Geomorphic Evidence for Compound Moraines, Ice Streaming, and Surging of the Laurentide Ice Sheet in Northern Wisconsin

Reconstructions of late-Wisconsin glacial events in the Lake Superior region have typically assumed broadly-synchronous lobes of the Laurentide Ice Sheet (LIS). End moraines are often thought to have formed by a single cycle of ice advance and retreat under steady-state flow conditions. However, our geologic mapping in northern Wisconsin during about the last decade shows a much more complex history for southern lobes of the LIS in this region (namely the Superior Lobe, Chippewa Lobe, Wisconsin Valley Lobe, and Langlade Lobe). Landform and stratigraphic evidence shows that terminal moraines produced by some of these lobes are composite features resulting from two or more ice-flow events. Associated with these events were large changes in ice-flow direction (sixty degrees or more) and the development of landforms that indicate the former presence of ice-margin offsets and intralobe shear margins, similar to those described on several modern surging glaciers and ice streams. More specifically, although these LIS lobes were broadly synchronous when advancing southward out of the Lake Superior basin, cross-cutting relationships of end moraines show interlobe variability in the timing of ice advances. Especially in the case of the Chippewa and Wisconsin Valley Lobes, ice-surface-slope and ice-flow indicators (i.e. drumlins) show large changes in ice-flow direction (approximately ninety degrees) during at least two distinct flow events of each lobe. Broad, hummocky, compound, terminal moraines indicate two or more flow events of each lobe into their marginal zones. The terminal zones of all the lobes must have carried a substantial volume of englacial and supraglacial sediment to result in such broad, hummocky moraines (which include many ice-walled-lake plains). In summary, the landforms and landscapes produced by southern lobes of the Laurentide Ice Sheet in northern Wisconsin are broadly consistent with criteria recently proposed for recognizing paleo-ice streams and are also consistent with features observed on modern surging glaciers. Additionally, reconstruction of paleo-ice-flow directions shows asynchronous advances of the LIS lobes in northern Wisconsin along with complex intralobe flow. Such ice-sheet behavior was likely in direct response to changes in the Superior Lobe in the Lake Superior basin; however, multiple factors still need to be evaluated.