Late Cretaceous Subsidence in Wyoming

Dag Nummedal, Colorado Energy Research Institute, Colorado School of Mines, Golden, CO 80401, phone: 303 384-2506, [email protected], Hongjun Luo, Institute for Energy Research, Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, and Shaofeng Liu, Faculty of Geosciences and Resources, China University of Geosciences, No. 29, Xueyuan Rd. Haidian District, Beijing, 100083, China.

The Farallon plate convergence with the western margin of North America during the late Cretaceous directly controlled rates and patterns of subsidence across the Rocky Mountains and Great Plains, through three linked mechanisms: 1) dynamic subsidence related to mantle convection above the subsiding slab – this subsidence mechanism operated on a wavelength of a few 1000s of miles and was in-phase along strike across most region, 2) flexural subsidence in the retroarc foreland basin landward of the Sevier orogenic belt – operating on a wavelength of less than 200 miles and probably asynchronous along strike, 3) dynamic subsidence or uplift related to plate convergence rate and subduction angle – in control of the temporal distribution of basement involved (Laramie) tectonism.

Quantitative modeling of subsidence induced by Sevier-belt flexure allows this component to be subtracted from the total subsidence across the region. One such detailed separation exercise has been performed across southern Wyoming, demonstrating that the Sevier-belt flexure influenced only the western parts of the Greater Green River basin, and that late Cretaceous subsidence from there eastward to Iowa was dominantly a product of dynamic subsidence. 3D modeling of the flexural forebulge in response to Sevier and Wind River thrusting demonstrates that this tectonic feature migrated southeastward in response to shortening on the Wyoming-Idaho salient of the Sevier thrust and the Wind River thrust, but rarely extended much farther east than the (tectonically younger) Rock Springs uplift.

This quantitative subsidence reconstruction reveals that most of the late Cretaceous Western Interior Seaway lay well to the east of the Sevier foreland basin; a finding that also is supported by mapping the forebulge as a zone of thin strata throughout the region.