NORTHWEST FLANK OF THE ERIE LOBE IN INDIANA: WHERE HAVE ALL THE MORAINES GONE?

Several prominent moraines of the Erie Lobe die out northwest of the Wabash-Erie Channel in a transition between landsystems. In Indiana north of Fort Wayne the moraines and intervening till plains consist mainly of fine-grained till referred to as the Lagro Formation. An existing hypothesis is that the Erie Lobe was halted in this region by increased basal traction and a steeper land slope. To understand this glaciological transition, particularly basal ice processes, we studied three rotosonic sediment cores obtained from the Wabash and Salamonie Moraines west of the channel. We determined grain size distribution, magnetic susceptibility, density, clast lithology, and, in X-ray radiographs, structures. These data were used to delineate lithofacies, lithofacies assemblages, and glacial events. Variations in magnetic susceptibility are particularly coherent and aid unit definition.

The Lagro is considered to be composed of two units, the lower unit having a higher sand content, suggesting an ice-marginal depositional environment. However, we found several different till units and lithofacies that are more consistent with a subglacial environment. Structures such as clast lines, deformed mud beds, and clast-preferred fabric provide evidence for subglacial lodgment. Based on the stratigraphic relationships, we infer that several different subglacial sequences are present within the cores. We explore different hypotheses for subglacial deposition of parts of the moraines and the implications for Erie Lobe history.