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Evidence for a relict glacial landscape in Quebec-Labrador
PAIAEO, ELSEVIER
Palaeogeography, Palaeoclimatology,Palaeoecology111 (1994) 217-228
Evidence for a relict glacial landscape in Quebec-Labrador Johan Kleman, Ingmar Borgstr6m, Clas Hattestrand Department of Physical Geography, Stockholm University, S-10691, Stockholm, Sweden Received 2 November 1993; revised and accepted 9 February 1994
Abstract
Two major glacial landform systems occur in Labrador. These are the radial lineation and esker swarm reflecting Late Wisconsinan decay, and the Ungava Bay lineation and esker swarm reflecting convergent northward flow. In some sectors the two landform systems give conflicting evidence regarding deglaciation pattern. We have interpreted the ice sheet dynamics in Labrador from morphological data, using a new inversion model that treats spatial patterns of deglacial meltwater landforms separately from lineation patterns. In the George River area we have found that the Ungava Bay swarm of deglacial landforms has been overprinted at a right angle by a younger regional meltwater pattern from the last deglaciation. A similar overprint also exists along the intersection line in west-central Labrador. These relations show that the previously accepted relative ages of the two landform systems (Hughes, 1964; Boulton and Clark, 1990a,b; Klassen and Thompson, 1993) have to be reversed. We interpret the Ungava Bay lineation and esker swarm to represent a 0.25 x l 0 6 km 2 pre-Late Wisconsinan relict landscape, formed during the deglaciation of an older ice sheet and later preserved in a dry-based central zone of the Labrador dome.
I. Introduction
Two major glacial landform systems (Fig. 1) occur in Quebec-Labrador. One is the radial swarm of drift lineations and eskers, that in most sectors can be traced inwards from the peripheries of the peninsula. The other one is the Ungava Bay swarm, comprising lineations and eskers that converge towards Ungava Bay. Both of these systems were previously interpreted as formed by the Late Wisconsinan Laurentide Ice Sheet, and used to reconstruct the retreat pattern of that ice sheet (Prest, 1970; Boulton et al., 1985; Dyke and Prest, 1987). Considerable uncertainty, however, still exists regarding the late-glacial Laurentide Ice sheet dynamics in central Quebec-Labrador. A key problem is that the location of some glacial lakes is in direct conflict with the retreat pattern inferred from glacial lineations and eskers. This is the case south of Ungava Bay, where traces of two 0031-0182/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0031-0182(94)00016-2
large glacial lakes, Glacial Lake Naskaupi and Glacial Lake McLean, were described by Ives (1960). Reconstructions that do account for the existence of Glacial Lake Naskaupi involve either a tortuous ice margin outline, with a narrow "finger" projecting in a N N E direction from the main ice mass (Prest 1970), or involve a p o o r fit between postulated flow lines and observed lineation and esker pattern south of Ungava Bay (Dyke and Prest, 1987). Other ice sheet reconstructions (Boulton et al., 1985; G r a y et al., 1993) show configurations that are incompatible with the extent and location of these lakes. Ice-dammed lakes in the Arnaud, Feuilles and Mel6zes River valleys on the Ungava Peninsula (Lauriol and Gray, 1987; Vincent, 1989) required damming of ice in the east of these lakes. Yet, current reconstructions show a late-glacial Ungava Peninsula ice divide placed so far west that the damming of these lakes is unaccounted for.
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J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
Fig. 1. The eskers and till lineations in Quebec-Labrador, somewhat simplified after Prest et al. (1968). Boxes show the location of Figs. 4 and 6, based on new air photo interpretation, which give evidence that the convergent flow traces towards Ungava Bay were formed by an older ice sheet, unrelated to the Late Wisconsinan ice sheet which formed the major radial landform system. A n o t h e r key p r o b l e m concerns areas in central Q u e b e c where n o r t h w a r d - d i r e c t e d a n d southwestw a r d - d i r e c t e d flow traces intersect (i in Fig. 1). C r o s s c u t t i n g striae led H u g h e s (1964) a n d K l a s s e n a n d T h o m p s o n (1993) to c o n c l u d e t h a t flow
t o w a r d s the n o r t h p o s t d a t e d the flow t o w a r d s the southwest. H o w e v e r , H u g h e s (1964) d i d n o t e t h a t channel systems n e a r M a l a p a r t L a k e i n d i c a t e d t h a t the last ice r e m n a n t s were situated n o r t h o f the intersection zone.
J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
Boulton and Clark (1990a, b) reconstructed the evolution of the Laurentide ice sheet on the basis of drift lineations mapped from Landsat images. They considered the Ungava Bay swarm to be younger than the radial swarm, which they assigned to near-maximum stages. This interpretation suggests that the eskers in the radial swarm are not deglaciation landforms, but were instead formed underneath the accumulation area of the ice sheet. We consider the evidence for nearmarginal retreat-stage formation of eskers overwhelming, and must therefore reject this specific part of the Boulton and Clark reconstruction. We have tried to resolve the Labrador ice dome dynamics, using an approach with the following components: --Some photointerpretation over the whole peninsula served to evaluate the reliability of the Glacial Map of Canada (Prest et al., 1968), which, in turn, served as the primary data for delineation of flow trace fans. --Detailed photointerpretation over two key intersection areas established relative chronologies. - - A systematic search in aerial photographs for "transparent" meltwater landform patterns, that may indicate cold-based deglaciation. Such patterns may reveal deglaciation iceflow different from ice flow directions inferred from older subglacial land forms. --Reconstruction of successive ice configurations and deglaciation events. For the reconstruction chain, going from landforms to landscapes, and resulting in reconstructed ice sheet configurations along a relative-time axis, we used a new inversion model outlined below. This inversion model was developed as a consequence of the recognition that old landform systems can be preserved in frozen-bed zones of ice sheets (Sugden and John, 1976; Dyke, 1983; Lagerb/ick, 1988; Kleman and Borgstrrm, 1990; Kleman, 1992; Dyke et al., 1992).
2. Method
The inversion model is based on the recognition that an ice sheet interacts with its substratum in three fundamentally different ways, related to wet-
219
bed conditions, dry-bed conditions and meltwater release. A frozen bed involves little or no change of preexisting basal landforms, but a thawed bed leads to continuous reshaping of the substratum. Large-scale meltwater traces such as esker and channel swarms reflect marginal retreat during decay phases. Flow traces form only under wet-based conditions. The substratum is then continuously reshaped and flow-aligned lineaments (till lineations, striae) are produced and destroyed. The amount of reshaping of older forms increases with ice velocity and time. This continuous reshaping process is terminated either by change to a frozen bed or by deglaciation (Kleman, 1990). The preservation potential of lineations is scale-dependent. Large, old, "ghost" lineations may occasionally be discernible despite later wet-based conditions at the site (Lagerb~ick, 1988; Clark, 1993), but we consider it unlikely that small-scale fluting could survive in wet-bed zones for any length of time. Swarms of lineations that lack aligned meltwater traces, such as eskers, were probably created well inside the ice margin (Kleman, 1990). Meltwater traces. The grain sizes and sedimentary structures in eskers indicate that they form by fast rhythmic sedimentation (Sugden and John, 1976), in an inward-transgressive fashion close inside retreating ice margins (Hebrand and b~nark, 1989). Beaded eskers (Norman, 1938) and varved distal esker sediments (DeGeer, 1940) demonstrate forcing by the yearly melt cycle. As the yearly cycle directly affects the ice sheet surface, but not its base, it is evident that the discharge in the conduits is largely from surficial meltwater. Surface meltwater release is typically high near the ice sheet margin and during decay phases but negligible near dome centres. On the basis of the clear link to near-marginal ablation and the evidence for continuity between esker sediments and proglacial sediments (Hebrand and .~mark, 1989) we believe that they formed a few tens of kilometres inside the ice margin at most, and therefore consider eskers to be deglaciation markers. Glacifluvial channels occur in both wet-bed deglaciation landscapes dominated by eskers (Borgstrrm, 1989), and in areas deglaciated under frozen-bed conditions (Dyke, 1993). Where chan-
J. Kleman et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
220
nels are the only linear meltwater traces, a coldbased deglaciation is indicated. The regional meltwater pattern indicates the approximate ice slope direction. Glacial lake traces well inside the maxi-
mum-stage ice sheet margins are part of the inward-transgressive deglaciation meltwater landform system (BorgstrOm, 1989). Glacial lake traces do not provide information of ice flow pattern in
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Fig. 2. The figure shows how we packed swarms of glacial landforms into the landscape-level fans shown in Fig. 8a.
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Fig. 3. A dry-bed consists only of a see-through pattern of meltwater traces overprinted on a relict surface. Channels are dominant, eskers are scattered and small or completely lacking. The relict surfaces may be former subaerial surfaces or may contain (usually non-aligned) flow traces from an older flow stage or glacial. A consists of a flow trace fan with an overlain and aligned esker fan. These "classic" fans are interpreted to represent inward-transgressive preservation of flow traces (Kleman, 1990). Preservation is by deglaciation. Such fans do not represent true flowlines, but reflect near-marginal flow during decay and deglaciation phases. A "synchronous" fan contains abundant flow traces but lacks aligned meltwater traces. In some cases such a fan may be interpreted as the site of a former ice stream, in other cases it may instead have formed by slow sheet flow far inside the margin. As the internal age gradient (if any) is usually unknown these fans are as a first approximation treated as being synchronously formed.
wet-beddeglaciationfan
J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
221
the same sense that lineations or eskers do, but they do give a coarse control on segments of the ice margin outline and the approximate location of shrinking dispersal centres. The main components in the inversion model are called fans. These are the enveloped map representations of glacial landform swarms. Coherent fans are defined on the basis of spatial continuity and/or the possibility to form a meaningful pattern (Fig. 2). The sides of the envelopes may be stepped but the segments are only allowed to be aligned or transverse to flow, thus facilitating directional correlations. The three fan types we have found important in Labrador (other types may exist elsewhere) are shown in Fig. 3. After delineation and classification of the directional glacial landforms into fans, we sorted them into a relative-age stack according to cross-cutting relationships. Thereafter they were assigned to stadials, according to the principle that a specific area can only be occupied by one deglaciation fan per stadial, but any number of non-deglaciation fans.
3. Results
The results of the photointerpretation of the two key areas are shown in Fig. 4 and 6. Figs. 5 and 7 show key sites with crosscutting relationships. The fans we have defined are shown in Fig. 8. Fan A (Fig. 8a) is the radially arranged dominating landform system in Labrador. It has invariably been associated with the Late Wisconsinan ice sheet (Prest, 1970; Dyke and Prest, 1987; Vincent, 1989). The fan comprises directionally integrated eskers, and is thus an inward-younging deglaciation fan. Nowhere have we found it overprinted by any younger system. It is the only fan indicating an ice sheet shape capable of damming Glacial Lake Nasakaupi (Ives 1960) (Fig. 4). At the intersection zone, cross-cutting till lineations, mainly WSW-directed fluting superimposed on N-trending drumlins and crag-and-tails (Fig. 6 and 7), clearly show that fan A is younger than B. The ice-directed channel systems in the intersection zone (open arrows in Fig. 6) all slope towards
Fig. 4. BetweenBaleineRiver and GeorgeRiver, two deglaciation drainage systems crosscut at right angle. As eskers are near-marginaldeglacialfeatures,interveningice-freeconditions are indicated.The overprintingchannelswarmindicatesfrozenbed conditions during the last deglaciation. (5) marks the location of Fig. 5.
the SW and indicate a northeastward retreat of the ice front during the last deglaciation. These channels form the proximal dry-bed deglaciation extension of fan A. The size of eskers and lineations decreases from SW to NE towards the intersection zone, indicating that this line is a formational instead of an erosional limit of fan A. A NNW-SSE trending last ice divide over the west shore of Ungava Bay is indicated by this Late Wisconsinan fan. Fan A1 is a proximal extension of the deglaciation fan A. During the photointerpretation we discovered that the till lineations of fan B are regionally crosscut, at approximately right angles by a coherent system of ice-directed meltwater
J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
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Fig. 5. A series of ENE-trending marginal channels south of Lake Ninawawe. The drumlins and crag-and-tails which they cut are interpreted as having formed during a pre-Late Wisconsinan glaciation. The overprinting Late Wisconsinan deglacial meltwater landform system indicates westward retreat of the ice front and can be traced over a 40,000 km 2 large area west of George River. See Fig. 4 for location. Canadian National Air Photo Library A17762-56.
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Fig. 6. In the intersection zone, small scale fluting formed by southwestward ice flow is superimposed on crag-and-tails and drumlins formed by older northward ice flow. (7) marks the location of Fig. 7.
channels (Fig. 4 and 5). This channel pattem is incompatible with the retreat pattern indicated by the eskers around Baleine River. The A1 fan area is interpreted as having had frozen-bed conditions during the last deglaciation, hence the preservation of older landforms and the predominance of channels over eskers in this area. Fan B is formed by drumlins and fluting converging towards the Ungava Bay. Slight angular unconformities exist in the fan, indicating formation in steps. Most eskers within the fan are aligned with the flow traces, although some divergence occurs in the western part. It is interpreted as an inwardyounging deglaciation fan. This fan is directionally incompatible with the A and A1 fans in the Glacial Lake Naskaupi area. The cross-cutting lineations in the southeastern comer of Fig. 4 show that fan B is older than A. As both fans A and B are deglaciation fans it is thus apparent that fans A and B were formed by different ice sheets, i.e. fan B must have experienced subaerial conditions before the onset of the last glaciation. Traces from the fan B event only exist in the core area of the A (Late Wisconsinan) ice sheet.
J. Kleman et aL /Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
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(a)
i. . . . .
Fig. 7a. Partly attenuated northward-trending drumlins in the intersection zone. The direction of the younger overprinting flow is indicated by the arrow. NAPL A23592-19. b. Southwestward-trending lateral glaciofluvialchannels (arrow) south of Lake Malapart indicate northeastward retreat of the ice margin across the intersection zone. The northward-trending drumlins and crag-and-tails, and thereby the Ungava Bay swarm (fan B), are unrelated to the Late Wisconsinan retreat. NAPL A23592-88. For location of photos see Fig. 6. A pre-A age for fan B, instead of the reverse, allows simplification of the flow event scheme of Klassen and T h o m p s o n (1993) in central Labrador. It permits their spatially complementary and directionally matching events I and Va to reflect flow during one and the same phase. Fan C is formed by scattered NE-trending striae N N W of Schefferville. It does not contain aligned meltwater traces and is thus interpreted as a nondeglaciation "synchronous" fan. The event that formed this fan (correlative with the Klassen and T h o m p s o n event Va) is interpreted to be older than A and younger than B. Fan D comprises a zone of E-trending drumlins SW of Schefferville. Like fan C it lacks aligned meltwater traces. It is interpreted to be older than A and younger than B, but the age relative to C is unresolved.
4. Interpretation The landform systems in central L a b r a d o r reflect two main events, the decay of the Late Wisconsinan ice sheet, and the decay of an older and yet undated ice sheet. The older ice sheet had an east-west trending ice divide in central Labrador, probably located as far south as 53 ° north. The morphological traces (fan B) from the older ice sheet only escaped destruction in the cold-based central part of the Late Wisconsinan L a b r a d o r dome. The non-deglaciation traces from either ice sheet appear to be insignificant compared to the two decay records. Fig. 8b shows the decay patterns and Fig. 9 the interpreted sequence of events in the George River area. The deglaciation ice sheet B was marked by convergent flow and calving in the Ungava Bay
Fig. 8a. The major glacial fans in Labrador-Ungava. Fan A is an inward-younging wet-bed deglaciation fan reflecting Late Wisconsinan decay, while fan A1 is a proximal extension of A. This channel-dominated fan lacks eskers, indicating dry-based deglaciation in its area. The C and D fans reflect non-deglacial Late Wisconsinan ice flow. The B fan reflects wet-based deglaciation from an older undated ice sheet. Traces from the B, C and D events are interpreted to have been preserved in frozen-bed core area of the Late Wisconsinan Labrador dome. b. The Late Wisconsinan retreat pattern. Successive ice margins have been reconstructed normal to the fan lines in Fig. 8a. The ice margin outline during the older ice sheet deglaciation is indicated.
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Fig. 9. The interpreted sequence of events in the George River area.
basin and separation into residual ice masses, one elongated in the north-south direction and located south of the Torngat mountains, the other with its eastern margin trending roughly along the 72 ° W meridian. Earlier, flow of this ice sheet, as reflected by the oldest lineations in the area of fan B, was northwards towards the Hudson Strait, but less convergent than during the decay phase. Although we cannot prove complete deglaciation from this ice sheet, the much retracted positions of its proximal eskers indicate that it had lost the main part of its mass. The first traces of Late Wisconsinan flow in central Labrador are fans C and D, both of which
lack aligned meltwater traces and indicate easterly and northesterly flow from a dispersal centre situated more westerly than the last ice sheet remnants. The final decay of the Late Wisconsinan is reflected by the wet-based deglaciation fan A and the drybed deglaciation fan A1. We have drawn the successive marginal outlines of the decaying ice sheet (Fig. 8b) normal to the fan lines, which for these fan types reflect inward-younging nearmarginal flow directions, see Kleman (1990). We infer that the late decay phase was characterized by a gradual retreat of the Labrador dome, and with no evidence of large and rapid ice-divide shifts. According to our interpretation the
J. Kleman et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
226
U-shaped and often referenced "ice-divide" in central Labrador never existed but instead represented the innermost position reached by the proximal boundary of an inward-migrating wet-bed zone during the Late Wisconsinan deglaciation. The approximate extent of dry-bed areas during two stages of the Late Wisconsinan deglaciation are shown in Fig. 10. It can be noted that the major Rogen moraine areas (Prest et al., 1968) in Quebec-Labrador are concentrated in the areas interpreted to have undergone a transition from a frozen bed to a thawed bed during the final stages of deglaciation. We suggest this spatial relation to indicate that Rogen moraine forms when a rising phase change surface (pressure melting isotherm), rises to the till/bedrock contact, thereby permitting detachment and later stacking of still frozen debris sheets. The deglaciation of the core area occurred under frozen-bed conditions, as indicated by the absence of eskers. DeGeer moraines, characteristic of wetbed deglaciation in the marine environment, are lacking along the Ungava Bay shores but are ubiquitous along the Hudson Bay shore (Gray and Lauriol, 1985; Lauriol and Gray, 1987). This supports the interpretation of frozen-bed deglacia-
tion in the Ungava Bay area. The last ice divide on Ungava peninsula ran NNW-SSE across the lower reaches of Arnaud, Feuilles and Melrzes Rivers. Such an easterly position of the last ice remnants is in accordance with the ice-dammed lakes west of Ungava Bay (Vincent, 1989; Gray et al., 1993), which are unaccounted for in the reconstruction by Gray et al. (1993), (their fig. 14 b).
5. Discussion and conclusions
The maps in Boulton and Clark (1990a,b) in Labrador include a number of regional lineation sets of which no or only fragmentary traces have been reported on relatively detailed maps by e.g. Hughes (1964), Gray and Lauriol (1985) and Klassen and Thompson (1993). Lineation set 23 (Boulton and Clark's 1990b designation system, their fig. 8) plays a key role in their age assignment of the Ungava Bay swarm. As we ourselves have failed to confirm set 23, we are sceptical about the existence of this set as a true glacially related regional feature. We believe that the ice-divide locations F and G2 (Boulton and Clark's terminol-
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Fig. 10. The inferred extent of frozen-bed areas (shaded) during two stages of the Late Visconsinan decay.
J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
ogy) in fact existed, but in opposite order from that postulated by Boulton and Clark (1990a,b), and related to two different ice sheets. A consequence is that fan B (related to Boulton and Clarks ice divide location G2), has no role to play in the reconstruction of late-glacial events in the Ungava Bay/Hudson Strait region. Our results have some implications for the development in the outer Hudson Strait area, where contrasting views postulate that the carbonate till on Meta Incognita Peninsula and outer Frobisher Bay was either deposited by Hudson Strait and Frobisher Bay ice streams (England and Smith, 1993), or was deposited by northward directed flow of the Labrador dome during two late-glacial readvances (Miller and Kaufman, 1990; Kaufman et al., 1993). On the basis of the spatial pattern of young striae on southern Baffin Island (Kaufman et al., 1993) and the necessity of northward-sloping ice over Hudson Strait to explain the meltwater gorges (Mercer, 1956; Blake, 1966) across Meta Incognita Peninsula, we favour the basic interpretation of Kaufman and Miller (1993), that Labrador ice flowed across Hudson Strait. We postulate a final ice divide over the west shore of Ungava Bay, a more northeasterly location than in current reconstructions, and consider this configuration compatible with the flow trace patterns described by Kaufman and Miller (1993). The wet-based zone responsible for the carbonaceous till on Meta Incognita Peninsula may have been restricted to the deep trough in outer Hudson Strait and the areas down-ice of it. Dry-based ice sheet core areas, similar to that described here, have previously been described from ice caps in Arctic Canada (Dyke, 1993) and the Fennoscandian Ice Sheet (Sollid and Sorbel, 1988; Kleman, 1992; Kleman and Borgstr6m, in press). One implication of a central dry-based zone is that the Late Wisconsinan Ice Sheet in the Labrador sector was characterized by high basal shear stresses and a thick rather than a low, completely basal sliding one. Our reconstruction explains the Labrador glacial landscape as a result of subglacial landform development under outer thawed-bed parts of the Late Wisconsinan Labrador dome, and preservation of
227
a residual pre-Late Wisconsinan landscape in a dry-based central area of the dome. We argue that this two-ice sheet interpretation better explains the observed lineation and meltwater landform patterns than previous one-ice sheet reconstructions.
Acknowledgements We thank A.S. Dyke, G. Hoppe, T.J. Hughes and M.J. Hambrey for valuable comments on the manuscript, and the staff of the Canadian National Air Photo Library for their generous assistance.
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J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
from geomorphologival evidence. Arct. Alp. Res., 17: 289-310. Gray, J.T., Lauriol, B., Bruneau, D. and Ricard, J., 1993. Postglacial emergence of Ungava Peninsula, and its relation to glacial history. Can. J. Earth Sci., 30: 1676-1696. Hebrand, M. and,~mark, M., 1989. Esker formation and glacier dynamics in eastern Sk~ne and adjacent areas, southern Sweden. Boreas, 18: 67-81. Hughes, O.L., 1964. Surficial geology, Nichicun-Kaniapiskau map-area, Quebec. Geol. Surv. Can. Bull., 106, 20 pp. Ives, J.D., 1960. Former ice-dammed lakes and the deglaciation of the middle reaches of the George River, LabradorUngava. Geogr. Bull., 14: 44-88. Kaufman, D.S., Miller, G.H., Stravers, J.A., Manley, W.F. and Duvall, M.L., 1993. Late glacial ice margins and deglacial chronology for southeastern Baffin Island and Hudson Strait, eastern Canadian Arctic: 1. Reply. Can. J. Earth Sci., 30: 1753-1758. Klassen, R.A. and Thompsson, F.J., 1993. Glacial history, drift composition and mineral exploration, Central Labrador. Geol. Surv. Can. Bull., 435, 76 pp. Kleman, J., 1990. On the use of glacial striae for reconstruction of paleo-ice sheet flow patterns--With application to the Scandinavian ice sheet. Geogr. Ann., 72A: 217-236. Kleman, J., 1992. The palimpsest glacial landscape in northwestern Sweden--Late Weichselian deglaciation landforms and traces of older west-centered ice sheets. Geogr. Ann., 74A: 305-325. Kleman, J. and Borgstr6m, I., 1990. The boulder fields of Mt. Fulufj~illet, west-central Sweden--Late Weichselian boulder blankets and interstadial periglacial phenomena. Geogr. Ann., 72A: 63-78.
Kleman, J. and Borgstrrm, I., 1994. Glacial landforms indicative of a partly frozen bed. J. Glaciol., in press. Lagerb~ick, R., 1988. The Veiki moraines in northern Sweden widespread evidence of an early Weichselian deglaciation. Boreas, 17: 469486. Lauriol, B. and Gray, J.T., 1987. The decay and disappearence of the Late Wisconsinan ice sheet in the Ungava peninsula, northern Quebec, Canada. Arct. Alp. Res., 19: 109-126. Mercer, J.H., 1956. Geomorphology and glacial history of southernmost Baffm Island. Bull. Geol. Soc. Am., 67: 553-570. Miller, G.H. and Kaufman, D.S., 1990. Rapid fluctuations of the Laurentide ice sheet at the mouth of Hudson Strait: New evidence for ocean/ice sheet interactions as a control on the Younger Dryas. Paleoceanography, 5: 907-919. Norman, G.W.H., 1938. The last Pleistocene ice-front in Chibougamau District, Quebec. R. Soc. Can. Trans. Section 4, 32: 69-86. Prest, V.K., 1970. Quaternary Geology of Canada. In: R.J.W. Douglas (Editor), Geology and Economic Minerals of Canada (Econ. Geol., 1). Geol. Surv. Can., 5th ed., pp. 675-764. Prest, V.K., Grant, D.R. and Rampton, V.N., 1968. Glacial map of Canada. Geol. Surv. Can., Map 1253A. Sollid, J.L. and S~Srbel, L., 1988. Influence of temperature conditions in formation of end moraines in Fennoscandia and Svalbard. Boreas, 17: 553-558. Sugden, D.E. and John, B.S., 1976. Glaciers and Landscape. Edward Arnold, London, 376 pp. Vincent, J.-S., 1989. Quaternary geology of the southeastern Canadian Shield. In: R.J. Fulton (Editor), Quaternary Geology of Canada and Greenland (Geol. Can., 1). Geol. Surv. Can., pp. 249-275 (also in The Geology of North America, K-I, Geol. Soc. Am.).
Palaeogeography, Palaeoclimatology,Palaeoecology111 (1994) 217-228
Evidence for a relict glacial landscape in Quebec-Labrador Johan Kleman, Ingmar Borgstr6m, Clas Hattestrand Department of Physical Geography, Stockholm University, S-10691, Stockholm, Sweden Received 2 November 1993; revised and accepted 9 February 1994
Abstract
Two major glacial landform systems occur in Labrador. These are the radial lineation and esker swarm reflecting Late Wisconsinan decay, and the Ungava Bay lineation and esker swarm reflecting convergent northward flow. In some sectors the two landform systems give conflicting evidence regarding deglaciation pattern. We have interpreted the ice sheet dynamics in Labrador from morphological data, using a new inversion model that treats spatial patterns of deglacial meltwater landforms separately from lineation patterns. In the George River area we have found that the Ungava Bay swarm of deglacial landforms has been overprinted at a right angle by a younger regional meltwater pattern from the last deglaciation. A similar overprint also exists along the intersection line in west-central Labrador. These relations show that the previously accepted relative ages of the two landform systems (Hughes, 1964; Boulton and Clark, 1990a,b; Klassen and Thompson, 1993) have to be reversed. We interpret the Ungava Bay lineation and esker swarm to represent a 0.25 x l 0 6 km 2 pre-Late Wisconsinan relict landscape, formed during the deglaciation of an older ice sheet and later preserved in a dry-based central zone of the Labrador dome.
I. Introduction
Two major glacial landform systems (Fig. 1) occur in Quebec-Labrador. One is the radial swarm of drift lineations and eskers, that in most sectors can be traced inwards from the peripheries of the peninsula. The other one is the Ungava Bay swarm, comprising lineations and eskers that converge towards Ungava Bay. Both of these systems were previously interpreted as formed by the Late Wisconsinan Laurentide Ice Sheet, and used to reconstruct the retreat pattern of that ice sheet (Prest, 1970; Boulton et al., 1985; Dyke and Prest, 1987). Considerable uncertainty, however, still exists regarding the late-glacial Laurentide Ice sheet dynamics in central Quebec-Labrador. A key problem is that the location of some glacial lakes is in direct conflict with the retreat pattern inferred from glacial lineations and eskers. This is the case south of Ungava Bay, where traces of two 0031-0182/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0031-0182(94)00016-2
large glacial lakes, Glacial Lake Naskaupi and Glacial Lake McLean, were described by Ives (1960). Reconstructions that do account for the existence of Glacial Lake Naskaupi involve either a tortuous ice margin outline, with a narrow "finger" projecting in a N N E direction from the main ice mass (Prest 1970), or involve a p o o r fit between postulated flow lines and observed lineation and esker pattern south of Ungava Bay (Dyke and Prest, 1987). Other ice sheet reconstructions (Boulton et al., 1985; G r a y et al., 1993) show configurations that are incompatible with the extent and location of these lakes. Ice-dammed lakes in the Arnaud, Feuilles and Mel6zes River valleys on the Ungava Peninsula (Lauriol and Gray, 1987; Vincent, 1989) required damming of ice in the east of these lakes. Yet, current reconstructions show a late-glacial Ungava Peninsula ice divide placed so far west that the damming of these lakes is unaccounted for.
218
J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
Fig. 1. The eskers and till lineations in Quebec-Labrador, somewhat simplified after Prest et al. (1968). Boxes show the location of Figs. 4 and 6, based on new air photo interpretation, which give evidence that the convergent flow traces towards Ungava Bay were formed by an older ice sheet, unrelated to the Late Wisconsinan ice sheet which formed the major radial landform system. A n o t h e r key p r o b l e m concerns areas in central Q u e b e c where n o r t h w a r d - d i r e c t e d a n d southwestw a r d - d i r e c t e d flow traces intersect (i in Fig. 1). C r o s s c u t t i n g striae led H u g h e s (1964) a n d K l a s s e n a n d T h o m p s o n (1993) to c o n c l u d e t h a t flow
t o w a r d s the n o r t h p o s t d a t e d the flow t o w a r d s the southwest. H o w e v e r , H u g h e s (1964) d i d n o t e t h a t channel systems n e a r M a l a p a r t L a k e i n d i c a t e d t h a t the last ice r e m n a n t s were situated n o r t h o f the intersection zone.
J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
Boulton and Clark (1990a, b) reconstructed the evolution of the Laurentide ice sheet on the basis of drift lineations mapped from Landsat images. They considered the Ungava Bay swarm to be younger than the radial swarm, which they assigned to near-maximum stages. This interpretation suggests that the eskers in the radial swarm are not deglaciation landforms, but were instead formed underneath the accumulation area of the ice sheet. We consider the evidence for nearmarginal retreat-stage formation of eskers overwhelming, and must therefore reject this specific part of the Boulton and Clark reconstruction. We have tried to resolve the Labrador ice dome dynamics, using an approach with the following components: --Some photointerpretation over the whole peninsula served to evaluate the reliability of the Glacial Map of Canada (Prest et al., 1968), which, in turn, served as the primary data for delineation of flow trace fans. --Detailed photointerpretation over two key intersection areas established relative chronologies. - - A systematic search in aerial photographs for "transparent" meltwater landform patterns, that may indicate cold-based deglaciation. Such patterns may reveal deglaciation iceflow different from ice flow directions inferred from older subglacial land forms. --Reconstruction of successive ice configurations and deglaciation events. For the reconstruction chain, going from landforms to landscapes, and resulting in reconstructed ice sheet configurations along a relative-time axis, we used a new inversion model outlined below. This inversion model was developed as a consequence of the recognition that old landform systems can be preserved in frozen-bed zones of ice sheets (Sugden and John, 1976; Dyke, 1983; Lagerb/ick, 1988; Kleman and Borgstrrm, 1990; Kleman, 1992; Dyke et al., 1992).
2. Method
The inversion model is based on the recognition that an ice sheet interacts with its substratum in three fundamentally different ways, related to wet-
219
bed conditions, dry-bed conditions and meltwater release. A frozen bed involves little or no change of preexisting basal landforms, but a thawed bed leads to continuous reshaping of the substratum. Large-scale meltwater traces such as esker and channel swarms reflect marginal retreat during decay phases. Flow traces form only under wet-based conditions. The substratum is then continuously reshaped and flow-aligned lineaments (till lineations, striae) are produced and destroyed. The amount of reshaping of older forms increases with ice velocity and time. This continuous reshaping process is terminated either by change to a frozen bed or by deglaciation (Kleman, 1990). The preservation potential of lineations is scale-dependent. Large, old, "ghost" lineations may occasionally be discernible despite later wet-based conditions at the site (Lagerb~ick, 1988; Clark, 1993), but we consider it unlikely that small-scale fluting could survive in wet-bed zones for any length of time. Swarms of lineations that lack aligned meltwater traces, such as eskers, were probably created well inside the ice margin (Kleman, 1990). Meltwater traces. The grain sizes and sedimentary structures in eskers indicate that they form by fast rhythmic sedimentation (Sugden and John, 1976), in an inward-transgressive fashion close inside retreating ice margins (Hebrand and b~nark, 1989). Beaded eskers (Norman, 1938) and varved distal esker sediments (DeGeer, 1940) demonstrate forcing by the yearly melt cycle. As the yearly cycle directly affects the ice sheet surface, but not its base, it is evident that the discharge in the conduits is largely from surficial meltwater. Surface meltwater release is typically high near the ice sheet margin and during decay phases but negligible near dome centres. On the basis of the clear link to near-marginal ablation and the evidence for continuity between esker sediments and proglacial sediments (Hebrand and .~mark, 1989) we believe that they formed a few tens of kilometres inside the ice margin at most, and therefore consider eskers to be deglaciation markers. Glacifluvial channels occur in both wet-bed deglaciation landscapes dominated by eskers (Borgstrrm, 1989), and in areas deglaciated under frozen-bed conditions (Dyke, 1993). Where chan-
J. Kleman et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
220
nels are the only linear meltwater traces, a coldbased deglaciation is indicated. The regional meltwater pattern indicates the approximate ice slope direction. Glacial lake traces well inside the maxi-
mum-stage ice sheet margins are part of the inward-transgressive deglaciation meltwater landform system (BorgstrOm, 1989). Glacial lake traces do not provide information of ice flow pattern in
Delineation of flow t r a c e fans Flow traces
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Fig. 3. A dry-bed consists only of a see-through pattern of meltwater traces overprinted on a relict surface. Channels are dominant, eskers are scattered and small or completely lacking. The relict surfaces may be former subaerial surfaces or may contain (usually non-aligned) flow traces from an older flow stage or glacial. A consists of a flow trace fan with an overlain and aligned esker fan. These "classic" fans are interpreted to represent inward-transgressive preservation of flow traces (Kleman, 1990). Preservation is by deglaciation. Such fans do not represent true flowlines, but reflect near-marginal flow during decay and deglaciation phases. A "synchronous" fan contains abundant flow traces but lacks aligned meltwater traces. In some cases such a fan may be interpreted as the site of a former ice stream, in other cases it may instead have formed by slow sheet flow far inside the margin. As the internal age gradient (if any) is usually unknown these fans are as a first approximation treated as being synchronously formed.
wet-beddeglaciationfan
J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
221
the same sense that lineations or eskers do, but they do give a coarse control on segments of the ice margin outline and the approximate location of shrinking dispersal centres. The main components in the inversion model are called fans. These are the enveloped map representations of glacial landform swarms. Coherent fans are defined on the basis of spatial continuity and/or the possibility to form a meaningful pattern (Fig. 2). The sides of the envelopes may be stepped but the segments are only allowed to be aligned or transverse to flow, thus facilitating directional correlations. The three fan types we have found important in Labrador (other types may exist elsewhere) are shown in Fig. 3. After delineation and classification of the directional glacial landforms into fans, we sorted them into a relative-age stack according to cross-cutting relationships. Thereafter they were assigned to stadials, according to the principle that a specific area can only be occupied by one deglaciation fan per stadial, but any number of non-deglaciation fans.
3. Results
The results of the photointerpretation of the two key areas are shown in Fig. 4 and 6. Figs. 5 and 7 show key sites with crosscutting relationships. The fans we have defined are shown in Fig. 8. Fan A (Fig. 8a) is the radially arranged dominating landform system in Labrador. It has invariably been associated with the Late Wisconsinan ice sheet (Prest, 1970; Dyke and Prest, 1987; Vincent, 1989). The fan comprises directionally integrated eskers, and is thus an inward-younging deglaciation fan. Nowhere have we found it overprinted by any younger system. It is the only fan indicating an ice sheet shape capable of damming Glacial Lake Nasakaupi (Ives 1960) (Fig. 4). At the intersection zone, cross-cutting till lineations, mainly WSW-directed fluting superimposed on N-trending drumlins and crag-and-tails (Fig. 6 and 7), clearly show that fan A is younger than B. The ice-directed channel systems in the intersection zone (open arrows in Fig. 6) all slope towards
Fig. 4. BetweenBaleineRiver and GeorgeRiver, two deglaciation drainage systems crosscut at right angle. As eskers are near-marginaldeglacialfeatures,interveningice-freeconditions are indicated.The overprintingchannelswarmindicatesfrozenbed conditions during the last deglaciation. (5) marks the location of Fig. 5.
the SW and indicate a northeastward retreat of the ice front during the last deglaciation. These channels form the proximal dry-bed deglaciation extension of fan A. The size of eskers and lineations decreases from SW to NE towards the intersection zone, indicating that this line is a formational instead of an erosional limit of fan A. A NNW-SSE trending last ice divide over the west shore of Ungava Bay is indicated by this Late Wisconsinan fan. Fan A1 is a proximal extension of the deglaciation fan A. During the photointerpretation we discovered that the till lineations of fan B are regionally crosscut, at approximately right angles by a coherent system of ice-directed meltwater
J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
222
Fig. 5. A series of ENE-trending marginal channels south of Lake Ninawawe. The drumlins and crag-and-tails which they cut are interpreted as having formed during a pre-Late Wisconsinan glaciation. The overprinting Late Wisconsinan deglacial meltwater landform system indicates westward retreat of the ice front and can be traced over a 40,000 km 2 large area west of George River. See Fig. 4 for location. Canadian National Air Photo Library A17762-56.
:
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Fig. 6. In the intersection zone, small scale fluting formed by southwestward ice flow is superimposed on crag-and-tails and drumlins formed by older northward ice flow. (7) marks the location of Fig. 7.
channels (Fig. 4 and 5). This channel pattem is incompatible with the retreat pattern indicated by the eskers around Baleine River. The A1 fan area is interpreted as having had frozen-bed conditions during the last deglaciation, hence the preservation of older landforms and the predominance of channels over eskers in this area. Fan B is formed by drumlins and fluting converging towards the Ungava Bay. Slight angular unconformities exist in the fan, indicating formation in steps. Most eskers within the fan are aligned with the flow traces, although some divergence occurs in the western part. It is interpreted as an inwardyounging deglaciation fan. This fan is directionally incompatible with the A and A1 fans in the Glacial Lake Naskaupi area. The cross-cutting lineations in the southeastern comer of Fig. 4 show that fan B is older than A. As both fans A and B are deglaciation fans it is thus apparent that fans A and B were formed by different ice sheets, i.e. fan B must have experienced subaerial conditions before the onset of the last glaciation. Traces from the fan B event only exist in the core area of the A (Late Wisconsinan) ice sheet.
J. Kleman et aL /Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
223
(a)
i. . . . .
Fig. 7a. Partly attenuated northward-trending drumlins in the intersection zone. The direction of the younger overprinting flow is indicated by the arrow. NAPL A23592-19. b. Southwestward-trending lateral glaciofluvialchannels (arrow) south of Lake Malapart indicate northeastward retreat of the ice margin across the intersection zone. The northward-trending drumlins and crag-and-tails, and thereby the Ungava Bay swarm (fan B), are unrelated to the Late Wisconsinan retreat. NAPL A23592-88. For location of photos see Fig. 6. A pre-A age for fan B, instead of the reverse, allows simplification of the flow event scheme of Klassen and T h o m p s o n (1993) in central Labrador. It permits their spatially complementary and directionally matching events I and Va to reflect flow during one and the same phase. Fan C is formed by scattered NE-trending striae N N W of Schefferville. It does not contain aligned meltwater traces and is thus interpreted as a nondeglaciation "synchronous" fan. The event that formed this fan (correlative with the Klassen and T h o m p s o n event Va) is interpreted to be older than A and younger than B. Fan D comprises a zone of E-trending drumlins SW of Schefferville. Like fan C it lacks aligned meltwater traces. It is interpreted to be older than A and younger than B, but the age relative to C is unresolved.
4. Interpretation The landform systems in central L a b r a d o r reflect two main events, the decay of the Late Wisconsinan ice sheet, and the decay of an older and yet undated ice sheet. The older ice sheet had an east-west trending ice divide in central Labrador, probably located as far south as 53 ° north. The morphological traces (fan B) from the older ice sheet only escaped destruction in the cold-based central part of the Late Wisconsinan L a b r a d o r dome. The non-deglaciation traces from either ice sheet appear to be insignificant compared to the two decay records. Fig. 8b shows the decay patterns and Fig. 9 the interpreted sequence of events in the George River area. The deglaciation ice sheet B was marked by convergent flow and calving in the Ungava Bay
Fig. 8a. The major glacial fans in Labrador-Ungava. Fan A is an inward-younging wet-bed deglaciation fan reflecting Late Wisconsinan decay, while fan A1 is a proximal extension of A. This channel-dominated fan lacks eskers, indicating dry-based deglaciation in its area. The C and D fans reflect non-deglacial Late Wisconsinan ice flow. The B fan reflects wet-based deglaciation from an older undated ice sheet. Traces from the B, C and D events are interpreted to have been preserved in frozen-bed core area of the Late Wisconsinan Labrador dome. b. The Late Wisconsinan retreat pattern. Successive ice margins have been reconstructed normal to the fan lines in Fig. 8a. The ice margin outline during the older ice sheet deglaciation is indicated.
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J. Kleman et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994)217-228 ~:
225
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.................!iiiii?iiiiiiiii)iiiii;iiiil;i!iiiiiii!iiiiiiii;ii)?ii;iii:~::~ ~i~iiiii[iiiiiii!iiii~iiiiiii;ii!iiiii!iiii!ili[!!ii~' ~ ::"
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Fig. 9. The interpreted sequence of events in the George River area.
basin and separation into residual ice masses, one elongated in the north-south direction and located south of the Torngat mountains, the other with its eastern margin trending roughly along the 72 ° W meridian. Earlier, flow of this ice sheet, as reflected by the oldest lineations in the area of fan B, was northwards towards the Hudson Strait, but less convergent than during the decay phase. Although we cannot prove complete deglaciation from this ice sheet, the much retracted positions of its proximal eskers indicate that it had lost the main part of its mass. The first traces of Late Wisconsinan flow in central Labrador are fans C and D, both of which
lack aligned meltwater traces and indicate easterly and northesterly flow from a dispersal centre situated more westerly than the last ice sheet remnants. The final decay of the Late Wisconsinan is reflected by the wet-based deglaciation fan A and the drybed deglaciation fan A1. We have drawn the successive marginal outlines of the decaying ice sheet (Fig. 8b) normal to the fan lines, which for these fan types reflect inward-younging nearmarginal flow directions, see Kleman (1990). We infer that the late decay phase was characterized by a gradual retreat of the Labrador dome, and with no evidence of large and rapid ice-divide shifts. According to our interpretation the
J. Kleman et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
226
U-shaped and often referenced "ice-divide" in central Labrador never existed but instead represented the innermost position reached by the proximal boundary of an inward-migrating wet-bed zone during the Late Wisconsinan deglaciation. The approximate extent of dry-bed areas during two stages of the Late Wisconsinan deglaciation are shown in Fig. 10. It can be noted that the major Rogen moraine areas (Prest et al., 1968) in Quebec-Labrador are concentrated in the areas interpreted to have undergone a transition from a frozen bed to a thawed bed during the final stages of deglaciation. We suggest this spatial relation to indicate that Rogen moraine forms when a rising phase change surface (pressure melting isotherm), rises to the till/bedrock contact, thereby permitting detachment and later stacking of still frozen debris sheets. The deglaciation of the core area occurred under frozen-bed conditions, as indicated by the absence of eskers. DeGeer moraines, characteristic of wetbed deglaciation in the marine environment, are lacking along the Ungava Bay shores but are ubiquitous along the Hudson Bay shore (Gray and Lauriol, 1985; Lauriol and Gray, 1987). This supports the interpretation of frozen-bed deglacia-
tion in the Ungava Bay area. The last ice divide on Ungava peninsula ran NNW-SSE across the lower reaches of Arnaud, Feuilles and Melrzes Rivers. Such an easterly position of the last ice remnants is in accordance with the ice-dammed lakes west of Ungava Bay (Vincent, 1989; Gray et al., 1993), which are unaccounted for in the reconstruction by Gray et al. (1993), (their fig. 14 b).
5. Discussion and conclusions
The maps in Boulton and Clark (1990a,b) in Labrador include a number of regional lineation sets of which no or only fragmentary traces have been reported on relatively detailed maps by e.g. Hughes (1964), Gray and Lauriol (1985) and Klassen and Thompson (1993). Lineation set 23 (Boulton and Clark's 1990b designation system, their fig. 8) plays a key role in their age assignment of the Ungava Bay swarm. As we ourselves have failed to confirm set 23, we are sceptical about the existence of this set as a true glacially related regional feature. We believe that the ice-divide locations F and G2 (Boulton and Clark's terminol-
HudsJn
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Fig. 10. The inferred extent of frozen-bed areas (shaded) during two stages of the Late Visconsinan decay.
J. Kleman et al./Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 217-228
ogy) in fact existed, but in opposite order from that postulated by Boulton and Clark (1990a,b), and related to two different ice sheets. A consequence is that fan B (related to Boulton and Clarks ice divide location G2), has no role to play in the reconstruction of late-glacial events in the Ungava Bay/Hudson Strait region. Our results have some implications for the development in the outer Hudson Strait area, where contrasting views postulate that the carbonate till on Meta Incognita Peninsula and outer Frobisher Bay was either deposited by Hudson Strait and Frobisher Bay ice streams (England and Smith, 1993), or was deposited by northward directed flow of the Labrador dome during two late-glacial readvances (Miller and Kaufman, 1990; Kaufman et al., 1993). On the basis of the spatial pattern of young striae on southern Baffin Island (Kaufman et al., 1993) and the necessity of northward-sloping ice over Hudson Strait to explain the meltwater gorges (Mercer, 1956; Blake, 1966) across Meta Incognita Peninsula, we favour the basic interpretation of Kaufman and Miller (1993), that Labrador ice flowed across Hudson Strait. We postulate a final ice divide over the west shore of Ungava Bay, a more northeasterly location than in current reconstructions, and consider this configuration compatible with the flow trace patterns described by Kaufman and Miller (1993). The wet-based zone responsible for the carbonaceous till on Meta Incognita Peninsula may have been restricted to the deep trough in outer Hudson Strait and the areas down-ice of it. Dry-based ice sheet core areas, similar to that described here, have previously been described from ice caps in Arctic Canada (Dyke, 1993) and the Fennoscandian Ice Sheet (Sollid and Sorbel, 1988; Kleman, 1992; Kleman and Borgstr6m, in press). One implication of a central dry-based zone is that the Late Wisconsinan Ice Sheet in the Labrador sector was characterized by high basal shear stresses and a thick rather than a low, completely basal sliding one. Our reconstruction explains the Labrador glacial landscape as a result of subglacial landform development under outer thawed-bed parts of the Late Wisconsinan Labrador dome, and preservation of
227
a residual pre-Late Wisconsinan landscape in a dry-based central area of the dome. We argue that this two-ice sheet interpretation better explains the observed lineation and meltwater landform patterns than previous one-ice sheet reconstructions.
Acknowledgements We thank A.S. Dyke, G. Hoppe, T.J. Hughes and M.J. Hambrey for valuable comments on the manuscript, and the staff of the Canadian National Air Photo Library for their generous assistance.
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