Sandstone provenance and biostratigraphy of Modino Unit stratigraphic succession (Lutetian-Aquitanian): a key stage of the Northern Apennine foreland basin

The Northern Apennine foredeep is one of the most classic examples of deep-water turbidite basin associated to the development of an orogenic wedge. The sediment fill of this basin mainly consists of deep-water sandstone that were progressively incorporated into the frontal part of the orogen during its eastward thrust propagation and depocentre migration. The Eocene-Oligocene succession of the Modino tectonic Unit (Tuscan Domain of the Northern Apennine) is a turbiditic sequence deposited during the early collisional and post-collisional stages of the orogen. The meaning of this unit in the Northern Apennine foredeep-belt system of the is not yet clear. In this work a multidisciplinary approach, consistig of biostratigraphy, high-resolution stratigraphy and provenance analysis, was applied on Modino Unit turbiditic succession in order to understand the main characteristics of its basin and try to give a paleotectonic reconstruction. The nannoplancton analyses indicate that Modino Unit succession ages is comprised between MNP15 (Lutetian) and MNN1d (Aquitanian) biozones. The petrographycal analysis on arenites show a change in sandstones compositions from the basal to the upper part of the Modino Unit succession, underlined by the decreasing of ophiolitic rock fragments, radiolaritic rock fragments and extrabasinal carbonates (from Lutetian to Aquitanian sandstones). The data coming from the detailed stratigraphycal analysis suggests that the lower part of Modino Unit is influenced by sediment failures and submarine landslides from a proximal and intermittent source, that is represented by the Apenninic Prism. We think that the lower part of Modino Unit succession (Lutetian-Rupelian) probably represents a stage of dominant Apenninic sedimentation influenced by an high tectonic activity. Although the presence of ponding structures in the basal part of this unit suggests the confinament of the flows, and these evidences can be interpreted like a stage of piggy-back or wedge-top basin sedimentation for the older part of the succession. While the upper part of the unit, Rupelian-Aquitanian in ages, seems to be less influenced by the “Apenninic source”, and confinament structures are not recognized. A comparision with the literature data suggest that only the upper part of Modino Unit succession can be compared with the Macigno Fm. of Tuscan Nappe. This new data suggests that the turbiditic sedimentation with “Alpine provenance” for the Modino Unit occurs not late than Rupelian (biozone MNP21).