Prograding muddy shelves in the Paleogene Wilcox deltas, south Texas Gulf Coast

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Core facies analysis integrated with subsurface well-log correlations and seismic horizon mapping document two thick mudstone-dominated deltaic successions in the Wilcox Group along the south Texas Gulf Coast. Meters to tens of meter-thick upper slope to outer shelf mudstones form units that lie at the base of upward-coarsening successions which are interpreted as fluvial or storm/wave dominated shelf deltas. The muddy character of the deposits is observed in cores and inferred from well logs, whereas the larger depositional architecture is interpreted from well log correlations and seismic data.Physical sedimentary structures described from a lower Wilcox core reflect deposition on a mud dominated prodelta controlled by hyperpycnal processes. Normally and inversely graded beds are present indicating sustained flows that waxed and waned. Upward-coarsening facies successions contain current ripples, organic matter, and siderite cemented bands, low trace fossil abundance and low diversity, which suggest deposition in a fluvial prodelta to shelf environment. Centimeter-thick interbeds of normal and inversely graded mudstones, siltstones and very fine-grained sandstones with low intensities of bioturbation, reflect the high physical and chemical stresses of hyperpycnal environments. Heterolithic stratification with lenticular, wavy and flaser bedding indicates some tidal influence.On the other hand cores of the middle Wilcox reflect deposition on a muddy wave-storm dominated shelf. Bioturbation intensities tend to be high, with unstressed, fully marine ichnological suites. Pervasively bioturbated sandy mudstones and muddy sandstones with Cruziana ichnofacies, wavy parallel and low angle lamination, and structureless sandstones with Ophiomorpha record deposition in a wave-influenced prodelta to shelf environment. Hyperpycnal flows associated with river floods might have transported mud across the shelf to the outer shelf where prevailing wave and storm conditions facilitated suspension and transport of mud beyond the shelf edge.Previous studies have shown that Wilcox delta sandstones are largely trapped on the shelf by contemporaneous growth faults. However, this paper illustrates that at times the Wilcox shelf margin prograded through mud accretion.