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3-D mapping of Quaternary deposits in the southern part of Simcoe County

A 3-dimensional (3-D) Quaternary sediment mapping project in the southern part of Simcoe County is one of several investigations being undertaken as part of a broader groundwater geoscience initiative in the Greater Golden Horseshoe region. Outputs of the project are designed to provide
the basic geologic foundation for groundwater studies in a part of the province that is expected to experience significant population growth over the next few decades. A basin analysis approach similar to that used by the GSC in the Oak Ridges Moraine to the south was undertaken in southern Simcoe
County to construct the 3-D model. Following the capture and standardization of all available subsurface information (much of which was provided by the Conservation Authorities Moraine Coalition), a series of geophysical surveys (ground gravity, airborne TDEM, downhole logging and reflection seismic
profiling) were undertaken to assist with the refinement of bedrock topography and the characterization of the geometry and contact relationships of overlying units. Continuous-coring of Quaternary sediments down to bedrock was undertaken at 25 sites, guided by the results of the geophysical
surveys, to establish the regional Quaternary stratigraphic framework and support the subsequent modelling exercise.
The southern part of Simcoe County overlies the Laurentian Valley, a broad, bedrock depression extending from Georgian Bay southward to Lake Ontario. The maximum depth to bedrock encountered in the OGS boreholes exceeded 175 m. Surficial sediment thickness increase to the south of the study area,
over the Oak Ridges Moraine, and diminish to the west as one approaches the Niagara Escarpment. Much of the study area is characterized as a till-capped upland incised by a network of deep valleys that are partially infilled with postglacial sediments. Uplands consist of an upper, Late Wisconsin
till (Newmarket Till) overlying a thick sequence of glaciolacustrine deposits correlative with the Thorncliffe Formation (Middle Wisconsin). An unconformity characterized by deep subaerial weathering and organic-bearing alluvial deposits at the base of the Thorncliffe Formation is AMS dated at 37.9
to >52.8 14C ka BP. The unconformity, which possibly spans Middle Wisconsin through to Sangamon time (based on fossil evidence) is developed on an older sequence of tills and stratified deposits overlying bedrock and presumed to be Illinoian in age. The valleys, which are in some cases incised
completely through regional upland strata, have been previously interpreted to be tunnel channels. They are partially infilled with fine-textured glaciolacustrine deposits capping variable thicknesses of coarse glaciofluvial sand and gravel either resting on, or incised through, Newmarket Till.
Streamlined till has been observed on the upland surfaces as well as on the flanks and bottoms of several valleys suggesting continued deposition by active ice following erosion of the valleys.
Significant groundwater resources are restricted primarily to the highly permeable aquifers at the base of tunnel valleys. Thick, productive aquifers are rare within most of the uplands with the exception of the extreme northern part of the study area where the Thorncliffe Formation coarsens and
sandy and gravelly facies are common. Waters from deeply buried aquifers often contain extremely high dissolved solids and some may contain methane limiting their use for municipal water supply.

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