Geochemical evidence for transient karstic water/rock interaction in evaporites of the northern Delaware Basin, New Mexico, USA

Abstract

Permian evaporites in the northern Delaware Basin (southeastern New Mexico) have experienced varying degrees of water/ rock interaction, locally resulting in karstic landforms such as topographic depressions, sinkholes, and blind valleys. Meteoric groundwaters and the rocks with which they have interacted bear geochemical signatures that delineate such interactions even in the absence of surficial indicators. Based on fresh-water potentiometric heads alone, hydraulically confined groundwater appears to flow north to south, at steady state across the site of the Waste Isolation Pilot Plant (WIPP), but freshens from an ionic strength of 0.3–1.6 to <0.1 molal south of the site. This abrupt change in hydrochemical facies is not caused by younger vertical recharge preferentially occurring in the south; residence times based on radiocarbon (∼14 ka) of less saline Ca-SO4 waters in the south are no shorter than those of more saline Na-Cl waters in the north.234U/238U activity ratios (A.R.’s) monotonically increase from west to east, correlative with a 104 decrease in fracture-permeability, but normal to the that of inferred modern flow. Preservation of high A.R.’s argues against rapid oxic recharge from shallower units. These geochemical constraints suggest that modern groundwater flow is transient, and that the original paleoflow direction at the time of recharge in the late Pleistocene had a significant west-to-east component. The geochemical record of paleoflow has not yet been obliterated by Darcian flow, whereas the potentiometric contours have responded to post-Pleistocene potentiometric changes. The records of interactions between these groundwaters and adjacent evaporite rocks are preserved in the characteristic87Sr/86Sr ratios of gypsum altered from anhydrite and the18O/16O ratios of calcite altered from dolomite. These zones of alteration occurring at less than 300 m depth are interbedded with less altered zones bearing geochemical signatures more characteristic of marine evaporites, suggesting stratabound rather than vertical flow. Combinations of these geochemical parameters provide the basis for distinguishing hydrologic regimes in which the water/rock ratios have been higher (karstic) from those where evaporite dissolution has been less active.