MAPPING BURIED ESCARPMENT VALLEY AQUIFERS IN SOUTHERN ONTARIO

Buried valleys have proven to host high yield aquifers across North America. In Southern Ontario buried valleys have been exploited for groundwater since the late 1800’s. Nevertheless there remains a poorly developed understanding of the distribution, scale, and aquifer characteristics of this hydrogeological setting in the region. On the basis of identified buried valleys in Southern Ontario, three distinct settings can be recognized, i) escarpment troughs, ii) escarpment re-entrant valleys, iii) steep-walled gorges.

Escarpment re-entrant valleys may form the most complex setting of the three as they straddle a distinct break in topography and consist of three recognizable elements, i) buried valleys east of an escarpment, ii) partially infilled escarpment re-entrants with bedrock exposed in the valley walls, and iii) western buried segments. On the basis of bedrock surface DEM a series of re-entrant valleys can be identified from the Bruce Peninsula in the north to the Niagara Peninsula in the south. The valley geometry, burial depth and general fill characteristics can be mapped from archival water well data. At a few sites reflection seismic reflection data and continuous core allow the character of the valley geometry and fill to be identified.

Two contrasting valley fill successions are currently recognized. Type one consists of a fining upward succession of gravel and sand with burial by a fine grained lacustrine succession. The entire succession is attributed to infill and burial in the late Wisconsin. A second style consists of a more complex fill succession of sand and gravel and diamicton that may record infill and burial over a longer geological time span dating back to the early Wisconsin. For groundwater studies it is important to recognize which of these two end member settings has been intercepted. The contrasting hydrostratigraphy of these fill and burial successions will strongly control the scale and heterogeneity of the valley aquifer, and also may have significance for aquifer recharge. Furthermore, in the absence of higher quality data, the depth and scale of the buried valley system may be underestimated leading to an under appreciation of the potential groundwater resource in the area.