Stratigraphy of Lacustrine Sediments Cored in 1996, Bear Lake, Utah and Idaho Open file rept

摘要

The overall goal of our research on Bear Lake is to create records of past climate change for the region, including changes in precipitation (rain and snow) patterns during the last 10,000 years and longer. As part of the project, we are attempting to determine how the size of Bear Lake has varied in the past, to assess the possibility of future flooding and drought. We also seek to understand human influences on sediment deposition, chemistry, and life in the lake. The overall structure of the basin, as reflected by the bathymetry of Bear Lake (Denny and Colman, 2003) and the pattern of deposition of its sediments (described herein), is that of an eastward tilting half-graben, controlled by a master normal fault along the eastern margin of the lake. This fault, the East Bear Lake fault, is steep at the surface and becomes listric at depth. It has experienced 46 episodes of faulting in the last 40,000 years, the youngest of which occurred about 2,500 years ago (McCalpin, 2003). The western margin of the graben is largely a hinge or a ramp, but it also is marked by normal faults, the youngest movement on which occurred about 7,000 years ago (McCalpin, 2003). This report describes the stratigraphy of sediments beneath Bear Lake along a transect of cores obtained in 1996. The stratigraphy is derived mostly from high-resolution acoustic-reflection profiles, supplemented by analytical information from cores that indicate correlations among various horizons. The deep drill hole obtained in 2000 (BL00-1) also is along the same acoustic profile, so the stratigraphy of its upper part also is described in relation to the 1996 cores.