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

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.

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