Evaporite karst features and processes at Nash Draw, Eddy County, New Mexico

Abstract

—Nash Draw, about 30 km east of Carlsbad, NM, is a karst valley developed on Upper Permian evaporite rocks of the Rustler and Salado Formations. Early studies found sodium chloride brine along the draw axis above halite of the Salado Formation, with the brine flowing toward and into the Pecos River. The draw was interpreted as the consequence of erosion, solution, and fill. Later studies report solution of sulfate units of the Rustler, yielding more complicated surficial structures. More drillhole logs are now available, showing that the eastern margin of Nash Draw (Livingston Ridge) overlies the position where the elevation of upper Salado halite changes, as deos the thickness across the upper Salado and the elevation of overlying units. These elevation changes reflect dissolution of Salado halite. Depressions along Livingston Ridge result from subsidence that has caused recent regrading of arroyos. At Laguna Grande de la Sal, elevations of the top of Salado halite indicate a depression under the lake, and lower halite south of the lake is associated with elongate valley-like depressions as well as rounded basins. The movement of brine at depth may be directed along this surface. Surface karst features in the southeastern arm of Nash Draw develop on and within sulfate beds of the Rustler Formation and surficial gypsite. Collapse sinks and coalescing collapse in small karst valleys appear to show some evolutionary trends based on period since erosion exposed sulfate beds. Vertical-walled collapse sinks without fill are young, while karst valleys are older. These features are developed along stratigraphic trends. Some of the recharge in this sulfate-dominated environment discharges year-round in local springs, indicating fluid storage within the system. Alluvium in sinks and karst valleys are proposed as part of the storage system. FIGURE 1. General location map of Nash Draw with identified features. Numbers refer to brine lakes informally named Laguna Uno (1) through Cinco (5). The Waste Isolation Pilot Plant (WIPP) site outline does not include the entire WIPP area. The area shown in Figure 3 is outlined here. 254 POWERS, BEAUHEIM, HOLT, AND HUGHES