TEXTURES OF CLASTIC SEDIMENTARY ROCKS

TEXTURES OF CLASTIC SEDIMENTARY ROCKS

 

        When sediment is transported and deposited, it leaves clues to the mode of transport and deposition. For example, if the mode of transport is by sliding down a slope, the deposits that result are generally chaotic in nature, and show a wide variety of particle sizes. Grain size and the interrelationship between grains gives the resulting sediment texture. Thus, we can use the texture of the resulting deposits to give us clues to the mode of transport and deposition.

 

Figure 1.1 - Type of sorting

Sorting :-

 The degree of uniformity of grain size. Particles become sorted on the basis of density, because of the energy of the transporting medium. High energy currents can carry larger fragments. As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density.

If the particles have the same density, then the heavier particles will also be larger, so the sorting will take place on the basis of size. We can classify this size sorting on a relative basis - well sorted to poorly sorted. Sorting gives clues to the energy conditions of the transporting medium from which the sediment was deposited.

Examples :-

 Beach deposits and wind-blown deposits generally show good sorting because the energy of the transporting medium is usually constant.

 Stream deposits are usually poorly sorted because the energy (velocity) in an stream varies with position in the stream and time.

GRAIN CLASSIFICATION : -  grain size and shape, among other factors.

Name of Particle	Size Range	Loose Sediment	Consolidated Rock
Boulder	>256 mm	Gravel	Conglomerate or Breccia      (depends on Cobble 64 - 256 mm Gravel rounding)
Cobble	64 - 256 mm	Gravel
Pebble	2 - 64 mm	Gravel
Sand	1/16 - 2mm	Sand	Sandstone
Silt	1/256 - 1/16mm	Silt	Siltstone
Clay	<1/256 mm	Clay	Claystone, mudstone, and shale

 Based on grain size the porosity, permeability, pore space, sorting, grain arrangement of rocks and layers are decided.

Rounding :-

 During the transportation process, grains may be reduced in size due to abrasion. Random abrasion results in the eventual rounding off of the sharp corners and edges of grains. Thus, rounding of grains gives us clues to the amount of time a sediment has been in the transportation cycle. Rounding is classified on relative terms as well.

Figure 1.2-  Stages of rounding/sphericity

Sediment Maturity:-

Sediment Maturity refers to the length of time that the sediment has been in the sedimentary cycle. Texturally mature sediment is sediment that is well rounded, (as rounding increases with transport distance and time) and well sorted (as sorting gets better as larger clasts are left behind and smaller clasts are carried away. Because the weathering processes continues during sediment transport, mineral grains that are unstable near the surface become less common as the distance of transport or time in the cycle increases. Thus compositionally mature sediment is composed of only the most stable minerals.

For example :-

 a poorly sediment containing glassy angular volcanic fragments, olivine crystals and plagioclase is texturally immature because the fragments are angular, indicating they have not been transported very far and the sediment is poorly sorted, indicating that little time has been involved in separating larger fragments from smaller fragments. It is compositionally immature because it contains unstable glass along with minerals that are not very stable near the surface ( olivine and plagioclase ).

On the other hand a well-sorted beach sand consisting mainly of well-rounded quartz grains is texturally mature because the grains are rounded, indicating a long time in the transportation cycle, and the sediment is well sorted, also indicative of the long time required to separate the coarser grained material and finer grained material from the sand. The beach sand is compositionally mature because it is made up only of quartz which is very stable at the earth's surface.