Lower Cretaceous Albian platform mixed carbonate/siliciclastic strata of the Upper Glen Rose D Member in the East Texas Houston Trough intrashelf basin : regional setting, depositional systems, and lithofacies
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In East Texas, a thick section of Upper Glen Rose strata was deposited in a newly defined intrashelf basin behind the Stuart City reef-margin complex. The Houston Trough is a half graben within the larger Brazos Basin. The stratigraphic section in this intrashelf basin forms a complex hydrocarbon system where porous grainstones are redeposited through sediment gravity-flow processes into the basin coevally with organic-rich mudstones. The depositional setting within the Houston Trough appears to have been deeper water and below storm-wave base. The water column was stratified where dysaerobic conditions existed, allowing for organic matter to accumulate and be preserved in low-energy muddy sediment. Grain-rich gravity-flows rich in ooids intraclasts, and mollusks derived from the shallower shelf to the northwest were deposited in the basin and provide the matrix porosity and permeability for the Alabama Ferry and Fort Trinidad fields. The Upper Glen Rose interval is composed of lithofacies that vary widely within the Houston Trough. Five lithofacies are recognized on the basis of sedimentary features, texture, fabric, biota, minerology, and organic-matter content: (1) laminated to non-laminated argillaceous mudstone to wackestone; (2) laminated to non-laminated terrigenous mudstone to siltstone; (3) intraclast–ooid skeletal packstone; (4) intraclast–bivalve–ooid grainstone; and (5) oyster rudstone/wackestone. A majority of allochems, such as ooids, intraclasts, and mollusks within the grain-rich lithofacies, were derived from the shallow-water, higher energy, oxygenated shelf. The intraclast–bivalve–ooid grainstones interbedded with organic-rich argillaceous and calcareous mudstones were previously interpreted as in-situ tidally influenced, high-energy, shallow-water shoals,however reconstruction of the depositional setting and interpretation of the vertical and lateral distribution of lithofacies now suggest they were deposited as grain-rich density flows into the deeper water Houston Trough. This new model can provide a fresh approach to exploration and production of similar juxtaposed mudstones and grainstones throughout the southern East Texas Basin.