Record of Large, Late Pleistocene Outburst ﬂoods Preserved in Saanich Inlet Sediments, Vancouver Island, Canada A

Home , Boundary Bay, Cowichan River, Fort Langley, Fraser Lowland, Lillooet, Saanich Inlet

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Record of large, Late Pleistocene outburst ﬂoods preserved in Saanich Inlet sediments, Vancouver Island, Canada A. Blais-Stevensa,*, J.J. Clagueb,c, R.W. Mathewesd, R.J. Hebdae, B.D. Bornholdf a Geological Survey of Canada, 601 Booth Street, Ottawa, Ont., Canada K1A 0E8 b Department of Earth Sciences, Simon Fraser University, Burnaby, Canada BC V5A 1S6 c Geological Survey of Canada, 101-605 Robson Street, Vancouver, Canada BC V6B 5J3 d Department of Biological Sciences, Simon Fraser University, Burnaby, Canada BC V5A 1S6 e Royal British Columbia Museum, P.O. Box 9815 Stn. Prov. Gov., Victoria, Canada BC V8W 9W2 f School of Earth and Ocean Science, University of Victoria, P.O. Box 3055, Stn. CSC, Victoria, Canada BC V8W 3P6 Received 23 December 2002; accepted 27 June 2003

Abstract

Two anomalous, gray, silty clay beds are present in ODP cores collected from Saanich Inlet, Vancouver Island, British Columbia, Canada. The beds, which date to about 10,500 14C yr BP (11,000 calendar years BP), contain Tertiary pollen derived from sedimentary rocks found only in the Fraser Lowland, on the mainland of British Columbia and Washington just east of the Strait of Georgia. Abundant illite-muscovite in the sediments supports a Fraser Lowland provenance. The clay beds are probably distal deposits of huge floods that swept through the Fraser Lowland at the end of the Pleistocene. Muddy overflow plumes from these floods crossed the Strait of Georgia and entered Saanich Inlet, where the sediment settled from suspension and blanketed diatom-rich mud on the fiord floor. The likely source of the floods is Late Pleistocene, ice-dammed lakes in the Fraser and Thompson valleys, which are known to have drained at about the time the floods occurred. r 2003 Elsevier Ltd. All rights reserved.

1. Introduction Three unusual beds occur within the indistinctly laminated mud unit: a thin layer of Mazama ash, about During Ocean Drilling Program (ODP) Leg 169S in 7700 years old (Bacon, 1983); and two gray, silty clay 1996, a continuous sequence of sediments spanning the beds of latest Pleistocene age. The origin of the gray last 15,000 years was cored at two sites in Saanich Inlet, silty clay beds is the subject of this paper. We argue that an anoxic fiord on southern Vancouver Island, British they are the distal deposits of huge outburst floods, Columbia (Fig. 1; Bornhold et al., 1998). The cores were probably caused by the sudden draining of Late collected using a hydraulic piston coring system to Pleistocene, ice-dammed lakes in the British Columbia maximum depths of 105 and 118 m below the seafloor at interior, more than 200 km east of Saanich Inlet. the two sites. The uppermost part of the sediment sequence comprises varved, olive-gray, diatom-rich marine mud intercalated with beds of massive, olive- 2. Saanich Inlet gray mud deposited by debris flows (Blais-Stevens et al., 1997; Blais-Stevens and Clague, 2001). The varved Saanich Inlet (Fig. 1) is 26 km long, up to 8 km wide, sediments are underlain by indistinctly laminated, and has a maximum depth of 238 m. Its main source of olive-gray, diatom-rich marine mud of middle and early freshwater and sediment is Cowichan River, which flows Holocene age, which in turn is underlain by gray into Cowichan Bay north of the mouth of the inlet glaciomarine sediments deposited at the end of the Last (Herlinveaux, 1962). A bedrock sill (Fig. 1) separates Glaciation (Blais-Stevens et al., 2001). Saanich Inlet from Cowichan Bay and Satellite Channel and restricts deep-water circulation in the fiord, creating *Corresponding author. anoxic conditions below depths of 70–150 m (Gross E-mail address: [email protected] (A. Blais-Stevens). et al., 1963). Satellite Channel extends eastward, via a

0277-3791/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0277-3791(03)00212-9 ARTICLE IN PRESS 2328 A. Blais-Stevens et al. / Quaternary Science Reviews 22 (2003) 2327–2334

128ûW 54ûN Cowichan River

Cowichan Bay

British Fraser Satellite Channel Columbia Saanich 100 mbsl Inlet

Coast Mountains r 1034

Rive 1033 200 mbsl

River

son p Goldstream River Victoria

Thom Lillooet Kamloops Vancouver Spences Bridge 0 10km St Vancouver rait Island of Fraser Lowland Geor gia A Pacific Fort Langley CANAD Saanich U.S.A. Ocean Inlet Nooksack River 0 200 km Strait of Victoria Puget Washington 48ûN Juan de Fuca Sound 48ûN 128ûW 114ûW Fig. 1. Map of southern British Columbia and northwestern Washington showing localities mentioned in the text. Inset map of Saanich Inlet shows ODP core sites (site 1033, 4835.4380N, 12330.2010W, 238 m depth; site 1034, 4838.0000N, 12330.0000W, 200 m depth). The sill (black box in inset) reaches to within 70 m of the sea surface. series of islands, into the southern Strait of Georgia, clay bed has gradational upper and lower contacts. The which is 200–300 m deep. The abundance of freshwater, clay mineral fraction of the silty clay beds is dominated the high primary productivity in Saanich Inlet during by illite-muscovite, which is abundant in modern Fraser spring and summer, and the sluggish estuarine circula- River sediments (Mackintosh and Gardner, 1966). tion contribute to anoxia of bottom waters (Herlin- veaux, 1962), an absence of benthic fauna, and, 3.2. Microfossils consequently, excellent preservation of seasonally deposited laminae. The gray silty clay beds contain freshwater diatoms and silicoﬂagellates. The microfossil assemblage con- trasts sharply with the typical marine assemblage in the 3. Gray silty clay beds bounding indistinctly laminated mud (McQuoid and Hobson, 2001). 3.1. Description Most palynomorphs in the Saanich Inlet cores are well-preserved Quaternary types such as pine pollen Two gray silty clay beds are present within the lower (Fig. 5a; Pellatt et al., 2001). Pre-Quaternary pollen and part of the indistinctly laminated, diatomaceous mud spores are present in only trace amounts throughout the unit about 3 m above its base (Figs. 2 and 3). The lower cores, except in the gray silty clay beds. The silty clay silty clay bed is about 30 cm thick and has an abrupt, beds contain diverse and abundant, pre-Quaternary non-erosional basal contact and a gradational upper pollen and spores. The pollen assemblage in these beds is contact (Fig. 3). This bed contains no macroscopic similar to that reported from Tertiary sedimentary rocks structures, but it is micro-laminated, and particle-size in the Fraser Lowland (Hopkins, 1968, 1969; Mustard analyses reveal that it is normally graded (Fig. 4). A and Rouse, 1994). The Eocene to lower Oligocene second, thinner (o10 cm) and less conspicuous bed of Huntingdon Formation (Mustard and Rouse, 1994)is gray silty clay occurs several tens of centimetres above the most likely source for most of the reworked pollen. the ﬁrst and is separated from it by indistinctly Examples of these pollen are shown in Fig. 5, including laminated, olive-gray mud (Figs. 2 and 3). The upper exotic ferns (Figs. 5c and d), Pterocarya (wingnut, ARTICLE IN PRESS A. Blais-Stevens et al. / Quaternary Science Reviews 22 (2003) 2327–2334 2329

cm 0 indistinctly laminated mud

gray silty clay

indistinctly laminated mud

90 gray silty clay

120

indistinctly laminated mud 128

Fig. 3. Photograph of two beds of gray silty clay interstratiﬁed with Fig. 2. (a) Schematic stratigraphic summary of ODP core 1033. indistinctly laminated, olive gray, diatom-rich mud (core 1033B). Note Numbers to the right of column are radiocarbon ages (14C yr BP). Bar the gradational upper contact and abrupt basal contact of the lower at the left side of the column indicates approximate stratigraphic silty clay bed. intervals shown in Fig. 2b. (b) Detailed stratigraphy of the parts of cores 1033 and 1034 containing the gray silty clay beds. Ages are in radiocarbon years before AD 2000. mainland source where such rocks are exposed at or near the surface. Some Upper Cretaceous pollen grains were recovered from the gray silty clay beds and are Figs. 5e and f), Liquidambar (sweetgum, Fig. 5g), Ulmus/ possibly derived from rocks of this age in the Cowichan Zelkova (elm family, Fig. 5h), Carya (hickory, Fig. 5i), River valley. However, they just as well could have come and Tilia (basswood or linden, Fig. 5j). Not illustrated from Upper Cretaceous rocks at the northern edge of but also prominent are Fagus (beech) and Juglans the Fraser Lowland in Vancouver (Rouse et al., 1975). (walnut). The presence of the extinct form genus Fig. 5l shows an Upper Cretaceous pollen grain of the Paraalnipollenites (Fig. 5k), a Paleocene taxon, suggests form genus Nudopollis, a type recorded by Rouse et al. a source in the lower part of the Huntingdon Forma- (1975) at Vancouver, but not in Cretaceous rocks on tion. Some of the pollen, however, may have come from Vancouver Island. the Miocene Boundary Bay Formation (Hopkins, 1968), including pollen of Cedrus (true cedar) (Fig. 5b), a 3.3. Age middle and Late Tertiary taxon that is no longer native to North America. The ODP cores contain a record of the last 15,000 Paleocene and Eocene rocks do not occur on southern years of sedimentation in Saanich Inlet (Blais-Stevens Vancouver Island, which suggests that the pollen have a et al., 1997, 2001). Among the 71 AMS radiocarbon ages ARTICLE IN PRESS 2330 A. Blais-Stevens et al. / Quaternary Science Reviews 22 (2003) 2327–2334 on the cores are 16 on shell and wood collected indistinctly laminated mud between the two clay beds just above and below the gray silty clay beds (Fig. 2, at site 1033. Two fragments of wood just below the Table 1). Reservoir-corrected ages on shells above the lower clay bed at site 1034 yielded ages of 10,110750 higher clay bed range from 10,190770 to 10,800770 and 10,410760 14C yr BP. The age of the two silty 14C yr BP. A radiocarbon age of 10,480750 14CyrBP clay beds thus is about 10,500 14C years (10,900– was obtained on shell fragments recovered from 11,110 cal yr). The silty clay correlates with a similar deposit of the same age, recently identiﬁed in three sediment cores from the central Strait of Georgia, 70 about 100–120 km northwest of Vancouver (Conway et al., 2001).

4. Discussion

) 90 The abrupt, non-erosional, basal contact of the gray silty clay indicates that deposition began suddenly. The ﬁne particle size and presence of micro-laminae suggest

Depth (cm deposition from suspension, perhaps influenced by tidal ebb and flow. Two episodes of clay deposition were separated by a brief period during which indistinctly 110 laminated olive-gray mud accumulated on the fiord floor. Based on an average varve thickness of 4 mm in the upper section of the core at site 1033 (Blais-Stevens 120 et al., 1997), we estimate that approximately 110 years 0 10203040 separated deposition of the two silty clay beds. % silt The microfossil content and mineralogy of the clay Fig. 4. Silt abundance in the lower silty clay bed, core 1033. The bed is beds show that the sediment was carried into Saanich normally graded. Inlet from a source external to southeastern Vancouver

Fig. 5. Selected pollen and spores recovered from the lower silty clay bed in Saanich Inlet ODP core 1034B. Magniﬁcations are 700 Â unless stated otherwise. (a) Pleistocene Pinus (pine) pollen grain overlapping a spiny dinoﬂagellate cyst of the marine Spiniferites type (400 Â ); both are common in the core. (b)–(k) Redeposited spores and pollen typical of Tertiary rocks in the Fraser Lowland: (b) Cedrus (true cedar) pollen grain (500 Â ), a typical Miocene indicator; (c) Osmunda (fern) spore; (d)=Gleichenia (fern) spore; (e) Pterocarya (wingnut) pollen grain; (f) Pterocarya grain under phase contrast to highlight the six pores; (g) Liquidambar (sweet-gum) pollen grain; (h) Ulmus/Zelkova (elm family) pollen in phase contrast; (i) Carya (hickory) pollen; (j) Tilia (linden or basswood) pollen; (k) extinct form-genus pollen of Paraalnipollenites, a Paleocene guide fossil; and (l) Nudopollis, an Upper Cretaceous indicator pollen. ARTICLE IN PRESS A. Blais-Stevens et al. / Quaternary Science Reviews 22 (2003) 2327–2334 2331

Table 1 Accelerator mass spectrometer (AMS) radiocarbon ages

Core, section, Depth below Position relative 14C age Calibrated CAMS#d Sample Taxon interval (cm) seaﬂoor (m) to clay beds (m)a (yr BP)b 14C age range material (yr ago)c

1033B- 6H4, 110 55.5 0.51 aub10,190 770 10,354–10,929 30,701 Shell Axinopsida serricata 6H5, 135 57 0.12 blb 10,600760 11,002–11,502 30,702 Shell Macoma calcarea 6H5, 141 57.1 0.17 blb 10,700750 11,053–11,693 30,703 Shell M. calcarea 6H6, 13 57.5 0.4 blb 10,710760 11,050–11,737 30,704 Shell M. calcarea 6H6, 88 58.2 1.15 blb 11,050760 11,718–12,386 30,705 Shell M. calcarea 1033C- 6H1, 134 55.8 0.2 blb 10,220760 10,405–10,944 33,484 Shell fragment 1033D- 6H2, 52 52.8 2.27 aub8890 760 9714–10,037 33,775 Wood 6H4,14 56.2 0.22 bb 10,480750 10,914–11,123 33,487 Shell fragment 6H4, 101 57.2 0.05 blb 10,630760 11,017–11,554 33,488 Shell fragment 6H4, 137 57.5 0.41 blb 10,890760 11,433–12,152 33,489 Shell M. carlottensis 1034B- 8H2, 57 72.8 4.2 aub9790 750 9964–10,235 30,714 Shell M. calcarea 8H3, 53 74.5 2.74 aub10,280 760 10,479–10,977 30,693 Shell 8H5, 16 76.5 0.11 aub10,800 770 11,148–11,994 30,715 Shell M. calcarea 8H5, 134 78.0 0.07 blb 10,490750 10,920–11,133 30,694 Shell 8H6, 66 78.7 0.89 blb 10,710750 11,059–11,712 30,695 Shell 8H7, 10 80.1 1.83 blb 11,070760 11,752–12,411 30,696 Shell 1034D- 8H1, 44 73.5 4.31 aub9900 760 10,033–10,389 33,630 Shell fragment 8H3, 143 77.5 0.32 aub10,690 750 11,048–11,664 33,632 Shell Nuculana fossa 8H4, 96 88.4 0.39 blb 10,610750 11,015–11,495 33,633 Shell fragment 1034E- 8H1, 12 70.45 9.6 aub9450 760 9541–9920 33,496 Shell M. calcarea 9H1, 19 81.1 0.19 blb 10,110750 11,136–12,155 33,786 Wood Taxus brevifolia 9H1, 69 81.6 0.69 blb 10,410760 12,088–12,559 33,787 Wood 9H1, 133 82.2 1.33 blb 11,450760 12,435–12,798 33,497 Shell M. calcarea

a aub—above upper bed, blb—below lower bed, bb—between beds. b 13 Laboratory-reported error terms are 1s: Ages corrected to d C ¼ 25:0%PDB with a marine reservoir correction of 801723 years (Robinson and Thompson, 1981). Datum is AD 2000. c Determined from dendrocalibrated data of Stuiver and Brazuinas (1993) and Stuiver and Pearson (1993). The range represents the 95% conﬁdence interval (72s) calculated with error multiplier of 1. d CAMS—Center for Accelerator Mass Spectrometer Number, Lawrence Livermore National Laboratory, California.

Island, probably the Fraser Lowland. The sediment glaciers, however, may have persisted until 11,000 cal yr must have been transported in suspension because BP in some mountain valleys and possibly in the eastern underflows from the British Columbia mainland coast Fraser Lowland (Kovanen and Easterbrook, 2001, 2002; cannot cross the Strait of Georgia, at depths of 200 m or Friele and Clague, 2002a, 2002b; Kovanen, 2002). Large more, and overtop the sill at 70 m depth at the north end masses of dead ice were probably present in the British of the inlet. Because very little Fraser River sediment Columbia interior at this time (Fulton, 1969, 1971). For reaches Saanich Inlet today (Thomson, 1994), we argue example, the Fraser River valley north of Lillooet was that the beds are products of very large flows, probably blocked by ice until near the end of the Pleistocene floods that swept through the Fraser Lowland where (Ryder, 1976; Huntley and Broster, 1997). This ice they eroded Pleistocene sediments and Tertiary sedi- impounded a large lake in central British Columbia, mentary rocks. termed Glacial Lake Fraser (Clague, 1987; Huntley and When the gray clay beds were deposited, about 11,000 Broster, 1997). An ice dam near Spences Bridge in the years ago, Vancouver Island, much or all of the Fraser Thompson River valley likewise impounded Glacial Lowland, and the Strait of Georgia were ice-free Lake Deadman, which extended north and east to (Clague, 1981; Huntley et al., 2001). The Cordilleran Kamloops (Fig. 1; Fulton, 1969). Smaller glacier- ice sheet had retreated from these areas between a few dammed lakes existed in some valleys in the Coast hundred years to as much as 3000 years earlier. Active and Cascade Mountains bordering the Fraser Lowland ARTICLE IN PRESS 2332 A. Blais-Stevens et al. / Quaternary Science Reviews 22 (2003) 2327–2334

Fraser

River

400 m

Fig. 6. Stereo photo pair showing large ﬂutes (arrowed), which are eroded into late Pleistocene glaciomarine sediments near Fort Langley, British Columbia (Province of British Columbia photos A27396, pp. 160–161).

(e.g., Saunders et al., 1987). These lakes may have Armstrong and Hicock, 1980). Clague and Luternauer drained catastrophically, either individually or in (1982) suggested that a large flood might have produced tandem, when their ice dams weakened during deglacia- the flutes, although they did not specify a possible tion to the point that they could no longer retain the source for the floodwaters. impounded waters. Jokulhlaups. from Glacial Lake Jokulhlaup. deposits possibly associated with a cata- Fraser and Glacial Lake Deadman may have discharged strophic drainage of Glacial Lake Fraser occur in the many cubic kilometres of water down the Thompson Fraser River valley north of Lillooet (Fig. 1). The and Fraser River valleys, through the Fraser Lowland, deposits extend over a large area beneath a terrace about and into the Strait of Georgia. The huge overflow 250 m above the Fraser River. They comprise blocks of plumes from any such floods would have extended reddish-brown till up to 10 m across, floating in a widely over the Strait of Georgia and could have chaotic gravel matrix (Fig. 7). The reddish-brown till reached into Saanich Inlet, depositing silty clay on the blocks are derived from outcrops in the Pavilion area in fiord floor. the Fraser River valley about 20 km north of Lillooet. Definitive evidence of these floods has not yet been Any outburst floods must have had a large impact on found in the Fraser Lowland or Fraser River valley. the lower Fraser valley and Strait of Georgia. Large Much of the flood path across the Fraser Lowland was volumes of sand and gravel probably were deposited an arm of the sea at the end of the Last Glaciation along the seaway that extended eastward from the Strait (Clague et al., 1982). This seaway later became filled of Georgia across the western Fraser Lowland at the end with marine, deltaic, and fluvial sediments (Clague et al., of the Pleistocene. The flood deposits may form the core 1982, 1983), consequently the flood deposits would be of the postglacial fill in this area. Finer flood sediment, deeply buried. However, landforms that may have been perhaps of considerable thickness, also underlies Holo- created by the floods occur on Pleistocene surfaces cene sediments in the southern and central Strait of bordering the Fraser River. Flutes up to 9 m high, 200 m Georgia (Conway et al., 2001), and thinner, distal wide, and 2 km long, oriented in a west-northwest sediments like those in Saanich Inlet could possibly direction and eroded into Late Pleistocene glaciomarine extend farther south and west into Puget Sound and sediments, occur at Fort Langley (Figs. 1 and 6; Juan de Fuca Strait. These catastrophic floods were ARTICLE IN PRESS A. Blais-Stevens et al. / Quaternary Science Reviews 22 (2003) 2327–2334 2333

Fig. 7. Exposure of late Pleistocene jokulhlaup. sediments in the Fraser River valley north of Lillooet, British Columbia. Large clasts of till (arrowed) are supported by a matrix of coarse gravel. unique events in the 15,000-year postglacial history of across the Strait of Georgia into Saanich Inlet, where it southwestern British Columbia. rained out onto the fiord floor. We attribute the floods to the sudden draining of ice-dammed lakes in the lower Fraser River basin, possibly Glacial Lake Deadman 5. Conclusion and/or Glacial Lake Fraser.

Two beds of silty clay occur within the dominantly diatomaceous sediments in ODP cores collected from Saanich Inlet in 1996. The silty clay beds are unique in Acknowledgements the continuous, 15,000-year sediment sequence. They abruptly overlie, and are gradationally overlain by, We thank K. Conway for his technical support and indistinctly laminated, diatomaceous mud, and they discussion of results. Steve Hicock, Lionel Jackson Jr., contain Tertiary pollen, which are exotic to eastern Thierry Mulder, and Jim Teller reviewed drafts of the Vancouver Island, and abundant illite-muscovite, which paper. H. Pass, A. Ledwon, and K. Robertson assisted are common minerals in Fraser River sediments. with core analyses, and J. Percival carried out clay Radiocarbon ages on shells and wood from just above mineralogy analyses. T. Barry, R. Franklin, J. Jorna, and below the silty clay beds show that they are about and R. Lacroix drafted some of the figures. 11,000 cal yr old. We hypothesize that the silty clay beds were deposited during large, sediment-charged floods that swept across References the Fraser Lowland just before the end of the Pleistocene. The floods eroded Tertiary rocks and Armstrong, J.E., Hicock, S.R., 1980. Surficial geology. New Westmin- Pleistocene sediments in the Fraser Valley. Some of ster, British Columbia. Geological Survey of Canada, Map 1484A, the entrained sediment was carried in overflow plumes 1 sheet, scale 1:50,000. ARTICLE IN PRESS 2334 A. Blais-Stevens et al. / Quaternary Science Reviews 22 (2003) 2327–2334

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