The Corsica–Sardinia Massif as source area for the early northern Apennines foredeep system: evidence from debris flows in the "Macigno costiero" (Late Oligocene, Italy)
Abstract. Large isolated gravity flows (debrites) are widely present in the stratigraphic record of the northern Apennines foreland-basin system. These strata may be useful for provenance signals and dispersal pathways during foreland evolution. This paper examines a cohesive debris flow bed interbedded with turbidite strata of the Macigno Formation (Late Oligocene, Tuscany, Italy), in order to obtain new data on the provenance of the clastic material. Clasts in the debris flow are predominantly plutonic (granodiorite, tonalite, and S-granite) and subordinately metamorphic (gneiss and schist) and sedimentary calcareous clasts. The composition of the clasts within the debris flow is similar to the clastic composition of the interbedded turbidite sandstones of the "Macigno costiero." The depositional features of the debris flow suggest that it traveled for a short distance within the basin before it was deposited not far from the slope. The absence of a high-pressure/low-temperature (HP/LT) paragenesis in the plutonic and metamorphic clasts of the debris flow indicates a provenance from a crystalline basement not involved in the high-pressure phases of the Alpine Orogenesis. Previous studies have indicated the Central-Western Alps as potential source areas for the Macigno Formation sediments. The lack of HP/LT metamorphic signatures in our studied samples excludes the Pennidic and Austroalpine nappes of the Western Alps as possible sources for the debris flows of the "Macigno costiero." These new data (sedimentological, petrographical, and microstructural) suggest that the Corsica-Sardinia Hercynian basement, lacking a HP/LT paragenesis, is the more accredited source area of the debris flow and of the related turbidite sandstones of the "Macigno costiero" succession. These foredeep-feeding sediments were probably before deposited within an episutural basin developed close to the northern Apennines orogenic wedge.