ABSTRACT

Many tributary valleys to major streams in Illinois, Indiana, and Wisconsin were sites of mud deposition in late Wisconsinan periglacial lakes. Today the flat lacustrine plains of these valleys contain freely meandering streams. Surveys during low-flow conditions have enabled documentation of the geomorphic and sedimentary features of the sinuous channels of several of these streams.

The subaqueous bed material is a complex, variable mixture of quartzose sand (main component), mud, mud blocks, equant mud clasts, rock fragments, and plant debris. Point-bar topography is restricted to bends of low or moderate curvature (r_c/w₂ > 3); bends of greater curvature show dominantly erosional inner banks. An organic-rich ooze lies beneath the stagnation zone that develops near the inner bank of highly curved sectors. Subaerial sediment on depositional surfaces is primarily a muddy sand with variable content of plant detritus.

The lateral-accretion deposits near the downstream end of bends were divided into a fine member and a coarse member, the former two to four times as thick as the latter. The fine member consists of three lithofacies: 1) upper interval of centimeter-thick beds of sandy mud and thinner beds of rooted sand with mud and some plant debris; 2) underlying interval of subequal thicknesses and numbers of beds of clean sand (mostly fine, likely with small-scale cross-stratification) and sandy mud; and 3) basal interval very similar to 2 but with distinctive beds of black, finely divided plant debris.

The coarse member is dominantly a fine-to-medium quartzose sand, contains mud clasts towards the base, and is inferred to be large-scale trough cross-stratified. Logjam debris appears near the thalweg in places.

Lateral-accretion bedding in the form of intercalated sand and mud-sand is inferred to extend throughout the fine member. Substantial local variations in cross-sectional geometry and channel lithofacies, especially near logjams, reduce the precision of existing paleohydrologic relations and the comprehensiveness of a single lithofacies model.

This class of meandering stream produces lateral-accretion deposits which are closer analogues in lithofacies to the classical upward-fining model than are deposits from the graveliferous sand-bed streams that have received primary emphasis from sedimentologists.