--> New Insights on the Variability of Submarine Lobe Deposits

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New Insights on the Variability of Submarine Lobe Deposits

Abstract

Submarine lobes are high aspect-ratio, sand-rich deposits fed by sediment gravity flows via channels. Basin-floor lobes, which form at the distal terminus of the system, have been a focus for recent studies. However, variability in lobe morphology in other geographical positions (e.g. intraslope), and the influence of subtle and dynamic topography and stratigraphic position on submarine lobe geometry, stacking patterns, and sedimentary facies, have received less attention. Multi-kilometer-scale outcrops and near-outcrop research boreholes from paleogeographically well-constrained fine-grained submarine lobes in the Karoo Basin (South Africa) allow variability in sedimentary facies and architecture to be constrained. The dataset includes 170 outcrop logs (~6.9 km) and 11 core logs (~ 1 km) from Units A, D/E and E, Laingsburg depocenter, and Fan 4, Tanqua depocenter. Integration of these data enables the recognition and analysis of different lobe types in the context of a physical hierarchy; this has enabled detailed facies distribution trends, stacking patterns and depositional models to be determined. Three main results have arisen from this research. 1) Establishment of recognition criteria for three distinctive lobe fringe settings: frontal, lateral and aggradational lobe fringes. Typically, frontal fringes contain high proportions of hybrid beds and transition from thick-bedded sandstones over length-scales of 1 to 2 km. In contrast, lateral fringes tend to be current ripple-laminated and transition to thick-bedded sandstones in the lobe axis over several kilometers. Observed differences between frontal and lateral lobe fringe deposits are controlled by flow processes. Aggradational lobe fringes form in response to subtle confinement by intrabasinal slopes through differential confinement of grain-size stratified flows. 2) Hybrid-bed distribution indicates strong geographic but weak stratigraphic trends at different hierarchical scales. Flow transformation processes and lobe stacking patterns are shown to be major controls on these trends. 3) The development and comparison of depositional models for basin-floor and intraslope lobe deposits that show distinct differences in facies, dimensions, sand percentage and architecture. Constraining the variability of submarine lobe deposits is important for understanding heterogeneity distribution during reservoir modelling and decreases uncertainty during hydrocarbon field appraisal and development.