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Geologic Publications for Mount Rainier

Holocene lahars and their by-products along the historical path of the White River between Mount Rainier and Seattle

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Author(s): Paul H. Zehfuss, Brian F. Atwater, James W. Vallance, Henry Brenniman, Thomas A. Brown

Document Type: Field Guide 4
Publisher: Geological Society of America
Published Year: 2003
Pages: 209 to 223
DOI Identifier:
ISBN Identifier:
Keywords: lahar delta Holocene Mount Rainier Seattle

Clay-poor lahars of late Holocene age from Mount Rainier change down the White River drainage into lahar-derived fluvial and deltaic deposits that filled an arm of Puget Sound between the sites of Auburn and Seattle, 110–150 km downvalley from the volcano's summit. Lahars in the debris-flow phase left cobbly and bouldery deposits on the walls of valleys within 70 km of the summit. At distances of 80–110 km, transitional (hyperconcentrated) flows deposited pebbles and sand that coat terraces in a gorge incised into glacial drift and the mid-Holocene Osceola Mudflow. On the broad, level floor of the Kent Valley at 110–130 km, lahars in the runout or streamflow phase deposited mostly sand-sized particles that locally include the trunks of trees probably entrained by the flows. Beyond 130 km, in the Duwamish Valley of Tukwila and Seattle, laminated andestic sand derived from Mount Rainier built a delta northward across the Seattle fault. This distal facies, warped during an earthquake in A.D. 900–930, rests on estuarine mud at depths as great as 20 m.

The deltaic filling occurred in episodes that appear to overlap in time with the lahars. As judged from radiocarbon ages of twigs and logs, at least three episodes of distal deposition postdate the Osceola Mudflow. One of these episodes occurred ca. 2200–2800 cal. yr B.P., and two others occurred ca. 1700–1000 cal. yr B.P. The most recent episode ended by about the time of the earthquake of A.D. 900–930. The delta's northward march to Seattle averaged between 6 and 14 m/yr in the late Holocene.

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Suggested Citations:
In Text Citation:
Zehfuss and others (2003) or (Zehfuss et al., 2003)

References Citation:
Zehfuss, P.H., B.F. Atwater, J.W. Vallance, H. Brenniman, and T.A. Brown, 2003, Holocene lahars and their by-products along the historical path of the White River between Mount Rainier and Seattle: Field Guide 4, Geological Society of America, pp. 209-223.