ML20151P816

Implications of Late Holocene Salt Marsh Stratigraphy for Great Earthquake Recurrence Along Coast of South Central Oregon, Presented at 880506-08 Meeting in Seattle,Wa

ML20151P816
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Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/06/1988
From:	Alexis Nelson INTERIOR, DEPT. OF, GEOLOGICAL SURVEY
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References
NUDOCS 8808100194
Download: ML20151P816 (1)

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l IMPLICATIONS OF LATE HOLOCENE SALT-MARSH STRATIGRAPHY FOR GREAT EARTHQUAKE RECURRENCE ALONG THE COAST OF SOUTH-CENTRAL OREGON i Alan R. Nelson, Branch of Geologic Risk Assessment, U.S. Geological Survey, PO Box 25046, MS 966, Denver, Colorado 80225 l

l Repeated, great plate-interface earthquakes have been postulated for the Cascadia subduction zone in western Washington and Oregon. The best evidence of the coseismic subsidence to be expected near the coast during great earthquakes is found in southwestern Washington where many i exposures record repeated episodes of submergence of late Holocene marshes. Atwater and others l have used consistent stratigraphic relationships, liC ages, and plant macrofo:sils from sequences of interbedded marsh peats and intertidal muds to show that the 6 marsh peats buried in the last 4000 l

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years throughout southwestern Washington were submerged suddenly. The late Holocene estuarine record in the central part of the subduction zone in Oregon is more difficult to interpret; there are very few good exposures, and coring at some of the 12 marsh sites investigated has produced i

+vidence of a gradual rise of late Holocene sea level while sea level rise appears to be jerky at other sites.

At sites in the eastern arms of Coos Bay and in the Umpqua River estuary, one probable buried marsh surface is found in the upper 1.5 m of most cores overlying 4-6 m of uniform mud.

i in some cores both upper and lower contacts of peaty units are gradational, but in most cores the thickest peat bed has a fairly abrupt upper contact suggesting sudden submergence of a marsh. A spruce root from this buried surface in Shinglehouse Slough was dated at 340 li C yrBP. One interpretation of this type of marsh sequence is that sediment deposition rates in most tidal inlets have teen low during all but the last few hundred years of the late Holocene and that for this reason no evidence (buried marsh surfaces) of earlier sudden submergence events has been preserved. Another interpretation is that no sudden changes in sea level have occurred.

Coring in South Inlet, an arm of the Siuslaw River estuary, shows that 4 m of fairly uniform peat overlies 4 m of mud. This type of marsh sequence suggests that late Holo:ene relative sea-level rise was gradual with no abrupt changes in the type or rate of sedimentation. Subtle, gra lithologic changes within the peat section suggest only small, gradual changes in sea lesel. A lithologic flood events.changes found in some cores farther up the valley of South inlet probably record stream In contrast to the above records, at two sites in South Slough in western Coos Bay, cores show 6-8 abruptly buried marsh surfaces that are 0.4-1.2 m apart. Exter.sive coring in a small marsh along Winchester Creek revealed up to 8 buried marsh surfaces in sections 5-8 m thick. The 4 best developed surfaces can be correlated across the inlet. The uppermost buried surface has a modern li C age; it must have been buried by sedimentation following diking of the marsn. Lower surfaces date at 460 (2.2 m) and 2880 (2.8 m)Cli yrBP, indicating highly non-uniform sedimentation rates. A core from Day Creek,4 km to the north, had a similar sequence of 6 buried surfaces. These sites are near the axis of the South Slough syncline, and tilted marine terraces on the west limb of the syncline document continued late Pleistocene folding of this folding rather than regional deformation of the central Oregon co earthquakes. Alternatively, sudden slip on flexure slip faults within the syncline might also occur primarily as a response to large subduction zone earthquakes.

G800100194 880615 ADOCK 05000344 i PDR P PDR j

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