Tsunami Deposits Beneath Tidal Marshes on Vancouver Island, British Columbia

Home , Port Alberni, Tofino, Ucluelet, Vancouver Island

Tsunami deposits beneath tidal marshes on Vancouver Island, British Columbia

JOHN J. CLAGUE Geological Survey of Canada, 100 West Pender Street, Vancouver, British Columbia, Canada V6B 1R8 PETER T. BOBROWSKY Geological Survey Branch, Ministry of Energy, Mines, and Petroleum Resources, 1810 Blanshard Street, Victoria, British Columbia, Canada V8V1X4

ABSTRACT they pose a hazard to people and property in as the rest of the coast, subsided suddenly some coastal areas. during large earthquakes. The earthquakes Thin sheets of sand occur within Holo- triggered tsunamis that deposited sand on cene mud and peat deposits beneath tidal INTRODUCTION subsided marsh surfaces that subsequently marshes at Tofino, Ucluelet, and Port Al- were buried by intertidal mud. berni on Vancouver Island, British Co- Historical, great earthquakes (magnitude This paper documents the stratigraphic lumbia. The sand sheets are extensive and 2:8) in the Pacific Ocean have produced setting, physical characteristics, and age of have sharp upper and lower contacts. In tsunamis that have caused widespread de- anomalous sheets of coarse sediment in in- most cases they consist of moderately struction along the coasts of the Americas tertidal sequences near Tofino, Ucluelet, sorted, massive sand and silty sand with and Asia (Iida, 1983; Lockridge and Smith, and Port Alberni on Vancouver Island, Brit-

abundant wood and other plant detritus. 1984). The 1964 Alaska earthquake (Mw9.2), ish Columbia. We argue that some of the At Port Alberni, the thickest sheet has for example, generated a tsunami that sheets were deposited by tsunamis and that gravel and is graded. The upper two sand caused >$300 million damage (1964 Cana- one probably records a large earthquake on sheets in the Tofino-Ucluelet area, and dian dollars) and killed 130 people, some of the Cascadia subduction zone <500 yr B.P. possibly the third, are also present at Port whom were 2600 km from the epicenter Alberni. (Hansen and others, 1966). SETTING Eyewitness accounts and 137Cs analysis The main sources of destructive, far-trav- suggest that the uppermost, thinnest sand eled Pacific tsunamis in the twentieth cen- The Tofino-Ucluelet and Port Alberni ar- was deposited by the tsunami triggered by tury have been the Nankai Trough east of eas (Fig. 1) were chosen for study because the great Alaska earthquake in 1964. The Japan, the Aleutian Trench south of Alaska, they were struck by the 1964 Alaska tsu- next oldest sand sheet has been radiocarbon and the Nazca Trench west of South Amer- nami. Sediment deposited during this event dated at <500 yr old. At Tofino and Uclue- ica (Rogers, 1988). The Cascadia subduc- is preserved in tidal marshes in both areas. let, it sharply overlies a former marsh sur- tion zone, west of Oregon, Washington, and Tofino and Ucluelet are located on a low- face that probably subsided suddenly dur- British Columbia, has been aseismic during lying coastal plain on the central west coast ing a large earthquake; the sand was the historical period, but there is abundant of Vancouver Island (Fig. 2). The plain is deposited by a tsunami generated by this geologic evidence that it has produced large underlain, in part, by Pleistocene glacioma- earthquake. The third sand sheet at Tofino earthquakes and tsunamis in late Holocene rine sediments deposited at the end of is 500-800 yr old. The tsunami that depos- time prior to European settlement of the the last glaciation. East and southeast of ited it may have been caused by the penul- region (Atwater, 1987, 1992; Grant and Tofino are the broad mud flats of Grice timate, great subduction earthquake in McLaren, 1987; Reinhart and Bourgeois, Bay, Browning Passage, and Meares Island southern Alaska, a previously unrecognized 1987, 1989; Darienzo and Peterson, 1990; (Figs. 2 and 3). The upper parts of the flats earthquake on the Cascadia subduction Atwater and Yamaguchi, 1991; Grant and are vegetated, mainly by sedge (Cara; lyng- zone, a great earthquake elsewhere in Minor, 1991; Clarke and Carver, 1992). Sub- byei), grass (Deschampsia caespitosa), rush the North Pacific, or a large submarine sidence attributed to these earthquakes is (Juncus balticus), and the forbs Triglochin landslide. recorded in Pacific coast tidal marshes from maritimum, Potentilla pacifica, and Salicor- Our data suggest that large tsunamis northern California to central Vancouver Is- nia sp. These areas are sheltered from the have struck the southern British Columbia land. Within the marshes, buried peaty soils open Pacific Ocean by Esowista Peninsula coast several times during the late Holocene similar to the present-day marsh soil are and by numerous islands north and west of and that some were much larger than the sharply overlain by intertidal mud. Sand lay- Tofino. Similar, although much smaller tidal 1964 tsunami, which caused about $10 mil- ers with sharp upper and lower contacts sep- flats fringe parts of Ucluelet Inlet, northwest lion damage (1964 Canadian dollars) to arate some of the buried soils from overlying of Ucluelet (Fig. 2). mud units. The favored interpretation of communities on Vancouver Island. Because Tides in the Tofino-Ucluelet area are this stratigraphy is that tidal marshes, as well such tsunamis can be expected in the future, mixed semidiurnal with a mean range of 2.8

Data Repository item 9421 contains additional material related to this article.

Geological Society of America Bulletin, v. 106, p. 1293-1303, 11 figs., October 1994.

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riod. The second and most destructive wave had a maximum height of ~7 m high and covered the tidal marsh west of the town to a maximum depth of 4-5 m. A 1.3-m-diameter pipeline, elevated 2.5-3.8 m above the marsh surface, was broken by the force of the wave or by the impact of logs (Fig. 6). Residents of the town reported sand and mud in gardens and houses that had been flooded. In addition, sediments on the sea- floor near the head of Alberni Inlet were eroded and redeposited in deeper water to the south (Morris and Leaney, 1980).

METHODS

Stratigraphy was described in detail at —110 sites in the Tofino-Ucluelet area and 50 sites at Port Alberni during the summers of 1990-1993. Shallow (0.5-1.5 m) pits were excavated with shovels in the upper vegetated part of the intertidal zone and in the adjacent forest. The deeper stratigraphy at many of these sites was inferred from 20- mm-diameter, 0.4-m-long solid cores taken with an Oakfield soil corer. Additional stratigraphic and sedimentological observations were made at exposures in the banks of tidal channels. Finally, continuous 10-cm-diameter core was collected to a depth of 11 m at a site in the Port Alberni marsh with a truck- mounted sonic drill. Figure 1. Map showing the locations of the Port Alberni and Tofino-Ucluelet study areas. Observations made at each site included Inset map shows lithospheric plates. the texture, structure, color, and organic composition of sediments, and the depth and characteristics of contacts. Samples were collected for grain-size, organic and in- m and a maximum range of 4.1 m (Fisheries 2.7 m and a maximum range of 4.0 m (Fish- organic carbon, I37Cs, 14C, and microfossil and Oceans, 1992). Waves in Browning Pas- eries and Oceans, 1992). The largest waves analyses. sage, Grice Bay, and Ucluelet Inlet are small in upper Alberni Inlet are — 1 m high and Elevations were determined with a sur- (<1 m) because of the protected setting and occur after extended periods of strong, sus- veying level by measuring differences be- limited fetch. tained, southerly winds (Morris and Leaney, tween sites and sea level at known times. Port Alberni lies at the head of Alberni 1980). Published Tofino and Port Alberni tide Inlet, a fjord that deeply indents the west gauge records (Fisheries and Oceans, 1992) coast of Vancouver Island (Fig. 1). The EFFECTS OF THE 1964 TSUNAMI were then used to calculate absolute eleva- study area is an intertidal marsh that is —50 tions. Absolute elevations are probably ac- ha in area, located west of the town, and The 1964 Alaska tsunami damaged To- curate to ±0.3 m. —1.5 km north of the mouth of Somass fino, Ucluelet, and Port Alberni. The Tofino Forty-three samples of wood, conifer River (Figs. 4 and 5). The dominant plants water supply was cut when the pipeline cones and needles, marine shells, and in situ in the marsh are sedge (Carex lyngbyei), crossing Browning Passage from Meares Is- rhizomes of intertidal plants were radiocar-

grasses (mainly Deschampsia caespitosa), land was destroyed. At Ucluelet, log booms, bon dated by conventional (C02 gas pro- rushes (Eleocharis palustris, Juncus balticus, an underwater telephone cable, and the wa- portional counting, liquid scintillation) and Scirpus spp.), and the forbs Lilaeopsis occi- ter pipeline were damaged, and at least one accelerator mass spectrometry methods. dentalis, Plantago spp., Potentilla pacifica, fish trawler sank. Port Alberni was particu- Most of the samples were collected from the Triglochin maritimum, and Typha latifolia. larly hard hit: 260 homes were damaged, 60 sand sheets that are the focus of this study. Somass River and its tributaries drain a extensively; estimates of total damage range Others came from the top of a peaty pa- broad low-lying valley underlain by thick from $5 million to $10 million (1964 Cana- leosol directly underlying the youngest pre- Quaternary sediments. The lowermost 4 km dian dollars; Wigen and White, 1964; Murty historic sand sheet, from muddy sediments of Somass River is tidal; tides are mixed, and Boilard, 1970; Thomson, 1981). Three between sand sheets, and from stumps in mainly semidiurnal, with a mean range of main waves struck the town over a 3 hr pe- growth position in the present intertidal

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muddy peat that is similar to the peaty soil of the contemporary marsh (Fig. 8). In places, this contact is erosive. In others, however, delicate leaves of herbaceous plants that presently inhabit the highest part of the marsh, including Juncus balticus and Potentilla pacifica, extend upward into and through the sand or are matted down beneath the sand, indicating rapid burial. Other delicate plant remains, including conifer needles and cones, are locally common at the top of the peat directly below the sand, notably at sites near the forest edge. Where there has been no postdepositional bioturbation, the sand is sharply overlain by olive gray (5Y 4/2) mud that grades upward through yellowish brown (10YR 3/2) peaty mud into the surface peat. There is a major change in the composition of foraminiferal assemblages at the level of the sand (J.-P. Guilbault, written commun., 1992; Clague and Bobrowsky, 1994). Muddy peat directly below the sand contains an upper marsh assemblage with high percentages of Trochammina macre- scens. In contrast, mud just above the sand has a middle marsh assemblage character- Figure 2. Tofino-Ucluelet area. Numbered sites correspond to sections shown in Figure ized by abundant Miliammina fusca. The fo- 7. Intertidal area is stippled. Topographic contour interval = 305 m (1000 ft). raminiferal data and stratigraphy indicate that peat deposition was abruptly termi- nated by submergence. The exact amount of zone. Radiocarbon ages, labeled "14C yr turbed by bioturbation. The source of most submergence is uncertain, although a com- B.P." (A .D. 1950 datum), were converted to of the sand is the tidal flats and tidal chan- parison of fossil foraminiferal assemblages calendric ages (cal yr B.P.) using the cali- nels of Browning Passage and Ucluelet Inlet and modern analogues suggests a sea-level bration curves and methodology of Bard (Fig. 3). change of several decimeters (Clague and and others (1993), Pearson and Stuiver The youngest of the three sheets is a thin Bobrowsky, 1994). (1993), Pearson and others (1993), Stuiver (1-5 mm), discontinuous layer of silt and A third, older sand sheet was encountered and Braziunas (1993), and Stuiver and sand. It is present in the root zone of the in several of the pits dug at Tofino and Pearson (1993). contemporary marsh at Ucluelet and locally Ucluelet (Fig. 7). It is similar to the second on southwestern Meares Island and along sand sheet but does not overlie peat; rather, STRATIGRAPHY AND Browning Passage. it sharply overlies, as well as underlies, olive SEDIMENTOLOGY A lower, thicker (<1-30 cm) sand sheet, gray mud. which was studied in much greater detail, The Holocene succession is almost every- Tofino-Ucluelet occurs at depths ranging from 10 to 50 cm at where <2 m thick and sharply and uncon- Ucluelet, on Meares Island, throughout formably overlies late Pleistocene glacioma- We have found three sand sheets in tidal Browning Passage, and, discontinuously, in rine sediments. Up to 1 m of gravel and sand marshes near Tofino and Ucluelet (Fig. 7). Grice Bay (Figs. 7 and 8). It extends from rests on the unconformity and may have The sheets consist of moderately well the seaward edge of the marsh to the edge of been deposited during a transgression sorted, medium to very fine sand and silt, the forest. On average, the sand is thinner -6000-7000 yr B.P. (Friele and Hutchin- mainly quartz, feldspar, and lithic frag- (1-2 cm) and shallower (<20 cm deep) at son, 1993). These coarse sediments probably ments. Also present are rare foraminiferal Ucluelet than in Browning Passage. In the were reworked during the subsequent late tests of the same species that occur in inter- latter area, it is thickest on tidal flats near Holocene regression. tidal sediments in the area. The sands are the heads of broad, north-facing bays and is generally massive, although at most of the thinnest in west- and southwest-facing bays Port Alberni sites we examined they are conspicuously and inlets. Locally along Browning Passage, mottled and may have been extensively bio- the sand rapidly thins and fines to silt in the Many sand layers are present in the peaty turbated; if so, any original sedimentary forest adjacent to the marsh. and muddy sediments underlying the Port structures have been destroyed. No conspic- At nearly all of the sites that we examined, Alberni marsh (Fig. 9). Not all of the sands uous grading was observed, even at sites this sand abruptly overlies a rooty, fibrous, are necessarily products of tsunamis, but the where the sediments have not been dis- dusky brown (moist Munsell color 5YR 2/2) upper two almost certainly are and the oth-

Geological Society of America Bulletin, October 1994 1295

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/10/1293/3381895/i0016-7606-106-10-1293.pdf by guest on 23 September 2021 Figure 3. Aerial photograph of the Tofino area showing the tidal flats bordering Browning Passage and Grice Bay. Numbered sites are those in Figure 2. Canada Department of Energy, Mines and Resources photos A24504-155 and -156.

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mate sand sheet at Tofino and Ucluelet and that contains abundant rhizomes of herbaceous marsh plants, notably Triglochin mari- Sewage timum. The peat, in turn, grades down into . pond organic-rich mud. The upper and lower contacts of the sand layer are sharp, except where the sediment has been disturbed by Port bioturbation. Alberni A thick (12-68 cm) unit of sand and pebble gravel was found at many sites below the second sand sheet, at depths ranging from 60 to 140 cm. These coarse sediments are sharply overlain by organic-rich mud and are sharply underlain by sandy silty mud and muddy sand. This is the thickest and coars- est of the anomalous sediment sheets observed at Port Alberni. It also is well strat- Figure 4. Port Alberni area. Numbered sites in (b) correspond to sections shown in ified, unlike the two younger sand sheets. At Figure 9. site 8, for example, pebble gravel grades up into fine sand without an obvious break. At site 9, pebble gravel is more abruptly overlain by medium to coarse sand. At still other ers may be as well. The sediments probably minor silt and is moderately well sorted. sites (1 and 2), an upward-fining subunit of were derived from the foreslope of the So- The sediment is generally massive, although coarse to fine sand with scattered granules mass River delta or from broad unvegetated there are scattered granules and pebbles and pebbles sharply overlies massive fine tidal flats seaward of the marsh. near the base of the sand at some sites. In sand. These relationships suggest that the The uppermost sand sheet, which is addition, at site 8 (Fig. 9), a thin lamina at sediments were deposited during two sepa- within 11 cm of the marsh surface, consists the base of the sand is distinctly finer than rate, closely spaced events or during a single of massive, moderately well sorted, fine to the overlying sediment. event under markedly fluctuating energy conditions. medium sand. Twigs, branches, logs, bark, At most sites, the sand is overlain and un- seeds, and conifer cones and needles are derlain by yellowish brown to olive gray or- Eight additional, older sand layers were scattered through the sediment and also oc- ganic-rich mud. In a few places, however, it penetrated in the sonic drill hole (site 1, cur locally in a mat at the top of the sand lies on a dusky brown muddy peat that is Fig. 9). Drilling was discontinued at this site sheet. The sand also contains rare tests of similar to the peat underlying the penulti- at a depth of 11.5 m (elevation = -8.5 m), intertidal foraminifera. This sand is present throughout the marsh south of the water pipeline that supplies the Alberni pulp mill, from unvegetated tidal flats to the upper limit of tides (Fig. 10). The layer is relatively uniform in texture and thickness (average = 1-2 cm) over much of this area, although it is unusually thick (up to 15 cm) at some sites just south of the pipeline. In contrast, north of the pipeline, the sand is relatively thin and discontinuous. The uppermost sand sheet is sharply overlain by dusky brown (5YR 2/2) rooty peat containing minor interstitial mud. The peat becomes increasingly muddy and less rooty below the sand, grading into yellowish brown to olive gray (10YR 3/2, 5Y 4/2) organic-rich mud 10-30 cm below the marsh surface. An older sand sheet is widespread both north and south of the pipeline at depths ranging from ~30 to 80 cm. It generally is thicker (average = ~5 cm) than the uppermost sand and does not vary in thickness in Figure 5. Oblique air photo of the Port Alberni intertidal marsh; view northeast. Num- the vicinity of the pipeline, as does the latter. bered sites are those in Figure 4. The water pipeline that was breached by the 1964 tsunami It consists of medium to very fine sand and (see Fig. 6) and subsequently repaired is indicated by an arrow.

Geological Society of America Bulletin, October 1994 1297

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110 14C yr B.P. (Table l1). The unconformity at the top of the Pleistocene sequence appears to span much of the Holocene. A branch directly overlying the gravel lag associated with the unconformity yielded an age of 700 ± 60 14C yr B.P. At another site, Triglochin rhizomes in growth position, recovered from sediments a few tens of cen- timeters above the unconformity, gave an age of 480 ± 50 14C yr B.P. The age of the uppermost sand sheet in the Tofino-Ucluelet area is uncertain, although it is in the same stratigraphic position as the youngest sand sheet at Port Al- berni and may have been deposited by the 1964 Alaska tsunami (see next section). Ten radiocarbon ages help date the penultimate sand sheet (Table 1, Figs. 7 and 11). Conifer needles from the top of the peat directly below the sand were dated at 180 ± 50 14C yr B.P. Plant rhizomes in growth po- Figure 6. Breached pipeline in Port Alberni marsh shortly after the 1964 tsunami. View sition at the same stratigraphic level as the west-southwest toward the Alberni pulp mill (upper right); up-valley is to the left. Note needles gave ages of 160 ± 60 and 440 ± 60 14 sections of pipe and the large number of logs that were transported up-valley by the C yr B.P. Other rhizomes, rooted in peat tsunami. Alberni Valley Museum photo PN 4383. and mud 5-22 cm below the top of the peat, yielded ages of 410 ± 50, 480 ± 50, and 480 ± 50 14C yr B.P. A cone at the base of still within the dominantly muddy sequence. CHRONOLOGY the sand gave a radiocarbon age of 1140 ± 14 Elsewhere beneath the marsh, the mud se- 50 C yr B.P., and branches and a log at the quence is thinner and overlies deltaic sand Tofino-Ucluelet and gravel (Fig. 9; see also Hardy BBT, 1990) deposited by the ancestral Somass Articulated pelecypod valves, recovered 'GSA Data Repository item 9421, Table 1, is River when relative sea level in the Port Al- from glaciomarine sediments near Tofino, available on request from Documents Secretary, berni area was lower than today. have been radiocarbon dated at 13 780 ± GSA, P.O. Box 9140, Boulder, CO 80301.

PEAT (P> © SITE NUMBER (SEE FIG.2 FOR LOCATION)

fc-Z-Z-d MUD (m) 1,2,3 SAND SHEETS |i:i;;;:;| SAND (s) u PLEISTOCENE-HOLOCENE UNCONFORMITY iy/o^l GRAVEL (g) 340 RADIOCARBON AGE (14C YR B.P.) STUMP

)V» SAND-FILLED BURROWS Figure 7. Representative stratigraphic sections from tidal marshes near Tofino and Ucluelet (see Fig. 2 for locations).

1298 Geological Society of America Bulletin, October 1994

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served bark, collected from the top of peaty mud directly below the lowest sand, were dated at 630 ± 50 and 680 ± 50 ,4C yr B.P., respectively. The ages are in general agree- ment with the previously mentioned age of 700 ± 60 yr B.P., 16 cm lower in the sequence, and correspond to calibrated ages of 500-800 cal yr B.P. (Fig. 11).

Port Alberni

Four lines of evidence indicate that the uppermost sand sheet at Port Alberni was deposited by the 1964 Alaska tsunami. First, peaty sediments bounding the sand have high concentrations of 137Cs and thus probably date to the time of atmospheric nuclear testing in the 1950s and 1960s (Clague and others, 1994). Second, eyewitnesses reported that the tsunami deposited sand and Figure 8. Massive sand (s) sharply overlying a peaty soil (so) in a pit along Browning silt in Port Alberni. Third, the sand sheet is Passage (site 3, Fig. 2). The sand is abruptly overlain by intertidal mud (m), which grades unusually thick along the south side of the upward into peat of the present-day marsh (p). pipeline crossing the marsh, suggesting that this structure influenced deposition. Fourth, there have been no floods or storms large top of the sand yielded ages of 190 ± 60,340 cal yr B.P., suggesting that sand deposition enough to deposit sand in the Port Alberni ± 50, and 6060 ± 70 14C yr B.P. occurred in the last 500 yr (Fig. 11). This is marsh during the historical period. Radiocarbon ages on detrital material compatible with calibrated ages of <650 cal The age of the penultimate sand sheet is from the top of the buried peat and overly- yr B.P. on growth-position fossils at the top constrained by radiocarbon ages. Growth- ing sand are maxima for the time of sand of the peat. position fossils 23-25 cm below the sand deposition. The youngest of the detrital ages Two radiocarbon ages help fix the age of were radiocarbon dated at 400 ± 50 14C yr (180 ± 50 and 190 ± 60 14C yr B.P.) corre- the oldest sand sheet in the Tofino-Ucluelet B.P. (Table 1). Four pieces of detrital wood spond to calendric ages of no more than 470 area. A conifer cone and a twig with pre- 1-32 cm below the sand yielded ages of

-1- -0.5

£ -2- e 30 —

1 U-J - or -

-6- 0.5- -7- 44 0 ESS3 fill (T) SITE NUMBER (SEE FIG. 4 FOR LOCATION) -8- 4730 •• PEAT (p) 1,2,3 SAND SHEETS

ËxH^j MUD (m) 840 RADIOCARBON AGE (14C YR B.P.) SAND (s)

lo O O O GRAVE1 L (g) SHELLS Figure 9. Representative stratigraphic sections from the Port Alberni tidal marsh (see Fig. 4 for locations).

Geological Society of America Bulletin, October 1994 1299

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A peaty soil also underlies the penultimate sand layer at some of the Port Alberni study sites, strengthening the correlation of this sand with its counterpart in the Tofino- Ucluelet area. The Port Alberni soil, however, is not as peaty or common as the soils at Tofino and Ucluelet. Geophysical models predict that Port Alberni lies near the east- ern margin of the zone of coseismic subsidence associated with great earthquakes on the Cascadia subduction zone (Hynd- man and Wang, 1993). Consequently, little crustal subsidence may have occurred there during plate-boundary earthquakes. Minor subsidence at Port Alberni could also result from differential compaction or liquefaction of sediments induced by seismic shaking. The thick sheet of sand and gravel at Port Alberni (the third anomalous sediment sheet in the sequence) is probably younger than 1000 cal yr B.P.; the third sand sheet at Tofino and Ucluelet is younger than 800 cal yr B.P. It is possible that the two correlate and date to -500-800 cal yr B.P. The Port Alberni deposit, however, is much coarser than any of the sand sheets in the Tofino- Ucluelet area, and, as discussed below, there Figure 10. Thickness of the uppermost sand sheet at core and pit sites (open circles) is some uncertainty about its origin. the Port Alberni marsh. The 137Cs sample site is starred. No great Cascadia earthquakes are known to have occurred between 500 and 800 cal yr B.P., but the penultimate great subduction zone earthquake in southern 270 ± 50, 400 ± 80,410 ± 60, and 570 ± 70 CORRELATIONS Alaska has been dated at —700-950 cal yr 14C yr B.P, Wood within and at the top of B.P. (Plafker and others, 1992; Combellick, the sand gave ages of 270 ± 60, 360 ± 50, We correlate the thin discontinuous layer 1993). It seems unlikely that an Alaskan tsu- 810 ± 70,840 ± 50, and 980 ± 60,4CyrB.P. of silt and sand in the root zone of the nami could transport and deposit gravel at As mentioned earlier, radiocarbon ages on marshes near Tofino and Ucluelet with the Port Alberni, assuming for a moment that detrital wood are maxima for the time of uppermost sand sheet at Port Alberni, the third sheet in that area is indeed a tsu- deposition of the enclosing sediments. Ac- largely on the basis of stratigraphic position. nami deposit. The 1964 tsunami, for exam- cordingly, the youngest wood ages cited Both probably were deposited by the 1964 ple, deposited only sand in the Port Alberni above are likely to be closest to the true age tsunami. marsh. Moreover, the inferred age of the of the sand. They correspond to calibrated The penultimate sand sheet at Tofino, penultimate Alaskan earthquake barely ages of <500 cal yr B.P. (Fig. 11). Ucluelet, and Port Alberni can be correlated overlaps the possible age range of the third The age of the third anomalous sediment on the basis of radiocarbon ages and strat- Tofino-Ucluelet sand sheet. An alternative sheet at Port Alberni is limited by the fol- igraphic position. The sand sheet contains explanation is that one or both of these lowing radiocarbon ages on detrital mate- abundant wood and other reworked plant sediment sheets are products of an as-yet- rial: 1010 ± 6014C yr B.P. on wood from the material, but the youngest radiocarbon ages unrecognized Cascadia earthquake. Yet an- top of the sand-gravel layer; 880 ± 90 and indicate that it is no more than 500 yr old other possibility is that the tsunami was 1160 ± 90 14C yr B.P. on a conifer cone and (Fig. 11). The peaty soil that underlies the caused by a large submarine landslide, ei- a twig from the bottom of the layer; and sand in the Tofino-Ucluelet marshes records ther related or unrelated to an earthquake. 1570 ± 70 and 1560 ± 60 14C yr B.P. on sudden submergence, probably during the Eight older sand layers, encountered in wood from sediments 16 and 31 cm beneath most recent earthquake on the northern the sonic drill hole at Port Alberni, range in the layer. Assuming that the youngest of part of the Cascadia subduction zone age from -2000 to 5000 cal yr B.P. They these ages is reliable, the sand-gravel layer is (Clague and Bobrowsky, 1994). Similar sed- resemble sands higher in the sequence, but -600-1000 yr old (Fig. 11). iments are present in many estuaries in we cannot currently test that they are tsu- The ages of older sand layers are less well Washington and northern Oregon (Dari- nami deposits because their spatial extent is known. Eight such layers underlie the coarse enzo and Peterson, 1990; Atwater, 1992; and unknown. sand-gravel unit at the sonic drill site; these references therein) and record either the The older sand layers at Port Alberni have are bracketed by radiocarbon ages of 2130 ± same earthquake or one that preceded or no known counterparts at Tofino and Uclue- 60 and 4730 ± 80 14C yr B.P. (Fig. 9). followed it by no more than 200 yr. let. The reason for this is that the outer coast

1300 Geological Society of America Bulletin, October 1994

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TOFINO deposited in south-central Chile in 1960 (Wright and Mella, 1963; Bourgeois and -co- Reinhart, 1989; Atwater and others, 1992). They also are similar to deposits in Wash- ington State that have been attributed to prehistoric tsunamis (Reinhart and Bour- geois, 1987, 1989; Atwater, 1992; Atwater •• • and Moore, 1992). The Tofino, Ucluelet, -OCK)-® and Port Alberni sands are widespread sheets that range from a few millimeters to .w -OCC^-O several decimeters thick. They overlie peat and mud and can be found throughout the i i r "i r i—r~ i r vegetated part of the intertidal zone. At 1500 1000 500 Tofino, one of the sand sheets was observed CALENDRIC AGE (CAL YR B.P.) to thin and fine into the forest beyond the marsh edge. The sands are sharply bounded at the top and bottom, and these contacts PORT ALBERNI commonly are conformable. Although the above characteristics are w compatible with a tsunami origin, some consideration must be given to other possible depositional mechanisms. Some explana- tions, such as the sudden breaching of sand and gravel barriers, deposition related to the migration of tidal channels, and eolian deposition, can be ruled out fairly easily. There are no barriers at Tofino, Ucluelet, and Port i—r i—r ~i i r Alberni that could have breached during 2000 1500 1000 500 late Holocene time; the sand sheets are thin CALENDRIC AGE (CAL YR B.P.) and areally extensive, and thus not tidal channel deposits; and the sands contain logs SECOND SAND SHEET BELOW SURFACE W- WOOD (TWIG, BRANCH, LOG, FRAGMENT) and branches that could not have been de- # SAMPLE IN OR ON SAND posited by wind. Nor are the sand sheets R- PLANT RHIZOME, IN GROWTH POSITION O SAMPLE BENEATH SAND beach sediments deposited during a transgression or regression: they are too thin and THIRD SAND/GRAVEL SHEET C- CONIFER CONE OR NEEDLES • SAMPLE IN OR ON SAND/GRAVEL are sharply but conformably overlain by sed- A SAMPLE BENEATH SAND/GRAVEL iment of markedly different character and origin. Furthermore, delicate, growth-posi- Figure 11. Plot of calibrated radiocarbon ages (Table 1) fixing the ages of the second and tion fossils of intertidal plants are present at third sand sheets below the surface at Tofino and Port Alberni. The ages are maxima for the top of the peaty soil beneath one of the times of sand deposition; the youngest ages in each suite are closest to the true age of the sand sheets at Tofino and Ucluelet; this in- respective sand. Note that most of the dated wood from the sands is older than the enclosing dicates rapid burial without the erosion ex- sediment and thus must be reworked. Radiocarbon ages were calibrated using the method pected in a beach environment. and curves of Stuiver and Pearson (1993). Some of the mean radiocarbon ages correspond Another mechanism that can be dis- to two, three, or four calendric ages; this is indicated by multiple symbols along one line. counted, at least at Tofino and Ucluelet, is The horizontal lines represent the 2a age range, calculated using an error multiplier of 2.0. river flooding. No significant streams enter (Note: error multipliers expand laboratory-quoted errors to cover uncertainties in repro- Ucluelet Inlet, Grice Bay, and Browning ducibility and systematic bias; for a discussion, see Stuiver and Pearson, 1993.) Passage. A river does flow into Alberni In- let, and it might be argued that some of the coarse sediments in the Port Alberni marsh of Vancouver Island has risen at an average sea-level rise, thus preserving the succession could be flood deposits. Somass River, how- rate of ~1 m/1000 yr during the late Holo- of anomalous sand layers extending back at ever, has not flooded the marsh during the cene (Clague and others, 1982; Friele and least 5000 yr. historical period and, as we have shown, Hutchinson, 1993). Sands that were depos- there is good evidence that the uppermost ited in tidal marshes at Tofino and Ucluelet sand sheet was deposited by the 1964 Alaska >1000-2000 yr B.P. have been either ORIGIN OF SAND SHEETS tsunami. Furthermore, it is unlikely that the eroded or elevated above sea level. At Port river could deposit widespread, sharply Alberni, in contrast, there has been a rela- The sand layers at Tofino, Ucluelet, and bounded, thin sheets of moderately well tive rise in sea level during the late Holo- Port Alberni share many of the characteris- sorted sand like the upper two in the Port cene. Aggradation has kept pace with this tics of historical tsunami sands, such as those Alberni sequence.

Geological Society of America Bulletin, October 1994 1301

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We cannot rule out a flood origin for the (1964 Canadian dollars). In addition, a Cas- we have no information on their areal extent older sand and gravel layers at Port Alberni. cadia tsunami might reach the outer coast of and thus cannot preclude alternative depo- The third layer appears to have a sheet-like Vancouver Island <30 min after earthquake sitional mechanisms, notably river flooding. form but is thicker, much coarser, and more shaking stops (Hebenstreit and Murty, 1989; Large tsunamis in the North Pacific are localized than the two above it. In addition, Ng and others, 1990), perhaps not enough sufficiently frequent to pose a major hazard unlike the younger sands, it is internally time to allow orderly evacuation of low-lying to people and property in low-lying areas on stratified and locally graded. Although these areas. western Vancouver Island. Some prehistoric characteristics do not preclude a tsunami or- Some prehistoric tsunami deposits on tsunamis probably were more powerful than igin, and indeed do not fully meet our ex- Vancouver Island may be the result of dis- the 1964 tsunami, the largest and most de- pectations for flood deposits, we prefer to be tant earthquakes. Possible sources include structive of the historical period. Unlike the cautious and thus leave open the interpre- subduction zones off Japan, Kamchatka, and 1964 event, a large Cascadia tsunami would tation of this layer. Alaska. Numerical simulations of a tsunami strike the outer coast of Vancouver Island Older sand layers penetrated during drill- generated by a hypothetical great earth- with little advance warning. ing are similar in texture and sorting to the quake south of the Aleutian Islands younger sands and, like them, have sharp (Shumagin Gap) indicate that wave heights ACKNOWLEDGMENTS upper and lower contacts. Their areal extent on the west coast of Vancouver Island might and form, however, are unknown. exceed those of 1964 (Dunbar and others, This work was funded by the Geological A final alternative mechanism is deposi- 1989). Survey of Canada and the British Columbia tion during storm surges. This possibility Geological Survey. Assistance in the field cannot be completely excluded but is un- CONCLUSIONS was provided by A. Blais, H. Blyth, A. likely because of the very protected loca- Clague, W. Jackaman, and C. Smith. Drill- tions of the studied marshes, especially Tsunamis best explain some of the thin ing at Port Alberni was done by R. Roussy those in the Tofino-Ucluelet area. In addi- sheets of sand beneath tidal marshes at (Sonic Drilling). L. D. Arnold (Alberta En- tion, a storm surge does not explain the sub- Tofino, Ucluelet, and Port Alberni. A tsu- vironmental Centre), R. P. Beukens (Iso sidence that immediately preceded deposi- nami origin is inferred from the form and Trace Laboratory), R. M. McNeely (Geo- tion of one of the sand sheets in the Tofino- sedimentological characteristics of the de- logical Survey of Canada Radiocarbon Lab- Ucluelet area. posits and from their presence over large oratory), and M. Tamers (Beta Analytic) In conclusion, consideration of available areas in protected, relatively low-energy provided radiocarbon ages. T. Oliveric evidence and possible depositional mecha- settings. The sands are generally massive, drafted the illustrations. Thoughtful reviews nisms suggests that the three sand sheets at have sharp upper and lower contacts, and by B. F. Atwater, D. F. Belknap, J. Bour- Tofino and Ucluelet and the upper two at contain abundant detrital wood. geois, and D. A. Hodgson improved the Port Alberni probably were emplaced by The uppermost sand sheet at Port Alberni paper. tsunamis. Older sand sheets at Port Alberni and a thin discontinuous layer of silt and

may also be tsunami deposits, although sand at Tofino and Ucluelet probably were REFERENCES CITED other origins are possible. deposited by the 1964 Alaska tsunami. The Atwater, B. F., 1987, Evidence for great Holocene earthquakes penultimate sand sheet is <500 yr old. At along the outer coast of Washington State: Science, v. 236, p. 942-944. HAZARD IMPLICATIONS Tofino and Ucluelet, it sharply overlies a Atwater, B. F., 1992, Geologic evidence for earthquakes during the past 2000 years along the Copalis River, southern coastal peaty soil that subsided suddenly, most Washington: Journal of Geophysical Research, v. 97, The results presented in this paper show likely during a large earthquake on the Cas- p. 1901-1919. Atwater, B. F„ and Moore, A. L., 1992, A tsunami about 1000 that large tsunamis have struck western cadia subduction zone. A similar peat un- years ago in Puget Sound, Washington: Science, v. 258, p. 614-617. Vancouver Island during late Holocene derlies the sand at some of the Port Alberni Atwater, B. F., and Yamaguchi, D. K., 1991, Sudden, probably time. Numerical simulations suggest that study sites. coseismic submergence of Holocene trees and grass in coastal Washington State: Geology, v. 16, p. 706-709. some prehistoric tsunamis probably were The oldest recognized sand sheet at Atwater, B. F., Jimenes Nunez, H., and Vita-Finzi, C., 1992, Net late Holocene emergence despite earthquake-induced sub- larger than the 1964 event, which itself was Tofino dates to 500-800 cal yr B.P. It prob- mergence, south-central Chile, in Ota, Y., Nelson, A. R., and Beriyman, K., eds., Impact of neotectonics on Quater- extremely destructive. Ng and others (1990) ably was deposited by a tsunami, but the nary coastal evolution: Quaternary International, v. 15/16, calculated wave run-ups to 5 m above sea earthquake source is unknown. One possi- p. 77-86. Bard, E^rnold, M., Fairbanks, R. G„ and Hamelin, B., 1993, 23l 234 14 level (asl) at Tofino and 16 m asl at Port bility is that the tsunami was triggered by the ^rh- LJ and c ages obtained by mass spectrometry on Alberni for a hypothetical M8.5 plate- corals: Radiocarbon, v. 35, p. 191-199. penultimate great subduction earthquake in Bourgeois, J., and Reinhart, M. A., 1989, Onshore erosion and boundary earthquake at the north end of the southern Alaska, dated between 700 and 950 deposition by the 1960 tsunami of the Rio Lingue estuary, south-central Chile [abs.]: Eos (Transactions, American Cascadia subduction zone. The predicted cal yr B.P.; alternatively, it could be a prod- Geophysical Union), v. 70, p. 1331. Clague, J., Harper, J. R., Hebda, R. J., and Howes, D. E., 1982, wave heights are about four to five times uct of a previously undocumented earth- Late Quaternary sea levels and crustal movements, coastal those recorded in the areas during the 1964 British Columbia: Canadian Journal of Earth Sciences, quake on the Cascadia subduction zone or a v. 19, p. 597-618. tsunami (Hebenstreit and Murty, 1989, their large submarine landslide. A sheet of sand Clague, J. J., and Bobrowsky, P. T., 1994, Evidence for a large earthquake and tsunami 100-400 years ago on western Table 1). and gravel at Port Alberni may also have Vancouver Island, British Columbia: Quaternary Research, v. 41, p. 176-184. Some perspective on the consequences of been deposited by this tsunami, but it may Clague, J. J., Bobrowsky, P. T„ and Hamilton, T. S., 1994, A sand be older and a flood origin cannot be ruled sheet deposited by the 1964 Alaska tsunami at Port Alberni, a future large Cascadia tsunami is provided British Columbia: Estuarine, Coastal and Shelf Science, out. v. 38, p. 413-421. by the fact that the 1964 tsunami, which oc- Clarke, S. H., Jr., and Carver, G. A., 1992, Late Holocene tectonics curred at a time when the coastline of Brit- Eight still-older sand layers were pene- and paleoseismicity, southern Cascadia subduction zone: Science, v. 255, p. 188-192. ish Columbia was much less developed than trated during drilling at Port Alberni. Some Combellick, R. A., 1993, The penultimate great earthquake in today, caused about $10 million damage or all of these may be tsunami deposits, but southcentral Alaska: Evidence from a buried forest near

1302 Geological Society of America Bulletin, October 1994

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R., and Rubin, M., 1992, Determining re- MANUSCRIPT ACCEPIED FEBRUARY 1, 1994 Columbia, File NX00777A, 22 p. currence intervals of great subduction zone earthquakes in GEOLOGICAL SURVEY OF CANADA CONTRIBUTION NO. 23793 Hebenstreit, G. T., and Murty, T. S., 1989, Tsunami amplitudes southern Alaska by radiocarbon dating, in Taylor, R. E., BRITISH COLUMBIA GEOLOGICAL SURVEY BRANCH from local earthquakes in the Pacific Northwest region of Long, A., and Kra, R. S., eds., Radiocarbon dating after four CONTRIBUTION NO. 009

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Geological Society of America Bulletin, October 1994 1303

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