Natural subsidence of the Venice area during the last 60 myr

Abstract - Integrated geohistory analysis performed on high-resolution stratigraphy of Venezia1 and Lido1 wells (Quaternary-Pliocene interval) and low-resolution stratigraphy of a simulatedwell extending Lido 1 down to the base of Cenozoic (Palaeocene-Miocene interval) is used to reconstruct the interplay between subsidence and sedimentation that occurred in the Venice area (eastern Po Plain) during the last 60 Myr, and to discuss the relationships between calculated subsidence rates and time resolution of stratigraphic data. Both subsidence and sedimentation are mostly related to the tectonic evolution of the belts that surround the Venice basin, in£uencing the lithosphere vertical motions and the input of clastic sediments through time. In particular, two subsidence phases are recorded between 40-33.5 and 32.5-24 Myr (0.13 and 0.14 mm/year, respectively), coeval with tectonic phases in theDinaric belt.Vice versa, during the main South-Alpine orogenic phase (middle-late Miocene), quiescence or little uplift ( 0.03 mm/year) reflects the location of the Venice area close to the peripheral bulge of the South-Alpine foreland system. Early Pliocene evolution is characterised by a number of subsidence/uplift events, among which two uplifts occurred between 5-4.5 and 3-2.2 Myr (at 0.4 and 0.2mm/year, respectively) and can be correlated with tectonic motions in the Apennines. During the last million years, the Venice areawas initially characterised by uplift ( 0.6mm/year rising to 1.5mm/year between 0.4 and 0.38 Myr), eventually replaced by subsidence at a rate ranging between1.6 and1.0mm/year up to 0.12 Myr and then decreased to 0.4mm/year, as an average, up to present. Our results highlight that time resolution of the stratigraphic dataset deeply influences the order of magnitude obtained for the calculated subsidence rate.This is because subsidence seems to have worked through short-lived peaks (in the order of 100.000 years), alternating with long relatively quiescent intervals.This suggests caution when components of subsidence are deduced by subtracting long-term to short-term subsidence rate.