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

Abstract

The Yonge Street Aquifer (YSA) in the Greater Toronto Area of south-central Ontario is a prolific municipal supply aquifer. It has been considered to be channelized sand and gravel linked to a bedrock valley. Despite considerable work, the fundamental conceptual model for the YSA is not well developed and documented. Based on high-quality data, a revised conceptual model of the aquifer is presented. Seismic profiles define the geometry of the regional stratigraphy with four distinct units: bedrock, Lower sediments, Newmarket Till, and Oak Ridges Moraine (ORM) sediment. Seismic data reveal two generations of roughly north–south channels: older sub-Newmarket Till channels within Lower sediments (termed Thorncliffe channel) and ORM-related channels (termed ORM channel) that incise both Newmarket Till and Lower sediments. The YSA is interpreted to occur within a Thorncliffe channel, with possible vertical connection to younger ORM channels and lateral connection to inter-channel Lower sediments. Thorncliffe channel deposits consist of fining-upward transitions from coarse gravel, to sand, to rhythmically bedded mud interpreted to be deposited within a channel – esker – subaqueous fan complex. Upper Thorncliffe channel mud facies and overlying Newmarket Till provide a capping aquitard. The YSA conceptual model benefits from a strong understanding of facies changes in the Thorncliffe Formation. The deposits with highest permeability occur within up to 80 m thick gravel and sand sequences at the base of the Thorncliffe channel, with transmissivity ranging from 1500 to 4500 m2/day. Groundwater level response to municipal pumping confirms connection along the channel with muted hydraulic response laterally. Thorncliffe channels are interpreted to be up to 20 km long and approximately 2 km wide.

Geological framework of the Laurentian trough aquifer system, southern Ontario

“…9b). This channel sequence is similar to the 20 km long north-southtrending 20-80 m thick channel sand and gravel sequence, mapped from Queensville south to Green Lane and Aurora, which is overlain by ϳ400 rhythmites (Table 4; Gerber et al 2018).…”

“…Larger channels (SV1, oldest sediment valley) occur across the study area in a range of landform and topographic settings (Figs. 6a, 6b, 6c, and 6e; Table 3; Pugin et al 2011), whereas smaller channels are most clearly imaged in the Queensville area (Gerber et al 2018). To the north of the study area, channels are wider (>5 km) and shallower (<100 m deep), truncating the northern margins of the thick Lower sediment package (Pugin et al 2018); and in places, dome sediment structures occur on the inferred channel floor (e.g., Wasaga Beach seismic profile; Table 3).…”

“…7a and 7b). TF channels (B2) form an ϳnortheast-southwest-oriented network with inferred channel spacing at ϳ10-15 km (Sharpe et al 2011), eroded into fine-grained TF and other Lower sediment (B1); these packages are defined by the integration of seismic, sedimentary facies arrangement and, in places, by hydraulic data (e.g., Gerber et al 2018).…”

Comment on “Converging ice streams: a new paradigm for reconstructions of the Laurentide Ice Sheet in southern Ontario and deposition of the Oak Ridges Moraine”

“…Building on the recognition by Gwyn and Cowan (1978) that Newmarket Till extended beneath the ORM, the subsequent cumulative body of work has refined and thoroughly documented the stratigraphic units and relationships that clearly place constraints on any process orientated interpretation of the ORM. Regionally extensive, streamlined Newmarket Till (e.g., Pullan et al 2002;Sharpe et al 2002;Sookhan et al 2018) is incised by a series of valleys that are exposed north of the ORM (Brennand and Shaw 1994;Brennand et al 2006) and that are locally buried by ORM sediment (e.g., Barnett et al 1998;Sharpe et al 2002;Russell et al 2003aRussell et al , 2004Brennand et al 2006;Gerber et al 2018). To the south of the ORM, Halton Till extent and sedimentology has been refined and demonstrated to overlie and be gradational from ORM sediment (White 1975;Sharpe and Barnett 1997;Russell and White 1997;Russell et al 2004;Sharpe and Russell 2016).…”

Quaternary geology of southern Ontario and applications to hydrogeology

“…The final paper on the buried valley setting, by Gerber et al (2018), focuses on a sediment-hosted channel beneath and north of, and stratigraphically older than, the Oak Ridges Moraine, in the northern Greater Toronto Area. Continuous core, downhole geophysical data, and reflection seismic profiles were used to develop a channel -subaqueous fan model of the Thorncliffe Formation.…”