Sediment characterization and porewater isotope chemistry of Quaternary deposits from the St. Clair Delta, Ontario, Canada

Walpole Island is part of a large freshwater delta located at the St. Clair River mouth in southwestern Ontario. Quaternary deposits on the island con sist of Lake Nipissing to Lake St. Clair stage deltaic sediments underlain by a thick sequence of Late Wisconsin glacio-lacustrine rhythmites and mass ive clayey till. These sediments overlie a thin freshwater aquifer resting on Paleozoic bedrock. Porewater delta(18)O, delta D, and delta(13)C(DIC) pr ofiles of cores taken from northern and central Walpole Island show that ol der (>10,000 yr B.P.), deeper glaciogenic porewaters have mixed with, and h ave been displaced by, younger modern surficial water. In contrast, isotopi c profiles from a core taken from southern Walpole Island indicate that mod ern St. Clair River waters have penetrated the entire 20-m core via fractur es, effectively displacing all glaciogenic porewater. Fracturing and faulti ng are clearly visible throughout the southern core, and their presence may be attributed to its location on the trend of the Electric Fault. These fe atures are interpreted as the product of renewed Holocene fault movement. P orewater Na+, K+, Ca2+, and Mg2+ concentration gradients are consistent wit h the presence of upwardly diffusing deep brine, for which the Electric Fau lt may serve as a conduit. (C) 1999 University of Washington.