SEDIMENTOLOGICAL EVIDENCE FOR AN ENRICHED GLACIAL OUTBURST FLOOD IN THURSTON COUNTY, WASHINGTON

Lacustrine sediments observed in the Carbon River Valley by Crandall (1963) indicate that it was the site of a lake blocked by the Puget Ice Lobe during the Last Glacial Maximum (LGM) in the Puget Lowland. The area of study presented in this research is centered around the town of Tenino, in Western Washington, and includes portions of the Maytown, Bucoda, East Olympia, and Tenino Quadrangles. In this region, the presence of matrix-supported rounded to sub-rounded andesite boulders contrasts with the igneous intrusive and metamorphic sub-angular boulders found in the Puget Lobe-derived glacial till of the area. Using the relationships between the positions of moraines, their outwash plains, and the elevations of successive river terraces incised into them, it is possible to constrain the time of deposition of the boulders between the LGM and the first recessional position of the Puget Lobe. In compiling the field observations it can be asserted that the mechanism of deposition for the distinctive matrix-supported andesite-rich diamicton was actually a debris flow resulting from the catastrophic release of Glacial Lake Carbon. Additionally, the subsequent undercutting and remobilization of the Lily Creek Formation and other andesite-bearing Mt. Rainier-derived units located along the pathway of the flood acted as the source material for the debris flow. Today, remnants of this debris flow exist in the mapping area at a maximum elevation of 370 ft. Also, the distinctive rounded to sub-rounded andesite boulders were found to exist in the matrix of the Mima Mounds of Rock, Violet, Mound, and Rocky Prairies as well as the Mima Mounds around Offutt Lake. Because these prairies all occur on top of outwash plains located only on the river terrace corresponding to the first recessional position of the Puget Lobe, and Mima Mound areas are found only on this terrace level (confirming the relationship first recognized on the East Olympia Quadrangle by Walsh and Logan, 2005) it is possible to use the presence of Mima Mounds as a criterion for the positive identification of the debris flow pathway within the study area.