BASIN-AND-RANGE TECTONICS AND QUATERNARY LAKE-LEVEL CHANGE IN BEAR LAKE VALLEY, UTAH AND IDAHO

Bear Lake resides in the southern end of Bear Lake valley, a complex, N-S trending graben in the eastern Basin and Range province. Dated shoreline deposits show that the elevation of Bear Lake fluctuated up to 25 m higher than modern between ~ 400 – 120 ka. As the level of the lake rose, it captured the Bear River (which currently bypasses the lake to the north) and transgressed to an outlet at the north end of Bear Lake valley. The elevation of Bear Lake was then controlled by the outlet elevation, which is currently 12 m below modern lake level. The outlet elevation must have been at least 37 m higher in the past to allow highstands of Bear Lake. However, no evidence has been found for a temporary obstruction, such as a landslide, that could have raised the outlet high enough to impound a lake 25 m above modern lake level. Movement on valley-bounding normal faults has undoubtedly changed the geometry of Bear Lake valley and affected the elevation of its outlet; therefore, tectonic activity influenced the elevation of Bear Lake. The morphology of range fronts in Bear Lake valley and slip-rate calculations on the east Bear Lake fault support the regional tectonic model that active normal faulting has migrated outward away from the Snake River plain during the Quaternary. The local expression of this regional faulting pattern in Bear Lake valley involved the southward migration of active displacement on the (N-S striking) east Bear Lake fault, which, along with stepwise motion on cross-valley (E-W striking) normal faults, caused differential lowering of the valley floor to the southeast. This pattern of normal faulting may have ultimately caused lake-level change by either 1) uplifting the Bear Lake valley outlet relative to the southern valley, and/or 2) reversing or reducing the gradient of Bear River, causing it to flow southward into Bear Lake or aggrade in northern Bear Lake valley and raise the elevation of the outlet. The first of these two mechanisms may have led to transgressions of Bear Lake up to 25 m between ~ 400 – 120 ka, and the second, combined with climate change, may have led to more minor transgressions since ~ 120 ka that interrupted an overall 25 m drop in lake level. After rising to the Bear Lake valley outlet, downcutting by an out-flowing Bear River caused lake level to drop and the north shoreline of Bear Lake to retreat southward.