The middle Wilcox subgroup in the Texas coastal plain of the Gulf of Mexico basin is consistently defined by maximum flooding surfaces associated with the Big shale at the bottom and the Yoakum shale at the top, dated at 56.5 Ma and 54.3-55.0 Ma, respectively. Two high-resolution genetic stratigraphic sequences of the middle Wilcox were delineated within the interval time span of 1.5-2.2 m.y. based on detailed correlation of approximately 700 well logs. The sequence boundaries are identified as regionally continuous, low-resistivity markers within thin marine shale beds and are located stratigraphically between upward-fining and upward-coarsening log responses in the platform environment. Across the growth-faulted shelf margin, the sequence boundaries lie within thick mar ne shale wedges of the expanded downthrown sections.

Each middle Wilcox sequence has an average duration of about 0.75-1.1 m.y. and provides a high-resolution stratigraphic framework for reconstructing depositional systems. Two high-resolution genetic stratigraphic sequences A and B (in ascending order) are useful map units for facies mapping because they show gradational changes without pronounced relative base-level falls, displaying aggradational-dominated stacking styles. The interval-isopach and net-sandstone distributions of sequences A and B show no major sediment supply shift along strike during middle Wilcox deposition. Downward shifting of deposition beyond the shelf margins occurred during slumping or relative sea level lowstand periods or when differential subsidence occurred within growth-fault zones. The middle Wilcox sequ nce A contains an updip fluvial belt, the Calvert delta system, the Fayette strand-plain system, the San Marcos barrier-lagoon system, the La Salle delta system, and downdip muddy shelf-slope systems. The middle Wilcox sequence B resembles sequence A in that it includes an updip fluvial belt, the Calvert delta system, and the La Salle delta system. However, sequence B depocenters have prograded seaward from sequence A locations in the San Marcos and Rio Grande areas. The La Salle delta system in sequence B is more extensive than that in sequence A, and a third deltaic system, named the Wilson delta system, localized in the San Marcos area has replaced the interdeltaic barrier-lagoon system of sequence A.