CORE-BASED STUDY OF ERIE LOBE TILL STRATIGRAPHY IN NORTHEASTERN INDIANA: IMPLICATIONS FOR ERIE LOBE HISTORY

The origin of fine-grained massive till deposited by the Erie lobe in prominent moraines across northeastern Indiana is problematic because of its significant thickness. The till in each of these moraines is commonly more than 20 m thick and usually explained as deposited from a deforming bed at the former ice margin during a single stillstand. Yet, most studies indicate that an individual till sheet deposited from a deforming bed is less than a meter thick. We report results from a stratigraphic study of Erie lobe till, the Lagro Formation in Indiana, based on several cores retrieved from morainal features that indicate the presence of a stratigraphy that constrains depositional mechanisms.

Descriptions of the cores draw on over 120 particle-size analyses combining sieve and laser diffraction data, magnetic susceptibility data with 2 cm vertical resolution, 23 m of x-ray radiographs, and borehole gamma radiation data. X-ray radiographs were used to distinguish structures, clast imbrication, major texture changes, and bed contacts. Magnetic susceptibility correlated principally with texture. Massive beds are more common than beds inferred to exhibit a preferred clast fabric.

Considering all properties, Lagro sediments in each core are divisible into several major units deposited in both subglacial and ice-marginal environments. The major till units are made of subunits on the scale of a meter in thickness that differ sufficiently in character as to reflect separate sedimentation conditions and depositional events. We suggest that the relatively homogenous subglacial Lagro facies accreted vertically from different subglacial deforming till beds that developed successively under the Erie lobe as it evolved. Overall, the evidence justifies additional examination of Lagro sediments to resolve a complex ice lobe history.