Abstract

The Catahoula Formation of the Texas Gulf Coastal Plain consists of two depositional systems -- the Gueydan bed load fluvial system of the Rio Grande embayment and the Chita-Corrigan mixed load fluvial system of the Houston embayment. Each system contains distinctive fluvial channel-fill, crevasse splay, flood-plain, and lacustrine facies, which tend to persist vertically through the section. The paleoclimate varied from subarid in the Gueydan system to humid in the northeastern parts of the Chia-Corrigan system. The clay composition reflects alteration to montmorillonite and kaolinite of large volumes of volcanic ash deposited in both systems in response to pedogenesis and shallow burial diagenesis.Diagenetic features, distribution of trace uranium in fine-grained tuffaceous facies, and reconstructed ground-water flow history in the Catahoula provide the basis for an interpretation of a terrigenous coastal-plain uranium cycle.1. Uranium was leached from volcanic ash soon after deposition by pedogenic processes and moved into shallow ground-water circulation cells.2. Oxidizing uranium-enriched waters entered semiconfined aquifer sands in areas of regional recharge.3. Geometry of the flow system was determined by the three-dimensional facies and structural framework of the aquifer system.4. Uranium was concentrated by aqueous geochemical gradients as discrete mineralization fronts that were closely associated with iron oxidation fronts. Uranium was preferentially deposited where facies changes or faulting induced cross-stratal flow from permeable fluvial channel facies into interbedded or less permeable overbank facies.5. Postmineralization decrease of regional and local ground-water flux has resulted in reequilibration of large parts of the aquifer with the regionally reducing subsurface environment.The inferred uranium cycle provides criteria that can be used to compare the uranium potential of the Gueydan and Chita-Corrigan fluvial systems and to determine the possible distribution and nature of mineralization within each depositional system. These criteria apply similarly to other potential coastal-plain uranium host systems.

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