The present study aims to recognize and quantify the influence of basin morphology on stratal patterns and facies variabilityof eight turbiditic sandstone lobes with well exposed onlap terminations, in order to unravel the contribution of sediment supply anddepositional processes that respond to external controls on sedimentation. The studied sandstone bodies form the Late Oligocene CengioTurbidite System (CTS) of the central Tertiary Piedmont Basin (Northern Italy). The study combines a physical stratigraphic andsedimentological 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 from5 to 10¡. Turbidite inflows entered the basin from the south and ran parallel to the western gentle branch of the paleoslope, then weredeflected to a more E-W trend along its northern side. Gradual and rapid pinchouts characterize the terminations of the sandstone lobesonto the gentle and the steep slopes respectively. The different flow–slope interactions are interpreted to be responsible for the trend ofaway-from-slope thinning of the sandstone beds and decrease of the NTG and AR ratios observed in sandstone bodies I to VI. An oppositetrend of the same variables was quantified in the uppermost sandstone bodies VII and VIII, and it is interpreted to be a consequence of twoconcurrent intrabasinal and external controls: (1) widening of the depositional area, bounded by gentle slopes, due to basin flooraggradation; 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. Therepeated coupling of a lower well-bedded sandstone–mudstone unit with an overlying amalgamated sandstone bedset has beeninterpreted in two ways. In the lowermost couple (lobes I and II), the segmented plot of cumulative bed-thickness distribution shows athreshold (2 = 134 cm) that is interpreted to correspond to the thickness of beds deposited by flows contained within an area of enclosedbathymetry, too small to permit the development of flow transitions. All of the other lobes show segmented thickness plots, with stepscorresponding to low thicknesses (45 and 90 cm for bodies III–VI and VII–VIII, respectively). In sandstone bodies III–VI, the step separatesthe beds with an aggradational tendency from those deposited by dilute turbulent flows after having undergone grain-size segregationand flow transformations. This is dependent on the magnitude and time duration of the parent flows. A repeated increase of this latterparameter could be recorded by the repetitive development of the thick and amalgamated bedsets in the upper part, or at the tops of thesandstone bodies. A comparable pattern of increasing frequency and time duration of the sustained parent flows is thought to beresponsible of the origin of the thickest and most amalgamated sandstone body VIII.
|Titolo:||Trends in bed thickness and facies trends of turbiditic sandstone bodies : unravelling the effects of basin confinement, depositional processes and modes of sediment supply|
|Settore Scientifico Disciplinare:||Settore GEO/02 - Geologia Stratigrafica e Sedimentologica|
|Data di pubblicazione:||1-dic-2009|
|Tipologia:||Book Part (author)|
|Appare nelle tipologie:||03 - Contributo in volume|