Quantification of the degree of confinement of a turbidite-filled basin: A statistical approach based on bed thickness distribution

Abstract

Bed thickness distribution within turbidite systems is related to basin geometry and to magnitude and duration of depositional events. Cumulative distribution of turbidite bed thicknesses is often interpreted in terms of a power law. Alternatively, these distributions have been described by a lognormal mixture model. Changes in the power-law exponent can be related to the rheological properties of the gravity-driven flows and to the geometry of the basin. This study attempts to quantify the degree of confinement of well-exposed turbidite units of the Oligocene Cengio and Bric la Croce–Castelnuovo Turbidite Systems (Tertiary Piedmont Basin, Italy). Focus is on the behaviour of bed thickness distribution. The succession is suitable for this study because it represents the infill of a progressively widening basin, during quiescence of intrabasinal tectonics. In this basin the cumulative frequency distribution of turbidite bed thickness follows a segmented power-law relationship. The deviation from the trend observed for the thickest beds is associated with a deficiency of beds in a certain thickness range. This behaviour suggests existence of a thickness threshold that segregates the deposits of the fully confined flows that could deposit thick aggrading beds, with minor downcurrent flow transitions, from the rest of the bed thickness population. This threshold decreases from the lowermost (and most confined) to the uppermost units of the Cengio Turbidite System (CTS). No thresholds were identified for the uppermost Bric la Croce–Castelnuovo Turbidite Systems (BCTS) that spread over a wider basin than the CTS, suggesting unconfined deposition. The BCTS was presumably larger than the area that could be covered by the turbidity currents. When integrated with data on sedimentology and basin morphology (well-log and seismic data), bed thickness statistics can provide general guidance about interpretation of depositional settings, degree of confinement, erosion/amalgamation and bypass of turbidites, improving and assisting the development of reservoir models.