Results of the Orangeville-Fergus 3-D sediment mapping project

The Orangeville-Fergus 3D sediment mapping project was undertaken between 2008 and 2015. The 1550 km2 study area is situated above the Niagara Escarpment between Waterloo Region and the city of Orangeville. The study area is centered on the sand and gravel Orangeville Moraine which forms the headwaters of the Nottawasaga Valley, Grand River and Credit Valley Conservation Authorities. Forty-three new boreholes were drilled and these, together with new geophysical surveying, surface sampling and analysis of a significant volume of legacy data, were the basis for the creation of a 3D model. The model is underpinned by a conceptual geological framework which subdivides the Quaternary sediments into 16 hydrostratigraphic units. The units are identified on the basis of age and sediment characteristics resulting from deposition in different environments. Each unit typically contains a range of sediment textures resulting from depositional facies changes and short-term or localized changes in ice-margins, lake levels, sediment sources and water velocities. Undifferentiated southward dipping Paleozoic bedrock forms the basal hydrostratigraphic unit across the area. A new bedrock topographic surface significantly refines previous provincial-scale mapping studies. Broad lows and resistant bedrock highs, deeply incised re-entrant bedrock valleys extending back from the Niagara Escarpment and narrow rectilinear buried valleys characterise the surface. A series of older tills confine the bedrock aquifer and small sediment aquifers in the western portion of the area. Catfish Creek Till, deposited during the last glacial maximum, is mapped across most of the area, thinning then pinching out towards the Niagara Escarpment. The till forms a regionally significant aquitard that can be detected in many low quality records and is considered to be an important stratigraphic marker. As the ice thinned and began to break up, a lake formed within the interlobate zone between the retreating ice fronts. Small sand and gravel fans were buried by up to 20 m of silt and clay in the central part of the lake. The readvance of lobate ice initiated deposition of the thick centrally located Orangeville Moraine. The moraine is characterised by a hummocky surface and coarse gravel to sand ice-proximal sediments in the northeast. The surface becomes progressively smoother and the sediments shift to fine-textured sand, sandy silt and silt towards the distal flanks. These fine-textured sediments can be expected to complicate local groundwater flow paths through the sediment package. The moraine forms an important groundwater recharge area, particularly in the hummocky and coarse-textured zones. The ice continued to advance, and partially overrode the moraine. A series of tills were deposited and these form the upper aquitard across much of the area. Outwash cobbles and gravels were deposited in channels incised into the till as the ice retreated and these form unconfined aquifers across the area. The final readjustment of the retreating ice lobes resulted in the construction of the hummocky, Paris and Singhampton end moraines in the southeast and northeast corners of the study area, respectively. The moraines are characterised by discontinuous beds of diamicton and course-textured stratified sediment, providing recharge potential despite classification as an aquitard. The development of an evidenced-based 3D hydrostratigraphic model in the Orangeville-Fergus area has provided the first opportunity for a detailed examination of the lateral extent, thickness and composition of regional-scale sediment packages. Surface and buried sediments forming aquifers have been identified and described. This information is intended to inform source water protection and land use planning.