Thorncliffe Formation: A proglacial to subglacial lacustrine basin sequence, Greater Toronto Region, Canada

Thorncliffe Formation (southern Ontario, Canada) sedimentary architecture, extent, facies trends, contacts and composition, combined with hydraulic data, define fundamental glacial basin depositional environments. Thorncliffe Formation (TF) comprises an ~50–100 m thick glaciofluvial–glaciolacustrine sediment sequence covering ~8000 km 2 east of Niagara Escarpment, from Lake Ontario north to the Canadian Shield. These sediments include aquifers that supply water to >200 000 people, with significant untapped resources in the Greater Toronto Area. A comprehensive regional dataset enables reassessment of TF character, origin and age within a 3Dmodel of the basin sediments. The TF is primarily basin mud (~90%) with associated source channels (<100 m deep) containing fining‐upward sequences (~10%) of gravel, sand, rhythmites and diamicton. TF deposition began as proglacial lake organic‐rich sediments accumulated in front of an advancing ice margin that obstructed Great Lakes drainage. Rising lake levels subsequently ponded meltwater against the ~100 m high Niagara Escarpment. When advancing ice overtopped the escarpment, TF basin evolved into a captured subglacial lake. Organic‐poor, stratified TF subglacial lake sediments were generally deposited conformably with overlying regional Newmarket Till (NT). Integration of high‐resolution subsurface data sets including seismic profiles, continuous cores, lake bluff sections and well monitoring data, enables the development of a robust conceptual framework for TF sediment architecture and depositional setting. This synthesis supports interpreting TF as high‐energy, subaqueous channel‐fan systems discharged from mapped inflow corridors into a muddy subglacial lake basin. The contact between glacial (NT) and inferred subglacial lake sediments (TF) exhibits interbedding, gradation, mounding and boulder scour features indicative of synchronous sedimentation beneath an ice‐covered subglacial lake. Episodic ice lifting from and lowering back to the bed likely occurred during meltwater discharge events. TF data support established theory that subglacial lakes were common beneath continental ice sheets. TF sediment characteristics can help to identify subglacial lake settings in other glaciated terrain.