Late Quaternary deglaciation and sea-level history of eastern Juan de Fuca Strait, Cascadia

ARTICLE IN PRESS

Quaternary International 121 (2004) 23–39

Late Quaternary deglaciation and sea-level history of eastern Juan de Fuca Strait, Cascadia David C. Moshera,*, Antony T. Hewittb,1 a

Geological Survey of Canada—Atlantic, Bedford Institute of Oceanography, 1 Challenger Dr, P. O. Box 1006, B2Y 4A2 Dartmouth, NS, Canada b Center for Coastal and Ocean Mapping/Joint Hydrographic Center, University of New Hampshire, 24 Colovos Rd., Durham, NH 03824, USA

Abstract Eastern Juan de Fuca Strait of southwestern British Columbia/northwestern Washington State is near the western and southern terminus of Late Wisconsinan glaciation in North America. Seismic-reflection profiles, bathymetric data, sediment core data and 41 new radiocarbon dates provide insight into the history of ice retreat and sea-level change in the eastern portion of the strait. The retreating ice margin appears to have passed through the area between 14,4607200 and 13,5957145 14C yr. Glacial marine sediments date between 12,889750 and 11,110750 14C yr BP. Wave-eroded unconformities, drowned prograded deposits and a partially isolated low-stand basin are evidence attributed to low-stand sea levels following deglaciation and five new control points on the local eustatic sea level curve are assigned based on radiocarbon dating of these features. Sea-level change was rapid: from +75 m immediately after deglaciation to possibly 60.4 m at 9920760 14C yr BP. After this time, gradual sea level transgression took place. Relative sea level reached its approximate present position by 54707115 14C yr BP. Since then, relative sea level has remained constant suggesting eustatic rise is balanced by crustal uplift. r 2004 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The eastern Juan de Fuca Strait in southwestern British Columbia and northwestern Washington State (Figs. 1 and 2) lies within the tectonically active Cascadia subduction zone near the southern and western terminus of late Wisconsinan glaciation. As a result, late-glacial and Holocene sea-level change in this region involves eustatic rise and deglacial isostatic rebound, forebulge migration and possible tectonic readjustments due to transpressional convergence of the Juan de Fuca and the North American Plates (Clague and James, 2002). The elevation and timing of high and low sea-level stands allows quantification of the total amount and rates of vertical motion, both important parameters in models of crustal dynamics. It is the intent of this paper to define the magnitude and timing of late Pleistocene and Holocene relative sea-level change for the eastern Juan de Fuca Strait and demonstrate the maximum extent of sea level lowering. To accomplish this *Corresponding author. 1 Present address: Fugro GeoServices, Inc., 6100 Hillcroft, Houston, TX 77081, USA.

objective, evidence obtained from seismic-reflection profiles, bathymetric data, sediment cores, and 41 AMS radiocarbon dates are integrated with published work to refine and provide new insight into the deglacial and sea-level history for the region.

2. Background 2.1. Geology Mosher and Johnson (2001) summarized the geology of eastern Juan de Fuca Strait in relative detail and the seafloor geology was recently interpreted by Hewitt and Mosher (2001). Quaternary deposits of the region comprise a stratigraphically complex basin fill of glacial and interglacial deposits that are locally as thick as 1100 m. On offshore seismic-reflection profiles, Pleistocene strata form distinctive seismic units, bounded below by pre-Tertiary or Tertiary basement and above by typically flat-lying latest Pleistocene post-glacial and Holocene deposits. There is little modern sediment input into the eastern Juan de Fuca Strait, so most Holocene sediments are composed of material reworked from existing shoreline

1040-6182/$ - see front matter r 2004 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2004.01.021

ARTICLE IN PRESS D.C. Mosher, A.T. Hewitt / Quaternary International 121 (2004) 23–39

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and marine bank deposits (Hewitt and Mosher, 2001). Thickest sediment accumulations are therefore found adjacent to cliff and bank exposures of unlithified

sediment. Radiocarbon dates from Holocene sediments fall between 10,130750 14C yr BP and 190750 14C yr BP, although most dates are older than 8000 14C yr BP (Hewitt and Mosher, 2001).

2.2. Glacial history During the Pleistocene, lobes of continental ice occupied the eastern Juan de Fuca Strait several times. Late Wisconsinan glaciation, known locally as the Fraser Glaciation, began after 28,8007740 14C yr BP (GSC-95) in British Columbia (Clague, 1981; Clague and James, 2002), reaching the vicinity of Victoria by 22,600 14C yr BP (Clague et al., 1980) and advanced westward and southward as the Juan de Fuca and Puget Lobes, respectively (Armstrong et al., 1965; Mullineaux et al., 1965; Armstrong and Clague, 1977; Hicock et al., 1982; Waitt and Thorson, 1983; Easterbrook, 1992; Porter and Swanson, 1998). Ice reached the northern Puget Lowland (48 N) around 15,000 14C yr BP (Porter and Swanson, 1998) and advanced to its terminal position in the southern Puget Lowland at about 14,150 14C yr BP (extrapolated date; Porter and Swanson, 1998). The Juan de Fuca Lobe reached the shelf edge through Juan de Fuca Strait shortly before 14,4607200 14C yr BP (Y-2452; Heusser, 1973; Herzer and Bornhold, 1982).

Study Area Pacific Ocean

Puget Sound Washington

Fig. 1. Location of the study area.

Vancouver Island

-50 -25

Victoria

Esquimalt Harbour

Haro Strait

-150

Trial Island

Co

Race Rocks

-100

-100

-50

nk

nk

Ba

ta ns

a -50e B nc

-25 Salmon Bank -50

-100

Unnamed Bank-50

-150

-100

-150

-150

Dungeness Spit

-100

Port Angeles

-50

Ediz Hook

-25

-50 -25

Green Point

-25

Partridge Bank

-25

-50 A

dm

-25

ira

lty

In

let

5000 m

Puget Sound

500 500 1000

Olympic Penninsula

Washington

500

1500

123˚45’

123˚30’

123˚15’

48˚15’

-50

-50

-50

-150

Smith Island

-25

-100

-100

-25

Larson Bank

-100

Eastern Bank-25 -50

-150

-50 -25

-25 -50

-25

-25 -50 -150

-150 -100

-100

Rosario Strait

in

-100

-100 -150

e dl id M ank B

He

-25 -50

Lopez Island

-150 -100

Discovery Island

Portage Inlet

48˚30’

San Juan Island

-200

Wh Isla idbey nd

Juan de Fuca Strait

Dallas Bank

Va nc o Is uve la r nd

McArthur Bank

British Columbia

123˚00’

48˚00’ 122˚45’

Fig. 2. Map of the study area showing place names referred to in the text. Selected topographic and bathymetric contours are in meters. The study area is approximately 80 km west-east, and 55 km north-south.

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Estimates of glacial maximum ice thicknesses for the Puget lobe are 1900 m near the international boundary to 1100–1200 m at the northeastern corner of the Olympic Mountains, to about 200–300 m at its terminus (Waitt and Thorson, 1983). For the Juan de Fuca lobe, thicknesses range between 1500 m near Victoria (Wilson et al., 1958; Alley and Chatwin, 1979), 1200–1100 m at its effluence from the Puget lobe to near sea level at its terminus on the continental shelf (Heusser, 1973; Alley and Chatwin, 1979). These differential ice thicknesses greatly influenced the amount of local isostatic depression due to ice loading. The Puget Lobe probably remained at its maximum limit for only a short time (100’s of years) before retreating to a position near Seattle by 13,6007280 to 13,7007150 14C yr BP (QL-4065 and QL-4067; Porter and Swanson, 1998). Deglaciation of the Juan de Fuca Lobe is not well known, but it seems to have begun around 14,4607200 14C yr BP (Y-2452; Heusser, 1973) and retreated rapidly, reaching Whidbey Island by 13,5957145 14C yr BP (BETA-1716; Dethier et al., 1995). Waitt and Thorson (1983) promote the concept that retreat of the two lobes was out of phase because glacial marine influences did not affect the two lobes equally; the Puget lobe being more land-locked.

This emergence has been evidenced by sub-aerially weathered marine sediments in Esquimalt Harbour at 9 m, unconformities and wave-cut terraces formed below the modern level of wave erosion (Linden and Schurer, 1988; Mosher and Johnson, 2001) and a peat layer sandwiched between marine sediments at Portage Inlet, near Victoria. This peat dates between 92507140 14 C yr BP and 54707115 14C yr BP (Foster, 1972; Clague et al., 1982). To date, there is no published evidence of maximum emergence. The greatest sea level lowering was inferred by Linden and Schurer (1988). They traced an erosional unconformity off Esquimalt Harbour near Victoria to a depth of 55 m that dated between 88707140 14C yr BP and 98507230 14C yr BP. They interpreted that this unconformity resulted from shallow water erosion, but there has been no supporting evidence prior to this investigation. As a consequence, the claim for a lowstand of this magnitude has not been accepted, generally.

2.3. Sea-level history

All radiocarbon dates used in this investigation are reported in 14C years before present unless otherwise noted (Table 1). Data were normalized to 25% d13C, (Stuiver and Polach, 1997) and ages on all marine shells were corrected for carbon reservoir effects ( 801723 yr; Robinson and Thompson, 1981) to account for differences between atmospheric and ocean carbon reservoirs.

In southwestern British Columbia and the Pacific Northwest, the relationship between glacio-isostatic depression, post-glacial rebound, and eustatic sea-level change is complex. The crust was isostatically depressed by glacial loading and rebounded rapidly following deglaciation. The amount of depression and rate of rebound appears to vary locally (Waitt and Thorson, 1983). Evidence for high sea level stands (crustal depression) is found within onshore exposures of marine sediments. Along the northwestern Washington coast marine sediments extend only as high as 10 m (Heusser, 1973). On the Olympic Peninsula, the maximum observed high-stand is around +50 m (Dethier et al., 1995). Near Victoria, on the Colwood delta, a marine limit of +75 m at 12,469760 14C yr BP is indicated (Mathews et al., 1970; Huntley et al., 2001; date from Blais-Stevens et al., 2001 (CAMS-33492)). In the Puget lowland at about 48 N latitude, marine sediments extend as high as 80 m. In the Fraser Lowland, they are higher than 200 m (Mathews et al., 1970; Armstrong, 1981). Isostatic rebound following removal of the glaciers caused relative sea-level regression, eventually reaching a sea level equivalent to today at 11,7007170 14C yr BP (I-3675; Mathews et al., 1970; Clague, 1980, 1981; Clague et al., 1982). Eustatic sea level was low at this time (although rising), so continuing rapid rebound resulted in emergence of areas that are now underwater.

3. Methods 3.1. Radiocarbon dating

3.2. Seismic-reflection High-resolution seismic-reflection profiles were collected using Huntec DTS and IKBt Seistec boomer systems (Hutchins et al., 1976; Simpkin and Davis, 1993; Mosher and Simpkin, 1999), and an airgun singlechannel system described in Mosher et al. (1998). A total of 2169 line-km of Huntec profiles were collected in 1996 and 1997 (Mosher and Johnson, 2001), and 225 line-km of Seistec data were collected along the Victoria waterfront (Mosher and Bornhold, 1995). During the 1997 survey, 800 line-km of single-channel airgun seismic-reflection and Huntec boomer data were collected simultaneously. Survey track-lines are shown in Fig. 3. All seismic data were digitally acquired, processed, imaged and interpreted. 3.3. Bathymetry/geomorphology The Canadian Hydrographic Service (CHS) and the National Oceanic and Atmospheric Administration (NOAA) of the United States provided hydrographic

26

Table 1 Radiocarbon dating results from this investigation Core number

Water depth (m)

Core length (m)

Sample number (CAMS)

Depth in core (cm)

14

Raw C agea (yr BP)

+/ Error (yr BP)

Correctedb C age (yr BP)

Calendar age range in yr BPc maximum (intercepts) minimum

Material dated

123.4879 123.4879 122.8341 122.8341 122.8367 122.8367 122.8367 122.8389 122.8389 122.8435 123.4263 123.4263 123.4263 123.4263 122.7888 122.9904 122.9904 122.9904 123.1682 123.1757 123.0517 123.0517 123.0396 123.4136 123.4136 -122.8079 122.8079 122.8079 122.8079 122.8079 122.9007 122.9007 122.9007 122.9539 123.43 123.422 123.425 123.425 123.427 123.427

48.36683 48.36683 48.2835 48.2835 48.28313 48.28313 48.28313 48.28305 48.28305 48.28283 48.41535 48.41535 48.41535 48.41535 48.38283 48.35807 48.35807 48.35807 48.23322 48.28287 48.29973 48.29973 48.29993 48.3995 48.3995 48.34905 48.34905 48.34905 48.34905 48.34905 48.27907 48.27907 48.27907 48.33597 48.4196 48.4088 48.4124 48.4124 48.4154 48.4154

71.3 71.3 118.9 118.9 120.7 120.7 120.7 122.5 122.5 124.4 41.7 41.7 41.7 41.7 85.7 153.6 153.6 153.6 155.7 121.6 156.5 156.5 173.7 57.97 57.97 103.6 103.6 103.6 103.6 103.6 114.8 114.8 114.8 132.5 30.71 48.72 46 46 41.2 41.2

3.76 3.76 3.63 3.63 5.83 5.83 5.83 5.65 5.65 3.47 2.36 2.36 2.36 2.36 4.48 5.13 5.13 5.13 1.65 3.47 4.57 4.57 0.1 3.73 3.73 5.11 5.11 5.11 5.11 5.11 3.37 3.37 3.37 3.83 2.17 1.69 2.05 2.05 2.93 2.93

58672 58673 58674 58675 58676 58677 58678 58679 58680 58681 58682 58683 58684 58685 58686 58688 58687 58689 58690 58691 58693 58692 58694 58695 58696 58697 58698 58699 58700 58701 58702 58703 58704 58705 62767 62768 62531 62532 62533 62534

20 360 63 319 63 210 373 85 479 64 31 45 96 224 13 114 123 205 31 33 93 133 182 246 325 43 179 192 342 504 61 206 320 221 213 164 44 77 55 132

1720 11900 1190 8310 8580 10630 10780 10690 10930 13390 190 1090 10640 9880 3660 10630 10950 13230 13250 4320 12670 13250 13470 10720 13690 2510 11910 13030 13150 13500 9340 9530 9840 10580 8910 3600 5020 6460 8490 13370

40 50 40 50 40 50 50 50 50 60 40 40 50 50 40 40 50 50 50 50 50 50 60 60 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50

919 11099 389 7509 7779 9829 9979 9889 10129 12589 190 289 9839 9079 2859 9829 10149 12429 12449 3519 11869 12449 12669 9919 12889 1709 11109 12229 12349 12699 8539 8729 9039 9779 8109 2799 4219 5659 7689 12569

953 (883) 749 13158 (13006) 12861 502 (464) 328 8467 (8364) 8257 8883 (8647) 8529 11591 (11131, 10958, 11698 (11305) 10867 11625 (11151, 10939, 12074 (11639) 10887 15523 (15067, 14747, 190 459 (321) 268 11595 (11134, 10955, 10545 (10252) 9833 3223 (3090) 2940 11588 (11131, 10958, 12080 (11661) 10889 15395 (14298) 14113 15411 (14307) 14120 4049 (3886) 3743 14047 (13807) 13437 15411 (14307) 14120 15603 (15195, 14646, 11663 (11164, 10930, 15849 (15431) 14381 1823 (1687) 1546 13160 (13014) 12865 15222 (14117) 13708 15330 (14240) 14085 15630 (15233, 14609, 9818 (9506) 9093 9962 (9808) 9495 10539 (10235) 9823 11576 (11122, 10969, 9312 (8966) 8854 3165 (2991) 2844 4952 (4835) 4727 6614 (6469) 6351 8722 (8548) 8404 15503 (15019, 14789,

Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell Wood Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell Fish bone Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell Shell

14

All analysis were conducted at the Center for Accelerated Mass Spectrometry at the Lawrence Livermore National Laboratory. a Raw 14C ages were determined using the Libby half life of 5568 yr and following the conventions of Stuiver and Polach (1997). b Corrected 14C ages were determined using the reservoir correction of 801723 (Robinson and Thompson, 1981). c Calendar age was determined using CALIB 4.3 (Stuiver et al., 1998) and employs the reservoir correction of 801723 yr.

10876) 10843 10886) 10855 14377) 14172

10878) 10845

10876) 10845

14429) 14203 10890) 10858

14455) 14213

10869) 10828

14365) 14166

ARTICLE IN PRESS

Latitude ( )

D.C. Mosher, A.T. Hewitt / Quaternary International 121 (2004) 23–39

TUL96-03 TUL96-03 TUL96-04 TUL96-04 TUL96-05 TUL96-05 TUL96-05 TUL96-06 TUL96-06 TUL96-07 TUL97-02 TUL97-02 TUL97-02 TUL97-02 TUL97-04 TUL97-05 TUL97-05 TUL97-05 TUL97-07 TUL97-08 TUL97-09 TUL97-09 TUL97-11 TUL97-12 TUL97-12 TUL97-16 TUL97-16 TUL97-16 TUL97-16 TUL97-16 TUL97-18 TUL97-18 TUL97-18 TUL97-21 TUL99-15 TUL99-16 TUL99-17 TUL99-17 TUL99-18 TUL99-18

Longitude ( )

ARTICLE IN PRESS D.C. Mosher, A.T. Hewitt / Quaternary International 121 (2004) 23–39

Tul99 Cores Tul97 Cores Tul96 Cores

15 18 17 16

27

Fig. 6

Fig. 13 Fig. 11

Fig. 12

Tul97007 - Huntec & Single Channel Seismic Tul96006 - Huntec and Multi-Channel Seismic Rev94002 - Seistec Seismic

Fig. 3. Core locations and seismic survey tracklines in the study area. Thick lines indicate the locations of selected figures.

data. There are over 600,000 single-beam soundings within the study region. In addition to the single beam sounding data, CHS, Geological Survey of Canada (GSC) and the Canadian Department of National Defense Acoustic Data Analysis Centre (DND-ADAC) conducted multibeam sonar surveys using Simrad EM3000 and EM1002 systems. The EM3000 is a highfrequency system (center frequency of 300 kHz) with 127 beams per ping in an angular sector of up to 130 . The EM1002 operates at frequencies of 98 kHz for the inner swath (nadir 750 ) and 93 kHz for the outer swath (>50 ), with 111 beams per ping in an angular sector of up to 150 . These systems can resolve seabed features with horizontal scales of about 3–8% of the water depth (Hughes Clarke, 1998).

4. Results 4.1. Seafloor geomorphology The present-day seafloor relief in eastern Juan de Fuca Strait is a consequence of the various Pleistocene glacial epochs and sea level changes. It consists of many shallow banks (some subaerial) and intervening deep troughs to 250 m below sea level (mbsl) (Fig. 2).

The majority of the banks have a roughly drumlin form, elongated in a northeast-southwest direction, changing to north-south in the easternmost part of the strait. In general, they have steep sides and relatively flat tops at o40 mbsl. Their steepest slopes tend to face east. In the northeast corner of the study area are two long narrow (B1 km) ridge-like banks (Fig. 2) that appear like moraines. McArthur Bank is B1 km wide and stretches for 12 km from Lopez Island to Smith Island Bank. A depression (B10 m deep) runs parallel to the bank along its western side. Lawson Bank is located in Rosario Strait, between Lopez Island and Whidbey Island. It is 11 km long and has a V-shaped plan-view. A similar narrow ridge segment spans the gap between Smith Island Bank and Partridge Bank, creating a small basin between the banks and Whidbey Island. Multibeam data along the Victoria waterfront reveal a series of terraces at depths of around 15, 35, 50, and 65 m (Fig. 4). The continuity of these terraces can be traced westward, but give way to a more gradual and continuous seafloor slope south of the entrance to Esquimalt Harbour. Similar terraces are apparent on the flanks of many of the shallow banks within the strait (Figs. 5 and 6), at depths of 30 to 60 m. In addition, many of the bank tops exhibit planar surfaces at depths of 30 to 65 m.

ARTICLE IN PRESS D.C. Mosher, A.T. Hewitt / Quaternary International 121 (2004) 23–39

Fig. 9

28

1000 m

Victoria

Delta . Fig 7

-5

0m

Fig. 8

Terraces

-15 m

-35 m -65 m

1000 m

Terraces

-35

m

-6 5

Dun

m

gen

t Spi ess

-15

-50 m

m Dallas Bank

Olympic Peninsula

Fig. 4. Multibeam bathymetry reveals four prominent seafloor terraces (highlighted by contours) south of Victoria (upper). Terraces are observed at similar depths in single-beam data from the vicinity of Dungeness Spit (lower).

There are several barrier-spits in the strait; the two largest, Ediz Hook and Dungeness Spit, occur along the Olympic Peninsula (Fig. 5). The seafloor appears terraced off Dungeness Spit, at roughly the same depths as the terraces along the Victoria waterfront (Fig. 4). A submerged feature at 15 m with a morphology resembling Dungeness Spit is also noteworthy. Similar features are present off Ediz Hook (Fig. 5), where three submerged protuberances occur on the eastern edge of a delta-shaped platform that extends 4.5 km offshore from Ediz Hook. These protuberances occur at depths of 6, 8 and 25 m. The seafloor morphology of Admiralty Inlet reveals NE-SW trending ridges and troughs, several kilometres long, giving it a braided-channel-like appearance (Fig. 2). This terrain style ends in the basin between Dallas Bank and Partridge Bank where two delta-like lobes of sediment appear to splay out onto the

ocean floor on either side of a small bathymetric high point. 4.2. Seismic stratigraphy The regional marine seismic and litho-stratigraphy have been developed by Mosher and Law (1996), Mosher and Hamilton (1998), Mosher and Moran (2001) and Hewitt and Mosher (2001). The seismic stratigraphy consists of four main units (units 1–4). Each unit is composed of a seismic facies or sequence of facies and is separated from other units by an unconformity or non-conformity. These seismic units correlate to the four main lithologic units observed in the region: bedrock (Unit 1), glacial diamict (Unit 2), glacial-marine sediment (Unit 3), and modern (Holocene) sediment (Unit 4). Hewitt and Mosher (2001) detail the distribution of seismic units within the eastern

ARTICLE IN PRESS D.C. Mosher, A.T. Hewitt / Quaternary International 121 (2004) 23–39

A

C

Dungeness Spit

- 25 m -8m -6m

B -15 m

Ediz Hook

Two way travel time (sec.)

5000 m

Two way travel time (sec.)

29

Contour interval =5m

Green Point

D

0.0 (A) 100 m

25 m

Clinoform unit

-48 m

0.1

Unconformity

-65 m

Glacial-marine

-75 m

Multiple

0.2 0.0

(B)

100 m 25 m Clinoform unit

Post-glacial

-60 m

0.1

-83 m

Glacial-marine

Mult 0.2

0

Unconformity

Ice-contact/proximal

iple

(C)

(D)

Depth (m)

20 40 60 80 100 120

1000 m

140 Fig. 5. The upper plate shows bathymetry along the Olympic Peninsula, and the depth of submerged ‘‘spit-like’’ protuberances offshore of present day spits, Ediz Hook and Dungeness Spit. The 60 m contour is highlighted. Lines A and B correspond to the airgun profiles in the middle two plates, and lines C and D correspond to the bathymetric profiles shown in the lowest plate.

Juan de Fuca Strait and interpret the near-seabed geology based on these results. In many locations, the depth of occurrence of particular units, the morphology of the deposits, the stratigraphic relationships and the characteristics of particular seismic facies provide evidence as to the glacial, post-glacial and sea-level history of the region. 4.2.1. Unconformities Throughout much of eastern Juan de Fuca Strait and Haro Strait and the Strait of Georgia, the Holocene unit (Unit 4) is divided into a lower post-glacial unit and an upper post-glacial unit (Hart et al., 1995) separated by a high amplitude reflection event that is sometimes an

apparent erosional unconformity (Figs. 7–10). This unconformity sometimes reaches to the top of the glacial marine section and extends down to depths of 70 mbsl (Hewitt and Mosher, 2001), below which the amplitude lessens and the relationship appears everywhere conformable (Fig. 7). Offshore of Esquimalt Harbour, this unconformity surface forms ridges or terraces as it deepens (Fig. 7). Core samples from immediately above the unconformity provide age control at various elevations (Figs. 8–10, Table 1) and show the unconformity to be early Holocene in age and possibly time-transgressive. Airgun reflection profiles across the shallow banks in the study area show a complex stratigraphy, with

ARTICLE IN PRESS D.C. Mosher, A.T. Hewitt / Quaternary International 121 (2004) 23–39

30

reflectors dipping in a variety of orientations and unconformable relationships between reflections. Most of the tops of the shallow banks are presently at about

25–40 m waters depth and is planar in form. Subbottom reflections truncate against the seafloor, suggesting an erosional surface (e.g., Figs. 6 and 11).

SW

NE

W

E Clinoform unit (-30) B

Terrace (-50 m)

Terrace (-50 m)

0.1

500 m

0.2

le ltip

Mu

0.3

Two-way Travel time (ms)

Two way travel time (s)

A

Clinoform wedge unit 50

0

metres

25 m

200

Multiple

Terrace 75

70

Upper post-glacial sediments Lower post-glacial sediments

100 m

50

Unconformity

80

60

90 70

Glacial-marine sediments

Terraces & Ridges

100

Water Depth (m)

Two way travel time (ms)

Fig. 6. Airgun profile through Middle Bank, oriented southwest-northeast (A) and west-east (B), showing a small clinoform unit at 30 m and terraces formed along its flank at 50 m. Reflections within sediments beneath the terraces appear truncated, as do the sediments in the shallow subsurface on the top of the Bank at 32 m. The inset is a Huntec boomer high-resolution seismic profile, showing details of the clinoform wedge unit and the eastern terrace of this same line. Depths in meters were provided by a precision depth recorder.

80 110

Two way travel time (ms)

Fig. 7. High-resolution boomer (Seistec) profile off Victoria, showing a series of erosional terraces within the post-glacial unconformity. The deepest terrace occurs at an elevation of 65 m. The high amplitude reflection off the unconformity becomes lower in amplitude and conformable below 70 m water depth.

- 60.4

60 70 80

100

100 m 5m

Upper post-glacial Lower post-glacial Glacial-Marine Victoria

90

TUL97-12 - 58.0 m

Water surface reflection

TUL96-12 - 65.1 - 64.8 m

Glacial-Marine Bedrock Water surface reflection

Fig. 8. High-resolution boomer (Huntec) profile off Victoria. The high amplitude reflection event within post-glacial sediments is an unconformity. A shell at 2.46 m (60.4 m below sea surface), just above the unconformity in core TUL97-12 yielded a date of 9919+/ 60 14C yr. BP (corrected). The same post-glacial stratigraphic sequence is seen in core TUL96-12.

ARTICLE IN PRESS D.C. Mosher, A.T. Hewitt / Quaternary International 121 (2004) 23–39 100 m

50

99-18 97-02

99-15

97-01

99-17

10 m 60

End of sruvey line

Two way travel time (ms)

31

70

Line A

N Esquimalt Harbour

Line A

20 Water depth (m)

100 m

- 27 m - 33 m

30

- 36 m - 39 m

40

- 44 m

99-17

97- 02 99-18

9079 +/- 50

97- 01

Profile through Line A

99-15

8109 +/- 50 -32.8 m

Post-glacial Unconformity

-44.0 m Glacial-marine

50

Fig. 9. High-resolution boomer (Seistec) profile (upper left), bathymetric perspective view (upper right), and profile (bottom; Line A) through ridge features near Esquimalt Harbour. The genesis of these ridges is unclear as to whether they are primary depositional features or post-depositional features as might be generated in a rotational slide failure. Black bars indicate locations of cores and their approximate depth of penetration. Several cores penetrate the unconformity between the post-glacial and glacial-marine sediments. Ages shown are corrected 14C ages. The unconformity is shown with arrows in the seismic profile.

0

Gravelly sand

Depth (cm)

50 100

Fine sand / shell fragments

Fine sand / shell fragments

Tul99-18 Lithology

150 200

Tul97-12 Lithology

Tul97-02 Lithology

Tul99-15 Lithology

Muddy medium sand

8109 +/- 50

Grading to Coarse sand

7689 +/- 50

Coarse Sandy mud / pebbles / shell fragments Silty-clay

Grading to Coarse sand

190 +/- 50 9,839 +/- 50

9079 +/- 50

-44.0 m

-32.8 m

Fine sand / shell fragments

Tul99-17 Lithology Fine sand / shell fragments Coarse sand

Silty-clay

12,569 +/- 50

4219 +/- 50 5659+/- 50

Coarse Sandy mud / pebbles / shell fragments

9,919 +/- 60

Unconformity Silty clay with thin silty layers

12,889 +/- 50

Silty-clay

-46.7 m

-60.4 m

-42.0 m Fig. 10. Five cores penetrate the glacial-marine unconformity off Victoria; each contains a layer of glacial coarse sandy-mud, pebbles, and shell fragments over silty clay. This contact is considered to be the unconformity observed in seismic reflection profiles (e.g. Figs. 7 and 8). Ages are given in radiocarbon years corrected for a reservoir effect. The depth illustrated at the bottom of each core is the depth of the unconformity below modern sea level. Radiocarbon dates show the time-transgressive nature of this unconformity.

4.2.2. Clinoform units At the edges of many of the shallow banks and some nearshore areas, airgun seismic reflection data reveal wedge-shaped deposits with underlying unconformities and clinoform-shaped internal reflections dipping at B8 –9 (e.g. Fig. 11). These features share several common characteristics: (1) clinoforms dip toward deeper water; (2), they appear to be post-glacial, deposited over units interpreted to be till and glacial marine sediment, and (3), a prominent reflection dipping toward deeper water at B4 defines the base of clinoform units. This reflector appears to sometimes truncate underlying reflections. In most examples, the

clinoform sequences begin at the bank top and terminate in 60–90 m water depth (e.g. Figs. 6 and 11). The two lobes of sediment between Dallas Bank and Partridge Bank, north of Admiralty Inlet, show internal clinoforms down-lapping at angles of 5–9 on both sides of the deposit (e.g. Fig. 12). 4.2.3. Isolated basin Bathymetric basins at appropriate water depths are prime targets for sea level studies because they may show the marine/brackish/freshwater transitions during sea level changes (e.g. Josenhans et al., 1997). Micropaleontologic evidence from drill cores in Saanich Inlet,

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32

500 m 25m

Clinoform unit (-41 m) Terrace (-55 m) Clinoform unit (-60 m)

Two-way Travel time (s)

0.1 -90 m Erosional Moat Unconformity

Truncated reflections

0.2 Multiple

0.3 W

E

Fig. 11. Airgun seismic reflection profile across Eastern Bank, demonstrating seismic facies of clinoform-shaped reflections on the flanks of the bank. The inset shows the sequence in more detail. These clinoform facies occur on the flanks of many of the banks in the Juan de Fuca Strait and at a variety of water depths down to about 60 m. Depths in meters were provided by a precision depth recorder.

Two-way travel time (s)

0.10 SW

NE -70 m

1000 m

-80 m

0.15

Post-glacial 0.20

Glacial-marine Multiple Multiple

0.25

Fig. 12. Airgun profile through two sediment lobes near the entrance to Admiralty Inlet showing a fan-delta shaped deposit topping in 70-m water depth. Depths in meters were provided by a precision depth recorder.

just north of the study area, showed that Holocene sea level could not have fallen below 75 m (McQuoid and Hobson, 2001; Mosher and Moran, 2001). The search for isolated basins within the strait was restricted, therefore, to basins with a perimeter depth of 75 m or shallower. The only candidate is a basin that lies between Partridge Bank, Smith Island and Whidbey Island (Fig. 13). The deepest location on the perimeter of this basin is about 75 m on the north side, and the sill at its southern side is at 55 m. A seismic reflection profile through this basin showed an opportunity to core a sequence to produce a composite stratigraphic section (Fig. 13). The lithostratigraphy of this small basin contained a unique lithologic unit. Glacial-marine sediments were overlain by a unit of light-green silty-clay and gray siltyclay (Fig. 13). Its color and fine-grained texture make it unique amongst other Holocene sediments identified in the strait (see Hewitt and Mosher, 2001). The green silt-

clay appears structureless and contains abundant mollusc fragments. It also lacks ice-rafted debris that glacial marine sediments contain. This unit was overlain by olive-green sandy mud that is recognized as part of the typical post-glacial sequence. Eight radiocarbon dates acquired from cores within this basin show this silty-clay unit to be between 9829750 and 10,3797100 14 C yr BP (corrected) in age (Tables 1 and 2; Fig. 13). The lower age bracket is an extrapolated age based on an average sedimentation rate of 14 mm/yr calculated for the unit.

5. Discussion 5.1. Deglacial history Given the narrow range of ages for glacial-marine sediments in Juan de Fuca Strait, ice retreat was

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0 0.50 1.00

Metres downcore

1.50

Tul96-07

Tul96-06

Sandy-mud Silty-clay scattered Pebbles Shells

12589 +/- 50

2.50 3.00

Sandy mud and shells

Gray Silty-clay Silt

9829 +/- 50 Increase in sand

Green Silty-clay

3.50

389 +/- 40

.

9889 +/- 50

2.00 Silty-clay/ thin silty layers

Tul96-04

Sandy-mud Coarse sand sandy mud 7779 +/- 40

Gray Silty-clay Silt

Tul96-05

33

7509+/- 50 Green 9979 +/- 50 Silty-clay

4.00 4.50 10129 +/- 50

5.00 5.50 6.00

10379 (calculated age, based on sedimentation rates in cores 96-05 and 96-06)

160

TUL96-07

TUL96-04 TUL96-05 TUL96-06

120

Post-glacial

“green silty-clay” Glacial-marine

210 Ice-contact/proximal W

W Isl hidb an ey d

Smith Island -6 00 m

Two-way travel time (ms)

100 m 10 m

Water depth (m)

80

110

E

-2 0 Pa r tridge Bank

2000 m

Fig. 13. Cores (upper plate) and high resolution boomer (Huntec) reflection profile (middle plate) from a basin near Whidbey Island (lower plate) that may have been an embayment if sealevel fell below 55 m. The sedimentary sequence is contrary to the typical stratigraphy of the Strait. Glacialmarine sediments are not directly overlain by the usual post-glacial sediments, but instead, a light-green silty-clay, followed by a thin silt bed and a layer of gray silty-clay, and then finally, the recognizable post-glacial sequence. Radiocarbon ages shown are reservoir-corrected dates (see Table 1).

presumably rapid. Deglaciation of the Juan de Fuca Lobe is not well known, but it seems to have begun around 14,4607200 14C yr BP (Y-2452; Heusser, 1973) and retreated rapidly, reaching Whidbey Island by 13,5957145 14C yr BP (Dethier et al., 1995). The oldest date on marine shell fragments in eastern Juan de Fuca Strait from this study is 12,889750 14C yr BP (corrected age, Core TUL97-12; Hewitt and Mosher, 2001). The youngest date from glacial-marine sediments in the study area is 11,109750 14C yr BP (Core TUL97-16), which is consistent with the range of dates in the Everson (Capilano) glacial-marine sediments throughout the region (Easterbrook, 1992). Waitt and Thorson

(1983) presume that the Puget lobe and Juan de Fuca lobe had retreated into a single lobe in northern Puget lowland by 13,600 yr BP. They infer rapid northward recession with continued iceberg calving into northern Puget Sound after this time. Sub-bottom reflection profiles across the shallow banks within the eastern Juan de Fuca Strait suggest they consist of diamict (cf. Davies et al., 1997), supporting the interpretation that ice was grounded and formed the moraines and drumlins that form the core of the banks. Occasional recessional stalls and small re-advances are interpreted by Waitt and Thorson (1983). Ice likely retreated from southwest to northeast,

34

Table 2 Radiocarbon ages used in compilation of the sea level curve (Fig. 14) Error

Age (max)

Age (min)

5470 8910 9670 9880 10630 10650 10720 11180 11700 12510 12850 13070 13070 13090 13110 13130 13150 13240 13270 13340

5470 8109 8869 9079 9829 9849 9919 10379 11700 11709 12049 12269 12269 12289 12309 12329 12349 12439 12469 12539

115 50 140 50 50 230 60 100 170 160 230 90 160 110 170 160 50 80 60 80

5585 8159 9009 9129 9879 10079 9979 10479 11870 11869 12279 12359 12429 12399 12479 12489 12399 12519 12529 12619

5355 8059 8729 9029 9779 9619 9859 10279 11530 11549 11819 12179 12109 12179 12139 12169 12299 12359 12409 12459

Elevation 1 33 55 44 55 55 60.4 55 6.1 0 8 38 28 65 20 26 53 60 90 23

Ref. no.

68.2% Prob. cal. yr max

68.2% Prob. cal yr min

95.4% Prob. cal. yr max

95.4% Prob. cal. yr min

I-3673 CAMS62767 RIDDL-265 CAMS58685 CAMS58677 RIDDL-258 CAMS58695 CAMS58677-58678 I-3675 GSC-1114 GSC-398 TO-9189 GSC-246 O’Donnel Bog GSC-418 GSC-763 Maltby L. TO-9192 CAMS33492 Beta-123075

6463 9293 10243 10413 11591 11803 11603 12703 14103 14103 15103 15153 15203 15153 15203 15203 15103 15303 15403 15503

6113 9053 9813 10233 10843 10803 11253 12003 13453 13503 13603 14103 13903 14103 14103 14103 14153 14203 14203 14303

6553 9323 10303 10453 11131 12403 11623 12903 15203 15203 15503 15503 15503 15503 15603 15603 15503 15503 15503 15603

5953 8833 9603 10203 10876 10603 11243 11803 13203 13203 13503 13803 13803 13803 13803 13903 14103 14103 14103 14203

All dates are from marine shells except I-3673 and I-3675 which are peat and plant debris, respectively. All ages are conventional (Stuiver and Polach, 1997) normalized to 25% d13C. A reservoir correction of 801 yr has been applied to the marine shells, as per Robinson and Thompson (1981). Radiocarbon dated samples are from the following references: CAMS33492, (Blais-Stevens et al., 2001; Huntley et al., 2001); TO9189, TO9190, TO9191, TO9192 (James et al., 2002), GSC246, GSC418, GSC763, GSC398 (Clague, 1980), GSC1114 (Lowdon et al., 1971), CAMS58677, 58678, 58695,58685, 62767 (Table 1, Mosher and Johnson, 2001), RIDDL258, RIDDL 265 (Linden and Schurer, 1988), I-3673, 3675 (Mathews et al., 1970; Clague, 1980), and Beta-123075 (S. Johnson, Pers. Comm., 2003–Port Townsend, Olympic Peninsula). CAMS=Center for Accelerated Mass Spectrometry at the Lawrence Livermore National Laboratory, Beta=Beta Analytic, Inc., TO=Isotrace Labs, I=Isotopes Teledyne Co., RIDDL=Radio-Isotope Direct Detection Lab, Simon Fraser University, GSC=Geological Survey of Canada Laboratory. 14C years were converted to calendar years before present using CALIB 4.3 (Stuiver et al., 1998). Text line in italics is an extrapolated date, based on sedimentation rates derived from the ages given in the two references indicated (see Table 1).

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changing to north-south in the easternmost portion of the strait, given the shapes of these banks. This change in direction is consistent with onshore morphological analysis using LIDAR altimetry (Haugerud et al., 2003). The morphology and internal structure of the extensive complexes that include McArthur Bank, Smith Island and Partridge Bank in the central strait, and Lawson Bank at the southern mouth of Rosario Strait, are interpreted as terminal moraines, marking examples of probable grounding lines. A date east of McArthur Bank indicates ice had retreated off the bank before 12,700750 14C yr BP (Core TUL97-16), and probably before 12,7957145 14C yr BP based on a date from glacial-marine sediments on Whidbey Island (BETA1716; Dethier et al., 1995). 5.2. Sea-level history In the following sections, three styles of submerged features are discussed as evidence for sea-level stands lower than present: (1) wave-eroded unconformities and terraces; (2) drowned prograded deposits; and (3) a partially isolated low-stand basin. 5.2.1. Wave-eroded unconformities and terraces Glacial-marine sediments in the eastern Juan de Fuca Strait were deposited as ice retreated and relative sea level was up to 75 m above present. Subsequent glacial isostatic rebound caused rapid local sea-level regression but its extent is in question. The unconformity that Linden and Schurer (1988) traced to a depth of 55 m on seismic reflection profiles, dated between 88697140 14 C yr BP and 98497230 14C yr BP. In the present study, this unconformity is recognized throughout the strait down to 70 m. It is interpreted as an erosional surface carved into early post-glacial and sometimes glacial-marine sediment by wave action during lower sea level. Core TUL97-12 penetrated this unconformity near its greatest depth ( 60.4 m) (Fig. 8), providing an age of 9919760 14C yr BP just above the unconformable surface. A series of cores taken off Esquimalt Harbour sampled this unconformity in successively shallower water depths (Figs. 8–10). There is significant scatter of ages because the material overlying the unconformity is reworked, but in general they show corresponding progressively younger ages, supporting its time-transgressive nature. This result is expected from a waveeroded surface generated during sea level rise. The unconformity surface is formed into a series of terraces or ridges at 60, 51, 46, 43, and 41 m as it shoals off Esquimalt Harbour (e.g. Fig. 7). Further evidence for terraces is present in the seabed morphology along the Victoria waterfront at about 65, 50, 35, and 15 m and off Dungeness Spit along the Olympic Peninsula at similar depths (Figs. 4 and 5).

35

These ridges and terraces are likely positions of lower sea-level stands formed during sea-level transgression. Emergent terraces of similar description were noted by Haugerud et al. (2003), representing former sub-aerial marine limits. Eroded surfaces and terraces are also common on the bank tops and down to 90 m (e.g. Fig. 5). They were probably formed during lower sea level stands. Although these surfaces occur as deep as 80 to 90 m, sea level was probably some meters above this elevation. Erosion would have begun at erosional wave base; the minimum depth where waves had sufficient energy to mobilize the available sediment. Komar and Miller (1975) show that the threshold of grain movement under waves is a function of water depth, wave period, wave height, and wave length. In the eastern strait under present conditions, waves are typically less than 2 m high, with periods of around 6 s, and wave lengths of 55 m (Thomson, 1981). These waves have the ability to mobilize coarse sand (2 mm) at depths of o25 m. The unconformities are not, therefore, a precise indicator of the past sea-level in which they formed, particularly in the more open areas of the strait, where waves are larger and the wave base deeper. Subtracting 25 m from the maximum depth of these unconformities yields a sea-level of around 55 to 65 m, which is close to the depths of the unconformities observed in more sheltered areas. In the approaches to Esquimalt Harbour, for example, the erosional unconformity probably records the corresponding sea-level more precisely because of the sheltered conditions. In any case, the reported depths indicated by the terraces and the unconformity surface should be considered a maximum for sea level lowering. 5.2.2. Drowned prograded deposits Wedge-shaped deposits with clinoform reflections occur adjacent to a number of banks within the strait and against the present coastline down to 60 m. They are interpreted as progradational deposits associated with sea level transgression. They may represent coastal deposits such as spits and beaches on the flanks of shallow banks; however, their position extending from the tops of the banks suggests they are probably bank spillover deposits as described by Shaw and Forbes (1992). As such, their relevance to sea level relates to the elevation of the bank and the local physical oceanographic regime, forming when the bank top was slightly submergent (within wave base). They are not precise sea level indicators, therefore, but are likely indicative of shallow water in this oceanographic setting. Some post-glacial clinoform deposits appear to be submarine fan deltas based on their morphology (e.g. Fig. 12). This interpretation derives from the larger volume of the deposits, their lobate form in plan view and the irregular basal reflector that separates

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36

55 m at 9829750 14C yr BP. The base of the unit has an age of about 10,3797100 14C yr BP, based on extrapolation of dates shallower in the core. This age should represent the time when sea level was below 55 m during the regressive stage.

clinoforms from underlying glacial deposits. They would likely form close to sea level as streams carrying sediment meet the sea and consequently deposit their entrained sediment load. Bathymetric data along the Olympic Peninsula reveal submerged features with similar morphology to the modern spits known as Ediz Hook and Dungeness Spit (Fig. 5). At Ediz Hook, these features occur at depths of 6, 8, and 25 m, and at Dungeness Spit another occurs at 15 m. The close proximity of these features and their similar morphology to the modern spits, leads to their interpretation as drowned barrier-spits resulting from sea level rise (cf. Rampino and Saunders, 1981; Oldale, 1985; Jensen and Stecher, 1992; Wellner et al., 1993; Roy et al., 1994; Forbes et al., 1995). These spits probably formed during transgression rather than regression because in the latter case they would have had to survive a period of sub-areal exposure and subsequent transgression. The tops of drowned spits are probably within a few meters of the sea level in which they formed.

5.2.4. Sea-level curve and rates of sea-level change Fig. 14 is an updated sea-level curve proposed for eastern Juan de Fuca Strait, with five new control points added to published data. Given the above discussion, these control points should be considered maximum for sea level lowering datum and it must be noted that there is likely significant local variability because of the region’s proximity to the ice margin. In addition, there is possible overprinting of sea level adjustments due to glacial tectonics, i.e., glacially induced fault deplacements (e.g. Thorson, 1989, 2000). The eastern strait was filled with ice before 12,889750 14C yr BP (the oldest glacial marine date in this study). Following deglaciation, the crust was isostatically depressed, reflected in a period of maximum marine submergence of approximately +75 m at 12,470760 14C yr BP (Mathews et al., 1970; Huntley et al., 2001; Blais-Stevens et al., 2001). Subsequent crustal isostatic rebound was rapid and caused relative sea level to fall, eventually reaching a level the same as today at around 11,7007170 14C yr BP (I-3675, Clague et al., 1982; GSC-1114 and GSC-1131, Clague and James, 2002; James et al., 2002). Terrestrial emergence continued beyond the present level while eustatic sea-level was still low, resulting in relative sea

5.2.3. Partially isolated low-stand basin The morphology and elevations surrounding a small basin west of Whidbey Island (Fig. 13) indicate it potentially was an embayment during a 60 m sea level stand. Cores within this basin contain a lithologic unit of Holocene green silty-clay not seen in any other core from the study. If this unit represents deposition while the basin was a marine embayment, then a date from the top of the green mud indicates sea-level was around 100

CAMS 33492 14

0

3

GSC 246 GSC 418 GSC 398

a rs

GSC 76

TO 9191 TO 9189

Beta123075

Elevation (m)

50

TO 9192 TO 9190

Ca len dar Ye

Max age ( C yrs) Min. Age ( 14 C yrs) Max. age (cal. yrs) Min. age (cal. yrs) Fairbanks (1989) Peltier (2002)

GSC 1114 I-3675 I-3673

Extrapolated (CAMS 58677 and 58678)

CAMS 62767

-50 CAMS 58685 RIDDL 265 RIDDL 258 CAMS 58695 CAMS 58677

-100 0

2

4

10 6 8 Age (ka before present)

12

14

16

Fig. 14. Relative sea level curve for the eastern Juan de Fuca Strait developed from radiocarbon dating. All dates are from marine shells except I3673 and I-3675, which is peat and plant debris, respectively. Minimum and maximum ages provided are based on the error data associated with each sample measurement. As these are marine dates, they likely represent age maxima. The ages assigned to these sea level positions may well be younger than presented, as a result. The Fairbanks (1989) and Peltier (2002) curves are global eustatic curves in 14C years, presented by these authors. Dates used in this compilation are shown in Table 2.

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levels lower than present. The magnitude of this sea level lowering has been questioned. Clague et al. (1982) argued sea level fell no more than about 10 m below present. Linden and Schurer (1988) suggested that an erosional unconformity off Esquimalt Harbour implied a maximum lowering of about 55 m. In this study, it is argued that this unconformity surface likely formed close to sea level. The apparent wave-cut terraces and possible drowned beaches in the morphology of this surface lend credence to this argument. At 60.4 mbsl, this horizon dates at 9919760 14C yr BP (Table 2; Fig. 10), providing the maximum sea level lowering attained during regression. A low stand of several tens of meters is supported by post-glacial rebound modeling (James et al., 2000; Clague and James, 2002; T. James, Pers. Comm., 2003). Interim submarine data points were added to the sea level curve based on direct and indirect evidence from the present study. The isolated basin cores place sea level at 55 m at about 10,379 yr BP during regression and again at about 9829750 14C yr BP during transgression. As an extrapolated point, the former date has a large degree of uncertainty. Linden and Schurer (1988) placed sea level at this elevation at 98497230 14C yr BP. The unconformity surface off Victoria also dates at 9079750 14 C yr BP at 44 mbsl and 8109750 14C yr BP at 33 m. Although these data points should be considered as maxima for the degree of relative sea level fall, it is encouraging that they align along a reasonable sea level curve trajectory when merged with other data sources (e.g. Clague et al., 1982; Linden and Schurer, 1988; Hutchinson, 1992; Fig. 14). Rates of sea-level change and crustal motion were calculated from the sea level curve shown in Fig. 14. The sea level curve shows a rapid regression after ice retreated, followed by a gradual transgression toward the present. The form of this curve results from the net effect of opposing global eustatic sea-level rise and local isostatic rebound. It does not point to significant glacial tectonic effects as discussed by Thorson (2000). This result may be a function of the fact that the data are of insufficient resolution to discriminate fault motions from regional glacial isostatic effects. Much finer age control on unconformity and terrace surfaces is likely required. The nature of the age control in this study are limiting, thus the rates reported are maximum. It appears as though, initially, isostatic rebound dominated, resulting in a rapid fall in relative sea level at a maximum rate of 58 mm/yr (150 m in 2550 years). The global eustatic rise during this time was between 28 and 42 m, or 10–16 mm/yr (Peltier, 2002; Fairbanks, 1989, respectively). The maximum rate of crustal uplift necessary to cause relative sea level fall and overcome eustatic rise was between 68 and 74 mm/yr, therefore. Relative sea level remained below the global eustatic sea level curve during its transgression between about

37

10,300 and at least 7500 14C yrs BP, but reached its present position by about 54707115 14C yr BP; 10 m above the Fairbanks (1989) eustatic curve and 3.5 m above the Peltier (2002) curve. Sea level remained at its modern level since that time while the eustatic level has risen slowly (about 1.7 mm/yr), implying crustal uplift has nearly balanced eustatic sea level rise.

6. Conclusions The eastern Juan de Fuca Strait is near the western and southern limits of late Wisconsinan North American glaciation. About 1.5 km of glacial ice covered the study area during the Vashon stade of the Fraser Glaciation between 16,000 and 13,750 yr BP. The strait deglaciated quickly, likely within a few hundred years. Drumlins and moraines that now form shallow banks in the strait are indicative of ice grounding, perhaps reflecting that glacier retreat was episodic. Sea level changed dramatically following deglaciation. The regional pattern of this change appears to be a function of glacial isostatic rebound. Finer scale motions, perhaps related to glacial tectonics (Thorson, 2000) have not been resolved. The lithosphere was isostatically depressed immediately following deglaciation and sea level reached its maximum high-stand of +75 m near Victoria at 12,469760 14C yr BP (Mathews et al., 1970; Blais-Stevens et al., 2001). Along the Olympic Peninsula the maximum observed high-stand is around +50 m, near Port Angeles (Dethier et al., 1995), so there is regional variability reflecting ice thickness variability as it thins towards the south and west. Subsequent rapid isostatic rebound caused relative sea level to fall, passing through the present 0 m isobath around 11,7007170 14C yr BP (Clague et al., 1982). Within the context of this study, marine data providing limiting age control on sea level fall, imply that by 9919760 14C yr BP, sea level had reached its maximum observed low-stand of 60.4 m below present. Relative sea level transgressed between that time and 54707115 14 C yr BP, but remained below the global eustatic sea level until at least 7500 14C yrs BP. Since 54707115 14 C yr BP, relative sea level has remained reasonably constant even though eustatic sea level has risen slowly (about 1.7 mm/yr), implying crustal uplift has nearly balanced eustatic sea-level rise.

Acknowledgements This research was supported by the United States Geological Survey (USGS) National Earthquake Hazards Reduction Program (NEHRP), Department of the Interior, under USGS award number 1434-HQ-96-GR02726, and by the Geological Survey of Canada (GSC).

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A. Hewitt received support from the National Oceanic and Atmospheric Administration (NOAA) of the United States through its sponsorship of the Center for Coastal and Ocean Mapping at the University of New Hampshire (NOAA award number NA97OG0241 and CICEET award number NA07OR0351). The authors wish to thank Larry A. Mayer for his support of this project. The efforts of the officers, crew and scientific support staff of CCGS vessels, John P. Tully, Revisor, and R. B. Young are greatly appreciated. The authors also wish to thank Kim Conway for arranging the radiocarbon dating, Robert Kung for assistance with GIS mapping, Glenda Rathwell for conducting the grain-size analyses, and Seismic Micro-technology, Inc., for their generous loan of the Kingdom Suite 2D/3D PAK seismic processing and interpretation software package. We thank D. Forbes, T. James, J. Clague, R. Thorson and V. Levson for critical reviews resulting in vast improvement of the manuscript, Geological Survey of Canada Contribution No. 2003052.

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