THE SUM IS WORTH MORE THAN THE PARTS: INTEGRATING 3D MODELS TO REFINE THE REGIONAL STRATIGRAPHIC FRAMEWORK FOR HYDROGEOLOGIC STUDIES ABOVE THE NIAGARA ESCARPMENT IN SOUTHERN ONTARIO

The Ontario Geological Survey (OGS) has been conducting 3D sediment mapping in southern Ontario since 2002. Projects are aimed at understanding the architecture and inherent properties of Quaternary sediment sequences for groundwater applications. To date, 181 continuously cored boreholes, many of which test features identified in supporting geophysical surveys, have been drilled within an area of roughly 10 000 km² above the Niagara Escarpment. Models centered over the Late Wisconsin interlobate zone reveal similar stratigraphic successions whereas both similarities and differences are found on the low-lying Niagara Peninsula beyond.

Tills and coarse- to fine-grained stratified deposits in the interlobate zone range from pre-Michigan Subepisode to late glacial (Erie, Port Bruce and Mackinaw phases) separated by Nissouri Phase Catfish Creek Till, an important marker bed and regional aquitard in southwestern Ontario. In contrast, on the Niagara Peninsula pre-Catfish deposits are confined to deep and narrow buried–bedrock valleys (BBVs) and late glacial fine-grained till and glaciolacustrine deposits dominate the sediment record.

Aquifers have been observed in several stratigraphic settings. Pre- and post-Catfish aquifers of variable thickness and in multiple stratigraphic positions have been identified within BBVs in all project areas. Partially buried, thick, sandy moraines deposited in the interlobate zone form the primary sediment aquifers. Gravelly outwash deposited during ice retreat and sands laid down in ancestral high-level Erie basin lakes cap the sequence and serve as important, but vulnerable, local and regional aquifers. Beyond the interlobate zone, sandy ice marginal fan – delta complexes are the most prevalent sediment aquifers and appear to have been preferentially deposited in close proximity to BBVs. Underlying thick, fine-grained glaciolacustrine deposits may prevent local groundwater recharge of bedrock and at least some BBV sediment aquifers.

The models provide valuable insights into the Quaternary history of a large portion of the Greater Golden Horseshoe highlighting the distribution and characteristics of important sediment aquifers. Knowledge gained can be transferred to surrounding areas and used to guide the on-going search for safe and sustainable water supplies.