OF 03-150 Home
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Bear Lake (Figure 1) is a tectonic lake that has existed for at
least several hundred thousand years. The lake basin
is a relatively simple half graben, a spoon-shaped depression
tilted toward the main fault on the east side of the lake. The U.S.
Geological Survey, in cooperation with researchers from several
universities, has been studying the sediments of Bear Lake since
1996 (http://climchange.cr.usgs.gov/info/lacs/index.html). The general purpose of this effort is to reconstruct past
limnological conditions and regional climate on a range of timescales,
from hundreds of years to hundreds of thousands of years. This research
relates to a variety of human concerns, including water usage in
the Bear River basin. Past work has included several coring operations,
a seismic-reflection survey, sediment-trap deployments, a barge-mounted
drilling operation with the GLAD800 drill rig, and a variety of
|Figure 1. Location map of Bear Lake, Utah-Idaho.
The objectives of the September, 2002 operations, preliminarily
reported here, were (1) to compile a detailed bathymetric map of
the lake using swath-mapping techniques, in order to provide baseline
data for a variety of applications and studies, and (2) to complete
a sidescan-sonar survey of the lake, providing a nearly complete
acoustic image of the lake floor. Limited amounts of subbottom acoustic-reflection
data (chirp) were also collected, along with samples of lake-floor
sediments representative of different kinds of backscatter patterns.
These surveys followed an earlier subbottom acoustic-reflection
survey (1997), using boomer and 3.5 kHz systems (S. M. Colman, unpublished
Past seismic-reflection work has indicated that faults secondary
to the east-side master fault cut the lake floor. These faults were
among the primary targets of the sidescan-sonar survey. Preliminary
interpretation of the data suggests that the morphology of the fault
scarps on the lake floor are too subtle to be imaged by the sidescan-sonar
system. However, some segments of the East Bear Lake fault at the
foot of the steep eastern margin of the lake, are visible in the
sidescan-sonar images. The other main targets of the sidescan-sonar
survey were possible springs discharging at the lake floor. Discharge
from such springs may be necessary to explain the chemistry and
mineralogy of the lake sediments. A number of structures that appear
to be related to spring discharge were observed in the sidescan-sonar
images, and sediments at some of these features were sampled.
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