OF-12-01 Geologic Map of the Antero Reservoir Quadrangle, Park and Chaffee Counties, Colorado


The purpose of the Antero Reservoir quadrangle map is to describe the geology, mineral and ground‐water resources, and geologic hazards of this 7.5‐minute quadrangle located in central Colorado. 69 pages. 2 plates (1:24,000). 16 figures. 1 table. Digital ZIP/PDF download. OF-12-01D

Excerpt from the Author’s Notes:

The Antero Reservoir 7.5-minute quadrangle lies in the southwestern part of South Park, a high-altitude intermontane valley in central Colorado. The map area covers approximately 57 mi2 in Park and Chaffee Counties. U.S. Highways 285 and 24 cross the quadrangle. Within the northeast section of the quadrangle, the Antero Reservoir is fed by the South Fork of the South Platte River. Rolling topography and closed depressions on terraces form karst topography derived from dissolution of evaporites in the Minturn Formation. This quadrangle includes the famous Saltworks Ranch, from which halite was produced by evaporation of water from a saline spring by the natives and then the settlers.

The oldest rocks in the Antero Reservoir quadrangle are Paleoproterozoic biotite gneiss and Mesoproterozoic or Paleoproterozoic granite porphyry that crop out on Kaufman Ridge. The lower and middle Paleozoic is represented by the Sawatch, Manitou, Harding, Fremont, Parting, Dyer, and Leadville formations, which were recognized and described in the Antero Reservoir quadrangle by Johnson (1934). Later the Dotsero Formation was recognized in the area by Myrow and others (2003). These limestone, dolomite, and sandstone formations were deposited in shallow tropical seas that periodically covered much of the western United States. They are separated by unconformities that developed when the seas retreated and exposed the rocks to erosion. Later, when mineral-rich fluids migrated through the area, these unconformities were preferred pathways where minerals were deposited. For this reason a number of prospect pits on Kaufman Ridge occur at formation boundaries.

There are no active permitted mines in the quadrangle. Schwochow (1981) reported four nonmetallic mining operations within the quadrangle, none of which are currently active or permitted. The reported operations included two sand and gravel mines near the southwest end of Antero Reservoir, a surface mine that produced crushed rock from the Wall Mountain Tuff, and the halite produced at the Saltworks Ranch.