EXPLORING BEDROCK MECHANICAL PROPERTIES AND GEOMORPHOLOGY OF A BURIED BEDROCK VALLEY IN ELORA, ONTARIO, CANADA USING SURFACE AND DOWNHOLE GEOPHYSICS

Preglacial, glacial, and glaciofluvial erosion has resulted in an extensive network of buried bedrock valleys in many northern mid-latitude environments. These valleys appear to influence groundwater flow systems by enhancing bedrock transmissivity, potentially increasing the susceptibility of underlying and adjacent bedrock aquifers to contamination. While bedrock river morphology is often influenced by bedrock physical properties, the relationship between bedrock lithological, mechanical, and hydrogeological properties and bedrock valley incision is not well understood. To examine this, the channel morphology and bedrock properties of a buried bedrock valley in Elora, Ontario, Canada was characterized using an airborne frequency domain electromagnetic and magnetics survey, along with two electrical resistivity tomography transects across the valley. Full-Waveform Sonic (FWS) logs were also completed in bedrock boreholes along the flank and thalweg of each transect. These data reveal a linear U-shaped valley in the northeast that deeply incises bedrock but becomes progressively shallower, broader, and more complex downslope towards the southwest. Variability in valley morphology appears to correspond to a systematic change in bedrock electrical resistivity, which decreases from northeast to southwest along the valley; lower resistivity may indicate a more weathered and less competent bedrock. The mechanical and/or hydrogeological conditions imposed by aquitard units during incision are hypothesized to further influence valley morphology, with the river channel forced to develop laterally as the riverbed reaches more mechanically resistive units. To explore the relationship between valley morphology and bedrock competency, FWS logs were examined in 4 boreholes within the valley, with P- and S-wave velocities used to calculate mechanical property parameters including V_P/V_s ratio and Young’s Modulus. FWS logs show distinct mechanical units, which are compared to lithology and resistivity units and their relationship along and across the valley. Findings provide a better understanding of buried bedrock valley morphology and associated spatial variability of bedrock properties that could have important implications for future groundwater resource assessments in this region.