A 40 KYR LACUSTRINE RECORD FROM THE TERRETON BASIN, IDAHO: FLOODS, CLIMATE CHANGE, OR BOTH? (Invited Presentation)

The Terreton Basin is a large (~1000 km²) but shallow (0-20 m) basin located on the Snake River Plain (SRP) roughly 60 miles northwest of Idaho Falls, Idaho. The basin is fed by mountain rivers which flow southward onto the SRP before their discharge is absorbed into a permeable basalt aquifer. Ubiquitous lacustrine deposits within the basin demonstrate that Lake Terreton has been large at multiple times in the past, making it a potentially valuable climatic archive. Lake Terreton is also of interest because it was apparently filled by the Big Lost River Flood at ~22 ka, creating a hydrologic analog for similar fill-and-spill basaltic craters on Mars. Our work focuses on identifying and dating cryptic shoreline deposits around the margins of the Terreton Basin. At the time of writing, results from 10 radiocarbon ages and roughly 20 OSL samples demonstrate that the lake experienced near overflow highstands at roughly 35, 22, and 15 ka. Sedimentary deposits in the southeast corner of the Terreton Basin confirm that the basin likely overflowed through a topographic low roughly 25 miles north of Idaho Falls during one or more of these highstands. The 22 ka highstand matches the age of the Big Lost River Flood as determined by cosmogenic ³He dating of flood-deposited boulders, suggesting that the lake basin was indeed filled by this flood. The 15 and 35 ka highstands appear more likely to be climatically driven, although this remains an open question. Numerous minor highstands have also characterized smaller portions of the Terreton Basin, including a period of sustained lacustrine conditions from 17-14 ka, and smaller (transient?) highstands around 11-10 ka, 3.2 ka, and 0.38 ka. Although this chronology is preliminary it showcases the power of combining multiple chronometers in a complex geologic setting. The oral presentation will compare results from multiple methods on both outcrop and regional scales, highlighting the strengths and weaknesses of ³He, ¹⁴C, and luminescence dating in this setting.