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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

Puyallup River Delta

Bathymetric map of Commencement Bay The Puyallup River delta discharges into Commencement Bay and is highly developed with industrial and port facilities. Dredging of deep waterways, channelizing the river, and landfills have destroyed over 17 km2 of coastal wetlands, virtually all that existed on the natural delta.

A submarine landslide over 100 years ago destroyed docks, created a local tsunami, and killed at least two people. Lead-line soundings after the landslide showed that " area of 1200 feet (364 m) in length, by at least 800 feet (242 m) wide, comprising at least 20 acres, suddenly dropped 25 to 60 feet (8-18 m) downward toward the earth's center (Tacoma Daily Ledger, December 7, 1894).

GPR image of channel-fill deposit on the Puyallup River delta. Deposits of massive, coarse sand clearly originated as lahars, based on the andesitic mineralogy and the presence of scoria, pumice, and lapilli (Pringle and Palmer, 1992). Small twigs from the deposit yielded an uncorrected age of 2,320 +/- 120 14C yr B.P. (Palmer et al., 1991), similar to a lahar deposit upstream (Crandell, 1971). The paleochannel probably represents the channel of the ancient Puyallup River GPR image and interpretation of Puyallup River Delta

Core section of general lithology and occurrence of diatoms

Click on figure to view larger image (48K)

Geotechnical boring in the Puyallup valley penetrated an upper sequence of alluvial sand, silt and gravel to a depth. Black, well-sorted sand occurs at several intervals. Sand near the bottom is compositionally all glass, with amphibole crystals, tephra, and virtually no lithic fragments. This sequence also contains rip-up clasts of the mud and probably originated as a lahar.

Diatom analyses show changes in depositional environments. In general, the diatoms progressed from a flora that has marine affinities at the base of the borehole to floras with more freshwater/fluvial affinities in the upper part.

Where is the Osceola Mudflow? Dragovich et al., (1994) place it at a depth of 80 m at this location. However, if we accept the radiocarbon dates from this boring (at left, in red) as representative of the time of deposition, the organic-rich sediment in the interval 21-27 m likely represents the Osceola time period.

For locations of GPR profile and boring, see map in Geologic Setting section

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last modified 1 December 2003 (lzt)
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