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6.14: Sorting

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    Sorting

    The ability of running water to move sediments also sorts particles by size and to a lesser degree by shape. This is called sorting (illustrated in Figure 6.52). Sediments exposed to longer transport or exposure to currents and waves tend to be more sorted by shape and size.
    The amount of sorting depending on the energy conditions and amount of time at which the stream currents or ocean waves works on the particles. For instance, particles of the same mineral that are more rounded and more sorted have traveled farther.

    Sorting
    Figure 6.52. Sorting of sedimentary particles. Beach sands the to be very well sorted. River sands tend to be moderately sorted. Deep ocean turbidity current sediments tend to be very poorly sorted.

    As transportation distance increases, sediment becomes more "mature" and:
    • Clay content decreases (clays are carried away and deposited in other quiet water settings)
    • Sorting increases (gravel and sand gets concentrated)
    • Non-quartz minerals decrease (quartz is both an abundant and is harder than other common minerals)
    • Grains become more rounded (sharp edge break off easier)

    The sediments sorting, roundness, and sphericity could act as a clue to following either modern or ancient alluvial rocks to their ultimate source (such as for finding gold and diamonds). For example, very well sorted and rounded materials may suggest a source from an older sedimentary rock rather than from freshly exposed igneous rocks. Sand from rivers and stream are very different from sands associated with beach and sand-dune deposits (see Figures 6-53 to 6-56).

    sand from an upland stream is rich in feldspars Beach sand rich in quartz grains Beach sand rich in microfossils Dune sand
    Figure 6.53. Sand from a mountain stream may be rich in poorly sorted and angular grains of feldspars, quartz, and other minerals. Figure 6.54. Beach sand is enriched in well rounded and consist mostly of well-sorted quartz grains. Fine materials are winnowed out. Figure 6.55. Beach sand in many tropical settings may be enriched in shell material, including microfossils (such as shells of foraminifera). Figure 6.56. Wind-blown dune sand is typically very well sorted and very well rounded, polished to frosted grains of mostly quartz.

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