Evaporite Karst of the Lower Pecos Region

About this guidebook

Mirror Lake, one of seven sinkhole lakes, or cenotes, in Bottomless Lakes State Park, is located at the downgradient end of the regional hydrologic system in the Roswell Artesian Basin. Recharge to the artesian aquifer occurs on the Pecos Slope west of Roswell by direct infiltration from precipitation and by runoff from intermittent losing streams flowing eastward across the San Andres limestone outcrop. Groundwater flows east and south, then upward through leaky overlying gypsum confining beds of the Seven Rivers Formation in the Pecos River Valley, where the potentiometric surface in the artesian aquifer is above ground level. Dissolution of gypsum beds caused by this upward artesian flow created and continues to enlarge the cenotes along the Seven Rivers Escarpment. Overflow from Lea Lake, the southernmost and largest of the Bottomless Lakes sinkholes, amounts to roughly 15,000 acre-ft/yr and has caused an expansion of wetlands to the west, which are now hydraulically connected to the Pecos River, resulting in a net gain in streamflow and an increase in salinity in the river downstream from the park.

The gentle eastward regional dip of the area is locally reversed along the escarpment, where strata of the Seven Rivers Formation dip abruptly southwest by as much as 40°. This local dip reversal, clearly visible in the walls of Mirror Lake, is probably not tectonic but the result of subsurface dissolution of gypsum by ascending artesian groundwater and consequent slumping of overlying beds.