Calcareous plankton high resolution biostratigraphy for the Langhian of the Mediterranean

High resolution quantitative and qualitative analyses of planktonic foraminifera and calcareous nannoplankton content have been carried out on three Middle Miocene sections from the mediterranean area. Such sections (Cretaccio section, Tremiti Islands, Southern Italy;Moria section, Marche Region, Central Italy; DSDP Site 372 succession, Balearic Basin), all well known in the literarture, have been choosen because of their high quality biostratigraphic potential. rematkable magnetostratigraphic data were provided by the Site 372 succession where all chrons and subchrons of the interval C5Br-C5AAn have been recognised. The investigate interval falls between the First Occurrence (FO) of Preorbulina glomerosa sicanaand the Last Occurrences (LO)of Sphenolithus heteromorphus and Globorotalia peripheronda. The LO of S. heteromorphus was detected in the uppermost part of the investigated sequence of the Site 672 at the same stratigraphic level as the G. peripheronda LO. A drastic decrease in abundance of S. heteromorphus (Last Common Occurrence LCO)was detected slightly below its last occurrence; this event is highly correlatable with the same event astronomically calibrated at Ras-il Pelegrin section (Malta Island, which has been recently ratified as the Global Stratotype Section and Point (GSSP)for the base of Serravallian by the International Union of Geological Science. The stratigraphic correlation of the studied section is based on first and last occurrences, abundance fluctuations of selected taxa and additional biohorizons. In particular the peculiar distribution pattern of same taxa, e.g.Paragloborotalia siakensis and Helicosphaera waltrans, offered the opportunity to increase the biostratigraphic resolution of the Langhian interval.The biostratigraphic correlation of the studied sections with the Langhian historical stratotype pointed out its low degree of reliability. On the other hand no one of the sections here studied is suitable to be proposed as candidate for defining the Langhian GSSP. Thus the problem of the finding, in the mediterranean area, a valid section which could yield a new GSSP for the Langhian Stage is still open.