Influence of basin physiography upon the character and distribution of hybrid event beds

It is increasingly recognized that sediment gravity flow deposits often defy classical turbidite-debrite models and are wide ranging in character (e.g., slurry beds, hybrid event beds and transitional flow deposits). Such deposits, here collectively referred to as hybrid event beds (HEBs) are of importance as they cause heterogeneities in reservoir quality distribution from intra-bed to system scales. HEBs are inferred to record deposition beneath flow characterised by discrete internal rheological zones, ranging from turbulent to laminar, whose relative proportions within the flow evolved both spatially and temporally. The high clay concentration in these deposits could reflect preferential deposition of the coarser sand fraction, and hence a relative clay enrichment, and/or an actual enrichment through entrainment of muddy substrate into the flow. In topographically-influenced systems it has been suggests HEBs may indicate proximity to confining slopes due to their localised occurrence and systematic depositional variation towards such features. New research from a range of systems developed within basins of differing physiography (e.g., unconfined, confined and contained [ponded]) has begun to highlight contrasts in the depositional character and distribution of HEBs between these settings. Specifically; 1) the relative degree of flow transformation undergone by the parent hybrid flow, as expressed by the degree of textural and compositional segregation within HEBs, and how argillaceous (clay-rich) their deposits are; 2) the degree of lateral variation in HEB depositional character over short (10s – 100s metres) and longer (100s – 1000s metres) length scales, and whether such variation is systematic or not; 3) whether HEBs are localized to confining sea-floor topography and exhibit depositional trends towards such features; 4) the range of potential stratigraphic stacking patterns developed in these systems. Characteristics of HEBs in different system types are thought to reflect variations in the initial flow character, substrate entrainment, flow confinement, flow containment and flow run-out distance achieved and their influences upon the processes that promote flow evolution and transformation. This work highlights the range of boundary conditions to be considered when attempting to predict the spatial occurrence and depositional character of these non-classical deposits, and thus reservoir quality distribution, within deep-water systems.