A preliminary report on the lacustrine succession of Sessano basin (Molise, southern Italy).

The lacustrine succession of the Sessano basin represents the filling of a N-S trending structural depression which is placed at the boundary between the Central and Southern Apennines thrust and fold belt. This belt was generated by a progressive north-eastward thrusting and stacking of passive margin sediments, essentially during the Cenozoic, and by the development of strike-slip tectonic regime during Late Pliocene and Early Pleistocene. A subsequent extensional tectonic phase, SW-NE trending, overprinted the compressional and transpressional structures. Detailed field surveys allowed us to establish that this extensional tectonic phase strongly controlled the geomorphologic evolution of the basin as well as the phases of lacustrine and fluvial sedimentation. In particular, structural analyses showed that the sedimentation was controlled by the activity of two sets of faults, N-S and NW-SE trending, which displaced and tilted the fluvio-lacustrine deposits towards ESE. To characterise the lacustrine succession, an 80 m deep core was drilled in the central part of the basin. The analysis of the core showed that the succession is made of lacustrine silts and clays, including various layers of black peat or fine sands. The latter become then dominant in the upper part of the core where a two metres thick bed of fluvial silicoclastic sands is present. Moreover, near the top of the sequence we can also observe whitish, 20-30 cm thick, volcano-clastic layers, some of which include pumices.The prevalence of coarser grained sediments in the upper part of the succession indicates the end of the lacustrine deposition and the establishment of a fluvial regime. In order to obtain further information on the paleoenvironmental evolution of the basin, 78 samples were taken along the core for pollen analysis. Preliminary results revealed the occurrence of at least two periods with contrasting climatic characters. In particular, in the basal part of the core pollen spectra show the dominance of herbaceous taxa, indicating typical glacial open environments. Towards the top this association is replaced by forest elements, both deciduous and altitudinal, which typically indicate an interglacial period. This glacial-interglacial cycle strongly resembles those already identified in other Middle Pleistocene paleolakes of southern Apennines. The floral composition suggests a late Middle Pleistocene age for the lacustrine cycle of the Sessano basin, according to our hypothesis that the basin took origin and was filled during Middle Pleistocene.