MAGNETIC RECORDS OF ALPINE GLACIATION IN THE WESTERN U.S.A. DURING THE LAST GLACIAL INTERVAL

At Bear Lake (Utah/Idaho) and Upper Klamath Lake (Oregon) magnetic properties of sediments provide sensitive measures of glacial-flour content and yield high-resolution records of glacial growth and decay during the last glacial interval. Creation and preservation of such records requires that (1) properties of glacial flour contrast with those of other sedimentary components and (2) magnetic minerals are neither formed nor destroyed after deposition. In the Bear Lake watershed, glaciers were largely confined to a small headwater area of the Bear River underlain by hematite-rich rocks of the Uinta Mountain Group (UMG), which are not exposed elsewhere in the catchment. Because UMG detritus is abundant only in Bear Lake sediments of glacial age, hard isothermal remanent magnetization (a measure of hematite content) provides an excellent proxy for glacial flour. In contrast, the entire Upper Klamath Lake catchment, which lies to the east of the Cascade Range in southern Oregon, is underlain largely by basalt and basaltic andesite. Nevertheless, magnetic properties of fresh titanomagnetite-rich rock flour from glaciers on a Cascadean composite volcano contrast sharply with those of detritus from unglaciated areas in which weathering destroyed some of the titanomagnetite. For Upper Klamath Lake, magnetic susceptibility and other measures of titanomagnetite concentration provide quantitative measures of rock-flour content.

Glacial-flour records from the two lakes are remarkably similar and indicate maximum glacial-flour content circa 19 cal kyr followed by rapid decline beginning prior to 18 cal kyr. At Bear Lake the age of the decline in glacial flour coincides with cosmogenic exposure ages of terminal moraines in the upper Bear River valley. This concurrence supports the interpretation that the maximum amount and subsequent decrease in glacial flour are indicative of maximum glacial extent and glacial retreat, respectively, and more generally that increases and decreases in rock flour in these lake sediments represent waxing and waning of glaciers.