Paleogeographic evolution and subsidence analysis from micropaleontological constraints: the case of a shared Cenozoic foreland basin

During Cenozoic, the Friulian-Venetian Basin (FVB, NE Italy) underwent a complex evolution, related to the inherited Mesozoic sea-bottom topography and the load exerted in different times by the three surrounding belts: the Dinarides to the east, the Southern Alps to the north and the Northern Apennines to the south-west. The location of this basin, at the tectonic knot between the most important Italian hydrocarbon regions (the Po Plain and Adriatic basins), makes the FVB as an interesting case-study from both a scientific and industrial point of view. The micropaleontological study of foraminiferal assemblages from over 500 samples collected from 13 key wells provides important constraints on the paleobathymetric changes occurred into the basin through time. The reconstructed paleo-water depths together with the sedimentological information derived by the drilled depositional units were used as input in geohistory analysis in order to reconstruct the subsidence/uplift trends affecting different sectors of the FVB as a response to collisional tectonics of three surrounding belts along the basin boundaries. Results show that the overall depositional architecture of the FVB is the result of six main tectono-depositional phases that occurred at different times and were characterised by paleobathymetric changes and subsidence/uplift trends reflecting both the tectonic control and the balance between subsidence and sediment supply. In particular, the easternmost sector of the FVB evolved as a Dinaric foredeep during Lutetian time. Since Chattian, a moderate subsidence characterised by faint flexure to the north occurred, which was followed by a Langhian accentuated subsidence phase, consistent with the development of the Southalpine foredeep system. The accommodation space was progressively filled by the sediment flux during Serravallian-Messinian time, as demonstrated by an overall shallowing upward trend recorded in this stratigraphic interval. A new basin configuration occurred during Piacenzian, when a complex geometry was developed due to the flexural effect of the concurrent loads of the Southern Alps, to the north, and of the Apennines, to the south. Unlike the previous stratigraphic intervals, the Quaternary sedimentary sequence eventually shows a broadly symmetrical geometry with fast and relatively homogeneous subsidence all over the basin, overbalanced by the increased sedimentary supply likely related to the Quaternary climate deterioration.