Well logs from the deep Wilcox trend of south Texas delineate the upper Wilcox Rosita delta system which comprises three locally developed delta complexes comparable in scale to deltas in the lower Wilcox Rockdale delta system of middle and upper Texas Gulf Coast.

Each delta comprises several lobes, some of which can be traced across the deep zone where they thicken as much as tenfold, owing to progradation over a series of active growth faults. Characteristic upward-coarsening progradational units are interpreted to include prodelta, delta-front, distributary-channel, and interdistributary facies. There is an overall change from delta-plain to prodelta facies basinward across the growth-fault zone.

Growth faults are large-scale slumps formed by basinward gravity gliding of huge masses of deltaic sediments. Faulting was initiated and maintained as sand and mud of the delta front prograded over previously deposited prodelta mud. The growth faults influenced the thickness of sediment which accumulated on each side of the fault by controlling the relative rates of subsidence (growth ratio). Constant growth ratios for a series of consecutive progradational sequences indicate that rates of subsidence were insensitive to local fluctuations in sedimentation. Log patterns suggest that sedimentary processes were continuous across fault traces; no important surface expression of faults is evident.

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