HYDROGEOLOGIC INSIGHT FROM THE GEOLOGY OF A BURIED BEDROCK VALLEY

Understanding the depositional and erosional history of buried bedrock valleys is a crucial first step in identifying how they may impact subregional flow-paths, recharge, and contaminant pathways. Six boreholes were drilled along two transects separated by 3.5 km over a buried bedrock valley near Elora, Ontario. Bedrock and Quaternary sediments were continuously cored using PQ wireline and Roto Sonic, respectively. These data are being used to reconstruct the geologic history of the valley and examine how it may influence the groundwater flow system.

Previous geophysical study identified a w-shaped transect in the SW and a u-shaped transect in the NE, and results from this study were used to validate the estimated depths to bedrock. High variability in depositional environment, dissolution, and bedrock stratigraphy were observed between the two transects, reflecting the complicated history of the Silurian seaway and post-depositional processes that affected these rocks. At the SW transect, low-porosity wackestone dominated, interrupted by sporadic zones of high porosity and large (>10 cm) conduits. At the NE transect, thick encrinite and grainstone with a higher average porosity but fewer large conduits were observed. In both transects, dissolution-rich intervals were not easily correlated between the nearby thalweg and flank holes. In contrast to the variability in the bedrock, the Quaternary infill was consistent between boreholes, with 3 m of gravel at the base, 30 m of homogenous sand filling the valley, and 20-30 m of interbedded diamict and mud units.

Previous studies of buried valleys in SW Ontario have hypothesized that a hydraulic gradient developed during incision could lead to dissolution enhancement in bedrock aquifers; the observed heterogeneity in dissolution features between borehole locations will be an important consideration when testing this hypothesis during future hydraulic characterization. It will also be important for future study to determine the vertical hydrogeologic integrity of the uppermost diamict units, considering the coarse-grained nature of the remainder of the valley infill. The findings of this study provide an important geological framework for the hydraulics of this buried valley, which will shed light on the geological evolution of the groundwater flow system.