Reconstruction of post-Iroquois shoreline evolution in western Lake Ontario

Abstract

When Lake Iroquois drained between 11.7 and 11.4 ka BP, lake level in the Ontario basin fell from a high of more than 40 m above present lake level to a minimum close to the then-existing sea level, which was approximately 40 m below present sea level. Since that time, lake level has been rising at an exponentially decreasing rate in the western portion of the basin as a result of postglacial and neotectonic uplift of the outlet near Kingston, at the eastern end. The published lake level history has been combined with other less well-known parameters (the post-Iroquois regional topography, erosion – deposition rates, and distribution of resistive shore materials) to reconstruct the evolution of the western Lake Ontario shoreline. Borehole, long piston core, and other subsurface data sources, primarily from the western portion of the lake near Hamilton Harbour, provide most of the physical constraints. Time references were provided by radiocarbon dates on shallow-water organics in the subsurface sediments. A computer program was designed to calculate and contour the changing elevations of the rebounding post-Iroquois topographic surface, allowing the time-dependent water-plane elevation to be superimposed. Semiquantitative allowance was made for differential erosion and deposition along the advancing shoreline. The reconstruction provides a perspective on past and future shoreline evolution in the basin and possibly on the location of potentially commercial offshore deposits of aggregate.