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...

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