Uranium was discovered near Tordilla Hill in Karnes County, south-central Texas, in the fall of 1954, in the upper part of the Jackson formation of late Eocene age. By July 1955, 14 uranium prospects were reported as far northeast as Fayette County, northeast of Karnes County, and southwest to Duval County, south Texas, over a distance of about 190 miles. Uranium minerals or radioactive rocks have been found in tuffaceous sand, silt, or bentonitic clay in at least seven different stratigraphic positions ranging in age from late Eocene to Pliocene. The known radioactive materials occur in the Goliad sand of Pliocene age, the Oakville sandstone of Miocene age, and the Catahoula tuff of Miocene (?) age in Duval County; in theOakville sandstone and the upper 500 feet of the Jackson formation in Karnes County; and in the Catahoula tuff in Gonzales County. This paper presents the results of a preliminary investigation of the stratigraphy of the uranium-bearing formations through Karnes County and the adjoining counties to the southwest and of the correlation of the potentially uranium-producing beds in drill holes and surface exposures. During this study the following contacts were mapped in reconnaissance through the area: the base of the Oakville sandstone, the base of the Catahoula tuff, the base of the Frio clay (Oligocene?), and the base of the Jackson formation. All the formations strike northeastward in the region and dip southeastward toward the Gulf Coast. They are locally cut by a number of major and minor faults, trending generally parallel to the strike of the formations. Uranium minerals have been found, to date , generally no deeper than 30 feet below the surface, chiefly filling interstices between, and replacing, grains of sandstone that lie above impervious clay. Some of the clay also contains uranium minerals, generally as a coating along joint and bedding planes. The richest concentrations of uranium, however, are found in sand that has a clayey matrix. Some of the sand beds contain clay pebbles and small fragments of carbonaceous material. Although detailed mineralogic studies have not been made, the following uranium minerals have been identified: autunite, carnotite, tyuyamunite, uranophane, and schoepite. Other epigenetic minerals such as pyrite, hematite, sphalerite, and ilsemannite are associated with the uranium minerals at one prospect near Tordilla Hill. Detrital minerals of the ore-bearing sandstones include many derived from igneous rock sources.
Eargle, D. H., and Snider, J. L., 1957, A Preliminary Report on the Stratigraphy of the Uranium-Bearing Rocks of the Karnes County Area, South-Central Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 30, 30 p.