Clay-poor lahars of late Holocene age from Mount Rainier change down the White River drainage into lahar-derived fl uvial and deltaic deposits that fi lled 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-fl ow 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) fl ows deposited pebbles and sand that coat terraces in a gorge incised into glacial drift and the mid-Holocene Osceola Mudfl ow. On the broad, level fl oor of the Kent Valley at 110-130 km, lahars in the runout or streamfl ow phase deposited mostly sand-sized particles that locally include the trunks of trees probably entrained by the fl ows. 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 fi lling 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 Mudfl ow. 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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