Abstract

A large outwash complex occupies a northwest-southeast trending intermorainic low-land in northeastern Illinois. The complex can be divided into four assemblages arranged in zones paralleling the margin of the West Chicago Moraine on the east. 1) The marginal assemblage occurs along the margin of the moraine and consists of gravelly mud flows, fluvial sand and gravel in large, deep channels, and lacustrine muds deposited in shallow basins developed on mudflow surfaces. 2) The proximal assemblage occurs in a 1-km-wide band lying west of the moraine. It consists of cobble gravels in crude, horizontal strata deposited on longitudinal bars and cross-bedded sand and fine gravel deposited in interbar channels as migrating dunes or accretionary wedges during low flow conditions. 3) The medial assemblage occurs in a 1-km-wide band in the center of the intermorainic lowland and consists of cross-bedded sands and fine gravel deposited on transverse bars or interbar channels, coarser gravels deposited on longitudinal bars, and ripple-bedded sands and silts deposited in inactive channels or pools of active channels during low flow conditions. 4) The distal facies underlies the other assemblages of the complex and consists of laminated lacustrine muds that accumulated in one or more large proglacial lakes, and fine-grained cross-bedded sands deposited in a braided stream system developed on the eroded lake plain after it was drained. Sediments were derived from the West Chicago Moraine and were carried perpendicularly away from it until they reached the center of the lowland where they were transported southeasterly down the axis of the intermorainic trough.

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