New Mexico Geological Society Annual Spring Meeting — Abstracts

Solution caves are formed in soluble rock such as limestone, dolomite, and gypsum. Acidic surface streams dissolve the soluble rock along small cracks, joints, faults, and bedding planes. While most limestone dissolution is driven by carbonic acid, certain circumstances can lead to corrosion by stronger acids such as sulfuric or nitric acid, which represent a net CO₂ source to the atmosphere. In New Mexico’s Black Range, solution caves occur in Paleozoic carbonates that are stratigraphically associated with sulfur-rich black shales. We are therefore exploring the role of carbonic and sulfuric acid weathering of carbonate rock in the North Percha Creek Watershed. Using geographic information systems (GIS) software, we analyzed the bedrock composition and relative amounts of black shale and limestone in North Percha Creek to determine the primary method and dominant control on chemical weathering in that region. We then analyzed water samples using colorimetric techniques to evaluate shale and limestone weathering. Two preliminary water samples collected in August, 2023 were supersaturated with respect to calcite, and had relatively high conductivities of 587 and 731 µS/cm. Sulfate concentrations for these two waters were 127.5 and 75 mg/L, which could indicate that shale weathering is occurring and may be contributing to limestone dissolution. We will report results from ongoing analyses of a larger number of samples to evaluate if chemical changes in North Percha Creek reveal the primary method of chemical weathering of carbonate bedrock in the watershed.