Sediment Characterization and Porewater Isotope Chemistry of Quaternary Deposits from the St. Clair Delta, Ontario, Canada

Abstract

Walpole Island is part of a large freshwater delta located at the St. Clair River mouth in southwestern Ontario. Quaternary deposits on the island consist of Lake Nipissing to Lake St. Clair stage deltaic sediments underlain by a thick sequence of Late Wisconsin glaciolacustrine rhythmites and massive clayey till. These sediments overlie a thin freshwater aquifer resting on Paleozoic bedrock. Porewater δ18O, δD, and δ13CDICprofiles of cores taken from northern and central Walpole Island show that older (>10,000 yr B.P.), deeper glaciogenic porewaters have mixed with, and have been displaced by, younger modern surficial water. In contrast, isotopic profiles from a core taken from southern Walpole Island indicate that modern St. Clair River waters have penetrated the entire 20-m core via fractures, effectively displacing all glaciogenic porewater. Fracturing and faulting are clearly visible throughout the southern core, and their presence may be attributed to its location on the trend of the Electric Fault. These features are interpreted as the product of renewed Holocene fault movement. Porewater Na+, K+, Ca2+, and Mg2+concentration gradients are consistent with the presence of upwardly diffusing deep brine, for which the Electric Fault may serve as a conduit.