NON-CONFINED HYPOGENE EVAPORITE KARST MANIFESTATIONS, GYPSUM PLAIN, SOUTHEASTERN NEW MEXICO AND WEST TEXAS

The Gypsum Plain of the Delaware Basin in southeastern New Mexico and West Texas is dominated by outcrops of Ochoan Castile evaporites that host significant karst development ranging from small-scale karren, to laterally-limited epigene caves, to extensive hypogene caves. Because the study area is on the northern edge of Chihuahua Dessert, the arid conditions significantly limit the effects of meteoric processes; however, intense monsoon events in late summer can induce significant groundwater recharge. While hypogene karst is traditionally defined as being formed in confined to semi-confined hydrogeologic settings where coupling of free and forced convection drives upward dissolution, occurrences of hypogene-like karst features occur within the Gypsum Plain that formed and/or are continuing to form in unconfined conditions.

Unconfined, hypogene features occur as venting structures associated with solutionally-widened fractures and uncemented breccia pipes. Both manifestations are coupled to the land surface, exhibit restricted venting of moisture-laden air, and are associated with hydration buckling proximal to the land surface. Hydration buckles associated with breccia pipes are unique from traditional tumuli as rock density is significantly lower and are composed of brecciated clasts. Venting fractures form topographic ridges as a result of near surface hydration buckling and often host cavernous porosity. In both scenarios, density convection of moisture-rich air, coupled with shallow to deep groundwater, has created hypogene-like features due to hydration buckling diverting overland flow away and thus severely limiting any epigene overprinting. Arguably, these features could be classified as condensation corrosion features, but they appear to be coupled with deep karst process in regions known to host significant, traditional hypogene development and represent an end-member of hypogene karst formation where semi-confinement is created not by lithology but instead by surface geomorphology that precludes meteoric recharge and associated dissolution.