OF-18-02 Geologic Map of the Milliken Quadrangle, Weld County, Colorado


Extending previous CGS mapping efforts along the Front Range region the state, this publication describes the geology of this 7.5-minute quadrangle located southwest of Greeley, Colorado. Includes GIS data and two PDF plates. Digital ZIP download. OF-18-01D

From the Geologic History:

The Milliken quadrangle is in the northern Front Range urban corridor, located approximately 40 miles (64 km) north of metropolitan Denver and approximately 24 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. Two regionally extensive unconformities may define the 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 Milliken quadrangle consists of Upper Cretaceous (66-100 Ma) sedimentary rocks that were deposited during transgressive and regressive episodes of the Western Interior Seaway (WIS), a large 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. This final regressive pulse of the WIS defines the near-surface stratigraphy in the quadrangle and is comprised of the Pierre Shale, Niobrara Formation, and Fox Hills Sandstone. As the Laramide orogeny progressed, sediments eroded from 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.