STATUS OF GEOLOGIC MAPPING IN THE ALAMOSA 1/2° BY 1° SHEET, SOUTHERN COLORADO

The area of the Alamosa 1:100,000-scale 1/2° by 1° map sheet lies just north of the New Mexico border in south-central Colorado. It lies astride the Neogene Rio Grande rift, is bordered on the west by the middle Tertiary San Juan volcanic field and on the east by the rugged Sangre de Cristo Mountains. The San Luis basin occupies the western part of the map area and the Sangre de Cristo Mountains and its foothills occupy the eastern part of the area. These two vastly different domains are juxtaposed by the Sangre de Cristo fault zone, which extends N-S about 300 km through Colorado and northern New Mexico. This fault zone has the highest late Quaternary slip rate (>0.17 mm/yr) in the Rio Grande rift. The basin is not a simple graben, but is complex both in geometry and temporal history. The San Luis Hills, a trans-basinal horst composed of mid-Tertiary volcanic rocks, forms a structural barrier that subdivides the basin. The northern subbasin contains as much as 5 km of Neogene fill and precious water resources. Fill in the southern subbasin is largely covered by 3-5 Ma flood basalts (Servilleta Formation). Recent mapping of the surficial deposits south of the town of Alamosa has lead to the discovery of shoreline deposits (spits, barrier bars and lagoons) of ancient Lake Alamosa, which persisted as a fresh-water lake until 300-400 ka.

Our new map compilation effort is built on previous USGS mapping in the Sangre de Cristo Mountains and San Luis Hills, and newer Colorado Geological Survey mapping along the west margin of the mountains. In addition to completing reconnaissance mapping of the remaining parts of the 1:100,000-scale sheet, we are refining our understanding of the volcanic stratigraphy using ⁴⁰Ar/³⁹Ar dating, geochemistry, and paleomagnetism, and deciphering the Laramide (~70 to ~40 Ma) structural geology of the Sangre de Cristo Mountains and its young (~26 Ma to present) western fault boundary.

About 80 percent of the sheet has been compiled. Continued mapping is focusing on 1) a tier of 7.5' quadrangles high in the Sangre de Cristo Mountains, 2) the surficial deposits and volcanic rocks on the northwest flank of the San Luis Hills (formerly unmapped), and 3) detailed mapping of the San Luis 7.5' quadrangle, which includes active faults, landslide hazards, and a major intrarift horst (San Pedro Mesa).