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

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.