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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 (η2 = 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.

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