Basal till and tillite: Sedimentary characteristics and inferred sediment sources, mode of transport, depositional environment, and general conditions of the glacier bed

Characteristics in till at locations near the former margin of the Green Bay Lobe of the Laurentide Ice Sheet during the late Wisconsin, and in tillite in northwestern Argentina in a paleovalley cut into the Pagano basin during the late Carboniferous, are used to infer general conditions at the base of the ice, mode of sediment transport, and mechanisms of ice movement. Characteristics of the Carboniferous deposits are comparable to the characteristics of Wisconsin-age tills and indicate a glacial origin of some of the Carboniferous deposits which previously were interpreted as being entirely glacial lacustrine and alluvial in origin.; During retreat from the last glacial maximum, Green Bay Lobe ice re-advanced over clayey lake sediment and older sandy till. At several locations in northeastern Wisconsin, clayey diamicton with a homogeneous appearance, uniform grain-size distribution, and strongly developed clast-fabric, overlies or grades into sediment that has pre-deformation features including laminations, color and texture contrast, and included organic material. Till matrix composition is uniform at individual locations but varies fairly widely on a regional basis. Widespread, pervasive deformation of the whole thickness of the glacier bed is not indicated by this evidence. Rather it is likely that deformation and sliding varied through time and space and was strongly influenced by subglacial hydraulic conditions.; A record of glacier advance and retreat is preserved in Carboniferous strata in Argentina exposed in an exhumed paleovalley. A striking U-shaped cross-section indicates a glacial origin of the paleovalley. Based on uniform appearance and clast-fabric and shape, some matrix-supported diamictite is interpreted to be tillite, the presence of which provides direct evidence of glaciation in the area. Deformation in the sandstone underlying the tillite indicates warm-based conditions as the glacier advanced over soft deformable sediment. Diamictite beds within a sequence of alternating sandstone and siltstone beds may have originated off the ice front, or subglacially, or both, and may record minor ice-front advances into the flooded valley. Processes other than glacial erosion and deposition, including slumping, rafting, sliding, and debris flow operated in the steep-sided valley and contributed large amounts of sediment to the valley fill.