Abstract
AbstractGlacial erosion produces vast quantities of fine-grained sediment that has a far-reaching impact on Earth surface processes. To gain a better understanding of the production of glacial silt and clay, we use automated mineralogy to quantify the microstructure and mineralogy of rock and sediment samples from 20 basins in the St. Elias Mountains, Yukon, Canada. Sediments were collected from proglacial streams, while rock samples were collected from ice marginal outcrops and fragmented using electrical pulse disaggregation. For both rock fragments and sediments, we observe a log-normal distribution of grain sizes and a sub-micrometer terminal grain size. We find that the abrasion of silt and clay results in both rounding and the exploitation of through-going fractures. The abundance of inter- versus intragranular fractures depends on mineralogy and size. Unlike the relatively larger grains, where crushing and abrasion are thought to exploit and produce discrete populations of grain sizes, the comminution of fines leads to a grain size, composition and rounding that is continuously distributed across size, and highly dependent on source-rock properties.
Highlights
Derived silt- and clay-sized grains influence a wide range of Earth surface processes from slope stability to chemical weathering rates
In this study we compare the mineralogy and microfracture characteristics of glacial silt and clay to rock fragments that were liberated through electrical pulse disaggregation
We find that grain and mineral cluster sizes within glacial fines are log-normally distributed with a minimum grain size that is likely below our submicrometer detection limit
Summary
Derived silt- and clay-sized grains influence a wide range of Earth surface processes from slope stability to chemical weathering rates. Crushing occurs when compressive forces cause through-going fractures to fail in tension (e.g. Hiemstra and van der Meer, 1997; Hooke and Iverson, 1995), predominantly exploiting gravel-sized clasts to produce sand- and silt-sized daughter grains Subglacial abrasion has been argued to produce grains that are predominately silt-sized, the so-called ‘terminal grade’ of Dreimanis and Vagners (1971). The exact size of the terminal grade depends on lithology (Dreimanis and Vagners, 1971; Haldorsen, 1981), but is thought to be no smaller than 1 μm Within this framework, comminution is described as being discrete, i.e. crushing and abrasion selectively exploit different grain-size modes to produce distinct daughter-size populations no smaller than silt (see Fig. 6 in Haldorsen, 1981)
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