Late Paleocene paleoenvironmental gradients in Wilcox Group strata, Big Brown Mine, Texas

Abstract

The Wilcox Group in Texas has been recognized as an important oil and gas reservoir for much of this century, and for the last 30 years has been commercially mined for its extensive shallow lignitic coals. More recently, exploration for coalbed methane in the Gulf Coast has focused in this area. For this new play to become successful, more information on the stratigraphy and depositional environments of the Wilcox Group is needed. The Wilcox Group in the Gulf Coast has been interpreted as primarily fluvial-deltaic in origin; however, since the 1980s, much evidence for tidal control on the deposition of the upper Wilcox (Calvert Bluff Formation) has been compiled. New data from the examination of sediments and fossils contained in the upper Wilcox Calvert Bluff Formation in and near the Texas Utilities' Big Brown Mine confirm that significant marine influence was present at the time of deposition. Most fine-grained sediments contain flaser, rippled, and laminated bedding, and palynological assemblages from non-coal sediments commonly contain Paleocene-Eocene marine dinoflagellates as well as brackish-water indicator palynomorphs. Sand deposits also contain marine dinoflagellates and are in gradational contact with underlying fine-grained sediments. Channel-fill deposits present in the mine area are similar to known examples of tidal flat and estuarine channels. These features indicate that brackish to marine conditions existed during much of the deposition of the upper Wilcox Group in northeastern Texas. The presence of non-fluvial-deltaic deposition in the upper Wilcox Group has implications for the location and extent of the coals, which will aid in the location of coalbed methane reservoirs.