ABSTRACT

Strong to weak clast fabric patterns were recognized in the analysis of preferred long-axis orientations of prolate clasts from lodgement and melt-out tillites, and rain-out and debris-flow diamictites in the basal glaciogenic sequence of the Dwyka Formation. Strong clast fabrics were either inherited from the basal ice or caused by shear stress in the subice sediment during glacial deposition. Weak clast fabrics are present in 1) lodgement and melt-out tillites on the stoss sides of bedrock knobs, 2) leeside melt-out tillite with the upper surface abraded by overriding ice, 3) fluted lodgement tillite, and 4) rain-out and debris-flow diamictites. The latter two diamictites commonly show low-strength to random clast fabrics due to their mode of formation, but the weak fabric patterns n the subglacial tillites are attributed to the influence of 1) bedrock topography on clast orientation in the debris-rich basal ice before deposition, and 2) plastic deformation on the inherited clast orientation in the sediment. Unless the effects of these two variables can be established in ancient basal diamictites, the application of clast fabric analysis as the only means to determine the paleoice-flow direction and the paleoenvironment must be treated with circumspection.