LATE PLEISTOCENE GLACIATION OF EASTERN SOUTH DAKOTA AND RELATIONS TO THE GEOLOGIC FRAMEWORK OF THE CENTRAL MISSOURI RIVER

Several features of the late Wisconsin glacial geology and geomorphology of eastern South Dakota form salient aspects of the hydrogeologic framework of the central Missouri River, the course of which is located at or near the Wisconsin limit of the James Lobe of the Laurentide Ice Sheet. A lowland valley of 80-100 km width, with the James River generally at its central axis, lies between upland areas of the Prairie Coteau and the Missouri Coteau and Trench that contained the margins of this 250 km wide ice lobe. The James Valley marks a generally southward direction of past ice flow and decreasing glacial water potential gradient. In contrast to the Prairie Coteau which forms a prow-shaped upland on the east flank and to most of the Missouri Coteau of North Dakota, the Missouri Coteau of South Dakota is breached by broad valleys of 4 sublobes that record a distinctive character of westward ice flow toward the Missouri River Trench. Each of the sublobe footprints (which have been suggested to originate as preglacial valleys trending eastward across the present course of the Missouri River) has a broad low transverse ridge within it that suggests a recessional or readvance moraine correlative to the DeSmet advance limit (Martin and others, 2004) which trends southward to Yankton. The James lobe advanced to this limit across a partly forested parkland that has yielded many ¹⁴C ages on wood of ~12,500 ¹⁴C years BP (~14,500 Cal) , thus providing somewhat younger outwash to the Missouri River Valley at Yankton and to the Missouri River Trench from the sublobes upstream. The re-advance was followed closely by calcite precipitation from groundwater, as constrained by U-series, Sr, C, and O isotope data. Postglacial deposits of the river valley are underlain by late Wisconsin glacial and glaciofluvial deposts. There is particular interest in understanding the relation of buried Pleistocene glaciofluvial gravel to the modern river channel; these gravel deposits may be a primary control on groundwater/surface water interactions as well as a source of gravel-cobble substrate needed for spawning of endangered sturgeon.