Ice Margin and Sediment Fluctuations Recorded in the Varve Stratigraphy

Home , Lake Ojibway

Ice margin and sediment fluctuations recorded in the varve

stratigraphy of Lake Ojibway

Gianna Lee Evans

B.S, Colorado State University 2000

A thesis

Presented to the University of Cincinnati

In partial fulfillment of the degree of

Master of Science

In the Department of Geology

College of Arts and Sciences

2011

Committee Chair: Dr. Thomas V. Lowell

Abstract

Deglaciation of the Laurentide Ice Sheet is recorded, in part, in proglacial lake sediments. For the Lake Ojibway sequence three proxies are examined within an annual resolution framework to better understand the timing of events leading up to the final demise of the lake. Varve thickness, magnetic susceptibility (MS) and ice rafted debris (IRD) were analyzed individually and together to characterize sediment and ice margin fluctuations recorded within the varve stratigraphy. Varve thickness is a direct measurement of sediment flux. MS is a function of bulk mineralogy and grain size, and is applied here for correlation and as a proxy for the sediment source proximity. The IRD record tracks changes in the ice margin. These three properties, considered together help clarify the operation of sediment flux, water flux, and ice margin flux as the ice sheet decayed. The varve stratigraphy is compiled from sediment cores from five remnant lakes within the Lake Ojibway basin; Reid Lake, Lac Duparquet, Lac Montbeillard, Lac de Courval and Lac Wawagosic. Three prominent events in the Lake Ojibway varve sequence are explored with these proxies: varve 1528 which is thought to be the influx of Lake Agassiz water, years ~1750‐2000 which is thought to correspond to the Cochrane Advance, and the Connaught varve series from years ~2060‐2090 (Breckenridge et al., 2011, in press). Sand ripples capping the varves in Lac Montbeillard, located in the southern drainage pathway, are possibly related to overspill from the influx of Lake Agassiz water. The shift from the southern outlet to a northern drainage outlet is recorded by a cessation of varve deposition in Lac Duparquet, which is located near the arctic drainage divide. IRD and MS demonstrate that the Connaught varve series was possibly deposited as a proximal to distal series during ice margin retreat or meltwater diversion. The fluctuations in IRD from Reid Lake help refine the timing of the Cochrane advance within the varve series. The IRD may also indicate two individual surge events within the Cochrane advance. Post glacial sediments from Reid Lake located above the varve deposits were dated at 8400 ± 53 cal BP. These new data will help refine the chronology and ice margin fluctuations within the Lake Ojibway‐Laurentide Ice Sheet system prior to the final demise of Lake Ojibway.

iii Acknowledgements

I would like to thank Andy Breckenridge for his thoughts and advice on this project. I would like to thank my committee; Tom Lowell, David Nash and Aaron Diefendorf for their help editing this document. Thanks to Warren Huff, Mike Menard, Bill Honsaker and Esteban Sagredo, and others for help with lab work. Thanks also to Kristina Brady and Anders Noren for logistics and assistance at University of Minnesota‐ LRC. I would also like to thank Aaron Lingwall at the Lac Core facility at UM‐Duluth for running all the x‐ray scans on short order. Funding for this project is thanks to NSF grant EAR‐0643144. A special thanks to Kate Cosgrove for making sure that all the analytical labs were paid on time.

Table of Contents

Abstract…………………………………………………………………………………………………………… II Acknowledgements…………………………………………………………………………………………. IV Introduction…………………………………………………………………………………………………….. 1 Prior work……………………………………………………………………………………………………….. 3 General stratigraphy……………………………………………………………………………. 3 Varve thickness occurrences………………………………………………………………. 4 Varve stratigraphy………………………………………………………………………………. 7 Methods ……………………………………………………………………………………………………….. 9 Results …………………………………………………………………………………………………………… 11 Varve stratigraphy………………………………………………………………………………. 11 Magnetic Susceptibility………………………………………………………………………. 14 X‐ray Diffraction (XRD)………………………………………………………………………. 16 Ice Rafted Debris………………………………………………………………………………… 21 Chronology………………………………………………………………………………………… 31 Discussion………………………………………………………………………………………………………. 35 Magnetic Susceptibility ……………………………………………………..………………. 35 Ice Rafted Debris ………………………………………………………………………………. 39 Varve 1528 ……………………………………………………………………………………….. 40 Closing the Southern Drainage Outlet (Varve 1776) ……….……….………… 41 Cochrane Advance …….………………………………………………………………………. 42 Connaught Varve Series ……………………………..……………………………………… 44 Conclusions…………………………………………………………………………………………………… 46 References……………………………………………………………………………………………………… 48

v List of Figures and Tables

Figure 1 ……………………………………………………………………………………………………. 2

Figure 2 ……………………………………………………………………………………………………. 5

Figure 3 ……………………………………………………………………………………………………. 8

Figure 4 ……………………………………………………………………………………………………. 13

Figure 5 ……………………………………………………………………………………………………. 15

Figure 6 ……………………………………………………………………………………………………. 17

Figure 7 ……………………………………………………………………………………………………. 18

Figure 8 ……………………………………………………………………………………………………. 22

Figure 9 ……………………………………………………………………………………………………. 24

Figure 10 …………………………………………………………………………………………………. 26

Figure 11 …………………………………………………………………………………………………. 27

Figure 12 …………………………………………………………………………………………………. 29

Figure 13 …………………………………………………………………………………………………. 30

Figure 14 …………………………………………………………………………………………………. 36

Table 1 ……………………………………………………………………………………………………. 32

vi Introduction:

At the onset of the Holocene, approximately 12 ky BP, warming temperatures are thought to have contributed to the disintegration of the Laurentide Ice Sheet (LIS), (Alley et al., 1997; Dyke, 2004).

Large meltwater lakes, such as Agassiz and Ojibway, formed in isostatically depressed basins at the ice margin during the final retreat of the LIS (Vincent and Hardy, 1979; Dyke and Prest, 1987; Veillette,

1994; Dyke, 2004). Further decay of the ice sheet led to catastrophic failure of ice dams within Hudson

Bay causing an outburst of meltwater from Lake Agassiz‐Ojibway that likely drained into the north

Atlantic via Hudson Straight (Teller, 1995; Clark et al., 1999; Leverington et al., 2002; Lajeunesse and St‐ onge, 2008; Haberzettl et al., 2010). Sediment sequences that formed in the ice contact Lake Ojibway basin record ice margin and meltwater fluctuations. These ice margin and meltwater fluctuations will serve to help reconstruct the basin dynamics in the Laurentide‐Lake Ojibway system, which preceded the final termination of Lake Ojibway.

The chronology of these large meltwater lakes can be reconstructed through analysis of varve stratigraphy (Antevs, 1925; Antevs, 1928; Hughes, 1965; Veillette, 1994; Breckenridge et al., 2011, in press), which provides a high resolution framework. This study focuses on the stratigraphy within the

~1200 years prior to the final drainage event. Five remnant lakes within the Lake Ojibway basin (Figure

1) form a ~300 km northeast transect and connect, by way of Reid Lake, to a northwest transect from a previous study (Stroup, 2009). Varve correlations were matched to Antev’s (1925, 1928) record to provide a floating chronology of the sedimentation record (Breckenridge et al., 2011, in press).

The annual resolution of the varves can be exploited to directly compare the factors controlling sedimentation and to determine if they operated in the same temporal and/or spatial context. In addition to providing a temporal framework varve sequences constitute a record of sediment flux.

These can be supplemented with magnetic susceptibility (MS) and ice rafted debris (IRD), which provide

1 Location Map

JAMES BAY Legend

Seismic sites collected in 2008-2009 LAC WAWAGOSIC LACLakes WAWAGOSIC cored in 2008-2009 Lillabelle Lake study LACPaulen De COURVAL (2001) LAC De COURVAL Hardy (1976) LAC DUPARQUET Hughes (1965) LAC DUPARQUET REID LAKE Antevs REID LAKE LAC(1925, MONTBEILLARD 1928) Lake cores LAC MONTBEILLARD collected in km 2010 0 50 100 Cochrane limit

Figure 1: Digital elevation map of the study area showing the Kinojévis phase shoreline (black line). The dashed brown line is the approximate location of the Cochrane limit. Adapted from Breckenridge, et. al (2008), Dyke (2004) and Stroup (2009).

2 additional insight about sediment source and proximity. Long term sedimentation patterns may become visible and easy to correlate from different lakes across the basin by correlating the MS data with the varve years. Because MS is a function of grain size and bulk mineralogy (Dearing, 1999) it may also be used as a proxy for terrigenous material deposited within the lake sediments (Thompson and

Oldfield, 1986; Nowaczyk, 2001; Teller et al., 2008; Lennox et al., 2010). This allows the MS curve from

Reid Lake, located near the paleoshoreline of Lake Ojibway (Vincent and Hardy, 1979; Veillette, 1994;

Stroup, 2009), to be compared with the MS curves resulting from deep water sediments from the other remnant lakes within the Lake Ojibway basin as a proxy for water flux as well as allowing its use as a correlation tool. Ice rafted debris (IRD) was measured from X‐ray imagery in Reid Lake to assess ice margin fluctuations (Bond et al., 1992; Grousse et al., 1993; Robinson et al., 1995; Mangerud et al.,

1998; McCabe and Clark, 1998; Knies et al., 2001; Stroup, 2009; Larsen et al., 2011). This multi‐proxy study of the Lake Ojibway‐Laurentide Ice Sheet system will provide a high resolution record of the events leading up to the termination of Lake Ojibway. In addition, these results will help refine sediment, meltwater and ice margin fluctuations within the floating varve chronology.

Prior work

General Stratigraphy

Within this study site, there are four major stratigraphic units present in the lake cores as shown in Figure 2 (Stroup, 2009; Breckenridge et al., 2011, in press). The lowest stratigraphic unit is the Lake

Ojibway varves. These varves have been correlated by Breckenridge et al (2011, in press) with the previously established varve record (Antevs, 1925; Antevs, 1928; Hughes, 1965; Hardy, 1976; Paulen,

2001), which represent the last ~1200 years of the history of Lake Ojibway. The varves will be discussed in more detail in the next section.

3 Overlying the varves is a clay pellet conglomerate, Stroup’s (2009) pellet unit. Stroup (2009) interpreted the unit as the final deposition before the drainage of Lake Ojibway. The pellets are unconsolidated clasts of material thought to be ice rafted debris. These pellets are recorded in the varve unit as well in lower concentrations. Stroup (2009) determined that this unit has a very slow sedimentation rate based on calcite dissolution and the presence of delicate ostracod shells. However, this unit may coincide with the drainage breccia unit reported by Roy et al (2010). The distribution of the pellet unit is within the ice proximal lakes in Stroup (2009) and in two of the lakes presented here:

Wawagosic, and Lac de Courval.

The clay pellet conglomerate is overlain by laminated clays that are deposited rhythmically, but without the classic varve structure. Stroup (2009) interpreted this unit to represent post‐glacial deposits formed after the final drainage of Lake Ojibway. In most lakes, the laminated structure grades into a massive gray clay as a result of increasing bioturbation.

The youngest unit is indicative of modern productivity in smaller lake basins and is an organic rich lacustrine sapropel. This unit tends to have a gradational contact with the massive post‐glacial clay below. It has a very low MS signal compared to the glacial and post‐glacial deposits, so the boundary between these units is delineated by where the MS values abruptly drop.

Varve Thickness Occurrences

There are three major events in the Lake Ojibway stratigraphy as expressed by increased varve thickness. These include varve 1528 which is thought to be the influx of Lake Agassiz water; years

~1750‐2000 which is thought to correspond to the Cochrane Advance; and the Connaught varve series from years ~2060‐2090 (Breckenridge et al., 2011, in press).

4 Wawagosic 2222 +/- 81

Duparquet 8400 +/- 53* Reid 19,288 +/- 278 de Courval 13,179 +/- 91 1635 +/- 63 2031 +/- 80 31,072 +/- 206 Final varves 14,392 +/- 237 Disconformity Montbeillard 30,046 +/- 333 1776 3735 +/- 88 1776

1776 Organic sapropel Scale Massive clay

Laminated clay Sand/Gravel 1375 1375 Varves Sandy laminations (Rhythmites) Figure 2: General stratigraphy of the lakes presented in this study. Dashed lines show

10 m correlative ties at specific varve years. Calibrated radiocarbon ages are shown for the Clay/pellet conglomerate laminated and organic units for all cores. Ages in red are from terrestrial plant material. Ages in black are from bulk samples that contain aquatic, terrestrial and old organic material from Clay/silt the ice sheet. *Terrestrial sample with one seed of aquatic pondweed present. This seed may have a reservoir correction, making the age appear to be 100-500 years older (C. Yansa, personal Slump comm.). Missing section due to coring

5 Varve 1528 is defined by an abrupt change in sediment thickness observed at multiple sites in the Lake Ojibway basin and is present in lakes Reid and Duparquet from this study (Breckenridge et al.,

2011, in press). It occurs on both sides of the Superior basin drainage divide implying a shared water plane, indicating that the southern drainage outlet through the Ottawa River was still open at this time.

Breckenridge et al., (2011, in press) hypothesizes that this increase in varve thickness is a result from the diversion of meltwater from Lake Agassiz. The cause of this meltwater diversion is believed to be caused by retreat of the ice sheet north of the Hudson Bay‐Superior basin divide (Thorleifson, 1996;

Leverington and Teller, 2003; Breckenridge et al., 2011, in press). Varve 1528 appears in Reid Lake and

Duparquet, although there are slump features in Lac de Courval and Wawagosic at varves ~1550 and

1574 respectively. The disconformity in Wawagosic at varve 1574 may mask this event because there are no varves counted from these cores below the slump and this to give an idea how much time is missing. The cessation of varve deposition in the Superior basin and subsequent diversion of Lake

Agassiz water into the Ojibway basin at 7950‐8250 cal BP gives an age for varve 1528 placing the end of the varve sequence at 8470 ± 200 cal BP (Breckenridge, 2004; Breckenridge, 2007; Breckenridge et al.,

2010; Breckenridge et al., 2011).

The Cochrane advance is thought to be a final surge of the ice sheet before the final deglaciation and is associated with an increase in varve thickness (Hughes, 1965; Hardy, 1976). The Cochrane advance is thought to occur from varve years ~1750‐1900, which are present in the three lakes Reid, de

Courval and Wawagosic (Breckenridge et al., 2011, in press). This was an advance of ~75 km along ~350 km of the ice margin recorded from varve chronology reconstructions (Antevs, 1925; Antevs, 1928;

Hughes, 1965; Hardy, 1976; Paulen, 2001; Breckenridge et al., 2011). The Cochrane advance is defined by a till sheet that overlies varve sequences to the north of the Cochrane limit. The Cochrane limit is defined by the southern edge of a smooth lineated surface mapped with 90mSRTM DEM data

6 (Breckenridge et al., 2011, in press). For full history and description of the Cochrane advance, see

Breckenridge et al., (2011, in press).

The Connaught series are varves that are in excess of 10 cm thick, which are followed by a dramatic thinning, from varve numbers ~2065‐2090 (Breckenridge et al., 2011, in press). These varves are present in Reid, de Courval and Wawagosic. From examination of core X‐ray images the thin varves at the top of the Connaught series continue into the pellet unit in de Courval and Wawagosic and the laminated unit in Reid. There are high quantities of clay pellet and rocky IRD present in these varves that continue into the pellet and laminated units (Stroup, 2009). It is unclear whether or not these varves can be correlated by thickness as the other varve series are, or if they represent a proximal to distal depositional regime that is time transgressive in the direction of ice margin retreat. These three increases in varve thickness can be directly compared in a temporal time frame using the high resolution varve chronology developed for the five lakes in this study (Breckenridge et al., 2011, in press).

Varve Stratigraphy

Figure 3 shows the general varve stratigraphy of all five lakes synthesized from Breckenridge et al. (2011). Working from the oldest varves to the youngest, the first event of note is the termination of the varves in Montbeillard capped by normally graded sand waves at varve number 1375. Montbeillard is located near what was initially the southern drainage outlet of Lake Ojibway, until the water flow was diverted north of the arctic drainage divide detaching this remnant lake from the main body of water

(Vincent and Hardy, 1979; Veillette, 1994; Teller, 1995; Dyke, 2004). At the base of the sand unit in

Montbeillard there is erosional scouring, indicating removal of an unknown amount of varves.

The Duparquet varve record ends at varve 1776 and is capped by a small sandy unit that transitions into post glacial rhythmites, similar in appearance to the pellet unit. Duparquet is located north of Montbeillard at the head of the southern drainage pathway, just north of the present arctic

7 Varve Year Stratigraphy Reid de Courval Wawagosic 2124 Final varve 010 ~ 30-50 more uncorrelated cm 2115 Final varve 05Transitional unit above 0 10 20 2150 cm varves above until pellet unit Changes to sandy rhymites above cm (similar to pellet unit?) 2100 2122 Final correlated varve

2050 Connaught Varves Disconformity, no gap in sediments Disconforming 2000 Youngest varve gap of 6.8 cm 1950 covered by Cochrane I Sandy IRD unit 3.4 cm

1900 Uncorrelated/missing varves between 1924 and 1852 1850 Duparquet 010 1800 cm Cochrane Advance Cochrane

1750 1776 last varve sandy layer 5-8 cm thick above 1700

1650

1600 Possible turbidite between 1582 and 1583 1574 First varve 1550 Slump 48 cm Lake Agassiz Sand 1528 1528 Pellets Slumped below 1573 Influx tilted varves below 1500 2.7 cm gap b/n 1492 &1493 Varves 1490-1495 1450 Slump Slumped & folded repeated, due to slump? Montbeillard ~ 105 cm 13.3 cm 1400 0 5 Erosional dis. 1414-1416 are visible 1418 First varve cm Normally graded sand bed but not counted

Sand Final varve 1375 1385 First varve Pellets Bedding structures 1350 and/or organics (summer layer) Sand Pellets Slump feature b/n 1300 1313 &1314, repeated section 8.6 cm 1250 Slump w/ some uncorrelated Stratigraphy varves. ~162.5 cm Missing varves 1200 Bedding structures (coring) 1.6 cm gap b/n and/or organics (summer layer) Bedding structures 1179 &1180 - Turbidite and/or organics (summer layer) 1150 Disconformities Slump 24.3 cm 1100 Slump 18.4 cm 5 cm sed. gap b/n 1050 1068 &1069 1049 First varve

Sand 1000 Pellets Varve 965 not recorded 950 3.2 cm gap b/n 964 & 966 Figure 3: Relative varve stratigraphy from all five lakes showing major disconformities and basinwide No space between varves 900 929 and 931in core events. Varve thickness for each lake is plotted on the left of the varve stratigraphy. Relative amounts of pellets, bedding structures and/or organic material and sand are represented by gray scale bars. 850 875 First varve Bedding structures and/or organics (summer layer)

8 drainage divide (Breckenridge et al., 2011, in press). Figure 1 shows the relative locations of lakes

Montbeillard and Duparquet.

Reid Lake, Lac de Courval and Wawagosic have varve records that span a similar time frame. All three lakes show a disconformity ending at varve ~2065, before the start of the Connaught series. The only difference is in the beginning of the disconformities. The disconformities in Reid Lake and Lac

Wawagosic start at the proposed end of the Cochrane advance (Figure 3), whereas Lac de Courval starts at varve years ~1900, which is during the advance.

In Lac de Courval and Wawagosic, the final varves transition into the pellet unit described by

Stroup (2009). Reid Lake fades into the laminated unit at this point, but there is a subtle color change in the rhythmites in the 20 cm of sediment above the varves in Reid Lake.

Methods:

Sediment cores were obtained during the winter of 2010 using Livingstone and Bolivian square‐ rod piston cores assisted by an electric winch to aid in the penetration and recovery of sediment.

Potential coring sites were examined with seismic profiles taken with a SysQwest Stratabox™ 10 kHz portable echosounder. From these data, lakes were chosen that showed laminated sediments without evidence of slumping or deformation (Breckenridge et al., 2011, in press). All Livingstone and Bolivian cores were processed at the Limnological Research Center (LRC), Department of Geology and

Geophysics, University of Minnesota. Whole cores were processed with the Geotek Corescan‐V for low field volumetric magnetic susceptibility (k) and wet bulk density and then split. Images of each split core section were taken with a DMT CoreScan Colour setup with a 10 pixels/mm resolution and polarizing filters. The split halves were then scanned on a Geotek XYZ MSCL analyzer with measurements at 0.5 cm intervals for high resolution magnetic susceptibility. One half of each core is archived at LRC and the other half is at the University of Cincinnati for active research.

The working halves of the cores were described and correlated with visual stratigraphy changes and magnetic susceptibility curves. Individual varve correlations and the establishment of the varve chronology were performed by comparing the varve thickness patterns. These varve thickness patterns were statistically correlated to find the best fit match following the techniques described in

Breckenridge et al (2011, in press).

Bulk samples were taken from sections of cores with high concentrations of organic material for

AMS radiocarbon dating. In addition to these selected sites, cores from Reid Lake taken during a previous study (Stroup, 2009) were sampled for organic material along their entire length. Individual samples with too little mass to be analyzed by the AMS facility, were combined with other samples in stratigraphic proximity to meet the minimum weight requirements. A total of nineteen 14C samples were processed by the Woods Hole NOSAMS facility. The samples underwent a series of heated acid‐ base‐acid leaches to remove inorganic carbon and mobile organic acid phases

(http://www.whoi.edu/nosams/page.do?pid=43497).

Magnetic susceptibility curves were correlated with varve years by matching the individual MS data points to each corresponding varve year. This was done using varve count raw data and varve images provided by Breckenridge et al (2011, in press). Each core was visually inspected for areas of stratigraphic warping that would cause anomalous data points resulting from the coring process; such as upwelling, faulting and sedimentation gaps. For each varve, all MS data points within the varve and the overlapping data points with the varves above and below were averaged together. This method provides a single averaged data point for each varve year with edge effects from the adjoining varves minimized (Nash, 2011, personal comm.).

10 To explain differences in the MS values related to sediment source lithology, bulk mineralogy samples from Reid Lake and Lac Wawagosic were taken from corresponding varves in each lake. X‐ray diffraction (XRD) scans for these samples were generated with a Siemens D‐500 powder diffractometer using CuKα radiation. Two sets of bulk mineralogy samples were made from each sampled location.

One set as oriented smear slides, and the another set done as a dried random oriented powder‐mount.

All samples were scanned from 2‐36° 2θ.

The archive core halves from Reid Lake were sent to the LacCore facility at University of

Minnesota‐Duluth for gray scale X‐ray scans on the ITRAX XRF core scanner. These radiographs were processed in ImageJ (Rasband, 1997‐2008) to look for grouped pixels of density contrast that may indicate small rocks or clay pellets thought to be IRD. These pixel groups that represent particles were measured to get an area and then each group was counted by varve number to get a total amount of particles per varve year. Only shaded areas greater than 0.5 mm2 in area were counted. The digital image of each core was examined to aid in the decision to count any pixel groups of unknown origin as

IRD. To check the results from ImageJ, varve samples at ten year intervals were wet sieved (number 35

– 0.500 mm) to determine actual rocky IRD concentrations. Total counts for both methods were then normalized with the sedimentation rate (the thickness of each varve in cm). This provides an index value for the number of IRD particles weighted by sedimentation rate.

Results

Varve Stratigraphy

In order to correlate the magnetic susceptibility, IRD deposition and varve thickness measurements together in the varve stratigraphy; three general characteristics were examined for relative abundance and are presented in Figure 3. Observations from digital core photos were made on

11 the occurrence of clay pellets, sand visible in the summer layer, and visible organic material and/or visible bedding structures.

General observations of the Reid Lake varves are presented here in addition to the observations from all the varves, recorded in the gray scale graph in Figure 3. Intermittent thick sand layers starting at varve year ~1693 show internal structure with visible organic material interbedded with the sand. At varve 1888 varves lose “clean” summer/winter structure and are dominated by fine grained sand. This coincides with the section of the varve stratigraphy that Breckenridge et al. (2011, in press) described as

“sandy rhythmites”. On the X‐ray digital image of these varves, there is a possible angular disconformity between varves 1960 and 1961. Between varves 1962‐2015 the varves begin to show a better defined summer/winter structure. There is a large amount of sand present in the summer layers, but the varves lack the visible internal structure of the previous section of varves. Varve 1980 is unusually thick and coincides with an abrupt drop in MS and the youngest varve covered by the Cochrane advance. This might indicate that a disconformity is present sometime between varves 1962 and 1980. After 2015 there is a 56 year long disconformity with no sedimentation gap. This starts the Connaught series that thins upward into the laminated unit. There is a large amount of pellets and IRD present in these varves, which can be seen as a large spike in IRD during the Connaught varves series in Reid Lake (Figure 4).

There is some fine sand in the interlayers of the thick varves, but it is not as dominate as in the “sandy rhythmites” described by Breckenridge et al. (2011, in press). IRD deposition continues into the laminated unit, post‐dating varve deposition and the termination of Lake Ojibway. The reason for the continuation of IRD deposition after the end of the varve sequence remains to be investigated.

Figure 3 shows the position of clay pellets present in all of the lakes in roughly the same time frame as varve 1528. In Reid and Duparquet there is a marked increase in the amount of pellets 8‐10 years before the varve thickness change at 1528 lasting until varve ~1590. There is also a small increase

12 Reid Lake- Varve stratigraphy

IRD X-ray Counts IRD Sieve Counts (10 year intervals) Varve year Counts w/ 11 Stratigraphy Magnetic Susceptibility Varve thickness point moving average Counts per Counts per (orange line) varve thickness Counts varve thickness 2150 2115 Final varve Changes to sandy Pellets Sand Bedding structures w/ organics 2100 rhymites above Connaught Varves 2050 Disconformity, no gap in sediments 30.0 +/- .3 2000 Youngest varve covered by Cochrane 1950 34.2 +/- .5 28.3 +/- .4 1900 Sandy rhymites 25.7 +/- .3 1850 38.1 +/- .6

Cochrane Advance Cochrane 35.9 +/- .7 1800

35.4 +/- .8 1750

33.9 +/- .6 1700 37.9 +/- .6 1650

1600

1550 Lake Agassiz 1528 Influx 1500

1450 Slumped & folded 13.3 cm 1400 1414-1416 not counted 1385 First varve 1350 02468101214 Radiocarbon Ages 0 20 40 60 80 100 120 140 0 102030405001020304050 01020 30 40 50 60 020406080100 (Cal BP) Κ (SI units x10-6) Varve thickness (cm) #/year #/cm #/year #/cm

Figure 4: Reid Lake varve stratigraphy with comparison of IRD record, magnetic susceptibility (MS) and varve thickness. Relative amounts of pellets, sand and summer layer bedding structures are also shown. IRD counts per varve year are plotted for both the X-ray and sieve methods. Density of the IRD counts normalized by varve thickness for both methods is also plotted.

13 in sand during this interval. Lac de Courval and Wawagosic show increases in pellets from 1573‐1590, though they both have slumps for the first part of this sequence.

The relative amounts of pellets, shown in Figure 3, fluctuate in the rest of the lakes and seem to be out of sync with Reid Lake until ~1790, which is around the start of the Cochrane advance. There is also a significant increase in pellets and sand in Duparquet at 1729. Varve 1729 in Duparquet marks the beginning of the thinning upward sequence ending at varve 1776, the final counted varve in this lake.

This thinning upward varve sequence is similar to Reid Lake, Lac de Courval and Lac Wawagosic, which all have large increases in pellets and sand during their terminal varve series (~2065‐2122).

Reid Lake is the only one of these lakes that has high quantities of sand present within the varves. This is thought to be from proximity to an esker ~2 km to the west (Breckenridge et al., 2011, in press). Reid Lake is also the shallowest of these lakes with an estimated depth of ~15‐60 m (Stroup,

2009). This proximity to the reconstructed paleoshoreline (Vincent and Hardy, 1979; Veillette, 1994;

Stroup, 2009) allows us to measure variations in terrigenous input with magnetic susceptibility

(Thompson and Oldfield, 1986; Dearing, 1999; Nowaczyk, 2001; Teller et al., 2008; Lennox et al., 2010) to get a proxy for lake level changes.

Magnetic Susceptibility

The magnetic susceptibility (MS) measurements plotted by varve year shows similar patterns in all five lakes (Figure 5). In Lac Wawagosic the pattern of the MS curve is similar to the other lakes, but has higher values until the start of the Cochrane Advance (~1750), when Wawagosic becomes out of sync with Reid Lake. Reid Lake shows a large increase in MS, which corresponds to the sandy rhythmites recorded by Breckenridge et al. (2011, in press). Tri‐colored pink laminations also begin in Wawagosic at this varve interval and the varves become difficult to correlate with the other records (Breckenridge et al., 2011, in press).

14 Κ (SI units x10-6) 100 120 140 160 180 20 40 60 80 0 800 Magnetic susceptibility vs varve year vsMagnetic susceptibility varve Figure year plotted 5: Magneticsusceptibility by for varve allfive lakesinthisstudy. 1000 1200

Varve 1375 Topped by sand waves

1400 in Montbeillard Varve Year 15

Varve 1528 Lake Agassiz influx 1600

End of Duparquest

1800 Topped by small sandy unit

Cochrane adveance ~1800 - 2000 2000

Disconformity PossiblyPossibl caused by lowstand

Connaught varves 2200 Wawagosic Reid Du Courval Duparquet Montbeillard Reid Lake has the only varve record that spans the entire Cochrane Advance. At varve ~1800 the MS drops, then rises sharply until varve 1980. This significant drop in the magnetic susceptibility curve in Reid Lake at varve ~1979 corresponds to an episode of basin deepening or potentially another disconformity. This is the same time frame as the youngest varve covered by the Cochrane Advance

(Breckenridge et al., 2011), and associated with a dropoff in IRD deposition. After this abrupt drop, the

MS values increase sharply again until the disconformity at the end of the Cochrane Advance (Figure 4).

The Connaught series (varves ~2065‐2125) are currently correlated based on their varve thickness patterns (Breckenridge et al., 2011), but it is possible to correlated this varve series differently with their MS patterns. Reid Lake and Lac Du Courval show a ~14 year phase shifted correlation with each other during the Connaught varve time interval (Figure 6). A less compelling correlation can be made with Wawagosic with a shift of ~19 years. These MS curves sections represent a relatively short time period, so it is also possible that these are random correlations.

X‐ray Diffraction (XRD)

Reid Lake and Lac Wawagosic both have a comparable varve record that spans the Cochrane

Advance and continues up into post‐glacial sediments. Reid Lake is located ~2‐5 km east of a small esker and Lac Wawagosic is located just to the west of the Harricana Moraine. These two lakes have the furthest spatial separation and were most likely receiving sediment from different sources within the ice sheet (Vincent and Hardy, 1979; Veillette, 1994). The sediment source for the ice stream near Reid Lake is derived from sedimentary basement rocks under western Hudson Bay, whereas the sediment source for the ice stream near Wawagosic is derived from Archean igneous and metamorphic rocks. These two separate sediment sources give these lakes differing mineralogic signatures, which should be expressed as different MS values. To see if MS values expressed the different mineralogy spatially, XRD patterns from bulk samples from selected varves in both lakes were compared in Figure 7.

16 Connaught varves MS curves Wawagosic 2086 2113

2104 Reid de Courval Reid 2095

2072 2095

2086

2100 2090

de Courval 2080 2081 2121 2081

2069 2065 2075 2093 Wawagosic

Figure 6: Magnetic susceptibility profile of Reid Lake, Lac de Courval and Wawagosic shifted to line up peaks and troughs. The varve year for the correlated peaks and troughs are noted above the curve for each lake (Breckenridge et al., 2011). Composite pictures of the cores with varve correlations (Breckenridge et al., 2011) with the same correlative lines as the MS curves demonstrating possible transgressive deposition.

17 XRD patterns for Reid Lake and Lac Wawagosic

Top of “Pellet Unit” 1200 Base of “Pellet Unit” 1600 1000 1400 Smear-Low-”PU”

1200 Smear-Top-”PU” 800 rtz/Illite/Biotite Feldspars

Dolomite Quartz Qua 1000 Quartz/Illite/Biotite Amphiboles Dolomite 600

Amphiboles 800 llite Chlorite

e ite Feldspars

Illite Calcite 400 Chlorite 600

llite

Chlorite Chlor

rtz Amphiboles Chlorit te Chlorite Illite llite Biotite/I 200 I Feldspars/Micas 400 Sapphirine group?

Calcite Qua Chlorite Wawagosic Amphiboles 1400 Chlori Biotite/I Feldspars 200 Illite 0 1200 Depth 0 0 1200

1000 2.8 m PM-Low-”PU” rtz/Illite/Biotite 1000

Qua tz/Illite/Biotite 800 800

Dolomite

Quar Feldspars

Amphiboles 600 oup?600 2

PM-Top-”PU” Quartz

llite Dolomite Calcite 400 rtz 400

ite

Amphiboles

Feldspars/Micas Chlorite Amphiboles Biotite/Illite Qua Calcite Chlorite 200 Feldspars 200 Feldspars/Micas Biotite/I

Chlor Sapphirine gr 0 0 Reid 4 Depth 1000 Varve 2071 Varve 2115 4 m 1500 800 Feldspars Smear-V2115 6 Smear-V2071 llite/Biotite 600 Quartz/Illite/Biotite

1000 te

Dolomite

llite ite

Chlorite

Dolomite rtz Chlori Quartz/I Amphiboles

Illite 400 llite 500 Feldspars Chlor

e Calcite Amphiboles

Qua Calcite Chlorite Chlorite Biotite/I Chlorite Illite Illite Amphiboles Feldspars/Micas 700 Quartz Chlorite

Amphiboles

200 Biotite/I 8 Pyroxene 0 Chlorit Feldspars/Micas 600 Illite 800 PM-V2115 500 0

rtz/Illite/Biotite PM-V2071 llite/Biotite Qua 400 600 Dolomite 10 300 Quartz/I

Dolomite Feldspars 400

Quartz

llite

Chlorite 200 Amphiboles Calcite

Calcite

ite Quartz Biotite/Illite

Feldspars/Micas 200 Illite

Feldspars/Micas Chlori Illite Chlorite Biotite/I 100 Chlorite

Chlorite

Chlor 0 12 0

Varve 1574 2500 Varve 2015 (RD) 800 and 1985 (WW) 14 Smear-V1574 2000 Smear-V2015/1985 600 tz/Illite/Biotite

ite

Feldspars tz/Illite/Biotite

r Quar

Chlor

1500 Qua 400 Calcite

ite rtz Amphiboles Chlori Illite

Dolomite

Chlorite

Amphiboles Qua Biotite/Illite

Feldspars/Micas 1000 16 200 Chlor

Illite

rtz te

Feldspars/Micas ite Length of core

Feldspars

Dolomite Amphiboles 500 Qua 0 alcite (m) Chlorite Chlori Illite Chlor C Biotite/

Amphiboles Illite Chlorite Calcite 300 e 0

1200 rtz/Illite/Biotite 250 PM-V1574

Qua llite/Biotit 1000 Feldspars 200 PM-V2015/1985 Dolomite

Feldspars Amphiboles

800 rtz 150

Quartz/I

ite

Qua

600 Chlorit 100

Biotite/Illite

Chlorite Feldspars/Micas Chlor Amphiboles rtz

Amphiboles

Dolomite

Feldspars/Micas e 400 50 Qua

Calcite

Biotite/Illite

llite I 200 0 Chlorit Chlorite

0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35

Figure 7: XRD patterns for six locations in Reid Lake (black) and Lac Wawagosic (red). Bulk fraction samples were taken from correlative stratigraphic positions and processed both as smear slides and powder mount slides. 18 The mineral groups observed in the XRD samples from both lakes include low MS value quartz, calcite, dolomite, illite, and high MS value minerals such as biotite, amphibole and chlorite (Thompson and Oldfield, 1986; Dearing, 1999). The XRD patterns also show minerals in the feldspar and mica groups, which can have a wide range of MS values (Dearing, 1999). The samples from the base and the top of the pellet unit in Wawagosic are stratigraphically equivalent to samples from the laminated unit in Reid and will be referred to collectively as the pellet unit samples.

Varve 1574 is the oldest varve in Lac Wawagosic and was deposited 44 years after the thick varve 1528 recorded in Reid Lake. Reid has elevated calcite and dolomite, whereas Wawagosic has very little calcite or dolomite but shows higher amounts of all the other major mineral groups.

Varve 2015 in Reid and 1985 in Wawagosic are just below the major disconformity recorded in

Reid, Du Courval and Wawagosic. There is a slightly elevated amount calcite or dolomite in Reid and little to none in Wawagosic, but in general there are very few carbonates present in both lakes at this time. Wawagosic has more chlorite, but otherwise Reid shows higher amounts of the other minerals.

There is a 30 year time difference in the deposition of these two varve years and it is conceivable that the sediment source could have shifted within this time frame, causing low correlation between these two samples.

Varve 2071 is just above the previously mentioned disconformity in both Reid and Wawagosic.

Both lakes show much higher amounts of calcite and dolomite compared to varves 2015/1985, with Reid having slightly more of these minerals than Wawagosic. Reid also has more feldspar, illite and quartz.

This increase of low iron bearing minerals coincides with a significant drop in the MS values of both lakes after the disconformity as shown in Figure 5.

Varve 2115 is the final counted varve in Reid before the transition to the post‐glacial laminated unit above and has a similar stratigraphic proximity to the transition to the pellet unit in Wawagosic.

19 Compared with Wawagosic, Reid has a very high occurrence of calcite, dolomite, and quartz with slightly elevated levels of biotite, illite, and chlorite. Wawagosic has almost no evidence of calcite or dolomite at this varve interval.

At the base of the pellet unit, Wawagosic has virtually no calcite and less dolomite than Reid.

Wawagosic also has a slightly higher amount of feldspar and illite, while Reid has a slightly higher amount of amphibole. In the samples from the top of the pellet unit, there is still no calcite in

Wawagosic and significantly higher amount of dolomite, amphibole and feldspars in Reid. The other minerals fluctuate in amounts in both lakes. From the base to the top of the pellet unit and the equivalent in the laminated unit in Reid, there is a decrease in the amount of calcite while the dolomite level is stable.

In Reid Lake in the time between varve 1574 and 2015, the relative amounts of calcite and dolomite decreased while quartz and chlorite increased. In the interval from 2015‐2071, calcite and dolomite were the same and quartz and chlorite decreased slightly. At the transition from the varves to the “pellet unit”, the relative amounts of all minerals were similar except for a slight drop in calcite.

During the transition from the “pellet unit” to the laminated unit, calcite and dolomite amounts drop.

Compared to Reid Lake, Wawagosic has much less evidence of carbonate minerals. For 1574 and 1985, there are none of the carbonate minerals and the relative amounts of quartz and feldspar stay steady in this interval. From 1985‐2071 both carbonate minerals increase with more dolomite present than calcite, while chlorite abundance drops and quartz holds steady. At 2115, both carbonate minerals decrease while the other minerals abundances stay the same. After 2115 in the pellet unit, there is virtually no calcite present and dolomite levels continue to drop.

Overall, the Reid Lake varves have more carbonate minerals present and the Wawagosic varves have more iron bearing minerals. This confirms that the different MS values from each lake are a result

20 of diﬀerent sediment sources. For both lakes, it is only at varve 1574 that there is more calcite than dolomite in the XRD proﬁles. The calcite levels in Reid Lake drop whereas the dolomite levels remain steady during the transition from varve 2115 to the overlying unit. Wawagosic has less calcite and dolomite present, but this lake does show a drop in the relative amount of calcite at this interval as well.

These results are consistent with calcite dissolution from slow sedimentation during the transition from the varves to the pellet unit (Stroup, 2009).

Ice Rafted Debris

The concentration of ice rafted debris (IRD) was assessed as a function of the raw counts of IRD per year and the IRD density per year, which is the raw count normalized by varve thickness (Figure 4).

Two diﬀerent methods of counting IRD were compared in Figure 4; areas of density contrast in X-ray images and wet sieved sediments every 10 years. Large varves, such as the Connaught series, usually have high counts of IRD but low concentrations of IRD within the thick varves. On the other hand, very small varves may have moderate IRD counts, but when normalized with varve thickness concentrations become high. For these reasons, both the raw counts and the normalized counts are considered in combination. These data from Reid Lake are plotted in Figure 4 together with MS and varve stratigraphy.

There does not seem to be good correlation between the X-ray images and the sieve data. The sieved data shows very high peaks with intervals of zero IRD deposition, whereas the X-ray data has more moderate peaks. However, differences in the two records are partially due to differing sample resolution and partially from the clay pellets, as described by Stroup (2009). These pellets are difficult to count because they disaggregate during the sieving process, and tend to have the same density as the surrounding clay matrix (Figure 8). The clay pellet IRD (Stroup, 2009) are the most difficult to count accurately. The sieve method is inadequate because the clay disaggregates, then is washed through the

21 Clay pellets

Figure 8: Photograph with X-ray overlay excerpt from core RD10-1B-9L with because they disaggregate during the sieving process and are the same density as the matrix material when X-rayed. 22 sieve. The X-ray method is better, but because the clay pellets have the same density as the surrounding material, the density contrast needed to spot the pellets may not be present (Figure 8).

There are other potential problems with the IRD data collection from the X-ray images processed with ImageJ. One is misrepresenting the size when drawing the polygons. If the particles are near the size of the 0.5 mm2 cut-oﬀ value, this can lead to over or under estimation. To counteract this problem the raw data from ImageJ was ﬁltered by size class in increments of 0.1 mm2 (Figure 9)

Figure 9 demonstrates that the overall pattern of relative IRD amounts is consistent in the varve stratigraphy, even if the cut-oﬀ value for counting the particles is changed.

Another problem is that particles may be “stacked” on top of each other. It may be impossible to distinguish diﬀerent particles from a 2-dimensional image in 3-dimensional space. This could make several small particles that would normally be below the cut-oﬀ size appear to be one larger particle.

Also, particles big enough to be counted may be located underneath many small particles that were not measured. Applying an 11 point running average to the data set smoothes the data so that anomalies from this problem are minimized (Figure 9).

A third problem is the appearance of indistinguishable varves on the X-ray image. It is difficult to tell which particles belong to each varve. Sandy varves may not be as well defined in the X-ray image leading to counting difficulties. Some of the sandy varves in Reid Lake appear to have a less well defined winter clay layer as in cores RD10-1A-4L and RD10-1B-4L. An overlay of the ImageJ counts on the core photos with the official varve counts (Breckenridge et al., 2011, in press) helps distinguish the different varves. Also, to make sure only the IRD is measured and not any detrital material washed into the lake from the nearby esker, the data were filtered and only the particles ≥ 0.5 mm2 are counted as IRD and plotted in Figure 4.

23 70 IRD Size classes

0.4 mm 0.5 mm 40 0.6 mm 0.7 mm 11 per. Mov. Avg. (0.4 mm) 30 11 per. Mov. Avg. (0.5 mm) 11 per. Mov. Avg. (0.6 mm) 11 per. Mov. Avg. (0.7 mm)

0 1350 1450 1550 1650 1750 1850 1950 2050 2150

Figure 9: cations maintain in the varve record.

24 The raw counts of IRD from the core X-rays show two small peaks in IRD intensity at varves 1466 and 1532, spanning the Lake Agassiz inﬂux at 1528. From the photographs of the cores the varves preceding 1528 appear to have mostly stony IRD with few clay pellets, while the varves deposited after

1528 have more clay pellets (Figure 10).

The next set of varves, from 1673-2015, covers the Cochrane Advance and the preceding 100 years. From 1673 to 1753, the IRD intensity is relatively ﬂat and thought to represent mostly background levels, in part, resulting from counting diﬀerences. After varve 1753 there is a marked increase in IRD intensity which is followed by an interval of background level IRD, then an extremely high amount of IRD from ~1850-1930. Based on these data, the maximum of IRD deposition was between varves 1753 and 1981. During this time frame, the varves between the peaks of IRD have sandy summer deposits with visible bedding and high organic concentrations (Figure 11). After varve 1981 the

IRD intensity drops back down to background levels.

Between varves 2015 and 2071 there is a disconformity representing a 56 year time gap that is stratigraphically equivalent to similar disconformities in Lac de Courval and Lac Wawagosic

(Breckenridge et al., 2011, in press). Varve 2071 is part of the Connaught varve series that ovelies the

Cochrane Till and is recorded in many other lakes in the Ojibway basin (Paulen, 2001; Stroup, 2009; Roy et al., 2010; Breckenridge et al., 2011, in press). These varves have a high amount of IRD, but because of the size of the varves (thicknesses in excess of 10 cm) they show moderate IRD concentrations that decrease to background levels towards the terminal varve 2115. This result is consistent with the IRD record from Lillabelle Lake (Stroup, 2009) which is also thought to be part of the Connaught varve series

(Stroup, 2009; Breckenridge et al., 2011, in press).

In summary, there is an increase in IRD abundance prior to varve 1528 that remains elevated until varve ~1700. The IRD signal peaks again starting at varves ~1750-1800, decreases at varve ~1800,

25 X-ray method of counting IRD

Figure 10: Lower image: high resolution photo of varves 1522 – 1530 in Reid Lake (Breckenridge et al., 2011). Top picture: X-ray image of same varves. Middle picture: same X-ray image processed in ImageJ showing counted particles.

26 Bedding structures in varves

1704 1703 1702 1701 1700

Figure 11: Excerpt from RD10-1B-6L showing bedding structures within the varve summer layers.

27 Even with these sources of error, the counts from the X-ray images provide a better assessment of the IRD ﬂux in Reid Lake. Of these two methods, the X-ray images are the only way the clay pellets can be counted. Also, the X-ray images are non-destructive and because of this, it is easier to preserve annual resolution of the IRD counts.

As a diﬀerent error assessment, the counts from the sieved cores were plotted against the counts from the X-ray images for the ≥ 0.5 mm2 and ≥ 0.6 mm2 size classes (Figure 12). The linear trend line of the ≥ 0.5 mm2 cross plot has an R2 value of 0.31. However, ﬁve samples from varve years 1789,

1799, 1829, 1869 and 1939 are undercounted with the X-ray images. This disparity is most likely a result of very high concentrations of IRD in each varve, which would negate the visible density contrast detectable in the X-ray image. When these samples are removed from the cross plots, the R2 value increases to 0.69. Results from the ≥ 0.6mm2 size class indicate a correlation of R2 =0.35 for all counts and R2=0.77 if the ﬁve anomalous points are removed.

There is also a high variability between cores with the same varve. IRD from varve 2079 was measured with both the X-ray and the sieve methods from overlapping core sections ~40 cm apart from each other. In the core RD10-1A-3L the count was 8 with the X-ray image and 10 with the sieve. In core

RD10-1A-4L, the counts were 42 with the X-ray image and 56 with the sieve (Figure 13). This variability from one varve is an indication that the counts from individual years are not as important as the relative changes in the counts over time.

To assess the IRD peak locations within the varve stratigraphy, the raw counts from the X-rays were delineated by 0.1 mm2 size classes. The size classes from ≥ 0.4 mm2 - ≥ 0.7 mm2 were plotted with an 11 point moving average demonstrating that, though the actual number of particles per varve year varied, the peak positions maintained their integrity (Figure 9).

28 Error graphs

All data points With undercounted data points removed

45 45 40 40 35 35 R² = 0.3076 R² = 0.6853 30 30 25 25 IMJ IRD/yr IMJ IRD/yr 20 20 15 15 10 10 5 5 0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Sieved IRD/yr Sieved IRD/yr

35 35 30 30 25 25 20 20 IMJ IRD/yr R² = 0.3454 IMJ IRD/yr R² = 0.7693 15 15 10 10 5 5 0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Sieved IRD/yr Sieved IRD/yr

Figure 12: 5 ingle layer ft have all

29 Varve 2079

1A-3L X-ray = 8 Sieve = 10

1B-3L X-ray = 42 Sieve = 56

Figure 13: Photographs and X-ray images with counted particles of varve 2079 from two overlapping core sections taken ~40 cm apart from each other. This is an example of the variability in IRD amounts in the same varve.

30 then increases dramatically again at varves ~1850‐1980 (Figure 4). There are high amounts of IRD in varves ~2065‐2075, but when normalized with the high sedimentation rate, the increase is not as remarkable as the increase for varves ~1750‐1980.

Chronology

Radiocarbon ages from all lakes are presented in Table 1. Sample locations from the organic and laminated units are marked on the stratigraphic columns in Figure 2. All of the varve samples from

Reid Lake are marked in the varve stratigraphy in Figure 4. The samples were inspected under a microscope and determined to be composed of bulk plant material including mixed aquatic and terrestrial plants (Levesque et al., 1988). For two of the samples, one in the transition of the organic unit to the massive clay unit of Lac de Courval (OS‐88624, OS‐88629) and one in the lower portion of the laminated unit ~50‐100 cm above the varve unit in Reid Lake (OS‐88631, OS‐88628), there was sufficient material from the same horizon to be divided into subsamples of aquatic or terrestrial material. This was done to see if there was any significant age difference between the two types of material present.

The samples from the organic unit were similar in age to a piece of wood at the top of the laminated/massive unit in Montbeillard. OS‐88624 is the terrestrial sample from the paired samples from de Courval, and is 330 years younger than the bulk material sample (OS‐88629) from the same horizon. This result may give the reservoir correction factor for the aquatic material for the post‐glacial time period of Reid Lake’s history. However, this correction factor might not apply to earlier times of glacial sedimentation because the basin conditions were vastly different during the glacial phase of Lake

Ojibway (Geyh et al., 1998).

The samples from the lower part of the laminated unit in Reid Lake were expected to be early

Holocene in age. The measured samples have older than expected ages, except for the one subsample of terrestrial material (OS‐88631) which dates at 8400 ± 53 cal BP. There is one aquatic seed that was

31 Table 1 - Radiocarbon dates

Sample # Lake Unit Material Uncalibrated age Error Cal BP age Error 1σ OS-88667 Wawagosic Organic Bulk material 2180 30 2222 81 OS-88624 Lac de Courval Organic Terrestrial plants 1730 40 1635 63 OS-88629 Lac de Courval Organic Bulk material 2060 45 2031 80

OS-88586 Montbeillard Laminated-Top Twig 3460 30 3735 88

OS-88631 Reid Laminated-Middle Terrestrial seeds* 7600 65 8400 53 OS-88630 Reid Laminated-Middle Bulk material 11300 100 13197 91 OS-88628 Reid Laminated-Middle Bulk material 16250 120 19288 278

OS-88929 Reid Laminated-Low Bulk material 26500 330 31072 206 OS-88930 Reid Laminated-Low Bulk material 12400 60 14392 237 OS-88931 Reid Laminated-Low Bulk material 25300 290 30046 333

OS-88750 Reid Varve-2007** Bulk material 25200 250 29991 289 OS-88746 Reid Varve-1964 Bulk material 29500 300 34150 452 OS-88949 Reid Varve-1942** Bulk material 23500 360 28270 379 OS-88985 Reid Varve-1932** Bulk material 21500 180 25735 303 OS-88748 Reid Varve-1893** Bulk material 33400 410 38130 571 OS-88640 Reid Varve-1853** Bulk material 31600 660 35943 675 OS-88636 Reid Varve-1783 Bulk material 30500 580 35392 801 OS-88637 Reid Varve-1699** Bulk material 29100 500 33854 632 OS-89770 Reid Varve-1697** Bulk material 33100 350 37886 574

*One seed of Potamogeton sp. (pondweed) was dated with the rest of the terrestrial sample, which makes the date older than its true age, at most 500 years too old, but perhaps more in the 100-300 year range (C. Yansa, personal comm.).

**Samples are mixed from several different varves in order to have enough material to analyze. To obtain the varve year in this table the varve years of the individual samples were averaged.

32 present in this otherwise terrestrial sample, which may have a reservoir correction offset making it appear older than the surrounding terrestrial material. The terrestrial material in the sample comprised most of the carbon weight, including a couple seeds of chenopods (C. Yansa, personal communication).

This would account for an offset of ~100‐500 for the age of the sample (C. Yansa, personal communication).

All of the samples taken from the varves in Reid Lake were mixed bulk material and yielded ages ranging from ~25.7‐35.1 ka Cal BP. Based on the previous work in this area (Hardy, 1976; Vincent and

Hardy, 1979; Veillette, 1994; Breckenridge, 2007; Stroup, 2009; Roy et al., 2010) these ages are older than expected. There are three plausible explanations that would explain why material from the varves would record old dates.

This first scenario, that the age of the varves is as old as recorded by these 14C samples, is extremely unlikely due to all the previous work done in the area (Vincent and Hardy, 1979; Veillette,

1994; Breckenridge, 2007; Roy et al., 2010). Hughes (1965) dated the Cochrane till that caps the lower portion of the varve sequence at 8275 BP, and Lajeunesse and Allard (2003) dated the Sakami moraine, which coincides with the eastern limit of Lake Ojibway, at 7.7‐7.9 uncalibrated years BP (~8641 ± 149 cal

BP). The transition from lacustrine deposits to marine deposit is dated at 8128‐8282 cal yr BP (Roy et al.,

2010). The red beds in Hudson Bay that also mark this transition from glacial to post‐glacial deposits are dated to ~8470 cal BP (Kerwin, 1996; Barber et al., 1999; Haberzettl et al., 2010). Stroup (2009) dates the laminated unit above the varves in Reid Lake to ~7776 cal BP. These dates are consistent with the

8400 ± 53 cal BP minimum age obtained from the sediments above the varve unit in Reid Lake in this study.

The second possible explanation for these ages is for the lake to have an extremely high reservoir correction. There was most likely a reservoir correction factor in Lake Ojibway, but in order for

33 there to be this much difference in samples containing bulk materials from a supposedly freshwater lake, it would have to be an unrealistic number. In the example by Geyh et al., (1998), high reservoir correction factors are achieved in lake basins with extremely brackish water with high amounts of inorganic carbon not equalized with the atmosphere. Lake Ojibway was an open basin system (Vincent and Hardy, 1979; Veillette, 1994; Teller, 1995; Leverington and Teller, 2003) and so it was very unlikely that dissolved inorganic carbon ever reached the level necessary make such a high carbon reservoir.

This leaves the third scenario, which is the most viable of the three. That is for a contributing factor of the material dated from the varves to be reworked material from the ice sheet. The most likely source for material in this age bracket is material between the Matheson till and older till from the

Wisconsinan glacial stage. This material has been dated to > 37.0 ka BP (GSC‐2148) (Brereton and Elson,

1979; Paulen, 2001).

Stuckenrath (1977) demonstrates that mixtures of infinite age carbon with modern carbon will alter the measured age. In order to estimate the true age of these samples, relative amounts of aquatic versus terrestrial material were measured from the sample photos. The terrestrial material is expected to be from the old carbon source below the Matheson till and the aquatic material is thought to be too delicate to survive glacial transport and should represent “new” material. This estimation was done for three samples: OS‐88628, OS‐88930 and OS‐88949.

The first two samples are from the laminated unit in Reid Lake. OS‐88628 is the paired sample from the laminated unit in Reid Lake. This sample is the leftover material after the terrestrial seeds were removed to make sample OS‐88630. The area estimates for this sample are ~80‐90% old carbon.

This plots the age of the non‐glacially transported material at ~6‐10 ka BP. OS‐88930 has an estimated

~40‐60% old carbon, giving an estimated age of ~6‐9 ka BP (Stuckenrath, 1977). These estimates are in a

34 consistent time frame with the expected true age of these sediments of ~6‐8 ka BP (Hughes, 1965;

Stroup, 2009; Roy et al., 2010; Breckenridge et al., 2011).

The final sample OS‐88949 is from the varve unit. The estimated old carbon from this sample is

~40‐60%. This places the “true” age for this sample at ~18‐21 ka BP. This is still older than expected, so it may be possible that some delicate organic material was preserved during ice sheet transport (Cuffey et al., 2000).

Discussion

The annual resolution provided by the varves makes it possible to directly compare the timing of stratigraphic events within the varve sequences presented in Figure 3, using varve thickness, magnetic susceptibility (MS) and ice rafted debris (IRD). Varve thickness is a direct measure of sediment flux and

MS is a proxy for the proximity to a sediment source. However, MS alone cannot distinguish if the sediment source is from proximity to the shoreline, indicating lake level variation, or from proximity to sediment from the ice margin. In all five lakes presented in this study, MS and varve thickness are compared to determine sediment flux and changes in the sediment source proximity. An annual resolution IRD record from Reid Lake is added and compared to the MS and varve thickness records to determine if source proximity is from the ice margin shifting or from lake level flux (Figure 14). The comparison of these proxies, in conjunction with the chronological time frame provided by the varves, offers insight into the sequence of events leading up to the termination of Lake Ojibway.

Magnetic Susceptibility

The magnetic susceptibility (MS) values are a function of grain size and bulk mineralogy composition (Dearing, 1999). Troughs in the MS record can show either an increase in non‐iron minerals such as quartz, calcite and feldspars, or an absence of iron bearing minerals, or an increase in organic

35 Varve year Magnetic IRD per Stratigraphy Susceptibility Varve thickness varve thickness

2115 Final varve

2100 Connaught Varves Disconformity, no gap in sediments Youngest varve 2000 covered by Cochrane e 1900 Sandy rhymites anc dv

ane A 1800 ochr C

1700

1600

Lake Agassiz 1500

Slumped & folded 13.3 cm 1400 1385 First varve

0246180 12 14 0 20 40 60 80 100 120 140 0 10 20 30 40 50 Κ (SI units x10-6) Varve thickness (cm) #/cm

36 material, or a decrease in overall grain size. In all likelihood, it is a combination of all of these factors that dictates the MS patterns in Figure 5. High values of MS are associated with increased grain size and more iron bearing minerals washed into the site, which indicated proximity to a sediment source, either the paleoshoreline or to the ice margin. The MS data for each lake, plotted in Figure 5, show a general agreement with a few exceptions noted here.

The deviation in MS values during the first 50 varve years at the beginning of the Duparquet series is attributed to ice proximal varve deposition. This is also consistent with thick varve deposition at the beginning of the Duparquet series, and is similar to other known ice proximal series (Antevs, 1925;

Antevs, 1928; Hughes, 1965; Hardy, 1976; Paulen, 2001).

The MS values for Lac Wawagosic are consistently higher than the values for the other lakes.

This difference in values is not thought to be due to closer proximity to a sediment source, but from a difference in the source lithology of the sediments. In Wawagosic, the sedimentary succession was located in deeper water within a small sub‐basin of Lake Ojibway (Stroup, 2009). It also had more sediment influence from the Harricana Moraine and from a local ice stream located near the Harricana moraine (Vincent and Hardy, 1979; Veillette, 1994) throughout the varve record. The hypothesis is that the ice stream that supplied the Wawagosic sub‐basin received its sediment from metamorphic and igneous Archean Canadian shield rocks, whereas the sediment supply to Reid Lake was from the proterozoic sedimentary rocks under Hudson Bay. This is supported by the comparison of X‐ray diffraction derived bulk mineralogy between Wawagosic and Reid lakes (Figure 7). The sediment deposited in Reid Lake has relatively high concentrations of calcite and dolomite, whereas Wawagosic has less carbonate minerals and more iron bearing minerals. The lack of carbonate minerals, and input from iron bearing minerals from the shield rocks explains why Wawagosic has overall higher MS values compared to the other lakes.

The deep water at Wawagosic (Stroup, 2009) allows for better mixing of fine grained material from the entire basin. This mixing may result in a relatively stable MS curve in Wawagosic compared to the high variability MS curves from Reid Lake. In Wawagosic the MS values stand alone from the other lake records, but the overall pattern of the curve is similar. This can lead us to conclude that while the

MS patterns are in sync in multiple lakes spanning the basin, the sediment sources were well mixed before deposition and thus have a similar grain size. Synchronous MS patterns indicate a well mixed, deep water environment (Vincent and Hardy, 1979).

Variability between the lakes is influenced by local sediment inputs, such as the proximity to eskers and different sediment sources from ice streams as discussed in Reid and Wawagosic. The Reid

Lake section formed in relatively shallow water compared to Wawagosic, and is located on the edge of the Lake Ojibway basin (Stroup, 2009). This proximity to a paleoshoreline makes Reid Lake likely to be sensitive to terrigenous inputs, which are interpreted here as a proxy for lake level fluctuations.

In Figure 5, from varve years ~1700‐1760 Reid Lake has a peak in MS values that show more variability and are asynchronous with Duparquet and de Courval. If the lake level had dropped, then the deposition in Reid Lake would have been subject to more influence from local sediments from sources other than the ice sheet. At times of higher lake levels, the sediment would represent a mix from the entire basin and the MS curves from all the lake should show better correlation. In Reid Lake at this time period, the MS pattern becomes out of sync with the other lakes and is associated with increased sand deposition. Consequently, this MS spike in Reid Lake is most likely the result of larger grain sizes and iron bearing minerals being washed into the lake from near shore during a low stand event.

Therefore, when the Reid Lake MS curve diverges from the MS patterns from the other lakes it may be an indication of changes in lake level.

38 Ice Rafted Debris

Magnetic susceptibility alone is insufficient to determine if proximity to the sediment source is due to lake level change or ice margin fluctuation. In order to get a better understanding of when the ice margin was advancing and retreating, ice rafted debris (IRD) was quantified in the varve stratigraphy of Reid Lake with X-ray images.

Increased IRD is associated with ice margin surges such as the Heinrich events (Bond et al., 1992;

Grousse et al., 1993; Robinson et al., 1995; Mangerud et al., 1998; McCabe and Clark, 1998; Larsen et al., 2011). It is unclear whether or not there are more icebergs at the onset of an advance or more from mass wasting as the surging ice dissipates. Ideally the peak in IRD material would coincide with the peak of ice margin advance. One hypothesis is that the peaks could represent surges in the ice stream resulting in more icebergs. This would show an increase in IRD from the initial surge, then a peak followed by IRD concentrations dropping oﬀ as the surge halted (Larsen et al., 2011).

In general, varves that were noted to have signiﬁcant IRD amounts had little to no organic material in them, as opposed to the layers with virtually no IRD which nearly always had some sort of organic material present, even if there was too little material to sample. The varves that have high amounts of IRD have a relatively normal summer layer, while the varves with high organic material content had high amounts of sand with some bedding structures in the summer layer.

During varve years ~1700-1950, which overlap with the Cochrane advance, the varves with high

IRD values are inter-bedded with varves that have thick sandy summer layers showing internal structure

(Figure 11). The structures are small ripple cross sections with layers of organic material in the beds.

This could indicate cyclic shifts between deep water varve sedimentation with IRD and an inﬂux of organic bearing terrigenous material. Increased input of terrigenous material may be an indication of lake level change (Thompson and Oldﬁeld, 1986; Dearing, 1999; Nowaczyk, 2001), although it is unlikely

39 that lake levels fluctuated that dramatically on a sub‐decadal scale. The alternative hypothesis to lake level flux is that the structures are from material deposited during inter‐annual storm events. When the bedding structures appear in the varve stratigraphy, this may indicate that weather conditions were unusual during the Cochrane advance as compared to the rest of the varve record.

Varve 1528

Varve 1528 is a sediment thickness change that has been recorded on both sides of the arctic drainage divide (Breckenridge et al., 2011, in press). It is recorded at Antev’s site 81, located in the

Timiskaming basin south of the arctic drainage divide, outside the spillway channels. This indicates that the southern drainage pathway through Lake Timiskaming and the Ottawa River was still active during the deposition of varve 1528, although the amount of water flow through this area is unknown. It is hypothesized by Breckenridge et al., (2011, in press) that this abrupt increase in varve thickness is due to a diversion of meltwater from Lake Agassiz caused by retreat of the ice sheet north of the drainage divide (Thorleifson, 1996; Leverington and Teller, 2003). This varve is present in two of the lakes in this study, Duparquet and Reid Lake.

In the ~100 years prior to the deposition of varve 1528, the MS values are increasing in Reid,

Duparquet and Lac de Courval (Figure 5). During this same interval varve thicknesses show a slight decrease (Figure 3). The increase in MS values may be an indication of sediment source proximity, indicating a slight lake level drop during the 100 year interval prior to varve 1528. At varve 1528, varve thickness increases and MS values decrease in lakes Reid and Duparquet. Lac de Courval has a slump in the varve sequence during this interval, which may or may not be related to excess meltwater propagating through the system or a change in basin dynamics from rebound. There is a moderate increase in IRD at this time, although not much above background levels, indicating that these events are most likely related to meltwater fluctuations and not a change in ice margin.

40 Because the southern drainage pathway to the Ottawa River is still active, if there was an abrupt increase in meltwater at the time of varve 1528, it would be expected that there would be an increased hydraulic gradient from Lake Ojibway through channels leading to the southern drainage pathway. This increased water flow would cause higher flow in the narrow spillway channels, then varve deposition south of this site as the water spread into the Timiskaming basin. The sand deposits in Montbeillard have a scoured erosional contact with the varves. The varve deposition indicates a stable lake environment, whereas the coarse grained sand indicates fast moving water. These normally graded sand beds could have been deposited during the influx of water from Lake Agassiz at varve 1528 as flow was pinched through the outlet channels, down‐cutting existing varves, before entering the southern

Timiskaming basin as water overfilled the basin.

The closing of the Agassiz outlet to the Superior basin would have lead to extra water entering the Ojibway basin, which would have then overflowed through the Ottawa River outlet (Vincent and

Hardy, 1979; Teller, 1995; Breckenridge, 2007). A significant source of fast moving water would be needed in order to deposit the very coarse grained sand with apparent ripples and normal grading in

Montbeillard. This is consistent with the hypothesis that the sand beds are the result of the diversion of

Lake Agassiz water into the Ojibway basin causing excess meltwater to flow through spillway channels towards the southern drainage outlet at a fast rate.

Closing the Southern Drainage Outlet (Varve 1776)

The end of the varve sequence in Duparquet is possibly related to the shift in water balance.

This shift could be from either a drainage outlet change from the southern Ottawa River outlet to a northern outlet, or a basinwide evaporation event. Both Montbeillard and Duparquet are located in the spillway of the southern drainage outlet. Montbeillard is located just south of the modern arctic drainage divide and Lac Duparquet is located just to north of the divide in the spillway to the drainage

41 outlet. Varve deposition thins, then ceases entirely at varve year 1776 in Lac Duparquet. This indicates that glacial meltwater from Lake Ojibway is no longer flowing past the Duparquet site into the spillway.

As this location disconnected from the main body of Lake Ojibway, the southern drainage pathway closed and water movement shifted north of the Superior basin divide. From Breckenridge et al., (2011, in press), there are no varves recorded in sites to the south of Duparquet after ~1780, consistent with a drainage shift away from the outlet through the Ottawa River.

The timing of the cessation of varve deposition in Lac Duparquet indicates that the sand waves in Montbeillard were deposited sometime between varve years 1375 and 1776, consistent with the hypothesis that these sand deposits are related to a large influx of meltwater at varve year 1528.

During this interval (varve years ~1700‐1760), the MS record in Reid Lake has a peak in values that is not observed in the other lakes. This MS peak is synchronous with low IRD deposition and a slight decrease in varve thickness. Because of the proximity of Reid Lake to the edge of Lake Ojibway, this is a possible indication of a localized lowstand event. This lowstand in Reid Lake could be from local rebound or a lowering of the water plane.

Cochrane Advance

The most significant ice margin fluctuation present in this varve stratigraphy is the Cochrane advance, estimated from changes in varve thickness to have occurred during varve years ~1750‐2000

(Breckenridge et al., 2011, in press). The Cochrane advance is the last advance of the ice sheet before the final termination of Lake Ojibway and the end of the varve sequence. In order to determine if the high MS values recorded during this interval are due to proximity to the ice margin or to the paleoshoreline, the ice rafted debris (IRD) record is used in conjunction with the MS curve from Reid

Lake. Increases in these two proxies at the interval of the Cochrane advance identify this event within the varve stratigraphy.

42 After the low values of IRD associated with an MS peak from ~1700‐1760, there is a moderate peak in IRD concentration from varves ~1740 ‐ 1790. This is similar to the earliest estimations of the beginning of the Cochrane advance (Breckenridge et al., 2011, in press). The largest peak in IRD concentration is from varve years ~1840 – 1925, which is coincident with an increase in MS values. This increase in MS values is associated with an increase in proximity to the sediment source. This is consistent with the ice margin advancing toward the Reid Lake site.

During the Cochrane advance from varve years ~1800‐1880 there are low values of MS and the amount of IRD decreases, with an increase in sediment thickness (Figure 14). This increase in varve thickness is similar to the thickness change that defines varve 1528. The decrease in MS values and IRD concentrations are indicative of a deep water event, coupled with an increase in sediment supply as evident from the increase in varve thickness. It is unclear if these events recorded by the proxies are associated with a cold period within the Cochrane advance or possible meltwater shifts. Further study is needed to identify the perturbation in the system during this interval.

The two peaks in the IRD record during the Cochrane advance years could be from two different ice advance episodes. This scenario is unlikely due to the distance the ice would have to retreat and then re‐advance in such a short time frame (Breckenridge et al., 2011, in press). It is possible that these peaks within the elevated signal represent either initial onset of the advancing ice followed by a large increase in IRD at the height of the advance or from distinct individual surging events within the

Cochrane advance.

After varve year ~1880 the sandy rhthymites appear in Reid Lake. These sandy deposits are associated with high MS values and high IRD, indicating sediment source proximity and that the ice sheet is a likely source of this sediment. Prior work shows that the Cochrane advance covered varve

1979 ~5 km north of the Cochrane limit (Antevs, 1925; Antevs, 1928; Hughes, 1965; Hardy, 1976;

43 Paulen, 2001; Breckenridge et al., 2011), and is no longer found in the varve record after varve year

~2000. This is the point in the IRD record where concentrations drop back down to the background level, consistent with the end of the Cochrane advance.

At the end of the Cochrane advance, the MS values in Reid Lake shows a sharp decline then an increase until the disconformity at varve 2015 (Figure 14). Other disconformities are observed during the interval between the Cochrane advance and the Connaught varves in lakes De Courval and

Wawagosic. These disconformities could indicate a widespread lowstand event given that the change in deposition is consistent throughout the basin during this time interval. These MS fluctuations in Reid

Lake, coupled with the disconformities observed in all three lakes could indicate a brief episode of water filling the basin followed by a partial drainage of Lake Ojibway before the deposition of the Connaught varve series (Breckenridge, personal comm.).

Connaught Varve Series

The current hypothesis for the Connaught varves deposition is increased sediment influx from the ice sheet (Breckenridge et al., 2011, in press) due to increased meltwater trapped by large ice dams in Hudson Straight (Teller, 1995; Teller et al., 2002). The thinning upward sequence indicates that the sediment flux had to exceed the water flux for the Connaught varves to be deposited. Then, as the sediment flux stabilized, meltwater volume increased leading to simultaneous deposition of the varve sequence. This hypothesis is consistent with being able to correlate the Connaught varves by varve thickness (Breckenridge et al., 2011). This correlation is also based on the assumption that the disconformities in Reid, Wawagosic and Lac de Courval all ended simultaneously.

An alternative hypothesis is that the Connaught varves represent a time transgressive series.

One way this is possible is for the Connaught varves to be a proximal to distal series. A transgressive

44 series could have been deposited as outlets shifted from rebound and overflow, altering the meltwater source transporting the sediment (Teller, 2004).

In the event that the thick varves were not deposited simultaneously, and the MS patterns are lined up so that the curves match, then the thick early Connaught varves in Wawagosic were deposited at the same time as the thin varves at the top of the series in Reid Lake (Figure 6). This illustrates that thick proximal varves could have been deposited first at Reid, then de Courval and finally at Wawagosic.

The thin distal varves show the same trend as the varves thinned and the series transitioned into the pellet unit. There is also a strong correlation between varve thickness and IRD concentrations during the Connaught series. These variables together show that grain size and IRD amounts both decreased as the varves thin up‐section, consistent with a model of ice margin or meltwater retreat. This retreat pattern is also consistent with the thinning upward sequence recorded in Duparquet, which is hypothesized in this study to be from a change in meltwater flow from the southern outlet to a northern outlet.

The final counted varves in the three lakes Reid, de Courval, and Wawagosic all end between

2115 and 2122. However, if the uncounted varves above 2122 in Wawagosic are factored into the counts then there is a transgression of the terminal varves from southwest to northeast. This transgression is consistent with an apparent offset in the MS curves of the Connaught series in these three lakes as demonstrated in Figure 6. This timing between the three lakes is also consistent with a transgressive series slumps observed in Lac de Courval and Wawagosic associated with the timing of varve 1528. If these events are both related by meltwater flux, these offset values could give an indication of how long it takes water and sediment to propagate through the system.

45 Conclusions

Varve thickness, MS and IRD can be used in combination to determine which process may be the dominate influence on a proglacial lake system such as Lake Ojibway. Varve thickness is useful for sediment flux and associated meltwater flux. The IRD record shows ice margin fluctuations and MS shows grain size and bulk composition fluctuation. All three attributes together give a clearer picture of the processes within the entire system leading up to the final drainage of Lake Ojibway.

• A radiocarbon date of 8400±53 cal BP located in the laminated unit above the varve stratigraphy

brackets the minimum age for the varve sequences. This is consistent with other dates for these

sediments.

• The sand deposits that have an erosional contact with the varve in Montbeillard may be from

the influx of Lake Agassiz water at varve 1528. The thinning upward varve sequence in Lake

Duparquet, located just north of the arctic drainage divide, reveals possible timing for the

closing of the southern drainage outlet through the Ottawa River at varve number 1776.

• A well defined annual chronology from the varve stratigraphy provides the opportunity to plot

the MS by varve year. This makes it easier to directly compare the signals from different

modern lakes within the Lake Ojibway basin. This comparison of MS signals can be used as a

proxy for sediment source proximity. This sediment source proximity can approximate water

level changes in the basin, or the proximity of the ice margin. The patterns can also help

determine if a varve series is deposited in a transgressive pattern such as the Connaught series.

• An offset in MS curves from the thinning upward Connaught sequences in Reid Lake, Lac de

Courval and Lac Wawagosic along with decreasing IRD densities are a possible indication that

these series proximal to distal time transgressive deposits.

46 • The changes in MS in Reid Lake and the disconformity in Reid, De Courval and Wawagosic after

the Cochrane advance indicate a basin deepening event followed by a partial drainage. This

event precedes the deposition of the Connaught varve series which leads up to the termination

of Lake Ojibway.

• The IRD record in Reid Lake can help pinpoint when ice margin fluctuation was a major influence

on the depositional system. This record vindicates ice margin flux during the time of the

Cochrane advance within the varve chronology provided by Breckenridge et al., (2011, in press).

The results also suggest that there may have been two distinct surging events within the

Cochrane advance.

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