REASSESSING THE BIOSTRATIGRAPHY AND THE PALEOBATHYMETRYOF THE GONFOLITE LOMBARDA GROUP IN THE COMO AREA(NORTHERN ITALY)

摘要

Calcareous nannofossil and foraminiferal analyseshave been carried out on outcrops from the type-area of the GonfoliteLombarda Group (Como, northern Italy). In these marine fine- tocoarse-grained elastics, rapidly accumulating at the southern front ofthe uprising Alpine range during the Oligo-Miocene, a scarce, butreliable, sequence of calcareous nannofossil events has been observed,allowing to refine the previous age assignments. Planktonic foramini-fera were found to be extremely rare and provided limited biostrati-graphic information. The Villa Olmo Conglomerate and the ChiassoFormation contain the Last Occurrence (LO) of Sphenolithus disten-tus and the First Occurrence (FO) of Triquetrorhabdulus carinatus,which are characteristic of the nannofossil zones NP24 and NP25(Chattian), respectively. The lower part of the Como Conglomeratewas deposited during the zone NP25, whilst the upper part of theComo Conglomerate straddles the Chattian/Aquitanian boundary inzone NN1. The deposition of the Prestino Mudstones also occurredduring zone NN1. However, the upper part of this formation hasbeen dated as Burdigalian during nannofossil zone NN2.