SPELEOGENETIC EVOLUTION FROM RISING TO DESCENDING FLUIDS: GYPSUM KARST OF SOUTHEASTERN NEW MEXICO AND FAR WEST TEXAS

The Gypsum Plain of Southeastern New Mexico and Far West Texas is dominated by outcrops of the Castile Formation gypsum facies (~1,800 km2), which rapidly responds to climatic variability as a result of high calcium sulfate solubility. Modern surface denudation rates range from 10 to 35 cm/kyr based on current standard tablet studies, with variability driven by seasonal and local precipitation patterns. The geomorphic landscape is dominated by extensive karst development with abundant sinkholes, incised arroyos, collapsed karst valleys, caves, residual breccias and resistant calcitized masses. However, much of the outcrop region is mantled by thick, eolian, gypsic soils. Dense clusters of genetically related caves, brecciation and calcitization occur throughout the western portion of the Castile outcrop area in regions of extensive surficial bedrock exposure. Genetically related clusters indicate that hypogene speleogenesis driven by rising fluids (i.e. water and hydrocarbons) originating from the underlying Bell Canyon Formation has dominated the regional diagenetic evolution.

The karst landscape of the Gypsum Plain is directly related to modern, surficial processes and regional dip, ~3° to east / northeast, with greatest Castile denudation occurring on the updip, western margin of the outcrop area. Therefore, speleogenesis associated with rising fluids, which is best developed proximally to the lower source aquifer, is best expressed at the surface where denudation has been the greatest. Although evidence of hypogene processes is most abundant in the western portion of the outcrop area, it is probable that extensive hypogene diagenesis occurs throughout the Castile Formation, but is not exposed because of surficial mantling by overlying strata or gypsic soils. Extensive breccia zones, breccia pipes, calcitized masses and economic sulfur deposits identified in the subsurface throughout the Delaware Basin further support the widespread dominance of hypogene processes throughout the region. Although hypogene karst within the Castile Formation is likely related to mid-Tertiary tectonism, the geomorphic expression of hypogene karst development is probably the direct result of surficial processes and epigene overprinting throughout the Quaternary.