LANDSCAPE SIGNATURES OF LATE HOLOCENE LAKE-LEVEL CHANGE ALONG LAKE SUPERIOR'S SOUTHEASTERN SHORE

Causal links between high stands of Lake Superior and Grand Sable Lake and dune building, stream damming and stream piracy are made from an investigation of late Holocene landscape history along the southeastern shore of Lake Superior. Radiocarbon ages of wood fragments from in-place stumps and soil O horizons, recovered from the bottom of 300 ha Grand Sable Lake, suggest that the near-shore inland lake was formed or deepened by dune-damming of ancestral Sable Creek at ~3000, 1530 and 300 cal BP. Peaks in probability distributions of 14C ages, obtained from paleosol surfaces within nearby channel-fills (n=15) and dune-fields (n=30), suggest the possibility of similar dune-damming and eastward diversion of Sable Creek onto abandoned glaciofluvial terraces at ~5500-5300, 3000, 1700-1500, 670-500 and 350-300 cal BP. Dune-building events that blocked the creek were probably driven by late Holocene high-stands of Lake Superior. Available bathymetry of Grand Sable Lake suggests that its shoreline configuration and depth varied in response to events of dune-damming and subsequent stream piracy. Rising waters of Grand Sable Lake appear to have undermined the eastern slopes of the ancestral valley of Sable Creek. Deflation from resultant west-facing bluffs may explain localized, small-scale episodes of dune-building and stream diversion. Repeated dune-damming and stream piracy caused Sable Creek to alternately occupy the north-trending late Pleistocene valleys of Town Creek, on the east, and Sable Creek, on the west, 4300-3800 cal B.P. One episode of inter-drainage stream diversion is documented in the exposure of a point-bar sequence, lying stratigraphically above early post-Nipissing lacustrine sediments in a bluff facing Lake Superior. The interrelated late Holocene events apparent in our study area suggest that climate-driven, century-scale variation in the levels of the upper Great Lakes has imposed complex signatures on both late Holocene and contemporary clastic coasts.