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Shale Diapir Emplacement in South Texas: Laward and Sherriff Examples
R. S. Bishop (1)
Diapirism generally is assumed to be an intrusive process whereby less-dense material (usually salt) rises into more-dense overburden, due to buoyancy. Shale diapirs in the Gulf Coast, although similar in shape and geologic setting to salt domes, do not behave as predicted by buoyancy theory. Shale density inversions occur in large areas of both South Louisiana and South Texas yet subsurface piercement shale diapirs occur only in South Texas. To explain this anomalous behavior required integrating field data, a loading model of diapirism, a model of the compaction of thick shales, and a numerical simulation of compaction history in South Texas.
Interpretation of the results is that two South Texas shale diapirs, Laward and Sherriff, formed primarily by extrusion and subsequent burial, rather than by intrusion. In these diapirs Jackson (Eocene) and Vicksburg (Oligocene) shales extruded upward with contemporaneously deposited Lower Frio (Oligocene) sands. Piercement of both diapirs ceased during Middle Frio time and was followed by a small amount of nonpiercement folding.
A simulation of the South Texas shale compaction history, combined with a loading model of diapirism, rather than a buoyancy model, provides the basis for a general model of shale diapir emplacement. The compaction simulation, using sedimentation rates and material properties representative of South Texas, shows that a shale density inversion could have occurred during the initial burial of the Vicksburg-Jackson shales by the Frio sands. Importantly, however, the density inversion is confined to the upper portion of the shale, not from uppermost to lowermost levels as would be the case for salt. Both the simulation and mapping show that extrusion occurred only during the time the shale density inversion was present (Lower to Middle Frio time). Extrusion stopped, however, due to shale compaction during the accumulation of Middle and Upper Frio overburden. Disappearance of the density inversion did not necessarily cause diapirism to cease but rather caused the emplacement process to change from extrusion to intrusion. Intrusion of the mapped diapirs, however, apparently was negligible. These diapirs may have ceased to move either because the diapiric pressure was insufficient to fracture the overburden or because the shale lost is thixotropic character and ceased to flow easily. This may also explain why deep shale density inversions, such as those in South Louisiana, apparently do not form late piercements.
In general, rapid deposition of a sandy overburden on a thick, montmorillonitic shale is the condition most conducive to forming shale density inversions. The sandy overburden is important because it is more dense than an alternating sand-shale overburden. The montmorillonite substrate is important because it is the least permeable of the clays and therefore compacts the most slowly. In addition, montmorillonite can flow more easily than other clays due to its lower angle of internal friction (5°-9° or montmorillonite, 20°-30° for kaolinite). The conditions of rapid, sandy deposition on montmorillonitic substrate in South Texas are very similar to those forming modern shale diapirs (mud lumps) at the mouth of the Mississippi River.
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