EVOLUTION OF SMALL BACK DUNE LAKES ALONG THE LAKE MICHIGAN COAST, NORTHERN ALLEGAN COUNTY, MICHIGAN

Gilligan and Kelly Lakes are small lakes (< 0.1 km2) that occur where streams encounter the inland edge of the Lake Michigan coastal dune complex and the flow direction is diverted from westwards (towards Lake Michigan) to northward (parallel to Lake Michigan). The longest of 4 vibracores from Gilligan Lake encountered a soil profile overlain by aquatic sediment at an elevation of ~ 182 meters above mean sea level. Radiocarbon dates indicate that aquatic deposition began during the Nipissing peak in Lake Michigan water levels. The bottom of a core from a somewhat deeper part of the lake contains aquatic sediments deposited at the same time at an elevation of 180 meters. Our single core from Kelly Lake indicates that aquatic sedimentation began about 1,000 years earlier at an elevation of ~ 175 meters. The basal units of all the cores consist of fine to medium grained sand with layering defined by variations in the amount of intermixed carbonaceous organics. The middle and upper units of the Gilligan Lake cores and the middle units of the Kelly Lake core are dominated by carbonaceous organics. A carbonate rich layer occurs near the base of the middle unit in the Kelly Lake core. The upper unit of the Kelly Lake core is predominantly fine silt and clay. Radiocarbon ages indicate that the transition from predominantly clastic inorganic sediments in the basal units to predominantly biological sediments in the middle units of the cores coincides with the early stages of the main phase in the growth and development of the adjacent dunes. The onset of sedimentation in these lakes is probably linked to the estuarine backflooding of westward flowing rivers during the rise in the Lake Michigan basin water levels from the Chippewa low to the Nipissing high. The growth of the coastal dunes later dammed the mouths of these drowned valleys diverting the drainage northwards and creating small back dune lakes in which predominantly organic sediments accumulated. A Preliminary study of one Gilligan Lake core suggests that the transition to a back dune lake may have been accompanied by a change in diatom assemblage from one in which eccentric monorapaphids and eunotoid species are prominent to an assemblage dominated by naviculoid forms.