The outcrop area of the Gueydan (Catahoula) Formation of South Texas is characterized by geologic features which are common to provinces of sedimentary volcanism. Evidence for sedimentary volcanism includes relict mud-volcano vents (silica knobs) along deep-seated faults and fractures; mudflow deposits containing relict gas vesicles and gasoclasts; erratic igneous blocks and boulders, limestone blocks, and orthoquartzite blocks as much as 4,500 cu ft in volume; a diapiric, limestone-bearing, serpentine mass; and numerous structurally controlled clastic dikes of varied composition.

Natural gas provided the explosive energy required to transport, via gas-filled mudflows, many large, erratic blocks and boulders upward for distances of thousands of feet to the surface upon which Gueydan sediments were deposited. Mudflows of clayey, vitric tuff and sandy, pebbly, and conglomeratic clay in the Gueydan were deposited contemporaneously with fluvial sandstone and, more rarely, airborne ash beds. Most of the mudflow material (nonmarine clay and volcanic ash) probably was derived from the Jackson Formation (Eocene); erratic material probably was derived from much greater depths.

The South Texas mud diapirs are related to regional, deep-seated faults rather than to shallow depositional phenomena. They were emplaced as intrusive or extrusive masses by rapid, very fluid flow rather than by the relatively slow plastic movement common to many salt diapirs.