SEDIMENTARY INDICATORS OF EXTREME SHALLOWING OF BEAR LAKE (UTAH AND IDAHO) IN THE LATEST PLEISTOCENE

Bear Lake is a 60-m-deep, 282-km² lake occupying a half-graben basin. Three cores collected along an east-west transect were in water depths of about 50 m (BL96-1), 40 M (BL96-2), and 30 m (BL96-3). A fourth core on the north end of the lake was in about 6 m of water (BL2K-2). Core BL96-1 recovered about 5 m of tan aragonitic mud. Core BL96-2 consists of tan aragonitic mud (0 to 1.8 m), interbeds of gray calcitic clay and tan aragonitic mud with abundant ostracodes (1.8 to 2.2 m), and carbonate-poor gray clay (2.2 to 3.9 m) becoming mostly red clay (3.3 to 3.9 m). Core BL96-3 consists of ostracode-rich tan aragonitic mud with gray calcitic clay interbeds (0 to 0.1 m), gray or greenish-gray clay with abundant ostracodes (0.1 to 0.4 m), then carbonate-poor gray clay (0.4 to 0.7) that is predominately red with greenish bands (0.7 to 3.9 m). BL2K-2 is a shell-rich sand in the upper 5 cm, tan aragonitic mud from 5 to 30 cm, gray mud and shell-rich sand from 30-55 cm, and carbonate-poor interbedded clay and sand to the base of the core at 3.5 m. The transition from clay-rich sediment to aragonitic mud records the change from a lake dominated by inflow of the Bear River during the Pleistocene to a groundwater-fed lake during the Holocene.

Features resembling root casts occur in the upper part of the clay-rich sediments in BL96-2 and -3. These are hollow tubes with rinds of pyrite crystals and thin, oriented clay linings. The tubes range from hair widths to about 0.5 mm in diameter, and, in places, larger tubes branch to smaller tubes. In each core, the tubes comprise four 5-to-15-cm-thick zones with larger tubes grading downward to smaller tubes. The lowest zone in BL96-3 also has mud-filled tubes as much as 0.5 cm in diameter that taper and branch downward and have partial sulfide lining. In BL2K-2, pyrite crystals coat plant fibers with morphologies similar to the hollow tubes in BL96-2 and -3. These comprise a single 15-cm-thick zone that is sharply capped by shell-rich sand.

Root structures in sediments require very shallow water or subaerial exposure. Carbon dates from BL96-2 indicate that they formed before 7000 BP and two dates within the rooted zones give dates of 10,300 and 12,700 BP. These dates are similar to periods of severe drought in other Great Basin lakes. The low lake depths may also be related to Late Pleistocene diversion of the Bear River and/or drought conditions slowing groundwater inflow.