Generation of a heavy-mineral glacial indicator dispersal train from a diabase sill, Nipigon region, northwestern Ontario

The heavy clinopyroxene mineral pigeonite forms a glacial indicator dispersal train originating from diabase intrusions in the Nipigon region of northwestern Ontario. Analysis and interpretation of the pigeonite dispersal pattern adjacent to the up-ice portion of the diabase provides a number of insights into the nature of glacial erosion of bedrock and the generation of heavy-mineral dispersal trains. Bedrock erosion and entrainment rates at the time of pigeonite dispersal train formation were high (3-14 mm a (super -1) ), suggesting that bedrock erosion was rapid yet spatially and temporally restricted. Contrasting erosion rates between the diabase and surrounding greenstone lithologies suggests that modern shield topography is not an assemblage of equilibrium bedforms with respect to the ice sheet. This agrees with hypothesized low total erosion of shield bedrock during the Pleistocene. Pigeonite grain size coarsens over the diabase source, indicating that most of the pigeonite was quarried from outcrops as coarse diabase fragments. Down-ice of the diabase source the mean particle size of pigeonite recovered from till decreases, suggesting most of the pigeonite was liberated from bedrock by the comminution of coarse diabase clasts during glacial transport. While the conclusions drawn from this study may not necessarily apply to all heavy-mineral dispersal trains, the interpretive framework provides a foundation for comparative studies.