Sedimentary Textures

 What is a sedimentary texture?

    The term texture refers to the pattern formed within a sedimentary rock by the constituting grains. This can best be studied under a microscope.

Classification of sedimentary texture

    In sedimentary rocks of Mechanical origin, formed by weathering, erosion, transportation, deposition followed by diagenesis & Lithification, the framework grains are joined together, either by finer fragmental particles or by chemically precipitated cement. The former is called matrix and the latter, cement. The pattern formed by framework grains, matrix and cement is called (a)Clastic texture. Sandstones show this kind of texture.

    The texture of a rock of Chemical origin is produced wholly by chemically precipitated materials. No inter-granular space is to be found in such a rock. Limestones of orthochemical origin show this kind of (b)Non clastic texture.

    Allochemical limestones, produced by fragmental pieces of calcareous material joined together either by fine carbonate mud or by calcite spars, show a clastic texture.

    Clastic textures are defined mainly by size, shape and roundness of the framework grains. Other features which play important roles in clastic texture are the nature of grain contact (grain-to-grain relationship) and grain packing. Porosity and permeability of a clastic rock depend on these parameters.

Grain size:

    The definition of particle size is related, at least in part, to the technique used for measuring it. When a particle is large enough to allow direct measurement of its diameters, say by calipers or by a micrometer scale fitted to the microscope oculars, size can be expressed in terms of the diameters (long, short and intermediate). A problem arises when a grain is too irregular to allow direct measurement of its diameters. In such cases the size is measured indirectly, firstly by measuring its volume, by the process of displacement of water, and then by working out the diameter of the sphere of equivalent volume by the following formula:  V = 4/3 лr3,  where ‘V’ and ‘r’ are the volume and radius of the sphere respectively. The diameter thus obtained was named ‘true nominal diameter by Wadell in 1932. Some other grainsize classification are shown below.

Grain size measurement:

   The techniques commonly used for grain-size analysis are: (i) direct measurement (in macro- or microscale), (ii)sieving and (iii) settling, Grains larger than sand can be measured directly. Sieving is generally used for particles ranging from granule to fine sand, Settling techniques are used for sizes finer than sand (silt and clay). More sophisticated methods such as image analysis of SEM pictures, electrical resistive pulse and light blockage counters, disc centrifuge and laser scattering devices can also be used.

Roundness & Shape

Roundness and shape of clastic particles are two separate parameters, independent of each other. Roundness implies rounding of the edges and corners of a clastic grian. Shape is expressed as the ratio between the three measurable diameters of an irregular grain: length, breadth, and width. Other measures of shape, including sphericity and flatness, are also used.

  Following Wadell, roundness is defined as the ratio of the average radius of curvature of the edges and corners of a grain to the curvature of the maximum inscribed sphere.

  According to Wadell’s definition, the roundness of a sphere is 1.0. The nearer a natural particle is to this value, the more rounded it is. A chart giving photographs of pebbles of different roundness values was published by Powers (1953) to provide a quick aid to roundness determination by visual comparison (Fig. 4.9).

Why it is important?

Applications & Limitations

•Rounding of sand-size particles of quartz and feldspar is negligible during fluvial transport. Whatever rounding is attained during transportation is confined only to the first few kilometres and that, too, observable only in the pebble-size grains.

•Beach action is an effective agent for rounding of pebbles but not for sand.

•Rounding of a sand grain is believed, on the basis of experimental studies, to be essentially due to eolian action.

•The effect of solution in the rounding of quartz grains has been shown to be negligible (Kuenen 1960 a, b and 1964)

•For grains larger than sand, size and roundness generally show a positive correlation. This is so because the smaller grains, which are produced by chipping of the corners of a large pebble, are more angular than the remaining central part.

•The fact that roundness is controlled by size of the pebble rather than distance of transport plus a serious limitation to the use of pebble roundness as a palaeo-current  indicators.

•Size and roundness relation in sand size is more complicated For example, Banerjee (1964) showed a negative correlation between size and roundness of sand and explain this as being the result of mixing of fist-cycle and second cycle sand grains.

Want to know more check out this video.

• References:
• Sengupta S M., Introduction to sedimentology (Second edition)
• Pettijohn, F.J. – 1967 – Sedimentary Rocks, Harpers and Bros
• Gary Nichols, 2009 – Sedimentology and Stratigraphy, John Wiley and Sons
• Sam Boggs, 2006 – Principles of sedimentology and stratigraphy, Pearson Prentice Hall