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Shoal Bight

USGS Topo Sheet: Lopez Pass

Geographic coordinates: 48°27.63' N, 122°49.25' W

Paleoseismic record: Insufficient fieldwork has been completed to evaluate the paleoseismic potential of this site.

Setting: Salicornia-dominated marshes as much as a 100 m wide fringe the mudflats of Shoal Bight between Lopez Island and the island of Henderson Lake to the north. The marshes appear relatively undisturbed, although construction of the road along the low berm on the east edge of the mud flats may have accelerated marsh progradation. This marsh was mentioned, but not specifically discussed, by Rigg (1958, p. 161).

Stratigraphic description: We examined six gouge cores (2.5 cm diameter) along a transect across the southern fringing marsh from the road west as far as an old, north-south, gravel road embankment. Four cores showed 0.2-1.0 m of peaty mud over sandy gravel or clayey silt. But the two cores nearest the old road embankment showed 0.4-0.6 m of salt-marsh peat over >1.2 m of soft, gray clayey silt. This is the only site we investigated in the Lopez-Fidalgo region with a non-sandy intertidal record >1.5 m thick. Rigg (1958, p. 162) noted that none of the 35 salt marshes he visited in the San Juan Islands contained peat deposits >1 m thick.

Paleontology/Paleoecology: No paleontologic work has been completed.

Geologic interpretation: Salt marshes along at least the southern part of the bight are underlain by intertidal sequences that suggest that relative sea level is rising faster than is apparently the case at other sites that we examined on Lopez and Fidalgo Islands. Possibly, aggradation of the mud and peat seen in the two longer cores was accentuated by construction of the road embankments. Nevertheless, the longer sequences appear capable of preserving an intertidal record of prehistoric coseismic land-level changes, tsunamis, or storms.

Date of last work at site: September 1997

Status of work: Additional gouge coring, radiocarbon dating, and diatom studies were planned for 1998 and have been rescheduled for 1999.

Investigators: A. Nelson, H. Kelsey

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

Nelson, A.R., Kelsey, H.M., Shoal Bight, 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/shoalbit.htm
Modified July 29, 2002 by Susan Rhea