NEW CONSTRAINS ON THE INTEGRATION OF THE BEAR RIVER AND CUTTING OF ONEIDA NARROWS CANYON—IMPLICATIONS FOR THE BONNEVILLE RECORD

Lake Bonneville was notably larger than the preceding lakes in its basin, was the largest pluvial lake in the Great Basin, and was the only one to overflow its threshold catastrophically. Lake expansion relates to climate change of course, but it has also been recognized (though largely overlooked) that the late Quaternary diversion of the Bear River played a key role in the hydrodynamics of the system. Previous workers constrained the integration of the Bear River into the basin to 100-50 ka.

New mapping and OSL geochronology of deposits above, within, and below Oneida Narrows provides new constraints. A remarkably different Quaternary stratigraphy is recorded upstream versus downstream of the canyon. In the upstream Thatcher Basin, uppermost basin fill strata recording the arrival of the Bear River’s Sr isotopic signal is newly dated to ~50 ka, consistent with broader amino acid estimates. The Bear River then overcame internal drainage and was superimposed downstream over the Oneida hills at a point near present Oneida dam. 200-225 m of bedrock incision was subsequently accomplished before the deposition of ~26 ka Bear River gravels near modern grade. This rate of incision rivals that in the most active orogens on Earth. The 18 km-long Oneida canyon was then backflooded by the Bonneville highstand, with newly dated ~20 ka lake deposits preserved both within the canyon and atop the fluvial gravels upstream. This is a remarkable defeat of the largest river in the Great Basin by the apparently rapid rise of Lake Bonneville.

The entrance of the Bear River to the basin at ~50 ka falls between the penultimate Cutler Dam and most recent Bonneville cycles, matching a freshening of waters recorded by ostracodes in the basin. River integration therefore can account for the anomalous height of the Bonneville highstand and outlet flooding at Red Rock Pass.