INVESTIGATING THE LATE-GLACIAL TANWAX FLOOD—A LITHOLOGIC STUDY OF SEDIMENTS IN SELECTED MOUNDED TERRACES IN THE PUGET LOWLAND

For 20 years researchers have tracked the late-glacial Tanwax flood from glacial lake Carbon to Mima Prairie in the Puget Lowland. The flood incised the drumlinoidal topography of the Vashon Drift in the southernmost Puget Lowland and deposited coarse gravels and diamictons atop recessional outwash terraces locally, including those that now host Mima Mounds such as Rocky and Mima Prairies (Pringle and Goldstein, 2002; Goldstein et al., 2010). Overlying slightly-imbricated, coarse-bedded gravels, the Puget Lowland Mima Mounds are composed of a loamy diamicton that is lithologically and stratigraphically correlative with “upstream” deposits produced during a late debris flow phase of the Tanwax flood. This coincidence of Mima Mounds and Tanwax flood stratigraphy suggests a common origin. To further investigate the higher concentration of andesite and other Cascade Range lithologies within Tanwax flood sediments noted by Goldstein et al. (2010) and Tanji and Goldstein (2014), we collected more than 100 rocks from Mima Mounds in multiple locations at Rocky, west Rocky, and Mima Prairies as well as 100 rocks from the basal gravels at Mima Prairie to ascertain if those units were deposited by the Tanwax flood or by some other mechanism. Our provisional analysis of the rocks from west Rocky Prairie show that more than 85 percent of clasts are of Cascade Range provenance (largely andesitic) and 44 percent of clasts at Mima Prairie originated in the Cascade Range, while the Mima Prairie basal gravels contain more than 40 percent andesitic rocks. The relatively high concentration of andesitic rocks in the basal “outwash gravels” at Mima Prairie, distinct in lithology from till and outwash deposits of the Puget Lobe, suggests deposition by the pre-debris flow phase of the Tanwax flood. Further, the abundance of andesitic rocks in the Chehalis River valley at Centralia indicates an arm of the Tanwax flood likely reached the Chehalis River valley via the Skookumchuck River. This preliminary analysis strongly supports the hypothesis that diamictons composing the Mima Mounds at several Puget Lowland prairies were deposited by a debris flow phase of the Tanwax flood. Studies of the percentages of Cascade Range rocks in late-glacial terraces of the lower Chehalis River valley will further assess the downstream extent of the flood.