DETRITAL ZIRCON AGE ESTIMATES OF SELECT UPPER CRETACEOUS AND NEOGENE SEDIMENTARY ROCKS, DENVER BASIN AND NORTHEASTERN, COLORADO

The Colorado Geological Survey (CGS) collects and evaluates geochronologic data to estimate the ages of geologic units to enhance the geologic knowledge of the state and for use in geologic hazard investigations, hydrogeologic studies, geologic mapping, and mineral resource evaluations. Recent CGS investigations within the Denver Basin and northeastern Colorado included the analyses of select samples using detrital zircon U-Pb geochronology to provide age estimates and for provenance analysis. Samples from the Late Cretaceous Fox Hills Sandstone, the Late Eocene Castle Rock Conglomerate, and the Miocene-Pliocene Ogallala Formation were analyzed using these methods. Two samples from the upper part of the Fox Hills Sandstone (collected near Limon, CO) yielded youngest single-grain (YSG) ages of 74 ± 2 Ma and 67 ± 3 Ma with youngest-age populations (YAP) of 76.6 ± 1.2 Ma (mean-square weighted deviation [MSWD] = 0.40; N = 3) and 72.2 ± 1.0 Ma (MSWD = 0.75; N = 9), respectively. Two samples from the Castle Rock Conglomerate (collected from Castlewood Canyon in Douglas County), collected near the base and upper part of the unit, contain YSG ages of 33.1 ± 2 Ma and 36.7 ± 2.5 Ma, while the YAP is 36.5 ± 0.85 Ma (MSWD= 0.96; N=6) and 37.4 ± 1.2 Ma (MSWD= 0.28; N=5), respectively. The upper part of the Ogallala Formation (collected from Cedar Point near Limon, CO) yielded a YSG age of 23 ± 1 Ma (MSWD = 1.05; N = 3), and a YAP of 29.3 ± 1.1 Ma. The YAPs for the units are generally in accordance with other published detrital zircon ages in the area; however, the 33.1 Ma YSG from the Castle Rock Conglomerate is younger than other published ages and may place part of the unit, which is suspected to be Late Eocene in age, into the lower Oligocene. Similarly, the 67 ± 3 Ma YSG age from the Fox Hills Sandstone near Limon, may straddle the K-Pg boundary. Additional dating of the Castle Rock Conglomerate and upper part of the Fox Hills Sandstone is warranted to refine the ages of these formations and to better define their contacts. General provenance analysis using kernel density estimation plots of these data is ongoing and will be provided in the session.