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Distal records of sandy Holocene lahars from Mount Rainier, Washington

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Author(s): Paul H. Zehfuss

Document Type: Ph.D Thesis
Publisher: University of Washington
Published Year: 2005
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Clay-poor lahars of late Holocene age from Mount Rainier change down the White River drainage to lahar-derived fluvial and deltaic deposits that filled a former arm of Puget Sound between Auburn and Seattle. Lahars in the debris-flow phase left cobbly and bouldery deposits on valley walls and in the valley bottom within 70 km of the summit. At distances of 80-120 km, transitional and hyperconcentrated flows deposited pebbles and sand that coat terraces in a gorge incised into glacial drift and the mid-Holocene Osceola Mudflow. The deposits contain abundant water-escape structures, including consolidation laminae, dish structures, and several types of pillars. On the broad, level floor of the Kent Valley at 110 to 130 km, hyperconcentrated flows or stream flows laden with laharic sediment deposited mostly sand-sized particles in channels and on the flood plain. Beyond 130 km, in the Duwamish Valley of Tukwila and Seattle, hyperpycnal flows left laminated andesitic sand derived from the lahars and built a delta northward across the Seattle fault. This distal facies, warped during an earthquake in A.D. 900-930, buries estuarine mud to depths of up to 20 m.

The deltaic filling occurred in episodes that appear to overlap in time with the lahars. At least three episodes of lahars post-date the Osceola Mudflow (5490-5600 cal yr B.P.). The Summerland episode (2200-2800 cal yr B.P.) consisted of at least two lahars probably produced during the euptive period of the same name. Two later episodes, the Twin Creek episode followed by the Fryingpan Creek episode, occurred btween 1000 and 1700 cal yr B.P. Each of these episodes concisted of at least one lahar. A lahar of the Twin Creek episode was the largest in post-Osceola time. delta progradation in the Kent and Duwamish Valleys in response to these lahars averaged between 7.2-14.3 m/yr in the late Holocene.

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In Text Citation:
Zehfuss (2005) or (Zehfuss, 2005)

References Citation:
Zehfuss, P.H., 2005, Distal records of sandy Holocene lahars from Mount Rainier, Washington: Ph.D Thesis, University of Washington,