HOLOCENE DEVELOPMENT AND PARTIAL DESTRUCTION OF A MASSIVE BARRIER ALONG THE SOUTHEASTERN SHORE OF LAKE SUPERIOR

The derivation of Holocene lake-level histories from studies of beach ridges and terraces along the upper Great Lakes has spawned interest in the response of other coastal features (e.g. dunes, barriers) to lake-level change. Spatial and temporal variability in littoral sediment supply appears to be one key factor in predicting coastal response to such change. While massive littoral sand drift dominated Holocene coastal development at the southern end of Lake Michigan, eastern Lake Superior is not known for its Holocene depositional features.

Here, we document the Holocene development and partial destruction of a massive coastal barrier in the vicinity of Grand Marais, MI along the southeastern shore of Lake Superior. Ground penetrating radar (GPR), stratigraphy exposed in lake-facing bluffs, soil and lake-bed stratigraphy, and 14C and optical stimulation luminescence (OSL) dating are employed in reconstructing the barrier’s history.

Littoral sediment for the barrier was derived from the early and mid-Holocene destruction of lake-edge kame terraces of differing heights during the Nipissing transgression of the upper Great Lakes. Since the height of the terraces increases landward, the timing of their destruction by bluff recession can be related to stages in the development of the barrier and to local signals of dune building, dune-damming, and stream diversion.

Rising levels of Lake Nipissing destroyed several low (5-10 m high) kame terraces and cut prominent bluffs into bedrock headlands southeast of present Grand Marais prior to emplacement of the barrier. As subsequent wave action undermined a much higher (70-80 m) kame terrace lying west of Grand Marais, large amounts of colluvial sand, gravel and cobbles were delivered to the base of the bluff. Wave transport of this material eastward built an ~2000 ha, 300 million m3 barrier that enclosed a lagoon lakeward of the bedrock headlands. Soil development on the barrier and on dunes that cap it suggest that the barrier may have been breached after ~2000 cal BP, when the falls of the Saint Mary’s River emerged to create a separate Lake Superior.