Upper Wilcox Rosita Delta System of South Texas: Growth-Faulted Shelf-Edge Deltas

Abstract

ABSTRACT The Rosita delta system, a heretofore unrecognized, major depositional system preserved in the deep upper Wilcox of south Texas, was delineated by detailed correlation of approximately 500 well logs. The Rosita delta system comprises at least three delta complexes, each of which can be traced up to tens of miles along strike and up to approximately 15 mi (24 km) downdip. Basinward, across the growth-fault zone, each delta complex thickens from about 600 ft (180 m) to more than 3,000 ft (900 m). The growth faults were activated by progradation of deltas over unstable prodelta-slope muds at the contemporary shelf margin. The three upper Wilcox delta complexes studied in detail are, from oldest to youngest, the Duval, Zapata, and Live Oak deltas, named for the counties in which they are centered. Each complex consists of several lobes, some of which can be traced across the deep zones where the thickness increases by as much as tenfold owing to progradation over active growth faults. Characteristic coarsening-upward progradational units are interpreted from electric log patterns to include prodelta shales, delta-front sandstones, distributary channel and channel-mouth bar sandstones, and interdistributary shales and sandstones. Appreciable variability in sandstone distribution in the deltas may reflect changing importance of fluvial versus marine currents in distributing sediment along the delta front. However, all of the deltas prograded abruptly toward the shelf margin. The gulf ward extent of these delta complexes is unknown, and it is concluded that appreciable quantities of sandstone remain to be explored in the deep subsurface of the trend.