EVIDENCE FOR ROCK MECHANICAL LAYERS INFLUENCING CHANNEL MORPHOLOGY IN A BURIED BEDROCK VALLEY

An extensive network of buried bedrock valleys exists throughout southern Ontario, Canada, which formed as a result of preglacial, glacial, and glaciofluvial erosion as well as subsequent dissolution within and around these features. Buried bedrock valleys can influence groundwater flow systems, playing a role in recharge of bedrock aquifers and may even act as preferential pathways that enhance a deeper aquifer’s susceptibility to contamination. The morphology of bedrock rivers is often influenced by bedrock physical properties. With erosion resistant units often acting as aquitards, a detailed assessment of the relationship between bedrock lithological properties and bedrock valley incision will contribute to a better understanding of the role buried bedrock valleys play in groundwater flow systems. The channel morphology of a buried bedrock valley in Elora, Ontario was characterized using an airborne frequency-domain electromagnetic and magnetics survey, along with two high-resolution, surface geophysical transects oriented orthogonal to the valley consisting of co-located gravity, seismic refraction, and electrical resistivity tomography surveys. Results revealed a linear U-shaped valley in the NE deeply incising bedrock but becoming progressively shallower downslope towards the SW. The thalweg elevation may be controlled by bedrock mechanical properties, aligning with the erosion resistant Ancaster Member of the Goat Island Formation. A myriad of evidence indicates this unit possesses high mechanical strength and may act as an aquitard (e.g., head profiles in the area). The erosion resistive mechanical properties imposed by this unit during incision may have influenced valley morphology, with the river channel widening to maintain the cross-sectional area for flow. Geophysical measurements also revealed variability in the physical properties of the incised bedrock, seen here as another bedrock property influencing valley morphology. A systematic reduction in bedrock competency along the valley thalweg likely contributed to a broader valley profile with a more pronounced weathered zone. Enhanced understanding of bedrock valley morphology and associated spatial variability of bedrock hydraulic properties proximal to buried bedrock valleys could have important implications for future groundwater resource assessments in this region. Continuous bedrock coring below the Quaternary sediment interface at these two transect positions is in progress with results helping to evaluate these hypotheses.