HIGH-RESOLUTION DEPOSITIONAL FRAMEWORK OF THE PALEOCENE MIDDLE WILCOXSTRATA, TEXAS COASTAL-PLAIN

The middle Wilcox subgroup in the Texas coastal plain of the Gulf of M exico basin is consistently defined by maximum flooding surfaces assoc iated 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 correl ation of approximately 700 well logs. The sequence boundaries are iden tified as regionally continuous, low-resistivity markers within thin m arine shale beds and are located stratigraphically between upward-fini ng and upward-coarsening log responses in the platform environment. Ac ross the growth-faulted shelf margin, the sequence boundaries lie with in thick marine 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 reconstruc ting depositional systems. Two high-resolution genetic stratigraphic s equences A and B (in ascending order) are useful map units for facies mapping because they show gradational changes without pronounced relat ive base-level falls, displaying aggradational-dominated stacking styl es. 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 m argins occurred during slumping or relative sea level lowstand periods or when differential subsidence occurred within growth-fault zones. T he middle Wilcox sequence A contains an updip fluvial belt, the Calver t delta system, the Fayette strand-plain system, the San Marcos barrie r-lagoon system, the La Salle delta system, and downdip muddy Shelf-sl ope 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 are as. The La Salle delta system; in sequence B is more extensive than th at in sequence A, and a third deltaic system, named the Wilson delta s ystem, localized in the San Marcos area has replaced the interdeltaic barrier-lagoon system of sequence A.