Document Type
Presentation
Publication Date
1-1-2019
First Page
1
Last Page
16
Abstract
The Bear River’s course has shifted over Quaternary time, and its late Pleistocene integration into the Bonneville basin long has been recognized as a possible explanation for why Lake Bonneville was apparently larger than the preceding lakes in its basin, and the only one to overflow its topographic threshold.
The middle-Pleistocene Bear River joined the Snake River to the north, likely via the Portneuf River drainage. Then an episode of volcanism in the Blackfoot-Gem Valley volcanic field ~100–50 ka diverted the Bear River southward into Gem Valley. Previous chronostratigraphic and isotopic work on the Main Canyon Formation in southern Gem Valley indicates internal-basin sedimentation during most of the Quaternary, with a possible brief incursion of the Bear River ~140 ka. New evidence confirms that the Bear River’s final diversion at ~55 ka led to its integration into the Bonneville basin by spill-over at a paleo-divide above present-day Oneida Narrows dam. This drove rapid incision of 200 m of bedrock in the canyon and excavation of southern Gem Valley in the subsequent millennia, before the rise of Lake Bonneville back flooded the area, as constrained by new optically stimulated luminescence dates above, within, and below the canyon.
Bear River integration into the Bonneville basin early during marine isotope stage 3 seems to postdate the Cutler Dam lake cycle, although that penultimate pluvial lake is incompletely dated and understood. It is also possible the Bear River’s hydrologic addition relates to the recently recognized but poorly constrained Pilot Valley shoreline that predates the main Bonneville lake cycle. Regardless, the Bear River certainly contributed to the rise of Lake Bonneville, culminating in the Bonneville flood.
Recommended Citation
Pederson, Joel L.; Rittenour, Tammy M.; Jänecke, Susanne U.; and Oaks, Robert Q. Jr., "The Bear River's Diversion and the Cutting of Oneida Narrows at ~55-50 ka and Relations to the Lake Bonneville Record" (2019). Geosciences Presentations. Paper 8.
https://digitalcommons.usu.edu/geology_pres/8