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USGS Topo Sheet: Belfair
Geographic coordinates: 47° 26.38' N, 122° 50.48' W
Paleoseismic record: Uplift, tsunami
Setting: The Lynch Cove site is comprised of a roughly 9 hectare saltmarsh and adjacent heavily wooded upland at the landward end of Hood Canal near Belfair, Washington. Tidal channels and the Union River dissect the marsh and form eroding banks of peat locally as much as 2 m high. A broad nearly planar surface extends from the seaward edge of the marsh to the foot of a sharp break in slope in the heavily wooded area bordering the marsh. The break in slope is 3 m above present mean higher-high water, which provides an estimate of the amount of uplift.
Stratigraphic description: Locally, as much as 2 m of peat overlies a sharp contact with an underlying meters thick sequence of mud and sand. Shells of Ostrea lurida dated 2700 ± 85 14C yr B.P. are abundant in the mud about 50 cm below the peat-sediment contact. A discontinuous layer of well-sorted very fine-grained sand inferred to be a tsunami deposit lies on top of the underlying mud and sand. Two high-precision radiocarbon ages bracketing the peat-sand contact have been determined by Minze Stuiver (Quaternary Isotope Laboratory, Univ. of Washington; a sample from the trunk of a small tree embedded in the sand gave a radiocarbon date of 1159 ± 14 14C yr B.P. (QL-4881), and leaf bases of the first plants to colonize the uplifted surface (Triglochin maritima) gave a radiocarbon date of 1132 ± 17 14C yr B.P. (QL-4658).
Paleontology/Paleoecology: Several stratigraphic sections at this site have been sampled in detail for paleontological analyses of diatoms, pollen, and seeds. Analyses are complete and some results have been reported in the reference cited below. More detailed reports are pending.
Geologic interpretation: This site has clear evidence of sudden uplift about 1100 years ago. The primary evidence for this uplift is the abrupt superposition of freshwater peat on tidal mudflat deposits. Prior to about 1100 years ago, the area now occupied by the Lynch Cove marsh was a mudflat that resembled the modern mudflats exposed at low tide seaward of the marsh. Sudden uplift, which raised the ancient mudflat above the reach of tides, was accompanied by a surge of water (tsunami) that deposited the discontinuous layer of fine-grained sand that encloses the radiocarbon dated log. Plants became established on the uplifted surface and eventually decomposed to form layers of peat. The presence of abundant wood, and diatoms, seeds, and pollen in the basal layer of peat are characteristic of moist upland meadows, freshwater marshes, and swamps and show that the peat formed above the highest tides. A subsequent rise in relative sea level has led to the formation of peat containing plant rhizomes and diatoms characteristic of tidal salt marshes.
Date of last work at site: August 1998.
Status of work: Fieldwork complete.
Published information on this site: Bucknam and others, 1992
Investigators: R. Bucknam, E. Leopold, E. Hemphill-Haley, B. Sherrod
Please cite information on this page as:
Bucknam, R.C., Leopold, E.B., Hemphill-Haley, E.,Sherrod, B.L., 1999, Lynch Cove, in Bucknam, R.C., compiler, Atlas of reconnaissance data fro m paleoseismic studies of the Puget Sound region, Washington: U.S. Geological Survey Web site, http://earthquake.usgs.gov/regional/pacnw/paleo/atlas.html.