SURFICIAL GEOLOGIC MAPPING IN THE ONEIDA NARROWS AREA, SOUTHEASTERN IDAHO

The rise of Lake Bonneville to its late Pleistocene highstand (30-15 ka) appears anomalously large. The lake grew ten-fold in area and is the only great Basin pluvial lake to overflow its threshold catastrophically. Lake expansion has traditionally been attributed to a climate shift where southward displacement of the jet stream brought cooler, wetter conditions to the region. Alternatively, the Bear River may have played a key role by contributing to the lake's expansion following its diversion south by young basaltic volcanics. Despite decades of speculation, little effort has been directed towards mapping and dating Bear River deposits in the key region where it enters the basin. Pleistocene deposits and volcanics near Oneida Narrows in southeastern Idaho record the arrival of the river to Lake Bonneville, but the timing remains imprecise. Ambiguities on the order of 50,000 years exist. Previous geochronology work in Bonneville Basin is abundant, but mostly focused on indirect dating by radiocarbon methods. The Bear River's input appears to have substantially raised the level of Lake Bonneville during the Pinedale (~110-9.6 ka), but not the Bull Lake (~200-130 ka) glaciation. Precisely when the Bear River began emptying into the lake and the hydrological impact on lake levels has not been factored into Great Basin paleoclimate models. New surficial geologic mapping in Thatcher Basin, Oneida Narrows gorge, and Bear River valley below the gorge sheds new light on episodes of middle and late Pleistocene lake filling and river incision. The mapping provides the framework for future chronologic investigations and paleohydrological interpretations for this area located at the transition between Great Basin and Rocky Mountains.