The syntectonic evolution of foredeep turbidites related to basin segmentation: Facies response to the increase in tectonic confinement (Marnoso-arenacea Formation, Miocene, Northern Apennines, Italy)

The Marnoso-arenacea Formation (MAF) records the progressive closure of the foredeep due to the north-eastward propagation of thrust fronts. The latter produced a complex foredeep characterized by the uplift of the basin inner portions and by the north-eastward shifting of the main depocentres. This work discusses the evolution of the MAF inner basin and the first deposits of the outer basin in a transect perpendicular to the paleocurrents and the main tectonic structures that are NW-SE-oriented. Specifically, this study focuses on the upper Serravallian transitional stage, which records the closure of the inner basin and the deposition shifting to an outer basin due to the growth of an important regional structure represented by the NW-SE-oriented M. Castellaccio thrust-related fold (CTRF). The growth of this structure produces progressive segmentation of the foredeep through the progressive isolation of the inner basin that becomes, in the upper Serravallian phase of basin narrowing, a ponded piggyback basin. During this evolution, the structural control and associated morphologic confinement increased over time, controlling the lateral and vertical distribution of turbidite facies that is very similar to that of intra-slope minibasins. In such a situation, vertical and lateral facies changes, associated with evident stratigraphic pinchings and onlap relationship, have played a key role in showing this phase of strong foredeep segmentation and basin confinement. This phase is marked by a well-determined vertical facies variation: contained-reflected beds pass upward into huge accumulations of sandstone lobes featuring the appearance of Ophiomorpha trace fossils and coarse-grained massive sandstone beds often overlain by mud-draped scours and tractive structures (megaripples and traction carpets). These beds are interpreted as related to flow decelerations induced by structurally-controlled basin confinement that favour the deposition of massive sandstones through hydraulic jumps and flow decoupling with bypass of upper turbulent flows that can produce well-developed tractive structures.