NEW INSIGHTS INTO THE OUTLET CONDITIONS OF PLEISTOCENE LAKE BONNEVILLE SOUTHEASTERN IDAHO, USA

Geologic, geomorphic, and geophysical analyses of landforms, sediments, and structures document a complex history of pluvial Lake Bonneville and its outlets. The lacustrine sill of Lake Bonneville shifted south from Zenda, Idaho, first 11, then 23 km during the Provo occupation. The subsequent Holocene establishment of the drainage divide at Red Rock Pass, just south of Zenda, resulted from an alluvial fan that dammed the north-sloping valley floor. Weak Late Cenozoic sediment formed the sill for all overflowing stages of the lake, including the highstand prior to the Bonneville flood. The Riverdale normal fault (landslide?) offset Bonneville deposits, but not younger Provo deposits ~25 km southeast of Zenda. We infer that rapid changes in water level may have induced slip on the Riverdale fault (landslide?) shortly before, during, or after the Bonneville flood. Although other processes may have played a role, it is possible that reservoir-induced-seismicity was the main cause of the Bonneville flood.

The flood scoured ~25 km of Cache and Marsh valleys, initiated modest landslides, and exhumed a new sill near Swan Lake. Lake Bonneville dropped ~100 m and stablilized south of the Swan Lake sill at the main ~4775 ± 10 ft (1456 ± 3 m) Provo shoreline. Later Lake Bonneville briefly stabilized at a lower Provo sill (4745 ± 10 ft (1447 ± 3 m), near Clifton, Idaho, >10 km farther south. An abandoned meandering riverbed in Round Valley, Idaho shows major flow of the Bonneville River northward from this youngest sill of Lake Bonneville.