SEDIMENTOLOGY AND DIAGENESIS OF THE UPPER CRETACEOUS AUSTIN CHALK FORMATION, SOUTH TEXAS AND NORTHERN MEXICO
DRAVIS, JEFFREY JAMES
Doctor of Philosophy
The Austin Chalk Formation in south Texas and northern Mexico represents an "impure" onshore depositional chalk deposited within an environmental framework characterized by distinct paleobathymetric variations. Relatively shallower water, highly fossiliferous chalks containing appreciable amounts of primary metastable carbonate material were generated on a platform coincident with the ancestral San Marcos arch. Deeper water basinal settings off this platform were sites of more typical chalk deposition which periodically received influxes of adjacent platform sediments. Abundant primary metastable carbonate constituents significantly complicated Austin Chalk diagenesis. Porosity trends and isotopic geochemistry confirm this fact and indicate the Austin Chalk underwent a greater degree of diagenesis compared to typical European chalks. Porosity in the buried Austin Chalk was destroyed relatively quickly earlier in its burial history and at shallower burial depths relative to typical North Sea chalks. Overall, the Austin Chalk exhibits an isotopic imprint distinctly different from European chalks but similar to marine limestones. Major processes that promoted Austin Chalk diagenesis and porosity reduction were physical compaction, fresh water dissolution/cementation, burial stabilization of primary aragonite, pervasive pressure solution and concomitant cementation, and tectonism. Where these processes operated depended on depositional setting, burial history and proximity to regional tectonism. Outcropping Austin Chalk in south Texas underwent extensive porosity reduction due to early physical compaction and later fresh water dissolution and cementation by non-ferroan calcite. Burial diagenetic effects are not conspicuous. Very low bulk iron and strontium concentrations support petrographic evidence for fresh water diagenesis. Buried chalks in south Texas underwent early physical compaction, early burial stabilization of aragonite (when present) and related cementation, and later deep burial pressure solution. Cements are predominantly ferroan calcite. Mexican outcropping chalks show a burial/diagenetic history similar to that for Texas subsurface chalks but were more deeply and deformed by Tertiary folding. Intense deformation of Parras Basin chalks partially to nearly totally obliterated primary depositional and secondary burial diagenetic fabrics and produced a matrix of micrite/microspar crystals elongated parallel to tectonic stress direction. Some fresh water cementation occurred attendant to this tectonism. Austin Chalk exhibits increased destruction of primary matrix components with progressive burial diagenesis. Concomitantly, Oxygen-18 becomes progressively depleted in response to pressure solution. Anomalous (delta)O('18) values at similar burial depths imply that factors other than burial depth influenced Austin Chalk diagenesis. Early burial stabilization of abundant primary aragonite in some samples may have created a lithified sediment framework sufficient to retard later pressure solution effects. Distribution of fractures in the Austin Chalk appears controlled by the abundance of argillaceous material. Highly argillaceous chalks do not fracture.