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.
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“Diapir” and “diapirism” come from the Greek diapeirein, which means “to pierce.” Diapirism sensu lato is a process by which earth materials from deeper levels have pierced, or appear to have pierced, shallower materials; it is divided into magmatic intrusion and diapirism sensu stricto on the basis of the temperature at which piercement occurs. Diapirs s.s. are composed of evaporites, argillaceous sediments, coal, peat, ice, serpentine, or other earth materials which have the critical characteristics of low equivalent viscosity and low density. These materials range in age from Precambrian to Recent. Diapirs are found in all parts of the world except the shield areas. They have many forms, ranging from smoothly rounded pillows to complexly injected laminae, are either connected with or disconnected from the “mother” bed, and are present either at the surface, where they form distinctive features, or at considerable depth. Diapirs have well-developed internal structures indicative of an origin by flow. Strata around a diapir may be strongly affected structurally and/or stratigraphically by the diapir, or they may be unaffected. Field and model studies indicate that diapirs have developed as a result of horizontal compression, gravitational instability, or both. Diapiric structures of various types contain large quantities of oil and gas, sulfur, salt, and potash and are important for underground storage and nuclear testing.