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

Abstract

Larson and Mooers 1753 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. We thank Barnett for his comments, and feel they serve in part to highlight the variety of problems regarding glacial erosion and entrainment and transport to which we believe our conceptual model of glacial indicator dispersal (Larson and Mooers 2004) is able to contribute. Barnett (2006) brings attention to the fact that tills exposed at surface in the Beardmore–Geraldton region contain a wide range (< 1 up to 23%) in carbonate content (Thorleifson and Kristjansson 1993). High carbonate contents in these tills are a clear signal of long-distance transport of carbonate-bearing debris from the Hudson Bay Lowlands. The occurrence of high-carbonate tills in patches of thick, streamlined drift has been interpreted by Hicock (1988) and Hicock and others (1989) to mean that all surface tills in this region were deposited at the base of a fastflowing ice stream. However, the ice stream model does not explain the widely disparate values in till carbonate occurring over distances of only a few kilometres; we believe a different interpretation explains the same data (Larson and Mooers 2005a). Tills sampled in our limited study area contained virtually none of the Paleozoic carbonate and Proterozoic greywacke clasts characteristic of high-carbonate tills found in the region. We believe this observation is inconsistent with an origin of the till-forming material by longdistance transport of debris from the Hudson Bay Lowlands by an ice stream, and rather that it reflects local erosion and entrainment and short-distance transport. However, the origin of the tills we sampled in our study area is not relevant to the interpretations we presented. Determining whether the till in our study area was generated by erosion and entrainment into the englacial basal debris layer of the ice sheet, by erosion and entrainment in a subglacial deforming debris layer, or by another process was beyond the scope of our study. We deliberately made no explicit assertion regarding the processes by which diabase was eroded and entrained, and transported. We did, however, attempt to document the rates at which the erosion and entrainment process worked. The only explicit assumptions we incorporated into our interpretations were of the debris mass per unit bed area (m) and of the velocity at which the basal debris layer was transported (u). 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. Hallet et al. 1996). Barnett (2006) questions our conclusion that the diabase sill in our study area was significantly more resistant to glacial erosion and entrainment than the Archean greenstones into which it was intruded. Notwithstanding the higher susceptibility of gabbro–diabase to physical and chemical