Multiphase flow of the late Wisconsinan Cordilleran ...

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Multiphase flow of the late Wisconsinan Cordilleran ice sheet in western Canada

Andrew J. Stumpf* Bruce E. Broster Quaternary and Environmental Studies Group, Department of Geology, University of New Brunswick, P.O. Box 4400, Fredericton, New Brunswick E3B 5A3, Canada

Victor M. Levson British Columbia Ministry of Energy and Mines, Geological Survey Branch, P.O. Box 9320 STN PROV GOVT, Victoria, British Columbia V8W 9N3, Canada

ABSTRACT In central British Columbia, ice flow during the late Wisconsinan Fraser glaciation (ca. 25–10 ka) occurred in three phases. The ice expansion phase occurred during an extended period when glaciers flowed westward to the Pacific Ocean and eastsoutheastward onto the Nechako Plateau from ice centers in the Skeena, Hazelton, Coast, and Omineca Mountains. Initially, glacier flow was confined by topography along major valleys, but eventually piedmont and montane glaciers coalesced to form an integrated glacier system, the Cordilleran ice sheet. In the maximum phase, a Cordilleran ice divide developed over the Nechako Plateau to 300 km inland from the Pacific coast. At this time, the surface of the ice sheet extended well above 2500 m above sea level, and flowed westward over the Skeena, Hazelton, and Coast Mountains onto the continental shelf, and eastward across the Rocky Mountains into Alberta. In the late glacial phase, a rapid rise of the equilibrium line caused ice lobes to stagnate in valleys, and restricted accumulation centers to high mountains. Discordant directions in ice flow are attributed to fluctuations of the ice divide representing changes in the location of accumulation centers and ice thickness. Ice centers probably shifted in response to climate, irregular growth in the ice sheet, rap*Present address: Illinois State Geological Survey, Natural Resources Building, 615 East Peabody Drive, Champaign, Illinois 61820-6964, USA; e-mail: [email protected].

id calving, ice streaming, and drainage of proglacial and subglacial water bodies. Crosscutting ice-flow indicators and preservation of early (valley parallel) flow features in areas exposed to later (cross-valley) glacier erosion indicate that the ice expansion phase was the most erosive and protracted event. Keywords: Cordilleran, divides, ice flows, ice sheet, Quaternary, western Canada. INTRODUCTION Colder temperatures and increased precipitation in the North Pacific region during the late Quaternary caused glaciers to expand outward from ice accumulation centers over high mountains into valleys (Davis and Mathews, 1944; Clague, 1991; Hall et al., 1996). At the climax (period of maximum ice expansion) of the last glaciation, an interconnected system of confluent alpine, intermontane, and piedmont glaciers, collectively known as the Cordilleran ice sheet, covered most of British Columbia, the southern parts of the Yukon Territory, and the state of Alaska, and extended eastward into Alberta and southward into the northwestern United States. The orientation of glacial striations, drumlin features, and the distribution of glacigenic sediments document several phases of ice movement controlled mainly by topography (Tipper, 1971a; Fulton, 1991; Ryder and Maynard, 1991; Levson and Giles, 1997), the location of ice accumulation centers (Clague, 1989), major outlet valleys (Hicock and Fuller, 1995), and possibly the development of proglacial lakes (cf. Dredge and Cowan, 1989).

GSA Bulletin; December 2000; v. 112; no. 12; p. 1850–1863; 12 figures.

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A new ice-flow data set from a large area in central British Columbia (Fig. 1) records a sequence of glacier movements much more complex than previously proposed. The data are evidence of multiple flow events, during which ice flowed under the control of both regional and local influences. The elevation and geographic position of directional indicators of glacier flow (crag and tail landforms, roches moutonneés, rat-tails, and striae on stoss and lee bedrock surfaces), together with crosscutting relationships, provide evidence of multiple flow events. The new ice-flow data are used herein to delineate the location of major ice centers and divides, the areal extent of major late Wisconsinan reversals in flow, and possible outlets for the Cordilleran ice sheet during the last glaciation. CORDILLERAN ICE SHEET Timing of the Fraser Glaciation In British Columbia, climatic deterioration associated with the Fraser glaciation began as early as ca. 29 ka (Clague, 1980), but extensive ice advance from mountains into valleys and fjords probably did not occur until sometime after ca. 25 ka (Fig. 2). In some areas of southern British Columbia glacier advance did not occur until after 17 ka. Following an extended period of ice buildup in interior valleys and over plateaus, the Cordilleran ice sheet attained its maximum expansion; the ice sheet was as wide as 900 km and had an upper surface above plateaus and valleys extending to 2000–3000 m asl (above present sea level) (e.g., Clague, 1989). Glacial

Downloaded from gsabulletin.gsapubs.org on September 15, 2015 MULTIPHASE FLOW OF THE LATE WISONSINAN CORDILLERAN ICE SHEET

Figure 1. Location map of central British Columbia. Thick dashed line delineates the study area. Thinner black solid lines define boundaries of six physiographic divisions of Holland (1976): Nechako Plateau, Hazelton Mountains, Skeena Mountains, Coast Mountains, and Omineca Mountains. Gray lines outline topography above 1000 m above sea level. The contour interval is 500 m.

maximum conditions were not achieved synchronously across the province. Along the central Pacific coast of British Columbia, the ice sheet reached its maximum extent between 16 and 15 ka (Blaise et al., 1990), whereas in southwest British Columbia and in northern Washington State glaciers reached their maximum limits between 14.5 and 14 ka (Hicock and Armstrong, 1985). Between 13 and 10 ka, glaciers retreated to early glacial positions in alpine areas and ice disappeared from fjords

and interior valleys. By 9.5 ka, the valleys and plateaus of British Columbia were ice free and glaciers were confined to alpine environments. Models of Cordilleran Ice Flow Previous workers used data from surficial geological studies to construct two main multiphase models of Fraser glaciation ice flow. One model implies that inception and growth of the Cordilleran ice sheet were sequential:

(1) outward expansion of alpine glaciers into valleys, (2) coalescence of alpine ice into piedmont glaciers and plateau ice sheets, and (3) further growth into a continental ice sheet comprising a single center of accumulation in the interior (Kerr, 1934; Davis and Mathews, 1944; Fulton, 1991; Ryder and Maynard, 1991). A second model argues for wide-scale glaciation by a series of mountain ice caps or domes separated by saddles (Tipper, 1971b; Clague, 1989).

Geological Society of America Bulletin, December 2000

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Downloaded from gsabulletin.gsapubs.org on September 15, 2015 STUMPF et al.

Figure 2. Time scale and summary of Quaternary events and deposits in British Columbia, southeast Alaska, and northern Washington State (compiled from Fulton and Smith, 1978; Clague, 1981; Blaise et al., 1990; Mann and Hamilton, 1995; Barrie and Conway, 1999; Easterbrook, 1992; Porter and Swanson, 1998).

BACKGROUND TO STUDY Location, Physiography, and Bedrock Geology The study area is within the central part of the Interior System of the Canadian Cordillera and comprises six physiographic divisions (Fig. 1). Much of this area is within the Nechako Plateau, which is characterized by low surface elevations ranging between 450 and 1500 m asl. An extensive blanket of glacial drift mantles bedrock, and well-developed flutings and drumlinoid ridges are dominant landform features. The Nechako Plateau is bordered to the west by the Hazelton and Coast mountains, to the north by the Skeena Mountains, and to the east by the Omineca Mountains (Fig. 1). These mountains contain both rugged peaks, with surface elevations to 2755 m asl, and lower rounded ridges at elevations between 1350 and 1900 m asl. Cirques, areˆtes, serrated peaks, and U-shaped valleys are common features of high mountains. On low mountains, discontinuous deposits of till overlie bedrock. The study area is underlain by rocks of the Coast Plutonic Complex, and the Stikine and

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Cache Creek terranes (Fig. 3). The Coast Plutonic Complex, underlying the western margin of the study area, consists of a series of midCretaceous to older plutons and associated high-grade metamorphic rocks. The Stikine terrane underlies the central part of the area and includes assemblages of Lower Devonian to Eocene volcanic, volcaniclastic, and plutonic rocks, and sedimentary strata (MacIntyre et al., 1998). Permian to Triassic rocks of the Cache Creek terrane underlie the eastern margin of the area; they consist of metasediments and metavolcanics, limestone, and chert units (Schiarizza and MacIntyre, 1999).

tional data on glacier flow were obtained from the orientation of grooves, flutings, and striae on stoss and lee bedrock surfaces. Although it has been suggested that some similar features are formed by subglacial water flow (e.g., Shaw, 1994), the majority of features identified in this study are parallel to, or associated with, welldeveloped striae formed by ice erosion. At sites with multiple ice-flow directions, crosscutting relationships were recorded, and the aspects of bedrock surfaces were measured to compare the relative preservation of glacial erosion features on inclined bedrock surfaces (e.g., Fig. 5). ICE-FLOW DATA

METHODS A large area of central British Columbia (nearly 35 000 km2) was traversed by truck, helicopter, and boat to provide as wide a geographic coverage as possible (Fig. 1). The directions of late Wisconsinan ice flow were deciphered from landforms such as rock drumlins, crag and tail landforms, and roches moutonneés identified on 1:15 000 and 1:63 500 stereo, color, and black and white aerial photographs. Interpretations made from the aerial photographs were verified at more than 200 sites (Fig. 4). Addi-

Babine and Takla Lake Valleys and Southern Skeena Mountains Valley-Parallel (East-Northeast–SouthSoutheast) Ice Flow In prominent southeast-trending valleys such as the Babine and Takla lake valleys, valley-parallel crag and tail landforms, drumlin features, and striae were formed when glaciers flowed east-southeastward from ice centers in the Skeena and Omineca Mountains (Tipper, 1994; Huntley et al., 1996a; Plouffe, 1997a,



Figure 3. Terranes and structural elements of central British Columbia (Gabrielse et al., 1992; CORDLINK, 1999). Gray lines outline major faults and structural lineaments. The thick dashed line delineates the area of study. 1997b, 1999; Levson et al., 1999). Directional indicators of ice flow in the Babine Lake valley record several different directions of eastsoutheasterly ice flow. Flow directions range from 1508 to 1808 in the northern part of the valley, 0908 to 1708 in central areas, and 0808 to 1908 in the southern part of the valley (Figs. 4 and 6A). Southeasterly ice flow in the Babine Lake valley is also recorded by the transport of weathered bedrock material down-ice (southeast) from possible source units. In the northcentral part of the valley, granodiorite and chert-pebble conglomerate clasts were found in till and glaciofluvial sediments, which are similar to lithologies in bedrock units mapped by MacIntyre et al. (1996, 1998) and McMillan (1995) to the northwest (sites 1 and 2, Fig. 7). At the Bell Mine (Fig. 1), mineralized volcanic and sedimentary rock clasts were transported southeastward from the main ore body and source bedrock units (Stumpf et al., 1997). Cross-Valley (West-Southwest) Ice Flow At more than 40 sites in the Babine and Takla lake valleys and in the southern Skeena

Mountains, directional indicators (rock drumlins, roches moutonneés, rat-tails, and striae) and the glacial transport of weathered bedrock material document cross-valley (west-southwestward) movement of glaciers. These features are best preserved in the Skeena Mountains and are rare in the southeast-trending Babine Lake valley. In the Skeena Mountains, indicators of the main direction of cross-valley ice flow, a westerly (2708) flow, are present to elevations of ;2200 m asl. The westerly ice flow trends obliquely across high ridges and mountains trending northwest-southeast. Locally, features of northwest-southwest (2308– 2908) flows were observed at high elevations in the Skeena Mountains north of the Babine River (e.g., site E, Fig. 4), and to the south on the sides of high mountain ridges and along major valleys aligned in an east-west direction. In the same area, glacial erratics of distinctive bedrock lithology were found west of source bedrock units, also supporting crossvalley ice flow in the area. For example, erratics of maroon Hazelton Group volcanic rocks identified on Mount Thomlinson were eroded and transported at least 55 km west-

ward of bedrock source units (site 3, Fig. 7). South of the Bulkley River valley, syenite clasts in till were observed to 25 km to the west of Eocene volcanic units containing similar rocks (site 4, Fig. 7). Crosscutting glacial erosional features at a few sites record glacier movements during at least two other events (sites A2D, Fig. 4). The state of preservation of these features on stoss and lee bedrock surfaces indicates a relative sequence of ice flow. For example, at Hearne Hill (Fig. 5), crosscutting striae and rat-tails on an unusual westward-directed roche moutonneé indicate that an early valleyparallel flow preceded both the westerly ice-flow event and a later valley-parallel (southeasterly) flow. On this outcrop, well-developed rat-tails and striae of early southeasterly flow are exposed on westward- and southwestward-facing bedrock surfaces. These southeast-directed features parallel large crag and tail landforms and most roches moutonneés in the area (Fig. 6A). Later westerly glacier flow rounded and eroded the outcrop, but did not totally obscure the features of earlier southeasterly ice flow on lee side (westwardfacing) surfaces. Fine, faint striae formed by later southeasterly ice flow are only preserved on the upper surfaces of the outcrop (Fig. 5). Indicators of the east-southeasterly flow events are also present on the same outcrop as cross-valley westward features, southeast of Dome Mountain (Fig. 8). Westward-directed rat-tails are preserved on the lee side (southeastward-facing surface) of a drumlinoid ridge formed by early valley-parallel flow. Later, weakly developed southeasterly striae (1308) crosscut the upper surface of the rat-tails. Directional indicators of multiple cross-valley (westerly and easterly) flow events are observed on opposing surfaces of the same bedrock outcrops in the Babine Lake valley and the southern Skeena Mountains. At Matzehtzel Mountain (site F, Fig. 4), features indicating westerly flow are present on northeastward- to eastward-facing surfaces (the stoss side to westerly flow), whereas indicators of east-northeasterly flow are preserved on the northwestward- and westward-facing bedrock surfaces (the lee side to westerly flow). Stosslee relationships along with crosscutting features suggest that the easterly ice flow preceded the westerly ice-flow event. Similarly, at Dome Mountain and in a U-shaped pass northeast of Smithers (sites G and H, Fig. 4), indicators of both westerly and easterly flow are present on opposing sides of bedrock ridges (westward- and eastward-facing surfaces).


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Figure 4. Ice-flow directions recorded in west-central British Columbia. The thick dashed outline delineates the area of study. Figures in the text are located in the map. The small inset map, lower right, is a simplified map of physiographic divisions (Coast Mountains [CM], Hazelton Mountains [HM], Nass Basin [N], Nechako Plateau [NP], Omineca Mountains [OM], and Skeena Mountains [SM]).

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east-northeasterly flow. Directional indicators of the west-southwesterly ice flow are present from the Hazelton Mountains eastward to the central part of the Ootsa Lake valley, but are less common in the eastern parts of the region (Figs. 1 and 4). Indicators of both west-southwesterly and east-northeasterly ice flows are exposed at single outcrops or in close proximity (e.g., sites L and M, Fig. 4). Crosscutting features at some sites indicate that the west-southwesterly flow preceded the eastnortheasterly flow. East of the central part of the Ootsa Lake valley, an extensively drumlinized and fluted landscape records the movement of glaciers toward the east-northeast (Tipper, 1971a, 1971b; Levson and Giles, 1997; Plouffe, 1997a, 1997b). This ice-flow direction is also indicated by the distribution of glacially transported material east and northeast of source bedrock units (Plouffe, 1995, 1999). Locally, some ice-flow features parallel major valleys, which are oblique in trend to the dominant (regional) westerly or easterly ice-flow directions. Areas Adjacent to Study Area

Central Nechako Plateau

Northern Skeena and Omineca Mountains The sequence of Fraser glaciation ice flow in the northern Skeena and Omineca Mountains is similar to ice flow identified in the study area (Fig. 10). During the early part of the last glaciation, glaciers expanded outward from alpine ice centers in the Skeena, Coast, and Omineca Mountains downslope along valleys (Ryder and Maynard, 1991). Ice-flow patterns early and late in the glaciation were complex; local shifts or reversals in flow occurred both when ice volumes changed near accumulation centers and where topography influenced glacier movement. At the climax of the Fraser glaciation, glaciers moved outward from an accumulation area (ne´ve´) over the northern Skeena Mountains (Ryder and Maynard, 1991). Several major ice streams flowed northeastward into Alberta, north-northwestward into the Yukon Territory, west-southwestward along coastal fjords to the Pacific Ocean, and east-southeastward through the Omineca Mountains onto the northern Nechako Plateau (Ryder and Maynard, 1991). At its maximum extent, the Cordilleran ice sheet flowed across ridges in the Skeena and Omineca Mountains at elevations to 2300 m asl (Armstrong and Tipper, 1948; Armstrong, 1949; Plouffe, 1997a).

Two main (regional) directions of ice flow have been identified on the central Nechako Plateau: a west-southwesterly flow and an

South of the Omineca Mountains To the south of the Omineca Mountains, the orientation of drumlinoid features, crag and

Figure 5. Chronology of late Wisconsinan ice flow at the Hearne Hill mineral property (Fig. 1; site A, Fig. 4) (lat 558119N, long 1268179W). Glacial erosional features exposed on a roche moutonneé on a valley-parallel ridge at an elevation of 1255 m above sea level record three phases of glacier movement. Features formed by the earliest ice-flow event are dominant in the area, and are only present at this site on westward- and southwestward-facing bedrock surfaces, which are on the lee side to later west-southwesterly ice flow. Westward-directed features of a second, later cross-valley flow event are present on east- and north-northeastward–facing bedrock surfaces. Westward-flowing ice did not remove or totally obscure earlier developed features on southwestward-facing surfaces. Features of the latest flow event (southeasterly flow) are weakly developed and occur only on the upper surfaces of outcrops.

Hazelton and Coast Mountains At more than 30 sites in the Hazelton and Coast Mountains, large-scale landforms (rock drumlins, crag and tail landforms, roches moutonneés) and small-scale glacial erosional features (rat-tails and striae) record multiple directions of westerly Fraser glaciation ice flow (sites I and J, Fig 4; Figs. 6, B and C, and 9). Erosional features of a major westsouthwesterly (2508–2708) flow are preserved throughout the Hazelton and Coast Mountains at elevations to 2440 m asl. Along the margin of some high mountains, and at lower elevations along valleys west of the Bulkley River, remnants of more variable ice flow are preserved. In these areas, ice flow varied between

southwestward and northwestward (2008– 3158). Numerous large glacial erratics (to 5 m in diameter) of distinctive lithology were identified in the Hazelton and Coast Mountains, west of source bedrock units, at elevations to 2070 m asl (sites 5 and 6, Fig. 7). For example, erratics of Triassic augite porphyry rocks were found in the Hazelton Mountains at least 125 km west of source bedrock units in the Babine Lake area (site 5, Fig. 7).


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Giles, 1997). Widespread crag and tail landforms, roches moutonneés, rat-tails, and striae on stoss and lee bedrock surfaces in the region record a major east-northeast flow event (Fig. 10). Locally, striae and other flow indicators oriented oblique to the regional flow direction suggest that glacier flow in some areas shifted to a more northerly direction and in other areas to a southeasterly direction. At the Fraser glacial maximum, glaciers covered most mountains below 2000 m asl (Tipper, 1971a). Remnants of a second, less extensive flow toward the west-southwest were identified by Tipper (1971a) and by the authors. Tipper (1971a) identified erratics of distinctive lithology (e.g., chert-pebble conglomerate) in the Coast Mountains, transported .90 km southwestward of source bedrock units (e.g., site 7, Fig. 7). Tipper (1971a) attributed evidence of this flow event to an earlier glaciation. We also identified southwestward flow features (rock drumlins and crag-and-tail landforms) on aerial photographs in the same area. These features are prominent, well-defined landforms exposed on mountains and along valley bottoms. DISCUSSION Phases of Fraser Glaciation Ice Flow

Figure 6. Photographs of ice-flow features in the study area: (A) low oblique aerial view of northwest-southeast–trending fluted bedrock ridges in the Babine Lake valley (lat 558099N, long 1268149W) and (B) a large west-southwesterly rat-tail present on a high mountain ridge west of Smithers (site J, Fig. 4) (548439N, 1278289W), and (C) elongated northeast-southwest–trending till ridges (to 6 m high) extending across low mountains at ;1675 m above sea level (site K, Fig. 4) (558589N, 1288009W). These ridges are aligned parallel to southwesterly ice-flow indicators in the region, and are oblique in trend to the direction of later valley-parallel ice flow. IF denotes ice-flow direction and VD is view direction. The locations of the photographs are denoted in Figure 4.

tail landforms, roches moutonneés, and striae (Figs. 10 and 11A) and the dispersal of glacially eroded material record a major eastnortheasterly ice-flow event (Armstrong and Tipper, 1948; Tipper, 1971a, 1971b; Gravel and Sibbick, 1991; Ryder and Maynard, 1991; Plouffe, 1991, 1992, 1995, 1997a, 1997b; West, 1997). Remnants of a later north-northeasterly ice-flow event were also recognized in the area, and likely occurred when ice flow-

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ing eastward from the Coast Mountains was deflected northeastward by northward-flowing glaciers from the Cariboo Mountains (Plouffe, 1997a, 1997b). Southern Nechako Plateau Two different directions of Cordilleran ice flow have been recognized on the southern Nechako Plateau (Duffell, 1959; Tipper, 1971a, 1971b; Ryder et al., 1991; Levson and

The record of glacier movement varies across the region and depends on the location, elevation, and aspect of the outcropping bedrock, and locally on the orientation of topographic features. Ridges and valleys had a significant influence on ice flow, especially where the ice sheet was thin or topographically confined, and consequently the sequence of ice flow is not the same at every location. Furthermore, striae from early events in many places have been obliterated by later events, favoring preservation only on the lee side of obstructions, down-glacier to later flow. It is only when ice movement is considered to have occurred in multiple phases, controlled by the orientation of confining topography and shifts in ice accumulation centers, that these variations can be explained. Glacier movement during the late Wisconsinan Fraser glaciation in central British Columbia is considered to have occurred in three main phases: (1) ice expansion phase, (2) maximum phase, and (3) late glacial phase. This multiphase model follows a similar framework for growth and decay of the Cordilleran ice sheet, as proposed by Kerr (1934), Davis and Mathews (1944), and Fulton (1991).



Figure 7. Directions of glacial transport of erratics of distinctive bedrock lithologies from source bedrock units (circled diamonds) in central British Columbia. Granodiorite (site 1) and chert-pebble conglomerate (site 2) boulders in till and glaciofluvial sediments in the Babine Lake valley were probably eroded from bedrock located to the northwest. At Mount Thomlinson (site 3), erratics of Hazelton Group maroon volcanic rocks have source bedrock units to the east. In the Houston area (site 4), syenite clasts were observed in till to the west of Eocene volcanic units containing similar rocks. Boulders of Triassic augite porphyry found on an unnamed ridge (site 5) are similar to augite porphyry in the Babine and Takla Lakes area. Tipper (1971a, 1994) reported the occurrence of maroon Hazelton Group pyroclastic rocks and chert-pebble conglomerate erratics in the Coast Mountains (sites 6 and 7, respectively), west of possible source areas. The location of site 6 is approximate. Geological mapping of source bedrock units is from Duffett and Owsiacki (1995), McMillan (1995), MacIntyre et al. (1996, 1998), Schiarizza (1998), and Schiarizza and MacIntyre (1999).

Ice Expansion Phase The ice expansion phase probably lasted late into the last glaciation, encompassing several subphases: the Alpine, Intense Alpine, and Mountain ice sheet phases of Kerr (1934), Davis and Mathews (1944), and Clague (1989). Initially, glaciers expanded outward from major high mountain accumulation centers along mountain valleys into interior and coastal areas. Glaciers flowed westward to the Pacific Ocean from the Coast and Skeena

Mountains, southeastward along the Babine, Takla, and Bulkley valleys from ice centers in the Skeena and Omineca Mountains, and eastnortheastward from the Hazelton and Coast Mountains (Fig. 12A). In most areas, the direction of ice flow was influenced by the orientation of major topographic features. In the interior, valley glaciers expanded to form an integrated system of piedmont or plateau ice sheets. Glaciers continued to flow along major valleys, and locally glaciers were

thick enough to flow across low (confining) ridges (Fig. 12B). Ice-flow data in the Skeena Mountains (e.g., Dome Mountain) suggest that late in this phase ice was thick enough to flow over low mountains and along passes at elevations to at least 1700 m asl. In major southeast-trending valleys, such as the Babine Lake valley, the preservation of erosional features at sites exposed to erosion or modification by later glacier flows (e.g., Hearne Hill) suggests that the ice expansion


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phase was the most erosive and protracted event, and later glacier flow was unable to significantly modify existing landforms or destroy the record of earlier (ice expansion) glacier flow. In contrast, indicators of the ice expansion phase are not present in valleys west of the Bulkley River (e.g., Morice, Telkwa, and Zymoetz river valleys and Trout Creek valley). The absence of these features suggests that later westerly flow overprinted or destroyed remnants of easterly ice expansion flow, which apparently occurred after a major reversal of flow late in this phase. During this reversal, ice flow shifted from eastward (flow direction 1, Fig. 12B) to westward (flow direction 2, Fig. 12B), probably as centers of ice accumulation moved to the east of the Hazelton and Coast Mountains. Westerly ice flow continued in these valleys well into the late glacial phase (see following).

Figure 8. Glacial erosional features of westerly and southwesterly maximum phase flow are preserved on the lee side of southeast-trending bedrock ridges below 1500 m above sea level in the Babine Lake valley (lat 548439N, long 1268259W). During the late glacial phase, southeastward-flowing glaciers eroded and striated bedrock surfaces, but did not remove evidence of earlier ice flow. The thick black line outlines the margin of a large bedrock ridge. The inset photo is a rat-tail formed by westward-flowing ice. The contour interval is 50 m.

Figure 9. Large 2858 trending rock drumlins on a mountain ridge southwest of Smithers at 1705 m above sea level (lat 548489N, long 1268259W). Province of British Columbia aerial photographs BCC 90073–85 and BCC 90073–86. IF denotes ice-flow direction and VD is view direction. The locations of these landforms are in Figure 4.

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Maximum Phase With continued expansion of glaciers, piedmont and plateau ice lobes coalesced, forming a single ice sheet, the Cordilleran ice sheet, during the maximum phase of glaciation. The maximum phase is correlative to the Continental ice sheet stage of Kerr (1934) and the fourth phase of Davis and Mathews (1944). During this phase, accumulation centers of the ice sheet shifted farther inland (Fig. 12C). Initially, ice flow was west-southwestward through the Skeena, Hazelton, and Coast Mountains along passes and over low ridges, and around major topographic barriers on high mountains. At the climax of the Fraser glaciation, the ice sheet attained a minimum surface elevation of .2500 m asl in the interior to overtop confining topography and flow upslope over high (at least 2440 m asl) mountains to the west. Outflow from the interior to the Pacific Ocean emanated from several centers of accumulation along an ice divide (Cordilleran ice divide) located east of the Babine Lake valley. The divide extended southward into the François and Ootsa Lakes area (Fig. 12C), and possibly northward over the northern Skeena Mountains (Ryder and Maynard, 1991). Glacier flow from these source areas was westward through fjords to the Pacific Ocean and eastward into the interior (Fig. 12C). During the maximum phase (Fig. 12D), a large area in central British Columbia to the west of the ice divide underwent a reversal in flow direction. The orientation of maximum phase flow indicators in this area is in some cases transverse or directly opposite to the directions of glacier advance (cf. Fig. 12, A and



Figure 10. Late Wisconsinan flow directions of the Cordilleran and Laurentide ice sheets, and the extent of proglacial lakes (dark gray shading) in central British Columbia and western Alberta (compiled from Fulton, 1995; Plouffe, 1997a, 1997b). At the glacial maximum, centers of accumulation of the Cordilleran ice sheet shifted to the east of the Hazelton Mountains and the Coast Mountains north of the Babine River and Bella Coola. Ice flowed westward from the Cordilleran ice divide (thick solid line) over major topographic barriers onto the British Columbian continental shelf and eastward across British Columbia, and past the Rocky Mountains into western Alberta. Glaciers advanced westward to the Pacific Ocean along the Nass River valley (A), the Skeena River valley–Dixon Entrance (B), the Kitsumkalum–Kitimat Trough (C), the Gardner Canal (D), and the Dean and Burke channels (E). Late Wisconsinan glaciers flowed eastward into western Alberta along mountain valleys and passes, including the Williston Lake–Peace River (F) and Fraser River– Yellowhead Pass–Athabasca River (G), and over low mountains (e.g., McGregor Plateau) (H). In the Jasper and Peace River areas, east-northeastward–flowing Cordilleran ice coalesced with southwestward-flowing Laurentide ice. The dashed line delineates the maximum eastern extent of Cordilleran ice. Labeled sites (triangles) denote the locations of aerial photographs shown in Figure 11. Arrows denote directional indicators of ice flow, and solid lines represent flutings where the direction of ice flow is unknown.

C) and of late glacial ice decay (see following late glacial phase discussion). The movements of this ice divide from high peaks in the Skeena, Hazelton, and Coast Mountains eastward over the interior, and locally back to high mountains were likely not constant along its length. Some portions of the divide were probably moving, while in other areas the divide remained stationary. Late Glacial Phase During the late glacial phase, climate amelioration triggered rapid downwasting and

thinning of the Cordilleran ice sheet. Centers of accumulation located in the interior began to shift back toward the Skeena, Omineca, Hazelton, and Coast Mountains (Fig. 12, E and F). However, the preservation of valley-parallel (westerly) ice-flow features in both westerly- and easterly-draining valleys, west of the Bulkley River and in the Tahtsa Lake valley (Fig. 4), suggests that ice centers persisted east of the Skeena, Hazelton, and Coast Mountains for much of this phase. These ice centers were probably located along the Babine and Bulkley valleys and in the François and Ootsa

Lakes area. Westerly ice flow would have continued along valleys in the Hazelton and Coast Mountains through low-elevation mountain passes open to the Pacific Ocean until the surface of the ice sheet dropped below the elevation of these mountain passes (,1525 m asl). At this time, glacier flow in many valleys would have ceased, or reversed direction into the interior. Widespread stagnation and rapid downwasting of ice lobes in valleys occurred when the equilibrium line rose above the elevation of the ice surface (Fulton, 1991). In the Babine and Takla lake valleys, re-


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Outlets for the Cordilleran Ice Sheet Throughout the Fraser glaciation, glaciers flowed from ice centers westward to the Pacific Ocean and eastward into eastern British Columbia and Alberta (Fig. 10). Along the Pacific coast, glaciers flowed westward and southwestward through a series of outlets: the Nass River valley (outlet A), the Skeena River valley–Dixon Entrance (outlet B), the southern part of the Kitsumkalum–Kitimat Trough (outlet C), the Gardner Canal (outlet D), and the Dean and Burke channels (outlet E)

Figure 11. Low-angle oblique aerial view of northeasterly to easterly ice-flow features in central British Columbia. (A) Crag-and-tail landforms and drumlinoid features in the Stuart River area (lat 548109N, long 1238509W), and (B) drumlinized terrain on the McGregor Plateau, east-central British Columbia (548459N, 1218509W) (see locations in Fig. 10). Province of British Columbia aerial photographs BC519–97 and BC761–71. IF denotes ice-flow direction and VD is view direction.

versal in ice flow from westward to eastward occurred as ice centers moved through the area (Fig. 12E). Initially, glaciers flowed across mountains and along passes, but as ice continued to thin, glacier flow shifted and ice flow was confined within valleys. Late glacial flow indicators that locally diverge from maximum phase flow directions probably reflect

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the influence of topography on ice movement. The preservation of ice expansion and maximum flow features—at low elevations and/or at sites unprotected from erosion by later southeasterly glacier flow (e.g., Hearne Hill, Fig. 5)—suggest that the erosional effects of the later topographically controlled ice flow must have been minimal.

Figure 12. Proposed multiphase model for ice expansion, maximum, and late glacial phases of Fraser glaciation ice flow in central British Columbia. At the onset of the glaciation during the ice expansion phase (A), ice centers (black asterisks) were located over high mountains in the Skeena, Omineca, Hazelton, and Coast Mountains. Continued glacier growth caused ice centers (gray asterisks) to shift east of the Hazelton and Coast Mountains (B). Ice flow in valleys west of the Bulkley River valley reversed from eastward (circled number 1) to westward (circled number 2). At the climax of the Fraser glaciation (maximum phase), ice centers shifted further into the interior (C). Glaciers flowed west-southwestward from major accumulation centers (large asterisks) along an interior ice divide. Smaller ice centers (small asterisks) probably continued to exist over high mountains in the Skeena, Hazelton, and Coast Mountains. Over a large part of central British Columbia (D), shifts in ice-flow direction occurred in the maximum phase; in some areas directions differed by as much as 1808 from flow during the ice expansion phase (shaded area). Rapid thinning of ice during the late glacial phase caused ice accumulation centers to shift back (westward) across the Babine and Ootsa Lakes area to high mountains (E and F). Early in the late glacial phase, the ice sheet was thick enough to flow across low mountains and ridges, but was forced to flow around higher mountains (E). With further drawdown of the ice sheet, glacier flow again became locally controlled by topography or stagnated (F). Arrows represent directions of ice flow. Asterisks with question marks denote the approximate locations of major and minor ice centers during the maximum and late glacial phases. Ice thicknesses in cross sections a, c, and e of the figure are schematic.


M



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(Clague, 1984, 1989; Hicock and Fuller, 1995; Barrie and Conway, 1999). At the glacial maximum, ice flowed through these outlets across the British Columbian continental shelf to the shelf edge (Josenhans et al., 1995) along a series of incised troughs (.400 m deep) (Luternauer and Murray, 1983). In eastern British Columbia and western Alberta, late Wisconsinan glaciers flowed eastward along mountain valleys and passes such as the Williston Lake–Peace River and Fraser River– Yellowhead Pass–Athabasca River (outlets F and G), and over low ridges and mountains in the Rocky Mountains (outlet H, e.g., Fig. 11B) (Rutter, 1977; Mathews, 1980; Catto et al., 1996; Levson and Rutter, 1996; Jackson et al., 1997). In the Peace River and Jasper areas, the orientations of glacial erosional features suggest that ice flowed over mountains to elevations of 2240 m asl. Cordilleran Ice Sheet: Flow Dynamics and Downwasting During the Fraser glaciation, subglacial and ice-marginal conditions would have been favorable for the rapid flow of ice. In the interior of British Columbia, rapid flow of the Cordilleran ice sheet was probably associated with glacier advance over deformable saturated sediments (cf. Patterson, 1998) and into proglacial water bodies (cf. Dredge and Cowan, 1989). Early in the glaciation, ice advancing into proglacial lakes, which developed along major valleys and low-lying areas of central British Columbia (e.g., Eyles and Clague, 1991; see Fig. 10), would have promoted extending flow conditions. Extending flow occurred as a result of a reduction in basal friction and periodic floating. Late in the glaciation, increased ablation along glacier margins and probable ice streaming along valleys in central British Columbia and Alberta would have contributed to rapid glacier drawdown. Draining of subglacial lakes may also have resulted in rapid thinning of the ice sheet (cf. Shoemaker, 1991). Along the Pacific coast, eustatic sea level dropped ;120 m (Fairbanks, 1989), exposing large areas of the British Columbian continental shelf. Glacier advance over saturated unlithified sediments on the shelf provided a soft deformable bed that likely facilitated rapid (ice stream) flow (Hicock and Fuller, 1995). High subglacial water pressures associated with the advance of thick ice could have promoted ice-bed separation (Brown et al., 1987), further increasing glacial flow velocities. Along the western margin of the Cordilleran ice sheet, rapidly rising sea levels late in the

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glaciation likely caused the ice margin to become buoyant, increasing calving and promoting ice streaming into coastal fjords, which contributed to a rapid lowering of the ice sheet (Clague, 1989; Ryder et al., 1991). SUMMARY AND CONCLUSIONS In central British Columbia, flow of the Cordilleran ice sheet during the Fraser glaciation occurred in three distinct phases: ice expansion, maximum, and late glacial. Directions of ice flow during the ice expansion phase, and to a lesser extent in the late glacial phase, were controlled predominantly by ice accumulation centers located over high mountains and by the orientations of major valleys and ridges. Glacier flow was generally westward to the Pacific Ocean and east-southeastward into the interior. Crosscutting ice-flow indicators and the preservation of features of earlier and later flows suggest that the ice expansion phase was the most erosive and protracted ice-flow event. Later glacier flow was unable to obscure or erode remnants of this phase. At the climax of glaciation (maximum phase), the ice sheet was sufficiently thick to flow unconfined by topography. Ice flow during this phase was outward from a source area in central British Columbia, the Cordilleran ice divide. Glaciers flowed westward onto the British Columbian continental shelf and eastward across British Columbia into western Alberta through a series of outlets. In a large area of central British Columbia, the direction of ice flow shifted and glaciers moved in directions transverse or opposite to valley-parallel flow. Features of the maximum phase are predominantly exposed on ridges and passes in the Skeena, Hazelton, and Coast Mountains, in valleys west of the Bulkley River, on the western part of the Nechako Plateau, and locally at low elevations along the Babine Lake valley. These ice-flow data indicate that westward-flowing glaciers were most erosive along valleys and passes parallel to ice flow and on mountains aligned transverse to ice flow, where compressive flow conditions developed (e.g., southern Skeena Mountains). In valleys west of the Bulkley River, erosion during an extended period of westerly ice flow was sufficient to remove or overprint ice expansion (eastward) features. However, in valleys trending obliquely to ice flow (e.g., Babine Lake valley) westerly flow was unable to significantly modify features formed by earlier glacier flow. The complex ice-flow record observed in some parts of the region resulted from shifts

in centers of ice accumulation along the divide, probably in response to regional and local changes in climate, increased precipitation or ablation of the ice sheet, and/or rapid calving of ice margins along the Pacific coast. Rapid glacier flow through ice marginal outlets was facilitated by advance over soft sediments and possibly high subglacial water pressures. Rising proglacial and subglacial water levels, increased calving, and ice streaming contributed to rapid thinning of the Cordilleran ice sheet late in the last glaciation. ACKNOWLEDGMENTS This project was funded by grants from the National Sciences and Engineering Research Council to Broster and Stumpf, the British Columbia Geological Survey to Levson, and the Nechako National Mapping Project (NATMAP) to Stumpf. This paper summarizes research undertaken by A. Stumpf as partial fulfillment of his doctoral study at the University of New Brunswick at Fredericton. We thank E. O’Brien, C. Churchill, A. Stuart, D. Meldrum, D. Huntley, G. Weary, and J. Hobday for assistance in the field, G. Klein for allowing us to publish his ice-flow data, and J. Attig, R. Fulton, J. Goodwin, A. Hansel, C. Patterson, A. Plouffe, and P. Stringer for reviewing and commenting on an earlier version of the paper. REFERENCES CITED Armstrong, J.E., 1949, Fort St. James map-area Cassiar and Coast District, British Columbia: Geological Survey of Canada Memoir 252, 210 p. Armstrong, J.E., and Tipper, H.W., 1948, Glaciation in north central British Columbia: American Journal of Science, v. 246, p. 283–310. Barrie, J.V., and Conway, K.M., 1999, Late Quaternary glaciation and postglacial stratigraphy of the Northern Pacific Margin of Canada: Quaternary Research, v. 51, p. 113–123. Blaise, B., Clague, J.J., and Mathewes, R.W., 1990, Time of maximum Late Wisconsin Glaciation, west coast of Canada: Quaternary Research, v. 34, p. 282–295. Brown, N.E., Hallet, B., and Booth, D.B., 1987, Rapid soft bed sliding of the Puget glacial lobe: Journal of Geophysical Research, v. 92, p. 8985–8997. Catto, N., Liverman, D.G.E., Bobrowsky, P.T., and Rutter, N.W., 1996, Laurentide, Cordilleran, and Montane glaciation in the western Peace River–Grand Prairie region, Alberta and British Columbia, Canada: Quaternary International, v. 32, p. 21–32. Clague, J.J., 1980, Late Quaternary geology and geochronology of British Columbia, Part 1: Radiocarbon dates: Geological Survey of Canada Paper 80–13, 28 p. Clague, J.J., 1981, Late Quaternary geology and geochronology of British Columbia, Part 2: Summary and discussion of radiocarbon-dated Quaternary history: Geological Survey of Canada Paper 80–35, 41 p. Clague, J.J., 1984, Quaternary geology and geomorphology, Smithers-Terrace-Prince Rupert area, British Columbia: Geological Survey of Canada Memoir 413, 71 p., Map 1557A, scale 1:100 000. Clague, J.J., 1989, Cordilleran Ice Sheet, in Fulton, R.J., ed., Quaternary geology of Canada and Greenland, Quaternary geology of the Canadian Cordillera: Geological Survey of Canada, Geology of Canada, v. 1, p. 40–42 (Geological Society of America, Geology of North America, v. K-1). Clague, J.J., 1991, Quaternary glaciation and sedimentation, in Gabrielse, H., and Yorath, C.J., eds., Geology of the Cordilleran orogen in Canada: Geological Sur-


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Geological Society of America Bulletin Multiphase flow of the late Wisconsinan Cordilleran ice sheet in western Canada Andrew J. Stumpf, Bruce E. Broster and Victor M. Levson Geological Society of America Bulletin 2000;112, no. 12;1850-1863 doi: 10.1130/0016-7606(2000)1122.0.CO;2

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