Calcareous plankton biostratigraphy of the fiume Orte section (Abruzzo, central apennines, Italy)

The uppermost Rupelian-Messinian portion of the Montagna della Maiella carbonate succession (Bolognano Fm. of Crescenti et al., 1969) outcrops extensively in the northern sector of the mass if. Most of the previous studies mainly dea! with facies analysis and sequence stratigraphy of these deposits (Accarie, 1988; Yecsei & Sanders, 1999), but their biostratigraphy is sti li poor investigated. The aim of thi s study is to improve the biostratigraphy of the hemipelagic marly deposits of the middle-upper portion of the Bolognano Fm .. These deposits are well exposed along the Fiume Orta valley, between Salle and Piano d'Orta villages. The section studi ed is located o n the right si de of the valley, 2 Km south-east of Piano d'Orta village. The lower portion of the section, which corresponds to the Briozoi superiori member of the Bolognano Fm. (Yecsei, 1991 ; Bellatalla, 1992), is made up of 35 meters of inner-ramp skeletal limestones arranged into stacked subaqueous dunes with cross-stratification associated with bed-concordant and bed-di scordant lamination. They are biocl as tic grain stones and packstones with abundant bryozoans, echinoderm debris and benthic foramjnifers, among which Nephrolepidina morgani and Miogypsina globulina were determined. These depos its are capped by a phosphorite hardground, 25-30 cm thick, rich in glauconite gra in s, phosphatized lithoclasts and bioclasts and fish teeth. Rare specimens of the nautiloid Aturia are al so present. The hardground is fol lowed by almost 80 meters of li mestones and marls rich in planktic foraminifers deposited in an outer ramp environment (Marn.e ad Orbulina member of the Bolognano Fm; Bellatall a, 1992). The interval starts with 2,5 meters of fine- gra ined bioclastic packstones, that up-section pass to 68 meters of cyclically bedded and thoroughly bioturbated hemipelagic marly limestones and calcareous marls. They are wackestones and subordinate packstones with abundant planktic foraminifers. The last IO meters of this interval consist of very fine to fine-grained biocla tic packsto nes, which show a clear shallowing-upward depositional trend characteri sed by both abundant benthic debris input (benthic foraminifers, echinoid remains, pelecypod fragments) and consistent decrease in planktic foraminifer content. The section ends with 10 meters of inner ramp thick-bedded bioclastic limestones. This portion belongs to the Calcari a litotamni member of the Bolognano Fm. (Crescenti et al., 1969). The biostratigraphic study presented here is focused on the Marn.e ad Orbulina member of the Bolognano Fm., that in this area shows the maximum thickness. The integrated study of planktic fo raminifers and calcareous nannofossil s, based on the analyses of 100 samples, indicate a Langhian to early Tortonian age for this interval. Praeorbulina glomerosa s. I. to Jower portion of Neogloboquadrina acostaensis planktic foraminifers Zones of Foresi et al. ( 1998), and the MNN5 to MNN7 calcareous nannofoss il Zones of Fornaci ari et al. ( 1996) were recognized. The Langhian l Serravallian and Serravallian l Tortonian boundaries were identified as well.ln agreement with Fornaciari et al. (1996), the Langhian l Serravallian boundary has been recognized by the LO of Sphenolithus heteromorphus. It is worth mention ing that the LO of Globorotalia peripheroronda occurs l ,5 meters above this boundary. Following Foresi et al. (1998), the Serravall ian l Tortonian boundary has been placed at the LO of Paragloborotalia siakensis. This bioevent predates the FO of Neogloboquadrina atlantica s.s. which occurs about 3 meters above. The section studied, characterised by severa! calcareous plankton bioevents, can be considered a reference section for the middle Miocene carbonate successions of the Maiella massif. Besides, the data coll ected allowed to state some biostratigraphic constrains useful for a better understanding of the sequence stratigraphic evolution of the Maiella un i t during Miocene times.