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Shale Tectonics Controlled Depositional History, Eocene Wilcox Group, Central South Texas Coast


Tectonic and stratigraphic architecture of the Wilcox Group along the central coastal plain of Texas (Bee/Goliad region) defines a series of growth fault–controlled subbasins similar to those in the Frio Formation in the Corpus Christi region, South Texas. A seismic sedimentologic study was performed to map higher-order (fourth- and fifth-order) systems tracts in a 2500-km2 3D seismic survey. Guided by third-order sequence- stratigraphic correlations from seismic and sparse core and 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 third-order lowstand subbasins. Recognized depositional systems include (1) incised valleys and relict on-shelf deltas on an exposed shelf; (2) lowstand prograding deltaic systems composed of lobate deltaic sandstone bodies at the shelf edge; and (3) faultcontrolled, off-shelf slope fans best characterized by point-source, fanlike channel/levee systems. Sand-dispersal patterns are controlled primarily by accommodation resulting from rollover topography associated with growth faulting initiated by shale-ridge movement. A sequential display of stratal slices reveals how the depositional systems responded to accommodation controlled by shale tectonics.