OF-22-07 Geologic Map of the Spinney Mountain Quadrangle, Park County, Colorado

The purpose of this publication is to describe the geology, geologic resources, and geologic hazards of the Spinney Mountain 7.5-minute quadrangle. This quadrangle is located within Colorado’s South Park basin, just south of Spinney Mountain State Park. Nearby completed CGS maps are the Hartsel, Antero Reservoir NE, and Agate Mountain quadrangles. Field work for this map was conducted during the 2021-22 mapping season. Digital ZIP download. OF-22-07D

From the abstract:

The Spinney Mountain quadrangle contains Proterozoic igneous and metamorphic rocks, Mesozoic sedimentary rocks, and Cenozoic volcanic and volcaniclastic rocks. Proterozoic rocks include Paleoproterozoic gneiss, schist, and amphibolite, and Mesoproterozoic granite. Mesozoic rocks include Cretaceous Dakota Sandstone and Pierre Shale. Cenozoic rocks include the Paleogene South Park Formation, Elkhorn conglomerate, Wall Mountain Tuff, Balfour Formation, Thirtynine Mile Andesite, and Tallahassee Creek Conglomerate. The quadrangle also contains a variety of alluvial and mass-wasting deposits, as well as a few human-made deposits. It is bisected by a major northwest-oriented fault zone including the West side, Elkhorn, Chase Gulch, and East side faults. The newly discovered northeast-trending faults on the quadrangle, especially the Gulch Road fault, may have transferred strain from the Elkhorn fault zone to the Currant Creek fault zone as the Elkhorn fault zone died out to the south. It is inferred that the faults in the map area were most active during the latest Cretaceous to Paleocene Laramide orogeny, the Eocene Epoch, and the Neogene to Quaternary opening of the Rio Grande rift. Geologic hazards in the map area include flooding, swelling soils, collapsing soils, landslides, debris flows, and falling rocks.