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 https://mc06.manuscriptcentral.com/cjes-pubs Page 1 of 61 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 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 https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 2 of 61 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. https://mc06.manuscriptcentral.com/cjes-pubs Page 3 of 61 Canadian Journal of Earth Sciences 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) https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 4 of 61 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 https://mc06.manuscriptcentral.com/cjes-pubs Page 5 of 61 Canadian Journal of Earth Sciences (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 https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 6 of 61 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