Sedimentary Rocks Textures

Some of the subjects in the field of sedimentology have been investigated more thoroughly than sedimentary textures. This important and powerful interest in sedimentary textures has apparently come out of the conviction of many workers that sedimentary texture is an important tool for the environmental analysis. The sizes, shapes, and arrangement of siliciclastic grains have been checked or observed and reexamined over the interval of decades in an effort to found through empirical and lab or tidal studies the validity of this assumption. This goal of environmental interpretation remains elusive, and different kinds of problems may be set investigators who try to use sedimentary texture as a kit for the environmental view. None the less, texture is a fundamental attribute of siliciclastic sedimentary rocks. Instead of other properties of that types of rocks, it is useful to characterize and separated them from other types of the rocks and it assists in their correlation. Derived properties of these rocks like porosity, permeability, bulk density, electrical conductivity, and sound transmissibility is affected by the texture of sedimentary rocks. This types of properties are of particular interest to petroleum geologists, hydrologists, and geophysicists. Three fundamental properties of sedimentary rocks are encompassed by Sedimentary texture like grain size, grain shape (form, roundness, and surface texture of grains), and fabric (grain packing and orientation). The properties of individual grains are Grain size and grain shape respectively. The property of grain aggregates is Fabric.

Sedimentary Rocks Textures

Most of the sedimentary rocks are formed by processes of weathering, transportation, deposition, and diagenesis. The final texture like grain size, shape, sorting, mineralogy, etc. in a sediment or sedimentary rocks is support on the process that occur during the each stage. The given points summarize the basic factors which affect the rock texture.

1. The rocks which are eroded to create the sediments. That is useful to determine the original shape and size of the grains and the mineralogical composition of the native sediment.
2. The force of the wind or water currents that carry and which deposit the sediment. This deposited sediment determines whether or not grains are transported or deposited. The process of deposition also controls structures that should be preserved in the sediment and thus give clues to the environment of deposition.
3. The distance transported or time in the transportation process. The longer shaped grains are in the transportation process the more likely they are able to change shape and they become sorted on the basis of size and mineralogy. This may also help to controls extent to which they break down to stable minerals during the process of transportation.
4. The chemical environment under which diagenesis occurs. During the process of diagenesis grains are pack, new minerals precipitated in the pore spaces, some minerals continue to react to produce new minerals, and some minerals recrystallize. The composition of fluids moving depends on what happen through the rock, the composition of the mineral grains, and the pressure and temperature conditions attained during diagenesis.

Concept of textural maturity

The term textural maturity is often used by Sedimentologists in reference to the textural characteristics of the individual sediment. Folk (1951) described that textural maturity of sandstones encompasses three textural properties: which are:

(1) The amount of clay-size sediment in the rock,

(2) The sorting of the framework grains, and

(3) The rounding of the framework grains.

Folk sees four stages of textural maturity they are immature, sub-mature, mature, and supernatural . Any sandstone which contains considerable clay, more than 5 percent, is in its immature stage. Also, the framework grains in immature sediments are loosely sorted and poorly rounded. The immature sediments have not undergone enough sediment transport and reworking to overcome the fine-size material and produce sorting and rounding of grains. With extra sediment transport and reworking, sediments which enter to the sub-mature stage, in which the sediments are characterized by low clay content but the grains are still not well rounded. The mature stage follows the this stage, in which clay content is low and framework grains become well sorted but they are not yet well rounded. In the super mature stage, Sediments are essentially free of clay, and the framework grains are both well sorted and are well rounded. The textural maturity concept is a helpful one for characterizing sediments. However, to associate with the progressive increase in textural maturity primarily to increasing total input of modifying kinetic energy is probably overly simplistic. For example, that the vadose infiltration of clays into sandy sediments is influenced by several variables in addition to the hydrologic conditions and depositional processes within a depositional environment. Also, the particles in many mature to super mature and stones may have been recycling done or more times. Thus, the total expenditure of energy necessary to generate rounding may not have come in a single depositional cycle.

References

Collinson, D J and B D Thompson. Sedimentary structures. Delhi: CBS Publishers and Distributors, 1989.

Ehlers, E G and H Blatt. Petrology: Igneous, sedimentary and Metamorphic. New Delhi: CBS Publishers and Distributors, 1987.

JR, Sam Boggs. Petrology of Sedimentary structures. New York: Macmillan Publishing company, 1989.

Pettijohn, F J. Petrology of Sedimentary rocks. New Delhi: CBS Publishers and Distributors, 1984.