DETRITAL ZIRCON PROVENANCE OF THE DENVER BASIN: EVOLVING SEDIMENT DISPERSAL PATTERNS DURING THE LARAMIDE OROGENY

Detrital zircon U-Pb ages from the latest Cretaceous-Paleocene fill of the Denver Basin provide new insights into the unroofing of the southern Front Range and evolving sediment transport pathways during the Laramide Orogeny. We analyzed six sandstone samples from the Kiowa drill core that was collected from near the center of the Denver Basin. Pre-orogenic samples of latest Cretaceous age (uppermost Pierre Shale and Fox Hills Sandstone) contain abundant 1.65-1.8 Ga zircon with additional age populations spanning Late Cretaceous through Archean time. These grain ages are consistent with a sediment source area in the western U.S., including Yavapai-Mazatzal basement and the Mesozoic Cordilleran arc. Three of the four samples analyzed from the overlying, syn-orogenic D1 Sequence are dominated by 1.05-1.15 Ga zircon grains that were likely derived from the 1.08 Ga Pikes Peak granite, thus recording a major provenance shift during uplift and denudation of the southern Front Range. However, one sample from the D1 Sequence yielded detrital zircon age populations more similar to the underlying Pierre Shale and Fox Hills Sandstone, with a major age peak at 1.7 Ga. This sample also lacks the 1.1 Ga age peak found in the other D1 Sequence samples. The anomalous D1 Sequence sample was collected from a coal-bearing zone within the middle of the D1 Sequence whereas the other samples were collected from coarser-grained units above and below. We speculate that the middle D1 Sequence sample might represent a separate sediment routing system that entered the Denver Basin longitudinally, likely from the north, pending corroboration from additional samples. This interpretation departs from previous studies that have largely interpreted the entire syn-orogenic fill of the basin to have been supplied by transverse rivers emanating from the southern Front Range. This study also has implications for the location of regional river systems that drained the Rocky Mountain region during the Laramide Orogeny.