ML20151P793

Evidence for Late Holocene Subduction Earthquakes Recorded in Tidal Marsh Deposits Along Nehalem & Salmon Rivers, Northern Oregon, Presented at 880506-08 Meeting in Seattle, Wa

ML20151P793
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Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/06/1988
From:	Grant W INTERIOR, DEPT. OF, GEOLOGICAL SURVEY
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References
NUDOCS 8808100189
Download: ML20151P793 (1)

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10 Evidence for late HNocene subduction earthquakes recorded in tidal marsh deposits along the Nehalem and Salmon Rivers, northern Oregon Wendy C. Grant (U.S. Geological Survey at Geophysics Program, AK 50, University of Washington, Seattle, WA 98195)

Subsurface stratigraphy in tidal marshes along the Salrnon and Nehalem Rivers in northern Oregon records at least one episode of sudden submergence that was probably caused by large thrust earthquakes on the Cascadia subduction zone. Along the Salmon River, up into both core and sandy-silt samples silt, and streameuts are characterized by fine-medium sand grading and gradually (over several centimeters) becoming very organic-rich, so that the upper part of the nnit consists of peat or muddy peat with an abrupt upper contact. These peaty units vary from several up to about 10 cm in thick-ness and are, in some cases, overlain by a fine medium sand unit. ,

(where present) from a trace up to approximately 10 cm in thickness and was overlain; by silt and silty sand grading up into the present marsh surface. Along the Nehalem River, the tidal marsh subsurface stratigraphy is similar to that of the Salmon River with the horizon. difference that there is no ' packet' of sand lying in direct contact over the peaty Instead, the peaty horizon is abruptly truncated and overlain by organic poor silt or sandy silt grading upward into the modern marsh surface.

At both tne Nehalem and Salmon Rivers, the subsurface stratigraphy is interpreted to represent the sudden lowering of a high marsh environment into an unvegetated tidal flat as a result of a large thrust earthquake on the Cascadia subduction zone. The fine-medium sand ' packet' that lies on top of the peaty unit along the Salmon River shows a  !

l distribution that thins both up the valley and away from the river channeh this distribu-tion by theindicates a seaward source and is interpreted as resulting from a tsunami generated rupture process.

Preliminary radiocarbon dating of the peaty horizons and preserved plant rhizomes both above and below the peat yields an approximate age of peat burial of 300 400 years before present at both the Nehalem and Salmon Rivers. This age is consistent with the age of the youngest buried unit at several sites on the Washington coast to the north I

(Atwater,1987). If the subsidence at the Salmon and Nehalem Rivers was coincident in I time with subsidence in southwestern Washington, the 240 km extent of the down-dropped featu res. zone indicates that an earthquake of magnitude 8.5 could have generated these REFERESCES l I

Atwater, Bstate, of Washington F. (1987).

ScienceEvidence for great Holocene earthquakes along the outer coast 236, 942 944.

along the northern Oregon 68.1239. coast EOS, Grant, W.C. and D.D. McLaren (19S7 I

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