ABSTRACT: Characterizing the Quaternary hydrostratigraphy of buried valleys using multi-parameter borehole geophysics, Georgetown, Ontario.

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Abstract

Thick Quaternary deposits are exploited widely in southern Ontario for groundwater but their stratigraphic complexity can present a challenge for groundwater exploration. In 2009, Halton Region and McMaster University initiated a regional investigation of the geology and hydrostratigraphy of Quaternary sediments near Georgetown, Ontario to assist the search for new high yield aquifers. The project involved drilling of new continuously cored boreholes (> 40) and surface and downhole geophysical investigations to characterize the infill stratigraphy of the Middle Sixteen Mile Creek (MSMC) and Cedarvale (CV) buried valley systems. Multi-parameter log suites (natural gamma, EM conductivity, resistivity, magnetic susceptibility, full-waveform sonic, caliper) were acquired in 30 new and 8 existing test wells (16 m to 55 m depth) to assist in subsurface correlation and the identification of potential aquifer and aquitard units. Characteristic log responses (electrofacies) were identified by cross-plotting of log parameters and correlated with core lithofacies. In the MSCM buried valley 7 distinctive lithostratigraphic units were identified within a thick (> 55 m) interbedded sequence of diamict (aquitards), laminated silts and coarse-grained glaciofluvial deposits (aquifers) overlying shale bedrock. Gamma, magnetic susceptibility, resistivity and conductivity logs were most useful for lithologic typing. Downhole changes in p-wave velocity and resistivity were also important for discriminating and correlating more compact diamict units. In a number of locations the lowermost diamict unit was thinned or erosionally truncated, allowing direct communication of the upper and lower aquifers. The infill stratigraphy of the CV valley comprised a complex succession of glaciofluvial sand and gravel up to 45 m thick. The CV sediments truncate the older MSCM stratigraphy across a well-defined erosional unconformity and were deposited in a sediment-hosted valley.The results demonstrate that the Quaternary sediments below Georgetown are complex and characterized by significant lateral and vertical sedimentary variability. The geophysical log responses of a number of hydrostratigraphic units were distinctive and provided useful subsurface marker horizons for correlation of the Quaternary deposits in areas where core data are unavailable. Electrofacies in the MSCM buried bedrock valley were generally predictable, whereas the sediment-hosted CV valley was more complex and variable. The differences in the two valley systems can be attributed to their contrasting depositional environments.

Bibliography of Canadian Geomorphology