Erie lobe fine-grained till in northeastern Indiana: Insights into Lagro stratigraphy and depositional mechanisms from cores in the Wabash moraine.

摘要

The moraines of the Erie lobe of the Laurentide Ice Sheet which are a prominent feature of northeastern Indiana are enigmatic because of their fine-grained textural composition and contrasting morphology north and south of the Wabash-Erie channel. The limb north of the channel is more deeply incised and broad. A study of the limb of the Wabash moraine north of Fort Wayne, IN has been undertaken to investigate the change of morphology as well as to provide a detailed analysis of the Lagro Formation from which the Wabash moraine is composed.;Results indicate that the ridge contains two distinct fine-grained diamicton deposits. These two diamicton units are separated by well-sorted sand and laminated mud beds, interpreted to be a proglacially deposited unit. This proglacial interval indicates that more than one ice lobe advance occurred to build the Wabash moraine. The two diamicton deposits are identified as till from their very poor sorting, average grain size of 6 to 8 &phis;, and the presence of preferred fabric. Where fabric and inclusions are not observed, the till units are largely homogenous. These structures and fine-grained sediment size indicate that till in this ridge was deposited subglacially. However, differences in surface contacts as well as sand and gravel content between the top and bottom fine-grained till deposits indicate two different environments of deposition. Stacked beds observed in the lower till reflect movement and aggradation of till near an ice margin. Multiple till deposits in the upper unit indicate basal changes under the Erie lobe resulting in thick deposits. The lower environment is ice-marginal and the upper environment is interpreted to be subglacial.