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Isostatic rebound in the northwestern part of the Lake Agassiz basin: Isobase changes and overflow
Palaeogeography, Palaeoclimatology, Palaeoecology 246 (2007) 23 – 30 www.elsevier.com/locate/palaeo
Isostatic rebound in the northwestern part of the Lake Agassiz basin: Isobase changes and overflow John A. Rayburn a , James T. Teller b,⁎ a b
Center for Earth and Environmental Sciences, SUNY Plattsburgh, Plattsburgh, New York, 12901, USA Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2 Received 10 January 2005; accepted 17 October 2006
Abstract Based on new GPS elevation data on the Upper and Lower Campbell strandlines in the northwestern part of the glacial Lake Agassiz basin, the trend of isobases representing differential glacio-isostatic rebound in that region is shown to bend and assume a nearly west–east orientation. This differs from the northwest–southeast orientation to the southeast of the study area, which others had projected into the northwestern corner of the Lake Agassiz basin; this means that there was more isostatic depression than previously thought north of ∼ 53°N. The difference in slopes on reconstructed water planes of the Upper and Lower Campbell beaches is less with these isobases, which better reflects the short period of time between their formation. It is likely that the change in orientation of isobases reflects the presence of a thick Keewatin ice center to the north. Our revised west–east isobase reconstruction indicates that when the lake was at the Campbell beach levels, ∼ 9900–9400 14C B.P., Lake Agassiz overflowed across the paleo-divide at Wycherley Lake, Saskatchewan, which controlled flow through the Northwestern Outlet of Lake Agassiz during part of its history; overflow would not have occurred through the Wycherley Lake channels at this time using the old isobase projections. © 2006 Elsevier B.V. All rights reserved. Keywords: Lake Agassiz; Isobases; Beaches; Northwestern outlet overflow
1. Introduction Glacial Lake Agassiz formed about 11,700 14C years B.P. when the Red River–Des Moines Ice Lobe retreated north of the Hudson Bay–Gulf of Mexico drainage divide (Upham, 1895; Fenton et al., 1983). As ice retreated, Lake Agassiz expanded from the United States (North Dakota, South Dakota, and Minnesota) into Canada (Manitoba, Ontario, and Saskatchewan) (Fig. 1). Differential isostatic rebound in the southern Lake Agassiz basin was described by Upham (1895) and others, and Johnston (1946) drew ⁎ Corresponding author. Tel.: +1 204 269 3851; fax: +1 204 474 7623. E-mail address: [email protected] (J.T. Teller). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.10.025
seven lines of equal isostatic rebound (i.e. isobases) through points of equal elevation on the rebounded strandlines between the southern outlet of the lake in South Dakota and about 53°N, near the town of Hudson Bay in east-central Saskatchewan. Along the southwestern side of the basin, these isobases have a NW–SE trend of approximately N 56°W (Fig. 1), showing that the direction of maximum isostatic rebound there (perpendicular to isobase trend) was toward the northeast. Teller and Thorleifson (1983) joined Johnston's isobases with isobase trends in the Great Lakes, and extended them across the region at 100 km spacing (Fig. 1); these isobases were then projected northward toward the Northwestern Outlet region from control
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Fig. 1. Total area covered by glacial Lake Agassiz south of the younger marine transgression of Hudson Bay, with isobases showing lines of equal post-glacial uplift spaced at 100 km (after Teller and Thorleifson, 1983, Fig. 1). Isobases are speculative outside of the southern Lake Agassiz basin and the Great Lakes. Location of Fig. 3 indicated by boxed area. NW = Northwestern Outlet overflow route; C–A = Clearwater–Athabasca River valley overflow route from Lake Agassiz to the Arctic Ocean; WW = Wapawekka Hills; PH = Pasquia Hills; DM = Duck Mountain; RM = Riding Mountain.
points to the south. It is important to note that Teller and Thorleifson (1983) stated that “isobases are speculative outside of the southern Lake Agassiz basin” (p. 264), and that virtually none of the isobases in the northwestern region were controlled by data points on beaches. Johnston's (1946) and Teller and Thorleifson's (1983) isobases in the northwestern region were controlled by elevations along the western shore only as far north as about Hudson Bay, Saskatchewan (53°N latitude), so isobases north of this were only projections. On the eastern side of the basin, trends were controlled by scattered shorelines only as far north as about 50°N latitude; a few islands in the southeastern part of the Lake Agassiz basin do provide controlling elevations on the strandlines that lie within the main outline of the lake. Previous attempts at strandline correlation in the northwestern corner of the basin have been uncertain around the 7th and 8th isobases shown in Fig. 1 (Fisher, 1993; Fisher and Smith, 1994). Thus, the details of the magnitude and configuration of isostatic rebound in the Lake Agassiz basin were poorly constrained across a
large area, and there was no control north of about the 6th isobase shown in Fig. 1. This paper (1) presents new elevation data and correlations for the Upper and Lower Campbell strandlines in the northwestern part of the Lake Agassiz basin, (2) re-configures the isobases in that region, based on the new strandline data and correlations, and (3) discusses the implications of the revised isobases on overflow from Lake Agassiz through the Northwestern Outlet of Lake Agassiz. 2. Methods Strandlines, deltas, and other shoreline features associated with the Upper Campbell and Lower Campbell levels of Lake Agassiz were identified along the northwestern margin of the basin from (1) published reports and maps, (2) aerial photographs, (3) 1:50,000 National Topographic System map sheets, and (4) field reconnaissance. These features were plotted on 1:50,000scale map sheets (25 ft or 10 m contour interval). Initially,
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elevations along these strandlines were taken from either published reports, benchmarks on the strandline, or were estimated from the 1:50,000 NTS map sheets (Rayburn, 1997). A field survey along the mapped Upper and Lower Campbell strandlines was done using a high-resolution (± 0.5 m) Global Positioning System (GPS) (Rayburn, 1997). The system used was an Ashtech Reliance submeter post-processing differential GPS with a claimed error of ± 20 cm plus 10 cm for every 100 km between the base and rover unit (1 ppm of baseline). Our survey extended from the southern end of Duck Mountain, Manitoba (∼ 51°N) to near La Ronge, Saskatchewan (Fig. 1), a distance of ∼500 km. Because of the limited time available for a single survey session, determined by the battery life in the rover unit (about 6 h), and the distance imposed by the baseline error, the study area was surveyed in 15 loops within 5 areas. Each area was centered on a base station with no points farther than 100 km from one of the 5 base receiver sites. Strandline survey loops incorporated as many Geodetic Survey of Canada benchmarks as possible to provide known references (see Rayburn, 1997). Data sites were accessed by road, track, and helicopter. The maximum instrument elevation error was estimated to be ± 0.4 m, determined by taking multiple readings at reference points in each loop. Maximum elevation error, from the Geodetic Survey benchmarks used as base references, was about ± 0.5 m based on comparison of all benchmark elevation data within the survey. Taken together, this gives a total potential measurement error of ± 0.9 m for a point. In determining the elevation of a former water plane, there is the potential for introducing another error, one that is based on selection of the morphological or stratigraphic entities that represent the actual water plane. To help minimize this problem, and to be consistent, we selected similar types of sites for each morpho-stratigraphic location as representatives of the paleo-water plane; specifically, the crest line on barrier beaches, the berm on beaches along the mainland, and the inflection point (break in slope) of a wave-eroded escarpment. 3. The Campbell strandlines 3.1. Introduction The Upper Campbell shoreline is the best developed of the more than 50 different strandlines in the glacial Lake Agassiz basin (Elson, 1967). In places it forms an offshore barrier beach composed of sand and gravel,
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which is as much as a kilometer wide and up to 8 m above the adjacent lacustrine deposits (see Fig. 2). In other areas this beach is a sand or gravel berm or erosional escarpment along the bounding mainland. These morpho-stratigraphic representatives of the lake at this stage are found discontinuously along the western margin of the lake from its southern outlet in South Dakota as far north as the Wapawekka Hills in Saskatchewan (Fig. 1), a distance of N1000 km. Along the eastern side of the lake, as far north as about 50°N (just north of the U.S.– Canada border), the Upper Campbell strandline is also well developed, and islands are outlined by this beach in some areas in northwest Ontario. Recent work by Fisher (2005) in the southern end of the Lake Agassiz basin has provided insight into the relationship of beaches at and above the Campbell strandlines. The Lower Campbell shoreline is also well developed in many areas and is extensive. It, too, is represented by barrier beaches, berms, and erosional escarpments, and is discontinuous. The Lower Campbell beach lies only 1.5 m below the Upper Campbell beach at the southern outlet of the lake in South Dakota (Fisher, 2005), but their vertical separation increases northward (e.g. Upham, 1895) because of differential isostatic rebound during the interval of time between the formation of these two shorelines. The Campbell strandlines were first mapped by Upham (1887, 1890, 1895) in North Dakota, South Dakota, Minnesota, and in southern Manitoba as far north as Riding Mountain; Tyrrell (1889) mapped these beaches along the escarpment of Riding Mountain and north to Duck Mountain (∼52°N) (Fig. 1). The name Campbell beach was given because it passes through the town of Campbell, Minnesota, near the southern end of the Lake Agassiz basin. Leverett (1932) mapped these strandlines in greater detail in Minnesota. Johnston (1946) studied the strandlines as far north as ∼53°N at the edge of the Pasquia Hills in Saskatchewan, and Elson (1967) discussed their relationship to other features in the basin. Some of the details of the Campbell strandlines have been mapped in Saskatchewan by Langford (1973, 1977), Christiansen (1979), Schreiner (1983, 1984), and others. Matile and Thorleifson (1996) have mapped the southeastern region of Manitoba, including the beaches of Lake Agassiz; Klassen (1979) mapped the beaches in the Riding Mountain and Duck Mountain areas of Manitoba. Lake Agassiz beaches were mapped by Nielsen et al. (1984) in the Swan River, Manitoba area, by Moran (1969) in the Hudson Bay, Saskatchewan area, and by Christiansen et al. (1995) in the Nipawin delta area of eastern Saskatchewan; see summary in Rayburn (1997). Other published sources of strandline information used
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Fig. 2. Aerial photo of Lake Agassiz beaches near Arden, Manitoba, southeast of Riding Mountain. Lower elevation (younger) beaches lie to the east (right).
include: Manitoba Soil Survey reports and maps (1:126,720 scale), Saskatchewan Soil Survey reports and maps (1:126,720), Saskatchewan Research Council Aggregate Resource Potential and Quaternary Geology maps (1:250,000 scale), and Manitoba Energy and Mines Aggregate Resources Compilation maps (1:250,000 scale). 3.2. GPS data and isobase contours Sixty-one new GPS data points were collected on the Upper and Lower Campbell strandlines in Saskatchewan and western Manitoba (Fig. 3). In many areas as far north as the Pasquia Hills, both the Upper and Lower Campbell shorelines are present and distinct; Johnston (1946)
mapped these beaches and measured their elevations. For more than 150 km northwestward to the Wapawekka Hills, however, these shorelines are less distinct and more discontinuous (Fig. 3), where the Upper Campbell strandline is mainly a wave-cut escarpment; the Lower Campbell strandline there exists as a sand spit below it. To the west of the Wapawekka Hills, there is only a single well-developed barrier beach (Fig. 3). Schreiner (1983) concluded that the well-preserved strandline south and west of La Ronge, Saskatchewan, was “probably below the main Campbell level”. Our aerial photograph studies and field reconnaissance of beaches in this region support this conclusion, and show that this beach extends nearly continuously northwest to ∼50 km west of La Ronge at Gregory Lake. West of this there are only discontinuous
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Fig. 3. New GPS elevations (m) on the Upper and Lower Campbell strandlines along the western shoreline of Lake Agassiz, showing revised isobases on both strandlines that bend westward from the N 56°W orientation projected by Teller and Thorleifson (1983) and shown in Fig. 1. Note how these new isobases parallel previous reconstructions (short dashed lines, ∼5th and 7th isobases of Fig. 1) south of the Pasquia Hills, and how independently-controlled isobases on the Upper and Lower Campbell strandlines parallel one another.
strandline fragments mapped by Fisher (1993), with uncertain relationships to the Campbell beaches. New elevation data on both the Upper and Lower Campbell strandlines were contoured to produce isobases at 10 m intervals between Duck Mountain, Manitoba, and the La Ronge, Saskatchewan, area (Fig. 3). The beach correlation and elevation data result in a change in orientation of isobases in the region north of ∼53°N (near the town of Hudson Bay) (Fig. 3), which is at the northern end of the shoreline elevations used by Johnston (1946) to control his isobases and also used to guide the
northward projection of isobases of the map by Teller and Thorleifson (1983) (Fig. 1). Our new elevation data show that there is a progressive shift from an orientation of N 56°W to an almost W–E orientation along the Wapawekka Hills. Isobase orientation to the south of about 53°N parallels that of Johnston (1946) and that shown in Fig. 1; our new GPS elevation values in that area are very similar to those shown by Johnston (1946), rarely varying by more than a meter (see Rayburn, 1997). The slope on the rebounded beaches, based on the revised isobase lines of Fig. 3, increases from southwest
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to northeast from ∼ 0.30 to 0.41 on the Upper Campbell beach and from 0.30 to 0.37 on the Lower Campbell. This compares to a slope between the previously drawn 5th and 7th isobases of Teller and Thorleifson (1983) shown in Fig. 1 of ∼0.42 to 0.50 (Upper Campbell) and ∼ 0.37 to 0.50 (Lower Campbell). Thus, the gradient of the isobases is more gentle in our revised reconstruction in the region between Duck Mountain and the Wapawekka Hills than in the old reconstruction in that area between isobases 5 and 7 of Fig. 1. Our isobase reconstructions indicate that the slopes on both the Upper and Lower Campbell strandlines diverge only slightly across the mapped area (see Fig. 3); separation of these two strandlines typically is 15–20 m except in the northern area where values exceed 20 m, and decrease southward to ∼ 12 m at the northern end of Riding Mountain. This small divergence is compatible with the conclusion that these two beaches were both formed within a few centuries or so of each other (e.g. Nielsen et al., 1984; Teller et al., 2000; Fisher, 2003; Teller and Leverington, 2004). However, because these revised isobases are oriented west to east, rather than N 56°W, total crustal depression in the Northwestern Outlet region of Lake Agassiz was greater in the new reconstruction (Fig. 3) than in the previous one (Fig. 1). 4. Discussion The revised isobase trends of the Upper and Lower Campbell strandlines better reflect the late-glacial geometry and thickness of the Laurentide Ice Sheet (LIS). All ice sheet reconstructions in recent years have shown a thick Keewatin ice center located ∼ 1000 km to the north of this region (e.g. Shilts et al., 1979; Dyke and Prest, 1987; Hughes, 1987; Licciardi et al., 1998); new models of Tarasov and Peltier (2004, 2006) increase the thickness of ice in the Keewatin ice dome over previous reconstructions. Thus, a westward inflection in isobase trend in the northern part of the Lake Agassiz basin would be the expected result of the presence of a thick Keewatin ice center. The revision of isobases in the northwestern part of the Lake Agassiz basin to a more W–E orientation means that isostatic depression in the region where Lake Agassiz overflowed into the Clearwater–Athabasca River system (Fig. 1) would have been greater than that resulting from the isobase projections of Teller and Thorleifson (1983). Although northwestward overflow from Lake Agassiz was carried by a complex network of channels and lake basins during the time when the Upper and Lower Campbell beaches formed, the paleoelevation of the Wycherley channel area, ∼ 200 km
northwest of the Wapawekka Hills, restricted outflow into the Clearwater–Athabasca system (Fisher and Souch, 1998; Fisher, 2007-this issue). This threshold and its history are discussed in Fisher and Souch (1998) and Fisher (2007-this issue); the geographic relationship of this region to the Wapawekka Hills can be seen in Fig. 1 of Fisher (2007-this issue). The modern elevation of the Wycherley Lake paleohydrological threshold is 429 m (Fisher and Souch, 1998). When the Upper Campbell strandline is projected to the isobase through that lake – using our revised isobases, and assuming that the W–E orientation and the same slope continues northwestward – the Upper Campbell water plane lies at 470 m, 41 m above the Wycherley channel threshold elevation. When the Lower Campbell strandline is projected northwestward to this threshold, its water plane lies at 456 m, 27 m above Wycherley Lake. In the previous N 56°W isobase projections of Fig. 1, the Wycherley Lake control elevation lies roughly on the 7th isobase line; this means that the Upper Campbell water plane would lie at ∼ 415 m in elevation, 14 m below the Wycherley Lake overflow threshold. The Lower Campbell strandline would lie nearly 30 m below that northwestern overflow elevation. Thus, it is clear that, if overflow did occur through the Northwestern Outlet during the Upper or Lower Campbell lake stages, isobase orientation must have been more similar to our W–E reconstruction than to the previous N 56°W projections into the outlet region. Overflow through the Northwestern Outlet would have occurred when water was at the Upper and Lower Campbell beach levels only if that outlet was no longer blocked by the LIS. Smith and Fisher (1993) and Fisher and Smith (1994) concluded that the outlet region was deglaciated by ∼ 9900 14C B.P., and most researchers since then have concluded similarly. From the time the Upper Campbell strandline formed ∼ 9900 14C B.P. until the time the Lower Campbell beach formed ∼ 9400 14C B.P. (e.g. Teller et al., 2000), northwesterly overflow from Lake Agassiz was continuous (Fisher and Smith, 1994) or nearly continuous (Teller, 2001). An earlier episode of overflow through the Northwestern Outlet, between ∼ 11,000 and 10,000 14C B.P. was suggested by Teller et al. (2005, p. 1900–1901) and Teller and Boyd (2006), separated from the 9900– 9400 14C B.P. overflow by a brief re-advance of the LIS at ∼ 10,000 14C B.P. that closed that route; more research is needed to test this hypothesis. After 9400 14C B.P., northwestern overflow was re-routed when the lake fell below the Lower Campbell level, because either rebound closed that route and forced the lake to
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overflow south into the Mississippi River, and/or because a lower route into the Great Lakes west of Lake Nipigon was deglaciated (cf. Fisher, 2003; Teller and Leverington, 2004). The general absence of older (higher) strandlines in the study area north of Riding Mountain (i.e. the Herman, Norcross, and Tintah beaches), except for scattered small beaches of uncertain affinity in the area west of our study (Fisher and Smith, 1994), suggests that (1) the region was not deglaciated until after those older beaches had formed elsewhere in the basin or that (2) those older beaches were largely destroyed when the LIS re-advanced as far south as Riding Mountain ∼ 10,000 14C B.P. (Thorleifson, 1996; Teller, 2001; Teller and Leverington, 2004); Rayburn (1997) mapped an end moraine along the northern side of the Wapawekka Hills, which may be related to this readvance. The apparent absence of distinct Upper Campbell beaches west of La Ronge is probably related to (1) a limited supply of unconsolidated sediment to waves in this mainly Precambrian bedrock area and (2) the fact that the largest beaches of Lake Agassiz were transgressive and did not form in the region where lake overflow was occurring (Teller, 2001), so poor beaches formed when water was at the Upper and Lower Campbell levels in the northwestern part of the basin (see Teller, 2001, Fig. 3C). The greater isostatic depression of the Northwestern Outlet region, indicated by our revised isobases, would have resulted in a greater depth of water dammed against the LIS. This greater “head” would have resulted in a larger drawdown of the lake when overflow breached the ice dam, with the potential for a larger (more “catastrophic”) outflow. 5. Conclusions South of ∼53°N, along the western side of Lake Agassiz, new elevation data on the Upper and Lower Campbell strandlines show a good fit to the regional isobases of Teller and Thorleifson (1983) shown in Fig. 1. However, to the north and west of about the Pasquia Hills at 53°N, GPS elevation data indicate that isobases bend to a more west–east orientation (Fig. 3), from the N 56°W orientation projected into that region by Teller and Thorleifson (1983) (Fig. 1). These revised isobases, when projected northwestward from the study area, indicate that water planes associated with the Upper Campbell and Lower Campbell beaches lie above channels at Wycherley Lake, where Lake Agassiz overflowed into the Clearwater–Athabasca system. This means that, if the Northwestern Outlet of Lake Agassiz had been deglaciated by the time the Upper and Lower
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Campbell strandlines formed, overflow to the Arctic Ocean would have occurred until after the lake fell below the Lower Campbell level; this northwesterly overflow began ∼9900 14C B.P. (Fisher, 2007-this issue), perhaps earlier (Teller et al., 2005), and continued until after 9400 14 C B.P. In contrast, when the N 56°W isobases of Teller and Thorleifson (1983) are extended to the Wycherley Lake overflow threshold, both the Upper and Lower Campbell water planes fall below the elevation of the Northwestern Outlet of Lake Agassiz, so northwestern overflow could not have occurred at that time. Our new isobases also result in a smaller difference in slope between the differentially rebounded Upper and Lower Campbell beaches in this region than do the isobases of Teller and Thorleifson (1983), which better reflects the known small difference in age between these two beaches. We conclude that the bend in isobase orientation in this region reflects greater crustal depression associated with the presence a thick Keewatin ice center to the north. Our new reconstruction produces deeper water in the northwestern part of Lake Agassiz, and would have resulted in a greater drawdown of lake level when the Northwestern Outlet was deglaciated. Acknowledgements Support for this research came from the Natural Sciences and Engineering Research Council of Canada (NSERC) and from the Geological Survey of Canada. Thanks to L. H. Thorleifson (Minnesota Geological Survey) and G. Matile (Manitoba Geological Survey) for facilitating the GPS survey. Discussions with J. Mann, T. Fisher, H. Thorleifson, and, especially, Mike Lewis were helpful in developing our ideas. Jason Mann helped with the GPS surveying. Reviews of an earlier version of this paper by J. Andrews, A. Kehew, and M. Lewis were helpful. Many thanks to A. Patterson for preparing the figures. References Christiansen, E.A., 1979. The Wisconsinan deglaciation of southern Saskatchewan and adjacent areas. Canadian Journal of Earth Sciences 16, 913–938. Christiansen, E.A., Sauer, E.K., Schreiner, B.T., 1995. Glacial Lake Saskatchewan and Lake Agassiz deltas in east-central Saskatchewan with special emphasis on the Nipawin Delta. Canadian Journal of Earth Sciences 32, 334–348. Dyke, A.S., Prest, V.K., 1987. Late Wisconsinan and Holocene history of the Laurentide Ice Sheet. Géographie Physique et Quaternaire 41, 237–263. Elson, J.A., 1967. Geology of Glacial Lake Agassiz. In: Mayer-Oakes, W.J. (Ed.), Life Land and Water. University of Manitoba Press, Winnipeg, pp. 37–95.
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Isostatic rebound in the northwestern part of the Lake Agassiz basin: Isobase changes and overflow John A. Rayburn a , James T. Teller b,⁎ a b
Center for Earth and Environmental Sciences, SUNY Plattsburgh, Plattsburgh, New York, 12901, USA Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2 Received 10 January 2005; accepted 17 October 2006
Abstract Based on new GPS elevation data on the Upper and Lower Campbell strandlines in the northwestern part of the glacial Lake Agassiz basin, the trend of isobases representing differential glacio-isostatic rebound in that region is shown to bend and assume a nearly west–east orientation. This differs from the northwest–southeast orientation to the southeast of the study area, which others had projected into the northwestern corner of the Lake Agassiz basin; this means that there was more isostatic depression than previously thought north of ∼ 53°N. The difference in slopes on reconstructed water planes of the Upper and Lower Campbell beaches is less with these isobases, which better reflects the short period of time between their formation. It is likely that the change in orientation of isobases reflects the presence of a thick Keewatin ice center to the north. Our revised west–east isobase reconstruction indicates that when the lake was at the Campbell beach levels, ∼ 9900–9400 14C B.P., Lake Agassiz overflowed across the paleo-divide at Wycherley Lake, Saskatchewan, which controlled flow through the Northwestern Outlet of Lake Agassiz during part of its history; overflow would not have occurred through the Wycherley Lake channels at this time using the old isobase projections. © 2006 Elsevier B.V. All rights reserved. Keywords: Lake Agassiz; Isobases; Beaches; Northwestern outlet overflow
1. Introduction Glacial Lake Agassiz formed about 11,700 14C years B.P. when the Red River–Des Moines Ice Lobe retreated north of the Hudson Bay–Gulf of Mexico drainage divide (Upham, 1895; Fenton et al., 1983). As ice retreated, Lake Agassiz expanded from the United States (North Dakota, South Dakota, and Minnesota) into Canada (Manitoba, Ontario, and Saskatchewan) (Fig. 1). Differential isostatic rebound in the southern Lake Agassiz basin was described by Upham (1895) and others, and Johnston (1946) drew ⁎ Corresponding author. Tel.: +1 204 269 3851; fax: +1 204 474 7623. E-mail address: [email protected] (J.T. Teller). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.10.025
seven lines of equal isostatic rebound (i.e. isobases) through points of equal elevation on the rebounded strandlines between the southern outlet of the lake in South Dakota and about 53°N, near the town of Hudson Bay in east-central Saskatchewan. Along the southwestern side of the basin, these isobases have a NW–SE trend of approximately N 56°W (Fig. 1), showing that the direction of maximum isostatic rebound there (perpendicular to isobase trend) was toward the northeast. Teller and Thorleifson (1983) joined Johnston's isobases with isobase trends in the Great Lakes, and extended them across the region at 100 km spacing (Fig. 1); these isobases were then projected northward toward the Northwestern Outlet region from control
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Fig. 1. Total area covered by glacial Lake Agassiz south of the younger marine transgression of Hudson Bay, with isobases showing lines of equal post-glacial uplift spaced at 100 km (after Teller and Thorleifson, 1983, Fig. 1). Isobases are speculative outside of the southern Lake Agassiz basin and the Great Lakes. Location of Fig. 3 indicated by boxed area. NW = Northwestern Outlet overflow route; C–A = Clearwater–Athabasca River valley overflow route from Lake Agassiz to the Arctic Ocean; WW = Wapawekka Hills; PH = Pasquia Hills; DM = Duck Mountain; RM = Riding Mountain.
points to the south. It is important to note that Teller and Thorleifson (1983) stated that “isobases are speculative outside of the southern Lake Agassiz basin” (p. 264), and that virtually none of the isobases in the northwestern region were controlled by data points on beaches. Johnston's (1946) and Teller and Thorleifson's (1983) isobases in the northwestern region were controlled by elevations along the western shore only as far north as about Hudson Bay, Saskatchewan (53°N latitude), so isobases north of this were only projections. On the eastern side of the basin, trends were controlled by scattered shorelines only as far north as about 50°N latitude; a few islands in the southeastern part of the Lake Agassiz basin do provide controlling elevations on the strandlines that lie within the main outline of the lake. Previous attempts at strandline correlation in the northwestern corner of the basin have been uncertain around the 7th and 8th isobases shown in Fig. 1 (Fisher, 1993; Fisher and Smith, 1994). Thus, the details of the magnitude and configuration of isostatic rebound in the Lake Agassiz basin were poorly constrained across a
large area, and there was no control north of about the 6th isobase shown in Fig. 1. This paper (1) presents new elevation data and correlations for the Upper and Lower Campbell strandlines in the northwestern part of the Lake Agassiz basin, (2) re-configures the isobases in that region, based on the new strandline data and correlations, and (3) discusses the implications of the revised isobases on overflow from Lake Agassiz through the Northwestern Outlet of Lake Agassiz. 2. Methods Strandlines, deltas, and other shoreline features associated with the Upper Campbell and Lower Campbell levels of Lake Agassiz were identified along the northwestern margin of the basin from (1) published reports and maps, (2) aerial photographs, (3) 1:50,000 National Topographic System map sheets, and (4) field reconnaissance. These features were plotted on 1:50,000scale map sheets (25 ft or 10 m contour interval). Initially,
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elevations along these strandlines were taken from either published reports, benchmarks on the strandline, or were estimated from the 1:50,000 NTS map sheets (Rayburn, 1997). A field survey along the mapped Upper and Lower Campbell strandlines was done using a high-resolution (± 0.5 m) Global Positioning System (GPS) (Rayburn, 1997). The system used was an Ashtech Reliance submeter post-processing differential GPS with a claimed error of ± 20 cm plus 10 cm for every 100 km between the base and rover unit (1 ppm of baseline). Our survey extended from the southern end of Duck Mountain, Manitoba (∼ 51°N) to near La Ronge, Saskatchewan (Fig. 1), a distance of ∼500 km. Because of the limited time available for a single survey session, determined by the battery life in the rover unit (about 6 h), and the distance imposed by the baseline error, the study area was surveyed in 15 loops within 5 areas. Each area was centered on a base station with no points farther than 100 km from one of the 5 base receiver sites. Strandline survey loops incorporated as many Geodetic Survey of Canada benchmarks as possible to provide known references (see Rayburn, 1997). Data sites were accessed by road, track, and helicopter. The maximum instrument elevation error was estimated to be ± 0.4 m, determined by taking multiple readings at reference points in each loop. Maximum elevation error, from the Geodetic Survey benchmarks used as base references, was about ± 0.5 m based on comparison of all benchmark elevation data within the survey. Taken together, this gives a total potential measurement error of ± 0.9 m for a point. In determining the elevation of a former water plane, there is the potential for introducing another error, one that is based on selection of the morphological or stratigraphic entities that represent the actual water plane. To help minimize this problem, and to be consistent, we selected similar types of sites for each morpho-stratigraphic location as representatives of the paleo-water plane; specifically, the crest line on barrier beaches, the berm on beaches along the mainland, and the inflection point (break in slope) of a wave-eroded escarpment. 3. The Campbell strandlines 3.1. Introduction The Upper Campbell shoreline is the best developed of the more than 50 different strandlines in the glacial Lake Agassiz basin (Elson, 1967). In places it forms an offshore barrier beach composed of sand and gravel,
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which is as much as a kilometer wide and up to 8 m above the adjacent lacustrine deposits (see Fig. 2). In other areas this beach is a sand or gravel berm or erosional escarpment along the bounding mainland. These morpho-stratigraphic representatives of the lake at this stage are found discontinuously along the western margin of the lake from its southern outlet in South Dakota as far north as the Wapawekka Hills in Saskatchewan (Fig. 1), a distance of N1000 km. Along the eastern side of the lake, as far north as about 50°N (just north of the U.S.– Canada border), the Upper Campbell strandline is also well developed, and islands are outlined by this beach in some areas in northwest Ontario. Recent work by Fisher (2005) in the southern end of the Lake Agassiz basin has provided insight into the relationship of beaches at and above the Campbell strandlines. The Lower Campbell shoreline is also well developed in many areas and is extensive. It, too, is represented by barrier beaches, berms, and erosional escarpments, and is discontinuous. The Lower Campbell beach lies only 1.5 m below the Upper Campbell beach at the southern outlet of the lake in South Dakota (Fisher, 2005), but their vertical separation increases northward (e.g. Upham, 1895) because of differential isostatic rebound during the interval of time between the formation of these two shorelines. The Campbell strandlines were first mapped by Upham (1887, 1890, 1895) in North Dakota, South Dakota, Minnesota, and in southern Manitoba as far north as Riding Mountain; Tyrrell (1889) mapped these beaches along the escarpment of Riding Mountain and north to Duck Mountain (∼52°N) (Fig. 1). The name Campbell beach was given because it passes through the town of Campbell, Minnesota, near the southern end of the Lake Agassiz basin. Leverett (1932) mapped these strandlines in greater detail in Minnesota. Johnston (1946) studied the strandlines as far north as ∼53°N at the edge of the Pasquia Hills in Saskatchewan, and Elson (1967) discussed their relationship to other features in the basin. Some of the details of the Campbell strandlines have been mapped in Saskatchewan by Langford (1973, 1977), Christiansen (1979), Schreiner (1983, 1984), and others. Matile and Thorleifson (1996) have mapped the southeastern region of Manitoba, including the beaches of Lake Agassiz; Klassen (1979) mapped the beaches in the Riding Mountain and Duck Mountain areas of Manitoba. Lake Agassiz beaches were mapped by Nielsen et al. (1984) in the Swan River, Manitoba area, by Moran (1969) in the Hudson Bay, Saskatchewan area, and by Christiansen et al. (1995) in the Nipawin delta area of eastern Saskatchewan; see summary in Rayburn (1997). Other published sources of strandline information used
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Fig. 2. Aerial photo of Lake Agassiz beaches near Arden, Manitoba, southeast of Riding Mountain. Lower elevation (younger) beaches lie to the east (right).
include: Manitoba Soil Survey reports and maps (1:126,720 scale), Saskatchewan Soil Survey reports and maps (1:126,720), Saskatchewan Research Council Aggregate Resource Potential and Quaternary Geology maps (1:250,000 scale), and Manitoba Energy and Mines Aggregate Resources Compilation maps (1:250,000 scale). 3.2. GPS data and isobase contours Sixty-one new GPS data points were collected on the Upper and Lower Campbell strandlines in Saskatchewan and western Manitoba (Fig. 3). In many areas as far north as the Pasquia Hills, both the Upper and Lower Campbell shorelines are present and distinct; Johnston (1946)
mapped these beaches and measured their elevations. For more than 150 km northwestward to the Wapawekka Hills, however, these shorelines are less distinct and more discontinuous (Fig. 3), where the Upper Campbell strandline is mainly a wave-cut escarpment; the Lower Campbell strandline there exists as a sand spit below it. To the west of the Wapawekka Hills, there is only a single well-developed barrier beach (Fig. 3). Schreiner (1983) concluded that the well-preserved strandline south and west of La Ronge, Saskatchewan, was “probably below the main Campbell level”. Our aerial photograph studies and field reconnaissance of beaches in this region support this conclusion, and show that this beach extends nearly continuously northwest to ∼50 km west of La Ronge at Gregory Lake. West of this there are only discontinuous
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Fig. 3. New GPS elevations (m) on the Upper and Lower Campbell strandlines along the western shoreline of Lake Agassiz, showing revised isobases on both strandlines that bend westward from the N 56°W orientation projected by Teller and Thorleifson (1983) and shown in Fig. 1. Note how these new isobases parallel previous reconstructions (short dashed lines, ∼5th and 7th isobases of Fig. 1) south of the Pasquia Hills, and how independently-controlled isobases on the Upper and Lower Campbell strandlines parallel one another.
strandline fragments mapped by Fisher (1993), with uncertain relationships to the Campbell beaches. New elevation data on both the Upper and Lower Campbell strandlines were contoured to produce isobases at 10 m intervals between Duck Mountain, Manitoba, and the La Ronge, Saskatchewan, area (Fig. 3). The beach correlation and elevation data result in a change in orientation of isobases in the region north of ∼53°N (near the town of Hudson Bay) (Fig. 3), which is at the northern end of the shoreline elevations used by Johnston (1946) to control his isobases and also used to guide the
northward projection of isobases of the map by Teller and Thorleifson (1983) (Fig. 1). Our new elevation data show that there is a progressive shift from an orientation of N 56°W to an almost W–E orientation along the Wapawekka Hills. Isobase orientation to the south of about 53°N parallels that of Johnston (1946) and that shown in Fig. 1; our new GPS elevation values in that area are very similar to those shown by Johnston (1946), rarely varying by more than a meter (see Rayburn, 1997). The slope on the rebounded beaches, based on the revised isobase lines of Fig. 3, increases from southwest
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to northeast from ∼ 0.30 to 0.41 on the Upper Campbell beach and from 0.30 to 0.37 on the Lower Campbell. This compares to a slope between the previously drawn 5th and 7th isobases of Teller and Thorleifson (1983) shown in Fig. 1 of ∼0.42 to 0.50 (Upper Campbell) and ∼ 0.37 to 0.50 (Lower Campbell). Thus, the gradient of the isobases is more gentle in our revised reconstruction in the region between Duck Mountain and the Wapawekka Hills than in the old reconstruction in that area between isobases 5 and 7 of Fig. 1. Our isobase reconstructions indicate that the slopes on both the Upper and Lower Campbell strandlines diverge only slightly across the mapped area (see Fig. 3); separation of these two strandlines typically is 15–20 m except in the northern area where values exceed 20 m, and decrease southward to ∼ 12 m at the northern end of Riding Mountain. This small divergence is compatible with the conclusion that these two beaches were both formed within a few centuries or so of each other (e.g. Nielsen et al., 1984; Teller et al., 2000; Fisher, 2003; Teller and Leverington, 2004). However, because these revised isobases are oriented west to east, rather than N 56°W, total crustal depression in the Northwestern Outlet region of Lake Agassiz was greater in the new reconstruction (Fig. 3) than in the previous one (Fig. 1). 4. Discussion The revised isobase trends of the Upper and Lower Campbell strandlines better reflect the late-glacial geometry and thickness of the Laurentide Ice Sheet (LIS). All ice sheet reconstructions in recent years have shown a thick Keewatin ice center located ∼ 1000 km to the north of this region (e.g. Shilts et al., 1979; Dyke and Prest, 1987; Hughes, 1987; Licciardi et al., 1998); new models of Tarasov and Peltier (2004, 2006) increase the thickness of ice in the Keewatin ice dome over previous reconstructions. Thus, a westward inflection in isobase trend in the northern part of the Lake Agassiz basin would be the expected result of the presence of a thick Keewatin ice center. The revision of isobases in the northwestern part of the Lake Agassiz basin to a more W–E orientation means that isostatic depression in the region where Lake Agassiz overflowed into the Clearwater–Athabasca River system (Fig. 1) would have been greater than that resulting from the isobase projections of Teller and Thorleifson (1983). Although northwestward overflow from Lake Agassiz was carried by a complex network of channels and lake basins during the time when the Upper and Lower Campbell beaches formed, the paleoelevation of the Wycherley channel area, ∼ 200 km
northwest of the Wapawekka Hills, restricted outflow into the Clearwater–Athabasca system (Fisher and Souch, 1998; Fisher, 2007-this issue). This threshold and its history are discussed in Fisher and Souch (1998) and Fisher (2007-this issue); the geographic relationship of this region to the Wapawekka Hills can be seen in Fig. 1 of Fisher (2007-this issue). The modern elevation of the Wycherley Lake paleohydrological threshold is 429 m (Fisher and Souch, 1998). When the Upper Campbell strandline is projected to the isobase through that lake – using our revised isobases, and assuming that the W–E orientation and the same slope continues northwestward – the Upper Campbell water plane lies at 470 m, 41 m above the Wycherley channel threshold elevation. When the Lower Campbell strandline is projected northwestward to this threshold, its water plane lies at 456 m, 27 m above Wycherley Lake. In the previous N 56°W isobase projections of Fig. 1, the Wycherley Lake control elevation lies roughly on the 7th isobase line; this means that the Upper Campbell water plane would lie at ∼ 415 m in elevation, 14 m below the Wycherley Lake overflow threshold. The Lower Campbell strandline would lie nearly 30 m below that northwestern overflow elevation. Thus, it is clear that, if overflow did occur through the Northwestern Outlet during the Upper or Lower Campbell lake stages, isobase orientation must have been more similar to our W–E reconstruction than to the previous N 56°W projections into the outlet region. Overflow through the Northwestern Outlet would have occurred when water was at the Upper and Lower Campbell beach levels only if that outlet was no longer blocked by the LIS. Smith and Fisher (1993) and Fisher and Smith (1994) concluded that the outlet region was deglaciated by ∼ 9900 14C B.P., and most researchers since then have concluded similarly. From the time the Upper Campbell strandline formed ∼ 9900 14C B.P. until the time the Lower Campbell beach formed ∼ 9400 14C B.P. (e.g. Teller et al., 2000), northwesterly overflow from Lake Agassiz was continuous (Fisher and Smith, 1994) or nearly continuous (Teller, 2001). An earlier episode of overflow through the Northwestern Outlet, between ∼ 11,000 and 10,000 14C B.P. was suggested by Teller et al. (2005, p. 1900–1901) and Teller and Boyd (2006), separated from the 9900– 9400 14C B.P. overflow by a brief re-advance of the LIS at ∼ 10,000 14C B.P. that closed that route; more research is needed to test this hypothesis. After 9400 14C B.P., northwestern overflow was re-routed when the lake fell below the Lower Campbell level, because either rebound closed that route and forced the lake to
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overflow south into the Mississippi River, and/or because a lower route into the Great Lakes west of Lake Nipigon was deglaciated (cf. Fisher, 2003; Teller and Leverington, 2004). The general absence of older (higher) strandlines in the study area north of Riding Mountain (i.e. the Herman, Norcross, and Tintah beaches), except for scattered small beaches of uncertain affinity in the area west of our study (Fisher and Smith, 1994), suggests that (1) the region was not deglaciated until after those older beaches had formed elsewhere in the basin or that (2) those older beaches were largely destroyed when the LIS re-advanced as far south as Riding Mountain ∼ 10,000 14C B.P. (Thorleifson, 1996; Teller, 2001; Teller and Leverington, 2004); Rayburn (1997) mapped an end moraine along the northern side of the Wapawekka Hills, which may be related to this readvance. The apparent absence of distinct Upper Campbell beaches west of La Ronge is probably related to (1) a limited supply of unconsolidated sediment to waves in this mainly Precambrian bedrock area and (2) the fact that the largest beaches of Lake Agassiz were transgressive and did not form in the region where lake overflow was occurring (Teller, 2001), so poor beaches formed when water was at the Upper and Lower Campbell levels in the northwestern part of the basin (see Teller, 2001, Fig. 3C). The greater isostatic depression of the Northwestern Outlet region, indicated by our revised isobases, would have resulted in a greater depth of water dammed against the LIS. This greater “head” would have resulted in a larger drawdown of the lake when overflow breached the ice dam, with the potential for a larger (more “catastrophic”) outflow. 5. Conclusions South of ∼53°N, along the western side of Lake Agassiz, new elevation data on the Upper and Lower Campbell strandlines show a good fit to the regional isobases of Teller and Thorleifson (1983) shown in Fig. 1. However, to the north and west of about the Pasquia Hills at 53°N, GPS elevation data indicate that isobases bend to a more west–east orientation (Fig. 3), from the N 56°W orientation projected into that region by Teller and Thorleifson (1983) (Fig. 1). These revised isobases, when projected northwestward from the study area, indicate that water planes associated with the Upper Campbell and Lower Campbell beaches lie above channels at Wycherley Lake, where Lake Agassiz overflowed into the Clearwater–Athabasca system. This means that, if the Northwestern Outlet of Lake Agassiz had been deglaciated by the time the Upper and Lower
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Campbell strandlines formed, overflow to the Arctic Ocean would have occurred until after the lake fell below the Lower Campbell level; this northwesterly overflow began ∼9900 14C B.P. (Fisher, 2007-this issue), perhaps earlier (Teller et al., 2005), and continued until after 9400 14 C B.P. In contrast, when the N 56°W isobases of Teller and Thorleifson (1983) are extended to the Wycherley Lake overflow threshold, both the Upper and Lower Campbell water planes fall below the elevation of the Northwestern Outlet of Lake Agassiz, so northwestern overflow could not have occurred at that time. Our new isobases also result in a smaller difference in slope between the differentially rebounded Upper and Lower Campbell beaches in this region than do the isobases of Teller and Thorleifson (1983), which better reflects the known small difference in age between these two beaches. We conclude that the bend in isobase orientation in this region reflects greater crustal depression associated with the presence a thick Keewatin ice center to the north. Our new reconstruction produces deeper water in the northwestern part of Lake Agassiz, and would have resulted in a greater drawdown of lake level when the Northwestern Outlet was deglaciated. Acknowledgements Support for this research came from the Natural Sciences and Engineering Research Council of Canada (NSERC) and from the Geological Survey of Canada. Thanks to L. H. Thorleifson (Minnesota Geological Survey) and G. Matile (Manitoba Geological Survey) for facilitating the GPS survey. Discussions with J. Mann, T. Fisher, H. Thorleifson, and, especially, Mike Lewis were helpful in developing our ideas. Jason Mann helped with the GPS surveying. Reviews of an earlier version of this paper by J. Andrews, A. Kehew, and M. Lewis were helpful. Many thanks to A. Patterson for preparing the figures. References Christiansen, E.A., 1979. The Wisconsinan deglaciation of southern Saskatchewan and adjacent areas. Canadian Journal of Earth Sciences 16, 913–938. Christiansen, E.A., Sauer, E.K., Schreiner, B.T., 1995. Glacial Lake Saskatchewan and Lake Agassiz deltas in east-central Saskatchewan with special emphasis on the Nipawin Delta. Canadian Journal of Earth Sciences 32, 334–348. Dyke, A.S., Prest, V.K., 1987. Late Wisconsinan and Holocene history of the Laurentide Ice Sheet. Géographie Physique et Quaternaire 41, 237–263. Elson, J.A., 1967. Geology of Glacial Lake Agassiz. In: Mayer-Oakes, W.J. (Ed.), Life Land and Water. University of Manitoba Press, Winnipeg, pp. 37–95.
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