Lower Cretaceous nannofossil biostratigraphy of the Calpionella Limestone and the Palombini Shale in Southern Tuscany (Italy)

Resumen

Four Lower Cretaceous sections of the Calpionella Limestone and the Palombini Shale have been investigated in Southern Tuscany (Italy). The Mt. Alpe Chert, the Calpionella Limestone and the Palombini Shale, belonging to the allocthonous Internal Ligurid Units, represent the typical sedimentary cover on top of the Western Tethys ophiolites. This sequence, widespread in the whole Alpine belt, is generally dismembered and metamorphosed. Samples collected from both siliceous and calcareous deposits, are often barren of calcareous nannofossils.

A careful field investigation allowed the identification of sections that are almost complete and not metamorphosed. Most samples are characterized by rich and well-preserved calcareous nannofossil assemblages. The following calcareous nannofossil Zones have been recognized: Nannoconus colomii, Cretarhabdus angustiforatus, Calcicalathina oblongata and the middle-upper part of Lithraphidites bollii. The stratigraphic contact between the Mt. Alpe Chert and the Calpionella Limestone lies in the upper part of the Cretarhabdus angustiforatus Zone (late Berriasian), whereas the contact between the Calpionella Limestone and the Palombini Shale lies in the lowest part of the Calcicalathina oblongata Zone (earliest Valanginian). The sampled uppermost part of the Palombini Shale is referred to the middle-upper part of the Lithraphidites bollii Zone (late Hauterivian-early Barrermian). In the investigated sector of the Ligurian-Piedmont basin, the Lower Cretaceous pelagic carbonate sedimentation of the Calpionella Lirnestone, overlying the Mt. Alpe Chert, starts during the early Berriasian and becomes increasingly shaly from the early Valanginian.