Terrigenous deposits

Terrigenous deposits are found near land, covering the continental shelf and upper parts of the continental slope. Much of this material is derived from weathering and erosion of exposed land surfaces, and consists largely of particles worn from the coast by wave action or carried into the sea by rivers or glaciers. Terrigenous deposits contain some organic material, often some 0.01-0.5 per cent of the dry weight, the finer-texture deposits usually having the greater proportion of organic matter. Microscopic examination sometimes reveals recognizable traces of various materials of biological origin, both terrestrial and marine. The former are mainly fragments of leaf and wood from land plants. The latter are very diverse, deriving from both benthic and pelagic sources, and often include small particles of seaweed, diatom cell walls, sponge spicules, polychaete chaetae, and fragments of the shells and skeletons of foraminifera, hydroids and corals, polyzoa, crustacea, echinoderms and molluscs. Minute animals may be seen in samples from the surface layers of the deposit; for example, flagellates, ciliates, foraminifera, nematodes and copepods. Superficial deposits from shallow water may also contain a microflora of benthic diatoms. The mud from shallow creeks is sometimes rich in fragments of marine angiosperms, for example Spartina.

Terrigenous deposits show considerable differences of composition from place to place, varying with the nature of the adjacent coastline, the movements of the water and the contours of the sea-bed. They range from large boulders close to rocky shores where they have been dislodged by violent wave action, through all grades and mixtures of pebbles, gravel and sands down to fine clay. These familiar terms are defined by the specific size range of their particles. The much-used Wentworth scale of particle sizes (see Table 6.1) is geometric, giving smaller intervals towards the finer end of the range. Using this scale, a pebble is a pebble only if its size lies between 4 and 64 mm diameter. A pebble larger than this is defined as a cobble or a boulder.

Geologists tend to use a different scale called the phi ($) scale. This converts the unequal steps of the Wentworth scale into an arithmetic series of equal intervals, thereby simplifying graphical and statistical treatment. The particle diameter in millimetres is written as the equivalent negative of the power of 2. So for example to find the phi size of a pebble of 64 mm diameter, the diameter is first written as a power of 2, i.e. 26. (64 = 26). The power of 2 in this case is 6 so the phi size is the negative of 6, i.e. a 64 mm diameter pebble has a phi size of -6.

Likewise, a very fine sand with grains of 0.125 mm diameter has a phi size of + 3. This is calculated as follows: convert 0.125 to a fraction = 1/8. 1/8 written as a power of 2 is 2-3. The negative of —3 is +3.

The sizes of the sediment grains in a sample are found by drying the sample and sieving it through a series of sieves with meshes of decreasing size. The sieves are precision tools, constructed with very accurate mesh sizes relating to the phi scale. The grains that pass through one sieve but not the next, have a diameter size range between the two phi sizes of the sieves.

Marine sediments are never uniform in composition but contain particles of many grades and types. If the particles are mainly of one size, the sediment is said to be well sorted. If there are many sizes of grains then it is a poorly sorted sediment. Sediments containing more than 10 per cent dry weight of silt and clay fractions are commonly termed 'muddy sands'. If more than 30 per cent of the

Table 6.1 Wentworth Classification of Particle Grades and phi Scale.

Grade name

Particle size range (mm)

Phi units






-6 to -8



-2 to -6



-1 to -2

Very coarse sand


0 to -1

Coarse sand


+ 1 to 0

Medium sand


+2 to +1

Fine sand


+ 3 to +2

Very fine sand


+4 to +3



+ 8 to +4




deposit is silt and clay, the term 'sandy mud' is applied, and deposits with silt and clay fractions exceeding 80 per cent are generally described as 'mud'.

Some terrigenous deposits are exploited by man. For example, the continental shelf is nowadays an important source of sand and gravel for building operations.

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  • frida
    What are terrigenous deposits?
    8 years ago

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