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Lower Toarcian ammonite biostratigraphy and sedimentary evolution of the Causses Basin (S France)

Morard A.

Volumina Jurassica

2006

Vol. 4, no. 1

193-194

Detailed palaeontological sampling of numerous sections in the Schistes Carton Formation from the Causses Basin (S France) revealed several important aspects for the biostratigraphy and palaeogeography of the Lower Toarcian in this epicontinental setting. First of all, a major hiatus is documented at the Domerian (Upper Pliensbachian) – Toarcian transition, spanning most of the NW European Hawskerense Subzone and Tenuicostatum Zone. A regressive trend is clearly documented in the Upper Domerian, capped by a red pyritic layer, interpreted as a hard-ground (Guex et al. 2001). Sedimentation resumes under transgressive dysoxic conditions at the very end of the Semicelatum Subzone. Further upward the ammonite succession is very similar to that of other NW European basins (e.g. Yorkshire), with the following recognizable faunal horizons: Semicelatum, Elegantulum, Exaratum, Elegans, Pseudoserpentinum, Falciferum and Douvillei. A stage of relatively low sea-level is still recorded up to the so-called “Fish-Bed” (Exaratum Horizon), which we interpret as an episode of episodic water column stirring due to tempests. The dysoxic conditions progressively disappear at the base of the Middle Toarcian, with the couches en échellons (Sublevisoni Subzone) where oxic/dysoxic conditions alternate. It should be noted here that the Lower-Middle Toarcian transition occurs within the Schistes Carton Formation. This boundary, as defined by the first occurence of the genus Hildoceras, does not exactly correspond to the Falciferum- Commune zone boundary as used in the British Isles (Howarth 1992). In our view, the top of the Bituminous Shales and the Ovatum Band already belong to the Sublevisoni Subzone, and hence to the Middle Toarcian. At the basin scale, the Lower Toarcian deposits are characterized by a wide and almost uniform spread of the Schistes Carton Formation, with thicknesses ranging from 50 cm to more than 32 m. This uniformity is due to the rapid and large trangression. Other sedimentary facies are only present in the most proximal areas, mainly as sands and ferrugineous oolites, with intercalated bituminous limestones. Very interesting faunas are in course of study from these localities, as they may represent records of uppermost Domerian and/or lowermost Toarcian limited to some few lenses.