Elsevier

Quaternary Science Reviews

Volume 223, 1 November 2019, 105942
Quaternary Science Reviews

High-resolution varve sequences record one major late-glacial ice readvance and two drainage events in the eastern Lake Agassiz-Ojibway basin

https://doi.org/10.1016/j.quascirev.2019.105942Get rights and content

Highlights

  • Two new varve sequences and available varve records document the late-stage lake history.

  • Compositional varve data confirm the occurrence of one synchronous basin-wide ice readvance.

  • The Cochrane ice readvance occurred ∼310 years before the final drainage of the lake.

  • One varve sequence comprises two lake drainage units separated by 65 years.

  • This is the first continental evidence for a two-step drainage of Lake Agassiz-Ojibway.

Abstract

Lake Ojibway developed at the southern Laurentide Ice Sheet margin during the last deglaciation and covered vast expanses of northeastern Ontario and northwestern Quebec (Canada). The late-stage history of the lake is complex, presumably marked by a coalescence with Lake Agassiz and by late-glacial ice readvances into the basin shortly before its final drainage 8200 years ago. However, these events are poorly defined in the Lake Ojibway varve record. Here we present evidence for the occurrence of one major ice readvance and two distinct drainage events from two varve sequences in northwestern Quebec. The Matagami section contains 231 varves that comprise an interval recording a late-glacial (Cochrane) ice readvance. The La Reine section spans 100 varves and the base of the sequence is characterized by a set of chaotic and coarse-grained (silty) rhythmites that marks the first lake drainage. These bottom rhythmites are overlain by a set of 65 thick varves, which are in turn capped by a thick silt bed associated with the final drainage of Lake Ojibway. Varve thickness measurements allow the correlation of these sections with the main Ojibway varve record that spans 2129 years in the region (where varve 1 represents the start of sedimentation). The Matagami sequence covers varve years 1644–1874 and the onset of the Cochrane readvance occurred in varve year 1817. The lower part of the La Reine sequence below the bottom chaotic rhythmites was deposited between varve years 1843–1877, while the overlying set of thick varves is correlated to varve years 2065–2129, thus indicating a hiatus of 188 varves. The distinct thick varves show strong compositional and stratigraphic similarities with the Connaught sequence reported elsewhere in the basin and point to a connection with the late-glacial (Cochrane) ice dynamics, which appear to have played an important role on the lake evolution. These results indicate that the late-stage history of Lake Ojibway was marked by the already known Cochrane major ice readvance which was followed by two drainage events separated by at least 65 years, consistent with North Atlantic sediment records that document a two-step drainage of Lake Agassiz-Ojibway.

Introduction

The impounding of meltwater caused by the retreat of the southern Laurentide Ice Sheet (LIS) toward James Bay and Hudson Bay during the 10,570 to 8,200–8,150 cal yr BP interval of the last deglaciation (Dyke, 2004) led to the development of a succession of large ice-marginal lakes that occupied the newly deglaciated and isostatically depressed terrains (Fig. 1A) (e.g., Elson, 1983; Teller, 1987). Lake Agassiz first submerged the eastern Canadian prairies (Elson, 1983), while Lake Barlow-Ojibway subsequently inundated part of northeastern Ontario and northwestern Quebec (Coleman, 1909). Lake Barlow refers to the meltwater body that developed south of the continental drainage divide between the Hudson Bay and the St. Lawrence River watersheds, while Lake Ojibway formed north of it (Coleman, 1909). Lake Agassiz and Lake Ojibway evolved independently throughout most of the deglaciation, with their areal extent being controlled by glacial isostatic adjustment of the land surface and the dynamics of the retreating ice margin that caused shifts in the location of outlets that regulated runoff from the decaying LIS margin (e.g., Thorleifson, 1996). Continued ice retreat presumably allowed their coalescence during late stages of the deglaciation, which were marked by readvances of the ice margin into the lake (Antevs, 1925; Hughes, 1959; Prest, 1970; Thorleifson, 1996). Rapid disintegration and the concomitant collapse of the LIS remaining over Hudson Bay allowed the drainage of Lake Agassiz-Ojibway into the Tyrrell Sea first and then into the Labrador Sea (Dyke and Prest, 1987; Veillette, 1994; Barber et al., 1999; Teller et al., 2002; Hillaire-Marcel et al., 2007; Kleiven et al., 2008; Jennings et al., 2015). Glaciological (hydraulic) modelling suggests that this flood may have been initiated through subglacial drainage (Clarke et al., 2004), a mechanism supported by geomorphic and sedimentary records in the southern Hudson/James Bay region (Josenhans and Zevenhuizen, 1990; Lajeunesse and St-Onge, 2008; Roy et al., 2011). The abrupt release of a massive volume of meltwater caused the freshening of the North Atlantic surface waters that disturbed the Atlantic Meridional Overturning Circulation (Barber et al., 1999; Ellison et al., 2006; Kleiven et al., 2008) and triggered a 160-year long climate anomaly centered ca. 8200 yr BP in Greenland ice cores (Alley et al., 1997; Kobashi et al., 2007; Thomas et al., 2007; Rasmussen et al., 2014). Although marine records suggest that the drainage of Lake Agassiz-Ojibway likely occurred through a multi-step drawdown (Ellison et al., 2006; Jennings et al., 2015), evidence for multiple drainage events in the sediment and geomorphological records of continental settings remain relatively limited, highlighting significant uncertainties in the late-stage history of this proglacial lake complex.

We present high-resolution analyses of two Ojibway varve sequences from northwestern Quebec that bring new insights on late-glacial ice dynamics and their influence on the late-stage evolution of Lake Ojibway (Fig. 1B). Sedimentological and geochemical data provide evidence for two distinct drainage events, which are inserted into the main Ojibway varve chronology using varve thickness measurements and other parameters. The results also show evidence for the occurrence of a single major late-glacial (Cochrane) readvance, which had a strong impact on the sedimentation of the last ∼310 years of the lake. Overall, this study refines the sequence of events that led to the final drainage of the lake and provides new data that should help to better correlate continental and marine records pertaining to the timing of early Holocene freshwater forcing events for North Atlantic climate change.

During its existence, meltwater overflow from Lake Ojibway drained southward via the Ottawa River and into the St. Lawrence River, through an outlet located along the continental drainage divide, which changed position through time due to glacial isostatic adjustment (Fig. 1B) (e.g., Vincent and Hardy, 1979). The physiography of the Ojibway basin is characterized by a gently-rolling clay plain with an elevation ranging from 290 to 320 m, which is broken in places by bedrock-cored hills and the crests of eskers. The main geomorphic feature in the basin relates to the last deglaciation and comprises a major glacio-fluvial complex – the Harricana Moraine, which is oriented roughly north-south and interpreted as an interlobate time-transgressive deposit (Hardy, 1976; Veillette, 1986) or as a subglacial deposit formed synchronously (Brennand and Shaw, 1996) (Fig. 1B). The northern part of the basin was submerged by the postglacial Tyrrell Sea, which reached up to 180 m to the west and 270 m to the east of James Bay (Fig. 1B). The incursion of marine waters substantially eroded, reworked or covered the Ojibway sediments as well as ice-marginal deposits and other deglacial landforms. The bedrock geology of the Ojibway basin is dominated by metavolcanic and metasedimentary rocks of the Abitibi Greenstone Belt and felsic plutonic and gneissic lithologies that form the core of the Abitibi Subprovince, which is part of the Archean Superior Province (MERQ-OGS, 1984; Ayer and Chartrand, 2011). The northern part of the basin is underlain by Paleozoic limestone and dolomite rocks of the Hudson Bay sedimentary platform (Fig. 1B) (Telford and Long, 1986). Poorly consolidated lignite and abundant white silica sands of lower Cretaceous age are also present in the Moose River Basin (Try, 1984; Telford and Long, 1991; Long, 2000).

The deglaciation of the greater James Bay region was highly dynamic, with ice streaming (Hicock, 1988; Veillette, 1997; Veillette et al., 2017), glacial readvances (Hughes, 1959, 1965; Hardy, 1977, 1982; Dredge and Cowan, 1989; Roy et al., 2011) and iceberg discharges into Lake Ojibway (Veillette et al., 1991; Veillette and Paradis, 1996; Paulen, 2001; Stroup et al., 2013). The late-glacial ice readvances, also known as the Cochrane readvances, were documented from criss-crossing fields of glacial flutings showing a general radial pattern originating from southeastern Hudson Bay and covering extensive areas of northern Ontario and northwestern Quebec (Fig. 1) (Prest, 1970). The Cochrane ice predominantly overrode fine-grained glaciolacustrine sediments that were mixed with bedrock fragments of the underlying Archean crystalline terrains and Paleozoic carbonates of the Hudson Platform (Fig. 1B). The partially grounded Cochrane ice left a distinctive pebbly, silty-clay and carbonate-rich diamicton (till) that overlies or truncates Ojibway varves (Smith, 1992). The occurrence of Cochrane Till and the presence of late-glacial flutings are commonly used to delineate the maximum extent reached by the Cochrane ice (Fig. 1B) (Boissonneau, 1966; Hardy, 1976; Prest, 1970; Paulen, 2001; Breckenridge et al., 2012). In spite of the distinct imprint on the regional stratigraphy and geomorphology left by the late-glacial dynamics, there is no consensus on the number and the timing of Cochrane readvances, underlying some important issues with the recognition and/or definition (classification) of these ice margin fluctuations (e.g., Breckenridge et al., 2012; Veillette et al., 2017).

Our current understanding of the late-stage evolution of Lake Agassiz and Lake Ojibway – including their coalescence and subsequent drainage – comes from raised shoreline sequences that provide insights on changes in areal extent and depth of the lakes (e.g, Thorleifson, 1996; Veillette, 1994; Roy et al., 2015), and from varve records that provide a floating annual chronology for ice-margin dynamics and LIS paleohydrology. Pioneer work on varve sections from several localities across the greater Ojibway basin allowed correlations based on varve thickness measurements that resulted in the establishment of a main varve series comprising a total of 2027 varves (Antevs, 1925, 1928). Subsequent investigations in Ontario confirmed the work of Antevs and provided evidence for an additional set of ∼60 anomalously thick varves containing inclusions of ice-rafted material (Fig. 1B) (Hughes, 1959, 1965). This set of younger varves – named the Connaught sequence – is separated from the underlying Timiskaming series by an unconformity and was attributed to renewed deposition in a shallow remnant of Lake Ojibway resulting from ice margin fluctuations (Hughes, 1959, 1965; Breckenridge et al., 2012). However, the interpretation of the significance of this sequence in the late-stage lake history is complicated by the lack of firm control on the position of the sequence within the overall Ojibway varve chronology. Indeed, in the absence of an unambiguous marker varve, correlations based on matching thickness patterns of short sequences of thick proximal varves (like the Connaught sequence) are less reliable than those involving long sequences of distal varves. Similar variations in both varve thickness and ice-rafted material content of late-stage Ojibway varves were later reported from clay bluffs exposed along the shorelines of Lake Matagami (Quebec) where two intervals of coarse, thick and carbonate-rich varves were identified in the otherwise thin and carbonate-free distal Ojibway varves forming the sequences (Hardy, 1976, 1982). These two sets of anomalous varves were associated with late glacial ice (Cochrane) readvances of the Hudson Lobe into Lake Ojibway of which the youngest set was considered correlative to the Connaught sequence based on a broad varve count and similar stratigraphic position (Hardy, 1976, 1982; Vincent, 1989).

A recent study of Ojibway sediment cores from the southern Ojibway basin (Fig. 1B) shed new light on previously published varve records and increased the understanding of the late-stage evolution of Lake Ojibway (Breckenridge et al., 2012). The review and analyses of several varve thickness records updated Antevs’ main varve chronology and refined the timing of sedimentological changes that occurred in the Ojibway basin, notably in the upper Ojibway sequence, for which some issues were resolved regarding the position (timing) of the Connaught varves in the main Ojibway series (Breckenridge et al., 2012). This work reported the occurrence of one Cochrane readvance (∼310 years before the end of the lake), in addition to infer the presence of a probable widespread unconformity that would have formed near the end of the ice readvance and prior to the deposition of the Connaught sequence, thereby arguing for a partial drainage of Lake Ojibway (Hughes, 1965; Breckenridge et al., 2012). These results underline the complexity of the late-stage evolution of the lake, showing the need to improve our understanding of the sequence of events that ultimately led to the final drainage of Lake Ojibway.

Information on the final lake stages also came from thick (up to 15 cm) whitish, silt bands present in the uppermost metre of varved clay sections along the shorelines of Lake Abitibi, in Quebec and Ontario, that were first reported during a sub-bottom coring and acoustic profiling program of the lake (Prévost et al., 1995; Veillette et al., 1999). A subsequent investigation within the upper Moose River watershed in Quebec showed that the marker silt bands occur sporadically inland in freshly dug ditches along roads over an area of ∼3600 km2 of the clay plain, mostly below 290 m asl (above sea level), in the same stratigraphic position as those found along Lake Abitibi (Ménard, 2012).

A recent stratigraphic investigation of one of these anomalously thick silt bands in the eastern Lake Abitibi region provided fresh insight into late deglacial events of the region (Daubois et al., 2015). Although this marker bed was not correlated with the main varve series, oxygen isotopes (δ18O) measured on ostracods across the sediment sequence showed a marked depletion in δ18O values indicating that the thick silt bed coincides with the end of glaciolacustrine sedimentation in the region (Daubois et al., 2015). It was interpreted as a drainage varve, which may be correlative to a similar drainage unit present in stratigraphic sections in the James Bay region and radiocarbon dated at 8205 (8128–8282; 1σ) cal yr BP (Roy et al., 2011). There, the presence of marine microfossils in varves directly underlying the drainage unit provided evidence for subglacial exchanges between Lake Ojibway and the marine waters in Hudson Bay (Roy et al., 2011), suggesting that the drainage of Lake Agassiz-Ojibway likely occurred through a multi-step drawdown, consistent with marine records that show multiple meltwater discharges across this interval of the deglaciation (Ellison et al., 2006; Jennings et al., 2015). In northeastern Ontario, the final drainage of Lake Agassiz-Ojibway is not marked by a distinct sedimentary unit, but is interpreted from a transitional contact between a clay-pebble (IRD-rich) unit and a laminated to massive gray silt unit (Stroup et al., 2013). Although these studies improved our understanding of the overall lake drainage, many issues remain, notably regarding the number (and chronology), duration and magnitude of the drawdowns, while the expression and timing of these drawdown events and the influence of ice dynamics in the Ojibway varve sequences also need to be identified. These uncertainties reflect the complexity of the late stages of the deglaciation and their relation with the development and evolution of these glacial lakes.

Section snippets

Sites and sediment sampling

A ∼3.6 m-long varve sequence was sampled from a natural bluff exposed along the eastern shore of Lake Matagami (Fig. 2A-B-C), in the same general area where compositional changes in varves were used to identify late-glacial readvances (Hardy, 1976). Another ∼2.2 m-long varve sequence was collected from a roadside excavation near La Reine (Fig. 2D-E-F), at the same locality where a thick silt bed resting on Ojibway varves was documented (Daubois et al., 2015). The sequences were sampled using

General physical and geochemical characteristics of varves

The two Ojibway varve sequences (Fig. 3; 4) are characterized by couplets of alternating light-coloured (bottom) and dark-coloured (top) layers typical of detrital fine-grained sediment deposited in proglacial lakes where sedimentation is controlled by seasonal variations in the sediment inputs (e.g. Antevs, 1925; Sturm, 1979). These color changes are accompanied by systematic textural (grain-size) changes that reflect differential settling of the detrital particles in the basin (Smith and

The Matagami varve sequence

The Lake Matagami sequence begins with a series of thin distal varves that shows sudden sedimentological and marked color changes at about 2.5 m depth (Fig. 3) suggesting a shift in sediment provenance. This is supported by the onset of compositional variations at the same depth, notably a significant increase in Ca/Sr. Because the Ojibway basin is predominantly underlain by crystalline lithologies, the presence of detrital carbonate in Ojibway sediments reflects sedimentary inputs deriving

Conclusions

High-resolution analyses of two key varve sequences in northwestern Quebec and comparisons with other published varve records document late-stage events in the Lake Ojibway basin that cover the last ∼310 years of the lake's existence.

  • The base of the Lake Matagami record documents marked sedimentological and compositional changes characterized by the sudden arrival of detrital carbonates and ice-rafted material associated with the onset of a late-glacial Cochrane readvance. The late Ojibway

Acknowledgments

This research was supported by a NSERC grant to MR and a FRQNT PhD fellowship to PMG. Sincere thanks are expressed to Gregory R. Brooks (Geological Survey of Canada) who provided unpublished information and for his helpful comments on the manuscript. We are also thankful to Andy Breckenridge (U. of Wisconsin-Superior) for sharing digital varve thickness data. Thanks to Guillaume Thiery and Claudie Lefebvre-Fortier for fieldwork and laboratory assistance, and to Édouard Philippe for helping with

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