LATE PLEISTOCENE CLIMATIC INFLUENCES ON THE SEDIMENTARY ARCHITECTURE OF THE UPPER MISSISSIPPI VALLEY: IMPLICATIONS FOR FOSSIL PRESERVATION AND PALEOENVIRONMENTAL RECONSTRUCTION

The interplay between climate, sediment deposition, and fossil preservation constrains our understanding of the late Pleistocene paleoenvironment within the Upper Mississippi Valley (UMV). Terrestrial gastropods are common fossils and are well suited for paleoenvironmental reconstruction. However, depositional characteristics such as fluvial style, hillslope activity, and sediment lithology can bias the fossil assemblage through preferential preservation or destruction of individual taxa. These depositional characteristics are, in turn, influenced by large-scale climatic patterns.

A survey of Late Pleistocene sediments from Minnesota and Wisconsin revealed relatively stable gastropod fossil assemblages from the UMV dominated by taxa with Cordilleran-Boreal ranges, such as Discus shimeki and Columella columella alticola, while Eastern Deciduous Forest taxa, including Discus whitneyi and Euconulus fulvus, were generally rare or absent. Drawing paleoenvironmental conclusions from these findings is difficult due to the probable taphonomic bias associated with reworking and preferential preservation. For example, radiocarbon dates indicate that erosion into the late Pleistocene alluvium was fairly rapid and formed the Savanna Terrace around 16.4 ka. This degradation is generally attributed to low sediment concentration flows from proglacial lake outbursts. However, this incision predates the outburst of glacial Lake Agassiz (ca. 13 ka), but closely corresponds with Heinrich event H1 and an increased reworking of nanofossils from the Gulf of Mexico.

Although the composition of UMV gastropod faunas during this interval did not appear to change, interpreting local, small-scale climatic changes may be confounded by taphonomic uncertainties that result from changes in depositional patterns due to larger scale climatic influences.