The Mosbeck Site: a Paleoenvironmental Interpretation of the Late Quaternary History of Lake Agassiz Based on Fossil Insect and Mollusk Remains

Abstract

The Mosbeck Site is in the southern part of the Lake Agassiz basin in northwestern Minnesota. The stratigraphic section at the site consists of seven lithologic units, which are from bottom to top (A) unsorted, pebbly, sandy, silty clay, (B) coarse gravel, (C) silty sand, (D) peat, (E) fine sand, (F) interbedded sand and gravel, and (G) unbedded dirty gravel. The lower few centimeters of unit E are unoxidized and contain black spruce and tamarack driftwood, which has been radiocarbon dated at 9940 ± 160 BP (I-3880). Units C-E contain numerous, well-preserved insect and mollusk remains. These fossils have been compared with modern species, and at least 76 insect and 15 mollusk taxa are present. Assuming that their ecological tolerances have changed little in the past 10,000 years, they provide valuable information about the environment of Lake Agassiz. Few of the insects are now found in the region, indicating that the environment has changed. With few exceptions the species present indicate that the climate and vegetation at the time were similar to the present-day climate and vegetation of southeastern Manitoba. The lithology and faunal contents of the sediment are interpreted as follows. Unit A is Late Wisconsinan glacial sediment. Unit B is a lag concentrate formed by wave action during a regressive phase of Lake Agassiz. Unit C is the sediment of a small body of water that formed when the level of Lake Agassiz had dropped below the site. The banks were covered with a spruce forest. Open water gave way to swampy conditions, and unit D was formed. Both units C and D were deposited during the low-water Moorhead Phase of Lake Agassiz. Units E and F are shoreline sediment deposited as the lake level rose, drowning the vegetation. Unit G is modern ditch spoil.