Fort Stanton Cave and the northern Sacramento Mountains: Regional geologic and hydrologic context

Fort Stanton Cave, located in the northern Sacramento Mountains of south-central New Mexico, is formed in the middle Permian San Andres limestone. The cave is situated on the west flank of the Mescalero Arch, a broad structural divide separating the gently dipping eastern slopes of the mountains from structurally low areas of the Tularosa and Sierra Blanca Basins to the west. Fort Stanton Cave is located downgradient from extensive outcrops of siliciclastic sedimentary rocks as well as igneous and volcanic rock exposed at higher elevations in the Sierra Blanca Basin. This complex geologic setting results in surface drainage systems that originate on non-carbonate bedrock and are thus probably undersaturated with respect to calcium carbonate, making downstream dissolution and cave formation more likely. Evidence of both accretionary and dissolutional processes are widespread in the region. Tufa mounds associated with active and relict springs are a common feature in the southern Sacramentos. The most distinctive accretionary feature in Fort Stanton Cave is the Snowy River formation, a pool deposit composed of white calcite that coats the floor of the Snowy River passage, and currently extends >17 km with its southern terminus still undefined. Core samples collected from the Snowy River deposit reveal a laminated internal structure, indicating episodic deposition of sub-millimeter scale calcite laminae during periods when the passage stream is activated. The age of the basal layer has been determined to be only 820 years old, suggesting an abrupt change in climatic or hydrochemical conditions within the past millennium. The origin of water flow in the Snowy River passage is unknown, but appears to be associated with extreme summer precipitation events or heavy winter snowfall in the northern Sacramento Mountains. Field observations and hydrograph records support a point source or sources for water in the Snowy River passage via sinkholes or losing streams upgradient from the southwesternmost mapped stations in the cave.