The purpose of this map is to describe the geology, mineral and ground-water resource potential, and geologic hazards of this 7.5-minute quadrangle located southeast of Delta in western Colorado. CGS staff geologists David Noe, Matt Morgan, and field assistant Shannon Townley completed the field work on this project during the spring and summer of 2007. The geologic map plates and report were created using field maps, field notes, structural measurements, and photographs generated by the investigators. Surficial and bedrock unit descriptions were coordinated between this area and the adjacent Delta and Olathe quadrangles (Morgan and others, 2007; 2008). This mapping project was funded jointly by the U.S. Geological Survey (USGS) and the CGS. USGS funding comes from the STATEMAP component of the National Cooperative Geologic Mapping Program, award number 07HQAG0083, authorized by the National Geologic Mapping Act of 1997, reauthorized in 2009. CGS matching funding comes from the Colorado Department of Natural Resources Severance Tax Operational Funds, from severance taxes paid on the production of natural gas, oil, coal, and metals in Colorado. Digital ZIP download. OF-13-07D
From the Author’s Notes:
The Olathe Northwest 7.5-minute quadrangle is located in Delta and Montrose Counties, Colorado, flanking the Uncompahgre River valley. No towns are located within the quadrangle. The town of Delta lies 4 miles to the west. U.S. Highway 50 passes through the southwestern corner. Few named features appear on the base USGS topographic maps, and we have applied some informal names to designate areas or features of specific interest. These names reflect local usage where possible and are signified by quotation marks. The highest elevation is 6,160 ft along a ridge in the northeastern corner. The lowest point is 5,035 ft along an unnamed arroyo near “Star Nelson Airport Mesa.” Five geomorphic areas comprise the quadrangle. The Gunnison Uplift (GU) is a structural geomorphic feature that forms a westward-dipping escarpment (“Dakota dip slope”) along the eastern edge. The Uncompahgre River valley (URV), consisting of the modern river flood plain and gravel-capped mesas that mark its former courses, cuts across the southwestern corner. Gravel-capped mesas associated with the former Gunnison River form the margin of the Gunnison River valley (GRV) across the northwestern corner. The dominant terrain within the quadrangle is typified by shale badlands, called the “Adobe hills” or, simply, the “Adobes” by local residents. This area can be divided into a high part (HAH), which contains moderately steep shale ridges and hills and deeply incised valleys, and a low part (LAH) that features broad basins and low shale hills. A nearby structural geomorphic feature of note, the Uncompahgre Uplift, forms a northeastward-dipping escarpment several miles to the west of the quadrangle. The foot of its dip slope lies along the western edge of the Uncompahgre River valley, in the adjacent Delta quadrangle.