Luca Maria Foresi

CALCAREOUS PLANKTON HIGH RESOLUTION BIO-MAGNETOSTRATIGRAPHY FOR THE LANGHIAN OF THE MEDITERRANEAN AREA

High-resolution quantitative and qualitative analyses of the planktonic foraminifer and calcareous nannofossil 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 literature, have been chosen because of their high-quality biostratigraphic potential. Remarkable magnetostratigraphic data were provided by the Site 372 succession where all chrons and subchrons of the interval C5Br-C5AAn have been recognised. The investigated interval falls between the First Occurrence (FO) of Praeorbulina glomerosa sicana and the Last Occurrences (LO) of Sphenolithus heteromorphus and Globorotalia peripheroronda. The LO of S. heteromorphus was detected in the uppermost part of the investigated sequence of Site 372 at the same stratigraphic level as the G. peripheroronda LO. A drastic decrease in abundance of S. heteromorphus (Last Common Occurrence -LCO) was detected slightly below its last occurrence; this event is well correlatable with the same event astronomically calibrated at Ras-il Pellegrin section (Malta Island), which has been recently ratified as the Global Stratotype Section and Point (GSSP) for the base of the Serravallian by the International Union of Geological Sciences. The stratigraphic correlation of the studied sections is based on first and last occurrences, abundance fluctuations of selected taxa and additional biohorizons. In particular the peculiar distribution pattern of some taxa, e.g. Paragloborotalia siakensis and Helicosphaera waltrans, offered the opportunity to increase the biostratigraphic resolution of the Langhian interval. The resulting integrated calcareous plankton bio-magnetostratigraphic scheme represents the downward extension of that one previously established for the Serravallian - Tortonian interval. The biostratigraphic correlation of the studied sections with the Langhian historical Stratotype pointed out its low degree of reliability. On the other hand, none of the sections here studied is suitable to be proposed as candidate for defining the Langhian GSSP. Thus the problem of finding, in the Mediterranean area, a valid section which could yield a new GSSP for the Langhian Stage is still open.

Middle Miocene high-resolution calcareous plankton biostratigraphy at Site 926 (Leg 154, equatorial Atlantic Ocean): palaeoecological and palaeobiogeographical implications

Abstract High-resolution calcareous plankton (planktonic foraminifera and calcareous nannofossils) biostratigraphy is presented from the Middle to early Late Miocene interval (from 14.45 to 8.86xa0Ma) at Site 926 (ODP Leg 154, equatorial Atlantic Ocean). The main bioevents used in the low-latitude zonal schemes, and also auxiliary events revealing potential biostratigraphic value have been recognised. The investigated succession ranges from N.10 to N.16 Zones based on planktonic foraminifera, and from NN5 (CN4) to NN10 (CN8) Zones based on calcareous nannofossils. The evolution of the planktonic foraminiferal Globorotalia fohsi lineage appears to be environmentally controlled. The main diagnostic features of the species of this lineage are not always evident, rendering problematic the definition of the N.9/N.10, N.10/N.11 and N.11/N.12 zonal boundaries. Calcareous plankton events have been calibrated on the basis of the Astronomical Time Scale of Shackleton and Crowhurst 〚 Shackleton, N.J., Crowhurst, S., 1997 . Sediment fluxes based on an orbitally tuned time scale 5xa0Ma to 14xa0Ma, Site 926. In: Curry, W.B., Shackleton, N.J., Richter, C., Bralower, T.J. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program) 154, pp. 69–82〛. The astrobiochronology obtained at Site 926 has been compared with that of the Mediterranean astronomically calibrated deep marine successions, allowing the evaluation of the degree of synchroneity and diachroneity of bioevents. Some bioevents, such as the last occurrence of Globigerinoides subquadratus dated at 11.55xa0Ma, the last occurrence of Sphenolithus heteromorphus dated at 13.51xa0Ma and the last common occurrence of Cyclicargolithus floridanus calibrated at 13.32xa0Ma, are near-synchronous events between the equatorial Atlantic and the Mediterranean area indicating their high biostratigratigraphic value in global correlation. The diachroneity of the last occurrence of Paragloborotalia siakensis , the first occurrence of Neogloboquadrina acostaensis and the last occurrence of Globorotalia peripheroronda between equatorial Atlantic and the Mediterranean reflect a different spatial and temporal distribution of these marker species probably due to a sharp definition of surface plankton provinces related to the latitudinal thermal gradient.

HIGH RESOLUTION CALCAREOUS PLANKTON BIOSTRATIGRAPHY OF THE SERRAVALLIAN SUCCESSION OF THE TREMITI ISLANDS (ADRIATIC SEA, ITALY)

The planktonic foraminifer and calcareous nannofossil content of two Middle Miocene sections of the Tremiti Islands (Southern Adriatic Sea) have been studied. The two sections are composed of marly limestones rich in calcareous plankton which show cyclic alternations of indurated (higher carbonate content) whitish and less indurated grey or reddish beds. The two sections represent a succession with a total thickness of 38 m. Samples have been collected at a mean spacing of 10-15 cm; qualitative analyses were performed on one sample per meter but quantitative analyses were made for each sample. The abundance fluctuations of several marker species proved to be a very useful tool to correlate the two sections. The astronomical calibration of the sedimentary cycles provided absolute ages for all the recognised calcareous plankton bioevents.

The Miocene successions of the Fiora Hills: considerations about the development of the minor basins of Southern Tuscany

The Miocene sequences of Southern Tuscany represent the first post-nappe sedimentary record of the Northern Apennines, and are linked with the opening of the Tyrrhenian Sea. The sequences are located in several basins, characterizing the hinterland of the orogen. They are settled on a stack that is composed of deformed tectonic units and are capped by Plio-Pleistocene deposits. The respective basins have been strongly controlled by tectonics, which have remarkably forced the sedimentation. The Fiora Hills represent the southernmost area of Tuscany, where there are some minor but significant examples of these basins. In particular, they are the Fiora and the Tafone basins, the infillings of which are characterized by Miocene successions subdivided into several depositional units separated by unconformity or correlative conformity surfaces. This study deals with the stratigraphic features of such Miocene basinal infillings, with the aim being to define the depositional architecture and the tectonic-sedimentation interplays. Moreover, the collected data also enables there to be a discussion of some of the aspects of the basins’ structuring that are linked with the Miocene evolution of the hinterland of the Northern Apennines. The entire Miocene succession of the Fiora Hills spans from the Langhian up to the late Messinian, with there being minor differences between the Fiora-Tafone basins and with the nearby Albegna Basin. In general, we recognize: the basal Ponsano P Unit (middle Miocene), which is referable to coastal-shallow marine environment; the Lignitiferous T Unit (late Tortonian-early Messinian), which is referable to fan-delta and lacustrine systems; the Acquabona-Spicchiaiola M1 Unit (early Messinian), which is referable to lagoonal fan-deltaic environment; the Castelnuovo M2 Unit (early Messinian), which is referable to shallow marine environment; and the “Lago-Mare” M3 Unit (late Messinian), which is referable to fan-deltaic lacustrine systems. The Miocene succession is overlaid by Plio-Pleistocene marine to continental succession. The Miocene basins have settled on deformed Ligurian allochthonous units, which are markedly structured in tectonic depressions and highs and coherently so with the “crustal lateral segmentation” model. The development of the Miocene sedimentation in this sector of the chain appears to be strictly connected to the tectonic evolution of the Tyrrhenian Sea rifting. The middle Miocene deposits may in fact be related to the first syn-rift shallow-marine basins, marking the beginning of the post-nappe phase. During late Tortonian-early Messinian, important lacustrine-fan-delta systems dominated in the basins and represented the development of the middle Miocene stages. They evolved during the early Messinian in lagoonal fan-deltaic systems and then in shallow-marine systems. The transition to the upper Messinian deposits is marked by a significant unconformity, which is marked locally by angularity, thus noting an intramessinian deformative episode. This characterizes the lacustrine fan-deltaic systems that are linked to the Messinian salinity crisis for the paleo-Mediterranean Sea. The stratigraphic differences between the Fiora Hills’ basins are therefore linked to the somewhat different basinal sedimentary evolution, which is connected to the development of morphological/tectonic ridges.

CALCAREOUS PLANKTON BIO-EVENTS IN THE MIOCENECASE PELACANI SECTION (SOUTH-EASTERN SICILY, ITALY)

The upper Serravallian-Lower Tortonian Case Pelacani section in Sicily is represented by the epipelagic sediments of the Tellaro Formation. Along the section, which was sampled in four easily correlatable segments, a more or less continuous sequence of lithologic couplets, with whitish marls and blackish levels in the lower part and whitish marls and whitish more indurated levels in the upper part are present. The distribution of the most common taxa of planktonic foraminifera and calcareous nannofossils was quantitatively estimated in 316 samples, collected every 20 cm along the section which is 66.35 m thick. This allowed to identify eleven planktonic foraminifera (but only the eight marker events were quantitatively estimated) and five calcareous nannofossil bio-events.xa0 Their astrochronology is reported by correlation with the cyclostratigraphic results obtained for this section in a different paper. The ages are well comparable with the ages published for the same events in other Mediterranean sections. Paleomagnetic analysis along 30 meters in the central part of the section gave puzzling results and therefore no reliable magnetostratigraphy could be obtained. The Case Pelacani section, straddling the stratigraphic interval between slightly below the first occurrence and slightly above the first regular occurrence of Neogloboquadrina acostaensis , is a good candidate for the definition of the GSSP (Global Stratigraphic Section and Point) of the Tortonian. Independently from the selected section, we suggest to define the Serravallian/Tortonian boundary in a level coincident or near the last occurrence of Paragloborotalia siakensis , as already proposed, or coincident with or near the last common occurrence of Globigerinoides subquadratus. This latter level virtually coincides with the first common occurrence of Globigerinoides obliquus obliquus and is slightly above the last common occurrence of Discoaster kugleri , which occurs only 2.25 meters (and 3 precessional cycles) below in the studied section.