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1 ISMAR-CNR, Sezione di Bologna, Via Gobetti 101, 40129 Bologna, Italy andrea.argnani{at}ismar.cnr.it
2 Dipartimento di Scienze della Terra, Università di Modena-Reggio Emilia, Largo S. Eufemia 19, 41100 Modena, Italy
3 Dipartimento di Geologia, Paleontologia e Geofisica, Università di Padova, Via Giotto 1, 35137 Padova, Italy
4 Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Via Zamboni 67, 40127 Bologna, Italy
The Upper Cretaceous Helminthoid Flysch (HF) of the Northern Apennines consists of thick and regionally widespread deep-water carbonate turbidite successions, deposited during the initial stages of Alpine collision. The HF spans the time from Turonian to Early Eocene and is mainly composed of intrabasinal carbonate ooze mixed with clay; siliciclastic terrigenous beds are also present, but they are a volumetrically minor component of the successions. Petrographic and sedimentological signatures indicate that the HF was deposited in distinct basins located below the carbonate compensation depth. Bulk composition and heavy minerals of terrigenous beds indicate provenance from different crustal levels of the European and Adria plates. The petrographic and palaeobathymetric characteristics of these turbidites indicate the coexistence of an active-margin tectonic setting, a palaeogeographical position suitable for carbonate ooze production and storage, and limited supply of terrigenous detritus into the basin. Palaeotectonic reconstructions and stratigraphic data suggest that Adria represented a vast repository of penecon-temporaneous carbonate mud; the presumably intense seismic activity related to the pre-collisional Alpine orogeny promoted large-scale failures of shelf and/or slope biogenic muddy sediments, resulting in the deposition of a large volume of carbonate turbidites. Only occasionally, turbidity currents probably linked to exceptional fluvial floods generated pure terrigenous beds with different petrographic signatures for each HF succession.
