Sedimentary and tectonic evolution of a lower-middle Pleistocene lacustrine system in the Sant’Arcangelo piggyback basin (Southern Italy

A continental depositional sequence was developed in a NW-SE trending syncline basin, which was formed during the late tectonic-sedimentary evolutionary phases of the Sant'Arcangelo piggyback basin (SE Apennines). This continental sequence (San Lorenzo Cycle) is early-middle Pleistocene in age and can be divided into three units: - a lower sandy conglomerates (alluvial dominated) unit, - a middle silty claystones (lacustrine or palustrine) unit, - an upper sandy conglomerates (alluvial dominated) unit. A stratigraphic study of the middle lacustrine unit of the San Lorenzo Cycle has been carried out and preliminary results are shown. In the northern and southern sectors of the middle unit of the San Lorenzo Cycle the stratigraphic features and facies are different. The studied southern 200 m-thick succession, in the depocentral area, is represented by elaystones and silty claystones with interbedded calcareous and volcaniclastic layers, and organic-matter rich beds. The fossil content is represented by Ostracodes, Gastropods, plant and Vertebrate remains. This succession records the sedimentation developed in the inner zone of a terrigenous dominated diluite fresh water lake; the minor occurrence of organic matter-rich laminites suggests the episodic setting of meromictic, water stratified conditions, while minor carbonate deposits may record a more concentrated lake water body. The northern succession, some tens of meters thick, located in a marginal area, in its lower part displays similar facies to those observed in the SE basin zone; in the upper part shows, on the contrary, a clear change testified by sand-mudstones beds and the occurrence of thin coal seams which bear fossil plant remains and mammal bones. This succession records the late evolutionary stage of encroachment of a fan delta-alluvial fan and marginal lacustrine environments over the inner lacustrine zone. Moreover the observed overall features of the lacustrine system (thickness, structural arrangement, feeding axis trends, soft-sediment deformation structures) as well as its regional setting suggest a likely syntectonic sedimentation.