SEDIMENTOLOGICAL INVESTIGATION OF QUATERNARY SEDIMENTS EXPOSED ALONG THE CREDIT RIVER VALLEY, ONTARIO: ANALOGUES FOR BURIED VALLEY AQUIFERS

Thick successions of Quaternary sediment are exposed along bluffs fringing the Credit River in the community of Glen Williams near Georgetown, southern Ontario. In this region the modern Credit River occupies a north-south oriented bedrock valley bordered to the west by the Niagara Escarpment and to the east by low relief Quaternary sediments. The valley is carved into Paleozoic sedimentary rocks of the Queenston Shale and up to 30m of overlying Quaternary sediments.

Several sections along the Credit River were logged using standard logging techniques to record sediment characteristics including texture, sedimentary structures, paleocurrent directions, bed thickness and geometry. The sediments exposed in the bluffs include channelized successions of trough and planar tabular cross bedded gravels and sands, horizontally bedded sands, and thick successions of fine-grained rippled sands. These sediments record transport and deposition by traction currents under variable flow conditions and water depths and probably represent deposition in ice proximal braided fluvial to delta front environments. The occurrence of thin silt units and fine-grained diamicts interbedded with laminated silts and clays in several sections indicate (glacio)lacustrine depositional conditions. These deposits may have formed when meltwater was ponded between the Niagara Escarpment to the west and an ice margin lying to the east. The high degree of vertical and lateral facies variability shown in the logged sections is suggestive of dynamic environmental conditions affected by changing ice marginal positions and meltwater flow regimes. Some sections show a fining-upward trend that passes from gravels at the base to silty fine sands up-section, and may record ice withdrawal from the area at the end of the last glaciation. However, channelized gravels also cap some sections suggesting that fluvial conditions persisted in the bedrock valley for a considerable period of time. The importance of the Credit River valley as a long term fluvial host has implications for the search for buried aquifers and the detailed sedimentological analysis of exposures in the modern valley provides valuable information that can be used to better understand the characteristics and geometry of more deeply buried aquifers.