DECIPHERING THE RECORD OF SUB-ICE CONDITIONS AT THE QUATERNARY SEDIMENT–BEDROCK INTERFACE: A CASE STUDY FROM SOUTHWESTERN ONTARIO, CANADA

The complex interaction of Quaternary glacial processes and the underlying bedrock type plays a vital role in drainage at the sediment–bedrock interface and consequently glacial dynamics, erosion, and deposition. In addition, this interface is commonly the site of a modern contact aquifer system, which, depending on the protection afforded by the overlying sediments, can be an important source of drinking water in both rural and urban settings. Thus, analyzing the nature of that sediment–bedrock interface may help us to better understand and predict the sedimentary record of glacial and hydraulic conditions in sub-ice systems as well as their effects on modern groundwater resources at the sediment–bedrock interface.

Current research is focused on Southwestern Ontario, Canada, where Quaternary glacial and interglacial deposits are found on top of Paleozoic clastic and carbonate bedrock. Ongoing groundwater resource mapping by the Ontario Geological Survey has generated numerous high-resolution core logs and modelled stratigraphic surfaces across this area; these sedimentary records were used to examine the nature, distribution, and origin of interface sediments and how these relate to the underlying bedrock type. The objective is to visualize the nature of depositional conditions recorded at the interface and understand its relationship to the paleo-drainage system and glacial dynamics.

Analysis of the interface in 160 high-resolution borehole logs covering an area of ~11670 square km reveals different stratigraphic units recording subglacial, glaciofluvial, glaciolacustrine and non-glacial conditions from the Illinoian to Late Wisconsinan. Many boreholes exhibit interface sediment associated with the Illinoian and Late Wisconsinan (Michigan–Nissouri) regional ice advances. These in turn are often associated with specific bedrock units, namely the Guelph, Salina, and Lindsay formations for the Illinoian deposits and the Guelph, Salina, and Dundee formations for the Nissouri deposits. Using detailed core descriptions and our understanding of the hydrogeological characteristics of the underlying bedrock, this analysis provides insights on sub-ice deposition, and the expected coupling and decoupling processes and associated hydraulics at the ice–bed interface.