New geological map of the Northern Apennine front – Po Plain transition zone (CARG 160 – Pavia sheet): surface-subsurface relationships and tectono-stratigraphic evolution

The activities for the implementation of the CARG 160 – Pavia Sheet resulted in a brand new 1:10.000 scaled geological map of the hillslope sector, located between the towns of Casteggio and Stradella in the Oltrepo Pavese area (Northern Italy). Here, the transition between the outcropping sector of Northern Apennine Front and its foredeep basin, the Po Plain, takes place, within the framework of the central-western flank of the Emilian Arc structures. The study area, extensively exploited for vine cultivation, is characterized by the scattered distribution of outcrops and the pervasive development of weathering profiles and soils, making it very challenging to trace continuous limits between formations. Therefore, the approach used consisted in an accurate description of the few available outcrops, their sampling for petrographic and biostratigraphic analysis and the description of the alteration profiles associated with the lithotypes. Weathering profiles develops on the unaltered rock rarely exceeding 1 or 2 metres of thickness, so the recognition of the distinctive features of alterites associated with substrate units often allows to assess and trace the extent of formations despite their limited outcrop conditions. The sedimentary succession preserved in the area represents a typical active margin sequence. It includes an Upper Cretaceous to Miocene turbiditic succession, characterized by extensive arenaceous and marly deep-marine turbidites of the Val Luretta Formation which tectonically overlies younger units along a south-dipping, north-verging thrust fault with a general WNW–ESE orientation. The thrust is intersected by N–S-trending transfer faults, displacing the structure northward in the eastern part of the study area. North of the thrust fault, the Tortonian to Pliocene sedimentary succession extends across the area with a generally sub-planar attitude. Locally, this sequence is affected by gentle folding systems, with fold axes trending WNW–ESE. Moreover, the facies association indicates a shallowing-upward trend in the sedimentary environment, transitioning from deep to shallow marine conditions. This evolution led to the development of evaporitic sedimentation within restricted basins, followed by the reworking of these deposits by transitional deltaic and, later, continental fluvial systems associated with the Messinian Salinity Crisis. Eventually, shallow marine conditions were reestablished during the Pliocene. This work will be the baseline for future investigation of this sector of the Northern Apennine Front and for the understanding of the temporal and spatial relationship between its outcropping and buried portion. The integration of field and subsurface data will contribute to a more detailed reconstruction of the tectono-stratigraphic evolution of the area with particular focus on possible recent (Pleistocene) tectonic activity related with the debated seismogenic sources mapped close to the study area.