Box 72 Particlesize Distributions And Transport Distances

Dreimanis and Vagners (1971) argued on the basis of crushing experiments that the relative magnitude of the two particle-size peaks of subglacial sediment should change with transport distance. As transport distance increases, the finer mode (composed predominantly of mineral grains) should grow at the expense of the coarser mode (composed of predominantly lithic fragments). Dreimanis and Vagners (1971) demonstrated this by examining the particle-size distributions in three till samples in the Hamilton Niagara region of southeast Canada. The particle size of only one clast lithology (dolomite) was analysed in order to keep the results relatively simple. The results, illustrated below, show that with increased transport distance the finer mode increases in importance, although the amount of transport necessary will depend on the lithology. The patterns are of course more complex when multiple lithologies are considered and where new material is introduced to the glacier by debris entrainment. More recently, Hooke and Iverson (1995) have argued that particle-size distributions can be used to distinguish between those sediments that have been subjected to subglacial sediment deformation and those that have not.

20 -

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220 km transport of igneous

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and metamorphic fragments

15 -

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0-3 km transport of

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dolomite

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Particle size

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1 200 km transport of igneous and metamorphic fragments ^

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75 km transport of dolomite

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Particle size

Source: Dreimanis, A.and Vagners, U.J. (1971) Bimodal distribution of rock and mineral fragments in basal tills, in Till: a Symposium (ed. R.P. Goldthwait), Ohio State University Press, Ohio, pp. 237-50. Hooke, R.L. and Iverson, N.R. (1995) Grain-size distribution in deforming subglacial tills: role of grain fracture. Geology, 23, 57-60. [Modified from: Dreimanis and Vagners (1971) in Till: a Symposium (ed. R.P. Goldthwait), Ohio State University Press, Ohio, figure 4, p. 243].

7.4 Debris Transfer Between Low and High Levels 197

Figure7.10 A subglacially transported clast showing faceting and striations [Photograph:

Figure7.10 A subglacially transported clast showing faceting and striations [Photograph:

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