Sedimentary Record of Glacial Lake Mackenzie, Northwest Territories, Canada: Implications for Arctic Freshwater Forcing

Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Sedimentary record of glacial Lake Mackenzie, Northwest Territories, Canada: Implications for Arctic freshwater forcing

Andrew G. Couch, Nick Eyles ⁎

Department of Geology, University of Toronto at Scarborough, 1265 Military Trail, Scarborough Ontario Canada M1C 1A4

ARTICLE INFO ABSTRACT

Article history: The global oceanographic impact of large volumes of freshwater entering the Arctic Ocean from North Received 11 July 2007 America during Laurentide Ice Sheet deglaciation is the subject of much discussion. The model of lateglacial Received in revised form 20 April 2008 runoff-forced changes in ocean thermohaline circulation (THC) is constrained in part due to a lack of detailed Accepted 16 June 2008 study of the relevant terrestrial glacial sedimentary and geomorphic record. This paper reports and interprets sedimentary facies that accumulated in the former glacial Lake Mackenzie, which formed a long (~600 km) Keywords: Glacial Lake Mackenzie and deep (max: 200 m) lateglacial basin ponded along the lower Mackenzie Valley in Canada's Northwest 14 Northwest Territories Territories. This is a key study area because the lake was dammed between approximately 10,500 Cyrbp 14 Freshwater runoff and 9100 C yr bp when most of the runoff from glacial Lake Agassiz was routed northward through the Arctic Ocean Mackenzie Valley to the Arctic Ocean. Detailed outcrop descriptions and geomorphic mapping in the glacial Lake Mackenzie basin fails to identify a clear record of abrupt shortlived flood outbursts but a regionally extensive deposit of massive and laminated mud at least 55 m thick, records trapping of large volumes of suspended fine sediment. Recent work shows the importance of suspended sediment content in freshwater runoff in influencing THC; too much sediment and freshwater enters the ocean hyperpycnally with little effect on surface waters. The role of Lake Mackenzie may have been to trap suspended sediment that otherwise would have been released to the Arctic Ocean, thereby acting to increase the effect of freshwater runoff on thermohaline circulation. © 2008 Elsevier B.V. All rights reserved.

1. Introduction been implicated in major weakening of the THC (Peltier, 2007). It is in this broader context that we conducted a detailed analysis of glacial The effects of freshwater runoff events in forcing changes in sediments across a large (11,250 km2) area of the former glacial Lake ocean thermohaline circulation (THC) are much debated (Broecker Mackenzie basin (Figs. 2 and 3) seeking the sedimentary record of et al., 1989; Clark et al., 2001; Fisher et al., 2002; Lowell et al., 2005; distinct freshwater runoff events. Teller et al., 2005; Peltier, 2007). This is, in part, because the pathways, sedimentary record and timing of suggested flood events 2. Regional geologic setting of study area are not well known (Smith, 1992, 1994; Dyke and Brooks, 2000; Duk- Rodkin and Couch, 2004; Lewis and Teller, 2007). The Mackenzie The Mackenzie River is Canada's largest and flows along the eastern- Valley in Canada's Northwest Territories is a key area for study most flank of the Cordillera north toward the Mackenzie Delta (Mackay because it was a major outlet for runoff (precipitation and glacial and Mathews, 1973; Dinter et al., 1990; Hill, 1996; Hill et al., 2001; McNeil meltwater) during the closing stages of the Late Wisconsin glaciation et al., 2001). During late Wisconsin deglaciation, northward drainage was (Fig. 1). During ice sheet recession, glacial Lake Mackenzie (named by blocked and glacial Lake Mackenzie formed a deep (max: 200 m), long Smith, 1992) formed along the lower valley during ice sheet retreat (~600 km long) and relatively narrow (~25 km) water body (Fig. 1). It (Figs. 2 and 3) and was fed by runoff from other large glacial lakes such as spilled over a limestone ridge in the vicinity of the present day Rampart glacial Lake McConnell and glacial Lake Agassiz (Fig. 1)(Teller and rapids near Ft. Good Hope (Fig. 2). The lake filled what may have been a Thorleifson, 1983; Teller, 1990a,b; Smith, 1994; Smith and Fisher, 1993; glacio-isostatically depressed moat peripheral to the ice sheet margin Fisher et al., 2002; Teller et al., 2002, 2005; Teller and Leverington, 2004; extending southwards to present day Fort Simpson (Savigny, 1989; Smith, Lowell et al., 2005). In turn, water overflowed from glacial Lake 1992). The route taken by the Mackenzie River prior to the last glacial MackenzietotheArcticOcean(Fig. 1) where meltwater pulses have maximum is only poorly known and older abandoned channels lie east of the study area (Duk-Rodkin and Hughes, 1995). Beaches along the lower slopes of the Mackenzie Mountains to the west and the Franklin ⁎ Corresponding author. Mountains in the east mark the former extent of the lake (Duk-Rodkin E-mail address: [email protected] (N. Eyles). and Couch, 2004; Figs. 2 and 3). The study area lies in the central portion of

Fig. 1. Maximum extent of glacial lakes along the margin of the Laurentide Ice Sheet (after Smith, 1994; Teller et al., 2005). Numbered lines indicate ice margins during deglaciation in thousands of 14C ka from Dyke (2004). Glacial Lake Mackenzie drained after 9.1 14C ka (this paper). Note that this is a composite map that shows the maximum extent of glacial lake Agassiz (ca. 7.7 14C ka; Smith, 1994; Teller et al., 2005), which occurred after the ﬁnal drainage of glacial lakes McConnell and Mackenzie. theglacialLakeMackenziebasinwhereitshowsa‘Y' shaped divergence just north of the community of Tulita (Figs. 2 and 3).

3. Methods

The history of Glacial Lake Mackenzie is recorded in deposits exposed in substantial river bluffs up to 8 km long and 120 m high cut by deep postglacial incision of the Mackenzie River and its tributaries. The large outcrops extending from Gaudet Island in the northwest to Petroglyph in the south (Fig. 3) were accessed by helicopter and logged in detail (Figs. 4 and 5) using conventional and well-tried facies description and analysis techniques (e.g., Eyles et al., 1983)(Table 1). Facies information at individual outcrops was integrated with the spatial distribution of surface sediment types identified from regional mapping by the Geological Survey of Canada (e.g., Duk-Rodkin and Couch, 2004), to construct stratigraphic cross-sections (Fig. 6). A broad range of facies types is present in the basin fill (Figs. 7–10)and is readily grouped into five packages of genetically related facies (facies associations; FA). To constrain the ages of the deposits, wood was selected from sediments containing organics (twigs with bark attached and with no observable rounding or reworking) and sent for AMS radiocarbon analysis. The location of samples and the corresponding radiocarbon ages are shown in their stratigraphic positions on Figs. 4 and 5. Facies associations are described and interpreted below followed by a discussion of their significance in terms of resolving lateglacial paleodischarges of freshwater to the Arctic Ocean.

4. Description and interpretation of facies associations

4.1. Facies association 1: fan delta deposits

This association has a channeled geometry and consists of sands and gravels. It occurs below and above a regionally extensive till deposited by Fig. 2. Study area location and extent of glacial Lake Mackenzie (see also Fig. 1). 28 A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38

Fig. 3. Detailed location map for outcrops described in this paper. the Laurentide Ice Sheet (Fig. 6) and consists of multi-storeyed channel (1990), Bennett et al. (2002) and Lønne et al. (2001). Graded gravel and complexes where master channels are as much as 300 m wide and 15 m sand facies provide key evidence of a subaqueous setting and the deep. These are filled with smaller channels (b50 m wide and 5 m deep) downslope transport of coarse sediment by turbidity currents and nested one upon another. Channel fills are dominated by massive gravel hyperconcentrated flows (Eyles et al., 1987; Mulder and Alexander, and sand (Gm, Sm respectively), normal and inversely graded gravel and 2001; Amy et al., 2005). The same deposits are found locally at the sand (Gg, Sg), and planar cross-bedded gravel and sand (Gp, Sp). Clast surface within relict lateglacial fan deltas along lake shorelines where lithologies are dominantly quartzite, chert and limestone typical of rivers entered the main basin (Duk-Rodkin and Couch, 2004). Cordilleran mountain sources to the west but eastward-derived Common deformation structures such as slumps suggest high Canadian Shield Archean igneous and metamorphic clasts also occur. sediment depositional rates. Diamict and chaotically bedded sand Beds of small-scale, trough cross-bedded sand (St), rippled (Sr), massive and gravel facies are interpreted as debris flow deposits where the (Sm) and horizontally bedded sand (Sh) are common. In the case of incision of channels into underlying till resulted in sidewall collapse channels incised into till, thin (b2 m) units of massive diamict (facies and in-channel slumping of till. Dmm) occur at the base of many channels and consist of chaotic admixtures of gravel and cobbles with rafts and irregular masses of till 4.2. Facies association 2: till and deformed sediments (Fig. 7C. Some channels are filled with massive glaciolacustrine mud containing outsized (ice-rafted) clasts (Fig. 7D). Throughout FA 1, This facies association consists of a regionally extensive diamict paleocurrents are dominantly northward parallel to the long axis of (facies Dmm) resting unconformably on deformed fan delta sediments the lake basin (Fig. 6). Syndepositional faulting, slumps and soft sediment of FA 1 or bedrock (Fig. 6). The same deposit is mapped at surface beyond deformations are very common. Infinite radiocarbon dates of N44,800 the confines of the lake basin and displays streamlined landforms and N49,500 14C yr bp was obtained from detrital woody material at the (narrow flutes and larger drumlins) recording westward flow of the Petroglyph and St. Charles Creek sites respectively (Fig. 5B). Laurentide Ice Sheet toward the lower slopes of the Mackenzie The deposits of FA1 are readily interpreted as deposits of coarse- Mountains (Figs. 2 and 3)(Duk-Rodkin and Couch, 2004). The diamict grained fan deltas (Fig. 11) such as described by Nemec (1990), Postma is overconsolidated and massive with a dominantly silty-sand matrix A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38 29

Fig. 4. Detailed sedimentologic logs at locations identified on Fig. 3. See Table 1 for facies codes. Location of radiocarbon dates (in 14C yr bp) is shown. that supports striated and glacially shaped ‘bullet’ boulders up to three 4.3. Facies association 3: glaciolacustrine massive mud meters in diameter having a preferred long axis orientation from east to west. Clasts are of mixed provenance being derived from both the Facies association 3 is the thickest and most extensive sedimentary Cordillera to the west and the Shield to the east. Intraformational deposit within the glacial Lake Mackenzie basin consisting of massive, pavements of large striated boulders are common (Fig. 8A). Sediments poorly consolidated silty-clay (mud; facies Fm) (Fig. 5A). Continuous exposed immediately below the diamict show folds and thrusts (Fig. 8B, outcrops of this distinctive blanket-like grey coloured deposit are a C) consistent with deformation in an east to west direction. conspicuous mappable feature of the high cliffs that flank the modern This diamict deposit has been mapped as a subglacially deposited river (Fig. 9A). It is at least 30 m thick along the basin axis thinning till (Duk-Rodkin and Couch, 2004) because of its overconsolidation, toward the basin margins where it rests conformably either on till (FA 2) the occurrence of striated and oriented glacially shaped clasts and or fan delta facies (FA 1). Large-scale cross-bedded fluvial sands of an distinct streamlined landforms on its surface (e.g., Hart et al., 1990; early Holocene Mackenzie River (FA 5; see below) truncate the upper Benn and Evans, 1996). Underlying deformed sediments record surface of the mud (Figs. 6 and 10B) indicating that the original thickness glaciotectonic stresses as ice flowed to the west. of this deposit was substantially greater. Massive mud facies show 30 A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38 sparse lonestone boulders and locally, large rounded ‘pillows’ (up to 4 m 4.4. Facies association 4: rhythmically laminated glaciolacustrine mud in diameter) of white-coloured rippled fine sand and silt (Fig. 9B,C, D). Facies association 3 is interpreted as deep-water glaciolacustrine Facies association 4 consists of thick and monotonous successions of mud deposited on the floor of glacial Lake Mackenzie by the ‘rain out' ripple cross-laminated sand (Sr) with horizontally laminated sand (Sh) out’ of suspended silts and clays brought into the basin by turbulent and mud laminae (Fl) resting directly on the massive muds of FA 3 runoff (Fig. 11). A lack of well-defined sedimentary structures, bedding (Fig. 9E). The thickness of this association is as much as 25 m. Rippled and/or lamination, indicates rapid deposition and a quiet basin floor sands occur in beds up to 1 m thick and displays upward transitions from setting well below wave base free of current activity. Analogous massive A-type ripples to B-type ripples capped by draped lamination (C-type mud facies accumulate today in waterbodies in contact with wet based ripples) (Ashley et al., 1982). Horizontally laminated sands (Sh) occur glaciers that release turbid, sediment-laden meltwaters (e.g., Elverhoi either as a transition from drape laminations or are interbedded with et al., 1983; Cowan and Powell, 1991). The presence of isolated mud horizons ranging in thickness from a few millimeters to about lonestones (interpreted as dropstones) indicates floating icebergs and 15 cm. FA 4 is unambiguously the product of meltwater-sourced a calving ice sheet margin along part of the lake basin perimeter. underflows and comprising a classic glaciolacustrine facies association Isolated pillows of rippled silts and fine sands in mud facies (Ashley, 1975; Gilbert and Butler, 2004). Compared to the massive muds likely record episodic but short lived progradation of silt-laden of FA 3, it records a reduction in the volume of suspended sediment underflows (quasi-continuous turbidity currents) across the muddy entering the basin and a seasonal control on runoff akin to classical basin floor from fan-deltas along the basin margin (e.g. Smith and glaciolacustrine ‘varves’. Outcrops of this facies at Tulita contain wood Fisher, 1993; Bennett et al., 2002). Deposition of these distal silts dated from 9325+/−25, 9255+/−30, 9215+/−20 to 9130+/−80 14Cyrbp and sand was sufficiently rapid to create a reverse density gradient (Fig. 4). Wood material is located in a thick (~7 m) rippled sand facies and induce loading into underlying massive mud (e.g., McCarroll resting on rhythmically laminated facies (Sh, Fm; Fig. 4)thatlikely and Rijsdijk, 2003). records deltaic progradation into glacial lake Mackenzie.

Fig. 5. A/B. Detailed sedimentologic logs at locations identiﬁed on Fig. 3. See Table 1 for facies codes. A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38 31

ca. 35,000–22,000 14C yr bp for this event in the northwest Mackenzie Mountains when trim line data from the mountains indicate ice was about 1.5 km thick in the study area (A. Duk-Rodkin, Pers. Comms, 2005). Lake ponding during glacial expansion prior to the last glacial maximum was a common process in Cordilleran valleys when advancing ice blocked rivers resulting in thick sub-till glaciolacustrine deposits (e.g., Eyles and Clague, 1991). Dyke (2004) summarized lateglacial ice retreat from the Mack- enzie Valley in the form of paleogeographic maps showing the successive (but approximate) positions of the ice margin at 11,500 and 10,500 14Cyrbp(Fig. 1). According to this reconstruction, glacial Lake Mackenzie began to fill around 10,500 14C yr bp. Lemmen et al. (1994) and Smith (1994) cite an age of 10,600 14C ybp (reported earlier by Savigny, 1989) from organics in glaciolacustrine deltaic sands near Tulita. Dates reported by us from deltaic glaciolacustrine deposits at Tulita (Fig. 4) are younger ranging from 9325+/−25 to 9130+/−80. An older age of 10,290+/−180 14C yr bp was reported by Smith (1992) from fluvial-deltaic facies from a site 250 km to the south but may reflect local progradation of deltas at that site rather than the end of regional lake ponding. Sedimentation has been suggested to have been markedly diachronous in the highly elongated basin (Smith, 1992). Combining the recent work of Dyke (2004) with the ages reported here, it is suggested that glacial Lake Mackenzie existed in the study area for at least 1400 years from about 10,500 to at least c. 9100 14C yr bp. These dates clearly do not fully constrain the history of the entire 800 km lake body given strong diachroneity in ponding and sedimentation. Glacial Lake Mackenzie formed first in the north near The Ramparts and expanded as the valley was exposed by

Fig. 5 (continued ). Table 1 Facies types and codes for lithological logs

4.5. Facies association 5: sandy braided river deposits

Facies association 5 occurs throughout the study area and in outcrop forms a distinct stratigraphic cap of large-scale planar cross- bedded sands (Sp; Fig. 10A). These are typically rusty yellow in color and rest on a regionally extensive unconformity cut across fine- grained glaciolacustrine sediments of FA 3 and 4. (Figs. 6, 10). The mean thickness of this deposit is about 10 m. Planar cross-bedded sands occur in sets up to 2.5 m thick and are bounded by low angle (b10°) reactivation surfaces (Fig.10). Measurements of foresets dips on cross-beds and ripples indicate that flow was generally to the northwest (Fig. 6). These deposits are readily interpreted as the deposit of a sandy braided river similar to the modern Mackenzie River where planar cross-bedded sands are produced by the downstream-accretion of migrating dunes (e.g., Miall, 1977, 1992). This association records lateglacial drainage of the ice-dammed lake and 1} the beginning of the postglacial, northward-flowing Mackenzie River. Fluvial facies of FA 5 commonly show downward tapering wedge- shaped fills of chaotic pebbly sand, with warped bedding and normal faults on their margins (e.g., Dry Island: Fig. 5A). These are typical of ice- and sand-wedge casts and indicates the former presence of permafrost in these sediments and ground contraction and cracking (Mackay and Mathews, 1983; Fisher, 1996).

5. Glacial Lake Mackenzie: age and depositional model

Glacial sedimentary deposits of glacial Lake Mackenzie suggest a four-part history of the basin within the study area (Fig. 12). The oldest event recorded is lake ponding during the entry of the Late Wisconsin Laurentide Ice Sheet margin and deposition of gravelly fan delta facies (FA 1; Fig. 12A). These deposits were then overrun by the westward expanding ice sheet and smeared by till (FA 2; Fig. 12B) during the last glacial maximum. Zazula et al. (2004) established bounding dates of Modiﬁed from Eyles et al. al. (1983) (1983).. 32 A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38

Fig. 6. Simpliﬁed regional stratigraphy for the glacial Lake Mackenzie basin based on detailed measured sections (Figs. 4, 5). A: 135 km long south–north section. B: 40 km west–east section; see Fig. 3 for location of outcrops.

southeastward ice retreat. The lake also drained first in the north as a 6. Discussion result of sediment infilling and glacioisostatic uplift (Smith, 1992). The radiocarbon dates reported here simply demonstrate that a large lake 6.1. Implications for freshwater routing and run off events to the Arctic was in existence at the time when much of the freshwater runoff from Ocean northwest North American interior passed through lakes McConnell and Mackenzie en route to the Arctic Ocean (Fig. 1). During the Emerson Our study in the glacial Lake Mackenzie basin is the most detailed stage of Glacial Lake Agassiz (9900–9500 14Cyrbp;Smith and Fisher, glacial sedimentological investigation completed to date along the 1993; Fisher et al., 2002; Teller and Leverington, 2004; Lowell et al., Mackenzie Valley. We are not able to identify any deposit that 2005) the Mackenzie valley was the outlet for meltwater and runoff unambiguously records major runoff events entering the basin and overflowing from Lake Agassiz (Tripsanas et al., 2007; Peltier, 2007). yet, as related above, during the lifetime of this lake (and thereafter Final drainage of Lake Agassiz took place into Hudson Bay at 7700 14Cyr during the establishment of an early Mackenzie River) much of the BP (Barber and Dyke, 1999). runoff from glacial Lake Agassiz passed through the basin en route to The fourth and final stage of basin evolution in the study area the Arctic Ocean. To some, this freshwater efflux triggered major (Fig. 12D) was the establishment of the modern north-draining oceanographic effects (Smith and Fisher, 1993; Fisher et al., 2002; Mackenzie River, associated with erosion and downcutting into the Teller and Leverington, 2004; Teller and Boyd, 2006; Peltier, 2007) but older sediment fill and the deposition of braided river deposits. the routing and timing of these flows remains uncertain (see The former presence of Holocene permafrost in these sediments is discussion in Lowell et al., 2005; Teller et al., 2005; Fisher et al., indicated by relict sand and/or ice wedge casts (Fig. 5A); in the 2006). Our sedimentological work combined with regional Quatern- case of ice-wedges, thawing may be related to Hypsithermal ary and geomorphological mapping does not identify any exception- warming which created many thaw ponds throughout the Mack- ally coarse-grained bouldery facies or major channel incisions that enzie Valley (e.g., Dallimore et al., 2000; Huang et al., 2004; Smol could be interpreted as a record of large overflow events (e.g., Reuther et al., 2005). et al., 2006). There is no particular reason to link fan delta deposits A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38 33

Fig. 7. Facies Association 1: Fan delta deposits. (A) Channelised gravels (base arrowed) at the Petroglyph outcrop; note person for scale in circle. (B) Multi-storied channel complex at Petroglyph with multiple stacked beds of graded gravels. Note sharply erosive lower contact at bottom of spade and multistorey channels above. (C) Large rip-up of till in gravel channel at Petroglyph (D) Massive mud with outsized clasts ﬁlling abandoned channel at Old Fort Point. (E) Incised channel of massive to crudely stratiﬁed gravel. (F) Imbricated crudely graded gravels at base of channel.

Fig. 8. Facies Association 2: Till and glaciotectonized sediment. (A) 20 m thick outcrop of diamict (starting at shovel handle) with cluster of boulders (outlined) in the centre (arrowed) of the exposure on Johnson Creek. (B) Glaciotectonically deformed gravel overlaid by till. (C) Raft of deformed gravel at base of the till. 34 A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38

Fig. 9. Facies Associations 3 and 4: Glaciolacustrine mud. (A) Dry Island exposure of massive and deformed muds between fan delta gravels (FA 1) and uppermost ﬂuvial sands (FA 5). (B) Deformed pillows of sand in mud. (C) Beds of slumped silt and ﬁne sand in mud. Note erosive contact with FA 5 at top of picture (outlined). (D) Large ball and pillows within massive mud. Mature trees for scale. (E) ‘Varved’ rippled (Sr) and horizontally laminated sand (Sh) with mud (arrowed) of FA 4 at Tulita. Scale in upper left is 15 cm.

(FA 1) to abrupt high discharge drainage events into the basin since overflow downstream. This is significant because both Knies et al. these form around the margins of any glacial lake. (2007) and Peltier (2007) highlight the important role of suspended It is possible that the record of major discharge events along the sediment in either amplifying or suppressing the impact of fresh- Mackenzie Valley is provided not by coarse-grained facies (which water flows to the Arctic and Atlantic oceans. Water containing tend to dominate existing discussion in the literature; e.g., Teller substantial volumes of fine sediment enters the ocean hyperpycnally et al., 2005) but by mud. The fill of glacial Lake Mackenzie basin is (below surface waters) and is ineffective in affecting thermohaline dominated volumetrically not by coarse-grained sediments or till circulation. The Mackenzie Valley was the major lateglacial outlet for but by thick glaciolacustrine muds (FA 3 and FA 4) that record freshwater to the Arctic Ocean (the ‘Mackenzie outflow’ of Peltier, substantial inputs of suspended fine sediment (Fig. 6). Smith (1992, 2007) and glacial Lake Mackenzie may have played an indirect but p. 1759) also noted the preponderance of fine-grained glaciolacus- key role in global climate change by trapping suspended sediment in trine sediment outcropping along the Mackenzie Valley. It is worth runoff thereby enhancing the effect of such outflows on thermoha- emphasizing that regional mapping for aggregate deposits for line circulation. construction projects confirms the lack of coarse-grained sediment It is possible that the sedimentary record of younger postulated in the basin (Duk-Rodkin, 2005). Coarse-grained sediment may have Agassiz flood event(s) after the drainage of glacial Lake Mackenzie been trapped further upstream in large waterbodies such as glacial could be preserved within sandy braided fluvial sediments of an early Lake McConnell (Fig. 1) allowing only suspended sediment to postglacial Mackenzie River (FA 5) but these too, do not reveal any A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38 35

Fig. 10. Facies Association 5: Sandy braided river facies. (A) Large scale planar cross-bedded sand exposed between two prominent reactivation surfaces (arrowed) at Old Fort Point. (B) Stratigraphic cap of planar cross-bedded sand on mud of FA 3. obvious or unusual evidence that might indicate large abrupt being indistinguishable from ‘normal’ runoff deposits. Indeed, the discharges through the basin. Braided river systems may not preserve point has been made that only a relatively small change in base flow recognizable ‘flood’ deposits because the depositional system simply discharge if maintained over long time periods, may be sufficient to expands laterally to accommodate larger flows. In this regard, we effect significant oceanographic change (e.g., Licciardi et al., 1999; recognize that large discharges need not be catastrophic in nature but Clark et al., 2001). In which case linking oceanographic responses to could be sustained over much longer time periods thus essentially any unique terrestrial ‘flood’ deposit may be fruitless.

Fig. 11. Depositional model for facies preserved within glacial Lake Mackenzie basin ﬁll. 36 A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38

Fig. 12. A–D. Four-part depositional model for facies preserved within glacial Lake Mackenzie basin. Arrows are ice ﬂow direction.

We note the importance of the observation that the glacial Lake oceans recorded in the Mississippi delta and fan system well before Mackenzie basin records formation of an earlier ice dammed lake as the late Wisconsin maximum e.g. Tripsanas et al. (2007). the Laurentide Ice Sheet expanded and advanced westward into the valley (Fig. 12A). Earlier depositional environments just before the last 7. Conclusions glacial maximum may have been the mirror image of late glacial conditions in interior North America (Fig. 1). Ponding of large The Mackenzie River Valley in the Northwest Territories of Canada proglacial lakes occurred in northern interior valleys before the last guided freshwater runoff from interior North America to the Arctic glacial maximum (e.g, Eyles and Clague, 1991). In discussing the work Ocean during deglaciation of the Laurentide Ice Sheet. During the of Knies et al. (2007), Peltier (2007, p. 1147) suggested that freshening lifetime of glacial Lake Mackenzie (at least 10,500–9130 14C yr bp) of Arctic Ocean waters were an enduring feature of glacial-to- large flood discharges of freshwater escaping northwards from glacial interglacial transitions over much of the last million years. Conversely, Lake Agassiz into Lake Mackenzie are postulated to have had major we suggest that the formation of large glacial lakes during the oceanographic effects on thermohaline circulation of the Arctic Ocean. transition from interglacial-to-glacial conditions was also likely. Recent models indicate the critical importance of suspended sediment Greatly enlarged Great Lakes formed in mid-continent during the in either suppressing or enhancing these effects; too much fine growth of the Laurentide Ice Sheet (Eyles et al., 2005 and references sediment and inflowing waters are delivered hyperpycnally and have therein) and likely triggered the freshwater runoff events to the little influence on surface water circulation. Detailed sedimentological A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38 37

Fig. 12 (continued ). studies reported from the glacial Lake Mackenzie basin, show that the Ashley, G.M., Southard, J.B., Boothroyd, J.C., 1982. Deposition of climbing ripples beds: lake acted as a large sediment trap as indicated by a thick (55 m +) a flume simulation. Sedimentology 29, 67–79. Barber, D.C., Dyke, A., 1999. Forcing of the cold event of 8200 years ago by catastrophic mud blanket in the basin. Lake Mackenzie may have played an indirect drainage of Laurentide lakes. Nature 400, 344–348. but key role in global climate change by reducing the amount of Benn, D.I., Evans, D.J.A., 1996. The interpretation and classification of subglacially- suspended sediment entering Arctic Ocean waters thereby increasing deformed materials. Quaternary Science Reviews 15, 23–52. fl Bennett, M.R., Huddart, D., Thomas, G.S.P., 2002. Facies architecture within a regional the climatic impact of such out ows. glaciolacustrine basin: Copper River, Alaska. Quaternary Science Reviews 21, 2237–2279. Acknowledgements Broecker, S., Kennett, J., Flower, B., Teller, J.T., Trumbore, S., Bonani, G., Wolfli, W., 1989. Routing of meltwater from the Laurentide Ice Sheet during the Younger Dryas cold episode. Nature 341, 318–321. Research was supported by a Natural Sciences and Engineering Clark,P.U.,Marshall,S.J.,Clarke,G.K.C.,Hostetler,S.W.,Licciardi,J.M.,Teller,J.T.,2001. Research Council of Canada grant to Eyles. We thank the Geological Freshwater forcing of abrupt climate change during the last glaciation. Science 293, – Survey of Canada for generous logistical support, helicopter time, and 263 287. fi Cowan, E.A., Powell, R.D., 1991. Suspended sediment transport and deposition of radiocarbon dates, and Alejandra Duk-Rodin for eld discussions. We cyclically interlaminated sediment in a temperate glacial fiord, Alaska, USA. In: are especially grateful to the many helicopter pilots who ferried us back Dowdeswell, J.A., Scourse, J.D. (Eds.), Glaciomarine Environments, Processes and – and forth. Andrew Miall and Marianne Douglas, Finn Surlyk, Timothy Sediments. Special Publication of the Geologists Association, vol. 53, pp. 75 89. Dallimore, A., Schroder-Adams, C.J., Dallimore, S.R., 2000. Holocene environmental history Fisher and an anonymous referee are thanked for their very useful of thermokarst lakes on Richards Island, Northwest Territories, Canada: thecamoe- comments. bians as paleolimnological indicators. Journal of Paleolimnology 23, 261–283. Dinter, D.A., Carter, D.L., Brigham-Grette, J.,1990. Late Cenozoic geologic evolution of the Alaskan North Slope and adjacent continental shelves. In: Grantz, A. (Ed.), The References Arctic Ocean Region, The Geology of North America. The Geological Society of America, Denver, Co., pp. 459–490. Amy, L.A., Peakall, J., Talling, P.J., 2005. Density- and viscosity stratified gravity currents: Duk-Rodkin, A., 2005. A GIS dataset of surficial geological features for the Fort Norman Insight from laboratory experiments and implications for submarine flow deposits. map area (96C), Northwest Territories. Geological Survey of Canada, Open File 4885. Sedimentary Geology 179, 5–29. Duk-Rodkin, A., Hughes, O.L., 1995. Quaternary geology of the northeastern part of the Ashley, G.M., 1975. Rhythmic sedimentation in Glacial Lake Hitchcock, Massachusetts central Mackenzie Valley corridor, District of Mackenzie, Northwest Territories. Connecticut. In: Jopling, A.V., McDonald, B.C. (Eds.), Glaciofluvial and Glaciolacustrine Geological Survey of Canada Bulletin 458, 45. Sedimentation. Society of Economic Paleontologists and Mineralogists, Special Duk-Rodkin,A.,Couch,A.G.,2004.Surficial Geology, Fort Norman, Northwest Publication, vol. 23, pp. 304–320. Territories. Geological Survey of Canada, Open File Map 4662, scale 1:250 000. 38 A.G. Couch, N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 26–38

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