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Downstream Grain-Size Coarsening in Proximal Lobe Deposits of the Karoo Basin Compared With Comparable Flume Experiments


Turbidity currents typically deposit lobate sediment bodies on the basin floor which may act as reservoirs for hydrocarbons. Their grain-size trends determine the reservoir quality and are therefore of interest for hydrocarbon production. Generally, lobes are thought to show a dominant downstream fining trend due to deposition of coarser grains in proximal settings as they settle faster. Subsequent depletion of coarser grains and later arrival of finer grains at the bed produce downstream fining. This trend has been described in several studies on deep-marine deposits as well as in flume tank experiments. However, downstream coarsening was documented in proximal lobes in the Karoo Basin, South Africa. The same coarsening pattern was reproduced in flume tank experiments, where it is shown to be related both to the spatial evolution of the current's grain-size stratification and capacity-driven sedimentation. Extensive exposures of deep-marine deposits of Unit E, Fort Brown Formation, Karoo Basin, comprise a zone of bypass followed downstream by an area of abrupt onset of deposition, most likely initiated by a change in slope gradient. The proximal region of the deposit is characterized by pronounced thickening of sandstone from a few m to 40 m over a distance of ca. 400 m. Analysis of thin sections (>100) throughout the deposit show a downstream coarsening trend – associated with increasing sorting – over a distance of 1.2 km. Comparable Shields-scaled experimental turbidity currents were released into a 2D flume that mimics the slope to basin floor transition; flows on the slope are bypassing, and deposition is initiated at the break-of-slope due to a decrease in shear velocity. The deposits show a similar coarsening trend as observed in the sediments in Unit E. Vertical grain-size profiles of the current show a grain-size increase at the flow base with distance downstream of the break-of-slope. Simultaneous capacity-driven sedimentation results in downstream coarsening of the deposits. These results are encouraging for reconstruction of stratification effects within turbidity currents through time and their impact on grain-size partitioning in the deposits. Comparison and integration of natural outcrop deposits with Shields-scaled synthetic deposits allow the reconstruction of flow properties of past turbidity currents. This improves the verification of reservoir characterizations derived by numerical modeling of turbidity currents and related deposits.