The purpose of the CGS Open-File Report OF-15-02, Geologic Map of the Castor Gulch Quadrangle, Moffat County, Colorado, is to describe the geology, geologic resources and geologic hazards of this 7.5-minute quadrangle located in north-western Colorado. CGS staff geologist Peter E. Barkmann, along with William Curtiss, Christopher J. Carroll, Daniel R. Hosler, Nathan T. Rogers, Zachary D. Logan, and Michael J. Zawaski completed the field work on this project during the summer and fall of 2014. Digital ZIP download. OF-15-02D
Location (excerpted from USGS Open-File Report 79-820, 1979)
The Castor Gulch quadrangle is located in southeastern Moffat County in northwestern Colorado, approximately 1 mile (2 km) south of the town of Craig via Colorado Highway 13 (also known as Colorado Highway 789), and approximately 43 miles (69 km) southwest of the town of Steamboat Springs via Colorado Highway 13 and U.S. Highway 40. The town of Craig South Highlands is in the north-central part of the quadrangle. A few scattered ranches occur throughout the remainder of the quadrangle.
Previous geologic mapping
The first geologic description of the general area in which the Castor Gulch quadrangle is located was reported by Emmons (1877) as part of a survey of the Fortieth Parallel. The decision to build a railroad into the region stimulated several investigations of coal between 1886 and 1905, including papers by Hewett (1889), Hills (1893), and Storrs (1902). Fenneman and Gale (1906) conducted geologic studies of the Yampa Coal Field and included a description of the geology and coal occurrence in the Castor Gulch quadrangle in their report. Hancock (1925) includes the entire area within the Castor Gulch quadrangle and is the most comprehensive work on the area. Tweto (1976) compiled a generalized regional geologic map which included this quadrangle.
Structural Geology (excerpted from this report)
The quadrangle is located on the southwest flank of the northwest-trending Sand Wash Basin. Generally, strata in this region strike southeast to northwest, and dip gently to the northeast. This produces an overall pattern with oldest formations exposed in the southwestern corner of the quadrangle and youngest to the northeast. However, several notable features disrupt this regional pattern in the Castor Gulch quadrangle. A complex northwest- to west-trending anticline crosses the northern part of the quadrangle, creating the Big Bottom syncline. The Buck Peak anticline is the eastern segment of the anticline and enters the northeast corner of the quadrangle where it is truncated by the Great Wall fault. The later feature is a northwest-trending normal fault, down to the northeast, with evidence of Neogene movement in the adjacent Breeze Mountain quadrangle (Barkmann and others, 2015). The complex anticline continues to the west, on the south side of the Great Wall fault, as the Craig Dome. A series of northeast-trending faults cross-cuts the Craig Dome as it follows a serpentine axis to the west. Subsurface elevation data for the top of the Twenty Mile Sandstone Member of the Williams Fork Formation indicate a reverse fault, northwest side up, separates the Craig Dome from the Big Bottom syncline. This fault may not continue east beyond the series of northeast-trending faults.