AN UPDATED CHRONOLOGY OF THE JAMES LOBE, SOUTH DAKOTA AND IMPLICATIONS FOR LATE-GLACIAL FLUCTUATIONS OF THE SOUTHERN LAURENTIDE MARGIN

During the last glacial period, the southern terrestrial-terminating lobes of the southern Laurentide Ice Sheet exhibited vastly different behavior, with early (>20 ka) advance and retreat in the eastern sector (Wisconsin to New England), and relatively late (<18 ka) advance and retreat in the western sector (Iowa and westward). At present, the chronology of the western sector is relatively sparse, consisting of radiocarbon ages from the Des Moines Lobe in Iowa. Therefore we seek to bolster this data set to further test this apparent pattern of asynchrony across the southern Laurentide margin.

The James Lobe was the westernmost lobe of the southern Laurentide Ice Sheet and occupied much of eastern South Dakota, reaching the Missouri River at its western and southern margins, and the Prairie Coteau to the east. We focus here on the Pierre Sublobe, the largest of several sublobes that formed along the James Lobe’s western flank, which terminated at the Missouri River near Pierre, SD. We collected boulder samples from moraine surfaces for ¹⁰Be exposure age dating. We present here new exposure ages from four sites across the Pierre Sublobe that fall on either Tazewell, Cary, or Mankato drift as identified in previous studies (i.e. Flint 1955). These new ages agree with existing ¹⁴C ages from sediment cores across the James Lobe and are used to reconstruct the lobe’s behavior during the last glacial period. Taken together, this updated chronology suggests the James Lobe exhibited similar behavior to the Des Moines Lobe, experiencing relatively late (<18 ka) fluctuations in ice margin position.

We find that our new chronology challenges the traditional view of Tazewell, Cary and Mankato phases of advance as all three drifts appear to be very close in age. Further, our new chronology supports the apparent asynchronous behavior of the terrestrial margin of the largest ice sheet of the last glacial period.