The Brazos basin : deep basement structure and sedimentary fill, central east Texas

Abstract

Detailed mapping of central eastern Texas using deep well log and seismic data indicate that the region is underlain by a thickened sedimentary section, referred to here as the Brazos basin. Recognition of this basin indicates that the East Texas region, which has traditionally been divided into two geologic provinces, is best described using a three basin model. The basins include, from north to south, the East Texas basin, Brazos basin, and Houston embayment. Basement structures which separate the basins include the Houston arch and the Angelina-Grimes terrace. The Houston arch is a present day structural feature which separates the East Texas and Brazos basin. Salt is absent, and Late Jurassic and Lower Cretaceous strata thin across the arch. The Angelina-Grimes terrace separates the Brazos basin and Houston embayment. The terrace is expressed as a flattening of regional dip in Lower Cretaceous and Upper Jurassic strata. However, prior to the Tertiary it was a northward-dipping paleo-monocline. Slat is interpreted to be absent, and Jurassic and Cretaceous strata thin across the terrace. The Brazos basin appears to have formed as a large, complex half-graben between two transfer faults in association with Late Triassic through Middle Jurassic rifting that opened the Gulf of Mexico. The basin trends northeast-southwest, and is approximately 120 miles long and 50 miles wide. It existed as a unique structural unit from its inception until the end of the Early Cretaceous, accumulating 3,000 to 4,000 feet of Louann Salt, and over 20,000 feet of post-rift sediments. Initial subsidence within the basin was rapid, and gradually diminished with time. By the end of the Early Cretaceous, differential subsidence within the basin had diminished to the point that it ceased to exist as a unique structural unit. During the Tertiary, the Angelina-Grimes terrace subsided and reversed the former northwest dip along the southeast flank of the Brazos basin. Present day structure across the Brazos basin is characterized by monoclinal southeast dip, the existence of the Brazos basin indicated only by the stratigraphic thickening of Jurassic and Lower Cretaceous strata.