Recognition of trigger mechanisms for soft-sediment deformation in the Pleistocene lacustrine deposits of the SantʻArcangelo Basin (Southern Italy): Seismic shock vs. overloading

The lacustrine deposits of the San Lorenzo Cycle belong to the infill succession of the SantʻArcangelo Basin, a Pliocene to Pleistocene satellite basin, developed close to the front of the south-Apennines thrust belt. The lower to middle Pleistocene lacustrine succession is made up of siltstone and claystone interbedded with sandstone, carbonate and volcaniclastic beds, arranged in fining-upward sequences. The upper to middle part (about 50 m thick) of the succession (about 200 m thick), has been investigated in detail in the depocentral sector of the lacustrine basin. Here, soft-sediment deformation structures occur in fine-grained sandstone and claystone alternations and show a wide morphological variability (deformed laminations, slumps, load structures, large vertical water-escape structures and neptunian dykes). Their formation occurred during and after sedimentation, with different mechanisms of deformation: some structures are related to liquefaction and fluidization processes (viscous fluid behaviour) while others occurred when sediment had already undergone lithification and its behaviour was plastic and/or brittle.Facies analysis and detailed morphologic study of the soft-sediment deformation structures indicate that the main trigger agents for deformation were seismic shocks and overloading induced by sudden deposition of coarser sediments on clays due to the arrival of density currents. Thus, the soft-sediment deformation structures provide a continuous record of the tectonic and sedimentary processes that acted in the lacustrine basin from sedimentation until diagenesis.