--> Muddy Shorelines of the Paleogene Wilcox Deltas, South Texas Gulf Coast

AAPG ACE 2018

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Muddy Shorelines of the Paleogene Wilcox Deltas, South Texas Gulf Coast

Abstract

Contrary to the general belief that Wilcox shelf margin deltas catch most of the sandstone in growth faulted depocenters on the outer shelf to upper slope this paper shows that at times the shelf margin prograded through mud accretion. 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.

Fine-grained, low-density turbidites of the Lower Wilcox were generated by low-magnitude floods, but with a sufficient discharge to create hyperpycnal flows. The preponderance of thin-bedded and laminated heterolithic strata is typical of muddy distal shelf settings. Salinity fluctuations related to freshet conditions and river flood stages, high sedimentation rates, episodic heightened water turbidity due to hyperpycnal river plumes lead to general reductions in bioturbation intensities. The presence of large outer shelf to shelf-edge incisions in the lower Wilcox might had been initiated and sustained by hyperpycnal flows going off the shelf.

On the wave-dominated muddy shelves of the Middle Wilcox bioturbation intensities tended to be high, with unstressed, fully marine ichnological suites. Pervasively bioturbated sandy mudstones and muddy sandstones with Cruziana ichnofacies and structureless sandstones with Ophiomorpha record deposition in a wave-influenced distal prodelta to outer shelf environment. Amplitude maps show a strike-elongated geometry suggesting increased wave reworking of the shoreline.