L’évolution paléogéographique et géodynamique du Bassin aquitain au Paléogène : enregistrement et datation de la tectonique pyrénéenne

Abridged English version

The effects of the Pyrenean collision have been identified over many years in the Paleogene sedimentary sequence of the Aquitaine Basin (Kieken, 1973; Schoeffler, 1973; Feuillée et al., 1973; Plaziat, 1984, 1986; Razin, 1989;  Deramond et al., 1993, Tambareau et al., 1995; Cavelier et al., 1997 (Fig. 1). However, sequential analysis provides tody a better understanding of the sedimentary processes within an active tectonic setting. The stratigraphic correlation of third-order deposition of sequences at the biozone scale and not at stage scale as formerly proposed for the Paleocene and Eocene of the Aquitaine Basin (Sztrakos et al., 1997, 1998 - Figs. 2, 3 and 4) provides new insight into the understanding of the geodynamic development in the Aquitaine Basin.

This operation can be carried out at various scales:

- tectonic structure, by comparing the deposition history between the top and flanks of the structure (short-deformation wavelength, at kilometre scale; in the present paper the tectonic movements are described as "normal" when in agreement with the present nature of the tectonic structure and "reverse" in the opposite case);

- isopic region, by detecting the sedimentary units and discontinuities of regional extension (middle wavelength);

- basin and neighbouring areas, by elaborating paleogeographic maps of the maximal transgression of the deposition sequences and comparing with other basins such as the Paris Basin (long wavelength).

The variable wavelength deformations identified fit adequately with the model of imbricated prisms proposed by Lacombe and Mouthereau (1999) for the Pyrenean orogen. The short- to middle- wavelength deformation regimes that succeeded in the various domains of the Aquitaine Basin throughout the Cenozoic largely controlled the sedimentation (Fig. 2). The comparative tectonic history between the Aquitaine Basin and the Paris Basin make it possible to evaluation the long-wavelength deformations that affected the whole of the western Europe continental platform (Fig. 14).

During the Lower Danian, the development of the North-Pyrenean trough is a direct consequence of the flexure of the continental plate initiated during the Senonian, in the fore zone of the Pyrenean thrust front. In the Paris Basin tectonic tightening was initiated resulting in a general emersion at the end of the Cretaceous and almost total erosion of the Maastrichtian (Pomerol, 1989). The long-wavelength deformation at the Cretaceous-Tertiary boundary is however associated with a short-wavelength deformation that is suspected when considering the distribution of Danian deposits In the Paris basin, as in the Aquitaine basin, subsidence starts very locally during the Lower Danian represented by the Verus-Mont Aimé deposits (P1 sequence, Bignot, 1993).

The North-Pyrenean trough is initiated in the southern Aquitaine Basin during Danian times, west of the Lannemezan shoal (Fig. 5). In the southern part of the Adour Basin and the Basque thrust zones, the permanent presence of turbidites provides evidence for the instability of the southern slope of the North-Pyrenean trough. In the Paris Basin, short-wavelength deformations have been widely identified as syn-sedimentary normal faults in the Vigny Upper Danian (P3 sequence, Montenat et al., 1997).

In Aquitaine, the structural scheme of the Danian globally persists throughout the Lower Thanetian. However, the extension of the North Pyrenean trough east of the Lannemezan shoal during the Lower Thanetian is a direct consequence of the tectonic episode occurring at the P5/P6 boundary. Abrupt slowing down of the subsidence results, at best, in reduced accomodation but, in most cases, a middle- wavelength deformation generates a strong erosion of the Danian sequences locally expressed by the P6-sequence polygenic conglomerates.

Renewed subsidence occurs during the Middle Thanetian (Fig. 6) in relation with a long-wavelength deformation also expressed in the Paris basin when the first Thanetian deposits uncomformably overly the Senonian and the Danian (P6 sequence, Janin and Bignot, 1993).

At the Paleocene-Eocene boundary, subsidence decreases altogether in the western part of the Aquitaine basin and in the Paris Basin, accompanied by middlewavelength deformation, including emersion and erosion periods separating four short-duration cycles (top of NP8 zone and NP9 zone) identified in both basins (P8 sequence, both parasequences of the P9-E1 sequence and P10-E2 sequence, Sztrakos et al., 1997). Obviously, these four sedimentary cycles may be of eustatic origin while of being part of a period of active tectonics.

Renewed regional subsidence occurs during the Lower Ilerdian extending into the Bordeaux area from the Atlantic boundary (P11-E3 sequenec, Fig. 7). During the Middle Ilerdian, middle- to short-wavelength deformation is marked by polygenic breccias in the southern part of the North Pyrenean trough. This event is expressed in the Paris Basin by the reworking of the Thanetian and Lower Ypresian (Lower Sparnacian) sands within the Sinceny Sands. Renewed subsidence then occurs in both basins during the Upper Ilerdian (E4 sequence for the Aquitaine Basin and lewer part of the Laon Sands in the Paris Basin (Dupuis et al.,1982).

The first major tectonic phase extend from the upper part of E4 to the whole of E5 at the end of the Ilerdian; the North Pyrenean trough, that underwent strong accomodation during the whole of the Ilerdian, is progressively filled by thick lagoonal deposits followed by detrital continental deposits originating from the structuration of the axial zone of the the eastern Paleo-Pyrenees (beginning of the detrital sedimentation: Lower Palassou or Palassou 1). The subsidence pole, located in the North-Pyrenean trough since the Early Cenozoic (Fig. 8), migrates to the north-west into the Adour Basin at the E8/E9 boundary (Fig. 9). This migration together with the progradation of the detrital bodies towards the Atlantic boundary will persist until the Recent. In the Paris Basin, however, no deformation is known during the long deposition period of the Laon Sands, which are dated from the top of NP10 to the base of NP12.

The second major tectonic phase is dated at the E9/E10 limit, corresponding to the Ypresian/Lutetian boundary. This phase is marked by the beginning of the detrital sedimentation (Middle Palassou or Palassou II). The Pyrenean closure results in pre-Lutetian folds identified in the Peyrehorade unit, the first significant shortening of the southern slope of the North-Pyrenean trough. This is followed by massive deposition of conglomerates and polygenic breccias on the southern slope that is being deformed (Fig. 10). An eastwest, middle-wavelength structuration affects the Adour Basin and extends into the North-Aquitaine platform (Fig. 11). Short-wavelength tectonics also affect the Paris Basin as the Lower Lutetian transgression overlies the deformed and eroded Middle Ypresian (Gély, 1966). In both basins, renewed subsidence occurs at the beginning of the Lower Lutetian together with shot-wavelength synsedimentary tectonics during the E10 sequence and at the E10/E11 boundary.

During the Upper Lutetian-Lower Bartonian interval, the tectonic activity in the Aquitaine Basin is mainly represented by middle-wavelength deformation including shore migration toward the Atlantic border. This type of deformation is also found in the Paris Basin whose marine polarity changes: the marine gulf opened towards the North Sea up to the Middle Lutetian becomes an appendix of the Atlantic, up to the Recent (Gély, 1996).

The third tectonic phase is dated Upper Bartonian (beginning of the Upper Palassou - Palassou III detrital sedimentation); structuration of the whole North- Pyrenean trough results in uncomformities in the marine sequences as well as the molasse deposits around the E14/E15 boundary (Crochet, 1974, 1984, 1991; Fig. 12). It is also during this period that pre-nappe structuration occurs in the Corbières. The short-wavelength deformation extends northward as far as the Paris Basin, including synsedimentary fault tectonics during the Upper Bartonian (Middle Marinesian of the Paris Basin, Wyns, 1978). This phase is followed by a tectonic event of lesser extent at the E15/E16 boundary.The Priabonian appears as a calmer period (Fig. 13). The fourth and last major tectonic phase is dated Oligocene. The Rupelian is often uncomformable in the Aquitaine Basin while erosion occurs at the Priabonian (Ludian)-Rupelian (Stampian) boundary in the Paris Basin. Oligocene tectonics culminate between the Lower Rupelian (post N0P21 zone) and the Upper Chattian (pre NP25 or pre NP24 if the Mugnon sandstone is included) when the Basque nappes are emplaced into the Aquitaine Basin, probably accompanied by the northwest thrust of the eastern- Corbieres nappe. The north-west movement of the Basque Pyrenees has been estimated between 30 and 40 km (Zolnai, 1971, 1975; Muller and Roger, 1977). The comparison between the Paleogene sequences from the autochthonous and the nappes provides a significant paleogeographic marker represented by brown-red clays at the top of the P11-E3 sequence. In the Autochthonous, these clays are distributed along a narrow east-west band between the Goudon 1, Ger 1 and Artigueloutan 101 drillholes. In the allochthonous, these clays are present 35-40 km to the north-northwest in the Sorde 1 drillhole. The shifting of this isopic zone proves that, during the Middle Rupelian, the Peyrehorade unit was thrusted over the foreland for about 40 km (Fig. 1). During the Chattian, these tangent tectonics are sealed in Aquitaine as well as in the Languedoc.

Short-wavelength deformation also occurs in the Paris Basin during the NP23 interval: the Pierrefitte and Vauroux "faluns" transgressive over a structured basement; shortly after middle-wavelength deformation results in the sea moving out of the Paris Basin into the Ligerian channel (Gély and Lorenz, 1991; Debrand-Passard et al., 1997. The paleogeographic setting established then continues during the Miocene in the Aquitaine and Paris Basins. Upper Oligocene and Aquitanian extension affects the Languedoc area, among others, while a compressive regime including a main north-northwest constraint (Rocher et al., 1998) continues in Aquitaine from the Upper Oligocene to the Recent.

This study shows that sequential analysis of the Paleogene of the Aquitaine Basin has been properly dated at biozone scale as a result of a detailed biostratigraphy work. This analysis made it possible to determine the deposition dynamics of the sedimentary bodies separated by large discontinuities. Deformations of various wavelength account for the Pyrenean tectonics as shown by synsedimentary processes. It appears that the main tectonic phases are first associated with short-wavelength deformation identified in the whole foreland, then are rapidly overimposed by middle- and long-wavelength deformations. The emplacement and subsequent erosion of thrust nappes in the inner then outer zones of the Pyrenean Belt have a direct impact, making it possible to precisely date the major stages of the shortening. Although the collision is included in a deformation continuum, it is possible to differenciate and date, at geological scale, shortening periods, known of for a long time in the Pyrenees and that deeply alter the paleogeography in the forebelt basins. The continuity and classification of the deformation from the core of the collision belt to the outskirts of the foreland submitted to a compressive regime, should make it possible to establish a precise deformation model for a continental plate.

Keywords: Geology, Aquitaine Basin, Languedoc, Paris Basin, Paleocene, Eocene, Stratigraphy, Tectonic, Palaeogeography.