Nouvelles observations sur le Toarcien inférieur de la région lyonnaise. Comparaison avec les régions voisines

Abstract

The sedimentary and faunal hiatus at the Domerian-Toarcian boundary, which affects the whole of northwestern Europe, is particulary marked in southeastern France, especially along the Massif Central margins where Toarcian deposits are highly irregular Near Lyons, new observations made in the Lafarge quarries (Belmont, south Beaujolais) have demonstrated for the first time the presence of the Tenuicostatum and Serpentinus zones in a succession of limestone and marl. Work for the building of a new track in the quarries has revealed a stratigraphic section which, from bottom to top, consists of : 1 - blue-grey clay becoming progressively yellow and reddish (1.50 m). 2 - a dismantled bed with ferruginous incrustations (0.20 - 0.30 m): Pleuroceras spinatum (BRUG.), Amaltheus margaritatus (MONT.). 3 - yellow bioclastic limestone (0.30 - 0.40 m) topped by a rubefied coating: rich fauna of belemnites, gastropods, pelecypods and brachiopods. These first three levels belong to the Domerian. 4 - yellow calcareous bed (0.20 - 0.40 m) with abundant belemnites and Dactylioceras gr. of D. semicelatum (SIMPS.) in its lower part. This bed belongs to the Tenuicostatum zone. 5 - succession of yellow calcareous beds (10 cm or 30) separated by thin marly intercalations (0.70 m). At the bottom, numerous calcareous nodules contain rich fauna of the Serpentinum zone: Harpoceras serpentinum (REIN.), Hildaites subserpentinus BUCK., Dactylioceras mitis BUCK., etc. These fossils are associated with accumulations of Young ammonites ("calcaire à Ammonitella"). 6 - 9 - succession of calcareous beds and clay (more than 5 m) without fossils. 10 - calcareous bed with Hildoceras of the Bifrons zone. Study of the faunas underlines the important faunal replacement that accompanied the regressive then transgressive episode at the Domerian-Toarcian boundary. A series of gaps and reworked and condensed deposits are associated with this episode, especially at the top of the Domerian and in the lower part of the Toarcian. At Belmont, the stratigraphic gap covers at least the Hawskerense subzone and the Paltus subzone. A comparison with the neighbouring areas (Ain, Isère, Loire, Saône et Loire) shows that the outcrops at Belmont are the most complete in the Lyons region s.l. Elsewhere, in the few areas where Early Toarcian deposits exist, it is only represented by deposits of the Serpentinus zone; the Tenuicostatum zone is generally missing.