DANAHER, MICHIGAN: A PATH OF INTERBASIN MELTWATER EXCHANGE DURING EARLY HOLOCENE TRANSGRESSIONS OF LAKE MINONG?

Recent studies have partially clarified the terminal history of Lake Minong, the last glacial lake to occupy the Superior basin. Mapping of a well-defined shoreline and optical dating of adjacent sand dunes show that Lake Minong stood at ~220 m just prior to its rapid drop ~9.5-9.0 ka. Sediment cores from Fenton Lake, Ontario, document a ≥ 12 m transgression of Minong that led to the lake's breaching. An earlier, more obscure, ~18 m transgression of Minong is suggested by work west of Marathon, Ontario. If the rim that confined Minong along its SE periphery ("Nadoway Barrier") was not breached prior to ~9.5-9.0 ka, by what path might meltwater have escaped the Superior Basin during the earlier transgression? We use a four kilometer GPR transect and ¹⁴C and optical dating near the interbasin drainage divide at Danaher, Michigan to test the hypothesis that an earlier transgression drove meltwater from Lake Minong across that divide to Lake Chippewa in the Lake Michigan Basin. GPR data suggest four radar facies which we interpret (in parentheses), lowest to highest: 1) reflection-free (Algonquin silts and clays); 2) semi-continuous to discontinuous reflections (sandy glaciofluvial sediments discharged from early Lake Minong, confined by Marquette ice); 3) semi-continuous to continuous, sub-horizontal to southward dipping reflections, truncated along their upper surface (Marquette age glacial fans built southward across Danaher ~11.4-11.2 ka and horizontally truncated at their upper surfaces); 4) a channel-like reflection with an asymmetrical fill pattern, set within unit 3 and extending across the southern 1.7 km of the transect (a channel cut by waning of interbasin discharge and filled by peripheral sloughing). Since alluvial fans (unit 3) apparently filled the valley to a depth of 10-15 m, an OSL age of 10.6 ka from their truncated upper surface is consistent with westerly interbasin meltwater flux across Danaher at that time. Given extant topography and a terminal shoreline at ~220 m, a rising Lake Minong ~9.5-9.0 ka must have reached a point just east of Danaher, drowning trees at Kaks Lake ~9.3 ka, but did not course to Lake Chippewa. Meltwater flux across Danaher ~10.6 ka may explain anomalous oxygen isotope data from the southern Lake Michigan Basin.