Abstract

The lower part of the Wilkes Formation (uppermost Miocene) exposed along lower Salmon Creek in Lewis County, Washington, consists of volcaniclastic-dominated deposits. The section contains a stacked series of volcanic runout mudstone beds overlain by more distal runout mudstone beds, interlayered with carbonaceous mudstone and lignite-woodmat beds that are in turn overlain by poorly sorted volcaniclastic mudstone and sandstone. The section contains a record of forested lowlands inundated by volcanic mudflows, followed sequentially by rising water level associated first with deposition in swamp and lake-margin environments and later by deposition in a lake environment. The lakebed sediments contain common siderite concretions of varied form, including coprolite-shaped concretions that are confined to lakebed deposits. The volcanic mudflow deposits are similar to deposits of mudflows-lahars of modern Cascades stratovolcanoes. Two volcanic ash fall beds contained within lignites in the middle of the section yield 40Ar/39Ar radiometric dates. After separation of plagioclase crystals into populations of cloudy appearance (inherited) and clear appearance (newly crystallized), a best age of 6.13 ± 0.08 Ma is determined for the lower ash bed. Sediments contain abundant and well-preserved pollen and spores that document botanical changes progressing from Nyssa-dominated to Taxodium-dominated to mixed forest assemblages. The existence of Taxodium and other warm-climate taxa in the Wilkes Formation indicates the presence of a wet, warm temperate climate in the Puget lowlands during the latest Miocene.

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