Stratigraphy of Jackson Group (Eocene), South-Central Texas

Discovery of uranium and renewed exploratory activity for oil and gas in the outcrop area of the Jackson group of Eocene age in south-central Texas require setting up a standard nomenclature for rocks of the group in that area. In general, the names which Ellisor proposed in 1933 have been used, but modified to agree with current usage by most geologists. The Jackson group, as proposed, consists of four formations, in descending order: Whitsett, McElroy, Wellborn sandstone, and Caddell. The Whitsett formation, comprising the Fashing clay, Calliham sandstone, Dubose, and Stones Switch sandstone members, consists chiefly of bentonitic clays and tuff near the top and sandstones near the base. The McElroy formation consists of the Conquista clay (a new name substituted for the pre-empted name Falls City shale of Ellisor, 1933), the Dilworth sandstone, and the Manning clay members. The Wellborn sandstone and the clayey Caddell formation are indivisible into members in south-central Texas.