USE OF DENDROCHRONOLOGY TO STUDY A SUBFOSSIL FOREST AT KENT, WASHINGTON USA THAT WAS BURIED BY LAHAR-DERIVED SEDIMENTS FROM MOUNT RAINIER ABOUT 530 CE

We conducted dendrochronological investigation of subfossil tree specimens found during excavations in 1996 along the Green River floodplain in Kent, Washington. The trees were buried in an andesite-rich, lahar-derived sand and gravel from Mount Rainier and dated at ~1500 yr BP by Vallance and Pringle (2008). The Kent buried forest is located about 120 km (72 mi) downstream of Mount Rainier. Two thin tephra layers at Mount Rainier associated with distinct small or moderate eruptive events ~1,600–1500 yr B.P. are inferred to be correlative with clay-poor lahar deposits found locally along the White River and with the andesitic sediment that buried the trees at Kent (Sisson and Vallance, 2008). We studied wood samples from eleven of the trees, eight of which were Douglas-fir (Pseudotsuga menziesii). The samples were mounted, polished, and sanded before being scanned at 0.01mm resolution. Use of ImageJ software to measure the annual growth rings and program Cofecha for analysis of the measurements, as well as visual examination and cross-dating methods, revealed likely correlations among at least five of the subfossil trees. Four of the trees had begun growth of early wood before their death, indicating the trees likely died in the spring.

We investigated undated lahar deposits upstream (southeast) of Enumclaw at two locations: Mud Mountain Dam and in a terrace along the White River near MP 32 of WA State Route 410. At the latter location, two seeds recovered from a peat deposit 2–4 cm (~ 1 in) below a clay-poor lahar yielded radiocarbon ages of ~2,400 yr B.P. Using rates of peat accumulation of about 2.5 cm per century measured lower at the same outcrop, we infer that the MP 32 deposits are likely from one of two earlier Summerland eruptive episodes of Mount Rainier at either 2,500–2,400 yr B.P. or 2,200 yr B.P. and thus not correlative with the deposits that buried the Kent forest at least a half millennium later. We plan to continue our tree-ring analysis of the Kent buried forest in hopes of assembling a provisional floating chronology for this cohort of subfossil trees.