ERIE LOBE TILL STUDIES IN INDIANA REVEAL A DYNAMIC ICE MARGIN

The prominent end-moraine landsystem in northeastern Indiana made up of Erie lobe tills fades out 20 km north of the Wabash-Erie channel and is replaced by a composite landsystem that covers the extensive northern margin of Erie lobe deposits. We studied this composite landsystem to improve understanding of Erie lobe and adjacent Saginaw lobe history. This work facilitates mapping of Erie lobe tills in Indiana where they are referred to as the Lagro Formation. The principal data consist of geomorphic observations, microstratigraphic studies on several high-quality cores, and 12 km of high-resolution shear-wave seismic reflection profiles.

The composite Lagro landsystem is dominated by sinuous meltwater channels of several scales that are discontinuous across the landscape and cut through most topographic features made of Lagro till. Their morphology and distribution, as well as the seismic profiles intersecting one channel, are indicative of subglacial formation. Likewise, the characteristics of the Lagro subunit that surfaces parts of the composite landsystem are consistent with subglacial deposition. We surmise that the composite landsystem is largely subglacial in origin.

Subglacial landsystem elements cut ridges that are cored by an ice-marginal facies of the Lagro Formation as indicated by core stratigraphy. Seismic profiles over one such ridge are consistent with deposition of a deep Lagro subunit as ice-marginal moraine. We infer that several ridges in the composite landsystem are palimpsest Erie lobe ice-marginal moraines.

We suggest that the composite Lagro landsystem reflects a sequence of Erie lobe events that are older than the Erie lobe stillstands represented by the end-moraine landsystem to the south. If correct, the margin of this lobe fluctuated on a significant spatial scale more rapidly than currently believed.