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

Abstract

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–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.

Quaternary glaciations of northern Europe

“…This reflects the highly dynamic nature of the glacial environment, the range of erosional, depositional and glaciotectonic processes that occur during successive phases of ice movement, and processes of mass-wasting and landscape inversion that occur during and after deglaciation (Aber et al, 1989;Ballantyne, 2002;Evans and Twigg, 2002;Everest and Bradwell, 2003;Van der Wateren, 2005;Benn and Evans, 2010). Perhaps the most significant development in our understanding of glacial processes over the past 30 years has been the realisation of the role played by 'subglacial deformable beds' in controlling ice dynamics and spatial variations in till composition (Boulton and Jones, 1979;Benn and Evans, 1996;Boulton, 1996;Murray, 1997;Kjaer et al, 2003;Larson and Mooers, 2005;Evans et al, 2006). Thus, whilst tills are often considered as laterally persistent datum horizons, spatial complexities in their composition and genesis often leads to their composition being heterogeneous at local or regional scales (Perrin et al, 1979;Kjaer et al, 2003;Boston et al, 2010;Scheib et al, 2011).…”

Genesis of dispersal plumes in till

“…Most tills are sampled using shallow pits dug at the surface (e.g., Larson and Mooers 2005;McClenaghan et al 2002), which precludes identifying the plume geometry. In addition, many till sheets are relatively thin; plumes in these are also likely 3-D, but this would be difficult to demonstrate.…”

Reply to discussion by P.J. Barnett on "Generation of heavy-mineral glacial indicator dispersal trains from a diabase sill, Nipigon Region, Northwestern Ontario"

“…These values were used to estimate the erosivity (E) recorded in the till sheet, and indirectly the absolute erosion and entrainment rate (e) of diabase into the basal debris layer. Examination of the equations presented in Larson and Mooers (2004), and the modified equations presented in Larson and Mooers (2005b), shows that increasing the estimated m results in increased estimates of the values of E and e. Increasing u likewise results in increased estimates of e. However, estimates of the erosion length scale (λ) and the relative rates of erosion of the diabase and greenstone are unaffected by varying these quantities.We feel that the m and u estimates that we incorporated into our interpretation of the heavy-mineral dispersal data represent reasonable values for an ice sheet, and that the resulting e estimates are likewise reasonable. It is important to note that the absolute values of erosion and entrainment we have estimated for diabase and greenstone lithologies lie within the range documented for modern glacial systems (cf.…”

“…Barnett (2006) poses a number of questions regarding some of the field observations and assumptions we used in our interpretation of a heavy-mineral dispersal train derived from an isolated eastern outlier of diabase of the Nipigon sills in northwestern Ontario (Larson and Mooers 2005b). He also brings attention to some details of the regional glacial geologic setting we did not address in our analysis and interpretation of the dispersal train, and highlights new field data (Barnett and Dyer 2005) relevant to interpretation of past glacial processes acting in the broader Nipigon region.…”