Insights from combined interpretation of sediment cores and geophysical logs in the Niagara Peninsula, southern Ontario

Abstract

In 2013, the Ontario Geological Survey (OGS) initiated a three-dimensional (3-D) mapping project encompassing the Niagara Peninsula in support of source water protection and land use planning. Key project goals are to 1) build a regional scale 3-D framework model of Quaternary deposits that form both regional and local aquifers and aquitards, and 2) characterize the internal properties of the modeled units. Between 2014 and 2017, 99 continuously cored boreholes were drilled, geologically logged, photographed and sampled in the field. Detailed sedimentological observations and field penetration test results on clay-rich intervals of core have now been augmented by pebble lithology counts and laboratory grain size and carbonate content analysis. Records were also kept regarding drilling methods, core recovery and intervals with drilling fluid losses. The Geological Survey of Canada collaborated in the borehole study by acquiring a suite of geophysical logs in 14 boreholes which had been converted to PVC-cased monitoring wells. Natural gamma and induction (apparent conductivity and magnetic susceptibility) logs were acquired to investigate lithological variation within the sediments. Downhole seismic logs were acquired to measure material velocities. High-resolution fluid temperature logs identified regional groundwater temperature trends. Significant changes in geophysical log responses were evident at major stratigraphic boundaries while subtle variations in responses provided insights into the geochemical and physical properties of the sediments within stratigraphic units. Key geological and geophysical logs from three boreholes have been selected to illustrate the benefits of an integrated approach. Borehole BH05 is characterised by glaciolacustrine mud and muddy diamicton, BH13 is dominated by glaciolacustrine mud, and BH14 intercepted gravel, sand and mud. Thinner beds of overconsolidated sandy silt diamicton and variably dolostone, gypsum and shale bedrock were also observed. Gamma and induction responses are sensitive to changes in the proportion of sand, silt and clay within both diamicton and glaciolacustrine deposits. Potentially water-bearing sandy units are clearly distinguished by high magnetic susceptibility and low conductivity values while the reverse records units rich in clay-sized grains. On the Niagara Peninsula, it can be difficult to distinguish between glaciolacustrine mud and muddy till. However, decreases in velocity correspond to a change from grounded ice to a glaciolacustrine depositional setting, while increases in magnetic susceptibility relate to ice-rafted debris. The integrated examination of sediment cores and geophysical logs together provides an improved understanding of the complex local geological processes, and thus aided in the search for aquifers and the characterization of regional aquitard units.