitoba. Mem. geol. Surv. Brch Canada, Ottawa, 281: 1-162.
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The Ordovician—Silurian boundary in the Rocky
Mountains, Arctic Islands and Hudson Platform,
Canada
B. S. Norford
Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada,
3303-33rd St N.W., Calgary, Alberta T2L 2A7, Canada
Synopsis
The Ordovician-Silurian Boundary is developed within sequences of platform carbonates at Pedley Pass
(southeastern British Columbia) and in the Kaskattama well (northeastern Manitoba). At Snowblind
Creek (Arctic Islands), the boundary is documented within a transitional facies between platform carbon-
ates and basinal rocks, but access to the locality is difficult and expensive. Further detailed palaeontol-
ogical studies are needed to establish the precise position of the boundary at all three localities.
The Rocky Mountains
Silurian carbonates are widespread in parts of the Rocky Mountains (1400 km long, 50-140 km
wide) and a graptolitic facies is locally present in the northwestern and west-central parts.
Access is expensive except close to the very few roads. In the graptolitic facies (Road River
Group), the Ordovician-Silurian boundary interval has not been studied in detail. Exposures
are not good and unconformities are present within or below the Llandovery part of the
sequence. The Ordovician ornatus Zone and the Silurian cyphus Zone are well documented
(Cecile & Norford 1979; Jackson et al. 1965; Davies 1966) and taxa identified by Davies may
indicate some of the intervening persculptus, acuminatus, atavus and acinaces Zones.
The carbonate facies consists of resistant dolomites almost throughout the Rocky Moun-
tains. North of Peace River an unconformity is present below Silurian dolomites of the Nonda
Formation. South of the Peace, the Beaverfoot Formation appears to span latest Ordovician
and most of Llandovery time.
The section at Pedley Pass is typical of many in southeastern British Columbia, except that
access is simple and inexpensive. The locality is within a carbonate platform, a considerable
distance inboard of the platform-front. Exposure is excellent along a steep ridge above the
timberline, and complete through more than 500m of Upper Ordovician and Lower Silurian
limestones and dolomites. Retreat of glaciers was relatively recent and the rocks are essentially
unweathered. The terrane is folded and thrust but structure is simple within the thrust plates,
with moderate dip parallel to the ridge. Disconformities have not been recognized within the
boundary interval, but discontinuities could be present within the sequences of shallow water
carbonates. Conodont alteration indices (CAI) of 4 are known from just above the Beaverfoot
Formation near Pedley Pass (Goodarzi & Norford 1985: 1091, sample D) and thus the rocks of
the boundary interval have high thermal maturity and are quite unsuitable for palaeomagnetic
and many geochemical studies.
At Pedley Pass, 130 m of poorly fossiliferous dolomites separates an Upper Ordovician coral
and brachiopod fauna (Bighornia—Thaerodonta Fauna of Ashgill age) from the lowest brachio-
pods (Nondia sp.) and corals (Rhegmaphyllum sp., Streptelasma sp.) confidently dated as Silurian
(Eostropheodonta Zone, part of Virgiana fauna, upper Lower to Middle Llandovery). Macro-
fossils are present in the intervening rocks but are poorly preserved. Conodont studies of these
beds have not been completed, but preliminary data (T. T. Uyeno in Norford 1969: 39) from a
corresponding interval at Mount Sinclair, 25km north of Pedley Pass, indicate that the
Ordovician—-Silurian Boundary lies somewhere within the upper 75m of the poorly fossiliferous
interval at Pedley Pass.
Bull. Br. Mus. nat. Hist. (Geol) 43: 259-263 Issued 28 April 1988
260 B. S. NORFORD
Thus, the Beaverfoot Formation seems to show sedimentation across the Ordovician—
Silurian Boundary but the problems are those of precisely locating the boundary and the high
thermal maturity (CAI 4) of the rocks. The region is not suitable for a stratotype of more than
local application.
The Arctic Islands
The Arctic Platform and the Inuitian Orogen comprise a vast region (2000 by 1000km) in
which Ordovician and Silurian rocks are widely distributed, both in outcrop and subsurface.
Exposures are mostly good, but logistic dependency on aircraft makes access expensive and
then only possible during the short summer. A carbonate shelf is bounded to the northwest by
a graptolitic facies, locally stratigraphical sections show the interfingering of the two facies in
great detail, for example, along Snowblind Creek, Cornwallis Island (Thorsteinsson 1959).
Broad open folds characterize the structure in most of the Arctic Platform; thermal maturities
are low on Cornwallis Island (Conodont Alteration Indices 1 to 2, Uyeno 1981 and in
Goodarzi & Norford 1985: 1091, sample B). Macrofossils are not common in the carbonate
facies, but the graptolitic facies is very fossiliferous, locally with exquisite preservation of
graptolites in full relief within limestone nodules. Palaeontological studies of both macrofossils
and microfossils are only at a reconnaissance level at present, but the region has great promise
for the achievement of detailed correlations of zonal schemes based on various phyla.
Carbonates of the Allen Bay Formation, the Baillarge Formation and correlative rocks
contain corals, cephalopods, brachiopods, gastropods, trilobites and receptaculitids. Ashgill
faunas resemble those of northwestern Greenland and the Hudson Platform. Conodont faunas
indicate Fauna 12 of the United States with the same fauna present in latest Caradoc rocks;
Fauna 13 may also be present below conodont faunas indicative of the mid-continent Lower
Silurian kentuckyensis Zone (Ryley 1984). Very early Silurian macrofaunas have not yet been
collected from these formations, and, similarly, the conodont faunas are poorly known.
Most probably, all of latest Ordovician and earliest Silurian time is represented within the
Cape Phillips and Ibbett Bay Formations of the graptolitic facies. However, the graptolite
faunas have not yet been described taxonomically and the presence of the pacificus, extraordi-
narius, persculptus and acuminatus Zones have not been established. Cephalopods, radiolarians,
sponge fragments, ostracodes, polychaetes and trilobites are associated with the latest Ordovi-
cian graptolite faunas and allow correlation into the carbonate facies.
Thus, the Late Ordovician and Early Silurian macrofaunas and microfaunas have yet to be
described, but the intricate facies relations of carbonates and graptolitic rocks make it a region
of international importance for the discrimination of the Ordovician—Silurian Boundary. The
section at Snowblind Creek on Cornwallis Island is eminently suitable as a key section for
intercontinental correlations except for its difficult access. The variety of fossil groups within
the graptolite zones provides for detailed correlation of shelly benthic zones with the standard
graptolite zonation.
The Hudson Platform
The Hudson Platform is a large remnant (1600 by 1000km) of a sequence of Palaeozoic
carbonates and evaporitic rocks that once covered much of the Canadian Shield. The platform
now floors Hudson Bay, but the rocks extend onshore in the Hudson Bay and James Bay
Lowlands to the south and on Southampton, Coats and Mansel Islands to the north. Access to
all of these areas is difficult and costly. Outcrop is very sparse in the Lowlands and limited to
the major rivers and some intertidal regions; exposures are less rare in the northern islands but
stratigraphical sections are few and incomplete. The rocks are essentially flat-lying with rare
faults. Thermal maturities are low. A number of wells have been drilled in the Lowlands and
offshore in the central regions of Hudson Bay; these provide the best stratigraphical sections
and several (including Sogepet-Aquitaine Kaskattama Province No. 1) took continuous slim
core through the Ordovician—Silurian Boundary.
261
ROCKY MOUNTAINS, ARCTIC ISLANDS AND HUDSON PLATFORM
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Two sets of nomenclature have been used for an interval of shallow water dolomites between
the Churchill River Group (Ashgill) and an unconformity at the base of the Severn River
Formation (basal beds high Lower or Middle Llandovery). The Port Nelson Formation
(Savage & Van Tuyl 1919; Norford 1971) is based on an outcrop on Nelson River, northern
Manitoba; the Red Head Rapids Formation (Nelson 1963, 1964; Sanford 1974; Heywood &
Sanford 1976) is based on two outcrops on Churchill River in the same region. The relations
are uncertain between these three outcrops and an interval recognized in the subsurface
between the Churchill River Group and the Severn River Formation. Nomenclatorial priority
suggests the use of the term Port Nelson Formation. The interval reaches 35m in the sub-
surface of northern Manitoba and is more than 60m thick on Southampton Island, where some
additional younger strata may be present beneath the sub-Severn River unconformity.
The Silurian Severn River Formation rests on 32m of the Port Nelson Formation in the
Kaskattama well (Norford 1970). The contact is apparently conformable in the well but else-
where there is evidence of an erosion surface beneath the Severn River Formation. The lower
part of the Severn River Formation can be dated as late Early or Middle Llandovery, primarily
on the presence of Virgiana decussata (Whiteaves). In the well, the Port Nelson Formation
consists of dolomites and dolomitic limestones with mudstone partings and nodules, isolated
crystals and thin beds of anhydrite and locally halite. In the Kaskattama well, corals and
brachiopods in the basal 11m indicate a very late Ashgill age and correlation with the lower
GRAPTOLITE |CONODONT|MACROFOSSIL} SOUTHERN ROCKIES ARCTIC ISLANDS HUDSON PLATFORM
ZONES FAUNAS PEDLEY PASS SNOWBLIND CREEK KASKATTAMA WELL
convolutus ? c
SEVERN RIVER
VIRGIA m
= FORMATION (9
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BEAVERFOOT
PHILLIPS
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Pacificus
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Fig. 2 Correlation diagram of the three areas.
<#—— ORDOVICIAN-SILURIAN BOUNDARY ——3
ROCKY MOUNTAINS, ARCTIC ISLANDS AND HUDSON PLATFORM 263
and middle part of the Stonewall Formation of southern Manitoba. The upper 21m contains
only fragmentary fossils in the well, but sparse conodonts (Conodont Alteration Index | to 1-5)
have been recovered from outcrop 5:5m below the top of the Port Nelson Formation in its
type section. T. T. Uyeno has identified Panderodus cf. P. simplex Branson & Mehl s.f. and
tentatively dates the horizon as early Llandovery, but comments that the form shows some
transitional features to those of the Middle and Upper Ordovician form-species Panderodus
compressus Branson & Mehl.
Thus, in the Hudson Platform the Ordovician—Silurian Boundary lies either within the Port
Nelson Formation or within a regional unconformity below the Severn River Formation. The
sediments that formed the Port Nelson Formation were inhospitable to animal life, and
although one can hope for more refined dating of the upper beds and thus more precise
positioning of the Boundary, the region is not suitable for a stratotype of more than local
application.
References
Barnes, C. R., Norford, B. S. & Skevington, D. 1981. The Ordovician System in Canada, correlation chart
and explanatory text. Int. Un. geol. Sci., Paris, 8: 1-27.
Cecile, M. P. & Norford, B. S. 1979. Basin to platform transition, Lower Paleozoic strata of Ware and
Trutch map areas, northeastern British Columbia. Geol. Surv. Pap. Can., Ottawa, 79-1A: 219-226.
Davies, E. J. L. (1966). Ordovician and Silurian of the northern Rocky Mountains between Peace and
Muskwa Rivers, British Columbia. Univ. Alberta, unpubl. Ph.D. dissertation.
Goodarzi, F. & Norford, B. S. 1985. Graptolites as indicators of the temperature histories of rocks. J. geol.
Soc. Lond. 142: 1089-1099.
Heywood, W. W. & Sanford, B. V. 1976. Geology of Southampton, Coats and Mansel Islands, District of
Keewatin, Northwest Territories. Mem. geol. Surv. Can., Ottawa, 382: 1-35.
Jackson, D. E., Steen, G. & Sykes, D. 1965. Stratigraphy and graptolite zonations of the Kechika and
Sandpile Groups in northeastern British Columbia. Bull. Can. Petrol. Geol., Calgary, 13: 139-154.
Nelson, S. J. 1963. Ordovician paleontology of the northern Hudson Bay Lowland. Mem. geol. Soc. Am.,
New York, 90: 1-152, pls 1-37.
— 1964. Ordovician stratigraphy of northern Hudson Bay, Lowland, Manitoba. Bull. geol. Surv. Can.,
Ottawa, 108: 1-36.
Norford, B. S. 1969. Ordovician and Silurian stratigraphy of the southern Rocky Mountains. Bull. geol.
Sur. Can., Ottawa, 176: 1—90.
—— 1970. Ordovician and Silurian biostratigraphy of the Sogepet—Aquitaine Kaskattama Province No. |
well, northern Manitoba. Geol. Surv. Pap. Can., Ottawa, 69-8: 1—36.
—— 1972. Silurian stratigraphy of northern Manitoba. Spec. Pap. geol. Ass. Can., Toronto, 9: 199-207.
Ryley, C. C. (1984). Late Ordovician and Early Silurian conodont taxonomy and biostratigraphy, lower Allen
Bay Formation, Cornwallis Island, NWT. Univ. Western Ontario, unpubl. B.Sc. dissertation.
Sanford, B. V. 1974. Paleozoic geology of the Hudson Bay region. Geol. Surv. Pap. Can., Ottawa, 74-1B:
144-146.
Thorsteinsson, R. 1959. Cornwallis and Little Cornwallis Islands, District of Franklin, Northwest Terri-
tories. Mem. geol. Surv. Can., Ottawa, 294: 1-134.
— 1963. Ordovician and Silurian stratigraphy. Mem. geol. Surv. Can., Ottawa, 320: 31—SO.
—— & Tozer, E. T. 1970. Geology of the Arctic Archipelago. Econ. Geol. Rept. Geol. Surv. Can. 1 (5th
edn): 547-590.
Uyeno, T. T. 1981. Systematic study of conodonts. In Stratigraphy and conodonts of Upper Silurian and
Lower Devonian rocks in the environs of the Boothia Uplift, Canadian Arctic Archipelago. Bull. geol.
Surv. Can., Ottawa, 292: 1—75, pls 1-10.
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Ordovician—Silurian boundary, northern Yukon,
Canada
A. C. Lenz’ and A. D. McCracken?
‘Department of Geology, University of Western Ontario, London, Ontario, N6A 5B7, Canada
?Department of Geology, Laurentian University, Sudbury, Ontario, P3E 2C6, Canada.
Synopsis
The Ordovician-Silurian boundary is described from three graptolite and conodont-bearing sections of
northern Yukon. Upper Ordovician graptolite biostratigraphical units comprise the Dicellograptus
ornatus, Pacificograptus pacificus and tentatively the Glyptograptus persculptus zones; that of the cono-
donts being the Amorphognathus ordovicicus Biozone and the North American Fauna 12. A strati-
graphical hiatus between the P. pacificus Zone and the G. persculptus Zone?, and probably equivalent to
the Diplograptus bohemicus and Climacograptus? extraordinarius graptolite Zones, and to North American
conodont Fauna 13, appears to be present everywhere in the region.
Introduction
The presence of excellently exposed graptolite-bearing sequences in the Richardson and Ogilvie
mountains of northern Yukon has been recognized for more than 20 years. Graptolitic strata of
the Road River Formation are known to be widely distributed throughout the northern Cor-
dillera of Canada and adjacent Alaska (e.g. Lenz & Perry 1972; Lenz 1972, 1982; Churkin &
Brabb 1965).
For the purpose of this paper, three key sections are discussed; these are Peel River, Pat
Lake and Blackstone River, the first in the Richardson Mountains, the latter two in the Ogilvie
Mountains (Figs 1, 2). The Peel River section is chosen because the Ordovician—Silurian
boundary beds are completely exposed and are well! studied, and are defined on both grapto-
lites and conodonts; Pat Lake contains a thick conodont and shelly fauna-bearing limestone of
probable latest Ordovician age in an otherwise entirely graptolitic sequence; and Blackstone
River is a much thicker boundary sequence containing both graptolites and conodonts. These
sequences have already been discussed in Lenz & McCracken (1982).
The three sections discussed are in remote and isolated areas of northern Yukon, the field
season is relatively short, seldom more than two and a half months, and the cost of access is
high. Access to any of the three localites is via regular scheduled aircraft service to Whitehorse
in southern Yukon, and then to the villages of either Mayo or Dawson City in central Yukon
(Fig. 1), and by privately chartered helicopter thereafter. Weather in the region can vary
considerably, but is generally pleasant in July and early August.
Stratigraphy
The Road River Formation, the type of which is in the Richardson Mountains (Jackson &
Lenz 1962), is a thick basinal sequence of dominantly dark grey to black shales and cherts with
minor dark limestone beds and a few relatively thick-bedded debris-flow carbonates. Grapto-
lites are common to abundant in the shales and conodonts occur in some of the thin, dark
limestone beds. The Peel River section is a more or less typical Richardson Mountains bound-
ary sequence, but is without significant carbonates.
The Road River strata of the Ogilvie Mountains, of which the Pat Lake and Blackstone
River sections are representative, are characterized mainly by thinly bedded dark shales and
calcareous shales, much greater amounts of dark limestone beds and laminae, and much less
chert. The shales and calcareous shales contain abundant graptolites, while the dark limestones
Bull. Br. Mus. nat. Hist. (Geol) 43: 265-271 Issued 28 April 1988
266 A. C. LENZ & A. D. MCCRACKEN
4
poo
Pa)
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Je
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1S)
of)
O
Zz
150 KMS.
_———
DAWSON 100 MILES
———
Fig. 1 Index map of northern Yukon showing localities. 1 = Blackstone River; 2 = Pat Lake; 3 = Peel
River.
ORDOVICIAN-SILURIAN BOUNDARY IN N. YUKON 267
4
BLACKSTONE RIVER 3
62°56'N, 137°20'W PEEL RIVER
65°53’'N, 135°42'°W
2
: PAT LAKE
4 65°O9'N, 136°42’W
ing
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= eee ., Eee L. acinaces Zone $
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=~. G. persculptus a Zone a
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4
iat LIMESTONE ia) CONODONTS
oO CORALS
Fig. 2 Correlation of graptolite zones of localites 1—3 (Fig. 1), using the base of the Silurian as a datum.
may contain conodonts and, rarely, trilobites (e.g. Lenz & Churkin 1966; Ludvigsen 1981). A
relatively thick sequence of light-coloured, probably shallow water, conodont and coral-bearing
limestone of probable latest Ordovician age (Glyptograptus persculptus Zone?) occurs in the Pat
Lake section (Fig. 1). The presence of the limestone is anomalous, and its origin may be related
to the widely recognized latest Ordovician glacially induced regression (e.g. Lenz 1976, 1982;
Lenz & McCracken 1982).
Graptolites
Ashgill graptolite faunas of the northern Cordillera are divisible into two biostratigraphical
units, a lower Dicellograptus ornatus Zone and the upper Paraorthograptus pacificus Zone. The
uppermost Ordovician, the G. persculptus Zone, is less well developed and is clearly absent
from the Peel River section, but is tentatively recognized in the Pat Lake and Blackstone River
sections. Lowest Silurian (Llandovery) strata, represented by the Parakidograptus acuminatus
Zone and the overlying Atavograptus atavus and Lagarograptus acinaces Zones are widely
recognized (Lenz 1982).
The D. ornatus Zone is characterized by the index species, and by D. minor, Glyptograptus
latus, Climacograptus longispinus, C. latus, C. hvalross, C. hastatus, C. supernus, Orthograptus
abbreviatus, O. cf. fastigatus, Orthoretiograptus denticulatus, Arachniograptus laqueus and Lepto-
graptus spp. Dicellograptus is common, as are most of the diplograptid species.
The P. pacificus Zone is taxonomically a much more impoverished fauna and is characterized
by an abundance of C. supernus and P. pacificus. Most of the species of diplograptids noted in
the D. ornatus Zone are present, but in much lesser numbers, and dicellograptids are rare. In
268 A. C. LENZ & A. D. MCCRACKEN
addition, the exotic Diceratograptus cf. mirus is represented by two specimens in the Peel River
section (Chen & Lenz 1984).
The supposed G. persculptus Zone, which was considered to be lowest Silurian in Lenz &
McCracken (1982), is characterized by a fauna of low diversity, and is only tentatively recog-
nized. The index species has not, to date, been recovered from the northern Canadian Cor-
dillera, although it does occur in southeastern Alaska (Churkin et al. 1971). This bio-
stratigraphical unit is distinguished by the relatively sudden appearance of narrow forms of
Climacograptus normalis and C. miserablilis, a very spinose form of ?Paraorthograptus and
Orthograptus cf. abbreviatus. Other species appearing in the interval, but not confined to it,
include Diplograptus modestus, Glyptograptus tamariscus, G. gnomus, G. cf. laciniosus, and G. cf.
lanpheri. Monograptids have not been recovered. The G. persculptus Zone? is absent in the Peel
River section.
The P. acuminatus Zone, the lowest Silurian biostratigraphical unit, is readily recognized by
the appearance of the index species, as well as Climacograptus cf. trifilis, ? Akidograptus ascen-
sus, Cystograptus vesiculosus and Diplograptus modestus diminutus. Monograptids have not been
found. The A. atavus and L. acinaces Zones are discussed together since they witness the
incoming of monograptids, particularly Atavograptus and Pribylograptus, as well as being
characterized by Dimorphograptus confertus swanstoni, D. physophora (and subspecies) and
common Cystograptus vesiculosus.
Graptolite correlation
The graptolitic sequences of the northern Cordillera are directly comparable to those in central
China and the Kolyma and Kazakhstan regions of the U.S.S.R., and indirectly with that of
southern Scotland (Lenz & McCracken 1982; Chen & Lenz 1984). The D. ornatus Zone is
directly comparable to the C. longispinus Subzone of Koren et al. (1979), more or less compar-
able with the D. szechuanensis Zone and possibly the Amplexograptus yangtzeensis Zone of
central China (Chen & Lenz 1984), and probably with the D. complanatus Zone of Scotland
(Williams 1982).
Correlation of the P. pacificus Zone of Yukon is almost certainly directly with the P. pacificus
Subzone of U.S.S.R., but comparison with the Chinese succession is more difficult. Faunally,
the P. pacificus Zone is most similar to the D. szechuanensis and A. typicus Zones; however, the
presence of rare Diceratograptus in the Peel River section suggests correlation with strata as
high as the Paraorthograptus uniformis Zone of China. The latter correlation would appear to
be even more reasonable if P. uniformis of China is, as suggested by Williams (1982), synony-
mous with P. pacificus.
Correlation of the G. persculptus and P. acuminatus Zones 1s relatively straightforward, and it
therefore appears that strata equivalent to the Diplograptus bohemicus Zone of China, and the
Climacograptus? extraordinarius Zone of U.S.S.R. and Scotland are unrepresented by grapto-
lites or missing from the Yukon sections.
Conodonts and conodont correlation
Ashgill conodonts from the Blackstone and Peel River sections are regarded as being within the
Amorphognathus ordovicicus Biozone and the North American Fauna 12. The conodont fauna
at Blackstone River (Figs 1, 2) occurs 3m below the supposed G. persculptus Zone and 13:7m
above the last occurrence of graptolites of the P. pacificus Zone.
Significant taxa include A. ordovicicus Branson & Mehl, Belodina confiluens Sweet, Besselodus
n. sp., Gamachignathus ensifer McCracken et al., Icriodella superba Rhodes?, Noixodontus
girardeauensis (Satterfield), Oulodus ulrichi (Stone & Furnish), Panderodus? sibber Nowlan &
Barnes, Plectodina florida Sweet, P. tenuis (Branson & Mehl), Protopanderodus sp., Scabbardella
altipes (Henningsmoen) and Walliserodus amplissimus (Serpagli). Not all of these species were
initially listed by Lenz & McCracken (1982) and some have since undergone taxonomic
revision.
ORDOVICIAN-SILURIAN BOUNDARY IN N. YUKON 269
Fig. 3 Ordovician-Silurian boundary section on Peel River. Arrow on upper photograph is Ordovician—
Silurian boundary. Lower photograph is a close-up of the boundary beds; the lower arrow is the top of
the P. pacificus Zone and the upper arrow is the base of the P. acuminatus Zone.
270 A. C. LENZ & A. D. MCCRACKEN
One of the most noteworthy species, N. girardeauensis, was also found by McCracken &
Barnes (1982) in Missouri in association with Aphelognathus grandis (Branson, Mehl &
Branson) and A. ordovicicus. The recent work of Sweet (1984) established the A. grandis
Chronozone; the nominal species not only occurs in the Missouri fauna, but also in the
Richmondian Vauréal Formation of Anticosti Island (Nowlan & Barnes 1981). This species was
not recognized in the Gamachian Fauna 13 by McCracken & Barnes (1981), but they recorded
the related species A. aff. A. grandis. The range of A. grandis is reported to be from the upper
Maysvillian through much of the Richmondian A. divergens Chronozone; it does not appear to
range into post-Richmondian, pre-Silurian strata (Sweet 1984).
The close stratigraphical proximity of the Blackstone conodont fauna to the G. persculptus
Zone? graptolites does not necessarily imply that it is latest Ordovician. The rare co-occurrence
on Blackstone River of G. ensifer with A. ordovicicus, B. confluens (= B. compressa of Lenz &
McCracken 1982), O. ulrichi, P.? gibber, P. florida and P. tenuis is comparable to the upper
Vauréal Formation fauna (late Richmondian) of Nowlan & Barnes (1981). Unless the upper
limit of A. grandis is younger than is at present known, the co-occurrence of N. girardeauensis
and A. grandis in Missouri may indicate the Richmondian or, possibly, the late Maysvillian
(based strictly on published microfossil data). Hence, the occurrence of N. girardeauensis at
Blackstone River may favour a late, rather than the latest, Ordovician age. The Lower Llando-
very shale and chert from both the Blackstone and Peel River sections have not been collected
for conodonts.
A single f element of G. ensifer co-occurs at the Peel River section (Figs 1, 2) with some of the
species listed above for the Blackstone River; I. superba?, N. girardeauensis, O. ulrichi and P.
florida are absent from this fauna, whereas O. rohneri Ethington & Furnish and Pseudobelodina
vulgaris vulgaris Sweet are present only at the Peel River section. Unlike the Blackstone section,
where the Ordovician conodont fauna is within a thick, 16-7m interval barren of graptolites,
the Peel River conodont-bearing stratum is within a thin, 2.5m interval bounded by shales
containing graptolites of the P. pacificus Zone, and hence this conodont fauna is regarded as
late, but not latest, Ordovician. The fauna occurs in strata 1:6-1:9m below the systemic
boundary.
Lenz & McCracken (1982) did not report Ashgill conodonts from the Pat Lake section
Figs 1, 2). The sparse faunas there comprise poorly preserved conodonts that were originally
assigned an early Silurian age on the basis of ramiform elements and on their stratigraphical
SOUTHERN
SCOTLAND
P. acuminatus
CORDILLERAN
CANADA
KOLYMA and
KAZAKHSTAN, USSR
CENTRAL CHINA
P. acuminatus P. acuminatus P. acuminatus
=|
G. persculptus ?
G. persculptus
G. persculptus persculptus
Climacograptus D. bohemicus C.?extraordinarius
Z extraordinariuS fF
D
< | P. uniformis 9
O D D. mirus
> P. pacificus Z P. pacificus | se ee
O D> T. typicus P. pacificus
S Qe D. anceps
na oa
O ez D. szechuanensis ? Fi
= Climacogr D. complanatus
D. ornatus O longispinus
A. disyunctus >
2 yangtzeensis
I J} aaa
Fig. 4 Correlation of Ordovician—Silurian strata of Yukon with those of central China, U.S.S.R. and
Scotland.
ORDOVICIAN-SILURIAN BOUNDARY IN N. YUKON 271
position with respect to G. persculptus Zone? graptolites. Ozarkodina sp. A Lenz & McCracken
has a definite Silurian aspect, although this does not demand an assignment to that system
since the genus has elsewhere been occasionally recognized from Upper Ordovician strata.
The poor preservation of the coniform elements limits their biostratigraphical value; they
could be assigned to either Ordovician or Silurian taxa. Thus an age determination for these
post-P. pacificus Zone and pre-persculptus Zone? conodonts may depend upon the positive
identification of Ozarkodina sp. A. An unequivocal age based solely on the conodont taxa
cannot be determined for the Pat Lake conodont faunas. Their occurrence below the G.
persculptus Zone? suggests an Ordovician age.
Llandovery and younger conodont faunas are much more diverse and better preserved than
those discussed herein; study of these faunas is in progress.
Acknowledgements
Assistance provided in the field by the Geological Survey of Canada, and particularly by A. E. H. Pedder
and D. G. F. Long, is acknowledged. Financial support for the project was provided by a National
Sciences and Engineering Research Council operating grant to Lenz, and in part to McCracken by the
Northern Research Group, the University of Western Ontario.
References
Chen Xu & Lenz, A. C. 1984. Correlation of Ashgill graptolite faunas of central China and Arctic Canada,
with a description of Diceratograptus cf. mirus Mu from Canada. In Nanjing Institute of Geology and
Palaeontology, Academia Sinica, Stratigraphy and Palaeontology of Systemic Boundaries in China.
Ordovician—Silurian Boundary 1: 247-258, | fig.
Churkin, M., jr & Brabb, E. E. 1965. Ordovician, Silurian and Devonian biostratigraphy of east-central
Alaska. Bull. Am. Ass. Petrol. Geol., Tulsa, Ok., 49: 172-185.
——,, Carter, C. & Eberlein, D. E. 1971. Graptolite succession across the Ordovician-Silurian boundary in
southeastern Alaska. Q. JI geol. Soc. Lond. 126: 319-330, 1 pl.
Jackson, D. E. & Lenz, A. C. 1962. Zonation of Ordovician and Silurian graptolites of northern Yukon.
Bull. Am. Ass. Petrol. Geol., Tulsa, Ok., 46: 30-45.
Koren, T. N., Soboleyskaya, R. F., Mikhailova, N. F. & Tsai, D. T. 1979. New evidence on graptolite
succession across the Ordovician—Silurian boundary in the Asian part of the USSR. Acta palaeont. pol.,
Warsaw, 24: 125-136.
Lenz, A. C. 1972. Ordovician to Devonian history of northern Yukon and adjacent District of Mackenzie.
Bull. Can. Petrol. Geol., Calgary, 20: 321-361.
1976. Late Ordovician—Early Silurian glaciation and the Ordovician—Silurian boundary in the
northern Canadian Cordillera. Geology, Boulder, Colo., 3: 313-317.
—— 1982. Llandoverian graptolites of the northern Canadian Cordillera: Petalograptus, Cephalograptus,
Rhaphidograptus, Dimorphograptus, Retiolitidae, and Monograptidae. Contr. Life Sci. R. Ont. Mus.,
Toronto, 130: 1-154.
—— & McCracken, A. D. 1982. The Ordovician-Silurian boundary, northern Canadian Cordillera:
graptolite and conodont correlation. Can. J. Earth Sci., Ottawa, 19: 1308-1322, 2 pls.
—— & Perry, D. G. 1972. The Neruokpuk Formation of the Barn Mountains and Driftwood Hills,
northern Yukon; its age and graptolite fauna. Can. J. Earth Sci., Ottawa, 9: 1129-1138.
Ludyigsen, R. 1981. Biostratigraphic significance of Middle Ordovician trilobites from the Road River
Formation, northern Cordillera. Prog. Abstr. geol. Assoc. Can., 6: A36.
McCracken, A. D. & Barnes, C. R. 1981. Conodont biostratigraphy and paleoecology of the Ellis Bay
Formation, Anticosti Island, Québec, with special reference to Late Ordovician—Early Silurian
chronostratigraphy and the systemic boundary. Bull. Geol. Surv. Can., Ottawa, 329 (2): 51-134, 7 pls.
—— —— 1982. Restudy of conodonts (Late Ordovician—Early Silurian) from the Edgewood Group,
Clarksville, Missouri. Can. J. Earth Sci., Ottawa, 19: 1474-1485, 2 pls.
Nowlan, G. S. & Barnes, C. R. 1981. Late Ordovician conodonts from the Vauréal Formation, Anticosti
Island, Québec. Bull. geol. Surv. Can., Ottawa, 329 (1): 1-49, 8 pls.
Sweet, W. C. 1984. Graphic correlation of upper Middle and Upper Ordovician rocks, North American
Midcontinent Province, U.S.A. In D. L. Bruton (ed.), Aspects of the Ordovician System: 23-35. Uni-
versitetsforlaget, Oslo.
Williams, S. H. 1982. The Late Ordovician graptolite fauna of the Anceps Bands at Dob’s Linn, southern
Scotland. Geologica Palaeont., Marburg, 16: 29-56, 4 pls.
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The Ordovician—Silurian boundary in the United
States
S. M. Bergstrom’ and A. J. Boucot?
' Department of Geology and Mineralogy, The Ohio State University, 125 S. Oval Mall,
Columbus, OH 43210, U.S.A.
? Department of Zoology, Oregon State University, Corvallis, OR 97331, U.S.A.
Synopsis
Ordovician and Silurian rocks are widespread in the United States and there are numerous outcrops in
many regions displaying the systemic boundary interval. However, a regional review of key sections in all
the major outcrop areas shows that biostratigraphically closely controlled and stratigraphically complete
or nearly complete boundary successions are quite rare. Indeed, the Esquibel Island section in south-
eastern Alaska, where the systemic boundary is in a continuous graptolitiferous sequence, is not only the
only known occurrence in the United States of a typical P. acuminatus Zone fauna, but also the only
known place in the country where the systemic boundary can be established precisely on graptolites in a
continuous succession. Elsewhere, relatively complete, if not complete, boundary successions are present
in the Appalachians and in the Great Basin, as well as Alaska, but in virtually all cases the bio-
stratigraphical control is not good enough to establish the boundary level with certainty. Most of the
sections in these regions display a gap in the boundary interval, and this is the case also in most of the
many boundary sections in the Midcontinent region. The best known, and stratigraphically most nearly
complete, cratonic sections are in Arkansas, Oklahoma, Missouri, and Illinois, where strata having a
taxonomically varied Hirnantia fauna are overlain, with locally only a minor, if any, stratigraphical gap,
by rocks containing Llandovery fossils. No graptoloid graptolites are known from these sections, and the
precise level of the systemic boundary is uncertain in some sections. It is concluded that further studies are
urgently needed on fossils and rocks in the boundary interval, particularly to establish the precise age of
the conodont faunal turnover as well as to clarify the mutual relations between the distribution patterns
in time and space displayed by different groups, and their relations to the graptolite-based systemic
boundary.
Introduction
Ordovician rocks are present in the subsurface over much of the United States and they are
exposed in several major regions (Cook & Bally 1975). Although less widespread than those of
Ordovician age, Silurian rocks are likewise distributed over major parts of the country and
exposed over considerable areas. Accordingly, it is not surprising that the interval of the
Ordovician-Silurian systemic boundary is available for study at a large number of localities
from the Appalachians in the east to the Great Basin in the west. In many of these sections, the
faunal succession is incompletely known or fossils are absent in critical intervals, which applies
to the cratonic areas in the continental interior as well as to the geosynclinal areas along the
continental margins. Nevertheless, because in most sections, particularly the cratonic ones, the
systemic boundary is associated with a stratigraphical gap and a change in lithology, its level
in those sections can be readily recognized. As is the case elsewhere in the world, nearly
complete successions in continuously fossiliferous facies across the boundary interval are quite
rare in the United States both in shelly and graptolitic facies. For instance, we are not aware of
a single section outside Alaska where the precise level of the base of the P. acuminatus Zone,
that is the internationally accepted base of the Silurian, can be recognized by means of graptol-
ites or other fossils. It is quite clear that the choice of this level for the systemic boundary at the
present time makes its recognition difficult, if not impossible, in stratigraphically more or less
complete successions like those in the Great Basin (Ross et al. 1979; Leatham 1985; etc.) and in
the Mississippi Valley region (Amsden 1986).
Bull. Br. Mus. nat. Hist. (Geol) 43: 273-284 Issued 28 April 1988
274 S. M. BERGSTROM & A. J. BOUCOT
Fig. 1 Index map showing areas with Ordovician and/or Silurian outcrops (black) and systemic
boundary sections. 1, northern Maine; 2, eastern New York and western Vermont; 3, central
Appalachians (Pennsylvania and adjacent states); 4, eastern Tennessee; 5, Alabama and Georgia
(southern Appalachians); 6, the Cincinnati region and adjacent areas in Ohio, Kentucky, and
Indiana; 7, the Nashville dome in central Tennessee; 8, northern Arkansas (including the Batesville
district); 9, eastern Missouri and southwestern Illinois; 10, southern Oklahoma (including the
Arbuckle Mountains); 11, Black Hills (South Dakota and Wyoming); 12, North Dakota; 13,
Colorado; 14, west Texas; 15, New Mexico; 16, Bighorn Mountains, Wyoming; 17, Montana; 18,
Idaho; 19, Nevada; 20, Utah; 21, southeastern California; 22, southeastern Alaska (inset map).
The purpose of the present paper is to review briefly the biostratigraphy of the systemic
boundary interval in key sections in the principal outcrop areas. Page limitations make it
necessary to restrict ourselves to data essential for the understanding of the local and regional
geology of this interval in the United States. For convenience, we will deal with each of the
major outcrop regions separately, from the Appalachians in the east to the Great Basin in the
west. For the location of these regions, see Fig. 1.
Northern Appalachians
In large parts of the Northern Appalachians in the United States (Maine to New York State),
Silurian or younger rocks rest with a conspicuous, in many cases angular, unconformity on the
Ordovician (Berry & Boucot 1970: fig. 6). This stratigraphical gap varies in magnitude both
locally and regionally but includes in most cases portions of both the Ordovician and Silurian
systems. Conventionally, this gap is explained as a product of the Middle to Late Ordovician
Taconic orogeny, but it is evident that the apparently global drop in sea level during the latest
Ordovician (Hirnantian) contributed to emergent conditions, at least locally.
In this region, biostratigraphical control through the systemic boundary interval is, in
general, poor. This is partly due to the fact that the rocks were largely deposited in
environments with small numbers of shelly organisms, and those that became fossilized were in
many cases strongly affected by the subsequent metamorphism of the host rocks. That diagnos-
ORDOVICIAN-SILURIAN BOUNDARY IN UNITED STATES 275
tic fossils are present locally is shown by Neuman’s (1968) finds of shelly fossils of the Hirnantia
fauna in east-central Maine, the only occurrences of this type of fauna from the northeastern
United States. Another, and in terms of geology of the systemic boundary even more inter-
esting, sequence is that of the Carys Mills Formation of northeastern Maine and adjacent New
Brunswick. The lower part of this thick unit has yielded specimens of Glyptograptus persculptus
(Rickards & Riva 1981) and Llandovery age graptolites are known from higher parts of the
formation (Pavlides 1968). The Carys Mills has also produced well preserved conodonts of the
Icriodella discreta—I. deflecta Zone of probable Rhuddanian (early Llandovery) age (Barnes &
Bergstrom, this volume) but, unfortunately, the precise stratigraphical position of these cono-
donts within the formation is uncertain because of the scattered exposures, considerable thick-
ness, monotonous lithology, and structural deformation of the unit. At any rate, it appears
rather likely that the Carys Mills represents a stratigraphically complete succession from the
uppermost Ordovician to the lower Silurian, but further studies are needed to pinpoint the level
of the systemic boundary.
Central and Southern Appalachians
In southern New York and parts of eastern Pennsylvania and Virginia (Fig. 1), the Ordovician—
Silurian boundary is marked by an unconformity (Dennison 1976) and parts of the Ordovician,
and possibly also of the lowermost Silurian, are missing. From north-central Pennsylvania to
eastern Tennessee, the systemic boundary is somewhere in a succession, several hundred metres
thick, of near-shore to non-marine clastic sediments lacking shelly fossils of stratigraphical
utility. Although the precise level of the systemic boundary remains undetermined in these
successions, it has been common practice to classify the Juniata and Sequatchie formations as
Ordovician and the overlying Tuscarora and Clinch formations as Silurian.
Recent work by Colbath (1986) has raised the possibility of establishing a viable palyno-
morph (acritarch and chitinozoan) biostratigraphy useful for precise recognition and correla-
tion of the systemic boundary in these successions. Likewise, Gray’s work (1985) on higher land
plant spore tetrads permits recognition of the approximate boundary interval. Both the spore
tetrads and the marine palynomorphs occur in some abundance in near-shore marine sedi-
ments. The spores are also found in purely non-marine facies provided they have not been
destroyed by low-temperature metamorphism of the host strata. However, palynomorph work
in the systemic boundary interval in this region has not passed the pioneer stage, and much
additional study is needed to assess the local and regional biostratigraphic utility of these
fossils.
In the southernmost Appalachians, in the Birmingham area of Alabama, the systemic bound-
ary is marked by a conspicuous stratigraphical gap that includes the entire Upper Ordovician
and probably the lowermost Llandovery as well (Hall, unpublished; Berry & Boucot 1970).
Near the Alabama—Georgia boundary, the stratigraphical gap also includes the entire Middle
Ordovician (Dennison 1976), but in northwesternmost Georgia, Chowns (1972) considered the
systemic contact conformable on lithological evidence. The youngest Ordovician strata in much
of Alabama, which are referred to the Sequatchie Formation (Drahovzal & Neathery 1971), are
of Late Ordovician (Maysvillian and Richmondian) age. In Limestone County in northern
Alabama, the Devonian Chattanooga Shale contains reworked Late Ordovician (probably
Richmondian) conodonts (Bergstrom, unpublished) apparently originating from now-eroded
rocks that may be younger than the biostratigraphically dated parts of the Sequatchie Forma-
tion. Where dated biostratigraphically, the Sequatchie is separated from overlying rocks by a
stratigraphical gap corresponding not only to the uppermost Ordovician but also some part of
the post-Ordovician succession. Locally this gap is substantial and may include more than a
system.
Eastern North American Midcontinent
We include in this area the Cincinnati Arch region in Ohio, Kentucky, and Indiana, and the
Nashville Dome area in central Tennessee (Fig. 1).
276 S. M. BERGSTROM & A. J. BOUCOT
The Cincinnati region contains the Reference Standard of the North American Upper Ordo-
vician, the Cincinnatian Series. Both faunal and lithological evidence suggest an appreciable
hiatus between the Ordovician and the Silurian over the entire outcrop area in the Cincinnati
region. The stratigraphically most complete succession is apparently on the eastern side of the
Cincinnati Arch in southern Ohio and adjacent Kentucky. There is no record of Hirnantian
(latest Ordovician) age rocks anywhere in the Cincinnati region and the youngest Cincinnatian
stage, the Richmondian, is considered to be of pre-Hirnantian age. Based on the succession of
Anticosti Island, Québec, Canada, Twenhofel’s (1921) Gamachian Stage has in recent years
been recognized as a post-Richmondian, pre-Silurian standard unit (Barnes & McCracken
1981). Although rocks of Gamachian age are\ not known to be represented in the Cincinnatian
type area, the Gamachian is now classified as the uppermost part of the Cincinnatian Series
(Ross et al. 1982).
One of the best exposed and most representative sections through the Ordovician—Silurian
boundary interval on the eastern flank of the Cincinnati Arch is a series of exposures along
Ohio Highway 41 between West Union and Ohio Brush Creek, Adams County, Ohio
(Summerson 1963; Rexroad et al. 1965; Gray & Boucot 1972; Grahn & Bergstrom 1985). In
this section, the beds are horizontal, developed in fossiliferous limestone and shale, and there
are no structural complications. The topmost Ordovician unit, the Drakes Formation of Rich-
mondian age, is overlain conformably and without conspicuous lithological break by the
Belfast Member of the Brassfield Formation (Fig. 2). This unit has produced a relatively
undiagnostic conodont fauna of general early Llandovery type (Rexroad 1967) as well as
chitinozoans suggesting a C. cyphus Zone age (Grahn & Bergstrom 1985). Grahn & Bergstrom
(1985) interpreted the stratigraphical gap as corresponding to about four graptolite zones and it
is surprising that there is no channelling, development of a conglomerate, or other lithic
evidence of a sedimentary break. The major body of the Brassfield, that is, its post-Belfast part,
contains a rich megafossil fauna of early to middle Llandovery age (Berry & Boucot 1970) as
well as a stratigraphically diagnostic conodont fauna of the Distomodus kentuckyensis Zone
(Rexroad 1967; Cooper 1975) and chitinozoans (Grahn 1985). There are no graptolites known
from this succession.
In many other Cincinnati region sections, especially on the west flank of the Cincinnati Arch,
the stratigraphical gap associated with the systemic boundary is even greater than in the Ohio
Brush Creek sections (Rexroad & Kleffner 1984).
In parts of the Nashville Dome in central Tennessee, the Devonian Chattanooga Shale
unconformably overlies Middle Ordovician rocks (Dennison 1976). In other parts of the Nash-
ville Dome, strata dated as Richmondian are overlain unconformably by the Brassfield Lime-
stone of middle Llandovery age (Wilson 1949), which indicates the presence of a stratigraphical
gap of magnitude similar to that in the Cincinnati region.
Central North American Midcontinent
We include in this area Oklahoma and adjacent Texas Panhandle, Arkansas, Missouri, Illinois,
Minnesota, and Wisconsin (Fig. 1).
In a recent comprehensive study, Amsden & Barrick (1986) provided a useful summary of the
geology of the Ordovician-Silurian boundary interval in this region. Of particular significance
is the confirmation of the widespread occurrence of latest Ordovician strata having shelly
fossils of the Hirnantia fauna and conodonts of the Noixodontus fauna. The stratigraphically
most informative sections are in the Batesville district of north-central Arkansas and in eastern
Missouri. Both locally and regionally, the stratigraphical succession varies a great deal, and in
several cases, sections in close proximity to each other exhibit striking differences in lithological
and stratigraphical development. This is well illustrated by the conditions in the Batesville
district as well as in eastern Missouri.
In the Batesville district two sections are of particular interest. One of these sections is in the
Love Hollow Quarry (Craig 1968, 1986a, 1986b; Amsden 1968, 1986). In this large and recently
active quarry, the beds are horizontal and there are no notable tectonic complications. A
ORDOVICIAN-SILURIAN BOUNDARY IN UNITED STATES 277
| CONODONTS | |] CHITINOZOANS | SHELLY FOSSILS |
Platymerella Zone
Ancyrochitina sp
iklaensis
C. gregarius Z. equiv
Aeronian
Ozarkodina hassi
Oulodus sp
Conochitina sp. cf
(6.
Icriodella discreta
Zz
o
a
<x
=
zi
q}|O
a|¢
=| (2)
er
nv) wu
re
i)
ip)
<x
jag
a
Distomodus kentuckyensis
D. kentuckyensis Zone
Ancyrochitina primitiva
Ancyrochitina ancyrea
Conochitina sp. cf. C. electa
No diagnostic
megafossils
Distomodus sp. cf. D. kentuckyensis
BELFAST MEMBER
C. cyphus Z. equivalent
Upper Rhuddanian
Richmondian Fauna
A. divergens Zone
ORDOVICIAN
DRAKES FM.
Pre-Hirnantian
Anc. merga
Fig. 2 Vertical ranges of selected conodont and chitinozoan species, and the occurrence of index
megafossil assemblages, in the systemic boundary interval in exposures along Ohio Highway 41
northeast of West Union, Adams County, Ohio. Based on data from Berry & Boucot (1970),
Cooper (1975), Grahn & Bergstrom (1985), and Grahn (1985). Note that there is a prominent
stratigraphical gap between the Ordovician and the Silurian corresponding to the Hirnantian and
the lower Rhuddanian. Although this gap is about four graptolite zones, there is very little litho-
logical evidence of its existence in these sections.
stratigraphical column with fossil occurrences is given in Fig. 3. It should be noted that the
Cason Oolite as well as the overlying Triplesia alata beds were developed in a large limestone
lens which is now quarried away.
The Cason Oolite contains brachiopods that are used by Amsden (1986) for correlation with
the Hirnantian Keel Limestone of Oklahoma. The oolite also contains conodonts of the Noixo-
dontus fauna (Craig 1986a; Barrick 1986) that supports this correlation. The overlying pelmato-
zoan limestone, referred to by Amsden (1986) as the Triplesia alata beds and by Craig (1986b)
as the Brassfield Limestone, contains late Llandovery brachiopods and conodonts (Craig
1986b). No graptolites have been found in this succession. The contact between the Cason
Oolite and the overlying pelmatozoan limestone has been described as ‘stylolitic’ (Craig 1969).
It appears to represent a stratigraphical gap but its exact magnitude is uncertain, although
Barrick (1986) and Craig (1986b) report O. celloni Zone (late Llandovery) conodonts from the
Triplesia alata beds at this locality.
A similar succession (Fig. 3) is reasonably well exposed 0:‘5km NE of St. Clair Springs
(Amsden 1986) in which the Cason Oolite, which contains a Hirnantian age brachiopod fauna
similar to that of the Keel of Oklahoma and the Edgewood of Missouri, is directly overlain by
about 3m of crinoidal limestone classified as the Brassfield Limestone by Craig (1986b). The
278 S. M. BERGSTROM & A. J. BOUCOT
LOVE HOLLOW QUARRY
|_Ems._| CONODONTS
St. Clair Ls St. Clair Ls
Cason Shale
D. k k
: Brassfield’ entuckyensis
“Brassfield’’
Ls fauna
Ls. or
Cra 19
Triplesia (Craig, 1986)
alata
beds
Many Hirnantia
Noixodontus fauna brachiopods
Cason Oolite fauna incl
Cliftonia
tubulistriata
Rich conodont
Fernvale
fauna
Ls
Fig. 3. Vertical ranges of important conodont species, and the occurrence of Hirnantia fauna bra-
chiopods in two sections in the Batesville district, Arkansas. Based on Amsden (1986), Craig
(1986a, 1986b), and Barrick (1986). Note that there is a conspicuous stratigraphical gap in the
systemic boundary interval with a considerable portion of the Llandovery missing. The Love
Hollow Quarry exposure of the Cason Oolite and the Triplesia alata beds is now quarried away
(Amsden 1986).
Distomodus
staurognathoides
>
2/0
</>
“jo
DSla
a/z
n|<
a
el
z
<
x
S)
>
sy
Lu
ie
Fe
<
2
fe)
ea
uu
Aulacognathus bullatus
Pterospathodus celloni
Oulodus pelitus
Ozarkodina sp. cf. oldhamensis
Distomodus kentuckyensis
Pt. amorphpg
I— p staurognathoides
D. kentucky.
&
Pt. celloni
(Cason Oolite
Cason Shale
“Fernvale’®
Ls
undatus
HIRNANTIAN
Phragmodus
Protopanderodus
insculptus
ORDOVICIAN
ASHGILL
Am. ordovicicus
Noixodontus
Carniodus sp
Amorphognathus ordovicicus
girardeauensis
conodonts from this locality confirm that the Cason Oolite is of Hirnantian age and that the
overlying Brassfield is coeval with the Brassfield of the Cincinnati region (Craig 1986b; Barrick
1986). The systemic boundary is placed at the base of the Brassfield and is not strongly
expressed lithologically; it may be associated with a stratigraphical gap corresponding to the
lowermost Llandovery, but conodonts and other fossils do not provide sufficient stratigraphical
resolution to assess its magnitude precisely.
The Cason Oolite equivalent in southern Oklahoma is apparently the Keel Limestone
(Amsden 1986) that has yielded Hirnantian age brachiopods as well as conodonts of the
Noixodontus fauna (Barrick 1986). Its topmost part has also produced stratigraphically younger
conodonts of general Silurian aspect but no Silurian index species. Barrick assigned the latter
fauna to the Llandovery and placed the systemic boundary within the Keel. Amsden, on the
basis of his brachiopod studies, placed the entire Keel in the Ordovician (Amsden 1986: text-fig.
37) and noted that the unit is separated from the overlying Cochrane Formation by a large
stratigraphical gap corresponding to the lower and middle Llandovery. In our opinion, the
Silurian-type conodont fauna reported from the upper Keel by Barrick (1986) does not provide
firm evidence of Silurian age because, as shown by Barnes & Bergstrom (this volume, p. 325),
the turnover from an Ordovician-type to a Silurian-type conodont fauna may well have taken
place in very latest Ordovician (late G. persculptus Zone) time, within a time interval older than
ORDOVICIAN-SILURIAN BOUNDARY IN UNITED STATES 279
the base of the Silurian. Whether or not this alternative dating is correct can be solved only
after the conodont faunal turnover has been firmly dated in terms of graptolite zones.
As noted by Amsden (1986), there are two important outcrop areas of the systemic interval in
the Mississippi Valley, one in west-central Illinois and northeastern Missouri, and the other in
southwestern Illinois and southeastern Missouri. A considerable number of sections through
the uppermost Ordovician and overlying Silurian strata have been described by Amsden (1974,
1986) and Thompson & Satterfield (1975). The former also described the brachiopod faunas
and the latter reported on the conodonts (also cf. McCracken & Barnes 1982). The strati-
graphically most complete systemic boundary sequences are in the former area; in the latter
area, the Edgewood Group, of Hirnantian age at the top, is overlain directly and unconform-
ably by the Sexton Creek Limestone that contains brachiopods suggesting a late Llandovery
(late Aeronian—Telychian) age (see Fig. 4).
One of the biostratigraphically most instructive sections is along the west side of Missouri
Highway 79 at Clinton Springs at the south edge of Louisiana, Pike County, Missouri, where
the horizontal beds are easily accessible along a major highway. Good brachiopod collections
of Hirnantian age have been described from the Noix Oolite at this locality (Amsden 1974,
1986) and conodonts (of Noixodontus fauna type) studied by Thompson & Satterfield (1975).
The overlying Bryant Knob Formation has yielded a few brachiopods (Amsden 1974) and
conodonts interpreted as indicating early Llandovery age (Thompson & Satterfield 1975). The
OKLAHOMA | _S.£. MISSOURI N.E. MISSOURI
Coal Creek | Thebes N. Clarksville ] 4 mi S. of Clarksville
Bowling
Green
Dolomite
U
Llandovery
U
Llandovery
Undiagn
Bowling
Cochrane Fm Sexton brachs
SILURIAN
Green
shelly &
conodont
Creek Fm. }| brachiopods
Silurian-
aspect
conodonts
Conodonts Dolomite
P. celloni Z
?0.? nathani Z
faunas of Silurian
Undiagnostic
conod. faunas
aspect
r? | LLANDOVERY
a ee ee
[a
Bryant Knob
Em
e Hirnantia Noix Oolite Hirnantia Ni Hirnantia Ni Hirnantia N
=] Keel Oolite $
= and RB and 3 Noix Qolite 2 and 3
Zz o S 8 ~
Pag Noixodontus 2 Noixodontus $ Noixodontus $ Neix Oolite | Nosxodontus S
z 3 a) 3 S
z o faunas S faunas 9° fauna v faunas x
fj o/> 3 9 5 5
— a
ola Ideal Quarry S |\Girardeau Ls| $ 2 3
a Mbr ° s rs <
3 i] o 2
O15 = 5 5 5
a 3 & 2 2
5 y = S S
fe) <= is) is) °
2 € iS S
S St So zt
Tz
D Orchard ; Maquoketa Maquoketa
Sylvan Sh complanatus| & Creek Sh Sh Sh
Z
Fig. 4 Occurrence of key fossil assemblages, and general biostratigraphy, in the systemic boundary
interval at some localities in Oklahoma, southeastern Missouri, and northeastern Missouri. For
the location of these sections, see Amsden & Barrick (1986), and Thompson & Satterfield (1975),
and these papers provide most of the data upon which this diagram is based. As is clear from the
diagram, it is a review of the general stratigraphy in each of the areas and no correlation is implied
between a unit in one column and one at the same vertical position in another column. In
Oklahoma and southeastern Missouri, the systemic boundary is associated with a prominent
stratigraphical gap whereas in the illustrated sections from northeastern Missouri, the succession
across the systemic boundary may have only a minor, if any, stratigraphical gap.
280 S. M. BERGSTROM & A. J. BOUCOT
succession does not show any distinct lithic break between these units and it may be one of the
stratigraphically most nearly complete boundary successions in the Midcontinent region. A
stratigraphically similar section is present along Highway 79 about 6:5km south of Clarksville
and about 19km southeast of the Clinton Springs locality (Fig. 4; Amsden 1974). In his recent
reassessment of the data at hand, Amsden again placed the systemic boundary at the top of the
Noix Oolite but indicated (1986: 42) that ‘the brachiopod biostratigraphy requires no signifi-
cant interruption in the Noix-Bryant Knob sequence’. Interestingly, McCracken & Barnes
(1982) reported conodonts of Silurian aspect, by them interpreted as representing either the O.?
nathani Zone or the D. kentuckyensis Zone, from the lowermost 1:65m of the Bowling Green
Dolomite from a locality near Clarksville, Where this unit directly overlies the Noix Oolite,
which yielded a representative Noixodontus fauna. The conodont faunas from the Noix and the
Bowling Green are quite different, and there is obviously a faunal turnover between these units.
Unfortunately, as noted by Barnes & Bergstrom (this volume), the precise age of this faunal
turnover is currently unknown in terms of the graptolite succession, but it is quite possible that
it took place in the latest Ordovician. If so, it cannot be excluded that the systemic boundary is
above the base of the Bowling Green. However, the fact that the latter unit is overlain by the
late Llandovery Sexton Creek Limestone (Amsden 1986) makes it clear that the systemic
boundary must be below the base of the latter unit.
Western Midcontinent
Important outcrop areas in this vast region (Fig. 1) include the Black Hills in South Dakota,
the Bighorn Mountains in Wyoming, and areas in Montana, Colorado, southern New Mexico,
and western Texas. Most of the Upper Ordovician in these areas consists of shallow-water
carbonates with few megafossils but with taxonomically varied and biostratigraphically useful
conodont faunas (Sweet 1979). The biostratigraphy of the overlying beds is less well known. No
biostratigraphically well-controlled section is currently known that is stratigraphically reason-
ably complete in the Ordovician—Silurian boundary interval, and the data suggest that every-
where rocks of Ordovician age are separated from younger rocks by an unconformity
representing a significant stratigraphical gap (Ross et al. 1982). The most nearly complete
boundary section may be in the subsurface of North Dakota; however, data from the deposi-
tional basin extension in adjacent Manitoba, where the succession is quite similar to that in
North Dakota, suggest the absence of at least the lowermost Llandovery (Barnes & Bergstrom,
this volume).
Great Basin
We include in this region western Utah, Nevada, Idaho, and southern California (Fig. 1). There
are numerous excellent sections of Upper Ordovician and Lower Silurian rocks in carbonate
facies with virtually 100% exposure in the Great Basin, and most of these sections may be
reached by car and by foot under reasonable conditions. However, many localities are structur-
ally complex, and widespread secondary dolomitization, particularly in the Ordovician, makes
it difficult to obtain well-preserved megafossils. Furthermore, diagnostic shelly megafossils are
not common and graptolites are rare. Conodonts are known from some sections and they offer
great potential for detailed stratigraphical work in the widespread carbonates; unfortunately,
the problem of dating the conodont faunal turnover referred to above currently restricts their
use in establishing precisely the position of the Ordovician—Silurian boundary. Accordingly, it
is currently impossible to recognize with certainty the exact level of the systemic boundary, or
even to assess whether or not deposition was continuous, at those carbonate sections where
there is not a conspicuous unconformity in the boundary interval.
Much of the pertinent biostratigraphical information from megafossils was summarized by
Berry & Boucot (1970). Additional data from shelly fossils have been published by, among
others, Budge & Sheehan (1980a, 1980b) and Sheehan (1980, 1982).
ORDOVICIAN-SILURIAN BOUNDARY IN UNITED STATES 281
Although conodont work in the systemic boundary interval is still in the pioneer stage in the
Great Basin, it is apparent that conodonts offer greater potential than any other group for
detailed biostratigraphical work. Two recent conodont studies deserve mention in a discussion
of the systemic boundary. Ross et al. (1979) described the conodont biostratigraphy of the
Hanson Creek Formation near Eureka, Nevada. They suggested that this unit represents
continuous deposition from Ordovician to Silurian time. This is quite possible, but it is perhaps
equally possible that all the conodont samples referred to in their study are of Ordovician age
and that the systemic boundary is at a higher, as yet undetermined, level in the Hanson Creek
than that advocated by Ross et al. because, as noted by Barnes & Bergstrom (this volume),
none of their conodont collections contain index conodonts of definite Silurian age.
In another recent study, Leatham (1985) described the conodont biostratigraphy of the Fish
Haven Dolomite and immediately overlying strata in a section in northernmost Utah. He
identified a prominent conodont faunal turnover and a transitional fauna interval of 5-Sm
thickness in the uppermost Fish Haven. The systemic boundary was placed at the base of this
transition interval, but Leatham (1985) was uncertain whether or not there was a strati-
graphical gap at this level. He was also uncertain about the nature of the mixed faunal
association and suggested that it might be a product of reworking or stratigraphic leak. In our
view, it cannot be excluded that the interval with the mixed fauna, regardless of its nature, is of
Hirnantian age, and that the systemic boundary, as it is now defined by means of graptolites, is
at a somewhat higher stratigraphical level, in the lowermost part of the Laketown dolomite.
Of special interest in a review of the Ordovician—Silurian boundary biostratigraphy in the
Great Basin is Berry’s (1986) record of an uppermost Ordovician to lower Silurian sequence of
graptolite faunas in cherts and dolomites of the upper Hanson Creek Formation in the
Monitor Range, central Nevada. A quartz sand-bearing dolomite, which evidently represents a
period of shallowing near the end of the Ordovician, is underlain by strata having the Dicello-
graptus complanatus ornatus graptolite assemblage, and directly overlain by rocks containing
the diagnostic species association of the Glyptograptus persculptus Zone. Stratigraphically
higher beds contain species that may represent the P. acuminatus Zone but the zonal index has
not been found.
Graptolite-bearing shale sequences of Ordovician and Silurian age are widespread in the
mountain ranges in the Great Basin but the studied successions appear to be stratigraphically
incomplete and display a gap in the systemic boundary interval. For instance, in the carefully
studied and well-known graptolite shale succession in the Trail Creek area, central Idaho,
Llandovery beds older than the M. convolutus Zone are missing (Carter & Churkin 1977).
Alaska
With one important exception, little information is currently available concerning the geology
of the Ordovician—Silurian boundary in Alaska. This exception is the Prince of Wales region in
southeastern Alaska (Fig. 1) where in the long-ranging Descon Formation there is a quite
condensed succession through the systemic boundary interval, which displays a complete
sequence of late Ordovician—early Silurian graptolite zones. The best known succession is on
Esquibel Island (Churkin & Carter 1970; Churkin et al. 1971) where a few metres thick
sequence of cherty shales spans the systemic boundary without any indication of depositional
breaks. A less than 3m thick interval with the G. persculptus Zone fauna is overlain by about
1-5m of strata containing graptolites characteristic of the P. acuminatus Zone, including the
zonal index. The Esquibel Island graptolite species associations show close similarity to those
of coeval strata in the Birkhill Shale in the Ordovician—Silurian boundary stratotype at Dob’s
Linn in south Scotland, making it possible to recognize the level of the systemic boundary with
considerable precision. This may be the only place in the United States where the level of the
systemic boundary can be fixed conclusively on graptolite evidence in a stratigraphically con-
tinuous section, and one can only regret that this key locality is located in a remote region that
is likely to be visited by very few geologists.
282 S. M. BERGSTROM & A. J. BOUCOT
Zz
<
ac
=)
=
2p)
Zz
x
2
>
e)
Qa
oc
fe)
Conclusions
. The Ordovician—Silurian boundary interval is well exposed at numerous localities through-
out the United States from the Appalachians in the east to the Great Basin in the west.
. Available biostratigraphical and/or lithostratigraphical evidence suggests that in the vast
majority of these sections, there is a stratigraphical gap, of greatly different magnitude in
different sections, in the boundary interval (Fig. 5). Particularly in the shallow-water cratonic
successions, this gap reflects the global drop of sea-level near the end of the Ordovician, but
there is evidence that local uplifts have been of importance in some areas. Currently, we are
aware of only a single biostratigraphically closely controlled section in the United States, on
Esquibel Island, southeastern Alaska, which displays continuous deposition throughout the
boundary interval. However, such sections may exist elsewhere, particularly in the Appa-
lachians and in the Great Basin.
. Some of the best, and biostratigraphically most closely controlled, boundary sections are in
Arkansas, Oklahoma, Missouri, and Illinois where rocks having the Hirnantia shelly fauna
and the Noixodontus conodont fauna are overlain by Llandovery-age strata with, at least
locally, only a minor, if any, stratigraphical gap. Regrettably, no stratigraphically diagnostic
graptolites are known from these sections.
. A considerable number of well-exposed, thick, and apparently stratigraphically relatively
complete sections in shallow-water carbonate facies are known from the Great Basin. Dolo-
mitization has seriously affected the state of preservation of the megafossils, which are rather
scarce in most sections, but conodonts are moderately common and taxonomically varied.
Yet, because the conodont biostratigraphy is not tied reliably to the graptolite zone suc-
cession in the G. persculptus and P. acuminatus Zones, currently the conodonts cannot be
used to pinpoint the level of the systemic boundary in carbonate sections without a signifi-
cant stratigraphical gap.
. As far as we are aware, in the United States the P. acuminatus Zone has been identified with
certainty only on Esquibel Island, Alaska, and this is the only place where the level of the
systemic boundary can be established precisely by means of zonal graptolites. The suc-
cessions of shelly fossils, conodonts and palynomorphs are thus far calibrated only impre-
N.E. MAINE} APPALACH N. ARKAN.| S.OKLAH. JE.MISSOURI} N. UTAH NEVADA jS.E. ALASKA
Cochrane cs Sexton
[Foul Creek Fm
Brassfield
Ls (Tripl.
cs
? c
alata beds)
LLANDOVERY
eps
Brassfield
Tuscarora
and
Clinch Fms
Bowling Laketown
Green Dol
Descon
Juniata and Oolite & c«s
Sequatchie
c c
Sh
Fms csp
Richmond-
janice Sylvan Sh peg | Maquoketa
Sh
Fig.5 Summary diagram showing important formations and degree of stratigraphical completeness
of systemic boundary sections in nine important outcrop areas in the United States. Small letters,
which indicate the presence of biostratigraphical control by means of a particular index fossil
group, denote the following: c, conodonts; g, graptolites; s, shelly fossils (especially brachiopods);
p, palynomorphs (especially chitinozoans). For further data on each of these successions, see the
text. The section of northern Utah is that described by Leatham (1985). Vertical ruling marks
proved or assumed stratigraphical gaps. Only formations near the systemic boundary are listed in
the diagram.
ORDOVICIAN-SILURIAN BOUNDARY IN UNITED STATES 283
cisely and broadly with the graptolite zone succession, and therefore these fossils cannot yet
be used successfully to pinpoint the precise level of the Ordovician—Silurian boundary,
especially in sections without a significant stratigraphical gap in the boundary interval. If the
base of the P. acuminatus Zone is to be a viable and useful level for the base of the Silurian,
then it is clearly necessary to determine the precisely equivalent level in the successions of
shelly fossils, conodonts and palynomorphs. Because of the absence of graptolite control in
the critical sections in the United States, that biostratigraphically most important correla-
tion work will have to be carried out elsewhere in the world. However, the mutual strati-
graphical relationships between non-graptolitic taxa are well displayed in sections in the
United States. A detailed study of these relations no doubt will produce interesting and
useful information of regional significance.
Acknowledgements
We are indebted to Dr W. B. N. Berry for valuable information and to Ms Karen Tyler for technical
assistance with the manuscript. Special thanks are due to T. W. Amsden and J. E. Barrick for proof copies
of their important study on Hirnantian and associated faunas in the central United States.
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56: 1-407, pls 1-28.
The Ordovician—Silurian boundary in South America
A. J. Boucot
Department of Zoology, Oregon State University, Corvallis, Oregon 97331, U.S.A.
Synopsis
In South America late Ashgill rocks followed in the same succession by the early Llandovery are known
only in the Precordillera of San Juan, Argentina. Early Llandovery fossils are known from the Puna Well,
Argentina, the basal Trombetas Formation of Brazil, west of Lake Titicaca in Peru, and in the Merida
Andes of Venezuela. Glaciogenic deposits of presumed Ordovician-Silurian boundary age are known
from Argentina, Bolivia, Brazil and Peru.
Introduction
There are unfossiliferous and relatively unfossiliferous strata in South America whose assign-
ment to either the Ordovician or Silurian is a problem. But I am unaware of any South
American area where there are fossiliferous strata involved in real Ordovician—Silurian bound-
ary indecision. In South America the assignment of fossiliferous beds to either the Ordovician
or Silurian has been easy because there are no areas recognized to date where fossiliferous beds
of latest Ordovician and earliest Silurian age are present in conjunction with each other.
Discussion of the Ordovician—Silurian boundary in South America may be broken into two
parts: (1) the strata present on the shield areas; (2) the strata present in the structurally complex
Andean regions bordering the shield areas to the north and west. Recognized, fossiliferous
Ordovician rocks have not yet been shown to exist on the shield areas except for a few areas
very close to the Andean, disturbed rocks, whereas there is widespread Ordovician scattered
here and there in the Andean regions; fossiliferous Silurian rocks are widespread on the shield
areas, as well as in the Andean regions. There are potentially Ordovician, unfossiliferous strata,
possibly latest Ordovician, rocks on the shield areas, but until some means of dating them
precisely emerges it would be futile to spend time discussing them. For example, Caputo &
Crowell (1985) have described diamictites that may be tillites of Ashgill age, that occur not too
far below Silurian strata containing higher land plant spores of earlier Llandovery age (Gray,
unpublished data from the Amazon Basin). I will, as stated, not devote attention to such
difficult and biostratigraphically ambiguous beds.
In the following summary statement I will review, geographic region by geographic region,
what is currently known about the lowest Silurian and highest Ordovician fossiliferous rocks of
the continent. It should, however, be kept in mind that the later Ordovician and earlier Silurian
of South America are very poorly known, or known only in a rough reconnaissance manner,
when contrasted with rocks of similar age in Europe. Conclusions arrived at here, particularly
in the many poorly understood Andean regions, will certainly be subject to serious revision
during the next few decades as additional field and laboratory studies take place.
The Silurian correlation chart for South America (Berry & Boucot 1972) provides a good
summary of the data available up to about 1970, but can now be significantly supplemented by
additional published and unpublished data. Extra new data are also published by Cuerda et al.
and Baldis & Pothe de Baldis (this volume, pp. 291-295).
Argentina
Amos (in Berry & Boucot 1972) provided an authorative review of the Argentinian Silurian,
and its relations with the underlying Ordovician where present. The Argentinian Palaeozoic
may be easily divided into that associated with the Andes in the north and the west, as
contrasted with that present on the shield areas to the east. Much of the shield area Palaeozoic
in Argentina is present in the subsurface beneath Mesozoic and Cenozoic cover, but there are
Bull. Br. Mus. nat. Hist. (Geol) 43: 285-290 Issued 28 April 1988
286 A. J. BOUCOT
limited areas where high-angle faulting has brought Precambrian and Palaeozoic rocks to the
surface.
The shield regions in the Buenos Aires, La Pampa and Rio Negro regions (Amos in Berry &
Boucot 1972: fig. 2) have not yielded any body fossils of proved Ordovician age, although some
unfossiliferous units have been assigned for varied reasons to the Ordovician. Fossiliferous
Silurian rocks are present in these regions, but no fossils of proved Lower Llandovery age have
been demonstrated. The Silurian fauna consists of Malvinokaffric Realm brachiopods for the
most part, and, as is characteristic of that cool to cold climate Realm, few taxa are present. It is
presently unclear in these regions whether strata that could conceivably have crossed the
Ordovician-Silurian boundary are present.‘ The prevalence of late Ordovician to earlier Silu-
rian continental glaciation in the Southern Hemisphere opens up the possibility that any such
beds might well be in the non-marine category that can only be dated with a certain level of
uncertainty for this time interval. The presence in the Cape Mountain System (Gray et al. 1986)
of nearshore marine and possibly non-marine beds of probable Lower Llandovery or Ashgill
age, or both, has some bearing on the Argentinian shield type occurrences in the Sierra de la
Ventana, to the southwest of Buenos Aires in the Sierras Australes, which are commonly
considered to be a pre-Jurassic continuation of the Cape Mountain System by many. In any
event, it is reasonable to conclude (in the total absence of any dated Ordovician or early
Llandovery fossils) that non-marine, or very nearshore, relatively unfossiliferous boundary beds
might have been, or still might be present in the shield portions of Argentina. More subsurface
data could demonstrate this possibility, particularly through the use of palynomorphs.
For purposes of considering the Ordovician—Silurian boundary, the Andean regions of
Argentina should be divided into the Precordillera de San Juan, where the Cambrian and
Ordovician fossils have North American platform biogeographical affinities and occur in plat-
form carbonate type rocks, and the Andes proper with their Malvinokaffric Realm Ordovician
and Silurian faunas occurring in siliciclastic rocks.
Amos (in Berry & Boucot 1972) has provided a summary for the Silurian of the Precordillera
de San Juan. Nowhere are there fossiliferous Silurian rocks suspected to be older than Upper
Llandovery, and the underlying Ordovician is nowhere thought to be younger than Caradoc,
i.e. the Precordillera de San Juan is not a place in which to find a close approximation to the
Ordovician-Silurian boundary as far as was then known, but see Cuerda et al. and Baldis &
Pothe de Baldis (this volume). The only exception to this statement about the absence of the
Ashgill is in a limited area, where the Cantera Formation (Furque & Cuerda 1979: 473) has
yielded Ashgill trilobites and brachiopods (Baldis & Blasco 1975; Nullo & Levy 1976; Levy &
Nullo 1974), although interrupted above by ‘contacto tectonico’ with a Lower Devonian unit.
Tillites are not reported from this region, which suggests that the area may not necessarily have
been subjected to continental glaciation, and might have been the site of a major regression
associated with the terminal Ordovician—earliest Silurian glaciation.
Amos (in Berry & Boucot 1972) has summarized the Andean Silurian of northwestern Argen-
tina, chiefly in the Provinces of Salta and Jujuy. The fossiliferous Silurian is no older than
about Upper Llandovery based on available data, except for the single Lower Llandovery
fossiliferous occurrence in the Puna well to the west of the material summarized by Amos (see
Boucot et al. 1976). This fossiliferous Silurian is underlain by the tillites of the Mecoyita
Formation which lack diagnostic fossils, and have been commonly considered (Laubacher et al.
1982) to be of Ashgill age (although shown by Amos, in Berry & Boucot 1972, to be well up
into the Upper Llandovery). The underlying fossiliferous Ordovician is nowhere demonstrated
to be of Ashgill age, although Caradoc equivalents are recognized (Amos in Berry & Boucot
1972).
It is clear that there are few places anywhere in Argentina for a palaeontologically-based
close approach to the Ordovician-Silurian boundary.
Bolivia
Fossiliferous Ordovician (Hughes 1981, summary) and Silurian (Laubacher et al. 1982) rocks
are well known in the Andean portions of Bolivia. However, no proved fossiliferous Silurian of
ORDOVICIAN-SILURIAN BOUNDARY IN SOUTH AMERICA 287
Lower Llandovery age is known, nor fossiliferous beds of Ashgill age. Tillite separating fossil-
iferous rocks belonging to the two systems is widespread. The oldest fossiliferous Silurian at
present recognized is of Upper Llandovery age (Berry & Boucot 1972) from the Pojo region,
where both brachiopods and graptolites provide the date. It is likely that there is a major,
glacially correlated disconformity over most of Bolivia between the two systems (Berry &
Boucot 1972: fig. 2). There is no reliable palaeontological evidence for placing any of the
Andean tillites above the Llandovery: Berry & Boucot (1972: 26-27) summarize the graptolitic
and brachiopod evidence from the overlying Kurusillas and Llallagua Formations, which
contradicts that provided by Crowell et al. (1980); Crowell et al. (1981) suggest a Wenlock or
Ludlow lower limit based on palynomorphs. An Ashgill age is most consistent for these tillites,
in view of the overall emphasis on a glacial peak during that interval as contrasted with earlier
Ordovician and later Silurian times. Antelo (1973) described Llandovery fossils from the Canca-
niri, but the fossils actually come from above the tillite horizon (Cuerda & Antelo 1973) in beds
which at Pojo were assigned by Berry & Boucot (1972) to the Llallagua Formation, which
overlies the tillite proper.
Brazil
Fossiliferous Ordovician from the shield areas is unknown, except far to the west in the
Amazonian region in the subsurface close to the areas of Andean disturbance. Silurian (Lange,
in Berry & Boucot 1972) has been known from the Brazilian shield areas for over a century, but
the graptolitic Silurian featuring Climacograptus has been conventionally assigned to the Llan-
dovery, and not the latest Llandovery, because that genus was unknown above the Llandovery
in the classic European and North American areas. Since 1972 there has been an accumulation
of data indicating that Climacograptus can occur as high as the Lower Devonian (Jaeger 1978)
in Austria, and that the palynomorphs associated with the graptolite show that the graptolites
are no older than about Ludlow, rather than being of Llandovery age as had always been
assumed. The palynomorphs in the Amazon Basin, where they occur with the graptolite,
include acritarchs being studied by Luis Quadros, chitinozoans being studied by Florentin
Paris, and higher land plant spores being studied by Jane Gray. All three specialists concur in
assessing the age of the graptolitic part of the Trombetas Formation, the unit in question, as
being no older than Ludlow. There is a possible tillite beneath the Trombetas Formation
(Caputo & Crowell 1985). The tillite and associated strata are unfossiliferous, but an Ashgill
age has been inferred, largely because the overlying, fossiliferous Trombetas Formation was
concluded earlier to have been of Lower Llandovery age; this is now known to be an error. But
basal Trombetas Formation beds, strata lacking any marine megafossils or marine palyno-
morphs, have yielded spore tetrads to Jane Gray which are of earlier Llandovery age and which
also indicate in the absence of any marine organisms a possible non-marine environment.
Similar spore tetrads of similar age have been recovered from the Brazilian Parana Basin (Gray
et al. 1985) and from the Cape Mountain System of South Africa (Gray et al. 1986).
Silurian strata have been reported from the Parnaiba Basin (Lange in Berry & Boucot 1972
gives a summary) based on palynomorph studies. However, there is still uncertainty about the
precise parts of the Silurian present within this Basin, and no fossils of proved Ordovician age
are known.
Fossiliferous Silurian was unknown in the Brazilian part of the vast Parana Basin until this
decade (see Gray et al. 1985, for a summary, including the initial recognition of these beds and
their fossils by de Faria). Now, with the aid of both acritarchs and higher land plant spore
tetrads there is no doubt about the presence of shallow water, Benthic Assemblage 1, marine
earlier Llandovery on the northeastern flank of the Basin. Earlier Silurian, based on graptolites
from the southwestern flank of the basin in Paraguay, has been known for some time
(Harrington in Berry & Boucot 1972), but no trace of any fossiliferous Ordovician is known
anywhere to be associated with the Parana Basin.
In summary the Brazilian shield areas are not ones where the Ordovician—Silurian boundary
may be located by means of fossils, owing to the total absence of any Ordovician fossils
immediately beneath the available Lower Silurian fossils.
288 A. J. BOUCOT
Chile
Fossiliferous Silurian rocks are unknown in Chile. The rocks from the Salar de Atacama region
in northern Chile, assigned by Cecioni & Frutos (1975) to the Lower Palaeozoic (Ordovician,
Silurian and Lower Devonian) are probably of Lower Carboniferous age, due to the similarity
of their brachiopods to those found nearby (Bahlburg et al. 1986) which were assigned by
Boucot to the Lower Carboniferous (fossiliferous Devonian beds are known from this area,
yielding Tropidoleptus and Australocoelia, but these shells are unlike those figured by Cecioni
& Frutos 1975 as contrasted with the Lower Carboniferous brachiopods). Fossiliferous earlier
(Arenig) Ordovician is known in the Puna de Atacama, well to the east of the Salar de
Atacama, but unassociated with fossiliferous Silurian. The nearest fossiliferous Silurian consists
of a single Lower Llandovery locality in the Argentinian Puna, which yielded Cryptothyrella
among other things (Boucot et al. 1976), which is unassociated with any fossiliferous Ordovi-
cian. The fossiliferous Devonian beds in the Salar de Atacama region are no older than about
Siegenian—Emsian, and rest unconformably on an older basement complex. We do not know
whether there is any possibility of finding Ordovician—Silurian boundary region strata in Chile.
The older Palaeozoic rocks of Chile are almost unknown, although there are many suspect
regions that warrant careful attention.
Colombia
Fossiliferous Silurian rocks are presently unrecognized in Colombia, while none of the known
Ordovician has been shown to even reach the Caradoc, much less the Ashgill (Hughes 1981).
The presence in the Perija Andes, on the Colombian—Venezuelan boundary, of Lower Devon-
ian fossiliferous beds, resting unconformably on basement complex, indicates that at least in
some spots one would not expect fossiliferous Silurian or Ordovician strata to be preserved.
Ecuador
Fossiliferous Ordovician and Silurian rocks have not yet been recognized in Ecuador, although
there is no reason to doubt their potential presence in the Andean part of the country.
Paraguay
See discussion of the Paraguayan Lower Silurian occurring on the southwestern margins of the
Parana Basin under ‘Brazil’, p. 287.
Peru
There is widespread fossiliferous Ordovician and Silurian in southern Peru, both to the east
and west of Lake Titicaca (see discussion of the Silurian in Laubacher et al. 1982; Hughes,
1981, summarizes the Ordovician, which has reliable palaeontological evidence only up to beds
of Caradoc age). Laubacher et al. (1982) recognized Early Llandovery brachiopods to the west
of Lake Titicaca, in the absence of the tillite that so commonly separates fossiliferous Ordovi-
cian and Silurian rocks from each other in the central Andean region. But these fossiliferous
Early Llandovery fossils are removed stratigraphically some distance from the youngest Ordo-
vician rocks which have yielded fossils no younger than Caradoc. In southern Peru, therefore,
there is no locality known where a close approach to the Ordovician—Silurian boundary is
made within fossiliferous rocks. In central and northern Peru, as well as along the coast,
fossiliferous Silurian rocks are unrecognized. The lack of tillite to the west of the Titicaca
region does raise the possibility that an Ordovician-—Silurian transition may eventually be
discovered in southern Peru or adjacent Bolivia, since a major disconformity might be more
likely in the more easterly regions characterized by tillite.
ORDOVICIAN-SILURIAN BOUNDARY IN SOUTH AMERICA 289
Venezuela
The Ordovician and Silurian rocks related to the Ordovician—Silurian boundary are restricted
in their occurrence to the Merida Andes, well to the south of Lake Maracaibo. Hughes (1981)
comments that the Ordovician faunas of the Merida Andes are of Caradoc age; they are
structurally well removed by faulting from immediate contact with the Lower Llandovery
faunas of the Merida Andes described by Boucot et al. (1972). The Lower Llandovery faunas of
the Merida Andes are dominated by brachiopods that cannot be dated any closer than Lower
Llandovery; thus we are ignorant about whether or not these faunas are actually very close to
the Ordovician-Silurian boundary. Graptolites that might help to resolve the age problem are
unknown from the Merida Andes Llandovery. The shallow water nature of the Merida Andes
Lower Llandovery, the medium-grained sandstones of the Silurian portion of the Caparo
Formation with a Benthic Assemblage 2 set of communities dominated by such genera as
Mendacella, is, however, consistent with the concept that there might be a disconformity
between the two systems there, related to possible glacial regression, as is the case in many
other parts of the world. In any event, the recognition of a close approximation to the
Ordovician—Silurian boundary in Venezuela is as yet unknown.
Acknowledgments
I am indebted to Chris Hughes and Barrie Rickards for their friendly advice on a draft of the manuscript,
and to Jane Gray and the palaeontologists of the PETROBRAS office in Rio de Janeiro for their
assistance with the Brazilian data.
References
Antelo, B. 1973. La fauna de la Formacion Cancaniri (Silurico) en los Andes Centrales Bolivianos. Revta
Mus. La Plata, (Paleont.) 7 (45): 267-277.
Bahlburg, H., Breitkreuz, C. & Zeil, W. 1986. Palaozoisches Sedimenten nordchiles. Abh. Berliner Geowiss.
(A) 66: 147-168.
Baldis, B. A. & Blasco, G. 1975. Primeros trilobites Ashgillianos del Ordovicico Sudamericano. Actas I
Congr. argent. Paleont. Bioestratigr., Tucuman, 1: 33-48.
Berry, W. B. N. & Boucot, A. J. (eds) 1972. Correlation of the South American Silurian Rocks. Spec. Pap.
geol. Soc. Am., New York, 133: 1—59.
Boucot, A. J., Isaacson, P. E. & Antelo, B. 1976. Implications of a Llandovery (early Silurian) brachiopod
fauna from Salta Province, Argentina. J. Paleont., Tulsa, 50: 1103-1112.
——, Johnson, J. G. & Shagam, R. 1972. Braquiopodos Siluricos de los Andes Meridefios de Venezuela.
Boln Geol. Minist. Minas V enez., Caracas, Publ. Esp. 5 (Mem. 4 Congr. geol. Venez. 2): 585-727, 34 pls.
——., Rohr, D. M., Gray, J., de Faria, A. & Colbath, G. K. 1986. Plectonotus and Plectonotoides, new
subgenus of Plectonotus (Bellerophontacea: Gastropoda) and their biogeographic significance. N. Jb.
Geol. Palaont. Abh., Stuttgart, 173: 167-180.
Caputo, M. V. & Crowell, J. C. 1985. Migration of glacial centers across Gondwana during Paleozoic Era.
Bull. geol. Soc. Am., New York, 96: 1020-1036.
Cecioni, A. & Frutos, J. E. 1975. Primera noticia sobre el hallazgo de Paleozoico Inferior marino en la
Sierra de Almeida, Norte de Chile. Actas I Congr. argent. Paleont. Bioestratigr., Tucuman, I: 191—207.
Crowell, J. C., Rocha-Campos, A. C. & Suarez-Soruco, R. 1980. Silurian glaciation in central South
America. In M. M. Cresswell & P. Vella (eds), Fifth International Gondwana Symposium: 105-110.
Balkema.
——, Suarez-Soruco, R. & Rocha-Campos, A. C. 1981. The Silurian Cancaniri (Zapla) Formation of
Bolivia, Argentina and Peru. In M. J. Hambrey & W. B. Harland (eds), Earth’s pre-Pleistocene glacial
record: 902-907. Cambridge.
Cuerda, A. J. & Antelo, B. 1973. El limite Silurico-—Devonico en los Andes Centrales y Orientales de
Bolivia. Actas 5 Congr. geol. argent., Buenos Aires, 3: 183-196.
Furque, G. & Cuerda, A. J. 1979. Precordillera de La Rioja, San Juan y Mendoza. 2 Simp. geol. regional
Argentina, Cordoba, 1: 455—522.
Gray, J., Colbath, G. K., de Faria, A., Boucot, A. J. & Rohr, D. M. 1985. Silurian-age fossils from the
Paleozoic Parana Basin, southern Brazil. Geology, Boulder, Colo., 13: 521—S25.
290 A. J. BOUCOT
——, Theron, J. N. & Boucot, A. J. 1986. Age of the Cedarberg Formation, South Africa and early land
plant evolution. Geol. Mag., Cambridge, 123: 445-454.
Hughes, C. P. 1981. A brief review of the Ordovician faunas of northern South America. Actas II Congr.
argent. Paleont. Bioestratigr. (y I Congr. Latinamericano Pal.), Buenos Aires, I: 11—22.
Jaeger, H. 1978. Late graptolite faunas and the problem of graptoloid extinction. Acta palaeont. pol.,
Warsaw, 23: 497-521.
Laubacher, G., Gray, J. & Boucot, A. J. 1982. Additions to the Silurian stratigraphy, lithofacies, biogeog-
raphy and paleontology of Bolivia and southern Peru. J. Paleont., Tulsa, 56: 1138-1170.
Levy, R. & Nullo, F. 1974. La fauna del Ordovicico (Ashgilliano) de Villicun, San Juan, Argentina.
Ameghiniana, Buenos Aires, 11 (2): 173-194.
Nullo, F. & Levy, R. 1976. Consideraciones faunisticas y estratigraficas del Ashgilliano de Sudamerica.
Actas 6 Congr. geol. argent., Buenos Aires, 1: 413—422.
The Ordovician—Silurian boundary in Bolivia and
Argentina
A. Cuerda’, R. B. Rickards’ and C. Cingolani*
1 La Plata Museum, Paseo del Bosque, 1900 La Plata, Argentina
? Sedgwick Museum, Department of Earth Sciences, Downing Street, Cambridge CB2 3EQ
3 Centro de Investigaciones Geologicas, Universidad Nacional de la Plata, Calle 1, no 644,
1900 La Plata, Argentina
Synopsis
The Ordovician-Silurian boundary level has been identified in few areas, although there is considerable
potential for future work. The following sections are the best: 1 Lampaya, Bolivia; 2 the Don Braulio
Valley, Argentina; 3 Talacasto, Argentina. Recent fieldwork has established that Talacasto appears the
best of these, and a sequence of persculptus Zone, probably acuminatus Zone, and approximate equivalent
of the atavus Zone has been established. The base of the Silurian at Talacasto is taken at 60cm above the
base of the La Chilca Formation, following a persculptus Zone assemblage. Several stratigraphically
important graptolites are recorded from South America for the first time.
Introduction
In Bolivia undoubted low Silurian rocks are exposed in the Eastern Cordillera, and in Argen-
tina in the Precordillera (Fig. 1). The Cancaniri Formation is the basal unit of the Silurian in
Bolivia (Castanos & Rodrigo 1978) and consists of 105m of diamictites, shales and sandstones
yielding palynomorphs and, in some sections, scarce brachiopods. The Precordilleran Argentin-
ian Silurian is recognized as three facies types: the Eastern Facies, some 2500-3000 m of shales,
sandstones and conglomerates with associations of brachiopods, corals and graptolites; the
Central Facies, 450-500 m of green shales, orthoquartzites, and fine grained limestones, with
rich assemblages of brachiopods, corals, trilobites and graptolites; and the Western Facies,
restricted to the Calingasta region, approximately 1000m of shales and turbidite sandstones,
yielding some brachiopods. Each facies type (Cuerda, in press) is interpreted as having a
different palaeoenvironment, respectively: a N—S trough between Pre-Cambrian ridges; proxi-
mal to distal platform; distal platform to abyssal plain. The stratigraphically lowest formations
in these facies are the La Rinconade Formation, the La Chilca Formation, and the Calingasta
Formation.
Bolivia
The Lampaya section is located near Cochabamba. Three lithological units have been recog-
nized in the Silurian, the Cancaniri Formation at the base, and above it the Kirusillas and
Catavi Formations, a total of 1355m spanning the Llandovery to Ludlow. The Ashgill Series
seems to be absent in Bolivia so that the Cancanfiri Formation rests upon Caradoc or earlier
strata. At Lampaya the Cancafiri Formation consists of 105m of diamictites with shales and
sandstones intercalated as thin layers. A Llandovery age is supported by palynomorphs refer-
able to the Veryhachium rhomboidium Zone (Suarez-Riglos 1975). Macrofossils have been re-
covered including trilobites, brachiopods, corals and ostracods by one of us (A.C.). The
Cancaniri Formation at Lampaya rests upon the Caradoc.
Argentina
Villicum Hills Section. The Don Braulio Valley drains the eastern slopes of the Villicum Hills,
where the Ashgill black shales and grey sandstones are topped by a ferruginous oolite. The grey
Bull. Br. Mus. nat. Hist. (Geol) 43: 291-294 Issued 28 April 1988
292 CUERDA, RICKARDS & CINGOLANI
e\ Tucunuco
Talacasto --
'
WJ
o
<
c
ig
=F
a
(e)
a
=
<
-
o San Juan
f
ar lsazic
\
& deca ge
Tontal
s
EC Eastern Cordillera
Vary
- Pate
CF Frontal Cordillera
R.d. |
Sierra del
|P Precordillera
SP Pampean Ranges
Fig. 1 Distribution of Silurian facies in the Precordillera of San Juan, Argentina. The western facies
is shown around Calingasta, the central facies in the close stipple and the eastern facies in open
stipple to the right.
sandstones have yielded the trilobites Calymenella (Eohomalonotus) villicumensis Baldis &
Blasco and Dalmanitina sudamericana Baldis & Blasco (Baldis & Blasco 1974) and the brachio-
pods Fascifera punctata, Arenorthis cuyana, Villiscundella muozetici, Bagnorthis garrigoui and
Kjaerina (Neokjaerina) florentina (all Levy & Nullo 1977).
The Silurian commences with argillaceous sandstones and has a palynomorph assemblage
referable to the Llandovery, which Volkheimer et al. (1980) list as Ancyrochitina sp., A. cf.
ORDOVICIAN-SILURIAN BOUNDARY IN BOLIVIA AND ARGENTINA 293
ancyrea (Eisenack), Conochitina cf. chydaea Jenkins, Desmochitina sp., Cyathochitina cf. cam-
panulaeformis Eisenack, Euconochitina cf. filifera Tangourdeau, Rhabdochitina sp. ‘A’, Spathochi-
tina cf. clarindoi de Costa and Sphaerochitina sp. Above the argillaceous sandstones the beds
grade into medium and coarse sandstones of Wenlock and Ludlow age (Magotes Negros
Formation). Baldis & Pothe de Baldis (1988, this volume) have reviewed and revised this
section.
The Talacasto section (Figs 1, 2) is located some 16km WNW of Talacasto railway station and
has been studied by Cuerda et al. (1982). Recent collecting by the authors yielded several
hundred graptolites throughout the whole of the 3-65m of the La Chilca Shale Formation.
Collecting was done every few centimetres, as closely as the friability of the shale would allow.
Several confirmatory collections were made nearby. Glyptograptus persculptus occurs com-
monly, both flattened and in three dimensions, in association with equally common specimens
of Climacograptus angustus Perner and in addition Pseudoclimacograptus sp. nov., Glyp-
tograptus sp. (an undescribed form commonly seen in the persculptus Zone in other parts of the
world), Climacograptus cf. medius Tornquist, and Climacograptus normalis Lapworth. This
assemblage is taken to indicate the latest Ordovician G. persculptus Zone.
At 55cm above the base of the formation G. persculptus s.s. disappears, but the remainder of
the fauna continues. Rhaphidograptus sp. at 90 cm, and G. ex gr. persculptus (late forms, smaller,
and with a delayed median septum) also occur between 1-1 m and 1-38 m, where Pseudoclima-
cograptus sp. nov. is also especially abundant and dominates the fauna. The Pseudoclimaco-
graptus sp. nov. is close to P. fidus and P. pictus described from the acuminatus Zone of
Kazakhstan by Koren & Mikhailova (1980). From 60 cm to 1:7 m we have recorded specimens
Arenig limestones (San Juan
Formation), above thrust
se
CAAA
atavus z
zone AZ
BA La Chilca Shale Formation
(3°65m)
BB
mcrae basal Silurian conglomerate
<a GAA with Arenig chert pebbles
zone
a Zz
persculptus EA
zone A Z O° ese
ge SESS Arenig limestone
ZR ioe eee ses
ge as PEI IGE: (San Juan
5 ee ee limestones)
Se ee ee
SE
oe
SA
Fig. 2 Section through the Ordovician-Silurian boundary near Talacasto, San Juan Province,
Argentina. The ‘basal Silurian conglomerate’ also includes the persculptus Zone.
294 CUERDA, RICKARDS & CINGOLANI
of Climacograptus acceptus Koren & Mikhailova, also typical of the acuminatus Zone, and we
have found specimens possibly referable to Glyptograptus maderni Koren & Mikhailova from
60-90 cm. At 1-6 m there is a further change in the fauna, with the disappearance of glyp-
tograptids and the Pseudoclimacograptus, whilst there is an increase in abundance of C.
angustus, C. normalis and C. rectangularis and the appearance for the first time of the mono-
graptid Lagarograptus. Paraclimacograptus cf. innotatus (Nicholson) appears at 1-75 m. This
fauna is then maintained to the top of the section apart from the addition of a new diplo-
graptid.
The base of the acuminatus Zone, and hence of the Silurian, is probably best taken at 60cm
with the appearance of Climacograptus aéceptus. For reasons which we shall discuss in a
systematic paper elsewhere, we take the incoming of Lagarograptus to be roughly equivalent to
the atavus Zone.
Thus the Talacasto region at present affords the best recognition of the base of the Silurian
in South America. The potential is considerable for further precise subdivisions on other
sections in the same region. The authors’ recent fieldwork established the following strati-
graphically important forms for the first time in South America: G. persculptus, C. angustus, C.
normalis, C. acceptus, C. rectangularis, Rhaphidograptus, Paraclimacograptus, and Lagaro-
graptus.
Acknowledgements
The authors would like to thank CONICET and the Royal Society for supporting both the fieldwork and
subsequent laboratory work.
References
Baldis, B. A. & Blasco, G. 1975. Primeros trilobites Ashgillianos del Ordovicico Sudamericano. Actas I
Congr. argent. Paleont. Bioestratigr., Tucuman, 1: 33-48.
& Pothe de Baldis, E. D. 1988. The Ordovician—Silurian boundary in the Sierra de Villicum,
Argentine Precordillera. Bull. Brit. Mus. nat. Hist., London, (Geol.) 43: 295-297.
Castanos, A. & Rodrigo, L. A. 1978. Sinopsis estratigrafica de Bolivia. I—Parte Paleozoico. 146 pp., La
Paz.
Cuerda, A. J. 1971. Monograpten des Unter-Ludlow aus der Vorkoodi-Vere von San Juan, Argentinien.
Geol. Jb., Hannover, 89: 391—406.
(In press). El Silurico de la Precordillera de San Juan. Boln Yacimientos Petrolif. Fisc. Bolivianos, La
Paz.
——,, Furque, G. & Vliarte, E. 1982. Graptolitos de la base del Silurico de Talacasto, Precordillera de San
Juan. Ameghiniana, Buenos Aires, 19 (3—4): 239-252.
Koren, T. N. & Mikhailova, N. 1980. In M. K. Apollonov, S. M. Bandaletov & J. F. Nikitin (eds), The
Ordovician-Silurian Boundary in Kazakhstan. 300 pp. Alma Ata, Nauka Kasakh S.S.R. Publ. Ho.
Levy, R. & Nullo, F. 1974. La fauna del Ordovicico (Ashgilliano) de Villicum, San Juan, Argentina.
(Brachiopoda). Ameghiniana, Buenos Aires, 11 (2): 173-194.
Suarez Riglos, M. 1975. Algunas consideraciones biocronoestratigraficas del Silurico—-Devonico en Bolivia.
Actas I Congr. argent. Paleont. Bioestratigr., Tucuman, 1: 293-317.
Volkheimer, W., Pothe, D. & Baldis, B. 1980. Quitinozoos de la base del Silurico de la Sierra de Villicum
(Provincia de San Juan, Republica Argentina). Revta Mus. argent. Cienc. nat. Bernardino Rivadavia,
Buenos Ayres, (Paleont.) 2 (6): 121-135.
The Ordovician—Silurian boundary in the Sierra de
Villicum, Argentine Precordillera
B. A. Baldis and E. D. Pothe de Baldis
Avenue Cordoba 261, Este, 5400 San Juan, Argentina
Synopsis
The Ordovician-Silurian boundary is defined within the Don Braulio Formation at its type locality near
San Juan, Argentina. The boundary sequence consists of: 1, Upper Ashgill (Hirnantian) defined by the
presence of Hirnantia cf. sagittifera and Dalmanitina sudamericana; 2, a short stratigraphical interval of
10m of shales with unidentifiable graptolite fragments, perhaps Lower Silurian in age; 3, levels with
acritarchs, chitinozoan and graptolites which can be related with certainty to the Lower Llandovery.
The stratigraphical section which includes the Ordovician—Silurian boundary in the Sierra de
Villicum is perhaps the best known and palaeontologically controlled locality in South
America. The Sierra (Range) of Villicum is situated in the Argentine Precordillera, in San Juan
Province about 1100km northwest of Buenos Aires (see Fig. 1). Upper Ordovician and Silurian
sediments outcrop in the eastern flank of the range, and the best section is found in Don
Braulio Creek, 35km north of the city of San Juan. The section is well exposed, in a desert
climate area, and the following formations are present:
Mogotes Negros Fm Lower to Upper Silurian age
Don Braulio Fm Ashgill to Llandovery age
La Cantera Fm Llandeilo to Caradoc age
Los Azules Fm Llanvirn to Llandeilo age
San Juan Fm Arenig age
La Flecha Fm Upper Cambrian to Tremadoc age
La Laja Fm Lower to Middle Cambrian age
The first Ashgill macrofossils from South America were found in the Don Braulio Formation
(Baldis et al. 1982). The brachiopods were described by Levy & Nullo (1974) and trilobites of
the Dalmanitina faunal group by Baldis & Blasco (1975). Benedetto (1985) has reported the
presence of Hirnantia associated with Modiolopsis (Sanchez 1985), which gives an accurate
Upper Ashgill age for the top of the lower Don Braulio Formation.
The trilobites found in the lower part of the formation are Dalmanitina (D.) sudamericana
Baldis & Blasco and Calymenella (Eohomalonotus) villicunensis Baldis & Blasco, and brachio-
pods belonging to the genera Fascifera, Arenorthis, Bagnorthis and Kjaerina (Neokjaerina).
From the middle to the upper part of the lower portion of the formation are reported Hirnantia
Sagittifera (M‘Coy) and Dalmanella aff. D. testudinaria, associated with Modiolopsis, Nuculopsis
and Palaeoneilo. The lower part of the formation is separated by several metres of shales with
indeterminable remains of graptolites from the upper part.
In the base of the upper part of the formation, Volkheimer et al. (1980) determined a
chitinozoan microflora composed of Ancyrochitina cf. ancyrea (Eisenack) Eisenack, Conochitina
cf. chydae Jenkins, Desmochitina (?) sp., Cythochitina cf. campanulaeformis (Eisenack) Eisenack,
Euconochitina filifera (Eisenack) Tang, Rhabdochitina sp. A, and Spathachitina cf. clarindoi da
Costa. The Llandovery age of the association is indicated by the presence of Ordovician—Lower
Silurian chitinozoa together with Lower Silurian ones. The genus Spathachitina da Costa
indicates a Lower Silurian age in the Amazon Basin of Brazil. Pothe de Baldis (1980) described
a varied microflora of acritarchs from the same level, with 26 genera and 47 species, of which 34
are known from other countries, mainly northern Spain, Belgium, England and northern
Africa. The association shows a predominance of Veryhachium trispinosum, followed in impor-
tance by Eupoikilofusa tenuistriata (POthe de Baldis) aperturata n. var. The genus Eisenackidium
Bull. Br. Mus. nat. Hist. (Geol) 43: 295-297 Issued 28 April 1988
296
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10 Il 12 13 14
8 Modiolopsis
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l4Lw Sil Chitinozoa
The location of the type locality of the Don Braulio Formation (above), and a section
through the formation showing the distribution of the fossils mentioned in the text (below).
ORDOVICIAN-SILURIAN BOUNDARY IN ARGENTINA 297
was recorded for the first time from the Lower Silurian (formerly only described from the
Lower Devonian). Other forms such as Veryhachium tetraedron Deunff, Marrocanium simplex
Cramer et al., Multisphaeridium alloiteaui Deunff and M. cf. remotum (Deunff) are typical of
Ordovician sediments. The age of the association is based on the presence of Tunisphaeridium
tentaculaferum (Martin) and Domasia limaciforme (Stockman & Williére) whose first appearance
is in the Lower Llandovery of England and Belgium.
The graptolites appearing in all parts of the Upper Section of the Don Braulio Formation
were determined by Peralta (in press) as the typical Lower Silurian assemblage of Cli-
macograptus aff. C. hughesi (Nicholson), Monograptus sp., Glyptograptus sp. and Rastrites sp.
The Don Braulio Formation has 40 m thickness in the type locality. A brief description of the
section is as follows (also see Fig. 1):
A. Hematitic Member:
9 Pale green-greyish shales (strongly deformed), with spots of iron oxides and scattered ramous
CALDION IES oc nda qooe so da re RO ORER CEE ROH oad tEaae reader osese eaten cece con OONConC Ee aeee eee aes 2m
8 Oolitic sandstones weathering dark red; red brownish to green in fresh fracture. Acritarchs
GING! GNINCOAOCEIE SeacadoanposasmondddcoososccogsvoucUdmoanenentod loa menESdae Sooo Basen cr acO Cone: 0:50m
7 Pale green-greyish shales, intercalated with thin hard siltstones and fine-grained sandstones of
2 Gin WMS SNES nouboreute eenuusAeueubobon cs doobU OURO ROaUDHIORE OAR OE Hen tanud toad acne eee oares 2m
6 Oolitic sandstone similar to Bed 8 but with fewer oolites, with indeterminable monograptid
BUDS. coosvodprlb smactidhindguadaeseads leaaboaases eRe Ee OT CeCe eC nO ONU TS RTOT ROR cer ar eee a aiars aera 1m
5 Green-greyish hard siltstone with some shaley levels and Climacograptus hughesi .............. 3m
elematiticisiltstones (poorly bedded) oe ca..cececcswe nes ce cee eh asienearees occ aee sreeu cise esnes seeds 1m
B. Silty and Shaly Member:
3 Dark green shales and siltstones (highly deformed) alternating with pale green-greyish clay,
with fragments of unidentifiable graptolites (Monograptidae?) ................cec cece eee ee eee eees 10m
C. Conglomerate and Sandstone Member (Lower Section):
2 Dark green to green-greyish fine-grained sandstones (poorly bedded) with the trilobites Caly-
menella (Eohomalonatus) villicumensis, Dalmanitina sudamericana and the brachiopods Fas-
cifera punctata, Hirnantia cf. sagittifera, Dalmanella cf. testudinaria ............0.0.000cceeeeeeeees 12m
1 Basal oligomictic conglomerate with intercalations of green-greyish lenses ...................... 6m
Unconformity ~ ~ ~ ~ ~ ~
La Cantera Formation (Llanvirn to Caradoc).
From the above we may conclude that the Ashgill is well dated in the whole sequence with
trilobites, brachiopods and Hirnantia cf. sagittifera at its top. Ten metres of barren shales
follows this section, followed by a Llandovery graptolite fauna and acritarchs and chitinozoans
of Lower to Middle Llandovery age.
References
Baldis, B. A. & Blasco, G. 1975. Primeros trilobites Ashgillianos del Ordovicico sudamericano. Actas I
Congr. argent. Paleont. Bioestratigr., Tucuman, 1: 33-48.
Benedetto, J. L. 1985. El hallazgo de la tipica fauna de Hirnantia en el Ashgilliano tardio de la Sierra de
Villicum. Asoc. Paleont. Argent., Reun. Com. en San Juan, 1: 56-57. San Juan.
Levy, R. & Nullo, F. 1974. La fauna del Ordovicico (Ashgilliano) de Villicum, San Juan, Argentina.
Ameghiniana, Buenos Aires, 11 (2): 173-194.
Peralta, S. 1988. Graptolitos del Llandoveriano Inferior en el Paleozoico Inferior clastico del pie oriental
de la Sierra de Villicum. Act. J Jorn. Geol. Precord., San Juan (in press).
Pothe de Baldis, E. D. 1980. Lower Silurian acritarchs from Villicum, Province of San Juan, Argentine
Precordillera, Argentina. Abstr. 5th Int. Conf. Palynology, Cambridge: 315.
Sanchez, T. M. 1985. El Género Mediolopsis en el Ashgilliano de la Sierra de Villicum y la comunidad
Hirnantia—M odiolopsis. Asoc. Paleont. Argent., Reun. Com. en San Juan, 1: 58-59. San Juan.
Volkheimer, W., Pothe, D. & Baldis, B. 1980. Quitinozoos de la base del Silurico de la Sierra de Villicum
(Provincia de San Juan, Republica Argentina). Revta Mus. argent. Cienc. nat. Bernardino Rivadavia,
Buenos Aires, (Paleont.) 2 (6): 121-135.
Late Ordovician and Early Silurian Acritarchs
F. Martin
Département de Paléontologie, Institut Royal des Sciences Naturelles de Belgique, Rue Vautier
29, B-1040 Bruxelles, Belgium
Synopsis
The principal stratigraphical data for late Ordovician and early Silurian acritarchs are reviewed; at
present they do not justify any formal zonation on a broad geographic scale. The systemic basal boundary
stratotype at Dob’s Linn, southern Scotland, has not yielded index acritarchs. A preliminary selection of
taxa from correlative strata on Anticosti Island, Québec, eastern Canada, indicates that the area has the
most continuous palynological record from at least the Ashgill to the late Llandovery, with the best
potential for establishing detailed acritarch systematics and interregional correlation.
Introduction
In general, the biostratigraphical tool provided by the acritarchs is still only partly exploited for
interregional correlation, for the following reasons: (1) sufficiently detailed systematic descrip-
tions have become available only during the last fifteen years or so, through the use of SEM,
and a coherent taxonomic framework is still lacking; (ii) precisely defined taxa are most often
reported only from regions where their total stratigraphical range is not established; (iii) a large
number of data relate to dispersed samples, for which there is no macrofossil age control. In
particular, acritarchs of latest Ordovician and earliest Silurian age have received little docu-
mentation. This scarcity of data reflects the lack of palynological investigations rather than of
suitable marine deposits, for these probably planktonic, organic-walled microfossils appear to
be relatively weakly facies-controlled when compared with macrofossils. Nevertheless, the
Ashgill extinction that affected numerous other fossil groups also involved the acritarchs.
Differences in composition of assemblages between the end of the Ordovician and the begin-
ning of the Silurian are indicated in the following areas: Anticosti Island, eastern Canada;
southern Appalachians, U.S.A.; Belgium; and the Algerian Sahara. These differences are ampli-
fied by the absence of Hirnantian or Gamachian strata, except on Anticosti, where, on the basis
of preliminary data (Duffield & Legault 1981, and author’s personal observations), the disap-
pearance of numerous Ordovician taxa seems to occur in the Gamachian. A marked change
between acritarch associations from the late Ashgill and the Llandovery is mentioned briefly
(Le Heérissé 1984) for the subsurface rocks in southern Gotland. Colbath (1986) has reviewed
different hypothetical causes for these acritarch extinctions, ranging from the effects of sea-level
and climatic changes associated with glaciation to a bolide impact model.
Review of data
The map (Fig. 1) shows the distribution of late Ordovician and early Silurian acritarchs and
indicates detailed references. Numbers (see explanation of Fig. 1) refer generally to the most
recent publication that indicates previous data; exceptions are Anticosti and Great Britain, for
which further references are given. Anticosti and southern Scotland provided the two final
candidate sections for the Ordovician-Silurian boundary stratotype considered by the Subcom-
mission on Silurian Stratigraphy (Holland 1984). Since then the International Commission on
Stratigraphy (Bassett 1985) has chosen to fix the base of the Llandovery Series, together with
that of its lowest stage, the Rhuddanian, at Dob’s Linn, southern Scotland; the boundary
stratotypes for the two other Llandovery stages, Aeronian and Telychian, are located in the
type area of the Llandovery in Wales (Cocks 1985).
Areas from which no index acritarchs are known (for example, the Ashgill of southwest
France, Rauscher 1974) are omitted. Owing to the lack of agreement on precise correlation
between the North American and British upper Ordovician standard successions (Barnes et al.
Bull. Br. Mus. nat. Hist. (Geol) 43: 299-309 Issued 28 April 1988
300 F. MARTIN
Fig. 1 Generalized world map showing late Ordovician and early Silurian acritarch localities. The
following abbreviations indicate the information included in publications 1—37 listed below: (CA),
undifferentiated late Caradoc and Ashgill; A, Ashgill; P, Pusgillian; R, Rawtheyan; H, Hirnantian;
G, Gamachian; L, undifferentiated Llandovery, possibly including Rhuddanian; Rh, Rhuddanian;
Ae, Aeronian; T, Telychian; p.d., palynological dating only. Chronostratigraphic units groups
within parentheses are not differentiated from each other.
1, Hill 1974, Rh-T: 2, Aldridge et al. 1979, Rh-T: 3, Hill & Dorning in Cocks et al. 1984, Rh-T: 4, Downie 1984,
Rh-T: 5, Eisenack 1968, A: 6, Eisenack 1963, A: 7, Umnova 1975, A, L: 8, Gorka 1969, A: 9, Konzalova-
Mazankova 1969, (PR): 10, Vavrdova 1974, A: 11, Vavrdova 1982, H: 12, Martin 1969, (CA), Rh-T: 13, Martin
1974, (CA), Rh: 14, Elaouad-Debbaj 1981, (CA), A, p.d.: 15, Jardiné et al. 1974, (C ?A), (AeT), p.d. in part: 16,
Deunff & Massa 1975, ?C, p.d., 7Rh: 17, Molyneux & Paris 1985, A, p.d.: 18, Hill et al. 1985, (RhAe), p.d.: 19, Bar
& Riegel 1980, (AL), p.d.: 20, Brito 1967, L, p.d.: 21, Gray et al. 1985, L, p.d.: 22, Melendi & Volkheimer 1985, L:
23, Colbath in press, (CA), (PR), (RhAe): 24, Loeblich & Tappan 1978, (CA), (PR): 25, Loeblich & McAdam 1971,
(CA), (PR): 26, Loeblich 1970, (PR): 27, Colbath 1979, (CA): 28, Johnson 1985, L: 29, Wright & Meyers 1981,
(CA), p.d.: 30, Miller & Eames 1982, Rh: 31, Martin 1980, (PR): 32, Legault 1982, (CA), p.d.: 33, Staplin et al.
1965, (PR): 34, Cramer 1970, (AeT): 35, Duffield & Legault 1981, 1982, G, Rh-T: 36, Jacobson & Achab 1985,
(PR): 37, Martin in press and personal observation, G (at Anticosti only), Rh-T. [Since submission of this paper,
Whelan (1986) has commented briefly on the acritarchs from Dob’s Linn. ]
1981; Ross et al. 1982; Shaver 1985), palynological references for both late Edenian and
Maysvillian strata in U.S.A. are included. In the Llandovery Series, acritarch data given for the
Rhuddanian sometimes include those for the Aeronian and Telychian. Localities where the
sections begin only with the Aeronian or Telychian are omitted here and may be found in
Martin (in press).
LATE ORDOVICIAN AND EARLY SILURIAN ACRITARCHS 301
Europe
In Great Britain, no palynological work has been published on the Ashgill. The Ordovician—
Silurian boundary stratotype strata at Dob’s Linn (Cocks 1985) are composed of condensed,
deep-water, graptolitic shales, the base of the Llandovery being coincident with the base of the
P. acuminatus Zone. The whole succession, from the Climacograptus peltifer Zone (early
Caradoc) upwards, contains rare, blackish acritarchs, but these are too poorly preserved to
provide useful information. The type Hirnantian (Hirnant Limestone) at Cwm Hirnant quarry,
near Bala, North Wales, yielded rare acritarchs belonging to either poorly-defined or remnant
Arenig—Llanvirn taxa (personal observation). The Caradoc Series (Costonian to Onnian stages)
in the type area of Shropshire contains well preserved assemblages (Turner 1984) of Caradoc
age, associated with others derived from Tremadoc and Arenig—Llanvirn deposits. Rhuddanian
microfloras from near Llandovery are both poorly preserved and of low diversity but permit
(Hill & Dorning in Cocks et al. 1984) the recognition of three biozones characterized, on the
basis of published lists, by the successive appearance of taxa that, for the most part, are
long-ranging in the Silurian or are left in open nomenclature. The top of the Rhuddanian there
also contains reworked, pre-Caradoc Ordovician material (Martin in press).
In the same region, and especially in the the Welsh Borderland (Hill 1974), partly published
results for the Llandovery show, from the Aeronian onwards, a refined palynological zonation
that may be compared with that outlined for Belgium (Martin 1969). Of particular significance
are species of Domasia Downie, 1960 emend. Hill, 1974 and Dilatisphaera williereae (Martin)
Lister, 1970.
In the Massif of Brabant, Belgium (Martin 1974), moderately well preserved acritarchs,
mostly long ranging and including some known from the Tremadoc to the Arenig—Llanvirn,
are from boreholes. Parts of these rock successions are assigned a late Caradoc and/or Ashgill
age on lithological and structural grounds in the absence of diagnostic macrofossils; those of
the basal Rhuddanian are dated by graptolites and include strata of the P. acuminatus Zone.
In the Baltic region (Gotland, Estonia, Latvia—Eisenack 1963, 1968; Umnova 1975), Poland
(Gorka 1969) and Czechoslovakia (Konzalova-Mazankova 1969; Vavrdova 1974, 1982), as in
Portugal (Elaouad-Debbaj 1981), data are relatively few for the Ashgill and absent for the
Rhuddanian. The only Hirnantian acritarchs so far illustrated come from the Prague region
(Vavrdova 1982).
Africa and South America
Microfloras from boreholes in north Africa are well preserved. At the Grand Erg Occidental in
the Algerian Sahara (Jardiné et al. 1974) acritarch zone F corresponds to the Caradoc and
perhaps Ashgill; it also contains taxa characteristic of the Arenig—Llanvirn and is present too in
deposits of the Illizi Basin attributed doubtfully to the M. sedgwickii Zone of the Aeronian. In
Libya (Deunff & Massa 1975; Molyneux & Paris 1985; Hill et al. 1985) acritarchs from the late
Ordovician and early Silurian, cited and partially figured, are dated with particular reference to
palynological data from western Europe and central U.S.A. In Deunff & Massa (1975) the list
of taxa alleged to have been found in the early Rhuddanian C. vesiculosus Zone indicates a
post-Llandovery age and is not considered further here.
Acritarch data for the relevant interval in Ghana (Bar & Riegel 1980), Brazil (Brito 1967;
Gray et al. 1985) and Argentina (Melendi & Volkheimer 1985) are dispersed and mainly
without independent age control. The most noteworthy illustrated observation is that samples
from Ghana said to occur at the Ordovician/Silurian boundary share only a single species,
Dactylofusa marahensis Brito & Santos, 1965, with strata of the Maranhao Basin attributed to
the Lower Silurian. In both cases the age is based on structural and palynological arguments.
North America
Publications referring to the eastern and central U.S.A. deal mainly with numerous new late
Ordovician taxa from Oklahoma (Loeblich & McAdam 1971; Loeblich & Tappan 1978) and
the Cincinnati area (Loeblich 1970; Loeblich & McAdam 1971; Loeblich & Tappan 1978;
Colbath 1979); however, the acritarchs from the Richmondian Stage, which is correlated with
part of the Ashgill Series, are from isolated samples. In the southern Appalachians (southwest
302 F. MARTIN
Virginia, northwest Georgia and east Tennessee), a consistent acritarch correlation, based
largely on new taxa, is documented (Colbath, in press) for the passage from Ordovician to
Silurian; but the presence of the Gamachian and earliest Rhuddanian in the region is debat-
able. An acritarch assemblage of undoubted Rhuddanian age in western New York State
(Miller & Eames 1982) enables preliminary correlations to be made with assemblages in the
southern Appalachians, Anglo-Welsh area and Belgium. A very few Llandovery, including
perhaps Rhuddanian, acritarchs are known from central Pennsylvania (Johnson 1985).
In eastern Canada, except for palynologically dated latest Caradoc or Ashgill strata in a
borehole in the Labrador Sea (Legault 1982), data relate to the Province of Québec. Only
reconnaissance studies are available for the pre-Hirnantian Ashgill of the Perce area (Martin
1980) in the Gaspé Peninsula. The White Head Formation at White Head (Lespérance 1985;
Fig. 2 herein) has not yielded index acritarchs in the Hirnantian interval, and the basal Llando-
very portion (base of Unit 6; personal observation) contains specimens deformed by crystal
growth; some of the latter, for example Eupoikilofusa aff. E. ampulliformis, sensu Duffield &
Legault 1981, are very characteristic of the Rhuddanian at Anticosti, from the base upwards of
the Becscie Formation at Ellis Bay.
At Anticosti an Ordovician/Silurian boundary stratotype was proposed (Barnes &
McCracken 1981) in an allegedly continuous limestone-shale succession in the upper part of the
Ellis Bay Formation (sensu Petryk 1981) at Ellis Bay. The base of the Silurian is marked by the
appearance of the conodont Ozarkodina oldhamensis (Rexroad, 1967); Oulodus? nathani McCra-
cken & Barnes, 1981 is an auxiliary indicator for the boundary. However, Lespérance (1985)
places the boundary higher and in the Becscie Formation, on the assumption that the appear-
ance of the trilobite Acernaspis coincides with the base of the P. acuminatus Zone. The shallow
marine platform deposits there are very rich in microfloras and in micro- and macrofaunas,
except graptolites (see Lespérance 1981 for numerous contributions and earlier references). On
the whole, the Ashgill and Llandovery acritarchs of Anticosti are very well preserved and
relatively abundant, but have been described only partially (Staplin et al. 1965; Cramer 1970;
Duffield & Legault 1981, 1982), apart from strata dated as D. complanatus Zone, assigned to the
early or middle Ashgill (Jacobson & Achab 1985).
The Anticosti acritarchs
The quality of the palynological material at Anticosti and its age control based on shelly
macrofaunas and conodonts justify a preliminary synthesis. The ranges of some taxa there are
compared (Fig. 2) with those from other regions. The compilation is based on the references
given in the general distribution of data (Fig. 1) and for the post-Aeronian of the same regions,
following those assembled by Martin (in press; explanation of Fig. 1). This restricted choice of
taxa is conditioned by personal examination of twelve samples (see Appendix) from the upper
part of Member 4 of the Ellis Bay Formation, of Gamachian age, to the upper part of the
Jupiter Formation, correlated with the Telychian (C;) (Lespérance 1981). The choice could have
been different, but in the present state of knowledge the comments would probably have been
comparable with those below.
The observations of Duffield & Legault (1981) are confirmed with regard to the change in
composition of acritarch assemblages just above the base of the Silurian as defined on the basis
of the appearance of diagnostic conodonts (Barnes & McCracken 1981) within Member 7 of
the Ellis Bay Formation. If the correlation proposed by Lespérance (1985) is accepted, the
major change in terms of appearance of new acritarch taxa occurs within the late Gamachian,
rather than in the early Llandovery. At its type locality, on the west side of Ellis Bay, the entire
member, | to 4m thick, is very poor in acritarchs. In particular, the locally developed bio-
hermal bed, 1-5 to 2m thick, above the systemic boundary is sterile. Immediately above this
bed, from the base of the Becscie Formation (sensu Petryk 1981; sample A2B7) onwards, the
majority of taxa known from other regions and of Ordovician affinities are absent. Aremorica-
nium squarrosum Loeblich & McAdam, 1971 (see synonymy in Jacobson & Achab, 1985: 171) is
recognized in the early Richmondian, which is equated with latest Pusgillian to early Raw-
303
LATE ORDOVICIAN AND EARLY SILURIAN ACRITARCHS
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304 F. MARTIN
theyan by Barnes et al. (1981). The disappearance of Orthosphaeridium rectangulare (Eisenack)
Eisenack, 1968 (Figs 4a, b; see synonymy in Elaouad-Debbaj 1981: 48) and of O. insculptum
Loeblich, 1970 (Figs 3a, b) occurs within an unobserved interval in the Gamachian, between the
upper parts of Member 5 (about 5 m below its top; sample A2B3) and Member 6 (0-3 m below
its top; sample A2B4) of the Ellis Bay Formation. Baltisphaeridium plicatispinae Gorka, 1969
(Fig. 9) extends, according to Duffield & Legault (1981), into Member 7, below the biohermal
bed. The appearance of taxa of Silurian affinities, which occurs mainly and progressively from
the base of the Becscie Formation onwards, begins in the Gamachian, no later than the upper
part of Member 5 (sample A2B3), source of the present example of Multiplicisphaeridium sp. 1,
sensu Duffield & Legault 1981 (Fig. 16). The latter recalls the “M. forquiferum—M. forquillum’
group found by Cramer & Diez (1972) in the late Llandovery of Kentucky. Eupoikilofusa aff. E.
ampulliformis (Figs 14a, b), which appears at the base of the Becscie Formation (sample A2B7),
earliest Llandovery, is close to a Llandovery species known from the early Rhuddanian in
Belgium (Martin 1974). The entry of Domasia Downie, 1960, emend. Hill 1974 (Fig. 6) and
T ylotopalla Loeblich, 1970 (Fig. 10) on the one hand, and of Dilatisphaera williereae (Martin)
Lister 1970 (Fig. 5) on the other, occurs in the Jupiter Formation at levels that are correlated
(Barnes & McCracken 1981) respectively with the late Aeronian (C,—C,; sample A6A, about
3m above base of Member 3) and with the Telychian (C;; sample A7A1, 4 m below top of the
Jupiter Formation). As yet no diacrodian has been identified from the upper part of the
Gamachian, and no form suspected of being reworked from the Ordovician has been found in
the Llandovery of Anticosti.
The richness and variety of the microfloras in the Gamachian and Llandovery at Anticosti
will lead inevitably to the introduction of new taxa, some of which will be index fossils. As an
example, two forms from the Ellis Bay Formation (sample A2B3) are illustrated for the first
time here and left in open nomenclature: Pheoclosterium sp. nov. (Figs 7a, b) and Gen. et sp.
nov. cf. Rhopaliophora (Fig. 8). The only species formally assigned to the former genus, Pheo-
closterium fuscinulaegerum Tappan & Loeblich, 1971, is characteristic of the late Ordovician. Its
range (see Jacobson & Achab 1985 for all references) is from the Edenian of Indiana (Kope
Formation; Tappan & Loeblich 1971; Colbath 1979) and from the Onnian, highest Caradoc, in
Shropshire, England (upper part of Onny Shales; Turner 1984) to the Hirnantian in Czechoslo-
vakia (Kosov Formation, Vavrdova 1982). The second acritarch, cf. Rhopaliophora, differs from
that exclusively Ordovician genus in its opening and resembles ‘Hystrichosphaeridium’ wimani
Figs 3-16 Acritarchs from Anticosti. All figured specimens are in the type fossil collection of the
Geological Survey of Canada, Ottawa, and have numbers with the prefix GSC.
Figs 3, 4, 7-9, 12, 15, 16: sample A2B3, Ellis Bay; Ellis Bay Formation, upper part of Member 5,
Gamachian. Figs 11, 13, 14: sample A2B7, Ellis Bay; lowermost Becscie Formation, Llandovery,
correlated with Rhuddanian, A, ,. Figs 5, 6, 10: sample A7A1, 4km southeast of Pointe Sud-
Ouest; upper part of Jupiter Formation, Llandovery, correlated with Telychian, C;. Age assign-
ments according to Lespérance (1981).
Fig. 3 Orthosphaeridium insculptum Loeblich 1970. GSC 82877. Fig. 3a, x 400; Fig. 3b, enlargement,
x 3000, of base of left process. Fig. 4 Orthosphaeridium rectangulare (Eisenack) Eisenack 1968.
GSC 82878. Fig. 4a, enlargement, x 2000, of base of left lower process. Fig. 4b, x 200. Fig. 5
Dilatisphaera williereae (Martin) Lister 1970. GSC 82879, x 1000. Fig. 6 Domasia limaciformis
(Stockmans & Williere) Cramer 1970. GSC 82880, x 500. Fig. 7 Pheoclosterium sp. nov. GSC
82881. Fig. 7a, enlargement, x 3000, of upper median processes. Fig. 7b, x 750. Fig. 8 Gen. et sp.
nov. cf. Rhopaliophora sp. GSC 82882, x 300. Fig. 9 Baltisphaeridium plicatispinae Gorka 1969.
GSC 82883, x 300. Fig. 10 Tylotopalla sp. GSC 82884, x 750. Figs 11, 12 ‘Hogklintia digitata—H.
visbyensis.. Fig. 11, GSC 82885, x 250. Fig. 12, GSC 82886, x 100. Fig. 13 Goniosphaeridium
oligospinosum (Eisenack) Eisenack 1969. GSC 82887, x 250. Fig. 14 Eupoikilofusa aff. E. ampulli-
formis, sensu Duffield & Legault, 1981. GSC 82888. Fig. 14a, x 1000; Fig. 14b, enlargement,
x 5000, of lower right part of vesicle. Fig. 15 Diexallophasis remota (Deunff) Playford 1977. GSC
82889, x 500. Fig. 16 Multiplicisphaeridium sp. I, sensu Duffield & Legault 1981. GSC 82890,
x 500.
LATE ORDOVICIAN AND EARLY SILURIAN ACRITARCHS 305
306 F. MARTIN
Eisenack, 1968, determined by its author from the latest Ashgill of Gotland (Bornholmer Stufe
F2 from an erratic boulder at Oil Myr).
On Anticosti, in both the Ashgill and the Llandovery, there are geographically widespread
forms with long stratigraphical ranges that are difficult to define because of their wide, contin-
uous morphological variability within a single sample; examples are Diexallophasis remota
(Deunff) Playford 1977 (Fig. 15) and the “Hogklintia digitata—H. visbyensis’ complex (Figs 11,
12). The recurrent abundance in certain Ordovician and Silurian strata, notably on Anticosti
and in the Baltic region, of the latter complex and of, for instance, Goniosphaeridium oligospino-
sum (Eisenack) Eisenack 1969 (Fig. 13) probably results from particular palaeoenvironmental
conditions; the latter led Cramer & Diez (see 1974 for earlier references) to postulate a certain
degree of provincialism linked to palaeolatitudes for Silurian acritarchs.
Acritarch data for the latest Ordovician and earliest Silurian are as yet too disparate to
permit reliable palaeogeographic reconstructions. Data from Anticosti indicate affinities and
possibilities for correlation as follows. The Gamachian microfloras contain taxa known from
the late Ordovician of central U.S.A. and/or the pre-Hirnantian Ashgill of Gaspé, and from the
Ordovician of Europe (Baltic region and Portugal) and North Africa (Libya). In particular, the
evolutionary scheme proposed by Loeblich & Tappan (1971) for the genus Orthosphaeridium
Eisenack 1968, notably in part of the Cincinnatian of central U.S.A. and in the late Ashgill of
the Baltic region, Gotland and Estonia, may be applied to the Gamachian of Anticosti and the
late Ordovician of Portugal. The possibilities for correlation offered by the Llandovery acri-
tarchs of Anticosti concern affinities with, principally and in decreasing order, the Gaspé area
of Canada, England and Wales, Belgium and the U.S.A. In particular, the first occurrences of
Domasia and of Dilatisphaera williereae, the levels of which are still inadequately known on
Anticosti, should permit correlation with at least the Aeronian and the Telychian of the
Anglo-Welsh area. Palynological data for the Rhuddanian of the latter area allow only a local
zonation at present.
Conclusions
Owing to the dearth of published data, acritarchs have not been used directly as one of the
criteria for the choice of an Ordovician-Silurian boundary during the activities of the I.U.GS.
working group from 1974 to 1985. The Anticosti deposits are those likely to provide the most
reliable palynological correlations, not only in the immediate vicinity of the systemic boundary
but also at least from the early to middle Ashgill to the late Llandovery (Telychian, C;). This
view is supported by the indication both of relatively continuous data and of direct correlations
with the Gaspé area from the base of the Rhuddanian upwards, and the Anglo-Welsh area from
the Aeronian upwards.
Acknowledgements
I am indebted to M. G. Bassett (National Museum of Wales, Cardiff), G. K. Colbath (Smithsonian
Institution, Washington, D.C.) and J. A. Legault (University of Waterloo, Ontario, Canada) for critically
reviewing the manuscript.
Appendix
Locality data for Anticosti Island, Province of Québec, Canada. All locality numbers in
Lespeérance (1981: 1).
Loc. A-2A: Pointe Laframboise area. Sample A-2A1: Ellis Bay Formation, Member 7, 0-40 m above
oncolithic bed. Sample A-2A2: Becscie Formation, 0:60 m above base.
Loc. A-2B: west side of Ellis Bay, section proposed as Ordovician—Silurian boundary stratotype by
Barnes & McCracken (1981). Samples A-2B2 to A-2B6: Ellis Bay Formation; A-2B2: member 4, 3m
below top of member; A-2B3: member 5, 5 m below top of member; A-2B4: member 6, 0:30 m below
top of member; A-2B5 and A-2B6: member 7, respectively just above and 0:75 m above oncolithic bed.
Samples A-2B7 to A-2B9: Becscie Formation. A-2B7: immediately above the biohermal level of
LATE ORDOVICIAN AND EARLY SILURIAN ACRITARCHS 307
member 7 of the Ellis Bay Formation. A-2B8 and A-2B9: respectively 1-30 m and 25 m (approximately)
above A-2B7.
Loc. A-6A and sample A-6A: Cap Jupiter, north of mouth of Riviere Jupiter, Jupiter Formation, about
3m above base of member 3.
Loc. A-7A: 4km southeast of Pointe du Sud-Ouest. Sample A-7A1: Jupiter Formation, 4m below its top.
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Brachiopods across the Ordovician—Silurian boundary
L. R. M. Cocks
Department of Palaeontology, British Museum (Natural History), Cromwell Road,
London SW7 5BD
Synopsis
Most of the late Ordovician brachiopod superfamilies also extend into the early Silurian, although the
Gonambonitacea become extinct at or near the Ordovician—Silurian boundary and the earliest Cyrtiacea
are found very close above it. Faunas close to the boundary are reviewed and listed, and the Hirnantian
faunas of the latest Ordovician are found to be richer than the earliest Silurian Rhuddanian faunas in
both abundance and diversity.
Introduction
At the time the Treatise on Invertebrate Paleontology brachiopod volume (Williams et al. 1965)
was written, 44 brachiopod genera were recorded with ranges spanning the Ordovician—
Silurian boundary, and in addition there were various families and subfamilies whose ranges
spanned the boundary even if the recorded ranges of individual genera within them did not.
The superfamilies involved are the Lingulacea, Trimerellacea, Discinacea, Craniacea, Orthacea,
Enteletacea, Tripleciacea, Eichwaldiacea, Plectambonitacea, Strophomenacea, Davidsoniacea,
Chonetacea, Porambonitacea, Pentameracea, Rhynchonellacea, Atrypacea and Athyridacea—a
list which in itself demonstrates the morphological variability and diversity of the phylum in
Ordovician—Silurian boundary times.
However, rather than review each family, genus or species in turn here, it is more relevant to
consider the brachiopod faunas actually recovered from strata near the boundary. In general
the middle Ashgill was a period of great diversity among the brachiopods, but this diversity
was reduced when the Rawtheyan endemic faunas, for example of North America (the late
Richmondian) and Europe (e.g. the Boda Limestone of Sweden) gave way to the more cosmo-
politan, and hence in total less diverse, faunas of Hirnantian times. Similarly, the profound
effect of the Ordovician—Silurian boundary glacial episode made the subsequent recovery and
build-up of the brachiopod faunas rather slow, and thus, even where the earliest Llandovery
time is represented by rock (and not by the usual unconformity), the numbers and more
particularly the diversity of the brachiopod faunas were rather poor.
Latest Ordovician and earliest Silurian brachiopods
In the following lists the records are reproduced of reliable determinations from relatively
recent papers on brachiopods of Hirnantian and early Rhuddanian ages respectively. In most
cases they are as the original authors determined them, but with ‘aff. or ‘cf’ omitted, and
sometimes with genera or species updated by subsequent works. They are from the following
authors and localities: A, uppermost Ellis Bay and lowermost Becscie Formations, Anticosti
Island, Canada (Cocks & Copper 1981); B, Kosov Formation, Bohemia, Czechoslovakia
(Marek & Havliéek 1967; Havliéek 1977); D, Durben Horizon, Kazakhstan, USSR (Nikitin et
al. 1980); E, Lower Edgewood Group, Oklahoma, USA (Amsden 1974); G, High Mains Sand-
stone and Lady Burn Formation, Girvan, Scotland (Cocks & Toghill 1973; Harper, this
volume); H, St Martin’s Cemetery Horizon, Haverfordwest, Wales (Cocks & Price 1975); I, Hol
Beck, England (Temple 1965); K, Kildare, Ireland (Wright 1968); L, Bronydd Formation,
Llandovery, Wales (Cocks et al. 1984); M, persculptus and acuminatus Zones, Mirny Creek,
north-east USSR (Koren et al. 1983); O, Langgyene and Langara Formations, Oslo—Asker
district, Norway (Brenchley & Cocks 1982; Cocks 1982) and Myren Member (Baarli & Harper
1986); P, Unit 5, White Head Formation, Percé, Québec, Canada (Lespérance & Sheehan 1976,
Bull. Br. Mus. nat. Hist. (Geol) 43: 311-315 Issued 28 April 1988
312 L. R. M. COCKS
1981); R, Varbola Formation, Estonia, USSR (Rubel 1970); S, Stawy, Poland (Temple 1965); V,
Dalmanitina Beds, Vastergotland, Sweden (Bergstrom 1968); W, Hirnant Beds, Wales (Temple
1965); X, Hirnantian Beds, Keisley, England (Temple 1968); Y, Kuanyinchiao Beds, Yichang,
China (Rong 1984a); Z, Artchalyk and Minkutchar Beds, Zeravshano-Gissar section, Altai
Mountains, USSR (Nikiforova 1978).
The latest Ordovician (Hirnantian) records from these localities are as follows:
Lingulacea: Lingula sp. H, O; Lingulella sp. I, S; Palaeoglossa sp. V; Craniops/Paracraniops sp. H,
O, V, X.
Discinacea: Trematis norvegica Cocks O; Orbiculoidea concentrica (Wahlenberg) H, V, S; Orbiculoidea
sp. O. |
Craniacea: Acanthocrania sp. O, X; Philhedra grayii (Davidson) X; Philhedra sp. H, V; Philhedra?
stawyensis Temple I, S; Philhedrella cribrum Temple X; Philhedrella sp. A, O.
Orthacea: Comatopoma sororia Marek & Havli¢ek B; Comatopoma sp. O; Dolerorthis intermedius
Nikiforova M; Dolerorthis praeclara Temple X; Dolerorthis savagei Amsden E; Dolerorthis sp. O;
Geraldibella bella (Bergstrom) M, V; Geraldibella giraldi (Bancroft) H; Giraldibella subsilurica (Marek &
Havli¢ek) B; Glyptorthis sp. G, O; Hesperorthis sp. M, O; Nicolella sp. O; Orthostrophella sp. E;
Plaesiomys sp. G; Platystrophia sp. E, G, O; Skenidioides scoliodus Temple X; Skenidioides sp. H, O;
Toxorthis mirabilis Rong Y; Toxorthis proteus Temple X.
Enteletacea: Dalmanella biconvexa Williams H; Dalmanella cicatrica Nikitin D; Dalmanella edgewoodensis
Savage E; Dalmanella pectinoides Bergstrom B, V; Dalmanella testudinaria (Dalman) A, B, H, I, K, M,
O, P, S, V, W, Y; Dicoelosia sp. E, X; Diceromyonia? sera Amsden E; Draborthis caelebs Marek &
Havli¢ek B, V, X, Y; Drabovia agnata Marek & Havli¢ek B; Drabovia westrogothica Bergstrom V;
Drabovia sp. O, X; Dysprosorthis sinensis Rong Y; Epitomyonia sp. O; Hirnantia noixella Amsden E;
Hirnantia sagittifera (M‘Coy) B, D, G, H, I, K, M, O, P, S, V, W, X, Y; Hirnantia sp. A; Horderleyella
bouceki (Havliéek) S, W; Horderleyella fragilis Bergstrom V; Isorthis sp. M; Kinnella kielanae (Temple)
B, P, S, V, W, X, Y; Leptoskelidion loci Cocks O; Leptoskelidion septulosum Amsden E; Mendacella? sp.
E; Mirorthis mira Zeng Y; Onniella kalvoya Cocks O; Onniella? yichangensis Zeng Y ; Paucicrura sp. O;
‘Pionodema’ retusa Temple X; Ravozetina rava Marek & Havli¢ek B; Reuschella inexpectata Temple X;
Trucizetina subrotundata Havli¢ek B; Trucizetina yichangensis Zeng Y; Visbyella? sp. [= Kayserella sp.
nov. of Temple] X.
Gonambonitacea: Kullervo? sp. O.
Tripleciacea: Cliftonia psittacina (Dalman) B, H, K, O, V; Cliftonia obovata Chang Y; Cliftonia tubuli-
striata (Savage) E; Cliftonia sp. D, M; Onychoplecia sp. X, Y; Oxoplecia sp. O; Triplesia protea
Oradovskaya M;; Triplesia sanxiaensis Zeng Y; Triplesia sp. O.
Plectambonitacea: Aegiromena convexa Chang Y; Aegiromena durbenensis Nikitin D; Aegiromena ultima
Marek & Havli¢éek B, Y; Aegiromena sp. X; Anisopleurella novemcostata Nikitin D; Chonetoidea papil-
losa (Reed) H; Eochonetes sp. G; Eoplectodonta nesnakomkaensis Oradovskaya M; Eoplectodonta rhom-
bica (M‘Coy) O; Eoplectodonta oscitanda Cocks O; Eoplectodonta sp. D; Leangella cylindrica (Reed) O,
V; Rugosowerbyella ambigua (Reed) D; Sampo sp. O; Sericoidea? O.
Strophomenacea: Aphanomena parvicostellata Rong Y; Aphanomena schmalenseei Bergstrom V; Biparetis
paucirugosus Amsden M; Eopholidostrophia sp. G; Eostropheodonta bublitschenki Nikitin D; Eostro-
pheodonta hirnantensis (M‘Coy) including E. lucavica and E. siluriana A, B, G, I, K, M, O, P, S, V, W;
Eostropheodonta whittingtoni Bancroft H; Katastrophomena sp. O; Kjaerina? sp. O; Kjerulfina? sp. V;
Leptaena aequalis Amsden M; Leptaena martinensis Cocks H; Leptaena rugosa Dalman B, D, V;
Leptaena sp. E, O; Leptaenopoma trifidum Marek & Havlitéek B, D, K, V, Y; Paromalomena polonica
(Temple) B, D, I, S, X, Y; Rafinesquina? latisculptilis (Savage) E, M; Rafinesquina stropheodontoides
(Savage) E; Rafinesquina ultrix Marek & Havliéek B, D; Rafinesquina urbicola Marek & Havliéek B, D;
Titanomena grandis Bergstrom V.
Davidsoniacea: Coolinia convexa (Savage) E; Coolinia dalmani Bergstrom A, O, V; Coolinia propinqua
(Meek & Worthen) E; Coolinia sp. M, Y; Fardenia comes Marek & Havlitek B; Fardenia sp. G, X.
Porambonitacea: Parastrophinella gracilis Oradovskaya M; Parastrophina sp. O.
Pentameracea: Brevilamnulella kjerulfi (Kjaer) O; Brevilamnulella thebesensis (Savage) E, M; Brevilamnu-
lella undatiformis Rozman M; Holorhynchus giganteus Kjaer O; Tcherskidium unicum (Nikolaev) M.
Rhynchonellacea: Dorytreta sp. Y; Hypsiptycha sp. G; Rostricellula sp. G, O; Rhynchotrema? sp. M;
Stegerhynchus concinna (Savage) E, M; Stegerhynchus? sp. E, O; Thebesia admiranda Oradovskaya M;
Thebesia scopulosa Cocks O; Thebesia thebesensis (Foerste) E.
Atrypacea: Eospirigerina gaspeensis (Cooper) M; Eospirigerina prisca Oradovskaya M; Eospirigerina
putilla (Hall & Clarke) E; Eospirigerina sublevis Rozman M; Eospirigerina sp. G, O; ‘Homoeospira
BRACHIOPODS ACROSS THE BOUNDARY 313
fiscellostriata Savage E; Plectatrypa sp. M; Protatrypa sp. X; Protozyga gastrodes Temple X; Zygospira
fallax Marek & Havliéek B; Zygospira sp. O.
Athyracea: Cryptothyrella crassa (J. de C. Sowerby) incipiens Williams G, H, K, Y; Cryptothyrella ovoides
(Savage) E; Cryptothyrella terebratulina (Wahlenberg) M; Cryptothyrella sp. B, X; Hindella cassidea
(Dalman) O, ?P, ?A; Hyattidina sp. M; Plectothyrella crassicostis (Dalman) [ex platystrophoides
Temple] B, I, K, P, S, V, W, Y; Plectothyrella? mirnyensis Oradovskaya M.
Eichwaldiacea: Dictyonella sp. E.
The earliest Silurian (lower part of the Rhuddanian) records from these localities are:
Lingulacea: Lingula sp. G.
Discinacea: Orbiculoidea sp. H.
Orthacea: Dolerothis plicata (J. de C. Sowerby) O; Dolerorthis sowerbyiana (Davidson) L; Dolerorthis sp.
O, R; Giraldiella sp. L, Z; Hesperorthis imbecilla Rubel R; Platystrophia sp. R; Schizonema sp. L, O;
Ptychopleurella sp. R; Skenidioides scoliodus Temple M; Skenidioides woodlandensis Reed O; Skeni-
dioides sp. H, L, O.
Enteletacea: Dalejina sp. R; Dicoelosia osloensis Wright O; Dicoelosia sp. L; Draborthis? sp. M;
Epitomyonia sp. O; Fascifera sp. O; Howellites sp. O; Isorthis neocrassa Nikiforova Z; Isorthis prima
Walmsley O; Isorthis sp. A; Kinnella sp. O; Onniella mediocra Rubel R; Ravozetina sp. L, O; Resserella
sp. H, L; Reuschella sp. O; Visbyella sp. L.
Tripleciacea: Triplesia sp. L, O.
Plectambonitacea: Aegiria norvegica Opik O; Anisopleurella sp. L; Anisopleurella gracilis (Jones) H;
Eoplectodonta duplicata (J. de C. Sowerby) L, O; Eoplectodonta sp. H; Leangella scissa (Davidson) L, O.
Strophomenacea: Eopholidostrophia sp. A, L; Eostropheodonta sp. H; Furcitella sp. L; Katastrophomena
sp. L; Leptaena aequalis Amsden M; Leptaena contermina Cocks A; Leptaena haverfordensis Bancroft
O; Leptaena reedi Cocks L, O; Leptaena valentia Cocks L; Leptaena sp. H, O, R; Leptostrophia reedi
(Bancroft) A; Leptostrophia sp. L.
Davidsoniacea: Fardenia sp. G, L, R.
Porambonitacea: Parastrophinella sp. Z.
Pentameracea: Clorinda malmoyensis St Joseph Z; Clorinda undata (J. de C. Sowerby) H, L, O; Clorinda
sp. R; Stricklandia lens (J. de C. Sowerby) H, L, O, R, Z; Virgiana sp. Z; Virgianella sogdianica
Nikiforova & Sapelnikov Z.
Rhynchonellacea: Rhynchotrema sp. L; Rhynchotreta? sp. G.
Atrypacea: Alispira gracilis Nikiforova R; Clintonella aprinis (Verneuil) R; Clintonella sp. R; Eospirigerina
porkuniana Rubel R; Idiospira sp. O; Eospirigerina sp. H, O, Z; Meifodia recta alia Nikiforova Z;
Meifodia sp. L, O; Plectatrypa imbricata (J. de C. Sowerby) Z; Plectatrypa sp. L; Protatrypa malmoey-
ensis Boucot, Johnson & Staton O, Z; Protatrypa sp. M; Protozyga sp. L; Zygospiraella sp. M, Z;
Zygospiraella duboisi (Verneuil) R.
Athyracea: ‘“Atrypina’ gamachiana Twenhofel A; Cryptothyrella angustifrons (Salter) L, G; Cryptothyrella
crassa (J. de C. Sowerby) H, L; Cryptothyrella sp. A, R; ‘Hindella’ extenuata Rubel R; Hyattidina sp. M.
From these lists it can be seen that the cited faunas carried 90 genera in the Hirnantian and
only 54 in the early Rhuddanian, with 32 genera in common between the two lists. Part of this
numerical discrepancy can be explained by the greater number of faunal lists available for beds
of Hirnantian age (18), compared with only 8 for the early Rhuddanian; nevertheless that
discrepancy can itself be explained by the fewer number of early Llandovery age faunas that
can actually be found. Moreover, whereas the Hirnantian faunas can often be found in abun-
dance (for example in China—Rong 1984a, b), the early Rhuddanian faunas are often very
sparse both in numbers and diversity, and also in the actual size of the specimens, all of which
explains why monographic treatment of them has been rather neglected, particularly by com-
parison with the much richer and more diverse later Rhuddanian faunas, which are relatively
well described (e.g. Temple 1970). In addition, presumably because of the glacially-caused
eustatic lowering of sea level which peaked during the Hirnantian, there are many sections in
which only the Hirnantian is represented by shelly deposits and with the beds above and below
in which the only macrofossils are graptolites.
Missing from both of the above lists are representatives of the Trimerellacea, Acrotretacea,
Siphonotretacea and Chonetacea, all of which have reliable records from both late Ordovician
and early Silurian rocks, but not from beds very close to the boundary; and from the early
314 L. R. M. COCKS
Rhuddanian list the Craniacea and the Eichwaldiacea, which also yield representatives from
later horizons in the Llandovery. The only brachiopod superfamily which appears to have
become extinct at the end of the Hirnantian is the Gonambonitacea (although a few lower taxa
such as the Trematidae also disappeared then); and the only new superfamily to appear
anywhere near the base of the Silurian is the Cyrtiacea, whose earliest records, although not
accurately dated in detail, come from beds in Tasmania extremely close to the boundary
(Sheehan & Baillie 1981). In general, however, the degree of extinction across the boundary
appears to have been far less than previously reported, largely because earlier studies have not
concentrated on latest Ashgill and earliest Llandovery rocks. The extinctions at the end of the
Hirnantian do not appear to have been greater than at the end Caradoc or end Rawtheyan.
This is exemplified by a recent review of the atrypoids by Copper (1986), who states that only
two genera, Idiospira and Cyclospira, may have become extinct near the boundary, and even
these two have been reported (e.g. Baarli & Harper 1986) from early Silurian rocks. The strong
‘Silurian’ elements in the spire-bearer fauna, for example Hindella and Eospirigerina, actually
appeared in late Rawtheyan times.
Unfortunately no evolutionary gradation within a single genus has been adequately studied
across the boundary, and thus no perfect recognition of the boundary by brachiopods is yet
possible. The most striking changes in closely related groups are seen in the Pentameracea,
which can be found in virtually rock-building abundance in some beds both above and below
the boundary, although only rarely in the earliest Rhuddanian. In the Hirnantian, Holo-
rhynchus, Brevilamnulella, and others dominate the fauna, whereas in the Rhuddanian their
place is taken by Stricklandia, Clorinda, and a wide diversity of genera in the then tropical areas
of the USSR (Nikolaev et al. 1977) and, rather later, Virgiana and Platymerella in the USA. In
the east Baltic, Borealis is known from as low as the vesiculosus Zone (Boucot et al. 1969).
The exact age, in terms of graptolite zones, of the various brachiopod faunas from near the
systemic boundary, in particular the Hirnantia fauna, is also of great relevance in international
correlation. In continuous sections, most Hirnantia faunas underlie beds bearing persculptus
Zone graptolites, for example in the vast outcrop area in China, and in general the fauna is
undoubtedly of extraordinarius Zone age or older; it spans four graptolite zones in China
(Rong 1984b). However, in at least two places it occurs in beds with and above persculptus
Zone graptolites. One is in Kazakhstan, USSR (Apollonov et al., this volume, p. 145), and the
other is in the Lake District, England, where Locality 74/1 of Hutt (1974: 15) in Yewdale Beck,
Cumbria (National Grid ref. SD 30739858) has yielded to J. E. Hutt (registered numbers
BC 7217-7236), in order of abundance, Kinnella kielanae (Temple), Mirorthis mira Zeng, Plec-
tothyrella crassicostis (Dalman), Cyclospira sp., Hirnantia sp. and other indeterminate orthids
and dalmanellids, identified by the author and Rong Jia-yu. In addition the same bed has
yielded many graptolites (J. E. Hutt, pers. comm. 1986), including Climacograptus medius Torn-
quist, C. normalis Lapworth, C. miserabilis Elles & Wood, Glyptograptus persculptus (Salter),
Diplograptus ex gr. modestus Lapworth and Monograptus ceryx Rickards & Hutt. These new
records endorse the most preferable systemic boundary at the base of the acuminatus Zone.
Acknowledgements
I am most grateful to Rong Jia-yu and A. J. Boucot, who helpfully commented on the first draft of this
paper.
References
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352-367.
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Chitinozoan stratigraphy in the Ashgill and
Llandovery
Y. Grahn
Geological Survey of Sweden, Box 670, S-75128 Uppsala, Sweden
Synopsis
There is little published information on chitinozoan faunas from sections with continuous sedimentation
across the Ordovician—Silurian boundary. Most boundary sections from which chitinozoans have been
described include a hiatus. To aid determination of the extent of any hiatus, the ranges of thirty-one
diagnostic chitinozoan species from Ashgill and Llandovery strata are documented, with reference to the
British standard graptolite zonation. The composition of the chitinozoan faunas at the Ordovician—
Silurian boundary is discussed, and the influence of ecological factors is assessed.
Introduction
Chitinozoans are organic-walled microfossils known from marine sedimentary rocks of Ordovi-
cian to Devonian age. Although pertinent information is missing from, for instance, Australia
and the East Indies, it is no exaggeration to say that chitinozoans have great stratigraphical
potential on a world-wide basis. However, our knowledge of the chitinozoan faunas at the
Ordovician-Silurian boundary is still scanty. Chitinozoans from sections with continuous sedi-
mentation across the boundary are known only from Anticosti Island, Québec (Achab 1981),
Skane, Sweden (Grahn 1978) and probably Estonia (Nestor 1976, 1980a, 1980b, and personal
communication 1985; Nolvak 1980, and personal communication 1985). Faunas from sections
with a small hiatus have been described from Libya (Molyneux & Paris 1985; Hill et al. 1985),
the Cincinnati Region, midcontinent U.S.A. (Grahn 1985; Grahn & Bergstrom 1985; M. A.
Miller, personal communication 1985) and the Brabant Massif, Belgium (Martin 1973). Refer-
ences to other papers with relevant data on Ashgill and/or Llandovery chitinozoan faunas will
be made in context. To help in determining the extent of any hiatus, the ranges of selected
chitinozoan species from the early Ashgill to the late Llandovery are documented here. The
total range of each species (Fig. 1) is defined according to the British standards for the Ashgill
(sensu Williams 1983) and Llandovery (sensu Cocks et al. 1984) Series.
Diagnostic Ashgill chitinozoans
Many Ashgill chitinozoan species are long-ranging and persist from the middle or lower
Ordovician. Only a few species are restricted to the Ashgill (Fig. 1). The chitinozoans can be
divided into a pre-Hirnantian fauna, and a fauna that ranges into the Hirnantian (Figs 2-12).
Ashgill chitinozoans from Great Britain are virtually unknown. The type Hirnantian is barren
of chitinozoans. In older strata, Tanuchitina bergstroemi occurs in the Rawtheyan (F. Paris,
personal communication 1985).
In North Africa Armoricochitina nigerica and Calpichitina? lenticularis are very characteristic
for the late Ashgill (Elaouad-Debbajy 1984; Molyneux & Paris 1985; J. C. Jaglin, personal
communication 1985). These species are also known from SW Europe (Paris 1981). Acanthochi-
tina? rashidi, Ancyrochitina merga, Plectochitina sylvanica, and Sphaerochitina lepta characterize
the Ashgill in midcontinent U.S.A. (Jenkins 1970; M. A. Miller, personal communication 1986,
own observations). Ancyrochitina merga has a more spinose ornament in the lower Ashgill than
higher, and all specimens of Sphaerochitina lepta are smooth in the mid-Ashgill but are joined
by spinose forms in the upper Ashgill. No Hirnantian chitinozoans are known from the mid-
continent U.S.A.
So far, Conochitina gamachiana has only been reported from the upper Ashgill strata of
Anticosti Island (Achab 1978). Other typical associated species are Ancyrochitina longispina,
Bull. Br. Mus. nat. Hist. (Geol) 43: 317-323 Issued 28 April 1988
318
Y. GRAHN
yanica
gispina
ree sg
oa &
crenulata
griestoniensis
Telychian
turriculatus
Aeronian
Rhuddanian
pe rsc ulptus
D » extra- a
Rawtheyan
Llandovery
argente
magnus
triangulatus
acinaces
|
———
——
anceps
es s +4-+-—__-
Pusgillian
Fig. 1 Buiostratigraphical ranges of selected chitinozoans from the Ashgill and Llandovery.
Figs 2-12 Selected Ashgill Chitinozoa. 2, Tanuchitina anticostiensis, Vauréal Formation (early
Ashgill), boring RHS, Anticosti Island, Canada; SEM x 150. 3, Calpichitina? lenticularis, middle
Caradoc-—early Ashgill, boring E1-81 (791m), Libya; SEM x 230. 4, Armoricochitina nigerica, late
Caradoc—Ashgill, boring E1-81 (785-792m), Libya; SEM x 190. 5, Plectochitina sylvanica, early
Ashgill, boring J1-81A (3985-4000 m), Libya; SEM x 310. 6, Sphaerochitina lepta, Sylvan Shale
(early Ashgill), Arbuckle Mountains, Oklahoma, U.S.A.; SEM x 345. 7, Conochitina gamachiana,
Ellis Bay Formation (late Ashgill), boring A425, Anticosti Island, Canada; SEM x 270. 8, Lageno-
chitina prussica, Vormsi Stage (early Ashgill), Gotska Sand6n boring (93:30—-93-35 m), Sweden;
SEM x 360. 9, Tanuchitina bergstroemi, erratic of late Caradoc age, Oland, Sweden; SEM x 130.
10, Coronochitina taugourdeaui, Porkuni Stage (late Ashgill), Taagepera boring, Estonia; SEM
x 225. 11, Acanthochitina barbata, Vormsi Stage (early Ashgill), Gotska Sandon boring (96-34-
96:40m), Sweden; SEM x 140. 12, Plectochitina concinna, Vauréal Formation (early Ashgill),
boring AF6, Anticosti Island, Canada; SEM x 310.
Figs 2, 12 with permission of Aicha Achab (Ste-Foy), Figs 3-5 with permission of Florentin Paris
(Rennes), Fig. 7 with permission of Alain Le Herisse’ (Brest) and Fig. 10 with permission of Jaak
Nolvak (Tallinn).
CHITINOZOAN IN ASHGILL AND LLANDOVERY
320 Y. GRAHN
Coronochitina bulmani, Plectochitina concinna, and Tanuchitina anticostiensis (Achab 1978). In
contrast to other areas, Hercochitina species are very common in the Ashgill of Anticosti Island
and the midcontinent U.S.A.
Acanthochitina barbata is restricted to the upper Pleurograptus linearis Zone in Baltoscandia
(Nélvak 1980), but has a slightly longer range in north Africa and North America. Coronchitina
taugourdeaui is another excellent index fossil. It is one of the few chitinozoan species indicative
of the Hirnantian and is known from Baltoscandia and Anticosti Island (Eisenack 1968;
Noélvak 1980; Achab 1981). Lagenochitina prussica and Tanuchitina bergstroemi are Baltoscan-
dian species (Grahn 1982); the former is also known from the Ashgill in north Africa (Elaouad-
Debbaj 1984; Molyneux & Paris 1985),! the Brabant Massif, Belgium (Martin 1973; own
observations) and Podolia, U.S.S.R. (Laufeld 1971).
Chitinozoan faunas at the Ordovician-Silurian boundary
Chitinozoan faunas at the Ordovician-Silurian boundary are characterized by a complex of
Ancyrochitina (e.g. A. ancyrea, A. spongiosa) and Cyathochitina species (e.g. C. campanulaeformis,
C. kuckersiana). Nestor (1980a) described Conochitina postrobusta from the Juuru Stage in
Estonia. However, in Skane, Sweden (Grahn 1978) and the Brabant Massif, Belgium (Martin
1973: own observations) there is no difference between late Ashgill and Llandovery specimens
of Conochitina robusta. It is therefore uncertain whether C. postrobusta can be separated from
Ashgill specimens of C. robusta.
Some Ordovician genera (e.g. Acanthochitina, Hercochitina) and typical Ordovician species
(e.g. Desmochitina gr. minor, Conochitina gr. micracantha) disappear in the top Ashgill, but most
Ordovician genera persist into the Silurian. However, very few Ordovician species range into
the Llandovery.
Diagnostic Llandovery chitinozoans
Silurian chitinozoans (Figs 13-27) are more widely distributed than Ordovician ones (Laufeld
1979). Endemic chitinozoan faunas do occur, but not to the same extent as during the Ordovi-
cian. However, there is a difference in chitinozoan assemblages between north Africa, Anticosti
Island and Baltoscandia. Chitinozoans from the type Llandovery are poorly preserved and the
diversity seems to be low (K. Dorning, personal communication 1985). On the other hand,
Telychian faunas from Great Britain show a striking similarity to contemporaneous faunas in
Baltoscandia (Aldridge et al. 1979; Dorning 1981; Mabillard & Aldridge 1985).
Figs 13-27 Selected Llandovery Chitinozoa. 13, Coronochitina fragilis, Juuru Stage (early
Llandovery), Ohesaare boring (466:5m), Estonia; SEM x 300. 14, Conochitina armillata, middle-
late Llandovery, boring D1-31 (1895-1896 m), Libya; SEM x 160. 15, Conochitina edjelensis elon-
gata, middle-late Llandovery, boring E1-81 (606-612m), Libya; SEM x 160. 16, Eisenackitina
dolioliformis, Restevo Beds (late Llandovery), Podolia, U.S.S.R.; SEM x 420. 17, Conochitina
aspera, Juuru Stage (early Llandovery), Ikla boring (514-6m), Estonia; SEM x 430. 18, Conochi-
tina proboscifera, Upper Visby Beds (early Wenlock), Gotland, Sweden; SEM x 70. 19,
Pterochitina dechaii, middle-late Llandovery, boring D1-31 (1895-1896 m), Libya; SEM x 325. 20,
Plectochitina pseudoagglutinans, middle-late Llandovery, boring A1-81 (1154-1161 m), Libya; SEM
x 195. 21, Conochitina electa, Raikkiila Stage (middle Llandovery), Emmaste boring (41-2 m),
Estonia; SEM x 160. 22, Angochitina longicollis, Lower Visby Beds (late Llandovery), Gotland,
Sweden; SEM x 175. 23, Conochitina iklaensis, Raikkiila Stage (middle Llandovery), Ikla boring
(492:0m), Estonia; SEM x 160. 24, Coronochitina maennili, Raikkila Stage (middle Llandovery),
Ikla boring (462-9m), Estonia; SEM x 160. 25, Ancyrochitina convexa, Raikkiila Stage (middle
Llandovery), Ruhnu boring (536-0m), Estonia; SEM x 300. 26, Desmochitina densa, Upper Visby
Beds (early Wenlock), Gotland, Sweden; SEM x 345. 27, Ancyrochitina laevensis, Juuru Stage
(early Llandovery), Laeva boring (122-5 m), Estonia; SEM x 300.
Figs 13, 17, 21, 23-25, 27 with permission of Viiu Nestor (Tallinn), Figs 14-15, 19-20 with
permission of Florentin Paris (Rennes) and Figs 16, 18, 22, 26 with permission of Sven Laufeld
(Uppsala).
CHITINOZOAN STRATIGRAPHY IN ASHGILL AND LLANDOVERY
B22 Y. GRAHN
The appearance of Ancyrochitina laevensis and Coronochitina fragilis indicates lowermost
Rhuddanian strata (Nestor 1980a). Otherwise pre-cyphus beds have a low chitinozoan diversity,
and, apart from Conochitina aspera, there are very few diagnostic species above the acuminatus
Zone (Fig. 1). In the cyphus Zone Conochitina iklaensis occurs, and is joined in the topmost part
by Coronochitina maennili (Nestor 1980a). These two species disappear in the sedgwickii Zone
(Nestor 1980a; own observations) together with Conochitina edjelensis, a useful representative
of the Aeronian. The lowermost Aeronian is characterized by the presence of Ancyrochitina
convexa (Nestor 1980b). Eisenackitina dolioliformis and Conochitina emmastiensis (Nestor 1982a)
have their first appearance in the sedgwickii Zone and range into the Wenlock. A very charac-
teristic chitinozoan assemblage occurs in, the griestoniensis Zone, consisting of Angochitina
longicollis, Conochitina proboscifera and Desmochitina densa and is widely distributed (Dorning
1981; Nestor 1982b; Verniers 1982; Mabillard & Aldridge 1985; etc.).
The presence of Baltoscandian species among north African chitinozoan assemblages makes
it possible to determine the stratigraphical ranges of some north African taxa (Paris, in press),
such as Plectochitina pseudoagglutinans and Conochitina vitrea (Hill et al. 1985). These species
range from the lower Rhuddanian to the upper Aeronian (Fig. 1). Two other species, Conochi-
tina armillata and Pterochitina deichaii, range from the mid-Aeronian to the mid-Telychian
(Paris, in press).
Remarks on the boundary chitinozoans
In general the abundance and diversity of chitinozoans are comparatively low at the
Ordovician-Silurian boundary, irrespective of geographic area. This is probably due to the
Gondwana glaciation, which led to a eustatic sea-level drop, and the subsequent deposition of
shallow-water sediments in many cratonic successions. Chitinozoans are usually rare or absent
in rocks deposited in very shallow water (Laufeld 1974; Grahn & Bergstrom 1984, 1985). If
chitinozoans are present in these rocks, planktic forms often dominate and these were probably
transported inshore by currents and waves. This is demonstrated in the Belfast Beds of early
Llandovery age in the Cincinnati Region, where the planktic genus Ancyrochitina constitutes
about 99% of the chitinozoan fauna (Grahn & Bergstrom 1985).
Acknowledgements
I am indebted to Sven Laufeld (Uppsala) and Florentin Paris (Rennes) for critical reading and improve-
ments of the manuscript. Aicha Achab (Ste-Foy), Viiu Nestor (Tallinn), Florentin Paris (Rennes), Sven
Laufeld (Uppsala), Jaak Nolvak (Tallinn), and Alain Le Herissé (Brest) provided me with SEM-pictures,
and Francine Martin (Bruxelles) and Merrell A. Miller (Tulsa) with samples. Karin Feltzin (Stockholm)
finished my line drawing and Richard J. Aldridge (Nottingham) checked the English. My sincere thanks to
all these friends.
References
Achab, A. 1978. Les Chitinozoaires de ’Ordovicien Supérieur—Formations de Vauréal et d’Ellis Bay—de
Vile d’Anticosti, Québec. Palinologia, Léon, (num. ext.) 1: 1-19. £
— 1981. Biostratigraphie par les Chitinozaires de lOrdovicien Supérieur—Silurien Inférieur de I’Ile
d’Anticosti. Résultats préliminaires. In P. J. Lespérance (ed.), Field Meeting, Anticosti—Gaspe, Quebec,
1981 2 (Stratigraphy and paleontology): 143-157. Montréal (I.U.G.S. Subcommission on Silurian Strati-
graphy Ordovician-Silurian Boundary Working Group).
Aldridge, R. J., Dorning, K. T., Hill, P. J. Richardson, J. B. & Siveter, D. J. 1979. Microfossil distribution
in the Silurian of Britain and Ireland. Spec. Publs geol. Soc. Lond. 8: 433-438.
Cocks, L. R. M., Woodcock, N. H., Rickards, R. B., Temple, J. T. & Lane, P. D. 1984. The Llandovery
Series of the type area. Bull. Br. Mus. nat. Hist., London, (Geol.), 38 (3): 131-182.
Dorning, K. J. 1981. Silurian Chitinozoa from the type Wenlock and Ludlow of Shropshire, England. Rev.
Palaeobot. Palynol., Amsterdam, 34: 205—208.
Eisenack, A. 1968. Mikrofossilien eines Geschiebes der Borkholmer Stufe, baltisches Ordovizium, F2.
Mitt. geol. StInst. Hamb. 37: 81—94.
CHITINOZOAN STRATIGRAPHY IN ASHGILL AND LLANDOVERY 323
Elaouad-Debbaj, Z. 1984. Chitinozoaires Ashgilliens de l’Anti-Atlas (Maroc). Geobios, Lyon, 17: 45-68.
Grahn, Y. 1978. Chitinozoan stratigraphy and paleoecology at the Ordovician—Silurian boundary in
Skane, southernmost Sweden. Sver. geol. Unders., Stockholm, (C) 744: 1-16.
—— 1982. Caradocian and Ashgillian Chitinozoa from the subsurface of Gotland. Sver. geol. Unders.,
Uppsala, (C) 788: 1-66.
— 1985. Llandoverian and early Wenlockian Chitinozoa from southern Ohio and northern Kentucky,
U.S.A. Palynology, Dallas, 9: 147-164, 2 pls.
—— & Bergstrom, S. M. 1984. Lower Middle Ordovician Chitinozoa from the Southern Appalachians,
United States. Rev. Palaeobot. Palynol., Amsterdam, 43: 89-122.
1985. Chitinozoans from the Ordovician-Silurian boundary beds in the eastern Cincinnati
region in Ohio and Kentucky. Ohio J. Sci., Columbus, 85 (4): 175-183, 1 pl.
Hill, P. J., Paris, F. & Richardson, J. B. 1985. Silurian palynomorphs. In B. G. Thusu & B. Owens (eds),
Palynostratigraphy of North-East Libya. J. Micropalaeont., London, 4: 27-48.
Jenkins, W. A. M. 1970. Chitinozoa from the Ordovician Sylvan Shale of the Arbuckle Mountains,
Oklahoma. Palaeontology, London, 13: 261-288.
Laufeld, S. 1971. Chitinozoa and correlation of the Molodova and Restevo Beds of Podolia, USSR. Mem.
Bur. Rech. géol. minier., Brest, 73: 291-300, 2 pls.
1974. Silurian Chitinozoa from Gotland. Fossils Strata, Oslo, 5: 1-130.
—— 1979. Biogeography of Ordovician, Silurian and Devonian Chitinozoans. In J. Gray & A. J. Boucot
(eds), Historical Biogeography, Plate Tectonics, and the Changing Environment: 75-90. Oregon State
Univ. Press.
Mabillard, J. E. & Aldridge, R. J. 1985. Microfossil distribution across the base of the Wenlock Series in
the type area. Palaeontology, London, 28: 89-100.
Martin, F. 1973. Ordovicien supérieur et Silurien inférieur a Deerlijk (Belgique). Mem Inst. r. Sci. nat.
Belg., Brussels, 174 (for 1973). 71 pp., 8 pls.
Molyneux, S. G. & Paris, F. 1985. Late Ordovician Palynomorphs. In B. G. Thusu & B. Owens (eds),
Palynostratigraphy of North-East Libya. J. Micropalaeont., London, 4: 11-26.
Nestor, V. 1976. A microplankton correlation of boring sections of the Raikkula Stage, Estonia. Eesti
NSV Tead. Akad. Toim., Tallinn, (Keem. Geol.) 25: 319-324 [In Russian with Engl. summ. ].
— 1980a. New chitinozoan species from the Lower Llandoverian of Estonia. Eesti NSV Tead. Akad.
Toim., Tallinn, (Geol.) 29: 98-107 [In Russian with Engl. summ. ].
— 1980b. Middle Llandoverian chitinozoans from Estonia. Eesti NSV Tead. Akad. Toim., Tallinn,
(Geol.) 29: 136-142 [In Russian with Engl. summ. ].
—— 1982a. New Wenlockian species of Conochitina from Estonia. Eesti NSV Tead. Akad. Toim., Tallinn,
(Geol.) 31: 105-111 [In Russian with Engl. summ. ].
—— 1982b. Chitinozoan zonal assemblages (Wenlock, Estonia). In D. Kaljo & E. Klaamann (eds),
Communities and biozones in the Baltic Silurian: 84-96. Valgus, Tallinn [In Russian with Engl. summ. ].
Nolvak, J. 1980. Chitinozoans in biostratigraphy of the northern East Baltic Ashgillian. A preliminary
report. Acta palaeont. pol., Warsaw, 25: 253-260.
Paris, F. 1981. Les chitinozoaires dans le Paleozoique du Sud-Ouest de Europe. Mem. Soc. géol. miner.
Bretagne, Rennes, 26: 1—412, pls 1-41.
(in press). Biostratigraphy of selected Silurian Chitinozoa. In C. Holland (ed.), A global standard for
the Silurian System. Nat. Mus. Wales Press.
Verniers, J. 1982. The Silurian Chitinozoa of the Mehaigne area (Brabant Massif, Belgium). Prof. Pap.
Belg. geol. Dienst 1982/6, 192: 1-76.
Williams, S. H. 1983. The Ordovician-Silurian boundary graptolite fauna of Dob’s Linn, southern Scot-
land. Palaeontology, London, 26: 605-639.
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Conodont biostratigraphy of the Uppermost
Ordovician and Lowermost Silurian
C. R. Barnes’ and S. M. Bergstrom?
‘Geological Survey of Canada, 601 Booth St, Ottawa, Ontario K1A 0E8, Canada
*Department of Geology and Mineralogy, The Ohio State University, Columbus,
Ohio 43210, USA
Synopsis
A review of the conodont biostratigraphy of the Ordovician—Silurian boundary sections in North
America, Europe, and Asia shows that virtually all sections are either incomplete stratigraphically or have
intervals from which no diagnostic conodonts are known. The best known conodont succession across the
systemic boundary is on Anticosti Island, where, however, the precise level of the boundary remains
unknown because of the absence of diagnostic graptolites. Ordovician and Silurian conodont faunas differ
greatly and there is conclusive evidence that a conspicuous turnover in the conodont faunas took place
globally in the systemic boundary interval. This turnover involved the replacement of a fauna of Ordovi-
cian aspect containing more than 25 genera with one of Silurian aspect having fewer than 15 genera, eight
of which are known also from the Ordovician. A few coniform conodont species survived this extinction
event, but we have identified only one species with compound elements in the apparatus that may range
from the uppermost Ordovician to lowermost Silurian; however, even in the case of this form, there is
some question whether we are dealing with the same species in both systems. The dating of the conodont
faunal turnover in terms of standard graptolite zones is still somewhat uncertain, but available data
suggest that it occurs in an interval in the upper G. persculptus Zone but below the systemic boundary.
This extinction event is probably a result of the Saharan glaciation. In those cases where the origin of the
Llandovery stocks is known or can be postulated, they appear to be derived, in almost all cases, from
stocks that inhabited the tropical waters of the Midcontinent Province during the Ordovician. It is
concluded that further studies are urgently needed, particularly to date exactly the conodont faunal
turnover and to define the Ordovician-Silurian boundary in terms of the conodont succession.
Introduction
Extensive research during the last few decades has firmly established conodonts as a key zone
fossil group in Ordovician and Silurian rocks. The conodont zone successions now in use
within each of these systems provide a stratigraphical resolution which in many cases is
superior to that of other fossil groups, also including the graptolites. Furthermore, the fact that
conodonts are present in rocks representing the whole range of marine depositional
environments from basinal to intertidal, or even supratidal, makes them very useful for both
local and regional biostratigraphical work. This is particularly the case in the shallow-water
carbonate deposits that occupy vast areas on the cratons of all continents except Africa and
Antarctica but which contain only few and scattered occurrences of zonal graptolites.
In view of the significance of conodonts as zonal fossils in Ordovician-—Silurian strata, it is
hardly surprising that they played a major role in the lengthy discussions about the
Ordovician-Silurian boundary which were carried out within the Ordovician-Silurian Bound-
ary Working Group of the I.U.G.S Commission on Stratigraphy. Although it was ultimately
decided to define this systemic boundary on graptolites, the absence of diagnostic graptolites in
many boundary sections, particularly the cratonic ones, makes it necessary to use other fossils
for establishing the precise level of the systemic boundary. Conodonts have great potential to
serve in this capacity. The purpose of the present contribution is to summarize and assess
currently available conodont evidence that has bearing on the recognition and definition of the
Ordovician-Silurian boundary. Although we attempt global coverage, we will concentrate on
North America and Europe, where the most detailed studies have been carried out and from
which we have not only easily accessible information but also personal field experience of most
of the important boundary sections.
Bull. Br. Mus. nat. Hist. (Geol) 43: 325-343 Issued 28 April 1988
326 C. R. BARNES & S. M. BERGSTROM
Upper Ordovician—Lower Silurian Conodont Zonations
The striking faunal provincialism of Late Ordovician conodonts (Barnes et al. 1973; Bergstrom
1973; Sweet & Bergstrom 1974, 1984; Dzik 1983) has necessitated the use of separate bio-
stratigraphical zonal schemes for the North Atlantic and Midcontinent provinces. Although
Sweet & Bergstrom (1984) recently introduced more refined provincial units for the Upper
Ordovician of North America and Europe, in the present contribution, which is global in scope,
we use only these two provinces. Provincialism was not conspicuous during the Early Silurian
but several slightly different zonal schemes have been proposed. However, eventually it may be
possible to use a single zonal scheme globally for this part of the succession.
The Middle and Upper Ordovician zone succession for the North Atlantic Province devel-
oped by Bergstrom (1971a, 1971b, 1978, 1983, 1986) has been tested and used by many other
authors, e.g. Dzik (1976, 1983), Harris et al. (1979), Orchard (1980), and Schonlaub (1971, 1980).
This zonal scheme (Fig. 1) is based on the evolutionary lineage of Amorphognathus. The
successive zones of A. tvaerensis, A. superbus, and A. ordovicicus covers the Caradoc—Ashgill
interval. The A. tvaerensis Zone has three named subzones but no attempt has yet been made
to subzone the A. superbus and A. ordovicicus Zones although the restricted stratigraphical
range of some taxa (e.g. A. complicatus, Hamarodus europaeus, Sagittodontina robusta; Berg-
strom 1983: fig. 1) may eventually allow this (cf. Orchard 1980).
Conodont biostratigraphical classification of the Upper Ordovician of the Midcontinent
Province was first developed as a sequence of faunas characteristic of particular stratigraphical
intervals (Sweet et al. 1971; Sweet & Bergstrom 1976; McCracken & Barnes 1981). The interval
of Faunas 10-13 covered the Cincinnatian Series. Later work by Sweet (1979a, 1979b, 1984)
using graphic correlation methods has led to the establishment of a Composite Standard
Section and a formal zonal scheme with the successive Belodina confluens, Oulodus velicuspis, O.
robustus, Aphelognathus grandis, A. divergens, and A. shatzeri Zones. Because of regional migra-
tion of North Atlantic Province faunal elements into the Midcontinent Province during the
Late Ordovician (Sweet et al. 1971: fig. 3), it is possible to tie some of the zonal boundaries of
these two provincial zone schemes (Sweet 1984: fig. 2). Other studies documenting and support-
ing this scheme include those of Nowlan & Barnes (1981), McCracken & Barnes (1981, 1982)
and Nowlan et al. (in press). Outside North America, studies of cratonic conodont faunas have
been undertaken by, among others, Moskalenko (1983), An (1981), and An et al. (1983), and a
formal zonation has been proposed for Siberia (Moskalenko 1983). It is possible that other low
latitude Ordovician plates (e.g. Kazakhstan, north China and Australia) may require separate
zonal schemes because their conodont faunas include many endemic elements.
The first attempt to develop a conodont zonal scheme for the Lower Silurian was by Walliser
(1964, 1971) from work in the Carnic Alps. Work in this area was later undertaken by Schon-
laub (1971, 1980). Following descriptions of faunas from other regions, it gradually became
apparent that the Carnic Alps standard sequences were stratigraphically incomplete. Aldridge
(1972, 1975) established a new zonation in the Welsh Borderland, but non-productive clastics in
the lowermost Silurian there prevented the establishment of a complete zonal succession
through the Llandovery. In North America, Barrick (1977), Barrick & Klapper (1976), Cooper
(1975, 1980), Fahraeus & Barnes (1981), Helfrich (1980), LeFevre et al. (1976), McCracken &
Barnes (1981), Nowlan (1983), Pollock et al. (1970), Rexroad (1967), Nicoll & Rexroad (1971),
and Uyeno & Barnes (1983), among others, have documented faunas from important
sequences. Elsewhere, studies of Early Silurian conodonts include those of Mannik (1983) in
Severnaya Zemlya, USSR, Lin (1983) in China, and Igo & Koike (1968) from Malaysia.
As a result of these studies, two Lower Silurian conodont zone schemes have evolved for
North America and Europe (Fig. 1) and another for China. However, the phylogenies of
important lineages, such as those of Icriodella, Distomodus and Oulodus, have yet to be fully
documented, and the precise ranges of several key species, including platform taxa, are not yet
established. Once these have been clarified, particularly in sequences such as those on Anticosti
Island, a single zonal scheme should be applicable to most areas. There is also an urgent need
for further documentation of the conodont species succession across the Ordovician-Silurian
UPPERMOST ORDOVICIAN AND LOWERMOST SILURIAN CONODONTS 3y}T/
O? nathani
ui N. AMERICAN CONODONT
= 1U.K. CONODONT ZONES
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Fig. 1 Late Ordovician—Early Silurian chronostratigraphy and conodont zonation for U.K. and
North America. For new terminology of Silurian chronostratigraphy see Barnes (in press) and
Holland (1985); for zones see references in text.
328 C. R. BARNES & S. M. BERGSTROM
boundary. Even though faunal provincialism is much reduced in the Lower Silurian when
compared to the Upper Ordovician, the conodont faunas exhibit considerable differentiation
horizontally; hence there are significant biofacies differences between nearshore and basinal
environments (e.g. Aldridge & Mabillard 1981), and community patterns across shelf
environments can be deduced (e.g. LeFeévre et al. 1976; McCracken & Barnes 1981; Nowlan
1983; Uyeno & Barnes 1983).
North America
Conodont studies of strata close to, or across, the boundary interval have been undertaken in
many regions in North America, including Anticosti Island, Gaspé, the Michigan, Hudson Bay,
Williston and Illinois basins and adjoining arch areas, the western Midcontinent, the Cor-
dillera, Arkansas-Oklahoma, and the Canadian Arctic and its extension into northern Green-
land. The best section currently known is on Anticosti Island, Québec, where there is a
continuous and continuously fossiliferous sequence across the systemic boundary. Elsewhere,
there is a stratigraphical hiatus in the boundary interval, or the faunal sequence is incomplete.
The Anticosti Island conodont sequence (Fig. 2) has been documented by Nowlan & Barnes
(1981), McCracken & Barnes (1981), Fahraeus & Barnes (1981), Uyeno & Barnes (1983), and
Barnes (this volume). Conodont Fauna 13 is developed in Gamachian strata, and the Oulodus?
nathani, Distomodus kentuckyensis, D. staurognathoides, Icriodella inconstans, and
Pterospathodus amorphognathoides Zones (Fig. 1) are recognized in Llandovery strata. These
studies are based on intensive sampling and on the investigation of nearly 100000 superbly
preserved conodonts. Conodont Fauna 13 of McCracken & Barnes (1981), which contains the
distinctive genus Gamachignathus, is associated with Ordovician macrofossils such as Vellamo
and aulacerids. Through the overlying O.? nathani Zone there is a sequential occurrence of
Silurian brachiopods (Zygospiraella, Stricklandia, Virgiana) and the trilobite Acernaspis
(Lespérance 1985). From one locality on eastern Anticosti Island Cocks & Copper (1981)
reported a Hirnantia brachiopod fauna just below a level where Nowlan (1982) recovered
conodonts of Silurian aspect.
On the Gaspé Peninsula (Fig. 2), Québec, the White Head Formation exhibits a faunal
sequence similar to that of Anticosti Island. Gamachignathus (Fauna 13) is known from Unit 4
of this formation, the Hirnantia fauna and the Mucronaspis fauna are well developed and
associated with G. persculptus Zone graptolites in Unit 5, and Acernaspis occurs with Silurian
conodonts (D. kentuckyensis) in Unit 6 (Nowlan 1981, 1983; Lespérance 1985). In another part
of Gaspé, the O.? nathani Zone has been recognized in the Clemville Formation (Nowlan 1983).
On Anticosti Island there is a marked faunal change with a rapid replacement of a diverse
Ordovician conodont fauna with a distinctive, but less diverse, Silurian fauna. In an interval up
to two metres thick, a few Ordovician taxa co-occur with species of Silurian aspect. Unfor-
tunately, the absence of graptolites diagnostic of the P.? acuminatus Zone in the Anticosti
Island succession makes it impossible to establish the precise level of the systemic boundary,
and the relations between the faunal turnover and this level. The fact that the uppermost
interval of Fauna 13 has a Hirnantia fauna and graptolites of the G. persculptus Zone on Gaspé
(Lespérance 1985) shows that the conodont fauna below the turnover interval is of pre-Silurian
age, and the systemic boundary must be at a higher stratigraphical level in Anticosti.
Lespérance (1985) suggested that the appearance of Acernaspis may be coeval with the base of
the P.? acuminatus Zone and hence mark the systemic boundary; however, as noted below, the
reliability of the appearance of this genus regionally as a guide to the boundary level needs
confirmation, and its appearance on Anticosti Island might be at a higher stratigraphical level
than in some other areas.
In Ontario and Michigan in the Great Lakes region, conodont studies have revealed the
existence of a hiatus at the boundary that spans the Gamachian Stage and possibly parts of the
Richmondian and early Llandovery as well (cf. Barnes & Bolton, this volume). Fauna 13 and
the O.? nathani Zone are not recognized in this area. A similar hiatus exists to the north in the
Hudson Bay Basin (LeFevre et al. 1976) and to the south in the Cincinnati Region (cf. Sweet
UPPERMOST ORDOVICIAN AND LOWERMOST SILURIAN CONODONTS
SJIVHS 11IHXYIE
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Lanyon sane NVIYSINAW V HOWWW9|'GNOWHO
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Fig.2 Key stratigraphical sections across the Ordovician-Silurian boundary showing dominant lithologies of formation and their biostratigraphical
correlation based on presence of conodont (c), graptolite (g), and Hirnantia (h) faunas.
329
330 C. R. BARNES & S. M. BERGSTROM
1979a, 1984; Grahn & Bergstro6m 1985). The faunas of the latter region have been well docu-
mented in the last decades by W. C. Sweet and co-workers for the Ordovician and C. B.
Rexroad and co-workers for the Llandovery. Further to the west, in the Williston Basin, no
Gamachian Fauna 13 has been recognized (Sweet 1979b; Barnes, unpublished collections from
Manitoba), and the earliest Silurian conodonts compare well with those from the Manitoulin
Formation of Ontario discussed by Barnes & Bolton (this volume). These conodont successions
from the Midcontinent Region suggest that a major regression left the North American craton
largely emerged for at least the duration of the Gamachian, and possibly longer, at least in
some areas. The only exceptions to this, that is, areas where youngest Ordovician conodonts
are present, are in marginal basins (e.g. Anticosti Island), some intracratonic troughs (e.g. in
Arkansas—Missouri-Oklahoma; see Bergstrom & Boucot, this volume), outer miogeoclinal
areas (e.g. Utah and Nevada), and regions having offshore basin and slope deposits (e.g. Gaspé
and Arctic Canada).
In some Midcontinent areas (Fig. 2), incomplete stratigraphical successions produce intrigu-
ing conodont faunas of latest Ordovician age. Such faunas are known from the Cason Oolite of
Arkansas (Craig 1969, 1986; Barrick 1986), the Noix Oolite and Girardeau Limestone of
Missouri (Satterfield 1975; McCracken & Barnes 1982), and the Keel Formation of Oklahoma
(Barrick 1986). These units yield sparse faunas characterized by Noixodontus girardeauensis
(Satterfield). McCracken & Barnes (1982) assigned a Fauna 12 (Richmondian) age to the Noix
fauna, but Barrick (1986) suggests that the presence of a Hirnantia fauna in several of these
units indicates a latest Ordovician (late Gamachian, Hirnantian, Fauna 13) age. In the Yukon
(Fig. 2), Lenz & McCracken (1982) recorded both Noixodontus and Gamachignathus in strata
referred to the Pacificograptus pacificus Zone (the upper Climacograptus supernus Zone, equiva-
lent to the lower part of the interval of the Hirnantia fauna in China; Lenz & McCracken 1982:
fig. 6). In the Yukon, the overlying Climacograptus extraordinarius Zone is not recognized and
that interval may be represented by a hiatus. The latest Ordovician Glyptograptus persculptus
Zone is identified only with question, but significantly a Silurian conodont fauna is recorded
from 6:3—-13-3m below the top of the G. persculptus Zone? in the Pat Lake section (Lenz &
McCracken 1982, Appendix). With a hiatus below the G. persculptus Zone?, it is possible that
only the uppermost part of that zone is present in the succession.
In the Canadian and Greenland Arctic regions, several conodont studies have been com-
pleted, or are under way, but little has been published to date. Preliminary results (Mayr et al.
1980) suggest the presence of a regionally developed hiatus in the systemic boundary interval.
This is certainly the case in the carbonate platform facies (e.g., the Allen Bay Formation) and
probably in the basinal facies as well, where the G. persculptus Zone has not been recognized.
Finally, Leatham (1985) has described a section in carbonate facies across the systemic
boundary interval in the Great Basin. Absence of graptolites precludes recognition of the
precise level of the systemic boundary. However, Leathan recognized an interval with mixed
faunas between typical Ordovician and typical Silurian faunas, but he was inclined to believe
that these mixed faunas were due to stratigraphical leaks or reworking of Ordovician cono-
donts into basal Silurian strata near an unconformity associated with the systemic boundary.
In central Nevada, Ross et al. (1979) interpreted the Hanson Creek Formation as ranging
without significant gap from the Late Ordovician to the Early Silurian. Fauna 13 seems to be
represented in their collections but because they do not describe their Silurian conodonts, it is
not clear how the conodont faunal succession is developed in the boundary interval.
Great Britain
No continuous section across the Ordovician-Silurian boundary developed in a facies suitable
for conodont extraction is known from the British Isles. The boundary stratotype at Dob’s
Linn, Scotland (Fig. 2), as well as the lowermost part of the Llandovery reference standard in
south Wales, are both unpromising for conodont work. A few conodonts have been recovered
from shale bedding planes at the boundary stratotype, Dob’s Linn (Barnes & Williams, this
volume), and a single conodont collection is known from the lowermost Llandovery of the type
UPPERMOST ORDOVICIAN AND LOWERMOST SILURIAN CONODONTS 331
area (Cocks et al. 1984). Efforts to collect from strata near the systemic boundary elsewhere in
Britain have not been very successful; hence, only two productive samples are known from the
Hirnantian (Bergstrom & Orchard 1985), none of them with very diagnostic species although
the faunas are clearly of Ordovician aspect. Apparently, as in Scandinavia, the Hirnantian
rocks in Britain are very poor in conodonts.
Currently available information about British early Llandovery conodonts derives largely
from the work by Aldridge and co-workers. As noted by Aldridge (1985), very few conodonts
are currently known from the Rhuddanian although a sample from the lower part of the stage
at Llandovery contained a species association diagnostic of Aldridge’s (1972) Icriodella
discreta—I. deflecta Zone (Cocks et al. 1984). Aeronian strata in Wales and the Welsh Border-
land have yielded taxonomically varied species associations (Aldridge 1985), which include
Kockelella? abrupta, Ozarkodina oldhamensis, O. hassi, and Pterospathodus? tenuis. The upper
Aeronian is characterized by the appearance of Distomodus staurognathoides, Oulodus? fluegeli,
Pseudooneotodus tricornis, and Kockelella ranuliformis.The interval having this species associ-
ation is referable to the Distomodus staurognathoides Zone (Aldridge 1972).
Scandinavia
The few sections in Sweden (Vastergotland, Scania) and Denmark (Bornholm) where the base
of the Parakidograptus? acuminatus Zone, and hence the base of the Silurian, can be recognized
are all in dark shale facies from which no conodonts have been recovered. In other sections,
shallow-water strata with the Hirnantia fauna (Bergstrom 1968) are overlain, in places uncon-
formably, by Llandovery age shales and mudstones. In Sweden, the Ashgill conodont faunas
are known from several sections (Bergstrom 1971la; Sweet & Bergstrom 1984) but the early
Llandovery ones are virtually unknown. No conodonts have been recorded from the systemic
boundary interval in Denmark.
Biostratigraphically well controlled lower Llandovery successions have recently been
described from the Oslo region, Norway (Fig. 2). The conodont succession there is particularly
significant because it can be tied to the distribution patterns of key graptolites and shelly fossils
(Aldridge & Mohamed 1982). As is the case in Sweden, rocks of latest Ordovician (Hirnantian)
age have produced very few conodonts, the only reasonably common species being a form close
to, if not identical with, Ozarkodina oldhamensis, which is also characteristic of coeval strata in
Sweden (Bergstrom 1971a: fig. 4:11). Absence of close graptolite control makes it impossible to
establish the precise level of the systemic boundary in the Oslo region, but the graptolites
indicative of the upper Glyptograptus persculptus Zone or lower P.? acuminatus Zone present in
the lower Solvik Formation (Howe 1982) suggest that the systemic boundary is close to the
base of that unit, which is separated from the underlying Hirnantian strata by what appears to
be a minor gap. The recent suggestion that the appearance of the trilobite Acernaspis is coeval
with the base of the P.? acuminatus Zone is not well supported by the conditions in the Oslo
region where this genus makes it appearance in the middle Solvik Formation (6ba) in an
interval that on graptolite evidence appears to be no older than the Monograptus atavus Zone
(Howe 1982).
A summary of the conodont, shelly fossil, and graptolite biostratigraphy of the lower Llando-
very of the Oslo region is given in Fig. 3. The faunal succession is quite similar to that of the
Anticosti Island (Barnes & McCracken 1981; Lespérance 1985), Gaspé (Nowlan 1983;
Lespérance 1985), and the Rhuddanian and lower Aeronian of Britain (Aldridge 1985; Cocks et
al. 1984). In the lowermost Llandovery of the Oslo region, the presence of Oulodus? cf. O.?
nathani strongly suggests that the Oulodus? nathani Zone can be recognized (Aldridge &
Mohamed 1982), which is overlain by the Distomodus kentuckyensis Zone. In the uppermost
part of the Solvik Formation, representatives of Distomodus staurognathoides and other species
of the D. staurognathoides Zone make their entrance, which suggests correlation with the
middle Aeronian of Britain (Aldridge 1975) and the lower part of the Jupiter Formation of
Anticosti Island (Uyeno & Barnes 1983). Although the Llandovery conodont succession of the
Oslo region is one of the best biostratigraphically controlled in the world, it unfortunately
332 C. R. BARNES & S. M. BERGSTROM
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of the Oslo region, Norway. Based on many sources, particularly Howe (1982) and Aldridge & Mohamed (1982). Note that the Hirnantia
fauna-bearing upper Ashgill (5b), which is separated from overlying rocks by a minor unconformity, has yielded only a few conodonts and no
Fig. 3 Comparison of stratigraphical ranges of key conodonts, shelly fossils, graptolites, and conodont and graptolite zones in the lower Llandovery
diagnostic graptolites.
UPPERMOST ORDOVICIAN AND LOWERMOST SILURIAN CONODONTS 338
provides little information about the conodont sequence right across the systemic boundary
interval, despite the fact that much of this interval is developed in calcareous rocks that are
readily digestible in weak acids.
Carnic Alps and nearby areas in Austria and Italy
The Cellon section in the Carnic Alps has become classic as the reference standard of much of
the Silurian conodont zone succession (Walliser 1964) but this border region between Austria
and Italy has several other important sections that include Late Ordovician as well as Early
Silurian strata (Fig. 2) (Schonlaub 1969, 1971, 1979, 1980; Jaeger & Schonlaub 1977; Jaeger et
al. 1975; Serpagli 1967; Vai 1971; Flajs & Schonlaub 1976). Because graptolites diagnostic of
the P.? acuminatus Zone are unknown in the Carnic Alps, the precise level of the base of the
Silurian cannot be determined in sections with more or less continuous sequence. In other
sections, Silurian or younger strata rest unconformably on Ordovician beds and the systemic
boundary coincides with a conspicuous stratigraphical gap.
Many of the conodont data available from this region pertaining to the Ordovician—Silurian
boundary interval consist of lists of species, but there are also published descriptions and
illustrations of Ashgill (Serpagli 1967; Flajs & Schonlaub 1976) and Llandovery (Walliser 1964;
Schonlaub 1971) conodonts. Sweet & Bergstrom (1984) suggested some updating of the tax-
onomy of Ashgill species and additional taxonomic work on some of the faunas is clearly
needed.
The most distinctive Ashgill age unit in the Carnic Alps is an argillaceous limestone a few
metres thick, the Uggwa (Uqua) Limestone ( = Tonflaserkalk). Although its conodont fauna,
which was monographed by Serpagli (1967), includes some species currently unknown outside
Austria and Italy, it is clearly of Ordovician rather than Silurian aspect and represents the
Amorphognathus ordovicicus Zone. Some of its characteristic genera include Amorphognathus,
Ansella, Birksfeldia, Drepanoistodus, Hamarodus, Plectodina, Protopanderodus and Scabbardella,
which are all restricted to the Ordovician. In several sections, the Uggwa Limestone is followed
by a prominent stratigraphical gap that may represent a portion of the Silurian (or more) and
possibly also the uppermost Ordovician. At other sections, a part of this gap is filled by
calcareous sandstones and dark shales, commonly referred to as the ‘Untere Schichten’, that
locally, for instance at the Cellon section, contain megafossils of the Hirnantia fauna associated
with Ashgill conodonts. Walliser (1964) classified the “‘Untere Schichten’ as the upper part of his
Bereich 1 and referred this unit to the Lower Silurian. We believe that most, if not all, of the
‘Untere Schichten’ belongs to the uppermost Ordovician, if one follows the practice of having
the systemic boundary at the base of the P.? acuminatus Zone.
As shown by Walliser (1964), the beds on the top of the ‘Untere Schichten’ at Cellon contain
conodonts (Apsidognathus tuberculatus, Distomodus staurognathoides and Pterospathodus
celloni) of the P. celloni Zone, and a similar fauna is known also from beds just above the
Ashgill age limestone at the Mount Seewarte section (Schonlaub 1971, 1980). At both these
sections, the stratigraphical hiatus associated with the systemic boundary includes two-thirds of
the Llandovery (Rhuddanian and Aeronian stages). On the other hand, at other localities, such
as the Feistritzgraben section (Jaeger et al. 1975; Schonlaub 1980), the Uggwa Limestone is
directly overlain by dark shales that contain Glyptograptus cf. G. persculptus near their base.
This suggests a much smaller, if any, stratigraphical gap above the limestone, and the systemic
boundary is evidently at an unknown level in the clastic succession above the graptolite-
bearing interval.
Although earliest Silurian, and perhaps also latest Ordovician, conodonts are unknown from
the sections in the Carnic Alps and nearby regions, this area is of interest in discussions about
the conodont biostratigraphy near the systemic boundary because of its rich Ashgill and middle
and late Llandovery conodont faunas. Furthermore, in view of the local variations in both
lithological and stratigraphical development near the systemic boundary, it is not excluded that
further studies may lead to the discovery of stratigraphically more complete sections in a
lithology suitable for extraction of conodonts than those now known.
334 C. R. BARNES & S. M. BERGSTROM
Other areas
Outside North American and Europe, latest Ordovician and/or earliest Silurian conodonts are
known from Siberia, China and Malaysia. In her review of the Ashgill conodont bio-
stratigraphy of the Siberian Platform, Moskalenko (1983) recognized an Aphelognathus pyrami-
dalis Zone in the topmost part (the Burian Stage) of the Ordovician but she noted that the
succession is terminated by an erosional unconformity. Apart from the zonal index, the low-
diversity and apparently largely endemic conodont fauna includes, among others, Acanthodina
nobilis, A. variabilis, and Acanthodus compositus (Moskalenko 1973). Mannik (1983) recorded a
conodont succession through the Silurian of Severnaya Zemlya. The lowermost unit, the
Vodopad Formation, yielded in its lower part Ozarkodina oldhamensis, Icriodella cf. I. deflecta,
and Oulodus? cf. O. kentuckyensis, among others. This interval was referred to the I. discreta—I.
deflecta Zone and interpreted to be of late Rhuddanian to early Aeronian (=Idwian in
Mannik) age. The similarity to coeval faunas in the Oslo region and eastern Canada is striking.
In China, the uppermost Ordovician, where present, is in most places developed in a litho-
logy unsuitable for conodont extraction, and it has yielded only a few undiagnostic species (An
1981). Shelly facies of Llandovery age produce taxonomically varied and well preserved cono-
donts such as those from the Guizhou Province recorded by Zhou et al. (1981; also cf. Lin
1983) that provide correlation with the early Llandovery I. discreta—I. deflecta Zone, although
some of the published identifications need confirmation.
Another section of interest in a discussion of the conodont biostratigraphy across the
Ordovician-Silurian boundary is on Langkawi Islands, Malaysia (Igo & Koike 1967, 1968).
The latest Ordovician and earliest Silurian are represented by clastic strata (Lower Detritus
Band’), but rocks below and above this interval have produced well-preserved conodonts.
Although some of their identifications need reappraisal, it appears clear that the lowest Silurian
fauna recorded by Igo & Koike (1968) represents the Pterospathodus amorphognathoides Zone
and is of late Llandovery age (Fig. 1). A modern restudy of the Langkawi succession would be
of considerable biostratigraphical interest.
Changes in conodont faunas across the Ordovician—Silurian boundary
One of the most striking, if not the most striking, faunal turnovers during the 400 million year
long history of the Phylum Conodonta occurred near the Ordovician—Silurian boundary. As
recently shown (Sweet 1985: figs 7, 8), the total species diversity decreased from an estimated
75-100 species in the lower-middle Ashgill (Sweet & Bergstrom 1984) to about 20 species in
the lower Llandovery. This diversity reduction was not a sudden catastrophic event although
only a few species survived into the Silurian; rather, during the Ashgill there was a gradual
disappearance involving many characteristic and long-established stocks and the new taxa that
appeared were considerably fewer than those that became extinct. However, within a very
limited interval, probably in the latest Ashgill, most of the remaining Ordovician taxa were
replaced by forms of Silurian aspect, producing a very different appearance of the conodont
faunas. From both biostratigraphical and palaeobiological points of view, it is obviously of
considerable interest to establish the precise timing and detailed scenario of the conodont
faunal turnover. Unfortunately, conodont data from strata reliably dated as representing the G.
persculptus Zone, and particularly the upper part of this zone, are few and incomplete, making
it currently impossible to tie the turnover closely to the graptolite zone succession. As noted
below, we believe that the turnover occurred before the beginning of the Silurian (as defined by
the base of the P.? acuminatus Zone), but we admit that the evidence for this conclusion is not
yet conclusive. The best illustration of the faunal turnover is in the Anticosti Island succession,
where there seems to be no significant stratigraphical gap in the boundary interval. As
described by McCracken & Barnes (1981), the Ordovician-type conodont fauna in the
Hirnantian-age Ellis Bay Formation there includes some 38 species. Immediately above a thin
(0-5—2 m thick) interval having a mixed fauna, there is a Silurian-type conodont fauna of about
21 species, 16 of which are not known from older strata. Because no graptolites useful for
UPPERMOST ORDOVICIAN AND LOWERMOST SILURIAN CONODONTS 335
precise zonal classification are known from the turnover interval and the immediately overlying
strata, this interval cannot yet be classified in terms of standard graptolite zones and even the
base of the Silurian there cannot be tied to a specific stratigraphical level.
In Fig. 4 we illustrate the known ranges of significant conodont species in the Ashgill and
lower Llandovery. It should be stressed that a compilation of this type, involving data from
many different sources in widely different geographical regions, will necessarily be both incom-
plete and probably incorrect in some respects, especially as it is based partly on arbitrary age
assessments of some faunas. One interesting feature emerging from Fig. 4 is that apparently,
with the possible exception of a form in the still poorly known Ozarkodina oldhamensis
complex, not a single species with compound elements in the apparatus survived the faunal
turnover. Only a few generalized species of the coniform conodont genera Dapsilodus, Decori-
conus, Panderodus, Pseudooneotodus, and Walliserodus range into the Lower Silurian, but it
should be noted that the taxonomy of some of these taxa is still not very clear.
Figure 5 summarizes the known ranges of important genera in the Ashgill and lower Llando-
very. Significantly, only eight of the more than 25 Late Ordovician genera range across the
turnover interval. Among these, only three (Icriodus, Oulodus, and Ozarkodina) have compound
elements in the apparatus, whereas the five other genera have apparatuses composed of exclu-
sively coniform elements. In our interpretation, the Amorphognathus lineage, which may be
traced back to the Early Ordovician (Bergstrom 1983), became extinct in the Hirnantian. In the
past, some authors have referred the early Llandovery Pterospathodus? tenuis to Amorphog-
nathus, presumably on the basis of a perceived similarity in the Pa elements. However, the
ramiform elements of the apparatuses of the two genera differ markedly, and we question that
the Silurian species has any affinity at all with Amorphognathus.
The mutual relations, and possibly synonomy, of the two Ashgill genera Birksfieldia and
Gamachignathus are still unclear, and it is outside the scope of the present study to discuss
those matters here. However, it should be noted that it is conceivable that the ancestor of the
Silurian genus Distomodus is to be found among this group of Late Ordovician conodonts.
The Icriodella lineage can be traced, with no significant interruption, from the Llandeilo to
the Ashgill (Bergstrom 1983). We are not aware of any confirmed record of the genus in the
Hirnantian but several widely distributed species have been described from the Llandovery
(Aldridge 1972). The platform elements in the Silurian species are certainly similar to those in
the Ordovician forms, but the non-platform elements differ in some respects, and the relations
between the Ordovician and Silurian forms referred to Icriodella need further study; it is
premature to conclude that all these forms represent the same lineage.
The Late Ordovician and Early Silurian representatives of Oulodus exhibit close similarity in
morphology (Sweet & Schonlaub 1975) and they appear to represent the same stock. The same
applies to Ozarkodina but this genus is not well known from the Ordovician. Its strati-
graphically oldest species, O. pseudofissilis from the upper A. superbus Zone (lower Ashgill) of
Britain (Lindstrom 1959; Orchard 1980), is isolated stratigraphically from a Hirnantian species
in Scandinavia close to O. oldhamensis. The latter is so close morphologically to Llandovery
species of Ozarkodina that there appears to be no doubt that they represent the same lineage.
It may be significant that the genera that survived the turnover are widely distributed in
Ordovician rocks, and the species involved may have been ecologically tolerant. Sweet &
Bergstrom (1974: 20) noted that, when known, the ancestry of most Llandovery stocks appears
to be from among forms with particularly wide distribution in Midcontinent (warm-water)
Province Ordovician faunas, whereas the North Atlantic (cold-water) Province stocks virtually
disappeared in the Late Ordovician. A possible exception may be Dapsilodus, which in the
Ordovician is best known from, and most common in, North Atlantic Province faunas. The
severe regression reduced the space and range of environments available to the Midcontinent
faunas and presumably resulted in the demise of many stocks. Many coniform taxa seem
to have been less affected, particularly forms interpreted as pelagic rather than nektobenthic in
habit (e.g. McCracken & Barnes 1981). The North African glaciation would have created
different oceanic conditions, in terms of circulation, oxygenation and cooler temperatures. This
combination of factors probably reduced the diversity of late Ashgill conodont faunas and
336
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C. R. BARNES & S. M. BERGSTROM
ORDOVICIAN
autleyan| Rawtheyan
Hirnantian
SILURIAN
Ahuddan.
A. ordovicicus /
Culumbodina penna
Pseudobelodina torta
Belodina confluens
Aphelognathus shoshoensis
Aphelognathus flowers
Plectodina aculeatoides
~ — > —Plectod. florida
~— — —Plectodina tenuis
Pseudobelodina adentata
Pseudobelodina inclinata
Pseudobelodina ants
Staufferella
Pseudobelodina quadrata
Pseudobelodina vulgaris
Plegagnathus nelsoni
Rhipidognathus symmetricus.
Scabbardella a/tipes
Dapeiedus Mautatus
Aphelagnathus pyramidalis
Pseudobelodina dispansa
1
Panderodus feu/neri
'
‘’Spathognathodus *
discreta-/. deflecta
Oulodus?
kentuckyensis
Qulodus? nathani
Ozarkodina hassi
Distomodus
kentuckyensis
Icriodella discreta
Walliserodus curvatus
manitoulinensis
— “Spathognathodus”™ elibatus
Icriodella deflecta
— Coryssognathus nsp
— Kockelella? abrupta
Pterospathodus? tenuis
Llandoverygnathus
siluricus
Ozarkodina pirata
Pterospathodus
posteritenuis
Walliserodus sancticlairi
Decoriconus fragilis
Ozarkodina protexcavata
Oulodus? fluegeli
Distomodus
staurognathoides
Walliserodus amplissimus
Oulodus ulrichi
i
— Parabelodina denticulata
Icriodella prominens
Panderodus bergstroemi |
'
Protopanderodus insculptus
/strorinus erectus
—— Sagittodontina robusta
Plegagnathus dartoni
—— Aphelognathus Giveraens
—— Pristognathus Bighereners
Belodina Peetiorebista
Nordiodus italicus
'
— Belodina stone
Plectodina alpina
' 6
—— Aphelognathus shatzeri
1
Panderodus liratus
Phragmodus undatus
Frorcmondenades liripipus
Wamenactis europaeus
Aphelognathus grandis
Amorphognathus ordovicicus
Drepanoistodus suberectus
Gamachignathus ensifer
'
Panderodus Clinatus
1
Walliserodus cf .W. curvatus
Oulodus robustus
Ozarkodina
oldhamensis
5 abe cere!
Decoriconus costulatus
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UPPERMOST ORDOVICIAN AND LOWERMOST SILURIAN CONODONTS
ORDOVICIAN
CAUTL. | RAWTH. | HIRNANT. RHUDD. AERO
Amorphognathus
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|
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Birksfeldia & Gamachignathus | | |
pee ee ess oe ciodella
| ]
as Coryssognathus
Pseudobelodina Y
e@mme Spathognathodus”’
|
Llandoverygnathus
Belodina
|
EEE 00/0005
ioe
Plegagnathus
eee
Aphelognathus
Plectodina
Rhipidognathus
1
qm BP BSee ew Sagittodontina
Hamarodus
'
Strachanognathus
I
Phragmodus
| 1
Drepanoistodus
Walliserodus
Dapsilodus
Acanthodina |
Pseudooneotodus
|
Staufferella |
mums 0/00 S |
quem —/5(0//0US
Panderodus
337
338 C. R. BARNES & S. M. BERGSTROM
produced the profound turnover (Barnes 1986). In addition, plate motions may have aided in
these faunal changes since, by Hirnantian time, typical North Atlantic Province areas such as
Baltica had moved into the tropical belt, which caused extinction of stocks long adapted to the
conditions in high-latitude regions. If so, one would expect that perhaps some taxa of North
Atlantic Province aspect would have survived into the Silurian in high-latitude regions, provid-
ed conditions did not become too severe. Unfortunately, high-latitude early Llandovery cono-
dont faunas remain virtually unknown. Also, deeper-water early Silurian conodont faunas are
poorly known, and it seems likely that the enigmatic origin of some of the Early Silurian
platform genera may be discovered in such faunas.
Conodont Correlation of the Ordovician-Silurian boundary
As shown above, there is a profound conodont faunal change in the Ordovician—Silurian
boundary interval. This faunal turnover occurs in both shallow-water cratonic and deep-water
oceanic environments. The more detailed sampling and better faunal control that is feasible in
carbonate platform successions is likely to provide more precise correlation within the bound-
ary interval than can be expected in predominantly clastic deep-water oceanic deposits, which
tend to contain fewer conodont-producing beds, and which are now largely preserved in
structurally complex orogenic belts. Because diagnostic graptolites are largely restricted to the
latter deposits, there is an obvious need to be able to recognize the systemic boundary accu-
rately on the basis of fossils present in the cratonic successions. The geographically widespread
and rapidly evolving conodonts can be expected to be helpful for precise correlations across
facies boundaries also in the systemic boundary interval.
Three matters are of basic importance for the conodont correlation of the Ordovician—
Silurian boundary: (1) the relation between the conodont faunal turnover and the systemic
boundary in oceanic and slope sequences having zonal graptolites; (2) the relation between the
conodont faunal turnover and the systemic boundary in platformal successions having key
shelly fossils; and (3) if the conodont faunal turnover does not coincide with the graptolite-
based systemic boundary, how do we define this boundary in terms of conodonts? All these
matters involve several unsolved problems and, as will be shown below, we cannot now provide
a definite answer to the last question.
In the North American platformal sequences, graptolites are rare or absent in the boundary
interval. The informal units corresponding to Conodont Faunas 12 and 13, which are of Ashgill
age (Fig. 1), have recently been replaced by a succession of formal conodont zones based on
graphic correlation techniques (Sweet 1984: fig. 1), including the Oulodus velicuspis, O. robustus,
Aphelognathus grandis, A. divergens, and A. shatzeri Zones. Latest Ordovician strata have a
very restricted distribution on the North American craton due to a major regression associated
with the Saharan glaciation(s). Further, the biostratigraphical, palaeoecological and biogeo-
graphical distributional constraints of latest Ordovician key taxa such as Noixodontus and
Gamachignathus are not yet fully established. The interval of Faunas 12-13 corresponds
broadly to the North Atlantic Province Amorphognathus ordovicicus Zone and also correlates
with the lower Maysvillian to Gamachian stages, and the Cautleyan to Hirnantian stages (Fig.
1). On Anticosti Island as well as in Oklahoma—Arkansas—Missouri (Fig. 2), Gamachian cra-
tonic faunas (Fauna 13) are associated with shelly faunas of Hirnantia fauna aspect. Available
data show that this interval (that of Fauna 13) at least broadly correlates with the Pacific-
ograptus pacificus and Climacograptus extraordinarius and at least part of the Glyptograptus
persculptus Zones in the graptolite succession.
In the Yukon the first conodont faunas of Silurian aspect are found just below graptolites
assigned to the G. persculptus Zone? (Lenz & McCracken 1982) in an interval possibly coeval
with the upper part of this zone. If this interpretation is correct, it shows that the conodont
faunal turnover was in latest Ordovician time and not coinciding with the systemic boundary.
Lespérance (1985) has suggested that on Anticosti Island and Gaspé the level of appearance of
Acernaspis is coeval with the base of the P.? acuminatus Zone, that is, the base of the Silurian.
However, long-distance correlation of shelly fossils at the generic level is bound to be uncertain
UPPERMOST ORDOVICIAN AND LOWERMOST SILURIAN CONODONTS 339
and the appearance of this trilobite in eastern Canada could obviously be younger than the
base of the Silurian. On Anticosti Island, the level of the first appearance of Acernaspis is
30-70 m above the level of the first appearance of conodonts of Silurian aspect that mark the
base of the Oulodus? nathani Zone. If the systemic boundary correlation of Lespérance (1985) is
approximately correct, it is obvious that the horizon of the conodont faunal turnover is well
below the base of the Silurian; it is certainly unlikely that it is higher than that stratigraphical
level.
Some further data are available from other regions but unfortunately they are not decisive
for establishment of the exact relationship between the conodont faunal turnover and the
systemic boundary. Samples from the Carys Mills Formation of Maine and New Brunswick,
possibly representing the G. persculptus Zcne, contain faunas typical of the Silurian Icriodella
discreta—I. deflecta Zone (Nowlan 1983; Bergstrom & Forbes unpublished). Conodonts of the
Amorphognathus ordovicicus Zone are known from the D. anceps and C. supernus Zones at
Dob’s Linn, Scotland (Barnes & Williams, this volume) and Mirny Creek, northeast Siberia
(Barnes, unpublished), respectively. Few sections are known where conodonts can be extracted
from the G. persculptus Zone. At Mirny Creek, the basal P.? acuminatus Zone contains Silurian
conodont faunas of the /. discreta—I. deflecta Zone, but the underlying G. persculptus Zone has
not produced stratigraphically diagnostic conodonts (Barnes, unpublished).
We conclude that the precise correlation of the systemic boundary is uncertain in strati-
graphically continuous shelly successions. Although a series of zones has been distinguished in
graptolite-bearing successions, severe taxonomic problems involve several of the key species,
and graptolite-based correlation into sequences with shelly fossils and conodonts is rarely
possible, and conodont correlation into graptolitic facies is equally difficult. The degree of
stratigraphical resolution appears greater for graptolites than for conodonts. However, Sweet’s
(1984) new zonal scheme for the North American Midcontinent has a resolution approaching
that of the graptolite zone succession in China, and further refinements of the conodont zonal
schemes are possible. If our suggestion that the conodont faunal turnover is in the upper G.
persculptus Zone proves correct, the base of the Silurian, as now defined, will be above the
interval of the most significant event in the conodont evolution of the Lower Palaeozoic. A
future challenge is obviously to recover diagnostic conodonts from the G. persculptus Zone, and
preferably from adjacent zones as well, in continuous sections, but very few sections suitable for
this are known to us. In the meantime, a situation must prevail where the base of the P.?
acuminatus Zone defines the base of the system in graptolitic successions, and the base of the
Oulodus? nathani Zone defines a level near the systemic boundary in conodont sequences.
Because of the prominent unconformity that is associated with the systemic boundary in most
cratonic sequences, the latter level will in many, but not all, cases be the same as the systemic
boundary. In stratigraphically more complete sections, it is possible that the difference between
the graptolite-based boundary and the level of the conodont faunal turnover may correspond
to as much as half a graptolite zone.
Conclusions
1. Although the conodont succession is known in considerable detail in both the Ashgill and
the Llandovery, there are few data available from sections with rocks reliably dated by
graptolites representing the upper G. persculptus and P.? acuminatus Zones.
2. Most boundary successions from which conodonts are known are stratigraphically incom-
plete or have intervals from which no diagnostic conodonts are known. The best known
conodont succession across the boundary interval is on Anticosti Island, but the position of
the graptolite-defined systemic boundary is uncertain there as the boundary interval lacks
reliable graptolite control.
3. Ordovician and Silurian conodont faunas are strikingly different. The interval of faunal
turnover is less than 2m thick in the stratigraphically rather expanded section on Anticosti
Island. The precise position, in terms of graptolite zones, of this turnover is still uncertain,
but the available evidence indicates that it is likely to be in the upper part of the G.
340 C. R. BARNES & S. M. BERGSTROM
persculptus Zone, below the systemic boundary. Hence it seems unlikely that the profound
turnover coincides with the systemic boundary.
4. At the present time, the base of the P.? acuminatus Zone, that is the Ordovician—Silurian
boundary, cannot be identified precisely on conodont evidence in sections with continuous
sedimentation through the boundary interval. Further studies are needed in graptolite-
controlled sections to clarify the exact relations between conodont and graptolite zones at
the systemic boundary.
Acknowledgements
C.R.B. gratefully acknowledges continued financial support for conodont studies from the Natural Sci-
ences and Engineering Research Council of Canada.
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Graptolite faunas at the base of the Silurian
R. B. Rickards
Department of Earth Sciences, Downing St, Cambridge CB2 3EQ
Synopsis
The base of the Silurian System is globally defined by the appearance of a number of species of graptoloid
referable to the genera Akidograptus and Parakidograptus, as well as by a pronounced increase in species
diversity from the underlying persculptus Zone. The nature of this diversity is given in terms of distinctive
elements of the acuminatus Zone, in terms of its less diagnostic species, and in terms of species of local
occurrence. Contrasts are made with the graptoloid faunas of the persculptus and atavus Zones.
Introduction
The ratification by the International Commission on Stratigraphy of the base of the Silurian
System at the base of the acuminatus Zone at Dob’s Linn, Scotland, greatly facilitates interna-
tional correlation of the base in the graptolite facies. Even in sections with only moderately
abundant or diverse graptolite associations, the acuminatus assemblage can usually be identi-
fied, although not always the precise lower and upper limits of the zone: the approximate
correlative level is often quite clear. Furthermore, the distinctive nature of the acuminatus fauna
makes relatively simple the present task, namely that of defining the base of the Silurian in
terms of its graptolites. It should not be assumed that the base of the acuminatus Zone
corresponds with the beginning of the post-glacial evolutionary explosion in graptoloids
(Koren & Rickards 1979): that precise level is probably near the base of the persculptus Zone,
using that term in its broadest sense. The lowest graptoloid diversity corresponds roughly with
the extraordinarius Zone. This was followed by an increased diversity in the persculptus Zone,
and a yet greater increase in the acuminatus Zone. It is the last that now identifies the base of
the Silurian and which is described below.
It is helpful that the acuminatus Zone was originally defined at Dob’s Linn (Lapworth 1878).
However, he included at its base gingerbread-coloured shales which Jones & Pugh (1916)
considered equivalent to Jones’ (1909) persculptus Zone at Port Erwyd. This opinion was amply
reinforced by Davies (1929), Toghill (1968) and Williams (1983), so that the original concept of
the zone has been changed to mean the graptolite faunal assemblage between the persculptus
and atavus Zones or their equivalents.
Graptolites immediately preceding the acuminatus Zone
Low diversity characterizes both the extraordinarius and persculptus Zones. There is a total
absence of multistiped genera such as Dicellograptus and Tangyagraptus, and the extraordi-
narius Zone comprises only a few biserial types, including C. extraordinarius together with
diminutive climacograptids such as C. normalis, C. angustus (=C. miserabilis) and C. mirnyensis.
C. medius appears near the top of the zone in northeastern U.S.S.R. The persculptus Zone has a
fauna a little more diverse than that of the extraordinarius Zone, but apart from rare uniserial
scandent forms (Atavograptus ceryx and similar species) comprises biserials, including the three
just listed for the extraordinarius Zone, but excluding C. extraordinarius. Glyptograptus per-
sculptus itself and several closely related forms typify the persculptus Zone, but at least one
subspecies persists into the acuminatus Zone. Thus the persculptus fauna is similar to the
extraordinarius fauna, but differs in having the first uniserial scandent species, the very begin-
ning of a major evolutionary explosion of these forms, and more numerous biserial species,
especially glyptograptids.
Bull. Br. Mus. nat. Hist. (Geol) 43: 345-349 Issued 28 April 1988
346 R. B. RICKARDS
Distinctive features of the acuminatus Zone
The base of the zone is defined by the appearance of biserial graptolites with a characteristic
drawn-out, thorn-like proximal region involving elongate sicula, elongate early thecae and a
pronounced alternating arrangement of the thecal apertures. Two genera are involved: Akido-
graptus (type species A. ascensus Davies) with climacograptid-like thecae, and Parakidograptus
(type species P. acuminatus (Nicholson)) with orthograptid-like thecae. In the lower half of the
zone A. ascensus is usually much more abundant than P. acuminatus, the reverse obtaining in
the upper part of the zone. However, in sections with somewhat depleted diversity the two may
appear in sequence with a relatively short period of overlap. It cannot be emphasized too
strongly that in richly graptolitic sections the two species seem to occur throughout, with 4.
ascensus perhaps becoming extinct a little before P. acuminatus.
An additional parakidograptid P. acuminatus praematurus was described by Davies (1929)
from the lower half of the zone. Although this form has not yet been widely recorded, it has
considerable potential for correlation because it is a (morphologically) earlier form than the
type subspecies, having a less protracted proximal end which clearly indicates a typically more
robust biserial ancestor. It is likely that P. a. praematurus is restricted to the lower half of the
acuminatus Zone.
Another rare species occurring in the lower part of the acuminatus Zone is Atavograptus
ceryx, although this species is more common in the persculptus Zone. From unpublished
information and new specimens it seems likely that other, related, uniserially scandent forms
will be described from this zone. Subspecies of G. persculptus do occur at the base of the
acuminatus Zone, overlapping with Akidograptus and Parakidograptus, but there are also a
number of other undescribed glyptograptids in both the acuminatus and persculptus Zones,
often referred to as G. ex gr. tamariscus. Elucidation of these will clearly help refine correlation.
G.? avitus extends into the lower half of the zone from the persculptus Zone.
C. trifilis is recorded from the middle of the acuminatus Zone. This tiny form has a striking
three-fold spine at the base of the rhabdosome, presumably involving virgellar and antivirgellar
spines. Its relationship to C. tuberculatus from the persculptus Zone is not clear; and it should
be said that multispinose biserials in the Silurian are in general need of revision, as implied by
Rickards & Koren (1974). Cystograptus vesiculosus, which lends its name to the succeeding
zone in some broader zonal scenarios, occurs first of all in the upper part of the acuminatus
Zone, as does Climacograptus rectangularis, a presumed derivative of the earlier C. medius.
Finally in this section we should mention Orthograptus truncatus (=O. amplexicaulis), sensu
lato, which has been widely recorded from both the persculptus and lowest acuminatus Zones.
The taxonomic positions of these forms are uncertain: certainly forms I recently recorded from
Northern Ireland lack the proximal end spinosity of typical, earlier species, and in this sense at
least are more characteristically Silurian. The same is true of Hutt’s (1974) recordings of O. t.
abbreviatus.
Less diagnostic species of the acuminatus Zone
The most common species in most assemblages are relatively small climacograptids which
extend upwards from the Ordovician. Typical amongst these are C. normalis Lapworth, C.
angustus, C. innotatus Nicholson and the more robust C. medius. In addition the diplograptids
D. modestus and D. diminutus occur, the second possibly appearing in the acuminatus Zone,
though I hesitate to claim this with the certainty the literature suggests, simply because the
group is in dire need of revision. Other forms related to C. innotatus (sometimes referred to the
genus Paraclimacograptus) may occur, and I have already mentioned the undescribed glyp-
tograptids. In addition a number of sections round the world have a smaller number of forms
seemingly referable to the genus Pseudoclimacograptus (see next section). All the forms listed in
this section range upwards into the atavus Zone, and in some cases higher.
GRAPTOLITE FAUNAS 347
Species of local occurrence
In addition to the above species, modern work in several parts of the world has resulted in the
recognition of what are, at present, species of relatively local occurrence. Thus Pseudoclimaco-
graptus (P.) orientalis occurs in the Soviet Union, and may possibly do so in Poland (Rickards
1976: 159). In the Kolyma region Obut et al. (1967) record A. aff. priscus and Orthograptus
sinitzini as well as C. mirnyensis. The relationship of Orthograptus sinitzini to C. tuberculatus has
never been clarified and is another area worthy of further investigation, and in the recent
account of the geology of northeastern U.S.S.R. (Koren et al. 1983) P. aff. acuminatus praece-
dens is recorded. Of pseudoclimacograptids Koren & Mikhailova (1983) have recorded P. fidus
and P. pictus, and like forms have been found recently in the type Llandovery area (Cocks et al.
1984).
Waern (1948), in a careful revision of normalis-like climacograptids, described C. praemedius
and C. transgrediens, and also recorded C. indivisus Davies (previously only known from the
persculptus Zone).
The latest records from China are summarized by Mu (this volume), but it is worth noting
especially that several additional records of akidograptids have been made, such as A.
xixiangensis Yu et al. and A. parallelus Li & Jiao, as well as other biserial species as yet listed
only from China. It appears correct to say that China is the only country to date with a record
of the typical late Ordovician genus Paraorthograptus in the Silurian, i.e. in the acuminatus
Zone. Mu (this volume) also notes the presence of several subspecies of G. tamariscus, but
whether they are related to the later evolutionary burst of that group is not discussed.
Top of the acuminatus Zone
It is necessary by international agreement to define only the base of a zone. Nevertheless, it is
useful here to outline what distinguishes the acuminatus Zone from the overlying atavus Zone.
Basically the demise of the akidograptids is followed by increased diversification of the uniserial
scandent monograptids belonging to several genera (Atavograptus, Lagarograptus and
Coronograptus) as well as by numbers of dimorphograptids. Only in one section have akido-
graptids been recorded from the vesiculosus Zone, namely in Sardinia (Jaeger 1976). There is
some overlap, naturally, but the two faunas could hardly be much more different than they are.
Finally it is clear that the acuminatus Zone is capable of being subdivided in useful fashion, a
step already taken by Teller (1969) for example, and in effect, by Stein (1965; see also Jaeger,
this volume). In most sections a lower, middle, and upper part can be identified, not only upon
the occurrence of akidograptids and parakidograptids, but also on the occurrence of such
species as A. ceryx, C. trifilis, Cy. vesiculosus, C. rectangularis and so on. The revision of other
groups, so necessary at present, will undoubtedly increase the potential not only for interna-
tional correlation at this level, but also for subdivisions of the presently defined acuminatus
Zone.
Conclusions
The acuminatus fauna is not only distinctive and easily recognizable, but is widespread in the
world, as the other sections in this volume make clear. The akidograptids and parakidograp-
tids, whatever the species or subspecies, seem to be almost totally restricted to the zone. The
zonal assemblage forms not only a gradual change between the persculptus and atavus Zones,
but represents a distinctive stage in the evolution of Silurian graptoloids reflecting a very
advanced stage of post-glacial marine transgression and the development of widespread anaer-
obic black shales and the re-establishment of a rich, marine, tropical plankton.
(es
GRAPTOLITE FAUNAS 349
References
Cocks, L. R. M., Woodcock, N. H., Rickards, R. B., Temple, J. T. & Lane, P. D. 1984. The Llandovery
Series of the type area. Bull. Br. Mus. nat. Hist., London, (Geol.) 38 (3): 131-182.
Davies, K. A. 1929. Notes on the graptolite faunas of the Upper Ordovician and Lower Silurian. Geol.
Mag., London, 66: 1—27.
Hutt, J. E. 1974. The Llandovery graptolites of the English Lake District. Part I. 56 pp., 10 pls. Palaeon-
togr. Soc. (Monogr.), London.
Jaeger, H. 1976. Das Silur und Unterdevon von thiiringischen Typ in Sardinien und seine
regionalgeologische Bedeutung. Nova Acta Leopoldina, Halle a.S., 45 (224): 263-299, pls 1-3.
Jones, O. T. 1909. The Hartfell-Valentian succession in the district around Plynlimon and Pont Erwyd
(North Cardiganshire). Q. JI geol. Soc. Lond. 65: 463-537, pls 1, 2.
—— & Pugh, W. J. 1916. The geology of the district around Machynlleth and the Llyfnant Valley. Q. JI
geol. Soc. Lond. 71: 343-385.
Koren, T. N., Oradoyskaya, M. M., Pylma, L. J., Sobolevskaya, R. F. & Chugaeva, M. N. 1983. The
Ordovician and Silurian boundary in the Northeast of the U.S.S.R. 207 pp. Leningrad, Nauka.
—— & Mikhailova, N. 1980. In M. K. Apollonov, S. M. Bandaletov & I. F. Nitikin (eds), The Ordovician—
Silurian Boundary in Kazakhstan. 300 pp. Alma Ata, Nauka Kazakh S.S.R. Publ. Ho.
—— & Rickards, R. B. 1979. Extinction of the Graptolites. Spec. Publs geol. Soc. Lond. 8: 457-466.
Lapworth, C. 1878. The Moffat Series. Q. JI geol. Soc. Lond. 34: 240-346.
Rickards, R. B. & Koren, T. N. 1974. Virgellar meshworks and sicular spinosity in Llandovery graptoloids.
Geol. Mag., Cambridge, 111: 193-202.
Stein, V. 1965. Stratigraphische und paldontologische Untersuchungen im Silur des Frankenwaldes. N. Jb.
Geol. Palaont. Abh., Stuttgart, 121: 111—200, pls 1-2.
Teller, L. 1969. The Silurian biostratigraphy of Poland based on graptolites. Acta geol. Pol., Warsaw, 19:
393-501.
Toghill, P. 1968. The graptolite assemblages and zones of the Birkhill Shales (Lower Silurian) at Dobb’s
Linn. Palaeontology, London, 11: 654-668.
Waern, B. In B. Waern, P. Thorsland, G. Henningsmoen & G. Sadve-Sdderbergh 1948. Deep boring
through Ordovician and Silurian strata at Kinnekulle, Vastergotland. Bull. geol. Instn Univ. Uppsala
32: 337-474.
Williams, S. H. 1983. The Ordovician—Silurian boundary graptolite fauna of Dob’s Linn, southern Scot-
land. Palaeontology, London, 26: 605-639.
Fig. 1 Typical acuminatus Zone assemblage. Specimens in Sedgwick Museum, Cambridge. a, Cli-
macograptus medius Tornquist, A20150; b, Climacograptus normalis Lapworth, A20090; c, Paracli-
macograptus innotatus (Nicholson), A20226; d, Glyptograptus sp., X.9999; e, Climacograptus
rectangularis M‘Coy, A20067; f, Glyptograptus avitus Davies, A10019, figd Davies, 1929: 8, fig. 21;
g, h, part and counterpart of Akidograptus ascensus Davies, X.9996a, b; i, Climacograptus angustus
Perner (=C. miserabilis Elles & Wood), X.9993; j, Parakidograptus acuminatus (Nicholson),
A75394; k, Parakidograptus praematurus (Davies), A10023, figd Davies, 1929: 10, fig. 25; 1,
Orthograptus sp. (? ex gr. amplexicaulis Hall), X.9995; m, n, Diplograptus modestus Lapworth,
respectively A20425 & A20428; 0, p, Glyptograptus persculptus (Salter), sensu lato, figd Davies,
1929: 14, respectively figs 15 and 20 as ‘mut. omega’, A10013 and A10018, the latter being regarded
as holotype; q, Cystograptus vesiculosus (Nicholson), X.9994; r, s, Climacograptus trifilis Manck,
respectively X.9998 and biprofile view showing virgellar spine only, X.9997. All figures x S.
a BP x
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Land plant spores and the Ordovician—Silurian
boundary
J. Gray
Department of Biology, University of Oregon, Eugene, Oregon, U.S.A. 97405
Synopsis
The size of early tetrad spores can be used to differentiate in a general way between late Ordovician and
early Silurian rocks, although not to a fine degree of accuracy. No single trilete spores are found in
Ordovician or earliest Llandovery rocks. Spores measurements are presented from the Ashgill of
Bohemia, Canada and U.S.A. and the early Llandovery of U.S.A., Sweden, South Africa and Brazil.
Introduction
Early land plants can be traced through spores, having morphological analogues with spores
produced by some living hepatics, back to the mid-Ordovician, about Llanvirn—Llandeilo time
(Gray et al. 1982; Gray 1985), when recognizable remains, in terms of modern analogues,
disappear. Abundant spores occur in a number of Late Ordovician (Ashgill) and in many early
Silurian (Llandovery) rocks immediately above and below the Ordovician-Silurian boundary,
and in some successions straddling the boundary as defined by marine invertebrates and
phytoplankton. Spores occur in continental strata for this interval; they are principally abun-
dant, doubtless related to intense weathering and often extensive metamorphism of continental
rocks of this age (Gray & Boucot 1975), in shallow-water, nearshore marine rocks where other
biostratigraphically useful microfossils and invertebrates are absent or inadequate for correla-
tion. Land plant spores may ultimately prove to be the most useful fossils for helping to fix the
approximate position of the Ordovician—Silurian boundary in that environment.
Gray (1985) assigned Late Ordovician—Early Silurian spores to Microfossil Assemblage Zone
I. MA Zone I is a homogeneous assemblage of spores of a single morphological type: compact
tetrads arranged in a tetrahedral configuration with a mean size generally less than 35 microns,
and usually smooth-walled. No single, trilete spores are found in Ordovician or earliest Llando-
very rocks, although they appear, locally, in small numbers about midway through MA Zone I.
Tetrads can be assigned for the most part to Tetrahedraletes cf. T. medinesis, although this does
not necessarily mean that they all represent a single taxon, since spore ‘morphological species’
have different taxonomic values, representing anything from families and family groups to
species or subspecies. Spore tetrads are found in Late Silurian assemblages but they do not
dominate in the post-Early Silurian, where they are replaced by single trilete spores, smooth-
walled and with varied types of wall ornamentation, which find their closest morphological
analogue in spores of lower vascular plants. Locally, in Ordovician-Silurian rocks from the
central and southern Appalachians and the midcontinent of North America, tetrads with a
reticulate surface ornamention also occur in Microfossil Assemblage Zone I, beginning in the
Ashgill and continuing through the early and middle Llandovery and early part of the late
Llandovery. In North America, tetrads with other ornament types appear about midway
through the Llandovery (Gray et al. 1986: fig. 5). Tetrads with reticulate surface ornamentation
have also been found in samples from Gotland, Sweden, in earliest Silurian and Ordovician—
Silurian boundary rocks but have not otherwise been convincingly identified elsewhere below
the Silurian, although Vavrdova (1984) claims the presence of varied ornamentations among
spore tetrads from the Kosov Formation of Bohemia. I did not see these on spore tetrads
extracted in my laboratory from one rock sample kindly sent to me by M. Vavrdova.
Attention has focussed on the Ordovician-Silurian boundary, and the Ashgill, a time of
glaciation and widespread marine regression, as one of a small number of intervals of mass
extinction among marine invertebrates and phytoplankton. Spore tetrad assemblages show no
Bull. Br. Mus. nat. Hist. (Geol) 43: 351-358 Issued 28 April 1988
3s J. GRAY
clearly defined changes across the Ordovician—Silurian boundary to indicate that land plants
were in any way affected by the circumstances responsible for severe extinction in latest Ordo-
vician shallow seas. There is no basic change in spore assemblages at the systemic boundary, no
‘turnover’ related to first or last appearances of spore types, or change in relative frequency of
spore types on either side of the boundary.
The principal change that can be demonstrated for spore tetrads in Microfossil Assemblage
Zone I is an increase in size from tetrads with average diameters under 30 microns in the
Ordovician to tetrads with average diameters close to 50 microns near the end of Microfossil
Assemblage Zone I in the mid-late Early Silurian (Gray et al. 1986). The consistent change in
tetrad size is useful for determining the stratigraphical position within Microfossil Assemblage
Zone I; change in tetrad size is less useful for discriminating the precise age of rocks to either
side of the Ordovician-Silurian boundary, although tetrad size is useful for approximating the
position of the boundary and for discriminating rocks close to the boundary from units of
younger Llandovery age.
Spores are now known (Appendix, p. 356) from rocks deposited near the boundary from the
midcontinent and Appalachians of North America; Manitoulin Island, Ontario, Canada;
Brazil; Czechoslovakia; Gotland, Sweden; Libya; South Africa; and Arabia. At few of these
localities is there independent information based on fossiliferous facies, shelly or graptolitic,
bearing on the precise age relations of the rocks. However, marine palynomorphs (organic-
walled phytoplankton: including prasinophyte phycomata and ‘acritarchs’) show an ‘abrupt
turnover’ at the Ordovician—Silurian boundary related to change in phytoplankton
assemblages coincident with extinction of many Ordovician species, in some southern Appa-
lachian sections that are also spore-bearing. These have been used to position the systemic
boundary in the absence of invertebrate fossils (Colbath 1983, 1985). In the absence of indepen-
dent palaeontological evidence, the approximate stratigraphical position of measured spores
assemblages relative to the Ordovician—Silurian boundary can be fixed, at least in North
American sections, by the unconformity and lithological discontinuity at the systemic boundary
itself (see Bergstrom & Boucot, this volume, p. 273).
Elmina Sandstone, West Africa
Spore tetrads have also been recovered from the Elmina Sandstone (lower Sekondi Series) from
the vicinity of Sekondi—Takoradi, on the southwest coast of Ghana, West Africa. The Elmina
was believed to straddle the Ordovician—Silurian boundary by Bar & Reigel (1980), who based
their age assignment on marine phytoplankton (‘acritarchs’), and in particular Dactylofusa, a
taxon also found in strata assigned to the Itaim Formation, Maranhao (=Parnaiba) Basin,
Brazil by Brito (1967: 480). Brito correlated his Palynological Zone T, from the Itaim, charac-
terized by Dactylofusa maranhensis, with the Trombetas Formation of the Amazon Basin,
regarded as ‘probably Lower Silurian in its upper part and Upper Ordovician in its lower part’
from the occurrence of Climacograptus, a taxon then mistakenly believed to occur only in the
Lower Silurian. However, the marine, fossiliferous part of the Trombetas Formation can now
be regarded as post-Lower Silurian (post-Llandovery) and probably Ludlow to possibly Gedin-
nian in age (Gray, unpublished spore data; P. Janvier, unpublished vertebrate data; F. Paris,
unpublished chitinozoan data; L. Quadros, unpublished acritarch data 1985). Thus, Brito’s
assignment of Palynological Zone T from the Maranhao Basin subsurface and the coeval part
of the Trombetas Formation from the Amazon Basin to the Lower Silurian—Upper Ordovician
is in error. Moreover, I have recovered from the lower Trombetas, well below sections yielding
marine phytoplankton, chitinozoans and vertebrates, spore tetrads of Microfossil Assemblage
Zone I. Additionally Lange (1972: 38) concluded that strata from the Maranhao Basin which
Brito (1967: 480) correlated with the Trombetas Formation of the Amazon Basin on the basis
of shared acritarchs should be assigned to the Serra Grande Formation ‘probably of Silurian
age’ and possibly representing lower and part of the middle Llandovery. Colbath (personal
communication 1986) regarded the microfossil evidence provided by Bar & Reigel as inconclu-
sive: he wrote *... they haven’t illustrated any taxa which require an Ordovician age. They
appear to be on safe ground in concluding that the flora is pre-Devonian, but exactly where it
LAND PLANT SPORES 353
belongs in the Silurian is a bit tricky. The diversity of the assemblage suggests an age of
approximately middle Llandovery or younger (as does the presence of Veryhachium carminae),
but that may be an artifact of sampling... Their identification of Dactylofusa maranhensis
appears reasonable, and does suggest correlation with the Itaim Shale in Brazil.’
Spore tetrads in the Elmina Sandstone confirm a Llandovery age assignment and indicate
that the Elmina is older than Brito’s Palynological Zone T in the Maranhao Basin and the
marine upper Trombetas Formation in the Amazon Basin, but possibly correlative with lower
Trombetas that also yields spore tetrads. The large size of the Elmina tetrads (23 (37:8) 50)
based on 100 (G1473) measurements suggests mid-Llandovery rather than close to the
Ordovician-—Silurian systemic boundary. Finally, the sample of Elmina Sandstone collected by
Bar & Reigel and later by Gray & Boucot came from a fault sliver in a badly faulted zone (all
that was available). There is no assurance that this sample was near the Ordovician-Silurian
boundary and there is no palaeontological evidence that requires an age near the boundary.
Manitoulin Island, Ontario, Canada
Spore tetrads come from a palaeokarst sample at, or very close to, the Ordovician—Silurian
systemic boundary. The palaeokarst, represented by two surfaces, lies between the Late Ordovi-
cian (Ashgill) Kagawong beds and the basal beds of the Early Silurian (Llandovery) Manitoulin
Formation on Manitoulin Island, Lake Huron, Ontario, Canada (Kobluk 1984). The boundary
lies within the 0-5 m which includes the palaeokarst surfaces, but its exact position is controver-
sial. Kobluk, who collected the samples, interprets the palaeokarsts as erosional disconformities
which mark subaerially exposed surfaces that resulted from lowered sea-level at the close of the
Ordovician.
Midcontinental eastern North America
Spore tetrads have been noted (Gray & Boucot 1972) in latest Ordovician—earliest Silurian
beds to either side of the paraconformity that marks the boundary at Ohio Brush Creek, Ohio.
Grahn & Bergstrom (1985: 179) have indicated, from chitinozoans, that this stratigraphical gap
represents an interval from the Ashgill Didymograptus complanatus Zone to the early Llando-
very Climacograptus cyphus Zone and ‘hence corresponds to about four graptolite zones —the
upper Ashgill (Hirnantian or Gamachian stage) and three graptolite zones of the lowermost
Llandovery. Thus the uppermost tetrad-containing Preacherville is no younger than middle
Ashgill. Measured spore tetrads represent a single sample from the Preacherville Member of the
Drakes Formation (called Elkhorn Formation in Gray & Boucot 1972, Gray et al. 1986: fig. 5)
and two samples from the lowermost Silurian Belfast Member of the Brassfield Formation
(G1385, G1386 from the base of the lower bed; G1384 from 10 inches above G1385 and
G1386).
Eastern North America
In New York, north central Pennsylvania, southwestern Virginia, southeastern Tennessee, and
northwestern Georgia various rock units to either side and encompassing the Ordovician—
Silurian boundary have yielded measurable spore tetrads. These include various Llandovery
formations: Whirlpool (Niagara Gorge, New York: Bolton 1957; Martini 1971; Gray &
Boucot 1971), Tuscarora (Millerstown, Pennsylvania: Cotter 1982), Hagan Shale Member,
Clinch (Hagen, Virginia: Miller & Fuller 1954), Red Mountain (Ringgold, Georgia: Chowns &
Howard 1972), and Rockwood (Green Gap and Nickajack Dam, Tennessee: Milici & Wedow
1977). Ashgill Formations include: Red Mountain (Ringgold, Georgia), Shellmound (Nickajack
Dam, Tennessee) and Sequatchie (Ringgold, Georgia; Green Gap, Tennessee). There is little
independent invertebrate evidence for the age of these shallow-water, nearshore rocks to either
side of the Ordovician-Silurian boundary in most of these sections and the amount of section
missing at the systemic boundary may be both variable and considerable. The marked change
in phytoplankton in boundary rocks reported by Colbath (1983, 1985) is the basis for posi-
tioning the boundary within a number of these stratigraphical units, including the Hagan,
Nickajack Dam, Green Gap, and Ringgold Sections. Neither the Tuscarora Sandstone nor the
Whirlpool (Medina Group) contains diagnostic invertebrate fossils for correlation (Berry &
Boucot 1970), although field relations suggest that the lower Tuscarora, in the Millerstown
LAND PLANT SPORES 355
Section (Cotter 1982) and the Whirlpool, at Niagara Gorge, are early Llandovery (Gray &
Boucot 1971).
Brazil
The presence of Silurian rocks in the Parana Basin, Brazil, has long been at issue. Spore tetrads
and phytoplankton (acritarchs and prasinophytes) are both consistent in suggesting a Llando-
very age for the Vila Maria Formation, northeast Parana Basin, southern Brazil, although
Gray et al. (1985: 524) noted that the spore tetrads are similar in size to Late Ordovician and
earliest Silurian tetrads whose average sizes are 27 to 29 microns. The Silurian age of the Vila
Maria is, however, consistent with the regional geology, including the regional absence of
Ordovician rocks.
Sweden
In southern Gotland, well cores at Nar and Gr6tlingbo include the entire Silurian below the
Wenlock—Ludlow, based on age references provided by Monograptus spp., and penetrate the
Ordovician-Silurian boundary; in the Nar core at 380:50m (Snall 1977). However, lowermost
Silurian graptolites (M. cometa Zone?) are first found at 369m (S. Laufeld, personal communi-
cation to A. Le Herisse). According to Le Herisse (personal communication) acritarch
assemblages between 385-50 and 380:50m are Late Ordovician in age, but the interval 380—
372 m, characterized by red beds, is largely devoid of organic microfossils, and the ‘real Silurian
transgression’ begins at 372m where acritarchs and other organic microfossils are abundant.
Rare spore tetrads were recovered from Nar samples (379, 380, 380-50, 382-50, 384m) by A. Le
Herisse, who kindly provided photographs of specimens and small splits of the cores. From
three of these samples, 379m, 380m and 380:50m at the Ordovician—Silurian boundary as
positioned by Snall, and 380m, I recovered sufficient spores to measure.
Czechoslovakia
The Kosov Formation, at Hlasna Trevan near Beroun, on the Berounka River, central
Bohemia, has yielded spore tetrads illustrated and described by Vavrdova (1982, 1984). The
Kosov Formation corresponds to the latest Ordovician, Upper Ashgill Glyptograptus bohe-
micus Zone (Havli¢ek & Vanék 1966; Havli¢ek & Marek 1973). Vavrdova was kind enough to
provide a sample of the Kosov Formation from which abundant spore tetrads were recovered.
South Africa
Spore tetrads are known from the basal Soom Shale Member of the Cedarberg Formation,
Table Mountain Group, southwestern Cape Province, South Africa. As discussed by Gray et al.
(1986), the age of the Cedarberg Formation has been variously interpreted as latest Ordovician
(Ashgill) to earliest Silurian (Llandovery) on the basis of limited invertebrate information.
Cramer et al. (1974) bracketed the Soom Shale as latest Ordovician—earliest Silurian by chitin-
ozoans, but favoured an Ashgill age because of brachiopod data (Cocks & Fortey 1986). Spore
size is inconclusive. The measured eight samples also bracket the age of the basal Soom Shale
as latest Ordovician—earliest Silurian. However, J. N. Theron’s recent discovery of conodont
assemblages there, considered to be of late Ordovician age by a number of specialists, confirms
an Ashgill age for the unit.
Conclusions
These preliminary results, with size frequency measurements, show that the Ordovician—
Silurian boundary is bracketed by spore assemblages with spore tetrads having average sizes
Figs 1-6 Scanning electron micrographs of obligate tetrahedral tetrads of spores typical of Micro-
fossil Assemblage Zone I (Gray 1985). Magnification x 1500. All from the Ashgill Preacherville
Member, Drakes Formation, Ohio Brush Creek Section, Kentucky, U.S.A. (G1285). Most spore
tetrads from Microfossil Assemblage Zone I are smooth-walled (Figs 1, 5), and some have an outer
envelope that may be shed. The outer envelope is most commonly reticulate (Figs 2, 4, 6). Fig. 3
shows a spore tetrad with a smooth-walled envelope or possibly a degraded reticulate envelope.
356 J. GRAY
less than 30 microns. The average size of spore tetrads to either side of the boundary, as
positioned by palaeontological or micropalaeontological data, or by a stratigraphical gap and
change in lithology, is about 26-29 microns, although there are both smaller (Sequatchie
Formation) and larger spore tetrads (Manitoulin palaeokarst) known from rock units close to,
or at, the systemic boundary. Slight differences in spore tetrad size on opposite sides of the
boundary are inadequate, without other evidence, to distinguish latest Ordovician from earliest
Silurian age rocks, although the Ordovician—Silurian boundary is easily bracketed by spore
assemblage measurements.
I have no explanation for the relatively small size of the spore tetrads from the Sequatchie
Formation. The measured samples may be lower in the Sequatchie, i.e. older, than now recog-
nized in terms of their stratigraphical position relative to the Ordovician-Silurian boundary,
possibly related to the presence of a significant disconformity. I have no independently dated
assemblages from within the Ashgill for comparative purposes. With small microfossils, there is
always the possibility of independent size-sorting, since these fossils behave as clastic sedimen-
tary particles with hydraulic equivalents in the fine or very-fine silt size fraction (Stanley 1969;
Muller 1959; Brush & Brush 1972). Water turbulence can keep large quantities of pollen or
spores in suspension for extended periods, and it may be that the smaller spore tetrads of the
Sequatchie were winnowed from the spore assemblage through progressive sorting and depos-
ited with finer mineral particles, possibly in a more off-shore environment than represented by
the depositional environments of many of the other units, or in a pattern related to marine
currents or some other hydrodynamic factors. This phenomenon may also account for some of
the inconsistencies found in a few of the other measurements. The large size of the Manitoulin
tetrads is not consistent with the other results and a more serious threat to the utility of
spore-size measurements for discriminating the Ordovician—Silurian boundary, since the strati-
graphical position of the sample seems well fixed. The comparatively large size of these spores,
for which only relatively few measurements were available, may reflect the fact that this sample
was not originally extracted for spores, but for arthropod cuticle remains, so that smaller
tetrads may have been lost in the sieving process. This material is being re-extracted specifically
to recover spores and measurements repeated on a larger number of spore tetrads.
Acknowledgements
I would like to thank A. J. Boucot for discussion of stratigraphical data, A. Le Herisse and M. Vavrdova
for supplying sediment samples, and G. K. Colbath for information on the acritarchs from the Elmima
Formation.
Appendix
Size measurements of Ashgill and Early Llandovery spore tetrads
Lower Llandovery N Min. Aver. Max.
Robert Moses Power Plant Section, Niagara Falls, New York
Whirlpool Sandstone (G1189) 250 13 26:5 44
Millerstown Section, Pennsylvania
Tuscarora Formation (G1408) 100 18 27-0 41
Tuscarora Formation (G1407) 100 17 27:3 47
Tuscarora Formation (G1406) 100 i7/ 28-0 49
Tuscarora Formation (G1374) 150 16 27°5 45
Nickajack Dam Section, Tennessee
Rockwood Formation (ND70) 41 a) 33-6 51
Rockwood Formation (NDS54) 107 15 29-9 53
Ringgold Section, Georgia
Red Mountain Formation (RN570) 200 15 26-6 39
Red Mountain Formation (RN470) 86 17 29-4 48
Red Mountain Formation (RN420) 45 7/ 27:8 39
Red Mountain Formation (RN370) 148 13} 25-6 38
Red Mountain Formation (RN320) 98 13 26-9 45
LAND PLANT SPORES 397/
Hagan Section, Virginia N Min. Aver. Max.
Hagan Shale Member, Clinch Formation (HGII70) 200 13 29-2 49
Hagan Shale Member (HGIIS50) 135 19 32:2 54
Hagan Shale Member (HGII30) 87 18 29-7 49
Hagan Shale Member (HGII10) 215 13 DES 47
Ohio Brush Creek Selection, Ohio
Belfast Member, Brassfield Formation (G1384) 100 17 26:9 39
Belfast Formation (G1385) 100 18 DiS 45
Belfast Formation (G1386) 150 17 27-0 40
Narborrningen 1, southern Gotland, Sweden
Unnamed formation, 379:00 m (G1553) 25) 19 28:8 41
Unnamed formation, 380-00 m (G1549) 69 20 28-6 52
Unnamed formation, 380-50 m (G1548) 34 19 29-8 40
Fazenda Tres Barras Section, Brazil
Vila Maria Formation (G1391) 150 18 29-1 42
Ashgill
Swartleikloff Section, South Africa
Soom Shale Member, Cedarberg Formation (G1363) 100 15 27-5 40
Soom Shale Member (G1364) 100 17 28:5 37/
Soom Shale Member (G1365) 100 20 28-4 41
Soom Shale Member (G1366) 100 17 27:5 39
Soom Shale Member (G1367) 100 Dp 30-5 40
Soom Shale Member (G1368) 100 17 28:7 40
Soom Shale Member (G1369) 100 20 29-6 43
Soom Shale Member (G1370) 100 17 29:2 45
Combined average 800 15 28-8 45
Hlasna Treban Section, Bohemia 108 16 28-0 47
Kosov Formation (G1430)
Paleokarst at Ordovician-Silurian systemic boundary,
Manitoulin Island, Ontario (G1272) 45 22 33-0 46
Ohio Brush Creek Section, Kentucky
Preacherville Member, Drakes
Formation (G1285) 252 17 DIPS) 53
Green Gap Section, Tennessee
Sequatchie Formation (GG19) 58 12 22-0 32
Nickajack Dam Section, Tennessee
Shellmound Formation (ND33) 150 18 26:5 50
Shellmound Formation (ND20-5) 141 16 27-7 43
Ringgold Section, Georgia
Red Mountain Formation (RN210) 59 16 25:6 41
Red Mountain Formation (RN201) 89 14 23-9 46
Sequatchie Formation (RN195 = G1245) 200 11 23-2 40
Sequatchie Formation (G1245) 100 11 24-0 46
Sequatchie Formation (G1246) 100 14 23-7 50
Sequatchie Formation (RN139) 66 14 24-4 35
Notes: The samples are in stratigraphical order within each section, with the youngest at the top. G
numbers are Gray extractions; others are Colbath extractions. G1385, G1386 were measured from
samples collected along the strike.
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Trilobites
P. J. Lesperance
Département de Geologie, Université de Montréal, Casier Postal 6128, Montréal,
Canada H3C 3J7
Synopsis
Hirnantian (latest Ordovician) trilobite faunas are surveyed. Some are of restricted diversity, but others
are highly diverse. A coeval trilobite fauna from the Gamachian Stage of Anticosti Island is highly diverse,
but of different zoogeographical affinity. Dalmanitina—Mucronaspis occurrences, of putative Silurian age,
and usually with other shelly fossils, are discounted. The base of the acuminatus Zone may correlate with
the first appearance of the trilobite Acernaspis in shelly sequences but this awaits confirmation. The
systematics of spinose hypostomata within the Dalmanitidae are critically examined, and it is concluded
that the concept of Mucronaspis requires that spinose hypostomata be present before the generic name is
applied. A lectotype of Mucronaspis danai (Meek & Worthen 1866) is selected. Dalmanitina (Songxites) sp.
(nov.) from Dob’s Linn and Mucronaspis danai from Illinois and Missouri are illustrated and described.
Mucronaspis mucronata and Acernaspis norvegiensis are also illustrated, but only briefly discussed. Acer-
naspis (Acernaspis) salmoensis sp. nov. from Anticosti Island and Cryptolithus portageensis sp. nov. from
Percé are erected.
Introduction
The correlation of the base of Silurian, as defined at Dob’s Linn, Scotland, using trilobites is
difficult as major changes in trilobite faunas occur near, or at, this boundary. In the following,
the term Hirnantian (stage) will be used for the strata immediately underlying the acuminatus
Zone, while the Rhuddanian is the oldest Silurian stage; the Hirnantian, however, has not been
approved by the International Union of Geological Sciences.
The disappearance of many trilobite genera and families in the latest Ordovician is well
known. Thomas et al. (1984: 39) noted that the change from the Ashgill stages Rawtheyan to
Hirnantian, in England and Wales, entailed the disappearance of many genera and important
Ordovician families such as the agnostids, Trinucleidae, Remopleurididae, Telephinidae, Cyclo-
pygidae, Asaphidae, Dionididae and Phillipsinellidae. Trinucleidae are now known to extend
into the Hirnantian (see below). Asaphidae (from Scotland) and Cyclopygidae (from Ireland)
were, however, reported from the Hirnantian by Thomas et al. (1984: 41, 44). The Hirnantian
Stage is reputed for its distinctive impoverished trilobite faunas (see also Lespérance 1974),
although the degree and nature of impoverishment is variabie from region to region. It would
thus appear from these and other data that the major trilobite extinction was near the
Rawtheyan—Hirnantian boundary, and not at the base of the Silurian.
Lespérance (1985) attempted to correlate the base of the acuminatus Zone with shelly
sequences. He noted an ordered succession of appearances of faunas and taxa on Anticosti
Island and elsewhere: the Oulodus? nathani (conodont) Zone, followed upward by the brachio-
pods Zygospiraella, succeeded by Stricklandia, then the trilobite Acernaspis, and finally the
brachiopod Virgiana. Only the appearance of Acernaspis seemed to coincide with the base of
the acuminatus Zone, when compared with the Oslo region (Norway) and the USSR
(Kazakhstan and northeast USSR). This acuminatus—Acernaspis correlation has still to be
further tested and confirmed, but no additional data have since come to light to contradict or
reaffirm it; it is therefore accepted and used herein.
The recognition of trilobite faunas immediately younger than the base of the acuminatus
Zone is exceedingly difficult if one excludes Acernaspis. Trilobite genera recorded from lower-
most Silurian (Rhuddanian) strata consist of holdovers from the Ordovician, and show little
change from their ancestors. This apparent lack of change may, however, be due more to the
scarcity of monographic treatment, poor preservation and/or, more probably, to infrequent
Bull. Br. Mus. nat. Hist. (Geol) 43: 359-376 Issued 28 April 1988
360 P. J. LESPERANCE
preservation, than to lack of evolution. Stenopareia, aulacopleurids, proetids and calymenids,
although apparently common, seem to show little change, or, at least, stratigraphically useful
species have not been recognized. Lichids and odontopleurids are scarcer, but still widespread;
again stratigraphically useful species are not evident. Homalonotids are even scarcer. All these
lowermost Silurian taxa should be reviewed in the light of new material.
Rhuddanian trilobite faunas are perhaps notable by the presence of a limited number of
Ordovician holdovers. Examples are Cyphoniscus cf. socialis (Salter 1853) associated with Acer-
naspis (A.) primaeva (Clarke 1908) and other trilobites in the Matapédia Group north of Percé
(Lespérance in Ayrton et al. 1969: 476;,Dean 1972), and Hadromeros which has been widely
reported lately in Rhuddanian strata. Lane (in Thomas et al. 1984: 53) reports the presence of
Panarchaeogonus and Ceraurinella in the later Llandovery, so that these otherwise typical
Ordovician genera must also have been present in the Rhuddanian. Sphaerocoryphe has also
been reported from an unspecified level in the Silurian (Thomas & Lane 1984: 62). Thomas et
al. (1984: 52) state that the following genera are unknown from the Ordovician: Warburgella
(Warburgella), Harpidella (Harpidella), Dalmanites, Anacaenaspis, Podowrinella, Calymene s.s.
and Acanthopyge (but see below). All in all, early Silurian trilobite faunas appear to be charac-
terized by the absence of specialized Ordovician families and genera, and by the presence of
‘generalized’ forms, rare new ones (notably Acernaspis), and some holdovers from the Ordovi-
cian. The ‘generalized’ trilobites yield in many instances (in the later Silurian and Early
Devonian) specialized and distinctive descendants. The early Silurian trilobite faunas thus stand
between distinctive and specialized faunas, both older and younger.
This contribution will consequently focus on a certain number of biostratigraphically useful
taxa which were abundant, or at least well known, in the latest Ordovician or earliest Silurian.
Hirnantian and Gamachian trilobite faunas
Lespeérance (1974) surveyed Hirnantian brachiopod and trilobite faunas. Some of this is still
pertinent, but must be viewed in the light of the recently promulgated acuminatus boundary.
Subsequent data from the midcontinent of the USA (Amsden 1974), China (Nanjing Institute
1984), Wales (Cocks & Price 1975; Cocks et al. 1984), Norway (Brenchley & Cocks 1982), and
the USSR (Apollonov et al. 1980; Koren et al. 1983) have since been added.
Precise correlations of shelly faunas near and at the acuminatus boundary are hampered by
the lack of continuous thoroughly investigated sections possessing enough elements in common
to correlate. The Anticosti (and, accessorily, Percé) and Oslo region sections are at present
those that are easiest to correlate, and they permit, in turn, additional correlations with other
sections. The basal Oulodus? nathani Zone occurs in the lower part of member 7 of the Ellis
Bay Formation on Anticosti, and this zone also occurs very low in the Solvik and Selabonn
Formations of the Oslo region (Worsley 1982; Lespérance 1985). It is inescapable that the
ecologically complex and diverse faunas of the latest Ordovician ‘Sa’ and ‘5b’ of the Oslo region
(Brenchley & Cocks 1982) must correlate with strata below the lower part of member 7 on
Anticosti. Only ‘5b’ (Langeyene and Langara Formations) is Hirnantian, whereas the lower
boundary of the Gamachian (at the base of the Ellis Bay Formation) is older than the base of
the Hirnantian (it occurs 34m above the base of the 130m thick Burmingham Member in the
Percé area, Lespérance, this volume). To compare Hirnantian faunas on Anticosti and the Oslo
region, it is necessary to draw the base of the Hirnantian within the Ellis Bay. As no drastic
drop in diversity is apparent in the Ellis Bay (as present in the type Rawtheyan—Hirnantian),
quite to the contrary, members | and 2 are arbitrarily excluded from the following discussion
(representing a thickness comparable in proportion to the Percé strata). Pre-Oulodus? nathani
Zone trilobites common to Anticosti and the Oslo region are Platycoryphe and Toxochasmops.
Calyptaulax, Decoroproetus, Dicranopeltis, Harpidella, Illaenus, Mucronaspis, Panderia and
Stenopareia are only known from ‘Sb’, whereas Amphilichas (two species), Cyphoproetus,
Erratencrinurus (Celtencrinurus), Failleana, Hemiarges, Isotelus, Lichas, Nahannia, Otarion,
Paraharpes and Sphaerocoryphe are only known from Anticosti (Bolton 1981; Brenchley &
TRILOBITES 361
Cocks 1982; Chatterton et al. 1983; and the writer’s unpublished data). To compare post-basal
Oulodus? nathani and pre-acuminatus trilobite faunas from the same areas, all ‘6a’ and ‘6b’
occurrences are presumed to predate the first occurrence of Acernaspis (this is probably too
generous, as it first occurs in the upper half of “6ba’) (data are from Chatterton et al. 1983;
Helbert et al. 1982; and the writer’s unpublished data). Cyphoproetus, Diacalymene, Harpidella
and Stenopareia occur in both areas, but Amphilichas, Astroproetus, Failleana, Illaenoides,
Leonaspis and Primaspis occur only on Anticosti, while Arctinurus, Calymene, Dicranopeltis and
Hadromeros only in the Oslo region.
From the above survey, it is clear that there are few Hirnantian genera in common between
Anticosti and the Oslo region, which suggests significant zoogeographical differences. If one
tabulates the genera restricted to either region, throughout the whole Hirnantian, 23 are
counted. Of these, 9 can be considered long-ranging, and 11 seem to be typical Ordovician
genera at the end of their biozones (Amphilichas, Calyptaulax, Erratencrinurus (Celtencrinurus),
Failleana, Illaenus, Isotelus, Mucronaspis, Nahannia, Panderia, Paraharpes, Primaspis). The
remaining three (Arctinurus, Calymene and Illaenoides) are more typical of the Silurian, and
their biozones should consequently be extended downwards. The genera common to both in
pre-Oulodus? nathani strata are typical Ordovician ones, while those common to both in
post-Oulodus? nathani strata are long-ranging.
As the nearby Percé area was assuredly on the same platform as Anticosti and it has very
little in common with Anticosti (or the Oslo region), one must seek an explanation. The most
obvious reason for these differences is ecological control on these faunas, and, particularly,
depth of water and temperature. Depth, per se, appears insufficient to explain these differences.
Water temperature, particularly considered with an upward-moving thermocline (and
glaciations?), appears far more plausible an explanation for these zoogeographical differences.
Finally, what does a typical Hirnantian trilobite fauna contain? Benthic Assemblage 6 faunas
consist wholly or predominantly of trilobites, and can be composed of few or many taxa, but
shallower communities have far fewer trilobites, commonly with abundant brachiopods.
Excluding for the purpose of this discussion groups other than trilobites, two distinct trilobite
faunas apparently coexisted. A North American type appears evident (Anticosti Island, Ellis
Bay Formation; other faunas such as the Mackenzie faunas reported by Chatterton & Ludvig-
sen 1983, but sparingly developed in view of the profound disconformity between the Ordovi-
cian and the Silurian in most places in North America). The typical ‘Old World’ Hirnantian
trilobite fauna can be monospecific to highly diverse, but usually includes Dalmanitina or
Mucronaspis and a homalonotid (Brongniartella or Platycoryphe). The Oslo region faunas
appear to be intermediate between the two. On the other hand, this variation in diversity has
also been ascribed to nearness to the center of glaciation (Cocks & Fortey 1986), but the
problem appears more complex than that explanation suggests.
Dalmanitina—Mucronaspis taxa near the Ordovician—Silurian boundary
A bewildering number of species, particularly from China, and variously referred to Dalmani-
tina or Mucronaspis, have been reported from strata immediately above or below the previously
accepted or assumed Ordovician-Silurian boundary. Apart from the difficult systematics
associated with the generic assignment of the various species (a few are discussed at some
length below), some of them have been taken as indicative of a Silurian age. These putative
Silurian species are: Mucronaspis danai (Meek & Worthen 1866), Dalmanitina hastingsi (Reed
1915), D. kosyndensis Balashova 1966*, D. malayensis Kobayashi & Hamada 1964, D. brevispina
Temple 1952, D. nanchengensis Lu 1957, D. pamirica Balashova 1966* and D. subduplicata
zorbata Balashova 1966* [*: as cited by Kobayashi & Hamada 1971, but primary source
unverified by the present author].
It will be shown below that these occurrences are logically assigned to the Ordovician, if one
accepts the base of the Silurian as at the first appearance of the acuminatus Zone. This principle
of correlation by first appearances is at the heart of recent stratigraphical practice, and under-
362 P. J. LESPERANCE
lies the choice of ‘golden spikes’, as exemplified by the choice of the Silurian—Devonian bound-
ary. If this is followed, strata underlying the acuminatus boundary must be assigned to the
Ordovician, whatever the sedimentological and/or faunal succession may suggest.
The primary types of Mucronaspis danai occur in an erosional channel, assigned to the
Leemon Formation, within the Girardeau Limestone of southern Illinois. Conodonts within the
same beds as the trilobite are of the Amorphognathus ordovicicus fauna (Thompson & Satter-
field 1975), of undoubted Ordovician age. Whether this occurrence is of Richmondian or
Gamachian age is unknown. The species also occurs in the Edgewood Group of northeastern
Missouri (see below).
Dalmanitina hastingsi occurs in the lower, or trilobite, unit overlain by the upper or graptolite
unit, of the Panghsa-pye Formation (Bender 1983: 63) in Burma. This lower unit is only known
from the Panghsa-pye region itself, where it is underlain by the Nyaungbaw Limestone, which
is Late Ordovician on the basis of conodonts (Wolfart et al. 1984: 41). The graptolites from the
upper Panghsa-pye have been assigned to the Rhuddanian (but not as old as the acuminatus
Zone). The brachiopods from the lower trilobite unit are closely related, if not identical in many
cases, to Hirnantian forms (Temple 1965). There is thus no compelling evidence to consider D.
hastingsi Silurian, and it is here assigned to the Ordovician.
Dalmanitina malayensis occurs 1-4 to 1-8m above the base of the Detrital Band in the
Langwaki Islands, above graptolites (Kobayashi & Hamada 1971, 1974) of the persculptus
Zone. The topmost 4:7m of the 25m thick Detrital Band yields graptolites of the upper
Rhuddanian—Aeronian. There is consequently no reason to consider D. malayensis Silurian.
The primary types of Dalmanitina brevispina originate from Watley Gill (Lake District of
northern England), from a limestone of the ‘Silurian Basal Beds’. Graptolites of the acuminatus
Zone are welded (sic) on top of the “Basal Beds’ (Rickards 1970: 7). There is no evidence for
such a zonal assignment for the “Basal Beds’, or strata below them. The same species occurs at
Keisley, where it is known from strata below the persculptus and acuminatus Zones (Wright
1985). Thus both the Keisley and Howgill Fells occurrences of D. brevispina are probably
Ordovician.
The type material of Dalmanitina nanchengensis comes from southern Shaanxi, and it occurs
above beds yielding the graptolites Climacograptus angustus (Perner, 1895) and C. mirnyensis
(Obut & Sobolevskaya, 1967) (Lu & Wu 1983). Although D. nanchengensis is also known from
Sichuan—Guizhou (Szechuan—K weichow), it is the Shaanxi occurrence that is considered Silu-
rian, on the basis of C. mirnyensis which apparently occurs only in the acuminatus Zone. Koren
et al. (1983), however, report that C. mirnyensis occurs in the extraordinarius, persculptus and
acuminatus Zones, so that D. nanchengensis is herein assigned to the Ordovician, because of the
lack of diagnostic Silurian elements below it.
Apart from these species, Mucronaspis mucronata (Brongniart 1822) has also been claimed to
occur in Silurian strata. Disregarding the Scandinavian claims to this age, which are now
abandoned in Scandinavia itself, M. mucronata has been so cited in the Percé area and in
Kazakhstan. Lespérance (this volume) assigns the Percé occurrences to the Hirnantian, while
the Kazakhstan occurrences (which cannot be proven to belong to Mucronaspis), with other
shelly faunas, are in the persculptus Zone (Apollonov et al. 1980) and so they are pre-Silurian.
Dalmanitina sp. occurs in the ‘Protatrypa’ assemblage, which may reach a level as high as the
Coronograptus cyphus Zone (Mu 1983: 116-7) in China. In accord with Williams (1983: 611),
the base of the acuminatus Zone in China is higher than elsewhere, and hence the Dalmanitina
sp. is perhaps largely pre-Silurian in age; stratigraphical details are not sufficient for a more
extended discussion.
The Haverford Mudstone Formation of Wales has yielded in its lower 235m ‘Mucronaspis
mucronata’ (quotes are this writer’s) and other fossils (Cocks & Price 1975), assigned to the
Hirnantian, while the uppermost 140m yields a rich Rhuddanian fauna, containing, i.a.,
Acernaspis sp. Brongniartella sp., Hadromeros elongatus (Reed 1931) and Dalmanites sp. (Temple
1975); the generic assignment of the dalmanitacean is noteworthy, as are its associated
trilobites.
TRILOBITES 363
Systematic Palaeontology
Family DALMANITIDAE Vogdes, 1890
The distinction between the genera Dalmanitina and Mucronaspis, as well as the proper assign-
ment and distinctive characters of the numerous species referred to these genera, is difficult. The
most recent treatments are by Ingham (1977); Lespérance & Sheehan (1981); Owen (1982); Lu
& Wu (1983); Zhu & Wu (1984); Wu (1984); and Cocks & Fortey (1986). Zhu & Wu (1984: 89)
were uncertain whether a denticulate posterior hypostomal margin was diagnostic of Mucro-
naspis and, if so, no genuine Mucronaspis would be present in China. Hypostomata are conser-
vative evolutionary features and, potentially, powerful phyletic tools, which is a truism in
trilobite systematics. As both Destombes (1972) and Ingham (1977) stressed the presence of a
denticulate (spinose) hypostoma in Mucronaspis, a survey of Ordovician dalmanitacean hypo-
stomata is instructive.
Llanvirn spinose hypostomata are unknown. Three are known from the Llandeilo: Eodalma-
nitina macrophtalma (Brongniart, 1822) (the type species of the genus, Henry 1965: pl. 6, fig. 2),
Crozonaspis struvei Henry, 1968 (Henry 1980: 149) (but Crozonaspis morenensis morenensis
Hammann, 1972 (Hammann 1974) is not spinose), and Phacopidina micheli micheli (Tromelin,
1877) (Henry 1980: 128). These hypostomata have two small spines (or ‘denticles’) on their
posterolateral border. Caradoc spinose hypostomata also have two spines or denticles: Klouce-
kia (Phacopidina) aff. solitaria (Barrande, 1846) (of Destombes 1972), Mucronaspis zagoraensis
Destombes, 1972 (but hypostoma not illustrated), Dalmanitina (Dalmanitina) socialis (Barrande,
1846) (of Struve 1958: pl. 2, fig. 14), the one questionably referred to Eudolatites cf. angelini
(Barrande, 1852) by Struve (1958: 208; pl. 2, fig. 11), as well as the upper Caradoc and Ashgill
Baniaspis globosa Destombes, 1972. The following Ashgill spinose hypostomata have six spines:
Mucronaspis danai, Dalmanitina (Mucronaspis) termieri Destombes, 1963 (the type species of the
subgenus), and Mucronaspis mucronata (Brongniart, 1822). Except for Crozonaspis, and the aff.
solitaria of Destombes (1972; see below), the genera appear to be characterized by these spinose
hypostomata, but the hypostomata of most named species are unknown.
The hypostoma of Dalmanitina mucronata illustrated by Kielan (1960: pl. 20, fig. 6) is spinose,
but it is uncertain if two or six spines are present. Ingham (1977: 113; pl. 25, figs 3—4) described
a small holaspis of Mucronaspis mucronata which has marginal denticles; he compared this
specimen with Kielan’s (1960) illustration. Here again, it is not clear how many spines are
present; additional data are needed on these unique (?) Polish and northern English
occurrences. Eudolatites (Deloites) maiderensis Destombes, 1972 (the type species of the
subgenus) is said to have the beginnings of three small ‘denticles’, from a worn posterior border
of the hypostoma; again more data are needed to confirm this unique type of spinosity. These
three occurrences are apparently all Hirnantian.
From the spinose hypostomata previously enumerated, five appear to share common traits:
significantly greater width than length (ratio as 4:3), essentially identical shapes (strongly
curved posteriorly, lateral margins subparallel), a distinct lateral and posterior border, with two
or six denticles or spines. These five are: Crozonaspis struvei, Eodalmanitina macrophtalma,
Kloucekia (Phacopidina) aff. solitaria of Destombes 1972, Dalmanitina (D.) socialis of Struve
1958, and Mucronaspis termieri. However, significant nomenclatorial problems exist with two of
the above taxa. The lectotype of Sokhretia solitaria (Barrande, 1846) (the type of the genus) has
been illustrated (Snajdr 1982), and it is obvious that it is not conspecific with the Moroccan
species. This Moroccan aff. solitaria falls within the concept of the genus Phacopidina of Henry
1980, and is consequently better referred as Phacopidina n. sp. The second nomenclatorial
problem is, however, far more serious. Barrande’s (1852: pl. 26, fig. 21) illustration of the
hypostoma of Dalmanitina socialis (the type of the genus) shows no denticles, and Struve’s
(1958) illustration of the species appears to differ only in the presence of these hypostomal
denticles. Either hypostomata are sexually dimorphic, they are phenotypically variable, or
significant parallel evolution exists within the Dalmanitidae, with consequent polyphyly. Paral-
lel evolution appears much more plausible to this writer, if only to explain the notoriously
364 P. J. LESPERANCE
difficult systematics associated with some dalmanitaceans. If this explanation is correct, it also
necessitates a revision of many previously held taxonomic concepts. Be that as it may, Struve’s
(1958) socialis is better called Mucronaspis sp. (nov).
Denticles on hypostomata apparently appeared in the Liandeilo; originally two in number,
Ashgill representatives acquired six. Some denticulate hypostomata do not fit into the five taxa
quoted above, and one is led to conclude that a separate branch diverged in the Caradoc. These
considerations indicate that denticles, or spines, are diagnostic of the hypostomata of Mucro-
naspis, if only because a possible evolutionary path leads to it. If this is the case, the numerous
Hirnantian species which are problematically assigned to Dalmanitina or Mucronaspis should
accord with what the type species of the two genera in question possess: non-denticulate in
Dalmanitina, and denticulate (or spinose) in Mucronaspis. Other generic characters of Mucro-
naspis (as opposed to Dalmanitina) have been given by Ingham (1977) and Owen (1982). Mucro-
naspis should therefore be interpreted in a strict sense: the diagnostic spinose hypostoma must
be identified from a locality before the generic name Mucronaspis can be applied to the
specimens from the locality. Obviously this course of action creates complications, necessitating
in most instances open nomenclature.
Hirnantian, and some pre-Hirnantian, dalmanitaceans referred either to Dalmanitina or
Mucronaspis, and variously assigned to the species mucronata Brongniart, 1822, olini Temple,
1952, or other more recently erected ones, are almost impossible to assess, because many
reported occurrences of these latest Ordovician dalmanitaceans do not illustrate hypostomata,
or else the material is more or less severely distorted. A critical look at associated hypostomata
is needed to prove or disprove polyphyly in these dalmanitaceans, confirm generic assignments
and thus tabulate occurrences, before these trilobites are used for unequivocal dating of the
latest Ordovician, as yet impossible with the data at hand. Nonetheless, Dalmanitina (Songxites)
is apparently restricted to the Hirnantian.
Subfamily DALMANITININAE Destombes, 1972
SYNONYM. Mucronaspidinae Holloway, 1981.
Discussion. Holloway (1981) distinguished the Mucronaspidinae (Mucronaspis, Eodalmanitina,
Eudolatites (Eudolatites) Delo, 1935, E. (Banilites) Destombes, 1972, E. (Deloites), Retamaspis
Hammann, 1974 and ?Chattiaspis Struve, 1958) from the Dalmanitininae (Dalmanitina,
Crozonaspis) exclusively on thoracic and pygidial characters. Many characters listed by Hol-
loway (1981) are couched in jargon (well rounded as against not strongly rounded pleural
bands; thick and deep as against sharply impressed pleural furrows; shallow and sharply
impressed as against sharply impressed interpleural furrows), while other characters differ little
in each subfamily (posteriorly elongated posterior projections of thoracic pleural tips, which
may be spinose as against rounded; thoracic and pygidial facets (essential to enrollment), either
wholly as against essentially non-furrowed). If almost straight pygidial pleural furrows are
typical of the Dalmanitininae, none of the Chinese Dalmanitina are correctly assigned. While
pygidial doublures are said to be narrow in the Dalmanitininae, and broad in the other
subfamily, this feature is still contentious at the specific level, for example in Stenopareia
linnarssoni (Holm, 1882) (Lane 1979: 16). Of Holloway’s criteria between the two subfamilies,
perhaps the slope of the pleural bands is distinctive, but the same morphology is recurrent in
dalmanitaceans. In any event, this last criterion alone is insufficient for subfamilial distinctness;
at best, one could envisage tribal status for spinose hypostomata, but present data are insuffi-
cient for this taxonomic status.
Genus DALMANITINA Reed, 1905
TYPE SPECIES. Phacops socialis Barrande, 1846.
DiscussION. Two distinct subgenera are recognized within this genus: D. (Thuringaspis) (type D.
(Thuringaspis) osiris Struve, 1962) (recently discussed by Cocks & Fortey 1986) and D.
(Songxites) Lin, 1981, which has been accorded generic status by VandenBerg et al. 1984, as it
TRILOBITES 365
was assigned to the Mucronaspidinae. Until further data from Dob’s Linn (see below) are
presented, subgeneric status is preferable.
Subgenus SONGXITES Lin, 1981
TYPE SPECIES. Dalmanitina (Dalmanitina) wuningensis Lin, 1974.
Discussion. Siveter & Ingham in Siveter et al. 1980 indicated that the reduced palpebral lobe of
D. (Songxites) cellulana of these authors was the most distinctive feature of an as yet unnamed
genus, which would also encompass the Dob’s Linn dalmanitacean described below. Lin’s
(1981) erection of the subgenus D. (Songxites) appears to have pre-empted this question as D.
(Songxites) wuningensis, D. (Songxites) darraweitensis Campbell, 1973 (see VandenBerg et al.
1984) and D. (Songxites) cellulana are very closely related by the possession of reduced palpe-
bral lobes and eye ridges in contact with the axial furrow, opposite (tr.) the 3p lobes. The
hypostomata of D. (Songxites) darraweitensis and D. (S.) cellulana have approximately equal
lengths and widths, significant lateral and posterior borders, but are non-spinose, as is appar-
ently D. (Songxites) sp. (nov.) discussed below (Siveter & Ingham in Siveter et al. 1980: 201).
This suggests that an assignment to Dalmanitina (as opposed to Mucronaspis) is indicated.
Dalmanitina (Songxites) sp. (nov.)
Figs 1—2
1980 Mucronaspis sp. Siveter & Ingham in Siveter et al.: 200, 201.
MATERIAL. Material collected in 1979 by this writer consists of six complete cranidia (and five
less complete ones), three incomplete pygidia, one fragmentary thoracic segment, and a frag-
mentary hypostoma. It comes from a level 10cm below the extraordinarius Band at Dob’s Linn,
Scotland. Additional material has been alluded to, including librigenae (Siveter & Ingham in
Siveter et al. 1980: 201).
DISTINCTIVE ATTRIBUTES. Maximum (tr) width of fixigenae same as maximum width (tr) of
frontal glabellar lobe: fixigenae thus very wide. Lateral border furrow shallow, not reaching
more incised posterior border furrow. Genal spine short and stout, approximately as long along
its length as distal part of posterior border (exsag). Posterior branch of facial suture reaching
border at a point (tr) from middle of 3p lobe. Anterior branch of facial suture delimiting a
progressively narrower (tr) fixigena, merging into a narrow (exsag) frontal border, absent in
front of central third of frontal glabellar lobe. A slightly anteromesially elongated protuberance,
opposite (tr) proximal end of 3p furrows, slopes equally in all directions; in so doing, this
protuberance reaches the facial suture, which is not dorsally deflected. Protuberance presum-
ably an obsolete palpebral lobe, but librigenae or complete cephala essential to confirm this;
Figs 1-2 Dalmanitina (Songxites) sp. (nov.). Two differentially preserved inner moulds of cranidia,
Fig. 2 showing obvious shearing; from a level 10cm below the extraordinarius Band, Dob’s Linn,
Scotland. Figs 1a, 1b, BM(NH) It.20480; 1a, x 6-8; 1b, lateral view showing presumed obsolete
palpebral lobe and anterior fixigenal area, x 13 (counterpart, not illustrated, BM(NH) It.20480a,
shows an undamaged occipital segment). Fig. 2, BM(NH) It.20481, x 3-5.
366 P. J. LESPERANCE
eyes, presumably, degenerate. 2p furrows transverse, proximal end of 1p furrows slightly poste-
riorly directed, central part of occipital furrow shallower than distal parts.
Posterior part of hypostoma not preserved, with a distinct lateral border. Pygidial pleural
furrows twice as deep and twice as wide as interpleural furrows, anteriormost four pairs evenly
curved posterolaterally.
All the material consists of inner and outer moulds; exoskeleton probably very thin and
unornamented.
Discussion. The presumed obsolete palpebral lobe, the absence of an eye-ridge (as previously
noted by Siveter & Ingham in Siveter et al. 1980: 205), and a significant anterior fixigenal area
are the unique characters of this species, which should be named when the extant material is
brought together.
Genus MUCRONASPIS Destombes, 1963
TYPE SPECIES. Dalmanitina (Mucronaspis) termieri Destombes, 1963.
Mucronaspis danai (Meek & Worthen, 1866)
Figs 3-9
1866 Dalmania Dane Meek & Worthen: 264.
1868 Dalmanites Dane (Meek & Worthen) Meek & Worthen: 363; pl. 6, figs la-f.
1917 Dalmanites danai (Meek & Worthen) Savage: 147; pl. 8, figs 16, 17.
1940 Dalmanites danae (Meek & Worthen); Delo: 40; pl. 3, figs 24, 25.
Types. Meek & Worthen’s (1868) first illustrations of the species, along with the original
description marginally modified, were based on four distinct specimens: a cephalon, a
pygidium, an hypostoma, and an incomplete outstretched individual, with a major part of the
left side wanting. Two institutions now hold A. H. Worthen’s types. The University of Illinois
at Urbana-Champaign (UJ), under lot X-98 (and 11635), has (a) a complete individual, with the
posterior half of the thorax wanting (this specimen has never been illustrated and is not a type),
(b) a pygidium (illustrated in Delo 1940: pl. 3, fig. 25; not the original of Meek & Worthen
1868: pl. 6, figs 1d, le), and (c) a cephalon claimed to be a syntype of M. danai (original of pl. 6,
figs 1b, lc of Meek & Worthen 1868; Hansman & Scott 1967), reillustrated in Delo (1940: pl. 3,
fig. 24), but this writer has been unable to examine this specimen recently. Delo (1940) referred
to the complete individual above as the holotype, and the pygidium as a paratype (in the text),
but in the plate explanations the pygidium and the cephalon are treated as paratypes. This is
not, however, considered a designation of a lectotype (which would be invalid in any event).
The Worthen collection in the Illinois State Geological Survey, formerly Illinois State
Museum [ISGS(ISM)], holds a syntypic lot of five specimens (Kent 1982): (a) a complete
specimen, with much of the left side wanting (original of Meek & Worthen 1868: pl. 6, fig. 1a;
2184-1); (b) a teratological pygidium, with the right pleuron damaged, never illustrated or
referred to (2184-2); (c) a cephalon, with most of the right gena missing, never illustrated or
referred to (2184-3); (d) a cranidium, with most of the occipital segment broken off, never
illustrated or referred to (2184-4); and (e) a pygidium, very probably the original of Meek &
Worthen: pl. 6, figs 1d, le (2184-5). No hypostoma is thus present in these type collections; two
specimens can be identified as syntypes (ISGS 2184-1 and 2184-5), in addition, apparently, to
the cephalon in UI X-98. Meek & Worthen’s (1866, 1868) measurements refer only to the
complete individual (although mention is made in the discussion of an enormous pygidium five
inches in length). ISGS 2184-1 is herein designated lectotype of Dalmania danae (recte danai)
Meek & Worthen 1866; ISGS 2184-5 becomes a paralectotype, as apparently does the
cephalon in UI X-98. The syntypic hypostoma appears lost, which is not surprising in view of
the adventures of the Worthen collections (Kent 1982). From the preceding, it is clear that this
writer accepts as syntypes only those specimens illustrated or referred to in the original descrip-
tion of the species; it is possible that some of the specimens referred to above, but not
considered paralectotypes, were indeed syntypes. Formal indication that they were used by
TRILOBITES 367
Meek & Worthen (1866) must be presented, however, before they are added to the paralec-
totype list.
Savage’s (1917) drawings of hypotypes (lot UI X-910, topotype cephalon and pygidium) are
imprecise, the pygidium particularly so (notably the posterior part of the axis); the upturned
posterior spine can, however, be observed on the original.
Mucronaspis danai is commonly cited as being erected in 1865, but Hansman & Scott (1967)
have shown that the December issue of the Proceedings of the Philadelphia Academy of
Natural Sciences was published in 1866. Savage’s (1917) publication was also published as
an extract in November 1913 (Notice between pp. 66 & 67, Savage 1917), with a different
pagination.
OCCURRENCE. The syntypes are from an erosional channel of the basal Leemon Formation,
along the east bank of the Mississippi river, 5900 ft (1:8 km) NNW of the railroad track and
road intersection on the eastern edge of Thebes, Alexander county, Illinois. J. H. Stitt has
collected this species from the Late Ordovician Edgewood Group (probably from the Cyrene
Member), from a stream outcrop immediately south of ‘Ebenezer Church’ (Elsberry 15 minute
quadrangle, 1934 edition), Lincoln county, 18 mi (29 km) southeast of Louisiana, Missouri.
ESSENTIAL ATTRIBUTES. Maximum width of glabella anteriorly, slightly posteriorly of junction of
axial and lateral border furrows, 46% of width measured across (tr) occipital segment. 2p
furrows essentially transverse, but arched anteriorly, 1p furrows faintly and, more commonly,
distinctly posteriorly directed proximally, distal 1p lobes isolated by shallow inner (exsag)
furrows, more incised on smaller specimens. Frontal glabellar lobe with auxiliary impression
patterns, median posterior impression well developed, stellate, with apparently six rays. Palpe-
bral lobe forms highest part of cephalon; eyes with 37 (or 36?) dorsoventral files, commonly
with 10 lenses per file (for a total of approximately 300 lenses), but with as few as 8 lenses per
file in smaller specimens. Posterior branch of facial suture reaches marginal furrow at a point
across (tr) from 1p furrow, then turns sharply posteriorly across convex border and reaches
margin at a point across (tr) occipital furrow. Posterior border furrow deeply incised, meeting
marginal furrow, which is the junction of differently dipping border and inner parts of genae
(and thus not incised). Frontal border narrow, commonly more or less crushed. On a well
preserved topotype specimen, 32mm long (sag), frontal border consists of an inner portion
0:-5mm long (sag & exsag), separated from an outer portion (librigenae) by the dorsal suture;
outer portion ranging from a feather edge (sag) to 2mm (exsag) anterolaterally of the frontal
glabellar lobe. Genal spines half as long as sagittal length of cephalon.
Hypostoma subquadrate with six marginal denticles, incipient on a small individual. Border
somewhat convex, significantly longer posteriorly than laterally, set off by distinct furrows.
Thoracic segments deeply furrowed, with a stout posteriorly directed distal spine.
Pygidium with 8 (and an incipient ninth) deeply furrowed pleurae, posterior one exsagittal,
posterior bands sloping more steeply to interpleural furrow than anterior bands. Pleural and
interpleural furrows not reaching margin, former slightly more incised (longer exsag), anterior
bands slightly longer than posterior bands across border. Axis with 11 distinct axial rings and a
post-axial piece continuing into a posterior spine upturned at approximately 20°; length of
spine (sag) same as length (sag) of anteriormost 6 or 7 axial rings, depending on the specimen.
Spine and post-axial piece continuing at same height.
Ornamentation poorly known as observed only in the following instances. Pygidium prob-
ably smooth, hypostoma with scattered tubercles on anterior lobe of median body, rare to
absent on posterior lobe; genae, inward of posterior and marginal furrows, covered with
irregular shallow 0-8 mm depressions, lateral cephalic border with granules.
Discussion. This species is almost identical to M. mucronata dorsally, and may eventually be
synonymized with it when the relationships between M. mucronata and M. olini have been
redefined. M. danai differs from M. mucronata by its tendency to have a more flaring outward
(wider, tr) frontal glabellar lobe; maximum width of the glabella in M. mucronata is half width
across occipital segment. The hypostomata, though, differ more markedly: M. danai has fewer
368 P. J. LESPERANCE
tubercles and its anteriormost marginal denticles are opposite (tr) the proximal end of the
median furrow while in M. mucronata these denticles are more posterior, nearly opposite (tr)
the middle (sag) of the posterior lobe of the median body, and, furthermore, the tubercles tend
to coalesce. The median furrow of the hypostoma in M. danai is also more incised than in M.
mucronata.
TRILOBITES 369
Mucronaspis mucronata (Brongniart, 1822)
Figs 10, 11
1822 Asaphus mucronatus Brongniart: 24.
1822 Asaphe mucroné, Entomostracites caudatus de Wahlenberg; Brongniart: 144; pl. 3, fig. 9.
1952 Dalmanitina mucronata (Brongniart) Temple: 10; pl. 1, figs 1-3 , 5-8; pl. 2, fig. 1.
1981 Mucronaspis mucronata (Brongniart) Lesperance & Sheehan: 232; pl. 3, fig. 4; pl. 4, figs 1, 2, 4.
1982 Mucronaspis mucronata mucronata (Brongniart); Owen: 271, figs 1A, 1B.
Types. Lectotype cephalon and paralectotype pygidium selected by Owen (1982), Uppsala
University, from the ‘Dalmanitina’ Beds, Vastergotland, Sweden.
Discussion. The above synonymy list includes only those illustrated occurrences that can
obviously be referred to the species [but the Perce hypostoma included in this list (Lespérance
& Sheehan 1981), and reillustrated here for comparison with M. danai, with another from the
same locality (Figs 10, 11), could conceivably be M. olini (Temple 1952)].
Our understanding of this species must still be founded on Temple’s (1952) careful study. He
has detailed its intraspecific variability and occurrences, but did not record the spinose hypo-
stoma. He distinguished mucronata from olini almost exclusively on pygidial characteristics, and
in fact Lespérance & Sheehan (1981) could not distinguish cephala of the two species, although
this distinction is obvious using the pygidia. Because of this, this writer remains convinced that
careful bed by bed collecting may eventually prove or disprove suggestions that olini is only a
geographical variant (or ecologically controlled) subspecies of mucronata, and thus the two
species should be kept separate until conclusively proven otherwise.
A complete hypostoma of M. mucronata kiaeri (Troedsson, 1918) (Owen 1982, from the
Rawtheyan and Hirnantian of the Oslo region, Norway) is unknown, but at least ‘a small spine
base a short distance out from the sagittal line’ is known (Owen 1982: 274), indicating that
kiaeri is assigned to the proper genus.
Family TRINUCLEIDAE Hawle & Corda, 1847
Trinucleid trilobites occur within the Hirnantian, but they are very uncommon. Cryptolithus
portageensis sp. nov., described below, occurs in the Percé area. A trinucleid brim fragment has
been reported between extensive Hirnantian brachiopod and trilobite faunas and below the
persculptus Zone at Keisley, northern England (within unit 9 of Wright 1985: 267). Perhaps
more significantly, a fragment of a tretaspid (suggesting the Tretaspis seticornis (Hisinger, 1840)
group) occurs in northern Wales (in the type region of the Hirnantian) within a brachiopod-
Figs 3-9 Mucronaspis danai (Meek & Worthen, 1866). Figs 3-5, 7, and 9 types and topotypes from
north of Thebes, Illinois, Leemon Formation (formerly referred to the Edgewood Group); Figs 6
and 8, from stream outcrop near ‘Ebenezer Church’ (longitude 90° 53’ 19”, latitude 39° 12’ 57”),
northeastern Missouri, Edgewood Group (Late Ordovician). Fig. 3, pygidium, latex cast of outer
mould with exoskeleton showing upturned spine, posterior part preserved on original; 3a UMC
16590a, x 1 (outer mould UMC 16590, not illustrated); 3b, lateral view emphasizing spine, x 1.
Fig. 4, inner mould, paralectotype pygidium, ISGS 2184-5, x 1-2. Fig. 5, inner mould, incomplete
individual, lectotype (herein selected), ISGS 2184-1, x 0-7. Fig. 6, thoracic segment, outer mould
with exoskeleton, stout spine on pleural tips can be discerned, UMC 16591, x 1-2. Fig. 7,
cephalon, inner mould, UMC 16592, x 0:9 (partial outer mould with exoskeleton shows a com-
plete eye, UMC 16592a, not illustrated). Fig. 8, inner mould, small hypostoma with incipient
denticles, UMC 16593, x 3-9. Fig. 9, inner mould, incomplete hypostoma with six denticles, UMC
16594, x 1-4. [ISGS: Illinois State Geological Survey, Champaign, Illinois; UMC: University of
Missouri at Columbia, Columbia, Missouri. ]
Figs 10-11 Mucronaspis mucronata (Brongniart, 1822). Inner moulds of incomplete hypostomata,
C6te de la Surprise Member, White Head Formation, 17 km west-northwest of Percé, Québec. Fig.
10, showing three denticles on left side, posteriormost one present, GSC 83013 (GSC 83013a,
counterpart with exoskeleton, not illustrated), x 1-8. Fig. 11, showing a total of four denticles
(posteriormost two denticles present on counterpart with exoskeleton, GSC 21909a, not
illustrated), GSC 21909, x 1-9.
370 P. J. LESPERANCE
dominated [Hirnantia sagittifera (M‘Coy, 1851), Crytothyrella sp. and Plectothyrella platystro-
phoides Temple, 1965] community at the Graig-Wen quarry, Powys (SJ 1018 0930) (J. T.
Temple in coll. & personal communication 1985).
Genus CRYPTOLITHUS Green, 1832
TYPE SPECIES. Cryptolithus tessellatus Green, 1832.
Cryptolithus portageensis sp. nov.
, Figs 12-14
1974 Cryptolithus n. sp. Lespérance: 15.
1981 Cryptolithus n. sp. Lespérance & Sheehan: pl. 3, fig. 2.
1985 Cryptolithus n. sp. Lespérance: 845.
Types. Holotype: cephalon Geological Survey of Canada, Ottawa (GSC) 21914 (previously
illustrated in Lesperance & Sheehan 1981), paratype cephala GSC 82988 (ventral view of lower
lamella of fringe) and 82989. Also known from an additional six more or less complete cephala.
From a small tributary to the Portage River, 17km WNW of Percé, Cote de la Surprise
Member, White Head Formation, Hirnantian (Lespérance 1974, and this volume, p. 242).
DiaGnosis. A species of the genus without glabellar furrows or pits, but with auxiliary impres-
sion patterns. The species has complete E,, I, and I, arcs, but no I, arc. Sagittal and imme-
diately adjacent parts of glabella distinctly reticulated.
Figs 12-14 Cryptolithus portageensis sp. nov. Specimens with exoskeleton, same locality as Figs
10-11. Figs 12a, 12b, holotype, GSC 21914; 12a, showing length of genal spines, x 2-8; 12b,
showing well girder on left side, x 4-3. Fig. 13, lower lamella of fringe, paratype GSC 82988, x 3.
Figs 14a, 14b, incomplete cephalon showing ornamentation and glabellar auxiliary impression
patterns, paratype GSC 82989; 14a, x 3-4; 14b, lateral view, x 3-8.
TRILOBITES 37/1
DESCRIPTION. Sagittal length of cephalon twice maximum width measured across posterior
margin. Genal spines slender, flaring outward, then inward distally, 1:5 times length of
cephalon. Sagittal tubercle on glabella, slightly in front of glabellar mid-point (excluding occipi-
tal segment). Posterior margin of occipital segment entire, not drawn out by a spine, nor
possessing a tubercle. Occipital furrow and posterior margin furrow wide (sag, exsag), deep, but
occipital shallower. Glabellar furrows or pits absent, but three pairs of darker, slightly
impressed auxiliary impression patterns present on sides of glabella, a short distance from axial
furrow. Posterior pair comma-shaped, with a more strongly curved portion ventralmost, almost
touching occipital furrow, elongated essentially perpendicularly to axial furrow, approximately
1mm in greatest dimension; second pair circular, approximately 0:6mm in diameter; anterior
pair much as posterior pair, but ventral portion not posteriorly elongated, 0-6mm along its
greatest length, situated essentially transversely to glabellar tubercle (measurements taken from
paratype cephalon GSC 82989).
Prominent girder list present on upper lamella of fringe; another list, between I, and I, only
present on posterior half of fringe. Lower lamella of fringe with pseudo-girder between I, and
I,, girder continuous onto genal spine; both girder and pseudo-girder attenuated toward sagit-
tal line. Genae smooth, central and highest part of glabella (sag, exsag) reticulated for a width
of approximately 1mm (tr) (as present on GSC 82989), but ornamentation unknown on
anteriormost, and subvertical, portion of glabella.
Following the orientation suggested by Hughes et al. (1975: 547), frontal part of fringe
horizontal, laterally gentle sloping downward. Arcs E,, I, and I, complete; I, absent. Half
fringes with 24-25 pits in E, 18-20 in I,, and 18-19 in I, arc; 8-10 smaller flange pits present
posteriorly, and 6-8 occur along the posterior margin of the fringe.
DIMENSIONS. All the type material is slightly laterally compressed; measurements are in mm.
Length (sag) Width across posterior margin
GSC 21914 5-6 11-7 (est.)
GSC 82988 6:3 12:9
GSC 82989 — 11-5 (est.)
Discussion. Glabellar auxiliary impression patterns are known in Caradoc species of Crypto-
lithus (Whittington 1968: pl. 87, figs 6, 10; pl. 88, fig. 11; pl. 89, fig. 1). The low number of pits,
particularly the absence of an I, arc, as well as a different glabellar ornamentation, distinguish
C. portageensis sp. nov. from C. stoermeri Owen, 1980, from the uppermost Husbergoya For-
mation (upper Rawtheyan) of the Oslo region. C. portageensis sp. nov. is nearest C. kosoviensis
Marek, 1952 (uppermost Kraluv Dvir Formation, Rawtheyan?, Bohemia), which however has
a frontally incomplete I, arc; only the posterior half of the glabella of kosoviensis is reticulated,
as is part of the inner posterior cheeks (Pribyl & Vanék 1969: 104). Hughes et al. (1975) have
questioned the assignment of kosoviensis to Cryptolithus, but the similarity of portageensis to
kosoviensis suggests that the Bohemian species is correctly assigned to Cryptolithus.
Family PHACOPIDAE Hawle & Corda, 1847
Although the genus Acernaspis apparently first occurs with the onset of the acuminatus Zone,
Lespérance & Letendre (1982: 329) have drawn attention to a new genus of this family that first
occurs in the Belgian Ashgill.
Genus ACERNASPIS Campbell, 1967
TYPE SPECIES. Phacops orestes Billings, 1860.
REMARKS. Acernaspis (subgenus?) norvegiensis Lespérance & Letendre, 1982 is herein reillus-
trated (Fig. 15) to show its distinctness from other species of the genus. It is the only known
species within Acernaspis which has granules and pustules, many of the latter being perforated.
It may be noted here that this species is associated with another species of Acernaspis within
‘6b’ of the Asker region, Norway (Lespérance & Letendre 1982: 336).
372 P. J. LESPERANCE
Subgenus ACERNASPIS Campbell, 1967
DIAGNOSIS. Primitive phacopids with continuous vincular furrows, which may be anteriorly
shallower. Ornamentation variously with punctae or smooth, but more commonly granulose
(Lespérance & Letendre 1981: 199).
REMARK. The use of subgenera within Acernaspis has been amply discussed by Lespérance &
Letendre 1981, and need not be repeated here.
Acernaspis (Acernaspis) salmoenstis sp. nov.
Figs 16-19
1981 Acernaspis sp. Lespérance & Letendre: 197.
1982 Acernaspis sp. Lespérance & Letendre: 329.
1982 Acernaspis (Acernaspis) n. sp.? Lespérance & Letendre: 332; pl. 1, fig. 16.
1985 Acernaspis n. sp. Lespérance: 845.
Types. Holotype: GSC 69146, previously illustrated (Lespérance & Letendre 1982). Paratypes:
GSC 82990, incomplete cranidium; GSC 82991, a pygidium; and GSC 82992, incomplete
cephalic doublure.
Fig. 15 Acernaspis (subgenus?) norvegiensis Lespérance & Letendre, 1982. Incomplete cranidium
with exoskeleton, upper half of ‘6bo’ (Solvik Formation: Worsley 1982: 165), Spirodden peninsula,
Asker region, Norway; PMO 106-509, x 9-5. [PMO: Paleontologisk Museum, Oslo. ]
Figs 16-19 Acernaspis (Acernaspis) salmoensis sp. nov. Specimens with exoskeleton, Becscie Forma-
tion, Anticosti Island, Québec. Fig. 16, incomplete cranidium, paratype GSC 82990, x 7-1. Fig. 17,
pygidium, paratype GSC 82991, x 6-8. Fig. 18, cephalic doublure showing vincular furrow, holo-
type GSC 69146, x 7-3. Fig. 19, incomplete cephalic doublure, paratype GSC 82992, x 5-3. [GSC:
Geological Survey of Canada, Ottawa. ]
TRILOBITES 373
OCCURRENCE AND MATERIAL. Only known from the Rhuddanian Becscie Formation of eastern
Anticosti island, Québec. Paratypes from roadside outcrop on northern side of road parallel to,
and south of, Salmon River, from a level 4m above lowermost occurrence of the species. This
outcrop extends westward from a stream emptying into the river, and is 960m west of longi-
tude 62° 18’ 00” and 250m south of latitude 49° 24’ 00”. This level has yielded approximately
45% of the known material of the species, and the level 4m below it another 45%. This
lowermost level is 45m above the base of the Becscie Formation (Lespérance 1985: 845). The
species also occurs at the ‘major falls’ along the Salmon River, at ‘pool 16’ (9-5 km west of the
previous locality), and the holotype is from an outcrop along the road leading to Baie de la
Tour, 0-8 km north of the main road (approximately 27km to the northwest of the paratypes;
see also Lespérance & Letendre 1982: 334). Extant material of the species includes approx-
imately 10 cephalic doublures, 35 cranidia, 60 pygidia and a few incomplete thoracic segments
and librigenae.
D1AGNosis. A species of Acernaspis (Acernaspis) with a very shallow anterior vincular furrow
and a posterior vincular furrow with dividing walls between fossulae; dorsal sutures functional
and ornamentation consisting of microgranules.
DESCRIPTION. Glabella expanding forward, widest across frontal glabellar lobe, with a width
ratio of 8:5 with width (tr) of occipital segment. 3p furrows bicomposite, distal part impressed,
proximal part faintly, as 2p furrows. Distal 1p lobe isolated, below level of 2p lobe and distal
part of occipital segment. 1p furrow continuous, poorly incised and shallow sagittally. Occipital
furrow incised, continuous. Palpebral furrow incised, extending from axial furrow anteriorly to
a point transverse from occipital furrow. Posterior border furrow wide (exag), incised. Palpebral
lobes below level of central part of glabella, convex and thus bent downward distally. Dorsal
sutures functional. Eyes with a minimum of 14 dorsoventral files, with 3—5 lenses per file.
Anterior part of vincular furrow marginal and ventral, as anterior and anterolateral part of
subvertical doublure slopes very steeply posteriorly. Anterolateral section of anterior part of
vincular furrow broadly incised, but sagittally barely perceptible and very shallow. Posterior
part of vincular furrow scalloped, with 8 or 9 fossulae, with dividing walls between fossulae
reaching approximately the mid-point between the bottom of the fossulae and the bounding
walls. Anterior half of proximal bounding wall of posterior vincular furrow vertically below
adjacent/distal wall, while posterior half of proximal bounding wall of posterior vincular furrow
vertically shorter than outer, adjacent distal wall.
Pygidium wider than long (as 8:5), axis with 7 axial rings, not reaching posterior margin.
Axial ring furrows transverse, progressively shallower posteriorly. Pleurae with 4 pygidial ribs,
very faintly furrowed; distal third of pleural fields unfurrowed. Articulating half-ring cut in
middle by facet; furrow between this half-ring and anteriormost rib apparently continuous to
margin.
Ornamentation consisting of microgranules (densely packed 0-01—0-:04mm granules, better
developed on cephalic doublure, including the anterior part of the vincular furrow), probably
modified by surficial weathering.
DIMENSIONS. All lengths given are sagittal and all widths are transverse; measurements are in
mm.
GSC 69146 GSC 82992
Width of cephalon 7:3 —
Length of cephalic doublure 1:16 1:91
Paratype pygidium (GSC 82991) has a width of 5-0; its total length is 3-1, which includes a
length of 0-20 for the articulating half-ring; length of axis, including articulating half-ring, 2-6.
Paratype cranidium (GSC 82990) has a length of 3-8, and widths of 2:2 for the occipital
segment and 0-6 for the palpebral lobe.
Discussion. The very shallow anterior part of the vincular furrow sets this species apart from
all others within the subgenus. The taxon closest to it appears to be Acernaspis (Murphycops)
374 P. J. LESPERANCE
skidmorei (Lespérance, 1968) (Lespérance & Letendre 1981), which has no anterior vincular
furrow and in which the anteriormost part of the cephalic doublure is vertical. Acernaspis (A.)
salmoensis sp. nov., in this regard, appears as an ideal ancestor for A. (Murphycops) skidmorei,
of lower Idwian age. The lowest Acernaspis sp. from the Becscie Formation of western Anti-
costi, near Cap a Ours (Lespérance 1985: 845), is too poorly preserved for specific assignment.
Acknowledgements
The writer is indebted to T. E. Bolton (Geological Survey of Canada, Ottawa) and A. A. Petryk (Ministére
de l’Energie et des Ressources du Québec) who have provided specimens of Acernaspis with painstakingly
gathered stratigraphical data. J. H. Stitt (University of Missouri, Columbia) made available some Mucro-
naspis danai. T. L. Thompson (Missouri Department of Natural Resources, Rolla) guided the writer to the
type locality of danai. D. B. Blake (University of Illinois at Urbana-Champaign, Urbana) allowed access to
the type collections in his care. D. Mikulic and R. D. Norby (Illinois State Geological Survey, Champaign)
facilitated the loan of type specimens of the Worthen collection. Operating grants from the Natural
Sciences and Engineering Council of Canada are gratefully acknowledged.
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(eds), Autecology of Silurian organisms. Spec. Pap. Palaeont., London, 32: 55-69.
——, Owens, R. M. & Rushton, A. W. A. 1984. Trilobites in British stratigraphy. Spec. Rep. geol. Soc.
Lond. 16. 78 pp. + Index 25 pp.
Thompson, T. L. & Satterfield, I. R. 1975. Stratigraphy and conodont biostratigraphy of strata contiguous
to the Ordovician-Silurian boundary in eastern Missouri. Rep. Invest. Mo. geol. Surv., Rolla, 57 (2):
61-108.
VandenBerg, A. H. M., Rickards, R. B. & Holloway, D. J. 1984. The Ordovician-Silurian boundary at
Darraweit Guim, central Victoria. Alcheringa, Adelaide, 8: 1—22.
Whittington, H. B. 1968. Cryptolithus (Trilobita): specific characters and occurrence in Ordovician of
eastern North America. J. Paleont., Tulsa, 42: 702-714.
Williams, S. H. 1983. The Ordovician—Silurian boundary graptolite fauna of Dob’s Linn, southern Scot-
land. Palaeontology, London, 26: 605-639.
Wolfart, R. et al. 1984. Stratigraphy of the western Shan Massif, Burma. Geol. Jb., Hannover, (B) 57: 3—92.
Worsley, D. (ed.) 1982. I.U.G.S Subcommission on Silurian Stratigraphy. Field meeting Oslo Region 1982.
175 pp. Oslo (Paleont. Contr. Univ. Oslo 278).
Wright, A. D. 1985. The Ordovician-Silurian boundary at Keisley, northern England. Geol. Mag., Cam-
bridge, 122: 261-273.
Wu Hong-ji 1984. A species of Dalmanitina (Trilobite) from Deqing and Yugian counties, western Zhe-
Jiang. In Nanjing Institute of Geology and Palaeontology, Academia Sinica, Stratigraphy and Palaeon-
tology of Systemic boundaries in China. Ordovician—Silurian boundary 1: 455-466. Anhui Sci. Tech. Publ.
House.
Zhu Zhao-ling & Wu Hong-ji 1984. The Dalmanitina fauna (Trilobite) from Huanghuachang and Wangjia-
wan, Yichang county, Hubei Province. In Nanjing Institute of Geology and Palaeontology, Academia
Sinica, Stratigraphy and Palaeontology of Systemic boundaries in China. Ordovician—Silurian boundary 1:
83-110. Anhui Sci. Tech. Publ. House.
Note added in page proof. Additional topotype material of Cryptolithus portageensis sp. nov.,
previously not examined and from a different field collection number, contains three partial and
a complete cephalon, as well as a pygidium with a damaged axis. Ornamentation on the central
part of the glabella continues on the subvertical frontal lobe, but does not reach the fringe. The
pygidium has a width to length ratio of 4:1, three interpleural furrows not quite reaching the
steeply inclined border, and a fourth incipient and posterior one.
Environmental changes close to the
Ordovician—Silurian boundary
P. J. Brenchley
Department of Geological Sciences, University of Liverpool, P.O. Box 147,
Liverpool L69 3BX
Synopsis
Most late Ordovician to early Silurian sequences show evidence of a regressive phase followed by trans-
gression, reflecting glacio-eustatic sea-level changes. Continental glacial deposits are particularly well
known from Saharan Africa, and glaciomarine deposits from Iberia and Normandy. Rapid growth of the
ice caps at the beginning of the Hirnantian is reflected on clastic marine shelves by a change from
mudstones to a variety of shallow marine sand facies. Withdrawal of the sea to the edges of shelves fed
sand into basins to form submarine fans. Shallow carbonate shelves generally became exposed during the
Hirnantian, and karstic surfaces developed. A sea-level fall of between 50 and 100m is envisaged. The
regressive deposits are usually abruptly overlain by deeper-water deposits formed during a rapid trans-
gression. Graptolitic shales are widely developed on clastic shelves, but there is a return to shallow marine
limestones on carbonate shelves. There is local evidence of oscillations of sea-level within the main
Hirnantian glacial event, but it is uncertain whether these changes were eustatically controlled. It is
suggested that the climate during the Hirnantian remained cold in peri-polar regions, but may have been
variable in mid-latitudes and was tropical in equatorial regions. There is some palaeomagnetic evidence to
suggest that continents were moving unusually fast during late Ordovician times, which might have had
an influence on the growth and decay of late Ordovician ice caps.
Introduction
Most late Ordovician to early Silurian sequences show evidence of a regressive phase followed
by transgression. The regressive—transgressive interval is of the same age on plates which were
separate in the Lower Palaeozoic (Berry & Boucot 1973) and so satisfies the criteria for
identifying eustatic sea-level changes (Fortey 1984). The fall in sea-level started at the beginning
of the Hirnantian and the subsequent rise of sea-level had been largely completed before the
end of Hirnantian times. A major ice cap was present on the Gondwana plate at this time and
it is likely that the sea-level changes were related to the growth and decay of that ice cap.
The Ordovician-—Silurian boundary, as it is now placed at the base of the P. acuminatus Zone,
post-dates the late-Ordovician sea-level changes and falls within a period of environmental
stability. Thus the often striking facies changes in the Hirnantian, and particularly the change
from shallow to deeper water facies at the top of the Hirnantian, help to identify horizons
immediately below the boundary between the systems, but not the boundary itself.
Duration of the eustatic changes
Different ways of estimating the duration of Hirnantian environmental changes can be made,
and these produce somewhat different results. Estimates of the duration of the Hirnantian made
by dividing the duration of the Ashgill, based on radiometric age determinations, by the
number of stages (four) give 1-8 to 2:‘5my. If the duration of the Ashgill is divided by the
number of zones in the type area (eight) (Ingham 1966) the duration of the Hirnantian, which
has only one zone, is 1 to 1-25my. A value between | and 2 million years is probable, but more
radiometric dates close to the Ordovician—Silurian boundary are needed to give more accurate
estimates.
Changes in sedimentary environments
Continental glaciation. The deposits of continental ice sheets of upper Ordovician age in
Saharan Africa are well known through the descriptions of Beuf et al. 1971, Rognon et al. 1972,
Bull. Br. Mus. nat. Hist. (Geol) 43: 377-385 Issued 28 April 1988
378 P. J. BRENCHLEY
and others. They recognized nearly all the features characteristic of land-based ice deposition,
including glaciated pavements, striated pebbles, tillites, varved sediments and dropstones, and a
wide variety of fluvio-glacial sediments (Fig. 2, section 1), some of which are associated with
long esker-like ridges. Similar deposits have been recognized in South Africa (Rust 1982), and
glacial deposits believed to be of a similar age have been described from west Africa, South
America (see Spjeldnaes 1981 and references therein) and Saudi Arabia (McClure 1978). The
late Ordovician Gondwana glaciation was clearly of continental dimensions and appears to
have extended from the south pole through at least 40° of latitude. There is no evidence of a
contemporary ice cap in the Ordovician northern hemisphere, which, according to palaeogeog-
raphic reconstructions, had no continental areas near the pole at that time.
Glaciomarine environments. Tilloids of glaciomarine origin were initially identified by Dangeard
& Doré (1971) in Normandy, and by Hempel & Weise (1967) in Thuringia. Subsequently,
glaciomarine sediments, usually consisting of pebbly mudstones, have been recognized in Brit-
tany (Hamoumi et al. 1980), Celtiberia (Carls 1975), west central Spain (Robardet 1981) and
Portugal (Romano & Diggens 1973-74; Young 1985).
Most of the clasts in the tilloids can be matched with carbonate or coarse clastic horizons in
the underlying succession, indicating that at times the ice was grounded and caused erosion.
Striated clasts are recorded from Normandy (Dangeard & Doré 1971) and Navatrasierra,
western Spain (personal observation). Deposition, however, appears to have been from floating
ice, as indicated by the delicately laminated nature of some of the sediments, the presence of
dropstones in Brittany (Hamoumi 1981), but above all by the nature of the predominantly
massive sandy mudstones which lack associated sand deposits of fluvioglacial origin. In Spain
and Portugal there is evidence of regression and emergence prior to the deposition of the
tilloids (Fig. 2, sections 2 and 3), and there are variable proportions of normal marine sediments
interbedded with the glaciomarine sediments.
At the time of the maximum continental glaciation of the Gondwana plate, the adjacent
Armorican plate apparently lacked a continental ice sheet. Here, ice was locally grounded on
recently exposed shelf sediments but at times of slightly higher sea-level there was widespread
floating ice from which was deposited the mainly structureless sandy mud with its dispersed
clasts.
Clastic shelves. Many sequences which formed on clastic shelves show an upward passage from
mudstones to shallow marine sandstones. On shelves where there was an adequate supply of
sand complete upward-coarsening regressive sequences were formed starting with shelf muds
and passing gradationally upwards through various shoreface facies (Fig. 1, section 1), or
sometimes more abruptly into a variety of shallow marine facies (Fig. 1, sections 2 and 3). At
other places where there was channelling of the shelf, massive or cross-stratified sandstones lie
with a sharp erosional base on the underlying sediments (Fig. 1, section 4, might represent such
a situation). When a clastic shelf or relatively shallow basin was relatively starved of sediment
the regressive sequence is condensed, sometimes to as little as a metre, and may be partly
calcareous, as in Vastergotland (Fig. 1, section 5) where there is a thin oolite bed, or in the
Yangtze Basin where a thin bioclastic limestone caps graptolite shales (Fig. 1, section 6).
At most places shallow marine sediments of the regressive phase are succeeded abruptly by
mudstones with a benthic fauna indicating a deep shelf environment, or by graptolitic shales.
Facies formed during the rise of sea-level are usually less than a metre thick, suggesting that the
transgression was rapid.
Clastic basins. There is evidence from the Welsh Basin that the end Ordovician regression
caused sediments to be carried across the marginal shelves and produced an influx of coarse
clastics into previously mainly argillaceous basin environments. Pebbly mudstones of mass flow
origin, thick-to-thin bedded turbidites, some of which are channelled, and some slumped units
suggest the presence of substantial base-of-slope fans (Fig. 1, sections 7 and 8). At the north-
west margin of the basin, fan sediments with resedimented ooids and fragmented valves of a
Hirnantia fauna overlie trilobite-bearing mudstones, suggesting that this particular fan accumu-
lated at no great depth.
ENVIRONMENTAL CHANGES 379
Clastic shelves
1 2 3
Oslo Glynceiriog Girvan
S.Norway N.Wales S.Scotland
P.acuminatus Deep shelf
G. persculptus
Oolite shoal
HIRNANTIAN
50-70m Scie 20-70m b Low energy shallow
; Regressive shoreface sequence Inntelln Sele. ~30m Shallow marine sands
= Shallow shelf | Coquinoid limestones cx | ey
RAWTHEYANA Deep shelf ( BAS ) a Mid-shallow shelf a Deep shelf
Garth Vastergotland E.Yangtze Gorges
Mid Wales Sweden China
P.acuminatus Mid - : Deep shelf (G ) =
Ginereculpts id -deep shelf ( BA.2-5 ) Deep shelf (G)
b Mid-deep shelf
15-50m f°: : Shoreface or channel sands = Shallowloolitersheat
HIRNANTIAN | >0-3m Mid shelf ( BA.2-3 )
| Td
wap
b |
= ane Shallow shelf Mid-deep shelf tt
ae — |) |=
Deep shelf ( BA‘5 )
RAWTHEYAN A
Deep shelf ( BA.5 ) a Deep shelf (G )
Clastic basins
v 8 e)
Plynlimon Towyn-Corris Moffat
Central Wales N.W.Wales S.Scotland
P.acuminatus b Anaerobic basin shales
G_persculptus 1-6m
f Muddy debris flows | =
b ~ 260m | 1:17m
~500m
HIRNANTIAN | | Oxidized basin shales
a
a a Slope base debris flow 0-96m
RAWTHEYAN Basin floor
Fig. 1 Generalized sections to show the sequence of environmental changes near the Ordovician/
Silurian boundary. Data for the interpretations are to be found in the following references. Section
1: (a) Husbergoya Shale, (b) Langoyene Sandstone; Brenchley & Newall 1980. 2: (a) Dolhir
Formation, (b) Glyn Formation; Hiller 1981; Brenchley & Cullen 1984. 3: (a) Drummuck Group,
(b) High Mains Formation; Harper 1981. 4: (a) Wenallt Formation, (b) Cwm Clyd Formation;
Williams & Wright 1981. 5: (a) Dalmanitina Beds; Stridsberg 1980. 6: (a) Wufeng Formation, (b)
Guanyinqiao Formation; Geng Liang-yu 1982. 7: (a) Nant-y-Moch Formation, (b) Drosgol For-
mation; James 1971; Cave 1979; James 1983. 8: Garnedd-Wen Formation; James 1972; James
1985. 9: (a) Upper Hartfell Shale Formation, (b) Birkhill Shale Formation; Williams 1983.
380 P. J. BRENCHLEY
In some basins which were isolated from a source of coarse clastics there were no obvious
changes in pelagic sedimentation, as in some of the graptolitic shale sequences in the Yukon
(Lenz 1982; Lenz & McCracken 1982). In a rather similar graptolitic shale sequence at Dob’s
Linn in the Southern Uplands of Scotland, the end Ordovician regression cannot be identified
but the transgression is reflected in a change from grey mudstones, without graptolites, to black
graptolitic shales (Fig. 1, section 9). This change from oxidized to anoxic sediments might
reflect the change from the vigorous bottom circulation of the glacial period to the more
sluggish circulation following the melting of the ice caps.
The graptolitic shales, which commonly succeed the coarser clastics formed during the
regression in basin environments, may contain a G. persculptus fauna, but may in other
instances have P. acuminatus or even younger faunas in the lowest horizons. The local absence
of the lowest Silurian graptolite zones is probably the result of erosion or non-deposition.
Similar hiatuses are being increasingly recognized in DSDP cores in areas of pelagic sedimenta-
tion (Moore et al. 1978). For example, widespread deep-sea erosion in the Miocene is associ-
ated with periodic cold-climate events, lower eustatic sea-level and an intensification of bottom
circulation (Keller & Barron 1983).
Carbonate shelves. Most of the very extensive carbonate platforms in North America and Arctic
Canada appear to have been exposed at the end of the Ordovician, producing regional discon-
formities (Lenz 1976, 1982). The sedimentological effects of the regressive—transgressive cycle
are commonly not easily recognized in shallow marine carbonate sequences. Nevertheless a late
Ordovician, generally regressive, sequence culminating in a widespread oncoid bed has been
recognized in Anticosti Island (Petryk 1981a), and this is succeeded by generally transgressive
sediments (Fig. 2, section 5). At Manitoulin Island, Ontario, two karstic horizons separated by
15cm of sediment occur close to the Ordovician—Silurian boundary in a sequence of shallow
marine carbonate facies (Fig. 2, section 4). The effects of the end-Ordovician regression can also
be recognized in the more offshore facies associated with carbonate mud mounds. In two of the
carbonate mounds of the Boda Limestone (central Sweden) there is evidence of emergence of
the mound crests, with karst surface on one mound (Fig. 2, section 6), and dripstone calcite
lining fissures in the other. Graptolitic shales, formed after the transgression, mantle the
‘o°| Oolites
| Bioherms
Besa Limestone
Breccia
Pebble horizon
a4 Tilloids
Sandstones and shales
Sandstones
is Grey shales
BA Benthic assemblage
G _Graptolites Key to Figs 1-2.
ENVIRONMENTAL CHANGES
Glacial sequences
1 2
Central Sahara
North Africa
P.acuminatus
Central Portugal
Deep shelf
» Deep shelf
G. persculptus Fault
Fluvio-glacial sand
varved and slumped clay
Sub-glacial marine-tilloid
381
3
Celtiberia
N.E.Spain
HIRNANTIAN Fluvio-glacial sand b |
Varved clay
~ 200m Sub-glacial marine tilloid
Terrestrial tillite 50-80m
a Shallow massive sands |
Pro-glacial and sub-glacial | Bedded sandstones
melt-out sands
Regressive shelf sequence 1
SSS = ? karstified surface
RAWTHEYAN Calcareous tuff
a Mid-deep shelf
Carbonate shelves
4 S)
Manitoulin Anticosti
Ontario, Canada E.Canada
P.acuminatus
G. persculptus
T5em C7
ZS Shallow high eneray shelf
Karst
HIRNANTIAN a
RAWTHEYAN
Carbonate mud mounds
6
KalhoIn
Central Sweden
P.acuminatus Deep shelf
G.persculptus Karst
Coquinoid limestone/above
~25m storm wave base
HIRNANTIAN
a Carbonate mud mound/below
wave base
RAWTHEY AN
Patch reefs - Oncolites
Above wave base
Shallow shelf
Above wave base
Keisley
N.England
Deep shelf
Syn - sedimentary breccia
Shallow carbonate
sands with Girvanella
Carbonate mud mound
below wave base
Fig. 2. Generalized sections to show the sequence of environmental changes near the Ordovician—
Silurian boundary. Data for the interpretations are to be found in the following references. Section
1: “Unit IV’; Beuf et al. 1971. 2: (a) Porto de Santa Anna Formation, (b) Ribeira do Bracal
Formation, (c) Ribeira Cimeria Formation: Young 1985. 3: (a) Cystoid Limestone, (b) Orea Shale:
Carls 1975. 4: Georgian Bay Formation, (b) Manitoulin Formation; Copper 1978; Kobluk 1984. 5:
(a) Ellis Bay Formation (up to Oncolites), (b) Becscie Formation; Petryk 1981a, 1981b. 6: Boda
Limestone; Jaanusson 1979; Brenchley & Newall 1980. 7: Keisley Limestone; Wright 1985.
382 P. J. BRENCHLEY
mounds and fill fissures in both cases. In the carbonate mound at Keisley, in northern England,
the regression is reflected by the development of beds containing the alga Girvanella at the top
of the mound. There is a final capping of breccia, a few cm thick, and this is succeeded abruptly
by graptolitic shales, again marking the transgressive phase (Fig. 2, section 7).
Bathymetric changes. There is good evidence that most carbonate and clastic platforms and
shelves shoaled to near sea-level or became exposed during the Hirnantian regression. Some of
the platforms were already shallow before the start of the regression, but some muddy shelves
which were initially below storm wave-base, suggesting water depths of several tens of metres,
also became exposed (Brenchley & Newall 1980). The relief on an erosion surface below the
Silurian in Iowa, USA, suggests that sea-level dropped at least 45m (Johnson 1975). The
emergence of the crests of carbonate mud mounds and the lining of fissures to a depth of nearly
30 m implies a sea-level fall of about 70m (Brenchley & Newall 1980). A sea-level fall between
50 and 100m seems likely though a figure of ‘not more than 20m’ has been suggested by Geng
Liang-yu (1982).
The widespread presence of grey mudstones with deep shelf benthic faunas prior to the
regression, but graptolitic shales after the transgression, suggests that the sea-level rise might
have been greater than its fall (Brenchley & Newall 1980). However, the evidence from carbon-
ate platforms does not support this because in general early Silurian carbonates are similar to
those of the late Ordovician and both suggest shallow marine environments. It may be that the
development of early Silurian graptolitic facies is determined more by the preceding transgres-
sion which drowned many source areas, rather than by a substantial increase in water depths.
Although only a single regressive phase followed by transgression is apparent in many
sections there is some evidence for oscillations of sea-level within the Hirnantian. Two karstic
horizons representing two phases of emergence were recognized at Manitoulin Island (Kobluk
1984) and in a carbonate sequence near Oslo (Hanken 1974). Three regressive phases were
identified by Petryk (1981b) in the upper Ordovician sequence on Anticosti Island. It is possible
that these bathymetric changes might be related to phases of growth of the continental ice caps
reflected by three separate horizons of till in the Saharan and South African sequences. Epi-
sodes of ice advance and retreat are now well documented in the Pleistocene record. Changes in
the size of the Pleistocene ice caps produced cyclic changes in the 18O/!°O isotopic record in
oceanic sediments implying temperature fluctuations with a periodicity of about 20000, 40000
and 100000 years (Hays et al. 1976) similar to those predicted by Milankovitch (1938) on
astronomical grounds. A similar cyclicity might be expected in earlier glaciations, and might be
represented by the three sea-level oscillations and three tills in the Hirnantian. However, the
time-scale of these oscillations is still unclear.
Geochemical changes. There are very few studies of sediments close to the Ordovician—Silurian
boundary which might show if the geochemistry reflected the climatic and other environmental
changes. A pilot study in a relatively uniform sequence of argillaceous sediments in the type
Ashgill area of northern England showed changes in carbonate, Fe and P content and in Fe,O,
activity at the base and/or top of the Hirnantian, which were correlated with minor changes in
lithology and probably with changes in palaeobathymetry (Brenchley 1984). A study of carbon
and oxygen stable isotopes in a sequence through a Boda carbonate mud mound showed
changes in '°O values which suggested a fall in sea-water temperature during the Hirnantian
(Jux & Manze 1979). Both these studies suggest that further geochemical work might prove
valuable in determining changes in sea-water chemistry and temperature during the Hirnantian.
Climatic changes. The distribution of late Ordovician glacial deposits suggests that continental
ice sheets extended from the south pole through at least 40° of latitude and that there was
floating ice for another 10° of latitude. The temperature of peripolar oceans would have been
substantially depressed during such periods of glaciation. The effect of glaciation on the tem-
perature of surface waters in lower latitudes is less easy to predict. Studies of surface waters at
18000 years B.P., during the last interglacial, show marked differences between the Atlantic and
Pacific Oceans, indicating there is no simple global pattern of temperature (McIntyre et al.
ENVIRONMENTAL CHANGES 383
1976; Moore et al. 1980). Two points possibly relevant to the reconstruction of Ordovician
climate do however emerge; one is that water temperatures in some tropical and temperate
areas may actually be raised during a glacial episode, and the second is that notably cooler
waters can develop in both temperate and tropical areas.
The widespread extension of cooler surface waters during a glaciation might explain the very
broad distribution of the Hirnantia fauna, thought by some to be a cool-water fauna, through-
out most temperate and sub-tropical regions during some part of Hirnantian times.
The possibility of elevated temperatures during a glacial phase might partly account for the
apparently anomalous occurrence of Hirnantian oolitic horizons in sequences which were
hitherto wholly clastic (Oslo in Norway, and Garth and Bala in north Wales). It is not
necessarily a contradiction that the sequences which contain oolites also contain an Hirnantia
fauna, since the changes of sea-surface temperatures can be substantial between glacials and
interglacials, particularly in mid-latitudes.
A tentative construction of Hirnantian climate is that polar and peri-polar regions remained
cool to glacial throughout the Hirnantian, mid-latitudes had very variable climatic conditions
varying in time and space from cool to warm, while tropical areas in general remained hot. The
climate instability and geographic contrasts of the Hirnantian were succeeded by more stable
conditions in the Silurian. It is thought that the climate was in general similar to that of today,
but that climatic belts were more nearly parallel to lines of latitude because of the relative
absence of land in low latitudes (Ziegler et al. 1977).
Palaeomagnetism. The distribution of continents, based on palaeomagnetic evidence, has been
reconstructed for the middle Ordovician and for the early Silurian (Ziegler et al. 1977; Ziegler
& Scotese 1979; Scotese et al. 1979). Unfortunately there are no maps of comparable detail for
the Upper Ordovician. Early Silurian reconstructions show Gondwanaland lying in high
southern latitudes and other continents spread across the southern hemisphere and into mid-
northern latitudes. No continents are located in high northern latitudes.
There is some evidence from the shape of the apparent polar-wandering paths of the Ordovi-
cian that the continents must have moved unusually fast in late Ordovician times, to create the
Lower Silurian palaeogeography. Some confirmation of this rapid movement comes from a
wealth of palaeomagnetic data in north and west Europe, which shows upper Ordovician
(Caradoc and Ashgill) magnetism with steep inclination, implying a new polar position, con-
trasting with earlier and later data with significantly lower inclinations (Piper, 1987). Palaeo-
magnetic data from China also shows uppermost Ordovician poles differed in position from
those earlier and later (Wang Xiaofeng et al. 1983). If these proposed unusually high rates of
continental movement are confirmed they could have a significant bearing on the growth and
decay of the late Ordovician ice caps (Piper, 1987).
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Index
This is a selective index, for example the many references to the acuminatus Zone have largely been
omitted. Principal references are shown in bold type. In fossil names ‘aff.’, ‘cf.’ etc. have been left out.
Aalair 139 Aquitaine 77
Abbey-Cwmhir 66 Aratane 180
Abergwesyn 66 Arctic Islands 260
Abergynolwyn 66 Arctic Platform, Canada 260
Aberystwyth 66 Argentina 285-6, 291-7
Abteilli Group 180 Argentine Precordillera 295
Acanthochitina barbata 318, 320 Arina Formation 85
Acernaspis 359 Arkansas 276, 278, 330
(Acernaspis) salmoensis 372 Armoricanium 42
norvegiensis 372 squarrosum 302
primaeva 360 Armoricochitina nigerica 317-8
Achatella truncatocaudata 48 Arndell Sandstone 191
acritarchs 41, 299-309 Aroostook-Percé Anticlinorium 239
acuminatus Zone 5, 9, 14, 53, 128, 345-6 Arrkine Formation 171
Adavere Regional Stage 88 Artchalyk Beds 312
Adrar 177-80 Asaphidae 359
Africa, north 301; South 355, 357, 378 ascensus Subzone 96, 98, 127-8
Ain Oui n’Deliouine 165 Ashchisu River 145
Akidograptus acuminatus, see acuminatus Zone Asker District 82
ascensus 25, 346, 348; Subzone 96, 98, 127-8 Atavograptus atavus 27, 163
xixiangensis 127 ceryx 27, 346
Akuna Mudstone 186 atavus Zone 53
Alaska 268, 281 Aulacera (Beatricea) 197
Algeria 171-6 . Australia 12, 183-94
Algonquin Arch 247 Austria 107-15
Allen Bay Formation 260 Azrou 165, 168
Allt-g6ch Grit 67
Alpeis horizon 149 Baie des Chaleurs 239
Altai Mountains 139-43, 312 Baillarge Formation 260
Altai—Sayan fold belt 139 Bajaokou Formation 123
Amazon Basin 285, 287 Bala 12, 66
Amorphognathus 326 Baltic Syneclise 85
ordovicicus 33, 326, 333; Zone 326-7, 338 Baltisphaeridium plicatispinae 304
shatzeri Zone 326-7 sp. 42
superbus 33, 326; Zone 326-7 Banjiuguan Formation 123
tvaerensis 326; Zone 326 Bardo Range 93
Amplexograptus inuiti Zone 222 Barrandian area 95
latus 223-4, 226 Batesville district 276, 278
prominens 223; Zone 221, 224 bathymetric changes 382
Anceps Band 19; anceps Zone 25 Bavarian Facies 103
Ancyrochitina ancyrae 42-3, 320 Beaverfoot Formation 259, 262
convexa 320 Béchovice sections 96
laevensis 320, 322 Becscie Formation 196, 199, 222, 311, 373, 381
Anglesey 66 Belfast Member 276, 353, 357
Angochitina longicollis 320 Belgium 320
Angullong Tuff 189 Benambran Orogeny 183
Anti-Atlas 165, 167 Benjamin Limestone 191
Anticosti Island 11, 195-237, 311, 328, 360, 373, Berwyn Hills 66
381; acritarchs 302 Betkainar Formation 147-9
Anzhar River 152 Bighorn Mountains 280
Aphelognathus grandis 205 Bighornia—T haerodonta Fauna 259, 262
pyramidalis Zone 334 Birkhill Shale 22, 25-7, 35, 379
Appalachians 274 Bischofalm Quartzite 111
387
388 ORDOVICIAN-SILURIAN BOUNDARY
Blackstone River 265-8
Boda Limestone 380-1
Bohemia 95-100, 311, 357
Bolinda Shale 184
Bolivia 286-8, 291
Borealis borealis 83
Bou M’haoud 175; Formation 174
Bowling Green Dolomite 279, 280
Brabant Massif 320
brachiopods 126, 200, 311-15
Brassfield Formation 276, 353, 357
Brazil 287, 352, 355, 357
Brevilamnulella kjerulfi 81, 312
British Columbia 259
Brittany 378
Bronydd Formation 69, 311
Browgill 55
Brummunddal 83
Brush Creek sections 276
Bryant Knob 279
Bryn-glas Formation 68
Builth Wells 66
Burma 7
Burmingham Member 240-1, 360
Buroblyanka Creek 140
Cadia Group 189
Calapoecia 200
California 280
Calingasta Formation 291; region 291
Calpichitina lenticularis 317-8
Calymenella bayani 73
Camaret 76
Canada 195-271, 381
Canawindra 189
Cancaniri Formation 291
Canomodine Limestone 189
Cantera Formation 286
Caparo Formation 289
Cape Phillips Formation 260, 262
Carnic Alps, Austria 11, 107-15, 333
Carys Mills Formation 275, 339
Cason Oolite 277-8, 330
Cedarberg Formation 355, 357
Cellon section 333
Cerrigydrudion 66
Chagyrka Creek 140
Chalmak horizon 133-4
Charysh-Inya Zone 139
Chesnaie Formation 75
Chicotte Formation 196
Chile 288
China 117-31, 312, 361, 379
Chineta village 142
Chingiz Range 150
chitinozoans 41, 317—23
Chokpar Formation 145, 148
Chu-Ili Mountains 145
Churchill River Group 262
Cincinnati Arch 275
Cincinnatian Series 276
Cliftonia psittacina 312
climacograptids 346
Climacograptus angustus 348
extraordinarius 25; Zone 19, 128, 345
hastatus 25
incommodus 172
innotatus 224
medius 62, 348
miserabilis 25, 62, 74, 230
normalis 25, 62, 230, 346, 348
rectangularis 346, 348
scalaris miserabilis 25
transgrediens 82
trifilis 346, 348
climatic changes 382
Clinch Formation 275, 357
Clorinda community 88
Cochabamba 291
Cochrane Formation 277, 279
Colombia 288
Complanatus Band 19, 22; Zone 19
Conochitina armillata 320
aspera 320
edjelensis elongata 320
electa 320
gamachiana 317-8
iklaensis 320, 322
micracantha 320
postrobusta 320
proboscifera 320
tormentosa 43
conodonts 31, 201, 250, 268, 325-43
Conwy 66
Cormorant Lake 256
Cornwallis Island 11, 260
Coronochitina fragilis 320
maennili 320, 322
taugourdeaui 318
Corris 66
Céte de la Surprise Member 240-2
Cotton Siltstone 188
Craigskelly Conglomerate 46
Criccieth 66
Cross Fell Inlier 59
Crozonaspis struvei 363
Cryptolithus portageensis 370
Cryptothyrella angustifrons 50, 313
crassa incipiens 313
Cwm-Clyd Formation 69, 379
Cwm Hirnant quarry 301
Cwmere Formation 68
Cyathochitina campanulaeformis 42
kukersiana 42
Cyclopygidae 359
Cyphoniscus socialis 360
Cyrtiacea 314
Cystograptus vesiculosus 346, 348
Czechoslovakia 95-100, 355
Dactylofusa maragensis 301
Dalmanella testudinaria 312
Dalmanitidae 363
Dalmanitina 364; Fauna 88, 312, 379
brevispina 362
hastingsi 361-2
malayensis 362
mucronata 109, 363
nanchengensis 362
socialis 363
(Songxites) 365
Dalmanitininae 364
Dapsilodus obliquicostatus 35, 37
Darraweit Guim Mudstone 184
Darriwilian Zone 185
Deep Creek Siltstone 184
Delegate, New South Wales 184, 186
Des Jean Member 240-1
Descon Formation 281
Desmochitina densa 320
minor 320
Deuxiéme Bani Formation 169
Deuxiéme Rang, Percé 243
Dewukaxia Formation 124
Dhlou Chain 177
Dicellograptus anceps Zone 25
complanatus 25
complexus 25
ornatus Zone 267
Diceratograptus mirus Zone 125
Dictyotidium 43
Didymograptus uniformis Zone 128
Diexallophasis 42
remota 304
Dilatisphaera williereae 301, 304
Dinas Mawddwy 66
Diplograptus bohemicus Zone 125
fastigatus 74
kiliani 172-3
modestus 348
Distomodus kentuckyensis 213; Zone 205, 327
D. kentuckyensis—D. staurognathoides Zone 83
D. staurognathoides Zone 327, 331
Djanet-In Dyjerane Oued 171
Dobele Formation 88
Dobra Sandstone 104
Dob’s Linn 11, 14, 17-44
acritarchs 41-4
chitinozoa 41—4
conodonts 31-9, 330
graptolites 22-7
Dolgii Formation 134
Dolhir Formation 379
Domasia 301
limaciformis 304
Don Braulio Creek 295; Formation 295-7
Draborthis caelebs 312
Drabovinella erratica 73
Drakes Formation 276, 353, 355, 357
Drevnyaya River 133
INDEX
Drosgol Formation 379
Drummuck Group 45, 379
Drygarn 66
Durben 146, 148; Horizon 311
East Baltic 85-91
East Qinling trough 125
East Yangtze Gorges 379
Eastern Tassili-n-Ajjer sections 171
Ecuador 288
Edgewood Group 279, 311, 362
Eisenackitina dolioliformis 320
El Kseib section 174-5
Elkhorn Formation 353
389
Ellis Bay 306, 311; Formation 196-7, 203, 222,
225, 302, 360, 381
Elmina Sandstone 352
English Head 197
Eochonetes advena 48
Eodalmanitina macrophtalma 363
Eoplectodonta duplicata 313
Eospirigerina 125, 312-3
Eostropheodonta hirnantensis 48, 312
mullochensis 83
Esquibel Island 281
Estonia 85-91, 312, 319-20
Eudolatites (Deloites) maiderensis 363
Eupoikilofusa ampulliformis 304
eustatic changes 377
extraordinarius Band 25
Zone 19, 128, 345
Fish Haven Dolomite 281
Fisher Branch Dolomite 255
Fjacka Formation 88
Florentine Synclinorium 191
Valley 191-2
Forbes area 188
France 73-9
Gala Greywacke Group 19
Gamachian 197, 199
Gamachignathus ensifer 203
hastatus 203
Gangmusang Formation 124
Gaojiawan section 121
Gara Bouya Ali 177
Gara Foug Gara 178
Gara Tembi sandstones 171
Garat el Hamoueid Group 177
Garnedd-wen Formation 379
Garth 66, 379
Garth Bank Formation 69
Gasworks Sandstone Formation 69
Gell Quartzite 194
geochemical changes 382
Georgia 356-7
Georgian Bay Formation 247, 381
Ghana 301
Ghogoult 165
390 ORDOVICIAN-SILURIAN BOUNDARY
Girardeau Limestone 279, 330, 362
Girvan 33, 45-52, 311, 379
Girvanella 60, 382
glaciation 6, 158, 175, 377-83
Glenkiln Shale 18, 20
Glyn Ceiriog 379
Glyn Formation 379
Glyptograptus 62, 348
avitus 25, 348
bohemicus 96
hudsoni 226
persculptus 25, 62, 348; Zone 9, 19, 53, 128, 345
posterus 25
sahariensis 171-4
tamariscus 19
Gonambonitacea 314
Gondwana 6, 377
Goniosphaeridium oligospinosum 304
Gordon Group 191
Gotland 355, 357
Goulburn 189
Graig-wen Sandstone 67
Grand Erg Occidental 301
Grande Coupe beds 240-1
graptolites 22, 126, 345-9
Great Basin 280
Greenland 7
Grés de Kermeur 75
Grés de Lamm-Saoz 75
Guanyingiao Formation 379
Gun River Formation 196, 233
Hadeland 83
Hagan Shale Member 357
Hamerodus europaeus 35
Hanadir Shale 156
Hanson Creek Formation 281
Hart River 266
Hartfell Shale 18, 20-7, 33, 379
Harz Mountains 101
Haverford Mudstone Formation 69, 362
Haverfordwest 66, 311
Hedrograptus 225
janischewskyi 235
Helgoya Quartzite 83
Hemiarges extremus 48
Hercochitinia turnbulli 42
High Atlas 165, 168
High Mains Sandstone 45, 311, 379
Hiiumaa Island 85
Himmelberg Sandstone 107
Hindella crassa 48
Hirnant, Wales 312
Hirnantia 48; fauna 6, 45, 62, 67, 81, 96, 115, 125,
200, 314
sagittifera 312
Hirnantian 313, 359-60
Hodh escarpment 177, 179
Hogklintia digitata 304
Hoher Trieb section 109
Hol Beck 311
Holorhynchus 83, 314
giganteus 81, 135, 146, 312
Holotrachellus punctillosus 146
Holy Cross Mountains 93
Honorat Group 239
Howegill Fells 53-4
Hubei 11, 118
Hudson Platform 12, 260-2
Husbergoya Shale 81, 371, 379
Ibbett Bay Formations 260
ice cap 377
Icriodella deflecta 213
discreta 213
I. discreta-I. deflecta Zone 82, 252, 327, 331
inconstans Zone 327
Idaho 280
Ideal Quarry Member 279
Illinois 362, 367
Immouzer du Khandar 169
In Djerane Oued 171
Ina River 133
Interlake area 255—7; Group 255
Iowa 382
Iryudi Formation 134-5
Itaim Formation 352
Italy 107
Jbel Eguer-Iguiguena 165
Jbilet 165, 167
Jebel Serraf Formation 174-5
Jenhochiao Formation 124
Jerrara Beds 189
Jiancaogou Formation 123
Jumpersian 197
Juniata Formation 275
Jupiter Formation 196, 307
Juuru Regional Stage 90
Kabala Formation 88
Kagawong Member 247, 353
West Quarry 249
Kalholn 381
Kaliningrad 88
Kalochitina 43
Kalvsjo Formation 83
Kaochiapien Formation 123
Karasay River 149, 152
Karlik 97
Kaskattama well 262
Kazakhstan 12, 145-53, 311, 362
Keel Formation 277, 330
Keisley 59-63, 312, 363, 381
Limestone 59, 381
Kentucky 355, 357
Kerguillé Groupe 77
Kermeur Formation 77
Kiesselschiefer-Fazies 103
Kildare 311
Kinnella kielanae 312, 314
Kjorrven Formation 83
Kloucekia (Phacopidina) solitaria 363
Koichin Formation 147
Koigi Member 85
Kok Formation 111, 115
Kolyma Basin 133
Konglungen 82
Korgessaare Formation 88
Kosov Formation 96, 99, 311, 351, 355, 357
Kraluv Dvur Formation 95-6
Kuanyinchiao Beds 117, 312
Kuldiga Formation 88
Kurama Range 172
Kuznetsk Alatau 139
Kysylsai Formation 147
La Cantera Formation 295-6
La Chilca Shale Formation 291, 293
La Rinconade Formation 291
Labrador Sea 302
Lachlan Fold Belt 183
Lady Burn Conglomerate 46; Formation 311
Ladyburn Starfish Beds 46
Lagenochitina prussica 318, 320
Lake District 12, 53-7, 362
Lake Vyrnwy 66
Lande Murée Formation 74
Langara Formation 81, 311, 360
Langkawi Islands 334, 362
Langgyene Formation 81, 311, 360, 379
Latvia 85
Lederschiefer 103, 105
Leemon Formation 367
Leptaena rugosa 312
Leptaenopoma trifidum 312
Levaya Khekandya River 133
Liangshan 121
Libya 7, 301, 318
Linda Valley 191
Linhsiang Formation 120
Linn Branch 22-4
L’Irlande Member 240-3
Lithuania 85
Litohlavy Formation 97-8
Llallagua Formation 287
Llandeilo 66
Llandiloes 66
Llandovery 9, 11, 66, 311, 331
Llangollen 66
Llangranog 66
Llansawel 66
Llantsantffraid ym Mechain 66
Llanuwchllyn—Llanymawddy 66
Lodénice 97
Love Hollow Quarry 276, 278
391
Lugian Zone 101
Lukavy Creek 133
Lungmachi Formation 117—22
Macasty Formation 197
Machynlleth 66
Maine 275
Malaysia 7
Malvinokaffric Realm brachiopods 286
Manitoba 12, 255-97
Manitoulin Formation 247, 353, 381
Manitoulin Island 12, 247-53, 353, 357, 380-1
Maquoketa Shale 279
Martigné-Ferchsaud 75
Massif armoricain 73—7
Matapédia Group 239-44, 360
Mauritania 177-82
Maut Formation 134, 137
McAdam Sandstone 185
Mecoyita Formation 286
Medina Group 247
Melbourne 183
Ménez-Belair 73
Menierian 197, 199
Meriangaah Siltstone 186
Merida Andes 289
Michigan Basin 247
Midcontinent Province 326
Midcontinent Region, U.S.A. 12, 330
Millambri Formation 189
Minkutchar Beds 312
Mirny Creek 11, 128, 133-7, 311, 339
Mirorthis mira 312, 314
Missouri 11, 276, 279, 330
Mjoesa Limestone 83
Moffat 379; Shales 17, 20, 22
Mole Creek 191-2
Monograptus atavus Zone 53
cyphus praematurus 27
Montagne Noire 73, 77
Morocco 12, 165-70
Morriseau well 255
Moulay bou Anane 167
Mount Easton Shale 185; Province 185
Mount Kharkindzha 133
Mount Sinclair 259
Mount Wellington Belt 186
Mucronaspidinae 364
Mucronaspis 366; Community 96
danai 361-2, 366, 369
mucronata 362, 369
termieri 363
Mulloch Hill Conglomerate 46
Multiplicisphaeridium 304
Mynydd Cricor 66
Myoch Bay 33
Myren Member 82, 311
Nant-y-Moch Formation 379
392 ORDOVICIAN-SILURIAN BOUNDARY
Nanzheng Formation 121 Pabos Formation 240-1
Nashville Dome 275 pacificus Zone 25, 128, 146, 267
Neseuretus 162 Padun Formation 134
Nevada 7, 12, 281 palaeomagnetism 383
Newfoundland 12 palynomorphs 41, 201, 351
New South Wales 183, 186—9 Paraclimacograptus 225, 229
New York State 302, 353 decipiens 223—4, 226, 229
Neznakomka River 133 innotatus 229, 234, 348
Niagara Escarpment 247, 251, 356 manitoulinensis 223, 229
Noix Oolite 279, 330 Paraguay 288
Noixodontus girardeauensis 268, 330 Parakidograptus acuminatus 25, 62, 346, 348;
Nolblinggraben section 109 Zone 127
Nonda Formation 259 praematurus 348
Normandy 73, 378 Parana Basin 287, 355
North Africa 301 Paraorthograptus 225
North Atlantic Province 326 pacificus 234; Zone 25, 128, 146, 267
Norway 81-4 typicus 223, 234
Nova Ves 96 Parnaiba Basin 287
Paromalomena polonica 312
Pat Lake 265, 267
Ogilvie Mountains 265-6 Peace River 259
Ohio 275-7, 357 Pedley Pass 259
Ohne Formation 88 Peel River Section 265-70
Oklahoma 276, 301, 311, 330 Pennsylvania 302
Oman 156 Penwhapple Burn 46
Omuka Formation 134 Percé 12, 239-45, 311, 360
Omulev Uplift 133 persculptus Zone 9, 19, 53, 128, 345
Ontario 247-53 Peru 288
Orbiculoidea concentrica 312 Pheoclosterium 304
Orchard Creek Shale 279 Phragmodus undatus 203
Ordovician System 9 Pirgu Regional Stage 86, 90
Ordovician-Silurian Boundary 5, 13-14, 24-7 Plaesiomys porcata 48
Working Group 5, 9-15 plants 351-8
Orea Shale 381 Plas uchaf Grit 67
Orthograptus sinitzini 347 Plateau des Phosphates 165
truncatus 346 Plectochitina concinna 318
truncatus abbreviatus 73 pseudoagglutinans 320
truncatus pauperatus 73 sylvanica 318
truncatus socialis 25 Plectothyrella chauveli 175
truncatus truncatus 73 crassicostis 313-4
Orthosphaeridium insculptum 304 Plegagnathus dartoni 203
rectangulare 304 Pleurograptus linearis Zone 22
Osju Limestones 145, 148 Plécken Formation 109, 115
Oslo 12, 81-4, 311, 331, 360, 379 Plynlimon 66, 379
ostracode faunas 201 Pointe Laframboise 206, 21 1
Otyzbes Mountains 150 Pojo region 287
Oualata 181 Poland 12, 93, 102, 312
Oued Ali Formation 174-5 Pont Erwyd 12
Oued Chig Group 180 Porkuni Regional Stage 85, 90
Oued In Djerane Formation 171—4 Porsgrunn 83
Ougarta Range 174-5 Portage River 243
Oulad Said 165 Port Menier 195
Oulodus kentuckyensis 37, 213 Port Nelson Formation 262
nathani 213; Zone 205 Portfield Formation 69
robustus 205 Porto de Santa Anna Formation 381
rohneri 205 Portugal 378, 381
ulrichi 205 Prague Basin 95-100
Ozarkodina hassi 213 Preacherville Member 353, 357
olkhamensis 213 Precordillera de San Juan 286
Presqu ile de Crozon 75
Prince of Wales region 281
Proboscisambon Community 96
Proconchidium tchuilensis 146
Prostricklandia prisca 145
Protopanderodus liripipus 35
Pseudobelodina dispansa 203
vulgaris 203
Pseudoclimacograptus 225, 346
manitoulinensis 234
orientalis 347
Pterochitina dechaii 320
Pumpsaint 66
Québec 195-245
Queenston Delta Complex 247
Qusaiba shale 156-7
Ra’an shale 156-7
Raikkiila Formation 85; Regional Stage 88-90
Rectograptus abbreviatus 230
Red Head Rapids Formation 262
Red Mountain Formation 356-7
Reporyje 97
Repy 95, 97
Rhabdochitina gallica 42
magna 43
Rhayader 66
Rheinisches Schiefergebirge 101
Rhuddanian 7, 313
Riadan Formation 75
Ribeira Cimeria Formation 381
Ribeira do Bracal Formation 381
Rich Mel’ Alg 165
Richardson Mountains 265-6
Richea Siltstone 194
Richmondian fauna 200
Ringerike 82
Road River Formation 265
Rockdale Formation 189
Rockwood Formation 356
Rocky Mountains 259-61
Rouge Member 240
Rytteraker Formation 83
Saaremaa Island 85
Saelabonn Formation 83, 332, 360
Sahara 171, 381
Saint-Germain-sur-Ille Formation 73
Salamat Formation 149
Saldus Formation 88
Salmon River 206, 211
San Juan 292-3
Saskatchewan 255
Saudi Arabia 155-63, 378
Saxonia 101
Saxothuringian Zone 101
INDEX
393
Scabbardella altipes 35, 37
Scalarigraptus 225
angustus 225, 230, 232
normalis 230
tubuliferus 226
Scania 12
scolecodonts 41
Scotland 17
Scrach Formation 69
Sequatchie Formation 275, 357
Serra Grande Formation 352
Severn River Formation 262
Severnaya Zemlya 334
Sexton Creek Limestone 279
Shaanxi 121
Shalloch Formation 46
Shellmound Formation 357
Shelve area, Shropshire 66
Sierra de Villicum 291—2, 295-7
Siluro—Devonian Boundary Working Group 9
Skelgill section 53
Skien 83
Skoyen Sandstone Formation 83
Slade and Redhill Mudstone Formation 69
Snowblind Creek 260
Solisphaeridium nanum 42
Solvik Formation 82, 332, 360
Songxites 365
Soom Shale 355, 357
South Africa 355, 357, 378
South America 285-97
South Dakota 280
South Threave Formation 46
Southampton Island 262
Southern Uplands 20
Spain 378, 381
Spathognathodus manitoulinensis 213
Spengill 54
Sphaerochitina lepta 318
spores 351-8
St Martin’s Cemetery Beds 67, 311
Stacitnai Formation 88
Stawy 312
Stellechinatum brachyscolum 42
Stonewall Formation 255—7; Quarry 255
stratotype 27
Stricklandia lens 313
lens 82
prima 82
Sweden 312, 318, 331, 355, 357, 379, 381
Sylvan Shale 279
Tabberabberan orogeny 183
Tabuk Formation 155
Taconic orogeny 274
Tagant 177, 180
Talacasto section 293
Tamsal Formation 88
394 ORDOVICIAN-SILURIAN BOUNDARY
Tanuchitina anticostiensis 318 Veryhachium corpulentum 42
bergstroemi 317-8, 320 lairdii 42
Taoudeni Basin 172, 177, 179 reductum 42
Tasmania 191-4 rhomboidium Zone 291
Taucionys Formation 88 Victoria 183-6
Tazekka 165, 169 Vietnam 7
Tcherskidium ulkuntasense 146 Vila Maria Formation 355, 357
unicum 135, 312 Villicum Hills 291—2, 295-6
Tennessee 356-7 Virgiana 313
Tetrahedraletes 351 f barrandei 199
tetrad spores 351 decussata 262
Thailand 7 Vormsi Regional Stage 85, 90
Thebesia admiranda 312 Virginia 357
scopulosa 81, 312
Thuringia 101-6, 378 Wagga Metamorphic Belt 183, 186-7
Tibet 118 Wales 65-71, 311, 379
Tiger Syncline 191-2 Wanyaoshu Formation 124
tillites 6, 378 Warbisco Shale 186
Tinioulig 181 Watley Gill 362
Tirekhtyakh horizon 133-4 Welsh Borderland 301
Titicaca region 288 Welshpool 66
Tombong Beds 186 Wenallt Formation 69, 379
Towy anticline 67 Westfield Sandstone 191, 193
Towyn 66, 379 Whirlpool Formation 247, 353, 356
Trail Creek 281. White Head Formation 240-1, 302, 311, 328, 370
Tralorg Formation 46 Williston Basin 330
Tregarvan 76 Wolayer Limestone 109, 115
Trematis norvegica 312 Woodland Formation 46
Tridwr Formation 69 Wufeng Formation 117, 126, 379
trilobites 200, 359-76 Wulipo bed 117
Trinucleidae 359 Wyoming 280
Triplesia alata beds 277
Trombetas Formation 287, 353
Tuscarora formation 275; Sandstone 353, 356 Xainza Formation 124
T ylotopalla 304 SES Ve
Uggwa Formation 107, 115, 333 Yalmy Group 186
Ulkuntas Limestone 146, 152 Yangtze Basin 117, 378
uniformis Zone 128 Yewdale Beck section 53
United States of America 273-84 Yichang 118
Usbekistan 172 Yukon 12, 265-71, 380
US.S.R. 85-91, 133-53
Ust’-Chagyrka village 142 Zelkovice Formation 95, 97
Utah 281 Zemmour Noir 177-82
Zhalair Formation 146, 148-50
Varbola Formation 88, 312 Zhideli River 152
Vastergotland 312, 331, 378-9 Zwischengebirge Mountains 103
Vaureal Formation 196-7, 221 Zygospiraella 172
Venezuela 289 duboisi 83, 313
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Bulletin of the British Museum (Natural History)
Geology Series
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