New Mexico Geological Society Annual Spring Meeting — Abstracts

The San Luis basin (SLB) is the northernmost large basin of the Rio Grande rift. It extends about 240 km from Velarde, NM to Poncha Pass, CO, encompassing the San Luis Valley to the north and the Taos Plateau to the south. The basin is bounded on the east by the Sangre de Cristo Mountains, on the west by the Brazos uplift and San Juan Mountains, and on the south by the Picuris Mountains. Ranging from 30 to 70 km wide, with elevations between about 7000 and 8000 ft, the basin is said to be the world’s largest high-elevation valley. It is nearly the size of the state of Connecticut, and averages less than 7 people per square mile. The Rio Grande originates in the San Juan Mountains, flows through the valley to Alamosa, runs south through a gap in the San Luis Hills, and into New Mexico where it has carved the Rio Grande gorge.

The SLB is east-tilted, with about 8 km of Neogene throw along the normal Sangre de Cristo fault, and with a complex hinge zone along the western border. In the New Mexican portion of the basin, a deep north-south graben (the Taos graben) exists along the Sangre de Cristo fault. The deepest part of the graben is west of Taos Pueblo, where the depth to Precambrian rocks is estimated to be >5000 m. The west-bounding fault of the Taos graben, known as the Gorge fault, approximately coincides with the Rio Grande gorge. West of the gorge, a bedrock bench rises gently to the topographic basin boundary along the Tusas Mountains. To the north, the bench becomes a complex, intra-rift horst with pre-rift, Oligocene volcanic rocks exposed in the middle of the rift at the San Luis Hills of southern Colorado. The 65-km-long, NE-striking Embudo fault is a transfer (or accommodation) zone that forms the complex structural boundary that absorbs the differential extension between the east-tilted SLB and the west-tilted Española basin.

The southern part of the basin is a physiographically and geologically unique terrain known as the Taos Plateau. The Pliocene to Pleistocene Taos Plateau volcanic field is the largest (7000 sq km) and perhaps most compositionally diverse volcanic field in the rift, with at least 35 discrete vents that range from tholeiitic basalt to rhyolite, and that form vent morphologies including shield volcanoes, cinder cones, and volcanic domes. The plateau surface shows only minor dissection, with the Rio Grande and its major tributaries confined to deep canyons.

The SLB is host to a variety of recent and ongoing geological work. Through the STATEMAP program, the NMBGMR has mapped all of the 7.5-min quads in the southern SLB. The largest ongoing project is a USGS program designed to assess the influence of geology on the availability of ground water, natural resources, and hazards in the central SLB. As part of this study, much of the basin was flown for a high-resolution aeromagnetic survey. Other recent and ongoing NMBGMR projects include: a series of detailed hydrogeologic assessments of highgrowth areas in the Taos region; an inventory and geochemical study of the springs in the Rio Grande gorge; a paleoseismic trenching study of the southern Sangre de Cristo fault near Taos; and a study of the geology, hydrogeology, and hydrogeochemistry of surface water/ground water interactions along the Rio Grande in northern Taos County. The Taos Soil & Water Conservation District has inventoried water wells and mapped the ground water and water quality of the county. Over many years, Glorieta Geoscience, Inc. has investigated the hydrogeology, aquifer characteristics, and hydrogeochemistry of the Taos region.