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Seismic Sedimentology of Incised Valley, Lowstand Delta, and Slope Fan Systems in the Eocene Wilcox Group, Central South Texas Coast


Tectonic and stratigraphic architecture of the Wilcox Group along the central coastal plain of Texas defines a series of growth-fault-controlled subbasins similar to those in the Frio Formation in South Texas. A seismic sedimentologic study was performed to map fourth- and fifth-order systems tracts in a 2,500-km2 3D seismic survey in Bee and Goliad Counties. Guided by third-order sequence-stratigraphic correlations from seismic and sparse wireline-log data, we prepared stratal-slice maps to reveal high-resolution (10-m scale) sediment-dispersal patterns and associated systems tracts in a relative geologic-time domain, particularly in lowstand subbasins. The integration of core, wireline log, and seismic-geomorphologic patterns in stratal slices makes it possible to identify three types of depositional systems in a typical third-order lowstand systems tract: (1) incised valleys and associated fluvial systems on an exposed shelf; (2) lowstand prograding deltaic systems composed of strike-oriented and lobate deltaic sandstone bodies at the shelf edge; and (3) fault-controlled, off-shelf slope fans best characterized by point-sources, fan-like channel/levee systems or line-sourced, complex mudslide/gully systems. Sand-dispersal patterns are controlled primarily by accommodation resulting from rollover topography associated with growth faulting. Sandstone thickness and dispersal patterns can be predicted by integrating wireline-log measurements and seismic amplitude patterns. The high-quality, large 3D seismic data set provides a rare opportunity to visually inspect spatial relationships between incised valleys, self-edge deltas, and slope fans in a high-resolution stratigraphic framework. A sequential display of stratal slices reveals how the depositional systems respond to accommodation controlled by gravity tectonics.