Brief notes on the Texture of Sedimentary Rocks

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The word ‘texture’ refers to the size, shape, packing and fabric of the components of the rock. Since the sedimentary rocks are broadly classified as (1) exogenetic or clastic rocks and (2) endogenetic rocks or the chemically precipitated amorphous or crystalline rocks, accordingly their texture are also classified into two broad categories.

1. Clastic texture:

It includes elements like:

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(I) Size, (II) Shape, (III) Sphericity, (IV) Packing, (V) Fabric.

(a) Size. The grain size is dependent on the (i) mode of weathering, (ii) nature of the source rock, and (iii) kind and distance of transport and the nature of deposition.

Broadly, the size characters of the sediments are described as either coarse, medium or fine. The size grades of the clastic parti­cles, in the went worth scale are indicated as follows:

The sire analysis results are represented in form of tables. Histograms or frequency curves as well as by statistical methods.” In statistical methods the following measurements are made:

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(a) Measurement of central tendency:

Which determines the average size of the distribution and refers to the overall competency of the transporting medium.

(b) Measurement of dispersion:

It determines the turbulence of the transporting medium or the amount of reworking of the sediment has undergone prior to final burial.

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(c) Measurement of skewness:

It determines whether the coarser and finer admixture occur in same proportion, in a, sediment.

(d) Measurement of kurtosis:

It determines the peakedness or flat-toppedness of a distribution.

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Geological Significance:

The size analysis indicates the fol­lowing:

1. Provenance:

(i) Composition of the source rock is an important factor that determines the extent to which the component minerals are susceptible to weathering and liable to pass on to the products and reduced in size and shape.

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(ii) Besides the coarse or fine texture is also a function of the source area.

2. Transport:

As we know, more the distance of transport, finer is the grain size. Besides, the character of the sediments are also governed by the mode of transport, i.e., traction, saltation, suspension, which is a function of the kinetic energy of the trans­porting medium. Higher the energy, the coarser particles can be transported.

3. Depositional environment.

4. Palaeo current:

The coarser sediments carried in rolling are deposited in basin margin, whereas the finer sediments are gradually carried to the centre. As such there is a regular variation in grain size from the margin to the centre of the basin.

5. Transporting medium:

Graded sediments are the result of long continued transport, while ill-sorted sediments of a rapid and confused deposition like glacial deposits. Aeolian deposits are apt to be well graded and uniform.

6. Tectonics of depositional site:

With an increase in the rate subsidence of the area of deposition, the grainsize decreases and the average sorting is poor. Under stable condition, the grainsize is
determined by the texture of the material available for reworking sorting steadily improves.

II. Shape:

It is defined as the sharpness of corners and edge of a clastic fragment. Accordingly the shape may be angular, sub angular, sub-rounded, rounded, well rounded, etc. The shape of the sedimentary grain is determined by:

(i) Original shape of the mineral, (ii) Stability of minerals and (iii) amount and nature of transport.

III. Sphericity:

It is defined as the extent to which a particle approaches a sphere. It depends on (i) distance of transport. (ii) Mode of transport, and (iii) provenance.

(i) Longer the distance of transportation, more chances of being reworked and therefore more the degree of round­ness. Besides, wind produces perfect rounding, glacier does the least.

(ii) The mode of transport like traction, saltation and suspen­sion produces particles of variable roundness.

(iii) Generally the elongation quotient is maximum in metamorphic rocks, less in igneous rocks and very less in sedimentary rocks. Thus when the source rock is of metamorphic origin, the sphericity of the clastic grains is not that much pronounced.

IV. Packing:

It is the manner of aggregation of sedimen­tary grains, which are held together in place in the earth’s gravitational field. There are six methods of packing out of which the rhombohedfal packing is the most compact and tight whereas, the cubic packing is the loosest possible packing. It determines the porosity and permeability of sedimentary rocks.

V. Fabric:

It is the arrangement of the clastic particles in sediments. It is defined as the orientation of the grains or lack of it with which the sedimentary rock is composed. Pebbles, sand grains, mica-flakes etc. are the most useful fabric elements, also, some fossils like gastroped shells etc. It determines the palaeo-current direction.

2. Non-clastic textures:

It is formed as a result of deposition through chemical reaction. They are transported chemically by getting dissolved in the transported media but reappear due to precipitation or evaporation. It is of two types:

(a) Crystalline texture.

(b) Non-crystalline tenure.

(a) Crystalline texture:

They are formed due to direct precipitation from a saturated solution, and the result is an interlocks aggregate of crystals.

(b) Non-crystalline texture:

When colloids coagulate they form a gelatin like mass. This gelatinous mass may lose some of, he water in it and eventually harden to form an amorphous mass Nodular, oolitic, spherulitic textures are the examples. Many concretionary, botryoidal, reniform, nodular, oolitic and pisolitic textures are believed to be of colloidal origin and they show non­crystalline textures as described above.

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