The Oligocene/Miocene Catahoula Formation of the upper Texas coastal plain is a fluvial and lacustrine volcaniclastic unit composed of “normal” quartz-rich fluvial material mixed with distal rhyolitic air-fall ash from coeval volcanic source areas in Trans-Pecos Texas and northern Mexico. It consists of poorly sorted siltstones, sandstones, and mudstones. The silt-size fraction includes abundant, slightly altered, volcanic glass shards. Sandsize grains are mostly quartz, some of which are very coarse—the “rice sands”. The clay-size fraction is montmorillonite, an alteration product of the glass.
The narrow outcrop belt of the Catahoula Formation, which is somewhat less than 300 ft (100 m) thick in Jasper County, extends from central Mississippi, through Louisiana and Texas parallel to the present coastline, and into eastern Mexico where it is not well studied.
Catahoula deposition is characterized by sporadic influxes of air-fall ash into low-gradient fluvial and coastal lake environments. Stratigraphic studies of the Catahoula Formation began as early as 1857, and the formation has since been studied by numerous stratigraphers and sedimentologists, including Hilgard, Penrose, Dumble, Veatch, and Deussen. Dumble (1918) gives a summary of this earlywork. Bailey (1926) named the lateral equivalent in the lower Texas coastal plain of south Texas theGueydan, and along with Renick (1936) established the Catahoula as a mid-Tertiary fluvial unit with abundant volcaniclastic material. A more recent summary of stratigraphic nomenclature is given in Sheldt (1976). Other recent studies of the Catahoula are concerned with petrology, geochemistry, and uranium ore genesis.