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Guemes Island Road Marsh

USGS Topo Sheet: Anacortes North

Geographic coordinates: 48°34.03' N, 122°37.35' W

Paleoseismic record: Based on very limited reconnaissance, this site appears to record 3-4 rapid flooding events. Much further work is needed to determine if these events may be related to coseismic land-level changes, tsunamis, or storms.

Setting: This 300-m-wide, 400-m-long area of pasture is developed on former fresh-to-brackish water marshes and lagoons on the northeast coast of Guemes Island. A 100-m-wide, 1-2-m-high, sandy beach berm with a road and houses along its distal edge separates the pasture from the beach. Old, low (<1-m-high) tidal channels, berms, and/or dunes beneath the pasture create a subtle underlying topography.

Stratigraphic description: Three gouge cores (2.5-cm-diameter) collected from the southeastern part of the marsh along transect from the berm to the center of the marsh show a similar but complex, predominantly intertidal stratigraphy. The crumbly soil structure and/or peaty texture of four stratigraphic units in one of these cores (Core GI-01 on figure) indicate that they are the A or O horizons of soils that formed in the marsh or immediately adjacent upland. Sharp upper contacts on at least three of these soils suggest repeated rapid flooding of the marsh. An AMS 14C age on spruce needles, herb stems, leaf fragments, and a fir twig from the base of a humified fibrous peat in another core (which correlates with the peat near 104 cm in core GI-01) is 4040+/-50 14C yr BP (Beta-109813).

Paleontology/Paleoecology: No paleontologic work has been completed.

Geologic interpretation: The sandy substrate, low elevation, and complex geomorphology of this site suggest that it may be difficult to distinguish among the processes that could have caused the rapid flooding of this marsh over the past 4000 yearsæ coseismic land-level changes, tsunamis, or unusually large storms. Unless the dated sample is seriously reworked (which seems unlikely considering the range of fragile materials dated), the age yields a net rise in sea level for this coast of roughly 0.25 mm/yr. Thus, relative sea level appears to have been near its present elevation at this site throughout the late Holocene.

Date of last work at site: September 1997

Status of work: Additional work is planned.

Published information on this site: None

Investigators: A. Nelson

Please use the following format in citing information from this page:

Nelson, A.R., Guemes Island Road Marsh, in Bucknam, R.C., compiler, 1999, Atlas of reconnaissance data from paleoseismic studies in the Puget Sound region, Washington: U.S. Geological Survey Web Page, http://earthquake.usgs.gov/regional/pacnw/paleo/atlas.html.

This URL is: http://earthquake.usgs.gov/regional/pacnw/paleo/reports/guemes.htm
Modified July 29, 2002 by Susan Rhea