ABSTRACT

Hypogene Karst Porosity within the Central Basin Platform: Yates Field, Pecos County, Texas

Kevin Stafford^1,2 and Fred H. Behnken³

¹Cave and Karst Studies, Earth and Environmental Science Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA (kwstafford@juno.com)
² National Cave and Karst Research Institute, Carlsbad, NM 88220, USA
³ Kinder Morgan Production Company LP, Midland, TX 79701 (fred_behnken@kindermorgan.com)

Yates Field paleokarst is traditionally characterized as resulting from multiple subaerial exposure of San Andres Formation (Permian) grain-rich shoals during, or recently after, deposition with development of island karst, such as flank margin cave systems. However, no unequivocal evidence exists to support the subaerial exposure theory, instead fusulinid packstones and wackestones suggest that the San Andres was deposited as non-emergent, subtidal shoals, which is further supported by the absence of grainstone and peritidal facies, including definitive paleosols. Therefore, without subaerial exposure, syndepositional island karst could not have occurred. However, the spatial distribution of Yates Field karst porosity, including caves and associated collapse brecciation, exhibits patterns typical of hypogene speleogenesis. Cave distribution is extremely clustered, showing a relative decrease in depth of karsting towards the east, including similar karst development in the overlying Seven Rivers Formation and the Toborg Field (Yates Formation) along the northeastern margin of the Yates Field.

Current research indicates that Yates Field karst porosity is due to hypogene speleogenesis, where a mixed convection system (free and forced flow), enhanced by thermal waters with a sulfuric acid component drove carbonate dissolution. Thermal cooling and fluid saturation density gradients established free convection flow, while Pecos River entrenchment provided the regional hydrologic gradient driving forced convection. Associated with karst porosity development, secondary mineralization accompanied hypogene processes. Calcite spar, subaqueous speleothems and native sulfur indicate precipitation in confined settings, while uranium enrichment appears greatest in brecciated, collapse features where increased confined fluid migration occurred. Exposed caves within the region (e.g. Ess Cave, Amazing Maze Cave and Caverns of Sonora) indicate hypogene speleogenesis is the dominant regional process.