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JEFFERSONIANA

Contributions from the
Virginia Museum of Natural History

Number 19 10 January 2009

Unusual Cambrian Thrombolites from the
Boxley Blue Ridge Quarry, Bedford County, Virginia

Alton C. Dooley, Jr.

ISSN 1061-1878

Virginia Museum of Natural History
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Jeffersoniana, Number 19, pp. 1-14
Virginia Museum of Natural History

Unusual Cambrian Thrombolites from the Boxley Blue
Ridge Quarry, Bedford County, Virginia

Alton C. Dooley, Jr.

Virginia Museum of Natural History
21 Starling Avenue
Martinsville, Virginia 24112, USA
alton.dooley@vmnh.virginia.gov

ABSTRACT

Three unusual thrombolites were collected in June 2008 from the Late
Cambrian Conococheague Formation at the Boxley Materials Blue Ridge
Quarry in Bedford County, Virginia. These specimens are isolated low domes
with a thrombolitic core and a pustulate, stromatolitic outer layer. The two
largest domes have a distinctive thickened rim around their margins. There are
apparent traces across the upper surfaces of the domes that may indicate grazing
by invertebrates.

The overall structure and morphology of the Boxley specimens is reminiscent
of modem thrombolites forming in Lake Thetis, a saline lake in southwestern
Australia. The low domes and thickened rims in Lake Thetis specimens seem to
be a result of growth in a protected setting, with shallowing water levels. Based
on the similarities with the Lake Thetis specimens, the Boxley thrombolites may
have formed in a protected lagoonal setting with gradually dropping water levels,
followed by relatively rapid inundation and burial.

INTRODUCTION

Stromatolites are a eommon oeeurrenee globally in Proterozoie and
Cambrian sediments, and oeeur less frequently in post-Cambrian deposits
up to the present day. Examination of modem forms has demonstrated
that stromatolitie and thrombolitie mounds are eonstmeted primarily by a
variety of blue-green algal eommunities (Blaek, 1933; Logan, 1961; Sharp,
1969; Reid et al., 2000), although laminated stmetures reminiseent of algal
stromatolites ean be fonned by other organisms or by abiotie proeesses
(Logan et al., 1964; Ahr, 1971; Walter, 1976; Grotzinger and Rothman,
1996). Stromatolitie stmetures ean assume a variety of morphologies

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that are influenced both by the taxa present and environmental conditions
(Black, 1933; Logan et al, 1961; Dill et al., 1986), and most of these have
been observed in both modem and ancient stromatolites (Logan, 1961;
Hoffman, 1976).

Modern stromatolites are largely restrictedto hypersaline environments,
with a few exceptions (Dill, 1986). The extensive geographic range of
stromatolites in the Proterozoic and Cambrian suggests that stromatolites
previously lived in a greater variety of habitats than do modem examples,
and that the presence of stromatolites is not necessarily an indicator of an
intertidal setting (Aitke, 1967; Playford and Cockbain, 1969). The decline
of stromatolites beginning in the late Cambrian has lead to suggestions
that the rise of grazing and burrowing metazoans in the Cambrian resulted
in the restriction of stromatolites to marginal enviromnents after that
time (Awramik, 1971; Walter and Heys, 1985), although open-marine
stromatolites have been found in Devonian deposits (Playford and
Cockbain, 1969). The presence of burrowing metazoans may have also
led to the appearance of thrombolites (unlaminated algal mounds) in the
Cambrian (Walter and Heys, 1985).

GEOLOGIC SETTING

The Eate Cambrian Conococheague Formation in Virginia and
Maryland is a thrombolite- and stromatolite-rich limestone that has been
interpreted as a shallow subtidal to peritidal deposit (Demicco, 1983;
Osleger and Read, 1991). Regressive cycles up to 10 m thick tend to have
basal thrombolitic beds overlain by cross-stratifled oolitic grainstones
grading into bioturbated and mud-cracked rhythmites (“ribbon rock” of
Demicco, 1983). The cycles are capped by laminated and stromatolitic
limestones and dolostones. A section of the Conococheague is exposed
in the Boxley Materials Blue Ridge Quarry in Bedford County, Virginia
(Demicco, 1982). This quarry is located within the Blue Ridge fault
zone, and a highly deformed allochthonous block of the Conococheague
Formation, overlain by the overthrust Elbrook Formation, has been
exposed during quarrying operations over the last century (Figure 1). The
lithologies and cycles present in the Blue Ridge Quarry are comparable to
other Conococheague localities in Virginia and Maryland.

Dooley: Boxley Thrombolites

3

Figure 1. Section of the Conococheague Formation at the Blue Ridge Quarry. The black
line is approximately 2 meters, and indicates one of the thrombolitic beds. Inset: map of
Bedford County, Virginia showing the location of the Blue Ridge Quarry.

DESCRIPTION

In June 2008, Boxley employee Riehard Benge diseovered a large,
isolated thrombolite in blast-produeed tailings during operations at the
Blue Ridge Quarry (VMNH 160000; Figure 2). After diseovery of this
speeimen, additional examination of tailings from that blast revealed
two additional, mueh smaller bioherms (VMNH 160001; Figure 3). The
largest of these speeimens is a low dome, subeireular in plan view with a
long axis diameter of 190 em and a short axis diameter of 180 em. While
mueh larger bioherms have been deseribed (see for example Ahr, 1971),
the Boxley speeimen represents one of the larger eomplete and intaet
speeimens eolleeted, and is unusual among Conoeoeheague thrombolites
in that it represents a eomplete bioherm.

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Figure 2. VMNH 160000 thrombolitic bioherm. A, surface view. B, profile view. Scale = 50 cm.

Dooley: Boxley Thrombolites

5

Figure 3. VMNH 160001 thrombolitic bioherms. A, surface view. B, cross-section. Arrow
indicates thickened rim. Scale = 10 cm.

The internal waekestone - to - mudstone fabrie with Renalcis algal
stmetures of the Conoeoeheague thrombolites was deseribed in detail
by Demieeo (1982). Internally, all the Boxley speeimens have a mottled
appearanee; throughout most of the strueture laminations are sparse,
diseontinuous, and loeated only near the tops of the domes. The outer ~5
mm of the domes are stromatolitie, with distinet knobs eovering the upper
surfaee of the dome (Figure 4). These knobs are generally eireular to oval
in outline, and are up to approximately 3 em in diameter.

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Jeffersoniana

Renalcis are not obvious in these thrombolites, but filaments that eould
represent algal struetures are visible in plaees in the stromatolitie layer
(Figure 5).

While the internal strueture of the Boxley speeimens is similar to that of
typieal Conoeoeheague stromatolites/thrombolites, the overall morphology
is quite different. Most Conoeoeheague bioherms are widening-upward
struetures, whieh are fiat-topped and merged together at their tops, often
reaehing thieknesses of many meters (Demieeo, 1982; Osleger and Read,
1991; Figure 1). The thrombolitie units are very notieeable in most of
the deposits in the Blue Ridge Quarry. In eontrast, VMNH 160000 and
160001 are low-profile domes that narrow upward. The two small domes
are eoaleseed only at their bases, while the largest speeimen is isolated.

Figure 4. Knob on the surface of VMNH 160000, showing stromatolitie layers.

A unique feature of the Boxley thrombolites is the presenee of a
thiekened rim around the margin of the mounds. This rim eompletely
eneireles the mound in VMNH 160000 and at least partially surrounds

Dooley: Boxley Thrombolites

7

Figure 5. Scanning electron micrograph of a section of the stromatolitic layers of VMNH
160000, showing a possible algal filament. Scale = 10 pm.

the larger mound in VMNH 160001 (Figures 2 and 3); the smallest dome
seems to laek this rim. In VMNH 160000, the thiekened rim is thrombolitie
with a pustulate, stromatolitie outer layer, just as in the main part of the
dome.

The upper surfaees of the thrombolites exhibit a small number of
apparent traees, in the form of meandering grooves (Figure 6). These grooves
are approximately 3 mm wide and eut aeross both the stromatolitie knobs
and the troughs between adjaeent knobs. As eaeh traee is restrieted to the
stromatolitie layers (the living part of the dome), and runs tangential to the
surfaee of the thrombolite, they are more eonsistent with feeding traees than
residenee burrows. The large, primitive Matthevia\ms been interpreted

as a grazer of stromatolites and is known from the Conoeoeheague Formation
(Runnegar et al., 1975), although it has not been identified at the Blue Ridge
Quarry. Matthevia would be expeeted to leave a feeding traee approximately
3 mm wide, assuming that the relative proportions of its mouth and valves
were eomparable to those of modem ehitons.

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Jeffersoniana

Figure 6. Possible feeding trace on the surface of VMNH 160000.

DISCUSSION

In general, the Conoeoeheague thrombolite units have been interpreted
as forming in the shallow subtidal zone (Demieeo, 1982, 1983), eonsistent
with general suggestions about thrombolite distributions suggested by
Aitken (1967). These thrombolitie beds are often several meters thiek,
with the individual thrombolitie heads eoaleseing into a eontiguous mass
at the top of the unit. This pattern is typieal throughout the Conoeoeheague,
ineluding at the Blue Ridge Quarry. However, VMNH 160000 and 160001
differ from this pattern, in that the heads are low in profile and isolated,
and have unusual thiekened rims. In the seetion of the quarry where these
speeimens were eolleeted there is no obvious thrombolitie unit, and no
eomparable isolated thrombolites were found in plaee. This suggests a
somewhat different history for the Boxley speeimens, eompared to typieal
Conoeoeheague thrombolites.

The overall morphology of the Boxley thrombolites is reminiseent of
modem stromatolites/thrombolites forming in Lake Thetis, a saline eoastal
lake in Australia (Grey et al., 1990). The morphology of the Lake Thetis
thrombolites appears to be a result of unique eonditions found in that
environment.

Dooley: Boxley Thrombolites

9

Lake Thetis is a small saline lake fed by rainfall and groundwater
input, with a maximum depth of about 2.25 m (Grey et al., 1990). There
is a eomplex and diverse array of algal struetures present in and around
the lake, ineluding lithified stromatolites along the southwest shore
that were deseribed in detail by Grey et al. (1990). These stromatolites
are low steep-sided domes eomparable in size to VMNH 160000, with
diameters up to around 1.5 meters, and extend well above the lake surfaee
(Figure 7). The eenters of these domes are eroded and often eollapsed,
with aetive growth only oeeurring along the wetted sides of the domes.
The inside of the domes are thrombolitie, with a stromatolitie outer layer.
The stromatolitie layer is eomposed of laminated eolumns about 1.5 em in
diameter and approximately 5 to 15 em thiek, whieh give the outer surfaee
of the dome a pustulate appearanee. This is very similar to the stmeture
of the Boxley thrombolites, although in the Boxley speeimens the outer
stromatolitie layer is only a few millimeters thiek. The Lake Thetis domes,
and partieularly the stromatolitie layers, are limited to the south shore of

Figure 7. Thrombolitie domes from Lake Thetis, southwestern Australia. Individual domes
are approximately 1 meter in diameter. Image by Ruth Ellison, retrieved on 18 November
2008 from http://flickr.com/photos/laruth/153584043/. Used with permission.

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Jeffersoniana

the lake where wave aetivity is low, possibly beeause a gelatinous eoating
does not develop under high-energy eonditions (Grey et al, 1990).

Some of the Lake Thetis thrombolites have a thiekened rim reminiseent
of the Boxley speeimens (Figure 7). In modem stmetures this rim seems
to have formed as a result of the drop in water level in Lake Thetis over
the last several thousand years (Grey et al., 1990; Figure 8). The exposed
stromatolitie domes erode and eollapse when desieeated, even as growth
eontinues along the wetted margins of the dome. This suggests that a drop
in relative water level exposing the eenter of the dome is neeessary for the
formation of the thiekened rim.

Figure 8. Google Earth image of Lake Thetis. Black arrows indicate past shorelines of
receding lake. White arrow indicates the position of modern thrombolitic domes. Image
retrieved on 18 November 2008.

The Boxley thrombolites differ from the Lake Thetis examples in that
the thiekened rim is eombined with a large, well-developed thrombolitie

Dooley: Boxley Thrombolites

11

dome, suggesting a more eomplex history of water level variation than
the steady drop observed in Lake Thetis. The unique morphology ean be
explained by a seenario in whieh stromatolite growth was initiated in a
proteeted setting, sueh as a baek-barrier lagoon. The shallow, low-energy
water would prevent the development of large biohenns while allowing the
growth of broad, low domes, with simultaneous deposition of earbonate
muds. A slight drop in water level restrieted growth to the margins of the
dome, resulting in the development of the thiekened rim. A subsequent
inerease in water level (perhaps due to a breaeh of the isolating barrier)
buried the largest thrombolites under eoarser-grained earbonates. Efforts
to loeate isolated domes in situ have so far been unsueeessful, even though
their approximate position is known. That this seetion of the quarry also
seems to laekthe basal thrombolitie beds typieally found in Conoeoeheague
eyeles suggests that this limited area experieneed somewhat different
eonditions from the rest of the formation.

ACKNOWLEDGMENTS

I would like to thank Boxley Materials Company, and partieularly Ab
Boxley, Bill Hamlin and Tom Roller, for their donation of these speeimens
to the Virginia Museum of Natural History and their eontinued aeeess to
the Blue Ridge Quarry. Ruth Ellison provided images of the Lake Thetis
stromatolites. I would also like to thank James Beard, Brett Dooley,
and Andrew Moore for helpful diseussions, Mary Catherine Santoro for
assistanee in obtaining referenees, Mary Carmen for SEM images, and the
Jeffersoniana editorial board and outside reviewers for helpful eomments.

REFERENCES CITED

Ahr, W. M. , 1 97 1 . Paleoenvironment, algal structures, and fossil algae in the Upper
Cambrian of central Texas. Journal of Sedimentary Petrology, 41 :205-216.
Aitke, J. D., 1967. Classification and environmental significance of cryptalgal
limestones and dolomites with illustrations from the Cambrian and Ordovician
of southwestern Alberta. Journal of Sedimentary Petrology, 37:1163-1178.
Awramik, S. M., 1971. Precambrian columnar stromatolite diversity: refiection of
metazoan appearance. Science 174:825-827.

Black, M., 1933. The algal sediments of Andros Island, Bahamas. Philosophical

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Jeffersoniana

Transactions of the Royal Society of London, Series B., 222:165-192, Plates
21 - 22 .

Demicco, R. V., 1982. Upper Cambrian Conococheague Limestone, in P. T. Lyttle
(ed.). Central Appalachian Geology. Geological Society of America Northeast-
Southeast Joint Section Field Trip Guidebook, American Geological Institute,
pp. 217-254.

Demicco, R. V, 1983. Wavy and lenticular-bedded carbonate ribbon rocks of the
Upper Cambrian Conococheague Limestone, central Appalachians. Journal
of Sedimentary Petrology 53:1121-1132.

Dill, R. R, E. A. Shinn, A. T. Jones, K. Kelly, and R. P. Steinen, 1986. Giant
subtidal stromatolites forming in normal salinity waters. Nature 324:55-58.

Grey, K., L. S. Moore, R. V. Burne, B. K. Pierson, and J. Bauld, 1990. Lake Thetis,
Western Australia: an example of saline lake sedimentation dominated by
benthic microbial processes. Australian Journal of Marine and Freshwater
Research 41:275-300.

Grotzinger, J. P. and D. H. Rothman, 1996. An abiotic model for stromatolite
morphogenesis. Nature 383:423-425.

Hoffman, P, 1976. Environmental diversity of middle Precambrian stromatolites,
in M. R. Walter (ed.). Stromatolites. Elsevier, Amsterdam, pp. 599-611.

Eogan, B. W., 1961. Cryptozoon and associated stromatolites from the Recent,
Shark Bay, Western Australia. Journal of Geology 69:517-533.

Eogan, B. W., R. Rezak, andR. N. Ginsburg, 1964. Classification and environmental
significance of algal stromatolites. Journal of Geology 72:68-83.

Osleger, D. and J. F. Read, 1991. Relation of eustasy to stacking patterns of
meter-scale carbonate cycles, Eate Cambrian, U.S.A. Journal of Sedimentary
Petrology 61 : 1225-1252.

Playford, P. E. and A. E. Cockbain, 1969. Algal stromatolites: deepwater forms in
the Devonian of western Australia. Science 165:1008-1010.

Reid, R. P, P. T. Visscher, A. W. Decho, J. F. Stolz, B. M. Bebout, C. Dupraz, I.
G. MacIntyre, H. W. Paerl, J. E. Pinckney, E. Prufert-Bebout, T. F. Steppe,
and D. J. DesMarals, 2000. The role of microbes in accretion, lamination and
early lithification of modern marine stromatolites. Nature 406:989-992.

Runnegar, B., J. Pojeta, Jr., M. E. Taylor, andD. Collins, 1975. New species of the
Cambrian and Ordovician chitons Matthevia and Chelodes from Wisconsin
and Queensland: evidence for the early history of polyplacophoran mollusks.
Journal of Paleontology 53:1374-1394.

Sharp, J. H., 1969. Blue-green algae and carbonates — Schizothrix calcicola
and algal stromatolites from Bermuda. Eimnology and Oceanography
14:568-578.

Dooley: Boxley Thrombolites

13

Walter, M. R., 1976. Geyserites of Yellowstone National Park: an example of
abiogenic “stromatolites”, in M. R. Walter (ed.). Stromatolites. Elsevier,
Amsterdam, pp. 87-112.

Walter, M. R. and G. R. Keys, 1985. Links between the rise of the Metazoa and
the decline of stromatolites. Precambrian Research 29:149-174.

Parts published to date

1 On the taxonomy of the milliped genera Pseudojiilns Bollman, 1887, and Georgiiilus,
gen. nov., of southeastern United States. Richard L. Hoffman. Pp. 1-19, figs. 1-22.
1992. $2.00

2. A striking new genus and species of biy'ocorine plant bug (Heteroptera: Miridae) from

eastern North America. Thomas .1. Henry. Pp. 1-9, figs. 1-9. 1993. $1.00.

3. The American species of Escciryus, a genus of Holarctic centipeds (Geophilo-morpha:

Schendyhdae). Luis A. Pereira & Richard L. Hoffman. Pp. 1-72, figs. 1-154, maps
1-3. 1993. $7.00

4. A new species of Piito and a preliminary analysis of the phylogenetic position of the

Puto Group within the Coccoidea (Homoptera: Pseudococcidae). Douglass R. Miller
& Gary L. Miller. Pp. 1-35, figs. 1-7. 1993. $4.00.

5. Cambanis (Cambanis) angularis, a new crayfish (Decapoda; Cambaridae) from the

Tennessee River Basin of northeastern Tennessee and Virginia. Horton H. Hobbs, Jr.,
& Raymond W. Bouchard. Pp. 1-13, figs, la-ln. 1994. $2.00.

6. Three unusual new epigaean species of Kleptochthoniiis (Pseudoscorpionida:

Chthoniidae). William B. Muchmore. Pp. 1-13, figs. 1-9. 1994. $1.50.

7. A new dinosauromorph ichnogenus from the Triassic of Virginia. Nicholas C. Fraser &

Paul E. Olsen. Pp. 1-17, figs. 1-3. 1996. $2.00.

8. “Double-headed” ribs in a Miocene whale. Alton C. Dooley, Jr. Pp. 1-8, figs. 1-5. 2000.

$ 1 . 00 .

9. An outline of the pre-Clovis Archeology of SV-2, Saltville, Virginia, with special

attention to a bone tool dated 14,510 yr BP. Jerry N. McDonald. Pp. 1-60, figs. 1-19.
2000. $3.00.

10. First confirmed New World record of Apocyclops dengizicus (Lepishkin), with a key

to the species of Apocyclops in North America and the Caribbean region (Cnistacea:
Copepoda; Cyclopidae). Janet W. Reid, Robert Hamilton, & Richard M. Duffield. Pp.
1-23, figs. 1-i 2002. $2.50

1 1 . A review of the eastern North American Squalodontidae (Mammalia.Cetacea). Alton C.

Dooley, Jr. Pp. 1-26, figs. 1-6.2003. $2.50.

12. New records and new species of the genus Diacyclops (Crustacea: Copepoda) from
subterranean habitats in southern Indiana, U.S. A. Janet W. Reid. Pp. 1-65, figs. 1-22.
2004. $6.50.

13. Acrojwiuici yuchi (Plecoptera: Perlidae), a new stonefiy from Virginia, U.S.A. Bill P.
Stark & B. C. Kondratieff. Pp. 1-6, figs. 1-6. 2004. $0.60.

1 4. A new species of woodland salamander of the Plethodon cinereiis Group from the Blue

Ridge Mountains of Virginia. Richard Highton. Pp. 1-22. 2005. $2.50.

15. Additional drepanosaur elements from the Triassic infills of Cromhall Quarry, England.

Nicholas C. Fraser & S. Renesto. Pp. 1-16, figs. 1-9. 2005. $1.50.

16. A Miocene cetacean vertebra showing partially healed compression fracture, the result

of convulsions or failed predation by the giant white shark, Carcharodon megalodon.
Stephen J. Godfrey & Jeremy Altmann. Pp. 1-12. 2005. $1.50.

17. A new Crataegus-fQQd'mg plant bug of the genus Neolygiis from the eastern United
States (Hemiptera; Heteroptera; Miridae). Thomas J. Henry. Pp. 1-10. $1.50.

!8. Barstovian (middle Miocene) Land Mammals from the Carmel Church Quarry,
Caroline Co., Virginia. Alton C. Dooley, Jr. Pp. 1-17. $2.00.

19. Unusual Cambrian Thrombolites from the Boxley Blue Ridge Quarry, Bedford County,
Virginia. Alton C. Dooley, Jr. Pp 1-12, figs. 1-8, 2009. $ 3.00.

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