Deconstructing the Newmarket Till in south-central Ontario

Abstract

The Newmarket Till (NT) records the advance of the Laurentide Ice Sheet over south-central Ontario during the Late Wisconsin. The till occurs across a wide area of the province, including three prominent physiographic regions. The NT caps and flanks the Niagara Escarpment, a regional scale bedrock scarp 200-300 m high, and occurs across drumlinized sediment-cored uplands underlain by >150 m of pre-Late Wisconsin sediments as well as beneath intervening lowland plains and tunnel valleys incised into pre-Late Wisconsin sediments and Paleozoic bedrock. Detailed surficial mapping over 3750 km2 in Simcoe County and surrounding areas has identified consistent internal facies within the uplands, but significant variability, both spatially and vertically, in matrix texture, colour, consolidation, and clast content, fabric and lithology characterizes the NT within the lowlands and in areas along the Niagara Escarpment. This variability led to previous interpretations of multiple till sheets involving repeated glacier advance and retreat phases during the Late Wisconsin. Combined analysis of internal sedimentology, stratigraphic information gained from 58 continuously-cored boreholes, and morphological data from recently-released high-resolution terrain models, indicates that the NT sediment package is characterized by a wide variety of distinct diamict units and interbedded stratified sediments that are linked to changes in substrate lithology and topography. The varying composition of the NT records evolving subglacial erosional and depositional conditions related to fluctuating subglacial stress regimes, porewater pressures and meltwater drainage. The wide range of properties that characterize the NT has significant implications for understanding and predicting its local hydraulic function as a 'leaky aquitard'. In the future, the groundwater initiative aims to enhance integration of its three core activities thereby providing a holistic approach to assessments of the provincial groundwater resource. Continued investigations will allow for the synthesis of information from individual regional studies to scales.