A sketch of the tertiary Formations of the Bordelais

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  • 153

    A SKETCH OF THE TERTIARY FORMATIONSOF THE BORDELAIS.

    By A. P. DUTERTRE , A SSISTANT AT THE FACULTY OF SC IENCES OF THE USIVER SITY OF L ILLE,M E M BE R OF THE S OCltTt LIN N E N N E OF B ORDE AUX.

    A. INTRODUCTION.THE Bordelais occupies the north-western portion of the basin

    of Aquitaine, in which were accumulate d an im portantserie s of Tertiary beds containing representatives of all the st agesof the Nummulit ic an d the Neogene from the Ypresian up to theMiddle Vindobon ian . These formations are rem ark able for thevariety of the facies they present , and especiall y for the wealthof their fauna.

    These several stages , as well as the Cretaceous whi ch appearsat the periphery of the basin , are covered over wide areas by thesands an d gravels of the landes ; along the Atlantic shore whichbounds this region on the west , a belt of sand-dunes, some times4 or 5 kilometres wide, st retc hes from the mouth of the Girondeto that of the Adour, and even a little farther to the south ;behind these dunes there is a series of ponds, somet imes of greatwidth.

    B. STRATIGRAPHICAL SUCCESSION.NUMMULITIC.

    Ypresian,This stage has very limited outcrops. On the righ t bank of

    the Gironde, near it s mouth, it is represented by fine, greenish-ye llow, glauconitic sands containing Nummulites planulaius andfilling pockets in the chalk of Dordonian [ = Maest richt ianJ agenear Meschers, S.E. of Royan. On the left bank the Verdonboring, near the Pointe de Grave, penetrated for a th ickn ess of47m . a coa rse, quartziferou s limestone with Nummulites planul atusand A lueolina oblonga, resting on quartz-san ds of unknownthickness.

    Lutetian.At the mouth of t he Gironde fragmentary outcrops of Lute tia n

    appear on both banks. At St. Palais, N.W. of Royan , the stage(IO-Izm .) begins with a ca lca reous conglomerate resting on theabraded surface of the Dordonian chalk , and composed offragments of the underlyin g chalk, of blocks of calcar eousglauconitic sandstone of Ypresian age, quartz pebbles and rolledfragments of bones of cetaceans , reptiles and fishes. Above thisis a soft white lim est one containing a fine sea-urchin fau na-

    Transla ted by Prof. A. MORLE Y DAVIES, D .Se., F .G.S., to whom th e Author expresseshis sincere thanks. In the preparation of this sketch the Author has utilized the sheets of, andexplanatory notes on, the Carte Geologique detaillee de Ia France, on the scale of 1 : 80,000 ,as well as th e chief pu blished works on the region.F ROC. G EO L. Assoc ., VOL. X L., P ART 2 , 1 9 2 9. I I

  • 154 A. P. DUTERTRE,Ccelopleurus delbosi, Schizaster archiaci, Brissopsis elegans,Echinocardium subcentrale and an interesting series of species ofEchinolampas (E. dorsalis, E. douvillei, E. archiaci, E. heberti) ;then comes a bed of limestone packed with Orbitolites complanata,miliolids and alveolines, followed by unfossiliferous sands passinglaterally into coarse calciferous or quartzose grits enclosing Ostreafiabellula, Venericardia planicosta, Orbitolites complanata ; theseries ends with sandy micaceous marls, white, grey or greenish,thin-bedded, containing Ostrea d. fiabellula and broken shells.At Terre Negre (N.W. of St. Palais) the white limestones containRimella fissurella, a Paris Basin species. At the rocks ofSt. Nicolas (Pointe de Grave) and those of Cordouan (beyondthis point) the bed of Miliolid limestone forms the surface of aplatform exposed at low tide, and rests on the zoft. limestonewith Echinolam.pas archiaci.

    The Lutetian of the Gironde shows three principal divisions.The lower series consists of grits and sands, more or less glauconiticwith Nummulites per/orata, Nilucasana, N. ramondi, N. aquitanica-girondica, Assilina spira, Orthophragmina submedia recognizedat depths in the materials brought up from borings in Bordeauxand its environs. These arenaceous beds proved very thickin the boring of the Bordeaux Park, which reached a depthof 504 m. without touching the Lower Eocene. Num. periorata,Ass. spira and Orthophragmina scalaris were found between430 and 464.5 metres in the Abatilles boring near Arcachon.

    The middle zone is composed of marls, sands and sandylimestones, containing in abundance Orbitolites complanaia,Rotalina saxorum, Alveolina elongata, as well as Orthophragminastellata, Truncatulina variabilis and abundant Bryozoa.

    The upper division is formed principally by the Calcairede Blaye, a littoral deposit composed of beds of coarse or compactlimestone, enclosing grains of glassy quartz, and rich inforaminifera such as Orbitolites complanata, Alveolina elongaiaand Miliolidse, and containing numerous molluscs, such as Fimbrialamellosa, Lucina elegans, Cerithium tricarinatum, Diastomacostellatum and above all a fine echinoid fauna-Echinolampasstelliierus, E. similis, E. blaviensis, Echinanthus desmoulinsi,Echinocyamus atJinis, Sismondia marginalis.

    On the left bank of the Gironde, the Lutetian outcropsbetween Queysan and Valeyrac, appearing either as a clayeyor earthy limestone, as a coarse quartzose limestone or as acompact limestone with sometimes a crystalline cement.

    Auversian.On the right bank of the Gironde this stage comprises two

    divisions, separated by a band of limestone containing in abun-dance an oyster (Ostrea cucullaris of Bordelais authors) allied toO. fiabellula and O. cubitus. The lower division resting directly

  • TERTIARY FOR;\IAT ION S OF TH E BORD ELAIS. ISS

    on th e eroded sur face of the Lutetian (of which it contains derivedlimestone blocks) is compose d of marls with Voluta athleta ,D iastem a costellatum, Cerithium cordieri, Corbula angu lata , Cyrenacampsa. The upper division is formed of marly or calcareoussands, rich in Anomire and oysters . Near Plassac it contains aninteresting fauna characterised by Laganum tenuissimum,

    On th e left bank, in Bas-Medoc, the stage is remarkabl e forthe abundance of foram inifera such as Orbitolites complanata.The beds with N umm, striatus penetrated by the Abatillesbo ring, between 395 an d 430 m . depth, have been referred tothis stage.

    Bartonian.The Calcaire de Plassae (7-8 m .) which outcrop s on the right

    bank, represents the cont inental inland-terrestrial facies ofthis stage: it consists of greenish clays with calcareous con-cretions and bones of Palaiotherium, overlain either by marlsand marly limestones or by hard compact , very white, at timessub-lithographic limestones, containing rather rarely Limncealongiscata ; to the south-east of Plassac the upper part passeslat erally into sandy formations.

    Priabonian.In Medoc th is stage presents a developm ent (12-20 m.) of

    formations of rather varied pet rographic character, un itedun der the name of Calcaire de S t. Estephe. The lower zone isespecially va riable : it consists of limestones which are compact,coa rse-grained, earthy or even clayey, passing at times intosa nds or into mas ses of shells; it is rich in foraminifera (orbito-lites and rotalines) and contains a mixture of Bartonian andLutetian molluscs: Diastema costellatum, Natica studeri ,Turritella imbrieataria and a rich fauna of echinoids : E chino-lampas oualis , E. subsimilis, E chinanthu s elegans , S ismondiaoccitana. The up per zone is formed of clays , marl s, sands orlimest ones with Echinolam pas similis and E . ovalis ; near th etop A nomia girondica abo unds an d Sismondia occitana is some-t imes common.

    The Abatilles boring between 3 2 0 and 395 m . passe d throughbeds containing fragments of echinoids, globigerinse andnummulites (N . incrassata) which led to their being referred toth e Priabonian.

    On the right bank the Upper Eocene crops out be tweenRoque de Tau and the neighbourhood of Blaye. At the ent ranceto th e Bois de Barbe th ere appear beds of calcareous grit withAnomia girondica (referre d to th e Sannoisian) resting on grey ishlimestones packed with miliolids which belong to the series ofth e Calcaire de St. Estephe . At th e hill of St . Lu ce thePriabonian beds rest on th e lacustrine limestone of P lassac and

  • A. P. DUTERTRE,

    contain some echinoids (Sismondia occitana, Echinopsis elegans)and oysters (0. bersonensis).

    The marine facies of the Upper Eocene disappears bit bybit towards the east of the basin and is replaced by continentalformations, the" mollasses de l' eocene superieur." The limestoneof St. Estephe, overlain by blackish clays, appears for the lasttime in the N.W. angle of the Libourne Sheet, where it consists.of a hard limestone with Orbitolites. The Upper Eocene mollasseis formed of sands, grits and clays, and constitutes the middlepart of the hills between Libourne and Ste. Foy.

    In the eastern part of the Dordogne valley, between Ste. Foyand Velines, north of the river, and between Razac and Eynnesseon the south side, appears the calcaire lacustre d' I ssigeac (om. 50-2 m.) much better developed on the neighbouring sheets ofLa Reole (where it attains 5 m. in thickness), Bergerac andVillereal : this is a soft white limestone or a travertine. some-times siliceous, and passing into millstone imeul. ere). At the footof the Tertre de Fronsac, near Libourne, it has yielded Melano-psismansiana and Melanoides sp.; at Ligueux (La Reole sheet)Tschurostoma [ormosum, Limncea pyramidalzs. To the samehorizon is attributed the Calcaire des Ondes which outcrops onthe Villereal sheet and contains bones of Xiphodon gracile,Palceotherium girondicum, P. magnum, P. medium, tortoises andcrocodiles.

    Sometimes the Upper Eocene is entirely mollassic, andit then becomes very difficult to subdivide it or even to dis-tinguish it from the mollasse du Fronsadais (Sannoisian). More-over, to the north it passes insensibly into the detrital sables duPerigord; and again to the east (Agen sheet) appears theterrain siderolithique, consisting of red quartzose sands withbright red clays and limonite concretions; these formationswhich are referred to the Upper Eocene and Sannoisian occur asscattered outliers on the .Jurassic plateaux, filling pockets inthe Kimmeridgian limestone.

    Sannoisian,On the right bank of the Gironde, near Roque de Tau, the

    bed of sandy clay (1-2 m.) with Anomia girondica, already notedat the entry of the bois de Barbe, represents, according to G.Vasseur and other authors, the base of the Sannoisian, thoughit is closely connected with the Priabonian limestone withSismondia occitana.

    In Medoc the stage consists of a complex series of bedsbeginning either with a mollasse or a marl in which Anomiagirondica abounds; these beds are overlain by argillaceous orcalcareous marls passing at times into a coarse or compactlimestone with Echinolam.pas and Cerithium (c. plicatum, C.trochleare). Above is an apparently lacustrine marl; finally,

  • T ERTIARY FORMA TI ONS OF THE BORDEL AIS. 157

    a small outl ier (Cieurac) of siliceous limestone with Nystiadu chastelii probably represents in th is region th e vestiges of thefreshwater limestone of Castillon which is well developed on theLibourne sheet. The marine facies disappears, indeed, com-pletely to the east, and the whole stage is then composed entirelyof cont inent al form ations-the mollasse du F ronsadais andthe calcaire de Castillon.

    Th e mollasse du Fronsadais consists of fine, micaceous sandsof a light grey colour , of more or less hard sandstones, and ofsandy micaceous clays ; these sediments are usually poor infossils, but at Auri ac th ey have yielded Paloplotherium m inus,A uratherium , A1.achcerodus bidentatus, Cebochcerus minor, rodentsan d lan d-tortoises : this remarkable fauna shows the firstassociation of Upper Eocene mammals with more modernAmerican types.

    Th is mollasse ap pears in the valleys of the Garonne andDordogne and their tributaries; in the north it passes insensiblyinto th e sables du Perigord; it extends through the Villerealand Agen sheets.

    The calcaire de Castillon is white , soft , chalky or marl y ,sometimes silicified or cherty. It is accompanied by marls ,green ish clays and by sandy horizons which may completelyreplace it (St. Emilion) ; sometimes it is divided into two bedsseparated by a mollasse which is well-developed on the Villerealsh eet . It attains it s maximum thickness (12 m.) near Ste . Foy-la-Grande. The fauna is poor-Nystia duchasteli and a Limn seanear to L. longiscata are the commonest fossils and have ledto it s being considered the equivalent of the calcaire de B rie"inthe Paris Basin. On th e Agen shee t the calcaire de Trentel et deSt. Georges, which also contains N . duchasteli appears to standat essen tially the same horizon.

    RupeJian .This stage is composed of two groups of different formations

    correspondi ng rather to two facies than to stra tigraphicaldivisions-a-the calcaire a A steries and the mollasse de I'Agenais.

    The calcaire a Asteries takes its name from the abundanceat certain levels of fragments of a starfish, Crenaster lcevis . TheRupelian usually begins with a bed of clay containing Ostreacyathula and O. longirosiris ; the calcareous series consists ofbeds of either compact, fine-grained limestone, or coarse lime-stone with abundant casts of fossils, or gritty or sandy limestone,sometimes soft an d t hen containing well-preserved fossils.

    On the left bank of the Gironde the calcaire !/ A steries formsseveral outcrops from Vendays in Medoc as far as Bordeau x ;in Medoc its thickn ess is reduced (10-12 m. ), on account bothof th e irreg ularity of the surface on which it rests and of

  • A. P. DUTERTRE,

    subsequent erosion. Above Bordeaux it reappears in thevalleys of the left bank tributaries of the Garonne.

    On the right bank of the Garonne outcrops of the limestoneappear some kilometres south-east of Blaye and continue acrossthe Bourgeais towards Bordeaux. At Cenon, near that city,several quarries show the following upward succession :-(r)soft marly limestone, bluish grey or yellow, crowded withmiliolids; (2) limestones with intercalations of bluish marl;(3) rather hard yellow limestone; (4) a bed of hard limestonewith corals (Cladocora manipulatum); (5) white, hard,cavernous limestone, rich in Lithothamnion and Bryozoa, showingalso casts of gastropod and lamellibranch shells, with rareScutellee and fragments of crustaceans.

    The calcaire it Asteries attains its greatest development(40 m.) in the region of Entre-Deux-Mers, where it forms theescarpment bearing the citadel of La Reale; on the right bankof the Dordogne, near St. Emilion, it is scarcely more thanro--r5 m. thick; it thins towards the northern area of theLibourne sheet, and in the neighbourhood of S1. Savin containsa conglomerate marking the proximity of the shore-line.

    The rich fauna of the calcaire it Asteries includes notablyEchinoderms-Scutella striatula, Amphiope agassizi, Fibulariapiriformis; Molluscs-Cerithium charpentieri, C. plicatum,Natica crassatina, Turbo parkinsoni, Venus aglaurce, Pectunculusangusticostatus, and Vertebrates-A nthracotherium magnum,Hyopotamus bovinus, Rhinoceros, Halitherium.

    The mollasse de l' Agenais is formed of grits, arkoses, sandswith beds of clay and gravels. This continental formationcontains bones of mammals (Aceratherium, Dremotherium). Itthins gradually to the west and is replaced progressively by thecalcaire a Asteries. In the centre of the basin, on the Agensheet, the whole stage consists of continental deposits; belowoccur the calcaires lacustres de Cieurac already containing Helixramondi. The overlying mollasse has yielded bones of A nthra-cotherium magnum, A. minimum, Rhinoceros minutus, Palceo-chcerus, Ccenotherium. In the neighbourhood of Bergerac, thecalcaire de Montbazillac with Helix cieuracensis and H. corduensisis a lacustrine deposit intercalated in the mollasse.

    Chattian.In the eastern part of the basin the continental regime

    already established was maintained during the final divisionof Nummulitic time and the opening of the Neogene. In theneighbourhood of Agen the calcaire blanc de l'Agenais wasdeposited in great lakes. This formation, which reaches itsmaximum thickness (25-30 m.) in this region, is there composedof beds of white or greenish argillaceous marl, alternating withbeds of limestone which may be either hard, compact and

  • TERTIARY FORMATIONS OF THE BORDEl_AlS. 159

    white or grey, or brecciated and concretionary, or even soft.It contains shells of continental molluscs such as Helix ramondi,H. oxystoma, Limneea pachygaster, Cyclostoma antiquum, Planorbiscornu. The Agenais white limestone thins progressively to theW. and S.W.; in the Sauternes district, at the base of theformation, there are days with calcareous nodules containingUnio lacazei, In the neighbourhood of Bordeaux (La Brede-Saucats) it is reduced to a thin bed of white marl in whichH. ramondi occurs.

    The Agenais white limestone has been eroded from wide areas,as can be dearly seen on the la Reole sheet, in Entre-Deux-Mers, where it occurs as isolated outliers on small buttes. Itcontains a numerous and interesting vertebrate fauna includingcarnivora (Mustela elegans, Cynodon lamilloquense, Amphicyonmajor), cervidre (A mph~tragulus elegans, Dremotherium feignouxit) ,suidze iPalaochcerus typus) , artiodactyla- (A nihracotheriumlamilloquenses, rodents (.4 rctomvs arcernensis, A rchceomvs.laurillard i), insectivora (Erinaceus ariernensis, Talpa aniiqua) ; allthese mammals are distinctively Oligocene.

    The determination of the precise age of this formation is adelicate matter. The Agenais white limestone seems to standat the limit of the Upper Nummulitic and Lower Neogene; itmay have continued to be deposited in the east of the basinwhen the first marine Aquitanian transgression had alreadyinvaded the western region. On this interpretation it wouldbe partly Chattian and partly Aquitanian.

    NEOGENE.Aquitanian.

    The stratigraphy of this stage is rather complex, on accountof the diversity of facies which it presents, and especially byreason of the alternation of marine and continental deposits,the development of which varies greatly across the basin.

    In the Bordeaux district the outcrops visible at La Bredeand Saucats give the following succession :-

    (1) Lower Aquitanian: Falun de La Brede with Ncritinapieta, Turritella desmaresti, Cerithium calculosum, Pirenellainconstans, Lucina globulosa, Phacoides columbella, Meretrixundata ; at the Bernachon mill (Saucats) the series begins withbluish or whitish days with Neritina [erussaci, Cer. calculosum.Pirenella pticata, resting on white marls with Helix ramondiand overlain by a marine falun with intercalated grit-bands,containing the fauna of La Brede.

    (2) Middle Aquitanian: yellow, argillaceous and calcareoussands of the Bordelais valleys, poorly fossiliferous.

    (3) Upper Aquitanian: calcaire lacustre de la route du Sonforming two bands, separated either by a brackish-water falun

  • 160 A. P. DUTERTRE,

    with Potamides (Moulin de l'Eglise), or by a marine falun (Lariey).The lacustrine limestone contains the fauna of the calcairegris de I'Agenais, of which it is the Bordelais representative.Among the principal fossils, the shells of which are white, maybe mentioned Planorbis mantelli, Helix girondiea, Limnceagirondica, L. paehygaster, Hydrobia aturensis. The falun ofLariey, wedged in between the two beds of lacustrine limestone,is very fossiliferous: Pirenella plieata, N assa aquitaniea, Ostreaproducta, M ytilus aquitanieus, A rca cardiijormis, Cardita hippopcea,Meretrix undata, Lutraria sanna, '1 imoclea sub-spadicea, Corbulacarinata, Balanus eoneavus. This falun is covered by greenishclays containing in abundance Potamides margaritaeeus andPirenella plicata, species found also in the brackish-water bedsof the Moulin de I'Eglise. The npper bed of lacustrine limestoneis accompanied by marls with an abundance of Hydrobiaaturensis, overlain by a bed of brackish-water falun containingPirenella inconstans and Cerithium subcorrugatum.

    The subdivisions shown in this classic section, especially thoseof the upper part of the stage, are not so clearly defined every-where in the Bordelais. At Merignac the falun which caps theseries contains a fauna which is a mixture of typical Aquitanianspecies such as Melongena lainei, Turritella desmaresti, Lueinaglobulosa, Cytherea undata, Cardita hippopcea, with LowerBurdigalian forms, as Euthrioiusus burdigalensis, Voluta rarispina,Calyptrcea dejormis, Cardium burdigalinum, Cytherea erycina.

    The marine Aquitanian formations, well developed in theBordeaux district, are progressively replaced to the east andnorth-east of the basin, by lacustrine deposits. In the Bazadaisthe stage is composed principally of a calcareous grit (gres deBazas) sometimes hard and poorly fossiliferous, sometimes softwith hard lenticles and scattered fossils (Ostrea aginensis,Seutella bioeulata) , passing at certain points (Moulins de Gamachotet de Fortis, La Saubotte) into every fossiliferous faluns, theprecise horizon of which is still under discussion. In thisregion the lower marine beds of the Bordelais (faluns of LaBrede and Moulin de Bemachon) do not seem to be representedby the marine facies. The marine series of the Aquitanian restson the Agenais white limestone and begins either directly withthe gres de Bazas or with a shelly falun; the Agenais greylimestone reappears in the form of a blackish, earthy limestone,not very consistent, or even of a whitish or grey marl with thesame white-shelled fossils as in the Bordelais. This limestone(0.8 to 3 m.) is intercalate with the gres de Bazas (Ciron rivervalley).

    At the Moulin de Gamachot the section shows several layersof marine falun, one of which is remarkable for the richness ofits fauna; it contains many corals, vermetids and verynumerous well-preserved shells-Turritella desmaresti, Area

  • TERTIARY FORMATIO~S OF THE BORDELAIS. 161

    cardiiiormis, Chione ambigua, Chama aquitanica. This falun isoverlain by brackish-water beds with Potamides inconstans andCyrena brongniarti and these in turn by marl with Hydrobiaaturensis.

    The falun of La Saubotte (outcrop of the Maison Lasserre)seems to come above the calcaire gris which outcrops in theCiron valley (Villandraut, Noaillan) : it also yields a very finefauna-Ervilia pusilla, Meretrix undata, Lucina dentata, Areacardiijormis, Ostrea produeta, Turritella desmaresii, N eritinafirussaei, Potamides inconstans, P. margaritaeeus.

    In Entre-deux-Mers the stage consists at the base of marlsand clays (s-rom.) with Ostrea aginensis and O. producta, en-closing towards the top the fossils of the Bazadais faluns ; theupper zone is formed of calcareous grits and hard mollasseswith Amphiope and Seutella. The grey limestone reappears atcertain points in the same district. Finally, near the centreof the basin, in the area of the Agen sheet, the stage comprisesat the base beds of marl and clay with Ostrea aginensis and sandymollasses : this series, resting on the white limestone, is 40m.thick on the borders of the Gers and diminishes to 20m. nearPort Ste. Marie. Towards the N.E. the oyster-marls thin outprogressively and are replaced by the white limestone. Theupper division is formed by the caleaire gris de l'Agenais, whichin this region is finely developed (rom.) : it takes the form of amarly white limestone or a fetid, cavernous, fossiliferous lime-stone, the surface of which is in some places excavated byholes filled with day containing O. aginensis.

    Burdigalian.In the immediate neighbourhood of Bordeaux (Latin

    Burdigalas, the type-locality of this stage, it is composed ofsediments of the "mollasse.' and above all, of the 'falun'type, celebrated for the abundance and beauty of their fossils.The series begins with the faluns of Pontic (Merignac), Thibau-de au (Leognan), Le Haut Bouscat (Canejan) and Le Peloua(Saucats). The boundary between the Burdigalian and thepreceding stage is sometimes difficult to trace: rolled blocks ofAquitanian freshwater limestone with molluscan borings arefound in the faluns at the base of the series (Le Peloua,Merignac). The lowest division also includes the mollasseossijere de Leognan, rich in Cetacean bones (Squalodon, Zeuglodon).

    The middle zone is represented typically by the falun ofLe Coquillat with its rich fauna: Xenophora burdigalensis,Calyptrtia depressa, Naiica 'burdigalensis, Turritella terebralis,Proto cathedralis, Ficula burdigalensis, F. condita, Euthriofususburdigalensis, Melongena cornuta, Cancellaria acutangula, Pectenburdigalensis, Pectunculus cor, Cardium burdigalinum, Meretrix

  • 16 2 A. P . DUTERTRE ,

    erycinoides, Donax transversa , Corbula carinata mut . hornesi ;Vag inella depressa , Scu tella leognanensis.

    In the upper part of the stage comes the falun ofPont Pourquey (Saucats) containing also a remarkable faunaincluding most of th e ab ove spec ies : the domin ant forms arenot abl y Dorsanum baccatum , Olicancillaria basteroti, Donaxtra nsversa , Tellina zonaria, T , bipartita , Solen burdigalensis;M actra substriatella, Phacoides columbella, Dicaricella ornata ,Grateloupia irregularis . At Pont Pourquey th e upp er part ofthe falun contains brackish-water shells such as Pirenella picta,which is abunda nt , Potamides papa ocraceus, and some species.seemingly derived from th e mollasse of I 'Armagnac, as wellas bones of birds. The falun of Cestas belongs to th e samehorizon and presents similar charac ters.

    In the neighbourhood of Villandraut, the Burdigalian isrepresented by the gris mollassiques de St. Leger with Pectenfraa si and Ostrea crassiss ima.

    The marls and mollasses of I' Armagnac, as well as thecalcaires de l'Armagnac with Helix larteti and H . leym eriei, seem ,in the East of the basin, to represent the continental facies ofthe stage.

    Helvetian.This stage appea rs in the neigh bourhood of Sa uca ts (La

    Sime) and at Martignas, but around th e town of Salles th efinest exposures occur . There it is represented especially bythe faluns of th e Moulin de Debat and the Moulin du Minoy,which contain a fine fauna : Pecten vasatensis, Chlamys pinorum ,Peciunculus car (of large size), Venericardia [ouanneti, Giycimerismenardi , The mollasse of La Levr e contains the same fossilsas well as bones of cetaceans i nd sharks. Th e falun of]'Argileyr e (at Salles) contains a somewhat different fauna andshould perhaps be placed at a rather higher horizon : it wasform erly classed as Tort onian , a stage not recognised in theBordelais, but occurring near Saubrigues (Landes) .

    On the Grignols shee t th e Helvetian is represented by theferruginous sands of Xaintrailles, as well as by th e huff sandsand the faluns, marls and rnollasses of Baudignan with Ostreacrassissi ma, Cerithium lignitaruni , Ancillaria glandilorm is,Venericardia [ouanneti ,

    C. SUMMARY OF GEOLOGICAL HISTORY.TECTONICS, MOVEMENTS OF 'IHE SEA.

    Th e Basin of Aquitaine includes a series of Mesozoic andCainozoic formations appearing like aureoles around the ancientmassifs which frame it . This dispositi on recalls that of theParis Basin , with which Aqu itaine has strong analogies: thus

  • TERTIARY FORMATIONS OF THE BORDELAIS. 163

    the Cretaceous of this region forms a series of parallel undulationsaccompanied by disturbances trending N.W.-S.E. as in theParis Basin.

    In a general way, the folds and faults of Aquitaine have adirection parallel to the boundary faults of the Central Massifand to the Hercynian folds of its western part; further, they havethe same direction as the Hercynian folds of South Brittany;moreover, the ante-Oligocene folds and faults may be con-sidered as the repetition of the Permian and post-Jurassic dis-turbances. Thus, in the Basin of Aquitaine, as in the ParisBasin, there has been continuity in the phenomenon of folding,on the same general plan, but each movement represents a.particular style. Again, certain disturbances have been renewedrepeatedly, as has also happened in the Paris Basin and notablyin the Boulonnais.

    Towards the end of Cretaceous times, in the Upper Danianepoch, an elevation occurred and the sea withdrew from theBordelais region. To all appearance it was this movementwhich gave birth to the undulations in the Cretaceous bedsseen on the northern border of the Basin and disposed in a generalN.W.-S.E. direction. They were accompanied by fracturessuch as the faille de la Gironde, of which the N.E. upthrow,eroded by the river, has left as a vestige of its former state theline of cliffs extending from St. Bonnet to Royan, while the down-throw side, dropped some hundreds of metres, has led to theformation of the deep depression of the Blayais.

    On the left bank of the Garonne, to the S. and S.E. ofBordeaux, the Maestrichtian inliers of Villagrains and Landirasare, doubtless, evidence of an anticlinal, parallel to the Pyreneanchain, the origin of which seems connected with the same move-ment. The fact that these Cretaceous outcrops are directlyoverlain either by Rupelian (Landiras) or Aquitanian (Villa-grains), and are not surrounded by any traces of Eocene, seemsto show that their upheaval goes back to pre-Nummulitictimes and is connected with the movement which caused theundulation in the Cretaceous rocks on the northern border ofthe Basin. Thus the marine regression at the end of Cre-taceous time seems to be a consequence of the phase of tectonicactivity which gave to the Bordelais its principal structuralfeatures.

    This movement raised a series of wrinkles, composed ofMesozoic rocks and approximately parallel to the chain of thePyrenees, which divided the Aquitaine basin into a series ofsecondary basins or gulfs: (1) the most northerly and largestextends from the Cretaceous margin of the Central Massif tothe fold of St. Sever, which is probably the continuation of thePetites Pyrenees of Ariege and Haute-Garonne ; the bottom ofthis gulf reached in the S.E. the col of Naurouze and com-

  • A. P. DUTERTRE,

    municated with the Herault basin: the Bordeaux basin occupiesits N.W. portion. (2) The gulf of the Chalosse de Montfort,much narrower than the preceding, extends between St. Severand Dax and includes the Chalosse de Montfort and the Orthezneighbourhood. (3) The most southerly, situated betweenDax and Biarritz, includes the Chalosse de Pouillon and theneighbourhood of Peyrehorade.

    As a consequence of the folding which it underwent at theend of Cretaceous time, the Basin of Aquitaine acquired a certaintopography which the agents of denudation modelled duringthe period of emergence that persisted through part ofPalreocene time except, perhaps, on the western margin. TheCretaceous foundation was sculptured by erosion and thematerials transported by the water-courses accumulated indepressions of the surface. In certain areas concretionary tufaswith plant-remains were deposited: such is the tufa of Passignacnear St. Maigrin (Jonzac sheet), which rests on the Campanianand contains ferns (Alsophlla thelipteroides, A. perneli) identicalwith those of the travertine of Sezanne (Paris Basin), as well asrepresentatives of the genera Protoficus, Cissus and Cinnamomumrecalling also the types of the same deposit.

    In the Ypresian epoch the sea advanced towards the interiorof the Bordelais gulf; on the northern border it directly coveredthe Cretaceous foundation and deposited various beds withNummulites planulatus, N. globulus and Aiucolina oblonga, nowsurviving only as very small outliers or more generallv as residualmaterial, in the neighbourhood of St. Palais and Meschers onthe right bank of the Gironde estuary. In the centre of theBordelais basin the deposits of this sea accumulated to a con-siderable thickness; several borings in the Bordeaux area havepenetrated beds which seem to be of the same age as thosein the Verdon boring (see p. 153) ; further, N. elegans and N.planulatus appear to have been found in a boring at Barbotannear Condom (Gers), so that the Ypresian sea must havepenetrated far into Aquitaine.

    During the next epoch immersion increased: a large partof the Bordelais must then have been covered by the sea which,in advancing transgressively over the margins of the basin,Tavined the earlier deposits (see p. 153). The planed-down surfaceon which it rests has been compared to that shown by theCarboniferous Limestone in the Boulonnais, around Marquise,where it is directly followed by Bathonian [and in the Mendips],

    The deposits left by the Lutetian sea in the Bordelais arelittoral sediments, in which are found, mingled with distinctivespecies, numerous elements of the Paris Basin fauna, such asNatica cepacea, Hipponyx cornucopias, Rimella fissurella, Ostreaflabellula, O. plicata, Chama calcarata, Meretrix elegans, Corbis-lamellosa. The Bordeaux Park boring shows that the sea

  • TERTIARY FORMATIONS OF THE BORDELAIS. 165

    deposited beds with Nummulites aquitanica, Assilina granulosa"Ac leymeriei, Orthophragmana submedia, then a horizon with Num,girondiea, N. aquitanica, Assilina planospira attributed to theupper part of the stage.

    Like the Bordelais basin the Gulf of La Chalosse de Montfortwas invaded by the sea at the opening of Lutetian time ; thetransgression came a little later in the southern gulf, and theregion of Biarritz was not submerged until the Upper Lutetianepoch.

    With the Auversian epoch a new movement of the sea madeitself felt on the northern margin of the Bordelais gulf: the surfaceof the Lutetian bears numerous and strongly-marked traces oferosion, and blocks derived from this substratum are found inthe basement beds of the new series. In this region, however,the Auversian formation retains the littoral and even brackish-water facies, as indicated by the fauna in which cerithids,cyrenids and oysters dominate. This regime becomes moremarked and, a little later, in the Bartonian epoch, lakes coveredthe areas abandoned by the sea and in these the Calcaire dePlassac was deposited.

    In the centre of the gulf, where the waters were deeper,the whole of the Nummulitic appears to be a conformablemarine series: thus at Arcachon (Abatilles boring) beds with,Num, striata and echinoids ascribed to the Auversian (35m.)are overlain by Priabonian beds with Num. inerassata,globigerin

  • l.66 A. P. DUTERTRE,

    with anomias (A. girondica) , oysters and potamidines. A slowmovement of elevation of the western region of the CentralMassif then took place, and as a consequence a hydrographicsystem appeared: watercourses descending the western slopesof the massif carried along the detritus of the Archaean andgranitic rocks and emptied it into the shore-lagoons. Thesearenaceous and argillaceous sediments, sometimes redistributedby the sea, produced the mollasse type of sediment.

    In the Bourgeais, between Cars and Bourg-sur-Gironde, themollasse du Fronsadais seems to pass laterally into the Anomia-beds ; in Medoc the marine facies of the Lower Oligocene is alittle more marked, in the limestones with Gonocardium andEchinolampas, which, however, are overlain by brackish-wateror lacustrine marls.

    The mollasse du Fronsadais is most typically and finely deve-Ioped in the Dordogne valley near Libourne and along thevalley of the Isle; near La Clotte it rests directly on the Chalk.

    The formations of varied character grouped as sables duPerigord include sands, sometimes consolidated into sandstones(Bergerac), with gravels of quartzite derived from the CentralMassif, and in part of Jurassic and Cretaceous rocks; theyalso include clays, sometimes very ferruginous, even passing intopisolitic iron-ores, the source of which must be sought in theCentral Massif; also lignites, and china-clays brought by streamsfrom the granites of the Massif. Traced from east to west theSables du Perigord become less and less detrital: sometimesthe different types of deposit are interdigitated, showing plainlythat they were laid down under the influence of currents ofvarying extent, the clays with lignite indicating quiet waters.The first formation of all these sediments must date from theUpper Eocene, as is clearly shown by the passage of the Sablesdu Perigord into the mollasses of the Dordogne and the lacustrineand marine deposits of the west of the Bordelais basin: theycorrespond to the various phases of a single formation in relationto the earth-movement of the Central Massif.

    In the Rupelian epoch a new positive oscillation took placein Aquitaine with a consequent progressive submergence of theregion. The sea must have entered the basin of Aquitaine fromthe S.W. and afterwards extended to the North: in theBordelais the Ostrea longirostris beds must have been depositednear the shore-line of this sea, which deepened to the south of thebasin, as is shown by the argillaceous facies of the Dax district(Gaas). In the Bordelais the Rupelian sea covered the sitesof the estuaries and lagoons filled up by the Mollasse duFronsadais and the lakes in which the limestones of Castillonand Cieurac were deposited; probably the westward com-munications in the Blayais were afterwards widened: thus atRoque de Tau the Rupelian is transgressive on the Priabonian.

  • TERTIARY FORMATIOKS OF THE BORDELAIS. 16]

    In the Bordelais the Rupelian marine formations have thecharacters of littoral or shallow-water deposits: the lower bedsoften rich in oysters (0. longirostris, O. girondicai, are overlainby the Calcaire a Asteries forming the main mass of the stage.This formation, which shows several facies, contains at certainhorizons fragments of a diversified series of organisms-stellerids, bryozoa, lamellibranchs, corals, echincids, andcalcareous algse, evidently showing the slight depth of thewater: the presence in this deposit of bones of Sirenians(Halitherium) and of land-mammals swept down by streams(p. 158) confirms this observation. On the Libourne sheet theCalcaire Ii Asteries thins gradually to the North, while its detritalcharacter becomes accentuated; the St. Savin conglomeratesare the vestiges of the Rupelian shore-line in this district.

    Then began a slow and continuous movement of elevation,and the sea, which had reached its widest extension at theopening of Rupelian time, began to withdraw little by little:the retreat of the sea was probably slow, like the oscillation ofthe land, for oyster-beds often link the Calcaire (i, Asteries to thecontinental deposits which replace it. Near Landiras the bedsof the calcaire it Asteries rise up at their contact with the Cre-taceous which appears in a little" button-hole" inlier, and thisfact seems to show that the anticlinal uplift already existingin the Rupelian was renewed towards the end of Nummulitictime or even in the Miocene. In the Chalosse the Rupelianstrata show a dip which may be as high as 25 at Banos, 45 atArcet, and even, exceptionally, 85 at Goss (Moulin de la Pelette).Similar observations may be made in the Landes at Lesperonand Gaas (near Dax), as well as in the cliffs north of Biarritz.All these facts reveal the effects of connected post-Rupelianmovements in the region bordering the Central Massif as wellas the Sub-Pyrennean region of France.

    Rivers flowing down from the regions laid dry by elevationonce again carried sand, gravel and clay, as well as bones ofvertebrates (p. 158) : the deposit of these materials constitutedthe Mollasse injerieure de l' Agenais, which is developed fromwest to east at the expense of the Calcaire (i Asteries and passeslaterally into the detrital formation of Perigord: these obser-vations clearly show the progressive retreat of the Rupelian seato the West.

    Near the end of Nummulitic time the western part of theGirondin region suffered brackish-water conditions: thus yellowor greenish clays, with or without calcareous concretions, therecover the Calcaire a Asteries directly without interposition ofthe mollasse injerieure de IAgenais. To the east, near thebottom of the basin, earlier emerged, considerable areas wereoccupied by great lakes in which were laid down the Agenaiswhite limestone (calcaire blanc de l' Agenais) with Helix ramondi

  • 168 A. P. DUTERTRE,

    (Agenais, neighbourhood of Nerac, Entre-Deux-Mers), or clayswith calcareous nodules containing Unio lacazei (Bazadais) andthis continental regime must have lasted in these regions duringEo-Aquitanian times.

    In the immediate neighbourhood of Bordeaux the sea didnot retire for long, for from the opening of Aquitanian timeits waters penetrated little by little into the lagoons whichhad been formed in the preceding epoch, and this positiveoscillation must have taken place slowly, for the passage fromthe brackish to the marine facies is insensible, and withoutravinement. Thus the Aquitanian series begins with bluishand white marls, linked to the Helix ramondi marls, and con-taining many brackish-water fossils mixed with marine shells(see p. 159). Gradually the marine facies becomes dominant,marine fossils becoming more and more numerous and varied.

    But this invasion, confined at first to the western part of theBordelais basin, spread greatly in the Middle Aquitanian epoch.The sea then advanced towards the centre of the basin andcovered the lake-sites in which the white limestone with H.ramondi had been laid down. But if this meso-Aquitaniantransgression was far-spread, the deposits which it left indicateshallow waters: they are, in fact, clays, with oysters (0. aginensis),faluns with comminuted shells and corals, arenaceous sediments,such as the yellow sands of the Bordelais valleys, the gresinjerieur du Bazadais, the moll asses with Scutella and Amphiopeof Entre-Deux-Mers,

    Then with the opening of Upper Aquitanian times a move-ment of general regression is shown. The sea-water withdrewto the west and the Bordelais basin was covered with lakesin which the grey limestone with Helix girondica and Limnceagirondica was laid down. But soon the sea re-took possessionof a large part of this abandoned territory and penetrated anewfar into the Bordelais gulf, but its deposits were still those oflittle depth. While the sea was depositing the [aluns of Larieyand la Saubotte and the sediments, which on consolidationformed the gres superieur du Bazadais, the grey limestone con-tinued to be formed in the lakes remaining in centre of thebasin (Agenais, Nerac region). Near the end of the Aquitanianepoch, lakes were again formed in the neighbourhood of Saucats,where a second bed of grey limestone with Helix girondicawas formed, covered by faluns with Potamides. During the finalAquitanian times the sea, which had withdrawn towards thewest, deposited the mixed faluns of Merignac and le Haillan.

    With the dawn of Burdigalian times a new marine invasiontook place in the Girondin region, but it was, this time, muchmore restricted than the earlier invasions. This advance ofthe sea was marked by the ravinement of the beds just formed:the falun of le Peloua (Saucats) contains fragments of the

  • TERTIARY FORMATIONS OF THE BORDELAIS. 169

    Aquitanian grey limestone, rolled and perforated by pholades.The series of mollasses and faluns of Leognan and Saucats was,then deposited in shallow waters while farther east the mollassede l' A rmagnac was formed.

    Finally, the sea deposited the Helvetian Beds of Salles and,retreating more and more in consequence of the continuedele:ation produced from N.E. to S.Vl., abandoned the Girondinregion.

    To sum up: the history of Nummulitic and Neogene timesin the Bordelais basin comprises a succession of positive andnegative oscillations marked by a series of episodes in the" struggle between the fresh waters and the sea waters." Thestudy of the changes of facies through this basin supplies re-markable examples of lateral variation. I have ventured toco-ordinate in a general table the relations of the variousTertiary formations of the Bordelais after the scheme of MM.Ph. Glangeaud and J. Repelin and the interpretation suggestedto me by the observations I have made during some years inAquitaine.

    The movement of elevation expressed by the retreat of thesea in the Middle Vindobonian epoch must have continuedto be felt in the Bordelais basin during the rest of Neogenictime. The deformations suffered by the Aquitanian beds ofEntre-Deux-Mers and in the Bazadais shows the continuity ofthe phenomenon of folding in these regions.

    [The table of the Tertiary formations in the Bordelais willbe published with the report of the Excursion, in the next partof the Proceedings.-ED.J

    PROC. GEOL. Assoc., VOL. XL., I'ART 2, 1929. J2'