Glacial indicator dispersal processes: a conceptual model

Abstract

Interpretation of indicator dispersal trains preserved in till sheets is widely used to investigate past glacial processes and to explore for buried bedrock mineralization. We present a conceptual model of erosion and entrainment and transport of indicator material in a glacial system. Indicator concentration in an individual size fraction of till is controlled by dilution and comminution. Dilution is the result of incorporation of additional material to the glacier's debris load down-ice of the indicator source, and is described in terms of erosivity and erosion length scale. Erosivity describes the amount of bed material eroded along a flowline, and is a function of both bed properties and the erosive power of the glacier. Erosion length scale describes the persistence of an indicator dispersal signal during transport, and controls both the maximum total indicator concentration and the eventual length of apparent dispersal. We adapt a modified batch grinding particle comminution model to describe breakdown of indicator material during transport and modification of the indicator particle size distribution. Indicator dispersal concentrations are the product of dilution and comminution processes.

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.…”

“…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.…”

“…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).…”

Timing and flow pattern of the Orta Glacier (European Alps) during the Last Glacial Maximum

“…However, these works are mostly related to gravel‐size clasts, while boulders can have been affected by different shaping and clustering processes (Boulton ). The abundance of a particular lithology is also an expression of the distance of the source outcrop from the marginal position (Larson & Mooers ). By identifying bedrock source regions (Fig.…”

Exploring the use of zircon geochronology as an indicator of Laurentide Ice Sheet till provenance, Indiana, USA

“…Local-scale dispersal will result in till composition that reflects regional bedrock whereas continental-scale dispersal (100s km) results in tills with compositions that are more challenging to track back to bedrock sources (Clark, 1987). In an ideal system where ice flow direction remains constant, glacial debris concentration in ice decreases exponentially downstream from the bedrock source, dispersing the concentration of debris from the original bedrock source over local (kilometers) to continental scales (100s–1000s km; Clark, 1987; Shilts, 1993; Boulton, 1996; Hooke et al, 2013; Larson and Mooers, 2004). The transport distance is controlled by a number of factors including till dynamics, sliding velocities, frictional resistances, basal debris concentration, and erosivity (Clark, 1987; Boulton, 1996; Larson and Mooers, 2004).…”