Analysis and correlation of growth strata of the Cretaceous to Paleocene Lower Dawson Formation: insight into the tectono-stratigraphic evolution of the Colorado Front Range

Abstract

Despite numerous studies of Laramide-style (i.e., basement-cored) structures, their 4-dimensional structural evolution and relationship to adjacent sedimentary basins are not well understood. Analysis and correlation of growth strata along the eastern Colorado Front Range (CFR) help decipher the along-strike linkage of thrust structures and their affect on sediment dispersal. Growth strata, and the syntectonic unconformities within them, record the relative roles of uplift and deposition through time; when mapped along-strike, they provide insight into the location and geometry of structures through time. This paper presents an integrated structural- stratigraphic analysis and correlation of three growth-strata assemblages within the fluvial and fluvial megafan deposits of the lowermost Cretaceous to Paleocene Dawson Formation on the eastern CFR between Colorado Springs, CO and Sedalia, CO. Structural attitudes from 12 stratigraphic profiles at the three locales record dip discordances that highlight syntectonic unconformities within the growth strata packages. Eight traditional-type syntectonic unconformities were correlated along-strike of the eastern CFR distinguish six phases of uplift in the central portion of the CFR.is area. The correlation of the syntectonic unconformities shows diachronous development of emerging structures that formed the CFR. The structures first developed in the South, then propagated in a northward direction along the eastern side of the CFR. Lithofacies and paleocurrent analysis within the growth strata record the transition from fluvial (confined) deposition to unconfined fluvial/megafan deposition. Sediment entry points for the fluvial (confined) and unconfined fluvial/megafan depositional systems were controlled by the lateral linking of along strike thrust faults (i.e., transverse or transfer zones) that bound the CFR. Provenance analysis supports the linkage of thrust structures controlling the provenance and sediment entry points to the Denver Basin. Petrographic analysis of twelve thin sections within the lower Dawson Formation shows two distinct petrofacies indicative of two fluvial megafan systems when considered with lithofacies and paleocurrent analysis. An unroofing signal was also identified that developed in response to the removal of Phanerozoic cover and Precambrian basement that covered the CFR due to emerging Laramide structures. The study has implication for predicting clastic sediment distribution in punctuated foreland basins, which ultimately controls reservoir presence for conventional plays and clay content for unconventional shale plays.