Acervulinid macroid and rhodolith facies in the Eocene Nummulitic Limestone of the Dauphinois Domain (Maritime Alps, Liguria, Italy)

摘要

The Eocene Nummulitic Limestone of the Dauphinois domain in the Argentina Valley (Maritime Alps, Liguria, Italy) is characterized by the local presence of carbonate ramp facies rich in acervulinid macroids, rhodoliths and larger foraminifera. The development of these particular facies is mainly controlled by palaeomorphology of the substratum, tectonics, type and amount of terrigenous supply and global sea level changes. The Upper Cretaceous to Eocene succession outcropping in the Argentina Valley shows differences in facies and age if compared to the typical succession of the Maritime Alps: 1. the Cretaceous substratum is younger (early Maastrichtian) and is followed by an unconformity that is interpreted as a submarine discontinuity surface; 2. the first Eocene carbonate deposits are older (late Lutetian); 3. the Nummulitic Limestone is characterized by the development of carbonate facies deposited in a deep infralittoral-circalittoral setting of a carbonate ramp, sheltered from terrigenous input; in these facies encrusting foraminifera (Solenomeris) replace calcareous red algae in nodules similar to rhodoliths (acervulinid macroids); 4. the Nummulitic Limestone is thicker than usual, reaching 110-160 m of thickness. The Eocene tectonostratigraphic evolution can be summarized as follow: (1) synsedimentary tectonic activity that causes the development of a carbonate ramp with an adjacent structural trough where ramp-derived bioclastic material is deposited (late Lutetian); (2) interruption of the tectonic activity and uniform deposition of deep circalittoral sediments, characterized by deepening upward trend (late Lutetian?); (3) regression indicated by an abrupt shallowing of the depositional setting (Bartonian); and (4) deepening of the deposi-tional setting, ending with the drowning of the carbonate ramp (late Bartonian).