ML20151P861
| ML20151P861 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 05/06/1988 |
| From: | Leopold E WASHINGTON, UNIV. OF, SEATTLE, WA |
| To: | |
| Shared Package | |
| ML20151H012 | List:
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| References | |
| NUDOCS 8808100209 | |
| Download: ML20151P861 (1) | |
Text
l 21 AESTRACT POSTGLACIAL TILTING OF LAKE WASHINGTON: SEDIMENTARY AND POLLEN j
EVIDENCE Estella E.
Leopold Apparent tilting of the Lake Washington basin in postglacial time is based on the difference in elevation of subsurface volcanic ash deposited at the lake outlet (at Renton) and that at Mercer Slough marsh in lacustrine sediment. The sites are 10 km apart, and the difference elevations of the tephra is 5 m.
Mullineaux's (1970) work established that the building of a fan (
36 m in thickness) bw the Cedar River followed the deposition of marine shell beds that mark a period about 13,200 wears ago when the Lake Washington basin was connected to Puget Sound. A volcanic ash i
layer occurring at about midsection is 20 meters above the highest i
marine shell beds. Intermittent gravel, sands and peat suggest that the fan was deposited in shallow water. He suggested the ash was of nild Holocene age and probably represented the Ma:ama tephra.
At Mercer Slough the position of the Ma:ama ash (7000 we B.P.)
is 13 m below the present marsh surface and 7 m below present sealevel; it occurs in lacustrine silts (Leopold 1986). In contrast the purported Ma:ame ash in the Renton cores is about 12 m below present
-sealevel (Mul l i neau:: 1970), some 5 m lower than the Mercer tephra. A line connecting the ash layers at the two points slopes southward 5m in 10 km. Since the lake outlet at the time of deposition could not have been lower than the lake sediment at Mercer Slough, the basin has probably endured deformation.
i According to Thorson's (1981) net postelacial deformation curve the Mercer Slough site should have been deformed 10 m more than that at Renton. The position of the subsurface volcanic ashes accounts j
for half of that amplitude. This could mean that half of this i
deformation took place in the last 7000 wears.
At Mer cer Slough the occurrence of shallow water and marsh peatt begin about 2 m above the Marama ash and contine to the sur f ace.
These sediments and their abundant pollen of marsh plants indicate that the lake level was near the peat surtete at Mercer Slough through most of the late Holocene; the data indicate that the level of Lele Washington was rising slowlw during mid and late Holocene time. During that tinie alluviation of the fan of the Cedar Riser built to its present level.
- Leopold, E. B. 198d. Pertinence of Late Washington to postglacial rise in sealevel. Geol. Soc.
Am.,
Annual Meeting, Abst.
- Thorson, R.M., 1081. Isostatic effects of the last glacletion an the Fuget Lowland, Washington.
U. S. Geological Survew open-File Repor t 81-370.