POST-LARAMIDE RIVERS, UPLIFT, AND NEW AGE CONSTRAINTS ON THE ROCKY MOUNTAIN EROSION SURFACE IN WESTERN COLORADO

Following the Laramide Orogeny, Paleogene rivers carved the widespread Rocky Mountain Erosion Surface (RMES) in Western Colorado and surrounding regions. In Western Colorado, the RMES is overlain by poorly dated river deposits as well as Oligocene volcanic units. New detrital sanidine (DS) ⁴⁰Ar/³⁹Ar dates for the river deposits help constrain the age of the RMES.

Near Black Canyon of the Gunnison, ~30 m of river gravel unconformably overlie the RMES and Precambrian basement rocks at an elevation of ~2500 m. The river gravels are a localized basal facies of the West Elk Breccia, which originated from the West Elk stratovolcano. Two samples of the basal river-gravel matrix produced 353 DS dates. 45% of the dates are ca. 31 Ma and the youngest age population produced a maximum depositional age (MDA) of 29.2 Ma. These dates suggest that burial of the RMES by West Elk Breccia occurred ~31-29 Ma. West Elk river gravels also mark an important re-organization of drainage patterns. During the Laramide Orogeny, rivers drained the Gunnison uplift and flowed northward into the Piceance Basin whereas by ca. 31-29 Ma, rivers flowed west-southwest across the eroded Gunnison uplift and away from the newly formed stratovolcanoes and shallow intrusive bodies of the West Elk Mountains.

On Cimarron Ridge located west of Montrose, Colorado, river deposits of the Telluride Conglomerate unconformably overlie the RMES, and are overlain by the volcanic Oligocene San Juan Formation. The RMES is at an elevation of ~3000 m. Three samples of Telluride Conglomerate produced 331 DS dates and a MDA of 34.5 Ma. Paleocurrent data and the presence of granitic clasts indicate that Telluride Conglomerate rivers flowed west away from the Sawatch Range and the Gunnison uplift.

In summary, new DS ages suggest that the RMES was forming ca. 35-30 Ma during a time of regional uplift and river erosion. This timeframe coincides with major caldera-forming eruptions in the Sawatch Range, which suggests the possibility that uplift was preceded by early stages of the ignimbrite flareup in Colorado. The difference in elevation (~500 m) between the ca. 35-30 Ma river gravels overlying the RMES suggests that either several hundred meters of incision occurred between the time represented by the river gravels, or that there has been significant differential uplift of the two study areas since the river gravels were deposited.