INTERPRETING FLUID DYNAMICS OF THE LATE-PLEISTOCENE TANWAX-OHOP DEBRIS FLOW BY CLASTIC SEDIMENT ANALYSIS IN THE PUGET LOWLAND, WA

This study is a sedimentological analysis of late-glacial debris flow deposits, put in motion by a short-term, high-velocity glacial outburst flood at the Last Glacial Maximum. The flood source was Glacial Lake Carbon, which formed behind an ice dam in the Carbon River Valley on the southwestern flanks of Mount Rainier, WA. Glacial melt water was released at the initial phase of retreat of the Puget Lobe, undercutting and incorporating Pleistocene-aged Mount Rainier mudflow derived diamicton such as the Lily Creek and Wingate Hill Formations. The Tanwax-Ohop debris flow can be traced from the Carbon River Valley, east through Eatonville and Tenino, WA. Previous attempts to delineate the pathway of the debris flow have shown that the Mima mounds and other mound prairies of the Puget Lowlands lay within the debris flow pathway, on the first glacial recessional terrace. A combination of field measurements such as clast size, shape and orientation along with provenence and bulk grain size analyses will be used to interpret fluid dynamics and water content of the debris flow as it washed through the Ohop Valley and westward towards Tenino, WA and ultimately the Pacific Ocean. This portion of the study focuses on the region between the towns of Eatonville and Maytown, WA. Sediment samples from the mounds at Mima and Violet prairie are examined along with debris flow sediments, in attempt to confirm whether the mounds contain a downstream lateral facies of the same debris flow. This additional information is essential to our broader understanding of the behavior of the Tanwax-Ohop debris flow, and our overall understanding of the geological history of the Puget Lowland at the Last Glacial Maximum.