ORGANIC PETROGRAPHY OF HOOPER FORMATION COALS, CENTRAL TEXAS, USA

Upper Wilcox Group sub-bituminous coals are well known and exploited for steam coal, as coalbed methane reservoirs, and as aquifers across Texas. Less well known are the lower Wilcox Group “Hooper” coals. The “Hooper”has been used as a member or formation name in the central Texas region since it was proposed in the 1950’s and is a succession of grey mudstones, sandstones, and coal; the clastic rocks tend to weather into friable brown exposures except where ironstone concretions occur, and the basal portion may be locally glauconitic. While not formally designated, the original type locality for the Hooper Formation occurs in McKinney Roughs Nature Park along the Colorado River northwest of Bastrop, TX. In this region, the Hooper overlies the Caldwell Knob and is overlain by the Simsboro Sand. A single sub-bituminous coal seam occurs in the upper member, overlying claystones that have been interpreted as lagoonal and in turn is overlain by fluvial sandstones. In the study area within McKinney Roughs, the ca. 2 m-thick coal contains two splits: the lower being an ashfall deposit which preserves fragmentary fossil leaves and is of varying thickness, likely representing redistribution by moving water during accumulation; the upper split is a decimeter-scale siliciclastic parting. The coal above the upper parting is better termed a carbonaceous mudstone. New petrographic data are presented in the context of paleoecological investigations underway near the type locality.