ABSTRACT

Lithofacies and their relationships, sedimentological and glaciotectonic structures, pebble fabrics, and laboratory data were used to delineate seven facies associations in a classic upper Wisconsinan glacigenic succession near Wedron, Illinois. Each association is interpreted to represent a different depositional environment. Proglacial facies include massive and laminated silt (lacustrine), bedded sand (fluvial), and massive silt (eolian). Subglacial facies consist of homogeneous diamicton (lodgement till) and variable diamicton and sorted sediment deformed by folding and shearing (deformed till). An ice-marginal facies of heterogenous diamicton and sorted sediment (supraglacial redeposited sediment) and a paraglacial facies of laminated silt (lacustrine) also are present. Stratigra hic relationships among the facies allow the delineation of three glacigenic sequences from which three glacial events are inferred. Each sequence consists of proglacial fluvial and/or lacustrine sediment, subglacial till and fluvial deposits, and supraglacial and ice-marginal sediment-flow, fluvial, and lacustrine deposits. The complete nature of these glacigenic sequences suggests that the area was located primarily in the depositional zone of the Laurentide Ice Sheet during the late Wisconsinan.

Erosional contacts and localized sorted sediment between three lithologically distinct till facies in the lowest sequence are interpreted to be subglacial in origin; the three facies were deposited during the same event, rather than separate events as previously inferred. Spatial and temporal variability among the subglacial processes of erosion, transportation, deformation, and lodgement is evident within that sequence.