What can sediments tell us?

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The sizes, size range and roundess of sediments are all related to how the sediments were transported, because wind, water and ice vary in terms of energy and viscosity

Fine grained, well rounded sediments have been transported for a longer time than coarse grained angular fragments.

The shape of grains depends on the type of rock or mineral from which they are made.

Sedimentary deposits in which all of the grains are of similar size have been transported in higher energy conditions than those where there is a wide range of sizes.

How sediments were transported is related to environmental factors, such as climate.

Sorting

A sedimentary deposit of grains that are all the same size would be described as very well sorted. One with a wide range of sizes would be termed very poorly sorted.

Sorting provides information about how sediments were transported. Wind does not transport very coarse grains. It deposits well sorted sands as energy and velocity decrease. Finer grains are blown further, forming deposits elsewhere.

Rivers flowing with high energy may transport coarse as well as medium and fine grained sediment. As energy levels fall, larger heavier grains are deposited first, medium sized grains are transported further and finer grains are deposited last. Poorly sorted sediment has usually not been transported far because the transporting medium lost energy quickly. A desert strem that rapidly dries up due to infiltration or high rates of evaporation will deposit the sediment very quickly. A meltwater stream from a glacier will lose energy as melting stops after the summer thaw has ended and sediment will be deposited over a short period of time.

Grain Size Analysis of Sediments

Grain size data is plotted as cumulative frequency curves.

Well sorted sediments have almost vertical curves. Poorly sorted sediments have curves that stretch over a wide range of sizes. The degree of sorting can be quantified.

Coefficient of sorting (p): (phi 84 - phi 16) divided by 2

Charactertistics of sediments related to transport

Wind blown dune sand in a high energy environment:

fine to medium sand

all quartz sometimes red due to iron oxide

well rounded

very well sorted, so that most of the sediment is in one sieve, as fine sand is easiest for the wind to carry

Ice - deposited as glacial till in a low energy environment:

varies from very coarse to very fine

varied - any rock fragments and clay

angular to subangular

very poorly sorted

River deposit of sand in channel - usually high energy with fast currents

coarse to fine sand

quartz and mica with rock fragments

angular near to source but subangular to subrounded downstream

poorly sorted near source of river - moderately sorted downstream gives wide range of sediment sizes

Beach or offshore bar in sea - high energy

medium sand - sometimes coarse (pebbles and gravel) close to shore and on beaches

nearly all quartz with some shell or rock fragments

subrounded to rounded

moderately sorted with sediment mainly in a few sieves

Gravity - low energy

varies from very coarse in rocks to very fine in soil creep

varied - any rock fragments

angular to subangular

very poorly sorted - larger fragments sometimes found at base of slope

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