Subsurface sediment profiles below Point Pelee: indicators of postglacial evolution in western Lake Erie

Abstract

An extensive drilling program, undertaken along the western barrier bar at Point Pelee National Park, Ontario, Canada, yielded considerable subsurface sediment data relevant to the nature and lateral geometry of sedimentary units below the Point Pelee foreland. Four major sedimentary units were identified: a basal clay-rich till, a fine-grained glaciolacustrine sand, a medium-grained sand unit (subdivided into a poorly sorted shoreface sand and an aeolian (dune) sand derived from the shoreface sand), and an organic marsh (gyttja) deposit. The present study confirms the existence of a planar, wave-eroded till surface below the southern portion of Point Pelee at an elevation of approximately 164 m asl. Following this low-water period in the basin, lake levels rose abruptly to an elevation several metres above 172 m asl. This resulted in erosion of the upper part of the glaciolacustrine sand during a later period of stable higher lake levels, perhaps coinciding with the Nipissing flood event (about 4000 BP). This resulted in a planar surface at approximately 169.5 m asl. Several radiocarbon dates on basal gyttja from the marsh (averaging 3200 BP) reflect a subsequent drop in levels to about 2-3 m below present levels. Though undated, the initiation of shoreface and dune sand deposition is roughly coeval with the basal marsh deposits.

Beach sediment magnetism and sources: Lake Erie, Ontario, Canada

“…It is unlikely, however, that material swept off the lake bottom several thousand years ago, as the central basin began to fill with water, could remain on the eroding, and westerly migrating beaches, rather than being carried to the sediment sink off its southern tip. Sediment may have been supplied from erosion of the underlying Pelee-Lorain moraine, but while subsurface data are lacking from the eastern coast, an extensive drilling program in western Point Pelee identified only basal clay-rich till and fine-grained glacio-lacustrine sand beneath several metres of wave deposited shore-face and aeolian sediments (Coakley et al, 1998). Although this aeolian and lacustrine material has not been analyzed yet, it is possible that the high concentrations of magnetic material on the eastern beaches are the result of the erosion and recycling of these sandy sediments in the nearshore system, as the Point migrates and rotates westwards.…”

Canadian Landform Examples ‐ 39 Point Pelee ‐ a large cuspate foreland on Lake Erie

“…when the rate of lake level rise was slowing. Radiocarbon dating of basal peat suggests that the interior marsh began to develop between 3,000 and 3,500 B.P., when the summit of the moraine had become sufficiently enclosed to exclude vigorous wave action (Terasmae 1970;Coakley et al 1998). The submerged, lower portion of the shoreface sand unit along the western side of Point Pelee is well below the marsh, however, and must therefore predate it.…”