Mudflow disturbance in latest Miocene forests in Lewis County, Washington

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 (super 40) Ar/ (super 39) Ar radiometric dates. After separation of plagioclase crystals into populations of cloudy appearance (inherited) and clear appearance (newly crystallized), a best age of 6.13 + or - 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.