OF-20-03 Geologic Map of the Bracewell Quadrangle, Weld County, Colorado

The Bracewell quadrangle lies in the northern part of the Front Range urban corridor, approximately 80 km northeast of metropolitan Denver and approximately 30 km southeast of Fort Collins. The quadrangle is located within the Colorado Piedmont section of the Great Plains physiographic province, an area in eastern Colorado where Neogene rocks were removed by erosion. The Colorado Piedmont is bounded by the Front Range foothills to the west, the High Plains to the east and north, and the Raton Basin to the south (Fenneman, 1931; Madole, 1991; Leonard, 2002; Smith and others, 2016). Two regionally extensive unconformities may define the onset of Piedmont deformation and uplift in eastern Colorado: 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). In eastern Colorado, Upper Cretaceous (100-66 Ma) marine sediments were deposited during transgressive and regressive episodes of the Western Interior Seaway (WIS), a large epeiric sea that existed during the Late Cretaceous. The beginning of the Laramide orogeny at ~70 Ma (Weimer, 1996) is roughly contemporaneous with the final regression of the WIS in eastern Colorado. This final regressive pulse of the WIS is responsible for the deposition of the Niobrara Formation, Pierre Shale, Fox Hills Sandstone, and Laramie Formation. As the Laramide orogeny progressed, sediments eroded off of the uplifting Rocky Mountains, filling the downwarped foreland basin to the east with detrital sediment. This sedimentary basin, known as the Denver Basin, is a strongly asymmetric structural basin, with steeply dipping strata along its western flank and gently dipping strata along its eastern flank.