SUMMARY OF THE OLYMPIA NONGLACIAL INTERVAL (MIS 3) IN THE PUGET LOWLAND, WASHINGTON

Deposits of the Pleistocene Olympia nonglacial interval, correlated to marine isotope stage 3, from 15,000 to 60,000 years old, are widely distributed but discontinuous in the Puget Lowland. These deposits, herein called "Olympia beds", are most easily identifed by provenance of clastic detritus, depositional environments, and age. Criteria for recognizing Olympia beds include presence of andesite-rich sand, lavender color, organic layers and paleosols, diatomites, and tephras. The topographic relief on the Olympia beds is likely greater than 230 m, ranging as high as 170 m above sea level and 60 m below modern sea level. Thickness, elevation, grain size, and composition all vary significantly over short lateral distances. The thickest Olympia beds (> 25 m), found near Tacoma, include multiple tephra, lahar, peat, and diatomite layers. The lack of Olympia-age lahars and andesitic sand on the west side of Puget Sound suggest that Olympia-age equivalents of Colvos Passage and the Tacoma Narrows channels were present near their current locations during that time. Forty new radiocarbon dates on peat and paleosols confirm their correlation with the Olympia interval. At least five Olympia-age tephras and lahars have been identified near Tacoma, with source areas including Mt. St. Helens and Mt. Rainier. Paleoecological analyses indicate a wide range of paleoenvironments for the Olympia interval. Many locations of Olympia beds yield excellent pollen preservation with a predominance of pine and spruce; freshwater diatomites suggesting clear, shallow lakes and large littoral areas; and macrofossils including mammoth teeth and tusks, Pinus sp.? cones and needles, branches, leaf prints, and in-situ tree roots. Olympia beds are locally deformed by folding and possible liquefaction, such as at Mee Kwa Mooks Park in Seattle and near the Tahlequah ferry terminal in Tacoma. Some of the deformation certainly has a tectonic origin. Since the Olympia beds are a mappable unit, they may provide limiting ages on tectonic deformation in the Puget Lowland, and on the timing of the subsequent Vashon glacial advance.