A reconstruction of Moorhead and Emerson Phase environments along the eastern margin of glacial Lake Agassiz, Rainy River basin, northwestern Ontario

Abstract

Mapping of Quaternary geology in the Rainy River lowland, northwestern Ontario resulted in discovery of several fossil-bearing localities. Organic remains are associated with both the Moorhead and Emerson phases of glacial Lake Agassiz. Wood samples recovered from Moorhead Phase deposits have radiocarbon ages ranging between 10.8 and 9.9 ka BP. The wood is detrital and cannot be used to date the beginning of the low-water phase. Nearshore, deltaic, alluvial, peatland, open-water wetland, and upland soil environments are represented in the Moorhead Phase sediment records. Emerson Phase transgressive deposits overlie Moorhead Phase sediments and erosional surfaces. Wood samples recovered from flotsam layers in Emerson Phase nearshore deposits have yielded radiocarbon ages ranging between 10.5 and 9.5 ka BP. Stratigraphic and sedimentologic evidence suggests that the transgression began approximately 9.9 ka BP. Pollen, plant macrofossil, insect, and mollusc assemblages have been affected by long-distance transport, sorting, and reworking by fluvial and nearshore processes. They represent a wide spectrum of terrestrial and aquatic habitats indiscriminantly brought together during high-water periods. The Moorhead and Emerson phase assemblages indicate conditions similar to those in the region today, but there is a distinct component whose modern range is much farther north and west of the study region. In this respect, the assemblages are similar to the mixed communities described from other late-glacial sites of the mid-continent.