Tue R. Dordogne at Sotillac (A.07.) the Geology of the Causse De Quercy,\ with Particular

$Tue R. Dordogne at Sotillac (A.07.) the Geology of the Causse De Quercy,\ with Particular$

TUE R. DORDOGNE AT SOTILLAC (A.07.) THE GEOLOGY OF THE CAUSSE DE QUERCY,\ WITH PARTICULAR

REFERENCE TO THE PALAEOENVIRONMENTS OF THE MIDDLE

AND UPPER JURASSIC

E.M. DAUKORU

Thesis submitted for the Doctor of Philosophy

Degree of the University of London

Royal School of Mines,

Imperial College,

London, S.W.7. October 1970 CONTENTS

Page

ABSTRACT 1

ACKNOWLEDGEMENTS 3 Chapter 1 - GENERAL INTRODUCTION 6 Review of Previous Work 10 The Themes of the Thesis 20 Future Work 23 Chapter 2 - STRATIGRAPHY 27 A. General Obstacles etc. 27 B. Outline Stratigraphy 30 1. Floirac Shale 30 Age of Floirac Shale 32 2. Mirandol Oolite 34 Age of Mirandol Oolite 36 3. Gluges Calcilutite 40 Unit 1 40 Unit 2 41 Unit 3 42 Unit 4 44 Age of Gluges Calcilutite 48 4. Blagour Breccia 53 Unit 1 53 Unit 2 55 Age of Blagour Breccia 57 5. Souillac Oolite 58 Age of Souillac Oolite 59 6. Lacave Calcilutite 62 Unit 1 62 Unit 2 63 Unit 3 67 Unit 4 71 Age of Lacave Calcilutite 72 7.. St. Etienne Limestone 80 Age of St. Etienne Limestone 82 8. Lanzac Oolite 86 Age of Lanzac Oolite 92 9. Crezelade Beds 94 Age of Crezelade Beds 94 10. Cates Beds 95 C. Stratigraphic Significance of Breaks Above and Below Blagour Breccia 96

Chapter 3 - STRUCTURAL GEOLOGY AND GEOMORPHOLOGY 99 A. Regional Setting 99 General Structural Difficulties 95

The N.W. Province 100 The Massif Central (N. and W. Province) 103

S.E. and S. Province . 105 The North Pyrenees (S. Province) 108 The Aquitaine Basin (W. Province) 109 B. Present Research 109 Folds 110 Discussion of Selected Traverses 112 Major Folds 116 Minor Folds 124 Faults and Faulting 132 Major Faults 132 Minor Faults 139 Joints 140 Structural Synthesis 142 C. Special Structural Topic - Origin of Blagour Breccia 149 Tectonic Aspects of Brecciation 152 Comparison with Previous Ideas 154 Chapter 4 - PALAEONTOLOGY AND PALAEOECOLOGY 158 A. General 158 B. Class BIVALVIA 159 Superfamily Nuculacea (specific descriptions and ecology) 159 Superfamily Arcacea 163 Superfamily Mytilacea 165 ,Superfamily Pinnadea 175 Superfamily Pteriacea 180 Superfamily Pectinacea 184 Superfamily Limacea 200 Superfamily Ostreacea 204 Superfamily Trigoniacea 214 Superfamily Hippuritacea 216 Superfamily Glossacea 217 Superfamily Lucinacea 219 Superfamily Crassatelacea 223 Superfamily Cardiacea 229 Superfamily Tellinacea 233 Superfamily Arcticicea 237 Superfamily Myacea 241 Superfamily Pholadomyacea 248 C. Class GASTROPODA 271 I. Fresh-water Gastropods 271 II. Stenohaline Gastropods 275 D. Phylum BRACHIOPODA 283 E. Phylum ECHINODERMATA 291 Ecology of Echinodermata 291 F. Phylum COELANTERATA 293 Ecology of Corals 293 Corals in the Mirandol Oolite. 294 Corals in the Lanzac Oolite 296 Stromatoporoids (and Polyzoa) 297 G. FORAMINIFERA 300 H. ECOLOGICAL SYNTHESIS 301 Qualitative Biofacies 301 Quantitative Assemblages and Diversity Trends 313 Choice of Technique 313 Procedure in Pair-group Clustering 315 Significance of Assemblages 323 Comparison of Quantitative and Qualitative Assemblages 324 Vertical Distribution of Compromised Assemblages 328 A New Application of a Modified Jaccard's Coefficient 331 iii

Chapter 5 - SEDIMENTOLOGY 333 A. General 333 B. Techniques 333 C. Petrography 337 Ooid 337 Vertical Variation in Ooid Structure 339 Oncoids 342 Algal Clasts and Overgrowths 344 Vertical Distribution and Depositional Environments of Oncoids, Algal Crusts and Clasts 344 Lime Clasts, lntraclasts and Pellets 345 Vertical Distribution and Environment 346 Vertical Distribution and Environment of Pellets 357 Non-skeletal Compound Particles 358 Bioclastic Particles and Microfauna 361 i. Bivalve and Brachiopod 361 ii. Gastropod 368 iii. Echinoderm 368 iv. Sponge 371 v. Foraminifera 371 vi. OstracodS and Dasyclads 374 Matrix 374 D. Mineralogy and Geochemistry 375 Gypsum 375 Age of Gypsum and Halite 377 Dolomite 378 Age of Dolomitization 382 Quantitative Distribution of Dedolomite 382 Calcite 386 Shale (Clay Minerals) 387 Geochemistry and Distribution of Ca, Mg and Sr 392 Calcium 394 Magnesium 396 Strontium 398 Potential Usefulness of Ca, Mg and Sr in Correlation 400 E. , Grain-size Distributions 401 Entropy Calculation 402 Discussion of Entropy and Grain-size Histograms 404 Comparison of Grain-size Histograms of Lanzac Oolite, Souillac Oolite and Mirandol Oolite 408 F. Sedimentary Structure 411 i. Grain and Fossil Orientation 411 ii. X-Stratification and Rippling 414 iii. Bed-thickness 420 Vertical Variations 420 Investigation of Possible Depositional Cycles in Unit 2 of Lacave Calcilutite 423 Variation in Thickness of Stromatolitic Member of Lacave Calcilutite 423 Isolith Map for Stromatolitic Components 425 Interaction of Stromatolite, Pellet and Oncoid Limestone and Calcilutite 429 Harmonic Analysis of Stromatolitic Member of Lacave Calcilutite 438 Lacave Calcilutite and St. Etienne Limestone Isopachs 443 Regional Comparison of Formational Thicknesses Facies Types 444 iv. Comparison of Facies Contacts and Vertical Passages 446 iv

G. Special Diagenetic Topic - Shell Preservation 448 Differential Shell Preservation and Lithology 450 I. Collapsed Cavities in Calcilutite 450 II. Pressure Solution 451 Differential Shell Preservation of Nerineids 453 Chapter 6 - DEPOSITIONAL FACIES AND PALAEOENVIRONMENTS 458 A. General Statement 458 B. Carbonate Facies 458 1. Oolite Grainstone Facies (Lithology, Geochemistry,etc.)458 2. Shell Packstone/ Il II II Grainstone Facies 467 3. Lime Mudstone/ Wackestone Facies II ii II 470 4. Laminated Algal Facies II II II 475 5. Pellet Grainstone/ Packstone Facies li Il II 481 6. Intraclast Grainstone/ Il II II Packstone Facies 483 C. Non-carbonate Facies 484 1. Shale Facies (Lithology, Subfacies, Sed. Structure, etc.) 484 D. Palaeogeography and Interregional Comparisons 486. APPENDIX Bathymetric Distribution of Molluscan Genera Represented in the Challenger Expedition Takings and in the Jurassic 490 Check List for Fossils Cluster Analysed - 495 Measurements on Pholadomya laeviuscula 501 Point-Count Data 503

BIBLIOGRAPHY 509 ABSTRACT

An area of about 262 square kilometres ('t, 100 sq. miles) in

the north of the department of Lot, soutwest France, has been mapped on 1 a scale of 1:25,000 (2 /2" = 1 mile). The succession is wholly

Jurassic and consists largely of an alternation of oolites and calcilu-

tites and closely related limestones.

A series of gentle folds with a general southesterly trend,

and problems associated with the largely vertical to sub-vertical

faults are discussed. Field examples are given to show that several

genetic relationships are possible between the folds and the faults.

A brecciated horizon at the top of the Bathonian is shown to be ini-

tially sedimentary but with a subsequent tectonic modification.

More reliable relative ages are deduced for the main litho-

stratigraphical units recognisable in the field. This is largely

based on the brachiopod faunas, but oysters and gastropods, and at the

base of the succession, ammonites are also utilized. The distribution

of stages above the Bathonian, on the map by Mouret (1892-8) is shown

from the above to be now obsolete.

Systematic descriptions of a large collection of fossils, mostly bivalves, are accompanied by detailed palaeoecological dis- cussions at generic level, the genera being grouped in superfamilies.

These form the basis for detailed biofacies analyses, both quantitative and qualitative.

The petrography of the limestones is studied in detail, descriptively and from point-count results. These are combined with studies, sometimes also quantitative, on sedimentary structures (in a very broad sense) and geochemistry to yield evidence for palaeoenvironmental deductions. 2

A combination of the stratigraphical, palaeoecological and sedimentological studies led to the recognition of an extensive oolite barrier, which was occasionally breached but persisted for most of the

Middle and Upper Jurassic. This barrier ran in a crude arc from the northwestern extremity of the Massif Central to Aveyron in the south and separated off dolomitic, gypsiferous, lignitic and stromatolitic calcilutites with a characteristic benthonic fauna between itself and the then emergent massif, from an off-shore ammonite dominated fauna in shale dominated lithologies further west. 3

ACKNOWLEDGEMENTS

The author wishes to thank Shell B.P. Petroleum Development Co.,

Nigeria for sponsoring him, not only as an undergraduate, but in

extending this sponsorship, during a difficult time in the company's

activities in Nigeria,to cover the period of this research. Without

the special provisions that were included in the terms of the award

from time to time, it may well have been impossible to transport the

large bulk of samples or to procure the aerial photographs on which

the map was based.

Special thanks go to Professor Ager and Dr. Evans whose involve-

ment in all phases of the research will be apparent in the thesis.

This was both physical involvement as well as their making available

to the author their wide experience as well as laboratory facilities.

Their interest in the author's private and personal problems is deeply

appreciated, as are the numerous ways in which the author has gained

from their influence and their prodding.

The author is grateful to Dr. Wallace for her willingness to

help at all times and particularly for effecting the speedy cutting

of the slides. Mr. Ferguson (now Dr. Ferguson) was always willing

to discuss and share his knowledge on statistical and computer appli7

cations to geology. He is to be thanked for this, and more speci-

fically, for supplying the programs for entropy calculation and harmonic analysis.

Thanks are due to Dr. Muir for making possible the preparation of the photomicrographs, and the stereoscan photographs, of some of the

limestones studied; to Mr. Bush (Peter Bush) for getting the powdered samples X-rayed for the author; and to Mr. Shearman and Mr. Bush for advising on particular aspects of diagenesis (dedolomite and

calcite-after-halite) or on particular points of analogy between this

succession and Persian Gulf carbonate deposition.

Miss Pugh, as always, knew no official bounds in her willingness

to help, and wholly deserves the special regard and gratitude of the author. Madame Andral of Rocamadour (Lot), France, must be mentioned

for her deep concern over the very trying circumstances in which the author found himself in his final field-season.

Finally, the author wishes to acknowledge the untold help and encouragement of his wife Rachael, who in addition, showed a rare patienciduring the difficult periodsof camp-life. But for her compan-

ionship, the lengthy field-seasons would have been either impossible or cripplingly less productive. 5

+ + + + I ·' + + + + + + + + + + + + +

~ it a Ro ch e l le ------J..'-.,.",J~~ + .+ . + + + -+ + + + +MAS+SIF+ + +

.... + + + + . 4- . ''1' ...... Q .u . .. 4- . (/- . • ...(j . • , . . . . /V . . . . . $' ...... ~ ...... s.

~ .Ju r aa s i c Cd '1' erti a r y ~ o. 0 C fJ 0 11·~ Co m p 1 ex . - -'...... '. P a l a e o zo i c LC t!:±J Trias s ic F:I·G .. L ~ . GENER ALISED GEOLO GIC AL rAP OF S . W. FR1 :C E t.

CHAPTER 1

GENERAL INTRODUCTION

The area studied lies in the northern third of the depart-

ment of Lot, Central France, and is about 150 kilometres east of

Bordeaux (fig. 1). The general dip of the rocks is away from the

Massif Central and a corresponding succession of topographic forms

and human activities can be traced in the same direction. The much

denuded crystalline rocks of the massif towers above the Permo-

Triassic and the Jurassic which in turn stand above the Upper Cre-

taceouS and Tertiary (the Cretaceous and Tertiary are unexposed) of

the AquitaineBasin.

In the massif the soils are thin and siliceous except in

erosional pockets, and bushes of chestnut preominate. The Stepbbn-

ian and Rqetic give a more eventful relief with chestnut bushes

interspersed with moors of broom. The Lias is thickly wooded with

oak and chestnut when shaly, while small plateaux with deep ravines

abound,in the limy horizons. The latter feature is very similar

to that of the nearly uninterrupted limestone succession of the

Middle and Upper Jurassic. Springs develop at the contact of lime-

stones and shales and here human habitations abound. Charmouthian

shales around Miers and Alvignac are, in fact, known to contain

sodium sulphate, which may well be remotely connected with the cal-

cification of gypsum in the Bathonian. (p1. The 'Gausses' proper,,start with the Bajocian and persist

over the Bathonian and Upper Jurassic. Extensive poorly vegetated

plateaux dominate the terrain, cut by deep valleys, often dry, since

the drainage is mostly underground. The valley sides and hill tops

support only forests of stunted oak and some sheep and goats. Rock '7-

PL. TEE ALZOU VALLEY AT ROCAMADOUR, showing typical Causse charactics - highly shrunken river discharEe,steep valley sides, flat hill-tops & patchy vegetation. 8

exposures are near absent even though the soil cover itself is thin.

Surface alteration is commonly found to have completely effaced the

subtle differences between limestones, so that the isolated patches

of soil-free pavements still present a mapping problem. In the dry-

valleys, cultivation is intense, tobacco being the principal economic

crop, with some maize for poultry, while vines grow here as in the

higher reaches of the valleys. The Upper Bathonian often weathers

to a very rich red soil, but this is only sporadically developed.

Caves developing from the underground drainage are the main tourist

attraction. In the area mapped, these are confined to the Callovian

while further west they occur i the Cretaceous as well. Springs

are not very common but occur in the massive basal Bathonian calci-

lutites as at Gluges and below a marly unit at the top of the same

stage. A water-bearing horizon again occurs at the base of breccias of Kimmeridgian age. Phosphorites associated with areas marking

the maximum extension of the Tertiary on to the Jurassic are not developed in this area. The Miocene is likewise no more than a doubtful chert-boulder-rich soil cover.

The Upper Cretaceous supports a much more varied agriculture, yielding vines, fruits and cereals. The alluvium of the Dordogne and the other major rivers, however, support by far, the bulk of the population of the region. The major towns are strung along these rivers, growing tobacco as the main crop.

The best time for field-work is April and early May when the foliage is just budding and the undergrowth is absent. The summer is warm but can be too warm - often sufficient to melt poly- thene sample bags! Quarry faces are also highly reflective in the height of summer and hard to expose properly photographically, or peruse for very long. Being a tourist area, cheap hotels are few and camping is about the only alternative accommodation. Luckily 9

camping-sites abound.

The Cretaceous of eastern Nigeria was first considered for study

under the supervision of Professor Gill but a tragic civil war broke out

which ruledthis out. The French alternative was then adopted at the

suggestion% of Dr. Audley-Charles who had noticed some cliff and rail-

cutting exposures of limestones some years previously. Dr. Ager's

(now Professor Ager at University. College, Swansea) previous experience

with students working for Ph.D.'s in similar lithologies in the Jura

and his having taught the author stratigraphy as an undergraduate at

once made it apparent that he had better supervise the earlier stages

of the research. Regular 'conferences' with him proved an indispens-

able spur to the research, while the field supervision was undertaken

for Professor Gill by Dr. Evans after Dr. Ager left to take the Chair

of Geology at University College, Swansea.

The present research was started in the late summer of 1967

when the author was introduced to the Mesozoic of the department of

Lot by Dr. Ager. Dr. Ager spent a week thus with the author in the

first field-season which lasted five weeks and furnished laboratory

material for the first winter. Laboratory work turned out to be

nearly impossible that winter because the samples were lost to French

Railways and took a while to retrieve. In the spring of 1968, seven weeks were spent camping in Souillac and mapping the area accessible on foot around Souillac. In fact, more field-notes and fossil collecting and petrographic sampling were undertaken than actual mapping, as the available topographic maps were very unsatisfactory both in spatial accuracy and in the quality of paper on which photo- graphic enlargement of the original was made. Dr. Ager saw the author briefly again on a hot-footed return from Spain. A short period of laboratory work was followed by a long summer field-season. of ten weeks spent mapping, this time on aerial photographs, and

sampling around St. Denis and St. Sozy. Dr. Evans spent four days

with the author to have a general view of the area.

The pattern of work for 1969 would have been as for 1968,

but for the International Symposium on the Jurassic held in this

College in the spring. The delay caused by this, and a personal

tragedy made it impossible to spend more than a week in the Racamaaour

area. But the experience which had now accrued ensured quite satis-

factory mapping and bed-by-bed logging and collecting up the Upper

Bathonian and Callovian so well exposed across the Alzou Valley at

RocamadOur. The rest of this shortened and combined spring/summer

season of fourteen weeks was spent mapping from Lacave and St. Sozy

and also checking, as far afield as 13-14 km, areas mapped in previous

seasons. Dr. Evans paid a repeat visit of ten days, specifically

to have a close look at the succession since this was to be the author's

last field-season. Most of the bed-by-bed logging and section

measurements, the recognition of the three-fold subdivision of the

rather thick Lanzac Oolite (Oxfordian-Kimmeridgian] was accomplished

in this season as was much of the statistical work on the stromatolitic

member of the Lacave Calcilutite, and the draught assembly from the

aerial photographs. Checking the identification of the fossils

collected, detailed petrographic work, quantitative studies of both

fossils and measured sections, and final draughting of the geological

map were the sole preoccupation of the winter preceding the writing

of the thesis in January of 1970.

Review of Previous Work

The literature on the Mesozoic of the area studied is scanty

- for three reasons: the rather monotonous lithology (on first encounter), the very poor fauna (often also difficult to recover), and 1

the rather simple structure. In such circumstances, the area could have attracted neither the stratigrapher/palaeontologist nor the

structural geologist, nor yet the sedimentary (in particular carbonate) petrographer who was still to come into his own. Much of the early work was thus directed to such purely utilitarian and economic ends as the location and exploitation of phosphorites and other residual deposits, and coal- and water-bearing horizons. The search for caves often with pre-historic paintings, was probably the only non-profit- making preoccupation at the time, the commercialisation of such caves being still to come.

Thevenin probably implied all this, when he ruled out the pre-1800 literature as worthless geologically. Some of that literature was in fact'in Latin! 1800 was however a conservative estimate, and 1841 may more appropriately be considered the date of inception of serious geological work on the Jurassic between the Aquitaine

Basin and the Massif Central. In that year, Dufrenoy and Beaumont, in their first volume of the memoires for the geological map of France discussed the Jurassic in general terms in the introduction. A four- fold division of the Jurassic was adopted, and subdivisions of these were in fact given names of equivalent British groups like Fuller's

Earth, Cornbrash, Oxford Clay, etc. The basement areas and the

Carboniferous received sole attention in this volume, but the map showed a remarkably accurate distribution of the Jurassic in France. i 1 j3 The designations, j and j for the Lower and Upper Lias and j , j3 for the Bajocian, Bathonian and Oxfordian (all sensu lato) do not correspond to current practice excepting the continued use of j3 in a general way for the Oxfordian. There is no doubt that for the first time geologists were able to see their areas of interest in a national setting. The pre-1841 literature need not be dwelt upon for the reasons

adduced. The following names, taken from Thevenin (1903) will admir-

ably illustrate its utilitarian nature: Blavier (1806), Cordier (1807,

1809, 1817), Berthier (1810, 1818), and Combes (1823).

The years 1841-1855 and perhaps to 1860, saw a mixture of

both economic and academic publications. Important works during this

period were those of Dufrenoy and Beaumont (1848), Boucheporn (1848),

and Coquand (1855, 1860). In their second explanation of the geologi-

cal map of France 1 /500,000, the Triassic and Jurassic in S.W. France

were especially considered. The sole preoccupation was stratigraphic

and was'directed to the correct cartographic delineation of the Jurassic

rocks. The work admirably brought together in one volume, much of

what was then known. The terms Lias, Lower, Middle and Upper Oolite

were assigned to the then popular four-fold divisions of the Jurassic,

even though the Middle Oolite corresponding to Bathonian, can hardly

be termed Oolitic throughout the western border of the Massif Central

south of the R. Charente. The alleged mixing of fossils from the

Bajociarwith those of the Upper Lias is perhaps not as serious an error (1895) (1895) as Glangeaud/ would have one believe. Glangeaud/ indicates an earlier

(1830) publication of the contents of these two volumes. This is now

hard to locate and is probably only of historical interest now, as

indeed, these latter volumes are themselves.

In 1848; Boucheporn produced two of the earliest maps of the

region - for;:the departments of Correze in the north and Tarn-et-Garonne

in the south. Details from the latter map are now incorporated in the

separate sheets of Montauban, Toulouse and Albi, and details from the

former incorporated in the Brive sheet. Coquand ranged wider than most, and in the two years cited above published respectively on the'

Permian of the department of Aveyron in the near southern extreme of the Jurassic outcrop, and on the geomorphology, geology, palaeontology 13

and mineralogy of the department of Charente in the north. Coquand's

work was in the north and has since been checked by Glangeaud, and the

fossil lists largely found fictitious. He may be forgiven,for geology

was still largely a matter of opinion and beautiful prose. In all this

activity, the department of Lot , i.e. the area between R. Dordogne

and the R. Lot was largely ignored. Also, the general lack of interest

in the Mesozoic and the general trend of what interest there was was

very similar to the situation in the western Massif Central.

The years between 1860 and 1864/1865 saw little activity and

the next phase of serious geological work may be said to begin with

Harle's'(1965) publication on the manganese deposits of the Dordogne.

In a short but brilliant paper he corrected Dufrenoy's wrong identi-

fication of Gryphaea sublobata, and correctly placed the bed with this

oyster at the topmost Lias rather than much lower. The deposits of

manganese, which he considered entirely Tertiary turned out to be

nothing in comparison with the contribution to Jurassic Stratigraphy.

In the same year a regional geological conference was held in Rodez,

but the results of work presumed stimulated by it, were not to come

until about 1868, and particularly 1869 in which year Magnan published

the first paper having specifically to do with the area investigated

in this research. He gave a description with fossil lists of the

middle and upper Jurassic and the age trend of folding and faulting.

Thevenin did not think Magnan was verified by subsequent workers in all his conclusions. Nonetheless, Magnan's contribution was generally sound and can be a useful introduction to the region. A series of papers followed Magnan, up to about 1874. The most outstanding of these were contributed by Boisse (1870), Bleicher (1872a,b) and

Peron (1873). Boisse's outline geology of the department of Aveyron

is now unobtainable but was considered useful by Thevenin. It was followed by Bleicher's discovery, for the first time, of the now ct 14

familiar brackish or fresh water horizon in the Bathonian. This

appears to rest almost directly on the Bajocian, while the Terebratulid, cepholo Z. ornitho-is placed above it and the thickness of both stages seems

altogether small. Thevenin whowdsfamiliar with that area did how-

ever bear him out. Peron's work on the geology of Tarn-et-Garonne a11NC'S the was usefully incorporated in a map co-oditod by part-precursor of the

present Montauban Sheet (218)(Geze, 1942-1943). But Peron was more

interesting than this. He confirmed Harle's observations on the ease

of recognition of the G. beaumontibed, and was the first to dispute b the new identification of Magnan's Terebratula sella (now T. subsella).

He dpught it ought to be T. biappendi culata. The present author's

views about this species are similar to those of Fournier (see later),

and contrary to Peron's. Peron found no sufficient lithological or

faunal basis for subdividing the Upper Jurssic, his expressed scepti-

cism leading him to consider the entire Oxfordian to Portlandian

under Sequanien, while using the appropriate stages informally.

The period between 1874 and 1884 saw few geological articles,

but the work of the previous years seemed now to be getting usefully

employed in the production of departmental maps. Most of these are

hard to obtain now, but the trend started at this time, of map, and

explanations on the same sheet clearly laid started in this period.

From 1884 to Thevenin in 1903, there was the greatest burst

of activity that the area had known. The curtain-raiser for this

period was Gourret whose'Constitution Geologique du Larzaci in 1884

was an excellent work for the area south of the R. Tarn. A detailed

map on a scale of 1:320,000 was accompanied by a series of geological

sections. The literature was competently reviewed, and a detailed

description, with fossil lists, of over 30 sections was followed by

a synthetic statement for the various Jurassic stages. In this

synthesis, the then recognised Liassic to Oxfordian zones were listed 15

with their complete faunas. An attempt was even made to deduce the

palaeo-environments from the fossils. The approach taken can be i

detected to a lesser, greater degree in the major works of Glangeaud

and Thevenin. Stratigraphically, the relegation of the Callovian to

a substage of the Oxfordian was unfortunate, as it has inhibited all

subsequent stratigraphers in the area, and kept them talking of a

Callovo-Oxfordian to this day. Specific citation of authorities for

zones was much needed in his confused zonal scheme for the Bajocian,

Bathonian and Callovian. The misidentification of G. beciumontias

Ostrea (G.) calceola Gddf. was unforgivable after Harle's (1865)

excellent work on the correct position of the bed with this fossil.

There were now often several publications a year, with the

Permian and Carboniferous at the edge of the Massif Central receiving

as much attention as the Jurassic. The period before 1896 was how-

ever dominated by Mourfet and Bergeron. Bergeron was largely active

far to the south and in the Massif Central and can be ignored, while

Mouretsresearches culminated in the Brive Sheet (1892-1898) and in

the Figeac Sheet (1936) in collaboration with Boisse and Raguin. His crystallized stratigraphical ideas as they are set forth in Chapter 2.

A serious underestimation of the thickness of the Bathonian,a failure to free himself from the shackles of Gourret's view of the Callovian, and imprecise petrographic diagnosis of the different limestones may be considered the general shortcomings of his ideas.

The year 1895 was mother milestone in this period when geologists chased after hmmonites as they did after local faults. In that year, Glangeaud published his voluminous work on the Jurassic west of the Massif Central. Having benefited from the mistakes of his . predecessors, he was able to set forth what might be considered the geological primer for the region stretching from Vendee to the R.

Dordogne. Like Gourret, each Jurassic stage was described in general 16

terms, then various sections were described in detail with sketches of

the local column, then closed with a resume and a faunal list. This

way; he thumped his way through to the Upper Oxfordian. He follows

with a general consideration of the tectonics, and concludes with

an environmental history of the region. He is taken up in detail

at appropriate places in Chapter 2. His main contributions were five-

fold:(a) The firm establishment of the Callovian as a stage in its own

right for this region. (b) His attempt to make much wider inter-

regional correlations than had been previously attempted. (c) The

use of ammonites as zonal indices without neglecting the associated

fauna thereby enabling that fauna to be used independently in the area

between R. Lot and R. Aveyroiwhere ammonites are scarce. (d) His

recognition of lateral facies changes in most of the stages investi-

gated by him - thus laying the groundwork for the latest interpretative (1 968) regional studies of Delfaud and Gauthier,and the present author. This

needed good correlation which Glangeaud achieved with remarkable

accuracy. (e) His detailed local columnar sections will be inval-

uable for one working in the Charente area with which Glangeaud was

most familiar.

Even his shortcomings were not entirely his alone; they were

typical of the time. His complaint about the overlapping ranges of

zone-ammonites, particularly in the Upper Callovian was merely the

now-recognised futility of attempting to define the top and bottom of

a zone simultaneously. He turns this dilemma to good purpose by

making the quite sound suggestion that the entire fauna ought to be

considered. There was a tendency to be self-opinionated in the

setting up of zones and the absence, in a major work, of a general correlation chart or the citing of authors for zones meant that some of the zones were probably just local_ facies successions. Inter- regional faunal comparisons ought, perhaps, not to have been a sub- 17

stitute for the citing of authors for zones.

In 1896, Fournier undericlmodest title about the Cahors Fault

made detailed descriptions with faunal lists, of Upper Lias to the

Upper Oxfordian. Bleicher's observations on the lignitic and brackish

water facies of the Bathonianwere confirmed and the possibility was

pointed out that Magnan's Terebratula sella in the Callovian was more

likely T. subsella or its ancestor. He was in a way disputing

Peron's (1873) earlier remarks, as well. The Souillac Oolite and

Blagour BreccI-a, subsequently known to occur (particularly the breccia

in that area) were severely subordinated to the Callovian limestones.

The liihostratigraphic subdivision of the Callovian was done with some

difficulty, largely because of poor petrography. The stromatolitic

horizon in particular was not recognised for what it was, but in all,

the attempt was far more commendable than the subsequent tendencies

to leave the Callovian limestones undifferentiated.

With the basic geology and stratigraphy of most departments

now established the period from Glangeaud to Thevenin was largely

taken up by very local problems. Most of the major faults were worked out during this period. The faults of Figeac, Gourdon, Rodez and Brive continued to occupy successive geologists, not least

Thevenin who in 1903 gathered all this experience and more into a major work for the area south of the Dordogne.

This work was comparable in most respects to that of Glangeaud for the northwest region. Indeed, the layout itself bore the stamp of Glaugeaud's influence, but the work was in several respects less detailed than Glangeaud's. It starts with the Permian and Carbon- iferous and finishes with the Tertiary. The Middle Jurassic and

Upper Jurassic were discussed as single entities and their differentiation in the text into stages was only partially done. His major contributions were the assembly in one volume of the scattered work 18

of his predecessors, the compliation of maps that spanned several departments, and his detailed discussion of the Tertiary and related phosphorite deposits. The last was probably even more significant

than the parts on the Jurassic, as the Tertiary which so dominates most of the southern departments had never before been so coherently presented. His palaeoenvironmental succession for the Jurassic

is correct in very general terms like 'littoral' and 'shallow water' and 'rolled entities', but his assertion of a uniform Callovian and

Oxfordian facies (p. 166) from Charente to the south flies in the face of Glangeaud's work,and is contrary to the findings of this research. The actual significance in terms of modern ideas on carbonate barrier-lagoon complexes continued to elude Thevenin as it did his predecessors.

Thevenin may be said to close the ancient literature of the region. The stratigraphy and major lateral facies changes were now well established along the border of the Massif Central. The major faults were now also well established, but major folds were known only for the Aveyron area in the south and the Charente area in the north. The period after 1903 thus saw a continuation of the publication of improved interdepartmental geological maps as sheets of the

Geological Map of France 1:80,000, with in 1942 the much delayed paper by Bergounioux on the regional structure between the R. Dordogne and the R. Aveyron. Active during this time were Mouret, Boisse,

Raguint, Bergounioux and Geze. Bergounioux's contributions were largely structural and the presence of domes in the area defined above was first mentioned by him. The objections to his attempts to set up a vertical succession of tectonic episodes are given in

Chapter 3, and the stratigraphical defects in the maps of the other authorities are pointed out in Chapter 2. 19

The waning interest in the area nearly died off completely

after 1942. The region from the R. Charente to the R. Aveyron was

now assumed satisfactorily tied up, and yet Parent when listing the

instances of evidence of a late Bathonian emergence in France, did not

include the area between the R. Lot and the R. Aveyron. How easily

the region was forgotten after 19031

The modern phase of studies was entirely interpretative, and

was activated by the Compagnie Francaise des Petroles who are currently

drilling in the Basin of Aquitaine. Their direct contribution was in

making available subsurface details of the Jurassic towards the com-

pilation of the second edition of the Bergerac Sheet(Goguel J.M.,

1965) west of the exposed Jurassic mapped for this thesis. Together

with the Societe Nationale des Petroles d'Aquitain, Pau, knowledge

of the Jurassic below the Cretaceous and Tertiary in the Aquitain

Basin itself was made available (even if only to a closed circle),

that could not have been unearthed otherwise. Bouroullec, Deloffre,

Gauthier and Delfaud are the most regular outlets for this knowledge and it is to them that the idea of a Jurassic barrier -lagoon complex west of the Massif Central is largely due.

Surprisingly, the stratigraphy of the concealed Jurassic is probably a lot better understood by Delfaud and Gauthier (1968) than

that of the exposed. For instance, in the regional geochemical study of Delfaud and Gauthier (1968), in complete disregard of the wealth of local lithostratigraphic information and of Glangeaud (1895), the lithological column for the Upper Bajocian to the Lower Oxfordian in the Martel Grammat area (the exact area studied here) was distorted out of recognition. One major consequence was that the possibility, advocated in this thesis, of carbonate barriers sometimes either migrating en-masse (e.g. Lanzac Oolite) or sending tongues over their associated lagoonal sediments was lost sight of. Instead, several 20

facies are arranged in a horizontal row and each interpreted to evolve strictly vertically. The result was that within each of seven sedimentary rhythms, there were as many as four time equivalent barrier- lagoon couplets landward from the postulated major barrier. Other differences in the minutiae of the barrier-lagoon idea are pointed out in appropriate places in the text.

The work by Bouroullec and Deloffre (1969) established for the Jurassic of southern Aquitainewhat Delfaud and Gauthier did for the Worth only much better in many respects. Boreholes, used as controls were indicated, and since the deductions were petrographic and palaeontologic rather than geochemical as in the previous work, an up-to-date stratigraphical scheme was included, and clearly adhered to throughout the test except for the use of Lias, Dogger and Malm for Lower, Middle and Upper Jurassic (see Ager, 1964). Termier was cited to prove that timmonites can legitimately be listed with mollusc benthonic forms and bryozoa as being benthonic sessile and littoral.

But these are minor objections to a very competent and up-to-date contribution to the sedimentolody of the south-west region.

Very recently, Bouroullec and Deloffre (1970) have turned out an excellent work on the Jurassic algae of the region, so perhaps we are moving into a phase of very narrow and highly specialised investigations.

The Themes of the Thesis

The project was chosen for its potential to simulate the sort of many-sided investigation which a prospective oil-geologist was likely to run into. The production of an original geological map was thus a foremost condition of the research. Topicality was nevertheless avoided by the choice of a central theme that drew equally from the separate subjects investigated. Thus, the structural geology, 21

alone may be said to make no direct contribution to the elucidation

of the palaeoenvironments. Yet even this is reciprocally related

to the understanding of the focal lithostratigraphy, and moreover

contributed in a major way to accounting for the accommodation of the

rather thick inferred lagoonal sediments, and the minor depositional

cycles.

It took longer to work on the map than on any other single

aspect of the thesis. It is the first of its kind in this area both

in the scale (1:25,000 as compared to the earlier one of 1:80,000)

and in the structural detail, and exactitude of outcrop distribution.

The application of modern knowledge of carbonate petrographic types

and a fresh look at the stratigraphy ought, it is felt, to ensure

the reliability of the present map.

The thesis is probably unique in attempting, in one breath,

to effect in a balanced way the difficult marriage of palaeontology

to sedimentology, in a common search for a palaeoenvironmental syn-

thesis. This attempt, based as it must be on a drastically revised

stratigraphy of the area, is felt to be the major contribution of

this thesis. Yet, it is hoped that some original and useful result

or exactitude of approach will be found in each of the separate sub-

jects investigated. The chapter on the stratigraphy, particularly

that part of it dealing with the Callovian upwards,- is felt to head

the list of these contributions. The use of species-frequency-

depth curves for various genera, the role of the auricle in the

Pteriacea, and the section on palaeoecological synthesis might perhaps turn out to be quite a genuine contribution to palaeoecology and palaeo-b4ofacies analysis. Chapter 5 on sedimentology and sedimentary petrography may be said for the first time to establish the presence of a stromatolite member in the Callovian and exhaustively to exploit its significance in the sedimentary evolution of this area. 22

TAB.I: STAGES OF THE JURASSIC

Arkell 1956, etc. French authors

Purbeckian ) (Purbeckien) Tithonien ) Portlandien Portlandian (Bononien/ ) Bolonien

( Kimmeridgien (Virgulien ( ( Kimmeridgian ( (Pterocgrien ( ( Sequanien=Astartien) ( ) Lusitanien ( Rauracien/Argovien ) Oxfordian ( ( Oxfordien

Callovian Callovien

Bathonian Bathonien

Bajocian (Bajocien ( (Aalenien (commonly placed in Lias)

Toarcian Toarcien

(Domerian Domerien

Pliensbachjan Charmouthien

(Carixian Pliensbachien)

(Upper Lotharingien Sinemurian (Lower Sinemurien s.s.

Hettangian Hettangien

Rhaetian (in Trias) Rhetien (in Lias) 23

The evidence on which the section on shell preservation is based is

highly subjective, but the enquiry it gave rise to, and the conclusions

about the different modes of collapse of shell envelopes in grain-

stones and calcilutites may well turn out to be of more than theoreti-

cal significance.

The general statements at the beginning of each ehapter make

a more detailed synopsis of the thesis superfluous. The straight-

forward subject-rather than object-oriented layout makes this still

more so. Only a very few introductory matters on the stratigraphy

and palaeontology will be touched upon. Throughout the text, French

stage names are used wherever it is necessary to retain the original

sense of their usage by another author. Otherwise, current equiva-

lent nomenclature is used without explantion. Arkell's (1956)

equivalents of common stage

names are tabulated (Tab. I). Special care has been taken to cite

authors for zones whenever a concensus is in doubt. Fossil names are

mostly used in their most modern form without so indicating, whenever

the diagnosis of an earlier author is apparent, often in the specific

name (as in most of the brachiopods and some of the bivalves).

Current generic names for the former were largely due to Professor

Ager and those of the latter are after Cox and Arkell's (1948-50)

revisions on Morris and Lycett's and Lycett's monographs and less

pertinent recent sources.

Future Work

The main lines of possible future research appear to be:

(a) To trace the lithological units studied here towards the north and south, systematically doing similar work to that done on the present area. This way, some of the conclusions which are merely ) inferential here can be confirmed or refuted. Above all, a more 24

reliable dating of lithostratigraphical units in these areas is urgently needed, and can ensure a more reliable palaeogeographical synthesis for the region. A detailed study of the brachiopods is clearly the first requisite in this regard, since ammonites are very scarce to absent from the Bajocian to the Lower Kimmeridgian.

Special problems associated with these are pointed out in the text.

(b) The four lithological units recognised within the Gluges

Calcilutite (Bathonian) offer interesting vertical palaeoenvironmental changes that have only been studied in a general way. Detailed profile studies of these units can be rewarding. The Lanzac Oolite

(Oxfordian-Lower Kimmeridgian) can further be studied in a similar way to that suggested for the Gluges Calcilutite. Its coarser grain and dedolomite (at the top), and abundance of fossils in the basal units should recommend it to the petrographer as much as to the palaeontologist.

(c) Unit 1 of the Lacave Calcilutite (Lower Callovian), and the bed with Gryphaea beaumonti(Aalenian) are both easy to recognise and fossiliferous and should offer opportunities for detailed horizontal palaeoecological studies based on numerical abundance as easily as on number of occurrences.

CHAPTER 2

STRATIGRAPHY

The chapter aims at establishing a more precise and detailed

rock-succession and relative-age determination than has previously been

attempted for this area. Difficulties arising from international

differences in mapping procedure, as they bear on the present project,

are discussed, and this is followed by a general description and dating

of the important lithostratigraphic units in ascending order( fjgs.2 & 3)

A. GENERAL OBSTACLES TO LITHO- AND CHRONOSTRATIGRAPHY

In France stages rather than rock-units are mapped and the

consequent common misunderstanding between different workers have been

mentioned by Ager and Evamy (1963). The sources of misunderstanding

and error as they apply to this area are five-fold:-

(a) It is obvious that a stage-by-stage succession as set

up in a type locality become confused by the field geologist as all or

some of the corresponding rock-units change their physical character

laterally, the order of succession of the lithostratigraphic units

remaining basically unchanged.

The practice exposes the field geologist to equating stage

boundaries and lithologic boundaries. Four cases with reference to

this area may be illustrated, where there is no such correspondence.

(b) The lithostratigraphic units in a layered-:cake succession may be diachronous as they are traced away from the type locality.

(c) Tongues regarded in one locality as of stage status may pass laterally into the parent formation. Other things being equal, the parent formation is rightly given a compound stage designation such as 2B

.x-y _I • It is suspected however that the parent formation has often been

arbitrarily split up on the map so that the resulting constituent units

are simply coloured bands on a map and are not mappable lithologic units. (1892-0 One direct consequence being the multiplication on • Mour et's map

of coloured bands for which there are no adequate lithologic represen-

tations. Secondlyotages with very thick lithostratigraphic representa-

tions tend not to be split up even though mappable sub-units are present

such as in the Bathonian. The old map is therefore not a sufficient

basis for judging the amount of detail on the new. The converse occurs when a formation with mappable lithostratigraphic units each cif stage status splits laterally into several tongues. Here the stage boundaries would run into the lateral equivalent of the first formation in such a way that the contacts of the tongues may not coincide with the stage boundaries. Again such tongues may be given compound-stage designations that are of little field value.

(d) What is true for several tongues is true of a lithostratigraphic unit not splitting up, but just pinching out in an adjacent unit.

There is a special case' however where the pinch-out may be traced into a hiatus that has no obvious physical characteristic. It quite clearly is infinitely more difficult to map such relationships as stages than as rock-units which are only afterwards dated.

(e) There may be an overstep in which stage boundaries as they apply. to rock-units above and below the discordance become confused as more beds or fewer beds and lithostratigraphic units are exposed below the discordance as it is traced laterally.

These errors in the earlier maps are pointed out in appropriate places in the text. Also in assigning rock-units to zones the currently accepted procedure of defining the base only ( Ag e r (1964)) of the zones is adopted. Often the lithostratigraphic interval between the base of two widely separated zones was devoid of index fossfls of the 29

M.U4 M.U.3 M.U.2 MU. I

F.U.3

F.U.2

PL.. 2: GENERAL VIEW OF FLOIRAC SHALE & MI- RANDOL OOLITE, N.703 between Stk.. Denl, Varrac.

intervening standard zones. Assignment to these intervening zones is

only tentative and although missing zones need not be indicated by visible

breaks in the lithostratigraphic record (Donovan, 1966), the absence of

such visible breaks in the intervals referred to, did encourage the ten-

tative assignment.

B. OUTLINE STRATIGRAPHY

1. Floirac Shale. ( p1.2 )

These consist of blue shales which are sometimes scaly and often

form grassy or wooded hill-slopes. The top part of them which is exposed

in this area is unfossiliferous although fossiliferous exposures (probably

of the lower part) do occur just north of Vayrac. Their thickness at

the northern end of Floirac is about 4 m. The shales are succeeded at

Floirac and Fontaine d'OulIe (St. Denis) by about 1 m of blue shale-

impregnated limestone with thin impersistent shelly ribs. This weathers

to a reddish or yeilow-brown and forms undercut ledges at both localities;

these are sparsely fossiliferous with Homoeorhynchia cynocephala and un-

identifiable belemnites. These are succeeded by 3 m of cross-bedded

rough sandy-looking shell grainstones with irregular shaly-limestone

partings. They weather to rusty red. No whole fossils were recovered -

from them, but pectinids in a very fragmentary condition dominate. The

unit. is well exposed at the type locality where the top strongly bio-

turbated 30 cm forms a distinct unit. The underlying main unit also

shows traces of burrowing, the infill often in the form of diffuse

streaks, being a greenish marl probably with finely disseminated glauco-

nite. The lowermost sub-unit of 1 m contains interbedded shale.

Three distinct units are thus recognised in the Floirac Shales and may

be designated as Floirac Shale. Units 1 - 3C.

Table 2 : Correlation scheme for the Toarcian toAalenian

Glangeaud (1896) Mouret (1892-8) . Geze (1942) Bergounioux (1939)

L. concavum L. concava ), G. sublobata G. beaumonti ) 7, il (+ 0. beaumonti) ) c Pi ) ..414 H . murchisonae ) 2

H. opalinum A. opalinus ) . H. opalinum Gastropods H. aalense H. aalense Gastropods D. radiosa

H. fallaciosum

A N H. bifrons A. bifrons H. bifrons

H. falciferum ( A. serpentinus ) L. falciferum A RCI

TO ( A. holandrei ) C./ 32

Age of Floirac Shale 4a MouretNe2-8)distinguished a lower L substage of black 4 or grey clays with A. bifrens from an upper L substage of coarse sandy

dark-blue or yellow-pink limestones overlain by limestones with Ludvigia concava and Ostrea beaumonti (Ostrea sublobata, Herbert of Harle 186 5)0701b.2.). 4a Alteration of shale and blue limestone is mentioned for L which may

thus be identified as Floirac Shale. Units 1-3D inclusive. Three ammon- 4a ite and one gastropod zonesare set up for L , but the author doubts the validity of the topmost gastropod zone, after having had the opportunity

to look closely at this part of the substage. Glangeaud (1895) mentions

Turbo capitaneus, MUnst as the only gastropod in his fossil list for the entire Toarcian in his work, and the author has seen a Turbo-like gastropod in a local private collection, but the former is for the Lower

Toarcian and the latter is from a very heterogeneous source. The possible explanation is that the gastropods (unidentified) which are supposed to 4a define the topmost zone of L may have been no other than those occurring further up at the base of the Mirandol Oolite (see later).. 4 L is defined byMouret(1892-8)as limited at the top by a zone with Ludvigia concava and 0. beaumonti. The distinctness of this marker bed elsewhere had been noted by Harle (1865) and is equally distinctive

in this area. The interval between the top of Floirac Shale Unit 3A and this bed can therefore be considered as representing L4.

On the sheet to the south, beds of nearly identical description and delimited above by the O. beaumonti (called on the sheet O. sublobata) bed are attributed to the Aalehian and labelled L. The thickness of 10-

20 m also indicates that a considerable part of what was attributed above L4a-4 to is being lumped with the Aalenian of this sheet, where black or grey shales are again mentioned as passing into Toarcian. 33

On the sheet to the south-west (Gourdon) a basal Posidonomya

zone, two middle ammonite zones, marls with gastropods and marly lime-

stones with G. beaumonti (sublobata) are taken byBerspuribux(1939) to

represent Toarcian (L4). No Aalenian is interposed between Toarcian

and Bajocian, perhaps becauSe even French geologists sometimes include

the Aalenian in the Bajocian.

Glangeaud (1895) recognizes six zones in the Toarcian, three

of which have been found by him only in the Peitian Pass. Two of these,

opalinum and falciferum are in fact listed alone or together in Fournier

(1901, 1903, ... ).

Occurring in the marker bed with G. beaumonti are a pair of

ammonites which are very poorly preserved. The small one has been by Mr. E.F. Owen identified/of the British Museum (Natural History) as Pleydellia and the

other which is a giant one that is very difficult to extract could be

L. murchisonae, although Glangeaud considers his very large specimens as

very similar to murchisonae but actually concavum. Both species and G.

sublobata have been taken by Glangeaud to characterise the Lower Bajocian

immediately above Toarcian. L. concava and/or G. beaumonti are unanimously

taken in all sheets as a distinct Aalenian or Uppermost Toarcian. The

International Stratigraphic Commission has a recommendation from the Ager (196 4) Jurassic Sub-Commission to recognise the Aalenian/ Thts stage is there-

fore taken to compromise the zones of L. concavum,- H. murchisonae, and on

the evidence of the occurrence of G. beaumonti with L. concave and

A. opalinus on the Brive Sheet, also A. opalinus. In other words, the

Floirac Shale Units 3B-C represent a condensed Aalenian with the three zones listed above, while Units 1-3A fall into the upper part of an undifferentiated Toarcian comprising in its complete development, Glangeaud's five zones falciferum to aalense plus perhaps, underlying these, holandreiiTab.2

Small incomplete specimens referable to the latter species have indeed been recovered from the soft shales just north of Vayrac together with 34

others of the falciferum type. In this as in subsequent parts or th

text, stage boundaries are assumed to be coincident with zone boundaries

although this may not always be so (Donovan, 1966).

2. Mirandol Oolite

The base of the formation is taken at the bed with L. murchisonae,

Playdellia and G. beaumonti at les Courtils and the top of the beds which

mark (a) the top of the last coral bed, (b) the change from a grain-

stone to a wackestone, (c) high angle cross-bedding to a flat one

approaching rippling, (d) beds 1 /3 2/3 m thick to a metre or more

thick, (e) from ooids and bioclasts to a mixture of limeclasts, oncoids and pellets. The top and main unit are exposed at the quarry and road cuttingbelow Mirandol.

The lower sub-units, as recognised between les Courtils and

Mirandol are - at the base the Gryphea-gmmonite bed which is the bed of black shales which are 'locally bluish and form a distinct incised band on the cliffs on the St. Denis-Vayrac route and St.Denis-les Courtils roac(p1.2)Biocalcarenite nodules occur in the shales along.with casts of large Ceromyas sometimes forming pseudonodules, as do the flood of

Gryoheas and very poorly preserved ammonites, and Trigonias, and Pectens.

This is overlain by sandy-looking, thinly bedded (2 cm) bioclastic limestones with an ochreous iron stain. The thin beds are slightly fluted to form incipient nodules. Pectens, Ceromyas andbe1Emnite fragments sometimes with fibrouscalcite-lined cavities form the cores of poorly developed nodules. Unit 1 of the Mirandol Oolite just described is 21 /2 metres thick and is overlain by a very fossiliferous unit (2A & B).

The lower sub-unit is nodular throughout with only a weak stratification.

Individual nodules are 10 cm or less but average 2-5 cm. They have a characteristic cauliflower shape and have a coarse pisolitic7sition similar to that of the internodular material. No other algal activity 35

such as algal banding is obvious apart from Spongiostroma forming the

individual pisoliths. The nodules are therefore probably concretionary.

Traces of serpulid debris and a few terebratulids occur. The higher

sub-unit is about 4 m thick and is more fossiliferous than the lower.

The nodules show a more stratiform arrangement and have crusts of long,

intertwinned serpulid tubules which give them a matted appearance.

There is great abundance of echinoids, terebratulids, turreted gastro-

pods and rhynchonellids. A sub-unit may be recognised between Unit 2B

and Unit 4, formed of a rapid alternation of beds with fine and those

with large pisoliths. It .is clearly a lithologic transition to the

oolite proper of Unit 4. It forms a feature on the cliffs just after

les Courtils, where rather large subangular nodules and a fauna similar

to but more sparse than that of Unit 2B are present. A similar feature

slightly overhanging Unit 2B is present at Fontaine d'Oulie. On the

cliffs between the last village and Vayrac, it is distinguishable at

a distance from Unit 2B by its thicker bedding and from Unit 4 by the

rusty brown to bluish incised band which separates the two (see photo-

graph). At Floirac; the same unit is exposed at the first quarry in

the valley just north-east of Floirac. It is quite accessible here

and shows both the characteristic fauna of abundant pectinids, ostreas,

some terebratulids, and largernontl ivalti id corals; as well as the

vertical variations in grain size and amount of matrix. The'unit is

3-4 m thick. Units 1-3 all have a reddiih-brown iron stain; they are

together referred to for convenience as the sub-Mirandol Oolite.

The marker unit, Unit 4, is the medium-grained yellowy to off-white ooid grainstone. The cores of the ooids easily separate

from their jackets with the false impression of a poorly cemented free- stone. For the same reason, it can be flaked by hand. It is well- bedded, and dominates the skyline in most of the area, within the

Mirandol-Foirac-Vayrac triangle and north from St. Denis up to Turenne. 36

On most of the cliffs it weathers to joint bounded vertical towers.

Colonies, 30 cm or more across, of corals appear at three levels. The top

two of these are accessible at the quarry and road-cutting below Mirandol

and the topmost also at the top of the hill on the N.703 from St. Denis

to Martel. The growth forms of the coralla, especially the sub-

spherical ones, have been misidentified on thel.Mnmap as chert nodules.

The calcite cleavage is not obvious in the very fine-grained cores of

the crustose Thamnaqiea, but the vigorous fizz v4ith dilute hydrochloric

acid clearly settles the matter.

Age of Miranol Oolite

Mouret (1892-98) placed the oolite coming above the Gryphaea iv beaumonti bed% in j (Bajocian). Apart from the position of the oolite

in the succession/, the stage designation is largely arbitrary. The

downward passage of the oolite into subcrystalline or coarse-grained

dolomites is probably a very subjective characterisation, since the

X-ray and chemical analyses of the author show little trace of this

mineral, and the petrography shows only very indirectly the possible

presence of calcite-after-dolomite. The mistaking of corals for chert

has been pointed out above. The only palaeontologic characterisation

is Pecten pumilus, which clearly is very abundant at this level, but needs to be taken together with other fossils, in order to be a useful

index. The dating of Mouret(1892-98) is instructive only because of the possibility that the earlier geologists probably had the benefit of tracing this formation to the N.W. region where Qmmonites occur.

The lithological explanation of the Bajocian of the Cahors

Sheet suggeststhe possible inclusion in the Upper Bajocian of the basal

Gluges Calcilutite$. The red or yellow cavernous limestones referred to the base of the Bajocian have their lithologic equivalents in Units 3 and 2, while both Chlamyspersonatus and Entolium disciforme referred to (op.cit.) by Mouret / occur with varying abundance throughout the formation. 37

Stomechinus mentioned by him is matched by the abundant

occurrence of a similar form, Plesiechinus, in Unit 26. The dating is

more satisfactory than it is in Mouret (op.cit.) but still relies more

on lithologic•succession than on species known to be associated with

the zone iimelonlfes elsewhere.

Bergounioux(1939 *3) milkus a similar division of the Bajocian

into ferruginous and cavernous dolomites below and white limestones with

P. pumilus, P. oersonatus, etc. above.

There is broad similarity between the Mirandol Oolite Units l-4 and the.lithostratigraphic•units assigned to the Bajocian on the adjoin-

ing sheet. Glangeapd (1895) had discussed the uniformity of the Bajo- cian fauna for the area mostly north of Brive (but also south), although • conceding that this stage was littlp known in Charente and the Dordogne at the time of his writing in 1895. Glangeaud can with some caution now be used to augment the broad similarity referred to above - a similarity that is still largely lithologic.

The general shortcomings of Glangeaud's characterisation of the

Bajocian, as they bear on the ultimate validity of the dating aimed at here, and as admitted by him are mostly the difficulty of specific identification arising from poor preservation - limiting him to genera.

Among the terebratulids, he admits•that juveniles were preponderant and that identifications were therefore sometimes doubtful. Poor preservation is also noted for the echinoids, which he identifies only on the basis of spines which are said to be very abundant. He admits that his age determination is based largely on gastropods.

Glangeaud probably decided he was in the Bajocian and pressed his species into those of Huddleston - hence the similarity. The following species were listed by Glangeand as characteristic of the oolitic facies of his Bajocian: Nerinea (Ptygmatis) cotteswoldiae,

N. (P.) oppelensis, N. oolitica, Cylindrites bullatus, Patella (Helcion) 38

myosa, Scurria nitida, Trigoniacostata , T. duplicta, Pecten arcuatus,

Pleuronectites velatus, Lima bellula and Tancredia angulata. Half of

these are gastropods. By comparison, the following have been collected

from the present area - Nerinea Patella cf. cingulata, Psenomelania

sp., Plagiostoma Larpax, Pecten personata, Pecten pumilus, Chlamys lens

Saw., Entolium disciforme Ceratomya bajociana Loboidot hirispe rova 1 i s ,

Rhactorhynchia lacunosa, and Plesiechinus; Eopecten sp.,

Nerinella grachis, Modiola lonsidaAei and Modiola scalprum have been

found plus Thamnastraea, Montlivaltia, and Serpula socialis,

The comparison is not so close with his

characteristic forms and he anticipates this by noting the possibility

that some of his species could evenThe found in the Bathonian elsewhere

since they are not so invariably characteristic of Bajocian only.

The comparison is therefore extended to the rest of his faunal list and

those of the faunal lists of adjacent areas. The following character-

isation is arrived at - (a) abundance and diversity of pectinids,

common species being Pecten pumilus, Pecten personatus, and less so

Entolium" disciforme, (b)Loboidothyris perovalis (noted by Neaverson,I928

also as U. Bajocian) ranging from the basal G. beaumonti bed into Unit

4, (c) abundance, (sometimes also variety) of echinoids, Plesiechinus

or the related Stomechinus being easily identified, (d) abundance of

patellids, (e) greaty variety of c rals (locally even greater variety and more frequent in situ developmen than the 'Corallian' Oolites •

further up), (f) the invariable po ition of the oolite (UnitA) above the good marker bed with G. beaumont; commonly but not always with dolomitic and ferruginous beds between the two. Anexception not generally covered by the above characters is the succession of Cajarc given by Bleicher (cited by Glangeand 1896) in which crystalline limestones with Terebratula perovalis, Rhynchonella subtetraeda,

Ceratomya bajociana pass through Sandy limestones with a flood of 39

PL.. 3 t GENERAL VIEW OF THE GLUGES CALCILUTITE FROM THE D.43 BETWEEN GLUGES & COLOMBIER. (Note the nonoony of the succession.) 40

Pecten disciforme and Trigonia striata into very fossiliferous dark marls

and nodular dolomites. The author considers the discrepancy to be due

to one or both of two possibilities - a) Beleicher's succession covers

largely Units 1 - 3 only, b) Unit 4 has been masked by dolomitization

generally acknowledged as sometimes invading the entire stage. (op tiff A comparison with Neaversoymay even enable a finer subdivision

to be made of the Mirandol Oolite succession. Thus Unit 2B with its

echinoid abundance would be the base of the Vesulian (U. Bajocian in

modern usage) and the coralliferous upper Unit 4 can be referred to a

higher level of the Upper Bajocian (see fig.).

3. G.1 uges Calcilutite (p1.3)

This is by far the thickest (200 m) of the formations. The

difficulty of subdividing it lithologically is proclaimed by the enormous

belt of its outcrop on the map. The absence of marker horizons of

sufficient thickness to have a fair chance of frequent exposure has not

helped the general similarity of the beds that lie in between. Four

as yet non-mappablesub-units can however be delimited.

Unit I

The base of this unit is at Gluges Village but the character of

the transition from the Mirandol Oolite is best brought out on the cliffs

and by the contrast on either side of the minor fault-slips below Mirandol.

The criteria for this passage have already been listed. The thick lower

beds seen on hand specimen as pebbly oncolitic micrites of a yellow-white, dark-blue, reddish colour measure some 4-5 cm at Gluges, where the wacke-

stones pass up to a fine pellet grainstone with considerable rippling

and small-scale cross-bedding and burrowing. These beds (totalling 6 m)

of Unit 1 are thinner and take on yellowy to light-yellow-blue colours on 41

the water-worn lower cliffs just by Gluges. At their top is a distinctive bed of varying thickness (30-60 cm) with coarse sheared (and flattened)

lime intraclasts. This bed is also very cavernous. The fauna is extremely sparse compared with the underlying Mirandol Oolite. A few fragments of Entolium disciforme on the underside of the overhanging bedding surfaces are all that have been found. The sheared intraclast bed passes up into a creamy laminated bed that turns up again at one or two points higher in the succession. Above this are dark-grey lime mudstones occasionally punctuated by very thin horizons of 1-cm beds or papery and rusty partings, and with several beds of lime mudstone spotted with pseudomorphs of calcite-after-gypsum and calcite-after-halite.

13 m above the base on the Gluges-Creysse route, a cavernous greasy- yellow crystalline bed occurs and is followed within 15 m by a pair of

13 cm bands separated by 3.8 m of calcilutites. The bands are of rusty, papery calcareous shales and show the only sign of life at the base of the unit. They teem with crinoid, echinoderm and pectinid debris and well- preserved specimens of(Arcomytilus)asper, and an indeterminate tetra- bratulid. The higher of these two beds defines the top of Unit 1.

Unit 2

The top of Unit 1 is followed by the same mixture of dark-grey, now also tea-green,lime mudstones with or without gypsum pseudomorphs and ripples but with an increasing frequency of platy and shaly'horizons and shelly or fossiliferous ones, the larger bivalves represented by

Pholadomya, Ceromya and Pinna, and the smaller ones by Anisocardia,

Sphenia, Lucina and Pteroperna ,terebratulids (Aulacothyris) occasionally occur but are clearly rare, the terebratulids often clearly juvenile.

The succession described here can be traced on a cliff exposure from

Gluges towards Creysse up to and up-hill above Colombier. Fossiliferous beds of the type described are exposed in quarries and road-sections on 42

the D.43 near Colombier and near Legol-pres-Martel. A great abundance of all the types mentioned except brachiopods occur and in the latter area beds referred to Unit 2 contain the additional elements of Pseudodiadema, crinoid stems, small turreted gastropods, rare oysters and serpulid tubes.

Here also bleached creamy shale and limestone beds are frequent, as are thin coquinas of Lucina, Pecten and less commonly echinoid spines and disarticulated terebratulids. The top of the sub-unit is marked by the base of a shelly bed at the Rifle-range between Creysse and St. Sozy.

This bed is coarse granular and shelly with a dull earthy hue. The same bed is observed at the bottom of the cliffs below the church at Montvalent.

Here it is 50 cm thick and is preceded by well-laminated black impure limestones and shaly bands with some gypsum pseudomorphs within the beds and between them as a 'reef'. The immediately overhanging bed here and at

Creysse is full of Thalassinoides on the under surfaces. (See next Unit.)

The entire unit is about 48.4 m thick.

Unit 3

The beds immediately succeeding the shelly basal bed at Creysse are dark and broadly rippled with an alteration of (80 cm) flaggy and

(20 cm) regular beds with trails and thalassinoid impressions and a flood of Gervilleta acuta and Lopha costata and less abundant Chlamys lens,

Pinna, and pleurotomariid gastropods. Just, north of Monvalent on the

N.681, the top part of the 58 m of inaccessible blue-grey, regularly bedded and flaggy limestones and interbedded shales and paper limestones overlying the basal bed shows a diverse fauna of Orthinella bathonica,

Lucina sp., Mytilids, Chlamys lens, Ceromya and Anisocardia, followed by a thin dark shaly limestone band with Rhizocorallium.

Part of the inaccessible interval below this fossiliferous band is visually traceable on the cliff to a road-side exposure at the junction of the D.15 and N.681. The beds have a very diverse bivalve fauna of 43

PL. 4:SWARM OF TRAILS IN SOOTT- LAMINATED WACKESTONEx Gluges,Calcilutite Unit 3, S.E. of Meyronne..

- • • 11-47. • 4441)110"-144t4111111$:

PL. 5: COQUINA c2,Paphil=coglats & Liostrea herbridica. (Horizon & locatioa as abc71-5-77-- 44

, Trichites costata, Liostrea cf. Pholadomya laeviuscula

herbridica, Lucina cf. Bellona, Trigonia duplicata, Modiola sp., B.

Arcomytilus asper, Modiola bipartus, Sphenia Thracia vicelliacensis,

Anisocardia dieulafaiti, Cyprina davidsoni, Pteroperna costatula,

Ceromya plicata, C. concentrica, Trigonastarte 12:, Lima sP., Gervilleia

acuta, Viviparus, Pseudodiadema subcomplanatus (an echinoid), trails(p1.44)(111d

traces of carbon - all very closely associated spatially. The common

rusty brown or empty specks and arcuate streaks are actually leached

shells, the decalcification probably also responsible for the pink tinge

of the beds attributable to residual ferric oxide. The top of the unit below is exposed higher up the road f the base of a rippled pebbly and intra-

clastic packstone/grainstone containing oyster fragments as the main

bioclastic constituent. This horizon is described below under Unit 4,

but it rests on cindery algae and platy impure limestones with Sphenia.

These platy beds and the thicker beds with abundant Pholadomya laeviuscula

are traceable on aerial photographs for a long way on the D.15 towards

Raveillon. The whole unit is 60-65 metres thick.

Unit 4

The highest unit of the Gluges Calcilutite is also the most

distinctive, particularly in its upper part. The basal sub-unit is the

pebbly packstone/grainstone referred to earlier and containing algal

intraclasts clearly derived from the underlying cindery algae of the top

of Unit 3(p1.6).it grades up into laminated calcareous silts with a great

abundance of Thalassinoides. The silts are succeeded by regularly

bedded and gently rippled or flat-laminated limestones with abundant

gypsum pseudomorphs and interbedded shales and paper limestones. At

least one bed of lime flat-pebbles is present above the silts at

Meyronne, the gypsiferous thin calcilutite beds in which they occur being constant in character throughout the area, and occurring as such at 45

PL. 6 A : ciNDEnT ALGAE t Top of Gluges Calci- luti te Uni t3 , S of Meyronne .

PL. 6 B : CI:DERY ALGAE OF PL. A RE7:ORKED IrTo OVERLYI1TG PEBBLY BASAL BED OF GLUGES CALCI- LUTITE UEIT 3. Locality as above. 46

45 PL. 7: ASSORTED INTRACLAST GRAINS FROM RED- BEDS OF TEE BREAK ABOVE T:: GLUGES CALCILU- TITE.A: Polished spectmens showing pebbles reworked from Lypsiferous calcilutites, and an erosional contact. Bs Thin-section showiftg clean intraclast grainatone with truncated gypsum pseudomorpha. 47

Raveillon. The calcilutites are often dense and cherty. A fossiliferous zone occurs in their upper part with an oyster brash at the water edge on the St. Sozy-Creysse route and fauna very similar to the

Gluges Calcilutite Unit 3 beds with Pholadomya but also with Lingula ? on the D.33 between St. Sozy and Baladon. The passage from these lower, regularly bedded limestones to the marls above is marked by iron-stained play flat-pebble limestones with nodular replacement chert, lignitic debris (identified by Thenevin (1896) as Brachyphyllum in the Rodez area), as at St. Sozy, Timbor and Blagour. Broken surfaces and plates . show a variety of textures from crude grading of reddish intraclasts and bioclasts to a streaky confused arrangement of these coarse components.

The bedded limestones are surmounted by buff to creamy marls whose bedding is only rarely preserved as at Rocamadour, where a regular alternation (partly disturbed) of platy beds about 1-2 cm and thicker beds of marl occur. Here also the plates have a misleading coarse, granular appearance on account of the flood of small gastropods, in situ in some levels, reworked, broken and rounded in others, which at this locality partly replace the usually large Viviparus in the same beds elsewhere. Gypsum pseudomorphs continue in strength, often coalescing to form irregular earthy mauve patches. Lignitic traces also occur.

The brecciated variant is often rich in Viviparus, Planorbis and Crosso- stoma often broken and sometimes with a locally-derived marl infill.

Even the displaced specimens are clearly locally derived since a lot of in situ Viviparids do occur - sometimes with a well-preserved orifice.

The top of this bed and of the Unit is marked by the lowest.of a series of re-worked horizons with brown, black, grey and cream coloured intra- . (1)1.7) clastvand re-worked gastropods, particles ranging in size from coarse sand to 2 cm or more. These lime-conglomerates attain a thickness of over 3 m just north of the town centre at St. Sozy, although they are . barely noticeable at Rocamador. The sedimentary origin of the conglo-

merates is stressed in view of the subsequent brecciation they have been

subjected to.

Age of Gluges Calcilutite

The following correspondence in fauna and succession may be

noted (with comments) between the present succession and that which is

referred to by Mouret (1892-98) -

(a) Upward increase in interleaving of paper shales with hard-

grey regularly bedded limestones.

,(b) Bed with herineids and Ostrea costata in upper part of

stage: this corresponds with the abundance of L. costata from upper Unit

2 upwards, but herineids though present as at the water edge between

St. Sozy and Creysse, are not an invariable component.

Pseudodiadema subcomplanatus and platy layers with Anisocardia dieulafaiti:

(i) The abundant terebratulid at Monvalent bears a slightly closer resemblance to Ornithella (T.) ornithocephala (Treatise) than to 0. bathonica of Muir-Wood, the form to which Muir-Wood refers W. ornithocephala of

Mouret (1892-98). W. ornithocephala as O. bathonica, Muir-Wood dire probably , s ynonymous With"0. (T.) ornithocephala.

(ii) The more abundant bivalve in the platy limestones is actually

Sphenia E. but may have been mistaken for the present but very subordinate

Anisocardia dieulafaiti.

(iii) The latter beds lie too close to the pebbly basal bed of Unit 4 to leave room for the ornithocephala beds. The latter probably come between the L. costata-Pholadomva-Pseudodiademd subcomplanatus beds and the platy ones rather than overlie both the latter. A platy bed does in fact directly overlie the ornithocephala beds at Monivalent, although the pebbly bed can only be inferred at this locality, assuming the absence of 49

a lateral variation. It is significant that no lignites are mentioned,

although they are quite obvious even from a cursory acquaintance with the

brecciated marls at Blagon. It adds to the list of omissions that can be

expected from a mapping of imaginary units.

The order of succession fossil and lithological is not so clear on the Cahors Sheet,.but the lignites and thin breccias underlain by a p'redominanace of massive and flaggy beds with clay interbeds broadly

identifies with Units 2-4. The lignites are considered particularly diagnostic when they hold a single level. Two cases where they occur at two levels is noted on the Bergerac Sheet to the west and at Larzac

(Gou9et, cited in Glangeaud (1895)). Equal agreement is seen in the list-

ing of Zeilleria [=0.] ornithocephala, Alectryonia 1= Lophdl costata] and the brackish-freshwater fauna of Planorbis calculus, Paludina

Viviparus], Neritina Crossostoma] among others, with similar forms

in Units 3-4. The white or grey fine-grained sometimes oolitic limestones of the uppermost (jiv ) Bajocian of this sheet probably corresponds with the as yet unaccounted for Unit 1. On the evidence of this sheet therefore j is the equivalent of Units 2-4.

Bergounioux(1939-43) makes a tripartite division of the Bathon-

(i i-iii). ian The lower two, lithographic limestones with Z. (=0.) ornithocephala and platy lignitic limestones with P. [Discohelix] calculus correlate with Units 3-4. The relevance of the uppermost division of limestones with nerineid gastropods and L. costata is controversial without more lithologic information. This division is probably inserted i jii-iii to represent j , but of Mouret (1892-98) also cover a similar division with' nerineids and L. costata which the author considered not distinctive. Nerineids and L. costata do however occur in the breccias i overlying Unit 4, but j (probably equivalent to j of Mouret) could not refer to these beds because Mouret already separates the breccias from the L. costata beds of j It is concluded that the nerineids and

O. costata of the Gourdon Sheet are equivalent to the Blagourareccial

of the Brive Sheet, both representing j (or j i ) whereas the nerineids and

0. costata of the Brive Sheet (Mouret, 1892-98) are invalid as a zone

characterisation even informally. The great abundance of nerineids and Qs associated 0. costata mentioned,' associated by Glangeaud (1895)), with an equal

abundance of R. decorata,ahopkinsi andRelegantula in the Thivier area

bears out this point The lateral passage claimed to occur

on the N.E. of the sheet between the subcomplanatum-dieulafaiti beds and

the lignite-Planorbis beds is highly suspect as the author has been able

to see ttis succession in the Rocamadour-Ravalon area. A summary state-

ment for the adjoining areas would thus be:

(a) Brive Sheet: Unit 2 ? , Unit 3/4; iv (b) Cahors Sheet: j E Units 2-4, and j (upper part?)

Unit 1;

and ? S. 001ite.

Glangeavd (1895) cites and accepts Mouret's three divisions of

the Bathonian of the departments of Dordogne and Lot. But the (a)

lithographic limestones and platy marls, (b) breccia• and freshwater

limestones, (c) rocks with Rhynchonella elegantula covers the entire

Units 1-4 plus Blagour Breccia' and Souillac Colite and very much needs to

be subdivided. The very coarse partition is probably due to a gross

under-estimation of the enormous thickness of the Gluges Calcilutite.

The Brive Sheet covering the present area gives no thicknesses but the

thickness of 80-100 m given for j ig iii on the Cahors Sheet and the 189 m

given for the Bajocian-Bathonian on the Bergerac Sheet have probably been

taken as representative of the Dordogne-Lot region. This is an error of

over 70-100 m for the present area - a wide margin requiring further

subdivision of Gluges Calcilutite Units 1-4 has been attempted here. 51

Faunal similarities may be cited within the general correlation made with Glangeaud's (after Mouret) coarse zonation - Pholadomya murchisonae [E P. laeviuscula], Ceromya concentrica, C. plicata, Trigonia duplicata, Lucina bellona, Pteroperna (E P.costatula ) plana, Anisocardia

(E A. dieulafaiti), Zeilleria (2 0.) ornithocephala, Sphenia Sphenia

Paludina [E Viviparus], Planorbis [E Planorbis Cyrena [E Cyprina davidsoni].

The correlation with Glangeaud's Bathonian oolitic and brackish facies is considered sufficient to establish the present succession as belonging to the standard Bathonian of Arkell, Buckman and other British authors, since Glangeaud, as before, could trace a lateral passage into facies with ammonites in the region east of la Rochelle. Comparison could be made with a few other sections such as that of Bonte (1941) and

J-C. Fischer (1969) east of the Paris Basin and those of Thevenin , Harle, and Magnan, for the S.E. region (River Dordogne-River Lot), but these are all non-pelagic near-shore to tidal and supratidal facies and lead us no nearer to correlation with the standard, mostly ammonite zones.

Gastropod and bivalve assemblages are frequently used whenever brachio- 1933 pods and ammonites fail (see ArkellA p.249).

To summarize, it was observed earlier on faunal grounds (also perhaps lithologically if the fossiliferous pisolites of Mirandol Oolite

Units 2A-B could be considered a facies equivalent of the Pea-Grit), that Mirandol Oolite Units 2B-4 represent the parkinsoni and at least the lower zigzag ZDnes of the Upper Bajocian. The very indirect evidence for taking Gluges Calcilutite Unit 1 as upper j iv (see earlier) and the fact that Arkell's zigzag is Bathonian strengthen the conclusion that

Unit 1 is upper zigzag and basal Bathonian (of Arke11,1956)Gluges Calci- lutite-Lr. Bathonian-zidzaq; Gluges Calcilutite Unit 2 probably mostly

Opellia fusta (by inference); Gluges Calcilutite Unit 3 and upper 52 •

Gluges Calcilutite Unit 2 = Tulites subcontractus ? (0. ornithocephala is considered sufficiently closely related to O. bathonica of the Fullers'

Earth Rock (see Muir-Wood) and fragmentary specimens of Stiphrothyris - a form coming into its own in the S. Oolite and Lacave Calcilutite Unit 1

- lend supportive evidence, the occurrence of the two being mentioned by

Arkell (1933, p.257). Gluges Calciluttte Unit 4: Obovothyris perobovata considered by Bonte (1941) as equivalent to sub-Cornbrash does not come in until the Souillac Oolite. Burmirhynchia hopkinsi, abundant in the

Blagour Breccia$ and Souillac Oolite is also very scarce, and certainly doubtfully identified i the basal bed of Unit 4 and even worse so in the Burmirhynchia marls of Unit 4. The smaller, less inflated./ elegantula occurring B. with hopkinsi in the S. Oolite is taken as an index of the uppermost

Bathonian-Callovian, while A. Bonte (op. cit.) in a very similar facies places Hopkinsi in the asp:idoides Zone in E. Hirson and eleciantula in c.e discus the lower of his /Zones. All three considerations place Gluges Calci- lutite Unit 4 at least below the discus zone. Gluges Calcilutite Unit

4 E Opellia asp - idoides - Lr.Clydoniceras discus [or C. hollandi,

Torrens 1968] (See summary table.)

Mention must be made of the equation of Souillac Oolite with the lignitic marls in this area by the Research Centre at Pau (Delfaud and authier 1968). The work was concerned with Jurassic palaeoenvironments based mainly on geochemistry and attendant palaeosalinity deductions. Since neither the text nor the bibliography shows any close examination of the succession, one is compelled to question what the sources are that support what they appear so obviously to be taking for granted. The literature is both scarce and ancient including Glangeagd's primer on the area, and the author knows of no part of it, not to mention his own work, that can lead them to such a conclusion. The rejection of their sedimentary models based on this assumption is elaborated subsequently. 53

4. Blagour Breccia

These breccias are named after the village of Blagour, about

4 km due north of Souillac. However, they have an equally good develop-

ment about the latitude of Mayrac on the D.33, the cliffs due east of

Meyronne, where they characteristically form the prominent feature at the

top, at Rocamadour, below the Rochers St. Marie, on the N.20 just outside

Souillac and at numerous other points. The great variability of the

breccias. , their similarity in many ways with the overlying Souillac

Oolite and the very obvious sedimentologic and palaeontologic criteria

for taking them as —the start of a clearly different sedimentary epoch -

all led the author to map them with the Souillac Oolite under the latter

name, even though in places, the breccias are nearly as thick and even

form a topographic feature - something uncommon in the Oolite proper.

Two units have been set up, of which the lower is by far the

most consistently developed, while the upper is often either brecciated

and sheared out of recognition or appears in part to revert to the fresh

-brackish water marly facies of Gluges Calcilutite Unit 4 and can easily be so mistaken.

Unit 1

The intraclast grainstone basal bed passes very gradually over

4 - 8 m and with marly or superficial oolite intercalations into Unit 1.

This passage is well known in St. Sozy as well as in a continuous log up the cliffs east of Meyronne and at the top of the Roque d'Or route.

In its unbroken form it is well-bedded and consists of marly and chalky limestones, shell and ooid grainstones/packstones (especially at the top) and interclast and pellet grainstones/packstones which are probably reworked equivalents of all the other varieties. The micritised jackets of ooids as well as some of their original cores of marly intra- 54

PL.SA THE MAIN UNIT OF 1..t:LE SOUILLAC OOLI TE ax THE N.20 OUTSIDE SOUILLAC.

PL..8B THE CONTACT OF THE SOUILLAC OOLITE & THE SHALT TOP OF THE BLAGOUR BRECCIA Hochers St.. Marie. 55

clast give even the sparry beds a superficial chalky appearance. A fourth

type of interbedding is a dark-light grey calcilutite which appears on

large outcrops as thin lenses up to 2 metres or more long and about 5 cm

thick. Their outlines are gently and irregularly undulose on a large

scale and flamed and loaded in hand specimens. Lenses are crudely

arranged en echelon and occasionally run into each other. They can be

shown to be contemporaneous gypsum lenses now calcitised to microspar and

hence diagenetic in origin, and do not in fact constitute a different cate-

gory from the three above. The top part of the bed is also pebbly,

pebbles occurring both as fairly persistent trains and as small-scale

impersistent flasers. Towards the top also, the regular bedding gives

way to crowded and extremely irregular parting in chalky shell and ooid

packstone/wackestone with platy lenses 3 m. by 45 m. When cut by minor

,faults or joints, these look like rudite fragments. These beds abound

in excellent specimens of Burmirhynchioelegantula and disarticulated and

fragmented ones of chlamys fibrosus. The origin of the breccias is dis-

cussed in a special section at the end of the Chapter.

Unit 2 of Blagour Breccia.

The complete succession of this thin unit is only rarely seen.

It is often sheared and fractured at its contact with the Souillac Oolite.

The platy dark limestones, with thicker intercalations occasionally with

Viviparus-type gastropods can very easily be mistaken for sub Blagour

Breccia Unit 1, and faulting or pinching out of the breccias can be erron-

eously inferred. At the type locality at Roch45- St. Marie, the bio-

calcarenites of Blagour Breccias Unit I pass up into a blue and sludgy pb8B shale/ separated from about I m of platy limestones by a string of large

pebbles. The platy limestones are dark smoky with vague impressions of

small bivalves. They are probably carbonaceous and are overlain by 1 m 56

of cremay thinly laminated Solenhofen type limestones. They are partly fissile and twisted and depressed plates are common. The partial fissil-

ity is probably imparted by trace amounts of clay. The creamy limestones are capped by 4-6cmof closely parting flat-pebble bed. Individual pebbles are dark-light grey, brown or creamy-yellow. A thin 50cm unit of light-coloured calcilutite which is reworked into the top-most bed of the unit - a pebbly oosparite. The ooids are fine and uniform in size while the pebbles show a sparse shell-content with doubtful specks of gypsum and are rimmed by an altered zone seen under stereoscan as equi- granular with the core and therefore probably due to oxidation. An intermediate zone is occasionally present as a transition to the outer one. The contact between the Souillac Oolite proper and this pebbly band is deeply stylol itised,an - evidence of the measure of normal-to- bedding stresses that have operated. The 50 cm unit and its pebbly cap- bed are quite fossiliferous elsewhere within the area mapped. At St.

Sozy a variation of the successions above the platy lowermost sub-unit is exposed. It is represented here by a series of marly intercalations between thin fine oolitic beds often themselves platy and with varying amounts of marly matrix. The top parts develop platy lenses and have slender pebbles, Ostreas, Rhynchonellids and ribbed Chlamyids. Extensive in situ reworking is indicated by manly pebbles and ooid-circumscribed smaller marl fragments. This enormous variation below the Souillac

Oolite is volumetrically unimportant but seriously hinders the precise determination of the base of the Oolite in tectonically disturbed exposures. It must be said that the first appearance of fine oolite is not necessarily the fine oolite of the Lower Souillac Oolite. 57

Age of Blagour Breccia

czf The Brive Sheet shows a three-fold division making the stage j (topmost Bathonian) into white subchalky, clotted oolitic limestones; a fossiliferous lamstrine limestone; and a breccia below. The first is identified with the Souillac Oolite proper and the last two with the is Blagour Breccia Units 2 and 1 respectively. The Breccia% a-re not even mentioned in the Cahors and Gourdon Sheets, the former merely giving a thickness of 30-40 m to a group of massive limestones, unfossiliferous, placed with difficulty under j i , and the latter incorporating the Souillac

Oolite and Breccia% all together, as the topmost of roughly partitioned i-(iii)). Bathonian The earlier conclusion is recalled that the

Aerineid gastropods and L. costata of the Gourdon Sheet (but not those of the Brive Sheet) are a valid representation of the Souillac Oolite and Blagour Breccia$. Glangeaud's observation of an association of nerineids and rhynchonellids allied to and including B. elegantula at a comparable level is also recalled. More work is needed to separate

I...elegantula from forms which the author considers to be Kallirhynchia and Rhvnchonelloidea. B. elegantula however is definitely present in these beds - its first major occurrence, and the beds are thus correlated with Bonte's (1941) -aspidoides-discus (hollandi SubZone of

;Torrens ,1968).This is also in line with current stratigraphic practice in which the base only of a zone is defined using the first occurrence of the index fossil or fossils. In Britain, Neaverson's

Clydoniceratan faunas bear some resemblance to the Nerineai-Rhynchon- ellai-CoralrPectinid bivalve association here, but an exact analysis is hard on this basis. Microthyridina lagenalis and Obovothyris perobovata perhaps also LL. sidding- T/ihu wever occur in the Souillac Oolite only and strongly point to the overlap of Callovian and Bathonian in this formation. 58

These terebratulids though ranging quite high in this area, have an

abrupt lower range, are easy to identify and on their basis therefore

the Lr. Souillac 001ite is taken together with the Blagour BrecciaV as

representing the topmost Bathonian zones of Chydoniceras (B.M.) and on

Neaverson's Macrocephalitanfauna which is correlated with Souillac

Oolite.

5. Souillac Oolite

The Souillac Oolite rarely forms an overhang except artificially

at road-cuttings. All the prominent features which appear from a dis-

tance as Souillac Oolite overlying Gluges Calcilutite Unit 4 marls such

as at Lendour and east of Meyronne are in fact Lacave Calcilutite or

Blagour BrecciaX. This is in keeping also with the extensive solution pl.SA pockets found in'it such as at the waste ground outside Souillac ./ The

oolites are nearly as severely fractured as the underlying Breccias;

it is thought therefore that the greater solutioning in the former may

rather reflect the remobility of connate waters through the two. If the

Gluges Calcilutite Unit 4 marls underlying the breccias are taken as an

effective lower barrier to percolation, then water draining through

joints in S. Oolite may come to flood the breccias and facilitate vein-

cementation.

This Oolite differs from the higher and lower oolites in the complete absence of bedding. This factor is locally used critically

to determine its upper limit - i.e., the onset of bedding. 'The original

type locality was at the Souillac Gare where the contact with the marly beds of the breccias is well-exposed. The entire succession is however nowhere exposed. At St. Sozy the thin lower unit and its base are too badly fractured to be recognisable,and the same thing applying to the good quarry exposure at the waste ground (Souillac). The succession from Blagour Breccia Unit 1 - topmost Souillac Oolite is well exposed at 59

Rochi,s St. Marie but only the lower reaches are accessible on the vertical blasted surface. The lower 2-3 m of fine well-sorted oosparite are well- exposed here and are followed here as elsewhere by the much more extensive pellety, oncol itk and intraclastic mixed pure and micritic oosparite.

The clean oolitic texture is partly obscured by encrusting algal activity and in situ reworking, producing a subsidiary boundstone texture and

isolated large grains of intraclasts and shell and gastropod infills.

Irregular vertical changes in the proportions of these components occur.

It is possible likewise to notice a greater micrite content and corresponding darker earthy colour at Rocamadour Gare intimating a possible similar lateral variation. In contrast to the thin lower unit, fossils

in the form of the small cupolid coral Chomatoseries orbulites, fihynchonellids and terebratulids sparsely but visibly dot freshly blasted surfaces. The upward rise in micrite content renders the top passage to the Lower Lacave Calcilutite very chalky. This top part is also more fossiliferous with a diversity of pectinid bivalves rivalling that of the

Mirandol Oolite accompanied by very large well-preserved specimens of

Stiphrothyris and ribbed oysters. The onset of bedding marks the upper limit of the Souillac Oolite which is invariably 20-21 m thick.

Age of Souillac Oolite

The age of the Souillac Oolite has largely been covered under the Blagour Breccia%. Additional evidence for conclusions reached there is briefly argued. The overlying basal Lacave Calcilutite contains both

Stiphrothyris tumida and more elongate, subpentagonal and flatter speci- 0851-5) mens of tolDhrothyris subselici_. Davidson/ notes partly after de Loriol and Oppel that this latter form occurs in Kimmeridge Clay of the

Boulonnais and in the Sequanian , Pterocerean and Virgulian and Portlandian elsewhere in France. It is locally recorded on the Brive Sheet as ranging 60

Lusitanian from the U. -Virgulian to the Upper Virgulian - a range very nearly

identical with Davidson's citations, and cuts out in terms of the area

mapped, the entire Lacave Calcilutite and St. Etienne Lst. or over 150

m. of sediments. The reason for doubting the absence of ; record of

the species in these lower formations is probably due to g sketchy

collecting as evidenced in the complete absence (except the record of

B. elegantula) of a faunal list for these levels which have richly re- warded more detailed collecting by the author. Additional evidence for believing the possible downward extension of L. subsella comes from

L. Gentil and P. Lemoine (1905) who while aware of its post Rauracian

(Argoviari) occurrence note with expressed lack of temerity, that they have found it at a lower level than the Rauracian in Maroc. The lowermost unit of the Lacave Calcilutite as well as all succeeding units contains faunas that are distinctly post-Lower Callov:ian, even though

in detail a series of faunal-age repetitions occur at these overlying levels - repetitions whose sedimentological implications are dealt with later. Fterocardiat, Diceras arietimum, Grypipabi lobate. T inconstans, as (also intra-- S. Oolite), Thurmarrlla pU'rlicias well,/ the Microthyrids are some examples of immediately or distantly post-Souillac Oolite forms that strongly argue the at the latest sub-ealloviense (see invariable occurrence of G. bilobata in this zone in Callomon (1968)) age of the Souillac Oolite. More work is required to separate the

Obovothyris perobovata from Microthyridina lagenalis, but the former are probably better represented in the S. Oolite than higher up, arguing strongly a correlation with the obovata zone in the eastern Paris Basin 494 (Bontek )anda perhaps the topmost of the C. discus gub7one

(Torrens ,J968,). Stiphrothyris tumida is sharply limited upwards at the top of the Lacave Calcilutite Unit 1. Its stratigraphic value is doubtful here but if it is indeed a Bajocian/Bathonian form then, not a condensation as was indicated by its associations in 51

PL. 9 : GENERAL VIEW 01 LACAVE CALCILITTITE. s Tilted pho .:.:o at the type-lo Lacave cemetry. The base is marked. by the upper limit of the grassy slope formed characteristically by the Souillac Oolite whila the highest unit of Calcilutite forms the incised band towards the top. B: North side of the Alzou Valley at Rocamadour, showing the absence of an obvious definition of the top of formation. 62

Evamy (1963) but a strong facies control seems operative here.

In summary, the Souillac Oolite and Blagour Breccia% together

may be considered as of Clydoniceras discus age and the cumulative

evidence from the fauna of overlying beds and the first appearance of ina Obovothyris and Microthyrid5 used to place the main Souillac Oolite

in the discus SubRbne and the Breccia% perhaps in the hollandi sub-

Zone (' TorTenns,19ZEr- ).

6. Lacave Calcilutite (p1,9)

The largely brachiopod, but also oyster-Diceratid zonation

from the Souillac Oolite upwards is very unsatisfactory from a classi- ed cal "layer-cake" point of view, as has already been hinted in the pre-

ceding section. The classical picture is painted using all the avail-

able evidence, and the unresolved contradictions are reserved for

sedimentary facies interpretation.

Four mappable, members are recognised in the Lacave Calci-

lutite although the upper and lower pairs have respectively been mapped

together, owing to restrictions on map-scale. The four members are

described below.

Unit I

The lowest members of the succession is well exposed in so

many places that a type locality would be superfluous. Wherever

Souillac Oolite is over1Lin by Lacave Calcilutite, the bedded first 7-

8 m of the latter are clearly distinguishable as thick-bedded calci-

lutites strongly contrasting with the platy, chalky and banded stroma-

tolitic Lacave Calcilutite Unit 2 above. A rough upper passage can be traced from a chalky pelmicrite below, through dense light coloured calcilutite both regularly bedded to a relative frequency of shaly 63

and rubbly partings. The development of the three characters varies

from place to place. Near the Auto-Gare at Souillac, the topmost rubbly

character is not developed while the chalky transition from the Souillac

Oolite is poorly exposed and only assumed. The absence of this passage

to the Stromatolites could be characteristic of the Souillac region, as

it is observed again at the bridge, doubtfully at La Chapellone viaduct

and certainly so along the N.20 north of Blazy. At both St. Sozy and

Rocamadour the passage is present.

The member as a whole is very fossiliferous for a calcilutite.

The lower passage teams with thynchonellids, Stiphrothyris, Microthvridina,

L. subsel'la, ribbed oysters, Modiola perplicata, Chlamys_stricta,

Entolium demissium, Pecten sp.. In the middle unit, the pectinids

(except C. stricta), the ribbed oyster (except as isolated valves) and

Stiphrothyris drop off, and are replaced by Acrosalenia solitary

cylindrical corals, Protocaradia and/or Ceromya, Cardinia (Eomiodon?),

Arcomytilus, and small gastropods. L. subsella and fihynchonellids are

the most immediately obvious elements in most exposures. Where the

rubbly parting is developed, fossil recovery is facilitated, although

the abundance here of a rich assemblage of Pleuromyo, Corimva

Phoiadomyq, C. concentrica, Mytilus sue., Lucina (or Cardinia), Anisocardia,

Pinna, and Nerinea p. in addition to the elements of the preceding unit

is not entirely due to this. The full range of this„largely bivalve,

diversity is typified by the beds across the river valley at Rocamadour.

Unit 2

The base of this unit is sharply distinguishable from the top-

most Lacave Calcilutite Unit 1 by the contrast in bed-thickness (at

least five times thinner in Lacave Calcilutite Unit 2) and by the entry of either of two of the four lithological types that comprise Lacave

Calcilutite Unit 2 - namely a pale-coloured calcilutite with gypsum por- 64

PLXD: SPECIMENS FROM THE 'BEDS-WITH-IMPRINTS t. OF-BIVALVES' OF LOCAL AUTHORS. At Asterte var. sonamula in Crezelade Beds Unit 2, Les Patous. B: Sane species in similar litho-faciest Lacave Calcilutite ?, St. Sozy. 65

phyroblasts and with or without Astarte or a stromatolite (banded

algal limestone). The sharp passage contact is slightly obscured wherever

the rubbly parting (often usurping the thickness of an otherwise regu-

lar bed) is well-developed.

Lacave Calcilutite Unit 2 is a thinly bedded unit with an

alternation of four distinct lithologic types - (a) Calcilutite, (b)

Grainstones without banding (c) Banded Algal Limestone (d) Chalky

limestones. The last category is created to accommodate those less

coherent calcilutites probably with a better intercrystalline porosity

as well as those pelmicrites whose grainsize or grain frequency is too such low to be diagnosed as grainstoncs in the field. Likewise (a) occurs

either in a grainstone or intergrades with (d). A triangular diagram can

summarise the mutual intergradation between the three. Field diag-

nosis of the three categories (a), (b), (d) was biased by personal as

well as chance factors, like the degree of induration and the propor-

tions of end members in a specimen. The fields assigned to the three wbuld component s, reflect a weighting to compensate for this bias.

(a) Calcilutites - These and the chalky limestones unrelated

category are often millimetre laminated, reflecting very minute changes

in grainsize. They often split into centimetre units to expose sur-

faces flooded with single valves of Astarte(0O)Thicker shelly laminae often with a sharp base and shell strings are also present. The millimetre lamination is more apparent in the chalky variant.

(b) Grainstones comprise pellets, whole or broken small gastropods with some bivalves, encolithS:.and at Rocamadour even ooids or, a mixture of them all, in a sparry or micritic matrix. They form the thicker beds and are diagnosed also by their granularity. Thin beds of flat pebbles have also been assigned to them for purposes of quantitative analyses. C)ncolitic grainstones and flat-pebble grainstones grade into Banded Algal Limestones, the former by grain coales- 66

PL. I I : STROMATOLI TIC. (UNIT 2) LAC).VE CALC I LIITTTE AT ITS TOP CONTACT WITH UfI T 3, N. 20 to Sassondrics.. A s ti on of li tho to cal types . Bs Banded algal component. Lamination mostly horizontal but with slight crinkling. r. 67

cence and the latter by the close packing of flat-pebble strings.

except that the mud-crack features and the complicated types of

stacking described for many ancient and modern algal mats are absent.

Banded layers are sometimes hummocky but even this may be tectonic.

They are sometimes so fissile as to be mistaken for shales or shaly

paper limestones as is shown by the error on the Brive Sheet at Le

Chapellone, when not so fissile and dominated by gratnstone beds, they

can be missed altogether.

(d) The chalky beds like the very fine-grained grainstones develop

a characteristic serration due to jointing (see Structural Section).

Lacave Calcilutite Unit 2 is dominated by Astarte and

small cerithiid gastropods. Burmir-lignhigelegantula is very rare

but present.

The thickness of Lacave Calcilutite Unit 2 ranges from 0 -

24 m with an average of

Unit 3

The thickly bedded calcilutite of Lacave Calcilutite Uhit 3

rests with a sharp contrast on) the thin-bedded to platy Lacave Calci-

lutite Unit 2 (see photo) north of Blazy (N.20). At Rocamadour the

entire unit is physically distinct, resting with an overhang on Lacave

Calcilutite Unit 2. A similar feature is expressed at St. Sozy where

' the Lacave Calcilutite Unit 2 is easily located at a distance as a

broad notch between massive Lacave Calcilutite Unit I and Lacave

Calcilutite Unit 3. The type locality for the member is at Lacave,

where detailed sampling has been done and the greatest range of characters

is developed. Continuous sampling has likewise been done at Rocamadour

but the top of the member is not so clear-cut here. The commanding

pile of calcilutites at Le Chapellone rest on a humble exposure of PL. 12 : FLASK-SHAPED PHOLLD-TYPE )BITRE07:S, (Field-pho to cre.nh above and. hand.-specirien below), Lacave ealcilutite Unit 3, Sas:-.;onarics. 69

Lacave Calcilutite Unit 2 but their erroneous thickness (ti 100 m), the

highest in the region for this member, has occasioned the absence of the

topmost horizons.

These greyish-blue impalpable calcilutites are divided into

an upper and lower half by pellety micritic oosparite bed which is p1.12). intensely burrowed./ Thalassinoides occur doubtfully in this unit, while cemented flat oysters and crustose polyzoans are quite common.

The base of this facies is relatively sharper than the top which some-

times nearly invades the entire upper half of the member. Good ex-- posures of this bed - whose oolitic grains are easily distinguished from thoSe of the Souillac Oolite by sparry cores (of Trocholina) dotting the fracture surfaces - occur near Sassondrics on the N.20, two points between Souillac and Terregaye at the northern approaches to

Blanzaguet, on the first footpath branching from the minor St. Sozy-

Souillac route at Le Bastit, arid at Lacave. This horizon is very fossiliferous, and contains in addition to the elements of the lower and upper calcilutite forms like Trigonia Chomatoseris orbulites, and at its upper and lower transitions Pholadomya%, Pleuromya$, Cero- mya$'; Pterocardia, and Diceras and abundant sponge spicules.

Fossils recovered from the rest of the member are:- Gryp17a ding bilobata, Microthyri$ lagenalis and Obovothyris perobovata (both more abundant in the burrowed horizon), B. elegantula(?), Lucina 22y ,

Mactromya rugosa, Gerviliasue., Modiola perplicata, Arcomytilus asper,

L. subsella, and costate oysters and Chiamys stricta. The similarity of the fauna to that of Lacave Calcilutite Unit 1 is quite noticeable. 70

PL. 13:YEDDING-SURFACE'EXPOSURE,LACAVE CALCI- LUTITE UNIT 4, with well-preserved echinoid spines - quite e;:ceptinal for this level. ( Quarry at Sassondrics ). 71

Unit 1+

This is the thinnest member of the Lacave Calcilutite measur-

ing only about 4 m. It is a very prominent marker bed maintaining its

thickness throughout most of the region. Its type locality is again at

Lacave where superposition on Lacave Calcilutite Unit 3 is well shown,

and the maximum differentiation into three subunits is well shown at an

accessible position above the cave entrance. Not showing this differ-

entiation but accessible to close study are the exposures at Sassandrics

and the minor syncline after the Port de Souillac Fault. At the quarry

works nearby top (plus middle) and lower units occur. It is a thinly

and regularly bedded light-coloured calcilutite with ubiquitous, milli-

• metre-lamination sometimes gently rippled. The colour is also greyish- blue - so-called blue-hearts (relics of incomplete oxidation). This

lutitic member contains grains of three kinds - (a) bioclasts mosIlyof

small ostrei4 - valves with a debris of C. stricta and echonoid spines Phd occur on the surfaces of fissile units , echinoid spines(rUyncomplete

cidarids also abundant on weathered surfaces) and as frequent graded

centimetre laminae with a sharp base. (b) irregular concentrations

and whispy streaks of very poorly sorted elongate clasts of black lime-

stone in a bluish grey lutitic host matrix. The grains may have been

unequally distributed by organic activity. Both these- structures come

from the lower unit which is 1.9 m at Lacave, the middle and upper units

being 30 cm and 1 m respectively. The lime-clasts have been found only

at Sassondrics, their probable representation at Lacave being either

the presence in thin-section of a few dark grains or the pebbly re-

occurrence of the oolitic facies of Lacave Calcilutite Unit 3, or both.

The two possibilities arise from a rapid lateral variation within Lacave

Calcilutite Unit 3 at Lacave. 72

Apart from the small cemented oysters and fragmentary echinoid

and pectinid remains, fossils are virtually absent. One large Pleuromya

has however been found very near the base of the member at Sassondrics.

Age of Lacave Calcilutite

The problem of dating largely due to the strong facies control

on the fauna is reflected in the one concensus among the regional sheets

- their uncertainty. All of them have mapped the Callovian and Lower 2-1.2-1 Oxfordian as inseparable under the compound stage j The description

of the Brive Sheet broadly coincides lighologically with three .of the members mapped. The folloWing difficiencies may be mentioned, in addi-

tion to the omission of Lacave Calcilutite Unit 1 as a distinct unit-

(a) The platy or shaly limestones with imprints and moulds of bivalves should have been more precisely identified to prevent the type of mapping errors already mentioned.

(b) The coarse-grained limestone reported to be exploited at

Gignac is probably the same one that is exploited at Port de Souillac, but it lies entirely within Lacave Calcilutite Unit 3 with varying degrees of definition and not as a distinct member between Lacave Cal- cilutite Unit 2 and Lacave Calcilutite Unit 3 as suggested in the caption.

(c) The unit described as overlying the 'coarse-grained' limestone is not always sub-chalky. Its approach to chalkiness depends on the degree of its invasion of the 'coarse-grained' facies it harbours.

The divergences between Mouret's (1892-98) succession (itself not over-accurate) and that of the adjoining sheets may also be traced either to the failure to diagnose Lacave Calcilutite Unit 2 precisely so that fissility which is purely secondary in importance to the banding decides, as on the Gourdon Sheet, whether Lacave Calcilutite Unit 2 is a shaly, platy limestone (when fissile) or just another compact subchalky limestone. The wide range of interbedded grainstones become in 73

turn confused as on the same sheet with Lusitanian coraliferous oolites.

The mention that the beds in question 'seem well-attached to the

Callovian-Oxfordian' in the north of the Gourdon Sheet confirms the

author's observations in the same area at Rocamadour. The uncertainty J

of the marker bed between Lacave Calcilutite and the overlying St,

EtienneUrntSturre-however does not appear sufficient to create a compound 2-3 stage j once the oolitic intercalations have been rightly placed in

Lacave Calcilutite Unit 2 since (i) a physically distinct lithologic

unit of 29-30 cm rests on the Lacave Calcilutite Unit 2(at least at

Rocamadour) in the north of this sheet. Elsewhere, this is Lacave

Calcilutite (author's notation) of the classical j2-1 , and should be

so considered here. (ii) This unit is also distinguishable by its

higher mean bedding thickness. Slightly over 50% of all beds measured

were over 1 m. thick as compared to less than 25% for the interval

assigned to the overlying marker bed. The weakly defined master

bedding planes in the latter means that even thinner units could have

been measured - a feature that further supports the standard character

of this bed. Thinner bedding differentiates it equally effectively

from the basal calcilutite of the St. Etienne Limestone. The latter

has a 60% frequency of > 1 m. beds - also typical. lime&to\NQS Qlre. wkOict. Commort in t-qcci ye Czatitut;ixt., tivii tit. • (iii) Blue-hearted. The above reasons establish the recog-

nition in the north of the Gourdon Sheet of the top of the lithological

unit customarily considered as j2-1 in the Brive Sheet. The near-

tripling of the thickness of the marker bed in the Rocamadour area alone

remains to be resolved, but this is a sedimentologic particular - and is

dealt with subsequently.

The platy or shaly-marly bed considered on the Cahors Sheet .2-1 as effectively defining the, base of j , and as being overlain by fine-

grained sublithographic limestones in thick beds may properly be litho- 74

logically correlated with the beds mapped, especially so also from the

mention of R. elegantula - barring the omission of a mention of a litho-

logic equivalent of Lacave Calcilutite Unit 1 that one has come to

expect (on account probably of its thinness (7 m)). The overlying

formations are also effectively discriminated on this sheet. In con- (I869) trast Magnaras able to separate j3 from j2 by the obviously coralline

aspect of the latter, but besides the marls, Posidonomya could not

separate J2 from j1 .

Locally, therefore, the beds mapped are traditionally j2-1 or

Callovian-Oxfordian - but are they really so in the interregional con-

text, ana if so, can Oxfordian be differentiated from the Callovian?

The Callovian fossil lists of Glangeaud (1895) for the

Charente region are dominated by ammonites and even his recurring

terebratulids such as T. dorsoplicata, T. intermedia, Zeilleria pale

and Z. hypocirta do not seem to have any counterparts in this region.

He was clearly working in a different sedimentary facies. But he faced

the similar problem of having a lot of forms with extended vertical

ranges. He states that a number of aspidoides zone forms such as

M. macrocephalum,Sphaeroceras cf. bullatum , S. microstoma,

Perisphinctes subbackeriae also appear in the Lower Callovian and re-

commends that the entire fauna rather than such key-fossils be used for zonation. His findings in the Callovian may be summarised thus -

(i) Basal Callovian (macrocephal4 Zone) usually missing. (ii) Middle -

Upper Callovian also sometimes unrepresented. (iii) Reinekia anceps

Zone (probably covering the calloviense, Jason and coronatum Zones)

is the most fossiliferous and most widespread [O. the abundant non- ammonite fauna probably ensures easy diagnosis everywhere]. (iv) The

Oxfordian-Callovian passage is not precisely zoned because of the co-occurrence of Q.-mariae and Q. lamberti. (v) The anceps and 75

athleta Zones overlap through the co-occurrence of E. coronatum and P. athleta.

These frequent overlaps, particularly the extension of the aspidoides into the macrocephaluA Zone-, explains how Neaverson's Macro- cephalitan fauna comes to be Clydoniceratan in the present area since the occurrence of Macrocephalites with Obovothyrids could easily have led

Neaverson to dub the latter Macrocephalitan.

Arkell (1933) rejectinc, D'Orbigny's inclusion of the macro- cephalus Zone in the Bathonian, mentions that neither in England nor in

France the Nlacrocephalitan and Clydoniceratan faunas co-exist. The author would like to place against this generalization all that has been said about the vertical range of Obovothyris, Microthyridina and their co-occurrence in beds that are overlain by a Callovian fauna, as well (19b) as about Neaverson's (1928) Macrocephalitan fauna. Arkell1,himself (p.

328) notes the unpredictable distributions in numbers and species of

Kallirhynchia, Ornithella and Kutchirhynchia within the otherwise monotonous obovata fauna. It is possible that some such new and sporadic species have been mistaken for zonal forms, but it is equally possible on this basis that zonal forms have wandered into each others stronghold

- the interpretation adopted here.

In addition to the difficulties confihned by Glangeaud, the very unsatisfactory systematics of the often quoted Rhynchonellids may be briefly mentioned. It was in 1895 after the regional sheets had been completed that Glangeaud in his extensive work, recognised the inclusion of R. elegantula proper, R. hopkinsi, and R. decorata under the one elegantula species. His figure for eleqantula proper is easily recognisable from hopkinsi but hopkinsi is morphologically very close ? to R. decorata (E Stolmorhynchia decorata of Schlotheim (see Piveteau,

1952) save for the incurved beak of the latter. This possibility of two rather than three species within the traditional R. elegantula is 76

further complicated by the presence in Lacave Calcilutite Unit 1 upwards

(also Blagour Breccia. and Souillac Oolite) of forms which are quite

clearly more inflated, with a much more gently trilobed brachial valve

than the type R. elegantula of Desiongchamps (1862-96). M. Cossmann

(1900) identified as R. elegantula a form which is different from

Deslongchamprs'figure both in the number of ribs (r%., 15 to 25), in the

very weak as opposed to strong trilobation, and in the very shallow

hinge area. Cossmann himself admits a great variation in the size of

ribs. A side view is not given, but it appears that the tumidity could

also be stronger than Deslongchamprs'figure, approaching in this and the

previous'characters R. hopkinsi. The time separating the two, if there

is any, isceMinly_tenuous. These features strongly suggest a contin-

uous series between R. hopkinsi and R. elegantula particularly also in

view of their frequent co-occurrence. Aware of these difficulties, the

author has nevertheless considered most of the smaller fthynchonellids

from the Breccia: to the St. Etienne Limestone (in part) to be nearer

to B. (elegantula) elegantula rather than B. (elegantula) Hopkinsi a

fine specimen of which does occur in the Souillac Oolite. Keeping

Glangeaud's (1895) recommendations,one may compare Arkell's fauna at the

top of the abovata zone to the Lacave Calcilutite fauna also overlying

beds of obovata age: (a) The great diversity of bivalves in Lacave

Calcilutite Unit 1 and the flood of Astarte% in Lacave Calcilutite Unit

2 both find their equivalent in Arkell. (b) The button coral Anabacia I complanata is probably the same as Chomatoseries orbulites (in Treatise)

in Lacave Calcilutite Units 1 and 3 (also Souillac Oolite).

in transverse and horizontal profile with the forms in Lacave Calcilutite

Unit 1 through Lacave Calcilutite Unit 4 (also Breccia_ and Souillac

Oolite) referred to B. (elegantula) elegantula, the resemblance being

even closer with Deslongchampre type figure. 77

Less controversial than the ithynchonellids for interregional

correlation is Gryphea bilobata which occurs in Lacave Calcilutite Unit

3. This form is limited locally to Lacave Calcilutite Unit 3 and appears

to be so limited to the Sioaloceras calloviensr zone in the Midlands of

England (see Callomon's (1968) sections, and older works).

Just as the extended upward range of the Obovothyrids and in* Microthyrids is partially atypical, so is the downward range of the

rudistid bivalves. Only in the latter case, a recent view has come

from the Holy-Cross Mts. (Poland) and the N. Calcareous Alps to support

strongly the suspected influence of sedimentary facies. Preservation has made dental structures inaccessible, but external identity can be established between the Rudistia genera of the condensed Lacave Caici-

lutite Unit 3 below St. Etienne and Karczewski's figures of Pterocardia,

Macrodiceras and Diceras. Diceras arietinum considered to be Upper

Oxfordian (Rauracian/Argovian) (T. & G. Ternlierp. 263 (1959), Magnan

186.9) is probably among the collection. The condensation of Lacave

Calcilutite Unit 3 however has taken place at the expense of the Calci- lutites above and below the pseudo-oolitic horizon which is the probable repository of the hippuritidsas, is borne out by the occurrences of hippuritids and hegalodontids by Karczewski (1969) and Zankl (1968) respectively. Pte_rpcardia4 occursalso in Lacave Calcilutite- Unit 1.

The following conclusions emerge from the classical arguments above -

(a) The entire Lacave Calcilutite is Upper Oxfordian (Rauracian) or younger (t. subsella ranges from Lacave Calcilutite Unit 1 and top Souillac

Oolite).

(b) Lacave Calcilutite Unit 3 is lower Callovian (S. calloviense)

(G. bilobata).

(from Obovothyris and Microthyridina). 78

(d) Lacave Calcilutite Unit 1 is of Reineckia anceps (calloviense

to coronatum age (from Blangeaud's observations).

(e) Lacave Calcilutite Units 1 and 2 are of Clydoniceratan-Macro-

cephalitan age (comparisons with (Bathonian) Arkell).

(f) The entire Lacave Calcilutite is of asp:idoides-discus age,

contrary to the local assignment based on the same B. elegantula.

It is obvious that no classical zonation can be carried out that

will do justice to all the possibilities. The strong influence of the

sedimentary facies is obvious and the best approach is thought there-

fore to be to establish an upper and a lower age bound for, plus

any other reliable datum within the controversial interval, and then an d seek a sedimentological model based on the regional geometry, vertical

succession of rock-units, and other sedimentary-and bidfacies criteria.

This is done in a subsequent section, but a random illustration or two

would be appropriate here -

(a) If the Souillac Oolite is accepted as Clydoniceratan age then conclusion (e) above can be accounted for by considering Lacave Calci-

lutite Unit 1 (lagoonal) and Lacave Calcilutite Unit 2 (supra-inter-

tidal stromotolites) as time-equivalent sedimentary facies of an oolite barrier.

(b) The very similar fauna of Lacave Calcilutites Units 1 and 3 is likewise explained by a transgressive and regressive lagoonal cycle wrapping the supra-inter-tidal phase of Lacave Calcilutite Unit 2 - an interpretation that is borne out in detail by the regional geometry of the members in question.

For the type of chronological control envisaged -

(1) Conclusion (b) is wholly accepted because of the vertical restriction of G. bilobata. 79

P L s 14 GENERAL VIEW OF THE ST. E TIENNE L S T • AT THE TYPE-LOCAL' TY.(Note the ."overlying Lanzac Oolite causing the break-in-slope at the right-hand. side of the sky-line, and the increasing thickness. by splitting of the underlying Lacave Caicilutite). 80

(2) Conclusions (b), (c) and (e) do not at least contradict a lower

Callovian (sub-Callovienst) for the entire Lacave Calcilutite - an

assignment different from the traditional view but considered reasonable

here.

7. St. Etienne Limestone(p1.141)

This formation was first studied in quarries near the village

of Les Patods. Microscopic studies have since shown the pellety gross

character as due in fact to a high content of foraminiferal tests as

well as of pellets. This has however turned out to be nearly the top-

most part of the formation and further study has shown a very rhythmic

interbedding of finer-grained chalky limestones with the coarse obviously

granular (pellets, intraclasts, forams, rare ooids) ribs. Some doubt

now exists on the original texture of the finer-grained rhythms owing

(1) to the fineness of whatever grains-pellets that they may have had and

(2) even more unfortunate, there was considerable dolomitizatibn (now

dedolomitized) which has tended to efface original pellets and produce

pseudo-pelletoid fabrics. With this textural qualification and the

complete exposure and accessibility of this formation below St. Etienne, the

more appropriate name of St. Etienne Pellet-Chalk Rhythmite (or Lime-

stones) is proposed with the intention declared earlier. With accepted

loss of content, but gain in brevity, the name St. Etienne Limestone

will be used throughout the text.

The St. Etienne Limestones64ess vcIriabilitythan the Lacave

' Calcilutite, and is not divisible into mappable sub-units. Throughout

the region, the same character observed at the type locality is dis-

played - an unbroken rhythm of Chalky, sometimes pelletoid limestones

and thinner beds of lntra or bio-pelsparites and -micrites, the latter

weathering out to form regular ribs. The white chalky colour which is

common further up is replaced by pale-yellow tints below, the colours in 8

both cases approaching grey and buff with increasing dedolomite content.

Interesting as a confirmation of textured and sedimentary kinship is

the ribbing imposed on Lacave Calcilutite Unit 3 in the Alzou valley

south of Lacave - a ribbing so similar to that of the St. Etienne Lime-

stone that the exact position of Lacave Calcilutite Unit 4, the pro-

minent marker bed is confounded From a distance. Apart from this, the

physical limits of the formation are very well-defined by the marker

bed below and the steep capping and paler colour contrast of the Lanzac

Oolite above, the St. Etienne Limestone itself preferring to form the higher hill-slopes and roofing the sub-vertical river-side cliffs and gorges of Lacave Calcilutite. A gradual passage to the oolite is however shown in the increasing grain-size and ooid content.

The Limestone has a clumped fossil distribution, fossils coming mostly from the thin granular bands, recovery being especially good where these are rendered irregularly fissile and rubbly by a subordinate clay content, as was the case in the rubbly lenses of Lacave Calcilutite

Unit 1. In keeping with the increase in grain-size, a very diverse benthonic fauna is present at the very top where the beds become pebbly with a crude grading, recovery being facilitated in these circumstances by a regular intersection of minor thermal joints leading to a broken- plate weathering. The following fossils have been recovered from the

St. Etienne Limestone::.: (elegantula?), Microthyridino vicifo$04,2.is lagenalis, Obovothyris perobovata, Digonel la skid Terebratula IR:,

Ostrea gregarea, Ostreasue., Lucina cf. striata, Pinna rriitiS.

Ceromya concentrica, Ceromya sue., Panopaea cf. mandibula,:=_, Mytilus varians, Corbis a:, Chlamys stricta, Alaria palmata,

Natica cf. neritoidea, Nerinea (Rigauxia) varicosa, Pterocardia epargnensis, Osteomya dilata, Chomatoseris orbulites, Arliulospira

:sharper ., Textularia, Trocholina, Milliolids, Pseudocyclamina, coskinolinopsis, Burrows, Stromatopora arrabidensis. 82

Age of St. Etienne Limestone

The description of this formation on the Brive Sheet 'limestones

with a lithographic fracture in thick massive beds' is extremely sketchy. .

The lithologic and faunal correspondences between the author's Lanzac

Oolite and the Borreze Limestone and between his Lacave Calcilutite and

the Rignac Limestone, leave no doubt that the intervening St. Etienne a Limestone is what is being described under the designation j3 - Cassagne

Limestone. The faunal similarity claimed for the Cassagne and overlying

BorreXze Limestones also suggest that the subdivision of j3 into j3a and

j3b was made lithologically.with a strong temptation to consider the

chalky character of the former as the beginning of an Upper Oxfordian which

they decided must be corraline. All they needed to do was then simply to

separate the chalky lower formation from the oosparites above.

This is unfortunate as Rhynchonella thurmanni (i.e. Thurm'ella) which

they admit for the Borre:ie Limestone (Lanzac Oolite) is a Lower

(cordatum - transversarium, A. Childs (1966)) rather than entirely Upper

Oxfordian form, if one may be true to local tradition and consider j3a- is 3b as Rauracian (= Argovian). This form veryrare or absent in the

St. Etienne Limestone as is Torquirhynchia inconstan!s a Kimmeridge Clay species which is present in the Lanzac Oolite but absent in the St.

Etienne Limestone. Both considerations strongly indicate that the

St. Etienne Limestone cannot be younger than the cordatum zone (top of Lower Oxfordian).

The Cahors Sheet distinguishes under Argovian-Rauracian (j3) a variable upper (j3b) limstone which is sublithographic to subchalky oolitic or coralline from a lower j3alimestone of thinner beds forming a talus. Again St. Etienne Limestone may be lithologically correlated with beds that are dated as lower Argovian (j3a). In the Gourdon Sheet, the St. Etienne Limestone as a distinct

formation is all but present in the welter of stage fusions and confus-

ions on the true extent of the lithologic variations within the stroma-

tolitic Lacave Caicilutite Unit 2 (see earlier).

Magnan's (1869) succession for the regions between the Vere and

the Lot does not subdivide his j3-Corallian at all and again fusing

the Lower Oxfordian to the Callovian.

For all the immediate regions, therefore, the rejection of the

age of the beds generally considered as entirely j3 may be applied so

that what was hitherto j3a (topmost Rauracian) starts lower at j2 on

the evidence of T. inconstans and Thurmaella obtrita. The underlying

St. Etienne Limestone is therefore partly Callovian on the last evi-

dence as well as on the compromise evidence of G. bilobata at the top

of the Lacave Calcilutite (see earlier).

Glangeaud partly bears out these conclusions, for he lists T.

inconstans in the Rauracian-Sequanian but goes on to say that from the

region of Charente to the Dordogne he could demonstrate in the Rauracian

and basal Sequanian 3-4 levels of coralline limestones overlying chalky

ones, an observation which must mean that the rhythms here, overlain

by beds with T. inconstans must be older than those of Glangeaud, The

confidence of Glangeaud's dating based on Ammonites is compared with

the confidence here placed on Thurmarpla and other evidence. Before

comparison with the British standard, additional local evidence for the conclusions reached above is briefly presented. (a) The occurrence at the top of the St. Etienne Limestone of Stromatopora arabidensis

figured for the Lusitanian of Portugal by Dehorne (1920) may indicate a sub-Rauracian'age as the youngest for the bulk of the underlying St.

Etienne Limestone, just as Thurmaplla obtrita, Defrance and T. inconstans

limited it also to sub-Rauracian. (b) The extended upward range

(high into the St. Etienne Limestone) of 9bovothyris though going beyond 84

the G. bilobata 'zone may be partly excused by the pointers to a Callovian

age for the St. Etienne Limestone. (c) Most of the bivalves and a

brachiopods considered in most of the region as typical of the Upper

Oxfordian and later can be shown to occur further down also. e.g. (i)

The occurrence of L subsella in the Lacave Calcilutite is again con-

firmed in this context by the figures of Loriol et al. (1872), particu-

larly as this work was exclusively on the Oxfordian and higher and the

terebratulid species were sufficiently few and distinct to leave no doubt

about the morphologic identity of the present collection with Loriol's.

Thurmann and Etallon (1861-64) extended the upward range of L. subsella

by referring both T. subsella (Rauracian-Virgulian), T. biplicata (Red

Chalk) and T. sella (Lower. Greensand) to the common species T. suprajurensis. This extension is very much greater than the proposed downward

extension of L. subsella into the Lower Callovian. Indeed the present

collection is identical with their T. suprajurensis and with the T.

subsella of Davidson and there is no reason not to depend on morphologic

identity, for consistency.

The rejuvenation of T. subsella by de Loriol (1872) and Davidson, in no way satisfactorily removes the quite close resemblance (age not withstanding) between L. subsella and all the forms referred by Thurmann and Etallon to suprajurensis. T. bicanaliculata (E T. subcanaliculata) of Thurmann and Etallon is likewise very similar to the present collection of 1- subsella. The conclusion is compelling that age differences are being used subjectively to side-track the morphologic identity of all

these forms and the very long range that a single all-embracing name would

have. A similar situation is near at hand in Rollier's (1911) range of

sub-Bajocian?, Bathonian-sub-Calkvian for the globata group of Terebra-

tulids to which Stiphrothyris tumida belongs. The current view (Treatise)

of the sub-Callovian range of S. tumida could mean that Rollier had in

the Callovian morpholigic relations of S. tumida,relations that may in present fact interestingly turn out to be theAauthor's L. subsella. Rather it

may thus be confirming this occurrence in a different language.

(ii) Ostrea pulligera listed in the Brive Sheet under Virgulian is one

and the same tibbed O. gregarea occurring without appreciable break from

the Souillac Oolite. (iii) Zeilleria humeralis listed under Virgulian

on the Brive Sheet and figured and described for the Corallian-Virgulian

by de Loriol et al. (1872) occurs also in the St. Etienne Limestone.

(iv) Ostrea bruntrutana listed under Virgulian may be the same as (1M) Exogyra nana (right valves) of Arkell's,Osmington Oolite (basal Upper

Oxfordian?) and of the Upper St. Etienne Limestone. These instances

lead to the conclusion that it is not as though we merely had high zonal forms occurring very low down, and partly explicable by condensations and/or facial repetitions, but that in some cases, such as those above, the vertical ranges of single species and faunas may have been grossly underestimated, and that faunas and species like this cannot be used to reject the new zonation that has emerged here.

( v) Aulacothyris impressa, Bronn., a small trim brachipod and very easy to identify as evidenced also by the unanimity of its designation by no less than five authors (from D. Ilovaisky to de Loriol). Moreover none of the similar forms in Davidson (ranging from the Inferior Oolite) appear to display the median septum so clearly through the brachial valve as the present species does. Locally it is restricted to the upper half of the St.. Etienne Limestone and both de Loriol (1900) and

D. Ilovaisky consider it to be of Lower Oxfordian age, the former author 86

observing it to occur with R. thurmanni just as it does here. Conclusion

(a) is therefore corroborated and the upper half of the St. Etienne

Limestone is Lower Oxfordian.

It may be concluded from the cumulative positive and negative evidence above that (a) the lower half of the St. Etienne Limestone is roughly Middle-Upper Callovian (Jason-lamberti inclusive). (b) The upper half of the St.. Etienne Limestone plus at least the lower third of the overlying Lanzac Oolite is Lower Oxfordian.

8. The Lanzac Oolite

On cliff exposures this oolite is easily recognisable from the St.

Etienne Limestone by the criteria already mentioned. On more gentle hill-sides, the contact may be ascertained from the first consistent occurrence of ooid grainstones and/or coarse cencolitic limestones.

In detail, the character of the passage is quite variable within the limits defined above. Just before the large quarries near the type- locality of Lanzac, about 13 m of St. Etienne Limestones are exposed and with them a nearly uninterrupted passage to Lanzac Oolite. Crudely graded very fossiliferous, fine white and soft (almost marly though about 100% pure) 2 m beds alternate with coarser less fossiliferous and competent beds up to the four metres when the soft beds tend to platy and sharply pass up into the pellety and coralliferous oolite grainstone of Lanzac Oolite worked in the overlying quarries. The sharpness of the passage is only in contrast to the immediately underlying bed, for the entire Lanzac Oolite may be looked upon as an extended coarse bed in the coarse-fine alternation of the St. Etienne Limestone.

At the inter-departmental boundary on the way to Cazates from

Souillac, a modified passage is shown in which high dedolomite concentration has rendered the top 8 m. of St. Etienne Limestone as a light- 87

yellow-buff massive, smooth-fracturing pseudocalcilutite overlying and

dominating the more obviously granular, chalky shell grainstones.

Thinner but here pseudo-lutitic limestones again form the immediate con-

tact with the yellowy clean, and fossiliferous oolite in the small dis-

used quarry above the railway tunnel. A similar, pseudo-lutitic passage

demonstrable in thin-section to be a dedolomitic fine, foraminiferal

grainstone is exposed at Le Roc across the river.

These variations while helping to fix the base of the Oolite more

precisely under varying circumstances, also support the view of the St.

Etienne Limestone as originally largely pellety. The Lanzac Oolite is

regularly bedded unlike the Souillac Oolite and shows a flaky weathering

in its lower reaches contrasting again with the Souillac Oolite and the

broken-plate weathering of the St. Etienne Limestone. The flakes yield a rich fauna of Aulacothvrisis, and abundant eierineid (some giant forms) gastropods, Rhynchonellids and Ostrea gregarea, not to mention a variety of corals, branching polyzoans and encrusting algae and polyzoans forming a boundstone fabric on the pellety oolite grainstone. The (Mite is far from monotonous; quite definite variations are discernible even grossly consisting of pellety and shelly oolite grainstones and boundstones, pure well-sorted oolite grainstones and oncolitic oolite grainstones and oncolite grainstones often with branching stromatoporoids.

These basic types are modified by varying entries of intraclasts, concentrations of dedolomite leading to whole beds of greasy crystalline limestones and grain-size variations within each of the rock types.

All these interact to determine the small-scale mode of weathering such as flaking (already mentioned), solution-pitting very much like burrowing (at St. Etienne) and a peculiar greenish alteration product common at Lanzac. The last process is probably the result of leaching, leaving behind an insoluble powdery residue. Large-scale intraformational 88

features however are never apparent anywhere on the landscape as in the

Lacave Calcilutite and St. Etienne Limestone, the nearest to this type

of feature being a break-in-slope at the Eglise-St. Etienne formed in

soft white oolite which also gives the solution-pitting.

.The upper limit of the formation is not easy to define for three

reasons:

(a) A discordance can be shown (see subsequent summary and evidence) to exist towards the top of the Oolite though this has hitherto been unrecognised. It can be shown both from the mapping and the examination of continuous sections such as those at Lanzac and Les Bastit-Cales that the lime-conglomeratic bed at the top of the Oolite near Cabales is diachronous and is the same bed at the other two sections but only coming to rest on ever higher horizons of the Lanzac Oolite. This is an example of the possibility of stratigraphical error type (v) if stages rather than rock-units had been mapped.

(b) The succession above the discordance is poorly known owing to the fact that erosion and the westerly regional dip have preserved only the erosional feather-edge of this succession in the area mapped, the implication being that the recognition of the point of truncation of the underlying formation rests implicitly on a good development of the conglomerate - and this cannot be guaranteed for at all circumstances.

(c) At least in one locality, (Les Bastit-Cales), what is known of the transgressing beds bears a close resemblance to the underlying platy oolite/pale calcilutite alternation that forms the top of the Lanzac

Oolite at this point - a complication that emphasizes once again the recognition of the conglomerate or a good cliff exposure or cutting to demonstrate the discordance.

(d) A possibility which is at least partially demonstrable here is that a high concentration of dedolomite, if developed near the top of the 89

Lanzac Oolite can greatly confuse the top contact in the same way that the topmost St. Etienne beds are sufficiently diagenetically varied to tempt one to recognise new lithologic horizons.

The above instances dispel any doubt as to whether the observations of the varied character of the top of the Lanzac Oolite which follow are genuine intraformational variations or whether they represent distinct formations in their own right.

At Lanzac, the white and yellow-stained white oolites of the y= rt quarries pass upward like the Mirandol Oolite into smoother mud-spotted oncolitic oolites in thick regular beds. The dull earthy-grey colour and diminIshed granularity contrast with the pure oolites. Above the wackestones are platy impure limestones with a flood of Astarte followed by a succession of coarse earthy intraclastic oolite beds alternating with soft and dark shaly beds. This alternation is disharmonically folded and brecciated, all the way up to the top of the hill, 40-50 m above the wackestones.. The bed with Astarte is recognised throughout the region as definitely above the Lanzac Oolite or its equivalent, so that the top of the Oolite here may be taken at the base of the wackestone, the lime-conglomerate being absent here.

At Cabales, the discordance and the break at the top of the Oolite is better demonstrated by the extreme thinness of the Oolite to the north of the hamlet as compared to the south of it and the greasy recrystallised lime conglomerate bed which is overlain by very pale unfossiliferous platy calcilutites very similar to some platy types in Lacave Calcilutite

Unit 2. The conglomerate is patchily exposed on the left of the Cabals-

Souillac route just before the very sharp bend.

Towards the last third of the Le Bastit-Cales route, the varying

(as described) oolitic limestone passes up to platy finer oolite grainstones with a great abundance of small oysters, and thinly bedded to platy creamy calcilutites followed by a bed with blacl/chert fragments and 90

x 45

PL.. I 5 : GRAZ101 TONE A T IINCONPORMITY BETWEEN' LAN7AC GOLITE & CALES BEDS,, showing iron- a_tained intrac1asts,b0 th free & clacketed, cierived o and, squashed ES atro po • 91

lime-pebbles in a fine ooid grainstone. This bed, very similar in

gross appearance and microscopic character with some pebbly intraclast

grainstones at the Gluges Calcilutite Unit 4/Blagour Brecciaii.Unit 1

break, is interpreted as forming the sub-Crezelade discordance. Above

this bed are some more platy true oosparites overlain by the pale

calcilutites on which Cales stands.

The evidence for the sub-Crezelade discordance can be summarised

thus:-

(a) Evidence from petrography: (i) - The occurrence at the top of the

Lanzac Oolite of only one mixed pebble horizon upholds the use of this

horizon as a datum for comparing the top Lanzac Oolite succession from

place to place. (ii) The petrographic similarity with the Gluges

Calcilutite Unit 4/Blagour Breccia% Unit 1. break( p1.15 ).

(b) Evidence from mapping:(0The differential thickness of the Lanzac

Oolite below the conglomerate bed within a small area about Cabales.

(ii) The frequent instances on the adjoining Gourdon Sheet of the un-

certainty of the nature of the passage of j3 to VI, as evidenced in the

presence in one place and absence in another of an undifferentiated J3-4

at the j3/j4 contact. This is taken together with evidence (a).

massive calcilutites (wackestones) - platy laminated beds with Astarte

and intraclastic grainstones is so similar to the Souillac Oolite -

massive Lacave Calcilutite Unit 1 - Stromatilitic Lacave Calcilutite

Unit 2 with Astarte that the operation of an ideal environmental succession

becomes more than tempting. One may well therefore use this yardstick

as an ideal in the absence of a more reasonable expected standard

succession above the Lanzac Oolite in other areas such as Cales and

Cabales. The coincidence in these localities of the absence of the expected succession and the development of the mixed-pebble bed close to 92

the top of the Lanzac Oolite may be thought of as evidence of a break,

the question only being what type of break - angular or conformable or

both. Angularity is decided at Cabales thought the conclusions reached

here would not be altered if it were conformable in some places.

Fossils are hard to extract in the Lanzac Oolite but even allowing

for this, the Oolite appears to be less fossiliferous than the top beds

of the St. Etienne Limestone and the lower pellety beds of the Oolite.

The following fossils may be considered more typical of the Lanzac

Oolite although a great faunal similarity occurs between its lower beds

and those of the St. Etienne Limestone- Thurman)lla obtrita, Torqui-

rhynchia inconstans, Trochaeria cupuloides, Nerinea tuberculosa, ona Stromatopora arrabidensis , Cardium coralinum / corals. An important

key fossil occurring in the basal Lanzac Oolite but more characteristic

of the topmost St. Etienne Limestone is Aulacothyris impressa.

Age of Lanzac Oolite.

The Brive Sheet describes under the name of Borreze. Limestones

white clotted or oolitic limestones in thick, massive beds, with corals

overlain by subchalky limestones with nerineids. it liststwo

nerineids, two terebratulids, R. thurmanni and Ostrea solitaria. The

nearness of the type-locality to the area mapped and the approximately similar positions in the lithologic sequence to the Lanzac Oolite and

the recognition of the overlying Astarte Beds indicate lithologic equivalence between the Borreze Limestone and the Lanzac Oolite. The reported occurrence of 1. subsella is however very suspect as detailed collecting in this area has not unearthed it in the Lanzac Oolite. The age of the Borreze Limestone is given as. j3b - Upper Argovian (Rauracian), despite the probable long range of L. subsella; even granting its occurrence. 93

The Gourdon Sheet likewise correctly diagnoses the Astarte beds but does not recognise a separate formation or formations representing j3, which is partitioned between Ft and j2-1 in between the Callovian-

Oxfordian and the Upper Lusitanian. The Lanzac Oolite is mentioned as a coral facies at the bottom of the Upper Lusitanian Limestones. This

is surprising particularly as the northern part of this sheet has been examined by the author, and here at least, the Lacave Calcilutite/

St. Etienne Limestone contact is unequivocal at Gales and fixable by bedding character near Rocamadour. Also at least on both norther'n approaches to Cales from Le Bastit and Lacave, the contact between St.

Etienne Limestone (j3a of Brive Sheet) and Lanzac Oolite (j3b of Brive

Sheet) is sufficiently sharp to question the failure to recognise the two formations on the Gourdon Sheet. Also, following from this and the evidence for the sub-Crezelade discordance, it is felt that the Lanzac

Oolite (or its equivalent) has been unduly downgraded as a distinct lithologic unit on the Gourdon Sheet.

In the Charente area Glangeaud mentions coral reef development in the Oxfordian (i.e. Lower Oxfordian) - Rauracian as well as the three levels in the Rauracian to Lower Sequanian. There is therefore no justi- fication for the tradition in this region to consider the first truly coralline formation as invariably Rauracian (j3) or higher (Brive Sheet,

Gourdon Sheet); it could be Oxfordian (j2) at least in part, from

Glangeaud's evidence, which is all the more compelling because of the widespread parallel developments that he cites.

A reasoned age for the Lanzac Oolite based on (i) the position in regional successions, (ii) interregional facies comparisons, (iii) the • index fossils T. inconstans, Thurmarlla obtrita, Aulacothyris impressa and Stromatopora arrabidensis - appear to be (I) Lower Oxfordian

(j2 E mariae-cordatum) for the lower half up to the band with Thurmaipla sbuttof obtrita at the quarry. (x) Lanzac; (2) Upper Oxfordian - Lower Kimmer- 94

idgean (ba9lei) for the upper half and up to the beds with Astarte.

9. Crezelade Beds

These beds are named after the hamlet of Crezelade on the N.20

south of Lanzac. These are the highest beds exposed in most of the

region mapped except for the very thin outcrop of discordant beds over-

lying them at a few localities. The discordance has further restricted

their occurrence so that their best section occurs only in the neighbour-

hood of Crezelade, particularly from about the telescopic viewing table

southwards on the N.20.

Their similarity with Lacave Calcilutite Unit 1 and Lacave

Calcilutite Unit 2 above the Souillac Oolite has been mentioned. It will

suffice only to point out the most obvious differences between the two

successions. (1) Rippling in the massive Lacave Calcilutite Unit 1

is more clearly and frequently developed than its counterpart Lacave

Calcilutite Unit 1 of the Lacave Calcilutite. (2) Lacave Calcilutite

Unit 1 is demonstrably less fossiliferous than Lacave Calcilutite Unit

1. (3) Algal banding is less well developed in Crezelade Beds Unit 2

than in Lacave Calcilutite Unit 2, being observed largely by trains of flat lime-pebbles which were subordinate in Lacave Calcilutite Unit 2

(4) The regular plates with Astarte as well as the overall appearance of Crezelade Beds Unit 2 is less 'pure and shaly than comparable aspects in Lacave Calcilutite Unit 2.

(5) No passage of Crezelade Beds Unit 2 into massive calcilutites is observed as is the case with Lacave Calcilutite Unit 2.

Age of Crezelade Beds

No detailed collecting was done in these beds and the only clue to their age is. their stratigraphic position, lithology and the presence of Astarte. 95

The distinctive character of the platy beds with Astarte has

been mentioned. The Brive Sheet dates similar limestones above the

Borreze Limestone as j4 (Sequanian?). The fossiliferous oolitic or

subchalky, lithographic limestones, shaly marls and breccias overlying

these beds and correlating by lithology and stratigraphic position with

the discordant Cabales Limestone at Cales is however placed in j5a

(Lower Virgulian) so that the intervening Pterocerian appears to have

been merged with one or both of these stages. In view of the break • between the two groups of lithologies it is probable that the missing

Pterocerian can in fact be thus accounted for, at least in some local-

ities, bearing in mind the diachronous nature of the break. Implications

of this break are not extended beyond the area covered by the Brive

and Gourdon Sheets for the reason that so many years after Arkell

(1933) it still appears premature to fit a local stratigraphic break

into a universal scheme. A glance at Arkell is sufficient to show

that nearly every Jurassic stage has a stratigraphic break in one local-

ity or other. It is for this reason and the demonstrable local dia- the chronism of the break that the temptation to match it with/Deister Phase

(cited by Arkell' "-' as, . 'sub-Gravesia) of intra-Jurassic move-

ments is resisted.

10. Cales , Beds

These are the transgressing beds and are being mentioned purely

for completeness as no good exposures of them exist anywher-e except at

Cabales where the conglomerate is better developed and at Cales where

the beds above the break are well represented by the alternation of platy

very fine and fossiliferous ooid grainstones and thin-bedded to platy

pale coloured calcilutites.

The age of these beds at Cales is probably Virgulian from the

abundance of the small oyster Ostrea virqula. 96

C. STRATIGRAPHIC SIGNIFICANCE OF BREAKS ABOVE AND BELOW BLAGOUR

BRECCIAS

In view of the common occurrence in shallow-voter limestone

sequences of horizons with intraformational lime-pebbles of a wide

variety of sizes, derivations and of qualitative and quantitative signi-

ficance, it was thought necessary to appraise the specific significance

of this particular pair of pebbly horizons.

The physical character of the two horizons have already been

described. The reddening, and the identity of the pebble petrography

and fauna with the petrography and fauna of the subjacent Gluges Calci-

lutite Unit 4 marls-with-Viviparus, as well as the lignitic fresh-

brackish water aspect of the subjacent marls, clearly indicate that the

sub-breccia pebble-beds are the basal transgression deposit following

on a period of emergence and possibly soil formation. The significance

of the supra-breccia pebble bed is less clear and its thickness is

quite small. The sporadic occurrence of the top beds of the breccias

(Blagour Breccia Unit 2) may be partly due to the brecciation, but it

is thought largely to be due (a) to a rapid lateral facies variation

or (b)to disappearance of parts of it by reworking. The former explana-

tion is preferred as being consistent with the sedimentological argu-

ments. These arguments also indicate a shallow-lagoon to swamp environ-

ment for some beds at the top.

This view of the two pebble beds is supported largely by the com-

pelling enumeration by H. Parent (1944) of nine localities (Normandy,

Boulonnais, Ardennes, Haute-Marne, Lorraine, Cote d'Or, Jura,.M6connais

and Provence) in which a stratigraphic break occurs between the middle and upper Bathonian. Two of these instances are argued but the rest

appear to be quite obvious, being characterised variously by hard- grounds with or without lithophaga borings and by lignitic and driftwood fragments and rolled corals. Parent's list showing the Rhynchonellid 97

zones above and below the contact(s) demonstrates the general equivalence

of the present observations and those compiled by Parent. A second hiatus similarity is in the small for reasons of inadequate palaeontologic

material, indeterminate time interval (or both). In those cases where

the break separates S. hopkinsi from B. elegantula as in the Boulonnais,

perhaps the gap may be surmised as equivalent to the R. decorata Zone.

Some time gap may likewise be inferred from the contact of beds with Z.

diggona wi:th those of R. decorata. But even in these cases, it is

impossible to be sure about one missing brachiopod zone because of the considerable overlaps in these zones, not to mention the validity of some of the local diagnoses, as for instance between R. elegantula and

B. hopkinsi.

The above reasoning raises the questions: (I) if the time interval has been short, how could the phenomenon have been so widespread.

(2) If it was long enough for the event as is quite likely from observations in the Persian Gulf (Shinnet al.) and yet not long enough in the majority of cases to be datable by brachiopods, then preserved zones must correspond to even longer intervals of time and the rare cases of their absence (see above) need all the more to be taken seriously.

The last possibility is adopted that (1) the time interval corresponding to the breaks is long by modern standards, though not always datable by brachiopods.

(2) That it is possible to interpret these surfaces and those of

Parent as , representing a series of regressige-transgressive events whose complete cycle is of the duration of the maximum gap recorded, i.e., equivalent to the chronozone of R. decorata or more.

(3) That the regression probably approached in a broad front from the north, north-east and west, the present area mapped probably representing a development midway in completion between three. of the four breaks 98

in Normandy and one in the Jura. If the degree of development of the deep-water phase is an index of distance from palaeo-shore-line then regression towards the directions cited is indicated, and more strongly 0963) so by the findings of Evamy(1963) and Ager,in the Jura where the nodular shales and pisolites in the Middle Chanaz beds may be interpreted in this light as a deeper water reflection of this same hard- \ ground phenomenon. That the nodular shales and pisolites may be a sedimentologic break is confirmed by the very similar ferruginous pisolitic facies developed at the end of the Aalenian in the area mapped by the present author. It is safe to conclude here, as elsewhere in this text, that there were wide-spread epeirogenic movements at this time, resulting in nearly simultaneous withdrawals and advances of the sea or that the sea itself extended and withdrew. Local tidal-flat progradation can hardly be so simultaneous over a very wide area. 99

CHAPTER 3

STRUCTURAL GEOLOGY AND GEOMORPHOLOGY

A. REGIONAL SETTING

The details are filled in here of the general structural setting briefly discussed in the general introduction (Chapter 1). An introductory statement of the main difficulties is followed by a consideration of the major structural sub-regions.

General Structural Difficulties

The main sources of regional structural information are the Bulletin Carte Geologique de la France, the Bulletin Societe Geologique de France, the Institut Petroleum Francais, the Bulletin Bureau de Recherches Geologiques et Minieres and Memoir Carte Geologique de la France. A thorough combing of all these showed that most of the work that ostensibly treats of the area south-west of the Massif Central or the area variously designated east or north of the Aquitaine Basin has very little to offer on the area corresponding to the Jurassic outcrop between the River Isle,in the north, and the River -Aveyron, in the south. The area mapped by the author falls in the heart of this poorly known territory, and is separated by at least 100 kilometers from the stratigraphically better studied area to the north and the structurally and stratigraphically better-studied areas of the Spanish border and the . departements of the lower Rhone.

• Secondly, both the local and regional fold trends are very weak. Gentle domes and flexures predominate, dying out over relatively small distances (less than 12-15 kilometers in the present area). A systematic plot of local fold trends over a large area, as was attempted by Bergounioux (1942) for the area between the River Dordogne and the River Lot , would probably simply emphasize the lack of strong linearity of the flod trends. Moreover, a detailed regional structural study did not claim the emphasis of this research. 100

The isolation from well-studied areas and the weak alignment of fold trends rule out the possibility of accurately tracing systematic variations in regional fold style. The fault-trends show some systematic orientations and were found useful in inferring the age of folding.

The regional tectonic framework may conveniently be referred to five provinces: (1) on the basis of their og predominant fault trend and (2) on the basis of their overallpalaeogeTc evolution. The provinces are (a) Thy North-Western Province corresponding roughly to the north- west extension of the Jurassic outcrop up to and including the southern edge of the Armorican Massif, (b) The Massif Central Province in the north and west, (c) The South and South-Eastern Province which may be defined after Magnan (1869) as lying between the Rivers Lot and Vere, (d) The South-Western Province corresponding to the Spanish border, and (e) The Western Province corresponding to the Aquitaine Basin. These considered in turn, the basic tectonic information being taken from the tectonic maps by Launay (1921), Goguel (1941), Demay (1948), Auboin (1965) and the International Tectonic Map (Sheets 9 & 10).

The North-Western Province

Faults: The province is dominated by north-westerly faults, sometimes dipping at a high angle mostly to the north-east. Glangeaud (1896) believes after Mouret that most of these faults can be taken to lie in a continuous zone marking the western limit of the Massif Central. Although Launay (1921) indicates no continuity of the faults, a steady drop in the throw from 250 m. in the south to 60-80 m. in the north was suggested by Glangeaud (1896) on the belief of a continuity. Launay 101 •

(op. cit) shows some deviation from vertical, but this is not likely to be much and could perhaps be only of qualitative significance. The facing of slightly inclined faults is variable but appears more commonly to be south-westerly. The down-thrown side is more consistent and is to the south-west giving both slightly reverse and slightly normal faults, but more of the former. Glangeaud (op. cit) considered the faults as due to collapse of .gently folded beds. He is supported by Bourgueil and Carlow; (1964), but the expression 'elastic-release' fractures, as applied by De Sitter (1964, p. 104) is probably more appropriate. The major fault traces persist for up to 60-80 km. The age of these faults is Alpine (Oligocene-Miocene) from the Dordogne up to Poitiers, but Hercynian in the Armorican Massif (Goguel, 1941). An exception is the nearly by east-west Brive fault and,,,inference, the Rocamadour fault of a similar orientation which are Pyrenean (Cretaceous-Focene) Goguel, (op. cit).

Folds:

For the reasons already given, fold trends have been more difficult to trace without controversy, so that the same trend has sometimes been called a fold axis by one author and a fault trace by another (compare Goguel's and Launay's tectonic maps, The axis at Perigueux is one example. Launay's maps are preferred here on account of their greater information content, notwithstanding their earlier date and minor inconsistencies in the facing of the faults. Folds N. W. -S. E. and roughly N. E. -S. W. (Welsch after Glangeaud). The latter, if they are present, certainly do not show up on the very much later compilations by Launay and Goguel, probably on account of their weaker development.

The folds trend in the same direction as the faults, are essentially vertical, symmetric and open with closures of up to 4 km. and amplitudes of 60 m, or less. Glangeaud (op. cit) mentions a direction of N.W. -S. E. but it is probably too weak to show on the later maps of Launay and Goguel. Maximum dips on the limbs are generally. o 50 - 25 , while a horizontal persistance of 10 km-25 km. has been given 102

to the axes by Glangeaud (1896) who also attributed the difficulty of tracing individual fold trends to the crowding of several folds within short distances. Some of Glangeaud's sections and the author's experience suggest, however, that these may just be minor or parasitic folds on the major ones. The age of the folds is presumably also dominantly Alpine, as judged from the close relation between them and the faults. (Borgueil and Coriou, op. city.

Fault-Fold Relation:

The trends of the two are obviously parallel and so intimately related as to be sometimes considered physically continuous or coincident. The faults are genetically related to the folds as (i) collapse or elastic-rebound structures (ii) as rotational planes in the cores of chevron folds.

The Massif Central (North and West) Province

Faults:

Faults here take a N. N. W. orientation as distinct from the N. W. regions treated above. There is a N. N.E. direction further in the Massif towards Aurillac, which appears to be a continuation of trends in the S. E. region to be discussed, but the former direction appears dominant. There is also a clearer indication of the N. E. trend which was shown in the former region only as on the main faults. The faults face westerly and could superficially be considered as the result of a clockwise swing of the N. W. trend of the previous region. These are, however, Hercynian in age , and this relation is suggested only because there is nothing to disprove the possibility of a Hercynian control on the N. W. Alpine trends. Individual faults are 25-50 km. long while splay faults, shear belts and quartz veins are a unique feature here. The structures are typically those of an intruded and metamorphosed axial orogenic zone (Auboin, 1965). 103

Folds:

Fold trends are more persistent here but intersections occur on account of the superposition of Hercynian deformation on a pre-Hercynian metamorphic episode. An easterly line from Perigeaux divides this part of the Massif Central into a portion north of Auboin's Moldanubian Axis and a second one south of this axis. Folds north of the axis are presumed to face north towards the northern foreland while those to the south are roots of nappes whose facing is controversial. Auboin cites Thoral,(1933), Demay (1948), De Sitter and Trump;/ (1952) as all supporting a southerly facing and Geze (1949) as holding the opposite view. Auboin himself inclines to the former view. Near the axis itself, Demay's ir4 sections show a bilaterial facing swing round the axis.

The folds are essentially Hercynian in age, but partly affected by Tertiary events.

Fault-Fold Relation

The dominant relation here is one of nappes and isoclinal folds riding on low-angle thrusts or of overturned or asymmetric folds with steeply dipping limbs associated with shearing. These relations, with varying intensities, extend N. E. across the Vosges and Black Forest and Bohemia. The major difference between this and the preceding regions is in the absence here of a Mesozoic cover and hence of the weak imprint here of the Alpine and Pyrenean events of the former regions.

Demay (1948) has compiled a memoir with a detailed tectonic map and geologic sections for the pre-Stephanian of the entire Massif. Rogues (1941) has studied the metamorphism and of igneous activity in great detail, with petrologicAa.nd geochemical analyses, distinguishing among other things, five mig,iliatic belts coinciding in their arcuate trend with the tectonic trends of Demay (1948) and Auboin (1965). He argues a Caledonian (pre-Devonian) age for the regional metamorphism, at least of the crystalline schistes. A retrogressive persistance in the

104

ntvaTeTit

r- Rocamaaour- + + rarnat + +

...... ' "

Espedaillac • 4- Marcillac %.*Cdtus

Zot Ca hors

•Limogne + +

9 20 40km.

FIG.4 : TECTONIC MAP OP TILE Cretaceous EAUTE.QUERCY(After Bor- M.- U. Jurassic counioux, 1942 ). Lias •.• . • . • . • Trias + Basement Faults • • • ...... •• Folds Principal domes 105

regional metamorphism in some places.

The South-Eastern and Southern Province (.1149.4 )

This is defined as south of the River Dordogne to Toulouse, and east to the eastern extremity of Montaigne Noire and including the Causses of the departements of Aveyron, Cand and Lozere. Three subprovinces may be defined - the Causses east and west of the southern tip of the Massif Central Montaigne Noire, and the tertiary cover of the south-east extremity of the Aquitaine Basin.

Faults:

The following characterisation of the Haute Quercy is taken from Bergounioux (1942). Three trends are given for both faults and folds - N.-S. E. -W. and N. W. -S. E. The first and last direction appear to be more common, judging from hiS map. An average length of 20 km. is apparent, but some are so local that they have not been mapped. The throw on the major faults varies from 100-300 m. but no facing directions are given. Associated drag folds and brecciation may be present as on one faUltwith a high throw.

The age of the faults is recorded as post- Stephanian (E. -W. and N. -S.), but this is much too vague without an upper age limit. But if, by post-Stephanian, he means Hercynian then he can hardly be believed since (a) Bergounioux himself makes one or two qualifications: such as that the E. -W. faults are those at the border of the Massif Central (i. e. these would rightly be Hercynian) and then the N. -S. ones as an afterthought, as if these latter outside the Massif were not Hercynian. (b) Both these groups of faults, however, affect Jurassic sediments, but the necessary proof of a post-post-Stephanian rejuvenation is not given. (c) Such an age for faults at the extreme western edge of the Massif Central is at variance with Goguel (1941) whose fold ages Bergounioux respects sufficiently to adopt. Goguel definitely gives the age of the Padirac Fault as Cretaceous to Eocene, and that of the Villefranche 106

f• and Bergerac faults as Oligocene - Miocene. These ages are preferred for the E. -W. and N. -S. (also N. E. - S. S. W.) directions respectivly. Thevenin (1903) and Magnan (1869) have studied the area south of the River Lot. Here, as in the north, E. -W. and N. -S. (plus N. N.E. -S.S. W. ) directions predominate with throws of 150-200 m. The faults are again drawn as vertical, but Launay gives an easterly direction, the NNE faults of which the Villefranche one is by far the most persistent, connect with post-Stephanian (late Hercynian) synclines and faults all the way to the northern edge of the Massif, providing a good evidence of rejuvenation. Thevenin (1903) provides a singular evidence for additional post-Jurassic to pre-Cretaceous faulting and folding event. This possibility is retained here although its proof one way or the other is clearly prevented by the Cretaceous overstep of the area mapped. Faults on the Causses Mejac and the Causses du Larzac are aligned with the E. -W. Padirac one and dip north. The two sides of the Causse differ in the direction and in the age of the most dominant faulting: it is N. -S., Oligocene-Miocene in the west and E. -W., Cretaceous-Eocene in the east.

Folds:

Bergounioux (1942) distinguishes the same three directions for the folds as for the faults. Regional dips at the areas where he has documented his N. -S. folds are given as 30-400 towards the south, so this may be taken as the plunge of these folds. Often the same word 'plungementr is applied to dips and plunges - so making a distinction difficult. Also the examination of sections oriented at low angles (or even alone.) the fold trend to be demonstrated and also the gross approximation of fold directions (or disparity in text-quoted directions and map directions)• - cast some doubt on the scientific value of the work. However, the N. -S. folds comprise broad flexures, crowded asymmetric ruckles, minor box-folds (15-20 m. across), and minor monoclines. The proof for the ages of the folds is based on a series of folds with a discordant cover. These ages vary: Bathonian-Callavo- Oxfordian and Kirnmeridgian-Cenoraanian. E. -W, folds are either broad •

FIG. 5 : COMMON FOLD FORMS OF HAUTE RUERCY (from Bergouruoux,1942 A. AAsH1, B. RSH C. RAsH D. RA.s E. Mixe eeTab.3 108

fig.5 )• flexures, angular with geniculations or asymmetric flexures./ The age of the E. -W folds is thought to be Stampian reflecting a late arrival of orogenic events centered on the Pyrenees. The N. W. -S. E. folds are gentle flexures with a slight asymmetry. A whole series of repeated movements can apparently be traced: Hercynian - Kimmeridgian - Portlandian - Cenomanian - Oligocene. Bergounioux also records three E. -W. domes without discussion. Further south, Thevenin (1903) records no pre-Cretaceous events on account of the Cretaceous overstep and attributes most of his folds to the general Alpine episode. His sections show partial Jura-type structures - insipient asymmetric box-folds, with straight limbs.

Most of his structures are N. -S. (2869) or N. N. E. -S. S. W. with some domes. Magna= give s the same .A. direction for folds which split and recombine over relatively short distances (5 km.) . On the eastern Causses, not much appears to be known of the folding, and trends close to those of the faults can be assumed.

Structures in Montaigne Noire are reminiscent of those in the Pyrenees. They are Hercynian with some Cretaceous-Eocene influence. A common trend in E.N.E.

The stratigraphy is mainly Palaeozoic, and Trump...y (1952) and De Sitter have, in two N. N.W. -S.S.E. sections , worked out some interesting nappe structures which theyconsider to be

facing south, while Geze (1952) side by side with the other two in the same paper has demonstrated with careful palaeogeographic reconstruction and geologic sections his views to the contrary. • - Auboin (1966) inclines to the first view, perhaps because he would like it so but the controversy cannot be resolved here.

The North Pyrenees (Southern Province)

Bo th a 0,s te:cas ( 1933) and. La are( 1936) none recently rli re use ( 1.9 6 6 ) havo. Witi t synthese s N7:Ltb. "e:/zhatis tive niblio es..on t'..110 109 tectonics of the Pyrenees. The bibliography on this region could be extended indefinitely without altering the broad picture of a Hercynian folded geosyncline, subsequently partaking in the early phases of the Alpine Orogeny. M. qaoux (1950) agrees with Bertrand in acknowledging the presence of folded and granitized Coal Measures originally deposited in internal basins of the Hercynian Chain. This chain becomes apparent from the late Palaezoic, but developed a shallow North Pyrenian Basin north of its axis - this basin taking part in the Cretaceous-Eocene or Pyrenian episode in a manner similar in its sedimentary and tectonic detail to the Oligocene-Miocene or Alpine episode in the Alps.

The Hercynian history of the Massif Central, the Montaigne Noire subregion and the Northern Pyrenees is thus similar in the development of internal basins that were intruded and granitized and mobilized into huge nappes, recumbent folds, thrusts and decoulement. They differ, however, in the degree of response to the succeeding Alpine Orogeny. Magnan (1869) however, does not agree with the post-Stephanian emergence in the Massif Central.

The Aquitaine Basin (Western Province)

The lack of exposure in the Aquitaine Basin has probably accounted for the poor structural documentation. Deep boreholes reaching to the Jurassic and earlier are probably either few or not in the published literature. The International Tectonic Map of Europe (sheets 9 and 10), gives fold and fault trends for that area generally N. Mr. -S. E. in the north of the Basin becoming W. N. W. or nearly W. as the foot-hills of the Pyrenees are approached. The Aquitaine Basin is Tertiary in age and the structures are certainly Alpine.

B. PRESENT RESEARCH

The structures are discussed, for convenience, as folds, faults and joints, followed by the interrelations between them. There is no convincing vertical change in structures 116

attributable to disc-rdant tectonic events on account,perhaps, of the low intensity of deformation. Such changes as occur can be explained in terms of bed thickness and lithologic variations. Inclusion in the fold descriptions of lithostratigraphic horizons affected at outcrop is intended to demonstrate the two dominant controls.

Folds

The regional dip varies widely from W. S. W. to S. S. W. The mean direction is probably S.W. The south-east plunge of the folds mapped is probably an expression of a local fold culmination occasioned by an interference of fold trends and is not a persistent regional plunge at all. Of about 225 dips taken, only 17 or 18% o o were 25 or over in value. Dips of 10 and lower were the most frequent. Sheltered under the western percussion of the Massif Central, this area was remote from the Alpine fronts of the French Alps and the Pyrenees with consequent mild deformation. The bedding and lithology may have also determined the mild deformation,since the platy marly horizons with the strongest minor contortions were at the same time less common than the thin-bedded lime mudstones or the thin to thick-bedded mudstones and grained limestones. High dips associated with minor folds would also clearly be missed more often than those associated with the gently but large-scale folded enveloping beds.

The following are a consequence of the low values of dip: (i) Thin horizons that can scarcely be inserted on steep valley sides, become enormously wide outcrops on flat hill tops which are very common frt. 4e the area, and have partly earned for it the name of Causse. An attempt IDYczc-h-cdect to compensate the resulting loss in structural detail„by exposure on the flat hill-tops and by the gentle dips which - (ii) make the outcrop distribution highly unpredictable; strongly folded and deeply sculptured terrains could have been an advantage after all. III

FIG.. 6:CONTOURED BEDDING-POLE DISTRIBUTIOY ON EQUAL-AREA PROJECTION FOR THE .WHOLE AREA MAPPED. Contour interval -.4 112

(iii) Stereographic plots of bedding poles are highly erratic and give only poor indications of plunge direction and low values of plunge.( fi g.6 )• Also the segregation of nearly E. W. from the N. W. -S. E. fold trends is very poor even though individual outcrops display the former trend admirably. Some of the scatter is no doubt due to measurement errors occasioned by the low dip values. (iv) The presence at numerous points of anti-dips, some of them erratic, makes it difficult to demonstrate the major structure by a sketchy survey alone of isolated outcrops. Complete mapping and geologic sections become implicitly necessary for this purpose from the outset.

Discussion of Selected Traverses ( to illustrate points (iii) and (iv) )

Two roughly N. -S. and one nearly E. -W. traverses were made to ascertain how far the minor fold directions coincided with the regional N. -S, folds and the observed E. -W. ones. The aim was to analyse individual localities separately and thus minimise mutual interference of widely separated localities.

Fairly reliable plunges were obtained by making separate plots of dips for very limited traverses. But the regionally unrepresentative character of such results demanded a delicate balance to be struck, between this course and that of pooling widely scattered readings.

Evans (1963), commenting on the wide scatter of bedding poles associated with conical folding in Charnwood Forest states: "Clearly it is not possible to relate all the pole concentrations to one great circle and three great circles, which fit various concentrations, but ignore others, have been inserted on the figure to indicate the different trends and plunges of the fold-axis which could be inferred from this plot. Obviously it is unsatisfactory to choose any of these great circles as representative". Precisely this procedure was adopted here, as will be 113

Le Chapellone Lecol-pres-Creysse N N

Mo ntvalent Pooled Data

FIG. 7: EQUAL-AREA PROJECTIOIT OP BEDDING POLES. 114

apparent subsequently.

Le Chapellone - Le Pigeon Traverse (E. -W.) (figs.7 8 )

The length of this traverse is 3-4 km., and the dips span the lithostratigraphic internal Lacave Calcilutite Unit 1 - lower Unit 3, with a concentration in Lacave Calcilutite Unit 2. The bedding poles have a sub-circular distribution. At least four possible major circles may be fitted to them. Each of the four fitted here, however, contain only about 11 poles within a belt of 4° on either side, i.e. an allowed spread somewhat more than the plunge to be determined, and yet accounts for only 50% of the bedding poles. The plunge of 4°/83° is taken as an approximate average. An alternative interpretation is to consider the structure as a partial pericline. At the top of Le Chapellone, the beds n. appear to be dipping igedvei caltr?i but the angles are immeasurably low. This is simply a possibility.

Legol-pres-Creysse Martel Traverse (N. -S.) (fig. 7 )

The length of traverse is 2-3 km., and cuts through Gluges Calcilutite Units 2-3. The polar scatter is even more circular than in the previous traverse. With the same allowed spread, the percentage of poles accounted for is even smaller - 30%. One isolated fold plotted faithfully with a south-westerly plunge, but even within this short traverse it is obvious how the measurements from individual folds virtually cancel out. ,A rough average plunge may be taken as 70/282° , but with the same possibility of a dome or pericline or just an irregular surface.

Monvalent-Gluges (N.S. ) (fig. 7 )

The length of traverse here is 3-4 km., and measurements fall mostly in Gluges Calcilutite Unit 3. It is nearly impossible to fit a great circle and the best approximation appears to be a plane inclined 8°-10° to the S. W. Dips taken from visible folds are contrasted with the pooled data. This illustrates very well why more faith 1.15

PIG 4; 6: CONTOURED BEDDING-POLE DISTRIBUTION OJT EQUAL-AREA PRWECTION, Le Chapellone Travers Contour interval - 116

is placed on the inferred regional structure (from the map) and subjective inferences from minor fold directions, than on the dip at this or that locality.

Major Folds

Anticline 1:

This can be traced for a distance of about 12 km. in a S. E. direction from Presignac, west of Timbor, through Le Bastit. Its south-westerly limb forms the impressive hill-side and railway cutting of St. Etienne, the dip of 11°/234° remaining unchanged up the succession from Lac'ave Calcilutite Units 3 and 4 - Lanzac Oolite. It is down-faulted on the west side of the short tunnel below St. Etienne so as to accentuate the rapid disappearance of the Lacave Calcilutites, which can still be seen 5 m. below the road level on the cliffs of the river. At about the departemental boundary towards Cazoules, the dip on this limb is ameliorated by a pair of gentle minor flexures in St. Etienne Limestone which finally disappear under soil cover. The lowest formations in %a c.ortz.05 of the succession -en this fold, Souillac Oolite and Gluges Calcilutite Unit 4, can be picked up in the valley a few hundred metres west of Souillac Gare. At the west end of the Gare, steepening by rotational fault-folding is largely to blame for the brecciation of the lowermost Souillac Oolite. This is the eastern limb of the anticline. The much more rapid opening out of this limb when traced down-plunge, supports the observable local accentuation by rotational fault-fOlding at this point. This is not therefore the steep limb; it is the gentle limb of an asymmetric fold. (See Section A-4

From Presignac towards the bridge at Souillac, the fold is covered by alluvium, but the S.W. limb is again indicated by the dip at favourable points on the route from the bridge at (ai-burantie.) Cieurac. About here alsoA the fold is faulted transversely and longitudinally. On the Souillac-Cahors route further south at Les Patous and Chateau de Lanzac, south-westerly dips again confirm this limb. The north- 117

easterly limb at both these localities is nearly horizontal, and the fold o o nearly a monocline. This opening out is reflected in the very low (1 -5 ) values and erratic directions of dip between Souillac Bridge and Torre5aye , on the east bank of the river.

At Le Bastit, the limb and crest of the anticline are well displayed on the southern slopes of the hill west of the village. Lacave Calcilutite Unit 4, a marker bed, dips westward below Les Combes and is succeeded by St. Etienne Limestone and Lanzac Oolite further west. At Les Combes, the hinge zone of the fold can be visually traced, although modified by minor folding in Lacave Calcilutite Units.3 and 4, and at 250 metres south of Le Bastit, east-west trending stepped reverse faults and gentle minor folds similarly obscure the easterly limb of the fold on both sides of the valley.

The plunge of the fold continues south- eastwards younging up towards Lanzac Oolite and Cales Beds at Cales.

Syncline 1

This is best exposed at the hill facing the Gare at Souillac. The westerly dip on the eastern slope of the hill is on the easterly limb of the fold and is consistent with the presence of Souillac Oolite at the Gare. The disposition of the outline of Souillac Oolite,between the cemetry at Souillac and the road from the town centre to the Gare,may betreferred to this syncline. It rapidly opens out and is not encountered again until Cabales where a gentle sag may be inferred as signalling its reappearance and bringing in the much higher Lanzac Oolite and Cales Beds. This sag may also be inferred in the hill east of Le Bastit.

Anticline 2

The inferred axis of this fold runs N.E. - S.W. along the valley of the River Borreze through Timbor, but its north- west extension from this village is only inferred since the western valley side is poorly exposed and the Souillac Oolite accessible at about road level PL.I6 : THE MORE EASTERLY OF THE TWO FAULTS N. OF MA ZY..(Souillac Oolite in. quarry at top right- hand. of photo is is down-faulted. against boudi- naged top of Gluges Calciluti te )

PL . 1 7 : BOU=S ITT SAME BEDS AND OUTCROP AS A BOVE. 119

on the opposite valleyside is non-bedded and gives no dips. The highly fractured exposure of Souillac Oolite and BlagourBreccia at La Saule, south of Sonbros, may, however, be referred to the easterly limb of the fold. From here to Blagou; Gluges Calcilutite is encountered but it is either brecciated, marly and non-bedded or only gives conflicting dips. An almost imperceptible elevation in outcrop may nevertheless be traced northward and appropriately projected into the normal plane of the fold as evidence for a considerable levelling off of the easterly limb. At Le Blazy, shear-lenses occur along the three main routes out of Souillac. They are the result of minor thrusts inclined 30° towards the S.W. and striking 140° (S. E. ), ie. along the fold trend, and are interpreted as coeval Withthe main folding (see Ramsey 1967 p. 401). The foot-path leading from Le Blazy to the quarries at the hill-top, shows rare boudinage pjs,-168:17 ) structures and faulting while further on, overturned parasitic folds are associated with bedding dips of 30° and more. There are obviously steep dips here approaching 45° or more (although few suitable bedding surfaces are presented by Gluges Calcilutite Unit 4 and Souillac Oolite) and the • coincidence of boudinage long dimension and fold axis, both . point to a stretch origin of the boudinage observed here. The easterly limb steepens to 170-22° on the Souillac-St. Sozy route where it is exposed as Lacave Calcilutite Unit 2-4. The same limb is traceable to Roches St. Monges while the other limb corresponds to the steep hill-side exposure of St. Etienne Limestone at Garret, S.W. of Cabales. They are both very flat and only steepen somewhat at Belcastel where the down-dip of Lacave Calcilutite Unit 4 from the Chateau and Mayraguet route towards Lacave and Les Bertou clearly demonstrates the easterly limb. The downward succession takes in the topmost Sbuillac Oolite which appears transiently along the Lacave-Cales route below Belcastel . The other limb is confirmed by the westerly and north westerly dips at Min de Bourgnou on the D. 21 to Cales. The N. W: run of the tributary valley to the River Alzou is probably controlled by the fold crest. The plunge is once again S. E. at this point, bringing in the Lanzac Oolite after the crest has been has been reached at Belcastel. 120

The pair of anticlines so far described show a divergence in axial direction from N. W. to S.E. so that the complementary syncline opens out in the same direction.

Syncline 2

The trend of this syncline is not as persistent as the folds described above. It can be demonstrated at Le Chappellone and only reappears very much reduced in closure.at about Belcastel. Its westerly limb starts near the railway-cutting at Blazy. • It is faulted and modified by a pair of asymmetric minor folds whose easterly limbs, as are all the other minor folds on this limb, are• longer and more gentle. A minor anticlinal crossfold and a series of axial faults bring up the Souillac Oolite at the quarry and refuse dump on the N. 20, north of Souillac. A few building sites along the N. 703 before the viaduct expose the manly top-most Gluges Calcilutite, which is succeeded very near the viaduct by Souillac Oolite and Lacave Calcilutite Unit 1 giving conflicting dip directions, but obviously continuous with the exposures higher up the hill-side along the N. 20 and dipping into the valley of the N. 703. The Souillac Oolite is again thrown up by an E. -W. fault just after the viaduct across the N. 703, but after a few more minor folds and faults (some even bringing the manly and platy Blajour Breccias to view) finally dips under the valley before the diversion to Le Pigeon. The thickly bedded Lacave Calcilutite Unit 3 of the Le Chapellone is similarly seen to dip down into the valley. The fractured and plicated Lacave Calcilutite Unit 2 exposures along the tortuous main route to this village, at the eastern end of Le Chapellone, has been thrown up by a nearby E. -W. cross-fault and a geniculation of the easterly limb. The end of the Lacave Calcilutite Unit 2 exposure on this road shows the succeeding Lacave Calcilutite Unit 3 beds with very low dip values (2°). Similar low and variable dips are encountered in the railway tunnel and 121

cutting south of this hill. Both these exposures probably indicate the hinge zone of the syncline. Exposures north and east of these, i.e. in Le Chapellone - Baladou - Le Pigeon triangle and S. E. of this to Mayrac are patchy to absent. An almost continuous belt of pot-holes, some up to 200 m. across can be traced north-westward from Mayrac and defines the outcrop of Souillac Oolite. This and the overlying Lacave Calcilutite Unit 2 (Unit 1 is not identifiable below the soil cover) are also brought up by the plough in the fields on either side of the N. 703 between Le Pigeon and Baladou. These outcrops define the easterly limb of the syncline. The 1883 map places the Souillac Oolite - Lacave Calcilutite contact here under Miocene cover, but the ploughed fragments and pot-hole distribution enable it to be fixed to about 5 m. on either side.

This broad syncline dies out about south of Le Pigeon, whence its reappearance at Lacave is evidenced by the systematic drop in the Lacave Calcilutite beds from about Lacave Cemetry to the village and slightly beyond. This is the easterly limb and exposes as well as the Lacave Calcilutite, Souillac Oolites (underlain by a transient exposure of Blajour Breccia near Le Bougayrou) dipping in the same south-westerly direction and disappearing on the Lacave side of the cemetry. The westerly limb uns into Anticline 2 (see above). The axis of this latter fold is considerably shifted to the west. It is therefore only tentatively linked with the one at Le Pigeon on the basis of its continuity with Anticline 2 in the Lacave area. There is some indication of the easterly limb of the major syncline on the Le Bougayrou-Mayrignac road, where very flat-lying Lacave Calcilutite Unit 2 beds dip subtley but consistently south-west, and on the D. 32 and the R. Alzou valley, at Rocamadour, where the Gigues Calcilutite Unit 4 to Lacave Calcilutite Unit 4 succession shows a similar direction. The dips between St. Sozy and Creysse may also beconsidered. 122

Syncline and Anticline 3, Baladou

This is nearly a minor asymmetric flexure on the westerly limb of the next major anticline. The minor syncline in the quarry and railway cutting is interpreted as the hinge zone of a syncline whose easterly limb takes the succession Gluges Calcilutite Unit 4, Souillac Oolite and Lacave Calcilutite Unit, all of which can be noted within 200 metres in a south westerly traverse on the N. 703 towards Baladou. The same traverse comes down on Souillac Oolite again in a water-pipe ditch near the level-crossing at Baladou and ascends the succession to Lacave Calcilutite Unit 1 in the railway cutting S.W. of the village. The syncline just described rules out from consideration the possibility of a fault-repetitiion. A minor complementary anticline is preferred. These folds may be traced with some doubt east of the D. 33 between St. Sozy and Baladou.

Anticline 4, Martel

The Souillac Oolite and Elajour Breccia at Baladou are never encountered again on the N. 703 towards Martel. The succession is descended towards Martel but exposures are rare until the town is within view ,when some poorly exposed beds of Gluges Calcilutite dipping north to north-east confirm that the westerly limb and hinge zone of this asymmetric anticline have been left behind. The gentle south- westerly dip on the well exposed hill-side at Carman (north of Creysse) is taken to define the westerly limb which here passes up from Gluges Calcilutite Units 2/3 - Lacave Calcilutite Unit 1. The prominent capping on the hill is Lacave Calcilutite Unit 1 and not Souillac Oolite , as one might infer from a distance. The inferred axis of the fold runs S. E. from about Pouchou through Feneyrol, north of Monvalent. This is confirmed by the Trss. equal area projections whose mean (centroid) suggests a south-west dipping plane corresponding with the westerly limb. No such corresponence, however, was found between the 11ss- and the easterly limb which they should fall on - assuming the inferred axial position to be correct. The south-east trend of the valley at Feneyrol and the very subtle 123

north-easterly tilt of the Gluges Calcilutite Units 1 and 2 beds, as viewed from a distance from the Monvalent Cemetry towards Colombier, strengthen the likelihood of the postulated axial position.

This anticline connects with the syncline at Mirandol by a series of minor folds on the easterly limb. Two of these may be represented by the anticline (visible on the cliff face) and the syncline (suggested concealed at the Gare) at Cirque de Monvalent overlooking Foussac .

Anticline & Syncline 5, Mirandol - Foussac - Floirac

At the quarry below Mirandol, the oolite formation of that name starts to dip steeply west to south-west towards Gluges, forming the easterly limb of a small syncline. This limb is cut up by minor faults trending N. N. E. (also the dominant jointing direction) bringing down the thickly bedded basal Gluges Calcilutite in narrow fault- blocks on the eastern face of the quarry and probably doing the same at the N. 681 to Martel. The result is a false impression of an ever-steepening on this limb and a too rapid disappearance of the Mirandol Oolite below the Gluges Calcilutite, especially as the fault is presumably hidden in the scree- filled depression, presumably of erosional origin, near the foot-path from the quarry to Gluges. This limb is also affected by minor cross-folds trending N. E. -S. W. The westerly limb is very gentle as is shown on the cliffs of Gluges viewed from the first bend on the N, 681.

At the edge of the river-bank at Foussac, there is a small exposure of Mirandol Oolite which is interpreted as an extension of the easterly limb of the syncline complementary with the Foussac anticline. The easterly limb of this anticline is inferred from the ditch exposure of Mirandol Oolite at the northern outskirts of Floirac. The most northerly sag visible on the cliff at Cirque de Monvalent is taken as a continuation of the syncline, but the easterly limb here is a lot more 124

gentle and its culmination at the hinge zone of the Foussac anticline is not visible.

The folds east of this anticline tend to be asymmetric or monoclinal with their leading limbs systematically elevated north-eastwards in accordance with the regional dip.

Minor Folds

Observations on minor folds fall into (a) detailed ones made along traverses with continuous and accessible exposuies and (b) random ones. A third category of inferred minor folds could be set up for the smallest major folds already described, and not considered here. The closure of the structures is generally less than "-2 km. and commonly from 250 m. down to hand-specimen size. Both categories of observations are described in the order of the major folds they are positionally associated with. The order of description thus runs N. W. to S.E. and from W. -E.

(i) Souillac-Torreoaye Section; D.43

The section runs S. S.E. nearly parallel to the trend of the regional folding and on the inferred hinge zone and nearly horizontal easterly limb of Anticline 1. The dips are generally o less than 100, often 1-5 , and directionally erratic. A subjective assessment of the directions indicates a mixture of E. W. cross-folds and congruous N. W. - S.E. ones. The exposure is continuous. Asymmetric, rounded, open folds occur exclusively. Smaller structures probably die out rapidly upwards, larger ones range from Lacave Calcilutite Unit 3 - Lanzac Oolite .

(ii) Souillac-Le Pigeon Section; N. 703

The section is subdivided into a first part which is partly synthetic and runs N.E. and a second which is wholly. 125

exposed and runs N. N. E. Both subsections fall on the westerly limb of Syncline 2, the first more nearly normal to the axis of the syncline, and the second closer to the hinge zone. Direct measurements of minor fold trends support the westerly plunge given by the llss equal area projection. Mostly asymmetric, rounded open folds in Gluges Calcilutite Unit 4 - Lacave Calcilutite Unit 1 occur in the first subsection, restricted vertical persistance being probably due to fold size. Rotational fault-folds dominate in the second subsection with some rounded and open folds and angular inclined folds facing south. The beds involved are Lacave Calcattite Units 2 and lower 3.

(iii) Legol-pres Creysse-Martel Section

The section follows the road from Legol up the stream to its junction with the D. 23 to Martel, and 'falls within the hinge zone of Anticline 4. The N. W. -S. E. loop in the road is probably responsible for the good exposure of the rounded and 'kneed' concentric folds trending almost exactly E. -W. by direct observation and ii 11-ss projection. The northern limbs are nearly horizontal while the southern ones are subvertical, so that the asymmetry again faces south as at Le Chapellone (Traverse 2). The beds affected are Gluges Calcilutite Units 2/3.

(iv) Monvalent-Mercadire N. 681

The exposure of Gluges Calcilutite Unit 3 on the cliffs below the church at Monvalent may be included in this N. -S. traverse which descends in the direction of Gluges to increasingly older beds of Gluges Calcilutite Unit 3. A very broad synclinal flexure of low amplitude with congruent low amplitude anticlines passes north into concentric folds of higher amplitude. Paced closures are commonly about (Also see p1.23) 4 m. with one of 56 m. These folds die out vertically over about 5 m. This kind of feature is in the literature on Causses as establishing a hronology of folding 'episodes' and vertically distinct and identifiable 126

TAB. 3 Table of Classification of folds

4-,;-, ANGULARITY co .--8 c)„,

,..._ O .t40 Z 4 Angular (A) Rounded (R)

I 1

ic ASH RSH 1 1

tr (H1) 2 ASH (Rare- RSH (Absent) 2 2 mme (H2) Absent) Sy

U Ll 1 (H1) AAsH1 RAsH 1

>cri 2 2 RAsH <4 (H2) AAsH 2 127

tectonic facies. The author would rather attribute it to physical and mechanical factors in this area as well as in those areas where no more detailed tectonic study of the local succession has been carried out than is necessary merely to establish a local fault r. The axial trends are probably west both along the road and at the foot of the cliffs where one southerly one was also observed. The llss plot is inconclusive.

Significant Minor. Structures of Category b

Exposures with interesting structures are described, partly as a guide to the area. But to avoid the tedium of describing a lot of randomly distributed exposures, the structures themselves have been grouped on the basis of their morphology, and their location, have simply been listed. This grouping was not possible with the continuous traverses in which more than one morphologic fold-type was present. But the grouping here applies equally to minor structures on these traverses as well as to the major folds.

Minor folds were classified according to a scheme set up from the elements of fold morphology described by Ramsey (1967). The scheme is shown in ..tab.3 Symmetric and asymmetric folds were each subdivided into those with a single hinge and those with a double hinge (folds with more than two hinges were absent). Symmetric folds with double hinge, whether angular or rounded, were also rare or absent.

1. Angular, Symmetric, Single Hinge (ASHI)

These occur mostly at Le Chapellone (Section 2). They rarely occur without a rotational fault.

2. Angular, Asymmetric, Single Hinge (AAsH1)

These usually occur as a sudden steepening of otherwise flat-lying beds. Like 1 they are rotational fault- , 2 c

hi PL.. I 8 : TYPES OF ANGULAR ASVIETRIC SINGLE-HINGED FOLDS. A s- Minor overturned buckle in Lacave Calcilutite Unit 2; Diversion to N'.20 at Blau. Bs Steepened_ an& geniculated beds; Gluges Cal- cilutite Unit 1; A.681 overlooking Gluges. 129

(p? .18 B) folds./ The sudden drop could be mistaken for down faulting in areas of poor exposure. Striations and slickensides are common on the bedding surfaces indicating some degree of bedding slip. Typical exposures of this kind of structure occur— a) At the northern end of the Blajour Valley where the Souillac Oolite and particularly the bedded Lacave Calcilutite Unit 1 suddenly steepen o to over 20 with considerable fracturing. It is probably located on the westerly limb of Syncline 2. b) In the small quarry opposite Sassondrics on the N. 20, where a sub-angular (transitional to rounded asymmetric) terraced flexure occurs in Lacave Calcilutite Unit 4. The steep limb dips 30° due south. These also occur on the westerly limb of Syncline 2 but are probably a northerly extension of the belt of E. -W. structures behind Le Chapellone (Traverse 2). c) On the eastern side of the N. -S. valley at Le Bastit _ o where an angular, rotational type shows a limb steepening of 52 with considerable bedding slip, and passes north into a pseudo-angular variety with a reversely step-faulted middle limb. Both structures show Lacave .Calcilutite Unit 4 well. The facing of both the steep beds and the reverse faults is southerly. A probably similar structure with only the upper and middle limbs is exposed at water level along the river bank at The Chateau de Lanzac. The same bed is again involved. The steep limb here takes Lacave Calcilutite Unit 4 below the river•with a dip direction ° estimated by sighting as 208° or 190 . The re-appearance of these same beds at Le Bastit on the other side of the meander is therefore interpreted as describing an open double axial box-like syncline across Anticline 1. d) At Lacave, almost in a direct E. -W. line to Le Bastit, a steep limb ° in Lacave Calcilutite Unit 4 above the caves, dips with 58 towards 12° west-of-north on the easterly limb of Anticline 2. e) At Min.de Bourgnou, on the D.23 to Cales, where beds of Lacave

130

PL.1 9 : TYPICAL PARALLEL E.-W. FOLD, showing the maximum closure commonly observed for these foldm. Lacave Calcilutite Unit 3, Sassondrics.

__. -- • . .•., • — - - : A ...."'"' -*tie:- . :- -.,.: — •41:0-' ' ...• --4;:- — 4:-.41,e,' - - N.-. , r• ..A..... - ' ...,.,. ..., . .

-- - -' Nkii. --"l'' • : -.7 _ , 1.-"-- -

- --- ?'":7 - 'Ni, — N .1 , 441Y1.,,„- ati404...... 016:,•ar II.. r i.t... - tUe.?....

...,101.1it ,, ' . ''-

PL.,20: MINOR CONCENTRIC FOLD, showing some of the characteristic features of the style of deformation discussed in the text.. Gluges Calcilutite Unit 1, N6S1 overlooking Gluges. 131

Calcilutite Unit 2 have suffered intense bedding shear on the steep limb .dipping 30° to 164° (300/ 164) f) In the deep valley below Couderc, thick Gluges Calcilutite Unit 3 beds have suffered a similar steepening, accompanied by faulting. g) Overlooking the D. 43 south of Gluges, platy beds of Gluges Calcilutite Units 2/3 show a variable facing of the steep limb (342° and 0 225 ). these occuring near the hinge zone of Anticline 3. h) Near Blazy, and at Roches St. Monges, • where Lacave Calcilutite Unit 2 is affected, sometimes on a very small scale. The bearing of this phenomenon on the general tectonics of the area is dealt with later.

3. Angular, Asymmetric, Double Hinge (AADH2)

To this category may be referred some of the minor non-concentric ruckles figured by Bougouioux (1942). His are strictly sub-angular with a supression of the lag limb, and therefore not an ideal representation of this category. Some of the very minor ruckles behind Le Chapellone bear some affinity to those of Bougounioux.

4. Rounded, Symmetric, Single Hinge (RSH1)( pls:19gg20 )

Some of the major structures fall in this category when slight departures from asymmetry are ignored. The minor folds are usually conccentric with classical cuspate cores below the key-horizon. They are not very common and show best only near the quarry on the N. 681 at Gluges . . Some occur again on the same road near Monvalent. In the former case, Gluges Calcilutite Unit 1 is involved with a strong disharmony between adjacent horizons each . of which is concentrically folded. Almost perfectly concentric microfolds with wavelengths of about 2 cm. also occur in very thinly layered calcilutes of Gluges Calcilutite Unit 2 near Legol. They are obviously controlled by the layer-thickness. 132

5. Rounded, Asymmetric, Single Hinge (RASH1)

There is a complete gradation between these and the angular asymmetric ones; the boundary line is arbitrary. The majority of the major structures fall here, as well as the inferred minor folds in Souillac Oolite at Le Chapellone viaduct. In the Monvalent traverse a rounded asymmetric minor fold has slipped along a bedding thrust onto a high angled one, with some axial thickening. On a lesser scale, this category is poorly represented.

6. Rounded, Asymmetric, Double Hinge (RASH2),

These are best developed near Legol- pres-Creysse and on the D. 43 between Colombier and Martel. Both primary and dependent ones are developed. ' •. sometimes approaching a 'roof and wall' structure. The slight geniculation places in this category folds that would otherwise have considerable angularity, e. that development of polygonal form supresses angularity. The trends in this area are consistently E. -W.

The range of common fold-forms for this area and the Causses in general is tabulated. (See fig and caption). In addition to the above structures disharmonic folding in the thin incompetent, stromatolitic member of the Lacave Calcilutite, as at the Souillac Bridge. Faults and Faulting

Faults with throws of over 5 m. are not common but minor slip-blocks are ubiquitous. The latter are not discussed individually except where they bear a defininite relation to the tectonics in general.

Major Faults (i) Souillac-Lanzac Faults (pi. 21 )

A major fault cuts orthogonally across the regional fold trend between the bridge of Souillac and the first 134

Pl. 21:FAULTS AT PORT de SOITILLAC. A t Early high-art 1e reverse fault in. the stroma.toli tie (unit 2) Lacave caloilutite. Bs Late normal fault (See interpretative diagramat fig. En. 135

Z. Inception of monocline by stress aouple.Aszoi- tiated high-angle fractures.

II. Plication accentuated. Inception of reverse slip.

•III. Continued movement to form major reverse fault. Cogenetic micro fractures.

IT. Late normal faulting plustentional gaping of early reverse fault Dog-legging ofearly high-angle frdctures.. Raying of fractures. from. irregularities. on F3.

8/: SEQUENCE OF E7ENTS ON PORT de SOUILLAC FAULTS. 136

valley on the D. 43 across the bridge. On the Souillac end of the bridge (see interpretative fig (•8'.) and photograph) the complete sequence of events associated with this fault can be worked out. The total throw (reverse and normal) is about 15 m.

There are two faults on the west bank (see pi: 22 On the northernmost one, trending N.E. -S.W. (i. e. a dip-fault), St. Etienr4C_ L's..' ; Unit 3 on the north-west is down- faulted against Souillac Oolite on the south-east. The down-thrown succession on the north-west is exposed on the cliffs nearby while the succession on the south-east Souillac Oolite and Blagour Breccia are exposed in the small valley

On the south-east side of the valley Souillac Oolite is exposed and it is possible to go up-hill from here on to St. Etienne Limestone, without going through Lacave Calcilutite which appears to have been completely faulted out by a N. W. -S. E. fault orthogonal to the first one, and having a throw of about 80 m. Near the junction of the D. 43 with the N. 20, thick beds of Lacave Calcilutite Unit 1 rest on the Souillac Oolite, but again Lacave Calcilutite Unit 1 passes uphill and along the N. 20 onto St. Etienne Limestone without Lacave Calcilutite Units 2/4. The N. W. - S. E. fault can therefore be extended here (N.E. of Lanzac) with the same north-easterly throw of 80 m. Its south-east extension is probably buried in the alluvium of the River Dordogne, being continued merely as a master-joint on the road-cutting before Torreoaye.

(if Faults at Le Chapellone

On the N. 703 after the viaduct on inlies of-. marly and platy Gluges Calcilutite Unit 4 and Souillac Oolite is thrown up by a nearby E. -W. fault which is probably an extension of another one further along the road bringing up the Mlles of Lacave Calcilutite Unit 2. The throw is 20 m. or less. A second fault runs N.E. forming a narrow, positive fault-block with the first. The faults are 2 50- 500 m. long. 137

PL.22: THE FAULT AT DURANTIE, S.W. of Soulliac. ( Ceriopora diagnostic of the St. Etienne Limestone is common in the tumbled blocks. The Souillac 0,-Nlite is just to the left of the photograph, with the intervening Lacave Calcilutite wholly faulted out..) 138

(iii) Fault at Le Bougayrou

This trends N. W. for a quarter of a kilometer. It is easily demonstrated by the repeated succession of Lacave Calcilutite Unit 1 (with Stiphrothyris) and Souillac Oolite Blagour Breccia , Gluges Calcilutite Unit 4

From the east-west cliffs north of Le Bougayrou one can trace a younging succession westwards of brecciated, manly limestones with Burmirhynchia elegantula, creamy Solenhbfen-type limestones, platy dark-grey to black limestones, sludgy shales (all belonging to the Blagour Breccia4) Souillac Oolite, Lacave Calcilutite Unit 1, with Lophrothyris subsella. At the beginning of the bend on the D.23, the platy black limestones considerably sheared are suddenly brought up against the light-pink Lacave Calcilutite Unit 1 beds and then overlain by a poorly exposed Souillac Oolite at the fame building and just before. The fault does not extend as far as Le Limon but the jointing causing the oblique serrations on the cliff appear to be parallel to the fault and may suggest that the latter extends about half-way between the D. 23 and that village. The throw is of the order of the thickness of. the Souillac Oolite - 20 m. On Roch,rs St. Monges, across the River Dordogne, the tail-end of the fault is suggested by a pair of joints on the floor of a gentle syncline.

(iv) Faults at Blazy (see ' mop and sections)

The foot-path leading north up the hill to the large quarries passes through pellety and intraclastic bedded limestones with B. elegantula . The overlying basal Souillac Oolite is encountered in an overgrown small quarry after the double bend ( p1.16 ). A sharp fault running 160° is then encountered throwing up the dark- grey platy marls with the boudinage structure (see Anticline and Syncline 2). 139

A second fault running nearly along the strike of the beds completes the eastern face of a narrow fault-wedge with Souillac Oolite on either side (in the large quarries and in the overgrown, small ones). The throws are probably 15 m. or less.

A series of asymmetric block-faults distort the base of the Souillac Oolite on the N.20 between the first and second bends after Blazy. The faults are all normal with the low angle ones dipping about 300/230°. Manly, platy Blajour Breccias beds are brought ,up every other bldck until the quarry is reached at the second bend of the road. The coincidence in the facing of the drag-fold axial plane already described on the other brow of the hill after the large quarries and the low angle normal faults may suggest that the formation of the fold was accompanied by compressional joints with a horizontal principal stress, which later became tensional with a normal slip on them. The vertical faults which presumably arose in this later phase do show some evidence of being later than the low-angle ones.. - •-

(v) Minor Faults

Most of these have been figured on the principal traverses. These are not individually described. A few minor faults, however, involve the juxtaposition of lithologies that differ sufficiently subtley to be taken as a horizcn tal facies variation one of the other. The authors attention was called to a series of slip-blocks below Mirandol, where micritic but still quite granular oncolites of the topmost Mirandol Oolite are brought down frequently against the oolite proper. The throw itself is insignificant, but enough to illustrate the possible error just mentioned. A similar situation often occurs between the Souillac Oolite and the basal Lacave Calcilutite. The latter is sometimes sufficiently chalky to be mistaken for the Oolite proper, given only a slight throw as after refuse dump on the N.20 where, the onset of bedding infallably discriminates between Souillac Oolite and Lacave Calcilutite, 140

as at St. Sozy (west end of quarry).

Joints

A systematic study of jointing was not undertaken, partly because of paucity of bedding surface exposures with unambiguous joint-directions and partly owing to the claims made on the available time by other aspects of the field-work.

The following obvious features were noted: N. -S. Joints

The master joints trend between 150-20 0 on either side of north. They are dominant in the Mirandol Oolite especially at Mirandol, where some of the slip-blocks may actually have been let down on joints activated by blasting and in the Souillac Oolite at Roches St. Marie, St. Sozy and many other exposures. In both formations, the joints are locally observed to stop against the underlying shales, thick and of Tourcian age in the case of the Mirandol Oolite and only a metre or less, and sludgy in the case of the Souillac Oolite (see photo of Roche St. Marie). N. -S. joints occur at the cave at Lacave together with E. -W. ones, but on the whole they are rare in the succession overlying the Souillac Oolite. The Lanzac Oolite would have been expected to develop a similar trend to the other two Oolites, if gross lithology were the only control. The observation to the contrary suggests that in addition to gross lithology, other factors, among them the character of beds above and below, may be operative.

E. -W. Joints

These do not form a physical feature so strongly as the previous ones except for the location of the caves at Lacave. A rough census shows them to be most frequent in the succession from Lacave Calcilutite to Mirandol Oolite and particularly in the Calcilutite. o The direction varies 15-20 0about E. -W. in all the measurements made 141

at Lacave and between Souillac and Torreoaye. Isolated observations have also been made by them in Lanzac Oolite above Torreoaye, between Le Bastit and Cales and in many other localities. No convincing age relationships exist between these and the N. -E. ones. At St. Sozy joints (300-42°) and not N. -S. ones appear to stop against E. -W. ones. This, however, goes against the implication of age in the greater frequency of E. -W. joints in the younger formation. The overall evidence of age is scanty but supports an early E. -W. joint trend. The major tectonic events were all post-Jura.ssic, so no structural discordances would be expected in an entirely Jurassic succession.

Joint Frequency

A rose-diagram of joint directions for an area within a 4 km. radius from Souillac, corroborates the observations just discussed. The dominant E. -W. (actually W. N. W.) mode and subsidiary N. -S. mode are quite consistent with the greater abundance of exposures and outcrops of Lacave Calcilutite - Lanzac Oolite as compared to those of Souillac Oolite.

Lithologic Control of Jointing

In addition to the possible lithologic preference apparent in the above allocation of joints to their respective formations, there is a definite small-scale expression of preference in the chalky beds of Lacave Calcilutite Unit 2 and similar beds in the top part of the St. Etienne Limestone. In both of these, north-south and east-west joints intersect in an unbroken sequence along individual beds to produce a peculiar serrated appearance. Near the bridge at Souillac, and also elsewhere, the E. -W. ones are vertical while the N.-5. ones appear to o occur in sets with a dihedral angle of about 30 (dips: 600 and 88°). Bedding thickness is probably not so important since the interbedded plates develop a very small-scale pseudobrecciation which can be interpreted, on account of its regularity, as an expression in plan of the same serration. 142

Structural Synthesis

The interrelations of Folds, Faults and Joints are discussed interpretatively in the following sections.

Interrelation between Folds

The relations between the N.W. -S.E. and E. -W. folds is both geometrical and temporal. The E. -W. folds are mostly minor and therefore impossible to trace individually from place to place. The exposures at which they have been observed do, however, form an interrupted belt from just north of Souillac through Monvalent and Gluges. There seems to be no reason why this cannot extend further north and south. Throughout the region therefore, major N. W. -S. E. folds carry within them minor E. -W. folds which themselves have still smaller parasitic congruent ones. The disposition of the minor folds on the major ones (N. W. -S.E. ) shows no obvious localisation, with regard to the geometry of the latter. The former are equally well developed at all points on the latter, except in the Monvalent traverse where the very open minor synclines and subcuspate (=accentuated) anticlines could be due to an interference of the two directions It is not clear whether these structures are circumscribed or not.

Relative age of N. W. -S. E. and E. -W. Folds The absence of cleavage or other suitable planar or linear structures bearing unambiguous relation to the folds has hampered clear-cut fold chronology. The available evidence, however, points to an early E. -W. folding dated in other parts of the region as post-Eocene (Pyrenean) and a later N. W. -S. E. folding presumably post- Miocene (Alpine) (Goguel, 1941). The two trends are sometimes developed in a macroscopic scale The following is the only evidence of relative age here - 1) The bondinage developed in the steep limb of a N. W -S. E. fold at Blazy is not folded. It would have been if it was an early structure. 143

2) The absence of convincing axial culminations within this area rules out the possible interference of large wavelength E. -W. structures with the major N.W. -S. E. ones. The 1st-order and 2nd-order minor E.-W. structures can therefore be taken to occur within the N. W. folds without the confining beds being folded on an E. -W. trend of higher wavelength than 1st-order. The conclusion is reached that these are later than the E. -W. folds, i, e. the latter are being born in a higher wavelength late folding.

In the south of the present area (see Bergouraioux, 1942), mappable periclines occur, implying sizeable E. -W. fold wavelengths and the above argument would presumably be more difficult to apply. The periclines are, however, elongate E. -W. implying a much closer spacing of the E. -W. axes than the N.W. ones. This is taken to support the improbability of higher order E. -W. fold wavelength than 1st-order minor.

Folding, Lithology and Bed Thickness

Some of the fold peculiarities observed for particular lithologies have been due to layer thickness. To this might be due the rarity of minor folds in the Oolitic formations. When minor folds occur they are usually quite gentle,as in the Mirandol Oolite. The sudden disappearance of the Oolites under the overlying Calcilutites 0, w‘ino,,1-achAYQ.S (at Gluges; and at St. Sozy) is interpreted as slip fault folding characteristic of these formations on account of their high competence. This has both steepened the limbs and occasioned preferential erosion and land-slides. All three Oolites show a definite preference for fracturing than folding. The same is observed to hold true of calcilutites with a bedding thickness of nearly 1 m. or more.

Concentric folding or concentric micro-folds appear to have a preference for calcilutites with a layer thickness of less than a metre to about 1- cm. Beyond this, dicrete limestone layers pass into incompetent papery shaly limestones and shales. The pure calcilutites retain their competence sufficiently.throughout the 144

concomittant change in fold-wavelength with bed-thickness. In contrast, disharmonic folding dominates in the Lacave Calcilutite Unit 2 which is very chalky with gypsum pseudomorphs and interbedded platy algal beds. The minimum layer thickness, capable of retaining the mobile interbedded chalk, is probably higher than the average thickness of these beds. These rare, competent layers display. open concentric structures while the incompetent beds considerably distort the geometry of ruckles developed in beds below the critical thickness. Open structures die out over short vertical internals and their crests are staggered vertically. Disharmonic structures again occur in the Gluges Calcilutite Unit 4 marls east of Meyronne as a series of confused, overturned minor-folds, minor thrusts and breccias.

Folding and Faulting

Four modes of association are observed between folds and faults - a) Planar and angular, in which the fault-planes and fold axial planes are mutually inclined in ways that appear to satisfy the inferred directions of principal stress. Examples are the shear lenses and low angle thrusts in the core of anticline 2 • • . at Blazy; on the Monvalent traverse where a minor low-angle bedding thrust steepens up into an axial-plane of a minor cuspat e syncline. In both cases, Pmax and Pmin are horizontal with Pint presumed parallel to the fold axis. b) Linear, in which the fault is vertical or subvertical or unexposed and interest is in the axial and fault traces alone. Examples within this area are the E. -W. faults at Le Chapellone, at Durantie and at Le Bougayrou, and the N. W -S. E. one at Les Patous and N. E. of Blazy and again at Durantie. Each direction mimics its corresponding fold trend. This correspondence has been widely noted throughout the Causses and further north to Niort and the northern edge of the Central Massif (Glangeaud 1896 etc). The interpretation of the faults in those areas as due to stress release, is adopted here with the caution after De Sitter (1964) that too close a meaning should not be read into the parallelism, since the possible fault (or joint) directions associated with a given stress fold nearly amount 145

t o an uninterrupted girdle in the principal plane.on a stereogram. Moreover, he shows that E. -W. tension faults may not belong to E. -W. anticlines but to N. -S. synclines. c) Generative in which the fold has actually been formed by faulting while the beds remained rigid or it has been accentuated contemporaneously or subsequently by the faulting. Examples are the angular, symmetric single hinge category of folds, in which the axial plane coincides with a fault which may have been contemporaneous and compressional or subsequent and tensional or both.. Sassondrics, Le Chapellone, and rare competent bands in Gluges Calcilutite Unit 4 at several localities may be Cited as examples. Contemporaneous fold accentuation by faulting is shown on the Souillac-Torreoaye traverse where the small syncline bringing down Lacave Calcilutite Unit 4 near the other end of the island has been reversely and repeatedly down faulted at its northern limb. The slight mismatch on the fold outline on either side of the associated normal fault is attributed to fault-drag (see below). . The reverse faults associated with the minor monoclineat Souillac Bridge have been discussed. A similar fracture occurs at Le Bastit.. The basal Lacave Calcilutite Unit 2 on the N. 20 north east of Blazy shows a . minor graben formed by four vertically diverging normal step-faults only slightly modified by flexuring.

To the great fault at Au rillac may be Ad/ attributed some of the rotational bed-steepening such as at Le Bastit, Lacave, towards Cales (from Lacave), near Gluges and in many other places. The dips range from 26° and 30° to 50°. Sets of steep (500-52°) and gentle (260-30°) beds .can be envisaged for a vertical Pmax and a horizontal Pint, parallel to the fault trace if the E. -W. fault is assumed to be early and the associated structures (and tectonic cross) to have suffered a regional tilt subsequently. d) Fault-drag, in which the fault-fold relation is also generative but the faulting directly brings about the fold rather than accompany or 146

PL.2acri7131.1723SII7 JOINT-SET IN FLAT!' LIMESTONE, Top of Glugea, Caicilutite Unit 3,S.E. of Meyronne:, ( Note the atylolitic fringe of the joints - atylolite grain, is newly orthogonal to joint piano 147

accentuate it. Minor drag-folds are strictly local in occurence both as positive and reverse drags. They are unimportant and will not be discussed further.

Folding & Jointing & Stylotites

The joint directions probably also mimic the fold directions but the same caution about matching fault-fold directions is taken. The earlier age of the E. -W. joints, as observed at St. Sozy, would also be supported by an observable fold-axis/joint trend parallelism. The higher frequency of E. -W. joints in the younger formations is therefore attributed to other causes. Some stylotites with sutures orthogonal to joint planes (pl 23 ) clearly indicate a compressional origin of the joint. Moreover, joints with such stylotites have been observed in hand specimens to be in a plane perpendicular to the inferred Pmax of microfolds.

Faulting and Jointing

A strong relation probably exists between the two as the Le Bougayrou which appears to run into master joints on the other side of the river and at Roche St. Marie. But in all other cases no detectable relation exists as is also shown by the absence of increased joint frequency towards the eastern part of the present area, which is quite near the Alvignac fault.

The faults in the present area can be referred to three directions: N.W.-S.E., N.E. -S. W. and W.N.W.-E.S.E. fivst There is no evidence here of the relative age of then-e-c-and and third directions above, but the N. E. -S.W. ones, such as at Durantie clearly stop against the W.N. W. -E.S.E. ones, and can be dated as later than the latter. The area west of a north-south line through St. Sozy has no less than four W. N. W. faults - all having a southerly throw. All four probably 148

belong to a single system of parallel step-faults with a throw increasing southwards. These are roughly parallel to the faults at Le Bougayrou and Padirac which have an opposite sense of throw. Any matching of these two with the four would have to assume a pivotal fault-system. There is no other field evidence to support this assumption but, if it is right, then it is possible to consider such a system as the result of a compressive stress along the trace and in the plane of the complex pivotal fault. Such stresses would produce nearly north-south folds and it is not too far-fetched to refer the W. N. W. -E. S.E. faults to the same age as the N. W. -S. E. folds which dominate the major structure of the area. The complete sequence of tectonic events would thus run in younging order: E-W folds, northerly.trending folds and E-W (-W. N. W. -E. S. E . ) faults, northerly trending faults.

Structural Control of Geomorphology The drainage pattern shows a strong N. W. -S. E. alignment dAy_ing from either direction into the Dordogne. This is also the direction of the late,Tolding faulting and jointing. The pattern is most apparent in the Lias and Cretaceous areas east and west respectively of the area mapped, which is mostly M-U Jurassic Limestone. The latter show a striking lack of surface drainage on the 1:50, 000 topographic maps, perhaps even better on the Michelin 1: 200, 000 road maps. The same contrast in volume of surface drainage between M. Jurassic and Lias shows up remarkably well on either side of the Alvignac Fault, which could just as well have been defined on this basis. 149

C. SPECIAL STRUCTURAL TOPIC - Origin of Blajour Breccia

The scale of brecciation in this formation ranges from very large, where beds are snapped into segments a foot or more long, to microscopic where angular fragments partly of sedimentary and partly of tectonic origin can appropriately be called microbreccias. The large scale brecciation is 1of-1 fundamentally different in its origin from the smaller-scale tectonic brecciation, and the term broken beds applied to them in this text is purely an expression of the scale of the phenomenon. Broken beds are more common in Unit 4 of the Gluges Calcilutite but also occur in Blajour Breccias Unit 1 on the St. Sozy-Baladou route. At this exposure the broken beds of Gluges Calcilutite Unit 4 and Blajour Breccias Unit 1 clearly lie between undisturbed beds of Gluges Calcilutite Unit 3 below and Souillac Oolite and Lacave Calcilutite Unit 1 above.

Other features of the Blajour Breccia:.-; are summarised below, followed by a comparison with previous ideas on brecciation. (a) The breccias are probably regional in extent, occurring throughout the area mapped and probably on the adjoining sheets. Breccias were mentioned at a comparable stratigraphic level in the Rodez area (Durand, 1928-36). (b) Their thickness is more variable than that of the undisturbed formations above and below. (c) The component beds, both by their original lithology and by :the err, presence of diagenetic gypsum (pseudomorphs), are such as to equally brittle deformation aided by jointing and plastic deformation of flowage, as in salt tectonics. Specimens 4 and 5 show how a marly or chalky bed can vary in competence depending on the amount of gypsum in it. Specimen 4 has no gypsum and,even though subjected to compressive stress as shown by the gentle warp, has preferred to deform by fracturing. By contrast, specimen 5 with a lot of gypsum laths in the lower 5/6 of its thickness is dcvLioping an insipient matted fabric, localised shear planes and stylolites - all being a prelude to flowage. The upper 1/6 , 150

given a similar layer below,would have already been differentiated as a potential brittle envelope bed capable of breaking up to form a breccia or a joint-shear breccia. The insipient ductility is also obvious in the reduced ability to fracture sharply. (d) The planar distribution (primary,or secondary) solution planes and stylolites of the gyi5sum. Besides accentuating the attributes inherent on the distribution rock-types, it is thought to play a major role in the detailed mechanics of disruption, by localising planes of peeling or unsticking (not in the sense of a decollement). (e) (i) Within relatively unaffected stretches of bed subangular pebbles and intraclasts (of any of the 3 lithologic components), sometirr s themselves containing reworked pebbles of marl, are found, indicating that these are sedimentary pebbles. (ii) The large-scale grading (or fining up) at Blajour and the subgraded undisturbed fabric at Blazy where undisturbed Rhyuchonellids also occur - all support the presence of sedimentary breccias and microbreccias. (iii) The possibility (see fig. in note-book) of tracing the origin of some clasts back to what were undoubtedly original sedimentary structures, such as minor clay flasers, lends support to (i) and (ii). (f) Lime-clasts in brecciated bands show gaping fractures that conform to what would be mechanically expected if some clast impinging on it were acting as a fulcrum. The suturing of the contact between the two clasts • is a common feature and the implication of pressure-solution bears out the mechanics causing the fractures (g) The fractures referred to in (f) often are still open (even though overlain by fine debris) and sometimes are cemented with calcite indicating that a post-lithification, purely mechanical event is superimposed on the sedimentary breccia. Sometimes the gape is plugged by matrix, indicating that , in some instances, the incompetent bands have been mobile. Clasts bound partly or wholly by calcite frame usually also within or near to what are clearly recognisable as fracture zones suggest the same superposition. 151

PL. 24A:o RIGINAL ANGULAR LIME-C LAST Its GRAINSTONE FACIES CIF BLAGOUR BRECCIA, 3lazy.

PL. 248 :SPECILIEN SHOWING SOME ASPECTS OF THE LATE' BRECCIATION OF THE BLAGOITR BRECCIA .( Tote the swallow-tailed. gape of the fracture 5_4- the centre of the pliotocraph, and. the gape of the clast at the bottom-left in relation to the clast on which it inipingea).: Viaduct. Across: N.20 just N. of Blazy. 152

(h) Some of the larger clasts knock-off easily - to show an ironstand dented, gouge-coated contact establishing the artificial agglomeratic nature of the rest of the ensemble. (1) Considerable widening of fracture zones by wall-replacement of vein-calcite leads (1) to a still greater impression of fracture-intensity (2) to the formation of a fringe around fractures looking like a bleached zone, but actually only due to textural differences in cement. The effect is even more noticeable in the Souillac Oolite.

The Blalour Breccia/ therefore are true intraformational breccias and conglomerates formed by penecontemporaneous reworking, and only later (probably in Eocene-Oligocene times) subjected to tectonic brecciation which has considerably complicated and masked ( pi. 24 ) the genesis of the component fragment. The sedimentology will be dealt with in the appropriate section. While the tectonic component is account- able by a combination of existing ideas, certain minor features unique to these breccias are strongly suggestive of an exact mechanical mode of deformation hitherto not covered in the literature, at least not to the author's knowledge. Both these aspects are now dealt with.

Tectonic Aspects of Brecciation:- Summary

1. The presence of extraformational, competent beds above and below the breccias (b)., 2. The evidence that within the Blalour Formation there are beds that were either originally highly incompetent or secondarily rendered so by the presence of gypsum, acquired mobility and deformed as such (c). 3. The presence of interbedded competent chalky beds presumed to be well-jointed as are similar ones in Lacave Calcilutite Unit 2 (see photo) and of others capable of breaking to form go0ping rotational fractures (c). 4. The closely spaced planes of peeling (d) and 5. The observation of low-angle joints and shear-planes observed in connection with the large-scale folds - are thought to be sufficient for the view that disharmonic folding, itself of a low intensity, caused well-jointed brittle beds to break up into angular blocks which were rotated, sheared and carried away by the mobile Sutured contact -S E Q U ENCE 1 (sty io(ite)

SEQUENCE 2

SEOUE NC E 3

FIG. 9 : MODE OF SECONDARY BRECC1ATION IN THE BLAGOUR BRECCIA 154

medium in which they were embedded. Still more rigid non-jointed beds broke in a zig-zag fashion, increasing the superimcumbent pressure on the joint-blocks and causing them to break against obstructing blocks in a similar fashion - the process only being limited in time by the duration of the stress field and the length of the fragments. The zig-zag breaking, observed as broken beds and on a much smaller scale also, set in train the chain process in which a contiguous thinner bed whose breaking wavelength is out of phase with its thicker pair would unstick or peel from one limb of the angular fold, the free piece being concommittantly fragment to fill the axial gape of the parent fold (see fig-: 9 ). This process, and the previous ones, are as spatially unlimited as the formation of disharmonic angular (rotational) folds within the regional flexures. The last process, like the first, is also self-perpetulating so long as there is room in the axial gape and the length of the freed piece is equal to or longer than the span of the gape.

Structures generated by the three processes above are elaborated by low-angle compressional fractures syngenetic with the folding and by late tensional ones - the latter often extended by wall-replacement.

Comparison with Previous Ideas on Brecciation

Kerr 6,ncl. no pp (1958) examined five kinds of breccias recognized by various authors - Subaerial breccias Tectonic breccias, Glide breccias ,Dessjcation breccias,an Intraformational breccias They came to the conclusion that there was not even agreement on the general term breccia - a lot of people indiscriminantly using breccia and conglomerate. For this reason, and for the fact that the geometry and particularly the homogenity of their breccias differed significantly from the nearest possibility reviewed by them, they set up a sixth category, Salt Dome Breccias, to cover theirs. These were overlain by unbrecciated shales 155

which they thought were kept plastic during salt intrusion by precipitation. They attributed brecciation to pressure over and above that generated by normal overburden, and to the brittleness of a dehydrated part of the shale, but they did not consider the mechanics of the disruption..

Carozzi (1953) likewise distingUished four categories of breccias - sedimentary, tectonic, volcanic and physico-chemical. These he further divided into sub- and sub-sub- categories. Those types which are obviously out of context are not considered here. Of the rest, the 'Breches de pent& are similar to the present ones in the great range of particle sizes and very partially in the frequency of voids, but their sporadic occurence • and lack of stratification and interstratificatian rule them out as a possible analogy. The 'Breches de cavernes', usually the repository of phosphorites, are oft cited for the Causses de Quercy but the author has not encountered them. The frequent development of concretions and the sporadic occurence again rule them out. Such analogy is found between his 'Breches de pente sous marines' and certain angular, fining upwards breccias at the top of Gluges Calcilutite Unit 4 at Blajour, The fining up will, however, not be confused with the graded bedding that is quite in keeping with his geosynclinal setting. In detail also, there seems no evidence here of a turbidite succession.

The following categories are texturally and, in their dynamic origin (though not in the agent or in its detailed operation) similar to the sedimentary component of the Blajour Breccia- . These are the Breches intraformationnelles' and the le 'Conglomerats a ga6ts mous', the former referring mostly to contemporaneous marine reworking of consolidated to semiconsolidated sediments and the latter to seasonal fluvio-lacustrine reworking of dessicated point-bar clays. The rounded to angular fragments, the uniform fragment- type, and the parallel to isotropic fabric - all show that an agent (or agents) combining the characters of both of Carozzi's categories would • 156

satisfactorily explain the sedimentary origin of the breccias. It will be shown that the fractures in fragments especially when they are in contact with other fragments, though very near to Carozzi's observations, are not, in fact, of a similar origin. The obvious aspects of his 'Breches d'origine tectonique' - fault-breccias, etc., will not be dwelt upon, although a fracturing (a similar phenomenon to faulting) is considered partly responsible for the tectonic component of the brecciation. Of greater interest is Carozzi's mention of th. aedded cysts between massive beds breaking up under disharmonic folding to give regularly interstratified (though tectonic) breccias. An amplified version of this view is thought to be largely responsible for the tectonic component of the Blajour Breccias

P. Launey & R. Leenhardt (1960) have studied similar breccias of Lower ,Simornurian age located on an outcrop running 3-4 km. north of Figeac. They consider them to be penecontemporaneously formed on the hinge of a submarine talus slope leading down to a 'basin' facies of decreasing dolomitization on the west and up into a plateau facies of increasing dolomitization. The very narrow width of the talus belt (25 km.) and the- presence in it of contorted cascade-type beds make any analogy unlikely between theirs and the present breccias.

Shrock (1948) does not differentiate between breccias and conglomerates, but showed his awareness of the distinction between angular and rounded fragments in his subdivision of 'conglomerates' into sharpstone, roundstone and mixed stone types. Of his types two seem to be relevant to the present enquiry - (a) Residual Basal Sharpstone Conglomerate arising from subaerial weathering and cementation followed by a transgression or weathering and incorporation of products in the basal transgressive series. The latter definition finds some analogy in the Gluges Calcilutite Unit 4 cap-bed 157

and in the Blajour Breccias but in the latter case only in so far as some intergradation exists between the two , as at Blalour. The sedimentary aspect of the Blajour Brecciav proper would answer best to Shrock's. (b) Penecontemporaneous Sharpstone Conglomerates which may be summarised as due to the fra gmentation and fractional selling of the upper part of an essentially unconsolidated' deposit under the action of unusually strong tides, storm-waves, storm-produced currents and other similar phenomena whichcause temporary depression of the base-level of submarineerosion.

Pettijohn (1957) discusses a member of conglomerate and breccia types without multiplying categories. Under intraformational conglomerates and breccias he discusses early induration and reworking of limestone chips which are deposited locally in association with other shore-line facies or deposited by turbidity currents over wide areas. The former only is relevant here. As belonging either to pseudoconglomerates or cataclastic breccias and conglomerate, he discusses fold-breccias ( 'riebunssbreccias') in which, as Carozzi later repeated with variation, thin-bedded brittle layers within incompetent plastic layers break up under sharp folding. He notes that they 'are local, are confined to sharply folded strata and are likely to pass into unbroken beds' . It does not seem that they need necessarily be local if the bedding character and fold intensity are the requisites - both these can be regional in extent. As due also to folding he notes the formation of conglomerates by shearing action on joint blocks of closely jointed brittle rocks. Again a combination appears to agree best with the observed data. The good jointing of brittle layers within plastic beds would have to be invoked to produce what is observed here without counterbalancing the absence of 'sharp' (intense?) folding. 158

CHAPTER

PALAEONTOLOGY AND PALAEOECOLOGY

A. GENERAL

In this chapter fossils are briefly described, with as short a list of synonyms as possible. The description and main sources of

identification were considered to be the best compromise between the much needed taxonomic work in an area with a very poorly known fauna, and the heeds of the present research which are largely palaeoecologic.

Alternative identifications are sometimes possible because of the poor preservation of the material. These are indicated and are not synonymous but rather indications of the alternative palaeoecologic deductions that could be made.

Fossils and the environmental conditions they represent can be looked on as the end members in a multicomponent system with several biofacies. Proper fossil identification with preferably a brief descriptions is the only guarantee that the biofacies and their palaeoecologic interpretation stem from a proper understanding of the end members - the individual species. This approach is very much in evidence in almost all serious ecologic or palaeoecologic works - most modern but also ancient ones.

The description is followed by the mode of preservation and the 1ithology•and locality of occurrence but not the associated sedimentary structures. The aim was to work on the fossils and their immediate matrix and only later to integrate the deductions thus independently arrived at with purely sedimentologic deductions. Cyclic reasoning was thought to be best avoided in this way. In other words, the sediments (plus sedimentary structures) are not used to deduce the 159

palaeoenvironment of the fossils, but rather to use the fossils as an

independent source of evidence for the palaeoenvironment.

It will be seen that (a) the evidence from modern analogs

(b) in situ deductions made elsewhere on ancient analogs of the present

species and (c) inferences from the morphology of the present species -

form a sufficient basis for independent palaeoenvironmental deductions.

Sources of ecologic information are cited in the text. The

literature is so scattered that emphasis was naturally on standard Works

- as text-books, special publications by individuals or regular bio-

logical journals - rather than on incident references to the ecology of this or that species. The latter were drawn upon whenever practicable.

The order of taxonomic descriptions for the bivalves was largely that in Newell's (1965) classification of the Bivalvia. Other classes and phyla were not half as well represented in species and raised no special problems of arrangement.

Class BIVALVIA (Bonnani, 1681) Linnaeas, 1758, p. 654

Superfamily NUCULACEA

49, Nucula cottaldi, de Lor.

(Cypricardia nuculiformis)

Cossman (1907), P1. II, Figs. 10 and 11.

Dimensions: L 1.93, H 1.35, T. 1.25

Occurrence: An internal cast without a preserved dentition occurred in yellow-stained micritic ooid-grainstones. The shell had been dissolved.

160

Pectinidae -- Lucinidae 100 i.---., i ---- Limidae i \ ----- Cardiidae / • , Nuculidae /\ \, /ii ------1/4, 1. / \ , ‘ . cu ••• • •••••.:•••••-• ... , i')G0 •..---.-- - • - - _ • • • ...... • -•••• 4... • N. 1*.. \ \ O ..... • . • . • • .. \ \ ‘ \ N >N \ . . \ \ t 40 \ cr \ s...\1 . L...... e. i• ° 20 ► i• . t k \ '• L t

40 80 120(m• 160 200 Depth

FIG. K) BATHYMETRIC CURVES FOR FIVE MODERN BIVALVE FAMILIES EASED ON THE DEPTH- RANGES OF THEIR CONSTITUENT SPECIES. (Based on the obervations: of Cadeet 7'Palicers Tebble). 161

19. Palaeoneilo morissi, Desh.

Br. Geol. Survey Collection (75695), 1863

Dimensions: L 1.00, H 0.70, T ?

Diagnosis and Remarks: Only one right valve was found as a cast, the shell having been dissolved. It could have been mistaken for Sphenia sp. which occurs in great abundance in Gluges Calcilutite Unit 3, but the latter has a much longer and more obvious lunule giving a less full appearance to the outline.

Ecology of Nuculacea

Nucula is rare in the succession as a whole. Modern nuculids are deposit feeders generally thought to prefer deep water, but evidence on eight modern species indicates occurrence also in the depth range 0-30 m. , The Challenger results, in fact, indicate a definite rise in the number of nuculid species from about 100fthms. and less. Thorson (1957) records Nucula in the Syndosmya Community with a depth of (5) 10 - 30 m.

The frequency-depth curve (fig.lO) for the eight modern species was very similar to that for the Pectinidae, having a low frequency of species at 50 metres and peak frequencies at 20 - 40 metres and at 80 - 90 metres.

A higher depth limit of 160 metres (, 80 fthms.) can be assigned to nuculids on this basis, but they are clearly most frequent at less than

100 metres (rt, 50 fthms.) which is clearly inner shelf or infralittoral

(Hedgpeth,1557) The higher depth limit is probably influenced by low energy and a stable bottom, so that nuculids may be expected at much shallower depths if these conditions enable organic detritus to settle on the bottom. The scarcity of nuculids in the succession probably indicates the absence of quiet as well as fully marine or brackish

(the Syndosmya Community is partly estuarine - Thorson, 1957) water conditions - the sedimentology clearly indicating protracted barrier- lagoon conditions for most of the succession. The single occurrence 162

of N. cottalidi may very tentatively indicate quiet conditions with some

organic detritus, 5% carbon being considered ideal by Cadee (1968).

Other evidence actually suggests that quiet and high energy periods

alternated in the burrowed level of Lacave Calcilutite Unit 3. Modern

nuculids appear to favour mud, mud with shelly gravel or shelly gravel

according to species. The matrix here is washed and can be placed

between the last two bottom types. Articulated and tightly shut valves

(in this species) and gaping in species of other genera all indicate quiet conditions while these forms existed.

Von Zittel (1900) places Palaeoneilo in the family Ledidae

together With such modern forms as Ledaand Yolldia, but the divergences of Newell (1965), Piveteau (1952), Fischer (1887) and others make it hard to ascertain whether this form was siphonate or not and whether or not it ever burrowed. Its close relation to Nuculana is obvious from

McAlester (1969), who diagnosed the Nuculanacea as usually with a pallial sinus. Palaeoneilo was therefore probably a siphonate burrower.

Energy and bottom conditions may not have been far from those of Nucula, the occurrence of each just once and then together strengthen the deductions made for the bed in which they occur.

Both genera probably had gills that were able to pump (modern nuclids do possess them), thus enabling them to live in a medium that was more turbulent and a bottom coarser than ideal. This possibility is inferred from Atkin's (1936-8) interpretations of the relative development of labial palps, gills and gill cilia in bivalves. This is again consistent with the alternation of high and low energy depositional conditions for the bed in question. The solution of the shell probably reflects its aragonitic composition in Nucula and Nuculana. 163

Superfamily ARCACEA

84. Parallelodon Kyserlingi d'Orb

Arkell 1929-37 pl. 1, figs. 6, 6a.

Dimensions: L 2.4, H 1.2, T 1.1.

Occurrence: A cast of one valve with ribs barely noticeable was collected from fine pellet grainstones in the Upper St. Etienne Limestone.

185. Cucullaea

Dimensions: L 3.2, H 2.4, T 2.4.

Occurrence: One left valve occurred in impure shell packstones very

ac.twto close to the bed with Gryphaea sublobata to of the topmost Floirac Shale.

Abundance is probably more than this. The very thick shell is remarkably

intact notwithstanding some dissolution. The infill is like the rest of the horizon - greenish yellow, partly oxidised glauconitic ?

Ecology of Arcacea

The superfamily is poorly represented both in individuals and

in number of species.

Parallelodon often considered extinct in the Tertiary, is occasionally reported in the Antilles (apparently the home of extinct species, as are Australia and off South Africa!) and at abyssal depths elsewhere under the genus Macrodon (genus variously attributed to

Buckmann or Lycett). If Von Zittel (1900) is correct then the continuation of Parallelodon into modern arcaoid genera through Cucullaea is very similar to Cox's (1946) observation (based on H. Woods and others) of the probable unbroken connection between Mesozoic Panopaeai (and judging from Cox's own figure probably some QuenstedtiaJ also) and Caino- zoic ones. The affirmation elsewhere in this text, of the probable unbroken range of L. subsellz into the Cretaceous can likewise be recalled. 16'4

Commonly listed geographical occurrences of modern arcaoid

'genera are generally Warm Temperate or Outer Tropical. Like Nucula,

abyssal depths commonly attributed to the genus Arca and so indicated

for the two species A. corpulenta pompholvnx and A. nucleator by Parker

(19 64)and for small-sized species by Smith (1959), the detailed evidence

favours an upper depth limit of 100-150 m and a range of littoral and

infralittoral. The Challenger results show a threefold increase in

number of species as the edge of the shelf is approached at 100 fthms. 1960,1964 from greater depths. Parker (/), Tebble (1966), Wells (1957) and Wood-

ward (1887) and Medley (1896-1900) furnish detailed evidence for the p.2100 average depth range suggested above. Parker (1959) states that ancient

barrier deposits may be difficult to interpret faunistically because of

the presence in them of elements washed in from the shallow shelf and

beyond the surf zone, this itself offering the only evidence for their

diagnosis. We may say that Parallelodon would be not only shallow

shelf (infralittoral) but may without surprise be found in barrier or

sub-barrier deposits.

Area's are epifaunal suspension feeders requiring a firm rocky

bottom or just solid objects for byssal attachment, a requirement that

would be well-met by the sometimes pebbly matrix of the present specimen.

Their preference for shelter in rock crevices or dead shells and the

absence of cilia for dealing with coarse suspended material, both indi-

cate low turbidity as being the ideal, although some species occur in

low-salinity mangrove swamps where turbid waters must be prevalent.

Normal salinities are probably optimum for Arca% (Smith, 1959), but

poor aeration suggested for Parallelodon in the Lias by Webber (1968)

may indicate range of tolerance rather than an optimum requirement.

Cucullaea, unlike Parallelodon, is represented by a modern

species of the same genus in the Indo-Pacific Mauritius and Nicobar 165

(Woodward, 1887; Grasse, 1956). Not much is known of its ecology

and conclusions about Parallelodon based on Arca may be extended to

Cucullaea with due caution. Takai et al. (1963) list Cucullaea in

ancient rocks of Japan with the large pectinids, astartids, large Limids

as being a shallow subtidal coarse sandstone genus in contrast with the

deeper water forms Posidonia, small pectinids, small Limids and Inocera-

mus occurring in gmmonitic shales. Unlike Arca$-, Cucullaea may have

moved into deeper waters in recent times.

Superfamily MYTILACEA

149. Mytilus ungulatus, Arkell

Arkell 1929-37, pl. II, figs. 5-7.

Dimensions: L 3.9, H ?, T ?

Diagnosis and Remarks: The specimen is too incomplete to allow a firm

identification.

88. Mytilus cuneatus, Phil.

Phillips 1875, pl. XI, fig. 21.

Dimensions: L 2.7, H 1.85, T 2.0.

Diagnosis and Remarks: The specimen differs from Phillips' figure only in the presence of ribbing on the latter. The specific name should not be confused with Modiolus cuneatus, J. Sow now Modiolus anatinus,

Smith (Cox, 1965).

Occurrence: The species is very common from the Lacave Calcilutite to the St. Etienne Limestone, but the shell is often too thin to control the fracture of these impalpable limestones during extraction. Calci- lutites with some grain content seem preferred.

166

5. Mytilus varians, Roemer

Arkell 1929-37, pl. II, fig.8.

Dimensions: L 2.5, H 1.0, T ?

Diagnosis and Remarks: This is a small mytilid easy to identify from

the acute terminal umbones and the sharp postero-dorsal angle.

Occurrence: It occurs more often in coarse or fine grainstones in

the St. Etienne Limestone, but smaller out-of-home individuals occur

also in buff coloured calcilutites of Lower Gluges Calcilutite Unit 4.

183. Mytilus sublaevis, Sow

Occurrence: The species is common in Gluges Calcilutite Unit3in the

same impure carbonaceous calcilutites as Pteroperna costatula.

56. Brachiodontes (Arcomytilus) asper, J. Sow

Cox 1965, pl. 4, figs. 2a,b.

Dimensions: L 2.73, H 1.25, T 1.6.

Diagnosis and Remarks: Species with which this is likely to be con-

fused are B. (A.) laitmairensis, de Lor, B. (A.) subpectinatus, d'Orb,

and B. (A.) pectinatus, J. Sow. These were revised by Cox (1935a).

For palaeoecology at generic level, the results of a misidentification would probably be slight. Cox (1965) comments that B. (A.) asper is

exclusively Bathonian in England although occurring also in the Callo-

vian in other localities. The present area would be one of these

localities, as the species occurs both in the Bathonian and the Callovian.

Occurrence: Articulated valves are rare owing to the small articu-

lation area in the mytilids and the speed with which the ligament rots

(Cadee, 1968). The thinness of the shell may partly be due to the removal of its aragonitic component. This species occurs in the coarse granuipr or shelly seams and parting planes between the massive calci-

lutite beds of Gluges Calcilutite Unit 2 and Lacave Calcilutite Unit •3. 167

Muddy grainstones are probably the ideal for it.

Ecology of Mytilus and Brachiodontes

Like the other delicacies Ostrea edulis and the scallops, the habits of Mytilus are known in considerable detail through years of artificial culturing.

Mytilids are byssate epifaunal fillibranchs, although they can live attached but buried in soft mud, often in large banks and colonies, on bouys and concrete works and amid sea-weed on the fore-shore. The bottom character is thus varied but firm objects of attachment must be present. M. edulis is known to live down to 100 fthms, although cu- tomarily considered intertidal or shallow subtidal. Brachiodontes recurvus and B. exustus live from 20-60 m and 15-40 m respectively, this genus therefore probably indicating greater depths than the average for

Mytilus, nothwithstanding the occasional occurrence of B. recurvus in depths as shallow as 4 m.

Mytilus and Brachiodontes are usually brackish-water genera, but normal salinities are equally welcome.

Mytilids prefer strong currents which ensure a good food supply.

The food is taken in suspension, and in M. californianus consists of uni- cellular organisms. In some Modiola's (see under), the food is algae and occurrence is amid fucoids and filamentous algal patches - an observation which it projected to the mytilids, may explain the association in oncolitic wackestones in both the Lacave Calcilutite Unit 3 and the topmost St. Etienne Limestone. The scarcity of food is known to reduce directly the size of mytilids, which for this reason attain the maximum size just below low-water mark where a 24-hour supply is assured.

The presence of enlatero-frontal cilia in the Mytiliacea may suggest an adaptation to a coarse bottom but it does not explain the 168

likely high incidence of turbid waters in a brackish estuarine environ-

ment. In the event, they are more likely just an additional tool to

others dealing with excess fine of suspended matter.

Temperature limits for survival of M. edulis are 7°C and

40.8°C, a mean of 7-28°C for the genus being suggested by Smith (1959).

These temperatures are relevant to the incipient evaporite type dia-

genesis producing the halite gypsum and dolomites in the calcilutites,

for they may explain the shelly granular bands and partings with mytilids

as marking temperatures more nearly approaching the optimum of 28°C and

with this an amelioration of otherwise high salinities. Taken with

other evidence, the general paucity of mytilids may mean a widespread

failure to meet the dual element of shallow depths and normal tempera-

tures. When depth compensates for elevated temperatures (and salinities)

as is seen for instance in the absence of doloMite, gypsum or halite in

Gluges Calcilutite Unit 3, we have, not mytilids but the deeper-water

cousin Modiola$ which then as now are generally quite diverse at these

levels.

The ancient record of mytilids is more scattered than the

recent one, and isolated references to Mytilus are probably inexhaustible.

They support or at worst do not significantly contradict the above

observations and conclusions on modern mytilids. Gigneux (1950) con-

sidered Mytilus and three other forms to prefer sandstones and detrital limestones. Takai (1963) considered them estuarine. Termier and Termier

(1959) considered mytilids, Gervillella, nuculids and Protocardia to be

lagoonal although the use of the latter term in different settings else-

where in the text indicates that their lagoon could be any one of a brackish, hypersaline or normal one. It may be concluded from this brief census that over-generalisations probably worsened sometimes by

taxonomic inconsistencies in the use of Modiola and Mytilus contribute

very little to an objective comparison of ancient to modern data on the genus Mytilus. 169

Genus Modiolus, Lamarck (1799) p. 87

There appears to be a great variation in the Modiolus due

largely to insufficient systematic work The various forms that have

been referred to M. Sowerbyanus alone (some extremely elongate and

approaching M. scalprum, others very short and but for the straightness,

approaching imbricatus) would have deserved separate specific or sub-

specific status by the standards applied to modern members of the genus.

The information content of the specific names thus reduced, it has been

thought necessary to emphasize the morphology in the ensuing descrip-

tions in order not to perpetuate the same error. Cox and Arkell (1948- , 50) show the opposite view by fusing a lot of the species erected by

Morris and Lycett.,(1851-55).

218. Modiolus anatinus, Smith

Morris and Lycett (1853), p. IV, fig.2.

Dimensions: L 2.4, H 1.25, T 2.2?

Diagnosis and Remarks: The specimen is incomplete but adequate for

identification. The growth lines are clear but irregularly spaced and

the antero-ventral swelling approaches that of M. Lonsdalei Morris and

Lycett, which in turn is not sufficiently arcuate to accommodate the present specimen. The oblique antero-posterior ridge is rounded and suppressed, not sharp as in M. Lonsdalei. It may be distinguished

from M. subreniformis Morris and Lycett by its growth lines and perhaps

the inferred greater height/length ratio.

'Occurrence: The specimen is preserved in creamy-grey impure, almost manly limestones with a slight superficial ironstain. It occurs in

Gluges Calcilutite Unit 4, although a diminutive form occurring in the basal pisolitic Mirandol Oolite could have been incorporated in it were it not for the compelling intimacy in occurrence and identity and size 170

it bears to another species at that level.

214. Modiolus (Inoperna) sowerbyanus, D'Orb and

18. M. (I.) perplicatus, Etallon

Cox, 1965, pp. 38 & 39.

Dimensions: L 7.9, H. 1.9, T 1.25.

Diagnosis and Remarks: Cox (1965) gives a very convincing diagnosis of the two species, the splitting of the ribs into exactly two towards the oblique carina being characteristic of No.18.

Occurrence: The species was found intermittently from the lower bioclastic units of the Mirandol Oolite up to the St. Etienne Limestone, but occurrences above the bioclastics, were always fragmentary or articulated but widely gaping positive imprints lacking (?) the oblique antero-posterior ventral area. Such occurrences are in Lacave Calci-

lutites Units 1 and 2 and in fine pellety St. Etienne Limestone, in all of which it was probably out of place. It is also possible that these higher forms are M. perplicata of Etallon in which case they would have suffered little breakage and could be considered in place. This is a good example of the ecologic possibilities raised by inadequate specimens. Also the assumption that they were unbroken can be sustained by Cossmann's (1900) M. sowerbyana, D'Orb. As a compromise they are considered as unbroken specimens of a variant of M. sowerbyana.

171. Modiolus cf. buckmani, Rollier. 1951 -5 Morris and Lycett, Table IV, fig. 6.

Dimensions: L 1.9, H 1.3, T 1.15.

Diagnosis and Remarks: A diminutive wedge-shaped species that differs from the type figure only in the more uniform shape of the latter. The deceptive smoothness of the otherwise fine and regularly distributed growth lines is due to flaking off of large areas of the thin shell. 171

Occurrence: Its occurrence near the probably condensed Aalenian/

Bajocian contact together with Serpulids demands it be not confused fto ci with Lithodomus inclusus, another pigmy brac hloloid, but which has no

antero-posterior oblique ridge and is a borer.

This form was not found higher than the pisolitic Lower

Mirand01 Oolite, at which level it occurs,alwaysxarticulated,with

Terebratulids, Rhynchonelids, Echinoids, Gastropods and a mat of Ser-

pulids. A firm bottom for attachment would not have been lacking,

though with some post-natal dislodgement and limited local transport.

200. Modiolus virgulinus, Thurman and Etallon

Cox (1965), pl. 3, fig. 8.

Dimensions: L ?, H 2.0, T 1.4.

Diagnosis and Remarks: A giant species with a heat-shaped vertical section, the dorsal area being nearly horizontal, rather than the diamond or lensoid shape of most modiolinids. The antero-ventral area is very prominent and the growth lines on it gently turn dorsally as in M. Lonsdalei from which it differs in almost every other respect.

Only the anterior half of the specimen was secured.

Occurrence: The species is commonly observed in a vertical position in the massive bedded calcilutites of Gluges Calcilutie Unit 3, with the umbones,pointing downwards. Individuals of different size occur in this or an oblique position, not actually in oyster brashes but in vertical proximity to these. Rippling is common throughout.

It is concluded that the life position of this form was close to that of modern mytilids - attached with the antero-posterior axis parallel or oblique to the substratum - but that after death, being incapable of righting themselves they were pitched on their byssus by currents into vertical and subvertical positions. If this hypothesis is true the 172

dorsal side of the animal would be expected to face down current, but

the high symmetry of the ripples did not allow a determination of

palaeo-current direction. The strong alignment of echinoid spines in

these beds supports the view that currents were of sufficient compe-

tence to affect the rotation, the invariable near-life position of asso-

ciated Pholadomvai and PinnaX both living securely buried, being used

therefore as evidence of the epifaunal or near epifaunal habit of this species even without recourse to modern analogs.

The species was found only in these beds.

Myochonca rathieriana, d'Orb 1901 Loriol,/T. XXVIII, p.1 IV, fig. 1.

Dimensions: L 8.0, H 3.0, T 2.5.

Remarks: Although this genus belongs to a different family (Modiolor— sidae) from Mytilus and Modiolus it is described here because of its resemblance to the latter genus and because there are no modern representatives from which its ecology may be separately deduced.

Occurrence: The specimen is quite distinctive in its shape although the shell has been largely removed by solution. The species occurs only in the wavy muddy partings of Lacave Calcilutite Unit 1 where its frequency is only a little less than that of the Pleuromya's and comparable to or more than that of Modiolus sowerbyanus. It is found tightly closed either in a subvertical or horizontal position in lime- mud supported pallets and bioclasts.

Ecology of Modilous (and Myochonca)

There are more modern species or subgenera of Modiolasthan there are of Mytilus. Modiolus proper differ from Mytilids mostly in their greater depth preference implying also a prc'ference of more stable salinities, as for example in the Irish Sea (Jones, 1950). Because of the great range of ecologic conditions signified by the various sub- 173

genera of Modiola such as Botula,Lithodomus, Crenella, Modiolarca, Muscuiu:.,

to mention a few, the nearest morphologic approximation only to Modiolus

proper, i.e., Musculus, is included among the modern analogs. The sum-

marised depth ranges of three species of Modiola and four of Musculus

(see chart) give an average range of intertidal - 140 m, Modiola and phaseolinus can withstand low oxygen concentrations / occurs as such in

deepest zone of animal life in Black Sea .(Cadee, 1965)- 60 - 180 m,

Musculus niger being excluded as peculiar. Extensive beds of Modiolus

modiolus occurring in one instance at 29-60 m depth on a gravel bottom

may indicate the optimum depth and substrata) conditions.

'Modiola barbatus living on rocks or Laminaria, Musculus discors

under rocks and seaweed, M, marmoratus also on rocks, stones, and

Laminaria and costulatus amid algae all support the importance of a firm,

coarse bottom. Individual modioloids may however be quite capable of

living on a muddy bottom as long as a few objects of attachment are

available, and current bring a regular supply of food.

In ancient environments, Wobber (1969) thought that Modiola

hillanus and M. minima preferred a variable bottom, were fairly toler-

ant to intolerant of environmental changes, and low-moderate rates of

sedimentation, conclusions that support the deductions from the living

species, particularly-significant being the degree of tolerance, as the

coase shaly partings of Lacave Calcilutite Unit 1, the associated

Pholadomyas and absence of dolomite in Gluges Calcilutite Unit 3 and

the obviously marine associates in the pisolites of the Mirandol Oolite

all suggest close to marine salinities. McKerrow, Johnson and Jacob-

son ( 196.9 ) give an association of Modiola and Epithyris

as occurring in tidal channel floors in the Great Oolite, indicating

the same range of hydrodynamic, salinity, substrata] and sedimentation conditions to be expected from prior considerations. 174

A hitherto overlooked fact is the absence of rapier-like

Modiolas and Myochonca in the present day. In the succession here Cadee (1968,p1.2) treated they seem to be more tolerant than the short reniform species/

the latter apparently being replaced by mytilids (and brachiodontids)

at levels with elevated salinities while the sowerbyanus types con-

tinue to occur. Water-temperature, depth and salinity which seemed

crucial to the relative distribution of mytilids and short Modioles

may be ruled out,as the sowerbyanustype occurs with both of these thcat groups. It is thought the very long rudder-shape would have been,un-

like in the early Pteria's and Gervillia's, a disadvantage in condi-

tions of 'high current velocity, forcing these forms to lie either on

their side or to be buried subvertically, attitudes in which the Pinnas

already held sway. Also, whereas the gregarious attitude or ability

to hide in othershells of the usually deeper water (than Mytilus)

short Modiolab would have stood them in good stead in shallow rougher

conditions (where they would form banks), the longer ones were appar-

ently not able to adopt this condition. From all considerations, the

long Modiolab appear to have been displaced byforms

better adapted for each of the niches once occupied by them. This suggestion is in accord with the earlier occurrence of Modiolus(Devon-

ian) as compared to Mytilus (Tries), Gervillia (Trias), Pinna (Trias).

Bretsky Jr. (1969) in a survey of the evolution of Palaeozoic faunas similar suggests in a general way a series ofd ecologic replacements. 175

Superfamily PINNACEA, Leach 1819

107. Pinna cf. mitis Phil. 1875 Phillips,/Pl. V, fig. 7.

Dimensions: L 5.37, H 2.25, T ?

Diagnosis and Remarks: This species is hard to identify with exist-

ing figures. The hinge line and ventral line are not straight as in

P. sandfootensis Arkell, but curve outwards anteriorly, giving the out-

line a stalked appearance. The outline also differs from that of P.

cancelata, M.& L.and P.mitis,Phil. which it closely resembles in the

absence of an angular central line (present in P. sandfootensis) and the

presence of non-costatt fine ribs. The growth lines, seen from the

inside of the left valve are weaker than in P. mitis

Occurrence: One left valve only was recovered intact from near the

topmost St. Etienne Limestone at the Lot/Dordogne Dept. boundary on the

Souillac-Cazunles route, although, fragments are quite abundant on lam-

ination-surfaces, where also a variety of oysters and other bivalves and gastropods occur. The rock is a fine, chalky pelsparite.

220. Pinna cuneata Phil. 1875 Phillips, PL. IX, fig. 17; M.& L., Tab. V, fig. 11.

Dimensions: L 5.05, H > 2.05, IT 1.00.

Diagnosis and Remarks: In the absence of descriptions, it is hard to know the range of variation of Phillips' figure which this specimen resembles in the gently arcuate overall shape, the apparent lack of ribbing, and the well-marked angular central line. P. sandfootensis

Arkell is ribbed and straight-sided, but the curvature of this specimen is so gentle that it appears to be a good intermediate between P. cuneata and P. sandfootensis. 176

Occurrence: The internal cast with only residual traces of the shell

of the right valve was found in greenish-grey calcilutites at the base

of Gluges Calcilutite Unit 3 near Creysse. These particular beds are

fossiliferous with an abundant occurrence of Gervillia acuta, Chlamys

lens, a purpuroid gastropod, occasional trails, and small unidentified

oysters. The absence of the shell is probably due to a late solution

effect rather than to an early diagenetic removal of aragonite. The

Pinnacea are in fact mixed aragonite/calcite shells.

205. Trichites

Dimensions: (1) L ? , W > 6.3, T 3.55

(2) L 18.8?, w 6.3, T 2.75

Diagnosis and Remarks: This is a giant specimen with the dorsal line gently, broadly concave upwards as in T. plotii (Lhuyd ) Lycett, which it resembles also in the mytiliform shape, and the acute umbones, but the analogy ends. The umbones are extremely acute in this specimen and the carina present in T. plotii is incised by a central line as in some Pinnas. The radial lines of T. plotii are also absent here.

Occurrence: Three incomplete but articulated specimens were recovered from the fossiliferous calcilutites of Gluges Calcilutite Unit 3 near

Meyronne and near Legol-Pres-Martel. They are about as abundant as the

Modiola's and Pseudodiadem4near Meyronne, being dominated only by the

Pholadomya's and perhaps Anisocardia's and Cypricardies, with which they frequently occur. Like the ModiolA and Pholadomya's at the same locality, they are frequently in life position. A shell layer in the plane of symmetry of one specimen indicates that the animal came to lie on its side after death and was gradua1ly filled by sediment, a position that gives an idea of the competence of the currents invoked earlier to explain the probable post-natal orientation of the Modiola's at the same 177

site. Trichites were probably even more firmly and deeply implanted

than Modiolalg, but there must have been a sufficient projection above

the substratum to initiate scour and eventual disinternment. No com-

mensals were found on the Trichitids, so subsequent burial was either

rapid, or the environment did not favour them, assuming the above reconstruction to be valid.

Ecology of Pinnacea

Modern Pinnacea include Pinna rudis and Atrina vexillum

(Grasse, 1956), Pinna fragilis (Tebble, 1966, Cadee, 1968), P. rugosa

(Parker, 1964), Atrina tubercolosa (Parker, 1964), Pinna cranea,

A. serrata, A. rigida (Parker 1959,60), a very good modern representation estimated by Moodward (1887) at about 30 species distributed in the

U.S., S. Britain, Mediterranean, Austrlia, Pacific, Panama. Von Zittel gives another modern subgenus Cyrtopinna, Morch, while Fischer gives the form Briophila (setosa) as a small-sized subgenus living in algae.

The depth range (see Chart) of P. rugosa, P. carnea, A. tuberculosa, A. serrata and A. rigida is from uppermost intertidal to

60 m., P. fragilis being exceptionally cosmopolitan living from subtidal to over 220 m. A maximum species frequency occurs from 0-20 m. The

Challenger found no Pinna's at greater depths than 100 fthms and recorded only one species for depths less than 100 fthms, supporting what is considered to be the exceptional range for P. fragilis, supported by

Cadee recording P. fragilis in the Oceanic zone and outer bay only.

Pinna is one of about 10 bivalve genera living on an atoll lagoon floor

50-60 m by Wells (1957), while P. trigonalis Pease has been reported on the beach at Funafuti, (Medley 1896-1900) obviously derived from the 50-60 m lagoon floor. Piveteau (1956) gives a maximum depth of

100 m for Pinna. The overall average depth of Pinnacea is thus taken as intertidal - to 60 m. 178

The bottom type preferred by Pinna is reflected in the depth

ranges which are frequented by various species. P. fragilis, the

bathymetrically cosmopolitan form, living buried with the pointed end

in mud, sandy mud or gravel, attached to small pieces of stone or shell

(Tebble, 1966), presumption of a coincidence between these and water

depth being supported by J.Murray's (1895) use in the Challenger Expe-

dition of the concept of the 'mud-line'. Pinna rugosa (intertidal to 1964 10 m) lives on sand (Parker) as does A. tuberculosa from a similar

depth range, both reflecting the same coincidence expected also from

sedimentologic considerations. _Wells' lagoon floor is of coralgal

sand. The concensus would appear to favour fine, soft sand or muddy

sand rather than mud. The deductions from morphology however are

apparently in conflict with the bottom type. Newell (1965) places

the Pinnacea in Atkins' gill-type II, i.e. without coarse cilia and

presumably unable to deal with excessive coarse material. The same

conflicting evidence is presented from the ancient record on Phola- p.31 7-3 18 domya, Gineux (1950)and Russell (1943) attributing it to shell banks

and fine-grained nearshore sand facies (in Cretaceous) respectively

and H. & G. Termier (1959) making it a muddy-bottom dweller together

with 'Ceromy, Pleuromya and Goniomya.

It is thought that the significant factor which reconciles

the above divergences is the energy of the environment and in part by

reflex, the stability of the bottom. A stable bottom with little

sand in suspension (particularly if coarse) and with a few objects for

attachment may be more important for Pinna than the actual texture of

the bottom.

Salinity tolerance is little known, although Atrina occurs with Brachiodontes in a polyhaline environment in the Gulf of Mexico 1960 (Parker). The shallower water species are, probably fairly tolerant of

lowered salinities, such as one might expect there and in Cadee's (1968) 179

centre Bay of the Ria de Arose. It is notable that no brachiopods

occur at the very fossiliferous level o-f th-o Pinnalf. Other evidence

already partly touched on indicates near normal salinities, therefore

the absence of coarse objects of attachment is held to have favoured

those infaunal forms wholly or partially like Pholadomya, Pinna,

Modiola, Anisocardia which would be partly stabilised by some sur-.

rounding sediment in the face of fairly strong directed currents.

This may have been augmented by some salinity fluctuations.

In the ancient record, Webber (1968) notes that Pinna, occur-

ring in unlaminated sediments may indicate a sheltered bottom. Here

however there is current-ripple lamination and other biologic evidence

of current activity. He also infers a low suspended load as is indi-

cated here, but seems to place the emphasis on the control of distri-

bution by presence of coarse shells for attachment. There is no reason

whatever to suggest that the pure calcilutites of Gluges Calcilutite

Unit 4 have more coarse objects than the Lacave Calcilutite where PinnA

are rare to absent; indeed the latter is quite pebbly in the upper part.

The low sedimentary rate suggested by Webber is in accord with the de-

duced low suspended load.

In summary, Pinna and Atrina are suspension-feeders which

lived here at a depth between 0 - 60 m, on a mud bottom (but not

limited to this) which was probably not ideal, but sufficiently stable

in conditions of fairly strong currents with a low coarse (sand-size) suspended load, and salinities near normal but probably varying within narrow limits. Aeration was good but probably did not long remain so.

Temperatures. were probably cool as seen in temperate distribution of modern Pinna. 180

Superfamily PTERIACEA, Gray 1847

199. Pteroperna cf. costatula, Morris and Lycett

Cossmann 1900, P1. 5, fig. 5.

Dimensions: L 2.3, H 1.5, D. 0.3.

Diagnosis and Remarks: This small poorly preserved species appears

to show a less stout posterior ear than the figures 8 and 8a in Morris

and Lycett's type specimens, resembling in this respect the figures in

Cossmann. The suppressed anterior ear and the lack of strong differ-

entiation of the collar on the anterior ear together place it firmly

close to P. costatula than to any other. No wide size range was ob-

served in the sediments which appear from the evidence of co-occurring

forms and other sources to be deposited under fairly strong currents.

If the young of this epifauna form can be presumed to be more easily

transported in such a case then the lack of costation on these speci-

mens would make them the adults of Morris and Lycett who found costa-

tion only on the young.

Occurrence: The form is common on parting surfaces in foetid car-

bonaceous silty limestones, and calcareous silts, but they are so flat

and difficult to extract intact that it is hard to ascertain whether

the positive reliefs are still articulated. The shells are present

in patches probably after late solutioning; the Prteriacea have a mixed

calcite/aragonite shell. They occur with Gervillia acuta, Lucina,

Modiola and Anisocardia dieglafaiti and sparse Pholadomya! in Gluges

Calcilute Unit 3 between St. Sozy and Baladot4, near Meyronne, at Le

Limon and less commonly at Leyol-pres-Martel.

181

1 145. acuta, Sow. Jade C.

Morris and Lycett 1850, Tab. III, figs. 12, 12a, Mongin, P. 12, fig. 1-2

Dimensions: L 5.1, W 1.2, T ?

Diagnosis and Remarks: The species'is very similar to the type of

Morris and Lycett in the slightly curved body form, but there seemsto

be a good variation spreading to G. subcylindrica M & L which may truly

be a variant of G. acuta as the latter author. suspected. The

straight body form of G. acuta Phil. Pl. IX, fig. 36 may be diagnositic

of this species notwithstanding its similarity with these specimens of

G. acuta in the curved. outline of the anterior auricle

(a difference from the straight margin of the type of Morris and Lycett).

Occurrence: Specimens are quite abundant, dominating the assemblage

and nearly forming a coquina at the base of Gluges Calcilutite Unit 3 and

near the top, probably also sporadically occurring in between. The

valves are articulated but show some preferred orientation interpreted

as more likely in response to current-induced nutrient gradient. It

is very sparse but present in Lacave Calcilutite Unit I only on the east

side of the Blagour Valley, although a great variety.of bivalves plus

brachiopods and gastropods abound in this member here and elsewhere.,

Preservation is good, the shells being lostonly mechanically during ex-

traction.

211. Gervillia lata, Phil. [G. tortuosa Phil. PI.X1, fig. 36]

Phillips 1875, P1. XI, fig. 16.

Dimensions: L 3.4, H > 4, T 3.0.

Diagnosis and Remarks: A distinctive species from Pteria tutcheri 1929-37 Arkelypl. XXV fig. 1-lb) in size and double curvature of body axis

as well as by inflation of left valve. It is hard to separate it from

G. tortuosa Phil. (1874) which seems to display the anti-clockwise twist 182

of the umbones better than the figure of G. late. Perhaps this is an artifact arising from Phillips' perspective sketches. However the abrupt bend on the anterior auricle/body line in G. tortuosa is a smooth curve in G. late.

Only one specimen was recovered from the coarse biosparties over the Gryphea-Ammonite bed although it is probably quite common among the calcareous nodules often associated with large Ceromyas and

Trigonia. The shell is rarely preserved, being dissolved and supplying cement to the surrounding matrix which replaces piecemeal the space so vacated and forms a thin sediment replica.

Ecology of Pteriacea

The following deductions are made from the vertical di.stribu- tions and morphology of Gervillella acute G. late and Pteroperna cf. costatula in this area:

1. The occurrence of G. late only at one horizon with an open marine association74mmonites etc. is taken to indicate a lack of tolerance for restricted marine or lagoonal environments with abnormal salinities.

2. Pteroperna costatula seems to prefer calcareous clays and silts to pure limetones at the same outcrop. Its abundance and size in this lithology which is often sooty and containing a lot of Lucina's but fewer Pholadomya's and Modiola's and Gervillia suggests a tolerance for low oxygen concentration and restricted circulation. Lucinas are known to have a special adaptation for this kind of foetid bottom.

This lithology is rarely encountered outside this unit of the Gluges

Calcilutite as in Pteroperna, an observation that indicates, an inability of Pteroperna to compete with other forms in circumstances where its only adaptive advantage is absent or reduced. The large individuals 183

Auricle (serving as rudder)

As: Tethered mode of life of the. modern aviculid, Pteria hirund (After Tebble,19663- ----• rote slight twist of the umbones.

Cs Transverse section of G. lata B: Gervillia lata mi - excess moment on hoarier. left valve acting in clockwise diredtionabout axis of coiling when view, down-current from umbones. - water resistance on auric: set up by mi.

Coiling axis.

m1 Du Idealised sense of coiling of G. later. Sense of coiling is into current and. is such as. to augment the balancing effect of mQ .

FIG. n HYDRODYNAMICS OF -Csrvillia lata & RELATED PTERRIACIDS. 184

here may also indicate an absence of competitors for the same reason.

3. The occurrence of Gervillella coquinas above and below the bed with Pinna%, Modiola% and PholadomyaX and the presence immediately

above and below the coquinas of coarse, bioclastic intrasparite punctua-

tions of undoubted high-energy origin all suggest a shallower water habitat for Gervillia than the other forms (Phalodomya etc.).

4. Hydrodynamics of Pteriacea

The Pteria% as a group are inequivalve and byssate , and

Tebble (1966) has suggested a rudder role for the auricles of Pteria hirundo which he took to be-tethered on its byssus. The author suggests that the mode of attachment and the inequivalveLuild of Pteriacea are strongly related to the role of the posterior auricle. It is probable that this role was solely to balance the unresolved moment on the larger, heavier valve. If the relative size of the valves were to increase, there ought to be further enlargement of the auricle which must have a limit to its size or some other shell modifications to forestall that eventuality. Such a modification is thought by the author to have been the twist of the shell axis so well shown particularly on the left valve of Gervillia lata, assuming this form to have been tethered on a byssus at least in part. The twist corresponds to an attenuated logarithmic spiral (see fig•31) whose sense of coiling is such as to ease the yaw to the right caused by hydraulic forces acting on the'auricle from the right, to compensate the excess weight of the left valve. That the auricle is a response to the inequivalve condition of these tethered forms is supported by the absence of such a structure in the equivalve byssate forms such as the mytilids and Modiola% for which the ventral keel and the salient dorsal ridge were quite sufficient to prevent rotation about the byssal axis. An additional line of evidence is the very low frequency of upward-facing left valves of GervilliaA on the 185

bedding surfaces. In the absence of evidence for post-natal current disturbance, this is taken to indicate the stable position of rest

(heavy left valve down) of tethered inequivalve form. This could also have been the life-position, but to grant this does not detract from the conclusions reached above.

Modern Pterias given by Grasse without data on habitat are

P. scopali, P. hirundo, P. macroptera and Pinctada margarifera. These are ail form allies of Pteroperna, as is Meleagrina, both this and

Pteroperna being sub-genera of Avicula (= Pteria). Gervillella is perhaps represented by Crenatula; a form known to live among sponges

(Lankeste'r, 1906; Piveteau, 1956). Hedly (op. cit.) observed two species in Funafuti lagoon of which Pteria peasi lived attached in great numbers to the branches of Plexaura antipathes which lived in

2-3 fthms of water.

Meleagrina is a shallow water sub-genus (8-20 m) (Piveteau,

1956), while Pteria hirundo lives offshore to a considerable depth.

Thorp, E.M. records Pinctada from an unspecified position in the lagoon in Pearl and Hermes Reef.

Cadee (1968) found F. hirundo in the Central Bay to Oceanic environments. It was probably not considered an important element of this assemblage since he did not discuss it in the text. But evidence in the Central Bay indicates oxygen deficiency at the bottom (through over-consumption) and low current velocities at the bottom with high carbon up to 7% and black muds smelling of. H2S. . Depth 30 - 60 m.

The Challenger found four species of Meleagrina at less than

100 fthms. Crenatula would probably be within the photic zone as would most of the sponges (for. symbiotic reasons) among which it dwells. Ellison (1951) considers 50 m to account fof 95% of the incident illumination. 186

Considering the fine ciliation of the Pteriacea (Newell (1966)

after Atkins) the absence of coarse suspended load may be suggested for

modern as well as ancient Pteriids.

The overall evidence for modern representatives is however too

scanty for one to be firm about 50 m as a maximum depth for the beds in

which the ancient forms occur. In the fuller ancient record, Webber

(1968) considers Liassic Gervillella lanceolate to be associated with

deeper water sediments with framboidal pyrite indicating to him a fair,

not good, aeration. The association here is with flat oysters, Pinna,

purpuroid gastropods And one pectinid. He suggests a stable clay

bottom,a low rate of sedimentation and stable environmental conditions,

all of which may be applicable here. Farrow (1966) associates Ger-

villella with pure massive limestones offshore from shallower water

trace fossil assemblages in impure and gritty limestones, an observation

that is very striking in its resemblance to the bio-, intrasparite

-- Thal/assinoid casts horizontal burrows + Gervillia upward sequence

observed at Creysse in the basal Gluges Calcilutite Unit 3. This

succession is thought for independent reasons to deepen towards the

beds with Pholadomya at Meyronne (also see under G. acute). In the

absence of evidence of condensation, it is thought that the very small

rock thickness between intrasparite and Gervillia bed may suppbrt the

maximum depth of 50 m suggested, if the intrasparite matched with

Farrow% Grits is assumed to be not much deeper than low-water mark.

Farrow observed Pteroperna plane to form shell banks, while Meleagrin-

ella is associated from an examination of his logs and text with Astarte

minima in sandy flagstones, Shelly shales or fine shaly ironstones,

bituminous shales- - finer bottom than Pteroperna. The association with Astarte/ which live at about 30 m in the modern North Sea (Boeek-

schoten) may indicate a similar depth for Meleagrinella. Takai (1963) 187

considered Bakevillia to be estuarine and associated with Eomiodon and

bituminous shale. Russell (1943) considers Gervillia and Oxytoma (a

costate Pteriid) to occur in fine-grained near-shore sandy facies

probably partly washed in from offshore. Gignaux (1950) associated

Avicula (Pteria) with sandstones and detrital limestones and invaria-

bility that is rejected here for Pteroperna as for Pinna. He and other

Continental authors often associate Posidonomya (=? Steinmania) with

black shales. These thin-shelled pteriids are sometimes thought to

be pelagic. The latter assumption is not accepted here but the implied

foetid bottom is. Ferm and Williams (1968) associated Aviculopecten with a brackish environment with Linqula. Some Lingula-shaped casts

are indeed found with the Pteroperna's at one level and variable subnormal salinities are quite possible, though the main dominanace of

Pteroperna is attributed to falling oxygen supply.

In summary, the ancient Pteroperna's may have lived in water

less than 30-50 m deep on a bottom with suitable objects of anchorage but with a variable average grain-size. Oxygen supply could have been low to high, but the local dominance of Pteroperna particularly in a carbonaceous matrix is taken as sufficient evidence for reducing conditions. On the evidence of the apparent advantage of epifaunal

Pteroperna over siphonate types dependent on oxygen supply above the bottom, reducing conditions probably occurred above not below the sedi7 ment/water interphase. This and the prior postulates of an absence of coarse suspended load lock the argument for a reduced water circulation at depth, as in Cadee's (1968) Central Bay of the Ria de Arosa.

Parker, (1959) gives 9-30°/36° for comparable environments in the

Laguna Madre. This range covering also the 11-17°C of Cadee is below the values associated with penecontemporaneous dolomites, gypsum and halite, whose absence at the levels with Pteroperna confirms the likelihood of the range 9-30°C. Gervillella acute while similar to Pteroperna 188

in its depth and bottom requirements, probably preferred better aeration

and stable salinities. G. late was probably the most marine of the

three, also able to stand higher current velocities.

Superfamil PECTINACEA, Rapineque 1815

This is one of the most uniformly represented superfamilies

in the entire succession, rivalled only by the Ostreacea. The families

Oxytomidae and Aviculopectinidae of Newell (1965) have been discussed

under the Pteriidae for ecologic and morphologic reasons borne out by

the inclusion in the Pteriidae of constituent genera of Newell's fam-

ilies by Piveteau (1956). The genera described below belong to Newell's

Pectinidae and Entoliidae, but the grouping adopted below ignores this

because of the indiscriminate use in the literature of the generic

names Pecten, Chlamys, Entolium and Amissium The somewhat informal

and sceptically defined subgenera of von Zittel's Pecten are in fact

the basis of the grouping that follows. Together they accommodate all

the species except one.

16. Chlamys cf. stricta MUnst [Pecten Roederi Loriol]

Cossmann 1907, P1. II, fig. 5 and 6; Lorio l 1900, P1. VI.

Diagnosis and Remarks: A small pectinid whose ribs curve outwards

slightly instead of straight as in Cossmann's type. The fine growth

lines of that type are also absent here. The narrow, deeply notched. auricle, the regular external ribbing and the very acute-angled umbones make it very distinctive locally. Pecten roederi of Loriol may be a synonym. Very salient internal ridges on the specimens place

them close to Amussium-

Occurrence: This species occurs for the first time in the top beds of kluges Caicilutite Unit 4 as a finer ribbed form, but truly comes 189

into its own from the Blagour Breccia, up to the St. Etienne Limestone.

It is abundant in the chalky micritic oo- and pelsparites, occurring as such in the Blagour Breccia Unit 1 and the Souillac Oolite/Lacave

Calcilutite Unit 1 transition. Judging from the absence of other pectinids in calcilutite lithologies where C. stricta is patchily present, this form appears to be the most tolerant pectinid above the Gluges

Calcilutite. It was probably also more vagi•le than the others.

The positive casts rarely show a complementary valve which may have been transported, but it is not possible to tell the ratio of right/left valves because of the near equivalve symmetry and the difficulty of extracting the delicate auricle intact. Pectinid shells are partly aragonitic and dissolve more rapidly than those of Ostrea which are calcitic.

213. Camptonectes rigidus (J. Sow.)

Morris and Lycett 1853, Tab. I, fig. 18.

Diagnosis and Remarks: The outline and geometry of ribbing and size are identical with the specimens of C. cf. stricta from which they differ only in the tendency of the ribs to bifurcate near the posterior border and in the absence of the internal ridges. It may be the prototype of C. cf. stricta of the Blagour Breccia, and suggests the only evolutionary development across the sub-Breccia break.

P. arcuatus Morris and Lycett is a possible alternative.

Occurrence: The cast of the left valve with only the salient posterior auricle was found in the marly Gluges Calcilutite Unit 4 with Vivi— parus together with smaller unidentifiable bivalve casts. Large specimens occurring in the Gryphea-Ammonite bed)! at Les Courtils are placed here, although they could confound them with the smooth E. demissum.

19 0

131. Chlamys (Radulopecten) cf. hemicostatus (Morris & Lycett)

Morris and Lycett 1853, Tab. I, fig. I6a.

Diagnosis and Remarks: From the description of Morris and Lycett,

this could be the young stage as it lacks the strong costae attributed

to the adult. The shape is a 60° circular sector (ovately orbicular)

with an exaggerated curvature of the ventral line. The sloping lateral

lines being equal in length and merging insensibly with the ventral line

at about half-way down the height. Two prominent growth lines define

a narrow ventral area with very faint minor growth lines, a wide middle

area also with very fine minor growth lines and a dorsal area partly

covered by very regular, more distinct growth lines. Ribbing is fine,

moderately spaced and, straight.

Occurrence: The form is very rare - one occurrence in the fossil-

iferous Lacave Calcilutite Unit 1 at Souillac Auto Gare, where the dis-

tinct ribbing distinguishes it from the also abundant Chlamys fibrosus.

26. Chlamys cf. subfibrosus, d'Orb [C. fibrosus Sow., C. vagans, Sow.]

Phillips. (1895) P1. VI, fig.3; llovaisky 1903 P1. VIII, figs. 14,15a,15b.

Lanquine 1929-35, P1. X, fig. 8.

Diagnosis and Remarks: This form which is very common as disarticu-

lated valves in the lower Lacave Calcilutite Unit 1 is probably the form

identified at this level as P. fibrosus Sow. by Glangeand. The present species however has unequal auricles which are rarely preserved and ob- struct its right diagnosis as C. subfibrosus. C. vegans Sow. of Lan- quine and Ilovaisky's subfibrosus are nearly identical but both have delicate regular growth lines extending to the umbones as opposed to the

restriction of regular distinct growth-lines to the ventral area in the present specimens. There are also more plications (20) on these than

the few (rk, 5) of C. subfibrosus of those authors. The smaller, pre- 191

sumably younger specimens are more acute than the adults.

Occurrence: The shells are always preserved but partly rendered

chalky. Th._ species is limited to the lower Lacave Calciiutite Unit 1

and Upper Souillac Coolite, except for its patchy occurrence in the

chalky burrowed bed of Lacave Calcilutite Unit 3. The strong facies

control ensures its local usefulness as a guide fossil.

Camptonectes rosimon (Coss. ex. d'Orb) 159.

Morris and Lycett,1851-5Tab. I, figs. 17, 17a.

Diagnosis and Remarks: One of a group of small pectinids occurring

in the lower beds of the Mirandol Oolite down to the Gryphea-Ammonite

bed. The umbones are extremely sharp, and the anterior auricle

is broad. The posterior auricle is either absent or not recovered.

The shell is covered with an overlapping crowd of fine growth lamellae.

The lateral lines meet the anterior border lower down that in the ori-

ginal figure, and the junction between the two is also gradual, not as

abrupt as in the type. A variety occurs in which the anterior ear is

narrow and long and the umbones facing slightly anteriorly. This is

designated sp. No. 189: Chlamys vitreus , Roemer.

189. Chlamys vitreus, Roemer (var, C. personnatus?)

Ilovaisky 1903, P1. VIII, fig. 13.

(See above).

14. Camptonectes lens, Sow. [Pecten lorieriannus, Cotteau]

Cossman 1900, P1. VI, fig. 11; Peron, 1906 , P1. II, p. 2.

Diacnosis.and Remarks: This species is not to be confused with

Chlamys var. stricta in which the sloping lateral lines meet the ventral border much lower in that species, while in C. lens the ventral curva-

ture nearly completely usurps the lateral lines though the overall

192

pointed shape is retained.

Occurrence: The form occurs as a cast only in the Lower Gluges

Calcilutite Unit 3 and doubtfully in the Lower Lacave Calcilutite Unit

1. The large specimens of C. var stricta should be distinguished by

the greater height of the apical area and straight lateral lines from

C. lens. The patchy vertical distribution allows no deductions to be

made from local associations only.

157. Entolium (Pecten) demissum, Phil.

Phillips 1875, P1. 6, fig. 5.

Diagnosis and Remarks: The figure on P1. XIV of Morris and Lycett is

rather more pot-shaped and squat than either the present specimen or the

original figure in Phillips. The subequal pair of auricles is pre-

sumed present though rarely recovered.

Occurrence: This species clearly dominates all the others occurring SIselkt_ in all the units of the Floirac Bcdo as platy fragments and as whole

disarticulated valves. This coquina was observed in a shale intercala-

tion in the top Gluges Calcilutite Unit 2 with Lucina and occasional

valves occur in the burrowed pelsparite and Gluges Calcilutite Unit 1 at

Gluges.

22. Hinnites

Diagnosis and Remarks: A faintly and irregularly ribbed species

with irregular growth lamellae and a pair of subequal auricles. The

generic diagnosis might have been Chlamys on account of the well-

developed growth lines which are rare on Hinnites. Theivalve is also

remarkably free of distortions. Andreeva identified a similar form as

Entolium demissum as a subordinate form but the author sees no resem-

blance et all with Phillips' ficure. 193

Occurrence: It occurs in the upper passage of the Souillac Oolite together with other pectinids.

26b. Veleta tegulata, Morris and Lycett

[Hinnites cornueli, Lor.]

Morris and Lycett 1851-5, Tab. II, figs. 3 and 3a, Cossmann 1906,

P1. I, figs. 3-5.

Diagnosis and Remarks: The species is very similar to Sp. No. 159

Chlamys personnatus in outline but the -equal auricles and the ribbing togethei- with the arcuate depression near ventral border on some speci mens all are peculiar to V. tegulata. The ribs are stout and widely spaced with a series of fine secondary ones in between. The ribs are either curved or straight and faint crenulate growth lines are present on older specimens. The type of Morris and Lycett has no secondary ribs.

Occurrence: The form occurs as casts with or without an adhering nacreous coating in the chalky upper Souillac Oolite passage and chalky algal grainstones or Lacave Calcilutite Unit 2 but is most common in the comparable facies of the St. Etienne Limestone.

161. Hinnies sp. V. tegulata ?

Diagnosis and Remarks: This form differs from V.tegulata in the clearer secondary costae and the lack of crenulation on the growth lines.

Occurrence: It occurs in the clean Mirandol Oolite, undissolved single valves in varying degrees of fragmentation. 194

TAB. 4: Summary Partition of Species of Pecten into Subgenera: m c = m 1-,-

0 c . s_ M >,- E

C l. Q) E1)

0 --o l—,

c . e mar

_c ra cn C) s

+J C) /p .w o ,:: 4-, 4-7 ite. enou o

N — (o r — ite. ig 0 -- c.. a .- — o 0 U arry m 0 o icr &p Oosp m V) 0....0 c) Terr I

P. (Chlamys) 16. stricta . tolerant

213. var. stricta

131. hemicostatus int-"?. 6 26. subfibrosus int ? Sub.Mir.Ool, 159. personnatus marker bed 189. vitreus H

P. (Camptonectes) 14 lens 1 int.-a.

P. (Entolium)157. demissum 1 f.t.

P. (Hinnites) 22. sp. S.Ool.,int? not so clean grainst. 266. tegulatus 3 clean Mir. Ool. 161. var. tegulatus H

P. (Amussium)160., pumilus

Smooth Costate pectinids pectinids !95

160. Amilssium (Pecten) pumilus

Lanquine 1929-35, P1. V, fig. 7.

Dimensions: L ;.05, H 1.07, T?

Diagnosis and Remarks: A very sma]: species with a remarkably thick shell and stout internal ribs showing faintly through the smooth shell whose faint external striae are not coincident with the internal ribs.

The growth lines are extremely faint. The present specimens are narrower than the figure in Lanquine (1929-35).

Occurrence: The species is commonest in the clean upper unit of the

Mirandol Oolite.

Ecology of Pectinacea:

Table (4 ) shows the pectinid species as they fall into the informal subgenera of Von Zittel (1500). An examination of the lithologies most frequented by individual species shows some conformity with the presence or absence of costation and less obviously with shell thickness. Thus Entolium demissum and the smooth chlamyids C. personnatus and

C. vitreus occur from offshore shales with Ammonites to shallower-water

impure shell packstones overlain by coralline and grainstones. E. demissum is probably the most tolerant of the three, occurring also in what appears from other evidence to be outer barrier lagoon-type pelsparites and gypsiferous calcilutites as well as in shale intercalations in gypsiferous calcilutite. These latter shales in the Gluges Calcilu- tite Unit 2 may thus be episodes of a drop in hypersalinity. All the other pectinids except C. var.stricta are ribbed and occur with a consistent separation from the smooth forms along the ooid grainstone/ pisolith and shaly shell packstone line. The lithologies associated with.the ribbed forms are, from the general setting of the succession and from other lines of evidence, petrographic etc.,•are barrier lagoon- 196

type and are arranged roughly in the order of their proximity to a barrier oolite shoal. Cn this basis 8 out of the 9 ribbed forms

(usually also small), not counting C. var. stricta occur within the barrier and the lee of its transition to the calcilutitic central

lagoon. Population analysis in relation to the depositional setting

is treated elsewhere, but it is clear that the sharp drop in species from the barrier and outer lagoon to the inner lagoon, observed in the

Persian Gulf (Evans 1966), is well illustrated by the 9 pectinids.

For reasons of better aeration and more normal salinities as well as from modern observations of the frequency of shelly accumulations at the lee of coral knolls in the Persian Gulf-type environments, this pattern of distribution is a clear reflection of comparable modern environments. The more costate forms, A. (P.) pumilus (internal costae) and Hinnites var.tegulatus (better-formed secondary costae) may be separated from H. tegulatus (faint secondary costae), H.

Chlamys subfibrosus and Chlamys hemicostatus which are weakly costate.

The first group occurring in barrier-type oosparite and the second in a transitional lagoon-type lithology. Chlamys•stricta probably has a high salinity tolerance. The restriction of the thick-shelled

A. pumilus to oosparite is to be noted.

Modern pectinids live byssally attached or free for the whole or part of their lives according to species. Most are capable of swimming, and are thus able to avoid predators and adverse environmental conditions. The vagile character and the ease of post-natal transport (particularly of the smaller species) introduce some error into the depth range of pectinids obtained by trawling. The data on modern pectinids is given with this in mind.

The Challenger found pectini'ds up to 2000 fthms but the sharp rise in number of =ucies starts at 100-500 fthms, probably nearer 100 than 5CO. L':a from 20 modern species gives about maximum 197

species overlap from subtidal 0 - 90 m (19 species), an overlap of

10 species from 90-180 m and an overlap of 5 species from 100 m. down. see smoothed depth-frequency curve (ifig. 10 ) gives a negative skew favouring depths 80 m or less in accord with the 0-90 m obtained from the rough approximation. Since most of the limestones in the succession studied appear to come from depths less than 30-50 m, and no offshore calcilutites (thought by Hallam to come from 4,150 m) are known, the range 0-80 m would account for all the lagoonal and barrier limestones and the offshore shales of the Floirac Beds but satisfactorily exclude the offshore calcilutites (of Hallam).

Data from Tebble (1966) seems to favour various sandVbottoms

(clean, fine, shelly or muddy). This range would suitably exclude pure (presumably deep below 80 m) shales (non-shelly) with Ammonites but is otherwise not of predictive value as the lithologies associated with pectinids as well as all other fossils can be directly observed.

Because of the great diversity of pectinids, raw data on modern ones unrelated to some morphologic subdivisions of them

(pectinids) is hard to apply directly. Degree of costation, for example, is found to be environmentally sensitive by the author, and.

Doust, H., 1968 (also see below) made a major use of smooth and ribbed subdivisions applied to all his bivalves.

Salinity tolerance in modern pectinids is probably closely related to depth as most of the forms which frequent shallow water

(intertital: to about 20 m) are able to stand lowered salinities, and presumably a few could stand hypersaline conditions in shallow restricted bodies of water. Cadee (1968) gave 530/00 with winter dilution for his marginal zone (depth 5-5m - 20 m) in which Chlamys varia and C. opercularis occur, while Parker, H. (1959) gives 5-420/oo and 25-60/oo° for two geographic locations in which Aequipecten 192

irridans amplicostatus occurs in depths of 1-5 ft and 1-3 ft respect-

ively in the Laguna Madre area. The relation of salinity tolerance

to depth preference is obvious. It is not clear from these two works

which morphologic feature apertains to shallow with low salinities and

shallow with high salinities. C. varia and A. irridans amplicostatus

are both small (3 cm high) and coarsely ribbed. Bottom and salinity

tolerance deduced for C. stricta is supported by Cadee's observation

on C. opercularis.

A good water circulation to ensure food is generally thought

to be important for pictinids, but this is not peculiar to them only

and can be deduced in the ancient sections by a general rise in the

diversity of bivalves, not withstanding the occurrence in Liassic shales of giant pectinids and limids such as Lima gigantea.

In ancient environments, effort has been made to relate size of pectinids to habitat. Takai (1963) considered small Pectin-

ids to frequent deeper offshore waters (with Ammonites) than large

ones. The former he also associated with shales and the latter with coarse sandstones. Size is not thought to be critically related to

depth here and Entolium demissum appears in fact to be a deeper water form than the smaller pectinids. But in view of the possibility of susceptibility to easier transport from greater depths of the smaller ones which would explain their greater abundance in a barrier environment, Takai's observation is not ruled out here. Hedley (op. cit.) mentions four species of pecten in Funafuti lagoon (30-50 m) but the

limited depth range does not enable the lower limit to be ascertained.

Hinnites was observed attached to sheets of dead coral and to the brachiopod Thecidea maxilla. There may be some analogy in this with the association with calcareous algal pellet limestones observed by the author. Webber (op. cit.) thought that smaller individuals were in shale rather than limestone, implying, that shales were unfavourable, 199

but that his species clearly preferred near-shore conditions where currents were not strong enough to stir up large intraclasts. He cites Hallam as considering one pair of species, Chlamys subulata and

C. valoniensis as bathymetric equivalents, but no attempt was made to abstract from them morphologic differences that could be tried on pectinids as a whole. The feasibility of such a procedure is undoubted by the author, particularly as the group is quite widespread.

Webber's small individuals in shale were probably influenced by bottom toxity rather than depth. Doust, H. (1968) thought that the euryhaline

Chlamys was more widespread than the stenohaline Pecten and Flabelli- pecten, but his symbolic sketches show no morphologic difference, except a closer ribbing on Chlamys, and one is not sure here,as in other texts, what the basis of the generic appellation is. Allowing for this, the coincidence of ribbing with greater ability to penetrate shorewards fits into the present data on the costation/bathymetry and salinity relation, as it applies to a greater number of consistently determined subgenera of Pecten.

Summarising, the consistent subdivision of ancient pectinids into subgenera may be an aid to the better appreciation of the segregation into costate forms and smooth forms with notable ecologic preferences. The costate forms preferred coarse bottomed agitated environments and the quieter but well aerated area immediately to the lee of them. In these areas they were probably associated with corals or coralline algae. Depths probably ranged from near 0 to perhaps

20 m and salinities normal to above normal, depending on habit, the more vagile ones being able to move in the direction of even brief salinity changes, not reflected in the petrography of their present matrix. Texture of substratum was less important than depth, salinity, and energy as less vagile forms (groups already discussed) appear al-. ways to find one thing or other to cling to in otherwise pure calci- 200

luties, whereas the pectinids except Chlamys stricta are virtually completely absent in these sediments. Increase in costation may be used to delimit the outer lagoon from the barrier proper. Smooth pectin-

ids such as Entolium demissum preferred,on the other hand,an offshore environment with a greater bathymetric range of 20-50 or 80 m from where they are transported on-shore with considerable fragmentation.

Matthews, W.H., 1951, gives Pecten duplicosta as probably being washed into rudist reefs after death. Salinities here were normal marine although some penetrations to the outermost lagoon was possible.

Currents were strong enough to orient the valves flat (rather than angular) on the bedding or lamination planes, after death,,and ensure uninterrupted aeration.

Superfamily LIMACEA, Rafinesque, 1815

Family Limidae, Rafinesque, 1815

Limids are poorly represented in numbers and species. Six are described below.

209. Plagiostoma harpax, d'Orb [P. hellica]

Cossmann, 1906, Pl. 1, fig. 12-16; P1. II, fig. 1-2.

Dimensions: L 2.0, H 1.65, IT 0.5.

Diagnosis and Remarks: A small very expanded inequilateral species with the umbones markedly forward of the bisector of the ventral border, and slightly facing fowards. The leading and posterior dorsal sides are 0.95 and 1.4 cm respectively. Ribbing is very close and the Branco sulci are simply notched as in L. (P.) schimr.eri ' of Morris and Lycett

(Tab. XV, fig. 9a, 9b).

:201

Occurrence: One valve was found in the pale-coloured calcilutites

of Lacave Calcilutite Unit 1 near Le Bongayrou together with Terebratula

subsella, and a gently gaping whole specimen at the same level a few

feet above fossiliferous basal Lacave Calculutite Unit 1.

76. Lima (Plagiostoma), cardiformis, J. Sow.

Morris & Lycett 1850, Tab. III, fig. 8,8a.

Dimensions: L ?, H 2.9, T ?

Diagnosis and Remarks: The form is incomplete and identification is

imprecise but it can be distinguished from the other local species by its

resemblance to L. (P.) cardiformis.

Occurrence: One valve from near the top of the St. Etienne Limestone

in an algal oncolith calcilutite towards Le Roc from Souillac, occurring

with Mytilids.

94. Limatula rauracia, Cossmann

Cossmann 1907, P1. 11, figs. 12-13.

Dimensions: L 1.95, H 3.4, IT 0.85.

Diagnosis and Remarks: Ribbing fewer than in Cossmann's type about

6 and > 18 respectively, for the specimen and Cossmann's figure.

Occurrence: The shell has been removed by solution and the anterior

auricle may be lost in extraction. One valve was found in fine algal

pelsparites in the Upper St. Etienne Limestone. between Souillac and

Terregaye.

66. Limatula cf. corallina, Arkell

Arkell 1929-37, Pl. XII, fig. 6.

Diagnosis and Remarks: A finely ribbed form with very faint growth

lines. The auricles are not preserved. . it may be distinguished from

by the arcuate ribbing of the former. 202

Occurrence: One valve was found in micritic chalky biopelsparites

8 m below the top of the St. Etienne Limestone, together with small oysters,

Lucina, Pinna (fragmented) among others.

100. Pseudolimea sp.

Dimensions: L 1.85, H 1.9, IT ?

Diagnosis and Remarks: The outline is like some Limas and like them

the posterior dorsal line is longer than the anterior but is turned outwards to give a slight flair like an auricle. The umbones are shifted and turned anteriorly. Four to five stout ribs alternate with a pair of fine secondary ones.

Occurrence: Occurrence as for Limatula rauracia.

Lima sp.

Dimensions: L 3.65, H 3.9, T 1.0

Diagnosis and Remarks: Outline as in specimens of Plagiostoma harpax d'Orb but ribbing very coarse as in Ctenostreon, probably irregularly knotted also. Posterior dorsal line gently concave outwards unlike P.

harpax. Shell thick.

Occurrence: One articulated closed specimen was found in yellow ferruginous, coarse ooid grainstones in the Mirandol Oolite.

Ecology of Limacea:

The immediate evidence is scanty but all the species described appear to favour a coarse bottom with varying mixtures of calcilutite.

The small size of the two specimens of P. harpax may support their abhorrence of a fine bottom rather than that they are selectively transported young as they are still tightly closed. There appears to be a greater frequency of closed valves in this group than in the 203.

Pectinacea from a comparable or identical environment. This is attri-

buted to the ability of the Limids to build a protective nest.

Modern Limacea are represented by Lima liens, L. loscombi,

L. (Limatula) sulcata, L. (Limatula) subauriculata, Lima pellucida among

others. Limids appear to show the same sharp increase from 100-500

fthms shorewards as do the Pectinids, but persist to greater depths

than the latter in the data compiled from the Challenger results.

The character of the bathymetric percentage frequency curves

of Limids and Pectinids are also subtly different. The peak frequency

occurs at 10 metres deeper (i.e., at about 70 m) than in the pectinids,

while th'e tendency to flatten towards greater depths contrasts with the

steady drop in frequency for the pectinids. The shoreward spread of

the curve in the pectinids is on the other hand greater than in the

Limids. It may be concluded from 'the curves that the optimum depth

range for modern Limids is 30-120 m, the optimum single valve being 70

m, and that a few forms occur which outstrip the majority of Limids in

their ability to persist to great depths, as interpreted from the posi-

tive flattening of the frequency curve. A way (not yet found by the author) ought to be found for allowing for the possible presence of such forms in the ancient record, if direct extrapolation from modern ones

is to be precise. Limatula's appear to have a much greater depth range than the LimA (from data of Parker (1959) and Cadee (1968)), but only partly account for the shoreward flattening.

Without multiplying sources of evidence, Cadee (1968) (Appendix

pp. 118) and Parker in the Gulf of Mexico, readily show the bottom and salinity tolerance of modern limids, the former showing their confine- ment to the 'ocean' zone in obvious preference of a coarse bottom and a normal stable salinity in a setting showing the whole range from clay to coarse sand bottom and fresh to normal salinities and the latter author's data indicating the same selectivity for Limatula suicata. On 204

the recent evidence therefore, and in the absence of a change in the

habits of Limids with time, the coarse bands in the Mirandol Oolite and

the St. Etienne Limestone may be interpreted as fully marine to hyper-

saline bars and fully marine to hypersaline barrier respectively with

varying degrees of winnowing, but always with good aeration.

The ancient evidence, unlike the recent, shows exceptions

to the coarse bottom (sometimes with corals) preference of limids

(large ones specified by Takai 1963). Lima gigantea has already been

cited as occurring in shales in the Dorset coast although Webber (1968)

considers it to prefer a calcareous sea floor. Without benefit of a

more ri'gorous analysis of modern data, he did admit that the widely

variable substrata] preference made the ecology of limids difficult, but that they were (at least L. gigantea) unaffected by all but the

highest rates of sedimentation. The last point is in accord with the

author's observations, although the limids are, in fact, microcilio- branchiate (Atkins, 1936-8).

Superfamily OSTREACEA, Rafinesque 1815

Two families are represented, the Ostreidea obviously dominating the Gryphaeidea. Ostreidae occur throughout the succession, but because of the strong control of bottom microtopography on oyster morphology, fewer or more species may have been identified than are actually present. The description of identifiable species is followed by a discussion of the morphologic groups 'cupped' and 'flat' oysters.

53. Lopha 9regarea, Sow. [=? 0. pulligera, de Lor.]

Arkell 1929-37, P1. XXII, fig. 5.

Diagnosis and Remarks: The specimens are strongly ribbed radially on both valves and the margin is serrated, but Arkell's diagnosis of

205

right and left valves, based as it is on the facing of the umbones

gently arcuate or sickle-shaped specimens could be misleading in strongly

coiled specimens with incurved umbones, such as the present ones. It

seems more consistent to define right and left valves from the facing

of the outer terminus of the coiling axis. The result is the same

but the definition would be more consistent. Arkell's text and list

of synonyms indicating the range of variation of species were probably

similar to Lanquine's (1929-35) reasons for considering specimens with

a straight axis as Alectryonia gregarea and those with a curved axis as Heligmus (now Eligmus) polytypus Des. It seems more consistent to

refer L: gregarea, specimens with the shell geniculated near the ven-

tral margin and to E. polytypus those which have flattish ribs, with an ovate or flat and non-geniculate shell.

Occurrence: The shell is always intact and occurs in all grades of sediment from the Blagour Breccias upwards, but obviously most common in the pellety oosparites and boundstones of the upper St.

Etienne Limestone and Lower Lanzac Oolite. In the Souillac Oolite and Lacave Calcilutite,they are characteristically disarticulated.

They were probably transported from near-shore shoals into the Souillac

Oolite which also has more intraclasts than the Lanzac Oolite, and many have been a more mobile barrier. Valves in the Calcilutite may have been due to washover into a lagoon. Some L. gregarea in the

Blagour Breccia% may be Alectryonia asellus, Mer. of Lanquine, but no consistent identification was made.

Eligmus polytypus, Desl.

Occurrence: A form recovered subsequent to the completion of quantitative studies in the calcilutites of Lacave Calcilutite Unit 1 together with a great variety of bivalves and brachiopods. It is. 206

probably indigenous in this member.

187. Lopha costata, J. de C. Sow.

Morris and Lycett 1805, Tab. I, Figs. 5,5a.

Occurrence: This very small (barely 1 cm) constate species forms

laterally extensive coquinas 6 inches or less thick in between undulose rippled caicilutites in Gluges Calcilutite Unit 3 and Unit 4. Small gastropods, rarely nerineids, small smooth oysters and echinoid spines occur with it. The coquinas are interpreted as temporary pauses in sedimentation and the development of current-swept hard-grounds.

Salinities probably ranged from normal to high.

187b. Liostrea cf. herbridica, Forbes

British Museum.

A very small form in the beds with Pholadomya in Glugest Calci- lutite Unit 3, showing considerable inclined or vertical 'stacking' and thickening of the cemented valve is identical with a display specimen in the British Museum but the figure by the same name in "Britih

Mesozoic Fossils' (2nd Edition) is in fact different and rather identical with 0. sowerbyi, Morris & Lycett, which is considered in this publication to be a synonym. The author also has a specimen identical with O. sowerbyi of Morris and Lycett, but in the absence of any local evidence of possible . intermediate types between the markedly different local L. hebridica and O. sowerbyi, it is probably best to keep the two species.

The species is rare and occurs only in Gluges Calcilutite

Unit 3. 2.07

75. Ostrea sowerbyi, Morris & Lycett

Morris and Lycett 1805, Tab. I, figs. 3,3a.

Dimensions: L 1.8, H 2.15,

Occurrence: One cemented valve identical in shape with the type

figure was found in very pebbly poorly sorted lutitic wackestones near

the top of the St. Etienne Limestones. However there is nothing in

the morphology of O. sowerbyi of Morris and Lycett to indicate a cemented mode of life, hence the query.

138. Ostrea cotyhedon, Contegeau

Mem. Soc. Linn. Normandie T. XV, figs. 27, 27a - Lorioi, 1872.

Dimensions: L 1.85, H 2.1, T ?

Diagnosis and Remarks: The transverse corrugations and upturned distal edge are quite characteristic..

Occurrence: It is fairly common in the beds with O. sowerbyi, Morris and Lycett.

20. and 108. Ostrea sp.

Diagnosis and Remarks: These are two small cupped oysters that could be one species modified by the substrata] microtopography. Sp. 20 has a convexity in the centre of the cup and is larger, while spc. 108 is smaller and not distorted. If sp. 108 consistently sought out a slightly raised object or irregularity for settlement, then the differences between the two could be upheld. No decision can be made on the two possibilities. The two occur in the same beds in the St. Etienne

Limestone. 202

11. Ostrea delta ?

Diagnosis and Remarks: A thick-shelled fragment occurring in the

upper Lacave Calcilutite is tentatively assigned to O. delta.

9. Gryph7a bilobata Br. Geol. Survey Collection (K.1296 ) Dimensions: L 2.5, H 2.75, - 1.6

L 3.0, H IT 1.6

L ? H 3.7, IT 1.65

Diagnosis and Remarks: The specimens have an irregular concentric

ornament which is impaired by extraction from the indurated matrix of

reddish-grey mottled intraclastic oomicrite. The only variation in the

two complete specimens is the extent of the sulcus towards the umbo of the left valve which is more strongly arched than in G. dilatata J. Sow., a form from which it must be distinguished for reasons of stratigraphic zonation. The posterior lobation is always present unlike in G.lituola.

G. dilatata apart from its great range of variation is said by Arkell to have a height 90% - 106% of the length; these compare with 110% for the present specimens of G. bilobata. The less strong inflation is also shown by the inflation/length ratio of 20-50% for G. dilatata and

64% and 53.3% for these G. bilobata. Equivalent ratios for G. feituola confirm the visual differences from G. bilobata, the figures being 125% to 150% and 70% - 90%. Both sets of figures are well outside the range for both G. dilatata and G. bilobata.

The author has used this species as the index for his Callo- viense zone in the Lacave Calcilutite, on account of its reliable identification and its limited vertical range both here and in the British

Callovian. The form is thoughtto be a better index than the still poorly studied rhynchonellids traditionally assigned to S. elegantula.

209

The species occurs in the coarse burrowed oomicrite of Lacave

Calcilutite Unit 3 with terebratulids, rhynchonellids, Arcomytilus and

Cardinia. The hard-ground character of the horizon probably favoured

colonization by Gryp6ea.

Gryphea beaumont i

Occurrence: This strongly curved narrow form occurs in great abun-

dance at the Aalenian/Toarcian junction (see section on Stratigraphy).

Lift valves are always overturned, convex upwards indicating strong

current activity acting over a firm bioclastic bottom into which the

oysters presumably did not sink very much in life. The great abundance

of ammonites and belemnites clearly indicates a fully marine offshore

environment.

54. Exogyra nana, Sow.

ArkellS27-4P1. XVII, figs. 2-2], Pl. XIX, s. 4.4a,

P1. XVIII, figs. 3-11.

Diagnosis and Remarks: The diminutive species is identified on

local sheets as Ostrea bruntrutana, which Arkell considers, with the

author's complete agreement, to be synonymous with E. nana, Sow.

There are two depressed areas in the left valve, which if they are both

adductor impressions, would contrast with the one in ExogyraX, other-

wise the specimen is identical with Arkell's figures.

Indeterminate Oysters

Oysters probably non-ribbed ones form thin centimetre-laminae

in every formation. In Gluges Calcilutite Unit 3, small very shallowly cupped oysters occasionally occur in great abundance on fissility planes.

Some still have their free valves indicating very gentle currents. 210

Compared to the L. costata coquinas from the same locality as well as

elsewhere, these probably developed in a quieter subenvironment.

Serving partly as a diagnosis of the highest members of the

Lacave Calcilutite are extremely frequent centimetre shell-laminae

largely of small (1.3-.4 cm) oysters the cemented valves of which are

spirally twisted with incurved umbones. In section, the floating

valves (which dominate) are strongly contorted. Fissility planes are

given a nacreous coating. Small size in oysters was attributed to a

high rate of sedimentation by Imlay (1957) Selective transportation

of young free valves would be in keeping with the fragmentary pectinids

and sedimentary structures that also occur here.

In the Lower Mirandol Oolite, Upper Lacave Calcilutite Unit

1 and the St. Etienne Limestone, small patilliform oysters are very

common at occasional intervals. One such form, large enough to make

collecting worthwhile, occurs in the top St. Etienne Limestones. It

seems to be a new species; the author knows of no published figures

similar to it.

Ecology of Ostreacea:

The species described may be conveniently partitioned into

(1) Ribbed Oysters: (a) Flat - Eligmus polytypus, O. costata;

(b) Cupped -Lopha gregarea.

(2) Non-ribbed Oysters: (a) Flat - Ostrea sowerbyii; (b) Cupped

Liostrea herbridica, Ostrea sp. 20, Ostrea LE. 108, Ostrea cotyledon,

most small indeterminate forms.

In general, flat and non-ribbed oysters are rare, while flat.

ribbed forms such as O. costata form exclusive bands at a few levels and less abundant but equally transient was Eligmus polytypus. It i.s concluded that these forms were highly gregareous and did not occur at all if the environmental conditions did not fall within narrow limits 211

required for a certain minimum density of spat settlement. Cupped and

ribbed forms, judging from Lopha gregarea, were probably more cosmopoli-

tan - ranging from marine to hypersaline and clearly appear to prefer '

a medium-grained bottom rendered stable in the face of fairly strong

currents by encrusting algae. They are rare in the pure oosparite

bands which probably signify a high particle turn-over rate.

The cupped non-ribbed forms show a vertical distribution

parallelling that of the pectinids, being abundant at the transitional

horizons just above and below the pure oosparite members or formations.

These correspond to the inner and outer periphery of oolite bars and barriers. Their coincidence with levels of high fossil diversity is

interpreted as a preference for near-normal salinities, good aeration and a low-to-moderate sedimentary rate.

Modern oysters are thoughtby Smith to be very sensitive to temperature in their spawning habits, tolerating for this reason only the narrow range of 15°C - 26°C. He gave a salinity tolerance of 2.5

- 62 ppt with an optimum of 20-40 ppt. He thought that the bottom was variable, but that active currents were necessary to supply the food which was copepods, diatoms, rotifers etc., taken in suspension.

A wide variety of sources indicate a variable but fixed bottom.

(7)1- rt-P's were still very scarce (1 species) in the shallowed zone (400 fthms) investigated by the Challenger. All five of the 19,6 modern species of Ostrea and Crassostrea (Grass,r tninks Crassostrea is synonymous with ancient Gry4a) occur from intertidal to about 10 m.

Parker (1959) showed that in the Laguna Madre periods of high salinity produced reefs of Ostrea equestris and low salinity reefs of

Crassostrea virginica. The small costate O. equestris is almost morphologically identical with O. costata, the coquinas of which may have preferred similarly high salinities. 212

The turbidity tolerance of oysters is apparently a function of the cupping or degree to which the ventral margin is raised above the bottom. Thus Trueman and MacLennan(1?42)give the arched Portuguese oyster as more tolerant of turbidity than the flat English one, an observation that accounts for the observed vertical distribution of

Eligmus polytypus and 0. costata as compared with the cosmopolitan tendency of 0. gregarea and the small smooth cupped species. However the rarity of non-ribbed flat forms like 0. sowerbyi may not be due to a vertical persistence of turbid conditions but probably to the rarity of subsaline conditions of the type that the modern Crassostrea virginica prefers.

The ancient record is not examined in its particulars as there seems to be general agreement in the shallow-water preference of oysters.

Beyond this the variable bottom texture for the same and different species, the wide range of salinity, and the need for good aeration are all generalizations that do not necessarily apply in a specific setting. For example Gryphea is thought by Gignoux (1950) to prefer marly limestones and by Webber (1968)to prefer shale while in the present succession, it occurs in a shaly but highly bioclastic band. Lamb

(1968) gives an absolute depth estimate of < 20 fthms after Scott (1940), 4 for Gryphea. The association of Exogyra with a sandy bottom and its ability still to thrive on a muddy but agitated bottom (Jourdy, E.

1924) may be due to the prevention of toxity in the presumably high organic content muds by current removal of the organic content as inferred by Purdy.

From the local as well as ancient and modern evidence the following conclusions may be drawn about the oysters in this succession:

(1) The bottom texture was variable and less important that its

stability for all morphologic categories.

(2) The cupped varieties, ribbed and smooth when vertically outlast- 213

ing flat forms may be used to predict the onset of turbid or

hypersaline conditions. The presence or absence of penecontem-

poraneous dolomite, gypsum or halite and/or sedimentary structures

can decide which option is best.

(3) Ostrea costata coquinas indicate brief episodes of elevated salinity,

the rate of return to normal and subnormal may be judged from the

abrupt or gradual vertical persistance of this form.

(4) Both occurrences of Gryp0a in this succession coincide with sus- ; pected sedimentary breaks of persistent lateral extent. The

absence of the gregareous hypersaline O. costata and the high faunal

diversity at these levels is interpreted as evidence that Gryphpa

may be a useful guide of normal marine conditions in a succession

of protracted hypersaline lagoonal sedimentation.

(5) The depth of deposition of beds with in-place Ostrea's probably

less than 20 m but rarely actually intertidal. Gryphjea was pro-

bably 20 - > 40 m.

(6) Assuming no change in the habit of oysters since the Jurassic,

the optimum temperature for Ostrea of 15°C - 26°C may be accepted

without question since it controls spawning. Evidence for high

salinities associated with Ostrea either from petrography or from

0. costata coquinas may be sufficient proof of hypersalinity through

low precipitation rather than by elevated temperatures.

(7) All oysters preferred good circulation and aeration. •214

Superfamily TRIGONIACEA, Lamarck, 1819

13. Trigonia cf. impressa, Sow.

Occurrence: A small, neat cast of the species has been found articulated in the burrowed chalky oomicrite of Lacave Calcilutite Unit 3.

Because of stylolitization, the ribbing pattern' is in some doubt.

205. Trigonia cf. costata, Sow.

Agassiz (1842-5)Table 3, figs. 12-14.

Diagnosis and Remarks: The slight flexure in the ribs as they approach the anterior margin with perhaps the closer concentric ornament is the only convincing difference of this from T. similis, Ag. in the figures of Agassiz. Both these species differ from the co- occurring sp. 168, Trigonia sp. in the upturned concentric ornament and their tendency to crowd towards the anterior margin in the latter species.

Occurrence: The form is very abundant in the Gryphaea beaumc.•nti bed and the ferruginous impure shell packstones immediately above it, mostly as fragmentary or disarticulated casts of doubtful identification. It clearly disappears towards the overlying clean oolites.

206. Trigonia sp.

Dimensions: L 5.6, H 6.7 ?, T 7

Diagnosis and Remarks: The concentric ornament of this large specimen seem to run into each other as in the Cretaceous T. quadrate Ag. or T. geographica Ag. of the Corallian, but differs from these in detail by showing bifurcation towards both the anterior and posterior margins. A flat impression in ferruginous impure shell packstones occurs just above the G. beaumonti bed. It could be mistaken for a 215

helminthoid burrow, but the rounded turns in the loops of the latter

are characteristic.

(Myophorella)

168. Trigonia/cf. Phillipsi, Morris & Lycett

Morris and Lycett 1850, Tab. VI, fig. 1.

Dimensions: L 4.3, H 4.6, T ?

Diagnosis and Remarks: The concentric ornament slopes a long way

down postero-ventrally and diverges in the process, before turning up

slightly. When favourably preserved the ornament is minutely

tuberculated.

Occurrence: Occurrence is in ferruginous shell packstones of the

Lower Mirandol Oolite, as disarticulated left and right valve casts.

(Scaphotrigonia) 203. Trigonia/duplicata, Sow.J4E Trigonia bergeoni, Bigot.]

Morris and Lycett 1851 -5,Tab. VI, fig. 2;

Cossmann 1911, Pl. A, figs. 9-12.

The specimen needs no qualification of Morris and Lycett. A

small cast less than 1 cm high occurs occasionally with oysters,

Pholadomya, Pinna and regular echinoids etc. in Gluges Calcilutite Unit

3. Its small size in this horizon even with shelly beds with Echinoid

spines is thought to indicate absence of continuous oceanic conditions.

Ecology of Trigoniaeca:

In this succession Trigoni2s attain their greatest diversity

in the shaly biosparites; they are occasional in the chalky burrowed

oomicrite of Lacave Calcilutite Unit 3, but shunning the calcilutites

even when fossiliferous, and clearly absent in the fossiliferous Souillac

Oolite/Lacave Calcilutite passage. The observations suggest <1 coarse

.shelly bottom, open marine, well aerated conditions and depth and energy 216

optima falling between those for Gryphiea and associated ammonites, and pure ooid grainstones.

Grasse gives only two modern representatives, Neotrigonia pectinate and N. margaritacea, Woodward S.P. (1887) though there were three species mostly in Australia (abundant in Sydney Harbour).

The occurrence in Sydney Harbour and the absence of Trigonia in depths

> 100 fthms in the Challenger results both suggest the same shallow water habits of Trigonia now as in the Mesozoic. There appears however to be some possible tolerance of lowered salinity as Sydney Harbour is actually an estuary and since Takai (1963) associates Geratrigonia with estuarine conditions. No evidence exists in this succession for such a tendency and it is just possible that the vagile habit of

Trigonia enables it to keep always a step ahead on the marine side of the salinity gradient as this shifts in and out of any estuary.

Superfamily HIPPURITACEA, Gray 1848

31. Pterocardium cf. corallinum, Leymerie

Karczewski 1969, P1. XXVII.

Diagnosis and Remarks: The posterior lobation and fine striation are difficult to recover intact and the species can easily be mistaken for a large terebratulid, particularly also in the lack of proper ol)- servation of the prosogyrate umbones.. One small ribed bivalve described subsequently as Pterocardia eparayensis, J.-C. Fischer, is obviously no morphologic relation of the Diceratids as P. corallinum is.

Fischer's (1969) genus is probably synonymous with Protocardia.

Occurrence: The species is common in the thick regular bedded fossiliferous Lacave Calcilutite Unit 1 near Souillac town-centre.

217

Superfamily GLOSSACEA, Gray, 1847

5I. 1socardia (= Ceratomyopsis) inflate, Ag.

Agassiz 1842-5, Tab. 8e, figs. 13-21.

Diagnosis and Remarks: The right valve is slightly larger than the

left and the posterior auricle slopes further ventrally than in 1.

inflata. The genus is probably best referred to Ceratomyopsis on

account of the general similarity of the specimens to those now commonly

described under that genus.

Occurrence: One articulated specimen and a right and a left valve

were recovered from the borrowed granular bed of Lacave Calcilutite

Unit 3, where it is common. Agassiz also records its occurrence in

a littoral facies in the Portlandian and Pterocerian of several places

in France and Switzerland.

210. Pseudisocardia cf. cordata,(J. Buckman)

Morris and Lycett 1851-511. XV, fig. 5.

Dimensions: L 5.1, H 5.0, D. 2.65

L 4.5, H 455, IT 1.7

Diagnosis and Remarks: The umbones are more strongly incurved than

in the figure cited.

Occurrence: The species is abundant with Ceratomya bajociana in the sub-Mirandol Oolite and the bed with G. beaumonti.

Ecology of Hippuritacea and Glossacea:

Diceratids are now extinct, being represented in modern seas von only by the Chamidae which/Zittel (1900) considered sufficiently close to the Diceratids to include in one superfamily. (For opposite views Palsenear see Morris (1965) and / (19i1.) Modern Chamas are epifaunal, cemented 218

reef organisms, commonly associated with corals and coralline algae on

atolls and atoll lagoons. Parker (Gulf of Mexico) gives a depth of

13-40 m for Echinochama, while the Challenger data places Chama in a

depth range less than 100 fthm. But the phylogenetic controversies

make these no more than a rough guide to the habits of ancient Diceratids.

All the species described, either as undoubted Diceratids

or as lsocardia have ancient representatives confirming the very shallow

(< 40 m) sub-coralline conditions observed here. Karczewski (1969)

discussed the palaeoecology of rudists in respect to their radiation.

He concluded that diceratid species and genera were strongly environ-

menta119 differentiated, Diceras preferring oolitic (and presumably

high-energy and extremely shallow waters) and Macrodiceras preferring

marly and fine-oolitic limestones (probably akin to the chalky inter-

bar pel- and oomicrites of the St. Etienne Limestone). He interpreted

the great abundance of individuals of a few species in thin but later-

ally extensive horizons as an indication of strong sensitivity to environmental changes. Well-founded conclusions from elsewhere may

therefore• be applied with some uncommon certainty to the present succession. Agassiz (1842-5) thought that Ceratomya (including lsocardia) were littoral forms whose shapes were strongly affected by the resistance of the substratum. He repeated after Gresly that they were often fragmentary in coralline facies. Soft tidal-flat sands would appear to satisfy this condition, particularly in view of the lack of appreciable distortion in the lsocardia and Ceromya found here.

The lack of fragmentation would indicate that they are in situ.

Zankl (1968) associated Megalodontids with well-rounded calcarenites of the outer back-reef and laminated algal mats and dolomitic calcilutites of the inner back-reef in the Upper Triassic of the Calcareous

Alps. A depth of 0-10 m was assigned to the two.

219

To summarise-

(1) The few individuals and species of Diceratids (and Isocardia) indicate that conditions in the burrowed Lacave Calcilutite Unit 3 and

St. Etienne Limestone were neither ideal for the group nor for a few species.

(2) The two occurrences probably represent a series of bars in which the very high energy conditions favourable to Diceras and the lower energy ones favourable to Isocardia, Ceratomya (and Mecalodon) were developed alternately but never ideally. Such an environment would be shallow (< 10 m) subtidal with a soft but coherent and only slightly 'mobile bottom occasioned by gentle currents.

(3) The Ceratomyal and MacrodicerA probably flourished in periods of reduced turbulence, while the Diceras occurred in the higher energy periods. The disrupted coarse/fine laminations in the burrowed bed testifies to this deduction.

Superfamily LUCINACEA, Fleming, 1828

Although Newell (1965) includes the Mactromyaidae in this superfamily, the two species here would perhaps best be grouped with the

Pholadomyidae as von Zittel (1900) did. Fischer (1877) certainly thought the genus Mactromya was polyphyletic, and in any case, the two species here are quite unimportant. Only the Lucinidae are described here.

192. Lucille cardioides, d'Archiac [L. liv.ata, Phill. in Arkell].

Morris and Lycett 1851-5, Table VI, figs. 16 & 17.

Dimensions: L 1.5, H 1.3, T 0.4? . Diagnosis and Remarks: The species is distinguished by the very fine growth lines despite the presence of a doubtful umbone-ventral

220

line depression on the posterior of the shell as in some Lucina bellona.

Occurrence: It is small and occurs sparsely in grey calcilutites

of Upper Gluges Calcilutite Uni 3, near Montvalent. Pholadomya, small

Goniomya and Sphenia occur with it.

7 178. Lucina cf. bellona (var. depressa) 1851-5, Morris and Lycett,ITab. VI, fig. 15.

Dimensions: L 2.3, H 2.2, IT 0.35.

Diagnosis and Remarks: The hinge and lunule are nearly straight

and slope gently. The umbones slightly anterior, and behind them five-

six growth bands delimited by growth lines more strongly incised than

those within the bands.

Occurrence: It is abundant in impure paper limestones just below

the roof bed of Gluges Calcilutite Unit 3 near Le Limon. A host of

Sphenia plus Gervillia, Pter+a-, Anisocardia and Gyprina occur also.

4. Lucina ? sp. A.

Occurrence: A minute form occurring in regularly bedded Gluges

Calcilutite Unit 4 calcilutites at Timbor. It could be a young of

some other species not so far found at that locality:

83. Lucina ? sp. B

Diagnosis and Remarks: The umbo of this slightly squeezed semi-

circular form is straight, mesial. The umbonal area is large and

quite clear of growth lines which are faint but regular and occur in

concentric bands separated by stronger growth-lines. Three such bands occur. The umbo and dorsal areas are strongly depressed over the deeply looped ventral line. The anterior and posterior dorsal sides 221

are straight. The general resemblance to sp. 178 and some other

Lucinids is the only reason for placing it in Lucina.

Occurrence: It occurs in shelly pelsparites of the topmost St.

Etienne Limestone.

Ecology of Lucinacea:

In this succession Lucina is strongly associated with thin

platy calcilutites and shales but rarely with grainstones. They are

as noticeable by their absence in fossiliferous, high diversity grain-

stones as they are by their abundance with Sphenia and Anisocardia in

the platy calcilutites and'shales. They and their associates are

sometimes accompanied by very small juvenile terebratulids which are

interpreted as the result of early mortality rather than selective

transportation. These observations can be made from the second unit

of the Gluges Calcilutites to the fourth in the sections between Gluges,

Creysse and Martel and in the Meyronne-Montvalent area. The interpre-

tation from the internal evidence is that Lucina preferred a muddy the bottom with a good circulation but was very tolerant of / foetid condi-

tions that can be inferred from the carbonaceous, low diversity char-

acter of the host beds at Le Limon. The assemblage to which Lucina

belongs also appears specialised, while the still foetid smell of the host beds could be due to cryptocrystalline pyrite.

Modern data on Lucina has been extended to the related genera and subgenera Loripes, Lucinoma, Myrtea, Phacoides and Divaricella also. Nine species from the above genera gave a nearly symmetric depth frequency curve with a culmination at 80 m compared to 70 m for Lima and 40-80 m for Pecten. Lucina however is more strongly clustered around the culmination than either of the two genera, and therefore would probably make better depth indicators. The sharper seaward truncation is also indicated by the Challenger data for the three genera. 222

Data from Smith (1959) on four species of Phacoides not included in the

preparation of the depth-frequency curves led him to take 1-100fthms.

as the range for the entire genus. filing, Wells and Taylor ( 1 965)

observed the thin shelled Lucina edentula living in lagoonal muds under

< 5 ft. of water. It is thought that the ammonite-brachipod-free

restricted facies in the succession studied would favour 80 m to just

subtidal as the probable depth range of Lucina, in spite of Woodward

(1887) who has no upper limit for them at all.

The Lucinacea are thought to favour a quiet muddy, foetid

bottom (Lavington, 1933), but however this might agree with the obser-

vations the Gluges Calcilutite, the impartial census on modern forms

appears to favour a sandy bottom particularly as it seems that the mud-

dwellers like Myrtea spinifera also live in muddy sand or gravel while

the sand dwellers like Loripes lucinalis and Lucinoma borealis do not

on the evidence of Tebble (1966) live also in mud. It is evident that

the group characteristics of Lucinids would have to be dist1nguished

from what some species may be capable of adapting to.

Lucinids are double-siphoned, suspension-feeding deep

burrowers with a special adaptation for dealing with low oxygen concen-

trations. They are certainly able to stand high salinities from the

Persian Gulf evidence, and the direct observation in the succession studied, although Cadee("1968) shows that they can tolerate salinities otkov3, of 33% or lower (in winter). Like Pter4-a therefore, the dominance or abundance of Lucina at a particular level may indicate the inability of other forms to stand the restricted conditions. This interpreta- p- tion also suggesting that conditions of low aeration rarely existed above the Gluges Calcilutite. Such episodes of elevated salinities and reduced circulation under shallow water conditions are indicated for the Gluges Calcilutite. This is at variance with Lavington (1933) 223

who thought that ancient bioherms of Lucina occidentalis in Colorado were

determined by the preference for a mud bottom. Just as the Nuculacea

are macrociliobranchiate and still prefer quiet muds where food accretes

for them and very few other forms only, so it is thought that the macro-

ciliobranchiate Lucinids, though sandyforms from other modern evidence,

are able to dominate where competition is minimised by low oxygen con-

centration(more likely to coincide with a clay rather than a sandy bottom). The conclusions on competition were also reached by Allen

(1958) from a study of modern Lucinids.

The environmental zones over which Lucids were observed by

Cadee (1968) had temperatures ranging from 22°C - 18°C in the seaward zone. The palaeotemperatures, judging from the mineralogy of the interbedded limestones, were probably higher,at leastjust above and below the shale bands.

Superfamily CRASSATELACEA, Fe.rusrSac, 1822

The Astartidae clearly dominate the Cardiniidae in abundance.

Both families are described below.

121. Astarte ? ovata

Arkell 1827-39, P1. XXXII, Figs. 1-12.

Diagnosis and Remarks: The specimen is very much like fig. 1 o f

Arkell in the depression on the right valve at the ventral border. The left valve may have been squashed in but is too low for an astarte. The antero-ventral angle is also too acute, looking closer to that of a nuculid.

Occurrence: It seems to be restricted to the Upper St. Etienne Lime- stone at which level it has been collected in two localities. 224

71. Astarte var? squamula, d'Archiac

Morris and Lycett 1851 -5, Tableix, figs. 8a, 8b.

Diagnosis and Remarks: This species covers the surfaces of thin

(centimetre) plates of limestones in the Stromatolitic Lacave Calcilu- tite Unit 2, always as internal casts, and therefore hard to identify from growth-lines (E costae of Morris and Lycett). Occasional specimens show one prominent growth-line. The minor lines dorsal to this appear to abut on it but less obviously than in A. excentrica. The specimens also appear more expansiform than A. excentrica. A posterior carina is absent or present in varying degrees, the latter aspect re- calling the Anisocardia, from which this species may be distinguished by the absence of an inflection near the umbonal extremity of the posterior arc of the shell.

Occurrence: It should be noted that, although the species dominates at this level, and is cited as such (without specific identification), other small bivalves such as Sphenia and Isocardia occur with it and can easily be overlooked. Also the wide range of growth stages gives a deceptive appearance of absence of transportation, as thin reworked layer of 3-4 mm invariably underlies the surfaces covered by these forms which also lack the complementary valves in cross-sections. The left/right valve ratio is however only about 4:5 indicating that transportation may have been over a very short distance by a current regime well in excess of the threshold lifting forces for the largest valves.

The stable convex-up positions of the valves and the underlying erosional surfaces also suggest such a current regime. The species occurs also in micritic oncospartics above the Mirandol Oolite, where the carina is invariably developed. 225

(Leckhamptonia)

118. Astarte cf. interlineata, Lycett

Morris and Lycett 1851-5, P1JX, figs. 14,15a,b;

Cossmann 1900, P1. VII, fig. 9.

Diagnosis and Remarks: The form is extremely small, less than 1

cm, and the posterior part of the ventral line is less strongly turned

up than in A. interlineata. Also the arcuate contact of thelunule

and ventral line is truncated in this specimen, which occurs as a

cast in the fossiliferous St. Etienne Limestone. lutitic topmost beds

at Le Roc.

28. Astarte brompti

Mem. Soc. Geol. France No.14, Pl. IV.

The small size, more expansiform shape and more strongly

curved postero-dorsal line distinguish it from sp2,69, Corbicella sp.

It is common in the calcilutites of Lacave Calcilutite Unit 1 and the

pellety oomicrites and sparites of the burrowed Lacave Calcilutite

Unit 3, but also doubtfully throughout the St. Etienne Limestone.

It is probably cosmopolitan in habit.

186. Astarte squamula, d'Archiac

(see sp. 71)

Dimensions: L 1.5, H 1.15, T 1.20.

Diagnosis and Remarks: The specimen is highly inflated and cardi-

form with a small but deeply notched lunule and a gracefully curving

ventral line merging insensibly with the escutcheon. It is covered by fine growth lines (costae of Morris and Lycett).

Occurrence: It occurs sparsely in the impure biosparites above

the G. beaumonti bed, often still articulated, and also in the calci-

226

lutites of Gluges Calcilutite Unit 3 together with Pholadomya and

small bivalves, such as Sphenia and Anisocardia.

155. Astarte sp.

Diagnosis and Remarks: This is an indeterminate species occurring

subordinately among a flood of Sphenia in the Upper Gluges Calcilutite

Unit 3 calcilutites. It differs from A. excentrica in the absence of

growth lines. The two have similar associates in the calcilutites of

Gluges Calcilutite Unit 3 but A. excentrica occurs also in a coarse

facies lower down.

222. Opis (Trigonopis) sp.

Treatise in Invert. Pal. (Moore) fig. E.71 (3a-c).

Occurrence: Minute (0.4 cm) specimens occur in horizons domin-

ated by Gervillia acute and Pholadomya laeviuscula in Gluges Calci-

lutite Unit 3. It may support the evidence indicating absence of open-sea conditions at this level as inferred from Trigonia duplicate.

That absence of a coarse bottom is not alone responsible for the small size of Trigonia or Trigonia-like Opiids is confirmed by this species which actually occurs amidst a Gervillia coquina with some oysters.

Other Opiids like O. (Opis) and O. (Pachyopis) sp. are common in lime wackestones at the top of the St. Etienne Limestone at Le Roc.

They were recovered after completion of the faunal analyses.

Ecology of Astartacea:

The local occurrence of Astarte is spasmodic, being completely dominant in certain beds of Lacave Calcilutite Unit 2 and_the Crezelade

Beds, but equally obviously patchy in other formations. But even in 227

the latter case, the associated fauna tends to be biased towards small

Spheniids, Lucinids and Anisocardiids in decreasing frequency of domi-

nance. The oxygen deficient conditions in which the Lucinids dominate have been discussed and may be compared with the algal mat complex in which the Astartids dominate. Both are specialised environments favour-

ing a few forms peculiarly adapted to them. Within Unit 2 of the Lacave

Calcilutite the Astartids are almost wholly confined to the dense calci-

lutite and finely chalky components, and only occasional valves of them are found caught up sometimes edgewise in the Stromatolite subunits.

One Rhynchonella was found in the chalky facies. In the Crezelade

Beds, the Astartids again occur as disarticulated valves facing convex up. It is inferred that the Astartids lived in an area of near•-normal salinities within the lagoon, and were periodically transported by corn- -1 petent currents or waves on to the algal flats. The paucity of other transported marine forms supports the nearness of the 'Astarte Banks' to the lagoon, although there is a marginal chance that the Astartids were the smallest shells in an off-shore environment of much higher diversity on which selective transport operated. A modified view in which modern Crania were thought to be transported en masse was advanced by Cadee (1568) on the basis of bell-shaped size-frequency curves. 1965 Shinn, Ginsburg and Lloyd47)held storm and spring rides to be effective agents of transportation in the Bahamas.

Modern Astartids are principally temperate forms occurring in the Mediterranean, the North Sea and the east coast of America, and in

Australia and New Zealand. Woodward (1887) gives the depth as 30-112 fthms, although the author's census from Tebble (1966) shows that three of the four most common species A. triangularis, A. motagni and A. elliptica occur from low water to less than 90 m, only A. sulcata going beyond 200 m. The Challenger encountered only one species each at 100-500 fthms and at < 100 fthms, the majority of species probably 228

confined to still lesser depths. Astartids are apparently rare on the

Texas Coast, Parker (1959) listing only one species on previous evi-

dence, in 1-5 ft of water at an Open High-Salinity Bay Margin. Cadee

(1968) associated A. triangularis and Digitaria digitaria with about

60 m depth, concluding that they were limited by shallower depths and

fine sediments. Astarte also occurs in the North Sea at about 30 m.

A depth of 0 - 60 m is estimated, on the above evidence, for the

Astartids of the present succession.

There is general agreement on the preference of modern

Astartids for a sandy bottom, Cadee (1968) attributing this to their

suspension feeding habit. They are probably macrociliobranchiate

as the Crassatellacea and Arcticacea, for this same reason. Their

association with calcilutite is further evidence that they were

brought in post mortem. Boeckschoten (1966), Saleuddin (1965) both

indicate that Astarte is a shallow burrower living close to the sedi-

ment/water interphase. Most species are planted edgewise with the

lunule down and the postero-ventral angle nearly flush with the surface

except A. sulcata which lives buried with the left valve up

(Saleuddin, 1965). Dug up from such a position by currents of what-

ever origin, we would expect to find the original substratum also

at the site of deposition. The thin lenses of fine oncosparite and the

small gypsitised clasts of calcilutite in the erosional laminae under-

lying the Astarte shells may be such a bottom, but the clasts also show

that gypsitization probably occurred contemporaneously. Oncoliths

are generally interpreted as developing in the slightly protected zone

behind oolite barriers. Our Astartes may well have lived in the well-

.aerated moderately current swetp outer-lagoon where some calcilutite

(gypsitised) admixture would be expected. Boeckschoten (1966) also

deduced that Astartes were sluggish burrowers, a conclusion which, if

true, would explain why Astartids would rarely be able to right their 229

position after catastrophic transportation as does Mya arenica)

(Bradley, 1957) or Hydrobia even if only to die subsequently from other

causes. The possibility tallies well with the observations in the

present succession.

Modern astartids prefer normal salinities but evidence from

the Cretaceous (Voles, 1548) indicates that they can tolerate margin-

ally lowered salinities; it may be fair to extend some tolerance in

the other direction also. The choice between subsaline and hyper-

saline then falls unequivocally from the petrography and particular

lithostratigraphic succession on the side of hypersaline for the

stromatolitic member of the Lacave Calcilutite. Takai (1963) also

gives ancient inference on Astartids indicating near-shore coarse

sandstones as their home.

Astartids then, suggest nearness to a moderately agitated, wall mixed environment with a depth of 0-60 m and an oncomicrite-oncosparite

bottom in hypersaline to normal salinities. Conditions just below the sediment surface were sufficiently hypersaline to induce contemporaneous gypsitisation but did not adversely affect the shallow burrowing Astartids.

It is thought that Astartids would not settle in their new home because

they could not burrow quickly but more because the few intact and live ones found themselves on an intertidal to supratidal environment or in very hypersaline conditions coinciding, in the working model, -t-e the

interior of the lagoon.

Superfamily CARDIACEA, Goldfuss, 1820

Anisocardia and Cyprina are deliberately omitted from here.

They are described under the Arcticacea, for reasons of palaeoecology. 230

223. Protocardia schucherti, McLearn

Paterson, 1963, P1. XVII, Figs. 3a,3b.

Dimensions: L 2.9, H 2.8, T.1.0.

Diagnosis and Remarks: The shell is strongly inflated and higher on

the auriculated side. The umbo is submesial, moderately incurved. The

shell is very thick with a surface delicately reticulated by the inter-

section of growth-lines and striations both of which are very close and

regularly spaced. One or two major concentric incisions are present.

Occurrence: One broken valve was found in the topmost Mirandol

Oolite in well-sorted cold brainstones.

72. Protocardia cf. morinica, de Lor.

Neaverson, 1928 Fig. 50, p. 377.

Dimensions: L 0.66, H 0.67, T ?

L 4.25, H 4.1, T ?

Diagnosis and Remarks: The form is very symmetric, quadrate with

the curvature of the ventral line only slightly accentuated. Growth

lines are confined to a narrow band near the ventral border.

Occurrence: Two casts, one in a coarse bed in the St. Etienne Lime- stone and the other in the coralliferous Lanzac Oolite, were found.

The umbo is mesial, straight and the umbonal angle nearly 900 ± .

77. Protocardia sp. [=Chlamys? luciensis, d'Orb]

Cossman, 1900, P1. V, fig. 8.

Diagnosis and Remarks: The species may easily be mistaken for Chlamys stricta in the outline and in the approximate spacing of the costae, of which there are 18-20. The sole difference is in the stronger inflation and the very much subtler junction of the dorso-lateral lines and the ventral margin making this species markedly more ornate. 231

Occurrence: One cast occurs in pebbly oolitic pelmicrites in the

top St. Etienne Limestone with Ceratomya decuritata, Nytilus, Ostrea

sowerbyi among others.

139. Cardium ?

Diagnosis and Remarks: An indeterminate fragment nearer to sp. 77

than any other but certainly different in the wider spacing of the costae

and in the greater deviation from circularity of the ventral line.

62. Cardium subminutum, d'Orb. [=Pterocardia eparayensis ,J.C.-Fischer]

Cossmann, 1900, P1. VIII, Figs. 12-14; J.C.-Fischer 1969,

Diagnosis and Remarks: To this species are referred about three

varieties of small (1 cm or less) ribbed bivalves occurring most often

in the oolitic facies (Souillac Oolite and Lanzac Oolite). Protocardia

pectinata, J. Sow. and Cardium corallinum, Leymerie could accommodate

the other two of the three varieties, but extraction from the matrix is hard to effect. The usefulness of the generic name Pterocardia is doubtful considering the possible confusion with the ancestral Hippuri-

tacea.

93. Indet. Cardiid

A large fragile strongly inflated shell with a close-set ribbing irregularly emphasized and dying out rapidly towards the numbo.

Very faint growth lines are interspersed between three prominent ones.

A depression on the shell runs concentrically about a quarter of the height from the ventral line. It occurs sparsely just below the Mirandol

Oolite. 232

Ecology of Cardiacea:

Cardiidae are very well represented in modern seas the world

over from the shore to 140 fthms (Woodward, 1837); But this all-

inclusive depth range does no justice to the weighting of the depth-

distribution which is clearly in favour of a very much shallower value

than 140 fthms. H.M.S. Challenger encountered Cardium only patchily

between 100 fthms and > 2,500 fthms, the bulk of them occurring at < 100

fthms. G. Thorson (1957) included Cardium as a key representative

of the Macoma Community, the environmental conditions of which he gave

as estuarine, intertidal-10 m sometimes 60 m, a silt-clay bottom favour-

ing the deposit feeders and a sand bottom favouring the suspension-

feeding Cardium. The author's depth-frequency curve for Cardiidae

shows a bias for shallower depths than the Pectenidae, Limidae or

Lucinidae. There is a culmination at 20 m, and a minor levelling one

at 40 m and 60 m and a sharp drop to 100 m. A range of 0-60 m is con-

sidered reasonable although a cut-off value of 100 m may more safely

accommodate unaccountable geologic factors like changes of habit,

preservation and so on, as well as to take cognisance of the fact that

the peak frequency at 20 m accounts for only 75% of the species com-

pared with 90% and 100% in the Lucinidae and Limidae respectively.

Depths given by Cadee (1968) were 60 m or less for 10 species of Car- diidae. Cadee concluded that the cockle lived in shallow waters when

turbidity was higher. The limitation of the ancient Cardiids to the shallow depths indicated by the Oolites and pebbly pelmicrites need not, however, indicate turbid conditions. Modern Cardiids differ

in their preference for all environmental parameters but a coarse bottom, a factor which is thought to exert a decisive influence here, so that even those with an extended depth range were limited to a shallow oolitic bottom by the coincidence of calcilutites and the deeper lagoonal environment. On that bottom, they are thought to 233

have led a precarious existence in quiet pockets, being whipped up from their objects of attachment or just below the surface (depending on species) now and then by waves or tidal surges. The requirements of bottom stability were achieved for them as for Lopha gregaria by encrusting algae and polyzoa.

The salinity tolerance of Cariids varies according to species, the cockle preferring brackish waters while C. scabrum is very sensitive to lowered salinities (Tebble, 1966). Parker (1959) gave a few subgenera of Cardium which live up to 42% or more under very shallow (< 10 ft) conditions and high (30°C-35°C) temperature conditions in the Laguna Madre. In ancient environments, Termier & Termier

(1959) mentions Protocardia in a hypersaline lagoon in the Jurassic of Maroc. The choice between high salinities and low salinities for the present Cardiids is even more obvious than for the Astartids, in view of the dolomite common at the levels in question. The

Cardiids may be said at least not to contradict the evidence for hypersaline to normal conditions in the Mirandol Oolite, Souillac Oolite,

St. Etienne Limestone and Lanzac Oolite. The local distribution of

Cardiids shows them coming in when the diversity is high, therefore salinities were probably nearer normal than high, conditions that are more likely to be encountered in the barrier and its immediate shoreward margins. Unduly high energies rule out the seaward edge for Cardiids.

nperfamily TELLINACEA, Latreille, 1825

The superfamily is represented only by the Tancrediidae in this succession. It is not certain where Newell would have placed

Corbis and Corbicella, Whether as.a separate family or otherwise, the Lucinidae and Tancrediidae appear to vie equally strongly for 234

them (Piveteau, 1956 and Zittel, 1960). Both are described here as

Tancrediids partly after Piveteau (1956) and the Treatise (Moore),

the latter splitting various earlier species between the Tancrediidea

and Quenstedtiidae both in this superfamily.

216. Tancredia, sp.

Dimensions: L 2.6, H 1.3, IT 0.6.

Diagnosis and Remarks: The species has the characteristic umbo to

ventral line depression of the genus. The umbo is displaced posteriorly

and is sub-erect. • Occurrence: A cast of one left valve was found in the biocal-

carenites just below the uppermost Mirandol Ooolite unit.

158. Corbis sp.

Dimensions: L 3.1, H 2.45, T 1.05

L 4.05, H ?", T 1.75.

Diagnosis and Remarks: The shape is highly symmetrical about a plane through the right and left valves which are subequal. The ventral line

is scarcely curved and gives an equilateral triangular shape to the shell. The umbones are straight, mesial. Growth lines are distinct, regularly spaced.

Occurrence: Both casts are articulated and the species is probably always so. It occurs more often in the calcilutite with a few grains in Lacave Calcilutite Unit 1 at the upper fringes of the faunally highly diverse lower passage from the Souillac Oolite. The matrix is characteristically pale pinkish-grey. it occurs also in a similar facies in the passage beds of the St. Etienne Limestone towards Le Roc, again a faunally diverse horizon. 235

59. Corbicella E.

Dimensions: L 3.85, H 3.25, T ?

L 2.5, H ? , T 1.05.

Diagnosis and Remarks: The species is distinguished from sp. 158,

Corbis Le:, by the depressed lunule and the outward curving ,-,ostero-

dorsal line. The growth lines are very regular and clear and the

umbones submesial.

Occurrence: It is very common, both articulated and disarticulated,

as casts in the coarse bio-oosparites and current-laminated chalky bio-

pelsparites of the Upper St. Etienne Limestone. The disarticulation

coincides with fragments of Pinna and other forms in current-laminated,

micrograded pelsparites. It is less common in the Lower St. Etienne

Limestone.

127. Pleuromya tellina, Ag.

de Loriol 1872, P1. X, Figs. 6-8.

Dimensions: L.3.55, H 2.05, IT 0.4.

Diagnosis and Remarks: The specimen is very similar to de Loriol's

figure in the outline and disposition of growth-lines, but differs from

it in the gentle dorsal curvature of the ventral line and the develop-

ment of a subtle antero-dorsally slanting carina.

Occurrence: The specific name is revealing as modern Tellinids such as Psammobia .and Sanguinolar.ia livida are nearly mirror-images of

P. tellina. The left valve occurred in pellety ooid-grainstones of

the Upper St. Etienne Limestone.

27. Quenstedtia elongata, Hudleston

Arkell, 1927-39, Pl. XL, Figs. 6,7.

Occurrence: This slim and elongate species forms only 3% of the 236

assemblage of Lacave Calcilutite Unit 1 at Rocamadour, but is very con-

spicuous by its graceful shape.

110. Rosenbuschia typica Roed.

de Loriol 1872, Vol. XXVIII, P1. IV, Figures 3-5.

Diagnosis and Remarks: The species is distinguished from

Quenstedtia laevigata, Phillips, another giant species, by the more inequilateral outline of the present specimens.

Occurrence: It occurs in Lacave Calcilutite Unit 4 and in the upper half of the St. Etienne Limestone, being more abundant here, particularly in the pellety ooid packstones.

Ecology of Tancrediidae:

Tancredia and Corbicella are extinct genera about which little

is known. The intended procedure is therefore reversed and the sediments and associated forms are used to infer their habits. Corbis occurs in modern seas in India, China, N. Australia and the Pacific

(Woodward, 1887) to which it would appear to be limited. It is siphonate and probably a burrower, but even for Corbis, recent data on habits is scarce or absent. The genus is taken with Opis by Imlay (1957) to be associated with gypsum and red beds from which pectenl, tflya's,ctsunt oysters e4714 gryphaeas are missing. The occurrence of Conbis at the limits of the more fossiliferous Lacave Calcilutite Unit 1 where vertical sampling was unhampered could be significant, and this member is thought to pass into an intertidal/supratidal stromalotitic member.

Mya's occur above it and oysters, ptcten's and Lima's below it in the same member in this succession. Gardiner (1957) associated Corbis with the base of .a shelly Eocene beach sand that becomes less fossiliferous upwards. The author interprets this as indicative of shallow subtidal 2 37

conditions. Evidence from Russell (1943) and Irclay (1957)

from ancient records indicates the upper infralittoral (< 50 m) for

Tancredia and associated Linqula, Gryph7a, Ostrea, Mytilus and Echin-

itis (Implay op. cit.). The biosp4Zes in which Tancredia occurs

here in fact just overly Gr phla beds in what can be demonstrated petro-

graphically to be a shoaling sequence. Whispy greenish streaks are

interpreted as bioturbate pockets and finely disseminated glauconite.

A moderately winnowed, near-shore, open marine environment with a shell

consolidated bottom with low detrital sediment influx is inferred for

Tancred i a. The home of Corbicella was probably a coarse algal sta-

bilised bottom in a medium current regime environment of normal to high salinity in depths of 10 m or less, and near to a low deposition environment of high current regime, flat lamination and fine oo- and peisparties into which it was occasionally displaced, being fragmented and strongly oriented in the process.

Superfamily ARCTICACEA, Newton,l891

The family Arcticidae is probably equivalent to the Cyprin-

idae of Piveteau (1956) and the Pleurophoridae of von Zittel (1900), both of which include the genera Arctica (Cyprina) and Anisocardia which are described below.

79. Isocyprina depressiuscula (Morris & Lycett)

Morris and Lycett 1853Table. XIII. Fig. 4•

Dimensions: L 1.1, H 1.1, T ?

•Diagnosis and Remarks: No qualification is made on the identification except that it must be distinguished carefully from Astarte politula, Bean in Lycett Tab. XXXV, Fig.16. The latter has widely spaced growth lines and a rounded riot angular contact between the 238

postero-dorsal line and the ventral line.

99. Anisocardia cf.islipensis ,(Lycett)

Lycett , 1863 ,.P1. XXXV, Fig. 13.

Dimensions: L 4.15, H 4.0, IT 1.6.

Diagnosis and Remarks: The specimen is thought to be an incomplete

representation of A. idipensis with the area posterior to the carina

lost to the matrix. Even so, it appears slightly less expansiform

and the anterior turn of the ventral line more rounded than the figure

of Lycett.

Occurrence: It occurs in dolomitic fine chalky pellet-wackestone

in a sparsely fossiliferous band in the Lower and Upper St. Etienne

Limestone above a much more faunally diverse band with Naticids,

knynchonelbils, Obovothyris, Chlamys stricta (dominant), Modiola,

Nerinea and Corbicella. It is also present in a coarser micritic

ooid-grainstone of comparable diversity with large Arcomya and Nerinea

(or Cerithium) amongst others.

196. Anisocardia davidsoni,(Lycett).,

Lycett,1863,P1. XXXVI, Fig. 6, 6a.

Dimensions: L 0.95, H 1.0, LT 0.3.

Diagnosis and Remarks: It could have been sp. 71, Astarte excen-

trica if it was not for the slight down-turn of the ventral line imme-

diately on the anterior side of the carina; in A. excentrica it runs

antero-dorsally from this point.

Occurrence: One doubtful specimen occurs in the upper contact of

Lacave Calcilutite Unit 1 in a coarsely granular matrix. 239

194. Anisocardia cf. dieulafaiti (?)

Dimensions: L 2.9, H 2.5, IT 0.6.

Diagnosis and Remarks: This is a common species in Units 2 and 3

of the Gluges Calcilutite for which no original figure has been traced.

It seems best to accommodate the specimens in view of its common

occurrence in the local fossil lists. The umbones are posterior,

pointed and gently curving. The very clear striations along the

antero-dorsal line are characteristic. Growth laminae are present but vague, and the curvature of the ventral line is gentle. The shell is more often preserved than that of Lucina's.

194a. Isocyprina cyreniformis, Buvignier

Arkell, 1932, P1. XXXV, Figs. 2-8.

Dimensions: L 1.2, H 1.2, IT 0.4.

Diagnosis and Remarks: The specimen. is distinguished from Aniso- cardia or Lucina by its nearly circular outline, strong inflation, and lack of radial striation.

Occurrence: It is common as pink iron-stained casts in the calcilutites with Pholadomya of Gluges Calcilutite Unit 3 or in shalier horizons with Anisocarida, Lucina, and Sphenia throughout

Gluges Calcilutite Units 2 and 3. It is rare in the coarse bioclastic limestones in which Anisocardia and Lucina sometimes occur. Shaly but not carbonaceous calcilutites seem to be preferred.

Ecology of Articacea:

Isocyprina depressiuscula, Isocyprina cyreniformis and

Anisocardia cf. dieulafaiti in that order seem more reliable for local Anisocardia environmental reconstruction than islipensis on account of its doubtful taxonomic categorisation and A. davidsoni on account of the 240

doubt over the uniqueness of the specimen itself. T. depressiuscula

and A. islipensis both are more frequent in coarse pellet-oolite lime-

stones, with varying amounts of admixed calcilutite, the latter toler-

ating higher calcilutite content thanthe former. The conditions for

Corbicella are recalled with perhaps nearer normal salinities because

of the strong attachment to highly diverse horizons. Anisocardia was

probably tolerant of bottom texture and oxygen supply and salinity.

Large specimens of it are more likely in pebbly bioclastic beds than

in surfaces in platy calcilutite only one shell deep particularly if

dominated by a single form such as Sphenia. It may be concluded that

its optimum requirements were a well-aerated coarse bottom swept by

strong currents. Intercalated in a gypsiferous calcilutite sequence,

these conditions can hardly be ascribed to a greater depth than 30 m

- a common value for most modern shallow lagoons, the values 50-60 m

being extreme. Isocyprina cyreniformis was probably less tolerant,

and contrary to Arkell's (1952) observatios was always small. Its

requirements were a calcilutite bottom with good aeration and near normal salinities such as are associated with Pholadomya and Pinna.

Among modern Arcticidae, Arctica islandica is probably the best studied. According to Saleuddin (1964) it is a shallow burrower

(siphons 5-8 mm long) living vertically buried in firm sand and sandy mud feeding by suspension. it ranges from lower intertidal to a considerable depth (Tebble, 1966). Woodward (1867) thought it was a boreal form living in 5-80 fthms of water. Its absence on recent molluscan lists is attributed either to a natural paucity in numbers or to a preference for all-year round normal salinities, a demand not met by the estuarine influence in most well-studied areas, the latter possibility being supported by the absence of a significant freshwater influx into Sagadahoe Bay tidal (Bradley, 1957), an area in which

Arctica islandica was recorded by that author. The bottom here was 241

again loose sand near and below low-tide mark. Bradley had organic content in the sand about 10 times less than in the clays, being kept clean partly by currents 0.1 f.p.s. - 0.82 f.p.s. It may be inferred the ancient Isocyprina cyreniformis, given the same intolerance for putrid conditions, would have needed higher current velocities to effect a higher turnover of oxygen-laden waters to o:,,lidise the closer- packed calcilutites. Velocities of such an order are in accord with the inferences for Modiolag in a vertical position at the same exposure.

Superfamily MYACEA, Goldfuss, 1820

The Mactromya's omitted from the Lucinacea, for reasons given there, are described here, plus tha. 1phenia and Panopea. But in view of the great external morphologic similarity between a Panopeid such as P. brockworthensis of Arkell (1946) and Pleuromya, it is not without some hesitation that Panopea is described here, notwithstanding the similar shape of this Particular species to that of the Mactromya also described here.

95. Mactromya cf. globossa, Ag.

Agassiz,1842-5, Table 9d, Figs. 9-14.

Dimensions: L 1.85, H 1.55, T ?

Diagnosis and Remarks: The specimen is a small cast with submesial umbo with low auriculate areas on either side of it. The posterior of the valve is more pointed than the anterior. The growth-lines of the figured M. globossa are not evident.

Occurrence: It is rare, one occurrence each in thin coarse shelly lenses in the Lacave Calcilutite Unit 1 and in fine-grained pellet packstones in the St. Etienne Limestone,being all. In the former, associated forms are Pholadomya, Chlamys stricta, Obovothyris and TerebratUla 242

subsella, a small Nerineid and Corbicell

144, 113. Mactromya rugosa, Ag.

Agassiz,1842-5,Table 9c, Figs. 1-23.

Diagnosis and Remarks: Agassiz shows a very wide range of variation that could well take in his own M. globossa. Fig. 21 is represented by specimen 113 and figs. 5 and 14 by specimen 144.

Occurrence: Both are articulated casts occurring in the lower contact pale platy calcilutites of Lacave Calcilutite Unit 2 and the topmost Lacave Calcilutite Unit 1 on the east side of the BlagoUr Valley.

67. Panopaea ? cf. mandibula, J. Sow.

Neaverson ,1928, Fig. 55, p. 377.

Dimensions: L 1.3, H 1.15, T ?

Diagnosis and Remarks: The umbones are prosogyrate submesial and moderately incurved. A long, deep lunule extends over halfway towards the ventral border and the postero-dorsal line viewed from towards the interior of the left valve is a shallow 'S'. A perceptible depression runs from umbo to ventral line. Faint growth-lines are present..

Occurrence: An excellent cast occurs in the dedolomitic pseudo- calcilutites 8 in below the top contact of the St. Etienne Limestone and in a similar but visibly finely pelletal limestone just below the contact near Le Roc. The latter beds team with 2 species of.Opis, a mytilid, Pterocardia and in thin section a sizeable percentage of foraminifera as foraminiferal sand. The beds have shelly bases.

179. Sphenia sp.

Diagnosis and Remarks: This small (< 1 cm) bivalve is so common on the surfaces of fissile limestone plates in Gluges Calcilutite Units 243

2 and 3 that it hardly needs description. It is matched in its obvious dominance in these levels only by Astarte in Lacave Calcilutite Unit 2 and the lower Crezelade Beds. Its elongate shape and dominance readily distinguish it from any of the common forms at this level. The shape also distinguishes it from Astarte when it occurs very subordinately amidst the latter in Lacave Calcilutite Unit 2. It has been mentioned under its common associates earlier.

91. Homomya?

Occurrence: One arcuate cast looking as much like Tancredia as an incomplete Homomya gracilis, Ag, found in platy dedolomitic limestones

in the Upper St. Etienne Limestone.

o8• Arcomya latissima, Ag.

Agassiz 1842-9Tab. 9, Figs. 10-12.

In the absence of preserved dentition, the species may be diagnosed locally by the very large size (the largest bivalve in the succession). It differs from Pleuromya in the greater length/height ratio, in the commonness of a straight or gently dorsally arched ventral line, and in the strongly oblique dorso-ventral line at the anterior end of the valves. A pedal gape seems to be slight or absent. It is probably common in the fossiliferous coarse bio-oosparite bands of the lower and middle St. Etienne Limestone. Another articulated specimen comes from the generally unfossiliferous platy marker bed, Lacave

Calcilutite Unit 4.

146. Arcomya unioniformis, (M. & L.)

Morris and Lycett 1850, P1. X, Fig. 6.

Diagnosis and Remarks: Growth-lines are faint but closer spaced

244

than in the published figure. Homomya gracilis, Ag,(Agassiz (1842-5)

Tab. 20, figs. 1-3) may also accommodate the specimen, but the range

of variation casts doubt on the uniqueness of the species of Agassiz.

Occurrence: One articulated highly stylolitised specimen was re-

covered from grey calcilutites in Lacave Calcilutite Unit 1.

Ecology of Myacea:

All the forms described are burrowers that are very similar

in habit and sometimes in external morphology to Pholadomya, Pleuromya

and Ceromya. There are however perceptible differences in the local

distribution, among the Myacea and between them and the latter group.

The gregarious dominating habit of Sphenia may be compared with that

of Astarte, Ostrea costata, Gervillia acute and in a lesser way with

Lucina and Pteropern,. The controls were thought to be reduction of competition arising from high salinity for Astarte and Ostrea costata,

depth for Gervillia acuta, and reduced oxygen supply for Lucina and

Pterl-a. Sphenia is shown from the following evidence to have been controlled primarily by salinity and secondarily by oxygen supply;

(1) Glangeand (1895) and Mouret (1892-8) have observed it with obviously brackish or freshwater associates - Viviparus, Planorbis and

Cyrena and with lignites at a comparable level in regions adjoining this one. The occasional small bivalves occurring among the Sphenia here are however Cyprinas not Cyrenas.

(2) Sphenia .is most abundant at a level underlain by carbonaceous limestones with Pteroperna costatula and overlain by a fining-upwards pebbly oo-biospartie, suggesting a break of the type coming above the marls-with-Viviparas below the Blagour Breccia'. The marls contain and are underlain by lignite, and can therefore be compared with the

LELILLs,L, - cos which may also show a brackish influence. A continuation 245

into the Sphenia beds of the putrid conditions of the underlying

Pteroperna beds is unlikely because of the fall not increase in carbon

content upwards. But the juxtaposition of the two does indicate a

not unlikely residua :,-oxygen influence, confirmed also by -

(3)a.The absence of rippling in these beds, though common above and

below; b. The presence of attached valves and the 50:50 ratio of right

• to left valves..

(4) Sp!lenia's were rivalled in numbers by Anisocardia with small tere-

bratulids in the platy gypsiferous beds of Gluges Calcilutite Unit 2.

Their complete dominance here in the absence of gypsum is additional

evidence of the preference directly or through reduced competition,

for subsaline conditions. Gregarious rivals under near-normal con-

ditions were mostly Lucina and Terebratula ornithocephala if the term can

be extended to the abundance of the latter.

(5) Negative evidence is afforded by the apparent replacement of

Sphenia in the near-normal to hypersaline conditions (from petrography)

of Lacave Calcilutite Unit 2 and the lower Crezelade Beds by Astarte

whose dense extensive colonies differ from those of Spbenia only, but

significantly in the absence of gypsum and dedolomite in the host rock

of the latter. Normal to subnormal conditions are inferred as optimal

for Sphenia.

Modern habits of Sphenia are hard to apply directly to the

ancient because of taxonomic uncertainty. A number of modern genera

can be cited that represent a convergence of two or more ancient ones

that few authorities (recognising the fact of convergence) agree about.

Even the contents of the ancient (sometimes modern) genera such as

Arcomya, Mactromya, Panopaea and Homomya are son times doubtful. Again,

the Myacea, sensu lato,more than any of the groups treated so far show

a definite change of habit from the Jurassic to the Recent, as does 246

PL.-25:GAPINO VALVES OF Sphenip Ap.(Indica- ting aloes-energy life-assemb44.e), Top of Gluges, Calailutite IIhi t 3* S.E. of Meyronne. 247

the gastropod Pleurotomaria now largely a deep-water genus. Pholado-

raD is a well-known late convert into deep water, Panopea may be

another (Piveteati, 1956; Woodward, 16F7 (partially))and possibly

Cucullea (Woodward, 1867; Fischer, 1881). These possible reasons

for error account for the 'burrowing' [boring - present author] habit

of modern Sphenia in oyster shells and limestone compared with the

soft substratum of ancient Sphenia. The depth of modern Sphenia

(remarkably similar morphologically to ancient ones) of 10-25 fthms

(Woodward, 1887) presumably in brackish water as are other subgenera

of Corbula may however be extrapolated in view of the adaption of life

in this medium by the rest of the genus. The uniform layer of only

very gently disturbed (inference from p1.25.) Sphenia is even more

difficult to explain than the layers of Astarte in Lacave Calcilutite

Uni2 and the Mya shell pavements of Bradley (1957) who could not come

to a definite conclusion on his. No visible minor erosional surfaces

underlie them, and Sphenia was probably a deeper burrower than Astarte.

It is thought sedimentation was slow between successive Sphenia pave-

ments, bringing wave-induced turbulence to throw up the fine mud to

a depth at which the shells caved The cohesiveness of the mud

prevented sustained winnowing by unaided currents. The closeness

of the pavements suggests the operation of annual tidal cycles and storm-wave frequency. Wave base may have been sufficiently high above the sediment surface to be incapable of lifting the shells off the bottom too often. The depth of 10-25 fthms is the more plausible for this reason.

Panopaea,Mactromya and Arcomya together could be taken as ancient analogs of modern Panopaea and more distantly of Saxicava. Panopaea

is a very deep b.Jrrower in sand living from low-water to 90 fthms

(Woodward, 18 7). Imlay (1357) associated it with a soft bottom in the Jurassic of the American interior. Some measure of this range 248

in depth and bottom texture could perhaps be inferred from the occurrences

in the probable low-water mark to shallow lagoonal in the low-contact of the stromatolitic Lacave Calcilutite Unit 2 and the top of the fossil-

iferous fine-coarse bio-oosparites with or without foraminiferal sand of the St.e Etienne Limestone and the thin-bedded calcilutites of Lacave

Calcilutite Unit 4 which are thought from their setting in the succession to be deeper than the first pair. Other species occurring with Panopaea,

Mactromya and Arcomya, the faunal diversity at some of the relevant levels and the mineralogy suggest a preference for near-normal to slightly elevated salinities. The dominance of Arcomya latissima would be favoured in conditions where depth of burrowing was a premium. High temperature and a mobile bottom which may have caused the salutory occurrence of A. latissima in the unfossiliferous (except for erosional cm-laminae of small oysters and oriented echinoid spines), a mobile bottom is preferred from the strong rippling and erosional laminae but also purely from the state of the organic remains, i.e. the echinoid spine alignment and fragmentary occasional echinoids. It is inferred from A. latissima that a layer of sediment deeper than the range frequented by the majority of burrowers was often in motion. The passage from upward-finning erosional lamina to rippled calcilutite is interpreted as a passage from high current regime, with a high autosuspended load to a low-intermediate regime currents, low suspension load.

Superfamily PHOLADOMYACEA, Fleming;1828

190. Pholadomya laevinsula, Ag.

Agassiz 18425 Table 6, Figs. 8-10.

After much soul-searching, this supposedly Oxfordian Pholado- mya of Agassiz. has been chosen in preference to the common middle 249

Jurassic forms, of Agassiz (i8l2-5)and Moesch (1875). Pholadomygs

show so much variation, attributed by Agassiz to the stiffness of the

bottom, that there is scarcely any meaning in matching one specimen

with one or two published figures. Characters that have been used by

Agassiz and Moesch are: 1. Costation - multicostate or particostate.

2. Outline - triangular, cockle-shaped, heart-shaped, oval and flabell-

ate. 3. Growtn-lines - relative to costation considered es reticulation.

By far the commonest combination is the cockle-shape and costation -

the latter is used to subdivide the much more numerous cockle-shaped

forms. Because of the apparent inadequacy of this classical approach

with regard to the present collection of•31 specimens from the same

outcrop and the same beds only 3 metres thick, the author sought to

establish quantitatively that the entire collection belonged to one

species and then to see what unvarying feature may be added to the

ammunition of. Agassiz (op.dt) and Moesch (op.cit).

Procedure.

The enquiry being largely ecological and because of subsequent mechanical damage of the specimens onlyfiVe of a projected 10 parameters

(see Fig.12 ) were systematically scored. The angles and the linear measurements apart from the standard ones of length, height and thickness, should ideally be chosen to define the polygonal shape efficiently. On the figure all the outline can be generated from the parameters except the anterior portion of the ventral line whiCh may be completed approximately by a semicircle with a diameter at right angles to the point of intersection on the ventral line as defined by the inner tangent from the point S and the measure of height. The height was plotted against the thickness, lunule and escutcheon because these were the most frequent available measures and also the most straightforward - Also the analysis sought first to investigate the distribution of the parameters singly, before combin- 12 / \ , , 8 , i , ,..... i, , • i -..../ ‘ 4 / g: • CI 4 _ / , • • / • , \ • _ ._ __ / , •• ,, • , , 0 0 / 1 1 1 2 4 Height cm. 6 2 4Thickness cm. 6

\ • • . 4 •••• • • IL 4 ••• / / • V • • • \.

4 Length cm. 6 Lunule, Ele8utcheon cm 2.6

FIG. 12 FREQUENCY DIAGRAMS FOR Pholadornya 1 aeviuscutct Ag. 251

ing them in various ways.(fig.12)

Results and Discussion:

The followinD conclusions can be drawn from the plots - -

(1) that the height/thickness ratio is sufficiently consistent to

that a specimen should not be considered as falling outside the

range of the species on one pair of parameters only since there is

considerable diillerential sensitivity of the parameters to what are

thought to be environmental factors.

(3) the frequency diagrams for five of the measured parameters show

that (a) a perceptible size gap exists between long and short forms

and between high and low forms, but that highly inflated forms so

overwhelm thin ones that a differentiation of this parameter is less

convincing; a continuous variation is preferred. (b) the lunule

is also segregated into very short, intermediate and very long, the

intermediate lunule lengths constituting about 80% of the sample;

conclusions similar to those of the thickness variation are reached

for the escutcheon.

4. Bivariate Scatter Diagrams: (fig. 13)

Interactions of pairs of the measured parameters indicate

without formal statistical analysis like regression fitting, calcula-

tion and significance testing of correlation coefficients or calcula-

tion and comparison of standard deviations or mean/variance ratios -

(a) strength of correlation in decreasing order between height and

thickness; length and thickness; height and lunule, height and escutcheon; and height and length.

‘ , , 0 (

/ • kness ic

0 Th

t. lc.) 2 3 5 ) c^,

( 3 0 • ' heon tc u

3 Esc 2

2 2 3 4 5 6 2 3 4 6 1--?eight (cit^) Height (c,-) FIG.131131VAR1ATE SCAT T EP- DI AG-RAMS FOR P 1(..I:viuscuta , /19. 253

(b) the lunule and escutcheon are not interrelated.

It is inferred that the distortions in shape imposed presumbaly by

the environment acted most strongly but inconsistently on the height

sand length and least on the thickness, hence the greater approximation

to unimodality of the latter parameter and of bivariate distribu-

tions involving it; likewise the escutcheon appears more stable than

the lunule. The fields into which the bivariate plots involving the

more sensitive parameters - lunule vs height; length vs thickness do

not consistently enclose the same suite of specimens. Their validity

as a criterion of species or subspecies definition is doubtful.

There is'an unexplored possibility that the lunule vs escutcheon scatter

showing homogeneous distribution may define highly discrete

for different species. Specimens which appear visually different as

for example the siliquate P. puschi, Goldfuss type shapes were not

effectively and consistently separated by the bivariate plots from the

fuller bucardine shapes such as those of P. exaltata, Ag. This strongly

confirms the quite unpredictable presence of the former shape in

different species. The conclusion is reached that Pholadomyas deform

in shape with a constant length/height ratio and probably constant

volume.

Interactions of fours of parameters were pressed into a three-coordinate system, in which the ratios of the three basic measures, height, length, thickness and the lunule and escutcheon formed the end-members (see fig .14 ). The points thus plotted were projected parallel to each of the three axes, so that on each side of the triangle may be seen the relative contribution of the two end-members plus the contribution of the third end-member proportionately partitioned between the first two. The procedure was adopted as a quick graphic alternative to more sophisticated multivariate analyses. Renarkably L L E

Fl G-14:-Triple-coordinate plots of lunule &escutcheon vs. shell dimensions of Pholadomya laeviuscula Ag. 1-1=height, L= length; T=thickness; p= uncompleted projection-lines; numbers refer to speci mens of special interest

H/T 255

. stable results were obtained as summarised below - (a) For a

a projection in the manner described above gives the most stable defi-

nition of the species that embraces the entire collection. The spac-

ing of the projectiles on all three plots gives no convincing basis

for a subdivision of the collection except for one specimen in II.

An algebraic expression for this measure is derived below.

Let the ratio of any of the three basic measurements (L, H, T)

to each of the others be a/b

Let the lunule 1

Let the escutcheon = e

In a triangular system, every value of the lunule is expressed as:

1 1 + e + a/b

The other two end-members are likewise

a/b e and 1 + e + a/b 1 + e + a/b

a/b + e The sum of their ratios = 1 +e + a/b

The value of each of the two end-members weighted proportionately by

the first end-member may be expressed as:-

a/b I + e + a/b 1 1 a/b ._( + e + a/b a/b + e 1 + e + a/b 1 e + a/b

a/b \ a/b + + a/b = e + a/b s 1 + e + a/b

(For escutcheon substitUte e for a/b in numerator of bracketed

expression.) 256

a i \I a/b TD X ( (a/b + e)(a/b + e + 1)LI e + 1 + a/b

gr•

(b) For a 'splitter', specimens 2 and 8 may be separated consistently

•from the rest by a proportionate partition of the escutcheon between

the lunule and each of the three basic ratios. The expression

Fa a/b b (a/b + 1) (a/b e 1) e + 1 + a/b

has a consistent discontinuity between 0. 49 and 0.5 (maximum value

of expression = 1) and 0.47-0.55, i.e. a gap of .025 , 0.55 on all

three plots. There appears to be no obvious visual character that

uniquely applies to these two specimens, except that the extreme flat-

ness of specimen 2 may not be due to its youthfulness and slight

crushing alone.

field are in fact, dichotomised by interaction with the other end-

members in such a way that specimen nos. 2 and 8 are split one:one

between a group consisting of specimen nos. 1,3,4 and 5 and another

consisting of all the rest. This operates on all three plots. The

largest group (with one or two exceptions) is also the most cohesive

as compared to the point spacing of the groups 1, 3, 4, 5 and 2 and 8.

This is borne out by a visual inspection of the specimens. Blake as

early as 1907-09 was dissatisfied with existing diagnosis of Macro-

cephalites based on subjective assessment of only one or two parameters,

and proceeded instead to use five ratios of measured parami.tters. 257

Relative Sensitivity of Shell Measures to Microenvironment:

The tendency to a unimodal distribution of the thickness and

the escutcheon is thought to be more than fortuitously tied to the two

parameters whose increment receives the maximum opposition from the

surrounding mud. The outward inflation of the shell due to the addi-

tions at the shell-edge presents a broad surface upon which the ambient

stress of the mud acts uniformly. The restraining effect tends to

homogenise any subtle racial differences in this parameter. The

growing edge immediately contributing to the escutcheon likewise shows

little dichotomous variability because addition on it is very small

and because opposed edges are on a surface rather than an edge. In contrast, the contributions to the length and height depend on addi-

tions along a sharp cutting edge so that even when the investing matrix makes inflation impossible so addition can be made to these two parameters because of the cutting power in these directions. Processes of speciation are most likely to express themselves on these less restricted parameters, as is supported by the well-accepted view that the continued relative simplicity of the bivalves is largely due to their dependence on the immediate environment. The less dependrit length and height should, with no surprise, show greater tendency to diverging (or converging) changes.

In conclusion, there is reason to believe that overall shape alone as used by Agassiz and Moesch is ineffective in defining the sample of Pholadomya investigated, as it makes no distinction between changes of shape with constant ratios of standard body measurements, and a change in these ratios with no perceptible change in shape. Height, length and lunule (length) have greater freedom for variability than thickness, but for consistent definition of species or subspecies, they. need to be stabilised 1y a correction term as given earlier. It is in- 258

ferred that a combination of a greater number of parameters would pro-

duce still more consistent results, it being unknown at what point the

returns would begin to diminish. On the basis of this brief investi-

gation, Pholadomya laeviuscula may, in fact, consist of three sub-

species on the basis of the relative dominance of the lunule, escutcheon

or ratios of the basic shell measurements in a three-end member. system.

The synthesis of these three into one species need not be arbitrary

or subjective, as it can be precisely derived (see expression

30. Pholadomya cf. multicostata, Ag.

Agassiz,1842-5,Table 2111 , Figs. 1-12.

Occurrence: Internal casts occur intermittently from the Lacave

Calcilutite to the St. Etienne Limestone.

25. Pholadomya elongate

Agassiz,1842-5 Table I; Figs. 16-17.

Dimensions: L 4.7, H 4.4, T 2.9.

Occurrence: This is a distinctive form occurring from the fossil-•

iferous top passage of the Souillac Oolite to the topmost St. Ltienne

Limestone. It is always well-preserved and articulated. In the

latter formation its associates include stromatoporoids, oysers, tere-

bratulids and pebbly pelletylimestones and foraminifeFI4s.

It.is probably a muddy bottom form capable of surviving coarse bottoms,

in contrast to the apparent restriction of P. laeviuscula to a fine bottom.

34. Pholadomya protei, Bron. [Pholadomya lirata, Sow.]

Mem. Geol. Soc. Fr. No. IVA, P1V; Lycett (1863) P1. XLIII, Figs. 3-3a. 259

Occurrence: Articulated specimens occur intermittently from the

Lacave Calcilutite Unit 3 to the St. Etienne Limestone. Occasionally,

ribbed specimens may be found.

115, 111. Pholadomya sp.

Dimensions: L 3.2, H 3.55, IT 2.5.

Diagnosis and Remarks: The slim shape is similar to P. nuda, but

the smaller area anterior to the leading carina, the mesial umbo, and

the pronounced escutcheon adjoining a steeply sloping auricle-like area are features absent in P. nuda. Widely spaced growth-lines sometimes occur.

Occurrence: It occurs as casts in smooth calcilutites with a low fine pellet content in the St. Etienne Limestone at two separate local-

ities. It was disarticulated in both.

129. Pholadomya angustata, Ag.

Agassiz,1842-5 ;Table 3 1 , Figs. 4-6.

Dimensions: L 4.5, H 2.65, IT 1.4.

Occurrence: The form is sometimes very poorly preserved particularly in the chalky, pelsparites of the St. Etienne Limestone. It was disarticulated.

167. Pholadomya carinata, Ag.

Agassiz,1842-5, Tab. 4 1 , Figs. 4-6.

Dimensions: L 5.00, H 5.15, T 3.8.

Diagnosis and Remarks: it should be distinguished from P. murchisoni by the squatter shape and the higher posterior costa or carina. The difference between the two species of Ayassiz is so slight that less strongly carinate forms occurring in the fossiliferous beds of Lacave 260

Calcilutite Unit 1 with Eligmus polytypus and Quenstedtia elongata are

not referred to a separate species, and also because the faunas at

these two levels seem to be such mirror images of each other it is not

thought necessary to place the two varieties in different species.

Occurrence: This form is very common in the spicular pebbly upper

part of Lacave Calcilutite Unit 3 where it has been invaded by the

coarse burrowed bed. Oysters and long myacids are some of the common-

est associates.

116. Pholadomya sp.

Dimensions: L 2.45, H 2.3, T 1.60.

Diagnosis and Remarks: The form fits poorly into any known species.

It may be distinguished from Protocardia stricklandi by the absence of

a posterior striated area, and by the facing of the umbones which are

slightly towards the bluntly truncated end of the valves, not away

as in P. stricklandi. There are also faint costae on P. sp. The

outline, size and growth-lines of the two species are'however quite

close.

61. Indet., Pholadomya?

Strongly curving ribs on an incomplete fragment not belonging to any identified forms. Occurrence in Upper. St. Etienne Limestone dedolomites.

51b. C. var concentrica, Sow.

198. Ceratomya concentrica, Sow.

Morris and Lycett, Tab. X, Figs. 3a,b; Arkell Pl. XLIII, Fig; 10.

Dimensions: L 4.8, H 4.3, t.T 1.95. 261

Diganosis and Remarks: The author is not satisfied with the diagnosis

of the two common species of Cerat.omva, C. concentrica and C. excentrica.

Shape may be discounted as some C. excentrica of Agassiz do in fact

closely resemble C. concentrica in Morris and Lycett and Arkell. The

disposition of the oranment so useful in characterising C. excentrica

may not always be a good guide because the nature of the ornament

whether growth-lines, ribs or something else, has not been established

by any of the standard works. The author thinks this ornament is a

novel feature as it sometimes accompanies radial striations (= ribs) and/

or growth-lines on the same specimen (see figs. of C. excentrica in

Agassiz)'. (See also older specimens of Ceratomya plicata in Morris and

Lycett, 1850.) This view is different from that of Arkell (1927-30)

who calls the ornament ribs, and it leads to the conclusion that the

generally accepted wide variation in the disposition of this ornament

could in fact be due to its varying degree of development, and not to

an actual change in its orientation on the valve. Three sets of regu-

larly developed ornament may be present on Ceratomyas - (a) radial

ornament, (b) concentric or subconcentric growth-lines, (c) oblique

striae. Absence of (c) leads to the designation C. concentrica, and

its presence even with (a) and (b) leads to the designation C. excentrica.

Descriptively, it can be misleading to say growth-lines or costae are

concentric or oblique; one ought to say an oblique ornament is present

or absent.

Occurrence: One disarticulated cast each occurs in the muddier

finer upper reaches of the bioclastic cap-bed of Gluges Calcilutite Unit

3 and the fossiliferous calcilutites of Lacave Calcilutite Unit 1, plus

one highly inequivalve articulated cast-in the latter. It is obviously

better represented here (up to 2% of a very diverse assembly at Roc

.Amadour). 262

167. Ceratomya bajociana, d'Orb

Phillips, 1875 , Pl. Xl, fig. 40.

Diagnosis and Remarks: This form is nearly identical to sp. 198 except that it is larger, lacks the broad dorso-neutral depression, and occurs in the fossiliferous nodular shell grain/packstones•of the Lower

Mirandol Conte. The size and the stratigraphic position and resistance of shell to solution were used by Arkell to differentiate C. bajociana from C. concentrica, but the two were fused in.the cluster analysis.

The termination of the umbones in Phillips' figure is much too abrupt and gives the figure an ungainly look compared to the gracefulness of the present specimens.

50. Ceratomya cf. excentrica, Ag.

Agassiz,1842-5, Tab. 8a.

Occurrence: One collection in the fossiliferous burrowed Lacave

Calcilutite Unit 3, but they are probably quite common.

191. Ceratomya plicata, Morris and Lycett

Morris and Lycett, 1850, Tab. X, Figs. la,b and 2.

Diagnosis and Remarks: The geniculation in the oblique ornament appears to coincide with the subtle anterior carina instead of the posterior as in the figure of Morris and Lycett. The specimens are readily distinguished from Goniomya, with which they sometimes occur.

Occurrence: Rare, broken or small specimens occur with Gervillella acuta, Pholadomya and small TrigoniA in different combinations from

Gluges Calcilutite Unit 3 to Gluges Calcilutite Unit 4. 2 63

143. Ceratomya cf. goniophora, Cossmann, nov. sp.

Cossmann, 1900, P1. V111, Fig. 9.

Occurrence: One small doubtful specimen in the dolomitic pseudo-

calcilutites of the upper contact of the St. Etienne Limestone.

74. Ceratomya decuritata, Ag.

Dimensions: L 3.45, H 3.15, IT 0.6.

Diagnosis and Remarks: The growth lines are very regular, 13-14

per cm. and the umbones posterior, non-mesial and opisthogyrate. The

maximum elongation is along an axis sloping downwards posteriorly,

and about which the ventral line may be said to describe an ellipse.

The latter feature among others immediately distinguishes it from

sp. 516, Ceratomya var. concentrica with which it could temptingly be

lumped.

Occurrence: This very distinct, well formed species has been found

only in the pebbly oncolitic calcilutites at the top of the St. Etienne

Limestone.

201. Pleuromya sp.

Diagnosis and Remarks: This species should be distinguished from the equally large Arcomya latissima by the deeply incised lunule and the absence of a broad dorso-ventral depression on the valves.

Occurrence: A large articulated but strongly squashed cast with a small geopetal cavity in the lunule area of the left valve was foLind

in talus material traceable to Lacave Calcilutite Unit 1. The cavity

indicates either a sudden influx of sediment or a post mortem attitude different from; the life-attitude,.as the shell would need to. be mani- pulated in an attitude far from the known life-attitude of Pholadomya

264

or other myacids to get the cavity into a'horizontal position.

176. Pleuromya cf. ferruginea, A .

Agassiz,1842-5Tab. 24, Figs. 1-9.

Dimensions: L 3.95, H 3.05, T 2.3.

Diagnosis and Remarks: The specimen falls uncomfortably between

Gresslya and Pieuromya, tending more towards Pleuromya in the manner of

the growth-lines, and in the straightness of the shorter oblique dorso-

ventral line (tends to be concave outwards in Gresslya). The straight-

ness of the longer line balances perfectly between the sag in that of

the Pleuromya and the slight up-arching in +lie Pleuromya%.

Occurrence: It occurs articulated in the ferruginous lower Mirandol

Oolite, with a wide variety of pectinids.

207. Gresslya cf. ovata, g•

Agassiz,1842 -S Tab. 13, Figs. 1-3.

Dimensions: L 3.95, H 3.5, T 2.5.

Diagnosis and Remarks: The form is slightly inequivalve, the left

valve being smaller.

Occurrence: Only one occurrence in a thin band of impure biosparites

resting on, the Gryphea beaumonti Bed.

215. Thracia cf. depressa

Br. Mesozoic Fossils, Pl. 23, Fig. 4; Arkell Pl. L, Figs. 7-10.

Diagnosis and Remarks: The valves occur articulated. The umbones

are not so prosogyrate as in -G. depreSsa and the leading end of the

valves not so abruptly truncated. 265

Occurrence: A long-ranging form occurring here only in the same

level as G. ovate (above).

221. Corimya [Thracial pinauis, Ag.

Agassiz,1842-5Tab. 33, Figs. 1 -4.

Diagnosis and Remarks: Figure 1 of Agassiz approximates most to

the specimens.

Occurrence: Immature forms occur in patches in the calcilutites

with Pholadomya laeviuscula (Gluges Calcilutite Unit 3).

Thracia cf. vicelliacensis [=Thracia curtansata , Morris and Lycett]

Cossmann, 1905, Pl. II, Figs. 4-16.

Dimensions: L 3.5, H 3.25, T 1.55.

Diagnosis and Remarks: A late collection not included in the cluster

analysis. The extreme flatness of the left valve and the straightness

of the umbones are unique to the specimen which is articulated and

forms 20 of the assemblage of Lacave Calcilutite Unit 1 at Rocamadour.

T. curtansata lacks the strong curvature of the ventral line.

47. Unicardium parvulum, Morris and Lycett

Morris and Lycett,1851 -5,Table VIII, Figs. 6,6a.

Dimensions: L 1.7, H 1.69, L-T 0.3

L 4.3, H 3.3, IT 0.65

Diagnosis and Remarks: The smaller specimens considered to be the young of the species in the analysis could be .Nucula variabilis, Phill. 1875 (Phillips,/P1. IX, Fig. 11) which it very closely resembles. The larger specimen is described here with Thracia and with the samejusti- ficati as that by which Morris and Lycett separated fiat Nactromy2s

TAB. 5 : ECOLOGICAL PARTITION OF THE 266 PHOLADOMYACEA. ION T TY VA S I

CO ESER VER D I PR

190. Pholndomva laeviuscula dB M H-I A/L oa 30. P. multicostata H-I sA of 25. P. elonqata dB t;/mS H-I A of 34. P. protei dB mS H A of

115/111. 2. 5_12. PM L D of 129. P. anqustata mS H of 167. 2, carinat sM A/L oaf

116. P. LE. PM L A of 61. indet. P. H D of

198. Ceromya concentrica 1.1 I-L sA oa-f 167. C. baiociana mS/S oa 191. C. plicata M H-I of 143. C? goniophora pM H-I of 74. C. decuritata mS/sM H-I A of

201. Pleuromya dB M I-L A of 176. P. ferrugines dB S H A Of

207. Gressiya ovata mS H A of

215. Thracia depressa mS H A of 221. T. pinguis ti H-I sA oaf T. vicelliacensis sM H A of

47. Unicardium parvulum mS H D of

dB Deep burrower sB Shallow burrower L Life position E Epifaunal A Always articulated S Bio/oo-sparite sA Sometimes articulated mS Micritic Bio/Oo/pelsp. D Always disacticulated sM Reverse ua Uniformly abundant M Calcilutite of Uniformly few

H High diversity oa Occasional, abundant I Intermediate of Occasional, few L Low diversity (pM) Pseudomicrite of St.Et. Lst. 267

into Unicardium. The specimen for this species of Unicardium like

the comments of those authors is unique in the exaggerated length

occasioned by the long and nearly straight posterior dorso-ventral line

- the second reason for which it is described with Thracias.

Morris and Lycett considered the genus to be by nature

patchily distributed in clays, limestones and shelly oolite, being more

often broken or disarticulated in the last. The observation is largely

borne out here.

Occurrence: Both this and C. tenera proper are restricted to the

same burrowed level in Lacave Calcilutite Unit 3 and occur only once

individuilly and together, so the constitution of secondary Association

3 still have embraced the same specimens under whatever name.

Ecology of Pholadomyacea:

The genera described here seem so heterogeneous that the

following preliminary generic partition based on local observations

is made (see Tab. 5 From this the local ecology of the superfamily may be tabulated thus:

( 1 ) With the exception of a few species (Pholadomya laeviuscula,

Pholadomya carinata, Ceratomya bajociana, Ceratomya concentrica and

Thracia pinguis) which dominate or form a substantial fraction of the assemblage, the superfamily may be said to occur very patchily vertically and in small numbers of individuals. The association of these majority genera and species with fossiliferous horizons and the absence of numerical dominance referred to above make it impossible to establish from internal evidence, what the unique ecologic conditions of these genera and species were, in a way comparable to Sphenia, Astarte

Gervillia or Pteroperna. Postulated ideal conditions for the majority of bivalves may be invoked, i.e., near normal salinity, good aeration and food supply, a stable bottom, clear waters and a low fluctuation of 268

all these parameters, and a depth less than 100 m. Competition is

blamed for the patchiness as the occurrences of two deep-burrowers,

Pleuromya ferruginea and Gresslya ovate, may show that deep burrowers

can find solace in this habit in a fairly unstable bottom.

(2) Those that occur rarely but dominate when they occur, may

profitably be compared. Those that occur on a sandy bottom, all show

disarticulation while only 25% of those common on muddy bottoms show

that condition - a clear demonstration of a predictable relationship,

if it were not for the fact that C. hajociana was probably a very

shallow burrower like the modern lsocardia cor and would therefore

show a higher incidence of disarticulation even if they had the mud-

dwelling but deeper burrowing forms occurring with them. Left and

right valves of C. hajociana are probably equally represented (on the

basis of the six valves counted), so they were probably not exhumed

from greater, quieter depths where shallow burrowers would be favoured.

Periods of strong agitation may have been followed by quieter ones in which these two forms and other bivalves and brachiopods colonised the sediments overlying the G. beaumonti beds (see Selly, 1968 in reply

to Doust, H.). This is supported by the unlikely unremitted operation over a long period of the high energy conditions capable of over- turning the Grypheas, in view of the high bivalve diversity. Shallow burrowing and sporadic high energy may together better explain the above phenomenon. The enquiry considered only disarticulation and not fragmentation as difficulty of extraction from indurated limestones as well as Cadee's (1968) failure to show a relation between fragmentation and high energy were impossible to surmount. The presence of some disarticulation in a shilow burrowing mud-dweller like C. concentrica is, from the reasoning applied to the deep and shallow burrowers, quite legitimate. 269

(3) In the forms in which ribbing and its absence are developed in

the same or different species, a positive relation of strong ribbing

and carination to coarseness of bottom may be inferred in a comparison

of P. protei and P. carinata (also among specimens from lithologically

different'horiZons) on the one hand and P. laeviuscula on the other.

(4) The absence of associated diversities ranging from high to low

indicates a lack of tolerance for deteriorating environmental condi-

tions for the whole superfamily.

The habits of modern Pholadomya are limited to abyssal depths

and all that goes with that such as muddy bottom and uniform, fully

marine conditions, low illumination and a low suspended load mostly

of fine organic and inorganic detritus, conditions which may have

changed in absolute terms but apply to the present distribution in so

far as P. laeviuscula, P. carinata and less immediately P. orotei indi-

cate from the lithofacies succession and other evidence, nearness to

depth culminations at their respective levels. The deeper lagoonal

postulation for Lacave Caicilutite Unit 1, for instance, could have been made with at worst nothing to oppose it, on the evidence of P. elongata and P. carinata.

(5) Assuming the degree of streamlining to influence efficiency of bottom penetration (Nair and Ansel], 1968) the genera may be arranged 41- in order of increasing burrowing efficiency as C22yEIand isocardia,

Pholadomya, Pleuromya and Gresslya, Thracia (Unicardium unplaced).

Among the deep burrowers (i.e. excepting Ceratomya and Isocardia) this order reflects increasing bottom instability and a need to re-establish a hold quickly after dislodgement.

The maximum species-frequency of five modern Thracia's occurs. from intertidal to < 60 m with two species extending beyond 100 m,' partly account;i ,Lrhaps for the Challenger discovery of one Thracia

270

in the depth range 100-500 fthms, and four at less than 100 fthms.

ThraciA are usually few in number in modern environments, and are

absent altogether from the takings of Parker and others in the Gulf of

California, Gulf of Mexico and the Laguna Madre. Cadee (1968) lists

two species without discussion in a zone less than 20 m deep. Salinity

was 33-630/00. Sand or solid rock (for T. distorta) appears to be

important for species that occur in the intertital zone (after Tebble,

1966). This could indicate for Thracia depressa a shallower average

depth than T. pinguis and T. vicelliacensis. The biosparites of the

basal Mirandol Oolite would on this evidence be shallower water than

the middle Lacave Calcilutite Unit 1 and the middle Gluges Calcilutite

Unit 3. The ammonites occurring close to the first and absence in

the last two and seemingly contraverting the conclusions on depth could,

in fact, emphasize the overriding influence of facial situations - open

marine, barrier-front (lower Mirandol Oolite vs lagoonal (Lacave

Calcilutite Unit 1)) and lagoon with good circulation (Gluges Calcilu-

tite Unit 3) - over absolute depths. Isocardia partly represented now

by Glossus probably lived down to great depths.

For Woodring (1957), Thracia trapezoides was'deep, outer sub-

littoral' in the Pleistocene, while for Imlay (1957) Thracia, with an association very similar to the present one, was a mud bottom dweller.

For Thracia it is concluded that the depth ranged from intertidal to < 60 m, being deeper for the sandy bottom T. depressa than the muddy bottom T. pinguis and T. vicelliacensis. Salinity, aeration and turbidity were as for the superfamily and like it, with equipment to deal with coarse suspended particles if occasion arose.

Environmental conditions not immediately observable in the rocks for which the genera discussed above may offer at least limiting conditions may be summarised as - 271

(1) Salinity - all genera near normal.

(2) Aeration - good for all.

(3) Rate of sedimentation - probably low for all.

(4) Bottom stability - a series can be arranged parallel to another

from Pholadomya to Thracia.

(5) Energy level and duration of operation difficult as high energies

can operate on a muddy bottom and leave no trace - all genera may be used

individually and together; articulation, disarticulation and burrowing

capacity (depth and efficiency) may be effectively combined with sedi-

ment texture.

(6) Depth - all genera except Gresslya and Unicardium for which only

very general evidence of habit or none is available.

(7) Range (not rate) of fluctuation of conditions (1) through (6):-

Vertical distribution and associated faunal diversity of all genera;

can be checked from range of other associated textural characteristics.

C. CLASS GASTROPODA, CUVIER

Gastropods were generally few in number of individuals al-

though species-wise they rival the brachiopods. A few species occurring

in great abundance, sometimes to the exclusion of other fossils, will

be indicated where appropriate. A grouping is made into fresh-water or

near fresh-water forms and normal marine or hypersaline forms.

I.Fresh-water Gastropods

Viviparus sp.

Planorbis calculus, Sand

Diagnosis and Remarks: P. calculus is widely documented for this

level in other parts of the region, probably owing to the absence of controversy about I -Ls occu,-:--nct.; in the Mesozoic. 1t is a very small, 272

often blackened planispiral gastropod. Viviparus sp. by contrast,

was never documented previously (except in Glangeacid 1895) because of

its controversial occurrence in the Mesozoic. Hudleston (1387-1396),

Von Zittel (1900) and Piveteau (1956) illustrate the full cycle from

absent in the Mesozoic to present back to absent.

A wide variation is present in the specimens of Viviparus

sp. High- and low-spired forms occur as in some Wealden species, but

all share the tumid whorls separated by a deeply impressed, almost

canalicuiate-suture, the 'considerable umbilical furrow' and the absence

of ornament, other than growth-lines, which are rugose and varix-like

on the body-whorl - characters also used by Hudleston to characterize

his Paludina (= Viviparus of some authors) lanotonensis, Hudl. The whorls are about four in number. The aperture is very wide as in some Nerita's (N. costulata, Desh. and N. subrugosa, Hudl.). Without multiplying differences, the specimens are distinguished from Natica and the Neritidae by the inflation of the whorls, and more critically- by the presence of a deep umbilicus (absent or hidden in the latter two groups), and the subrounded aperture (lunate or biconvex in the latter two groups).

Ecology of Viviparus and Planorbis:

Viviparus and its close relation Valvata are widely recog- e.g. Arkell (1940). nised fresh-water dwellers Viviparus is sluggish and never burrows deeply into mud while Valvata prefers running water to static except in fairly large lakes; dwells and rasps among plants; and burrows into safety in winter, when weeds are absent (Cleland, 1354). Viviparus rasps but has a supplementary suspension mechanism - a unique factor among fresh-water gastropods in the opinion of Cleland. The association of Viviparus with lignites in this succession and the apparent ease of reworking into the basal (occasionally higher) Bloc:our Breccia$ is highlighted by the modern habits. The supplementary suspension- 273

feeding mechanism would have adapted Viviparus very well to areas

where prolonged inundation was possible and plant material less profuse.

Low-lying marshes and salt-pans with some fresh-water influx and tidally

inundated would account for the presence of brackish or normal marine

forms - oysters, Lucina, Lingula, Pholadomya (very occasional) and

Crossostoma prattii in the marls. The association of oysters particu-

larly in the uppermost reaches of the beds is significant in its simi-

larity to the lower tidal (just subtidal for oysters) mussel banks that

stand between tidal-flats and marshes, and open sea in modern estuaries

and estuarine bays such as the Wash (Evans, 1965).

' The complete absence of lamination in the marls containing

Viviparus and the high mottling as well as the good preservation of

the gastropods argues for burrowing occasioned by gastropods that

lived and died here. The blackening, particularly of the very small

Pianorbis is thought to be comparable to the iron monosulphide coating

common in recent tidal flat environments (also in off-shore slow-

depositional areas in the Persian Gulf (G. Evans, personal communica-

tion)). Associated bivalves are characteristically thin, minute and

disarticulated indicating that the gastropods lived at the upper limit

of tides, receiving only the finest of the transportable lower tidal-

flat and shallow subtidal fauna. Geopetal spar is present in Vivi-

parus but is less frequent than would be expected in a whorled organ-

ism unless the sediment was sufficiently soft or thixotropic to flow

freely. Dead conchs overturned by their living neighbours could thus

be easily completelyfilled with watery sediment. Hudleston's (1887-

96) observations on the occurrence of Paludina langtonensis are very

closely matched in substance by the present occurrence. The deposit

(of the Paludina Bed) is mortar-like (E- grey marl here) and contains other fresh-water gastropods (a? Crossostoma here?) and Chara. But

Cerithium, Nerinea and Amperieya (a mixed marine to brackish forms 274

listed above) also occur. The Paludina Bed underlies a gritty bed

containing teeth and plates of fish (E reddened lime-pebble bed with

reworked Viviparus at base of Blagour Breccia). The Purbeck Beds of

the Dorset coast likewise contain a Viviparus fauna considered from

the evidence of Brown (1963) and West (1964) to have been transported

into a hypersaline environment, or capable of standing high salinities.

The latter possibility is discounted (Fischer, 1887, p. 732).

The similarity of the infill of the gastropods to that of

the host rock and the presence of euhedral gypsum in both in this

succession shows that hypersaline conditions were imposed on .castropods

in place: The first alternative may also therefore be discounted

for this succession. The gypsum can thus be interpreted as forming during periods of low precipitation when fresh-water influx was negli- gible and high surface evaporation and the gastropod population was decimated, as happens in modern salt pans. Conclusions concerning the spawning temperatures of oysters may emphasize low precipitation over excessive temperatures, at least in the waters where oysters lived, conceived to be just subtidal. Rusty to creamy-yellow granules associated with Viviparus indicate that periods of low precipitation allowing fossil soils such as were invoked for the 'Dirt-Beds' of the.

Purbeck (Brown, 1963) to form at supratidal levels and were parly swept into the marshes when high run-off returned. Such material was worked into the sediment by the burrowing activity of Viviparus.

Crossostoma prattii is a much shallower form than Viviparus, is very much more symmetrical in shape and with normal sutures and uninflated whorls. It is completely smooth, lacking the coarse growth- lines of Viviparus. Smaller Viviparus may be distinguished by their umbilicus among other features. its occurrence only with Viviparus strongly suggests brackish-fresh water affinities. 275

Ii Stenohaline Gastropods:

These are described individually with ecological comments where

appropriate.

97. Nerinea cf. subcilabra, Hudleston, sp. nov.

Diagnosis and Remarks: Only the lowermost three whorls are present

and undoubtedly carry the 'granulations' of Hudleston's species. The

whorls are tco long to be those of Cerithium cerce, d'Orb.

Occurrence: One occurrence in oolitic pellet grainstones in she

same beds of the St. Etienne Limestone as Arcomya latissima.

86. Nerinea pseudocylindrica

Hudleston 1887-96, P1. XII/,Fig.

Diagnosis and Remarks: A very narrow species distinguished from sp.

97 by the fine spiral lines on the lower whorls, the upper ones being

usually unextractable.

Occurrence: It occurs in the shelly laminated and fossiliferous

pellet grainstones of the Upper St.Ettenne Limestone and occasionally

in the basal few non-stromatolitic beds of Lacave Calcilutite Unit 2.

48. Nerinea altivoluta ?

Hudleston,1887-96, P1. XIII, Fig. 10a.

Occurrence: An even narrower form than N. pseudocylindrica, it occurred as an etched mould in the burrowed Lacave Calcilutite Unit 3 below St. Etienne.

42. Nerinea tuberculosa, Defrance

British Museum Collection.

Occurrence:' One of two giant turreted gastropods in the Lai-izac

Oolite, this form is common in the. jety algal band lower beds. It 276

tends to occur in.nests just as the terebratulids associated with it, forming thin lenses of limestone 23 cm across. These flake off easily and show even on freshly broken surfaces, several layers of algal coat-

ing that seems to bind them on to the nearest particle. The whole fabric being obviously an algal boundstone. Oysters are other common associates as are crustose algal, polyzoa and corals..

Ecology of N. tuberculosa:

Nerineids are not represented today, but McKerrow, Johnson and Jacobson ( 969:. _ ) likened Great Oolite Nerineas and

Aphanootxxis to Tulitella which occurs in modern seas. If the assumption is correct than N. tuberculosa and other Nerineids in this succession were infaunal suspension feeders (note that the majority of gastropods, unlike the bivalves are either grazers, carnivores or bottom feeders rather than suspension feeders). Yonge (1946b)considered gravelly or shelly stiff mud to be ideal and Cadee (1968) implied the bottom texture could affect distribution of Tuitella. Woodward's (1887) depth ranging intertidal to 100 fthms may have been exaggerated on Cadee's

(1968) evidence for 50-60 m for T. communis. High current velocities in an inner neritic environment with a coarse algal stabilized bottom could consistently with the literature and other local evidence be postulated as ideal for colonies of Nerinea. A low suspended load, clear,warm, well-aerated waters could have prevailed. It is possible that patches of dead Nerineid colonies provided anchorage for terebratulids and Lopha gregarea. Most other Nerineids in the succession lack the abundance that would justify a rigorous extrapola- r. tion from Tur tella or other ancient Nerineids.

0 PtyrTiatis sp.

The form occurs profusely in tectonically disturbed beds outside ._ •- the mapped area but considered to be Lanzac Oolite. They are as large

277

as N. tuberculosa.and may consolidate the inferences for this formation,

based on N. tuberculosa.

Ampullospira sharpei,(Morris & Lycett) /58.

Morris and Lycett 1851 -5,P1. Xl, Fig. 22.

Diagnosis and Remarks: The last whorl is convex as in N. canali-

culata, Morris & Lycett, but the absence of channel at the upper angle

of the whorls does nevertheless distinguish it from N. canaliculata,

Morr. & Lyc. Various other euspirid (genuculated) naticids are readily

distinguished from this one-.

Occurrence: It occurs mostly in pellety dedolomitic calcilutites of

the topmost St. Etienne Limestone and less so in lower horizons - always

as casts.

166. Phasianella cf. umbilicata, d'Archiac

HUdleston 1887-96, P1. XIX, Fig. 15.

Diagnosis and Remarks: About five whorls are present and the ultimate whorl does not dominate the rest as in Natica where the spire is also lower than in this specimen. The sides of the spire show no change in slope as in P. umbilicata of Hudleston, and the ultimate whorl in the specimen is also larger relative to the rest.

Occurrence: The occurrence is in calcilutites of Lacave Calcilutite

Unit 3 with Pholadomya carinata and Quenstedtia elongata, and in the

Upper St. Etienne Limestone.

Ampullospira canaliculata,(Morris and Lycett)

Hudleston, 1887-96, Pl. XX, Fig. 16.

Occurrence: The form occurs as a cast in chalky oosparites in the coarse bands of the lower St. Etienne Limestone. 278

152!. Nat i ca neritoic1e_o

Occurrence: Two specimens occur one in each of two successive fine

pellety calcilutite bands of the Lower St. Etienne Limestone. One band

is faunally very diverse.

152b. Lyosoma enoda, Sohl

Paterson, 1968, P1. 11, Fig. 2a,b.

Occurrence: One specimen was found in very fine pelmicrosparties in

the basal Lacave Calcilutite Unit 2 near Souillac (Coach St.).

• 114. Nerita Le?

Diagnosis and Remarks: The whorls number 3-4, and the spire is much

lower than in most Natices. The aperture is partly mechanically dis-

placed but probably bounded by a partly convex inner lip, and a strongly

arched outer one.

Occurrence: One occurrence in pellety ooid packstones of Upper St.

Etienne Limestone.

225. Endiaplocus roissyi (d'Arch)sp.

Morris and Lycett,1851 -5,P1. VII, Figs. 14 and 14a.

Diagnosis and Remarks: The specimen lacks the weak radial striations

of E. roissyi, developing instead a very regular and close-packed spiral

ornament. The simplicity of the chambers and the high width/height ratio readily distinguish it from the Nerineids. The shell is preserved with a spar (polite infill.

Occurrence: One specimen was found in the oosparites of the Mirandol

Oolite. 279

224. • Nerinella gracilis, Lycett

Hudleston 1887-56, Pl. XII, Fig. 12.

Occurrence: This slender species occurs in a worn and broken condi-

tion together with coralline debris in the oosparites of the Mirandol

Oolite. in extreme cases it could be mistaken for slender coraila.

A very similar mode of occurrence of indeterminate species of Nerinea

occur in fossiliferous oosparite facies of the Upper Blagour Breccias4.

172. Pseudomelania sp.

Diagnosis and Remarks: The almond-shaped aperture and overall

shape mark it out from the Nerineids. It also appears to be the only

species of the genus at this level in the fossiliferous pisolitic beds

underlying the uppermost member of the Mirandol Oolite. It is chem-

ically much altered and blackened like most other fossils at this level.

Pleurotomaria sp.

Diagnosis and Remarks: This minute highly ornamented form occurs

among corals in the coral-beds of the Mirandol Oolite, probably their original home. In plan it is pentagonal, the ridges radiating slightly skewly from the apex. The spiral ornament forms knobs at regular

intervals •on the ridges.

Ecology of Natica, Nerita and Cerithium:

Woodward (1887) thought Naticds frequented sandy and gravelly bottoms from low water to 90 fthms, predating on smaller bivalves.

Natica, Tur,Itella and other gastropods were only patchy in depths greater than 100 fthms in the Challenger takings, the depth accepted by Woods (1897) for Natices of the Cha1:, Rock. Cadee (1368) considered the sediment texture to be crucial in naticid distribution on account of 280

the peculiar construction of the foot. Boekschoten (1566) thought

naticids frequented the area just beyond the surf zone where larval life

and adult bivalve life was profuse. Natica's were rare in Parker's.

areas where salinities were either high or fluctuated between normal

and low. From the brief survey, the relative commonness of naticas

and neritas in the St. Etienne Limestone may be taken as good evidence

of a strong marine influence attributed elsewhere to the ineffectiveness

of existing shelly oolite bars to isolate completely a lagoonal area.

Their occurrence in fossiliferous horizons further confirms the depen-

dence on a good bottom fauna for food. The local evidence suggests

a preference for a muddy sand bottom (E--- chalky, micritic pelsparties),

or pellety muds interpreted after Cadeels (1968) requirements for the

efficient use of the foot of NaticSs, as probably stiff. This should

not be pressed too far as the author has observed fresh-water gastro-

pods move with every ease on a cushion of mucus over the top loose

millimetre of pond mud.

Cerithium is a typically tropical gastropod (Woodward, 1887)-

that occurs also i.n norther latitudes. They range from marine to

highly saline - environments, while related forms like Potamides and

its subgenera are fresh-water. In the Persian Gulf cerithids form

large coquinas in the intertidal-supratidal zone, and occur also in-

the lagoon. In the Laguna Madre (Parker, 1959), a Cerithiid occurs

characteristically in a very shallow, grassy, hypersaline protected

lagoon. In the Niger Delta cerithiids very similar to Cerithidea

from Borneo (Wood,ward, 1887) and another more spinose form occur ex-

tensively on mud-flats on either side of the brackish-water lower

reaches of the distributaries, and are gathered for food. They are often swept by heavy tropical rains into the distributary channels, but crawl back in a few days - a very good an of the conditions under which Viviparus may have lived, and soils periodically washed in. 2 01

Centimetre-thick coquinas of unidentifiable cerithiids occur in their

,original state in the stromatolitic member of the Lacave Calcilutite.

Their concentration diminishes below the critical layer and bicturba-

tion• is traceable downwards from it. The associated algal structures

and gypsum confirm what could have been inferred from the modern ecol-

ogy of Cerithium - that a very shallow < 5 ft. of water or intertidal

to supratidal conditions of high salinity prevailed in Lacave Calcilytite

Unit 2.

106. Alaria palmate, Blake sp. nov.

Blake, 1905, Pl. VII, Fig. 9.

Diagnosis and Remarks: There seem to be two varieties, one in which

the spiral rise on the whorls is sharp and one in which it is not. The

former has never been found with a siphonal canal or an auricle, but

resembles Pitteia dussensis, Cox sp. nov. (Cox 1565, P1. 27, Fig. 16)

in all but the lack of granulation on the spiral ridge. Sometimes

the auricles are encountered more often than the individuals themselves,

as Blake also observed and specific diagnosis is hindered. There is

a third specimen which is probably a different species.

Occurrence: The species is common in the St. Etienne Limestone.

Ecologj, of Alaria:

Apporhais , the modern form equivalent of Alaria, is a bur-

rower in sand or mud according to species, and mud-dwellers are thought .

to have a broader auricle and lighter shell than coarse bottom dwellers

(Yonge, 1937). Woodward (1887) gives the depth range of Ar.)Dorhais

as 0-100 fthms. The wide auricle of the specimens compared to most

species of the genus, tallies with the occurrence in fine arnaceous

pelsparites. The siphonation may be interpreted as a pecularity of •

marine life. Aeration above and below the sediment surface was pro- •

282

bably good, as was bottom stability.

Ceritella lindonensis, Hudleston sp. nov. (P1. Xl, Figs. 3 & 4)

Cerithium polystrophum, (P1. X, Fig. 12)

Diagnosis and Remarks: These two species that flood the surfaces

of plates of limestone in the upper beds of Gluges Calcilutite Unit 4

and Lacave Calcilutite Unit 2 at Rocamadour. C. polystrophum, the

multispiral form is by far the most abundant, and probably very similar

or identical with small gastropods occurring in the same manner in

certain horizons of Lacave Calcilutite Unit 2 with algal-laminated

interbeds. They are here often broken and covered by oncolitic coats.

In neither horizon has burrowing been found with C. polystrophum.

Burrowing, the shoulder on the whorls and the nature of the chambers,

and the structure of the chamber partitions were the basis for con-

sidering the species. in the bioturbated horizons of Lacave Calcilutite

Unit 2 as distinct from the forms referable to C. polystrophum, but

akin to Ceritella lindonensis, save in the burrowing habit.

Occurrence: Both species were probably marine to hypersaline judg-

ing from the association with stromatolites in Lacave Calcilutite Unit

2 and gypsum in Gluges Calciluti'te Unit 4. The problem is complicated

in the latter case by the presence of extensive beds with Vivparus and

the question of the juxtaposition of fresh and hypersaline conditions.

Hudson, 1970, thoughtthat the association of his microtufa (inferred

by him as fresh-water) and gypsum (which is a hypersaline mineral) was

'understandable in the supratidal zone of a marginal marine environment'.

Here, periods of high precipitation and availability of fresh-water

probably alternated with periods of aridity and widespread hypersaline

salt-pan formation. Periods of torrential rain would discharge accumu-

lated laterised fossil soils on to Viviparus-inhabited muds and marshes. 283

180/181. Indeterminate small gastropods in

Gluges Calcilutite Unit 4.

D. PHYLUM BRACHIOPODA

in Brachiopods have not been studied r11-4 the same detail as the

bivalves and gastropods in palaeoecologic or palaeoenvironmental deduc-

tion. They were taken as indicators of marine or slightly hypersaline

conditions wherever they occur in the succcession, if evidence of appre-

ciable transport was absent. Their main value was in relative age

determination, a context in which some of them have already been con-

sidered in relevant detail In Chapter 2. What follows is little more

than a listing of brachiopods with pertinent palaeontologic and palaeo-

ecologic comments.

Because of their stratigraphic value, identification of the brachiopods was checked with the British Museum (Nat. Hist.), and Professor

Ager whose suggestions will be apparent in the specific descriptions

Details of horizon were withheld in the first instance in order to assess

the reproducibility of identifications and to avoid cyclic reasoning as far as was practicable.

208. Homoeorhynchia cynocephala, Richard

Davidson 1851-55, Pl. IV, Figs. 10-12; Ager 1956-60

Occurrence and Ecology: The species occurred as small fragile individuals in the black shales of the Floirac Shale and as large ones in coarse ferruginous shelly slabs probably coming from the above shale, but not found in place. Dr. Ferguson of this F.apartment kindly informed the author that the peculiar shape ;s an adaptation to strong currents.

If so, the black shales were obviously unfavourable in the absence of objects of attachment as much as in the sluggish currents leading to

284

reducing conditions. The ferruginous sheliy sediments would thus

have provided the ideal requirements of texture and good aeration

through strong currents. The central fold may have enabledit, through

efficient separation of the inhalent and exhalent currents, to cope in

the unfavourable reducing environment in which other brachiopods are

cui.te rate. It is important to note that serrations of the commissure

taken by Ager (1963) after Schmidt (1937) as efficiently excluding

large solid particles are developed only at what would appear to be

two lateral lobes serving the inhalent current, while the central fold

is as much reduced in area as it is free of serrations.

170. Rhectorhynchia? lacunosa (Schlotheim)

Diagnosis and Remarks: The species is uniplicate with a central

fold culmination for the right and coinciding with a restricted devel-

opment of the outer lobe of the shell. .The umbonal angle is rather

similar to that of Thurmari lla to which the specimen was indeed assigned

(without benefit of horizon details) by Owen of the B.M. The -adopted

identification was by Professor Ager.

Occurrence: The species occurs patchily in ferruginous shell grain-

stones of Mirandol Oolite Unit 2B together with Echinoids, bivalves,

PseudomelaniA and other Brachiopods.

152. Rhactorhynchia? subobsoleta (Davidson)

Davidson 1851-1;5, P1. XVII, Figs. 14, 14a,14b.

Diagnosis and Remarks: The species bearS a close superficial re-

semblance to. Prionorhynchia serrata, J. de C. Sow. (Ager, 1956-67,

Pl. II, Figs. 10a-c; Davidson 1851-55, PI-. 11:771 ) but the incurved beaks and rarity or absence of growth-lines observed for the latter p sp cies (Ager , 1956-67) make good negtive diagnosis of subobsoleta.

285

Occurrence: As for R. lacunosa.

151. Torquirhynchia inconstans, J. Sow.

Childs 1969, Pl. 10, Figs. 1-3.

Diagnosis and Remarks: The restricted range of this species

(Kimmeridgian - baylei - cymodoce Zones) made it very useful in the local zonation.

Occurrence: The species is common in the top of the lower third of the Lanzac Oolite.

123. Thurmannella obtrita (Defrance)

Childs 1969, P1. 5, Figs. 7-9.

Diagnosis and Remarks: The species is quite distinctive, the subpentagonal outline and obtuse beak angle distinguishing it from

Burmirhynchia elegantula (Desi.) occurring just below it in this succession. T. acuticosta, Childs, ranging one zone less than obtrita is without a concentric ornament (Childs, 1969) and therefore not to be confused with the present specimens.

Occurrence: T. obtrita is common-abundant in the pellety lower third of the Lanzac Oolite, but is scarce and larger in size in the pure ooid grainstones of the middle third. Good collecting localities are at the village of St. Etienne and on the hill-top form Lacave. to

Meyrihanc. The species probably preferred the quiet protected part of the oolite barrier although the thin string of large specimens in the pure ooid-grainstone may have been the result of an attempt to settle on the barrier crest which was foiled by the return of high- energy conditions with sorting to produce a residual concentrate of coarse shells. Childs (1969) surjgasts 10 f'::hms, nearness to shore- line and low osi on i'or tho s - a vthi;:h are in accord with the present data.

286

35/37. Burmirhynchia elegantula, Desl..

Diagnosis and Remarks: The specimens have a narrower beak angle

and a less well developed central fold than the figure of Deslongchamps

(1862). The revised genus was by Professor Ager.

Occurrence: The species is most abundant together with a wide var-

iety of bivalves in the top passage of the Souillac Oolite. They are

more abundant in the micritic coated grainstones of the Blagour Breccias,

but from the Souillac Oolite upwards they are consistently less abundant

than the terebratulids like Obovothyris. Even so, the terebratulid/

rhynchonellid ratio in calcilutites does favour the former to an extent

attributable to a greater tolerance for soft bottom and perhaps high

salinity by terebratulids. The observation-draws support from the

Gluges Calcilutite, where, throughout the gypsiferous lower units only

terebratulids manage to occur with obvious size reduction. With such sensitivity to facies, it is not surprising that this traditional discus (lower subzone) species should range here into the upper

Callovian.

153. Loboidothyris perovalis, J. de C. Sow.

Davidson,1874-82,P1. XXV, Fig. 11; Tr. Invert. Pal. H 782,

Fig. 6435a-d.

Diagnosis and Remarks: The species has already been discussed.

It only remains to add that there is much variation in the degree of plication and that very subtly plicated forms should not be mistaken for T. vari, Roll. (Lanquine 1929, T. XXXII, Pl. V, Fig. 6).

Ornithella bathon!ca, Roll.

Muir-Wood 1936, . V, FI • 3,7a-c. 2 87'

M. lagenalis D. siddingtonensis 4 0. perobovata

2 Th ickness( cm. )

_c 4-)

As above

2 Thickness( cm.)

BIVARIATE SCATTER DIAGRAMS of Microthyridina, Digonalla & Obovothyris 287

Diagnosis and Remarks: The species is probably equivalent to

Zeilleria and Waldheimia ornithocephala of local authors. The occurrence

and stratigraphic implications of the species have been discussed in

Chapter 2 under 1 Gluges Calcilutite'.

12. Obovorhyris peroborata, Digonella siddingtonensis and

132. Microthyridina

Distribution: These forms show a preference for the muddier parts

of oosparites/calcilutites passages. They are very abundant as such

in the upper passage of the burrowed level in Lacave Calcilutite Unit

3 throughout the area mapped. In the Souillac Oolite upper passage,

induration prevents a more favourable assessment. They are often

accompanied by oysters and encrusting polyzoans 'at the former level.

A well-aerated but protected and only slightly agitated environment

seems to be best for them, their pedicle being probably too slender

to cope with anything stronger. Oysters may have been resorted to as

objects of attachment on account of the mutual exclusiveness of high

energies and mud; the association could be thUs explained. Additions

• to the existing mud cover may have been slow as indicated from the

hard-ground aspect and the burrowing associated with these species.

The range is higher into the St..Etienne Limestone.

Remarks: A collection of seven specimens from the St. Etienne

Limestone and burrowed Lacave Caicilutite Unit 3 submitted to - the

British Museum confirmed, with inconsequential differences, the lack

•of a. vertical differentiation as well as the actual identifications

arrived at by the author. Three morphologic types appear to be

present both visually and from a plot of iength by thickness. The

width gave less satisfactory •results in combination with the other

two measurements 14' The lower thickness/length ratios

288

of specimens of group II are deceptive, as a t-test for significance

between unpaired samples (Ratcliffe, 1966) was negative: calculated

t = 0.38, t from tables = 2.78 (5%, • d.f.). It is with these quali- .

fications in mind that the highly inflated specimens are tentatively

assigned to Obovorhyris perobovata and the rest except one, to Digonella siddingtonensis. The odd one has a better defined pair of ridges

posteriorly and looks more elongate. It may not just .be a juvenile as specimen No.3 in group I is just as small but considerably wore spherical. Microthyridina lapenalis, Sch. (DaVidson, 1851-55, Pl.

VII, Figs. 1-13) seems the nearest to this specimen.

23. Lophrothyris subsella

Loriol 1872, Pl. XXV, Figs. 2-20.

Thurmann and Etallon, 1861-4, Pl. ? Figs. ? ; Davidson Sup. PL. XIX. Figs. 10-12 Diagnosis and Remarks: The arguments regarding the downward extension of this species need not be repeated except to say that the

British Museum identified it without aid as probably Kutchithyris and

Callovian in age, noting that the genus is as yet unknown outside

India. The age is quite in accord with the author's conclusions, although the obliquely truncate foramen, the greater angle between the beak ridges, the broad outer folds of the commissure and the more nearly . regular pentagonal outline are all features of Kutchithyris not shared by the specimens.

Occurrence: This species is much more abundant than the more tumid one identified as Stiphrothyris tumida (see below), although both are abundant in the chalky upper passage of the Souillac Oolite. The large pedicle foramen cannot be interpreted as a high-energy adaptation because of the common occurrence oF L. subsella in calcilutite, but ft ceJ;d have ensured a broad contact with the soft sediment, or 289

at least with that part of the sediment immediately around a solid

attachment, in similar fashion to the role of the auricle in inorrhais

serresiana (Purdy, in Imbric and Newell, 1564) or the web of mud-

dwelling birds. ( See Cooper, G.A. 1537 for similar, not identical

views about pedicle of Clidonophora and other Palaeozoic mud-dwelling

brachiopods.) The contrast with the very small foramen of Ornithella

bathonica could be a good confirmation of this view as the latter occurs

also in calcilutite but one with a great abundance of Lucina, Aniso-

cardia and a few Limas and Pectens which may have maintained a shelly

bottom.

188. Stiphrothyris? tumida

Treatise Invert. Pal. (More), Pt. H, Fig. 652.

Diagnosis and Remarks: A feature not generally included in des-

criptions of Stiphrothyris but present in several illustrations

(Davidson 1874-82, f1-55; Treatise on Invert. Pal. (le) and type

specimens (B.M.; College Collection), and which distinguishes it

from present collection of L. subsella is the relative shortness of

the beak ridges when straight or the tendency of these ridges to curve medic-posteriorly from about the level of the hinge-line, according to variety. The British Museum could not give a name but thought the specimens were Callovian in age, again entirely bearing out dating of the basal Lacave Calcilutite, but leaving unanswered whether a typically Bajocian/Bathonian genus (Ager, personal hints) was indeed ranging into the Callovian; the sedimentary arguments point a contemporaneous passage from Souillac Oolite (E barrier) through Lacave

Calcilutite Unit 1 (a- lagoonal calcilutite) to Lacave Calcilutite Unit

2 (E inner lagoonal and inter-/supra-tidal stromatolite and gypsum deposits).

290

32. Ornithella humeral's, ROmer [Davidson Suppl. P1. XXIV, Figs. i8 and 19]

109. Rugitella cadomensis (E.E.-Desiongchamps) [Muir-wood, 1936, P1. II, Fig. 171

140. Wattonithyris cf. pseudomaxillata, M.-Wood, sp. nov. (Pl. III, Figs. 8 and 5)

The three species are discussed together because the first two

have similar facies preference and the third is a doubtful cast not

deserving separate consideration.

Diagnosis and Remarks: The plates in de Loriol (cited in Davidson)

are closer to the specimens than those of Davidson (see above) for O.

humeralis. The specimens of R. cadomensis of which a fine specimen

with the dortal valve partly squashed in was collected in the top-most

St. Etienne Limestone, can be told from other dorsally carinated tere-

bratulids (sensu lato) of a comparable range by the subtlety or absence

of a depression on the dorsal valve, the absence of a septa] trace on

the dorsal valve, the rectimarginal commissure, or a combination of

all three. The inflation however seems rather less than in the figures

of Davidson. 0. humeralis as represented by the present specimens and

by de Loriol's figures, is distinguished from R. cadomensis by its sub-

orbicular outline.

Occurrence: O. humeral:s and R. cadomensis both occur occasionally

in unfossiliferous calcilutite, but attain their maximum abundance in

fine-grained pellet-limestones or oolite or a combination of both.

Both are abundant as such in the top passage of the Souillac Oolite and

in the coarser bands of the St. Etienne Limestone. 291

E. PHYLUM ECHIODIRHATA

Echinoderms formed such a small. proportion of the assemblages

that individual species are not described. Their ecology is discussed.

Ecology of ECHINODERNATA

Whole echinoids are sparse and very restricted vertically while crinoid thecae are rarer still. Regular echinoids Hemicidaris sp.

(no.39) and Pseudodiadema subcomplanatus (no.164) occur respectively' in the calcilutite of Lacave Calcilutite Unit land in a few lutitic beds with Pholadomya in Giuges Calcilutite Units 2 and 3. See G. Cotteau

1885, for list of Jurassic Echinoids of France. Hemicidaris E. occurs also as rare worn fragments in the thin-bedded uppermost member of the

Lacave Calcilutite

Irregular echinoids are represented only by Plesiechhus

(no.169) and only in the fossiliferous pisolitic serpulid bed of the middle member of the Mirandol Oolite particularly near Les Courtils

(pre St. Denis). All three spetimens recovered occurred in life attitude:

Echinoid spines and plates are much more persistent vertically, but still tend to a peak in the levels with whole specimens as in the

Gluges Calcilutite. The two often occur together on the same bedding plane. Spines often show strong orientation, as much as 66% of 59 measured spines on one slab falling within 20 degrees of each other and just less than 80% falling within 30 degrees (see fig; and Chapter

Sometimes the spines tend to be crowded along tracts 5-8 cm across on bedding planes. These tracts sometimes coincide with infilied grooveS containing spines and detached oyster valves.

The seemingly elaborate documentation of echinoidspines and the occurrence of whole echinoids among showerS of spines were designed to emphasize the similarity between the mode of occurrence of regular echinoids in this succession and in beds of a comparable ac to the 292

Middle Gluges Calcilutite studied by Aslin (1968). He thought that

echinoids of the Upper Estuarine Limestone were periodically swept

into depressions to be entombed amid semipiastic muds. The evidence

for strong currents often hard to find in fine uniformly grained rocks,

may be used to explain the grooving as due to minor

scour. One can then imagine the echinoids living on either side of

the scours on a muddy semiplastic well-aerated bottom in near-normal

salinity and being occasionally swept catastrophically (not directly

observed here) or gradually rolled after death into choked or choking

scours. Some of the lack of spine orientation on some patches may

be explained, on this basis, as due to local eddies. Sedimentary

structures in the topmost member of the Lacave Calcilutite, the strongly

oriented echinoid spines, and abundant detached oyster valves sometimes

on the same bedding planes, point to similar environmental conditions

at this level, although transport of whole echinoids may have been

over a greater distance than in the Gluges Calcilutite. Microscopic

echinoid remains are dealt with under a chapter on Microfacies.

Detailed information on the ecology of recent echinoids is rare, and

the group is demonstrably absent in organism lists of modern inshore •

environments. The depth however appears to be generally inner shelf

(< 50 m) from a perusal of compiled abstracts in the Treatise on

Marine Ecology and Paieoecology (Editor: Ladd, H.S.). Some indication

of shallow depths also comes from Hedl-ey (1896-1900) who found 7

species of Echinoids in Funafuti Atoll Lagoon from which a depth of

< 50 m may again be inferred, and also from modern coastal terrace

studies such as those on Aldabra and Andros Is. (Newell 1957. after

Ginsburg 1953) intertidal notches on solid rock are attributed partly•

to echinoderm rasping. Coral reefs are also known to be destroyed

by echinoids. !t must be said that the beds under consideration in

this succession have no evidence whatever of a 'hard' bottom, so that 293

the minor scours could not be attributed to our invertebrate sculptors.

It may be assumed that a depth of less than 50 m applied to the species

here considered. This depth also applies to Plesiechinus sp. with

the difference that - the latter probably always preferred a coarse

bottom partly consisting in this case of Serpulids that could with

some stretch of the imagination have supplied its food.

One doubtful crinoid theca each occurred in the top member of

the Miranda] Oolite and in pellets of the uppermost St. Etienne Lime-

stone, but stems in pieces usually less than 0.5 cm and single ossciles

can occur as minor constituents in almost any shell lamina throughout

the succession. A detailed consideration of. crinoids is clearly not

warranted, except to say that the high degree of fragmentation of

Stems, even granting the ease of fragmentation, and the rarity of

thecae, as well as the detrital condition of associated shells with

or without rippling,' all suggest that open marine conditions were rare

in the succession and that crinoid remains, light as they are, were either transported from a long way offshore or from nearby areas where salinity never remained normal lOng enough for a profuse growth.

F. PHYLUM COELENTERATA

Corals were the exclusive representatives of the phylum, bdt species were few and far between. Only the ecology of corals is discussed with respect to the present occurrences.

Ecology of Corals:

Corals form interrupted but traceable bands only in the topmost member of the Miranda] Oolite. In the Lanzac Oolite they form small (a few cm) colonies but these are visibly dotted irregularly in certain ill-defined horizons. They are sometimes hard to tell from branching polyzoans that occur with them, but both groups nave clear- 294

cut representative specimens. Various other irregularly celled col-

onial organisms encrusting shells and hard to assign either to corals

or to polyzoans without a disporportionately extensive search have

bean called stromatoporoids and treated as such. These encrusting

forms are again common in the main unit of the Biagour Breccia%,

certain beds of which have a very similar sparry and shelly compound-

.is are clearly not worth a consideration

.)ou .Iac Oolite and in the burrowed Lacave Calcilutite

Unit 3, except to mention the occurrence of the button-shaped coral

Chomatoseries ocbulites (Lamoureux) [S.C. -Fischer 1969, Pl. VI,

Figs. 11-13], the crenulated septae of which form initially curious

polyzoa-like fragments in the basal member of the Lacave Calcilutite.

Trochaeria cupoioides, J.C.-Fischer, sp. nov. [Pl. Vi, Figs. 4-6]

appears to be the Lanzac Oolite equivalent of Chomatoseries orbulites.

Corals in Mirandol Oolite:

The good development of corals in the Mirandol Oolite enable firm deductions to be made about the depositional environment of that

- formation, but first the species and mode of occurrence of the colonies are briefly considered. A large (> 2 cm) coarsely septate, cupolate solitary coral, Montlivaltia sp. (No. l77) occurs in small clumps in fine ferruginous oosparites of the Lower Mirandol Oolite. The inner ends of the septae are often destroyed by coarse, clear drusy calcite.

Some unfilled cavities persist. The base and wall are almost always missing so .that what at first appears to be an in situ gregareous -growth could, in fact, be a transported one. No supply. beds with criteria for an in situ origin have been found, while the commonness of the complete circumference argues a very short transport.

The corals beds in the upper half of the topmost Mirandol

Oolite member consist of three ,easily distinguishable species of corals 295

and clumps of Serpulid tubes that could be mists en for one of the coral

species. The three species are -

(a) A thecate solitary form with 12 widely spaced major septa with-

a central cup. Growth stages are clear on the theca. It is recrystal-

1ised at depth but the septa are still visible.

(b) Thecosmilia sp. A slender form 3-4 mm across with up to 30

delicate septae, and a worn theca, solitary at first, later fan- ning into 2 or 3 corallites. The vertical continuity of the septae should distinguish it from the weaving pattern of serpulid fragments.

(c) The bulk of the beds are formed of ramose colonies up to 60 cm across (see pi-26 ) with thamnastraeoid coralla. In less numbers are other forms of coralla such as spherical or crustose, the latter often with larger corralites. Exact identification has not been sought, butthe resemblance to Evamy's (1963) ramose and thamnastraeoid corals at a comparable level (Bajocian) in his Middle Amblean Limestone is worth mentioning. The same fine-grained calcite forms the periphery of the slender branching coralla of type (c) as was observed by Evamy (1963), but the lack of distinctness of the septa of the corallites seems here to be due to their extreme fineness (large corallites of the crustose form are well preserved) and the high ratio of the central depression diameter to the inter-corallite area where the septa should normally be readily visible. An initial aragonitic composition and susceptibility to solution -precipitation are not ruled out however.

The three types described above occur at precise levels within what appears to be a traceable environmental gradient (fig. 15 ). In a three-metre thick succession of the highest part of the member under consideration at the quarries/road-cutting overlooking Gluges, an upward passage is traceable that is summarised in fig.i 5 The general fall in enemy up into the Gluaes Cacilutite is obvious; it is the lower energy origin of the ohcoliths that could not have been obvious.

DESCRIPTION INTERPRETATION

...... *0.'0 " .. Oncolite wackestone. Low current—regime.

o-o-0 -0-0- Coral heads in growth position, 0 supplying bioclasts to foresets. 0 4% /4 0 X-stratified shell oolite grainstone OF Moderate current regime; • with graded foresets. intermittently high. N

Cr c. O 0000000 Oofite grainstone. Weakly or not O 000000 x-statified. High current regime; O 0000000 O 000000 sheet flow? 0. O 0000000 .•••/ \"/",

FIG.I5:DETAIL FROM TOP PASSAGE OF MIRANDOL OOLITE BELOW Mirandol 297

With deferring the matter until the section on sedimentology, the lower energy origin of the oncoliths emphasized here by the occurrence in micrite may be likened to the findings of Ginsburg (1960) on algal biscuits and stromatolites. Also it is significant that the slender non-colonial coralla are aligned with the foreset accretion planes, indicating . -;:re during periods of slow accreti, gut hab proJebly been carried down frdm amid the large heads prasumbaly by surge action.

For extensive in situ occurrence of colonial corals, the environmental requirements are so well-known that they need only be Sum- marised, s after Wells( 1956) for this level of the Mirandol Oolite:

Depth - probably < 20 m (because of occurrence in oolite); Temperature

Warm 25°C - 25°C (rather than 18°C); Salinity 36 ppm - 40 ppm rather than 27 ppm (because of upward passage into gypsiferous beds); Rate of sedimentation - Low with stable bottom but varying on either side of precise coral band.

The occurrence of random corals in fairly well defined strata suggests biohermal banks rather than biohermal reefs (terms after

Nelson et al., 1962).

Corals in Lanzac Oolite:

Four species of coral occur in the Lanzac Oolite, none of which form significant growth masses comparable to the ramose colonies in the Mirandol Oolite. Trochaerea anpoloides is solitary and found displaced at several levels in pure oosparites and oncolitic ones.

CladOphyllia [M. Edw. H., 1851] sp. occurs in phacelloid

Colonies of 90-100 corallites in pure oosparites. Corallites are .2 mm across with up to 26 septae visible only as very short stumps after destruction by very coarse clear calcite. The circular patches of calcite in horizontal section and their ramifications in vertical sec- 298

tion may be mistaken for burrows, but for the septation. Wells, J.W.

(in Treatise on Invertebrate Palaeontology, 1556) considers the sub- family Favinae to which Cladophyllia sp. belongs to be mostly berme- typic, so the lack of reefal development in the Lanzac Oolite may be considered as indicating a lack of persistence of the'requisite conditions, higher salinities than normal and/or destruction by algae and gastropods dependent on algae for food (Newell, 1557) being more suspect, as oncoliths and dedoiomite are both present in the Lanzac Oblite, but absent in the Mirandol Oolite where biohermal banks may have been present.

Unidentified meandroid and flabellate corals occur out of place in coarse laminae but are altogether very few.

The complete absence of circumscribed biohermal reefal structures or biohermal banks in the Lanzac Oolite. contrasts with the commonness of these structures in the Upper Oxfordian of Charente (

Glangeaud, 1895) and in the Haute Saone (Beauvais, L., 1960) and high- lights the thick calcilutite sequence of the present succession. It is suggested that the Dordogne-Lot region was predominantly a barrier lagoon from the Bajocian upwards, the development of biohermal reefs being further to the north-east in the Charente. This independent generalisation is borne out by other considerations.

STROMATOPOROIDS (AND POLYZOA)

ticizo) Stromatopora arabidensis, DehorneA a branching species about 2 cm across with distinct concentric bands' and vaguely radial structure is a very useful biologic marker for the basal Lanzac Oolite, as it occurs only from the top 2-3 metres of the St. Etienne Limestone upwards. In the latter it is sometimes found in a vertical position across graded pebbly laminae as below the quarries at the type- locality. A specimen showing the influence of currents in determining 299

the direction of growth and of maximum wear is illustrated .

In the Lanzac Oolite, particularly near St. Etienne, larger colonies

up to 4 or 5 stems occur in a coarse oncolitic band 3-4 metres thick

and flanked above and below by pure ooid grainstones. Ripple lamina-

tion is common throughout. Branching Stromatoporoids are few in the

oosparites and pellety beds of the Lower Lanzac Oolite and topmost

St. Etienne Limestone, are more abundant in oncolitic limestones of

the Middle Lanzac Oolite and are absent or few in the oosparites of the

Upper Lanzac Oolite. Lecompte (1956) observes that Lithothamnion-type algae occur amidst reef-forming Palaeozoic stromatoporoids while

Spongiosstroma, considered here as similar to oncolite-forming algae occur above and below. Wallace (1966) noted a passage from reef front to deep-lagoonal of massive stromatoporoids, less robust lamellar stromatoporoids,branching disphyllid corals and brachiopods in the

Givetian of the Boulonnais. The in-place growth of branching forms

in this succession seems from similarity of form more to indicate the shallow outer-lagoonal environment of disphyllid corals than it does that of the lamellar and massive stromatoporoids of the Devonian.

The coincidence of oncoliths and maximum branching stromatoporoid growth may support the view of a quieter environment for oncoliths compared to oolites as seen in the Mirandol Oolite and in the Middle

Crezelade Beds (see Stratigraphy). Other requirements for Stromato- poroids and applicable to the depositional environment of the relevant levels of the Lanzac Oolite were thought by Lecompte (1956) to be similar to those for corals.

Crustose stromatoporoids appear to abound in the same beds as encrusting algae. These levels in the main unit of the Blagour

Breccias, the coarser bands of the St. Etienne Limestone, the burrowed

Unit 3 Lacave Caicilutite and intermittently throughout the Lanzac 300

Oolite, but particularly at the base, all consist of algal encrusted

shells, grains of a multiple history such as ooids with differing modes

of subsequent algal encrustation (for exact terminology see Wolf, 1965,

Murray, 1969), an abundance of foraminifera often as cores of cir-

cumscribed grains, and sparry calcite. The Stromatoporoids them-

selves either rest on shell fragments, whole oyster valves or directly

on the other grains. It is not possible to use crustose stromato-

poroids as independent environmental indicators because of their.very

low abundance. The associated sedimentary textures and forams do

this much better.

Polyzoa occur as fragmentary debris and very sparse encrus-

tations on oyster-covered bottoms of pure calcilutite or oncolitic calcilutite in the basal unit of the Lacave Caicilutite and the bur-

rowed Unit 3 of the same formation. The regular hexagonal cells sometimes appear to be in their original position. A firm bottom, perhaps an incipient hard-ground with very low sedimentation and moderate currents are suggested for the relevant levels of the Lacave

Calcilutite, although the oysters, themselves firm bottom dwellers may have furnished objects of attachment. Fragmentary fenestrate polyzoans

in the biosparites of the 1.oWer Mirandol Oolite are considered in point7count analysis.

A stout branching polyzoan, Ceriopora leda, d'Orb (Annales de Paleontologic, 1967, T.II, Pi. X) is very common in the top third of the St. Etienne Limestone between Lacave and Maringrac to just to the east of the River Ouisse, and in most other localities. A short distance south of Lanzac they abound in a 1 /2 metre bed buff with dedolomite. Individual branches are 2-3 mm across and occur in oncolite as well as fine foraminifera-rich pellet-limestones. They probably required the same energy level as slender corals but may have 301

been commoner in deeper waters and fine sandy bottoms such as occur

in Atoll Lagoon floors. The Treatise on Marine Ecology and Paleo-

ecology (ed. Ladd) Vol.2, pp. 783-799 (Duncan, 1957) gives a copious

bibliography and Carozzi (1952) deals with the characteristic faunistic

associations of polyzoans, but the data does not demand a stringent

application of rather wide environmental range of these forms.

G. FORAMINIFERA

Forams will be considered in point-count analysis, and do

not belong to the macrofauna. Some genera however are sufficiently

abundant at certain levels to be obvious to the eye with or without

a hand-lens, and may be of interest to the casual investigator. These

are Trocholina sp. and a great variety of triserial and biserial

forms which the author has lumped together as Textulariids. The

former are very common to dominant in the oolitic-oncolitic pockets

of the burrowed Lacave Calcilutite where they are visible as sparry

specks forming the cores of the ooids and oncoids. With a high

lutitic matrix, the jackets are not so obvious. They are useful as a local index for this level and less so for the upper passage of the

Souillac Oolite by their great abundance and easy recognition, although

isolated individuals turn up at other levels in comparable lithologies from the Blagour Breccias to the Lanzac Oolite. Textulariid forams are common in fine white pellet-limestones of the basal St.

Etienne Limestone and higher up in the same formation, occur with

Ceriopora leda in a similar lithology. Interpretations are as under Microfacies. 302

H. ECOLO:C SYIITHESIS

QUALITATIVE B!OFACIES AND THEIR VERTICAL DISTRIBUTION

The reality of faunal assemblages arrived at by statistical

procedures is often in doubt to the statistician himself as well as to

the field-geologist inclined to trust what his intuition tells him.

The procedures adopted here, such as cluster analysis, lack formal

tests of significance (Parks, 1966), although individual values of the

correlation coefficient may be so tested (Miller and Kahn, 1962).

Again, even if a field-geologist was satisfied with statistical tests,

he may not himself have the time and/or the skill to undertake the

tedious process in a general palaeoecologic investigation. This sec-

tion is aimed at solving or lessening these shortcomings of the section

on quantitative biofacies synthesis; it is not a substitute.

The problem of characterizing an assemblage qualitatively

is not easy. Thorson (1957) suggested using a group of organisms

each of which represents a different trophic type or grade of-organisa-

tion. This is clearly impossible in ancient environments because of

the absence of soft-bodied forms like the polychaetes, although certain

burrows may partly meet this demand. Parker in several works (Gulf

of California, N.W. Gulf of Mexico and Texas Coast) has tended to use

one or two species for characterizing his assemblages. The present

investigation is like Parker's several in the preponderance of bivalves and gastropods, but is unlike his in the greater uncertainty of the

local range time- or facies-wise of species that may be chosen as bio- facial indices. This is readily attributable to the relative imper- fection of geologic sampling, to post-deposition homo- genisMcj. processes and to the impossibility of separating evolutionary from environmental influence. Acer (1963) illustrated several instances of c.r,lizio in recent years to support the view that opportunity was probably always a more important controlling factor in migration

than potential ability. in a single area of study with no evidence

of insurmountable geographic barriers, his findings may be used to

counter suggestions as to the possible influence of evolutionary poten-

tial in the vertical differentiation of the predominantly benthonic

assemblages suggested here. These assemblages often cutting across

generic boundaries (See• p.312) cl-et-

Assemblage I. Ceratomya - Trigonia - Brachiopod:

Composition: The assemblage consists of a great abundance of

large Ceratomya's such as C. bajociana or C. concentrica, a diverse

suite of large trigonias such as T. ccstata, T. phillipsi and T. sp.

(No. 206) and a much greater abundance of terebratulids than rhyn-

chonellids. Loboidothyris perovalis is a dominant terebratulid.

Qualitatively less important members of the association are large specimens of Gervillia late, Gryphea beaumonti, Modiola sowerbyanus,

Isocardia cor and two or three species of keeled ammonites.. G.

late, G. beaumonti and the ammonites occur at one horizon only, and

M. sowerbyanus occurs articulated only in this horizon. Isocardia cor is nearly as abundant as C. bajociana in the biosparites of the

Lower Mirandol Conte. Small specimens of Ceratomya plicate and

Trigonia duplicate occur in the same bed with. Pholadomya laeviuscula, the reasons for both dying young or undergoing size diminution have been discussed under the individual species. It is their togetherness and similar environmental response that is emphasized. The mutual absence in the fossiliferous top passage of the Souillac Oolite and the co-occurrence of T. imoressa and Ceratomya or Ceratomya-type

Isocardia both emphasize the togetherness. Terebratulids are a more variable constituent owing to stronger intrageneric pre-Jerences..

(Cr. deaLh assem'olage in Caicaires du Moutin

WI l Bert. (key 41^1dq(e.,19(3)) 304

PL 26: RAMOSE THALINASTRAEOID CORALS, rf.zando 1 Ooliter Below. Mirandol.. A s Colony in growth-position. B: Coral'a on, weathered surface.

305

Characteristic Lithc!ogy: impure shell grainstones.

Environmental Controls: Bottom texture and stability, salinity,

and agitation. Depth only of indirect importance, by limiting energy

Assemblage 2. Colonial Core ( D1.26 ) Composition: Ramose thamnastraeoid corafvcompletely dominate.

A wide variety of other growth-types may be present, but solitary troid

or cupoloid forms observed at various levels within the succession are

absent. Gastropods actually occurring among the coral-heads are

varied and characteristically too small to be properly assessed. The

small pleurotomarlids and cerithiids are characteristically ornamented. p1.27 ) Algal laminated pockets are sparse amid the corals, as is Patella cf.

rugosa. Above or below the coral colonies Nerineids, Pseudomelaniids,

Ptygmatis, smooth Cerithiids like C. roissi are common, the tips being

intact indicating absence of appreciable transport. Between coral

growth-heads any of these could occur but always broken and worn some-

times nearly beyond recognition. A small costate cardiid,.

Cardium subminutum sensu lato is more ubiquitous but still within the

oolitic formations. Rectimarginate inflated terebratulids like Micro- Ana thyris and Cbovothyris may be present in Nerinea dominated levels.

The best example of the assemblage is in the top ten metres

of the Mirandol Oolite, outside of which the role'of corals is sub-

ordinate to that of the larger gastropods, Nerineiids, Ptygmatis, etc.,

occurring with solitary corals like Trochaeria cupoloides and

-Chomatoseries orbulites, or meandroid and flabellate types. In a

laterally untraceable level of the topmost Blagour Breccial at the

beginning of the St. Sozy-Souillac route, a broken assemblage of slender

branching corals and Nerineiids with a host of flat - smooth oysers not

included in this assemblage, is well displayed. The gastropods appear 306

PL.,27 : LAMINAR ALGA FROM AMID CORAL COLONIES OF FL.26.

307

to be the most ubiquitous member of the assemblage, characterizing

what have liberally been called peri-reefal assemblages.

Characteristic Litholog,/: Oosparites when massive corals are

present with increasing content of algal coated oncoid-type grains,

pellets and encrusting Stromatoporoids when gastropods and solitary

corals dominate. The latter variant is exemplified by the main unit

of the Blagour Breccia% as at N. Souillac and by the Lanzac Oolite.

Suggested Environmental Controls: As for Assemblage 1, but

depth of indirect but decisive importance through influence on illu-

mination,. The assemblage and the characteristic lithology are pro-

bably the best absolute depth indicators in subtidal environments.

Assemblage 3. Pecten - Brachiopod:

Composition: Costate pectinids ..• and large-

medium, suiciplicate Stiphrothyris-type terebratulids and narrow

Rhynchonellids (the R. elegantula- type are considered here to be

narrow in relation to the TorGuirhynchia and Thurmannella-types).

In a random count at the top-passage of the Souillac Oolite in a

quarry at the north-western end of St. Sozy, the two constituents

of this assemblage formed 20 out of 29 fossils including other bivalves

and solitary corals. Rhyncheonellids outweigh —terebratulids. here

and in the main unit of the Blagour Breccia% at Souillac, Blagour

and several other localities. in the latter member the Pectinids

notably Chlamys stricta and the Rhynchonellids show some breakage

and wear suggesting some transportation. Rectimarginate terebratu-

lids like Obovothyris perobovata, Dieonela siddingtonensis and

MicrothyridiniangenaliS seem to link this assemblage to the cora)- gastropod one. hey seem however more strongly attached to oysters than to either assc.mblage. 308

Characteristic Lithology: Sperry 60-/pel-micrites such as occur in the Souillac Oolite-Lacave Calcilutite passage and suggesting energies intermediate between those for pure oosparites and calcilu-

tites. Pectinids dominate in higher energy environments such as the biosparite members of the Lower Miranda] Oolite where Ractorhynchia

lacunosa occurs as much with the Nerineiid dominated Coral-Gastropod

Assemblage as with the present one. Expanded Rhynchonellids and fragmented pectinids in the rippled or graded laminated pelsparites of the

Upper Lanzac Oolite emphasize preference for the sparry/micritic grainstones.

Suggested Environmental Controls: Salinity, aeration, and energy were probably more important than the grade of the. bottom sediment, as long as some shells or other objects are available for attachment. The assemblage is freer from the bottom than most, and is proportionately more useful for predicting the state of the aqueous medium which is often unfossilizable.

Assemblage 4. Deep Burrowing Bivalve - Mytilid - Cidarid Echinoid

Composition: A large group of bivalves represented by Pholadomya,

Arcomya, Pleuromya and Quenstedtia notably Q. elongate (sp. no. 27)-.

The Mytilids are represented by thick ModiciA as opposed to'the rapier- like forms like M. scalprum and M. sowerbyanus,and by Arcomytilus and

Mytilus cunatus. Arcomytilus QSper : . is often present in very small numbers but is conspicuous by its ornament and common occurrence in levels unsuspected of any life. It is taken to represent the

Mytilids. The. regular echinoids are represented by Pseudodiadema.

The assemblage is typified in the upper two-thirds of the basal member of the Gluges Calcilutite and in :.;nits I and 3 of the Lacave

Careful collection revealed a consistent passage of the Pectinid-

Brachiopod Assemblage of the upper passage of the Souillac Oolite into 309

this one. Good localities for this stratification are the south bank

of the R. klz.ou opposite Rocamadour, the east bank of the R. Blagour,

near Lacave Cemetry and at various other localities where the succession

is favourably exposed. Elicmus poiytypus or Gryhea may be present, as

well as Pinna.

Characteristic Lithology: Pure calcilutite free from gyp-sum

pseudomorphs or dedolomite is favoured by some Pholadomya like P.

laevinscula in the upper half-of the Gluges Calcilutite. Pseudodia-

dema is likewise favoured. Increasing grain content to a limit of

about 300 (from visual charts ) favours first

Quenstedtia elongata, then other myacids and tellinids, and Arcomytilus

and carinate- PholadomyA. Arcomya latissima is probably unusual in

occurring in the extreme lithologies sparry grainstones and. pure cal-

cilutite.

Suggested Environmental Controls: Aeration above the sediment/ water interphase, degree of bottom resistance (see experiments on bottom penetration by bivalves - Nair and Anseli, 1968), bottom stability, depth and saliniy. Texture is important probably only through control on the cohesiveness and resistance of the sediment, the attachment requirements of the byssate epifaunal members of the assemblage being easily met by a very low grain content as is apparent in most of the ecology of the individual species. There is no evidence from this succession that a higher grain-content would lead to extensive mussel- banks. Therefore the assessment necessarily pertains to individual mytilids. Ability to burrow deep enables the burrowing members to be unaffected by movement the top 1-2 cm of sediment (see occurrence of Pleuromya in Mirandol Oolite and Arcomva latissima in marker bed at top of Lacave Calcilutitc). The assemblages which follow are dominated in number by one

species which from previous arguments may be termed opportunistic.

They occur as accessories in a wide variety of environments and assom-

blages,l, t through some specialisation or an innate gregarious ten-

dency are able to exploit a rarely repeated favourable situation to

the fullest ( in. accord vah. recent ideas on community structure).

The characteristic lithology and environmental control and typical

occurrences are as for the listed species.

Assemblage 5. Astarte:

Astarte cf. excentrica (sp. no. 71).

Assemblage 6. Lucina:

Lucina cf. beliona (sp. no. 178) and Rhizocor. below sometimes.

Assemblage 7. Gervillia:

acuta and Thalassinoides just below.

Assemblage 8. Sphenia - Anisocardia:

Anisocardia dieniafaiti does occur without Sphenia and in some beds as Lucina, but it seems a more invariable minor constituent in Sphenia covered surfaces than in those of Lucina which tend to have small Chlamyids and Astartids. ,Dominant species - Sphenia sp.

Assemblage 9. Oyster Brash: e.g. Ostrea costata.

Assemblage 10. Viviparus - Planorbis.

E.g. Viviparus sp. (major constituent), Planorbis calculus; pionil remains (identified as Brachyphylium in South East of the area mapped - Thenevin, 1895). 311

PL.2 8 : S"IIBLAGE OF CERITHIID GASTROPODS IN LACPVE CALCILUTITE UNIT 2.. As Bedding—sur— face sample, Rocamadour. Bs Polished surface perpendiculr-r to bedding showing the maximum. ds!pth of sediment within gastropod. reworking $ icachers, Mon.ges,, St. So zy.

312

1-Aa. 6 : SYNOPSIS OF QUALITATIVE BIOFACIES

1. Ceromya-Trigonia-Brachiopod

C. bajociana,C. concenLrica, Isocardia cor T. costata, T. phillipsi, T. sp. Terebratula perovalis G. lata (Irregular Ech.) M. sowerbyana Terebratulids > Rhynchonellids

2. Colonial Coral - Gastropod

Nerineids, Pseudomelanias, Ptygmatis Cerithium roissi Pterocardium subminutus• Microthyridina, Obovothyris (Certain Rhynchonellids and Chomatoseris, Trochaeria Terebratulids, e.g.Thurmannella)

3. Pecten-Brachiopod

Costate pectinids, e.g. C. stricta Stiphrothyris-type terebratulids R. elegantula, Callirhynchia sp. (But not Thurmannelia or Torquirhynchia) Microthyridina, Obovothyris

4. Deep Burrowing Bivalve-Mytilid-Cidarid Echinoid

Pholadomya, Arcomya, Pleuromya, Quenstedtia elongata Arcomytilus, Mytilus cuneatus Pseudodiadema Eligmus polytypus, Pinna, Gryphea

5. Astarte: e.g. A. cf. excentrica

6. Lucina: e.g. Lucina cf. bellona

7. Gervillia: e.g. G. acuta

o. Schenia-Anisocardia

9. Oyster brashes: O. costata

10. Viviparus-Planorbis

11. Natica-Nertia-Phasianella

12. Cerithiid Coquinas 313

Assemblage 11. Natica-type Gastropods:

Natica-type gastropods nowhere form a coherent assemblage,

but are significant in showing a strong preference for the St. Etienne

Limestone a formation that is itself unique in breaking the thrice-

repeated pattern from Mirandol Oolite to Crezelade Beds of Oolite-

superposed-by-Calcilutite. Golite-Calcilutite pasSages are often a

matter of 1-2 metres, but the lithology of the St. Etienne Limestone

is such as to suggest an extended passage from Lacave Caicilutite to

Lanzac Oolite. The gastropods Natica, Nerita, Phasienella may be

thought of as characteristic of corresponding type of transitional •

environment.

Assemblage 12. Cerithiid Communities and Coquinas: (p1.2B

Ceritclia lindonensis in the stromatolitic Lacave Calcilutite

Unit 2 where its abundance coincides with bioturbation, being itself

in the minor burrows, and in the lignitic Gluges Caicilutite Unit 4 where its lower abundance coincides with absenCe of detectable burrow-

ing. Cerithium polystrophum is by far the more abundant of the two

in the same plates of limestone.

The opportunistic assemblages (5-12) which are largely monospecific or monogeneric are incapable of.detection in the cluster analysis which only considered presence or absence and did not reckon with abundance. The recognition of their distinctness; even at the danger of seeming to enumerate every fossil, is emphasized for this reason.

QUANTITATiVE ASSEM5LAGES AND D1VERSTY TRENDS

This section is directed towards grouping of the large number of recovered and described objectively. it is complementary to the section preceding. 314

The technique used is that of cluster analysis, discussed

below in detail. Its choice was diCtated by the nature of the avail-

able data and the relative simplicity of the technique itself.

Choice of Technique:

Two methods, cluster and factor analysis, are commonly used

to reduce a large number of components (fossils in this case) to a more

easily interpreted few. The similarity of the two multivariate pro-

cedures has been pointed out by Parks (1966), but one significant

difference immediately ruled out Factor Analysis in this investigation.

Whereas,Cluster Analysis is applicable both to data in which the para-

meters in question take up linearly ordered values as well as for data

in which the parameters take up only the values 1 or 0 (i.e. present

or absent), Factor Analysis is limited to the first kind of data in

which it is possible to consider the parameters as axes of a multi-

dimensional space. - - Because of the difficulty of extracting fossils,

from these indurated tests the small number of fossils visible on good

quarry faces, the paucity of fOssils on bedding surfaces and the irre-

gular distribution of outcrops it was unwise to have made counts of

each species either from collected samples (which moreover would have

been too costly to transport from shear bulk) or from quadrant or

stretched-line (Ager, 1963) estimates species of abundance. U'ncon-

'trolled counts in rock formations of varying degrees of exposure would

have been several times more biased than the preferred solution of

limiting the investigation to a present/absent score for each species.

As part of a very broad project, and in covering a thick succession

500-600 m, It has rapidity- to recommend it.. On a present/absent

basis, Factor Analysis naturally fi ll through.

Cluster inaiysis is .repetitive in the sense of involving

several cluster cycles, but is mathematically simpler than Factor 315 •

Analysis. Five cluster-cycles for instance were completed without computer aid for l91 fossils. Moreover the principal components that

could have been extracted from fossil-data with numeric abundance would still not have direct iy pin-pointed which species are significant

(ecologically or biostratigraphically) whereas the dendrogram from

Cluster Analysis leads step-by-step from species to super-groups of species.

In Cluster Analysis, the nature of the statistical distribu-

tion of the parameters (fossils) in question need not be known or even that the sample represents the whole population, (Parks, 1966). Tests

of significance are consequently absent. Since the correlation coefficient was not used, individual values of the associated coefficient could not be tested for significance. There are however several i stances in which the technique has produced practically meaningful results in the study of modern fisheries assemblages in tropical demersal fisheries (Longhurst, 1966), in Palaeoecology and biostratigraphy

(Lipps et ci., 1968), in bio- or sedimentaty facies analysis alone or

in conjunction with Factor Analysis in the Bahamas (imbrie and Purdy, analysis 1962) in carbonate facies (Banham-Carter, 1965), in the Devonian,

in taxonomic classification (where the raw data is not subject to much less sampling error (Sokal and Sneath, 1963)). The above other

literature covered by these authors but not read by the author are the sole basis of the confidence with which the results of the technique are assessed.

Procedure in Pair-Group Clustering by Average Linkage:

The procedure adopted draws heavily from Sokal and Sneath

(1963) who have discussed and appraised the different kinds of Cluster

Analysis. 316

1. At favourable outcrops throughout the area mapped, the species

present were entered. Species an: cutcrops were numbered, so that

acainst an outcrop the area mapper.`, the species present were entered.

Species and outcrops were numbered, so that against an outcrop number was a series of species code numbers, a new number being conferred whenever a new species was found. A refinement of the field listing was later done by fusing or splitting species as detailed identification dictated. Split species numbers with the designation a and b do not necessarily mean 'variety' each of the other in the taxonomic sense.

A subsequent re-ordering of code-numbers stratigraphically or otherwise was hot found to be necessary. Formal indentification and naming was done subsequent to the completion of the analysis. This way the procedure was both more rapid and free of subjectivity as to which species might best be associated with which, in cases where field entries were not clear or in other ways. controversial.• Some of the vertical sampling was direct for some favourable continuous parts of the succession, but a small percentage of it was synthetic owing to limitations of outcrop size, topographic disposition, and areal distribution.

2. A list of the summed occurrences of each species was drawn and also another in which each species had beside it all the other specie:, that occurred with it and the number of times they were so observed.

3. From the two lists a coefficient of association was calculated mostly mentally because of the low occurrence and co-occurrence values, and the simplicity of Jaccard's Coefficient that was adopted. The coefficient as given by Sokal and Sneath (op.cit.) was

S = n ,,/(n u) JK where S is the coefficient of Jaccard, n is the number of co-occurrences JK 317

[present author's modification], and u is the sum of the number of

lone occurrences for the two species.

The deominator reduces simply to the sum of the total num-

ber of occurrences of each species (together or not) minus the number

of co-occurrences. The value of this coefficient ranges from 0 - 1.

Sokal and Sneath (op.cit.) have a critical summary of a wide range of

coefficients.

4. A matrix (semimatrix strictly speaking) association coeff-

icients was then drawn up This way every species was

quantitatively related to every other species.

5. Pairs of species were, the extracted. Species A and B formed

a'pair-group' only if the highest association coefficient of A was with

B and that of B was with A. This is the first clustering cycle, and

pair-groups extracted were listed from 1 onwards with a prime, i.e.

1', 2', etc., followed by the unpaired species which retain their code

numbers. This list formed the basis for the next clustering cycle in

which the association coefficients of the unpaired species were trans-

ferred from the first matrix and two new values of the association co-

efficient were calculated - (a) between unpaired species and pair-groups

and (b) between paired-groups, i.e. between unprimed and primed numbers

and between primed numbers. The average linkage method of Sokal and

Sneath (partly after Rohlf) was used in which the (a)-type linkage

consisted of the sum of the association coefficients between an unpaired

species and each member of the pair divided by the number of species

(3 in this case), and (b)-type linkage consisted of the association

coefficients of each member of a pair with each of the other pair divided by 4, i.e. the arithmetic average of tho association coefficients. The

inapplicability of another kind of average, Spearman's sums of variables,

to association coefricients vies noted by Sokal and Sneath. Luckily

that is more complex and leads to a serious complication like the reversal 318

20 0 a) --Curve for no of species & sp.-groups U) - b) ----As(a) expressed as '10 no:of species in 1st. 0 matrix. L IJ II 8i preCeding rn c) matrix.

06 12 U) 100 *L5 8 -\\ 80 o 60

4 40 -6 20 •

0 . 1 2 3 4 5 6 Matrix ordinals

FZG.16 VARIATION IN NUMBER OF CELLS IN MATRIX WITH SUCCESSIVE CLUSTERING- CYCLES. 3i9

of association coefficients which the simpler calculation is free .of.

6. Two ambiguous situations arose during this and other cluster-

ing cycles: (a) Two species A and B have a mutually highest associa-

tion coefficient of the same magnitude with a third species, C, but not

between themselves. The solution was to form a-triple junction with

an association coefficient value that is the arithmetic average for

all three species. (b) Four-cornered linkages in which the sable value of association coefficient islnutually highest for three pairs of species, the remaining pair not having a mutually .highest coefficient.

It seemed the simplest solution appropriate to pair A with D and B with

7. Third and subsequent clustering cycles were described above except that higher order primed numbers representing larger groups appeared in the matrices. Five successive matrices were generated

in the entire process, from which pair-groups were extracted. The size of the matrices decreased from 154 in the 1st to 40 in - the fifth and final matrix (fig. l6 ), with the point of diminishing returns

(i.e. % drop in size of matrix less than the rate of increase of the number of clustering cycles) being reached at about the 3rd matrix.

On the basis of the curve in fig.16 , 13 matrices would have been needed to reduce the matrix size to the ultimate 2 elements, assuming the same rate of logarithmic decrease. The task proved overwhelming and the end did not justify this, so the following arbitrary procedure was adopted:

8. Cluster Termination of Residual Groups:

(a) Pair-groups were extracted from the 5th matrix in the normal

320 •

manner, and the super groups obtained from this and preceding matrices

considered as PRIMARY ASSEMBLAGES or GROUPS. The unextracted groups

in the 5th and final matrix were considered as'RESIDUAL ASSEMBLAGES or

GROUPS (fig.! 7 ) . 1st and 2nd Order Residual Groups were set up

as follows :-

(b) A species (no. 14) or pair-group was chosen from the resid-

ual matrix, that was associated with the greatest number of other pair-

groups. The multiple junction thus formed was given an average value

calculated from the sum of the association coefficients of the pair-

groups with the central species (no.14;. This accounted for 16 of the

30 residual pair-groups in the matrix. 95 more pair-groups (means

also single species after the 1st matrix) were extracted which were

associated either with the 16 centered on species no.14 or with the

primary assemblages of step 8(a), i.e. with pair groups already cluster-

ing around 14 and in the primary assemblages but not with those still

unclustered. The 21 pair-groups so far accounted for constituted 1st

Order Residual Assemblages.

Assemblages fell into three clusters according to the manner in which they were associated among themselves as shown below - ___ ------} 1 1 196 1:09 I I 27' I I I I -, i 4/ T.,,,7i i 5-, 17' 12 -

1 160 1 ___ - ' __.1 TT TTI

Association coefficient values for the multiple junctions were calculated by a simple arithmetic average as before. The emphasis was always on utility, and the rouncing off in the residual cssemolages often cut

Table. 7: The Differing Influence of Q-mode and R-mode studies

on Jaccad's Coefficient of Association.

To be compared Basis of Comparison Jaccad's Coff. Investigation influenced

Samples A & B No. of species No. of species in sample Lipps et al. (1968) common to A and B Sample size

i.e., Diversity of sample

Species A & B No. of shared No. of samples points at Present Study.

sample points which sp. occurs Specics =1

i.e, Sp. frequency 32:?,

across association coefficient values. The rule was systematically

to cluster the pair-groups around one that accounted for the maximum

number of pair-groups.

9. A dendrogram was drawn (fig. i7 ) by working backwards

from larger pair-groups to individual species which form the dendogram

terminations. Other information is summarised on the figure.

Significance of Assemblages at Different Levels of Association:

In the absence of statistical tests of significance for

Jaccard's Coefficient in the 1st matrix and of the calculated coeff-

icients df association in subsequent matrices, the amount of confidence

to be placed in the assemblages is based on the efficacy of the method

in other instances (already cited) in general and in detail. Lipps

et al. (1968) offer such particulars. In a Q-mode (sample oriented

rather than fossil oriented as in the present enquiry) study using

Jaccard's Coefficient, those workers found Jaccard's Coefficient to be

sensitive to the diversity of the samples being compared. To mini-

mise this error,they compared only those samples that fell within a

specified narrow range of diversity (75-50). The equivalent in an

R-mode study like this one would have been to compare only those species

that occurred in a specified number of sample points.(tGb7 ),

The range 1-3 occurrences would thus have accounted for 73% of the 154

species and that from 1-4 would have accounted for 60%. This would

have so severely limited the possible number of association coefficient

values (3 and 4 respectively) that the significance of values actually occurring would have been sacrificed for the stability of Jaccard's

Coefficient. Those species occurring in a large number of localities and fortunately also perhaps cosequently with a large number of other species were not eliminated and served to increase the significance of the association coefficients -by raisin thethe number of possiL)ie values 324

of the coefficient. Such species were No.16 Chlamys stricta (occurring in 41 sample points), no. 12 Obovothyrids (27 sample points) to name a few.

Lipps et al. (1968) and Longhurst (1969) both indicate how much meaning should be read into the value of the coefficient at which species and/or groups of species were associated. The former gave the values 5-15% and up to over 60% for collections from markedly different facies and for volumetrically identical samples from the same tense (bed) respectively, while the latter found good agreement between assemblages in which association coefficients were .05-.66 and assemblages Ohich were set up subjectively The two ranges compare very well with the ail-inclusive range > .04 - 1.0 and emphasize the intermediate range.0.1 - 0.5 which accounts for 80% of the cluster assemblages. in the present investigation.

Comparison of Quantitative and Qualitative Assemblages

Because qualitative assemblages cannot be sharply separated from one another, and their composition, apart from a few exceptions, is so unlimited, it is impossible to be very precise about their resem- blanceto quantitative assemblages. Granting the reality of the quantitative associations (see above), it may be said that the qualitative assemblages represent only the strongest and therefore obvious associations whereas the qualitative ones have brought out the subtleties.

The same conclusion may be reached by comparing the great number of quantitative assemblages to the. few (4-5) subjective level bottom communities recognised in modern seas. These subtleties are not easily pinpointed, but were probably every bit as real as shown from previous considerations, and as will be seen in some of the assemblages below.

No further exhaustive interpretation of the quantitative assemblages is attempted because the ecology of individual species, • 325

TAB. 8 CONTINGENCY TABLE FOR COMPOSITIONAL SIMILARITY BETWEEN QUALITATIVE AND

QUANTITATIVE ASSEMBLAGES

I I QUAL. BIOFACIES i I 1 2 3 4 5 i 6 1 7 8 1 9 10 11 12 /UANT. i I ASSEMBLAGES I 1 I. •

Al A2 A3 1 I I A4 X X A5 X .X X

A6 X- X X x A7 X -• X 1 A8 A , I A9 A10 All Al2

SA1 V X

SA2 IX X X XX SA3 (xx SA4

SA7 iX X SA8 X SA9, SA10

SA11 !X X x SA12 tX X X X X SA13

XXN-V. similar composition; Xxfairly similar compsition; X-some similarity.

Framed cells indicate mutually highest similarity. Qualitative Assemblages 1, 2, 3, 4, 7 and 5 may be considered on this basis to be capable of detection by the cluster method. Qualitative Assemblages with the greatest number of framed cells are probably the mast ill-defined in composition. Assemblages beyond SA13 are excluded from consideration. 326

genera and superfamilies has already been examined.

On the basis of the ecology of the species, genera and super-

families as discussed previously, generalisations are made on the quan-

titative assemblages as in (Table 8 ). For simplicity the headings

eipfaunal, infaunal, deposit feeding, suspension feeding, coral, echin-

old, terebratulid and rhynchonellid have been used. The epifaunal

condition is split into vagile as for Pectinids, Limids and Trigonia;

byssate throughout life or byssate and never vagile; and cemented.

Pinna is considered infaunal though byssate. Because the associa-

tions have probably been determined by a large number of factors, it

is possible to point out only a few cases where generalisations based

on these headings give a single and clear-cut environmental influence.

The subassemblage (SA 12) consisting of Phasianella cf. subumbilicata,

Cuculloea sp.,Loboidothyris perovalis, Entolium disciforma, Eopecten

sp., sp. 206, Pecten personnatus, Trigonia sp. and Plagiostoma ljbax

is clearly a vagile epifaunal one suggesting independence of the bottom

although living on or close to it. The adaptation to be either on

or off the bottom could not have been occasioned by grain size compo-

sition which would not fluctuate. Other factors could be listed and

eliminated. It appears that a mobile bottom would best account for

the association.

The list of quantitative assemblages and the qualitative

ones which they most resemble (Tab.8) shows (a) that several of the

former may be accommodated in one qualitative assemblage and (b) the

reverse. In the first instance, the cluster dendrogram places a

limit on the extent to which a qualitative assemblage may be stretched

to incorporate more species. Clearly species belonging to two quan-

titative assemblages which have not converged at 5% association should

not be lumped together, as could easily happen without such a well-

'defined limit. In the reverse case, one quantitative assemblage may "DUI 9 : VERTICAL DISTRIBUTION OF RECTIFIED QUALITATIVE ASSEMBLAGES

QUAL. ASSEMDL. No. of Assemblages No. of Assemblage (not including (including mono- 1 2 3 It 5 6 7 8 9 10 11 12 FORMATION' monospecific ones) specific ones)

U. LANZAC OOLITE • \\* \\* L.

U. !,\ 6 SI. ET. LIMEST. 2 4 4 LACAVE CALCIL. 2 4 2 SOUILLAC 00L.

\\: 3 8 GLUGES CALC IL. 2

MIRANDOL OOLITE • 328

be interpreted as the combination of two or more qualitative ones - a

circumstance in which unsuspected associations between species of

different qualitative assemblages are brought out by the clustering.

Through the procedure outlined in the contingency table,

it is clear that a combination of the cluster method and subjective

method can yield satisfactory compromise assemblages, whatever the

subsequent ecologic interpretations of those assemblages might be.

Qualitative assemblages which deny quantitative examination are in

decreasing order of reliability 7, 9, 4, 2; 3; and 1. It is inter-

esting to 'note that the remaining six of the twelve assemblages are

mostly the mono-generic or mono-specific ones which are obviously

beyond detection by clustering since numerical abundances were not

utilised.

Vertical Distribution of Compromised Assemblages:

Table 9 is constructed by shading under each qualitative assemblage t>>e stratigraphic horizon that is spanned by the quantitative assemblage(s) with which it has a mutually highest similarity as discussed in table . 8 Cross-hatching indicates two such quantitative assemblages occurring at the same stratigraphic horizon.

(a) About the stratigraphic units:

1. That the Upper St. Etienne Limestone, with five modified assemblages is the most diverse, followed by the Upper Lacave Calcilu- tite (4) and the Upper Gluges Calcilutite (3). Adding the monospecific assemblages gives the new order Upper Gluges Calcilutite,

Upper St. Etienne Limestone, and Upper and Lower Lacave Calcilutite.

2. That number of assemblages as derived here must not be confused with species diversity. A few species with simple and few interrelations could form more cluster modified assemblages than a great

329

number of species within one intricate ecological web. But even with

this in mind, the great number of assemblages in the Upper Gluges Cal-

cilutIte. calls for explanation. It is thought that the presence in

this part of the Giuges Calcilutite of the entire salinity range from

hypersaline to fresh could be responsible for the presence of a great

number of ecologic units.

3. The relatively small number of assemblages in the

oolitic formations is thought to emphasize the preceding conclusion that

those stratigraphic intervals which comprise a great variety of lith-

°logic or petrographic units representative of sufficiently sarong en-

vironmental conditions would be expected to have the greater number

of assemblages. In this view the Mirandol Oolite, Souillac Oolite

and Lanzac Oolite (probably partly under-sampled) may confidently be

considered as representing a uniform set of environmental conditions

as compared, say, to the Lower Lacave Calcilutite where the calcilutitic

unit 1 and its pellety lower passage and the stromatolitic unit 2

(itself with 2-3 ecologically distinct subunits) clearly comprises a

more varied set of environmental conditions. The distinction is

important because not all the differences in sedimentary conditions

(even'when these are obvious as for instance in the alternation of

oncolitic, pellety and oolitic textures in the Lanzac Oolite) are of

equal ecologic significance and the fossils and biofacies analyses

should have a chance to tell their own story.

(b) About the Assemblages:

1. That the maximum expression of the Colonial Coral-

Gastropod assemblage (2) in the Upper St. Etienne Limestone rather

than in the Mirandol Oolite (where large coral growth-masses occur)

should be interpreted as emphasizing the gastorpod-solitary coral sub-

assemblage rather than the colonial coral one. 330

2. That the Deep Burrowing Bivalve-Mytilid-Cidarid Echinoid

Assemblage is probably rightly typified in the Upper Lacave Calcilutite as the difficult choice between this and the Upper Gluges Calcilutite could on a subjective basis alone be wrongly tipped in favour of the

latter from the prominent occurrence in growth positions of a single species of Pholadomya (P. laeviuscula). Such a choice however would be unnecessary in a purely subjective investigation, as both horizons represent this assemblage quite well.

A palaeoenvironmental.interpretation of the stratigraphic units is deferred until the detailed sedimentology is dealt with.

It was intended up to now, only to describe the species and their ecology and to set up (as precisely as the data permitted) the biofacies and describe their ecologic controls - all with as little resourse to the sedimentology as possible. The sedimentology v0 next be considered and than a synthesis of the two is attempted. 331

A New Application of a modified Jaccad's Coefficient

Owing to the low numerical abundance of most of the species observed, the emphasis was always on number of occurrences. Jaccad's

Coefficient used in this regard in the Cluster Analysis, can however be used to generate a trend-surface that bridges the gap between associations among fossils, and similarity of sample locations based on faunal diversity.

The coefficient p is defined as n /n + n y where n is number xy x xy of time§ species x and y occur together; n and n' are the maximum x occurrences of x and y respectively; and 0 p < 1/2. p was calculated for all possible pairs of species under study and pairs with p < 0.25 were rejected. For each sample location, the number of high association species was divided by the total number of species occurring there. This ratio which can be percented, is evaluated for all sample locations in the defined stratigraphic interval under study. The underlying trend is shown by contouring, and can be called an 'Association

Trend'.

The application to this area suffered from inadequate stratigraphical control, but it is hoped to try this out under more favourable conditions.

A comparison of the diversity trend with the present one over the same sample locations did however show a very similar pattern for the two.

The association trend is interpreted as indicating the frequency at a locality of elements belonging to bispecific faunal webs. If species are considered in 3's, 4's or more, it would indicate the frequency of elements properly belonging to increasingly more complex faunal webs.

The main advantage of the association trend over a diversity trend is that the latter does not take into account the inherent tendencies of the species under study, so that a location with say indigenous 332

fresh-water forms plus transported marine ones may show higher diversity

than those with indigenous fresh-water forms only. Using the- associa-

tion trend, however, it is possible to eliminate the transported marine elements because they would show no consistent association and thus would have low values of p.