Geophysical surveys of Yellowstone Lake, Wyoming

Abstract

Reflection, refraction, and paleomagnetic data from Yellowstone Lake, Wyoming, were used to evaluate the Quaternary, tectonic, volcanic, and glacial history of this high mountain lake. Reflection data revealed a paleobasin beneath the main body of the lake containing up to 190 m of lake sediments deposited on intrusive or volcanic bedrock. Prominent reflectors are interpreted to correspond to glaciated layers, erosional surfaces developed during periods of desiccation, and sediments deposited during Recent lake impoundment by rhyolite flows. Volcanic units that outcrop at the south end of the lake are correlated with reflectors that plunge northward beneath the paleobasin, indicating at least 300 m of Recent subsidence of the Yellowstone caldera. Sedimentation rates of 10--13 cm/100 yr were deduced from paleomagnetic logs of shallow cores and suggest a maximum of 200,000 years of deposition within the paleobasin. Refraction data delineated a 1-km-deep, 5.0-km/s layer, interpreted as the top of a shallow crystalline intrusive or volcanic layer possibly associated with the Yellowstone caldera. A rapid decrease in the number of earthquakes occurring 4 km inside the geologically mapped caldera boundary corresponds to a zone of thermal gradients increasing from 200/sup 0/C/km to 800/sup 0/C/km and is interpreted to reflect the boundary ofmore » a deeper crustal structure, perhaps the edge of a cooling magma body. Contoured water depths obtained from high-resolution 7.5-kHz seismic reflection profiling provide a detailed bathymetric map of Yellowstone Lake.« less