Seismic-Stratigraphic Framework for Drill Cores and Paleoclimate Records in Bear Lake, Utah-Idaho

A 200-km network of high-resolution seismic-reflection profiles was collected in Bear Lake, Utah-Idaho, in order to explore the sedimentary framework of the lake's paleoclimate record as derived from cores and drill holes. In addition to a series of short cores, including piston cores as much as 5 m long, two deep holes were drilled at a site on one of the seismic-reflection lines. These continuously cored drill holes, 100 and 120 m deep, were part of the testing operations for the Global Lake Drilling-800 m (GLAD800) drilling system. The seismic stratigraphy below the lake indicates its principal origin as a simple half-graben, with a steep normal-fault margin on the east and a ramp margin on the west. Seismic reflections diverge toward the master fault, bounding eastward thickening sediment wedges. Secondary normal faults west of the master fault were imaged beneath the lake, and many of these faults show progressively increasing throw with depth and age. Several faults cut the youngest sediments in the lake as well as the modern lake floor. Although pinch-outs of sedimentary units are common in relatively shallow water, no major erosional or depositional features suggestive of shoreline processes were observed on seismic profiles in water deeper than about 5 m. The relative simplicity of the sedimentary sequence is broken in the northern part of the basin by what appears to be a large (2.5 x 9.5 km) bedrock landslide block, which overlies stratified lake sediment. Preliminary identification of volcanic ashes, gross correlation of climate proxies, and preliminary U-series data all suggest that the base of the deepest drill hole is about 250,000 years old, and that sedimentation has been relatively uniform at an average rate of about 0.5 m per thousand years. These data allow isochronous seismic-reflection horizons, tied to the drill hole, to be traced throughout the basin.