Paper No. 6
Presentation Time: 2:50 PM
GEOMORPHOLOGY AND OSL DATING OF EARLY LATE WISCONSIN TERRACES IN THE UPPER MISSISSIPPI RIVER VALLEY
Geomorphic mapping and LiDAR-based three-dimensional perspective correlation indicate there are over 15 terrace levels in the Upper Mississippi River Valley (UMV) between the mouth of the Minnesota River and the Minnesota – Iowa state line. Terrace levels range in geochronologic significance due to variability in genesis as expressed by terrace form and relationship to Late Wisconsin glacial fronts. Two of the higher UMV terraces are directly related to the east flank of the Des Moines lobe and southeast flanks of the Grantsburg sublobe and the Superior lobe via the Vermillion River Valley, the northernmost west-side UMV tributary south of Saint Paul. These two terraces are almost completely voided from the considered reach of the UMV proper due to subsequent incision by catastrophic flooding. However, the lower of these two high terraces is the most expansive terrace preserved within the mouths and lower reaches of tributaries downvalley where they are dominated by slackwater sediment assemblages. This most extensive terrace can be traced through the Middle Mississippi Valley, where it correlates with the St. Charles Terrace Group in the St. Louis vicinity, southward into at least the upper part of the Lower Mississippi Valley. Here we report on terrace geomorphology, chiefly in relation to the two terraces of interest, along with initial OSL and radiocarbon dating of the underlying sediment assemblages at their source.
At face value, OSL ages indicate that accumulation of coarse grain fluvial deposits was underway in the Vermillion drainage basin possibly as early as about 28000 OSL age yrs. B.P., and certainly by about 26000 OSL age yrs. B.P. If glaciofluvial in origin, the ages suggest a relatively early advance of glacial ice, most likely the Superior lobe. However, earliest sediment simply could have accumulated under severe climatic conditions. The higher of the two terraces post-dates about 22,000 OSL age yrs. B.P. Relatively shallow depths of older OSL ages between 25,400 and 22,000 OSL age yrs. B.P. beneath the lower high terrace suggests the lower high terrace level in the Vermillion Valley is primarily eroded into the higher terrace sediment assemblage. OSL ages, supported by radiocarbon ages, suggest glaciofluvial activity of the lower of the two high terraces continued until shortly after 18,000 cal. yrs. B.P.