WINDOWS INTO THE SUBSURFACE: SEDIMENTOLOGICAL ANALYSIS OF CORED QUATERNARY VALLEY INFILL SEDIMENTS, GEORGETOWN, ONTARIO

The Regional Municipality of Halton is undertaking an in-depth study of the subsurface sediments in the Georgetown region of southern Ontario with the aim of identifying sustainable sources of potable groundwater for the growing urban population. Georgetown is located at the base of the Niagara Escarpment where aquifers are hosted in thick successions of Quaternary sediment overlying an uneven bedrock surface ornamented with a network of buried valleys. Repeated cycles of glaciation in southern Ontario have resulted in highly complex Quaternary stratigraphies within valley infills and the 3-dimensional geometries of valley-hosted aquifers are difficult to determine.

A series of 10 fully cored boreholes have recently been drilled in the Georgetown area. The recovered cores are up to 51m long and consist of a series of interbedded gravels, sands, diamicts and muds resting on weathered bedrock of the Queenston shale. Detailed sedimentological logging of the cores recording sediment characteristics such as texture, sedimentary structures, bed thickness and clast lithology allows identification of a series of stratigraphic units. These include poorly sorted shale-rich diamicts containing angular clasts that rest on bedrock and are interpreted as residual weathered bedrock and slope debris. Overlying coarse grained units of gravel and sand are interpreted as glaciofluvial and fluvial deposits and contain rounded clasts of both local and far-travelled origin. Diamict units contain clasts of various lithologies (some of which are shaped and striated) within either a sand-rich or mud-rich matrix. Diamict facies are interpreted as subglacial deposits recording episodes of ice advance into the area. Units of fine silty sands and silty clays indicate localized lacustrine conditions, possibly resulting from the ponding of meltwaters between glacial ice to the south and east and the Niagara Escarpment to the west. The fine-grained matrix present in some of the diamicts may result from the overriding of lacustrine deposits by a fluctuating ice margin.

Correlation of stratigraphic units between boreholes and integration of the borehole data with sedimentological field data from the local area allows a preliminary interpretation of the basic structure and geometry of buried aquifers in the Georgetown area.