Résumé
Les séries du Toarcien supérieur et de l'Aalénien de la partie orientale du Quercy méridional s'intègrent dans deux formations : - la Formation de Lexos (Toarcien supérieur) qui comprend quatre unités lithologiques successives : les marnes silteuses, l'assise à gryphées, le niveau à brachiopodes et le niveau de remaniement du Toarcien terminal ; - la Formation d'Autoire (aaléno-bajocienne) qui comprend trois unités lithologiques différentes : les calcaires bioclastiques à lamellibranches, le niveau de remaniement aalénien et les calcaires bioclastiques à oncolithes. Durant la période étudiée, l'évolution générale en comblement et le passage progressif du domaine circalittoral, à l'infralittoral et à la base du médiolittoral soulignent l'installation d'une plate-forme carbonatée peu profonde. La distribution spatiale des faciès montre une sédimentation à caractère plus proximal en direction de l'est durant le Toarcien supérieur et l'Aalénien basal. À l'Aalénien, cette tendance s'inverse avec un caractère plus littoral vers l'ouest. L'étude biostratigraphique (200 ammonites) permet un découpage à l'échelle de la sous-zone et de l'horizon. Quatre sous-zones ont été définies : la sous-zone à Pseudoradiosa, la sous-zone à Mactra (horizon à Mactra et horizon à Sub-compta), la sous-zone à Lugdunense (horizon à Celtica, horizon à Lugdunense et horizon à Buckmani) et la sous-zone à Opalinum. Une hypothèse d'évolution géodynamique basée sur l'étude biostratigraphique et sédimentologique a été proposée. À la limite Lias-Dogger, une dynamique de blocs engendrée par une tectonique locale en décrochement et décomposée en sept stades paraît s'être instaurée dans le secteur étudié.
Abstract
Many outcrops near Villefranche-de-Rouergue in the Quercy region along the northeastern edge of the Aquitaine Basin, enable a precise analysis of the Upper Lias and Lower Dogger formations. The Lias-Dogger transition in French sedimentary basins constitutes an important period during which sedimentary regime changes, tectonic events and eustatic variations are superimposed. Through the analysis of 12 sections, our goal was to describe the stratigraphic succession, establish a biozonal division and define a paleoenvironmental and sequential evolution model. Study of the Lias-Dogger boundary in the Villefranche-de-Rouergue area has identified seven lithofacies that can be assigned to two formations: the top of the Lexos Formation (Cubaynes, 1986) for four lithofacies (silty marls, Gryphaea beds, brachiopod beds and an end-Toarcian reworked bed), and the base of the Autoire Formation for three lithofacies (basal Aalenian lamellibranch bioclastic limestone, an Aalenian re-worked bed, and the Toulzaine oncolithic limestone member [Pélissié, 1982]). The faunal abundance in the first five lithofacies of this lithological decomposition makes it possible to establish a precise biochronological division at horizon scale for the Upper Toarcian and at sub-zone scale for the basal Aalenian: - two zones are identified for the Upper Toarcian: the Pseudoradiosa zone (represented by the Pseudoradiosa sub-zone) and the Aalensis zone (represented by two sub-zones: the Mactra sub-zone with two horizons: the Mactra sub-zone with two horizons [Mactra and Sub-compta] and the Lugdunense sub-zone with three horizons [Celtica, Lugdunense and Buckmani]); - only one zone is identified for the Aalenian: the Opalinum zone (represented by the Opalinum sub-zone). This biozonation, which involves the use of new terminology (Elmi et al., 1997), presents some significant differences to that established in the Lexos aera by Rey et al. (1995). Based on the biochronological and lithostratigraphic divisions, a correlation has been established between the various sections. This correlation reveals a high shoal area to the northwest and north-northwest (east of the Villeneuve Fault), and an area of greater subsidence to the southeast and south-southeast (near the Villefranche-de-Rouergue Fault) for the Upper Toarcian and basal Aalenian. This pattern was reversed during the course of the Aalenian. In considering the paleoenvironmental evolution, the sedimentological and paleontological data reveal a regressive trend from the base to the top of the succession. The sedimentation was more proximal to the east during the Upper Toarcian and basal Aalenian, and to the west during the Aalenian. The Upper Toarcian and Aalenian formations are condensed in the Villefranche-de-Rouergue area. Nevertheless, two main boundaries of tectono-eustatic origin are identified, each at the base of a reworked bed; these constitute sequence boundaries and transgressive intervals. The first, dated to the Buckmani horizon, separates the To7 and To8 sequences; the second, dated to the Aalenian, separates the To8 and Aa1 sequences. Although eustatic variations appear to constitute the main factor controlling sedimentary evolution, the influence of extensional sedimentary tectonism is unquestionable (Pélissié et al., 1996). During the Lias and Dogger, the paleo-geography was affected by Tethyan rifting. The biostratigraphlc and sedimentological correlations suggest an influence of block dynamics broken down into seven stages.
Dernière mise à jour le 28.07.2015