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Textural and Compositional Trends in Transitional Flow Deposits of the Proximal Brushy Canyon Formation, Texas


The Salt Flat Bench of the Brushy Canyon Fm, Texas is a proximal deepwater outcrop in which transitional flow deposits (TFDs) are identified in a lobe fringe setting. Recent description of TFDs from deepwater lobe environments across the globe has fueled interest in understanding spatial flow transformations to better predict the distribution of reservoir quality sandstone facies and therefore maximize recovery in deepwater fields with limited well data, a situation not uncommon for plays like the Wilcox Fm, Gulf of Mexico. Previous studies of TFDs focused on longitudinal flow transformation and characterization of the distal pinchout of fan systems. In this contribution, we show an outcrop example of a lateral lobe pinchout in a proximal basin floor setting, where well-exposed facies transition laterally from clean sandstones to fine-grained, silt-rich deposits. Individual beds were traced between 20 measured stratigraphic sections on a 350m long oblique-strike exposure of the Salt Flat Bench, Upper Brushy Canyon Fm. Three distinct bed types were observed: (1) scour-amalgamated to tabular sandstones without bedcap preservation, (2) tabular sandstones with fine-grained bedcaps, (3) bipartite beds with a thin, clast-charged sand base overlain by a silty, clast-charged ‘cap’. Samples collected from individual beds and bed divisions were disaggregated for laser particle size analysis and thin sections were prepared to examine the textural properties of the different bed types. Results show that over a relatively short distance (<100m), grain sizes within the sand component of beds decreased from medium lower sand in an amalgamated turbidite to fine upper sand in the base of a bipartite TFD. Toward the pinchout, TFDs taper due to rapid sand loss and gradual thickness decrease in the silt-rich bed division. Integrated thin section and LPSA analysis shows the basal sandy portion of a TFD has at least 10% less matrix (<0.063mm) content than the silt-rich upper portion of the bed, which may change across a sharp or gradational transition. The observed vertical textural trends and lateral bed composition variations record the result of flow transformation resulting from rapid flow collapse. Previously unidentified in the Brushy Canyon Fm, the recognition of TFDs provides new depositional context for the Salt Flat Bench and adds critical insight regarding the spatial distribution of reservoir quality turbidites and associated TFDs in a proximal basin floor setting.