OF-19-03 Geologic Map of the La Salle Quadrangle, Weld County, Colorado


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 east of the Front Range uplift of Colorado. It is also directly east of and contiguous with our OF-18-02 Geologic Map of the Milliken Quadrangle, Weld County, Colorado. The geologic map plates were created using field maps, structural measurements, photographs, and field notes generated by the investigators. Includes 2 PDF plates and GIS data in a single zip file. Digital ZIP/PDF download. OF-19-03D

This mapping project was funded jointly by the U.S. Geological Survey through the STATEMAP component of the National Cooperative Geologic Mapping Program, which is authorized by the National Geologic Mapping Act of 1997, and also by the CGS using the Colorado Department of Natural Resources Severance Tax Operational Funds. The CGS matching funds come from the severance paid on the production of natural gas, oil, coal, and metals. Geologic maps produced through the STATEMAP program are intended as multi-purpose maps useful for land-use planning, geotechnical engineering, geologic-hazard assessment, mineral-resource development, and ground-water exploration.

From Plate 2, Geologic History:

The La Salle quadrangle lies in the northern Front Range urban corridor, located approximately 42 miles (64 km) northeast of metropolitan Denver and approximately 28 miles (39 km) southeast of Fort Collins. The area is located within the Colorado Piedmont physiographic province, an erosional area devoid of Neogene rocks that is bounded by the Front Range to the west and the High Plains to the east and north (Fenneman, 1931; Leonard, 2002; Smith and others, 2016). Two regionally expansive unconformities may define onset of Piedmont deformation and uplift: a late Eocene unconformity concurrent with the end of the Laramide orogeny, and an early Miocene unconformity that separates the Ogallala Formation from older strata below (Leonard, 2002). Bedrock within the La Salle quadrangle consists of Upper Cretaceous (100-66 Ma) sedimentary rocks that were deposited during transgressive and regressive episodes of the Western Interior Seaway (WIS), an extensive epeiric sea that existed in the Late Cretaceous. The beginning of the Laramide orogeny at ~ 70 Ma (Weimer, 1996) is roughly coeval with the final regression of the WIS in Colorado. The final regression of the WIS resulted in the deposition of the Cretaceous Fox Hills Sandstone and Laramie Formations on top of the marine sediments of the Pierre Shale. However, Pennsylvanian through Cretaceous rocks exist in the Denver Basin and rest unconformably on crystalline rocks of Precambrian age. As the Laramide orogeny progressed, sediments eroded off the uplifting Rocky Mountains, filling the downwarped foreland basin to the east with detritus. This structural basin, known as the Denver Basin, is strongly asymmetric, with steeply dipping strata along its western flank and gently dipping strata along its eastern flank. Based on outcrop data from the adjacent Milliken quadrangle (Palkovic and others, 2018) the regional dip in this quadrangle likely ranges from 1-5 degrees, dipping gently to the northeast. Many of the strata are important aquifers for communities along the Front Range in addition to being productive oil and gas reservoirs.