Humber River State of the Watershed Report – Geology and Groundwater Resources

3.0 GEOLOGIC SETTING

3.1 METHODOLOGY

As mentioned in Section 1 , the hydrogeologic information in this report is extracted from a wider study based on historical published information augmented by more recent investigations conducted by the YPDT-CAMC study team. These recent investigations included the compilation of a database of water related information (e.g., boreholes, wells, climate, stream flow), the refinement of the three dimensional stratigraphy provided by the Geological Survey of Canada (GSC), the construction of a three-dimensional numerical groundwater flow model (Kassenaar and Wexler, 2006) and field investigations to fill in key data gaps (Holysh and Davies, 2003; Davies and Holysh, 2004).

3.2 SURFICIAL GEOLOGY

The Humber River Watershed has a wide range of surficial geological features as shown on Figure 3-1. The upper reaches, particularly in the west, are characterized by significant thicknesses of sand and gravel associated with the Oak Ridges Moraine. These areas comprise the key recharge areas in the watershed. In some areas, the sand and gravel is overlain by a thin layer of silt till (Halton Till), but with the hummocky topography (Leney and Kenny, 2003 –Figure 3-2) and thin till, significant infiltration is still possible. In the upper east reaches, however, the till thickness increases to about 20-30 m, which limits the infiltration potential.

The surficial geology of the remainder of the watershed is dominated by low permeability silt, clay, and silt till of the Halton Till Formation, although there are sands in the lowest reaches near Bloor Street associated with the former Lake Iroquois shoreline and some isolated recent fluvial sand deposits along the Humber River.

3.3 STRATIGRAPHIC FRAMEWORK

The geology of the watershed generally consists of Quaternary sediments infilling a fluvial valley system incised into the bedrock surface. This bedrock valley system drained the upper Great Lakes basin to what is now the St. Lawrence River. This sedimentary package ranges in thickness from zero (bedrock outcrop) to 270 metres within the Laurentian bedrock valley system.

There are four main geologic features present, including:

A bedrock valley system that contains thick sand and gravel deposits;
The Niagara Escarpment that forms the western boundary of the Humber River watershed;
The Oak Ridges Moraine (ORM) that forms the headwater of the watershed; and
Areas where Quaternary sediments have eroded and largely in-filled with fining upwards sequences of sand and silt. This erosive action is attributed to tunnel channel formation beneath glacial ice (Sharpe et al, 2004).

13

Figure 3-1: Surficial Geology

Figure 3-2: Hummocky Topography Areas (Leney and Kenny, 2003)

Humber River State of the Watershed Report – Geology and Groundwater Resources

The various geologic deposits and their characteristics are described further in subsequent sections. It should be noted that there is still some disagreement and considerable discussion and research regarding the stratigraphy present within the study area. Historical work (e.g., White, 1975; Karrow, 1967) is still being re-interpreted by various groups including the OGS and the GSC (e.g. Barnett et al ., 1998; Sharpe et al ., 2004).

3.4 BEDROCK

In most of the Humber River watershed, the bedrock consists of Georgian Bay Formation (interbedded limestone and shale). These rocks were deposited over the Canadian Shield over a period of about 200 million years, beginning approximately 550 million years before present. The structures of the Paleozoic rock influence the groundwater resources and flow patterns. Glaciofluvial erosion may have enhanced these structures and valleys (Gilbert and Shaw, 1994).

Limestone bedrock units are generally more pervious than fine grained shale. Limestone aquifers can yield a considerable amount of groundwater via water stored within fractures and joints. However, such bedrock is only found in the extreme upper reaches of the Main Humber subwatershed, near Mono Mills.

Investigations have provided evidence that an underground buried channel called the Laurentian Valley exists in the Humber River watershed. This valley is part of a system that extends from Georgian Bay to Lake Ontario near Toronto (Spencer, 1881; Eyles et al ., 1993; Middleton, 2004), and is shown as blue-green areas on Figure 3-3. This valley system represents a preglacial, subaerial river system that carved a valley up to 1.5 km wide, and 70 m deep, with several associated tributary valleys. This has since been in-filled with sediments that form major regional aquifer systems (Davies and Holysh, 2004, Kassenaar and Wexler, 2006).

Beneath the ORM, the geometry of the bedrock surface remains poorly defined, as few wells have been drilled into the bedrock. Investigations using location corrected water-wells, hydrogeological borehole data, and seismic reflection profiles, indicate a trunk and tributary valley system (Brennand et al ., 1997). In the Town of Caledon portion of the Humber watershed, valleys eroded into the Niagara Escarpment form tributary valleys to the main Laurentian Channel. Two tributaries are interpreted to outlet groundwater along the shore of Lake Ontario with one outlet near Humber Bay. These bedrock valleys may contain productive aquifers.

16 Figure 3-3: Bedrock Surface Topography (Kassenaar and Wexler, 2006)

Humber River State of the Watershed Report – Geology and Groundwater Resources

3.5 OVERBURDEN SEDIMENTS

The overburden sediments deposited in the watershed 135,000 to 10,000 years before present comprise the bulk of the aquifer systems across the watershed. The stratigraphic framework for Quaternary sediments in the study area has been studied in detail over the last 12 years by researchers at the GSC (Sharp et al, 1999) and others (i.e., Karrow, 1989; Eyles, 2002). Quaternary glacial sediments within the study area consist of glacial and interglacial deposits formed within the last 135,000 years (Karrow, 1989; Eyles, 2002). Error! Not a valid bookmark self-reference. summarizes the Quaternary sediments generally found within the watershed. It is noted that all units do not exist through out the entire watershed. In the western portion, the Thorncliffe Formation lies in direct contact with the bedrock. Similarly, the moraine sediments are not present in the western part of the West Humber River subwatershed.

Figure 3-4: Quaternary Deposits and Paleozoic Bedrock Found Within the Study Area

(after Eyles, 2002)

18 Humber River State of the Watershed Report – Geology and Groundwater Resources

The unconsolidated sediments thin toward the northwest as the bedrock surface generally rises toward the Niagara Escarpment. The overburden thickens beneath areas of higher elevation such as along the Oak Ridges Moraine. Also, thick overburden sediments occur near the bedrock surface lows such as along the Laurentian bedrock valley system. The thickest overburden in the watershed occurs where these two features cross the ORM northeast of Nobleton . The different stratigraphic units in the watershed are described below.

Because of the thickness of the sediments under the Oak Ridges Moraine, and a general scarcity of deep boreholes in this area, uncertainty exists as to whether the lower overburden sediments occur in the north rather than just along the Lake Ontario shoreline. The term “or equivalent” represents an extension of the southern stratigraphy northward. For instance, the term Sunnybrook Drift (or equivalent) describes the aquitard material separating deposits of the Thorncliffe Formation from the Scarborough Formation.

3.5.1 Scarborough Formation

The Scarborough Formation is the oldest sediment unit found consistently across the Humber River watershed. The Scarborough Formation deposits are interpreted as a fluvial-deltaic system fed by large, braided, melt-water rivers draining from an ice sheet (Karrow, 1967; Eyles, 1997). The lower silts and clays are up to 60 m thick at the Scarborough Bluffs along Lake Ontario (Eyles, 1987). The upper sands are channelized in some locations, possibly resulting from fluvial erosion due to fluctuating lake levels. The delta, extending over an area of about 200 km2 was deposited by a large river flowing from Georgian Bay along the Laurentian River channel to ancestral Lake Ontario. Lake water levels must have been up to at least 45 to 60 m higher than present, possibly resulting from glacial ice damming to the east.

Figure 3-5 illustrates the interpreted thickness of the Scarborough Formation, where it is present. Although present in much of the study area, the unit is thickest in the bedrock valleys. The Scarborough Formation is absent along the Niagara Escarpment where the bedrock occurs at relatively higher elevations and in the western portion of the watershed where younger sediments lie directly on top of bedrock.

19 Figure 3-5: Thickness of Scarborough Aquifer

Humber River State of the Watershed Report – Geology and Groundwater Resources

3.5.2 Sunnybrook Drift

The Sunnybrook Drift, deposited some 45,000 years ago, separates the Thorncliffe Formation from the underlying Scarborough Formation. The Sunnybrook Drift is interpreted to be deposited on the floor of a glacially dammed lake, approximately 100 m deeper than the modern Lake Ontario. Sediments consist mostly of clay and silt (Eyles, 2002). Boulders and pebbles, found occasionally, represent drop stone from melting icebergs. The Sunnybrook Drift thickness generally ranges between 10 and 20 metres. The unit thins in the western part of the watershed where the bedrock elevation rises, while it thickens in areas where it fills valleys on the tops of the underlying units such as near High Park in the City of Toronto. Error! Reference source not found. illustrates the thickness of the Sunnybrook Drift.

3.5.3 Thorncliffe Formation

The Thorncliffe Formation deposits represent glaciofluvial deposition of sand and silty sand within lows on the underlying stratigraphy. Further to the south, the deposits comprise predominantly glaciolacustrine silt, sand and pebbly silt and clay deposited by glacial meltwaters entering a deep, ice-dammed ancestral Lake Ontario. The basal part of this unit is often marked by alternating thin layers of silt and clay. This unit was deposited approximately 30,000 to 50,000 before present (Barnett, 1992).

Figure 3-7 illustrates the thickness of the Thorncliffe Formation, where it is present. The thickest deposits occur beneath the Oak Ridges Moraine. These deposits are absent where tunnel channel activity has eroded down through the Thorncliffe Formation. The tunnel channel infill deposits, varying from gravel to clay, provide lateral hydraulic connection with the Thorncliffe Formation deposits. The top of the Thorncliffe Formation dips gently towards the south. The top of the formation generally occurs at an elevation of less than 260 metres above mean sea level and drops to an elevation of approximately 150 metres along the Region of York/ City of Toronto boundary. This general drop in elevation of the unconsolidated sediment stratigraphy from north to south, and towards the Laurentian Bedrock Channel, is typical for the area reflecting a general draping of the sediments over the bedrock surface.

21 Figure 3-6: Thickness of Sunnybrook Drift

Figure 3-7: Thickness of Thorncliffe Formation

Humber River State of the Watershed Report – Geology and Groundwater Resources

3.5.4 Newmarket Till

The Newmarket Till consists of a massive, stony (3-10 %) and consistently dense silty sand diamicton up to 60 m in thickness. Within the till, interconnected sand and silt lenses are commonly found. Fractures and joints are also observed that provide the bulk of the permeability within the till. Hydrogeologic investigations suggest that the bulk hydraulic conductivity (K) of the Newmarket Till is controlled by such structures or pathways (Kassenaar and Wexler, 2006).

The Newmarket Till exists up to 100 m thick ( Figure 3-8) locally with a general approximate thickness of 20-30 m. The till has been eroded in some parts in the north corresponding to the locations of interpreted tunnel channels where meltwater flow eroded down through the Newmarket Till into underlying sediments. The top of the Newmarket Till generally occurs at an elevation of less than 300 metres beneath the Oak Ridges Moraine and drops to an approximate elevation of 170 m southward towards Lake Ontario.

3.5.5 Tunnel Channels

The Oak Ridges Moraine deposits unconformably overlie the Newmarket Till, meaning there is gap between the depositions of the two units. During this gap of about seven thousand years, the Newmarket Till became partly eroded in some areas whereas complete erosion occurred in others. In some instances, the underlying Thorncliffe formation was also eroded. This erosional process created a deeply cut valley system which became subsequently infilled with sediments, attributed to decreased flows after a flood event.

Erosional features at this unconformity include an interpreted network of south-southwest- oriented “tunnel” channels (Figure 3-9) cut deep into or through the Newmarket Till. Information gathered by different researchers helped locate, extend and refine these channels. Although the surface expression of the channels disappears beneath the ORM, mapping, drilling, and seismic reflection profiling show that channels continue beneath the ORM (Kassenaar and Wexler, 2006). The channel width at surface varies from 1-4 km with a depth of tens of metres. In the subsurface, the channel width varies from 1-2 km with a similar depth in the order of tens of metres. The channels mainly consist of sand, however, some channels contain thick deposits of gravels that are 10-15 metres thick.

Three major channel systems trend generally north-south to northwest-southeast extending from north of the ORM into the area north of Lake Ontario. One such channel, within the Humber River watershed represents a major erosional system trending from Holland Landing east of Bradford southward to east of Nobleton and Kleinberg. This trend also approximates the location of the main channel of the Laurentian bedrock valley and major tributary up through Cooks Bay on Lake Simcoe. Another tunnel channel, herein termed the Oak Ridges Channel, passes only through a part of the watershed. This channel also trends southwest through Newmarket and Aurora, east of King City and appears to meet up with the Nobleton Channel near Woodbridge. An undisturbed (not eroded) Quaternary succession, described as the Woodbridge Cut further constrains the width of the two channels near the confluence.

24 Figure 3-8: Thickness of Newmarket Till

Figure 3-9: Tunnel Channel Locations

Humber River State of the Watershed Report – Geology and Groundwater Resources

3.5.6 Oak Ridges Moraine and Mackinaw Interstadial Deposits

The Oak Ridges Moraine (ORM) represents an extensive stratified sediment complex deposited about 12,000 to 13,000 years ago. It is approximately 160 kilometres long and 5 to 20 km wide, arranged as four sediment wedges, each widening westward. The ORM sediments occur along the northern part of the watershed in a west-to-east direction.

ORM sediments occur primarily within fan-shaped bodies on the scale of 10 to100 m thick, up to 5 km long and 1 km wide. Core logging shows moraine sediments consisting of two to three fining-upward sequences. Interbedded fine sand and silt deposits dominate the unit while coarse sands and heterogeneous gravels occur locally. Presences of a separate unit called Oak Ridges Aquifer Complex Silts, comprising of fine grained sediments and silts within the main Oak Ridges Aquifer Complex (ORAC), have been identified in certain parts of the Main Humber River watershed. Clay laminae are also present locally.

The moraine's sands and gravel deposits act like a giant sponge absorbing rain and snow melt. This underground water is then stored through layers of sand and gravel (aquifers), filtered and slowly released as cool fresh water to different rivers including Humber River and streams flowing north into Lakes Simcoe and Scugog and south into Lake Ontario.

The formation of the Oak Ridges Moraine progressed through four major stages, beginning with high energy subglacial channels depositing coarse gravels in east-to- west trending eskers, followed by the second stage formation of high energy subglacial fans. As the ice receded, glacial fan and delta formations emerged with the deposition of fine sands. The last depositional phase involved a lower energy environment with ice marginal deposits including glaciolacustrine stratified sediments and debris flow deposits (Barnett et al, 1998).

The borehole and water well record database compiled through the YPDT-CAMC Groundwater Management Project show the presence of significant sand bodies between the underlying Newmarket Till and overlying tills from the last glaciation. These sand bodies may or may not be hydraulically connected to the ORM sediments, but are within the same segment of the stratigraphic sequence, and are therefore considered together with the ORM sediments. They do not occur everywhere and are referred to as Mackinaw Interstadial deposits (Kassenaar and Wexler, 2006).

Figure 3- below shows the thickness of the Oak Ridges Moraine and Mackinaw Interstadial deposits. Generally, the Oak Ridges Moraine deposits are less than 90 m thick and especially thin along the north and south flanks of the moraine where they are covered by surface tills.

Figure 3-11 shows the thickness of Oak Ridges Aquifer Complex Silts within the main Oak Ridges Aquifer Complex unit. As evident from the map the silt deposits are also present in the extreme northern part of the West Humber River watershed.

27 Figure 3-10 : Thickness of Oak Ridges Moraine/Mackinaw Interstadial Deposits

Figure 3-11: Thickness of ORAC Silts

Humber River State of the Watershed Report – Geology and Groundwater Resources

3.5.7 Halton Till

The latest glacial ice advance over the southern part of the study area came from the Lake Ontario Basin about 13,000 years ago and resulted in the deposition of the Halton Till. Figure 3-12 shows the thickness of the Halton Till, which comprises sandy silt to clayey silt till interbedded with silt, clay, sand and gravel (Russell et al, 2002). In some areas it is very clay- rich where the Halton ice has overridden glaciolacustrine deposits. The Halton Till typically ranges in thickness from 3 to 6 m but locally exceeds 15 to 30 m in and around Nobleton and King City (Russell et al , 2002; White 1975). On the southern flanks of the Oak Ridges Moraine, it overlies the sand and gravel of the Oak Ridges Moraine deposits, as discussed in Section 3.2.

3.5.8 Surficial Lacustrine Deposits

The uppermost geologic unit consists of a sequence of glaciolacustrine deposits that form a thin layer above the underlying Halton Till, in places, overlying the Newmarket Till. These deposits vary from near shore sands and gravel beach deposits of the Lake Iroquois shoreline located within the southern part of the watershed, to the fine sands, silts and clays of glaciolacustrine pondings that occur north of the Lake Iroquois shoreline. These sediments generally form a thin veneer over the underlying deposits, although locally they can be several meters thick. These units represent local ponding of water, or higher water levels in Lake Ontario following retreat of the glaciers approximately 12,500 years ago. Glacial Lake Iroquois, the ancestor of Lake Ontario, had water levels that were at least 40 to 60 m higher than present levels of Lake Ontario due to ice blockage and damming of water along the St. Lawrence River (Anderson and Lewis, 1985; Eyles, 1997). Shoreline elevations continue to slowly change as a result of postglacial glacio-isostatic rebound (Eyles, 1997).

30 Figure 3-12: Thickness of Halton Till