sedimentary rocks

Sedimentary rocks are formed above the surface of the earth’s crust as a result of the deposition of fractures or crumbs of rocks of all kinds (igneous, metamorphic, sedimentary), which are disintegrated by weathering and by the influence of erosion factors (wind, water, freezing). ) Then this aggregate was moved and re-depositioned and cohesioned either with hardened materials such as (silica, calcium carbonate, iron oxides) or by pressure on them from other sediments and rocks. Sedimentary rocks are also formed through the deposition of chemical substances dissolved in aqueous solutions, and as a result of a change in the environment of the medium in which they are located, or as a result of an increase in the concentration of these lupus materials with evaporation crystallizes.

Sedimentary rocks may also form as a result of many biological activities, whether plant or animal, which lead to the formation of organic sedimentary rocks such as coal and phosphates.

The most important types of sedimentary rocks

clastic sedimentary rocks.

Chemical sedimentary rocks.

Organic sedimentary rocks.

Characteristics of sedimentary rocks

It is always in the form of layers.

It is characterized by the presence of fossils.

Fragile, loose-fitting rocks such as igneous and metamorphic rocks.

It consists of round grains of different sizes and may consist of mineral crystals if they are of chemical origin

It sometimes contains oil, gas, water and many natural resources.

It is distinguished by its light colours.

Contains special textures such as (Neem Marks, Mud Cracks, Cross Layer, and Graduated Lamination).

Conglomerate rock – one of the common clastic sedimentary rocks

1- Clatiform sedimentary rocks

consists of grains of rocks and minerals that were produced in the weathering process and transported by Running water, wind and thawing frozen water. Among its characteristics:
The size of its particles ranges from very large, such as gravel, to very fine, such as clay.
The transported materials are deposited when the velocity of the conveying medium decreases, and the large materials are first deposited, then the smaller ones, and so on.

The higher the velocity of the conveying medium, the larger the size of the transported granules and vice versa
The more the particles moved, the smaller their size and became in the form of Circular.
Fine materials are transported as suspended materials in the conveying medium. It has and transports larger materials on the river bed either by jumping, rotation or traction, and when the speed of the river water decreases at the estuary, the materials it carries are deposited, and the coarse materials are deposited near the estuary, then the smaller materials are deposited at further distances from the estuary.

Lithification

rock deposition The sedimentary layer is in the form of layers, and the process of solidification of these layers is called petrification, and it is carried out by a series of complex processes, the most important of which are:

1- Compaction

caused by pressure as a result Accumulation of sedimentary layers or as a result of pressures generated by earth movements. The pressure leads to convergence of the grains and the size of the pores decreases, which leads to the expulsion of porous water. Fine materials converge more than coarse materials, and this process begins immediately after sedimentation, and the effectiveness of the process decreases with time. As this process progresses, the porosity decreases, for example, the porosity of clay decreases from 250 % up to 5% when it changes To Shell Rocks.

2- Cementation

During the sealing process, some minerals and materials are deposited in the pores and spaces separating the grains and work to stick the rock grains with Some of them lead to the production of a cohesive rock, and groundwater may then intervene in this process.

Such as: calcite, silica, iron oxides, gypsum, clay materials.

The strength of the resistance of the rock depends on the strength of the adhesive material. Subsequent processes such as recrystallization may further harden the rock.

The texture of clastic rocks

The texture of this type of rock depends mainly on the size of the grains Its component, as well as the shape of these granules and the way they are interconnected. These granules have been classified based on size as shown in the table below.

Table The classification of sediments and clastic sedimentary rocks according to size shows:

The name of the rock

Name of the applicant

Radius

millimeter

Conglomerate

(rounded)

Breccia

(angular)

Gravel

Boulders

bigger

Cobbles

45-256

Pebbles

Sandstone

Sand

2- /1

Mud

Silt

187/1-

Claystone or Shale

Clay

less than 256/1

Detrital sedimentary rocks may consist of grains of similar sizes, and in this case they are called well-sorted rocks. If it contains granules of different sizes, it is called bad-sorted.

The size of the granules and the way they are sorted reflect the conditions Prevailing during the sedimentation process, the shape of the grains is either with sharp edges (angular) or rounded,

and the shape of the grains depends on:

1- The distance traveled during the transfer process from the source.2- The mineral composition of these granules. Structures of clastic sedimentary rocks1- Implementation BeddingThe sedimentary rocks are deposited in the form of layers, the thickness of the layer may be 1 cm and it may reach hundreds of meters. Each layer refers to a specific period of time during which it was deposited.

The layers are also consecutive from the oldest to the most recent, separated by levels called stratification levels These levels indicate the end of the sedimentation period of a layer and the beginning of the sedimentation of the next layer, and the stratification levels constitute areas of weakness in the sedimentary rocks.

2- Mud cracks

and it consists These cracks are in fine-grained sediments (clay) as a result of shrinkage when they lose water and are often pentagonal.

3- Ripple marks

This phenomenon is observed on the coasts of the seas and the shores of the rivers, and the ripples on the coasts of the seas are symmetrical, as the water crashes into the coasts and returns again. As for river sediments, they are usually asymmetric, as the current flows in one direction, and the ancient river ripples have a scientific benefit as they show us the direction of the old current. 4- Cross-bedding

It abounds in desert sediments as well as in river sediments and results from sedimentation in one direction and then changing the direction of sedimentation, which leads to the formation of sediments with false application and this type is observed in desert sediments. )

5- Graded-bedding

It occurs when the velocity of the transporting medium suddenly decreases, which causes the deposition of large and small materials in the same place and this leads to the formation of sediments containing gradient grains of larger sizes at the bottom and smaller ones at the top.

This species is found in the seas due to the so-called jostling currents, as seen in the sedimentation of rivers and lakes.

The most important types of clastic sedimentary rocks:

1- Conglomerates

It consists of round granules with a diameter of more than 2 mm and sometimes reaching larger than

mm and bonded with weld materials such as (silica, calcium carbonate, iron oxides). And when the grains that make up this type of rock are not round and sharp, it is called breccia.

2- Sandstone

It is a clastic sedimentary rock consisting of quartz and feldspar grains that are Diameter ranges from /1-2 mm and are held together by white welding materials consisting mainly of silica and calcium carbonate

3- Alluvial stone .

It consists of quartz grains in addition to clay minerals and mica, and its diameter ranges from

/1-312/1. These crumbs are transported by rivers and running water, where they are deposited as soon as their speed calms down and may reach the flat delta areas during floods, so it will be a cover of silt grains of great importance for agricultural soil.

4- Mudstone

It is the smallest sedimentary sedimentary rock in terms of diameter, as it does not exceed 349/1 mm. The rock consists of fine grains of quartz, clay minerals (hydrated aluminum silicate) and feldspar, in addition to a small amount of mica. And if the rock is in the form of flakes generated by the pressure on it during the sedimentation process, it is called a child and is often black in color. 2- Chemical sedimentary rocks

These rocks are formed as a result of the deposition of some dissolved compounds in saturated aqueous solutions when their concentration increases by the evaporation process. Chemical sedimentary rocks may also be formed by the deposition of soluble materials, and as a result of changing the medium containing them, as it turns from acid. to alkaline and this would cause the solution to lose its dissolved compounds in the form of chemical precipitates or layers. This group is divided into:

1- Limestone

It consists mainly of calcium carbonate (calcite) or calcium and magnesium carbonate (dolomite). Limestone rocks consisting of calcite mineral are precipitated from a solution of calcium bicarbonate (a dissolved substance) by the loss of water and carbon dioxide to produce calcium carbonate (a precipitate) in the form of layers of limestone.

The shape of limestone is one of the types of chemical sedimentary rocks

Salt rocks are chemical sedimentary rocks Common

3- Silicate rocks

It is formed by precipitation of silica from solutions rich in this substance, Where this is done in the form of knots or tubercles of flint or in the form of thin layers, and is often produced by increasing the concentration of silica between the gaps and cracks of limestone rocks, so they are accompanying these rocks and overlapping between their layers.

Silicate rock shape

3- Organic sedimentary rocks

Are those rocks that were formed as a result of the accumulation of organic materials left by the vital activities of living organisms (plant or animal) in the form of layers and include the process of decomposition that occurred during deep geological times such as charring of plant and tree remains, and the decomposition of the remains of reptiles and fish And be phosphorous rock

The most important types of organic sedimentary rocks

1- Organic limestone

It is formed as a result of the accumulation of marine animal skeletons consisting of calcium carbonate such as coral and chalk, which is formed from the accumulation of foraminifera skeletons.

2- Phosphates

It is generated as a result of the accumulation and decomposition of vertebrate animal skeletons, such as fish and reptiles, resulting in the deposition of calcium phosphate and other components in the form of tubercles or lentic layers.

The shape of phosphate rocks

3- Coal

It is formed as a result of the charring processes of plants that are subjected to quick burial, which shields them from interaction with atmospheric oxygen, and so this burial works on charring them and keeping carbon, which is the major component of charcoal.