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Sediment Dispersal Patterns in the Denver Basin Foreland: Insights From Detrital Zircon U-Pb Geochronology

Abstract

Detrital zircon U-Pb ages from the latest Cretaceous-Eocene fill of the Denver Basin provide new insights into evolving sediment dispersal patterns during filling of an intra-continental foreland basin. A suite of sandstone samples from the Castle Pines and Kiowa drill cores provide vertical transects through pre- and syn-orogenic Denver Basin fill in proximal and distal portions of the basin, respectively. Preliminary data from six sandstone samples from the Kiowa drill core reveal that pre-orogenic samples of latest Cretaceous age 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 underlying units, with a major age peak at 1.7 Ga and lacking 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 hypothesize that these samples represent the interplay between transverse and longitudinal sediment dispersal systems that can be distinguished on the basis of detrital zircon U-Pb age distributions. This preliminary interpretation departs from previous studies that have largely interpreted the 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 unroofing history of the southern Front Ranges and evolving sediment transport pathways during the Laramide Orogeny.