THE AGE AND SIGNIFICANCE OF THE RED CLAY BAND WITHIN THE GLACIAL LAKE AGASSIZ VARVES OF NORTHWESTERN ONTARIO AND NORTHERN MINNESOTA

There is a distinct band of red clay within the gray glaciolacustrine sediments in an area east of Lake of the Woods (within the modern Wabigoon, Seine, and Rainy River Basins.) Prior studies establish that the red clay is sourced from water from the Lake Superior Basin. Here we report on the stratigraphy and age of the sequence as found in four lacustrine sediment cores.

At these sites, the red clay is preceded by massive, gray silty-clay that appears to be deposited in localized lake basins that become isolated when Lake Agassiz levels dropped to a low level known as the Moorhead Low. The onset of the red clay is abrupt, and typically occurs within ~24 varves. The red varves are superseded by around 200 relatively thick gray varves, which were likely deposited in higher lake levels. At our site closest to the drainage pathway from the Superior basin, the ‘summer’ layers of the first red varve includes laminae of organic detritus with mosses and identifiable macrofossils (such as willow and dryas.) The macrofossils date to around the end of the Younger Dryas, ~11.3 cal ka. These dates overlap with radiocarbon dates that establish the timing for the end of the Moorhead Low.

The core stratigraphy and radiocarbon dates suggest that during the latter part of the Moorhead Low, water levels were maintained via a low elevation outlet into the Lake Superior basin, which became blocked by an ice advance. The advance caused a temporary flow of floodwaters from the Lake Superior basin into Lake Agassiz (depositing the red clay). The 200 gray varves suggest high lake levels were maintained for at least another 200 years. This interpretation of Lake Agassiz’s drainage history during the late Younger Dryas is similar to those that prevailed 20 years ago, but conflicts with more recent studies, many of which infer the drainage history of Lake Agassiz from marine oxygen isotope records.