8: Progressive development of basement-involved foreland thrust belts: Results from the NSF/EarthScope Bighorn Project applied to the Laramide orogeny in the Colorado Front Range
Eric A. Erslev, Karen Aydinian, Laura Kennedy, 2016. "Progressive development of basement-involved foreland thrust belts: Results from the NSF/EarthScope Bighorn Project applied to the Laramide orogeny in the Colorado Front Range", Unfolding the Geology of the West, Stephen M. Keller, Matthew L. Morgan
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Lithosphere-scale seismic experiments, structural geometries, and minor structures in the Bighorn region of the northern Rockies show that Laramide deformation was controlled by horizontal shortening driven by detachment in the lower crust. This field trip visits three exposures in the Colorado Front Range that helped generate the detachment hypothesis, and uses observations from both areas to generate a thrust belt model for basement-involved foreland orogens.
Starting north of Boulder, a traverse following the trace of the classic Six Mile fold reveals minor structures showing early layer-parallel shortening overprinted by a progressively tightening, non-self-similar fault-propagation fold. The overlying petroliferous Niobrara marls show how sequential deformation predicts the unit’s natural fractures, which are critical to the success of individual wells in this major resource play.
South of Boulder in Eldorado Canyon (known worldwide for near-vertical climbing), the trip traverses excellent exposures of the Paleozoic strata in the hanging-wall above the Golden thrust fault. Out-of-the-basin and into-the-basin thrusts allow the restoration of the Rocky Flats 2D seismic line just to the south. From a revealing overlook, Front Range and Bighorn (using the seismic and structural results from the newly completed Bighorn Project) arch geometries can be generalized into a 4D (3D space + time) model for basement-involved foreland orogens. The final stop visits the enigmatic Boulder-Weld County fault system in the Denver Basin to the east to discuss the relative importance of regional tectonic setting (e.g., low-angle subduction), stress, lithospheric rheology, gravity, and fluids to the formation of basement-involved foreland thrust belts.
Figures & Tables
Prepared in conjunction with the 2016 GSA Annual Meeting in Denver, Colorado, this volume contains sixteen guides to field trips in this rich geologic region. The four “Great Surveys” of the late 1800s ventured west to explore and document the region’s unknown natural resources and collect valuable geologic information. Many of the field guides in this volume, aptly titled Unfolding the Geology of the West, will cover the same hallowed ground as the early geologic expeditions. Organized into four sections, this volume spans some of the major subdisciplines of geology: (1) stratigraphy, sedimentology, and paleontology; (2) structure and metamorphism; (3) Quaternary landscape evolution; and (4) engineering and environmental geology.