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