Geological transect from the Ligurian Sea to the Po plain

A geological transect from the abyssal plain of the Ligurian Sea to the Po Plain, realised integrating seismic profiles (ENI, BG Group and CROP data set), well data and surface geology, is presented and discussed in order to highlight Messinian deposits and their structural-stratigraphic features in different structural domains, as a contribute to better define the geodynamic and tectonic implication of the Messinian salinity crisis (MSC) within the Northern Apennines (NA) evolution. In late Tortonian-early Pliocene time three main tectonic pulses caused the uplift of the emerging NA orogenic wedge and the outward migration of its associated basins (foredeep and wedge top basins), while a coeval extensional tectonic regime developed in hinterland areas (Tuscany region and Northern Tyrrhenian sea). The frontal zone of the NA orogenic wedge, thrusting over the west-dipping Adriatic monocline, is imaged in the NE part of the transect. Its thrust front is now buried under a thick succession of Plio-Pleistocene synorogenic deposits in the subsurface of the Po Plain. The Ligurian Units, the uppermost nappe of the tectonic pile, overly Oligo-Miocene foredeep turbidites which are tectonically stacked and accreted to the chain, together with their Mesozoic carbonatic substratum. In the NA foreland basin system two distinctive wedge-top basin types, with different morphostructural setting and Messinian stratigraphy, have been recognised: 1) closed marginal basins, where shallow-water primary evaporites, deposited during the first phase of the MSC, underwent deformation, uplift and erosion during the intra-Messinian tectonic phase (i.e. the Vena del Gesso basin, western Romagna area); 2) deeper and more subsiding basins where organicrich shales and thin limestones, coeval to marginal basins evaporites, were overlain by relatively deep-water resedimented evaporites during the intra-Messinian tectonic phase (i.e. the Cortemaggiore basin, in the Emila area). In deep basinal settings primary evaporites are never found. During the intra-Messinian tectonic phase the main foredeep depocentre shifted toward the NE and the late Tortonian-early Messinian foreland ramp areas were drowned. The axial zone of the chain occurs in the central part of the transect. No Messinian deposits are found here because this area underwent strong uplift and emersion during this time interval. Hinterland areas of the NA chain, characterised by peculiar Messinian deposits, occur in the SW portion of the transect: the Ligurian Basin and extensional basins pertaining to the Northern Tyrrhenian Sea and Tuscany region. The late Oligocene-early Miocene Ligurian Basin syn-rift sequences are highlighted in the Ligurian Basin. The Alpine units are interested by extensional west-dipping listric faults. An undifferentiated Messinian unit, including evaporites, salt and late Messinian deposits, has been recognised. Its deposition is related to a post-rift thermal subsidence regime that took place after the counterclockwise rotation of the Corsica-Sardinia Block. This well-stratified seismic unit shows evidence of deformations due to salt diapirism and gravitational tectonics, driven by shallow low-angle listrict faults that sole out at its base or immediately beneath. Moving NE-ward the intra-Messinian unconformity is sealed by the latest Messinian “Lago Mare” units, as recognized in the Gorgona High. The Northern Tyrrhenian Sea and Tuscany region in upper Tortonian-lower Pliocene time underwent extensional tectonic regime, with the developement of basins bounded by main west-dipping low-angle listric faults. In these basins, actually poor represented in our transect, primary evaporites are preserved under upper Messinian-Pliocene deposits. The Viareggio basin, imaged on this transect, displays evidence of important extensional tectonic regime only in Plio-Pleistocene time. At the base of these syn-rift deposits a probable undifferentiated Messinian unit has been traced. These sediments may represent the onset of the extensional tectonic phase in this area.