Electronic Supplement to
Evidence for liquefaction identified in peeled slices of Holocene deposits along the lower Columbia River, Washington
by Keita Takada and Brian F. Atwater
Appendix A
Overview of liquefaction features exposed at six islands along the lower Columbia River, Washington and Oregon
Figures
Tables (courtesy of Andrei M. Sarna-Wojcicki, written commun., 2002)
This appendix, written by Atwater and containing tephra data from Andrei Sarna-Wojcicki, summarizes data from outcrop surveys and penetrometer tests in 1993 and 1994 at six islands along the lower Columbia River.Ê The text refers to index maps (Fig.
A1) and graphs (Fig.
A2) that summarize many of the findings.Ê Full details from the 1993 surveys were compiled by Atwater (1994), and those from the 1994 field season appear in Appendix B.
The 1993 field party included, in alphabetical order:Ê Lorin Amidon, Brian Atwater, Patricia Atwater, Boyd Benson, Catherine Chatfield, John Bertelling, Scott Craig, Stephen Dickenson, Eileen Hemphill-Haley, Roger Lewis, Ian Madin, Richard McMullen, Mary Meek, Coby Menton, Lynn Moses, Stephen Obermeier, Stephen Palmer, Curt Peterson, James Phipps, Timothy Roberts, John Shulene, Maureen Soar, David Tabaczynski, Paul Travis, and Daryl Wieneke.
All the outcrops surveyed in 1993 are located on eroded banks of shrubby or forested wetlands that form islands among freshwater tidal channels of the Columbia River.Ê The length of the surveyed banks totals nearly 3 km.Ê The banks provided largely horizontal outcrops, totaling nearly 10,000 m2
(Fig.
A2a), between low tide and a gray silt layer about 500 years old.Ê This stratigraphic interval contains most of exposed liquefaction features from the 1700 earthquake.
At the time of the 1700 earthquake, the surveyed sites were probably hundreds of meters from large channels.Ê Subsequent erosion, estimated from comparison of historical maps, has widely exceeded 100 m in the past 130 years alone (Fig.
A1b).Ê The greatest erosion since the 1870Õs occurred at Marsh Island (up to 350 m), Price Island (up to 250 m), and upper Wallace Island (up to 350 m from the northwest, and about 600 m from the northeast).
Where surveyed in 1993 the islands had a sandy foundation and a mud cap.Ê This sequence records the shoaling of channels and subtidal bars (sandy foundation) and the subsequent construction and maintenance of vegetated wetlands (mud cap).
The sandy foundation consists mainly of fine and medium sand, as judged from vibracores at Marsh, Price, Hunting, and Wallace Islands.Ê The sand ranges from distinctly laminated to structureless.Ê Detrital wood in the sand, from vibracores collected in 1993 and 1994, gave ages of less than 2000
14C yr B.P. at Marsh and Hunting Islands (Fig.
A2d).
The mud cap ranges in thickness from less than 1 m (upper Wallace Island) to as much as 6 m (Price Island).Ê It started forming before 1100
14C yr B.P. at Price Island and by about 600
14C yr B.P. at Marsh Island (Fig.
A2d).
At all surveyed outcrops except upper Wallace Island, the mud cap contains a volcanic ash layer that was first recognized along the Columbia by Stephen Obermeier, David Tabaczynski, and Curt Peterson in 1992.Ê Phenocrysts of hypersthene and hornblende further suggest that the ash probably correlates with set W (Patrick Pringle, written communication, 1994), which erupted in 1479 (layer Wn) and 1482 (layer We) (Fiacco
et al., 1993; Yamaguchi, 1985).ÊCorrelation with Wn has been confirmed by Andrei Sarna-Wojcicki by means of major-element analysis of volcanic glass from pumice pebbles.
The sample analyzed by Sarna-Wojcicki was collected by Alex Bourdeau, of the U.S. Fish and Wildlife Service, from banks of Blind Slough, between Marsh Island and Knappa (Fig.
A1). As described by Sarna-Wojcicki (written communication, 2002), the color ranges from pale yellowish brown (10 YR 6/2) to white (N 9). It contains felsic glass and phenocrysts of hypersthene and hornblende, as well as opaque grains. Most of the glass shards moderately angular, highly vesicular, and pumiceous with rounded vesicles. A trace of shards are elongate with spindled vesicles. Many of the mineral grains have glass selvages.
Sarna-Wojcicki reported 13 electron-microprobe analyses of glass, measured by Jim Walker (Table A1). He recalculated mean of these analyses to total 100 percent, then made statistical comparisons with data of the U.S. Geological Survey tephrochronology project in Menlo Park, California. Among the closest 45 matches, the first six reference samples are Mount St. Helens layer Wn (samples 2 and 4-8 in
Table A2).
Downstream from Wallace Island the mud cap also contains a buried soil above the volcanic ash layer. This soil, commonly 1 m below the modern wetland surface, probably records subsidence during the 1700 earthquake.
Low-tide outcrops surveyed at the six islands contain at least 200 sand bodies that intruded the mud cap during the 1700 earthquake.Ê Most of these intrusions are easy to spot where etched by wind waves and boat wake.Ê Although none of the intrusions extend above the buried soil from the 1700 earthquake, some terminate at that level.Ê Such intrusions are illustrated by dikes surveyed at Marsh Island.Ê The highest level reached by many of the intrusions is unknown, because they are exposed only where truncated on an eroded bench.
Extruded sand crops out above the level of the highest intrusions at Marsh, Brush, Price, and Hunting Islands.Ê It forms lenses that mantle the buried soil.Ê The lenses are less than 2 cm thick and can be followed in outcrop for a meter or two at most.
Ten-meter-long stretches of the surveyed outcrops rarely contain more than five intrusions that are not visibly connected to one another (Fig.
A2g).Ê All but the short outcrops at upper Wallace Island have many 10-m stretches without any observed intrusions, despite ample horizontal and (or) vertical outcrop.Ê The largest intrusion-free stretch of surveyed outcrop, at Hunting Island, has a length of 70 m and an area of about 250 m2
(Atwater, 1994, p. 49, horizontal coordinates 600-670 m).
The greatest concentrations of intrusions in the surveyed banks are located at Brush and upper Wallace Islands.Ê These outcrops have at least 6 intrusions in outcrop areas less than 20 m2
(Atwater, 1994, p. 47, horizontal coordinates 220-230 and 300-310 m; p. 52, 10-20 m).
There seems no simple explanation for the overall variability in abundance of intrusions at the surveyed outcrops (Obermeier and Dickenson, 2000, p. 880).ÊÊ Abundance is not obviously controlled by thickness of the mud cap or by penetration resistance in the sandy foundation, as illustrated by data from Marsh Island (Appendix C).Ê It is not explained, either, by distance from the coast (Fig.
A2b). It does increase with outcrop area in some stretches of the surveyed bank atÊMarsh Island but not in a much larger bank (nearly 7500 m2; Fig.
A2a) at Hunting Island (Fig.
A2b).
Where seen in the surveyed outcrops, all but a few of the intrusions are dikes that dip within 20 degrees of vertical.Ê Though uncommon, exposed sills are present in the mud cap at Marsh and Hunting Islands.
The dikes vary greatly in strike across the length of each surveyed outcrop (Atwater, 1994, p. 55).Ê Dikes are similar in strike along hundreds of meters of outcrop at Hunting Island, but otherwise the dikes have little tendency to strike parallel to one another (Atwater, 1994, p. 46-52).Ê The dikes also show little tendency to strike parallel to modern or historical trends of nearby channels.Ê The widest dikes do have this tendency at Marsh and Brush Islands but not at upper Wallace Island.
Among the nearly 200 surveyed dikes, most are no more than 3 cm wide where seen in outcrop, and more than 90 percent are less than 5 cm wide along most of their observed length (Fig.
A2c).Ê Only eight are chiefly wider than 5 cm.Ê Two of these, one each at Marsh and Brush Islands, are as much as 30 cm wide; another, at Hunting Island, is up to 20 cm wide; and one at upper Wallace Island is 15 cm wide.
The upper few meters of the sandy foundation gave ranges of penetration resistance that are similar among all six surveyed areas (Fig.
A2d).Ê Deeper deposits, however, gave much higher blow counts at Price and Hunting Islands only.ÊÊÊ
Atwater, B.F. (compiler) (1994). Geology of Holocene liquefaction features along the lower Columbia River at Marsh, Brush, Price, Hunting, and Wallace Islands, Oregon and Washington,
U.S.
Geol. Surv. Open-File Rept. 94-209, 64 p.
Fiacco, R.J., Jr., J.M. Palais, M.S. Germani, G.A. Zielinski, and P.A. Mayewski (1993).Ê Characteristics and possible source of a 1479 A.D. volcanic ash layer in a Greenland ice core,
Quaternary Research
39,
267-273.
Obermeier, S.F., and S.E. Dickenson (2000).Ê Liquefaction evidence for the strength of ground motions resulting from late Holocene Cascadia subduction earthquakes, with emphasis on the event of 1700 A.D.,
Bull. Seism. Soc. Am.
90,
876-896.
Yamaguchi, D.K. (1985).Ê Tree-ring evidence for a two-year interval between recent prehistoric explosive eruptions of Mount St. Helens,
Geology
13,
554-557.
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