Abstract This 2½ day field trip will present an overview of a U.S. Geological Survey (USGS) project whose objective was to estimate pre-mining groundwater chemistry at the Questa molybdenum mine, New Mexico. Because of intense debate among stakeholders regarding pre-mining groundwater chemistry standards, the New Mexico Environment Department and Chevron Mining Inc. (formerly Molycorp) agreed that the USGS should determine pre-mining groundwater quality at the site. In 2001, the USGS began a 5-year, multidisciplinary investigation to estimate pre-mining groundwater chemistry utilizing a detailed assessment of a proximal natural analog site and applied an inter-disciplinary approach to infer pre-mining conditions. The trip will include a surface tour of the Questa mine and key locations in the erosion scar areas and along the Red River. The trip will provide participants with a detailed understanding of geochemical processes that influence pre-mining environmental baselines in mineralized areas and estimation techniques for determining pre-mining baseline conditions.

Mountain ranges in the southern Rocky Mountains, first uplifted during the early Cenozoic Laramide orogeny, have followed separate landscape evolutionary pathways in the late Cenozoic. We present a model that reconstructs the post-Laramide tectonic and geomorphic history of Sierra Nacimiento and the Taos Range, two nearly adjacent rift-flank ranges in north-central New Mexico that serve to illustrate the various processes shaping landscapes across the southern Rocky Mountains. The Sierra Nacimiento landscape reflects the exhumation of hard Precambrian rocks from beneath a softer Phanerozoic sedimentary cover. The exhumation is continuous, but not steady, being driven by distal base-level fall. Downstream diverging river terraces in the Jemez River valley on the eastern flank of Sierra Nacimiento and late Pliocene to Holocene fluvial deposits on the western Sierra Nacimiento piedmont document the base-level fall. The timing and contemporary rates of incision from these river systems suggest that exhumation is being propagated from south to north as knickzones work their way headward from the Rio Grande. In contrast, the Taos Range landscape reflects alternating active stream incision and aggradation astride, and throttled by, an active range-front normal fault. The distinction between the exhumation-dominated and tectonic-dominated mountain front is best quantified by analyses of first-order stream gradients and a watershed metric we call the drainage basin volume to drainage basin area ratio ( R va ). Gradients of first-order streams in the exhumation-dominated Sierra Nacimiento have a mode of 6.8 degrees, significantly less than the 17.7 degrees obtained from a comparable data set of Taos Range first-order streams. The distinct stream gradient and R va populations hint at an important change in the processes shaping hillslopes and low-order channels, which is supported by the lack of slope-clearing landslides in the Sierra Nacimiento landscape and the presence of such landslides in the Taos Range. Analogue and numeric models find that steep, rugged, faceted topography associated with tectonically active mountain fronts like the Taos Range can only be produced and maintained by creep and landslides where the sediment flux scales as a power law with respect to average hillslope or low-order channel gradient. Here, the fingerprint of active tectonics is recorded by both high R va values and steep modal channel gradients. By comparison, the Sierra Nacimiento landscape is shaped primarily by creep where the sediment flux has a linear relationship to average hillslope and low-order channel gradient. In this situation, the signatures of distal base-level fall are low R va values and relatively gentle modal channel gradients.