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Depositional Trends in the Paleogene Lower Wilcox Guadalupe Delta, South Texas Gulf Coast


The Wilcox Group in Texas is a thick sequence of clastic sediments deposited along the Gulf Coast during early Paleogene time. This study integrates core facies analysis with subsurface well-log correlation to document the sedimentology and stratigraphy of the Lower Wilcox Guadalupe Delta. Previous investigations interpreted these sediments as fluvial-dominated distributary channel and delta front deposits. However, detailed core descriptions show that bedding type, sedimentary and biogenic structures reflect a transition from fluvial- to tidal-dominated deposition. Current ripples, organic matter, low trace fossil abundance and diversity, synsedimentary microfaulting and soft sediment deformation structures suggest deposition in a prodelta to distal fluvial delta front environment. Upward-coarsening sequences in the upper part of the succession contain lenticular, wavy and flaser bedding. Sand streaks in mud, wave-ripple lamination and cross beds with mud or carbonaceous drapes are common. Bands or concretions of siderite typically occur in mudstones. The thin interbedded sandstones (less than 1 cm) are massive to parallel-laminated, whereas slightly thicker sandstones are both wave- and current-rippled. Trace fossils are represented by Cruziana ichnofacies with Planolites and Paleophycus being the most common. Low diversity, low density trace fossil suites that contain both dwelling and feeding structures suggest shallow brackish water. Tidal channels truncate upward-coarsening delta front deposits. Upward-fining successions of fine-grained sandstone have abrupt, erosional bases with large sideritized mudstone or carbonaceous clasts and wood clasts/debris. This transition from distal delta front to shallow brackish water implies relative shallowing due to deltaic progradation accompanied by a change in depositional style from fluvial to tidal deposition. Growth faults within the lower Wilcox section complicate the stratigraphy, controlling increased thickness of sedimentary units on the downdip sides of faults. Thick sandstone successions within rollover anticlines in these downthrown compartments represent potential hydrocarbon reservoirs. As a consequence of the tidal re-interpretation of these deposits, sandstone-body heterogeneity and the potential for increased reservoir compartmentalization are expected.