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Younger Dryas readvance in Squamish river valley, southern Coast mountains, British Columbia

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Abstract

Stratigraphy and landforms in the Brohm Creek basin and lowermost Mamquam River valley provide evidence for a readvance of a valley glacier in the southern Coast Mountains of British Columbia during the Younger Dryas interval. At Brohm Creek, till containing wood 10,500 and 10,700 14C yr old (ca 12,500–12,900 cal yr old), overlies an ice-contact terrace that is correlative with similar terraces at the mouth of Mamquam River, 11 km to the south and 900 m lower in elevation. A line linking the two sites has a slope of 5°, which is similar to slopes of termini of present-day valley glaciers in the southern Coast Mountains. We used these observations to reconstruct the Younger Dryas glacier in Squamish valley at the head of Howe Sound. The glacier deflected landslide debris, derived from steep slopes at the head of Cheekye River basin on the west flank of Mt. Garibaldi, southward to an ice-marginal lake at the mouth of Mamquam River. Foreset-bedded sediments in the core of a large fan at the mouth of Cheekye River indicate the glacier terminus had retreated up Squamish valley beyond Cheekye and Mamquam rivers by 10,200 14C yr BP (11,900 cal yr old). Radiocarbon ages on the oldest postglacial sediments in a kettle on the landslide debris show that stagnant ice in the study area had melted before 10,000 14C yr BP (11,500 cal yr BP). The readvance of the Squamish valley glacier during the Younger Dryas Chronozone was probably driven by a regional climate reversal.

Introduction

The southwest sector of the Cordilleran ice sheet disappeared about 11,000 yr ago, 6000 yr after achieving its maximum size (Clague, 1981; Porter and Swanson, 1998; Clague and James, 2002). Although the exact pattern and time of glacier retreat are not known, the continental shelf became deglaciated first, followed by lowlands bordering the Pacific Ocean, the Strait of Georgia, Juan de Fuca Strait, and Puget Sound. Ice persisted longest in fiords and valleys in the Coast Mountains (Friele and Clague, 2002). On a local scale, deglaciation was far more complex and was interrupted by still-stands and readvances (Armstrong et al., 1965; Clague et al., 1997). Clague et al. (1997), for example, documented two readvances of glaciers in the eastern Fraser Lowland (Fig. 1), east of Vancouver, during late-glacial time.

An issue of particular interest is the response of the wasting Cordilleran ice sheet to Younger Dryas cooling. It is now known that the Younger Dryas event had global effects (Peteet, 1993), although Younger Dryas cooling was much less on the west coast of North America than in the North Atlantic region (Mathewes, 1993). Did the Cordilleran ice sheet respond to Younger Dryas cooling by thickening and advancing? Did the ice sheet even exist during the Younger Dryas interval? Data to answer these questions are limited. A few studies in the North American Cordillera have documented late-Pleistocene end moraines, but good chronologic control is generally lacking (Davis and Osborn, 1987). The two readvances of the Cordilleran ice sheet that Clague et al. (1997) identified in the eastern Fraser Lowland predate the Younger Dryas. Kovanen and Easterbrook (2002a), Kovanen and Easterbrook (2002b), however, suggest that a later, less extensive advance in this area is Younger Dryas in age.

Friele and Clague (2002) showed that glacier retreat from Howe Sound, a fiord in the Coast Mountains north of Vancouver, lagged behind that in the western Fraser Lowland by about 2000 yr and that a glacier was present at the head of the fiord until after 12,800 yr ago. They also suggested that a limited glacier readvance or surge occurred just before final rapid deglaciation of the area.

This paper provides additional evidence for a late Pleistocene readvance in the lowermost Squamish River valley at the head of Howe Sound. New radiocarbon ages confirm that the readvance occurred during the Younger Dryas interval. We build on our previous paper by accurately delineating the terminal zone of the glacier and by closely constraining the time of its advance and retreat using geomorphic and stratigraphic evidence. The critical evidence comes from ice-contact terraces in the Brohm Creek and lower Mamquam River watersheds (Fig. 1), on the west flank of Mount Garibaldi, north of Squamish. Although based on only one glacier, our study suggests that ice in the westernmost Coast Mountains may have been restricted to local ice caps, mountain valleys, and fjord heads during the Younger Dryas interval.

Section snippets

Study area

Howe Sound extends 40 km north into the southern Coast Mountains from the Strait of Georgia (Fig. 1). At its head is Squamish River and its tributaries, which drain an area of about 3600 km2, ranging in elevation from 2700 m a.s.l. at Mount Garibaldi to sea level. The watershed is mountainous, and glaciers are common above about 1500 m a.s.l.

Our study area is in the lower part of the Squamish watershed on the west flank of the Mount Garibaldi massif (Fig. 1). Mount Garibaldi is a composite volcano

Brohm Creek terrace

The upper part of the Brohm Creek basin is filled with glacio-fluvial and glacio-colluvial sediments. The sediments underlie a complex ice-marginal terrace that extends from 1025 to 1100 m a.s.l. (Fig. 2, Fig. 3, Ryder, 1980). The terrace is bordered on the west by a north-trending ridge underlain by basement rocks of the Coast Plutonic Complex (Mathews, 1958), on the north by a late-Pleistocene lava flow, and on the east by the flank of Mount Garibaldi itself the terrace scarp faces south.

Squamish valley glacier position 12,800 yr ago

The Brohm terrace is correlated with two, matched, ice-contact terraces in the lowermost Mamquam River valley, 11 km south and 900 m lower in elevation (Mamquam–Mashiter terraces, Fig. 2). This relation is used to define the exact position of the Squamish valley glacier terminus about 12,800 yr ago (Fig. 2).

Friele and Clague (2002) described the sediments underlying the Mamquam-Mashiter terraces, thus only a brief summary is given here (see Fig. 3). At the base of the sequence are folded and

Time of final deglaciation

Final retreat of the Squamish valley glacier was accompanied and followed by construction of a large ice-marginal fan complex, a fan delta, and finally an alluvial fan, collectively referred to as the “Cheekye fan” (Friele et al., 1999). The Cheekye fan complex has been divided on the basis of morphology into three parts—the upper, middle, and lower fans (Mathews, 1952; Baumann, 1991). Here we present new data on the age of the upper, middle, and lower fans. These data place constraints on the

Ice extent in the Coast Mountains during the Younger Dryas

Our study provides a detailed snapshot of Younger Dryas glacier advance and retreat in a large valley in the southern Coast Mountains (Fig. 2, Fig. 4). Although we cannot confidently define Younger Dryas ice extent elsewhere in this vast mountain range, we can draw some general inferences about its extent based on our and others’ work.

The glacier that advanced to the present-day head of Howe Sound during Younger Dryas time was fed by ice flowing from both the upper Squamish and Cheakamus

Conclusion

Deposits along upper Brohm Creek and near the mouth of Mamquam River provide evidence for a readvance of the Squamish valley glacier in the southern Coast Mountains of British Columbia during the Younger Dryas interval. The readvance occurred shortly after 12,800 cal yr ago and was short-lived, lasting no more than several centuries. At its Younger Dryas maximum, the Squamish valley glacier covered the low, lake-dotted upland of the Squamish–Cheakamus divide and probably had a maximum thickness

Acknowledgements

Our research was funded by the Natural Sciences and Engineering Research Council of Canada (Research Grant 217051-99 to Clague). Thom Davis and Jerry Osborn provided helpful critical reviews of a draft of the paper. Rolf Beukens of IsoTrace Laboratory (University of Toronto) and Roger McNeely of the Geological Survey of Canada (GSC) provided radiocarbon ages, and Robert Kung (GSC) supplied the digital elevation model used in Fig. 4.

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