Early growth of the last Cordilleran ice sheet deduced from glacio-isostatic depression in southwest British Columbia, Canada John J. Clague a,b, * , Duane Froese a,1 , Ian Hutchinson c , Thomas S. James d , Karen M. Simon e a Department of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada b Geological Survey of Canada, Vancouver, BC V6B 5J3, Canada c Department of Geography, Simon Fraser University, Burnaby, BC V5A 1S6, Canada d Geological Survey of Canada, Pacific Geoscience Centre, Sidney, BC V8L 4B2, Canada e Department of Earth Sciences, Dalhousie University, Halifax, NS B3H 4J1, Canada Received 9 February 2004 Available online 19 November 2004 Abstract Relative sea level at Vancouver, British Columbia rose from below the present datum about 30,000 cal yr B.P. to at least 18 m above sea level 28,000 cal yr B.P. In contrast, eustatic sea level in this interval was at least 85 m lower than at present. The difference in the local and eustatic sea-level positions is attributed to glacio-isostatic depression of the crust in the expanding forefield of the Cordilleran ice sheet during the initial phase of the Fraser Glaciation. Our findings suggest that about 1 km of ice was present in the northern Strait of Georgia 28,000 cal yr B.P., early during the Fraser Glaciation. D 2004 University of Washington. All rights reserved. Keywords: Sea level; Glacio-isostasy; Quadra Sand; British Columbia; Canada Introduction The glacio-isostatic response of parts of northwest North America to decay of the Cordilleran ice sheet at the end of the Pleistocene has been well documented (Clague, 1983; Clague and James, 2002; Clague et al., 1982; Easterbrook, 1969; Fedje and Josenhans, 2002; Fulton and Walcott, 1975; Hetherington and Barrie, 2003; James et al., 2000; Josenhans et al., 1995; Mathews et al., 1970; Thorson, 1989). Relative sea level in coastal British Columbia and northwest Washington fell rapidly during deglaciation due to glacio-isostatic uplift of the crust. By the beginning of the Holocene, when the ice sheet was gone, isostatic uplift was nearly complete and sea level was below its present datum. Over the past 8000 yr, local sea level has followed the global eustatic trend. The rapid response of the crust to glacial unloading is attributed to an unusually low-viscosity, hot asthenosphere at the Cascadia subduction zone, which underlies the western margin of the North America plate (James et al., 2000). Although the sea-level history of coastal British Colum- bia during and after deglaciation is well documented, almost nothing is known about sea-level change in British Columbia during the early part of the last (Fraser, Late Wisconsinan) glaciation, when the Cordilleran ice sheet developed. Clague (1983) speculated that the crust beneath the main centers of ice sheet growth, for example, the Coast Mountains, began to subside about 30,000 cal yr B.P. and that the area of subsidence rapidly increased as glaciers expanded into lowlands. He suggested that the entire British Columbia landmass and adjacent continental shelf had become isostatically depressed by the time the Cordilleran ice sheet achieved its maximum extent about 16,000 cal yr B.P. Crustal depression was several hundred meters in 0033-5894/$ - see front matter D 2004 University of Washington. All rights reserved. doi:10.1016/j.yqres.2004.09.007 * Corresponding author. Department of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada. Fax: +1 604 291 4198. E-mail address: jclague@sfu.ca (J.J. Clague). 1 Current address: Department of Earth and Atmospheric Sciences, University of Alberta, AB T6G 2E3, Canada. Quaternary Research 63 (2005) 53 – 59 www.elsevier.com/locate/yqres