RECONSTRUCTING STREAMFLOW IN THE OLYMPIA NONGLACIAL DEPOSITS, PUGET SOUND REGION, WASHINGTON: AN ANISOTROPY OF MAGNETIC SUSCEPTIBILITY STUDY

The flow of proglacial streams during glacial retreat has implications for rates of ice loss in changing climates. The Puget Sound lowland has experienced several glacial advances and retreats, the deposits from which structured the region’s current landscapes. During a glacial retreat correlative with oxygen isotope stage 3 (15-60 ka), rivers draining from the Puget Lobe of the Vashon Ice Sheet deposited what are now known as the Olympia Beds. Although the paleotopography and sedimentology of the Olympia Beds have been studied, the paleocurrent directions and magnitudes are still incompletely characterized. Here we use anisotropy of magnetic susceptibility (AMS) to determine paleocurrent indicators of the Olympia Beds. Beds were correlated across discontinuous outcrops using distinctive mineral assemblages. The study was designed primarily as a proof of concept, to assess the effectiveness of AMS in mapping proglacial streamflow. In spite of the coarse grain size of the sediments and the difficulty sampling them, AMS measurements yield consistent and overall plausible results at a site level. AMS ellipsoids measured from outcrops along the SW shore of Commencement Bay are dominantly prolate, with major principal axes oriented N/S, suggesting an overall southerly flow direction. This paves the way for the use of AMS to reconstruct streamflow patterns within the Olympia Beds, through which we hope ultimately to gain insight into processes of modern glacial retreat.