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The Role of Confining Bathymetry Upon Hybrid Flow Development and Emplacement – Insights From the Confined Deepwater Castagnola System, Tertiary Piedmont Basin, NW Italy

Abstract

Mixed turbulent-laminar flows depositing hybrid event beds [HEB] are increasingly recognised as contributors of sand delivery to deep-water settings in a variety of basins. Bathymetry is often considered to be a key factor in triggering flow transformation by encouraging flow clay enrichment either through promoting incision of muddy substrate or through forced deceleration and progressive sand loss due to gradient reduction or reversals. Currently, predictive concepts for the character and distribution of HEB deposits in relation to increasing proximity to confining bathymetry are in their infancy with only a few outcrop examples. Greater understanding of the role of confining bathymetry upon flow transformation would improve prediction of the distribution of HEB mudclast- and or clay-rich facies, typically associated with poor reservoir quality, as well as their onlap character. New research from the confined Castagnola deep-water system of the Tertiary Piedmont Basin (∼10km), NW Italy, comprises high-confidence bed scale correlations along proximal-to- distal transects (5-8km) with increasing proximity towards a downstream confining basin margin. Preliminary results highlight; 1) repeated deflection of gravity currents adjacent to the confining basin margin with the zone of deflection migrating and tracking the base of slope during basin floor aggradation and onlap onto an inclined basin margin; 2) bed thickness often increases at or immediately downstream of the zone of deflection reflecting local flow deceleration, after which beds thin towards onlap onto the basin margin; 3) an apparent lack of systematic variation in deposit facies types and proportions towards the confining margin or at the zone of deflection suggests that bathymetric-forced deceleration had minimal effect upon flow transformation and resultant depositional facies in this case. Comparisons with other studies begin to highlight the likely importance of flow vs. basin size and subsequent implications for flow run-out length scales and scales of flow transformation.