SEDIMENTOLOGICAL FEATURES OF LATE WISCONSIN BASAL TILL

Re-advancing post-LGM ice of the Green Bay Lobe of the Laurentide Ice Sheet overrode sequences of soft sediment including saturated clay-rich lake sediment, wind-blown silt, and clayey and sandy older diamictons. We examined sedimentary sequences for indications of the soft sediments response to ice overriding. At several locations near the former ice margin, clay-rich diamicton with a homogeneous appearance, uniform grain-size distribution, and relatively strong clast-fabric, overlies or grades into sediment that has pre-deformation sedimentary features. These include textural laminations, color and texture contrast, included organic material, silt stringers, and crenulated beds of fine sand, silt and clay. We suggest that the basal till features, including relatively strong clast-fabric that is parallel to ice flow direction, developed in thick layers of clay-rich lake sediment due to the shear stress of the overriding ice. Lateral movement of sediment appears to have been minimal. Sediment was transported only a short distance and retains some primary structures and characteristics of the parent material. Laboratory analysis of the matrix of the clayey diamicton indicates a uniform grain-size distribution at individual locations, but fairly wide variation in matrix composition on a regional basis. Preserved clast-fabric parallel to ice-flow direction indicates the presence of little interstitial ice in the sediment as it was deposited by the glacier.

Fast ice flow is indicated by ice margin positions, and chronology based on stratigraphy and radiocarbon dates on organic material. Advance and retreat rates were on the order of a half a kilometer per year from 15,000 to 13,800 calendar years B.P. Because the fast-flowing ice did not move sediment long distances and produced local deformation of sediment, we infer that the ice coupled to the bed only locally and temporarily. At other times, high water pressure produced decoupling and sliding at the glacier sole. Widespread, pervasive deformation of the whole thickness of the glacier bed is contraindicated.