Facies and Late Triassic fossils in the Roisan zone, Austroalpine Dent Blanche and Mt Mary-Cervino nappe system, NW Alps

Abstract

The Roisan zone is a metamorphic cover unit exposed along the ductile shear zone between the Dent Blanche s.s. and Mont Mary-Cervino Upper Austroalpine outliers, Aosta Valley, north-western Italian Alps. It is characterized by the occurrence of dolostones, pure marbles, marbles with quartz, calcirudites and ophiolite-free calcschists. Locally, dolostones preserve alternances of thick massive beds and thinner levels of planar stromatolites and other sedimentary structures and textures typical of a carbonate platform. In Mt Grand Pays they contain Dasycladales and foraminifers referable to the Norian. Pure marbles and marbles with quartz grains are tentatively referred to the end of Triassic–Early Jurassic, thin-bedded marbles and calcirudites to the Early and Middle Jurassic, calcschists from Middle Jurassic to Late Cretaceous. This Roisan succession is quite similar to the one of Mt Dolin, in the Swiss part of the Dent Blanche nappe, where the same Triassic foraminifer association has been reported. There, the fossils were found only in reworked pebbles, contained in calcirudites of presumed Jurassic age. Some differences exist between the two successions: calcirudites are abundant in the Mt Dolin and sporadic in the Roisan zone, whereas calcschists are very thick in the Roisan zone. As consequence the Mt Dolin succession can be considered settled down in the proximity of the faults related to the pre-oceanic rifting of the Piedmont basin, whereas the Roisan zone could have been deposited in a more distal area.