⁴⁰Ar/³⁹Ar Geochronology of Laramide Basins on the Northeast Edge of the Colorado Plateau

M. E. Smith
University of Wisconsin-Madison, Dept. of Geology and Geophysics, Madison, WI

The aim of this project is to use modern ⁴⁰Ar/³⁹Ar methods to date tephra units within synorogenic Eocene strata in basins along the northeast edge of the Colorado Plateau in order to constrain the timing of Laramide deformation and sedimentation. The hypothesis that late Laramide deformation was characterized by north-south dominated shortening due to the northward movement of the Colorado Plateau is in debate at present. This project will test whether~4000 m of Eocene sediments in the “axial” North Park and Middle Park Basins record dextral transpression along the east side of the Colorado Plateau using detailed stratigraphy and temporal comparisons of sediments in the neighboring “ponded” Piceance Creek Basin.

Fieldwork will take place during summer 2001 in the Piceance Creek, North Park, and Middle Park basins. Detailed stratigraphic sections, paleocurrent measurements, and pebble counts will be made of the Green River, Coalmont, and Middle Park Formations. Stratigraphic sections will be plotted, time-correlated using tephra units, and analyzed using nonmarine sequence stratigraphic methodologies. Drill cores at the USGS repository in Denver will be utilized to correlate subsurface stratigraphy.

Single crystal laser-fusion measurements of sanidine grains from tuff samples will be conducted in the Rare Gas Geochronology lab at the University of Wisconsin. We have already successfully dated tuffs in the Green River Formation ofWyoming and Utah, producing age determinations from volcanic sanidine grains with 2r uncertainties of +0.13 m.y.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid