Sedimentology

Sedimentology is the scientific study of sedimentary rocks and the processes by which they were formed. It is concerned with the composition and genesis of sediments and sedimentary rocks, and the creation of predictive models that resemble and predict sedimentation. It's used to describe sedimentation processes by interpreting the facies in order to know the characteristics, methodology and principles of sedimentation. In addition, sedimentology is related to straigraphy which is the study of the physical relationships of sedimentary layers. The study of sedimentoloy and stratigraphy together gives clearer vision about the layers and provides us with any information we need.  

Sedimentary processes

Sedimentary processes lead to the development of sediments and they consist of 3 main processes:

• Physical processes (weathering and erosion)
• Chemical processes (precipitation of chemical compounds in water solutions)
• Biological processes

These 3 processes are important parts of the development of layers. Weathering and erosion are related to sedimentation processes through the material they provide. This material is transported to deposition place and imbedded there. Diagenesis processes transform sediments into sedimentary rocks due to the physical and chemical changes. The accumulation of these sedimentary rocks is subjected to temperature and pressure which take place in the chemical processes affecting the rocks. Then, they are subjected to biological processes which affect the growth of the rocks.

Environments and facies

Overview 

Facies of sedimentary rocks means that every rock has special specifications in which we can know the history by which it's formed. Consequently, engineers and geologists can use the history of these rocks in correlating and in predicting the existence of minerals. There are many factors that help in knowing the facies of rocks and the most three important of them are:

1. Minerology 
2. Grain Size and Sorting
3. Angularity

There are three sections that geologists use in facies identification: 

1. Facies Analysis (Intrepreting the layers of rock in the ground)
2. Facies Association (Establishing several facies according to their combinations)
3. Facies Successions (Facies associations with unique characteristics)

Spectrum of environments and facies  

The discription of sedimentary rocks through their facies and enviroments results in grouping every similar rocks to be unique and different from others, and hence, we can categorize the sedimentary rocks to make the process easier.   

Classification of Sediments and Sedimentary Rocks  

Sedimentary rocks can be classified into two main groups based on the processes by which they were derived Clastic and Non Clastic rocks.

Clastic rocks (>80%)

They are particles derived from the weathering and erosion of precursor rocks and consist primarily of fragmental material and are classified by grain size and composition. Clastic rocks are the most common  (i.e. mudstones (65%) and sandstones (20-25%)). They are classified into: 

1. Volcaniclastic 
2. Terrigenous clastic
3. Carbonates 

Non clastic rocks

Non clastic represents about 10 - 15% of sedimentary rocks.They are classified into:
 

1. Carbonates (in common with clastic rocks) 
2. Others (i.e coal) 
3. Evaporites (chemical depositions) 

Sedimentation principles, methodology and characteristics   

Principles of sedimentation

Principles of sediments is used to know more information about the characteristics of sedimentary rocks that are deposited in basins. Principles of Sedimentary rocks can be classified into:

Principle of superposition

Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top.

Principle of original horizontality

The deposition of sediments in a horizontal way which is their relief angle.

Principle of lateral continuity

The layers of sediments extend in all directions If there is no any restrictions on their ways.

Principle of cross-cutting relationships

Any intrusive/cutting rock or layer is younger than the layers of sediments.

Methodology of sedimentation

Methodology of identifying the sedimentation of rocks is based on collecting all possible information from the rocks including:

• Measuring and describing the outcrop and distribution of the rock unit
• Describing rock core
• Identifying sequence stratigraphy characteristics (Describes the progression of rock units within a basin )
• Describing the lithology of the rock  

Characteristics of sedimentation 

Characteristics of sedimentary rocks are very useful for identifying sedimentation methodology as it is possible to get more information about rocks. Examples of the characteristics include:

• Porosity is the volume of voids within a rock which can contains fluids.
• Permeability is the ability of water or other liquids to pass freely through a rock.
• Roundness indicates the roughness of sedimentary grains' surfaces.
• Sorting indicates to the range of particle sizes in the sediments or sedimentary rock.
• Matrix is a fine-grained material that's deposited originally with coarser-grained minerals.

Conclusion 

The world is always increasingly in need of more characteristics of rocks to know the exact place of oil and gas which are used in our daily lives. This can't be achieved if we don't focus on Sedimentology and widen this science. Sedimentology helps us know how the underground looked like in the past and the processes developed to reach its shape today and how we can get benefit from the sedimentary layers.