Volume 22, Issue No. 2, 1997

CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY OF THE CALPIONELLA LIMESTONE AND PALOMBINI SHALE (BRACCO/VAL GRAVEGLIA UNIT) IN THE EASTERN LIGURIAN APENNINES (ITALY)

Nicola Perilli* and Daniele Nannini**

** Dipartimento di Scienze della Terra, Universitˆ di Pisa, Italy, ** c/o Dipartimento di Scienze della Terra, Universitˆ di Pisa, Italy.

Keywords: Early Cretaceous, calcareous nannofossil biostratigraphy, Bracco/Val Graveglia Unit. Northern Apennine, Italy.

ABSTRACT

A detailed lithostratigraphy and calcareous nannofossil biostratigraphy was carried out on the Calpionella Limestone and the lower portion of the Palombini Shale (Bracco/Val Graveglia Unit). The studied sections crop out in the Rocchetta di Vara area (eastern Ligurian Apennine), precisely, southeast of Mt. Nero, south of Rocchetta di Vara, between Beverone and the Cuccaro Peak and at Casa dei Cipressi. The calcareous nannofossil assemblages of the Calpionella Limestone are referable to the lower part of the Calcicalathina oblongata Zone, whereas the assemblages from the upper part of the Palombini Shale are assignable to the upper part of the Calcicalathina oblongata Zone/lowest part of Litraphidites bollii Zone. The boundary between the Calpionella Limestone and the Palombini Shale lies in the lower part of the Calcicalathina oblongata Zone, before the disappearances of Rucinolithus wisei and Tubodiscus verenae. The Calpionella Limestone sampled at Mt. Nero, at Rocchetta di Vara and at Beverone-Cuccaro spans in age from ?latest Berriasian/earliest Valanginian to early/late Valanginian. The Palombini Shale sampled at Mt. Nero and at Beverone-Cuccaro ranges from early/late Valanginian to late Valanginian/early Hauterivian. At Casa dei Cipressi, the early/late Valanginian lower portion of the Palombini Shale stratigraphically overlies the Mt. Alpe Chert. These data suggest a ?latest Berriasian/earliest Valanginian beginning of the pelagic Calpionella Limestone deposition in the Rocchetta di Vara area. Sedimentation of the Palombini Shale, since the Valanginian, records an increased supply of terrigenous fines. The achieved results point to an inception of carbonate sedimentation slightly younger than that of the Calpionella Limestone cropping out in Southern Tuscany, which started during the late Berriasian. The beginning of the Palombini Shale deposition, instead, seems to have occurred during the Valanginian all over the Ligurian-Piedmontese Basin.