Abstract

A section of varved clays exposed by excavations for the University–Bloor–Danforth Subway in Toronto is described. The individual varves are noted, in general, for such irregular features as erosional structures, poor sorting and grading, sublaminations, and angular fragments of unconsolidated material scattered in the coarse-grained layer. The most characteristic property of an individual varve is the sorting and grading of the coarse-grained layer. On this basis three groups can be distinguished: graded, complex, and diamictic varved clays. Deposition from a special type of turbidity current, the meltwater bottomflow, offers the best explanation for the origin of these varves. The deposition of complex varves, which are the typical varves of the section, probably takes place from continuous meltwater bottomflows that vary in sediment load. Because of the continuity of meltwater currents, sedimentation occurs under the influence of three independent but overlapping processes: settling from a graded suspension, addition of coarse sediments rolled along the bottom, and deposition from a uniform clay suspension. Complex varve structures and textures reflect variations in the coarse sediment load that is ultimately controlled by short-term (commonly diurnal) and long-term temperature variations (chiefly seasonal) affecting the rate of melting at the glacier and consequently the discharge and competence of meltwater currents. The deposition of graded and diamictic varved clays, on the other hand, probably takes place from either single overflows or single bottomflows.

You do not currently have access to this article.