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The Morphology of Transgressive Ravinement Surfaces and Syn-Ravinement Deposition: Modern Analogues for Reservoirs: Examples from the Modern Texas Coast

Abstract

Along the northern Gulf of Mexico, direct and proximal hurricane and tropical storm strike are the primary drivers of shoreface ravinement. The high energy conditions that exist during direct and proximal strikes of hurricanes and tropical storms allow for unique, large-scale erosional features to develop during the pre-frontal and frontal passage of the storm and for sand deposits to become stranded during the post-frontal passage. Typical nearshore features resulting from the passage of the storm include the formation gutters, large scour pits and stranded offshore bar-trough systems. However, the ability for these features to remain intact and become preserved in the sedimentological record is not well documented. Using high-resolution side-scan imagery, swath bathymetry, and vibra-cores we show the transition of a featureless seabed before hurricane impact, to one with spatially complex morphodynamic features immediately afterward, to a seabed with little surface evidence of any catastrophic event after a multiple year recovery period. During Hurricane Ike, a large scale bar- trough system developed at an oblique angle to Galveston Island's shoreface on the order of 25 km long with individual bars nearly 1 km wide. This mega bar and trough system is accompanied by multiple scour pit fields, inshore of the bars. These pits are ellipsoidal with the longer axis shore normal and on the order of 100 m with the shorter axis varying from 50-100 m. The pits appear to have asymmetric depths with a maximum depth of 1.5 m. Multiple pit fields were found along the inner shelf of the island. Similar pit fields were found off of Matagorda Peninsula following Hurricane Claudette in 2003. These pits were monitored for one year and were found to have largely filled within 6 months, with little evidence of the pits after one year. The pits off of Galveston Island were still distinct three years after Hurricane Ike, as were the mega bar and trough fields. As transgression continues and accelerates due to enhanced sea level rise, these deposits will be preserved and additional texture will be added to the ravinement surface as additional storms strike. The traditional view of transgressive ravinement being a destructive erosional process that leaves little structure is challenged by the findings of this study. The potential for large sand deposits to be preserved as syn- ravinement deposits offer the potential for reservoirs in previously overlooked areas.