Bear Lake Geologic Background: Tectonism

Tectonism (land movement)

View of Bear Lake looking towards the fault-uplifted east side.

The Bear Lake basin developed as part of fault subsidence that continues today, slowly deepening the lake along the eastern side. The Bear Lake graben (valley bounded by faults) is 80 km long and 7-14 km wide. It extends across the Utah-Idaho border and involves faults on both eastern and western sides of Bear Lake. Faulting along eastern Bear Lake is similar and related to faulting that created other deep basins: Cache Valley, Utah and Star Valley, Wyoming (McCalpin, 1993). Hundreds of thousands and possibly millions of years of sediment have accumulated in the Bear Lake Valley. This potentially long sediment record was one of the first characteristics that excited our interest in studying Bear Lake.

The age of the lake itself is uncertain, but a 1975 seismic study revealed 380 m of lake sediments above bedrock along the eastern side of Bear Lake (Skeen, 1975). This trend was confirmed in an acoustic survey completed by the USGS in 1997. Our radiocarbon dates indicate a sedimentation rate varying from .85 mm/yr on the east side to .215 mm/yr on the west side of the lake. At that rate, 380 m of lake sediment means the lake is at least 455,000 years old.

The faults around the lake are still active, but large magnitude earthquakes are relatively infrequent. Three quakes of magnitude 7+ on the eastern fault and two on the western fault have shifted the valley floor by as much as 5.6 m (18.4 ft) in the last 6500 years (McCalpin,1993). The most recent earthquake of that size was about 2000 years ago. Earthquakes often trigger landslides and it is likely that a large quake led to a slide that blocked drainage of the Bear River to the north. The water from the river flooded the valley, forming a much larger Bear Lake. Eventually the river eroded down through the landslide, allowing more and more of the lake to drain out, until only an isolated pool of water was left in the deepest basin. For more information on recent earthquakes check out these earthquake sites:

Newspaper Articles for 1884 Bear Lake, ID Earthquake
SUMMARY OF 1884 BEAR LAKE, ID EARTHQUAKE

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