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

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.