OSL CHRONOLOGY OF LATE PLEISTOCENE GLACIAL OUTWASH AND LOESS DEPOSITS NEAR IDAHO FALLS, IDAHO

The late Pleistocene in the Snake River Plain of eastern Idaho was characterized by braided streams transporting gravel-rich outwash from glaciated headwaters and widespread deposition of loess. Here, we present 12 new quartz single aliquot OSL ages for outwash and loess from the vicinity of Idaho Falls.

OSL samples for the outwash were taken from gravel pit exposures of cross-bedded, lenticular sand beds interpreted to be bar-top deposits. These yielded ages of 25.2 ± 1.5, 23.3 ± 1.5, 14.4 ± 0.8, and 12.6 ± 0.7 ka. The older ages are from the surface of the Pinedale outwash plain at its maximal extent. We interpret the younger ages as final stages of outwash deposition before development of the Holocene Snake River.

Loess was sampled in two backhoe pits on lava flows adjacent to the Snake River. Samples were collected every 20 cm and analyzed for particle size, pH, % carbonate, electrical conductivity, magnetic susceptibility, and soil properties. OSL samples were restricted to C horizons. At the 5.2 m deep Michelson pit, loess >15 m thick overlies a ~580 ka basalt lava flow. A weak paleosol is present near the base of the pit. A sample below this paleosol has an age of 75.3 ± 5.2 ka, corresponding to the MIS4 Early Wisconsinan period of loess deposition recognized elsewhere in the Rocky Mountains. Samples from depths of 3.80, 3.12, 1.78, and 1.05 m have ages of 21.9 ± 1.3, 20.6 ± 1.3, 18.4 ±1.1, and 17.5 ± 1.2 ka, respectively. A regression age-depth model indicates a deposition rate of 0.63 m/ka and the end of deposition at 15.7 ka. The 4.0 m deep Griffin pit samples loess trapped between pressure ridges of a late Pleistocene lava flow. Ages from depths of 4.03, 2.70, and 1.90 m are 24.3 ± 1.9, 22.2 ± 1.6, and 20.5 ± 1.5 ka, with a model deposition rate of 0.56 m/ka and end of deposition at 17.2 ka.

Our outwash ages suggest initiation of Pinedale glaciation in the Snake River headwaters as early as ~25 ka and deglaciation at ~13-14 ka. This is consistent with terminal moraine ages from the eastern Tetons of 14.6 and 13.5 ka (Pierce and Licciardi, 2008, Quat Sci Rev 27, 814–831). The end of loess deposition at ~16-17 ka is similar to the timing of regional deglaciation in Idaho, the Uinta Mountains, and western Yellowstone.