Lake Bonneville's Tilted Shorelines Revisited: Implications for Late Pleistocene Climate

Abstract

Since G.K. Gilbert’s monumental work in the eastern Great Basin during the late 1800s, the late Pleistocene Lake Bonneville (30–10 ka) has been recognized as a natural laboratory for studies of both (1) deformation of the lithosphere and mantle due to surface loading and (2) climate–forced water balance changes since the Last Glacial Maximum (LGM; 21 ka). The predecessor to the Great Salt Lake, Lake Bonneville reached a maximum depth of 350 m and attained a surface area roughly equal to that of modern day Lake Michigan ( 50,000 km2). Remnants of this vast body of water are preserved as paleoshorelines and subaqueous depositional landforms that record a complex history of lake level changes induced by deglacial climate change. While shorelines normally are expected to exist at equal elevations along an equipotential surface throughout a lake’s extent, the paleoshorelines of Lake Bonneville instead exhibit a domed pattern, in which paleoshorelines near the center of the old lake reside 70 m higher in elevation than the paleoshorelines along the periphery (Gilbert, 1890; Currey, 1982). Although many studies have evaluated the deformation captured by these paleoshorelines in terms of the water load of the lake (i.e., hydro–isostasy), no published works have attempted to reconcile the relative contributions of the lake water load and the deflection from the coincident Laurentide ice sheet of North America. In this thesis, we simulate post–glacial rebound and hydro–isostasy of the Bonneville basin for a spherically symmetric, viscoelastic 3–D Earth model with laterally varying lithospheric thickness and topography. Although there currently are several limitations to the present version of the model, our order of magnitude calculations for the expected contributions of deformation by the Laurentide ice sheet and the water load of Lake Bonneville show that the present–day first–order pattern of the Bonneville paleoshorelines must be explained by a combination of both effects.