Biostratigraphie de l'Oligo-Miocène du bassin d'Aquitaine fondée sur les nannofossiles calcaires. Implications paléogéographiques

Abstract

Fossiliferous marine sediments of Chattian to Serravallian age are frequently outcropping in the Aquitaine Basin (Fig. 1). Because of their predominantly neritic origin, these deposits are generally rich in molluscs, largely collected by previous authors, but fairly poor in planktonic micro-organisms with high stratigraphic potential. Consequently, their age was often not well known (or still under discussion), especially from the Middle Miocene onwards, and this because of the disappearance of miogypsinids and other benthic larger foraminifera which, in Aquitaine, have been used to subdivide the preceding time intervals. Calcareous nannofossil investigations have led to the precise dating and palaeogeographical interpretation of about sixty Oligo-Miocene outcrops (Tabl. 1), and this despite the poorly diversified associations, the frequent absence of the classical marker species (particularly the marked scarcily of Discoaster and Sphenolithus species) and the high rates of reworking of Gretaceous and Palaeogene (mainly Eocene) material. Species are listed in alphabetic order at the end of the paper (addendum). Their stratigraphic distribution is given in tables 2-3 and 5-7. A few of the identifications were made by the late Hélène Manivit and Annick Pujos, to whom, as an expression of our sincere respect, we want to dedicate this work. The calcareous nannofossil zonation of Martini and Müller (1986) and the planktonic foraminiferal zonation of Blow (1969) are used in the present paper. I - The geological and structural framework of the Aquitaine Basin has conditioned the outline of the successive transgressions which have been more or less spreading out from the Atlantic in an eastern direction (Fig. 1, 4). Two major structural provinces can be recognized. The North-Aquitaine province consists of a stable platform with a few elevations (Villagrains ridge, , Fig. 4), whereas the South-Aquitaine area has been strongly deformed by the Pyrenean orogenic phases (Lutetian and terminal Oligocene in age). These tectonic events have led to the formation of a series of anticlinal ridges (among which diapirs) and subsiding basins, where marine sequences developed, often much more complete than in the north of the Basin. The studied area is bounded on the south by the North-Pyrenean subduction front. II - The Upper Oligocene is only out-cropping in the southwest of Aquitaine (Tabl. 1 and 2). Calcareous nannofossil zones NP 24 (or even 23 ?) and NP 25 have been identified at different sites in the Adour Basin, which agrees with the ages suggested by means of foraminifera (the Pontonx and St Paul-lès-Dax areas, the St Etienne-d'Orthe/Orthevielle/Bélus palaeocanyon). The Chattian succession, which, according to the "grade-datings", spans the interval from ca. 27 to 23 Ma, seems to be fairly complete; it corresponds to eustatic cycles TB 1-2 to TB1-4. The Upper Oligocene deposits are transgressive in the southwest of Aquitaine, having been deposited after a major regressive episode at the Stampian/Chattian boundary. This episode seems to correspond to the major global sea-level fall occurring at the base of cycle TB 1-1. The incision of the palaeocanyon of Saubrigues-Bélus, probably the eastward prolongation of the present "Gouf de Capbreton", may be related to this major sea-level fali (Fig. 2). North of this palaeocanyon, the Chattian coastline skirted along the western edges of the Pey/St Lon-les-Mines ridge and the Tercis ridge. Marls and calcareous sands accumulated in shallow bays (Escornebeou outcrops = NP 25) (Fig. 1). Coral reefs developed in the St Paul-lès-Dax area. III- The majority of the deposits in the Miocene Gulf of Saubrigues consists of marls with fairly rich macro-faunas, to which, according to the authors, different ages have been assigned, ranging from Burdigalian to Pliocene (Tabl. 4). The depositional history of the Saubrigues-Bélus palaeocanyon has been reconstructed through the investigation of many outcrops and the Saubrigues Sa H1 borehole section (Fig. 1-3 and Tabl. 3, 5). The eastern part of this canyon ("gouttière de Bélus") has been filled in during the Chattian (NP 24-25). As the area of marine sedimentation progressively shifted from SE to NW the oldest outcropping Miocene deposits, of Aquitanian and Burdigalian age (NN 1-3), were laid down on the southwestern edge of the gulf, the younger Late Burdigalian deposits (NN 3-4) in the central and eastern areas, and the Langhian deposits (NN 5) in the northwest. The Sa H1 borehole section, of which 12 cored samples have been studied (Tabl. 5), has proved to be indispensable for the completion and the understanding of the surface data. The marine Miocene, which is predominantly marly and about 360 m thick in this borehole, unconformably rests on Lower Eocene (and Lowermost Lutetian) strata (NP 12-14). It consists of a basal 156 m thick marly unit of Aquitanian age (NN 1), of which parts are outcropping at Haut-Bernet, which is overlain by 50 m of Lower Burdigalian marls (NN 2) and 20 m of Upper Burdigalian deposits (NN 3 or NN 4). The study of three additional borehole sections, drilled on the structural high of St Jean-de-Marsacq, the western prolongation of the Gretaceous ridge of St Lon-les-Mines/Pey (Fig. 3), provided evidence of a substantial hiatus, encompassing large parts of the Eocene and the entire Oligocene. Thus, the Stampian and the Eocene appear to have been deeply eroded during the incision of the palaeocanyon. Subsequently, the canyon was there filled in with Lower Miocene deposits, which, on top of the St Jean-de-Marsacq anticline, reach a thickness of over 450 m. (borehole Sa H2). IV- The marine Middle Miocene deposits, of which the age has been often intensely discussed until now, seem to have been formed during two distinct transgressive episodes (Tabl. 6 and 7): 1- A Langhian episode (NN 5, characterized by the presence of Sphenolithus heteromorphus and the absence of Helicosphaera ampliaperta and H. walbersdorfensis), identified in the "Bas-Adour" area and in central Aquitaine. Several outcrops rich in marine mollusc faunas have been recognized, which have led to the identification of the "golfe de Manciet", a small gulf between the Tennarèze Anticline and the Chalosse and Roquefort Anticlines (Fig. 4-5). These molluscs faunas are similar to those from the Langhian ("Pontilevian") of the Loire Basin (Northwestern France). The Langhian age has been confirmed on the basis of microplankton data, e.g. the presence of Praeorbulina. The first palaeogeographical sketch-map, illustrating this marine episode in the whole Aquitaine, is proposed here (Fig. 4). 2- A Serravallian episode (NN 6-7, even early NN 8), more widely recognizable. It is characterized by the absence of S. heteromorphus and the presence of H. walbersdorfensis. The latter, although fairly rare, seems to be a reliable marker for identifying the Serravallian in the Aquitaine Basin. This second episode has been recognized in the Soustons area, in the Chalosse and Orthez gulf along the northern margin of the Chalosse, as well as in central and eastern Aquitaine (Sos,...) as in the Gironde (the "Bordelais" gulf and the Salles and Mios areas). The outcrop at Sos, of uncertain age until now, is particularly characterized by the presence of Catinaster? sp., H. walbersdorfensis and Trochoaster concavus, indicating a Late Serravallian age (NN 7-8) (Tabl. 6). This age attribution agrees well with the planktonic microfauna (presence of Orbulina, beds at Sos are thus related to the "couches a Orbulines"), the Rodent fauna and with a Sr isotope dating of approximately 12,5 Ma. From the palaeogeographical data, it is clear that the Serravallian transgression was very widespread in south and east Aquitaine (Fig. 4), having flooded several previously eroded structural anticlines, and in east Aquitaine, having led to the formation of the "golfe de l'Armagnac". The latter is characterized by a generally highly detrital and very shallow marine depositional regime (highly hydrodynamic environments). In the north, the Serravallian sea skirted along the western edge of the Villagrains ridge (deposition of the bioclastic sands and coquinas of Salles and Mios). In the Bordelais, it formed a gulf with sandy and marly deposits rich in littoral macrofaunas. The investigation of the calcareous nannofossils has permitted to refine the stratigraphy and the palaeogeography of numerous classical sites from the Oligo-Miocene of Aquitaine, despite the unfavourable conditions of the deposits. The results presented here do not always agree with these obtained .through "grade-datings", especially for the Middle Miocene, for which this method seems to be less reliable. On the contrary, they generally fit well with information available from other disciplines (field observations, distribution patterns of planktonic and benthic foraminifera, macrofaunas, and isotope datings).