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

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

2020-04-24 | CGS Admin

As we continue to expand geological mapping coverage of Colorado, we recently uploaded another of our STATEMAP 7.5-minute, 1:24,000 quadrangle mapping products to our online store: OF-19-03 Geologic Map of the La Salle Quadrangle, Weld County, Colorado. Our STATEMAP series provides a detailed description of the geology, mineral and ground-water resource potential, and the geologic hazards of an area. You can see the current status of our mapping on the ON-004-01 StateMap Progression Map.

This release includes two PDF map plates (with a geologic map, cross-section with correlation, oblique 3D view, legend, and description) along with the corresponding GIS data package that allows for digital viewing and use, all in a single ZIP file. As always, it is a free download. To access the ZIP package go to the OF-19-03 Geologic Map of the La Salle Quadrangle, Weld County, Colorado publication page. You will need to create a user account before adding the map item to your cart. Once you place the order, you will receive an email with the direct download link.

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 small town of LaSalle, named for LaSalle Street in Chicago, provided the name for the quadrangle (note the spelling difference, though!). The quad extends north from LaSalle to encompass parts of Evans, Colorado, and includes a short stretch of the South Platte River. The highest altitude on the quad is at the far southern edge and just clears 5000 ft (1524 m).

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. 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. 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 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, 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.

Citations, Categories & Tags


Palkovic, Martin J., Kassandra O. Lindsey, and Matthew L. Morgan. “OF-19-03 Geologic Map of the La Salle Quadrangle, Weld County, Colorado.” Geologic. Open File Report. Golden, CO: Colorado Geological Survey, February 2020.


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