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The Effects of Volcanic Debris Flows (Lahars), Earthquakes and Landslides on Holocene Deltas at Puget Sound, Washington

Index Abstract Objectives Geologic Setting Nisqually River Delta Puyallup River Delta Duwamish River Delta Delta Model Conclusions References About these Web Pages Back to Home Page Conclusions
The geology of Holocene deltas at Puget Sound is primarily controlled by the geometry of receiving basins, sediment load, and tectonics. Elongate, glacially carved valleys have focused deposition and promoted the rapid growth of deltas. The rate of sediment supply was non-uniform and characterized by pulses of sediment, including lahars derived from Mount Rainier. Radiocarbon-dated, pumice-rich sand deposits indicate that lahars of intermediate size were not confined to upper reaches of the valleys, but have episodically flooded the valleys for their entire length. Lahars and/or hyperconcentrated flows have buried flood plains with thick sand deposits and changed the course of river channels. Although their full extent is unclear, the abandoned channels meander across the muddy deltas as narrow ribbons of sand. Evidence of liquefaction, subsidence, and uplift were observed on the deltas, and indicate significant risk of earthquake-induced failures of the delta front. Submarine landslides are considered a hazard common to steep, submarine slopes along the margins of deltas, although currently available data on prehistoric events is equivocal.

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last modified 1 December 2003 (lzt)
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