Tsunami Deposits Beneath Tidal Marshes on Northwestern Vancouver Island, British Columbia
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QUATERNARY RESEARCH 48, 192±204 (1997) ARTICLE NO. QR971911 Tsunami Deposits beneath Tidal Marshes on Northwestern Vancouver Island, British Columbia Boyd E. Benson1 Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada Kurt A. Grimm2 Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada and John J. Clague3 Geological Survey of Canada, 101-605 Robson Street, Vancouver, British Columbia V6B 5J3, Canada Received September 24, 1996 plate boundary that is storing elastic strain to be released in Two sand sheets underlying tidal marshes at Fair Harbour, Ner- a future earthquake, as is happening at subduction zones off outsos Inlet, and Koprino Harbour on the northwestern coast of Chile and Alaska (Heaton and Hartzell, 1986). Geophysical Vancouver Island, British Columbia, were probably deposited by models and geodetic measurements (Heaton and Hartzell, tsunamis. The sand sheets become thinner and ®ner-grained land- 1987; Rogers, 1988; Savage et al., 1991; Hyndman and Wang, ward, drape former land surfaces, contain marine microfossils, are 1993, 1995; Dragert et al., 1994) suggest that strain now locally graded or internally strati®ed, and can be correlated with earthquakes that generated tsunamis in the region. 137Cs dating accumulating on this 1000-km-long plate boundary may be and historical accounts indicate that the upper sand sheet was released either in a series of great, magnitude-8 earthquakes deposited by the tsunami from the great Alaska earthquake in or a single giant, magnitude-9 earthquake. 1964. Radiocarbon ages on plant fossils within and on top of the In the late 1980s and early 1990s researchers identi®ed lower sand sheet show that it was deposited sometime after about in coastal wetlands three types of geologic evidence best A.D. 1660. We attribute the lower sand sheet to a tsunami from explained by earthquakes at the Cascadia subduction zone the most recent plate-boundary earthquake on the Cascadia sub- (Atwater et al., 1995, and references therein): (1) buried soils duction zone about 300 yr ago, extending the documented effects of that were submerged suddenly, consistent with coseismic this earthquake north of the Nootka fault zone. The 1964 tsunami deposits differ little in thickness and continuity among the three subsidence; (2) sand sheets, interpreted to be tsunami depos- marshes. In contrast, the lower sand sheet becomes thinner and its, which overlie buried soils, thin and ®ne landward, and less continuous to the north, implying a tsunami source south of contain marine microfossils; and (3) intruded and extruded the study area. q 1997 University of Washington. sand bodies produced by liquefaction resulting from intense ground shaking. Studies of this geologic evidence have shown that intervals between successive earthquakes range INTRODUCTION from less than a few centuries to perhaps more than 1000 yr (Atwater et al., 1995; Atwater and Hemphill-Haley, in There are no written accounts of earthquakes along the press). The most recent earthquake or series of earthquakes offshore boundary between the North America plate and sub- occurred between about A.D. 1700 and 1720 (Atwater and ducting Juan de Fuca and Gorda plates (Cascadia subduction Yamaguchi, 1991; Nelson et al., 1995), probably in January zone, Fig. 1). The lack of historical seismicity has been attrib- 1700 (Satake et al., 1996). uted to aseismic creep (Ando and Balazs, 1979) or to a locked The purpose of this paper is to characterize tsunami depos- its in Holocene peat at tidal marshes on northwestern Van- couver Island, British Columbia. We studied the architecture, 1 Now at GeoEngineers, 8410 154th Avenue N.E., Redmond, WA, 98052. stratigraphy, and sedimentology of two sand sheets at Fair Fax: (206) 861-6050. E-mail: [email protected]. 2 Fax: (604) 822-6088. E-mail: [email protected]. Harbour, Neroutsos Inlet, and Koprino Harbour (Fig. 1) to 3 E-mail: [email protected]. understand their deposition, preservation, identifying charac- 0033-5894/97 $25.00 192 Copyright q 1997 by the University of Washington. All rights of reproduction in any form reserved. AID QR 1911 / a60d$$$$$1 09-24-97 15:41:13 qral AP: QR TSUNAMI DEPOSITS IN BRITISH COLUMBIA 193 FIG. 1. (a) Lithospheric plates and the seaward edge of the Cascadia subduction zone (study area outlined in box). (b) Study area; solid circles are study sites. teristics, and paleoseismic signi®cance. The two sand sheets Hutchinson, 1993; Clague and Bobrowsky, 1994a). This are similar in age, stratigraphic position, and morphology to sand and gravel may have been deposited during a transgres- tsunami deposits farther south that have been attributed to sion about 6000 to 7000 yr ago and then reworked during the 1964 Alaska earthquake and to the last great earthquake a late Holocene regression. The marshes, characterized by on the Cascadia subduction zone about 300 yr ago. salt-tolerant plants, are situated within the upper half of the intertidal zone between unvegetated tidal ¯ats and supratidal SETTING grass and forest. The mean tide range at the three study sites is about 2.9 m, and the maximum range is 4.5 m (Fisheries The study area is situated at the north end of the Cascadia and Oceans, 1995). subduction zone, a 1000-km-long area of plate convergence and subduction (Fig. 1). The Nootka fault zone, just south of METHODS the study area, may mark the northern limit of the Cascadia subduction zone (Rohr and Furlong, 1995). The small Ex- We studied sediments at the aforementioned marshes in plorer plate, which is separated from the Juan de Fuca plate channel-bank outcrops, pits, and cores. Gridded transects by the Nootka fault, is probably a broad deformational zone with data points 5 m apart were installed by tape and Brunton where convergence on the Cascadia subduction zone to the compass in areas of minimal ¯uvial in¯uence. The elevation south changes into strike-slip motion along the North of each grid point, relative to a temporary benchmark, was America±Paci®c plate boundary to the north (Rohr and Fur- measured with an automatic leveling instrument or a hand long, 1995). The subducting Juan de Fuca plate, south of the level. Benchmarks were linked to high tide levels which Nootka fault, is currently locked against the overriding North were determined from regional tide gauge data corrected America plate (Dragert et al., 1994; Hyndman and Wang, for local differences in time and magnitude (Fisheries and 1995). Evidence for locking and for the buildup of strain at Oceans, 1995). Depths of stratigraphic contacts, sediment the plate boundary includes geodetic measurements that reveal types, colors, and textures were recorded at each grid point uplift and crustal shortening in coastal British Columbia and from continuous cores retrieved with a 2.5-cm-diameter Washington (Savage et al., 1991; Dragert et al., 1994). gouge corer. Stratigraphic cross sections and three-dimen- Tidal marshes at Fair Harbour, Neroutsos Inlet, and Ko- sional fence diagrams were constructed from these data. prino Harbour were chosen for study because they are exten- Block samples of the sand sheets and enclosing peat were sive, easily accessible, and north of the area previously collected for laboratory study of sedimentology, internal shown to have been affected by earthquakes on the Cascadia stratigraphy, and microfossils. Samples of sand were col- subduction zone. The three marshes are underlain by about lected from the two sand sheets at each grid point at the Fair 1 m of peat containing two unique sand sheets. This peat Harbour marsh for grain-size analysis. Particle-size trends is underlain by Pleistocene glaciomarine clayey silt or by in the sand-sized fraction were determined with a semiauto- Holocene sand and gravel deposits similar to those found matic visual settling tube (Inter-Agency Committee on Wa- elsewhere on the coast of Vancouver Island (Friele and ter Resources, 1958). AID QR 1911 / a60d$$$$$2 09-24-97 15:41:13 qral AP: QR 194 BENSON, GRIMM, AND CLAGUE FIG. 2. (a) View north±northwest across Fair Harbour marsh; study site in the foreground. Bridges (B) were rebuilt after their destruction by the 1964 Alaska tsunami. (b) Bank of tidal channel at Fair Harbour; scale Å 30 cm. Fossil plant material for radiocarbon dating, including farther south on Vancouver Island described by Clague and sticks, conifer cones, bark, and leaf bases of growth-position Bobrowsky (1994b). herbaceous plants, was retrieved from near and within the Both sand sheets at each marsh become thinner and ®ner- lower sand sheet and at the base of the peat sequence. Eight grained landward and blanket paleosurfaces with relief like samples were radiocarbon dated at IsoTrace Laboratory, the that of the present-day marshes. Each sheet is commonly no Geological Survey of Canada Radiocarbon Laboratory, and more than 2 cm thick and consists of moderately sorted silt the Quaternary Isotope Laboratory of the University of and very ®ne to very coarse sand composed of quartz, feld- Washington. Calendric ages were calculated from the radio- spar, and lithic fragments. The sand also contains sticks, carbon ages using the decadal tree-ring dataset of Stuiver cones, bark, sparse shell fragments, foraminifera, and dia- and Becker (1993). toms. In grain size and lithology the sand resembles adjacent Ten samples from the upper 24 cm of the peat sequence tidal-channel sediments. at Fair Harbour were analyzed for 137Cs using an X-ray The sand sheets range from massive through normally spectrometer at the Paci®c Geoscience Centre (Geological graded to internally strati®ed. Locally, they consist of two Survey of Canada). The samples provided coverage from or three couplets of sand and mud. Each sand±mud couplet the marsh surface to a depth of 12 cm below the upper sand.