Palaeogeography of the Upper Cretaceous-Eocene carbonate turbidites of the Northern Apennines from provenance studies

The Upper Cretaceous Helminthoid Flysch (HF) of the northern Apennines are thick and regionally widespread deep-water carbonate turbidite successions, deposited during the initial stages of Alpine collision. The HF span from Campanian to Early Eocene in age and are mainly composed of intrabasinal carbonate ooze mixed with clay; siliciclastic terrigenous beds are also present, but they are a volumetrically minor component of the succesion. Petrographic signatures indicate that HF were deposited in distinct basins located below the carbonate compensation depth, probably corresponded to confined sectors of oceanic floor bounded by transform fault walls. Multiple sources of detritus controlled the basin-fill of the HF, in particular: a) major intrabasinal sources from outer shelf/slope areas and pelagic plateaus which supplied coeval carbonate, mainly fine-grained sediment of biogenic origin; b) minor siliciclastic terrigenous sources from hinterland rock units (crystalline basement plus minor sedimentary covers). Bulk composition and heavy minerals of terrigenous mode indicate provenance from different crustal levels of the European and Adria plates. These data allow to place the Helminthoid Flysch units within a Late Cretaceous palaeogeographic reconstruction of the Northern Apennine Tethys. The depositional characters of these turbidites indicate the coexistence of an active tectonic setting, a palaeogeographic location prone to carbonate ooze production and storage, and limited terrigenous sources outside the basin. Palaeotectonic reconstructions and stratigraphic data suggest that Adria represented a vast repository of penecontemporaneous carbonate mud; the presumably intense seismic activity related to the pre-collisional Alpine orogeny promoted large-scale failures of shelf/slope biogenic muddy sediments, resulting in the deposition of large volume of carbonate turbidites. Only occasionally, terrigenous turbidite currents, probably linked to exceptional fluvial floods, generated pure terrigenous beds with different petrographic signatures. The small amounts of extrabasinal siliciclastic materials entering the basin, compared to the dominant intrabasinal carbonates, indicates a limited availability of sediment sources outside the basin.