Preliminary Geologic Map of the Northwestern Part of the Alamosa 30 X 60 Minute Quadrangle, Alamosa and Conejos Counties, Colorado

This digital 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-minute quadrangles in the northwestern part of the Alamosa
1:100,000-scale quadrangle. The 7.5-minute quadrangles include the Alamosa
East, Alamosa West, La Jara, and Pikes Stockade. The map database, compiled at
1:50,000-scale resolution, provides geologic coverage of an area of current
hydrogeologic, tectonic, and stratigraphic interest. The mapped area is located
in Alamosa and Conejos Counties and includes the town of Alamosa in its
northern part. The map area is mainly underlain by surficial geologic materials
(fluvial and lacustrine deposits, eolian sand), but Tertiary volcanic and
volcaniclastic rocks crop out in the San Luis Hills, which are on the south
margin of the mapped area. The surficial geology of this area has never been
mapped in 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. Of particular
interest are newly discovered shoreline deposits (sands and gravels) associated
with the high 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 Ma). This
map provides new geologic information for better understanding groundwater flow
paths in and adjacent to the Rio Grande system.

[Summary provided by the USGS.]