THE TIMING AND PALEOENVIRONMENT OF THE MOORHEAD LOW WATER PHASE OF LAKE AGASSIZ IN THE SOUTHERN BASIN

The Moorhead Low Water Phase in the Fargo-Moorhead area is represented by 0-3 m of detrital organic beds of the Poplar River Formation. The deposits accumulated in shallow channels and lakes on the surface of the Moorhead Delta. Drowning of the delta and burial by 7 m of laminated sediments of the Sherack Formation occurred during the transgressive Emerson Phase of Lake Agassiz. Calendar dates for the MLWP range from 11,959 to 11,178 cal yr BP based on 11 AMS ages on non-aquatic seeds. Pollen, wood, seeds and fruits, insect and mollusc fossils, have been the subject of several paleoecological studies providing detailed information about the paleoenvironment. Earlier studies treated the organic beds as allocthonous but more recently they have been considered to be autochthonous, representing an extensive wetland of fens and bogs occupying the drained lowland of the southern Lake Agassiz basin.

The vegetation was similar to that existing today at Delta Marsh, southern Manitoba. The eutrophic wetland was especially rich in species of aquatic plants, insects and molluscs. Salix (willow) and emergent vegetation lined the channels and Picea (spruce), Populus ( aspen), Ulmus (elm) and Betula (birch) were present on better-drained soils. No vertebrate fossils have been found but it was a habitat perfectly suited for muskrat, beaver and moose. The paleoclimate, based on the presence of several species of plants and beetles that were south of their modern distributional limits, had cooler summers. MCR and other semiquatitative analyses of Coleoptera (beetles) species indicate a mean July temperature of about 17°C, or 4°C lower than today.

The MLWP occurred at the transition from the Younger Dryas to the Holocene. No differences in floral or faunal composition could be detected at the boundary suggesting minimal climatic impact in the region. Drowning of the MLWP in the Fargo-Moorhead area is the early part of the transgression that eventually led to water draining through the southern outlet. The AMS ages of the MLWP deposits support models for a younger rather than an older age for this event.