DEGLACIATION IN NORTHWESTERN ONTARIO: IMPLICATIONS FOR THE BEGINNING OF THE MOORHEAD PHASE OF GLACIAL LAKE AGASSIZ

The presence of glacial lake sediment above the sub-continental drainage divide (between the Superior and Agassiz basins) northwest of Thunder Bay, Ontario records a paleo lake at elevations higher than previously mapped. A series of hydraulically-assisted Livingston cores from small basins and channels penetrated the organic and inorganic contact. Numerous radiocarbon ages from terrestrial macrofossils at the base of the organic sediment indicate evolution of small ponds dominated by peat and gyttja between 10,200 and 10,000 14C yrs BP. Beneath the dated contact, approximately 200 rhythmites (possibly varves) indicate passage of the ice margin up to perhaps 200 years earlier.

The location of the coring sites atop the drainage divide requires dams to the east and west. To the east, both the Rainy and Superior lobes were likely close to the Dog Lake and Marks Moraines, respectively. To the west there is greater uncertainly as to what held the water in at this elevation. One possibility is that it was glacial Lake Agassiz, but this would require a waterplane to be ~50 m higher than previously suggested, and without independent dating control on strandlines, a difficult hypothesis to test. Alternatively, the dam to the west was the St. Louis sublobe, an eastern offshoot of the Des Moines lobe, and a lake was trapped in the Rainy River basin between the receding glaciers. Presumably this lake was glacial Lake Johnston (Antevs, 1951) in which 1250 varves were counted, but whether it is the same lake as glacial Lake Koochiching, as described by Hobbs (1983) in northern Minnesota, remains to be verified. Until ice retreated north of the McIntosh channel in west-central Minnesota, Lake Agassiz (Climax) could not have merged with Lake Koochiching, and sometime after that the lake dropped to the Moorhead low phase. At the Snake Curve section just north of the McIntosh channel fluvial gravel between lake clays dates at 9900 14C yr BP (Moran et al., 1971). Lake lowering at our coring sites in northwestern Ontario did not begin until ~10,100 14C BP and most of the 42 radiocarbon dates associated with the Moorhead Phase range in age from 10,200–9900 14C yrs BP. Reworking of some older material in Moorhead-aged deposits may explain the old age given to the Moorhead Phase (e.g., 10,960±300 W-723), and it is suggested that the Moorhead Phase is younger than previously supposed.