Sustained turbidity currents : evidence from the Pliocene Periadriatic foredeep (Cellino Basin, Central Italy)

The aim of this work is to investigate the nature of the numerous, very thick deep-water sheet-sandstones that dominate the lower portion of the Cellino Formation (Central Italy). The studied turbidite system (about 2,500 m thick) represents the Lower Pliocene turbiditic filling of the outer Abruzzo sector of the Periadriatic foredeep. The foredeep was affected by compressional deformation linked to the overall migration of the chain-foredeep system toward the east. Tectonic activity was mostly coeval with the sedimentation and propagated towards the foreland; thrusting became progressively younger from W to E. The Cellino Basin has been intensely explored, being the site of hydrocarbon-bearing sands. Very thick beds can be distinctively resolved in the well logs and correlated to the measured sedimentary sections on outcrop. Based on well log correlation, tens of individual beds up to 23 m thick have been traced along the axis of the basin over distance of at least 150 km and, perpendicularly to the basin, over distance of 30-40 km (Carruba et al., 2006). Palaeocurrent data taken from basal flute structures indicates southerly-directed flows, parallel to the depocentral axis of the basin. The thickest beds show a basin-wide extension, onlapping the basin margins without significant thickness variation. The Cellino Formation is mostly buried and crops out in a narrow belt a few kilometers west of the Cellino gas field. The internal organization of the studied megabeds provides evidence for occurrence of long-lived flows and suggests deposition by gradual aggradation from sustained currents (sustained turbidity current, Kneller & Branney, 1995). Assuming a quasi-steady flow scenario we can explain the nature of the numerous very thick megabeds within the Cellino Fm considering that the determining factors of the thickness of the studied deposits are the confinement of the basin and the rate and the duration of deposition, which may proceed as long as the current maintains a flux of grains towards the site of deposition. The origin of these large-volume turbidity currents and their high rate of occurrence can be related to an interaction of many factors and external controls, which are typical of the ancient foredeep basins (Mutti et al., 2003). The depocenter migration of the Periadratic Foredeep caused the accumulation of thick sedimentary bodies, which developed progressively within the depositional axes. Since the collisional margins were convergent, there was longitudinal migration of depocenters. Our data suggest that the studied megabeds could be originated from catastrophic floods and sediment failures during relative falling- and low-stand stage of sea-level forced by dramatic tectonic uplift of basin margins and turbidity systems already deposited in the northern areas.