HOLOCENE ALLUVIAL STRATIGRAPHY AND LANDSCAPE DEVELOPMENT IN SOAP CREEK WATERSHED APPANOOSE, DAVIS, MONROE, AND WAPELLO COUNTIES, IOWA

E. A. Bettis III and J.P. Littke


Iowa Department of Natural Resources, Geological Survey Bureau,
Open File Report 87-2, 1987, 170 p.

ABSTRACT


Soap Creek Watershed drains approximately 65,590 hectares (162,000 acres) of the Des Moines River Basin in the Southern Iowa Drift Plain landform region of southeastern Iowa. Detailed investigations of the late Wisconsinan and Holocene alluvial stratigraphic record in portions of the watershed were conducted in order to develop a landscape evolution model for the area. The purpose of the model was to assist archaeologists and planners in developing effective means for locating and evaluating the archaeological record preserved in the valley landscape. The present valley landscape is a mosaic of surfaces and underlying deposits of varying age and origin. Holocene-age alluvial fills in the watershed were grouped into four formal lithostratigraphic units. These were assigned member status in the DeForest Formation, which encompasses all Holocene-age alluvium in Iowa. The four members have distinct lithologic properties, and consistent landscape positions and stratigraphic relationships throughout the study area. Radiocarbon dates on organic material contained within the alluvial fills indicates that each lithostratigraphic unit was deposited during discrete intervals of the Holocene.

Maps of the distribution of the DeForest Formation members in portions of the watershed are presented. These provide the basis for formulating sample strategies necessary to evaluate the archaeological resource of the area. The sequence of deposits and their chronology in Soap Creek Watershed is compared to those identified elsewhere in Iowa. Factors responsible for the behavior of the fluvial system are discussed in order to provide a region-wide framework for evaluating the role of fluvial processes in shaping the archaeological record.