PRELIMINARY 10BE AGE EVIDENCE FOR PRE-SACAGAWEA RIDGE GLACIATION FROM SINKS CANYON, WIND RIVER RANGE, WYOMING

Two till units above Sinks Canyon, Wind River Range (WRR), Wyoming, lie beyond deposits mapped as Sacagawea Ridge moraines. Preliminary analyses of 10Be concentrations from six boulders on the highest of the two tills (Table Mountain) result in maximum ages ranging from ~270 to 736 ka (PRIME Lab RICH program; production rate of 6 + 0.3 10Be atoms gsuperscript-1 asuperscript-1 at sea level and high latitude). Alternate calculations (Stone, 2000) resulted in maximum ages of ~135 to 783 ka using a production rate of 5.1 10Be atoms gsuperscript-1 asuperscript-1. PRIME Lab maximum ages resulted from calculating boulder erosion rates of from 2.3 mm/kyr on the oldest boulder to 6.5 mm/kyr on the youngest boulder. Alternate maximum ages were calculated using a constant erosion rate of 2 mm/kyr and a different scaling factor from high latitudes sea level to the boulders at the Table Mountain altitude and latitude.

The 10Be ages from Table Mountain suggests the till was deposited prior to ~1 Ma if we assume the boulders were initially buried and since have been exhumed. We assume considerable erosion of the till from the combination of the boulder age estimates and the presence of meso-channeling and a ubiquitous cover of rock fragments on the soil surface.

No evidence presently exists for glacial events earlier than mid-late Pleistocene from the WRR. Recent work at Cedar Ridge (1999, GSA Bull., v. 111: 1233-1249) indicates deposits previously described to represent early-to-mid Pleistocene glaciations in the WRR (Cedar Ridge and Washakie Point) actually correspond to the Sacagawea Ridge glaciation (~730-600 ka). If additional cosmogenic nuclide analyses (26AL and 36CL) confirm the preliminary early-mid Pleistocene age for the Table Mountain unit, then it presently represents the only evidence for pre-Sacagawea Ridge glacial activity from the WRR.