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USGS Topo Sheet: Shelton
Geographic coordinates: 47°8.27' N, 123°4.96' W
Paleoseismic record: This site has stratigraphic evidence for abrupt subsidence of a Douglas-fir forest into the intertidal zone, interpreted as the result of a large earthquake in southern Puget Sound between 910-1170 cal years B.P..
Setting: The Little Skookum Inlet locality consists of a large brackish marsh and tideflat, bordered by lowland forest west of the marsh. Stratigraphic data from Little Skookum Inlet consists of gouge core descriptions and mapping of outcrops along a 305-m long transect across salt marsh and tideflat. The non-vegetated tideflat surface is at an elevation of ~3.1 m above Mean Lower Low Water (MLLW), well above the height of most low tides. Marsh vegetation is dominated by Distichlis spicata and Salicornia virginica in low marsh areas and Deschampsia caespitosa in high marsh areas. Picea sitcheneis (Sitka spruce) grows at the edge of the forest bordering the marsh. Tsuga heterophylla (western hemlock) and Thuja plicata (western redcedar) grow at elevations about 0.5 to 1 m higher than Sitka spruce. The lowest modern Pseudotsuga menzeisii (Douglas fir) was found at an elevation of 7.2 m above MLLW, slightly over 2 m above the lowest living Sitka spruce.
Stratigraphic description: Most deposits beneath the marsh and tideflat consist of peat or mud. The lowest deposit in the sequence is bluish-gray mud (LSI-1), massive-to-laminated in places, and barren of visible fossils in the field. The top of this unit has about 3 meters of relief over a distance of 150 meters, mainly from a 2-3 m deep depression in the western half of the transect.
A tan gyttja, (LSI-2), laminated in places, overlies the gray mud within the depression in unit LSI-1. A 2-3 cm gradational contact separates the gyttja from the underlying gray mud. The gyttja disappears in the eastern half of the transect where the underlying LSI-1 is highest.
A reddish-brown detrital peat (LSI-3) overlies above the tan gyttja (LSI-2), and appears to cap the fill of the small depression in the gray mud (LSI-1). A gradational contact 1-2 cm wide occurs between LSI-3 and the underlying tan gyttja. Wood and plant seeds are abundant in places within LSI-3. Two bulk peat samples from the upper 1 cm of LSI-3 provide radiocarbon ages of 7030± 70 14C years B.P. (Beta-97230) and 7520± 80 14C years B.P. (Beta-97231). Calibrated ages for these dates are 7660-7930 cal years B.P. and 8130-8420 cal years B.P. respectively.
A gray, fine to medium sand (LSI-4) with a gravelly base overlies the detrital peat (LSI-3) in most places. This sand is absent in the marsh subsurface beside the forest adjacent to the marsh and again disappears over the small rise in LSI-1 between 200 and 275 m of transect. In places, fining-upward sand layers are interbedded with layers of greenish-gray mud.
A massive, gray mud with orange mottles (LSI-5) is currently being exhumed on the modern tideflat, exposing dozens of sub-fossil Douglas-fir stumps rooted in the top of unit LSI-5. The contact between LSI-4 and LSI-5 is gradational over about 1 cm. The mud (LSI-5) is silty in places and is very stiff, making it hard to penetrate with a gouge corer. The stumps are associated with a buried soil (LSI-6), consisting of a dark brown to black detrital peat with conifer cones, logs, and abundant detrital woody stems. In several places, roots of alder and willow in growth position can be seen protruding from the surface of LSI-5.
Two samples of tree rings (from roots of separate Douglas-fir stumps) gave radiocarbon ages for burial of the forest soil at Little Skookum Inlet. The outermost 15-25 rings of a large Douglas-fir root (with attached bark), and yielded a radiocarbon age of 1090± 60 14C years B.P. (Beta-95912). The calibrated age for this sample is 910-1170 cal years B.P. A second sample, from the inner 16 rings of a Douglas-fir root (with a total of 192 rings), yielded a radiocarbon age of 1220± 50 14C years B.P. (Beta-102335); the calibrated age for death of this tree is 990-1220 cal years B.P. (offset by 184 years, the number of tree-ring years from the midpoint of sample and bark). The overlap in these two calibrated age ranges implies no statistical difference in the time of tree death.
Salt marsh peat (LSI-7) overlies the buried forest soil (LSI-6). A sharp contact (~1 mm wide) separates the peat from the underlying soil. The peat is slightly muddy in most places, and contains abundant Distichlis spicata rhizomes. Triglochin maritima rhizomes and leaf bases are rare in this unit. A radiocarbon age on Triglochin maritima rhizomes collected from 65 to 70 cm below the marsh surface (or 45-50 cm above the top of LSI-6) from an outcrop of peat at horizontal coordinate 305 m, yielded an age of 140± 60 14C years B.P. (Beta-117091), and a calibrated age of 290-0 cal years B.P.
Paleontology/paleoecology: Sherrod (1998) identified two biostratigraphic zones at this site. Zone BZ-1 (2.0-0.34 m) contained follicles of Spirea douglasii, a freshwater wetland shrub species, and an arboreal taxon, Betula papyifera, dominate macrofossils recovered from the lower part of BZ-1. The lower half of this zone contains seeds of Carex cf. leporina and Carex cf. aquatilus. Seeds of Juncus sp. are abundant in the middle and upper half of BZ-1. Arboreal and lowland-forest shrub taxa, namely Thuja plicata, Picea sitchensis, Taxus brevifolia, Rubus spectabalis, and Sambucus racemosa, dominate the macrofossils found in the buried forest soil (LSI-6). Stumps of Pseudotsuga menziesii (Douglas fir) in growth position are common and Alnus rubra (Red alder) stumps are rare at the top of this unit. A peaty soil (LSI-6) surrounds many of the stumps exposed in outcrop. Scattered Foraminifera were found in two samples just below the top of the zone.
Diatom samples from BZ-1 are dominated by several cosmopolitan and freshwater taxa. These include Fragilaria construens, F. virescens, F. brevistriata, Eunotia pectinalis, Aulacoseira italica, A. islandica, Epithemia turgida, and Gomphonema angustatum var. sarcophagus. Diatom assemblages are too poorly preserved for study (only small fragments observed) between 1.82 a.m. and 1.18 m below modern ground surface, and within the upper two thirds of the buried soil (LSI-6) between 0.45 and 0.34 m.
Zone BZ-2 (0.34-0 m) contained seeds of Juncus sp. and salt marsh plants, including Atriplex patula, Carex lyngbyei, and Deschampsia caespitosa, dominate the macrofossils of BZ-2. Needles and leaves of several arboreal taxa appear at the top of this zone. Large numbers of foraminfera are common throughout this zone.
Diatoms from BZ-2 are dominated by brackish water taxa, including Diploneis interrupta, Nitzschia bilobata, Melosira nummuloides, Tryblionella debilis, Paralia sulcata, and Caloneis westii. Cosmopolitan taxa are common, including Cosmioneis pusilla and Fragilaria construens. Freshwater taxa also appear in small numbers, the most common being Aulacoseira italica.
Geologic interpretation: Peat indicative of high marsh environments (with rhizomes of Distichlis spicata and Juncus cf. balticus) sank to lower elevations so that laminated mud deposited in lower intertidal environments begin to accumulate over the marsh peat.
Date of last work at site: 08/98
Status of work: Additional stratigraphic and paleontologic work is planned for June-August, 2000.
Published information on this site: Sherrod (1998)
Investigators: B. Sherrod
Please cite information on this page as:
Sherrod, Brian L., 1999, Red Salmon Creek site, in Bucknam, R.C., compiler, Atlas of reconnaissance data from paleoseismic studies of the Puget Sound region, Washington: U.S. Geological Survey Web site, http://earthquake.usgs.gov/regional/pacnw/paleo/atlas.html.