New insights into the outlet conditions of late Pleistocene Lake Bonneville, southeastern Idaho, USA

Abstract

Geologic, geomorphic, and geophysical analyses of landforms, sediments, and structures in northern Cache Valley, USA, document a revised history of flooding and recession of Lake Bonneville, the world9s premier pluvial lake. Crosscutting relationships suggest that the Riverdale fault produced a surface-rupturing earthquake ∼25 km southeast of Zenda shortly before, during, or after the Bonneville flood, as well as possible younger surface ruptures. Thus fluctuating stresses and pore pressure induced by changing lake levels may have triggered a large earthquake that, in turn, triggered the Bonneville flood. The flood scoured ∼25 km of Cache and Marsh Valleys and activated landslides during its ∼100 m incision to a new outlet near Swan Lake, Idaho. One to two thousand years of steady outflow produced the main ∼4775 ± 10 ft (1455 ± 3 m) Provo shoreline, ∼10 m above the commonly accepted altitude. Later Lake Bonneville oscillated below the main Provo shoreline, incised the Swan Lake outlet, rose and paused briefly at a new lower Provo sill (4745 ± 10 ft [1446 ± 3 m], P9?) 23 km south of Zenda, before reverting to a closed-basin condition. Correlation to the Blue Lake chronology of Benson et al. suggests that aridity during the Heinrich 1 event activated the lower Provo sill ∼15.9 ka. An abandoned, meandering riverbed, north of the lower Provo sill, records a large northward flowing river. The Great Basin9s modern divide at Red Rock Pass formed in the Holocene when a small alluvial fan filled the dry bed of this paleoriver.