SEDIMENTOLOGY AND STRATIGRAPHIC ARCHITECTURE OF COARSE-GRAINED SLOPE CANYON FILLS: MONTE DELL’ASCENSIONE AND CASTIGNANO SYSTEMS (PLIOCENE, CENTRAL ITALY)

The Apennines imbricate fold-thrust belt and associated foredeep system have been developing since the Neogene and propagated progressively toward east in relation to the eastward retreat of the subducting Adriatic plate. The Periadriatic basin, the central portion of the Apennines foredeep system, is oriented almost northwest-southeast and its infill consists of Plio-Pleistocene sediments partially incorporated into the frontal part of the orogenic wedge. During the Pliocene, gravel and sand originated from the uplifting Apennines were abundantly supplied from the west to the deepwater basin through a series of transverse to longitudinal erosional conduits. In the rock record these conduits appear as a well-exposed series of slope canyons separated by thick and extensive hemipelagic intervals. The coarse-grained, deep-water strata of the Monte dell’Ascensione canyon system and those of the younger Castignano canyon system accumulated within a narrow piggy-back basin bounded by the Sibillini Structure to the west and the buried, actively deforming Ortezzano-Bellante Structure to the east. The trends of canyon margins, gross sedimentation patterns, and palaeocurrent measurements from the canyon fills all converge to indicate a northerly-directed dispersal pattern, which is roughly parallel with regional tectonic trends. This suggests that the sea-floor expression of the developing Ortezzano-Bellante Structure strongly influenced the downslope transport direction of gravity currents and was sufficient to cause major diversion of canyon orientation. A detailed facies analysis suggests that the infill of these slope canyons comprises several channel-levee complexes and reveals that most of the sediments were deposited by a wide spectrum of subaqueous sediment gravity flows, including cohesive debris flows and high- and low-density turbidity currents. In the present study the architectural elements associated with the channel-levee complexes are documented and depositional models to account for the observed sediment distribution and its relationship to tectonic deformation are developed. Architectural elements include channelized conglomerates that are locally flanked by thinly interbedded sandstones and mudstones derived from gravity flows escaping channel confinement (internal levees) and grade downstream into thick, channelized turbiditic sandstones. Each of these channel-levee complexes is overlain by a laterally extensive, mud-prone mass-transport complex, resulting into a well-developed cyclic arrangement of the canyon fill.