COMPARISON BETWEEN ROCK SPRING UPLIFT AND DOUGLAS CREEK ARCH: EVIDENCE FOR BUCKLING OF THE LATE CRETACEOUS ROCKY MOUNTAIN FORELAND

The Rock-Spring uplift of Wyoming and the Douglas Creek arch of Colorado/Utah are both Laramide-age intrabasinal uplifts located in the Rocky Mountain foreland. The arches lie on different sides of the Cheyenne Belt, which separates Archean Wyoming province crust to the north from Proterozoic crust to the south. We hypothesize that structures on the Eastern Rocky Mountain foreland resulted from lithospheric folding, controlled by the strength of the upper mantle, and resulting from a horizontal end load on the margin North American craton.

The subsurface geometry of the two structures was compared using a Bouguer (gravity) anomaly map and two-dimensional forward and inversion models constrained with geologic and 2D seismic data. The Bouguer map shows a striking north-south alignment of a gravity anomaly high. The trend is linear, except for a sinistral bend near the Uinta Uplift, such that the Douglas Creek arch is displaced eastward relative to the Rock Spring uplift. The amplitude of the gravity anomaly and basement relief observed in the gravity models are similar for both arches. The timing and evolution of uplifting on the two arches was determined using sedimentary relations. Sedimentary onlaps and thinning of sedimentary units observed on seismic lines, in addition to subsidence analysis, indicates two main episodes of uplifting for both structures: One in the Late Cretaceous and the second in Late Paleocene-Early Eocene.

The similarities between structural style and history of the Rock Spring uplift and the Douglas Creek arch, as well as the continuous north-south structural trend of the gravity anomaly, suggest that they initially developed as a single continuous arch, in spite their differences in crustal nature. Although reactivation of structures occurred locally during the Laramide orogeny, the results of this study suggests that the crust played only a minor role in the larger-scale deformational patterns of the Eastern Rocky Mountain foreland.