NEW COSMOGENIC 10BE EXPOSURE AGE CHRONOLOGY FOR PAYETTE MORAINES, CENTRAL IDAHO, USA

The Payette moraines near McCall, Idaho are a classic site for weathering rind relative age determinations (1). New mapping (2) has refined and strengthened correlation of units. Here, we report a cosmogenic ¹⁰Be surface exposure chronology for the moraines based upon 18 erratic boulders. Heights of sampled erratics average 2.8 m, precluding significant shielding by snow or soil. Aplite dike relief indicates that erosion of surface boulders is proceeding at 1-2 mm/ka. Cited ages are error-weighted means of 3-5 analyses with 1σ analytical errors. The oldest dated unit is the massive lateral moraine of Timber Ridge, correlated to the Bull Lake glaciation. It yielded a mean age of 113.4 ± 5.5 ka, or 135.3 ± 8.0 ka when corrected for an erosion rate of 1.7 mm/ka. The moraine representing the local last glacial maximum of the Pinedale glaciation produced an uncorrected mean age of 17.1 ± 0.9 ka. Erosion corrections for this and other last glacial moraines are negligible and are neglected. An adjacent recessional moraine yielded a mean age of 15.7 ± 1.1 ka. A moraine about 2 km upvalley gave a mean age of 14.2 ± 0.8 ka. These results agree well with recent regional chronologies from the Uinta, Yellowstone, Sawtooth, and Wallowa areas indicating local last glacial maximums at 16-18 ka followed by deglaciation by about 14 ka.

(1) Colman, S.M. and Pierce, K.L., 1986, Glacial sequence near McCall, Idaho-weathering rinds, soil development, morphology, and other relative-age criteria: Quaternary Research v. 25, p. 25-42. (2) Breckenridge, R.M. and Othberg, K.L., 2005, Surficial Geologic Map of the McCall Quadrangle, Valley and Adams Counties, Idaho: Idaho Geological Survey Digital Web Map DWM-35, scale 1:24,000. This study supported in part by a USGS STATEMAP project.