EXHUMATION OF THE GRANITE MOUNTAINS, WYOMING, FROM COSMOGENIC DIPSTICK DATING

Early Tertiary sedimentation in basins of Wyoming and northern Colorado buried large areas of the Rocky Mountain landscape that have since been re-exposed by Neogene exhumation. The Granite Mountains in central Wyoming are thought to be among the most recently exhumed, and may represent an intact Paleogene landscape over which deposition continued into the early Pliocene. The mountains are composed of erosionally resistant rock; there is little evidence of recent erosion and there is only a sparse colluvial apron capping Mio-Pliocene lake beds around the mountains. Preservation is enhanced due to the porous basin fill that limits surface drainage and thus also fluvial erosion. If the Granite Mountains are indeed a relict landscape with very little erosion, then their exhumation rate should be measurable by surface exposure dating of the newly exposed slopes. The mountains could be used for ‘dipstick dating’ in much the same way that nunataks can be used to date ice sheet ablation.

To determine the exhumation rate we collected 8 samples for exposure dating from bare rock slopes in a 184 m vertical transect at Lankin Dome. The lowest sample was from the present-day mountain base, within 1 m of where granite slopes emerge from basin fill, while the highest sample is from above the highest preserved lakebeds in the adjacent basin. Cosmogenic ¹⁰Be concentrations increase systematically with elevation, and are consistent with a simple model of exhumation and slow bedrock erosion. The best-fit models constrain exhumation rates to a range of ~200-300 m/My and bedrock erosion rates to ~2.5-3.0 m/My. The data indicate that exhumation at Lankin Dome commenced recently, near 0.5-1.0 Ma, likely due to recent incision of the Sweetwater River and its tributaries.