Abstract The present study aims to recognize and quantify the influence of basin morphology on stratal patterns and facies variability of eight turbiditic sandstone lobes with well exposed onlap terminations, in order to unravel the contribution of sediment supply and depositional processes that respond to external controls on sedimentation. The studied sandstone bodies form the Late Oligocene Cengio Turbidite System (CTS) of the central Tertiary Piedmont Basin (Northern Italy). The study combines a physical stratigraphic and sedimentological approach with a statistical description, based on five selected variables: sandstone bed thickness, net-to-gross ratio (NTG), amalgamation ratio (AR), graded beds/massive beds ratio, and ratio of the complex bed facies to the sum of all other facies. The CTS depositional setting is a semi-enclosed base-of-slope basin, bounded by a westward-curved paleoslope, with dip ranging from 5 to 10°. Turbidite inflows entered the basin from the south and ran parallel to the western gentle branch of the paleoslope, then were deflected to a more E–W trend along its northern side. Gradual and rapid pinchouts characterize the terminations of the sandstone lobes onto the gentle and the steep slopes respectively. The different flow–slope interactions are interpreted to be responsible for the trend of away-from-slope thinning of the sandstone beds and decrease of the NTG and AR ratios observed in sandstone bodies I to VI. An opposite trend of the same variables was quantified in the uppermost sandstone bodies VII and VIII, and it is interpreted to be a consequence of two concurrent intrabasinal and external controls: (1) widening of the depositional area, bounded by gentle slopes, due to basin-floor aggradation; and (2) increase in frequency of the sand-rich, low-magnitude, long-lived sustained inflows. Basin-floor aggradation during syndepositional tectonic stability determined the flat stacking pattern of the eight tabular bodies. The repeated coupling of a lower well-bedded sandstone–mudstone unit with an overlying amalgamated sandstone bedset has been interpreted in two ways. In the lowermost couple (lobes I and II), the segmented plot of cumulative bed-thickness distribution shows a threshold (η 2 = 134 cm) that is interpreted to correspond to the thickness of beds deposited by flows contained within an area of enclosed bathymetry, too small to permit the development of flow transitions. All of the other lobes show segmented thickness plots, with steps corresponding to low thicknesses (45 and 90 cm for bodies III–VI and VII–VIII, respectively). In sandstone bodies III–VI, the step separates the beds with an aggradational tendency from those deposited by dilute turbulent flows after having undergone grain-size segregation and flow transformations. This is dependent on the magnitude and time duration of the parent flows. A repeated increase of this latter parameter could be recorded by the repetitive development of the thick and amalgamated bedsets in the upper part, or at the tops of the sandstone bodies. A comparable pattern of increasing frequency and time duration of the sustained parent flows is thought to be responsible for the origin of the thickest and most amalgamated sandstone body VIII. External Controls on Deep-Water Depositional Systems SEPM Special Publication No. 92 (CD version), Copyright © 2009 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-200-8, p. 303–321.

Abstract The aim of this work is to investigate the origin of the numerous, very thick deep-water sheet sandstones that dominate the lower portion of the Cellino Formation (central Italy), which represents the Lower Pliocene turbidite fill (about 2,500 m thick) of the outer Abruzzo sector of the Periadriatic foredeep. The Cellino Formation is mostly buried and crops out in a narrow belt a few kilometers west of the Cellino gas field. Very thick beds can be distinctly 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 a distance of at least 150 km and, perpendicularly to the basin, over a distance of 30–40 km. The absence of channelization, the infrequency of bed amalgamation, and the extreme sheet-like geometry of individual beds for long distances indicate a relatively uniform basin-plain setting with a low gradient and the ability of flows to traverse the entire basin from north to south; paleocurrent data taken from basal flute structures indicates south-directed flows, parallel to the depocenter axis of the basin. The internal organization of the studied megabeds provides evidence for occurrence of long-lived waxing and waning flows and suggests deposition by gradual aggradation from sustained turbidity currents. Considering the anomalous thickness of the beds (up to ∼ 20 m), we could assume that current was steady at the timescale of deposition of the constituents of the single depositional intervals (e.g., massive basal division or climbing ripples) for periods of at least several hours. The following features observed in the megabeds of the Cellino Fm. have been argued to be characteristic of sustained currents: (1) turbidite beds of extraordinary volume and thickness, (2) very thick (0.5–6 m) massive basal divisions, (3) very frequent alternations of structureless and laminated intervals interpreted as subtle internal scour surfaces, (4) thick massive mudstone caps (1–10 m) that terminate the vertical organization of the sedimentary structures, (5) crudely developed normally graded grain-size profiles, (6) abundant organic matter, and (7) extensive water-escape features. The occurrence of these large-volume turbidity currents, with volumes of the order of a few tens of cubic kilometers (10–80 km 3 ), can be explained as the result of submarine slope failures. The frequency distribution of bed thickness in the correlated interval follows a segmented power-law size distribution, indicating that sediment deposition was dominated by a small number of large-volume events. This distribution can be linked to earthquake triggers, with comparably distributed magnitudes and consequently emplacement of megabeds can be directly related to seismic activity. The interaction of other external controls may have contributed to the origin of these large-volume turbidity currents. Periods of low sea level are traditionally held to be particularly dynamic in terms of turbidite deposition because a significant portion of continental shelves are exposed as subaerial coastal plains at such times. This allows rivers to dump their sediment loads closer to the shelf edge, and hence in potentially more unstable conditions. Sandy, basinal turbidites could originate from sediment failures during relative falling stage and lowstand stage of sea level forced by dramatic uplift of basin margins. In this stage, the tectonically active Cellino basin margins would have reached their greatest instability because their elevation was at a maximum and their distance from the shoreline at a minimum.