Preliminary Geologic Map of the North-Central Part of the Alamosa 30' x 60' Quadrangle, Alamosa, Conejos and Costilla Counties, Colorado

Abstract

This geologic map presents new polygon (geologic map unit contacts) and line (terrace and lacustrine spit/barrier bar) vector data for a map comprised of four 7.5' quadrangles in the north-central part of the Alamosa, Colorado, 30' x 60' quadrangle. The quadrangles include Baldy, Blanca, Blanca SE, and Lasauses. The map database, compiled at 1:50,000 scale from new 1:24,000-scale mapping, provides geologic coverage of an area of current hydrogeologic, tectonic, and stratigraphic interest. The mapped area is located primarily in Costilla County, but contains portions of Alamosa and Conejos Counties, and includes the town of Blanca in its northeastern part. The map area is mainly underlain by surficial geologic materials (fluvial and lacustrine deposits, and eolian sand), but Tertiary volcanic and volcaniclastic rocks crop out in the San Luis Hills, which are in the central and southern parts of the mapped area. The surficial geology of this area has never been mapped at any scale greater than 1:250,000 (broad reconnaissance), so this new map provides important data for ground-water assessments, engineering geology, and the Quaternary geologic history of the San Luis Basin. Newly discovered shoreline deposits are of particular interest (sands and gravels) that are associated with the high-water stand of Lake Alamosa, a Pliocene to middle Pleistocene lake that occupied the San Luis basin prior to its overflow and cutting of a river gorge through the San Luis Hills. After the lake drained, the Rio Grande system included Colorado drainages for the first time since the Miocene (>5.3 Ma). In addition, Servilleta Basalt, which forms the Basaltic Hills on the east margin of the map area, is dated at 3.79+or-0.17 Ma, consistent with its general age range of 3.67-4.84 Ma. This map provides new geologic information for better understanding ground-water flow paths in and adjacent to the Rio Grande system. The map abuts U.S. Geological Survey Open File Report 2005-1392 (a map of the northwestern part of the Alamosa 30' x 60' quadrangle map) to the west and U.S. Geological Survey Scientific Investigations Map 2965 (Fort Garland 7.5' quadrangle) to the east.