he Saint-Antonin Conglomerate
in the Maritime Alps: A Model
for Coarse Sedimentation on
a Submarine Slope

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IARINE SCIENCES.

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SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES © NUMBER 5

The Saint-Antonin Conglomerate in
the Maritime Alps: A Model for
Coarse Sedimentation on a
Submarine Slope

Daniel Jean Stanley

SMITHSONIAN INSTITUTION PRESS
City of Washington
1980

AUBi Swi REA Gil

Stanley, Daniel Jean. The Saint-Antonin Conglomerate in the Maritime Alps:
A Model for Coarse Sedimentation on a Submarine Slope. Smithsonian Contn-
butions to the Marine Sciences, number 5, 25 pages, 12 figures, 1 table, 1980.—
The Upper Eocene to Lower Oligocene Saint-Antonin Conglomerate, a
formation more than 1000 m thick well exposed in the French Maritime Alps,
about 30 km north of the Mediterranean coast, comprises coarsening-upward
successions, or megasequences, of silty shale- siltstone, sandstone and conglom-
erate sections. The megasequences include coarse channelized deposits asso-
ciated with coarse lenticular and fine-grained sheet facies that are identified
as migrating channels and lobe and channel overflow deposits. Microfossils in
the finer-grained units indicate dispersal in an open marine, outer shelf to
upper bathyal environment where minimal depths ranged from 100 to 200 m.
The spatial and temporal distribution patterns of facies successions, assem-
blage of stratification types and sedimentary structures, and petrology of the
various textural grades indicate submarine progradation on a slope, or in a
slope basin, seaward of a fan delta system. The Saint-Antonin Conglomerate
is more similar to alluvial fans than to some of the gravel-rich submarine fan
deposits that accumulate on a gentle gradient at the base of a slope. The
coarsening-upward megasequences record a strong tectonic overprint, includ-
ing a northward shift of the basin margin on which these strata were deposited,
concurrent andesitic flows and structurally-induced fan delta switching on the
adjacent land. This latter phenomenon was largely responsible for the irregular
back-and-forth migration of the sandstone and gravel-rich tongues on the
upper slope. Emplacement of poorly sorted (disorganized) conglomerates and
pebbly sandstones, and of strata displaying crudely stratified inverse grading
or preferred clast fabric, was largely by debris flow and associated high-
concentration dispersions. Slumping, turbulent flows with some bed-load
traction and turbidity currents also were effective mechanisms for the trans-
port‘of sediment to proximal depositional sites on the slope. Modern counter-
parts of the Saint-Antonin Conglomerate are probably to be found on the
leading edge of plates, rift margins and other tectonically-active coastal chain-
bounded margins where coarse terrigenous sediments bypass narrow shelves
and are transported directly on steep mobile slopes.

OFFICIAL PUBLICATION DATE Is handstamped in a limited number of initial copies and is recorded
in the Institution’s annual report, Smzthsonian Year. SERIES COVER DESIGN: Seascape along the
Atlantic coast.

Library of Congress Cataloging in Publication Data

Stanley, Daniel J

The Saint-Antonin Conglomerate in the Maritime Alps.

(Smithsonian contributions to the marine sciences ; no. 5)

ene Pp.
1. Conglomerate—Maritime Alps. 2. Geology, Stratigraphic—Eocene. 3. Geology, Strati-
graphic—Oligocene. _ I. Title. II. Series: Smithsonian Institution. Smithsonian contribu-
tions to the marine sciences ; no. 5.

QE471.15.C6S7 551.78’ 0944°941 79-23779

Contents

Page

jC NCNOIN a o.d le ace 6 Banc SB PON'S QE Cat OU IY Gree en ae 1
NEKO WICC CTIIC MISE ee ett nT Tent RT UE hoe ea A 3
Geologicalasetuine mare me ere ere eA oe ley 3
Generalystratignaphic PramewOrk=s 23.5.4. 44.¢-2-2-e e te e e 4
Wowernsviemben about a00tmiuthick)e ss). 2. 14s.) l/s dee cea 5
MiddlesMiember:(aboutes50mmr thick). 362 ho. ode ce et 5
WropemVlembers( = 200i) ee vee ee es ke seen ies sehen 6
Haciess Organization: andeAssociatlonsir 20 4)252 0 4.) 22522, ee 8
CoarseyChannelizedvVAssociations.. =). ))j. 2. se ene es oy 9
Coarscepbenbticulary Associaton oer ce ee ake bene cee aes 9
ine craimedustieetyASSOCIAUOM (ec ce ec oe os ee ee. 13
Proximal slope—siope Basin’ Fan Model ......................2 6.4... Ie
SULMDEINY «5 gps dd ais 6 bo ine bah mee Re a See ee el ee 21
LaiereCUERD CHGS | bes) aracths eye e bebe aA SRE NBS Se cOer al ne 23

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The Saint-Antonin Conglomerate in
the Maritime Alps: A Model for
Coarse Sedimentation on a
Submarine Slope

Daniel Jean Stanley

Introduction

The origin of deep marine gravel facies remains
poorly understood. On modern subaqueous
slopes, pebble- and cobble-rich deposits constitute
channelized tongues in canyons and fan valleys
(Shepard and Dill, 1966; Whitaker, 1976; Stanley
and Kelling, 1978), talus on seamount flanks,
oceanic ridges, and arc settings (Heezen and Hol-
lister, 1971), progradational wedges off reefs and
carbonate shelves (Cook and Enos, 1977), and
ice-rafted patches at high latitudes (Brundage, et
al., 1967). Most studies of gravity-emplaced,
coarse terrigenous sediments on modern slopes
emphasize channelized settings where they are
frequently observed and dredged (Gennesseaux,
1966; Hersey, 1967; Stanley, 1974; Heezen and
Hollister, 1971), and recorded on high-resolution
subbottom profiles (Embley, 1976). Gravels, peb-
bly sands and pebbly muds also are recovered at
and beyond the base of slopes, most commonly in
the vicinity of submarine valleys (Shepard and
Dill, 1966). Coarse surficial deposits are difficult
to core, and in consequence, little is known of
their structure and fabric or of the transport

Daniel Jean Stanley, Division of Sedimentology, National Museum of
Natural History, Smithsonian Institution, Washington, D.C. 20560.

mechanisms responsible for their emplacement.
Attributes of pebble-rich slope sequences are
more readily investigated in the rock record, and
here most attention has been paid to deep marine
channelized canyon and fan valley conglomerates
and to coarse base-of slope, rise and fan lobe
sequences. Information on gravel-rich sequences
in more proximal and non-channeled submarine
slope environments is actually quite limited. The
present study identifies such a deposit, the Saint-
Antonin Conglomerate (anglicized from “con-
glomerats de Saint-Antonin” and “formations
detritiques de Saint-Antonin”; cf. de Laparrent,
1938; Stanley, 1961; Bodelle, 1971), a coarse Neo-
gene terrigenous formation in _ southeastern
France (Figure 1). This formation, the youngest
marine complex in this sector of the Maritime
Alps, crops out in an elongate syncline about 15
km wide near the villages of Saint Antonin, Col-
longues, and La Rochette (Figure 1). The large
size of metamorphic and igneous debris provides
some indication of proximality to source and
transport by powerful torrential flow (Vernet,
1964a). Most workers who previously examined
this formation have asserted that its conglomer-
atic and pebbly sandstone series were emplaced
close to a site of sediment entry on the margin of

D a oOo aye
20 0° Sq CONGLOMERATE FACIES

© 906

Vv
VOLCANIC BRECCIA

AC
UPPER MEMBER

MIDDLE MEMBER SAINT-ANTONIN

CONGLOMERATE
LOWER MEMBER

« thin calcareous sandstone

MARNES BLEUES
SILTY SHALES

NUMMULITIC LIMESTONE
AND SHALY LIMESTONE

MB
cs

MB

Ficure 1.—Geological map of Saint-Antonin syncline study area in French Maritime Alps
showing three stratigraphic members (1, 2, 3) of the Saint-Antonin Conglomerate and location
of major conglomerate sequences (simplified from Bodelle, 1971). (LP = La Penne; SP = Saint-
Pierre; A, B, C = simplified columnar depictions of three major upward-coarsening megase-
quences mapped along SSW-NNE transect in vicinity of village Saint-Antonin; MB = Marnes
bleues silty shales; CS = Calcareous sandstone; AC = first appearance of andesitic cobbles in
megasequence A.)

NUMBER 5

a tectonically active basin.

The coarse strata have been interpreted in sev-
eral ways: as alluvial and lacustrine deposits (de
Lapparent, 1938), fluvio-deltaic series (J.-P. Ber-
trand, pers. comm., 1974, 1976), marine near-
shore deposits (Boussac, 1912; Bodelle, 1971), and
marine delta-front platform and proximal margin
deposits (Stanley, 1975). The present study fo-
cuses on the temporal arrangement of the terri-
genous units as thick coarsening-upward succes-
slons, or megasequences, and on the spatial dis-

tribution comparable to some alluvial and sub-.

marine fan facies. On the basis of such lithofacies
successions and their assemblages of stratification
types and sedimentary structures, I propose a
marine slope depositional origin for the Saint-
Antonin Conglomerate. The depositional model
involves (a) shelf bypassing and direct accumu-
lation of gravelly fan delta sediment on the mar-
gin of a structurally mobile basin at depths
greater than formerly envisioned, possibly in a
depression or perched slope basin, and (b) fan-
like emplacement of coarse sediment by slump-
ing, debris flows and associated high-concentra-
tion dispersions, and by traction processes.

ACKNOWLEDGMENTS.—Appreciation is ex-
pressed to Mr. J.-P. Bertrand, Institut Francais
de Petrole, and to the 1978 Valberg Penrose
participants for stimulating discussions at the
outset that prompted further research; Mme.
G. Glagon, University of Paris, for identification
and interpretation of foraminifera; and Drs. J.
M. Coleman, Louisiana State University,
M. A. Hampton, and T. H. Nilsen, both of the
US. Geological Survey, J. Le Fournier, SNEAP
(Boussens, France) and R. D. Winn, Jr., Mara-
thon Oil Co. for their constructive critique of this
paper. Funding for the study, part of the Medi-
terranean Basin (MEDIBA) Project, was pro-
vided by a Smithsonian Research Award (grant
FY-79-89191028).

Geological Setting

The Saint-Antonin syncline is located approx-
imately 9 km north of the Cheiron Mountain

Chain, and about 4 km south of the Var River
and 30 km northwest of the city of Nice on the
Mediterranean coast (Figure 1). This structure
lies within a sector of the Arc de Castellane
tectonic belt where trends are oriented primarily
east-west (Goguel, 1936). Most workers have as-
signed an Upper Eocene (Priabonian) age to the
Marnes bleues marl and shale section underlying
the more than 1000 m of poorly cemented sand-
stone and conglomerate (de Lapparent, 1938).
The age of the coarse series is not as well defined.
On the basis of fauna and petrology Vernet
(1964b) attributes an Oligocene age to the mas-
sive bedded sandstones and pebbly mudstones
above the Marnes bleues, and a lower Miocene
age to the overlying coarser conglomeratic series
and associated extrusive volcanics. In a more
recent biostratigraphic analysis, Bodelle (1971)
assigns a latest Eocene-earliest Oligocene (Globi-
gerina gortanit zone) age to the sandstone, pebbly
sandstone, and lower conglomeratic series with-
out volcanics (his “premiere formation detri-
tique”’), and a lower Oligocene age to the thick
conglomerate sequences with volcanics (his “‘sec-
onde” and ““troisieme formation deétritique’”).
Field measurements of sedimentary structures
indicate a predominant SE to NW paleocurrent
trend (Kuenen, et al., 1957; Stanley, 1961; Bod-
elle, 1971). A southern provenance is corrobo-
rated by the composition of heavy and light sand-
size material and of cobbles and boulders (de
Lapparent, 1938; Stanley, 1961, 1964; Vernet,
1964b; Bodelle, 1971). Sources include both ig-
neous and metamorphic terrains in the Maures-
Esterel Massif located about 30 to 50 km to the
south of the Saint Antonin exposures, and sedi-
mentary (largely Mesozoic limestone and shale)
formations in the adjacent Maritime Alps. How-
ever, the origin of ophiolitic and other Saint-
Antonin cobble and boulder lithologies, particu-
larly granitic rock types that do not occur in the
Argentera-Mercantour and Maures-Esterel mas-
sifs (shown on the inset map in Figure 1), is
problematical: a derivation from once-exposed
crystalline terrains to the southeast and east, areas
presently covered by the Ligurian Sea or buried

by the Helminthoid Flysch, or both, has been
suggested by the above-cited authors.

On the basis of geodynamic and paleogeo-
graphic considerations it would appear that Cor-
sica and its northern and western extensions were
once much closer to the present French Provengal
coast (Kuenen, 1959; Stanley, 1961; Nairn and
Westphal, 1968; Bodelle, 1971; Alvarez, 1972;
Moullade, 1978). The pre-Miocene reconstruc-
tions that place the Corsica-Sardinia land mass
close to, or against, southern Europe would delin-
eate a much enlarged source area that could have
furnished the large volumes of coarse sediment
transported northward to the Maritime Alps and
northern Apennines at the end of the Eocene and
in the early Oligocene (Stanley and Mutti, 1968;
Stanley, et al., 1978, fig. 17). The best fit between
Corsica and the Provengal margin brings Hercy-
nian and Alpine series of Corsica to within 60 to
70 km south and southeast of the Saint Antonin
syncline (Bodelle, 1971, fig. 210). This scheme,
still a speculative one, might well account for the
large size of Corsican-type crystalline debris and
the recorded dispersal pattern toward the NW.

As presently envisioned, deposition of the lower
sandstone, pebbly sandstone, and the first coarse
conglomeratic sequences of the Saint-Antonin
Conglomerate was contemporaneous and geneti-
cally related to the thick sandstone series (“gres
d’Annot”’) to the north and northwest in the
marine Annot Basin (de Lapparent, 1938; Stan-
ley, 1961, 1975). The mid- and upper terrigenous
series of the Saint-Antonin Conglomerate record
the final stages of marine deposition in this part
of the Maritime Alps; as these sequences accu-
mulated in the Saint-Antonin region, the Num-
mulitic Sea had largely receded from the Annot
Basin (Bodelle, 1971, figs. 206-208). Large vol-
umes of andesitic debris introduced from local
volcanic centers, and unconformities within the
conglomeratic series, amply document local emer-
gence and structural displacement of this region
during the early part of the Oligocene. Following
deposition vigorous Middle and Upper Oligocene
folding and thrusting prevailed, and further in-
tense structural deformation affected the region

SMITHSONIAN CONTRIBUTIONS TO MARINE SCIENCES

in the Miocene and Pliocene during regional
uplift of the southern Alps and evolution of the
Ligurian Sea.

General Stratigraphic Framework

The coarse conglomerate series in the Saint-
Antonin syncline were deposited above a lower
Tertiary sequence consisting, from base up, of
Eocene Nummulitic limestone (‘“‘calcaires a Num-
mulites”) and shale-limestone series (“calcaires
argilo-sableux’”’), thick (about 200 to 300 m) Up-
per Eocene gray to bluish silty shales (““Marnes
bleues”’), and massive bedded sandstones (Log A
in Figure 1). These series tend to be best exposed
on the southern, more gently dipping (30° to 40°)
limb of the syncline. The stratigraphy, petrogra-
phy, and faunal content of these formations in
the Saint Antonin syncline and adjacent areas
have been detailed by Bodelle (1971) and Cam-
predon (1972).

The Marnes bleues marls and shales include a
variable number of thin (most 5-25 cm; a few to
100 cm), calcareous siltstones and sandstones and
calcarenites about half-way upsection in the
southern part of the syncline, southwest of Saint-
Antonin and northeast of Collongues (units CS
in log A, Figure 1). These coarser units, alternat-
ing with gray-blue silty shales, commonly contain
plant matter, mica, glauconite, mollusc frag-
ments, and foraminifera, including both large
and small benthonic and planktonic tests. Anal-
ysis of this facies and associated microfossil assem-
blages suggests deposition at neritic depths, pos-
sibly 100 m or less (Bodelle, 1971). The calcareous
sandstone and detrital limestone units accumu-
lated in an environment affected by bottom cur-
rents and favorable to benthic organisms as re-
corded by the bioturbated base, moderate to
highly cross-stratified internal structure, and rip-
pled upper bedding surface. A few of the calcar-
eous sandstone beds are not cross-stratified but
moderately graded with sole markings indicating
transport primarily toward the NW;; these units,
resembling poorly developed turbidites, record a
more rapid introduction and burial of sand on
the margin.

NUMBER 5

The massive sandstone and overlying conglom-

| erate facies in the eastern part of the syncline are
| well exposed along the small road (D-427) leading
- to Saint-Antonin. On the basis of this and other
cross-syncline transects, Bodelle (1971; 158-171)

has identified three members briefly described
below (their distribution is depicted on the map

| in Figure 1). The name Saint-Antonin Conglom-

erate is applied here as the formation name for
the stratigraphic section formed by the three
members.

Lower Memeser (about 400 m thick).—The

lower 120 m of this member comprises light-col-

_ored, friable (argillaceous and calcareous ce-
| ment), coarse-grained, poorly sorted sandstone
_ and pebbly sandstone. Common features include
_ cross-stratification (foreset laminations oriented
_ toward the W and NW) with thin shaly interbeds

(Figure 2A). Asymmetric ripple surfaces (current
toward the W and NW) and plant-rich layers are

common (Figure 2B). The massive sandstone

strata are amalgamated units that consist of in-
dividual layers 30 to over 100 cm thick. Erosional
features (cut-and-fill, small to moderate-size
channels, large rip-up clasts) that abound within
most units record migrating channel processes.
These sandstones display both horizontal and
forset stratification, and the small pebbles com-
monly concentrated along these surfaces provide
evidence of emplacement by traction mechanisms
or represent lag deposits. A few thin (< 1 m)
individual sandstone layers, poorly to moderately
graded with sole markings, including flame struc-
tures, also are observed. The first major conglom-
erate is a 5 m-thick, sharp-based, poorly sorted,
matrix-supported cobble and boulder mudstone
showing poorly developed vertical grading of the
larger debris (Figure 2C). Coarse debris of diverse
provenance include subrounded to well-rounded
granite, rhyolite, and other igneous and meta-
morphic lithologies, and minor amounts of sedi-
mentary clasts. This unit is followed by a thick
succession of sandstone, pebbly sandstone, and
conglomeratic strata of variable thickness, coarse-
ness, and configuration (Figure 2D); some are
sharp-based lenticular units and others chan-

nelized. Further description of the lithofacies in
this and the other two members are presented in
following sections.

MippLe Memser (about 350 m thick).—The
base of this member, a coarse debris, matrix-
supported conglomerate, is identified by andesitic
cobbles and boulders (Figure 3B) of local deri-
vation (Sant-Antonin extrusive volcanics have
been described by Vernet, 1964a and Bodelle,
1971) that complement polymictic crystalline and
sedimentary debris comparable to that of the
Lower Member. Sandstone strata and the sand-
stone matrix in conglomerates tend to be darker
(ochre) than those in the lower member, in part
reflecting a compositional change: higher propor-
tions of iron-stained grains, amphiboles, pyrox-
enes, and plagioclases. A thick (about 50 m) series
of alternating laminated siltstones and shales is
interbedded between conglomerate beds; the
finer-grained layers include a benthic and plank-
tonic foraminiferal fauna (Table 1) confirming
the marine origin of this unit. Shales, in turn, are
covered by a thick section of massive amalga-
mated and graded sandstone types and_ coarse
polymictic conglomerate lenses and channels. A
nearly 100-m-thick unit (Figure 2D) comprising
large blocks (>1 m in diameter) crops out north-
east of the village of Saint Antonin. This coarse
composite lens, consisting of laterally discontin-
uous strata, 1s in turn covered by about 150 m of
silty micaceous shales and laminated siltstones
(similar to the upper Marnes bleues shales) and
fine-grained sandstones. The base of thin sand-
stone layers display sole markings, including or-
ganic structures and some tool marks, oriented
toward the NW. The shales include a mix of fresh
and rather poorly preserved planktonic and ben-
thonic foraminifera; the planktonic/benthonic
ratio ranges from 0.5 to over 1.0. The shale-
siltstone section is partially covered and slightly
truncated by an andesitic flow breccia of the type
illustrated in Figure 3C. The breccia delineates
the top of the Middle Member; where these
volcanic flows are absent, the shales are in ero-
sional contact with conglomerates of the Upper
Member.

SMITHSONIAN CONTRIBUTIONS TO MARINE SCIENCES

Ficure 2.—A, B, Foreset stratification and sets of ripple lamination oriented toward W and
NW, in massive, poorly cemented sandstone section (megasequence A) above Marnes bleues
shales along road D-427, SW of village of Saint-Antonin; C, first conglomerate bed (sharp
based, matrix supported, crudely graded) above massive sandstone section in megasequence A,
D, interbedded sandstone and clast supported conglomerate strata forming coarse, upper term
of megasequence B, NE of Saint-Antonin. (Hammer = 28 cm; ruler scale = 15 cm; bar scale in
lower D = 100 cm.)

Upper MEMBER (> 200 m).—This upper term
comprises moderately to very coarse conglomer-
ates, both lenticular and channelized. The clasts
include most metamorphic and igneous rock
types found in the Lower and Middle members,
but, in addition, higher proportions of andesitic
debris (Figure 3A); the mauve hue to this member

is in part due to the abundance of this volcanic
component (usually weathered) and Mesozoic
and Neogene limestone lithologies. Microfossil
faunas in interbedded silty shales indicate an
Oligocene age according to Bodelle (1971); these
are mixed with tests reworded from mesozoic and
older Nummulitic series. Thin gypsum stringers,

NUMBER 5

Ficure 3.—Evidence of syndepositional volcanic activity in Saint-Antonin
syncline: A, Andesitic cobbles and boulders, in megasequence C, NE of
Saint-Antonin; B, same, in megasequence A, SW of Saint-Antonin; C,
andesitic breccia NNW of Collongues, along small road near Fontane.

Taste 1.—Foraminiferal assemblages in samples collected
in megasequences A, B, and C of Saint-Antonin Conglom-
erate in vicinity of Saint-Antonin (G. Glagon, pers. comm.,
1978)

1. Sample MED-78-2: shale sample in a 5-to-7-m-thick
fine-grained section, between 2 conglomerate layer in the
upper part of megasequence A, along road D-427, SW of
Saint-Antonin.

Planulina wullerstorfi (Schwager)

Uvigerina species

Cibicides pseudoungerianus (Cushman)
Cibicides aff. lobatulus (Walker and Jacob)
Robulus serpens (Seguenza)

Robulus species

Gyroidina species *

Siphonodosaria verneurl: (d’Orbigny)
Spiroplectammina carinata (d’Orbigny)
Globigerina sp.

2. Sample MED-78-3: shale sample in a thick siltstone-
shale section, base of the lower term of megasequence B,
along road D-427, W of Saint-Antonin.

Robulus serpens (Seguenza)

Robulus species

Eponides aff. umbonatus stellatus (Silvestri)
Catapsydrax species

3. Sample MED-78-7: shale sample in a thick siltstone-
shale section below the sandstone-conglomerate lenticu-
lar units, megasequence B, at the Saint-Antonin village
cemetery.

Eponides aff. umbonatus stellatus (Silvestri)
Globigerina species
Catapsydrax species

4. Sample MED-78-5: shale sample in a ‘flysch-like’ silt-
stone-shale section, lower term of megasequence C, along
road D-427, about | km NE of Saint-Antonin.

Trochamminidae
Globigerina species

5. Sample MED-78-4: shale sample in a ‘flysch-like’ silt-
stone-shale section, lower term of megasequence C, along
road D-427, about 1.7 km NE of Saint-Antonin.

Bulimina jarvist Cushman and Parker

Gyroidina girardana perampla Cushman and Stainforth
Osangularia mexicana (Cole)

Bolivina species

Nodosaria species

Vulvulina species

Catapsydrax unicavus Bolli, Loeblich and Tappan
Globigerina gortani praeturritilina Blow and Banner
Globigerina angiporoides Hornibrook

SMITHSONIAN CONTRIBUTIONS TO MARINE SCIENCES

locally exposed in the uppermost shale section
between Miolans and La Penne, record the end
of the marine regime in this region.

Facies Organization and Associations

The lithostratigraphic boundaries outlined
above serve as datum planes to delineate gross
regional facies variations, including strata thick-
ness and debris coarseness, along the E-W depo-
sitional strike, north-south transects, and opposite
syncline limbs. In the Lower Member, for exam-
ple, lowermost Oligocene conglomerate lenses are
thicker and somewhat coarser in the SE sector
(near Saint-Antonin) than to the west and north.
Important spatial and temporal changes are not
surprising in view of unconformities and stratal
pinch-out commonly observed in exposures of the
three members. With the exception of some an-
desitic flow breccias, most individual layers can-
not be traced laterally for more than several
hundreds of meters, and it is difficult to correlate
groups of strata for distances in excess of about 1
km.

A sedimentological interpretation of the Saint-
Antonin terrigenous series can be achieved in two
steps. The first involves organizing the strati-
graphic sections comprised between the upper
Marnes bleues shales and top of the Upper Mem-
ber as defined by Bodelle (1971) into major nat-
ural lithofacies successions, or megasequences.
Three coarsening-upward megasequences are rec-
ognized in the eastern part of the syncline where
the stratigraphic sections tend to be more com-
plete and better exposed: (A) silty shale-siltstone
alternations massive sandstone and_pebbly
sandstone— coarser, predominantly conglomer-
atic series; (B) shale-siltstone alternations— in-
terbedded sandstone and coarse conglomeratic
lenses; and (C) shale-siltstone alternations>
coarse conglomeratic lenses. Generalized columns
of megasequence A, B, and C as identified in this
study are shown by the logs in Figure 1.

The second step requires identification of the
basic components that form these megasequences.
Each megasequence includes coarsening-thick-
ening upward and fining-thinning upward sets

NUMBER 5

formed by three major sedimentary associations:
coarse channelized, coarse lenticular, and finer-
grained sheet facies.

CoarsE CHANNELIZED AssociATION.—This as-
sociation, accounting for 15 to 25 percent of the
coarse facies column, includes channel fills 1 to 6
m thick and 10 to over 100 m wide consisting of
poorly to moderately sorted, massive clast-sup-
ported (Figure 4A,B) and, to a lesser extent,
matrix-supported conglomerates (Figure 5A).
The fills are for the most part unstratified (dis-
organized), whereas the upper parts, in some
cases, display vague to moderate stratification of
the clasts. Clasts include subangular to well-
rounded pebbles, cobbles, and boulders over 50
cm in diameter; matrix is sandstone or sandy silty
shale. Channelized units truncate by as much as
2 to 3 meters the underlying pebbly sandstones
and conglomerates. The axial trend can vary
from E-W to S-N, but is most often oriented SE-
NW, or approximately parallel with paleocurrent
directions measured from stratification structures
and/or sole markings in the associated strata. The
sizes of the maximum and larger clasts sometimes
decrease upward; reverse grading is also noted in
some sections. Coarser lag deposits fill the base
and lower parts of channels (Figure 4D); shale
and sandstone rip-up clasts are common as well.
Channel fill units are generally interstratified,
and occasionally merge with, regionally more
extensive lenticular layers of conglomerate and
pebbly sandstone.

Coarse LENTICULAR ASsSOCIATION.— This, the
most abundant facies, may account for as much
as two-thirds of the coarse-grained column and
comprises interbedded conglomerate, pebbly
sandstone, and massive sandstone strata (Figures
4C, 5C) that ranged from | to 10 m thick. Trun-
cation of underlying units is not as extensive as in
the channelized association. Individual strata
that form this association can sometimes be traced
laterally for at least several hundred meters, un-
like channelized units that pinch-out abruptly.
As a group, these layers form lens-shaped se-
quences that exceed 50 m and sometimes 100 m
in thickness, and comprise very coarse debris; the

latter often consist of granite and rhyolite, usually
subrounded to well-rounded. Clasts are com-
monly 0.3m°, but blocks as large as 3 m° are
observed (Figure 4C). More continuous, moder-
ate to well stratified finer-grained sandstone and
shale layers (Figure 2D) occur in this association.
A broad diversity of coarse lenticular stratal types
is encountered: disorganized conglomerates and
pebbly sandstone, and organized conglomerates
and pebbly sandstones.

Coarse disorganized conglomerates, commonly
1 to 5 m thick, are blanket-shaped strata or
lenticular pods that do not show well-defined
internal structures, are usually very poorly sorted
and matrix supported (large subspherical, sub-
rounded clasts float in a sandstone or muddy
sandstone mass). The base of a bed may be flat
or slightly truncate underlying layers (Figure 6B);
basal cobbles and boulders penetrate the under-
lying sandstone (Figure 6A). Most elongate clasts
generally show no preferred organization.

Coarse organized conglomerates, in contrast
with above, display vague to moderately well-
defined stratification features and prefered fabric.
Larger clasts show inverse graded bedding, or
crude inverse-to-normal graded bedding (Figure
5A). The long axis, a, of flat elongate pebbles
may be parallel or subparallel to flow (Figure 7B)
and imbricated (dipping upslope, Figure 6A); in
other instances, as on the soles of some conglom-
erates, the long axis is normal to flow (Figure
7C). Disorganized matrix-supported units may
pass upward abruptly or gradually to organized
conglomerates; the former situation (Figure 7A)

is more commonly observed.

Disorganized pebbly sandstone constitutes a
significant proportion of the lenticular associa-
tion, and these are recognized by the random
dispersion of pebbles, typically smaller than 5 cm,
in massive bedded, coarse-grained sandstones or
muddy sandstones. In most cases clasts are irreg-
ularly dispersed (Figure 8C), but in some in-
stances show preferential orientation (Figure 8B)
with imbrication (upslope dip) of either the elon-
gate a axis or intermediate 6 axis.

Organized pebbly sandstone types comprise

10 SMITHSONIAN CONTRIBUTIONS TO MARINE SCIENCES

Ficure 4.—Examples of coarse channelized facies association: A, clast supported conglomerate
channel fill that truncates thin sandstone and siltstone section (megasequence B, just below
village of Saint-Antonin); B, more detailed photograph of channel fill shown in A; C, thick
channelized and lenticular sequence of coarse (arrow points toward block > 2 m*) conglomerate
and pebbly sandstone, megasequence B, NE of Saint-Antonin; D, small channel (lag filled,
clast supported conglomerate) truncating a sandstone layer, same location as C. (Hammer = 28

cm.)

NUMBER 5

i

Ficure 5.—Examples of coarse lenticular facies association: A, inverse-to-normal graded, matrix
supported conglomerate (arrow shows base of unit truncating underlying pebbly sandstone
layer); B, alternating pebbly sandstone and pod- and irregularly shaped conglomerate strata;
C, disorganized pebbly sandstone showing cut-and-fill structure (arrow). (All sections SE of St.
Pierre, about 4 km W of Saint-Antonin; hammer = 28 cm.)

several varieties: pebbles are distributed as dis-
tinct, often thin, layers within coarse sandstone,
and (usually) concentrated along horizontal (Fig-
ure 8A) or foreset stratification boundaries (clasts
are commonly imbricated); or pebbles and cob-
bles are inversely graded in the lower part of
sandstone units and progress upward as normally
graded series (Figure 9A) (clasts occasionally are
imbricated); or coarse clasts (pebbles usually <5
cm) concentrated as the base of beds are normally
graded, and then topped by coarse sandstone or
pebbly sandstone layers (Figure 9B).
Sedimentary structures commonly observed in

the lenticular association include large variably

shaped rip-up clasts of shale and sandstone in

both sandstone and conglomerate layers (Figure
10A, B). Also noted are armored mud-balls and
rip-up shale clasts enclosing large pebbles (Figure
10C), and large individual pebbles or cobbles
isolated in sandstone layers. Plant matter is ubiq-
uitous and locally concentrated as thin lignite
lenses or, more commonly, as small flakes, often
with mica, along stratification surfaces (Figure
2A, B). A relatively low proportion of sandstone
and conglomerate layers are deformed, and these
are interpreted as slumps (Figure 10D) and pos-

SMITHSONIAN CONTRIBUTIONS TO MARINE SCIENCES

Ficure 6.—Examples of structures in coarse lenticular facies association: A, matrix supported
conglomerate with clasts showing some imbrication of elongate pebbles (bars highlight the a
axis of clasts dipping upslope toward SE), basal cobbles, and boulders resting within top of
underlying coarse, poorly sorted sandstone, section SE of St. Pierre; B, sharp based (arrows),
clast supported conglomerate truncating underlying sandstone layer, megasequence B, NE of
Saint-Antonin. (Hammer = 28 cm.)

NUMBER 5

Figure 7.—Examples of fabric in coarse lenticular facies association: A, disorganized, matrix

supported conglomerate in contact with overlying unit showing some imbrication of large,

elongate clasts (S of La Penne); B, pebble orientation in sandy mud matrix, viewed from above,

elongate a axis of clasts oriented subparallel to flow (megasequence A, SW of Saint-Antonin);

C, pebble-produced tool markings (elongate a axis of pebbles oriented transverse to flow, arrows)
on sole of coarse conglomerate (megasequence B, NE of Saint-Antonin). (Hammer = 28 cm.)

sibly liquefied deposits (Figure 10E).

Both disorganized lenses and some channel
units comprise subrounded to rounded subspher-
ical andesitic boulders, particularly the upper
conglomeratic series of megasequence A (Figures
1, 3B) and many of the coarse layers of megase-
quences B and C (Figure 3A). The composition
of this extrusive volcanic material and of suba-
queous breccia flows between and within coarsen-
ing-upward series has been described by Bodelle
(1971).

FINE-GRAINED SHEET AssocIATION.—This_as-
semblage comprises several types of deposits. The

thickest (50 m to 150 m or more) are alternating
silty shale and laminated siltstone and sandstone

’ sections that form the base of coarsening-upward

megasequences (Figure 11E). Some thin, poorly
graded siltstone beds (1 to 5 cm) resemble Te
turbidites, and display trace fossils and tool mark-
ings oriented primarily toward the NW and
WNW. Similar but much thinner sequences of
fine-grained alternations and moderate to well-
stratified sandstone layers are interbedded within
coarser strata of the assemblage.
Graded sandstone strata with sole marks and
flame structures (Figure 11C) are identified as Ta,

lenticular

14

SMITHSONIAN CONTRIBUTIONS TO MARINE SCIENCES

Figure 8.—Examples of stratification structures in coarse lenticular facies association: A,
stratified pebbly sandstone; B, C, pebbles irregularly dispersed in coarse sandstone, all in
megasequence A, SW of Saint-Antonin. (Bar in B highlights imbrication pattern of some
pebbles; hammer = 28 cm; ruler scale = 15 cm.)

Ta» (Figure 11B) and Ta-- (Figure 11A) turbi-
dites. Most well-stratified sandstone and finer-
grained layers, truncated by channelized (Figure
11D) and coarser units (Figure 5B), cannot be
traced beyond several hundreds of meters within
the composite lenticular association.

Proximal Slope-Stope Basin Fan Model

A depositional model for the Saint-Antonin-
Conglomerate, including the coarsest series, is
formulated on the basis of several lines of evidence
summarized below. A very generalized schematic
depiction is presented in Figure 12.

The formation is marine, a conclusion rein-
forced by the faunal content. Foraminiferal as-

semblages in the upper part of the Marnes bleues
shales (listed in Bodelle, 1971), and fauna more
recently collected in the silty shales of megase-
quences B and C (Table 1) include both benthic
and planktonic forms. Caution is needed in inter-
preting these assemblages because both well-pre-
served autochthonous faunas and those reworked
from older sections occur together; the latter,
usually abraded, altered, and iron-stained, are
more commonly encountered in the coarser lay-
ers. The fresh pelagic tests, including Globigerina
and Catapsydrax (Table 1), probably accumulated
on a silty mud bottom in a relatively open marine
realm, away from the immediate influence of a
high-energy coastal or shallow water regime.
None of the benthic species are living today, but

a

—— —— Oe Oe,

———

NUMBER 5

Ficure 9.—Examples of stratification structures in coarse lenticular facies association, pebbly
sandstone units (megasequence A, SW of Saint-Antonin): A, reverse graded; B, normally graded

(note truncation of plant-rich sandstone layers, arrow, by coarse sandstone). (Hammer = 28

cm.)

a comparison at the generic level with modern
assemblages suggests that the finer-grained terms
of megasequences were deposited in an outer shelf
to upper bathyal setting, and minimal depths of
100 to 200 meters are proposed (G. Glagon, pers.
comm. 1978).

The formation appears to record the direct
depositional influence of a coarse alluvial system
on the adjacent land. It is recalled that laminated
siltstones forming the finer-grained sections below
the sandstones and conglomerates of megasequ-
ences A, B, and C comprise abundant plant
material and display structures indicative of both
traction and gravity flows, suggesting a pro-del-
taic slope environment. The poorly sorted, plant-
rich, current-stratified massive sandstone above
these are likened to modern delta front lenses and

associated distributary channels (Coleman and
Wright, 1975), rather than to more texturally
mature, shelly nearshore sand bodies (cf. Hechel,
1972). The overlying conglomerate series consti-
tutes a facies not commonly encountered in the
typical modern mud-to-sand deltaic construc-
tional model. The initial phase of cobble and
boulder transport would almost certainly involve
dispersal directly to an offshore margin from
mountain torrents and an alluvial fan system
positioned between a mountain base and the
coast; gradient, proximality to source and vigor-
ous periodic flooding are implied (Figure 12). The
large volumes of coarse material spread along
broad sectors of the basin margin also would
suggest input from a partially submerged fan
delta system backed by alluvial fans.

16

SMITHSONIAN CONTRIBUTIONS TO MARINE SCIENCES

Ficure 10.—Examples of structures in coarse lenticular facies association: A, large shale rip-up

clasts in sandstone; B, conglomerate; C, shale clasts enclosing rounded pebbles in a coarse
sandstone layer. (A, B, C, megasequence A, SW of Saint-Antonin); D, E, deformed pebbly
sandstone layers S of La Penne. (Hammer = 28 cm.)

The depositional environment of the Saint An-
tonin Conglomerate in the study area is a proxi-
mal subaqueous slope. (1) Some of the thinner,
better stratified sandstone sheets associated with
the massive sandstone and conglomerate lenses
are graded and display moderately developed Ta,
Tas, and Tac sequences, flame structures and
tool markings that for the most part are consist-
ently oriented toward the NW and WNW. This

consistency of sole mark directions at the base of
graded beds indicates emplacement by sediment
gravity flows, probably turbidity currents, on a
subaqueous slope rather than by storm stirring
on a marine platform. (2) A slope origin also is
favored by the association of slump deposits, very
large (>1 m diameter) crystalline boulders, and
inverse-to-normal and normal grading displayed
by some matrix- and clast-supported conglomer-

NUMBER 5

Ficure |11.—Examples of the fine-grained sheet association: A, graded sandstone layers, Ta-c
sequence; B, same, Ta-5 sequence; C, same, sole markings and flame structures (arrow) at base
of a graded layer; D, sandstone-shale sequence truncated by a pebbly sandstone unit; £,
laminated siltstone-shale section, lower term of megasequence C, NE of Saint-Antonin. (Ham-
mer = 28 cm.)

18

ates and pebbly sandstones. (3) The slope was
submarine as indicated by Bodelle’s (1971) clast
shape analysis that showed coarse, fluvially de-
rived debris mixed with pebbles that record abra-
sion in a coastal setting; these conglomerates are
covered directly by finer-grained facies contain-
ing marine microfossils. (4) The slope in the
eastern and central part of the study area was
oriented toward the NW on the basis of sole mark
and small channel axis orientation, foreset lami-
nation, and a or 6 clast axis imbrication. (5)
Interpretation of the interbedded volcanic cob-
bles, boulders, and andesitic breccia flows as la-
hars (Bodelle, 1971) would be consistent with a
subaqueous slope setting (Figure 12).

Any comprehensive sedimentation model must
take into account a potential tectonic component.
Regarding the Saint-Antonin Conglomerate,
there is indirect evidence for strong structural
control: accumulation was concurrent with pro-
gressive uplift of terrains south of the basin mar-
gin. This is recorded by a marked increase, in the
upper megasequences, of coarse sedimentary de-
bris that can be specifically related to adjacent
Mesozoic and early Tertiary terrains. Eocene
Marnes bleues shales and older Saint-Antonin
sandstone debris in megasequence B and C fur-
ther attest to the rapidly changing configuration
and accelerated erosion of exposed Eocene sec-
tions immediately south or southeast of the south-
ern limb of the Saint-Antonin syncline. This con-
current deformation maintained the gradient be-
tween source. terrains and the coast, thus prompt-
ing continued northward transport of sand to
boulder-size debris directly onto the alluvial fan-
delta front and adjacent coastal sector. Synde-
positional faults are not mapped, but unconform-
ities within the formation record uplift of adja-
cent land and the progressive shift of the coastline
and basin margin toward the north and north-
west; some depression of the slope in the form of
a perched basin also may have occurred during
deposition. The gypsum in the uppermost shales
above megasequence C is probably associated
with a progressive shallowing of the slope and
basin and their eventual emergence in the early
to mid Oligocene.

SMITHSONIAN CONTRIBUTIONS TO MARINE SCIENCES

The tectonic-sedimentation interplay would ac-
count for the appearance of conglomeratic series
that forms an integral part of the proximal sub-
marine slope sequence. The depositional model
envisioned here depicts fan delta growth directly
on a mobile tectonically-formed slope rather than
on a low-gradient shelf platform. Such conditions
would provide a substantial delta front slope that
facilitated failure of rapidly deposited metastable
sequences by overloading during flood and vol-
canic events, and by earthquake vibration. The
combination of periodic failure and structurally-
induced diversion of fluvial systems and delta
switching produced a back-and-forth migration
of coarsening-upward tongues downslope from
the delta proper. This process resulted in the
lateral spread of a series of coalescing terrigenous
cones across a broad sector (<10 km) of the upper
margin; the major sediment build-up (>1000 m)
appears to have occurred in the southeastern part
of the syncline, possibly in a slope basin.

The model for the Saint-Antonin Conglomer-
ate also should incorporate the observed spatial-
temporal organization of major facies compo-
nents, 1.e., channelized, lenticular, and sheet.
Each progradational coarsening-upward megas-
equence is dominated by a network of migrating
channels (the fills exposed in the study area are
too small and shallow to be submarine canyon
deposits) that intertongue with lenses consisting
of diverse fine-to-coarse-grained stratification
types. This arrangement broadly defines a chan-
nel and lobe arrangement that calls to mind a
moderately well-organized fan system. If such a
system is applicable, then the finer-grained sand-
stone sheets and _ siltstone-shale sequences in-
terbedded or interfingering with the coarser lenses
are identified as channel overflow, interlobe, and
more distal fan deposits. The thicker and region-
ally more extensive silty shale sequences accu-
mulated in sectors where active progradation was
reduced and lobes temporarily abandoned. Large
fan-head channel fills are not recognized. It is
presumed that these and associated nearshore
delta-front series deposited on the uppermost
slope in the sector immediately south of the south-
ern limb of the syncline have been removed by

NUMBER 5

19

COARSE CHANNEL

COARSE LENTICULAR FAN LOBE

SAND SHEETS (SOME GRADED )

SILTY SHALE

ANDESITIC BRECCIA FLOW
& DEBRIS

Ficure 12.—Scheme of coarse sedimentation, slope model based on study of Saint-Antonin

facies associations; diagram depicts active fan-like, prograding channel-lobe and abandoned
systems, below a fan delta, on a tectonically active submarine margin bounded by a coastal

range.

erosion.

The exposures in the Saint-Antonin syncline
record primarily the submerged portion of the
coalescing fan-progradational cone below the fan
delta front complex. On the basis of good to
excellent quality outcrops, it is possible to ascer-
tain that the facies organization pattern, domi-
nant stratification types, and the dimensions of
individual pebble- to boulder-rich layers, in some

respects, are different from previously described °

ancient conglomerate-bearing _ base-of-slope
(Ricci Lucchi, 1969; Shideler, 1970; Lowe, 1972,
Stanley and Hall, 1978) and coarse deep-sea fan
deposits (Davies and Walker, 1974; Carter and
Lindqvist, 1975; Piper, et al., 1978). The latter
generally comprise series of channel conglomerate
deposits, associated with well-defined fining- or
coarsening-up sandstone turbidite and mudstone
lobe sequences (cf. Mutti and Ricci Lucchi, 1972).
The Saint-Antonin facies distribution patterns
are considerably less well organized and appear

more similar to some modern alluvial fans (Blis-
senback, 1954; Bluck, 1964; Bull, 1964; 1972;
Hooke, 1967) and ancient coarsening-up alluvial
cycles (Steele, et al., 1977; Heward, 1978) than to
classic submarine fans. In the vicinity of the
village of Saint-Antonin, for instance, the pebble
and cobble channel-interchannel distribution
pattern of megasequences A and B is comparable
to the Trollheim Fan of California described by
Hooke (1967, fig. 4).

Subaqueous displacement of pebble- to boul-
der-size material involved high-concentration dis-
persion, probably with some bed-load traction.
As in the case of alluvial fans, it would appear
that the prevailing emplacement mechanism for
many conglomerates and pebbly sandstones, par-
ticularly the poorly sorted and irregularly strati-
fied units and lobes, is debris flow. This transport
process, which has received much attention in
recent years (Johnson, 1970; Hampton, 1972;
Beaty, 1974; Enos, 1977) implicates (1) large clast

20

support by a mixture of interstitial fluid and fine
sediment (“matrix”), (2) finite yield strength
(cohesion), and (3) flow motion as a series of
waves and surges. As generally conceived, debris
flows would best account for the disorganized
conglomerates in some channels and in many
coarse fan lobes. However, the diversity of strati-
fication and fabric displayed by clast- and matrix-
supported conglomerates and pebbly sandstones
strongly suggest that debris flow as used here
likely encompasses, and is associated with, a spec-
trum of high-density subaqueous gravity flow
mechanisms (Fisher, 1971; Middleton and
Hampton, 1973). Structures such as inverse grad-
ing, or inverse-to-normal grading, normal grad-
ing, and preferred fabric (orientation and imbri-
cation of clasts) indicate that at some point dur-
ing flow, clasts were able to move freely relative
to each other. Dispersive pressure (Hampton,
1972), release from suspension (Davies and
Walker, 1974) and a bed-load traction compo-
nent related to turbulence (Winn and Dott, 1977)
may be invoked, respectively, for inverse grading,
grading and imbrication, and the traction struc-
tures observed.

As in many natural physical systems, a grada-
tional sequence probably develops during a single
high-concentration dispersion event: changes oc-
cur with time and from one point to another
within the flow. Some examples of sequential
gravel facies evolution have been illustrated by
Walker (1975) and Carter and Norris (1977), but
these do not appear directly comparable with the
time-space organization patterns of Saint-An-
tonin pebble to boulder series observed in this
study. The prevailing association of disorganized
and graded-to-graded conglomerate
types, and the low proportion of graded-to-strat-
ified conglomerate types (terminology of Walker,
1975) are largely a function of deposition on a
steep slope in a very proximal setting unlike the
more distal, gentle gradient, base-of-slope or sub-
marine fan environments modeled by Walker and
other authors.

inverse

The presence of deformed conglomeratic strata
allows for, but in no way proves, the transfor-

SMITHSONIAN CONTRIBUTIONS TO MARINE SCIENCES

mation of slumps to debris flows and associated
highly charged dispersion flows following failure
at the fan delta front. Some evidence of progres-
sive distal facies changes is recorded by the some-
what higher proportion of finer conglomerate in
sections of the northern syncline limb. The short
transport distance between failure point and dep-
ositional site probably accounts for the low per-
centages of graded pebbly sandstones and well-
developed sandstone turbidites.

Although one tends to be most impressed by
the coarser conglomerate units, it should be noted
that failure of the fan delta front did not, in every
case, initiate highly charged en masse and debris
flow transport in channels or result in debris flow
emplacement in lobe or interlobe settings. The
graded sandstone and better stratified siltstone-
shale sections indicate that floods triggered less-
dense flows, including turbidity currents. Some
further insight on this phenomenon is provided
by the Annot Sandstone “‘gres d’Annot” sections
at Puget-Théniers, about 6 km to the north of La
Rochette, and at adjacent localities (Rouaine,
Scaffarels, etc.). These sections north of the Saint-
Antonin syncline are approximately time-equiv-
alent to megasequence A and are formed largely
of coarse sandflow and turbidite units that com-
prise significantly lower amounts of pebbles and
virtually no cobble-size debris; they are identified
as base-of-slope deposits that accumulated sea-
ward in deeper, more distal sectors of a relatively
large basin (Stanley, 1975). If, as earlier suggested
in the present study, minimal slope depth at La
Rochette was 100 m, a rough estimate of the
depth of the slope-basin interface at Puget-Then-
iers can be made: depths of about 400 m, 600m,
and 1200 m, respectively, are calculated if slopes
of 3°, 5°, and 10° are assumed between La
Rochette and Puget-Theniers. Steep slopes (prob-
ably at least 5° to 10°) are favored on the basis
of the assemblage of stratal types forming the
Saint-Antonin Conglomerate, and postulated ba-
sin depths of at least 1000 m at Puget-Theniers
conform with other paleogeographic analyses
(Stanley, et al., 1978; Stanley, 1980). The lack of
conglomerates in the deeper water-time equiva-

—_---— - oe

NUMBER 9

lent sandstones at Puget-Theniers suggests that
some coarse debris may have been trapped in
depressions, perhaps perched basins, on the slope.

The coarse facies described in this study repre-
sent an important, but often poorly exposed,
component of ancient mobile belt margins, and
as such the Saint-Antonin Conglomerate proxi-
mal submarine slope model as depicted on Figure
12 may be applicable to some similar sequences
in the rock record. Substantial refinement of this
depositional model is required, however, and
some insight is provided by evaluating potential
analogs. The Quaternary Var River—Var Delta
steep slope gravel complex west of Nice on the
Mediterranean coast southeast of the study area
(Figure 1), for example, presents some attributes
comparable to the Saint-Antonin Conglomerate.
At this locality, coarse debris from the adjacent
Maritime Alps and some reworked from adjacent
beaches are mixed with silty mud and accumulate
on a steep margin slope within a short distance
from the beach (Stanley and Unrug, 1972). Pe-
riodic displacement of coarse debris at the delta
front and upper slope is recorded, particularly at
times of flood, and Bourcart (1964) and Gennes-
seaux (1966) have shown that this material is
transported downslope by gravity-induced mech-
anisms ail the way to the Ligurian Basin plain.

The fan-like submarine slope progradation of
the Saint-Antonin Conglomerate, unlike that of
the modern Var gravel delta, minimizes the role
of submarine canyon channelization. The model
depicted in Figure 12 emphasizes the back-and-
forth lateral migration and build-up of coarse
tongues as well as the interplay of structural

mobility, associated volcanic activity and floods-

as basin margin triggering mechanisms. More
comparable depositional analogs are the Pliocene
Ridge Basin series of California where coarse
debris was shed directly into a deep lacustrine
setting (Link and Osborne, in press) and the
marine Eocene-Oligocene conglomerates of the
Santa Ynez Mountains, California, deposited on
structurally mobile slopes (Van de Kamp, et al.,
1974). Modern marine counterparts would likely
be encountered on the leading edge of plates and

ZI

a

orogenic settings such as rift margins where
coastal ranges provide large volumes of sand- to
boulder-size material directly onto margins where
shelves are narrow or absent (Crowell, 1974; Dick-
inson, 1974; Okada, 1974; Friedman and
Sanders, 1978: 302-306). Particularly favorable
conditions in such regions as the Red Sea or Gulf
of California probably occurred in the late Pleis-
tocene when, as a result of substantial lowering
of sea level, sediments were emplaced directly at
the shelf edge and uppermost slope. There is a
need to document both the deposits and processes
on modern narrow, steep submarine margins bor-
dered by coastal regions where coarse terrigenous
debris is being transported to deep settings.

Summary

The depositional history of the latest Eocene to
early Oligocene Saint-Antonin Conglomerate se-
ries in the French Maritime Alps is reviewed:

1. The three stratigraphic members that form
the Saint-Antonin Conglomerate (defined by
Bodelle, 1971) serve as datum references to iden-
tify the spatial and temporal distribution patterns
of silty shale-siltstone, sandstone, and conglom-
erate sections in the Saint-Antonin syncline.

2. These marine facies form coarsening-upward
successions, or megasequences (termed A, B, and
C in the eastern part of the syncline), that are
discontinuous along the E-W depositional strike.

3. The basal term of each megasequence con-
sists of silty shale-siltstone alternations that con-
tain microfossil assemblages indicative of open
marine, outer shelf to upper bathyal conditions,
and minimal depths of 100 to 200 meters; the
petrology and abundance of plant matter in silt-
stone layers suggest proximity to a fan delta
system.

4. The megasequences comprise coarse chan-
nelized, coarse lenticular, and fine-grained sheet
facies associations probably organized as a net-
work of migrating channels and sequences of
strata that resemble fan lobes and channel over-
flow deposits.

22

5. Debris flow and associated high-concentra-
tion dispersions are responsible for the emplace-
ment of poorly sorted, disorganized conglomer-
ates and some partially organized (crudely strat-
ified, inverse graded, prefered clast fabric) con-
glomerates and pebbly sandstones; failure at the
submerged delta front induced slumping and tur-
bulent flows with some bed-load traction, and
also turbidity currents that released graded silt-
stone, sandstone, and pebbly sandstone layers.

6. There are insufficient data to demonstrate
transformation of slumping to debris and turbid-
ity current flow within the study area, and the
lack of an obvious sequential transport-deposi-
tional pattern is largely a function of sedimenta-
tion in a proximal setting; it is possible, however,
that high-concentration dispersions evolved to
turbidity currents further downslope, based on
study of time-equivalent deposits in more distal,
deeper-water Annot Basin settings.

7. The facies successions, assemblage of strati-
fication types and sedimentary structures, and
size of individual strata recall alluvial fans rather
than the more commonly described and better
organized gravel-rich base-of-slope and subma-
rine fan deposits.

SMITHSONIAN CONTRIBUTIONS TO MARINE SCIENCES

8. Coarsening-upward megasequences are in-
terpreted as a progradational system of coalescing
fans deposited below an alluvial fan-backed sub-
merged fan delta complex; deposition occurred
directly on a slope, or in a proximal slope basin
rather than on a gentle base-of-slope to basin
gradient.

9. The proposed depositional model invokes a
strong tectonic overprint as recorded by progres-
sive uplift of terrains south of the basin margin,
northward shift of the coastline and basin margin,
andesitic flows, and the structurally induced di-
version of fluvial systems and delta-switching; the
interplay of these phenomena produced an irreg-
ular back-and-forth migration of the coarsening-
upward tongues on the structurally mobile slope.

10. Modern counterparts of the Saint-Antonin
Conglomerate fan-like progradational patterns,
involving tectonics and vigorous sedimentation
on a slope, probably occur on the leading edge of
plates and on other structurally-active, coastal
chain-bounded margins where coarse terrigenous
sediment is transported directly onto mobile
slopes. Definition of coarse deposits and processes
responsible for their emplacement in modern tec-
tonically mobile settings is needed.

|

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