WATER IN THE SAN LUIS BASIN — LAKE ALAMOSA'S LEGACY

The San Luis Basin (SLB) started to form about 26 Ma as a result of E.-W. opening of the Rio Grande rift. Early uplift produced an intrabasin horst (San Luis Hills) that divided the SLB into two subbasins. Syntectonic sediment (Santa Fe Group) and basalts of the Hinsdale Formation (ca. 12-15 Ma) dominated basin filling during the Miocene, but by 3.7 Ma widespread deposition of Servilleta Basalt had buried any possible south-draining channels on the structurally lower E. and W. margins of the SLB. Subsequently, the northern (Alamosa) subbasin constrained the extent of Lake Alamosa and lacustrine deposition of the Alamosa Formation, especially the hydrologically important thick “blue clay” layers within the confined (lower) aquifer. Sediment continued to fill the subbasin as Lake Alamosa grew and shrank with Quaternary climatic oscillations. Around 440 ka (marine oxygen-isotope stage 12), the lake overtopped a bedrock-cored sill in the San Luis Hills, cut a deep gorge, and released its water to New Mexico.

Today, Servilleta Basalt is at the surface in the SLB as far north as Antonito and Fort Garland,Colo., and drill holes penetrate this basalt at 100-200 m at least as far north as La Jara and Blanca. Aeromagnetic expression of the basalt becomes more muted northward as a result of burial by the lake sediment. Sedimentation of the Alamosa Formation was driven by climatic changes in a dynamic environment that controlled the distribution, depth, and flow characteristics of the confined intrabasin aquifer, which is the source of most agricultural, high-production water wells in the subbasin. Improvements in future hydrologic modeling will require analysis of water-well logs that consider (1) climatic cycles of Lake Alamosa, (2) internal structures within the basin (from new geophysical data), and (3) basin geometries driven by the deformation on the Sangre de Cristo fault system. The Alamosa subbasin is ripe for paleoclimatic modeling of basin sedimentation.