Paleoecology of late quaternary Molluscan-Ostracod assemblages from the Norwood site, southeastern Minnesota

Well-preserved fossils, including mollusks, ostracods, beetles and plant remains were extracted from sediments at the Norwood Site in southeastern Minnesota during July, 1977. Stratigraphic units, in ascending order, were: (1) sandy claystone, (2) clayey siltstone, (3) sandy siltstone, and (4) laminated peat. Unit 1 was interpreted to be till or sediment that slumped or flowed into the lake. Unit 2 and the lower part of unit 3 were interpreted to be lacustrine sediments. The upper part of unit 3 was interpreted to be a shoreline deposit. Unit 4 was interpreted to be a terrestrial or marginal lacustrine deposit. Total thickness of the section was 1.7 m. Wood from the boundary of units 3 and 4 yielded a radiocarbon date of 12,400±60 years B.P. Sediment samples yielded at least 2 species of sphaeriid bivalves, 3 species of gastropods, and 6 species of ostracods. The low diversity in number of species and the age of the sediments suggested that these individuals may have been the first species to inhabit the study area after the Grantsburg glaciation. Both mollusks and ostracods were most abundant and diverse in unit 2 and the lower part of unit 3. The fosil assemblage of these units indicated a permanent lake with vegetation. The lack of mollusk fossils and better sediment sorting in the upper part of unit 3 suggested a shoreline environment. Abundant plant remains and the absence of mollusk fossils in unit 4 indicated a terrestrial or marginal lacustrine environment. Pollen analysis indicated that the lake was surrounded by tundra and, later, a spruce forest