A FRESH LOOK AT LATE PLEISTOCENE MOUNTAIN GLACIATION IN CENTRAL IDAHO

The history of Idaho mountain glaciation has been studied for decades, but suffers a dearth of radiometric dates. We summarize chronologies from the region, and present new insights from OSL dating and fault scarp analysis.

Two 10Be chronologies exist for western central Idaho. The classic moraine sequence near McCall includes moraines dating to MIS 6, 4, and 2 (Phillips et al., 2007, GSA abstract). A moraine intermediate to the MIS 4 and 2 moraines remains undated. Sherard (2006, WWU MS thesis) and Easterbrook et al. (2011, in Easterbrook, editor) developed a 10Be chronology at Redfish Lake in the Sawtooth Mountains and concluded that the maximum Late Pleistocene glaciation occurred during MIS 2. Additional, outermost Late Pleistocene moraines in nearby drainages remain undated.

Recent OSL dating of outwash fans and terraces and analysis of fault scarp offsets indicate prominent MIS 4 and 3 glaciation in the Pioneer and Lost River ranges of east-central Idaho. Progressively higher fault scarps cut an outwash terrace sequence at Rock Creek in the Lost River Range. Applying published offset rates to those scarps yields age estimates in MIS 2-4, broadly correlative with a detailed OSL chronology on outwash fan sediments in adjacent Cedar Creek (Kenworthy et al., 2014). An OSL age from outwash in the Pioneer Mountains indicates an early MIS 2 advance of the Wildhorse Creek glacier, while 3-5 geomorphically older, Late Pleistocene moraines suggest extensive MIS 3 and/or 4 glaciation. A 30-35 ka outwash terrace age on outwash downvalley (Kenworthy et al., 2014) supports this inference. These findings bear implications for the timing of the Big Lost River outburst flood, sourced from this drainage.

Thus, there are indications of prominent MIS 4 and 3 glaciation in central Idaho that deserve closer scrutiny. In general, much remains to be learned about central Idaho mountain glaciation. The region represents a transition from wetter, Pacific-influenced climates and drier continental climate, and firm chronologies are likely to yield new insights into Late Pleistocene paleoclimatic patterns.