Snowhomish Delta - GSA 2001

Geologic evidence of earthquakes at the Snohomish delta, Washington, in the past 1200 years

Article published by the Geological Society of America**, 2001, GSA Bulletin Vol.113, p. 482-494

Joanne Bourgeois, Department of Geological Sciences, University of Washington, Seattle, WA 98195
Samuel Y. Johnson, United States Geological Survey, MS 966. Box 25046, DFC, Denver, CO 80225

ABSTRACT

Exposed channel banks along distributaries of the lower Snohomish delta in the Puget Lowland of Washington reveal evidence of at least three episodes of liquefaction, at least one event of abrupt subsidence, and at least one tsunami since about A.D. 800. Forty-five measured stratigraphic sections consist mostly of 2-4 m of olive-gray, intertidal mud with abundant marsh plant rhizomes. The most distinctive stratigraphic unit is a couplet comprising a 0.5- to 3-cm-thick, laminated, fining-upward, tsunami-laid sand bed overlain by 2 to 10 cm of gray clay. We correlated the couplet, which is generally about 2 m below the modern marsh surface, across a ~20 km2 area. Sand dikes and sand-filled cracks up to 1 m wide, which terminate upward at the couplet, and sand volcanoes preserved at the level of the sand bed record liquefaction at the same time as couplet deposition. Differences in the type and abundance of marsh plant rhizomes across the couplet horizon, as well as the gray clay layer, suggest that compaction during this liquefaction led to abrupt, local lowering of the marsh surface by as much as 50-75 cm. Radiocarbon ages show the tsunami and liquefaction date from about A.D. 800 to 980, similar to the age of a large earthquake on the Seattle fault, 50 km to the south.

We have found evidence for at least two, and possibly as many as five, other earthquakes in the measured sections. At two or more stratigraphic levels above the couplet, sand dikes locally feed sand volcanoes. Radiocarbon ages and stratigraphic position suggest one set of these dikes formed about A.D. 910-990; radiocarbon ages on a younger set indicate a limiting maximum age of A.D. 1400-1640. Also, we interpret a sharp lithologic change, from olive-gray, rhizome-rich mud to grayer, rhizome-poor mud, about 1 m above the couplet to indicate a second abrupt lowering of the marsh surface during an earthquake at about AD 1040-1400, but no conclusive liquefaction structures have been identified at this horizon. Two distinctive coarse-sand laminae, 30-80 cm below the couplet, may record tsunamis older than AD 800.

Thus study shows that in the last ~1200 years, this part of Washington's Puget Lowland has been subjected to stronger ground shaking than in historic times, since about 1870.

INTRODUCTION

The densely populated Puget Lowland of Washington State (Figs. 1, 2) occupies a dynamic geologic setting in the forearc of the North American plate above the Cascadia subduction zone. Complex plate interactions along this convergent continental margin are the driving force for a significant, yet poorly understood earthquake hazard (Ludwin and others, 1991; Rogers and others, 1996). Sources for moderate or larger earthquakes include slip on the Cascadia plate boundary (interplate), ruptures in the downgoing Juan de Fuca plate about 60 km below Puget Sound (intraplate), and movement on shallow crustal faults in the North American plate such as the Seattle fault or southern Whidbey Island fault (Figs. 1, 2) (Gower and others, 1985; Johnson and others, 1994, 1996, 1999a; Pratt and others, 1997). Only the downgoing plate has produced large historic earthquakes (e.g., 1949 and 1965; Langston and Blum, 1977; Baker and Langston, 1987; Chleborad and Schuster, 1998).

Paleoseismologic studies in southwest Washington suggest seven great earthquakes (M > 8) have occurred at the Cascadia plate boundary since about 3500 yr B.P. (Atwater and Hemphill-Haley, 1997), including the most recent (M ~ 9) event in A.D. 1700 (e.g., Nelson and others, 1995; Satake and others, 1996). In the Puget Lowland, Bucknam and others (1992) have shown that a large (M > 7) earthquake occurred on the Seattle fault about A.D. 900-930 (Atwater, 1999). Recognition of these events has boosted estimates of regional seismic hazard (Frankel and others, 1996) and demonstrates the need for a more complete catalog of the number, frequency, sources, magnitudes, and effects of large earthquakes affecting the Puget Lowland.

Here, we report geologic evidence for paleoearthquakes from the Snohomish River delta near Everett in the northern Puget Lowland (Figs. 1, 3), where there is minimal historical information on earthquake effects. Deltas are particularly good sites for paleoseismologic investigations because their young, loosely packed sediments are prone to coseismic liquefaction, compaction, and subsidence, and because their low elevation and proximity to a water body make them susceptible to tsunamis. Bank exposures along tidal distributaries of the lower Snohomish delta show evidence of at least three liquefaction events, inundation by at least one tsunami, and at least one episode of abrupt subsidence, all since about A.D. 800. We infer that the most distinctive stratigraphic horizon (~A.D. 800-1100) records strong ground motion and tsunami inundation associated with the A.D. 900-930 Seattle fault earthquake.

THE SNOHOMISH RIVER DELTA

The Snohomish River begins at the confluence of the Skykomish and Snoqualmie rivers and empties into Possession Sound (Fig. 1). At its lower end, the Snohomish flows through a wide (~4 km) postglacial valley bounded by morainal deposits of the last (Fraser) glaciation. About 12 km upstream from Possession Sound, at an elevation of less than 1.5 m above sea level, the main channel divides into several distributaries or "sloughs" (Fig. 3). Spring-tide range in the sloughs is as much as 4.5 m.

Snohomish delta lowlands are now primarily undeveloped wetlands or are used for agriculture. Lumber mills and storage, marinas, sewage treatment plants, and a hazardous waste site are also located on the delta plain. The delta is crossed by an interstate highway (I-5), the Burlington-Northern railroad, and a busy local highway. Agriculture on the delta is dependent on a system of dikes and levees, construction of which began in 1876 (Dunnell and Fuller, 1975). The cities of Everett and Marysville, flanking the Snohomish delta, were settled in the late 1870s and early 1880s. The northwestern part of the delta is on the Tulalip Indian Reservation.

Cutbanks along both the main river channel and the sloughs in the lower delta typically expose, at maximum low tide, 1-4 m of strata deposited in about the last 1500 years (Figs. 4, 5). These strata, in general, record the building up of the delta, from channel point bar deposits to intertidal mud flats to supratidal marsh deposits. Marsh deposits predominate in studied outcrops, and in-growth-position plant material and detrital wood debris are abundant in these sediments. Although tidal laminae are visible on weathered surfaces, they have been largely disrupted by bioturbation associated with modern and relict marsh vegetation.

Snohomish River delta channels and marshes appear not to have migrated much since about AD 800. Since a United States Coast Survey hydrographic map was made in 1884, there has been neither significant lateral migration of channels nor progradation of the mouth. Also, sedimentary facies, including fossil plant material, in cutbanks are at approximately the same elevations as facies presently being deposited at nearby locations. Thus sediment supply appears to be in approximate balance with the combined effects of subsidence and slow sea-level rise. Vertical aggradation rates are approximately 2 m/1000 yr, based on our radiocarbon dating (Table 1, Figure 4).

SNOHOMISH SEDIMENTARY ENVIRONMENTS AND FACIES

The lower, modern Snohomish delta comprises five basic sub-environments, in succession from deeper to shallower: 1) subtidal channels, 2) lower intertidal flats and point bars, 3) upper intertidal flats and point bars, 4) supratidal marsh, and 5) lower delta plain and levees. This same succession is present in late Holocene facies in outcrop, produced as distributary channels migrate laterally and as the delta aggrades. Facies are distinguished primarily by sediment texture and color, growth-position plant fossils (see Table 2), and sedimentary structures. Our observations of the modern delta environments and the outcrop facies, including modern and fossil vegetation, are summarized in GSA Data Repository (click to download pdf) (DR) Figures DR 1-3.

Subtidal

The major subtidal channel environments of the modern delta (i.e., the axes of the main channel and its distributaries) are constantly submerged, so we did not directly observe and describe them. Lower point bars at low tide expose sand, minor gravel, and mud drapes. Thus we infer that the channel floors are primarily sand and gravel. The subtidal facies is also rare in outcrop, presumably because it is below the present low tide level. By hand coring, we encountered sand beneath exposed facies at a few localities. A sandy unit, inferred to be a few meters below the surface, is the most likely source for sand-filled dikes and other liquefaction features described in this paper.

Lower intertidal

The modern lower intertidal zone has its greatest areal extent near the delta front and along channel point bars. This subenvironment is unvegetated and characterized by deposition of interbedded sand (fine to coarse), silt and mud. The lower intertidal facies is exposed at the base of many outcrops and along low banks adjacent to modern point bars. It comprises interlaminated silt, mud, and sand.

Upper intertidal

The modern upper intertidal zone is most prominent above lower intertidal flats near the delta front, and in upper point bar and lower cutbank positions along distributary channels. This subenvironment is characterized by deposition of olive-gray mud and is commonly covered with Carex lyngbyei. Other plants present, in order of their typical first appearance with increasing elevation in the tidal zone (Figure DR1) include Juncus balticus, Triglochin maritima, Lilaeopsis occidentalis, and Potentilla anserina. Deschampsia capitosa and Scirpus acutus, although most common on the supratidal marsh, are locally present at intertidal elevations. More salt-tolerant species such as Distichlis spicata and Scirpus maritimus, are present as fossils in some intertidal-facies outcrops, but were not observed on the modern marshes. The upper intertidal facies is the dominant facies in outcrops, and consists of bioturbated olive-gray mud with fossil vegetation in growth position. The most abundant plant fossils are Carex roots and stems, Triglochin rhizomes, and Scirpus rhizomes and stems. Detrital wood fragments are also common in this facies.

Supratidal

The supratidal marsh, which makes up most of the modern lower delta area, where not artificially drained, is submerged only during extreme high tide and river flooding, when mud may be deposited. The supratidal marsh surface is commonly littered with driftwood and heavily vegetated, characterized by all of the species in the upper intertidal zone plus Deschampsia capitosa, thick stands of Scirpus acutus, and Typha latifolia, Rumex sp. , and unidentified grasses. The upper ~30 cm of soil below the marsh surface is typically weathered and oxidized. In bank exposures, the supratidal marsh facies is distinguished from the upper intertidal facies by its greater organic content, more pervasive fossil roots, and brownish to reddish mottling. Individual plant fossils are difficult to identify in this facies. Where supratidal marsh deposits have subsided below the water table into a more reducing environment, some of the mottling has been destroyed, and this facies is difficult to distinguish from upper intertidal facies.

Delta plain

The lower delta plain subenvironments, where not significantly altered by modern agriculture, is characterized by immature soils developed on a peaty mud substrate. Surfaces are vegetated by grasses, as well as shrubs and trees, including primrose, alder, crabapple, cottonwood, willow, and blackberries. The highest surfaces on the areas of the lower delta we studied are also occupied by a native conifer, Sitka spruce (Picea sitchensis), which commonly appears unhealthy, with dead and dying foliage. In outcrop, the delta plain facies is most easily distinguished by the presence of growth-position root systems and trunks of trees and woody shrubs. Detrital wood fragments are also common.

PALEOSEISMOLOGY

There is evidence at the lower Snohomish delta for several prehistoric, late Holocene earthquakes (labeled A through E from oldest to youngest) based on our examination of outcrops at about 45 localities (Figs. 3, DR 2, 3). Below we describe and discuss each of these events, in order of the strength of the evidence supporting a paleoseismic interpretation. We consider events B, C and E, all with liquefaction, more reliable than Events A1, A2 and D. We report the latter three events to help develop a regional catalogue of possible paleoseismic indicators. (continued)

DISCUSSION | CONCLUSIONS | ACKNOWLEDGMENTS | REFERENCES

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