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

Abstract

Permianevaporitesin thenorthernDelaware Basin (southeastern New Mexico)have experienced varyingdegreesof water! rock interaction, locallyresultingin karsticlandforms such as topographic depressions, sinkholes, andblindvalleys. MeteoricgroWldwaters and the rocks with which they have interacted bear geochemical signatures that delineatesuch interactions even in the absenceof surficial indicators. Based on fresh-water potentiometric heads alone,hydraulically confinedgroundwater appears to flow north to south, at steady state acrossthe site of the Waste IsolationPilot Plant (WIPP),but freshens from an ionic strengthof 0.3-1.6to <0.1 molal southof the site. This abruptchangein hydrochemical facies is not causedby youngervertical rechargepreferentially occurring in the south;residence times basedon radiocarbon (-14 ka) of less salineCa-S0 4 watersin the south are no shorterthan those of more saline Na-O watersin the north. 234UfDlU activity ratios (AR.'s)moIlOtonically increasefrom westtoeast,correlative witha 1000decrease in fracture-permeability, butnormal to the that of inferredmodem flow. Preservation of high A.R. 's argues againstrapidoxic rechargefrom shallowerunits, Thesegeochemical constraints suggest that modern groundwater flow is transient, and that the originalpaleoflow direction at the time of rechargein the late Pleistocene had a significantwest-to-east component, The geochemical recordof paleoflow hasnot yet been obliteratedby Dareianflow, whereas thepotentiometric contourshaveresponded to post-Pleistocene potentiometric changes. The recordsof interactions between these groundwaters and adjacentevaporiterocks are preservedin the characteristic~Srf'6Sr ratios of gypsumalteredfrom anhydrite and the .'O! 16() ratiosof calcitealteredfromdolomite. These zonesof alteration occurring at lessthan 300m depthareinterbedded withlessalteredzones bearinggeochemical signaturesmorecharacteristic of marineevaporites, suggesting stratabound ratherthan verticalflow. 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 evaporitedissolution hasbeen less active.