OF-05-04 Geologic Map of the Como Quadrangle, Park County, Colorado


Describes the geologic setting and mineral resource potential of this 7.5-minute quadrangle, which is located in Park County. Field work for this project was conducted during the summer of 2004 by CGS staff geologists Beth L. Widmann and John W. Keller, consulting geologist Robert M. Kirkham, and field assistants Joel T. Poppert and Jason B. Price. Digital ZIP/PDF Download. OF-05-04D

From the Author’s Notes:

The Como quadrangle is located at the northern edge of South Park, just a few miles northeast of Fairplay in Park County, central Colorado. The small town of Como is in the east-central part of the quadrangle. U.S. Highway 285 traverses the southern half of the quadrangle. Several creeks and tributary streams flow south to southeast across the quadrangle and are part of the South Platte drainage basin. Within the quadrangle, the principal streams are Tarryall Creek and Trout Creek. Topography ranges in elevation from roughly 9,500 feet in the southeastern part of the quadrangle to nearly 13,600 feet on the eastern flank of Mount Silverheels in the northwestern part of the quadrangle. The forested land in the northwest part of the quadrangle is administered by the U.S. Forest Service (Pike National Forest). To the southeast, ranch lands are privately owned or controlled by the U.S. Bureau of Land Management or State Land Board. Numerous small parcels of land within the Forest Service boundaries along Tarryall Creek and in the northwest corner of the quadrangle are also privately owned.

The oldest rocks exposed in the Como quadrangle are the east-dipping Pennsylvanian to Permian redbeds of the Minturn, Maroon, and Garo Formations, which crop out in the western part of the quadrangle. The overlying sequence of rocks youngs eastward and includes the Jurassic Morrison Formation and the Cretaceous Dakota Sandstone, Benton Group, Niobrara Formation, Pierre Shale, Fox Hills Sandstone, and Laramie Formation, listed from older to younger. The Garo Formation and Dakota Sandstone form a north-trending hogback cut through by Red Hill Pass in the western third of the quadrangle. The low topography on either side of the hogback is due to the erosion of the less resistant fine-grained siltstone of the Maroon Formation to the west and Pierre Shale to the east. The Morrison Formation, Benton Group, and Niobrara Formation rarely crop out and are mapped on the basis of relative stratigraphic position and their characteristic residuum. Upper Cretaceous to Paleocene volcanic rocks of the South Park Formation form Reinecker Ridge and other low knobs in the southeast corner of the quadrangle. Clastic sedimentary rocks of the same formation overlie the volcanic rocks at Reinecker Ridge in the southern part of the quadrangle. Even younger Paleogene intrusive rocks crop out as sills, plugs, and stocks throughout the quadrangle. Most of these intrusive bodies are monzonite to quartz monzonite in composition and represent a period of intense magmatism about 41 to 43 Ma. Quaternary surficial deposits are widespread throughout the quadrangle and include modern stream alluvium and several older, broad alluvial surfaces, glacial deposits representing multiple glaciation events, alluvial fans, and mass-wasting deposits such as colluvium, talus, and landslide debris. Nearly everywhere above tree line (approximately 11,600 feet), bedrock is mantled by periglacial felsenmeer, which is a type of residuum resulting from freeze-thaw processes. These deposits were not mapped due to their ubiquity.