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.

Geochemical characteristics of glacial Lake Agassiz sediments and new ages for the Moorhead Phase at Fargo, North Dakota, USA

“…The OSL ages are older than the oldest radiocarbon ages for the Moorhead Phase in the Lake of the Woods region ( Fig. 1; Bajc et al 2000), the Redwood Loop site northwest of Grand Forks (Fisher et al 2008 and references therein), and ϳ1750 years older than the Poplar River Formation radiocarbon ages in the Fargo region (Fig. 4).…”

“…Most previously reported ages from the Lake Agassiz basin are not from in situ organics (Bajc et al 2000;Fisher and Lowell 2006;Fisher et al 2008), usually providing only maximum ages for the encasing sediment. Moreover, the start and finish of the regressiontransgression cycle remains poorly constrained in time because the dated sediment is located at elevations partway between the maximum lowstand, and highstand when flow returned to the southern outlet (Fig.…”

“…The following descriptions of the Brenna, Poplar River, and Sherack formations come from Harris et al (1974), Arndt (1977), and Bajc et al (2000). The Brenna Formation extends from the south end of the lake basin to at least the Canadian border (49°N) and is as much as 50 m thick.…”

Stratigraphic and chronologic analysis of the Warren Beach, northwest Ohio, USA

“…The difference in age ϳ2200 years between the OSL and radiocarbon ages is explained by deposition of older, reworked organics into younger shoreline deposits, not uncommon in glacial lake sequences (cf. Bajc et al 2000;Fisher et al 2008). Also, note range of ϳ2000 years for ages assigned to the Warren Beach in Table 1.…”

Geoarchaeology and late glacial landscapes in the western lake superior region, Central North America

“…Wood and charcoal radiocarbon ages associated with the Moorhead regression have a combined 2-sigma range of 10,720-9700 yr B.P. (Bajc et al, 2000;Fisher and Lowell, 2006; Table I).…”