Upper Wilcox deposits at Katy field in southeastern Texas are composed of terrigenous clastic facies. These rocks are the uppermost beds of the Wilcox Formation (late Paleocene to early Eocene), which crops out in central Texas and extends basinward to a known total subsurface thickness of 9,100 ft at Katy field. The upper Wilcox is 1,800 ft thick at Katy; however, only the upper 800 ft were included in this study. Analyses of the regional stratigraphy, structural trends, and paleobathymetric relations indicate that beds were deposited by high-constructive and destructional delta systems marginal to the subsiding Gulf of Mexico basin; resultant deltaic and interdeltaic facies ultimately were transgressed by open-shelf environments.

The depositional interpretation in vertical sequence begins with prodelta silty clay at the base and coarsens upward to very fine and fine-grained sands in a typical progradational delta-front sequence. Progradation was repeated after subsidence, yielding a second series of delta-front and overlying fluvial facies. The next vertical sequence consists of thinly laminated and burrowed silty clay and thin sand beds which locally are slumped and microfaulted, and sand units containing shale clasts. These thin units represent a period of minor sediment influx and consist mainly of interdeltaic marsh to offshore silty clay and destructional delta-front sand; local distributary channels cut into the bay-marsh transition facies. The third vertical sequence is largely offshore silty clay, thin glauconite beds, and thin silty sand units which transgressed the area as a result of increased subsidence. Local relict shoreline sand beds were deposited during temporary stillstands as the shoreline shifted landward.

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