Paper No. 85-1
Presentation Time: 8:15 AM
SHORELINE ARCHITECTURE DURING A LAKE-LEVEL RISE AND FALL OF THE UPPER GREAT LAKES OVER A PERIOD OF SEVERAL MILLENNIA
The Nipissing phase of the ancestral upper Great Lakes was the last pre-modern high lake level confluent to the lakes (Superior, Huron, and Michigan). This phase was preceded by a rapid lake-level rise from an extreme low during the early Holocene to 6,100 years ago, a slower rise to a peak elevation 4,500 years ago, and a rapid fall during the subsequent ~500 years. These water-level changes produced a series of barrier beaches, strandplains, and spits along parts of the coastline that were related to the rate of sediment supply to the shoreline and rate of lake-level change moderated by the slope of the predepositional surface. Numerous cores, cone-penetrometer soundings, and ground penetrating radar lines were collected to define the stratigraphy and characteristics of these depositional systems in several parts of the basin. Barrier beaches having internal aggradational and depositional transgression sequences occur in areas of moderate to high sediment supply and moderate- to high-angle predepositional slopes. Strandplains of beach ridges containing depositional regression and forced regression sequences occur primarily in embayments having high rates of sediment supply and gentle predepositional slopes. Spits are similar in architecture to strandplains and are present in downdrift areas of eroding promontories. Strandplains of beach ridges and spits also contain ravinements that are associated with shorter-term fluctuations in lake level during the overall rise and fall in lake level.