Abstract

Selected core sections from the lower San Andres (units 3, 4, 5), upper San Andres, lower Clear Fork, and the Salado-Tansill formations. Fibers or cylinders found from only the lower San Andres Formation units 4 and 5, the upper San Andres Formation, and the Salado-Tansill salt. The fibers are inorganic, light to dark reddish brown, pleochroic, highly birefringent, filamentary single crystals, <1 to approximately 5 mu m in diameter, with length-to-diameter ratios of at least 20:1 (some > 5000:1). Energy-dispersive analyses (SEM) and Gandolfi X-ray diffraction techniques have identified the fibers and cylinders as members of the cryptomelane-hollandite series. The fibers can be straight and/or curved, can bifurcate, can form loops, waves or spirals, and can be isolated or in parallel groups. Detailed petrographic analyses show no evidence for recrystallization or deformation of the enclosing salt after fiber formation. Suggestion that the fibers grew in situ by a solid-state diffusional process at low temperatures. The cylinders are pleochroic, highly birefringent, light to dark reddish brown, hollow, thin-walled, open-ended right cylinders, having a 1- to 2-mu m wall thickness and variable lengths (2 to 434 mu m) and diameters (6 to 55 mu m). The cylinders found in the recrystallized salt are perhaps a residue from dissolved chevron salt. The observation of cylinders partially or completely enclosed by salt stratigraphically above large fluid inclusions suggests that natural downward fluid-inclusion migration has occurred, in response to the geothermal gradient.--Modified journal abstract.

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