Contribution a l'etude stratigraphique et paleontologique du synclinal d'Ancenis (Massif armoricain)

Abstract

Abstract The schists and quartzites which form the northern and southern flanks of the Ancenis syncline on the right bank of the Loire river (France), have yielded sandy, micaceous nodules which contain a middle Ordovician trilobite fauna. This discovery indicates a similarity between the quartzites and the Calymenella bayani sandstone of the more northern folds of the Saint-Georges-sur-Loire synclinorium; but the volcanic schists which overlie the C. bayani sandstone are not represented in the Ancenis syncline. Contrary to what has been thought until now, the schists and quartzites rest unconformably on the Brioverian basement complex. The Brulis limestones, present as lenses interbedded in the schist-quartzite band on the south flank, are considered middle or upper Ordovician. The Ecochere jasper and calcareous schist with limestone lenses are directly transgressive on the middle Ordovician and contain a fauna typical of the Frasnian (upper Devonian). The Frasnian Cope-Choux limestone with Hypothiridina cuboides of the north flank seems to be directly related to fissile shales identical to those containing the Ecochere limestones. Continental deposits of the lower Culm, which contain Rhodea hochstetteri, occupy the core of the syncline and are principally Dinantian, but may extend into the lower Namurian. The absence of upper Namurian and Westphalian deposits suggests the intervention of a new orogenic phase (Erzgebirgian?). The Llandeilian trilobites Placoparia zippei, Colpocoryphe aragoi, and Pricyclopyge binodosa are described from the nodules. Pricyclopyge binodosa is the first representative of the Cyclopygidae reported from the Armorican massif.