WINDBLOWN CRETACEOUS DIATOMS IN UPPER MIDWESTERN QUATERNARY SEDIMENT
These fossil diatoms occur in loess deposits in lake basins, which implies glacial and eolian pathways. No in situ diatom-bearing biosiliceous units of the Late Cretaceous Pierre seaway are known to exist in the upper Midwest, so there is no obvious primary regional source for these fossils. However, all the loess samples are found to include calcareous foraminifera and Cretaceous marine diatoms. By contrast, no post-Paleozoic marine fossils have been found in Michigan Lobe tills. This implies that the Des Moines Lobe overran the original Late Cretaceous Pierre Seaway deposits, where the fossils became admixed into till and transported subglacially. As the ice retreated, many of the microfossils would have been carried by meltwaters and accumulated in outwash deposits, which provided the source for eolian distribution as loess.
Mapping concentrations of marine diatoms in Quaternary loess, till, and outwash deposits throughout the study area will be done to identify the likely source area for the original Cretaceous deposits. Given that the marine diatoms have been established as very distinctive tracers, their concentration and distribution in loess will provide important data on late Pleistocene/Holocene wind strength and direction.
We are currently analyzing microfossil assemblages in dated cores of 7760 - 17140 cal year BP from Cross Plains, WI, and undated sediment cores from DeKalb, IL, in order to better characterize and date these microfossil distribution events. The microfossils have been found in Wisconsin lake cores as deep as 200 cm beneath the surface. Diatoms common to both loess and lake cores include the unique Cretaceous diatom Gladiopsis sp. and many longer ranging taxa such as Stephanopyxis spp. Preliminary data suggests a south-easterly wind direction.
Anticipated results of this study are two-fold: (1) we will use the diatom distribution and concentration as a tracer of early Holocene winds, and (2) we expect to learn more about Pierre Seaway productivity events during the Late Cretaceous (Campanian).