Canadian Journal of Earth Sciences

The pre -Late Wisconsin stratigraphy of southern Simcoe County, Ontario: Implications for ice sheet build-up, decay, and Great Lakes drainage evolution

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2016-0160.R1

Manuscript Type: Article

Date Submitted by the Author: 20-Dec-2016

Complete List of Authors: Mulligan, Riley; Ontario Geological Survey, Earth Resources and GeoscienceDraft Mapping Section; McMaster University, School of Geography and Earth Sciences Bajc, Andy F.; Ontario Geological Survey,

southern Ontario, Stratigraphy, Quaternary geology, glacial history, Keyword: hydrogeology

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The pre-Late Wisconsin stratigraphy of southern Simcoe County, Ontario: Implications for ice sheet build-up, decay, and Great Lakes drainage evolution

Mulligan, R.P.M. 1* and Bajc, A.F 1.

Draft

*Corresponding author: [email protected]

and [email protected]

Ontario Geological Survey 933 Ramsey Lake Road, Sudbury, ON, P3E 6B5 Office: 705.670.5963 Fax: 705.670.5905

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Abstract

Recent three-dimensional mapping investigations in southern Simcoe County, Ontario allow refinement of the existing regional stratigraphic framework. Analysis of 25 continuously-cored boreholes has revealed a complex but consistent sediment succession that provides a record of the last two glacial cycles (Marine Isotope Stages 1-6). Five stratigraphic units (SU 1-5) comprise the pre-Late Wisconsin record. The stratigraphy is floored by a presumed Illinoian glacial complex consisting of a lower, coarse-grained till (SU 1), locally overlain by stratified glaciolacustrine and glaciofluvial sediments (SU2), but more commonly capped by a stone-poor, fine-grained till (SU 3) of the GeorgianDraft Bay lobe. A widespread subaerial unconformity developed on the upper surface of SU3 contains organic-bearing, non-glacial deposits (SU 4) ranging between 54 800 ± 3000 (considered beyond the limits of radiocarbon dating) and 37 450

± 590 14 C yr BP. SU 4 is abruptly overlain by a thick succession of rhythmically laminated lacustrine muds graded upwards into glaciolacustrine silts and clays interrupted by regionally continuous sand bodies (SU 5). The succession is capped (and locally truncated) by Late

Wisconsin Newmarket Till. The sedimentary record of southern Simcoe County is correlated with other well-studied reference sections in southern Ontario and contains information that informs reconstructions of former ice extents in the lower Great Lakes region following the

Illinoian glaciation. Several sediment units host aquifers, but limited thickness and spatial extent, as well as issues with naturally-occurring dissolved gases and solids, restrict their use for groundwater supply.

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KEYWORDS Quaternary geology, stratigraphy, southern Ontario, glacial history, hydrogeology

INTRODUCTION

Knowledge of changing paleoenvironments throughout the Quaternary Period in southern

Ontario has been evolving since the latter parts of the nineteenth century. Early workers focussed

on improving understanding of preglacial drainage systems (Spencer 1890), ice marginal features

(moraines) and postglacial shoreline records (Leverett and Taylor, 1915), as well as the

reconstruction of past glacial and interglacial conditions (Coleman 1939; Berti 1975; Karrow

1990) – work that is still ongoing today.

Due to the highly erosive natureDraft of the last major ice advance, much of the record of older (pre-Late Wisconsin) environments has been removed across wide areas of southern

Ontario. Understanding of previous climatic conditions was deciphered from decades of

investigations on isolated exposures of pre-Late Wisconsin sediments in pits and quarries

(Coleman 1932; Westgate and Dreimanis 1967), along prominent Lake Ontario shore bluffs at

Scarborough (Karrow 1967; Eyles and Eyles 1983; Sharpe and Barnett 1985; Eyles and Clark

1988) and Bowmanville (Brookfield et al. 1982; Brookfield and Martini 1999), as well as the

north shore of Lake Erie (Dreimanis et al. 1966; Dreimanis 1992; Dreimanis and Gibbard 2005;

Fig. 1a).

Increased surficial mapping investigations in southern Ontario during the 1970’s led to

the discovery of numerous, near surface, sub-till organic sites inland from the well-studied bluff

exposures (Fig. 1a; e.g. Burwasser 1974; White 1975; Cowan 1975). Recent government

mapping initiatives shifted from surficial studies to regional-scale, three-dimensional (3D)

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investigations aimed at characterizing the full succession of Quaternary sediments overlying bedrock in southern Ontario and providing new insights into the character and distribution of older sediment units across the province (Sharpe et al. 2002; Bajc 2004; Bajc and Dodge 2011;

Burt and Dodge 2011; Marich et al. 2011; Burt and Dodge 2016). 3D mapping projects in

Ontario integrate data collected from a variety of investigative techniques including: surficial mapping; continuous coring of boreholes; ground and airborne geophysical surveying; downhole geophysical logging and hydrogeological data collection (e.g. Burt et al. 2015). The high quality data collected as part of these investigations enhances reconstructions of paleoenvironmental conditions and greatly assists in informing groundwater-related decisions (Frind et al. 2014).

Recent 3D mapping investigations in the southern part of Simcoe County (Fig. 1c,d) have identified a complex, but consistent sedimentDraft succession underlying Late Wisconsin glacial deposits (Bajc and Rainsford 2011; Bajc et al. 2012; Bajc and Mulligan 2013; Bajc et al. 2014).

This paper presents an overview of the pre-Late Wisconsin stratigraphic succession identified and discusses the significance of this record to interpretations of changing environmental conditions and regional extents of the Laurentide Ice Sheet (LIS). The hydrogeological implications of the newly established hydrostratigraphic framework are also discussed as this region of southern Ontario is poised for accelerated population growth over the next few decades and existing models were based on limited high-quality data.

GEOLOGIC SETTING

Bedrock Geology

Southern Ontario is underlain by Paleozoic bedrock of the Michigan basin unconformably overlying Precambrian basement rocks of the western Grenville province

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(Armstrong and Carter 2010). In Simcoe County, the Paleozoic bedrock surface displays large

topographic variations and can generally be categorized into two sub-regions: the Niagara

Escarpment, a north-south trending bedrock cuesta (up to 250m high) stretching from Manitoulin

Island into Western New York State and the Laurentian Valley, a broad bedrock trough, 20-40

km wide, immediately to the east and below the Niagara Escarpment (Fig. 1b-e). The escarpment

formed as a result of differential erosion of resistive Silurian dolostones capping the cuesta

versus softer, underlying Ordovician shale and limestone to the east (Armstrong and Carter

2010). The position and general form of the escarpment pre-date Quaternary glaciations,

although its surface and the form of prominent re-entrant valleys cut into the face of the

escarpment have been significantly altered by glacial and glaciofluvial activity during the

Wisconsin Episode (Straw 1968; Kor andDraft Cowell 1998; Gao 2011; Eyles 2012; Mulligan 2015).

Unlike the Niagara Escarpment, the geometry and form of the Laurentian Valley is far less well

understood. Drift thickness above parts of the valley exceeds 200 m (Fig 1e), and there are

extensive areas where few if any borings reach bedrock (Gao et al. 2006). The Laurentian Valley

is generally believed to represent a former fluvial valley system that connected the Lake Huron-

Georgian Bay basins to Lake Ontario and the St. Lawrence River (Fig. 1b; Spencer 1890; Eyles

et al. 1985), although the preglacial fluvial origin for the bedrock trough has recently been

questioned by Gao (2011) who favors a subglacial meltwater origin, and recent work has

suggested a polygenetic origin for the valley (Sharpe et al. 2013).

Quaternary Geology

Paleozoic bedrock is overlain by a number of Illinoian tills in southern Ontario, the York

Till in Toronto (Terasmae 1960; Karrow 1967; Karrow et al. 2001; Fig. 2a) and the Bradtville

Till along the north shore of Lake Erie (Dreimanis 1992). The Bradtville Till underlies an

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accretion gley paleosol with warm climate indicators (Dreimanis 1992) and the York Till underlies non-glacial, alluvial and lacustrine sediments of the Don Formation, hosting warm (2-3

˚C above present average temperatures), presumably interglacial, fauna and flora (Coleman

1933; Karrow 1967; Eyles and Clark 1988; Richard et al. 2000). The accretion gley and Don

Formation are correlated to the Sangamon interglacial Episode and record low base levels in the

Lake Erie basin (greater than 15m below modern lake level) and a proto-Lake Ontario, 2 – 20 m above the present level of the lake, respectively (Fig. 2).

At Toronto, the Don Formation is overlain by the Scarborough Formation; a 50 m thick succession of laminated silt and clay passing upwards into trough cross-bedded sand (Fig. 3). The Scarborough Formation is interpretedDraft to record delta progradation into a lake standing at least 45 m above modern lake level, presumably due to glacier ice blocking the St. Lawrence valley during the Ontario Subepisode (Karrow 1967; Eyles et al. 1985; Fig. 2). Drainage was likely diverted initially to the southeast, through the Rome outlet (Fig. 1b) and into the Hudson

River. The Scarborough Formation has been mapped in the subsurface as far north as the Oak

Ridges Moraine (ORM; Fig. 1c) in river valleys north of Lake Ontario (Karrow 1967), in subway excavations (Watt 1954; Lajtai 1969), in continuously cored boreholes (Sharpe et al. 2003), and in seismic investigations (Sharpe et al. 2002). Eyles et al. (1985) identified coarsening-upward successions in downhole geophysical logs as far north as Barrie which they assigned to the

Scarborough Formation (Figs. 1,2).

The upper surface of the Scarborough Formation along the Lake Ontario bluffs is incised by large channels (up to 2 km wide and more than 50 m deep) interpreted to record either: a drop in base level causing subaerial erosion of the delta surface (Karrow 1967); enhanced scour by fluvial feeder systems and slumps off the delta front during ice advance from the northeast

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(Eyles and Eyles 1983); or subglacial meltwater erosion (Sharpe and Barnett 1985). Karrow

(1974) suggested that fossiliferous gravels (Pottery Road Formation) partially infilling these

valleys may be correlative to the St. Pierre beds in the St. Lawrence lowlands. However,

competing hypotheses and a lack of chronological constraints continue to obscure the

reconstruction and correlation of Early and Middle Wisconsin sedimentary units.

Fine-grained diamict of the Sunnybrook drift (Fig. 2) overlies the Scarborough Formation

and records the further encroachment of ice into the Ontario basin. Divergent views on the origin

of these deposits (i.e., glacial (Karrow 1967; Sharpe and Barnett 1985; Hicock and Dreimanis

1992) versus glaciolacustrine (Eyles and Eyles 1983; Westgate and Chen 1987; Eyles et al. 2005) have been presented, however, bothDraft interpretations are in agreement regarding the presence of glacial ice well within the Lake Ontario basin. Ice certainly advanced south and west

of the Rome outlets (Fig. 1b) and may have abutted parts of the Niagara Escarpment in Ontario

and/or New York State, causing a transgression in the Lake Erie basin and deposition of Member

B of the Tyrconnell Formation (Dreimanis 1992; Barnett et al. 1996; Karrow et al. 2000; Fig.

2c).

A period of ice withdrawal following the deposition of the Sunnybrook drift occurred

during the Port Talbot Phase. Ice may have retreated sufficiently to uncover the Rome outlets, re-

establishing drainage into the Hudson River valley. Water levels in both the Erie and Ontario

lake basins fell resulting in the deposition of Member C of the Tyrconnell Formation in Lake

Erie (Dreimanis 1992; Fig. 2c) and the lower Thorncliffe Formation delta (approximately 130 m

asl) at Toronto (Karrow 1967; Eyles and Clark 1986; Fig. 2a,b). Rising and possibly fluctuating

water levels are recorded by younger sediments of the Thorncliffe Formation, attributed to the

Middle Wisconsin Brimley and Farmdale Phases (Fig 2a). Iceberg scours into sands (Eyles et al.

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2005) and intervening Seminary and Meadowcliffe diamicts (tills?) (see Karrow 1967; Eyles and

Eyles 1983) record persistent ice in the Lake Ontario basin until the region was eventually covered by Late Wisconsin (Michigan Subepisode) ice depositing the Catfish Creek and

Newmarket/Northern tills sometime after 22.8 14 C ka BP (Hobson and Terasmae, 1969; Fig. 2).

METHODS

Field work in southern Simcoe County was conducted during the summers of 2010-2012 and the fall of 2013, commencing with surficial mapping investigations (Bajc and Rainsford

2010; Mulligan 2011; Mulligan and Bajc 2012), followed by geophysical surveying and drilling of continuously cored boreholes (Bajc and Rainsford 2011; Bajc et al. 2012; 2014; 2015; Figs.

3,4). Draft

A ground–based gravity survey was conducted in order to better constrain the poorly- defined bedrock surface of the Laurentian trough. Low residual gravity signatures were interpreted as possible depressions in the bedrock surface and guided the site selection process for subsequent drilling in an attempt to intersect the most complete sediment records within the study area (Fig. 4). 25 boreholes were drilled in southern Simcoe County with all but one reaching bedrock (Fig. 3). Continuous core (8.5 cm diameter) was drilled in 1.5 m intervals using a conventional mud rotary system equipped with a modified Christiansen core barrel retrievable by wireline to maximize core recovery and preservation of fine sedimentary structures. Core recovery for the entire project was >90%). Boreholes were logged using standard sedimentological logging techniques recording grain size, sedimentary structures, bed contacts, and lithology. Representative samples of all significant sediment units were collected at 1.0-1.5 m intervals and analyzed for particle size with tills being tested for total carbonate,

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calcite:dolomite ratios and heavy mineral composition. Individual facies are grouped into facies

associations that record evolving depositional systems. Fossils were isolated from organic-

bearing sediments by wet sieving. Residues were picked under a binocular microscope to isolate

wood, needles, leaves, seeds, molluscs, insect fragments, bones and teeth. Only wood, leaves of

terrestrial plants and, in one case, valves of the terrestrial mollusc Succinaeidae sp. were sent out

to the Illinois State Geological Survey Radiocarbon Dating Laboratory for radiocarbon age

determination by accelerator mass spectrometry (AMS). A standard acid-alkali-acid

pretreatment was performed on the wood and leaf samples prior to combustion to remove any

potential modern contaminants (humic and fulvic acids). Shells were treated using 0.2 M HCl to

remove secondary carbonates then were digested in 100% phosphoric acid. CO 2 recovered from

the combustion of wood/leaves and the Draftdigestion of shells was sent to the Keck Carbon Cycle

AMS Laboratory of the University of California-Irvine for AMS 14 C analysis using the

hydrogen-iron reduction method with d 13 C values measured on the prepared graphite.

Radiocarbon dates are reported as uncalibrated ages (Table 1). Only preliminary paleoecological

work has been carried out on pollen and macrofossil assemblages to date.

FACIES ANALYSIS A consistent sediment succession, consisting of 5 Stratigraphic Units (SU) is observed

throughout the study area. The stratigraphy is floored by a lower glacial package consisting of

two tills locally separated by intervening glaciolacustrine/glaciofluvial sediments (SU1-3; Fig.

4). The top of the upper till is marked by a distinct weathering profile at many locations and is

overlain by non-glacial, organic-bearing alluvial and lacustrine deposits (SU4; Fig. 4) which are,

in turn, buried by thick successions of lacustrine and glaciolacustrine deposits (SU5; Fig. 4). The

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Late Wisconsin Newmarket Till overlies and truncates the older sediment units in the study area

(Fig. 4).

SU1 – Lower Coarse-Grained Till

SU1 is a light to dark grey or brownish-grey, overconsolidated sand- and clast-rich diamicton, up to 34 m thick (Fig. 4), usually resting directly on the Paleozoic bedrock surface.

The matrix is generally massive but very crude stratification of clast-rich and clast-depleted facies is observed in some boreholes (Fig. 5a,b). Clasts range up to 50 cm in diameter and make up approximately 10-15% of the diamict by volume. They are sub-angular to sub-rounded and dominated by Paleozoic lithologies with common Precambrian clasts. Fine-grained limestone clasts are commonly faceted with flattened,Draft sub-horizontal, upper and/or lower surfaces (Fig. 5c). Small bullet-shaped clasts ornamented with gouges or fine parallel striae are also commonly observed. The fine sand fraction (0.125-0.25 mm) of the diamict matrix contains slightly higher concentrations of total heavy minerals including higher almandine garnet grain counts and lower concentrations of tremolite/actinolite and clinopyroxene diopside relative to the younger fine- grained till (SU3) suggesting the possibility for different source areas for the two tills.

The combination of clast faceting and parallel striations, both on clasts within the till matrix as well as on the underlying bedrock surface, is suggestive of subglacial transport

(Boulton and Deynoux 1981; Evans et al., 2006). The predominantly massive matrix likely reflects high cumulative strain (Boulton 1987) and complete homogenization of transported debris. Stratified horizons may record changes in subglacial dynamics (ice flow velocity, thermal regime, bed coupling and de-coupling, production of basal meltwater (Brown et al. 1987;

Meriano and Eyles 2009; Slomka et al. 2015), variations in the composition and stone content of

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debris bands at the base of the glacier or changes in the substrate fronting the advancing ice

margin (Roberts and Hart 2005).

SU2 – Lower Glaciolacustrine Complex

SU2 ranges between and greater than 30 m thick (Fig. 4), but remains poorly understood

as it was intersected in just 7 of 25 boreholes and facies types are highly variable. Silt, clay and

fine-grained diamictons are most commonly encountered but significant (10-16m thick)

packages of sandy sediment occur locally with no clearly identified spatial trends. The lower part

of SU2 is composed of horizontally-laminated and clast-free silt and clay with rare thin (10-

15cm) sandy interbeds (Fig. 6). Deformed horizons and interbedded diamict are increasingly common up-section (Fig. 6). Massive orDraft planar bedded sand and gravel facies commonly form the upper part of SU2 (Fig. 6) and directly underlie SU3. Detrital organic was recovered from

SU3 in SS-11-08 (Fig. 4); all samples yielded ages beyond the limits of radiocarbon dating.

The fine-grained and well-sorted nature of the fine-grained facies in SU2 suggests a quiet

subaqueous (lacustrine) depositional environment (e.g., Ashley 1975). Clasts and diamicton beds

within the fine-grained part of the succession may record delivery of coarse-grained material

from drifting ice, in an ice-contact lake (Smith 2000; Eyles et al. 2005). The sand-rich facies of

SU2 suggest a proximal sediment supply, which, given the stratigraphic context (see below) was

likely an advancing ice margin. SU2 is interpreted to record a brief period of ice recession from

the study area following the deposition of SU1 and prior to deposition of SU3.

SU3 – Lower Fine-Grained Till

SU3 has been observed to overlie either SU1, SU2 or Paleozoic bedrock. It is composed

of a grey to dark grey clayey silt to sandy silt diamicton up to 30 m thick (Fig. 4). The matrix is

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overconsolidated and generally appears massive (Fig. 7a), but sand, silt, or gravelly interbeds are observed in some boreholes, and vertical zonation of clast-poor and clast-rich facies are common. SU3 commonly displays coarser facies where it overlies bedrock, SU1 or the coarse upper part of SU2, and finer facies where it overlies lower SU2 deposits. Where it rests directly on bedrock, the upper surface of the rock is gouged and striated (Fig. 7c). Clasts make up less than 5% of the diamicton by volume and are dominated by local Paleozoic lithologies (up to

90%) (Fig. 8). Of note are high proportions of black, petroliferous shale clasts derived from the

Collingwood Member of the Lindsay Formation (Armstrong and Dodge 2010), which outcrops and subcrops to the northwest. This lithology is noticeably absent from SU1. Clasts are angular to subrounded and are generally less than 5 cm in diameter, although larger boulders have been observed. In some boreholes, rotationalDraft and linear mechanical disaggregation and shearing of clasts was observed (Fig. 7b). Many larger limestone clasts are faceted and striated. Carbonate analysis of the till matrix reveals lower calcite:dolomite ratios, and generally lower concentrations of heavy minerals. SU3 is typically found at lower elevations (140-150 m asl), and forms thinner successions (5-10 m thick) beneath lowland areas compared to regional uplands where it is commonly 10-30 m thick, lying between 180 and 200 m asl (Fig. 4). SU3 forms thick successions and is interbedded with stratified material in the south-central part of the study area (boreholes SS-11-01, SS-12-06; Fig. 4) but was not intersected in SS-11-02 or at a cored borehole just outside the study area in Schomberg (Figs. 3,4). The structural contour of

SU3 closely resembles that of the underlying bedrock surface. The upper 0.5 - 2 m of SU3 commonly displays mottled discolourations, with light grey, green, and orange staining reminiscent of soil formation (Fig. 7d). In these zones, matrix carbonate is severely depleted and redistributed downward into fractures (Fig. 7e).

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The poorly sorted, massive, overconsolidated matrix, striated clasts, and gouged bedrock

surface supports a subglacial origin for SU3 (Iverson 1991; Benn and Evans 1996; Hambrey and

Glasser 2012). The presence of black shale clasts of the Collingwood Member, Lindsay

Formation, suggest ice flow was sourced from the Georgian Bay basin to the north-northwest.

Broken and disaggregated clasts in the matrix likely record mobilized sediment and clast

crushing in the subglacial environment (van der Meer et al. 2003). Rotation and shearing of

clasts may indicate that transient deformation and lodgement processes were operating beneath

the ice that deposited SU3 (Evans et al. 2006; Piotrowski et al. 2006). Thick, stacked sequences

of diamicton and stratified sediment attributed to SU3 in the southern part of the county may

record ice-marginal sedimentation as debris flows (flow tills) during the retreat phase of SU3

(Evensen et al. 1977). Depletion of carbonateDraft and colour staining in the upper part of SU3

indicate prolonged subaerial exposure and incipient soil development following glacier retreat

(Karrow et al. 2001).

SU4 – Non-glacial deposits

A succession of non-glacial sand, silt, gravel, and gyttja with localized peat beds

unconformably overlies the paleosol developed on SU3 (Figs. 4,8). These deposits were

intersected in 14 of the 25 boreholes drilled as part of this project and can be up to 9 m thick

(Fig. 4) although intercepts of a few meters are far more common. Ripple-drift cross-laminated

and planar-laminated fine- to coarse-grained sand with silt drapes are the predominant facies

within SU4 (Fig. 8a,c). Planar bedded, pebbly, medium- to coarse-grained sand with gravelly

interbeds commonly occur near the base of the unit and are generally overlain by massive to

faintly laminated silt, clay, gyttja or peat (Fig. 8b), which are, in turn, overlain by the sand beds.

Organic remains occur in most facies observed within this unit.

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A rich and diverse assemblage of plant and animal macrofossils including wood, leaves, conifer needles, fungal filaments, molluscs (aquatic and terrestrial), bones, teeth and insect fragments were recovered from SU4 (Fig. 9). They are concentrated along ripple foresets, in planar laminated to massive units as well as in compacted and humified peat and silty gytjja beds that likely accumulated in situ (Fig. 8a,b). The recovery of fungal filaments (Fig. 9) from borehole SS-13-01 indicate the presence of aerobic soils (J. McAndrews, written communication,

2014). Preliminary analysis of macrofossils and pollen indicates interstadial conditions for all but one occurrence. Pollen concentrations are generally low if not absent and indicate sedge- dominated tundra communities (J.H. McAndrews, Professor Emeritus, University of Toronto, written communication). Well preserved leaves of Dryas integrifolia and Salix herbaceae , shrubs often associated with newly deglaciatedDraft landscapes (Anderson et al. 1992) were commonly recovered from the non-glacial unit (Fig. 9b). Several specimens of Pleurocera acuta , a distinctive freshwater gastropod currently restricted to the lower Great Lakes-St.

Lawrence region (Clarke 1981) and broken valves of unionid clams were recovered from gravelly, alluvial, non-glacial deposits in SS-12-03 (Figs. 3,4,9c) at 137 m asl. Both taxa have only been reported from Sangamon and mid-late Holocene sites in southern Ontario suggesting a possible interglacial assignment to this unit (Kerr-Lawson et al. 1992). Wood and Dryas integrifolia leaves from SU4 record ages ranging between 37.5 and greater than 54.8 14 C kyr BP

(Fig. 10). Generally, organic material encountered at higher elevations yielded progressively younger ages than those at lower elevations (Fig. 10; Table 1). Further, dates returned from organic material in SU4 appear to plot into three distinct elevation ranges (Fig. 10).

SU4 records non-glacial conditions potentially spanning the Sangamon through the Early and Middle Wisconsin. The wide variety of sediment types and facies record the many different

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subenvironments within the paleo-landscape following deglaciation, just as is observed today

within the region (e.g. Mulligan and Bajc 2012). Peats accumulated in poorly-drained

depressions or along the shallow, sheltered margins of lakes; gravelly facies record alluvial

sediment transport, likely in streams flowing across the pre-existing landscape (SU1-3); organic-

rich rippled sands likely record deltaic or nearshore sedimentation in standing water bodies that

inundated low-lying areas. Macrofossil assemblages changing from warm, possibly interglacial

conditions at lower elevations ( Pleurocera acuta and unionid clams ; Fig. 10c) to cold tundra

conditions ( Dryas integrifolia and Salix herbaceae ) at higher elevations is consistent with the

trend of decreasing ages with higher elevations observed in SU4 deposits across southern Simcoe

County, and together record climatic deterioration and progressive drowning of the paleo-

landscape spanning the Sangamon throughDraft to Middle Wisconsin. The apparent lack of sites with

warm assemblages at higher elevations is probably a result of vigorous erosion during the

subsequent transgressive event that deposited SU5 (see below). Grouping of dates into distinct

clusters along specific elevation ranges may reflect water plane stabilization during the regional

transgression, either due to stalls in the advance of the LIS, or rapid rise following outlet

blockage, followed by prolonged periods of relatively stable water levels until the next outlet

was overridden by the LIS (Fig. 10).

SU5 – Upper Glaciolacustrine Complex

A succession of dominantly fine-grained silty and clayey deposits containing occasional

zones with abundant ice-rafted debris, with subordinate very fine-grained sand to gravelly sand

comprises SU5. This unit reaches thicknesses of up to 125 m (Fig. 4). The thickest successions

were encountered beneath upland areas as compared to intervening lowlands (Fig. 5). Finely

laminated and highly bioturbated silt and clay rhythmites abruptly overlie SU4 and pass upwards

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into well bedded/laminated rhythmites, which make up the bulk of SU5 in most boreholes (Figs.

4, 11a,b). Rhythmite couplets are between a few mm to 15 cm thick (Fig. 11b). The coarse fraction generally has a sharp erosive base (Fig. 11b, overlain by fine planar laminated silt and sandy silt and grades upward into the finer fraction of each couplet, which is commonly massive silt fining upward into clay and silty clay (Fig. 11b). No consistent trends in rhythmite thicknesses are observed within the succession on a regional scale. The highest number of couplets that were counted in a single borehole was 1795 in SS-11-02 (Figs. 3,4). Discrete deformed horizons, 5-20 cm thick, are observed throughout the otherwise undisturbed laminated succession (Fig. 11c,d). In the northern and eastern parts of the study area, deformed zones are more common, thicker and overlain, in places, by massive or deformed silt with abundant granules and pebbles and light grey silt Draftor clay intraclasts or fine-grained debris-rich beds up to several metres thick. Large-scale (10-30 cm) folds, flames, and ball and pillow structures comprise the ductile deformation features; micro-faults with diverse orientations and offsets of up to 2 cm are abundant within larger deformation features (Fig. 11d). Abundant trace fossils

(tracks and burrows) are commonly observed along the parting planes of the rhythmites especially in the finely laminated, and locally bioturbated, lower few meters of SU5 (Fig. 11e).

Thin sand stringers or interbeds commonly host fine detrital organics and high concentrations of dissolved gases (Fig. 11f).

Thick sand bodies are common within SU5, particularly in the north near Barrie (Fig. 1) and in upper parts of the succession where continuous beds of sand and/or gravel approaching 30 m thick underlie the surficial Late Wisconsin till (Fig. 4). Sands display planar or ripple-scale cross-laminations and are deformed in many boreholes (Fig. 12). The sands generally form the upper parts of thick, aggradational, coarsening-upward successions readily seen in gamma and

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conductivity logs. The fine-grained rhythmites coarsen-upwards and are overlain by ripple-scale

cross-laminated very fine- to fine-grained sand to planar-bedded medium- to coarse-grained sand

with lesser pebbly sand or gravel (Fig. 4). Generally, sandy interbeds in SU5 are coarsest and

thickest in the northern and eastern parts of the study area (Fig. 4) as well as in the uppermost

parts of the succession, where they are mined for aggregate at a few locations (Fig. 12d).

Borehole SS-13-05 (Figs. 3,4) represents the extreme of this trend, where SU5 is composed

almost exclusively of coarse, cobble-rich gravel.

Fine-grained detrital organics are commonly encountered within the sand bodies, and

consist primarily of water-worn wood. Well preserved Dryas integrifolia leaves were, however, recovered from thin peat mats in a sandDraft body in SS-12-04 (Figs. 3,4,10). The organic material within SU5 deposits range in age from 28.06 to 51.8 14 C kyr BP, an age which is considered to

be beyond the limits of radiocarbon dating. The upper surfaces of sand bodies encountered in

boreholes drilled in uplands are commonly observed at similar elevations – the lowest unit

occurs at 190-200 m asl, the middle unit at 225-235 m asl, and the uppermost sands are

unconformably overlain by Newmarket Till. At borehole SS-12-04 (Figs. 3,4), the occurrence of

heavy iron-staining and thin peaty beds containing well preserved leaves of Dryas integrifolia

possibly suggest periods of subaerial exposure during deposition of SU5 (Fig. 12c).

The fine-grained nature of the finely laminated rhythmites suggests a quiet subaqueous

depositional environment (Smith and Ashley 1985). The finely laminated and bioturbated

lowermost part of the succession records suspension settling of fines during the earliest parts of a

regional transgressive event. Within the rhythmite succession, interbedding of the coarse (silt)

beds likely record density underflows along the lake bottom (Ghibaudo 1992), although the

sediment source remains unclear. Fining upward trends within the fine (clay) beds may record

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sediment shutoff, possibly due to ice cover in the lake. However, comparison of the range of reported radiocarbon ages (17 000 years) with the maximum number of couplets (clay caps) counted in SU5 (1795) reveals a large discrepancy, suggesting that a significant portion of the sediment record is missing at each location, or that seasonal ice cover was not the primary control on sedimentation rates/patterns. The abundance of trace fossils in the silt and clay layers throughout SU5, and preservation of fine detrital organics within some of the sand bodies (SS-

12-04), suggest that the lake that occupied the region remained a productive habitat (i.e., was proglacial) following the drowning event that buried the organic-rich SU4 deposits.

Clast-rich zones and diamict layers likely record sediment delivery to the basin by rainout from floating ice (Dowdeswell and DowdeswellDraft 1989). Since these intervals are far more common at the eastern end of the study area (SS-11-01 and SS-13-06) compared to the western side where rivers would have been feeding into the lake, it is likely that the source of ice and debris was the Simcoe ice lobe of the approaching LIS. The diamict beds may record slumping and/or downslope resedimentation possibly in response to oversteepening, seismic activity

(Eyles et al. 1983; Vardy et al. 2010), or more ice-proximal sedimentation as ice approached.

Coarsening-upward successions of fine-grained rhythmites passing upward into planar and ripple laminated sands are typical of progradational systems as occur in deltaic settings

(Ashley 1975). The occurrence of laterally extensive sand bodies, traceable for 10’s of kilometers at distinct elevation ranges may record regional base level changes linked to the opening and closing of ice-dammed outlets.

Together, the upward increase in rhythmite thickness, coarsening and thickening of sand bodies within the succession, and the occurrence and degree of deformation and ice-rafted material

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suggest that SU5 was deposited in an ice-dammed lake that spanned more than 17 000 years and

became increasingly glacially-influenced prior to eventual overriding by Late Wisconsin ice and

the deposition of Newmarket Till.

DISCUSSION The following discussion aims to place the environmental changes described above into a

regional paleogeographic context established by previous workers in other areas of southern

Ontario and the Great Lakes basins. Lack of finite dating control on sediments within and below

the deeply-buried non-glacial alluvial deposits (Table 1; Fig. 10) complicates direct correlations

between Simcoe County and regions to the south. Similar problems have existed for decades at various other pre-Late Wisconsin organicDraft sites across southern Ontario (Dreimanis and Karrow 1972; Berti 1975; Karrow et al. 1984; Karrow 2004; Bajc et al. 2015). We emphasize the

implications of the southern Simcoe County stratigraphy on interpretations based on classic ‘type

sections’ as well as additional characterization of processes and the nature and timing of

environmental change that is possible through detailed analysis of cored boreholes.

Regional correlations and implications for ice sheet reconstructions

The lower till complex (SU’s 1-3) in southern Simcoe County is interpreted as Illinoian

in age and equivalent to the York (Terasmae 1960; Karrow 1967) and Bradtville (Dreimanis

1992) tills of Toronto and the Lake Erie bluffs area, respectively (Figs. 2,13a,b). This correlation

is based on the stratigraphic position of the lower tills resting directly on Paleozoic bedrock and

the deep weathering profiles developed on the upper surface of the till suggesting prolonged

subaerial exposure prior to burial. Radiocarbon dating of deeply buried wood recovered from an

alluvial deposit containing a possible interglacial molluscan assemblage (SS-12-03; Figs. 3,4,9c)

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returned radiocarbon ages of greater than 51 700 14 C yr BP (Fig. 10) constraining the age of the underlying till. Although tills of presumed Illinoian age have been recognized in southern

Ontario for several decades, relatively little insight has been gained on the characteristics of the ice sheet(s) that deposited them. Classic exposures of York Till in Toronto (Terasmae 1960;

Karrow 1967; Eyles 1987; Karrow et al. 2001) are generally described as thin, weathered units that indicate conflicting paleo ice flows from the northeast (Karrow 1967) and/or from the south

(Terasmae, 1960). Watt (1954) and Lajtai (1969) reported York Till interbedded with and overlain by lacustrine sediments in subway tunnel exposures in downtown Toronto, but these are the only references to multiple till layers within the formation in southern Ontario. The

Bradtville Till in the Lake Erie basin, however, consists of multiple till layers, and appears to share similar sediment facies and architectureDraft to our SU1-3, but has only been observed in a few boreholes along the central north shore of Lake Erie (Figs. 1a, 2c; (Dreimanis 1992; Barnett et al.

1996).

Till facies observed in Simcoe County boreholes assist in characterizing the advance of ice that deposited the lower till complex, and help to constrain regional paleoenvironmental conditions. Consistent vertical facies variations suggest an early and late phase of ice flow (Fig.

13a,b). Lithologic analysis of clasts within the matrix suggests northeasterly flow for the lower facies rich in shield lithologies (SU 1) and a north-northwest provenance for the upper facies rich in Paleozoic lithologies, including the Collingwood member of the Lindsay Formation (SU 3).

The consistent coarser-textured and more poorly sorted lower till (SU1) facies compared to the finer-grained upper till (SU3) facies (Figs. 4,5,7) is similar to trends observed within Late

Wisconsin till successions in southwestern Ontario (e.g. Catfish Creek and overlying Port

Stanley or Tavistock tills), which have been interpreted to record shifts from regional ice flow

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during the main phases of the Late Wisconsin to more lobate flow during deglaciation (Barnett

1992; Bajc et al. 2015). The fine-grained nature of SU3 results from the incorporation of silty

and clayey glaciolacustrine sediments deposited up-ice within the Georgian Bay basin. Common

occurrences of finely laminated rhythmites at the stratigraphic position of SU2 north of Southern

Simcoe County support a readvance over lacustrine deposits in that area (Mulligan 2016), but

facies trends and future geochronologic work may support a significant age difference between

the two till units rather than a brief interstadial period separating them. Additional

sedimentological and/or palynological studies will assist in refining the age assessment of the

two tills and intervening laminated deposits. Bedrock topography within the Laurentian Valley at

the base of the Niagara Escarpment (Fig. 1) likely played a significant role in determining the

behaviour of ice flow during pre-Late WisconsinDraft glacial events.

Evidence of both lodgement and deformation within each of the tills may indicate

transient changes in subglacial dynamics and/or hydrology over varying spatial and temporal

scales (Slomka et al. 2015). The presence of intervening glaciolacustrine deposits between the

two compositionally distinct till facies suggests a period of ice withdrawal following the

deposition of the lower till unit. Additionally, a peculiar lack of glaciolacustrine deposits

associated with the retreat of ice responsible for the deposition of SU3 in southern Simcoe

County, compared to their common occurrence to the north (Mulligan 2016), suggests that no

large proglacial lakes fronted the retreating ice margin (Fig. 13b). One possible explanation is

that ice retreated north of the St. Lawrence valley uncovering low level outlets to the Hudson

River prior to retreat of ice lobes in the upper Great Lakes basins. This facilitated drainage of

Erie-Ontario, and possibly Huron lake basins, during the Illinoian deglaciation in contrast to the

extensive impoundments that occurred during the deglaciation of Late Wisconsin ice (Larson and

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Schaetzl 2001). The reader should note that in this discussion we suggest minimum and maximum water plane configurations based on the character and occurrence of sediments observed in southern Simcoe County boreholes, using local elevations only. Inferences on changes in drainage patterns for the Great Lakes basins during the Sangamon through to Middle

Wisconsin are based on the premise that glacial isostatic effects were similar then to those reconstructed during the last deglaciation.

Following retreat of the Illinoian ice sheets, the tills were exposed to extensive subaerial weathering and SU4 was deposited in low-lying areas of southern Simcoe County. The weathering interval is likely coincident with the development of the ‘accretion gley’ that formed above the Bradtville drift beneath the LakeDraft Erie bluffs (Fig. 1a; Dreimanis 1992), as well as weathering profiles that developed on colluvium overlying the York Till at Woodbridge (Fig. 1a;

Karrow et al. 2001) suggesting Sangamon Interglacial conditions. Regional base levels were at least 35 m lower than at present in southern Simcoe County during this interval. This hydrologic configuration is remarkably similar to the early Holocene Stanley low phase in the Lake Huron basin when waters were more than 100 m lower than present levels due to isostatic depression of the outlet at North Bay (Stanley 1936, Karrow et al. 1975; Lewis et al. 2008; Figs. 1b, 13c).

Meanwhile, climate had ameliorated sufficiently that temperate species were able to thrive in the region (McAndrews 1981).

It has long been held that the Laurentian Valley records the drainage pathways of preglacial fluvial systems draining Georgian Bay into Lake Ontario (Fig. 1b; Spencer 1890;

Karrow 1967; Eyles et al. 1985; Sharpe et al. 2013). Gao (2011) cited a lack of preglacial sediments exposed within the valley, as well as a lack of well-defined channel system(s) as evidence that the trough between Georgian Bay and Lake Ontario is a product of Quaternary

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glacial and subglacial meltwater erosive processes. These observations alone do not preclude the

use of the Laurentian Valley as a pre-glacial drainage pathway. Based on the relative elevations

of the bedrock surface within the Laurentian valley (80-135 m asl), compared to the significantly

higher southern outlets at Port Huron and Chicago, the Laurentian Valley remains the mostly

likely candidate for pre-Illinoian routing of drainage for the upper Great Lakes.

The occurrence of thick till successions (SU 1-3; Fig. 4) in the southern part of Simcoe

County may have been sufficient to form a drainage divide between the Lake Huron and Ontario

basins by the end of the Illinoian. Continuously cored boreholes in the ORM area at Nobleton

encountered the Don Formation at 88-101 m asl (Sharpe et al. 2003). Paleocurrent directional indicators for the Don Formation are onlyDraft available at Toronto and suggest former flow into the proto-Lake Ontario basin (Eyles and Clark 1988). Some (or all) of the fluvial systems recorded

by SU4 sediments in Simcoe County may have formed the headwaters of the paleo-Don River

that contributed to fluvio-lacustrine sediment deposition along the margins of a proto-Lake

Ontario 2-20 m above the present level of the lake (77-97 m asl; Karrow 1967; Eyles and Clark

1988; Fig. 13c). Dreimanis (1992) suggested the Laurentian Valley was likely still draining the

Lake Huron basin during the Sangamon, based on low water levels (at least 20m below modern

levels) in the Lake Erie basin permitting development of the accretion gley overlying the

Bradtville Till as well as minimal retreat of the waterfall in the Erigan valley. However, the

extent of the former catchment area for the paleo-Don River is unclear and until that can be

confidently established, through future detailed subsurface investigations, providing improved

constraint on the regional topography of the Illinoian glacial package (SU 1-3), assessing any

paleo-hydrological connection between Simcoe County and the Toronto area during the

Sangamon remains problematic.

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SU 4 shares many sedimentological and paleoecological characteristics with organic- bearing sediments of the Don and Scarborough formations of the Toronto area (Coleman 1932,

1941; Karrow 1967; 1990; Eyles and Williams 1992), but it is suggested here that SU4 spans the entire time interval from the end of the Illinoian to the Middle Wisconsin in southern Simcoe

County (Fig. 13c-f). The range in elevations and dates for the flooding surface represented by the transition from SU4 to SU5 in Simcoe County suggests that the event records a transgression that spanned several tens of thousands of years (Fig. 10), encompassing the entire time interval recorded by non-glacial sediments along the Scarborough Bluffs (Don through upper Thorncliffe formations; see discussion below; Figs. 1a, 2), without any intervening glacial incursions (Fig.

14c-f). The occurrences of Pleurocera acuta and unionid clam fragments in alluvial sediments containing wood fragments dated at greaterDraft than 51 700 14 C yr BP within Simcoe County suggest warm interglacial conditions generally associated with the Don Formation (Miller et al. 1985;

Kerr-Lawson et al. 1992). Significant climatic deterioration in southern Simcoe County is recorded by most occurrences of SU4 that contain plant macrofossils and pollen indicative of interstadial conditions within the region compared to the warm water specimens recovered from lower elevations (SS-12-03). The interstadial organic remains commonly yield finite radiocarbon ages ranging from 38 to 50 14 C kyr BP (Fig. 10), indicating non-glacial conditions persisted in the region until the Middle Wisconsin (Berti 1975; Warner et al. 1988; Bajc et al.

2015).

Eyles et al. (1985) used coarsening upward successions interpreted from geophysical profiles throughout the region in an attempt to map the northward extension of the Scarborough delta. The notion of widespread, water plane-controlled sand packages in the subsurface is supported by this investigation, although the units identified by Eyles et al. (1985) as potential

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correlatives to the Scarborough Formation south of Barrie appear to be younger (SS-12-04; Fig.

4). Assignment to SU5 (Thorncliffe Formation) in southern Simcoe County is proposed, but sand

bodies lying at lower elevations elsewhere in the study area may be chronostratigraphic

equivalents.

Initial flooding of SU4 deposits may have been initiated by isostatic uplift of northern

outlets (North Bay(?); Fig. 1b) and/or by blockage of the St. Lawrence valley by early advances

of the LIS that resulted in the construction of the Scarborough delta at Toronto (Karrow 1967;

Eyles et al. 1985; Kelly and Martini 1986; Fig. 14d-g). Rising water levels drowned low-lying

areas first and by the Middle Wisconsin, only areas above 175 m asl were still exposed (Fig. 10). Although Early Wisconsin ice advance Draftinto the Great Lakes basins is the cause for the deposition of SU5 in Simcoe County, there is no evidence to support ice cover in the study area during this

time. Ice advance into the Lake Ontario basin prior to the Late Wisconsin is recorded by the

Sunnybrook drift (till/glaciolacustrine diamict; see Karrow 1967; Sharpe and Barnett 1985 vs.

Eyles and Eyles 1983; Eyles et al. 2005; Fig. 13e). The advance of ice at this time must have

extended at least far enough south and west in the Lake Ontario basin to block the St. Lawrence

and Rome outlets (Fig. 1b) thus causing water levels to rise and flood the Ontario, Laurentian

valley and Erie basins (Fig 13e). Deposition of up to 115m of glaciolacustrine sediments of SU5

occurred within southern Simcoe County. Meanwhile, the Sunnybrook drift and Thorncliffe

Formation were deposited in the Toronto area and member B of the Tyrconnell Formation within

the Lake Erie basin (Dreimanis 1992; Fig. 13e).

An ice margin in the central Lake Ontario basin during this time interval is supported by

recent investigations in upper New York State (Young and Burr 2006; Karig and Miller 2013;

Kozlowski et al. 2016). Minor fluctuations of the ice front could have produced significant

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changes in water levels, due to repeated blockage and uncovering of southeastern outlets near

Rome, New York (Fig. 1b). A drop in base level (due to northward retreat of ice margins) following deposition of the Sunnybrook drift is suggested by the transition to the lower

Thorncliffe Formation delta at Toronto (Karrow 1967; Fig. 13f). Trough and hummocky cross- stratified sands along the Scarborough bluffs (Eyles and Clark 1986) suggest a water plane of approximately 130 m asl (similar to postglacial Lake Iroquois water levels, which also drained through the Rome outlet). Karrow et al. (2001) report dates of 45.0 ± 0.9 14 C kyr BP and 29.7 ±

0.9 14 C kyr BP for wood and bone, respectively, recovered from peat beds within a gravel unit overlying the Sunnybrook drift at approximately 152 m asl at Woodbridge (Figs. 1a,2). Member

C of the Tyrconnell Formation records a drop in water levels in the Lake Erie basin, to 10m below present (Fig. 2; Dreimanis 1992),Draft representing a drainage event, likely routed through the

Erigan Valley, that is potentially equivalent to that recorded by the lower Thorncliffe at Toronto

(Fig. 13f). Additional support for a low stand during this time interval is recorded at the Haight site where an organic-bearing alluvial deposit overlying glaciolacustrine deposits burying the

Sangamon geosol occur well below present lake level (Barnett et al. 1996). Reports of insect and plant assemblages from the Toronto and Lake Erie regions suggest a boreal to tundra environment during this time interval (Berti 1975, Barnett et al. 1996).

Subsequent changes in ice-marginal positions influenced water plane elevations as recorded in the Seminary, Middle Thorncliffe, Meadowcliffe and Upper Thorncliffe units at

Toronto (Figs. 2; 13g; see Eyles and Eyles 1983 vs. Dreimanis 1984 for discussion) and member

D of the Tyrconnell Formation along Lake Erie (Figs. 2,13g; Dreimanis 1992). Tills identified in

New York State may record the southeastern positions of ice margins (Karig and Miller 2013).

These deposits are likely correlative with the uppermost part of SU5 in southern Simcoe County,

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which may reflect ice front oscillations, possibly within the Lake Simcoe basin (Fig. 13g). Thick

sand bodies exposed in southern Simcoe County boreholes (SU5) were likely deposited in

subaquatic fan or progradational environments (e.g. Winsemann et al. 2007), and their vertical

stacking within the otherwise fine-grained succession may record still stands or slight regressions

during phases of ice retreat. The increased thickness and coarser grain size of sand bodies in the

north and northeast (Fig. 4) suggest the southern LIS margin likely supplied large proportions of

the coarse-grained sediment. This possibility is supported by the occurrence of waterlain

diamictons, abundant ice-rafted debris, and deformed horizons in the upper part of SU5 in the

eastern part of the study area, which are attributed to increasingly complex ice-proximal

glaciolacustrine depositional environments associated with the advance of the LIS. Draft The pre-Late Wisconsin stratigraphy in southern Simcoe County and the Toronto area is

capped by the Late Wisconsin Newmarket Till (formerly lower Leaside/Halton/Northern Tills;

Karrow 1967; Karrow 1974; White 1975), which was deposited sometime after 28 060 14 C yr BP

(Table 1; Fig. 13h). Recent studies in the Toronto area suggest that, locally, the ice flowed

initially from the southeast out of the Ontario basin (Mahaney et al. 2014), followed by more

regional southerly flow and a late-stage shift to northwest flowing ice during deglaciation (Boyce

and Eyles 2000; Maclachlan and Eyles 2013).

In summary, southern Simcoe County has a record of two, possibly Illinoian ice

advances, separated by a brief period of ice retreat. The younger till is deeply weathered and

overlain by alluvial deposits that record a transition from interglacial to interstadial climatic

conditions. Ice retreat north of the Lake Huron basin is required throughout this interval to lower

base levels well below the present level of Georgian Bay. A regional transgressive event

affected most of Simcoe County with water levels controlled by the positions of Early and

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Middle Wisconsin ice margins at the northern end of the Lake Huron basin and the eastern end of the Lake Ontario basin. Ice advance to a position south of Rome, New York resulted in the impoundment of a water body that likely covered most of Simcoe County as well as the Lake

Ontario and Erie basins with drainage to the Mississippi drainage basin via the Chicago outlet.

The chronology presented here should be viewed as an alternative hypothesis to existing reconstructions that invoke extensive ice advances into southern Ontario and beyond during the

Early and Middle Wisconsin. Tests of the reconstruction presented here can be achieved through further integration of pre-Late Wisconsin sediment records, particularly in the St. Lawrence lowlands, Lake Erie basin, and Mississippi Valley. If glacial advances rerouted drainage through Chicago during the Early and Middle Wisconsin,Draft there may be increased loess deposition associated with higher meltwater discharge into the Mississippi River system, although the relative connection of the Great Lakes and Mississippi basins, as well as pre-Late Wisconsin isostasy remain poorly understood. Identifying and characterizing the (glacio)lacustrine deposits that are commonly encountered to determine base levels and the outlet(s) that may have been in operation is essential. Improved absolute dating techniques and new subsurface data acquisition will also help to link processes and events in different parts of the Great Lakes region.

APPLICATION TO THE GROUNDWATER REGIME

Based on limited knowledge of the lower glacial complex (SU1-3) and underlying bedrock interface aquifers, it would appear that these lowermost units are a poor candidate for municipal water supply in southern Simcoe County. Both water quantity and/or quality are important issues to consider during the search for water resources within these lower sediments.

The overconsolidated nature of the tills and limited distribution of intervening coarse-gained

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sediment provide insufficient volumes of permeable sediment units, and their connection with

locally petroliferous bedrock formations appears to promote water quality issues. More

widespread coarse-grained aquifer units within this stratigraphic interval are suggested by recent

drilling to the north, and these may form important aquifers suitable for domestic use (Burt and

Dodge 2011; Mulligan 2016).

A second aquifer is hosted within the non-glacial deposits overlying the regional

unconformity (SU 4). This unit is generally thin and water within this unit commonly has high

concentrations of dissolved methane and has a strong sulfurous odour. Decomposition of organic

matter within the unit appears to be the source of high gas concentrations, which were sufficient to cause the community of Beeton to switchDraft from groundwater-derived municipal supply to a pipeline from Georgian Bay (Fig 1; Aravena et al. 2004).

Sand bodies within SU5 are not typically productive aquifers (due in large part to fine

texture and limited thicknesses throughout much of southern Simcoe County; Fig. 4). In places,

where they are thick, relatively coarse-textured and apparently laterally extensive, they form

aquifers capable of providing drinking water to smaller communities (e.g. Thornton aquifer;

Sibul and Choo-Ying 1971). However, SU5 aquifers throughout the majority of southern

Simcoe County are generally thinner and suitable for domestic water supply only. Water pumped

from aquifers hosted within these sand bodies is typically good quality, and are somewhat

protected from anthropogenic sources due to their confinement beneath the leaky Newmarket

Till and fine-grained rhythmite successions (Gerber and Howard 1996; Desbarats et al. 2001).

SU5 aquifers are largely absent beneath the broad valley systems in southern Simcoe County,

eroded during Late Wisconsin valley excavation (Sharpe et al. 2002; Mulligan et al. 2016).

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The recent investigations in southern Simcoe County provide the necessary detailed geologic information to provide new insights into the configuration of aquifers in the region.

Past studies utilized deep (70-170 m) water well records and static water levels to infer the presence of a connected deep aquifer system spanning much of the southern Simcoe County study area (e.g. Aravena and Wassenaar 1993). This aquifer was believed to be hosted within fluvial sands and gravels associated with the Laurentian River system that carved the bedrock valley from Lake Huron to Lake Ontario, although little detailed sedimentological or stratigraphic information was available at the time. Following extensive drilling and geophysical investigations, no continuous sandy unit has been identified. It is interpreted here that the

“Alliston Aquifer” is, rather than a single continuous sediment body, an assortment of saturated sand and gravel units occupying multipleDraft stratigraphic positions. In southern Simcoe County, organic-bearing sand and gravel within SU 4 provides the organic material for in-situ methanogenesis and well yields are enhanced where there is hydraulic connection with deglacial sand and gravel bodies near the base of valley infill successions beneath the lowlands (SS-12-03,

Figs. 3,4). This assessment is supported by reports of both old (13 000-25 000 years) and young water within the aquifer (Aravena and Wassenaar 1993) and the large range in elevations reported for the aquifer (119-265 m asl; Aravena and Wassenaar, 1993). Existing hydrogeologic conceptualizations (Post and McPhie 2015) were based on limited high quality subsurface data.

This new data provides an enhanced geologic framework to guide future groundwater flow models in the region.

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CONCLUSIONS

Analysis of 25 continuously-cored boreholes in southern Simcoe County has resulted in

the identification of a consistent pre-Late Wisconsin succession consisting of five stratigraphic

units. The stratigraphy is floored by a glacial package of variable thickness that records early

glacial events, attributed to the Illinois Episode (SU1-3). A prolonged non-glacial interval

ensued, represented by well-developed weathering profiles on the upper surface of the lower till

complex as well as the common occurrence of non-glacial, organic-bearing alluvial and

lacustrine deposits (SU4) that accumulated within and along the margins of (isostatically-

depressed?) low-lying basins and drainage systems. A single site well below the present level of

Lake Huron, containing a molluscan assemblage suggestive of interglacial conditions yielded

radiocarbon ages beyond the limits of theDraft method. Organic deposits lying along the same

unconformity and radiocarbon dated as young as 37,450 14 C yr BP have yielded rich fossil

assemblages indicative of sedge-dominated tundra communities. It is therefore suggested that

this non-glacial, subaerially exposed interval may have spanned the Sangamon through to the

latter part of the Middle Wisconsin. A regional transgressive event is recorded by lacustrine

followed by glaciolacustrine silt and clay rhythmite deposition (SU5) upon the regional

unconformity surface. The transgression event(s) spanned greater than 17,000 radiocarbon years,

depicted by younger sites along the unconformity occurring at higher elevations. Still stands or

lake level falls during the regional transgression are suggested by clustering of dates and the

apparent coincidence of sand bodies within the glaciolacustrine succession at distinct elevations.

Approach of the Laurentide ice sheet at the beginning of the Late Wisconsin is recorded by the

increasing glacial influence (increased clast content, interbedding of glaciolacustrine diamict

beds, resedimented units, and soft sediment deformation structures) in the upper parts of the

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glaciolacustrine package prior to the eventual overriding by ice and the unconformable deposition of Newmarket Till within the county.

Several producing aquifers are hosted within pre-Late Wisconsin sediments in the region.

Upper coarse-grained units are likely recharged by infiltration through the leaky surficial tills and fine-grained units, but the fine-grained nature, limited thickness and spatial extent usually limits these units to domestic supply, except in a few locations. Organic-bearing alluvial sands and gravels form a widely recognized aquifer deep in the subsurface, but in situ decomposition of this organic matter has resulted in the generation of methane leading to poor water quality.

Elsewhere, the contact aquifer at the sediment-bedrock interface may be a good producer of water for rural and domestic use but againDraft suffers from high levels of methane and very high total dissolved solids.

Results from this investigation suggest an alternative chronology compared to classical glacial reconstructions. Until better age constraints can be achieved for sediment units across southern Ontario, this should be viewed as an additional working hypothesis for glacial researchers in the region.

ACKNOWLEDGEMENTS

Discussions with many workers assisted in developing the concepts presented in this paper . Drilling of continuously cored boreholes was conducted by ProCore Drilling Ltd. (2011,

2012 drill programs) and Aardvark Drilling, Inc. (2013 drill program). This work was funded by the Ontario Geological Survey, Project unit 10-014. Components of this work formed the basis

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for part of a M.Sc. thesis by R. Mulligan at McMaster University. Reviews by Hazen Russell and

two anonymous referees assisted in clarifying data presentation and concepts presented in the

manuscript. Borehole logs, photos, and analytical data from this study are available at

http://www.geologyontario.mndmf.gov.on.ca/mndmaccess/mndm_dir.asp?type=pub&id=MRD324 .

This paper is published with the permission of the Director of the Ontario Geological Survey.

REFERENCES

Aravena, R. and Wassenaar, L.I. 1993. Dissolved organic carbon and methane in a regional confined aquifer, southern Ontario, Canada: Carbon isotope evidence for associated subsurface sources. Applied Geochemistry, vol. 8, p. 483-493.

Aravena, R., Wassenaar, L.I., and Barker,Draft J.F. (1995). Distribution and Isotopic Characterization of Methane in a Confined Aquifer in Southern Ontario, Canada. Journal of Hydrology, 173(1-4), 51-70.

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List of Tables:

Lab Borehole Easting Northing Collar Depth Material Age Stratigraphic Elevation ID Number (NAD83) (NAD83) (m asl) (m bgs) Age ( 14 C yr Position BP) A2053 SS-11-01 614442 4889524 267 55.9-56.1 Dryas Leaves 41800±600 SU4 A2443 SS-11-01 614442 4889524 267 56.1-56.2 Dryas Leaves 44400±1700 SU4 A2132 SS-11-01 614442 4889524 267 57.0-57.2 Wood 44000±1800 SU4 A2054 SS-11-01 614442 4889524 267 80.8-81.1 Wood >54700 SU2 A2131 SS-11-01 614442 4889524 267 80.8-81.1 Wood >50800 SU2 A2055 SS-11-01 614442 4889524 267 120.3-120.5 Wood 48800±1400 SU2 A2130 SS-11-01 614442 4889524 267 120.3-120.5 Wood >47500 SU2 A2133 SS-11-02 602975 4880338 256 70.25-71.10 Wood >45900 SU5 A2056 SS-11-02 602975 4880338 256 98.35-98.75 Wood 42600±700 SU5

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A2134 SS-11-02 602975 4880338 256 123.25-123.35 Wood 46400±2500 SU5 A2057 SS-11-02 602975 4880338 256 142.5-142.65 Wood 50100±1700 SU5 A2059 SS-11-04 586055 4878237 291 76.45 Wood 39800±470 SU5 A2058 SS-11-04 586055 4878237 291 90.05-90.40 Dryas Leaves 45200±900 SU4 A2135 SS-11-04 586055 4878237 291 90.05-90.40 Wood 48000±3000 SU4 A2445 SS-11-04 586055 4878237 291 90.05-90.40 Dryas Leaves 43900±1500 SU4 A2061 SS-11-06 594747 4898721 219 69.2-69.3 Wood 54800±3000 SU4 A2136 SS-11-06 594747 4898721 219 69.2-69.3 Wood >49200 SU4 A2062 SS-11-08 590082 4894188 264 63.65-64.35 Wood 49200±1500 SU5 A2138 SS-11-08 590082 4894188 264 83.2-83.25 Succineidae sp. >46900 SU4 A2064 SS-11-08 590082 4894188 264 98.3-98.9 Wood >54000 SU2 A2139 SS-11-08 590082 4894188 264 105.8-106.1 Wood >49500 SU2 A2392 SS-12-02 602163 4902748 292 114.20-114.45 Wood 49500±3100 SU4 A2447 SS-12-02 602163 4902748 292 114.45-114.75 Dryas Leaves 45400±1800 SU4 A2448 SS-12-03 593299 4906003 213 71.65-73.15 Wood >51700 SU4 A2393 SS-12-03 593299 4906003 213 72.00-72.85 Wood >50800 SU4 A2394 SS-12-03 593299 4906003 213 73.75-74.35 Wood >51700 SU4 A2395 SS-12-04 610758 4905514 286 50.40-50.95 Dryas Leaves 28840±240 SU5 A2449 SS-12-04 610758 4905514 286 50.40-50.95 Dryas Leaves 28060±230 SU5 A2450 SS-12-04 610758 4905514 286 53.30-53.80 Dryas Leaves 30540±300 SU5 A2451 SS-12-04 610758 4905514 286 53.3-53.8 Dryas Leaves >51200 SU5 A2396 SS-12-04 610758 4905514 286 115.80-117.35 Wood >48100 SU4 A2452 SS-12-04 610758 4905514 286 115.80-117.35 Dryas Leaves 48700±2800 SU4 A2397 SS-12-04 610758 4905514 286Draft 117.35-118.65 Wood 50700±3600 SU4 A2398 SS-12-05 599723 4894672 298 57.75-59.25 Wood >51700 SU5 A2478 SS-12-05 599723 4894672 298 59.25-60.80 Wood >47000 SU5 A2479 SS-12-05 599723 4894672 298 62.3-63.15 Wood >51200 SU5 A2453 SS-12-05 599723 4894672 298 123.60-123.75 Wood >46900 SU4 A2399 SS-12-05 599723 4894672 298 123.65-123.75 Wood >48500 SU4 A2400 SS-12-05 599723 4894672 298 124.15-124.25 Wood >49900 SU4 A2401 SS-12-06 606001 4885571 266 88.0-88.1 Wood 51000±3700 SU4 A2454 SS-12-06 606001 4885571 266 88.0-88.1 Wood 50800±3600 SU4 A2402 SS-12-06 606001 4885571 266 88.1-88.2 Wood >47800 SU4 A3087 SS-13-01 595574 4894319 219 68.40-69.05 Dryas Leaves >50300 SU4 A3034 SS-13-01 595574 4894319 219 73.3-73.5 Wood >51700 SU4 A3033 SS-13-01 595574 4894319 219 75.1-75.3 Wood >52200 SU4 A3088 SS-13-01 595574 4894319 219 75.50-75.65 Wood >52800 SU4 A3032 SS-13-01 595574 4894319 219 75.80-75.85 Wood >52801 SU4 A3086 SS-13-01 595574 4894319 219 75.85-75.90 Wood >52800 SU4 A3083 SS-13-01 595574 4894319 219 75.90-75.95 Wood >52800 SU4 A3031 SS-13-01 595574 4894319 219 76.05-76.20 Wood >52800 SU4 A3035 SS-13-02 591639 4878110 268 60.15-60.50 Wood 46800±1800 SU4 A3036 SS-13-02 591639 4878110 268 61.90-62.05 Wood 37450±590 SU4 A3037 SS-13-02 591639 4878110 268 61.90-62.05 Dryas Leaves 43500±1200 SU4 A3082 SS-13-02 591639 4878110 268 62.05-62.45 Wood 47180±1900 SU4 A3038 SS-13-02 591639 4878110 268 62.6-62.75 Wood >44800 SU4 A3042 SS-13-05 615203 4915966 250 68.55-70.10 Wood 46500±1800 SU4 A3043 SS-13-06 612674 4896493 302 53.35-54.90 Wood 51800±3400 SU5 A3044 SS-13-06 612674 4896493 302 100.6-102.0 Wood 38920±700 SU4 A3045 SS-13-06 612674 4896493 302 100.6-102.0 Dryas Leaves 37850±620 SU4

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Table 1: Radiocarbon dates obtained from pre-Late Wisconsin organic-bearing sediments in southern Simcoe County. Modified from Bajc et al. (2014).

List of Figures:

Fig. 1 : a)Map of southern Ontario showing location of the study area (red outline) and pre-Late Wisconsin organic- bearing sites (c: Clarksburg; g: Guelph; i: Innerkip; w: Waterloo; z: Zorra quarry; bw: Brantford-Woodstock area; n: St. David’s Gorge; interglacial sites shown in red; w: Woodbridge; t: Don Valley Brickyard), stratigraphic reference sections along the Erie (EB) and Scarborough (SB) Bluffs shown in thick red lines; b) Preglacial drainage pathways (blue lines) of the Great Lakes after Spencer (1890) and locations of former and present drainage outlets for the Great Lake region. C: Chicago (toward Mississippi R.); PH: Port Huron; NB: North Bay; E: Erigan; FF: Fenelon Falls; SL: St. Lawrence; R: Rome (toward Hudson R.). Note that the Laurentian Valley passes through the study area. Fig. 1c,d shown in red rectangle; c) topography and geography of study area (black outline) showing locations of regional geologic features, modern water bodies and place names mentioned in text. ORM = Oak Ridges Moraine (outlined in dashed black line); d) bedrock topography of Niagara Escarpment (short dashes) and Laurentian Valley (long dashes and shaded) in south-central Ontario.Draft Study area outlined in black. Location of cross-sectional profile in Fig. 1e shown in thick black line; e) cross-sectional profile showing bedrock topography and generalized lithology. Dip of unit contacts not to scale and shown for illustrative purposes only. Quaternary sediment cover shown in green. Modern Georgian Bay water level and approximate extent of Laurentian Valley for reference.

Fig. 2 : a) Time-distance diagram of late Pleistocene glacial-interglacial episodes showing location and ages of dated material. Dates from the present study are shown in red. OIS = oxygen isotope stage (modified from Karrow et al. 2000). Log at right shows generalized stratigraphy of Scarborough bluffs area with age determinations of units from radiocarbon ( 14 C; from Karrow 1984; Berti 1975) and thermoluminescence (TL) dating techniques (dates and log from Berger and Eyles 1994). Dashed line indicates classically held chronology of GTA sediment units. b) East- west cross-sectional profile through sediment exposed at the Scarborough bluffs (Fig. 1b for location), Modified from Karrow (1967); c) cross-sectional profile showing sediments exposed along the Lake Erie bluffs from Plum Point to Bradtville, Ontario. Modified from Dreimanis (1992).

Fig. 3 : Data distribution within the study area showing locations of boreholes overlain on modern topography. Boreholes shown in east-west (red outlines) and north-south (yellow outlines) cross-sectional profiles identified (see Fig. 4).

Fig. 4: Generalised N-S and E-W cross-sectional profiles showing configuration of SU’s and locations of selected samples with radiocarbon age determinations. See Fig. 3 for location and Table 1 for full radiocarbon sample list. Dashed red line marks the base of the late Wisconsin subglacial unconformity that truncates the pre-Late Wisconsin strata. Legend is for borehole logs only. Colours used in unit correlations are for illustrative purposes only. Valleys separating upland areas represent significant erosion of pre-Late Wisconsin strata. Their formation and characteristics are beyond the scope of the present paper, but are the focus of on ongoing investigations.

Fig. 5 : SU1 facies; a) 8 m (25ft) succession exposing typical SU1 facies near the base of SS-12-03; b) structureless, sand- and clast-rich facies; c) limestone cobble with flattened (abraded) upper surface, ornamented with well- developed parallel striae. Sediment cores in all photos are approximately 8.5 cm in diameter.

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Fig. 6 : SU2 facies; a) silt and clay with diamict interbeds and dropstones from 130.25-130.50 m (SS-13-08); b) ripple-scale cross-laminated sand containing fine detrital organics from 104.25-104.50 m (SS-11-08); c) gravelly sand, and gravel from 139.25-139.50 m (SS-12-06). Sediment cores in all photos are approximately 8.5 cm in diameter.

Fig. 7 : SU3 facies; a) structureless sandy silt diamicton with abundant small limestone clasts (SS-11-08). Core tubes are 1.5 m long; b) mechanical clast disaggregation structure (SS-13-02); c) gouged bedrock surface of the Lindsay Formation (limestone) directly underlying SU3 (SS-13-02); d) paleosol developed on the upper part of SU3; note orange, green, white colour banding in upper core and typical unweathered grey colour in lower core; e) organic staining and secondary carbonate in the upper part of SU3 (SS-11-06). Sediment cores in all photos are approximately 8.5 cm in diameter.

Fig. 8 : SU4 facies; a) 6 m succession of interbedded, sand, silt and gravel overlying the weathered upper surface of SU3 at top right and abruptly overlain by laminated clay of SU5 in the bottom left (SS-13-02); b) dense and compacted woody, humified peat (SS-13-01); c) ripple-scale cross-laminated sand with detrital organics along foresets (SS-13-06). Sediment cores in all photos are approximately 8.5 cm in diameter.

Fig. 9 : Macrofossils recovered from SU4. Grid is 2.5 x 2.5 mm; a) freshwater gastropod shells including Succinaeidae b ) Dryas integrifolia (top) and Salix herbaceae (bottom) leaves; c) Pleurocera acuta (top right) and unionid clam (middle) fragments; d) microscopic fungal filaments indicative of aerobic soil formation (SS-13-01).

Fig. 10 : Plot of local elevation versus radiocarbon age of organic deposits from SU4 and upper part of SU5. Modern Georgian Bay water level is shown for reference.Draft Elevation values have not been corrected for potential isostatic effects. See Table 1.

Fig. 11 : SU5 facies; a) sharp contact between silty clay of SU5 and sands of SU4 (top of black arrows; SS-13-02); b) well-bedded/laminated rhythmites. The coarse fraction has a sharp base (bent black arrow) and is generally interlaminated. The fine fraction has a gradational base and fines upward (grey triangles) into clay caps (SS-13-02); c) interbedded and deformed silt and clay with dropstones and thin diamict lenses (SS-11-01); d) deformed silt, clay with minor ice-rafted debris. Dashed lines denote fault planes cross-cutting near-vertical beds (SS-12-02) e) cruziana on silt planes of rhythmite; f) sand body within SU5, with detrital organics and high concentrations of dissolved gasses (SS-12-05). Sediment cores in all photos are approximately 8.5 cm in diameter.

Fig. 12 : SU5 facies; a) ripple-scale cross-laminated fine-grained sand commonly containing detrital organics along foresets; b) deformed very fine-grained sand, with loading/dewatering features characteristic of high sedimentation rates (SS-11-08); c) peat mats in sands showing signs of subaerial exposure in the form of oxidized zones (SS-12- 04) Sediment cores in all photos are approximately 8.5 cm in diameter, oriented with top toward the left; d) surficial exposure of sands and minor gravel underlying Newmarket Till (dashed red line denotes contact). Sands display paleocurrents toward the south and southwest (right in photo) and the upper parts of the succession are highly deformed and locally faulted. Black dashed lines outline a deformed silt bed that approximates the boundary between highly deformed (ductile) sediments and relatively undisturbed sediments except for minor faulting (black lines).

Fig. 13: Paleogeographic reconstructions of southern Ontario. a) main phase of the Illinoian glaciation: SU1 is deposited by ice that deposited the York and Bradtville tills, maximum extents in the northern USA; b) late stage of Illinoian glaciation: deposition of SU3 during ice readvance from the northwest out of the Georgian Bay basin following a minor(?) retreat. A proglacial lake likely fronted the advancing ice, but no significant lakes developed during final ice retreat, as meltwater drained through the St. Lawrence (SL) outlet; c) Sangamon Episode: deposition of SU4 recording warm climate conditions and drastically lowered base levels in the study area and throughout the Lake Huron basin, during isostatic depression of the North Bay (NB) outlet; d) Early Wisconsin: regional base level

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rise in southern Ontario. SU5 deposited in low-lying areas and continued deposition of SU4 on high grounds adjacent to drowned parts of the landscape. Scarborough Fm. deposited in the GTA as advance of the LIS blocks the St. Lawrence outlet and diverts drainage through Rome (R), NY; e) Early Wisconsin: continued drowning of the landscape in the study area during regional base level rise due to encroachment of the LIS into the Lake Ontario basin. Deposition of SU4 limited to highs on the paleolandscape, while SU5 is deposited contemporaneously with the Sunnybrook Fm. and member B of the Tyrconnell formation in the Lake Erie basin. Drainage was diverted south through Chicago (C); f) Middle Wisconsin: minor ice retreat within southern Ontario. Continued glaciolacustrine sediment deposition in southern Simcoe County (SU5) and base level fall recorded by the lower Thorncliffe Fm. in the GTA and member C of the Tyrconnell Formation in the Lake Erie basin. Ice retreat likely exposed the Rome outlet (R); g) Middle Wisconsin Episode: oscillatory ice advance into southern Ontario, promoting deposition of increasingly glacially-influenced lacustrine sediments (deformed intervals, abundant ice-rafted material and resedimented units) in the upper part of SU5 (Simcoe County) and in the GTA (Upper Thorncliffe Formation) and renewed glaciolacustrine sediment deposition in the Lake Erie basin (Tyrconnell D); h) Late Wisconsin Episode: ice advance into the northern USA and deposition of regional till sheets (Catfish Creek, Newmarket tills) overlying the pre-Late Wisconsin sediments in southern Ontario. Data from multiple sources, See references in text.

Draft

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Lab Borehole Easting Northing Collar Elevation Depth Material Age Stratigraphic ID Number (NAD83) (NAD83) (m asl) (m bgs) Age (14C yr BP) Position A2053 SS-11-01 614442 4889524 267 55.9-56.1 Dryas Leaves 41800±600 SU4 A2443 SS-11-01 614442 4889524 267 56.1-56.2 Dryas Leaves 44400±1700 SU4 A2132 SS-11-01 614442 4889524 267 57.0-57.2 Wood 44000±1800 SU4 A2054 SS-11-01 614442 4889524 267 80.8-81.1 Wood >54700 SU2 A2131 SS-11-01 614442 4889524 267 80.8-81.1 Wood >50800 SU2 A2055 SS-11-01 614442 4889524 267 120.3-120.5 Wood 48800±1400 SU2 A2130 SS-11-01 614442 4889524 267 120.3-120.5 Wood >47500 SU2 A2133 SS-11-02 602975 4880338 256 70.25-71.10 Wood >45900 SU5 A2056 SS-11-02 602975 4880338 256 98.35-98.75 Wood 42600±700 SU5 A2134 SS-11-02 602975 4880338 256 123.25-123.35 Wood 46400±2500 SU5 A2057 SS-11-02 602975 4880338 256 142.5-142.65 Wood 50100±1700 SU5 A2059 SS-11-04 586055 4878237 291 76.45 Wood 39800±470 SU5 A2058 SS-11-04 586055 4878237 291 90.05-90.40 Dryas Leaves 45200±900 SU4 A2135 SS-11-04 586055 4878237 291 90.05-90.40 Wood 48000±3000 SU4 A2445 SS-11-04 586055 4878237 291 90.05-90.40 Dryas Leaves 43900±1500 SU4 A2061 SS-11-06 594747 4898721 219 69.2-69.3 Wood 54800±3000 SU4 A2136 SS-11-06 594747 4898721 219 69.2-69.3 Wood >49200 SU4 A2062 SS-11-08 590082 4894188 264 63.65-64.35 Wood 49200±1500 SU5 A2138 SS-11-08 590082 4894188Draft 264 83.2-83.25 Succineidae sp. >46900 SU4 A2064 SS-11-08 590082 4894188 264 98.3-98.9 Wood >54000 SU2 A2139 SS-11-08 590082 4894188 264 105.8-106.1 Wood >49500 SU2 A2392 SS-12-02 602163 4902748 292 114.20-114.45 Wood 49500±3100 SU4 A2447 SS-12-02 602163 4902748 292 114.45-114.75 Dryas Leaves 45400±1800 SU4 A2448 SS-12-03 593299 4906003 213 71.65-73.15 Wood >51700 SU4 A2393 SS-12-03 593299 4906003 213 72.00-72.85 Wood >50800 SU4 A2394 SS-12-03 593299 4906003 213 73.75-74.35 Wood >51700 SU4 A2395 SS-12-04 610758 4905514 286 50.40-50.95 Dryas Leaves 28840±240 SU5 A2449 SS-12-04 610758 4905514 286 50.40-50.95 Dryas Leaves 28060±230 SU5 A2450 SS-12-04 610758 4905514 286 53.30-53.80 Dryas Leaves 30540±300 SU5 A2451 SS-12-04 610758 4905514 286 53.3-53.8 Dryas Leaves >51200 SU5 A2396 SS-12-04 610758 4905514 286 115.80-117.35 Wood >48100 SU4 A2452 SS-12-04 610758 4905514 286 115.80-117.35 Dryas Leaves 48700±2800 SU4 A2397 SS-12-04 610758 4905514 286 117.35-118.65 Wood 50700±3600 SU4 A2398 SS-12-05 599723 4894672 298 57.75-59.25 Wood >51700 SU5 A2478 SS-12-05 599723 4894672 298 59.25-60.80 Wood >47000 SU5 A2479 SS-12-05 599723 4894672 298 62.3-63.15 Wood >51200 SU5 A2453 SS-12-05 599723 4894672 298 123.60-123.75 Wood >46900 SU4 A2399 SS-12-05 599723 4894672 298 123.65-123.75 Wood >48500 SU4 A2400 SS-12-05 599723 4894672 298 124.15-124.25 Wood >49900 SU4 A2401 SS-12-06 606001 4885571 266 88.0-88.1 Wood 51000±3700 SU4 A2454 SS-12-06 606001 4885571 266 88.0-88.1 Wood 50800±3600 SU4 A2402 SS-12-06 606001 4885571 266 88.1-88.2 Wood >47800 SU4 A3087 SS-13-01 595574 4894319 219 68.40-69.05 Dryas Leaves >50300 SU4 A3034 SS-13-01 595574 4894319 219 73.3-73.5 Wood >51700 SU4 A3033 SS-13-01 595574 4894319 219 75.1-75.3 Wood >52200 SU4

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A3088 SS-13-01 595574 4894319 219 75.50-75.65 Wood >52800 SU4 A3032 SS-13-01 595574 4894319 219 75.80-75.85 Wood >52801 SU4 A3086 SS-13-01 595574 4894319 219 75.85-75.90 Wood >52800 SU4 A3083 SS-13-01 595574 4894319 219 75.90-75.95 Wood >52800 SU4 A3031 SS-13-01 595574 4894319 219 76.05-76.20 Wood >52800 SU4 A3035 SS-13-02 591639 4878110 268 60.15-60.50 Wood 46800±1800 SU4 A3036 SS-13-02 591639 4878110 268 61.90-62.05 Wood 37450±590 SU4 A3037 SS-13-02 591639 4878110 268 61.90-62.05 Dryas Leaves 43500±1200 SU4 A3082 SS-13-02 591639 4878110 268 62.05-62.45 Wood 47180±1900 SU4 A3038 SS-13-02 591639 4878110 268 62.6-62.75 Wood >44800 SU4 A3042 SS-13-05 615203 4915966 250 68.55-70.10 Wood 46500±1800 SU4 A3043 SS-13-06 612674 4896493 302 53.35-54.90 Wood 51800±3400 SU5 A3044 SS-13-06 612674 4896493 302 100.6-102.0 Wood 38920±700 SU4 A3045 SS-13-06 612674 4896493 302 100.6-102.0 Dryas Leaves 37850±620 SU4

Draft

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Draft

Fig. 1: a)Map of southern Ontario showing location of the study area (red outline) and pre-Late Wisconsin organic-bearing sites (c: Clarksburg; g: Guelph; i: Innerkip; w: Waterloo; z: Zorra quarry; bw: Brantford- Woodstock area; n: St. David’s Gorge; interglacial sites shown in red; w: Woodbridge; t: Don Valley Brickyard), stratigraphic reference sections along the Erie (EB) and Scarborough (SB) Bluffs shown in thick red lines; b) Preglacial drainage pathways (blue lines) of the Great Lakes after Spencer (1890) and locations of former and present drainage outlets for the Great Lake region. C: Chicago (toward Mississippi R.); PH: Port Huron; NB: North Bay; E: Erigan; FF: Fenelon Falls; SL: St. Lawrence; R: Rome (toward Hudson R.). Note that the Laurentian Valley passes through the study area. Fig. 1c,d shown in red rectangle; c) topography and geography of study area (black outline) showing locations of regional geologic features, modern water bodies and place names mentioned in text. ORM = Oak Ridges Moraine (outlined in dashed black line); d) bedrock topography of Niagara Escarpment (short dashes) and Laurentian Valley (long dashes and shaded) in south-central Ontario. Study area outlined in black. Location of cross-sectional profi le in Fig. 1e shown in thick black line; e) cross-sectional profile showing bedrock topography and generalized

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lithology. Dip of unit contacts not to scale and shown for illustrative purposes only. Quaternary sediment cover shown in green. Modern Georgian Bay water level and approximate extent of Laurentian Valley for reference.

545x735mm (96 x 96 DPI)

Draft

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Draft

Fig. 2: a) Time-distance diagram of late Pleistocene glacial-interglacial episodes showing location and ages of dated material. Dates from the present study are shown in red. OIS = oxygen isotope stage (modified from Karrow et al. 2000). Log at right shows generalized stratigraphy of Scarborough bluffs area with age determinations of units from radiocarbon (14C; from Karrow 1984; Berti 1975) and thermoluminescence (TL) dating techniques (dates and log from Berger and Eyles 1994). Dashed line indicates classically held chronology of GTA sediment units. b) East-west cross-sectional profile through sediment exposed at the Scarborough bluffs (Fig. 1b for location), Modified from Karrow (1967); c) cross-sectional profile showing sediments exposed along the Lake Erie bluffs from Plum Point to Bradtville, Ontario. Modified from Dreimanis (1992).

182x239mm (300 x 300 DPI)

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Draft

Fig. 3: Data distribution within the study area showing locations of boreholes overlain on modern topography. Boreholes shown in east-west (red outlines) and north-south (yellow outlines) cross-sectional profiles identified (see Fig. 4).

191x209mm (300 x 300 DPI)

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Draft

Fig. 4: Generalised N-S and E-W cross-sectional profiles showing configuration of SU’s and locations of selected samples with radiocarbon age determinations. See Fig. 3 for location and Table 1 for full radiocarbon sample list. Dashed red line marks the base of the late Wisconsin subglacial unconformity that truncates the pre-Late Wisconsin strata. Legend is for borehole logs only. Colours used in unit correlations are for illustrative purposes only. Valleys separating upland areas represent significant erosion of pre-Late Wisconsin strata. Their formation and characteristics are beyond the scope of the present paper, but are the focus of on ongoing investigations.

231x184mm (300 x 300 DPI)

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Draft

Fig. 5: SU1 facies; a) 8 m (25ft) succession exposing typical SU1 facies near the base of SS-12-03; b) structureless, sand- and clast-rich facies; c) limestone cobble with flattened (abraded) upper surface, ornamented with well-developed parallel striae. Sediment cores in all photos are approximately 8.5 cm in diameter.

561x497mm (96 x 96 DPI)

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Draft

Fig. 6: SU2 facies; a) silt and clay with diamict interbeds and dropstones from 130.25-130.50 m (SS-13- 08); b) ripple-scale cross-laminated sand containing fine detrital organics from 104.25-104.50 m (SS-11- 08); c) gravelly sand, and gravel from 139.25-139.50 m (SS-12-06). Sediment cores in all photos are approximately 8.5 cm in diameter.

266x232mm (96 x 96 DPI)

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Draft

Fig. 7: SU3 facies; a) structureless sandy silt diamicton with abundant small limestone clasts (SS-11-08). Core tubes are 1.5 m long; b) mechanical clast disaggregation structure (SS-13-02); c) gouged bedrock surface of the Lindsay Formation (limestone) directly underlying SU3 (SS-13-02); d) paleosol developed on the upper part of SU3; note orange, green, white colour banding in upper core and typical unweathered grey colour in lower core; e) organic staining and secondary carbonate in the upper part of SU3 (SS-11-06). Sediment cores in all photos are approximately 8.5 cm in diameter.

575x722mm (96 x 96 DPI)

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Draft

Fig. 8: SU4 facies; a) 6 m succession of interbedded, sand, silt and gravel overlying the weathered upper surface of SU3 at top right and abruptly overlain by laminated clay of SU5 in the bottom left (SS-13-02); b) dense and compacted woody, humified peat (SS-13-01); c) ripple-scale cross-laminated sand with detrital organics along foresets (SS-13-06). Sediment cores in all photos are approximately 8.5 cm in diameter.

568x558mm (96 x 96 DPI)

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Draft

Fig. 9: Macrofossils recovered from SU4. Grid is 2.5 x 2.5 mm; a) freshwater gastropod shells including Succinaeidae b) Dryas integrifolia (top) and Salix herbaceae (bottom) leaves; c) Pleurocera acuta (top right) and unionid clam (middle) fragments; d) microscopic fungal filaments indicative of aerobic soil formation (SS-13-01).

267x735mm (96 x 96 DPI)

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Draft

Fig. 10: Plot of local elevation versus radiocarbon age of organic deposits from SU4 and upper part of SU5. Modern Georgian Bay water level is shown for reference. Elevation values have not been corrected for potential isostatic effects. See Table 1.

180x115mm (300 x 300 DPI)

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Draft

Fig. 11: SU5 facies; a) sharp contact between silty clay of SU5 and sands of SU4 (top of black arrows; SS- 13-02); b) well-bedded/laminated rhythmites. The coarse fraction has a sharp base (bent black arrow) and is generally interlaminated. The fine fraction has a gradational base and fines upward (grey triangles) into clay caps (SS-13-02); c) interbedded and deformed silt and clay with dropstones and thin diamict lenses (SS-11-01); d) deformed silt, clay with minor ice-rafted debris. Dashed lines denote fault planes cross- cutting near-vertical beds (SS-12-02) e) cruziana on silt planes of rhythmite; f) sand body within SU5, with detrital organics and high concentrations of dissolved gasses (SS-12-05). Sediment cores in all photos are approximately 8.5 cm in diameter.

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Draft

Fig. 12: SU5 facies; a) ripple-scale cross-laminated fine-grained sand commonly containing detrital organics along foresets; b) deformed very fine-grained sand, with loading/dewatering features characteristic of high sedimentation rates (SS-11-08); c) peat mats in sands showing signs of subaerial exposure in the form of oxidized zones (SS-12-04) Sediment cores in all photos are approximately 8.5 cm in diameter, oriented with top toward the left; d) surficial exposure of sands and minor gravel underlying Newmarket Till (dashed red line denotes contact). Sands display paleocurrents toward the south and southwest (right in photo) and the upper parts of the succession are highly deformed and locally faulted. Black dashed lines outli ne a deformed silt bed that approximates the boundary between highly deformed (ductile) sediments and relatively undisturbed sediments except for minor faulting (black lines).

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Draft Fig. 13: Paleogeographic reconstructions of southern Ontario. a) main phase of the Illinoian glaciation: SU1 is deposited by ice that deposited the York and Bradtville tills, maximum extents in the northern USA; b) late stage of Illinoian glaciation: deposition of SU3 during ice readvance from the northwest out of the Georgian Bay basin following a minor(?) retreat. A proglacial lake likely fronted the advancing ice, but no significant lakes developed during final ice retreat, as meltwater drained through the St. Lawrence (SL) outlet; c) Sangamon Episode: deposition of SU4 recording warm climate conditions and drastically lowered base levels in the study area and throughout the Lake Huron basin, during isostatic depression of the North Bay (NB) outlet; d) Early Wisconsin: regional base level rise in southern Ontario. SU5 deposited in low-lying areas and continued deposition of SU4 on high grounds adjacent to drowned parts of the landscape. Scarborough Fm. deposited in the GTA as advance of the LIS blocks the St. Lawrence outlet and diverts drainage through Rome (R), NY; e) Early Wisconsin: continued drowning of the landscape in the study area during regional base level rise due to encroachment of the LIS into the Lake Ontario basin. Deposition of SU4 limited to highs on the paleolandscape, while SU5 is deposited contemporaneously with the Sunnybrook Fm. and member B of the Tyrconnell formation in the Lake Erie basin. Drainage was diverted south through Chicago (C); f) Middle Wisconsin: minor ice retreat within southern Ontario. Continued glaciolacustrine sediment deposition in southern Simcoe County (SU5) and base level fall recorded by the lower Thorncliffe Fm. in the GTA and member C of the Tyrconnell Formation in the Lake Erie basin. Ice retreat likely exposed the Rome outlet (R); g) Middle Wisconsin Episode: oscillatory ice advance into southern Ontario, promoting deposition of increasingly glacially-influenced lacustrine sediments (deformed intervals, abundant ice-rafted material and resedimented units) in the upper part of SU5 (Simcoe County) and in the GTA (Upper Thorncliffe Formation) and renewed glaciolacustrine sediment deposition in the Lake Erie basin (Tyrconnell D); h) Late Wisconsin Episode: ice advance into the northern USA and deposition of regional till sheets (Catfish Creek, Newmarket tills) overlying the pre-Late Wisconsin sediments in southern Ontario. Data from multiple sources, See references in text.

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