ABSTRACT

A subsurface study of the Midway-Wilcox section (Paleocene-Eocene) in Zapata County, south Texas, led to the recognition of several stratigraphic units of possible formation rank, within the Wilcox section. These units which, together with the Midway section, may reach a thickness of 15,000 ft (4.5 km) in extreme eastern Zapata County, were established mainly on the basis of depositional patterns as reflected on electric well logs. Four Wilcox units were identified and named, in ascending order, the Lopeno, the Volpe, the Vela and the Hinnant. Although gas sands are encountered in all four units, the more important ones occur in the Hinnant and Volpe units. Deposition of the Midway-Wilcox section appears to be closely controlled by synchronous structural activity initiated in connection with the formation of the Tertiary Rio Grande embayment. This structural activity caused extensive faulting, great variations in sedimentary thicknesses, and is also responsible for the formation of local and regional unconformities in the study area.