A subsurface study of the Midway-Wilcox (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 may reach a thickness of 15,000 ft (4,570 m) in extreme eastern Zapata County, were established mainly on the basis of depositional patterns as reflected on electric well logs. Four Wilcox units have been identified and named, in ascending order, the Lopeno, Volpe, Vela, and Hinnant. Although gas-bearing sandstones are present in all four units, the more important ones are in the Hinnant and Volpe. Deposition of the Midway-Wilcox appears to have been closely controlled by synchronous structural activity initiated in connection with th formation of the Tertiary Rio Grande embayment. This structural activity caused extensive faulting and great variations in sedimentary thicknesses, and is also responsible for the formation of local and regional unconformities in the study area.

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