OF-17-02 Geologic Map of the Johnstown Quadrangle, Larimer and Weld Counties, Colorado

The Johnstown quadrangle lies in the northern Front Range urban corridor, located approximately 40 miles (64 km) north of metropolitan Denver and approximately 20 miles (32 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 Foothills 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 (Leonard, 2002). Bedrock within the Johnstown quadrangle consists of Upper Cretaceous (100-66 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 (Weimer, 1996) 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 Niobrara Formation, Pierre Shale and Fox Hills Sandstone. As the Laramide orogeny progressed, sediments eroded off of 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. Bedrock exposure in the quadrangle is primarily limited to the flanks of the Little Thompson River, as well as the gullies of intermittent streams that feed these systems during heavy rains. When exposed, the bedrock is primarily composed of the Upper Transition Member of the Pierre Shale (Kpt), a Late Cretaceous, massive, buff-tan sandstone serving as a gradational member between the thick shales of the Pierre and the Fox Hills Sandstone. The Fox Hills Sandstone (Kfh), a white-beige, friable, massive sandstone, is also exposed on the quadrangle in the southeast corner of the map, in the intermittent gullies of St. Vrain Creek. In the subsurface, the Johnstown Wrench fault zone (JWFZ) (Weimer, 1996) runs through the quadrangle, creating an anticline in the north, and a graben in the south. The JWFZ is covered by quaternary sediments and the shallow bedrock (Kpt) in the area, and can only be seen on geophysical logs. Many of the strata are important aquifers for communities along the Front Range in addition to being productive oil and gas reservoirs.