The purpose of this map is to describe the geologic setting, mineral and water resources, and geologic hazards of this 7.5 minute quadrangle located in western Colorado. This publication includes a booklet, map plate (1:24,000) and GIS data. 18 pages. Digital ZIP/PDF download. OF-06-04D
From the Author’s Notes:
The Gunnison 7.5’ quadrangle is centered on the town of Gunnison, near the junctions of the Gunnison River, Ohio Creek, and Tomichi Creek in central Gunnison County, Colorado. The quadrangle lies on the southeastern margin of the Piceance basin between the Elk, West Elk and San Juan Mountains.
The Gunnison quadrangle contains a record of three phases of mountain building in Colorado that include the assembly of Precambrian basement terranes, the Pennsylvanian through Permian Ancestral Rockies uplift; and the early Tertiary Laramide Orogeny. Precambrian metamorphic and igneous rocks in the southwestern part of the quadrangle lie along the northern margin of the Gunnison annular complex, a doubly plunging synform that records polyphase deformation and intrusion that occurred between 1730 and 1700 million years ago during the early assembly of North America (Lafrance and John, 2001; Bickford and others, 1989). The quadrangle is located on what was the eastern flank of the ancestral Uncompahgre uplift active in Pennsylvanian and Permian time (Kluth and Coney, 1981). The Ancestral Rockies history is evident only in the nonconformity between the Jurassic Junction Creek Sandstone and Precambrian basement rocks, where the regional Paleozoic section was eroded from the Gunnison area during and after Ancestral Rockies uplift. The early Tertiary Laramide Orogeny is principally represented by the general gentle northward and westward dip of Mesozoic strata across the quadrangle. A discrete zone of Laramide reverse faulting and folding is exposed along the Gunnison fault and the Gunnison syncline in the southwestern part of the quadrangle. A second area of possible Laramide folding is located in the northwestern part of the quadrangle where a zone of east-dipping Mancos Shale interrupts the otherwise uniformly gentle westward dip in this area. This folding may result from slip on an underlying Laramide fault at the level of the Precambrian basement, or it could indicate the presence of a post-Laramide intrusion at depth.