Geology of the Aurora high-quality stratigraphic reference site and significance to the Yonge Street buried valley aquifer, Ontario

Abstract

The Geological Survey of Canada (GSC) Aurora cored borehole intersects the Yonge Street aquifer (YSA), an important groundwater source in Ontario. The borehole, sited along a 7 km long seismic profile, was drilled in order to provide data to improve sustainable use and management of groundwater in Aurora well fields and the regional aquifer system. Results provide high-quality hydrostratigraphic reference data, geological context, and a prospecting model for this significant buried-valley aquifer. The improved conceptual hydrogeological model offers a plan to effectively stress and assess the YSA system. A 130 m sedimentary succession unconformably overlies subhorizontal Whitby shale and a low-relief bedrock surface with no defined valleys. Gas seeping from the shale is trapped in Thorncliffe Formation, a regional aquifer below confining aquitards. The regional aquifer occurs below ~209 m a.s.l. and is a 80 m thick, fining-upward, sand and gravel sequence. It appears to represent a portion of a northeast-southwest-oriented channel, esker, and subaqueous fan system that fed Thorncliffe Formation aquifer sediments to the south. The newly identified aquifer system occurs above a regional unconformity within the succession. A 25 m thick Newmarket Till and silt-clay rhythmite sequence confines this aquifer. This aquitard drapes into Aurora basin, a possible pre-existing sediment valley. High clay content in aquitard rhythmites may allow conductivity logs to map it as a marker horizon. The overlying Oak Ridges Moraine (ORM) aquifer occurs as a 30 m thick gravel-sand-mud sequence above a second regional unconformity within the succession. This channel fill sequence is thinner than nearby 100 m thick ORM channel sediments.

Conceptual hydrogeological model of the Yonge Street Aquifer, south-central Ontario: a glaciofluvial channel–fan setting

“…These datasets have a wide range in quality and reliability. Higher quality geologic datasets include cored boreholes, and seismic surveys along with borehole geophysics collected from various studies (e.g., Sharpe et al 2003aSharpe et al , 2003bSharpe et al , 2011 Table 1). Interpretations from these higher quality datasets have been augmented with other geologic and hydrogeologic information for regional interpretations (e.g., Ontario Ministry of the Environment and Climate Change (MOECC) water well records).…”

“…Downhole seismic data indicates p-wave seismic velocities of 3000 m/s (Pullan et al 2000) indicative of gravel. Overlying the hummocky facies is a 30-50 m thick package of weaker reflectors that extend across the channel and have a consistent density and a p-wave velocity of 1800 m/s (Sharpe et al 2011). Weak inclined seismic reflections within this upper unit appear to be foreset bedding with a component of dip toward the west.…”

“…3). The classic Quaternary stratigraphy of south-central Ontario (north of Lake Ontario) has been described from the Scarborough Bluffs (e.g., Karrow 1967) and mapped in the subsurface northward to the Alliston and Barrie area (e.g., Eyles et al 1985;Sharpe et al 2011;Bajc et al 2014b) and eastward to the Bowmanville area (Kelly and Martini 1986;Martini and Brookfield 1995). Preserved Quaternary sediments represent an assemblage of glacial and interglacial units that have been deposited over the last approximately 135 000 years (Mulligan and Bajc 2018).…”

“…Seismic reflection data at Aurora and Schomberg reveal truncated seismic stratigraphy. Channels that erode tabular Lower sediment in a general north-south direction Sharpe et al 2011). (a) Age, lithostratigraphy, and chronostratigraphy; note that a regional (channel) unconformity (long sloping red line) is present beneath both Oak Ridges Moraine (ORM) and upper Thorncliffe Formation (TF) sediments; Lower sediment (blue sequence) comprises a series of formations and units below Newmarket Till and resting on bedrock, which are difficult to trace separately across the area.…”

“…are greater than 2 km wide, can be greater than 80 m deep, and are likely of Upper Thorncliffe Formation age (e.g., Sharpe et al 2011). Channel fills consist of fining-upward sequences of gravel, sand, and mud (Sharpe et al 2011).…”