Fractures in the Field: Surprising Insights into Rocky Mountain Deformation from Faults and Joints

Title: Fractures in the Field: Surprising Insights into Rocky Mountain Deformation from Faults and Joints

Author: Jeff Glossa

Publication: The Outcrop, July 2009, p. 26

Note: Field trip hosted by: Eric A. Erslev and Cody Allen, Department of Geosciences, Colorado State University, Fort Collins, (Now at: Department of Geology and Geophysics, University of Wyoming, Laramie, WY) ; Coordinated by Jeff Glossa.

Summary of Field trip
Dr. Eric Erlsev of the University of Wyoming has spent the better part of the last 25 years investigating complex fracture relationships occurring in Phanerozoic rocks along the Rocky Mountains in Wyoming, Colorado and Northern New Mexico. Studies conducted by past and present Erslev graduate students, have collected over 20,000 minor faults along with a growing database of extensional fracture measurements. Recent work integrating surface data on Laramide deformation trends shows the overwhelming importance of ENE-WSW horizontal shortening by thrust faulting, with structural variability caused by reactivation of basement weaknesses and 3D geometric constraints.

The purpose of this field trip was to examine the mechanisms and timing of fractures in the northeastern Front Range, an area with arguably the most abundant fracture data in the Rockies. The foothills west of Fort Collins, Colorado are underlain by east-dipping hogbacks of Paleozoic and Mesozoic sedimentary rocks. These rocks cap Precambrian metasedimentary and igneous rocks, which form the higher peaks of the Front Range. Under Fort Collins, the sedimentary strata are nearly horizontal and are part of the broad Denver Basin.

Over the course of five stops participants were exposed to syn­Laramide strike slip faults evidenced by slickensided surfaces in the Horsetooth member of the Dakota Group, syn-Laramide NNW striking thrustfaults in the Lyons and Owl Canyon Formations, complex multistage jointing in the Lykins Formations, and finally recent gravity slides of the upper Dakota group initiated by road construction and heavy rains of the late 9Os. The analysis of minor faults in this area has provided crucial tests of Laramide hypotheses, with results favoring uni-modal shortening directions consistent with Laramide fold axes. However, some Rocky Mountain joints appear to have been formed by splitting during Laramide compression, while others have clearly formed due to a host of other mechanisms. Overall Eric Erslev’s expertise concerning Rocky Mountain deformation in combination with good weather made for an interactive learning session that may provide valuable insights into the Denver Basin’s development in the near future. Current Erslev graduate students Cody Allen (Noble Summer Intern), and Wes Clary provided field assistance and shared some insights into their individual research and knowledge of the general area.