FISSION-TRACK EVIDENCE OF LATE TERTIARY COOLING AND INCISION OF THE WIND RIVER BASIN, WYOMING

Two deep wells (total depths of 3,700 and 6,000 m) penetrating southwest-vergent basement thrust sheets on the edge of the Wind River Basin, one through the Owl Creek Mountains and one through the Casper Arch, were sampled for apatite fission-track dating to constrain local heating and cooling events in this part of Wyoming. Fission-track age profiles from these wells show two breaks in slope. A near surface break in slope (indicating rapid cooling) represents the local onset of the Laramide Orogeny. A second, younger break in slope, seen at deeper levels records post-Laramide cooling.

Laramide cooling ages are widely reported from ranges surrounding the Wind River Basin, however they don’t show evidence for post-Laramide uplift. Perched remnants of Tertiary tuffaceous deposits in the region as well as vitrinite reflectance data from the basin suggest that 1 to 3 km of post-Laramide basin fill was present, consistent with our fission-track results. The implication is that maximum burial, and hence heating, was post-Laramide in age and predates final exhumation of the basin. The younger break in slope indicates when final exhumation and cooling of the area began.

These results are consistent with those of published fission-track studies from the northern Green River Basin and the Powder River Basin also in the central Rockies that indicate post-Laramide (Miocene-Pliocene) final cooling. Up to 1 km of erosion has taken place in both of these basins, since ~4 Ma in the northern Green River Basin and since ~12 Ma in the southern Powder River Basin. The thermal history outlined here supports the view that significant incision of the central Rocky Mountains took place beginning in late Miocene or younger time, not dissimilar to the inferred history of Colorado Plateau to the south.