ML20151P775

Testing Tsunami Hypothesis at Willapa Bay,Washington: Evidence for Large Scale,Landward-Directed Processes, Presented at 880506-08 Meeting in Seattle,Wa

ML20151P775
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Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/06/1988
From:	Bourgeois J, Reinhart M WASHINGTON, UNIV. OF, SEATTLE, WA
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References
NUDOCS 8808100184
Download: ML20151P775 (1)

Text

h TESTING THE TSUNAMI HYPOTHESIS AT WILLAPA BAY, WASHINGTON:

EVIDENCE FOR LARGE-SCALE, LANDWARD-DIRECTED PROCESSES t

MARY A.

REINHART and Joanne Bourgeois :

)

Department of Geology, University of Washington, Seattle, WA.

The occurrence of great-subduction earthquakes in the Pacific Northwest and their related tsunamis has been postulated by several workers. Geologic evidence for rapid co-seismic subsidence of coastal wetlands ( e.g., Atwater, 1987) at Willapa Bay consists of seven supratidal surfaces alternating with beds of estuarine mud, three of which are overlain by anomalous layers of sandy silt interpreted by Atwater to be tsunami-laid deposits.

In testing the tsunami hypothesis we must consider other mechanisms by which the sands could have been deposited:

1) seiches, 2) severe storms and 3) flooding and/or migration of tidal channels.

To date, our analysis has been conducted in Willapa Bay and has focused on the sandy unit overlying the youngest buried wetland surface.

This unit consists of planar-laminated sands alternating with muddy laminae, indicating l

deposition from suspension; commonly, a lamina of mud occurs directly above the buried peat, evidence that the coastal platform was submerged prior to deposition of the first sand.

The unit is capped by two thin laminae which can be correlated regionally. Detailed mapping of the unit at the NiaWiakum ' River shows that the unit is thickest (>60mm) near the mouth of the river and thins to less than 1 mm about 3 km upstream, and that l

the unit is thicker on the downstream sides of point bar/ wetland surfaces than on the upstream sides.

The results of our p-^11minary work have led us to conclude that 1) the sandy unit was eposited by an event of regional i

scale; 2) the depositional event was composed of several pulses, i

3) the event was landward-directed; 4) flooding of the channel and/or migration of tidal channels could not have produced this pattern of sand distribution.

Analyses are underway to determine provenance of the sand and to measure settling velocities of the sediment.

REFERENCES

Atwater, B.F.,

1987, Evidence for great Holocene earthquakes along the outer coast of Washington State:

Science, v.

236,

p. 9 4 2 -9 4 4.

Heaton, T.H. and Kanamori, H., 1984, Seismic potential associated with subduction in the northwestern United States, Bulletin of the Seismological Society of America, v. 94,

p. 9 3 3-941

Terwindt, J.H.J., and Breusers, H.N.C.,

1972, Experiments on the origin of flaser, lenticular, and sand-clay alternating bedding, Sedimentology, v.

19, p. 85-98 l

M bhD 4

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