Quaternary geology and hydrostratigraphy of the central Simcoe County area, southern Ontario

Abstract

The population of communities bordering southern Ontario's greenbelt is increasing. Effective growth plans require an understanding of the state of regional water resources and groundwater flow systems. To address these needs, the Ontario Geological Survey (OGS) initiated a three-dimensional (3D) sediment mapping investigation in the central part of Simcoe County in 2014 to help provide the regional context and necessary geoscience data to guide planning and resource management decisions. OGS 3D sediment mapping studies aim to characterize the architecture of Quaternary sediments, better define bedrock topography, delineate potential recharge areas and aquifer extents, and identify potential surface water-groundwater interactions. Surficial mapping from 2014-2016 assisted in establishing sediment properties and variability, refining the shallow subsurface sediment architecture, and developing an understanding of shallow groundwater flow systems in the region. A regional gravity survey was undertaken in 2014 to improve the understanding of the bedrock topography beneath thick drift underlying the study area. Seismic reflection surveys carried out by the Geological Survey of Canada in 2015 and 2016 assist in delineating the architecture and facies variability of Quaternary sediments and bedrock. Sediment drilling programs conducted in 2015, 2016 and 2017 have produced 26 continuously-cored boreholes. Additional boreholes from previous OGS mapping (5) and other geologic investigations (2) bring the total number of cored boreholes in the study area to 33. Most boreholes (26) reach bedrock and, in many cases, are the only sources of information on the deeper subsurface geology within several kilometers. Four major sediment packages characterize the Quaternary stratigraphy overlying Paleozoic and Precambrian bedrock in the study area. In ascending order, these include: older tills and glaciolacustrine deposits, Thorncliffe Formation equivalents, Newmarket Till, and postglacial deposits. Seismic data assist in deconstructing complex architectural and facies relationships observed within the major packages. Downhole geophysical data provides necessary ground-truth and an improved understanding of the variability of physical properties of regional sediment units at borehole locations. Significant aquifers are encountered at various intervals within each major sediment package except the Newmarket Till, which forms a surficial and subsurface leaky aquitard throughout the region. To support efforts to delineate groundwater recharge areas and identify major flow systems, 17 boreholes have been converted into monitoring wells in collaboration with local conservation authorities, municipalities and associations for long-term analysis of groundwater levels. In addition, a new initiative by the OGS, with field programs beginning in 2018, aims to integrate additional hydrogeological characterization with the geologic and geophysical data associated with 3D sediment mapping. This hydrogeological characterization will comprise groundwater sampling and chemical analyses, following the protocols of the OGS ambient groundwater geochemistry program, as well as the integrated analysis of in-situ pumping tests and hydrologic information from high-quality boreholes and local water well records to better understand the properties of aquifers beyond the borehole sites. Preliminary results indicate that refinement of the existing hydrostratigraphic framework is required for the region. The new data will provide additional constraints on the function and properties of regional aquitards, the stratigraphic setting and hydrochemistry of producing aquifer zones, the occurrence and geometry of previously unidentified aquifer, long-term climatic fluctuations and water level responses in the Great Lakes basins.