Paleohydrology and dolomitization in the Permian San Andres Fm, Guadalupe Mountains and Algerita Escarpment

Garcia-Fresca, Beatriz¹, F. Jerry Lucia² (1) The University of Texas at Austin, Austin, TX (2) The University of Texas at Austin, Austin,

Prediction of dolostone patterns requires knowledge of the hydrology of dolomitizing fluids. Reflux seepage of evaporated water is one the mechanisms invoked to explain dolomitization in restricted carbonate settings. We hypothesize that the source of dolomitizing fluids was located in coastal carbonate depositional environments (tidal flats) and that a hydrologic model can be constructed on the basis of tidal flat-related paleotopography and the permeability distribution in the underlying facies. High evaporation rates result in the generation of highly concentrated brines in the peritidal zone. The concentration gradient between these brines and fluids in the underlying sediments results in density-driven circulation of fluids. Shoreline migration in response to sealevel fluctuations results in the repositioning of the fluid source. Progradation of the peritidal succession places hypersaline environments over subtidal sediments of normal marine salinity providing an optimum situation for downward circulation of dolomitizing brines.

We have chosen the Permian San Andres Formation as the geologic model to study reflux. Ramp geometry, sequence-stratigraphic framework, depositional environments, and distribution of the main lithologies are constrained from outcrop and well log data. Dolostone/anhydrite cycles occur in the inner ramp. The formation is extensively dolomitized but limestone occurs in the inner ramp and ramp margin. Field observations of the limestone/dolostone relationships suggest that dolomitizing fluids sink vertically under the intertidal zone and then travel horizontally following the bedding and higher permeability facies. This hypothesis is tested by numerical modeling of the hydrologic system.