TERRACE DEVELOPMENT WITHIN THE LOWER RED CEDAR VALLEY

Terraces preserved within the lower Red Cedar and Chippewa River valleys in western Wisconsin are the result of complex relationships between internal and external influences on fluvial evolution. Recent fieldwork within the lower Red Cedar valley provides a glimpse of the fluvial dynamics in this drainage network. The highest terrace preserved in this region is the Wissota Terrace, with at least six intermediate terraces intermittently preserved within the lower Chippewa and Red Cedar River valleys. Investigation focused on terrace exposures along Irving Creek, a tributary of the Red Cedar River.

We constructed stream and terrace profiles using a LiDAR elevation data set to guide field investigations and interpret the fluvial dynamics for the region. Field descriptions and laboratory analyses were used to give an indication of the sedimentary source, texture, depositional context, and paleoecological setting. Charcoal from 1.65 m below an intermediate terrace at Irving Creek yielded an age of 9.4 ka (8390±40 radiocarbon yrs BP; Beta-301425). The exposure consisted of about 2.5 m of late Pleistocene alluvial sand and silt with occasional, thin beds of imbricated gravel. Based on the thickness of the alluvium and irregular contact with the underlying Cambrian sandstone, we interpret this as a strath terrace. Charcoal morphotypes obtained from the alluvium have a predominantly woody texture, suggesting the presence of forest at this time. If our radiocarbon date is correct, erosion of this strath occurred after 9.4 ka, about the same time as the forest to prairie transition within the Upper Mississippi Valley.

The terrace system of the lower Red Cedar valley is a complex set of preserved terrace treads that are the result of changing patterns in aggradation and erosion. Previous researchers have suggested the Wissota Terrace along the lower Chippewa River formed as a result of a change in sediment supply. However based on the slope of the long profile of the lower Red Cedar and its tributaries the formation of the Wissota Terrace appears to mainly be the result of a drop in base level. Recent work by Faulker et. al. (2011) suggests the middle terraces formed less than 5 ka, making these terraces significantly younger than our data suggest. Further research is needed to decipher the history of terrace formation in western Wisconsin.