1 2 The cordilleran ice sheet 3 4 Derek B. Booth1, Kathy Goetz Troost1, John J. Clague2 and Richard B. Waitt3 5 6 1 Departments of Civil & Environmental Engineering and Earth & Space Sciences, University of Washington, 7 Box 352700, Seattle, WA 98195, USA (206)543-7923 Fax (206)685-3836. 8 2 Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, Canada 9 3 U.S. Geological Survey, Cascade Volcano Observatory, Vancouver, WA, USA 10 11 12 Introduction techniques yield crude but consistent chronologies of local 13 and regional sequences of alternating glacial and nonglacial 14 The Cordilleran ice sheet, the smaller of two great continental deposits. These dates secure correlations of many widely 15 ice sheets that covered North America during Quaternary scattered exposures of lithologically similar deposits and 16 glacial periods, extended from the mountains of coastal south show clear differences among others. 17 and southeast Alaska, along the Coast Mountains of British Besides improvements in geochronology and paleoenvi- 18 Columbia, and into northern Washington and northwestern ronmental reconstruction (i.e. glacial geology), glaciology 19 Montana (Fig. 1). To the west its extent would have been provides quantitative tools for reconstructing and analyzing 20 limited by declining topography and the Pacific Ocean; to the any ice sheet with geologic data to constrain its physical form 21 east, it likely coalesced at times with the western margin of and history. Parts of the Cordilleran ice sheet, especially 22 the Laurentide ice sheet to form a continuous ice sheet over its southwestern margin during the last glaciation, are well 23 4,000 km wide. Because most of the marginal environments suited to such analyses. The approach allows interpretation of 24 of the Cordilleran ice sheet were not conducive to preserving deposits and landforms at the now-exposed bed of the former 25 an extensive depositional record, much of our understanding ice sheet, and it also suggests likely processes beneath other 26 of this ice sheet has come from limited areas where preser- ice sheets where reconstructions are less well-constrained. 27 vation is good and access unencumbered, notably along its Finally, expressions of the active tectonics of western 28 lobate southern margin in northern Washington State and North America are now widely recognized across the 29 southern British Columbia. marginal zone of the Cordilleran ice sheet. Such conditions 30 Arrival of geologists into Puget Sound late in the were little appreciated at mid-century. Only since the 1980s 31 19th century initiated study of the Cordilleran ice sheet. have the extent and potential influence of recent tectonics 32 The landscape displayed unmistakable evidence of past on the landscape of western Washington been appreciated. 33 glaciations, but a sporadic sequence of deposits along valley The regional setting for repeated glaciations owes much of 34 walls and coastal bluffs only hinted at a long and intricate its form to those tectonic influences; conversely, deformation and offset of ice-sheet deposits may be critical in unraveling 35 history of ice-sheet occupations. By the mid-20th century the Quaternary expression of the region’s tectonics. 36 extensive field studies had developed a framework for Pacific Perhaps the greatest development in recent study of the 37 Northwest Quaternary history. Evidence of four glaciations, Cordilleran ice sheet, especially its southwestern boundary, 38 summarized by Crandell (1965) and detailed by Armstrong has been the focus of scientific attention on this region – 39 et al. (1965), Mullineaux et al. (1965), and Crandell (1963), not only by geoscientists but also by resource managers, 40 followed the precedent from the American Midwest: four land-use planners, and the general public. In the last several 41 continental-scale glaciations, correlated across broad regions. decades, this glacial landscape has become a region of rapid 42 In the Pacific Northwest, the youngest ice-sheet glaciation population growth. In part because of these social pressures, 43 (Fraser), was constrained by radiocarbon dates and correlated the level of scientific study here has rapidly increased, which 44 with the Wisconsin Glaciation of the mid-continent. Earlier will likely render the story of the Cordilleran ice sheet 45 glaciations (given the local names Salmon Springs, Stuck, presented in this synoptic paper even more quickly outdated 46 and Orting) were identified only in the southeastern Puget than its predecessors. 47 Lowland. Crandell (1965) suggested that they spanned early 48 through late Pleistocene time. 49 In the latter part of the 20th century, improved under- 50 standing of global and regional stratigraphy, and emphasis Chronology and the Stratigraphic Record 51 on geomorphic processes, have brought new information 52 from studies of the Cordilleran ice sheet. These advances Quaternary Framework 53 are the topics of this chapter. The record of global warming 54 and cooling recorded in deep-sea cores shows that there More than one hundred years after Bailey Willis published 55 were many glaciations during the Quaternary Period, not just “Drift Phenomena of Puget Sound” (1898), geologists 56 four. Global perspectives on past sea-level variations prove continue efforts to identify and correlate the Quaternary 57 critical to understanding tidewater glacier systems like the stratigraphic units across the area episodically covered by 58 southwestern part of the Cordilleran ice sheet. New dating the southern part of the Cordilleran ice sheet (Fig. 1). Nearly DEVELOPMENT IN QUATERNARY SCIENCE VOLUME 1 ISSN 1571-0866 © 2003 ELSEVIER SCIENCE B.V. DOI:10.1016/S1571-0866(03)01002-9 ALL RIGHTS RESERVED 17 18 Derek B. Booth et al. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Fig. 1. Map of southern extent and lobes of the latest Pleistocene advance of the Cordilleran ice sheet in Washington and British 29 Columbia. 30 31 32 a half century of field investigations in the southern Puget Recognition of nonglacial environments in the deposi- 33 Lowland (Armstrong et al., 1965; Crandell et al., 1958; tional record is essential to unraveling the chronology here. 34 Mullineaux et al., 1965; Noble & Wallace, 1966; Waldron The present Puget Lowland may be a useful analog for earlier 35 et al., 1962) and in the northern Puget Lowland (Clague, nonglacial periods. Areas of nondeposition, soil formation, or 36 1981; Easterbrook, 1986, 1994) show that ice sheets have minor upland erosion dominate most of the lowland (Fig. 3). 37 advanced south into the lowlands of western Washington at Sediment is only accumulating in widely separated river 38 least six times. The global climatic template of the marine- valleys and lake basins, and in Puget Sound. Were the present 39 isotope record illustrates the likely number and frequency lowland again invaded by glacier ice, it would bury a complex 40 of glacier advances. It suggests that the current half-dozen and discontinuous nonglacial stratigraphic record. Thick 41 known glacier advances do not include every advance into sedimentary sequences would pinch out abruptly against 42 the region in the last 2.5 million years. The last three ice valley walls. Sediment deposited in valleys could be 100 m 43 advances correlate with marine oxygen isotope stages (MIS) lower than coeval upland sediment or organic-rich paleosols. 44 2, 4, and probably 6 (Fig. 2). The most recent advance Thus, the thickness and lateral continuity of nonglacial 45 was the Fraser glaciation, discussed in detail later in this sediment of any one nonglacial interval will be highly 46 chapter. variable owing to the duration of the interval, subsidence and 47 Little is known about the climate in the lowlands of uplift rates, and the altitude and surface topography of fill 48 southern British Columbia and western Washington during left by the preceding glacier incursion (Troost, 1999). 49 most of the Pleistocene; but recent research has focused West of the Cascade Range, Cordilleran glaciations were 50 on either MIS 2 (Hansen & Easterbrook, 1974; Heusser, typified by the damming of a proglacial lake in the Puget 51 1977; Heusser et al., 1980; Hicock et al., 1999; Mathewes Sound basin, the spreading of an apron of outwash, deep 52 & Heusser, 1981; Whitlock & Grigg, 1999), MIS 2 and 3 subglacial scouring and deposition of till, formation of 53 (Barnosky, 1981, 1985; Grigg et al., 2001; Troost, 1999), large recessional outwash channels, formation of ice-contact 54 or MIS 5 (Heusser & Heusser, 1981; Muhs et al., 1994; terrain, and deposition of glaciomarine drift in the northern 55 Whitlock et al., 2000). From these studies we know climate lowland. Glacial periods were marked by a change to colder- 56 during MIS 3 was somewhat cooler than today and sea level climate vegetation and increased deposition and erosion. 57 was lower. The climate of MIS 5 was similar to today’s, with Thick glaciomarine, glaciolacustrine, and outwash deposits 58 marine deposits commonly found near modern sea level. accumulated in proglacial and subglacial troughs, capped The Cordilleran Ice Sheet 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Fig. 2. Comparison of the marine oxygen-isotope curve stages (MIS) using the deep-sea oxygen-isotope data for ODP677 from 49 Shackelton et al. (1990), global magnetic polarity curve (Barendregt, 1995; Cande & Kent, 1995; Mankinnen & Dalrymple, 50 1979), and ages of climatic intervals in the Puget and Fraser lowlands. Ages for deposits of the Possession glaciation through 51 Orting glaciation from Easterbrook et al. (1981), Easterbrook (1986), Blunt et al. (1987), and Easterbrook (1994).