ML20151P744

Buried Holocene Wetlands Along Johns River,Southwest,Wa, Presented at 880506-08 Meeting in Seattle,Wa

ML20151P744
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Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/06/1988
From:	Atwater B INTERIOR, DEPT. OF, GEOLOGICAL SURVEY
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References
NUDOCS 8808100173
Download: ML20151P744 (1)

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Buried Holocene wetlands along the Johns River, southwest Washington Brian F. Atwater U.S. Geological Survey at Department of Geological Sciences University of Washington AJ-20 Seattle, WA 98195 Low tides along this tributary of Grays Harbor expose as many as five buried marshes and swamps younger than about 3100 sidereal years before present. These buried wetlands, the likes of which abound in coastal southwest Washington, seem best explained by the aperiodic recurrence of great earthquakes on the Cascadia subduction zone. Saturday's field trip permits scrutiny of six hypotheses on which this interpretation depends.

(1) THE WETLANDS WERE BURIED BECAUSE OF SUBMERGENCE MORE LASTING THAN STORMS OR FLOODS. Well-preserved soils of buried wetlands bear rooted stems and leaves of the marsh herbs Deschampsia caespitosa and Potentilla pacifica, or stumps of Sitka spruce ar.d western red cedar--plants that flourish today only near or above highest-tide level. But mud that buried the wetlands commonly contains the below-ground stems (rhizomes) of Triglochin maritima or Carex lyngbyei, plants that today colonize intertidal mudflats (Weinmann and others, 1985). Collectively, such plant fossils in grcuth position show that burial entailed changing wetlands to tideflats, with a relative sea-level rise on the order of 0.5-2.0 m.

(2) THE SUBMERGENCE, AND SOME OF THE CONSEQUENT BURIAL, OCCURRED RAPIDLY. Submergence taking decades would not permit the preservation of stems and leaves on the herbs that are rooted in the buried wetlands.

(3) RAPID SUBMERGENCE RESULTED FROM SUBSIDENCE OF THE LAND, NOT FROM EUSTATIC OR ISOSTATIC RISE IN RELATIVE SEA LEVEL. Eustatic and isostatic rise in sea level on other mid-latitude coasts has been sufficiently gradual in the past 5000 years for tidal wetlands to build upward apace with the sea, thick.

thus producing peaty tidal-wetland deposits many meters Such deposits are unknown in coastal southwest Washington.

(4) AT LEAST THE MOST RECENT JERK OF SUBSIDENCE COINCIDED APPROXIMATELY WITH A LANDWARD-DIRECTED SURGE OF SANDYThis surge, WATER.

about 300 sidereal years ago, deposited sand around the stems and leaves of D. caespitosa and P. pacifica at the onset of marshland burial. Possible analog:

the deposition of sand on freshly subsided Chilean lowlands by the tsunami from the 1960 Chile earthquake (Wright and Mella, 1963).

(5) EACH JERK OF SUBSIDENCE INVOLVED LARGE AREAS. Individual buried wetlands along the Johns River extend tens to hundreds of meters in outcrop.

In addition, they resemble in stratigraphic sequence and radiocarbon age the buried wetlands of most other estuarine areas in coastal southwest Washington.

YE ARS.(6) JERES RECURRED AT IRREGULAR INTERVALS WHOSE AVFRAGE IS ABOUT 600 Aperiodic recurrence is indicated not only by radiocarbon ages but also soil and stage ofdifferences by regional forestation.among buried wetlands in their maturity of Weinmann, Fred, and others, 1984, Wetland plants of the Pacific Northwest:

U.S. Corps of Engineers, Seattle, 85 p.

Wright, Charles, and Mella, Arnoldo, 1963 Modifications to the soil pattern of south-central Chile resulting from seismic and associated phenomena during the period May to August 1960: Bulletin of the Seismological Society of America, v. 53, p. 1367-1402.

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