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Turners Bay Marsh USGS Topo Sheet: Anacortes South Geographic coordinates: 48°27.35' N, 122°32.70' W Paleoseismic record: Insufficient fieldwork has been completed to evaluate the paleoseismic potential of this site. The stratigraphic sequence at this site may span a major part of the Holocene. Setting: A narrow, 50-m wide, 500-m-long tidal marsh fills a narrow valley at the head of Turners Bay, a northeast extension of Similk Bay. Although an east-west road separates the upper (northern) marsh from the lower marsh south of the road, tides have access to the northern marsh through a culvert. The upper marsh is covered with Distichlis and some Salicornia. Large parts of the lower marsh are covered with logs, which have accumulated against the bay side of the embankment. In spite of the road embankment, the upper marsh appears relatively undisturbed. The landowner informed me that the marsh has primarily been used for grazing throughout most of this century. Stratigraphic description: Although his published location is incorrect, Rigg (1958. p. 169) reported a meter of fibrous peat over a meter of mixed clay and diatomite over clay with clam shells, from the upper marsh. A cross section across the marsh north of and parallel to the road based on four gouge cores (2.5-cm-diameter) shows a more complex, thicker sequence of salt marsh peat over freshwater peat and pond deposits over marl with snail shells over shallow marine sediments. The deepest core (4.8 m) is shown in the figure. Two of 6 samples collected for AMS 14C analysis yielded ages of "post-bomb" (AA-26605; grass stem at 435 cm depth) and 12,730+/-50 14C yr BP (Beta-109812; marine shell fragments at 453 cm depth). Paleontology/Paleoecology: Shells in a sample from 435 cm depth in the marl are similar to cf. Fossaria obrussa, although they could also be Catinella sp. Helical shells of cf. Gyralus parvus were abundant. Both forms are found in or immediately adjacent to fresh water. Shell fragments in the marine sediments from 453 cm depth consist of at least three genera including Macoma, Mytilus, and Balanus, all of which are characteristic of shallow littoral environments. Geologic interpretation: The 12-ka marine sand near the base of core TB-01 suggests that relative sea level was close to present sea level at that time. A thin organic-rich bed within the sand may be a lagoon deposit or a soil; in either case, sea level must have regressed slightly for this deposit to form before it was again covered with marine or intertidal mud. The mud gradually transitions upward into a bright yellow marl deposited in a lake rich in carbonate. Abundant snail shells in the marl are typical of freshwater. Although regional sea level was probably rising during this period, this site was apparently rebounding from the load of the Puget Lobe at a much faster rate of rise. As relative sea level fell below the elevation of the site, a small lake formed, probably behind a log-strewn barrier similar to that of today. The 1.5-m-thick marl records a long period of uniform lake conditions, probably in the early Holocene. Later in the Holocene, freshwater marshes around the lake gradually contributed more and more detrital herbaceous and woody material to the lake sediment. Eventually, the lake filled in completely forming a freshwater marsh, as evidenced by about 50 cm of rooted freshwater peat between 70-120 cm. Near 70 cm depth, a rapid transition to salt marsh peat is the first evidence of a marine transgression that continues today. Although the transition to intertidal sediment could be the result of sudden coseismic subsidence, it more likely records the overtopping of a berm by gradually rising sea level about 500-1000 years ago. Problems with bent gouge corers are highlighted by my "post-bomb" age for a grass stem from 435 cm. Although this stem was horizontal in the center of the gouge at this level, its age shows that it was pushed down to this depth during repeated attempts to recover core from this stratigraphic level with bent core barrels. Lithologic changes identified so far at this site are more likely caused by rapid isostatic rebound and slow sea-level rise than by coseismic uplift or subsidence. At the very least, further study of this site will help constrain the postglacial history of land-level movements in this region. Date of last work at site: September 1997 Status of work: Additional stratigraphic investigations planned for 1998 have been rescheduled for 1999. This will include more detailed gouge coring along transects, possible study of diatom assemblages and macrofossils, possible vibracoring, and radiocarbon dating. Investigators: A. Nelson Please use the following format in citing information from this page: Nelson, A.R., Turners Bay 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.

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