Location of Gilmer Shelf margin, Upper Jurassic, East Texas Basin

Regional stratigraphic correlations, based on wireline logs, have defined a distinctive sequence of shales within the Bossier Formation. This lower Bossier shale member, of near constant thickness, extends across the East Texas basin into Louisiana. The overlying upper Bossier shale member thickens markedly to the south and east but northward and westward intertongues with Schuler Formation siliciclastics. Below the lower Bossier shale member a shale-dominated sequence with carbonates exists in some parts of the East Texas basin. This Gilmer shale member thickens to over 500 ft (150 m) in Panola and Harrison Counties and Louisiana, but is thin or absent over the Gilmer shelf margin, where lower Bossier shales rest on Gilmer limestone buildups. Across the center of the East Texas basin the Gilmer shales average 100-150 ft (30-45 m). The continuation of the conformable, uniform lower Bossier shale member across the basin implies some time equivalence of, at the least, the thicker Gilmer shales with the massive undifferentiated Gilmer/Smackover carbonates within the East Texas basin. The Gilmer shelf margin is situated where this carbonate succession passes laterally into eastward-derived shales; the margin is east-facing and east of the basin depocenter, trending through Upshur, Smith, and Rusk Counties. A subsidiary trend in the Gilmer limestone on the northwestern and western flanks of the basin is controlled by the presence of an underlying Buckner carbonate barrier. Only in the northeast of the basin is the Gilmer shelf margin coincident with earlier shelf edges. This reevaluation of the Gilmer shelf margin presents a major target for future hydrocarbon exploration.