THE GLACIAL/DEGLACIAL HISTORY OF SEDIMENTATION IN BEAR LAKE (UTAH AND IDAHO)

Bear Lake, in northeastern Utah and southern Idaho, lies in an active half graben at an elevation of about 1800 m, covers an area of 280 km², and is 63 m deep. The Bear River, the largest river in the Great Basin, enters the Bear Lake Valley about 15 km north of the lake but did not flow into the lake during historic times. Several 4 to 5 m cores provide a lake sediment record extending back about 32 cal. kyr. The penetrated section can be divided into two parts: a lower unit composed of quartz-rich clastic sediments and an upper unit composed largely of endogenic carbonate. Data from modern fluvial sediments provide the basis for interpreting changes in provenance of detrital material in the lake cores. Sediments from the local catchment are characterized by abundant dolomite, high magnetic susceptibility (MS) related to aeolian magnetite, and low values of hard isothermal remanent magnetization (HIRM, indicative of hematite content). In contrast, sediments from the headwaters of the Bear River in the Uinta Mountains (which makes up a very small part of the Bear River catchment) lack carbonate and have high HIRM and low MS, whereas sediments from lower reaches of the Bear River contain calcite but little dolomite and have low values of MS and HIRM. These contrasts in catchment properties allow us to interpret the following sequence from variations in mineralogy, chemistry, and magnetic properties of the lake sediment: (1) 32-24 cal. kyr – quasi-cyclical, millennial-scale variations in the concentrations of hematite-rich glacial flour derived from the Uinta Mountains and dolomite- and magnetite-rich material derived from the local Bear Lake catchment (reflecting variations in glacial extent); (2) 25-24 cal. kyr – maximum content of glacial flour; (3) 24-18.5 cal. kyr – constant input of Bear River sediment but declining input of glacial flour from the Uinta Mountains; (4) 18.5-17 cal. kyr – decline in Bear River sediment and increase in content of sediment from local catchment (indicating the initial withdrawal of the Bear River), and (5) 17-14.5 cal. kyr – increase in content of endogenic calcite at the expense of detrital material (transition from lower to upper unit as the lake became evaporative).