LATE GLACIAL ONTARIO LOBE ICE ON THE NIAGARA PENINSULA: HOW FAR DID IT GO?

Lobate ice, characterised by relatively short-lived oscillations, dominated southern Ontario after the breakup of the main late Wisconsin ice sheet. During the late glacial Port Huron phase, ice advanced radially out of the Lake Ontario basin depositing muddy Halton till. Digital seamless surficial geology maps, released by the Ontario Geological Survey (OGS), can be used to quickly define the surface extent of this till around the western end of Lake Ontario. Numerous regional-scale and site specific studies by the Geological Survey of Canada, OGS and university researchers have explored the subsurface characteristics and depositional history of sediment related to the advance of Halton ice to the north and west of the lake. Until recently, however, comparatively few studies have been completed southwest of the lake, particularly at a regional scale.

High-resolution digital elevation data, collected as part of the southwestern Ontario orthophotography project (SWOOP 2010, 2015), were released as digital surface models in 2015. These datasets reveal complex landform relationships that could not be detected on older elevation models. Within the Niagara Peninsula area, encompassing 5000 km² between lakes Erie and Ontario, the extent of older drumlins, a series of till cored moraine ridges located above the Niagara escarpment, ice-contact delta complexes and grooved and fluted bedrock along the escarpment edge provide insight on the most recent ice flow direction and extent. Preliminary results from 95 continuously cored boreholes drilled by the OGS from 2014 to 2016 show that Halton Till and associated deposits (stratified ice-contact sediments and interbedded diamictons) form a prominent marker horizon in the subsurface, separating fine-textured glaciolacustrine sediments into upper and lower packages. The influence of the ice advance is also apparent beyond the former ice margin as increased sediment loads resulting in deposition of coarser (silty) rhythmites.

By comparing the surficial morphological and sedimentary characteristics with high-quality drilling results, the extent of the Port Huron phase ice advance can be confidently mapped into the subsurface with more accuracy than was possible using water well and monitoring well records.