The Turonian rudist-bearingcarbonate platforms of the Charentes and Perigord areas, Aquitaine basin (France)

THE T U R O N I A N R U D I S T - B E A R I N G CARBONATE P L A T F O R M S OF THE C H A R E N T E S A N D P E R I G O R D A R E A S , AQUITAINE B A S I N (FRANCE) JEAN-PIERRE P L A T E L PLATEL J.P. 1998. The Turonian rudist-bearing carbonate platforms of the Charentes and Pgrigord areas, Aquitaine basin (France). [Les plates-formes carbonat4es ~ rudistes du Turonien des Charentes et du P~rigord, bassin d'Aquitaine (France)]. GEOBIOS, M.S. n ° 22: 295-311. ABSTRACT - One of the most favourable periods for the expansion of rudists on the northern rim of the Aquitaine basin took place during the middle and the late Turonian within the Charentes and Pgrigord areas. This major period of rudist congregation development benefited from highly favourable conditions during the end of the first early Cenomanian - late Turonian 2nd order sequence or "transgressive-regressive" cycle of the Aquitaine Late Cretaceous: an almost permanent regressive trend, sea-level highstand prograding wedges, and different palaeoenvironments from inner shelf to shoreline. About twenty species of rudists, particularly Radiolitidae and some scarce Hippuritidae contributed to the establishment of two successive extensive carbonate inner-shelves, conformably overlying Lower Turonian rocks without rudists. Their total thickness ranges from 35 m up to 70 m (northeastern Charentes area), and they extend over an area 100 km x 25 kin. The organization and the geodynamic evolution of these rudist-bearing formations were conditioned by the opposing influences of the open-sea, littoral or distal shoals and siliciclastic supplies, surrounding the inner-shelf The rudists adjusted to the major biotope variations by changing their specific associations three times. The different phases of ground colonization, then proliferation and finally desertion of rudist congregations reflect the successive sedimentary facies within a shallow-water inner-shelf, protected by a barrier, some 30 km wide at least, made up of high-energy grainstone facies, often located along structural highs. Siliciclastic supply within the P~rigord Noir area generally disrupted the development of the rudist assemblages during the late Turonian; however Hippurites requieni persisted and built widespread biostromes while small isolated Praeradiolites lived in muddy biotopes. KEYWORDS: RUDISTS, TURONIAN, PALAEOENVIRONMENTS,CARBONATEPLATFORM,AQUITAINE BASIN, FRANCE RI~SUMI~ - Pendant le Crfitacfi sup~rieur, l'une des 4poques les plus favorables/t la proliferation des rudistes sur la plate-forme nord-aquitaine correspond au Turonien moyen et supgrieur, en particulier dans les r~gions des Charentes et du P~rigord. Cette p~riode de d~veloppement maximum des peuplements ~ rudistes a b~ngfici6 de conditions extrgmement favorables pendant la fin de la premiere s~quence de 2~me ordre (cycle "transgressif-rggressif") du Crgtacg sup~rieur aquitain (C~nomanien inf~rieur-Turonien sup~rieur): ~volution r~gressive presque continue, prismes de haut niveau marin progradants, isolement par une barri~re d'une vaste plate-forme interne, zone /~ subsidence mod4rge compensant les effets de l'abaissement du niveau marinet des apports sfidimentaires. Environ vingt esp~ces de rudistes, surtout des Radiolitidae (Biradiolites lumbricalis et Durania cornupastoris en particulier) et de plus rares Hippuritidae (Hippurites requieni, Vaccinites petrocoriensis) contribuent ~ r~laboration de deux grandes formations carbonat~es successives de plate-forme interne avec bioconstructions vari~es, normalement superpos~es aux calcaires crayeux sans rudistes du Turonien inffirieur. Elles s'~tendent sur une r~gion de 100 km de long sur 30 km de large environ et leur puissance cumul~e est comprise entre 35 m/~ l'Est en P~rigord et 70 m dans le Nord-Est des Charentes. Uorganisation et l'gvolution ggodynamique de ces formations /~ rudistes 4taient conditionnges par les influences contraires de l a m e r ouverte et des environnements littoraux qui entouraient la plate-forme interne avec, vers I'Est, des apports silicoclastiques. Les rudistes se sont adapt~s aux variations des biotopes en changeant par trois fois leurs associations spgcifiques. Les cliff, rents facies sont le reflet des phases successives de colonisation des substrats par les peuplements de rudistes, puis de proliferation et enfin de dgsertion, d'une plate-forme interne carbonat~e protegee par une barri~re plus ou moins continue, large de 30 km environ, constitute par des facies de calcar~nites de milieux haute-4nergie, souvent gtablie sur des zones de pal4oreliefs structuraux. Les apports silicoclastiques qui se sont rgpandus sur le P~rigord Noir pendant le d~but du Turonien supfirieur ont perturb~ le plus souvent le d~veloppement des communaut~s de rudistes; cependant dans des milieux de d~pSts plus boueux, Hippurites requieni a rgussi ~ construire des biostromes assez 4tendus et des Praeradiolites vivaient de fa~on isolges. MOTS-CLI~S: RUDISTES, TURONIEN, PALI~OENVIRONNEMENTS,PLATE-FORME CARBONATI~E,BASSIN D'AQUITAINE, FRANCE

296 FIGURE 1 - Major events and sedimentologicfeatures during the Late Cretaceous on the Northern Aquitaine platform. Principaux dv~nements et dvolution sddimentologique sur la plate-forme nord-aquitaine pendant le Crdtacd supdrieur.

Stage

65 I ~:

Rudist bearing platform

Siliciclastic Tectonic inputs events

ne Southern area of t h e ~

713

5

S/M

SG

?

SF

? end of oceanic accretion

Vl

i

z
'~ c)

SW

s

6

=

--,-- transgression regression --,-

Age 3 th-order MA sequence

3

-

in Gascogne

8o,8

-

2

~

SE

~

.o, MFS

SD

s 83,5 SANTON. ~ I 85,8

, 2

SC

~

SD

~--iv

S

CONIACIAb

-~ 890 S TURONIAI~ _

925 93#

S

SA TB TA CE (31)

,

,,

~o.o~MFS /////////~/////////////j

cc CENOM.

CB 98,9

p

cA

MFS" Maximum flooding surface deposit hiatus Ages according to GRADSTEIN et aL (1994) Sequence definition sensu HAQ et al. (1987)

INTRODUCTION

The Upper Cretaceous sedimentary rocks of the n o r t h e r n Aquitaine platform are made up of two major t r a n s g r e s s i v e - r e g r e s s i v e cycles (secondorder sequences), the first from Cenomanian on to Turonian times and the second from Coniacian on to M a a s t r i c h t i a n times (Fig. 1). This Upper Cretaceous succession, which can be as thick as 700 m in places, is cut by a major unconformity corresponding to a period of local emergence at the end of the Turonian due to an important fall in eustatic sea-level added to by the effects of tectonism (Fig. 1).There were four major periods of rudist

1 scarce 2 common 3 abundant 4 highly abundant imput origin

development (middle Cenomanian, middle-late Turonian, late Santonian, and latest Campanian to early Maastrichtian - Platel 1987, 1996) within favourable palaeoenvironments in all periods, such as inner shelf or littoral, sea-level highstand prograding wedges and regressive conditions. The middle-late T u r o n i a n period is t he most interesting due to its diverse facies associated with barrier-platform zonation and n u m e r o u s rudist species (Radiolitidae and Hippuritidae), the proliferation of which contributed to the establishment of thick carbonate successions with varied bioconstructions.

297 The two main rudist-bearing platforms of the middle-late Turonian period will be presented, with an aim of identifying their local palaeoenvironmental context and demonstrating the relationships with their fringing facies. GEOGRAPHICAL AND STRUCTURAL SETTING

The Turonian rocks crop out in a belt oriented SENW about 250 km long and 30 km wide stretching from the Lot Valley to Rochefort on the Atlantic coast. From east to west, this belt passes through the regions of P~rigord Noir, PSrigord Blanc, Angoumois and Saintonge, the two latter forming the Charentes area (Fig. 2). Turonian rocks also crop out further south in NW-SE anticlines at Jonzac, Mareuil, Tour-Blanche, P~rigueux, Saint-Cyprien and Sauveterre. Complexity of the present structural setting is a result of: 1) extensional tectonism, that caused

major faulting and block tilting in the BordeauxArcachon region and the northern platform, from Early Cretaceous until probably ConiacianSantonian times and, 2) compressive tectonism related to the Pyrenean orogeny, from Late Cretaceous to the Eocene paroxysm (Platel 1987, 1996). Compressive tectonism gave rise to superficial folding with more than forty anticlines embossing the whole platform, apparently due to displacement along major strike-slip faults in the crystalline basement, that deformed the sedimentary cover along their strikes. Dextral movement is deducted for the NNE-SSW strike-slip Dronne fault, whereas westwards the major NW-SE Jonzac anticline could be associated with a sinistral strike-slip fault. Miocene Alpine deformation probably also affected this area. The palaeostructural setting had a strong influence on facies distribution during the middle and late Turonian. Since the middle Turonian, high-energy grainstones have been thus accumulated on most

MASSIF CENTRAL

t= $3

Satt

o O

o

Ry

c~e,, Mareuil anticline

:,- 4 R SAINTONGE . Ps

o

~/~ La-Tour-Blanche ~ anticline

Bb Mm >,

M r ~

NO

"rr

vt ~" ~

' Y::.i'L

Regional diping Jurassio outcrop

Late Cretaceous outcrop ~

'-"7"...

Tertiary cover

.','.

Garonne ".'.,.:..:

Ch 0

50

Montauban paleohigh

FIGURE 2 - Geological setting and distribution of Turonian outcrops in the P~rigord and Eastern Charentes area• Ab: Aubeterre Ag: A n g o u l ~ m e - Ac: A r c a c h o n - Bb: B a r b e z i e u x - Bg: B e r g e r a c - Bt: B r a n t S m e - Bv: B e l v ~ s Br: B r i v e - Cg: C o g n a c - Ch: C a h o r s CI: C h a l a i s - C n : C h a t e a u n e u f / C h a r e n t e - Ct: Coutras - Gd: Gourdon - Fro: Fumel - Jz: Jonzac - Lg: Langon - LB: Le Bugue - Li: Libourne - Lp: Lesparre - Md: Mussidan - Mm: Montmoreau - Mn: Marennes - Mr: Mareuil - Mt: Montendre - Nt: Nontron - Ps: Ports - Px: P 6 r i g u e u x - Rb: R i b 6 r a c - Rf: R o c h e f o r t - Ry: R o y a n - Sa: S a i n t e s - SA: S a i n t - A g n a n t - SI: S a r l a t - Tn: T h e n o n - Vt: V e r g t .

Cadre gdologique et situation des principaux affleurements du Turonien dans le Pdrigord et dans la partie orientale des Charentes.

298 of the future Eocene structures (Jonzac, Mareuil, La Tour-Blanche anticlines). That can be interpreted as very shallow sedimentation corresponding to palaeohighs on uplifted parts of tilted blocks.

Sedimentary facies

Conia. SA

~

[,H ..am J

4th order 3rd order sequence sequence

T

STRATIGRAPHICAL AND SEDIMENTOLOGICAL

Z <

SETTING

Recent stratigraphical and sedimentological studies of northern Aquitaine Turonian rocks (Platel 1982, 1987), which further refine and complete previous works (Cassoudebat & Platel 1973, 1976), are based on biostratigraphic subdivisions proposed by Alcayd~ (1980), Robaszynski et al. (1982) and Amedro et al. (1982): - Early Turonian: M a m m i t e s nodosoides (ScHLOT.) and Inoceramus labiatus SCHLOT. - Middle Turonian: Collignoniceras woolgari

TB2 80

~O

F--

70 TB1 60.

50.

t~ eq

TA4

(MANT.)

- Late Turonian: Romaniceras deverianum (D'ORB.)

Z < 40-

But the entire stage is not represented, because Coloradoense and Neptuni cephalopod zones are missing, as is the Giganteus rudist zone. Vertical and horizontal facies evolutions gave rise to the definition of more than t w e n t y formations along the outcropping area, the study of which was based on a dynamic approach involving sequence analysis of facies using t w e n t y composite sections (Cassoudebat & Platel 1973, 1976).

The first 3d-order sequence, 20 to 60 m thick, begins at the regional base of the Lower Turonian (within the Nodosoides Zone), with open-marine deposits containing cephalopods (TAI-TA2 4thorder sequences), and ends at the top of the Middle Turonian with rudist facies from inner shelf (TAa-TA4 4th-order sequences). The second 3d-order sequence, 10 to 50 m thick, corresponds to part of the Upper Turonian (TB1-TB2 4th-order sequences) and reflects an increasingly shallowing environment with successively fine-grained, rudist-bearing wackestone, sandy limestone, coastal sandstone and marls in the P6rigord Noir area.

44.

I-TA3

g I''

30. ,'Jo-I I i '12 ,sA il ~

TA21 ~Z

10.

rr

VERTICAL EVOLUTION The northern Aquitaine Turonian series correspond to the transgressive maximum and to the regressive p a r t of the first Late Cretaceous second-order sequence. The Turonian environmental evolution (Fig. 3) is represented by two transgressive-regressive cycles or third-order sequences sensu H a q et al. (1987). These are separated by a discontinuity that is fairly marked at the platform scale, with local unconformity on the Turonian highs such as at Mareuil in P6rigord Blanc.

•

TAt

E

CE

O cO

.

MW I I

i

Explanation of sedimentary facie on the figure 4 showing the palaeoenvironments organization

Accommodation sequence • oo-,f~ooo mrs: maximumfloodings u r f a c e ~ sequenceboundary deepening upward \~:{![ shoaling upward and landward stepping V and seaward stepping

FIGURE 3 - Sequence evolution of the T u r o n i a n P l a t f o r m of the N o r t h e r n Aquitaine basin. Evolution s~quentielle de la plate-

[brme turonienne nord-aquitaine.

HORIZONTAL FACIES DISTRIBUTION Six palaeoenvironmental domains can be reconstructed from all the Aquitaine Turonian marine rocks (Cassoudebat & Platel 1976; Platel 1974, 1982, 1987), five of which occurred across the northern platform. Only the general characteristics of the sedimentary units will be mentioned here; major facies types are indicated in bold (Fig. 5):

299

OUTER-SHELF DOMAIN Carbonate environments

- Chalky wackestone with quartz silt and bryo: - Chalky wackestone with cherts ammonites a,Dunaantsponges , , ' , ' - bna,Ky muastone, with saitary corars ammonites, bryozoans and gastropods Marly environments - Mady glauconi~c mudstone with oysters

BASINDOMAIN - Marly mudstone with radiolarian, sponge spicules and planktonic foraminifer~ "~'~

Continental slope, with deposit hiatus

FIGURE 4 - Palaeogeography and facies organization during the early Turonian. Paldog~ographie et organisation des facies pen-

dant le Turonien infgrieur.

1. The basin domain: marly mudstone sedimentation within the Arcachon-Parentis area (outside figure left boundary of). 2. The outer-shelf domain: low-energy environment in the lower subtidal zone not favourable for rudists. This domain is characterized by marly mudstone, chalky wackestone and mudstone with chert, bryozoans, planktonic foraminifera, inoceramids and ammonites (facies G1 and secondary G2, G'2). 3. The barrier domain: characterized by high-energy palaeoenvironments such as shoreface, distal shoals and tidal channels with cross-bedded grainstone. This barrier corresponds to a series of submarine rises forming a fairly continuous barrier that protects to a certain degree the internal domain from swell action (facies E1 and secondary E2, F). 4. The inner-shelf domain: sedimentation of shallow-marine bioclastic wackestone and packstone with rudist biostromes in the upper subtidal zone. The most impressive rock is the white Angoul~me Formation, a thick homogeneous accumulation made up almost exclusively of Biradiolites lurebricalis (D'ORB.) debris which, during the middle Turonian, formed the first platform (facies D1). A more-proximal facies of early late Turonian age is made up essentially of wackestone to packstone with rudists, building biostromes or isolated

(mainly Durania cornupastoris (DES MOUL.), Radiolites praesauvagesi Touc., Vaccinites petrocoriensis) (Dour.), gastropods (nerineids, acteonellids) and chaetetids (facies D2); close to the littoral domain, fiat widespread biostromes were built by Hippurites requieni while small isolated Praeradiolites and Vaccinites lived in m u d d y biotopes (facies D3). 5. The littoral domain: included the littoral shoals, tidal channels, mixed carbonate-siliciclastic flat, foreshore and beach environments where siliciclastic cross-bedded sandstone, silty marl and silicicarbonate g r a i n s t o n e a c c u m u l a t e d (upper subtidal to intertidal zones - facies B, C). Siliciclastic supplies within the P6rigord Noir area generally disrupted the development of the rudist congregations during the late Turonian. 6. The backshore facies are scarce behind the shore, e.g. organic clay, lacustrine limestone, lignite and deltaic sandstone (supratidal zone - facies A1, A2, A3). MAJOR RUDIST-BEARING FORMATIONS AND RELATIONSHIPS WITH F R I N G I N G F A C I E S

Overlying the Lower Turonian series, t h a t was devoid of rudists (Fig. 4), five rudist-bearing for-

300 Southwest

Northeast f

/~--y

"'-'d,

"'"

o .,.

~ "7

,-

~

<'j'

/

. i~k,'

,

',--<>

tk~#{o.:

... ,..:z •

l

°

.,i

ll'~i

i

i

~

/L

'o •

-

~

A1

D

(ii, . . . . . . . . . . . . . . . :1

". . . . .

,ill E 2

i

!

•-

//////I/i/i/i/i'"

-,x/z/i//////////i/////////i//-- .....

Ol

O2

B

C

D3 0 I

Oo,or-s.e,,

I B.rr,ordom.

I

,n.er-s.e,,

I L,,tora,

I

1

,

20 km I

oma,nloo.s,a'p,.'°

FIGURE 5 - Organization of palaeoenvironments during the middle and late Turonian on the Northern Aquitaine basin. Associated sedimentary facies: C o a s t a l p l a i n - AI: Organic clay and lacustrine limestone - A2: Lignite - A3: Deltaic coarse sand• L i t t o r a l d o m a i n - B: Siliciclastic sand, cross-bedded sandstone and carbonate sandstone - C: Cross-bedded sandy grainstone and carbonate grainstone. Inner-shelf - DI: Chalky packstone and wackestone with small rudists (Biradiolites, Praeradiolites). D2: Wackestone and packstone with biostromes (loose-aggregates grading to fioatstone) of large rudists (Durania, Vaccinites, Radiolitidae), gastropods (nerineids, acteonellinids), chaetetids - D3: Marly and silty wackestone with rudists (Praeradiolites, Vaccinites), dense-aggregates of Hippurites, bryozoans, brachiopods. B a r r i e r d o m a i n - El: Cross-bedded grainstone (shoreface and tidal channels) - E2: Packstone with rudist floatstones and biostromes (loose-aggregates) - F: Chalky peloidal paekstone, with chert and solitary corals. Outer-shelf - GI: Chalky wackestone and mudstone with cherts,ammonites, bryozoans and planktonic foraminifera G2: Chalky peloidal wackestone - G'2: Marly mudstone with oysters. Organ/sat/on des paldoenvironnements pendant le Turonien moyen et supdrieur sur la plate-forme nord-aquitaine. m a t i o n s a c c u m u l a t e d , t h a t c o r r e s p o n d to t h e v a r i o u s p h a s e s of r u d i s t d e v e l o p m e n t (Fig. 6). T h r e e of t h e s e f o r m a t i o n s a r e r e p r e s e n t e d on F i g u r e 7. EARLY MIDDLE TURONIAN CHALKY PELOIDAL PACKSTONE AND GRAINSTONE (La Couronne a n d Villars f o r m a t i o n s - TA3 - 4thorder sequence)

A f t e r t h e s e a l e v e l h i g h s t a n d of t h e e a r l y Turonian, during which the most transgressive facies ( c h a l k y w a c k e s t o n e a n d m a r l y m u d s t o n e ) a c c u m u l a t e d on t h e whole p l a t f o r m (Fig. 4), t h e r e g r e s s i o n s t a r t e d a t t h e end of t h i s period. At t h i s t i m e t h e s e a b e d w a s located in t h e shoreface w i t h d i s t u r b a n c e of t h e s e d i m e n t s due to h e a v y swell a n d s t o r m waves. D u r i n g t h i s period, w h i c h c o r r e s p o n d s to t h e phases of substrate p r e p a ration a n d establishment, a few m e t r e s of fineg r a i n e d g r a i n s t o n e to p a c k s t o n e (La C o u r o n n e F o r m a t i o n ) a c c u m u l a t e d in t h e C h a r e n t e s a r e a

a n d of c h a l k y peloidal p a c k s t o n e (Villars F o r m a tion) in t h e P~rigord region (TA3 - 4 t h - o r d e r sequence). T h e s e deposits c o n t a i n a b u n d a n t b/oclasts of b r y o z o a n s , bivalves, g a s t r o p o d s a n d echin o d e r m s , b u t not m a n y r u d i s t s , t h e m o s t c o m m o n species b e i n g S p h a e r u l i t e s p a t e r a A r n a u d (Fig. 6). MIDDLE TURONIAN CHALKY BIOCLASTIC PACKSTONE WITH BIRADIOLITES L UMBRIC A L I S (Angoul~me F o r m a t i o n - TA4 - 4 t h - o r d e r sequence) The striking Angoul~me F o r m a t i o n s t r e t c h e s f r o m C h a t e a u n e u f to P 6 r i g u e u x covering a n a r e a 100 k m long and 25 k m wide (Fig. 7, 8, 9). Its b a s e is m a d e up of a few m e t r e s of peloidal p a c k s t o n e a n d fine-grained g r a i n s t o n e w i t h h u m m o c k y cross-stratification in places, but m o s t of the f o r m a t i o n is a v e r y soft w h i t e chalky p a c k s t o n e to wackestone. In this first i n n e r shelf e n v i r o n m e n t , Biradiolites lurebricalis (D'ORB.) d o m i n a t e d the sea floor (proliferat/on phase). T h e i r b r o k e n t e s t s c o n s t i t u t e m o s t of

301 FIGURE 6 - Changes in associations and abundance of the major Turonian rudists on the Northern Aquitaine basin. Variations des associations et des frdquences des principaux rudistes pendant le Turonien sur Ia plate-forme nordaquitaine.

Formations

G°ur-de'l'Arche Fm I B°urg'des-Mais°ns F~m I Angou~lmFm e

La Couronne Fm Villars Fm

Hippurites requieni H. requieni v a r . subpolygonia

oZ

Vaccinites rousseli V. praepetrocoriensis V. petrocoriensis V. a f f . beaussetensis

[L

o

Agria salignacensis Sphaerulites patera Durania cornupastoris

{D co

Biradiolites quadratu~ B. angulosus B.lumbricalis Praeradiolites ponsi P. pailletei P. praesinuatus P.'praecoquandi P. praerequieni P. praetoucasi Radiolites peroni R. lusitanicus R. socialis R. praesauvagesi R. beaumonti R. radiosus R. trigeri

a

o < n-

Z

oE3 ~, UJ "1-

~o~

-~-~ GO

©

~o._~

~ o

=

~

o

~

~ .= ~

-~

o

.~

i

] EA.L_ TURONIAN

t h e d e p o s i t s (floatstone) laid d o w n as a m a s s i v e l a y e r b e t w e e n 7 a n d 15 m thick. A l t h o u g h B. lurebricalis is t h e e s s e n t i a l o r g a n i s m in this f o r m a t i o n ,

and almost totally disappears at the top, the main a s s o c i a t e d r u d i s t s are (Fig. 6, 7): P r a e r a d i o l i t e s p o n s i (D'ARcH.), D u r a n i a c o r n u p a s t o r i s (DEs MOUL.),

302 NORTHWEST

BARRIER DOMAIN

INNER-SHELF

I I I

DQMAIN

L-ITTORAL DOMAIN

IR'EN

COGNAC CHATEAUNEUF LA-TOUR-SLANCHE TAILLEBOURG]POiS ST-T EME I IANGOULEME 1 MAREUILI I PERIGUEUXI

ROCHEFORTI

Petleteriel J I

t i BUSSaCFrn

20mt St-Agnant Fm

St-Meme Fm Jonzac Fm

i

A..... ..,,.. .... Bourgodes-MaisonsFm

DOMME MONTIGNAC GOURrN

SAINT-CYPI St-Cirq

Ste-C~'oix Pauslsac Chancelade

St.M~me

SOUTHEAST

LaRouquelle

Goufdon

I I Grazes Pmst.Cir Fm 5t-Cr~pin fm q Ste-MondaneFm

TB2

4th-order sequence LATE TURONIAN

-

Building up of the second rudist-bearing inner-shelf 'o I I

TB1

20m t

St-Agnant Fm

Jonz3c/BussacFm

St-M~me ~'. . . .Fm . ChateauneufFm Bourg-des-Maisons Fm

~Grazes Fm

. . . .

s,0km

I I

I

. St-Cirq rm

Ste-MondaneFm

4th-order sequence

Early LATE TURONIAN -

Enlargement of barrier environments Maximum of siliciclastic supplies I

St-M~me Fm

TA4:1 J.th-order sequence

S

'I

~t ....,~,..

-

g

n

o

m a.

o

n

.

.

.

.

.

o ....

t,~ i u.c~ ~m,t ~

. . .~

MIDDLE T U R O N I A N -

Cephalopod

~1 Rudist

Chert

~

Hummocky T~lal channel

A F

~ ~

Building up of the first rudist-bearing inner-shelf

Chaetetidae

Cross-bedding

~

Chalkywackestone

~

Bioclasticgrainstone

[7~7] Chalkypackstone ~

~

Maltywackestone

~

Siliciclasticgrai ......

Chalkypackstone/wackestone ~

Sandstoneandsand

~

~

withrudists Wackestone/packstone withrudists

Siltymarl

Clinoform

FIGURE 7 - Palaeoenvironmental evolution through the Northern Aquitaine platform. Variations latdrales des paldoenvironnements & travers la plate-forme turonienne nord-aquitaine.

Radiolites beaumonti (BAYLE), R. socialis D'ORB. and H i p p u r i t e s requieni MATH., this last species for-

ming small biostromes. The homogeneity of this limestone reflects very stable sedimentation conditions, the accumulation rate remaining in equilibrium with subsidence. The f r i n g i n g f a c i e s of the low-energy Angoul~me Formation are well identified (Fig. 7, 9): - T o w a r d s the open sea: thick deposits of crossbedded grainstone of the high-energy barrier zone (St-M~me Formation); the largest B. l u m b r i c a l i s biostromes built up against the internal side of this barrier, especially in the Chateauneuf area, generally as loose aggregates, less commonly close packed fabrics. - T o w a r d s the coast, the rudist-bearing formation grades rapidly over about 500 m into grainstones without rudists, but which have cross-bedding with low-angle foresets, high porosity and in places recrystallization typical of a beach rock. Study of the distribution of these deposits between P~rigueux, Nontron and Angoul6me (Platel & Dubreuilh 1979; Plate] & Paris 1988) has revealed a dominant NE-SW orientation, probably related to the Jurassic palaeorelief framework and to

Turonian positive s t r u c t u r e s such La TourBlanche (Paussac Formation; Fig. 7, 8, 9). - In P~rigord Noir, TA3 type sedimentation persisted with the Domme Formation, which contains sparse A g r i a salignacensis (BAYLE)(Fig. 6). LATE TURONIAN BIOCLASTIC WACKESTONE AND PACKSTONE (Bourg-des-Maisons Formation - TBI-TB2 - 4th order sequences) After a major break in the sedimentation, represented in certain anticlines by a minor unconformity, e.g. at Mareuil, a few metres of channelled grainstone accumulated forming a new third-order sequence (TB), dominated by a regressive evolution. This grainstone was overlain by wackestone and packstone that generally constitute the substrate of the second Turonian platform, an extensive carbonate inner shelf. Its development was fairly restricted by the high-energy barrier that extended seawards and landwards during deposition of TB1 - 4th-order sequence (Fig. 6, 7, 8, 10); but the geographic coverage of the inner shelf was slightly greater during deposition of TB2 - 4th-order sequence than during the middle Turonian.

303 W

E

Bourg-des-Maisons

St-Just

Paussac

St-Vivien

Bourdeilles

I I

u nco

~ . ~ s ~ Fm

'

liit

m

%.

I L~Tour-Blanche "L.r-i~ . V . I~ R°~"'a*"""~t~ ant.cllne

s ~

(~ ~ ~ ~ .,~ ~ ~ (-.~ P,.ssac. J('~-~" ~ "~ 0 village ~ . °.~.,.-,_," _j

OF--

I

51kin

Coarse to medium-grained

cross-bedded grainstone Bioclastic grainstone

"Lnero~

~.._~i"

..

.

, '~.

~<

"

B

~J

.~__,

~

.~

"

Chalky packstone/wackestone with Biradiofites lumbricalis Wackestone/packstone with Durania, Vaccinites and Radiolites / ~ Rudist ~ Hummocky Chaetetidae ~ Cross-bedding

~

FIGURE 8 - H i g h e n e r g y s h o a l w i t h i n t h e m i d d l e / l a t e T u r o n i a n i n n e r s h e l f in t h e P a u s s a c area. Organisation des facies & haute-

dnergie sur le haut-fond de Paussac p e n d a n t le Turonien moyen et supdrieur.

Middle TA

TURONIAN

3rd-ofder s e q u e n c e - T A 4

4th-order sequence

.LITTORAL DOMAIN - Beach-lock: cross-bedded grainslone.

MASSI[ CENIRAI

INNER-SHELF DOMAIN

f

- Chalkypackstolm aP~lwackestone with small rudists (abundant Biradiofites, P[aeradioli[es). ~

i

N~,J

Rudlst hioM¢omes.

I~RRIER DOMAIN . Sandwaves and ~dal channels: Coarse- or medium-grained grainstone with foreset$ ot clos$-hedding. Scattered rudists m eastern area OUTER-SHELF DOMAIN +Pelok~alp~stone and wacheslone with abclndanl cherts. - Peloidal bioclastic wackeslone A wilh cherts, echin~s, sponges and bc/'ozoan$. ~ Chalky w ~ s t o n e with sotitai'y corats. Z-~ ammoniloS, b,'yozoansand gastiopods.

o°

. ~

I

.

- \.

"

-

~

~-~-i

I~ASIN DOMAIN

~.

- Marly mudstone wilh radiolarlan. I Spot, s spicules and planklonic Ioraminifer

"~+~¢rContinent=lslope, with deposit hiatus Transectofthe fKJ. 7 ]ransect of the fig. 8

I O

modified from PLATEL 1987 50 km

FIGURE 9 - P a l a e o g e o g r a p h y a n d facies o r g a n i z a t i o n d u r i n g t h e m i d d l e T u r o n i a n . Paldogdographie et organisation des facies pen-

dant le Turonien moyen.

304

LII-rORAL DOMA1N

-

Beach and lidal Ilal: Cross-bedded sand and sandslone. - Proximal lens: Cfoss-beddsd sandy grainstone and carbonate g,ainstone,

INNER-SHELF DOMAIN - Wackestone and packstone with biostromes [~ ( loose-aggregatesandfk~atsrone) of lar rudists (Durania, Vaccinites. Radlolitk gastropods (nerioids, acteonellids) and BARRIER DOMAIN - Biostromes wilh Radiofitldae and bioclastio p~ckslo~e and greinstone - Sand waves and tidal channels: Coarse- or medium-gralned grainston, with O ese s o C oss-beddinQ. OUTER-SHELF

DQM/MB

. Peloldal blOClastic wackestone with echinlds and bryozoans. - Chalky wackeslone with solilary corals, b~'Ozoans and gastropods. BASIN DOMAIN ] - Marly and chalky mudstone wllh planktonic loraminifela. ~

Continental slope, with deposit hiatus Direction ol sillciclastie supply

/:~*:" Probable

limit of Ihe shoreline

~ / " ' ~ * " ~ T r a n s e c t of the fig. 7 ~f

Transect of the fig, 8 and fig 1 I

1 0 - Palaeogeography and facies organization during the late Turonian. Paldog~ographie et organisation des facies pendant le Turonien supOrieur.

FIGURE

WNW

ESE

Aucors Castle _

Aucors

~

4 |

P;~

slump

Quaternary

erosion

%

"~-- ..... .., .

o I

---*Ladosse

cllnoforms

J = ~ - . ~ ~ = ~ - " ~ - " ~ - " ~ I

quarry

~

200 m ,

,

~

-

.........

'

'

~ p:

" . . . . .

Kaaiodtid denseto Ioose-aggfegate~

-

"

4

~

"~-:"__

~"

~

jz

/ Bourg-des-Maisons

Fm

\

, F f o a t s t o n e s and isolated Radiolitids

1 1 - Relationships between high energy littoral shoal prograding into the late Turonian inner shelf in the Aucors area (Beaussac region). Relations entre les facies littoraux it haute-dnergie progradants dans la plate-forme interne pendant le Turonien supdrieur dans le secteur d'Aucors (rdgion de Beaussac). FIGURE

Wackestone and packstone bioclastic limestone sequences, about 30 m thick in total, capped by floatstone with Radiolitidae and containing thin fine grainstone beds, constitute the Bourg-desMaisons Formation (TB1-TB2 - 4th-order sequences). A second p r o l i f e r a t i o n p h a s e developed in this low-energy depositional environment with, in addition to the acteonellids, nerinids, chaeteti-

dids, miliolids and rotalids, a new rudist assemblage of mainly elevator forms (Fig. 6): Durania cornupastoris, Vaccinites petrocoriensis (Douv.), V. praepetrocoriensis Tout., Hippurites requieni, Radiolites praesauvagesi Tout., R. trigeri (CoQ.), R. peroni (CHoFF.), R. radiosus D'ORB., Sphaerulites patera ARNAUD, Biradiolites angulosus D'ORB. and B. quadratus (D'ORB.), plus Praera-

305 diolites ponsi that almost entirely disappears in TB2 - 4th-order sequence. Polyspecific biostromes are common to the north of P~rigord Blanc, as are large biostromes (dense to loose aggregate growth fabric) of Radiolites sp. and Durania cornupastoris in the Beaussac region (Aucors section; Fig. 11) and of Biradiolites lumbricalis to the west of Mareuil (La Rochebeaucourt). The f r i n g i n g f a c i e s of the Bourg-des-Maisons Formation are (Fig. 7, 10): - Towards the open sea: high-energy calcarenite similar to that of the middle Turonian (St-M~me Formation) with large biostromes on the internal side (Chateauneuf Formation); -. Towards the south Pdrigord littoral domain: thin-bedded bioclastic packstone, cross-bedded grainstones and packstones (about 20 m thick) constitute the lower part of the Grazes Formation (TB1 - 4th-order sequence). Radiolitids are abundant, building small biostromes (loose aggregates) and reworked in floatstone beds. Large oblique bedding and some clinoforms, observable close to P~rigueux, indicate the proximity of high- energy barrier environment retrograding into the inner shelf. F u r t h e r east, a large detrital fan with sparse rudists grades into the previous facies as far as La Tour Blanche. However, near Saint-Cyprien where the deposits are less sandy, rudists flourish in the St-Cirq Formation. To the west of the V~z~re River, several layers of lignite-bearing marl characterize emergence of an area that separated this eastern region from the carbonate inner shelf; this is r e f e c t e d too by the lack of coarse detrital sediments within this carbonate shelf. Around P~rigueux, marly wackestone containing Vaccinites petrocoriensis and Hippurites requieni deposited towards the end of this period. - Towards the northern littoral domain: there are few data concerning the coastal facies in the north after Quaternary erosion, but at Beaussac (to the north of Mareuil), slumped thin-bedded grainstone is overlain by coarse to medium-grained grainstone with clinoforms. These last, which are exposed in the cliff beneath the Aucors Chateau, prograded eastwards into the proximal inner shelf where large Radiolitidae biostromes accumulated (Fig. 11). LATE TURONIAN MARLY WACKESTONE (Gour-de-l'Arche Formation, top of St-Cirq Formation - top of TB2 - 4th-order sequence) The last unit of the Turonian a g e , which is better preserved in P6rigord because pre-Coniacian erosion was less intense, is only 2-7 m thick and corresponds to the e x t i n c t i o n p h a s e of the rudist community and the d e s e r t i o n p h a s e of the platform. This environment was not very suited to rudist development with silty and pyritic marl

deposition, but an ultimate association of small species (clinger forms) continued to survive: Praeradiolites praesinuatus Touc., P. praecoquandi Touc., P. praerequieni Tout., P. praetoucasi Tout., P. pailletei (D'ORB.), Sphaerulites patera, Radiolites. trigeri, Vaccinites rousseli (Douv.) and V. aff. beaussetensis (Fig. 6). In P~rigord Noir, a final biostrome (closed-packed frameworks to floatstone) containing Hippurites requieni var. subpolygonia associated with rare Radiolites praesauvagesi and Durania cornupastoris caps the Turonian deposits.

PALAEOECOLOGY OF THE NORTHERN AQUITAINE TURONIAN RUDISTS MORPHOLOGICAL AND ECOLOGICAL DIVERSITY The Turonian rudists of northern Aquitaine display a large diversity of shapes, that were related to different ways of life and adaptation to specific environments (Platel 1974, 1982). They can be divided into two main morphological groups, as described by Skelton & Gili (1991), which in turn can be subdivided according to the classification of forms distinguished by Philip (1972): elevator forms (cylindrical or conical) and clinger forms (broad conical or conical with a fiat anterior part). In the Aquitaine Turonian series, there are two major rudist settlement modes, gregarious and isolated. G r e g a r i o u s r u d i s t s - These types form biostromes, in thin beds of 0.2 to 1 m-thick, that extend over large distances between 50 and 200 m in flattened lenses. The biostromes are generally made up of individual rudists growing side by side, within dense- or loose-packed fabrics (comprising one or two stacked generations), grading laterally into floatstone commonly associated with packstone-grainstone deposits. The former fabric type was sometimes called "framestone" whereas the latter one was assimilited to "bafflestone" by several authors (Bilotte 1985; HSfling 1985; Grosheny & Philip 1989), following the classification of E m b r y & Klovan (1971), created for modern coral frameworks. But this use has been criticized by Skelton & Gill (1991) and Gill et al. (1995), because these terms, inappropriate according to these latter works, could imply that rudist aggregates could have been upstanding frameworks, growing above the seafloor and baffling more or less transported sediment. On the contrary for Gili et al. (1995), most of rudist congregations could have been probably living in embedded growth fabric, within in situ generated sediment matrix: so these authors pro-

306 FIGURE 1 2 - 1. Middle Turonian Angoul6me Formation (TA4 sequence). Chalky packstone with loose-aggregates of Biradiolites lumbricalis at La Pelleterie Quarry (scale bar is 10 cm long). 2. Middle Turonian Angoul~me Formation (TA4 sequence). Small cluster with dense-packed congregation of Biradiolites lumbricalis at La Pelleterie Quarry (scale bar is 5 cm long). 3. Middle Turonian Angoul6me Formation (TA4 sequence). Loose-packed congregation of Biradiolites lumbricalis, upper face of the biostrome (scale bar is 10 cm long). 1.

Formations d'Angoul~me du Turonien moyen (sdquence TA4). Packstone crayeux avec associations & faible densitd de Biradiolites lumbricalis dans la carri~re de La Pelleterie (gchelle = 10 cm). 2. Formation d'Angoul~me du Turonien moyen (sgquence TA4). Petit bouquet avec une association dense de Biradiolites lumbricalis dans la carri~re de La Pelleterie (gchelle = 5 cm). 3. Formation d'Angoul~me du Turonien moyen (sdquence TA4). Accumulation en biostrome & faible densitd de Biradiolites lumbricalis, face supgrieure du biostrome (dchelle = 10 cm.

2

3

307 FIGURE 13 - 1. Late Turonian Bourg-des-Maisons Formation (TB1 sequence) Dense-aggregate of Durania cornupastoris and others radiolitids at the Aucors Quarry. 2. Late Turonian Bourgdes-Maisons Formation (TB1 sequence) Loose-packed congregation of Durania cornupastoris, Biradiolites lumbricalis, and others radiolitids at the Aucors Quarry. 1. Formation de Bourgdes-Maisons (s~quence TB1). Biostrome it haute densitd de Durania cornupastoris et autres radiolitidds dans Ia carri~re d'Aucors. 2. Formation de Bourgdes-Maisons (sdquence TB1). Biostrome it faibIe densit~ de

Durania cornupastoris, Biradiolites lumbricalis et autres radiolitidds dans la carri~re d'Aucors.

posed to refer to a "constratal fabric" for rudist aggregates, as opposed to a "superstratal fabric" for coral frameworks. The most favourable shapes for this settlement mode are the elongate narrow cylindrical shape and the long conical shape. The former one creates biostromes with a high density of individuals (dense aggregate fabric), most commonly of H i p p u r i t e s r e q u i e n i in the littoral environments around Gourdon (Nadaillac) or B i r a d i o l i t e s I u m b r i calis in the inner shelf (Chfiteauneuf, Angoul~me, Mareuil, Rochebeaucourt; Fig. 12.1,3). The latter shapes built looser packed biostromes (Radiolitidae

in inner shelves at La Maillarderie, Maine d'Euche, Pons, Aucors, etc.). The horizontal proliferation could have been assisted by a low sedimentation rate (Fig. 13.1,2). Rather than forming continuous beds, gregarious rudists sometimes are found in bunched and bundled aggregates ("cluster" and "thicket" of Kauffman & Sohl -1974). Instead of being fixed on the same horizon in a coalescent manner, the individuals "root" themselves onto one another ( B i r a d i o l i t e s l u r n b r i c a l i s at Chfiteauneuf-La Pelleterie, and H i p p u r i t e s requieni at Gourdon, Gour-de-l'Arche and Jonzac; Fig. 12.2). This could be the case, either

308

2 FIGURE 14 - 1. Coniacian-Turonian unconformity Early Coniacian marly wackestone overlying unconformably a late Turonian marly floatstone of radiolitids and hippuritids of the Saint-Cirq Formation at La Roquette section. 2. Late Turonian Saint-Cr6pin Formation (TB2 sequence) Dense aggregative thicket ofHippurites requieni var. subpolygonia. 1. Discordance Coniacien/Turonien. Wackestone marneux du Coniacien infdrieur recouvrant en inconformitd un floatstone de radiolitidds et d'hippuritidds dans les marnes de la formation Saint-Cirq dans la coupe de la Roquette. 2. Formation de Saint-Crdpin (sdquence TB2). Buissons & haute densitd d'Hippurites requieni vat. supolygonia..

w h e n t h e r e w e r e few firm s u b s t r a t e s for the l a r v a e to settle (using the older r u d i s t shells), or w h e n the s e d i m e n t a t i o n r a t e was quite high w i t h vertical growth being quicker t h a n horizontal colonisation to e n s u r e the congregation's survival. Four or five g e n e r a t i o n s so succeeded in the thickets of the littoral d o m a i n in the Gourdon region (Pont du GuG Nadaillac), w i t h i n t r a n s g r e s s i v e deposits of a final 4 t h - o r d e r sequence (Fig. 14.2). G r e g a r i o u s f o r m s are p a r t i c u l a r l y w i d e s p r e a d on

f i r m - g r o u n d s , m o s t c o m m o n l y m a d e u p of shell f r a g m e n t s , e.g. G o u r d o n r u d s t o n e . rudists - T h e s e r u d i s t s reflect d i f f e r e n t s e t t l e m e n t conditions. M o s t i n d i v i d u a l s b e l o n g to t h e long conical form, to t h e b r o a d conical f o r m or to t h e fiat conical form. I n t h e N o r t h e r n A q u i t a i n e T u r o n i a n f o r m a t i o n s D u r a n i a , V a c c i n i t e s a n d certain Praeradiolites b e l o n g to t h e f i r s t f o r m , Sphaerulites can be a s s i g n e d to t h e second one a n d o t h e r s P r a e r a d i o l i t e s to t h e l a s t one.

Isolated

309 The horizontal basal area of these latter organisms does not enable them to group together to build biostromes. The large contact surface of their lower valve with the sea-floor ensured a suitable foundation. These rudists were particularly adapted to low sedimentation rate in medium to low energy palaeoenvironments; in the Northern Aquitaine Turonian deposits, they could live on unconsolidated substrates such as wackestone soft grounds (Praeradiolites pailletei, P. praesinuatus, P. praecoquandi, P. praecoquandi, P. praerequieni of the St-Cirq Formation), but it seems that clinger forms were able to settle on different types of substrates from hard to soft, according to Skelton & Gill (1991). COLONIZATION OF THE PLATFORM Four phases of a sedimentary succession can be distinguished as the rudist-bearing assemblages took over the northern Aquitaine platform during the middle to late Turonian time. Succession of the phases was induced by changes in environmental conditions and can be recognized at the scale of several sequential orders. The phases could be diachronous over the platform as a whole. This pattern is well marked, when the regressive trend was sufficiently continuous as for the establishment of highstand systems tracts.

Substrate preparation phase - Over chalky deposition from the open-marine environment, a medium-energy zone can be depicted, that began to define a more-sheltered area as a result of a sea level fall (either eustatic or related to tectonic uplift). The elements formed were transported into this internal zone where bivalves, gastropods, etc. proliferated. The initially chalky and soft substrate became coarser grained and more solid (firm-ground).

Establishment phase - Since the high-energy barrier zone developed, the platform became increasingly protected; consequently isolated clinger rudists adapted to unconsolidated substrates such as Sphaerulites, Praeradiolites, etc. developed. Debris from these pioneer organisms led to a shelly substrate favourable for the rooting of next settlements.

Proliferation phase - Partly isolated from the open sea by a large grainstone barrier, the inner shelf became populated by a highly varied fauna of gregarious and isolated rudists (elevator and clinger forms) and other benthic organisms such as gastropods, acteonellids, nerineids and chaeterids. As conditions became optimal (inner shelf environments, low to medium water energy, very shallow depth of the sea), there was a bloom of the forms. This led to the establishment of thick bioconstructed deposits that constitute aggrading (or

prograding) highstand wedges with loose-packed to dense-packed rudist assemblages, generally built at the top of fourth-order sequences or shorter ones (Fig. 12.1, 13.1,3). However some occurences of hippuritid biostrome exist too in transgressive systems tract a within formed perideltaic environment.

Extinction and desertion phase - A sudden input of detrital material in the inner shelf caused the almost total disappearance of the rudist congregations. Hippurites requieni seems to have survived the best, because of the dense biostromes (clusters and thickets) that it formed in the transgressive base of the last genetic sequence in the P6rigord. Sparse Sphaerulites, Praeradiolites and Durania survived in the zone distal to input. By another way this last phase could have been caused by a rise in the relative sea level and re-establishment of an outer shelf. COMPARISONS WITH OTHERS LATE CRETACEOUS PLATFORMS

In some Mesogean regions, rudist-built formations clearly separate an internal environment from an open-marine one, such as in the Turonian series of the Pyr6n6es Audoises (Bilotte 1974, 1981, 1985); in contrast the middle-late Turonian northern Aquitaine platforms were organized into rudist-bearing complexes, as demonstrated for the same period in Provence by Philip (1972, 1974, 1994), Jolet (1996) and Jolet et al. (1994) and in the Turonian and U p p e r Santonian series of Northern Spain by Floquet (1991). However, the originality of this Aquitaine platform organizatien lies in the well marked separation of the inner upper subtidal rudist-bearing platform from the open-marine lower subtidal domain by well developed high-energy facies commonly related to tectonic highs, within which rudists are extremely sparse, reworked or completely broken. This major characteristic distinguishes too these T u r o n i a n formations from other N o r t h e r n Aquitaine r u d i s t - b e a r i n g formations (middle Cenomanian, late Santonian and latest Campanian to Maastrichtian in ages). These latter environments were distributed over less diverse platforms (carbonate ramp), with gradation between a relatively open shallow-marine shelf and the proximal shelf with siliciclastic supplies (Platel 1987, 1996). Rudists here were scattered over the entire platform except during the Santonian, when they were restricted to the P~rigord. In addition, there was no major biostrome apart from locally in Charentes during the middle Cenomanian (with Caprina adversa and Radiolitidae) and the latest Campanian (with Hippurites radiosus and Lapeirousia jouanneti).

310

CONCLUSIONS On t h e N o r t h e r n A q u i t a i n e C r e t a c e o u s m a r g i n , t h e m i d d l e to l a t e T u r o n i a n t i m e s w e r e t h e m o s t f a v o u r a b l e period for t h e d e v e l o p m e n t of r u d i s t a s s e m b l a g e s , b e c a u s e of t h e e n v i r o n m e n t a l specifity of t h e n o r t h e r n p l a t f o r m at this period. As a l m o s t c o n t i n u o u s e u s t a t i c r e g r e s s i v e conditions o c c u r r e d d u r i n g m o r e t h a n two million y e a r s a n d w i t h t h e s h o a l i n g e n h a n c e m e n t on t h e h i g h s of t h e p a l a e o s t r u c t u r a l f r a m e w o r k , a wide g r a i n s t o ne b a r r i e r s u r r o u n d e d a p r o t e c t e d s h a l l o w - m a r i n e inner shelf well-established within the Eastern C h a r e n t e s a n d t h e W e s t e r n P~rigord. Overlying the Lower T u r o n i a n deposits b a r r e n of rudists, five r u d i s t - b e a r i n g formations were deoisited w i t h several successive f a u n a a s s e m b l a g e s replacing one another. About t w e n t y species of rudists, p a r t i c u l a r l y Radiolitidae (especially Biradiolites lumbricalis and Durania cornupastoris) and sparse H i p p u r i t i d a e (Hippurites requieni, Vaccinites petrocoriensis) contributed to the e s t a b l i s h e m e n t of two m a j o r successive carbonate inner-shelves. The f o r m e r shelf is characterized by the a b u n d a n c e ofBiradiolites lumbricalis, the debris of which led to the accumulation of the striking m a s s i v e white c h a l k y A n g o u l ~ m e F o r m a t i o n . The second one (Bourg-des-Maisons Formation) corresponds to the b a s a l U p p e r T u r o n i a n deposits; their facies are m a d e up of sequences with a l t e r n a t i n g grainstone and bioclatic packstone/wackestone in more proxireal e n v i r o n m e n t s t h a n t h a t of the middle T u r o n i a n platform.To the E a s t of the platform the siliciclastic supplies covered the whole P~rigord Noir since the beginning of the late Turonianonward. So rudist a s s s e m b l a g e s deserted this area, but Hippurites requieni persisted in dense-packed fabrics. T h e n , d u r i n g t h e t w e n t y - f o u r million y e a r s of t h e r e m a i n i n g C r e t a c e o u s times, r u d i s t a s s e m b l a g e s n e v e r f o u n d on t h e N o r t h e r n A q u i t a i n e p l a t f o r m s u c h f a v o u r a b l e conditions a n d e n v i r o n m e n t s for t h e i r d e v e l o p m e n t ; Middle to U p p e r T u r o n i a n series t h u s r e p r e s e n t for t h i s region t h e b e s t e x a m p l e to u n d e r s t a n d t h e r e l a t i o n s h i p s b e t w e e n r u d i s t - b e a r i n g f o r m a t i o n s a n d f r i n g i n g deposits w i t h i n a m o d e l of c a r b o n a t e p l a t f o r m w i t h barrier. A c k n o w l e d g e m e n t s - The author is pleased to thank Rowena Stead (BRGM) for her help in the translation of this paper. He also wishes to acknowledge Marc Floquet (Universit~ de Provence) and Eulalia Gill (Universitat Autonoma de Barcelona) for their constructive comments on the manuscript.

REFERENCES ALCAYDEG. 1980 - Le Turonien. in Les ~tages fran~ais et leurs stratotypes. Mdmoire du BRGM, 109: 139-144. AMEDRO F., BADILLETG. & DELVAQUECh. 1982 - Biostratigraphie et biozonation des ammonites du Turonien

fran~ais. M~moire du Museum National d'Histoire Naturelle, s~rie C, 69: 167-173. BmOTTE M. 1974 - Exemple d'environnement p~rir~cifal dans le Cr~tac~ sup~rieur des Corbi~res: le r~cJf-barri~re du Turonien de Rennes-les-Bains (Pyr~n~es-Audoises). Comptes-Rendus du 96kme Congrgs des Soci~tds Savantes, Toulouse, 1971, Sciences, 2:239 - 244. BILOTTEM. 1981 - Quelques biotopes ~ rudistes pendant le Cr~tac6 sup~rieur. Exemples pyr~n~ens. Comptes-

Rendus du 106~me Congrgs des Soci~tgs Savantes, Perpignan, 1981, Sciences, 1: 167-175. BILOTTE M. 1985 - Le Cr~tac~ sup~rieur des platesformes est-pyr6n~ennes. Th~se Doctorat d'Etat esSciences, Toulouse. Strata - Toulouse, 2, 5, 438 p. CASSOUDEBATM. & PLATELJ.P. 1973 - Le Turonien de la bordure septentrionale du Bassin aquitain. Etudes s~dimentologique et pal~og~ographique - Th~se 3 ° Cycle, Universit~ Bordeaux III, 2 tomes, 226 p. CASSOUDEBAT M. & PLATEL J.P. 1976 - SSdimentologie et palSog6ographie du Turonien de la bordure septentrionale du Bassin aquitain. Bulletin du BRGM, (2), i, 2: 85-102. EMBRY A.F. & KLOVAN J.E. 1971 - A Late Devonian reef tract on the northeastern banks island, N.W.T. Bulletin

of Canadian Petroleum Geologists, 19, 4: 730-781. FLOQUET M. 1991 - La plate-forme nord-castillane au

Cr~tac~ sup~rieur (Espagne). Arri~re pays ib~rique de la marge passive basco-cantabrique. S~dimentation et vie. Th6se Doctorat d'Etat es-Sciences, Dijon, 2 tomes, 925 p. GILl E., MASSEJ.P. & SKELTONP.W. 1995 - Rudists as gregarious sediment-dwellers, not reef-builders, on Cretaceous carbonate platforms. Palaeogeography, Palaeoclimatology, Palaeoecology, 118: 245-267. GRADSTEIN F.M., AGTERBERGF.P., OGG J.G., HARDENBOLJ., VAN VEEN P., THIERRY J. & HUANG Z. 1994 - A Mesozoic time scale. Journal of Geophysical Research, 99, B12, p. 24. GROSHENY D. & PHILIP J. 1989 - Dynamique bios~dimentaire de bancs ~ rudistes dans un environnement p~ridelta~que: la formation de La Cadi~re d'Azur (Santonien,SE France). Bulletin de la Socidtg gdologique de France, (8), V, 6: 1253-1269. HAQ B.U., HARDENBOLJ. & VAIL P.R. 1987 - C h r o n o l o g y of

fluctuating sea levels since the Triassic. Science, 235: 1156-1167. HOFLINGR. 1985 - Faziesverteilung und Fossilvergesellschaftungen im karbonatischen Flachwasser-Milieu des alpinen Oberkreide (Gosau Formation). Mi~nchen Geowissenschaftliche Abhandlungen (A), 3, 141 p. KAUFFMANE.G. & SOHLN.F. 1974 - Structure and evolution of Antillean Cretaceous rudist frameworks.

Verhandlunge der Naturforschende Gesellschaft in Basel, 84: 399-467. JOLET P. 1996 - Etude stratigraphique et s~dimentologique d'une plate-forme mixte carbonat~e-silicoclastique: le Turonien de Provence (S.E. France). Th~se 3 ° Cycle, 1973, Universit~ de Provence, 221 p. JOLET P., PHILIPJ. & TRONCHETTIG. 1994 - Stratigraphie s~quentielle d'une plate-forme mixte (carbonat~esiliciclastique). Le Turonien inf~rieur et moyen de Provence (S.E. de la France). 1st international mee-

311 ting on P e r i m e d i t e r r a n e a n carbonate platforms (1994, Marseille). Ggologie Mdditerrandenne, 21, 3-4: 93-95 PHILIP J. 1972 - Pal~o~cologie des formations calcaires Rudistes du Cr~tac~ sup~rieur. L'exemple du SudEst de la France. Palaeogeography, Palaeoclimatology, Palaeoecology, 12: 205-222. PHILIP J. 1974 - Les formations calcaires ~ Rudistes du Cr~tac~ sup~rieur provencal et rhodanien: stratigraphie et pal~og~ographie. Bulletin du BRGM, (2), 1, 3: 107-151. PHILIP J. 1994 - Sequences and systems tracts of a mixed carbonate-siliciclastic platform-basin setting. The C e n o m a n i a n - T u r o n i a n of Provence (SouthE a s t e r n France). 1st i n t e r n a t i o n a l m e e t i n g on P e r i m e d i t e r r a n e a n carbonate platforms (1994, Marseille). Gdologie Mdditerrangenne, 21, 3-4: 149152. PLATEL J.P. 1974 - Un modSle d'organisation des biotopes ~ Rudistes: l'Angoumien de l'Aquitaine septentrionale. Bulletin de la Socidtd Linndenne de Bordeaux, 4, 1: 3-13. PLATEL J.P. 1982 - Les formations ~ Rudistes du Turonien moyen et sup~rieur de la plate-forme nordaquitaine. Mdmoire du Musdum national d'Histoire naturelle, C, 69: 91-96. PLATEL J.P. 1987 - Le Cr~tac~ sup~rieur de la plateforme s e p t e n t r i o n a l e du bassin d'Aquitaine. Stratigraphie et 5volution g~odynamique. Doctorat d'Etat es-Sciences, Bordeaux III, 573 p. avec atlas. Documents du BRGM, n ° 164 (1989).

et ~volution gdodynamique de la plate-forme carbonatde du Cr6tac~ sup~rieur du nord du bassin d'Aquitaine. Gdologie de la France, 4: 33-58.

PLATEL J.P. 1996 - Stratigraphie, s~dimentologie

PLATELJ.P. • DUBREUILHJ. 1979 - Carte g~ologique de la France (1/50 000), feuille P ~ r i g u e u x - E s t (759), Orl~ans-BRGM. Terrains c%tac~s et Notice explicative, 28 p. PLATELJ.P. & PARIS J.P. 1988 - Carte gSol. France (1/50 000), feuille P~rigueux-Ouest (758), Orl~ans-BRGM. Notice explicative p a r Platel J.P., Celerier G., Duchadeau-Kervazo C., C h a r n e t F., Chigot D. (1989), 82 p. ROBASZYNSKI F., ALCAYDE G., AMEDRO F., BADILLET G., DAMOTTE R., FOUCHER J.C., JARDINE S., LEGOUX O., MANIVIT H., MONCL~RDINI C. & SORNAY J. 1982 - L'~tage Turonien dans sa r~gion-type "de Saumur ~ Montrichard'. Biostratigraphie, s~dimentologie, coupures. Mdmoires du Musdum National d'Histoire Naturelle, C, 69: 15-28. SKELTON P.W. &GILI E. 1991 - Palaeoecological classification of rudist morphotypes, in 1st International Conference on Rudists - (1988, Beograd). Serbian Geological Special Publication, 2:71-86 (reprint from unpublished volume). J.P. PLATEL BRGM SGN/SGR/AQI French Geological Survey, Aquitaine Branch, F-33600 Pessac