GEOLOGIC MAPPING IN THE CARLSBAD AREA, SOUTHEASTERN NEW MEXICO: IMPLICATIONS FOR WATER ISSUES IN THE LOWER PECOS REGION

Results from recent geologic mapping (1:24,000 scale) integrated with subsurface data in the Carlsbad area of southeastern New Mexico can contribute to the understanding of the surface and groundwater hydrology and improve water management of the Lower Pecos region. Bedrock units include the basin and shelf deposits of the Permian reef complex (Guadalupian and Ochoan). The basinal deposits (100s-1000s m thick) are dominantly halite and anhydrite with lesser gypsum and mixed siliciclastic-carbonate intervals, and the shelfal deposits (10s-100s m thick) are dominantly carbonates with lesser siliciclastic and evaporate intervals. The reef (Capitan Aquifer) and most basinal deposits are expressed solely in the subsurface. Shelfal facies are in a belt paralleling the paleoshoreline to the west and north and commonly exhibit extensive mesoscale (m) to map-scale (10s m-kms) doming/folding related to mobilization of evaporates. Deformation is also observed in basinal deposits, yet scale and style are hard to determine—it is reflected by chaotic bedding orientations and is also due to salt movement. Surficial units include the Neogene to Quaternary Gatuña Formation and Quaternary deposits of the Pecos River and piedmont drainages, mostly from the Guadalupe Mountains/La Cueva Escarpment. The Gatuña Formation (0->130 m thick) is a conglomeratic sandstone to siltstone exposed in areas adjacent to and eastward of the modern Pecos river. This enigmatic deposit represents an alluvial valley-filling episode of the paleo-Pecos River. Quaternary piedmont gravels (0-75 m thick) cover a significant fraction of the landscape, have a remnant bar-and-swale topography showing east-northeastward flow, and grade laterally into paleo-Pecos River gravels. It is unclear whether these gravels represent only a few major, or several inset, depositional episodes of regional streams. All pre-Holocene units have 1-2 m of complex multistage calcrete development. Calcretes show deformation in the form of doming and reflect recent salt tectonics in the region. Many of these observed geologic features likely influence hydrologic pathways to and from the Pecos River and related Captian Aquifer.