Sedimentary Rocks
SEDIMENTARY ROCK
When disintegrated products of preexisting rocks are transported and deposited they get lithified forming sedimentary rocks.
Weathering
The process by which rocks and minerals break down at or near the earth’s surface.
Types Of Weathering
Physical Weathering: In this type of weathering rocks or minerals break into smaller pieces.
Frost Wedging: Water expands in volume by about 9% when it freezes. In case when water enters and freezes into pores or cracks of rock it exerts a great force in surrounding areas. This causes loosening or dislodging fragments of rocks.
Thermal Expansion & Contraction: Rock surface that are exposed to high daytime and low nighttime temperatures their components minerals expands with repeated heating an contract upon cooling, which should eventually cause the rock’s outer layer to break apart.
Mechanical Exfoliation: When erosion of overlying rock and soil exposes a wide area of a large plutonic mass, pressure on the mass is reduced and it expands. As the rock in the structure expands, it fractures into sheets parallel to its exposed surface.
Chemical Weathering: In this type of weathering changes in chemical composition of minerals or rocks takes place.
Dissolution: Process of removal of ion or ion groups by water, layer this water eventually evaporates leaving behind the dissolved substances.
Oxidations: In oxidation, a mineral’s ion combines with oxygen to form an oxide.
Hydrolysis: In hydrolysis, H positive and OH positive ions from water molecules displace other ions from a mineral’s structure, forming a different mineral.
Carbonation: The reaction with atmospheric carbon dioxide to form a carbonate.
Products Of Weathering
Sediments (from Latin sedimentum, meaning “setting”): fragment of solid material.
Detrital Sediment: Composed of transported solid fragments.
Chemical Sediment: When dissolved minerals either precipitate from solution or are extracted from water by living organisms and converted to sells, skeletons, or other organic substance.
The process by which moving water, wind, ice or gravity carries pieces of rock and deposits them elsewhere.
Composition
When transporting media is unable to carry on transporting weathered product, it deposits them at suitable places.
Diagenesis
Heat, pressure and the ions alter the physical and chemical nature of both detrital and chemical sediments by a set of processes known collectively as diagenesis.
Lithification OR Consolidation:
Lithification (Greek lithos, meaning rock, and latin facere, meaning to make) process of conversion of loose sediment into solid sedimentary rock.
1 – Compaction & Dehydration
The process by which pressure reduces the volume of sediments by squeezing out excess water which increased the cohesion of sediments.
2 – Cementation
Material originally dissolved during chemical weathering precipitate from water circulating through sediment, creating a chemical cement that binds the sediment grains together.
Sediment Texture
Grain Size: Different rocks produce grains of different size, shapes and resistance to weathering.
Grain Shape: The more vigorous collision a particle experiences and the father it moves away from its parent rock, the more rounded it becomes.
Sorting: The process by which a transport medium selects particles of different sizes, shapes or densities.
Sedimentary Structure
Physical features that reveal the conditions under which sediment deposition occurred.
1 – Stratification
The deposition of sediments into layers or beds is called stratification.
2 – Lamination
Thin bedding, less than one centimeter in thickness are called lamination.
3 – Grade Bedding
During deposition when particles settle at different rates, depending on their sizes, densities and shapes, they produce beds of coarse particles at bottom and fine particles at top.
4 – Cross Bedding
It is consist of sedimentary layers deposited at an angle to the underlying set of beds.
Ripple marks are the symmetrical or unsymmetrical undulations which may be seen on the surface of some sedimentary deposit. Produced by waves and currents in shallow water.
Classification Of Sedimentary Rocks
On the basis of mode of origin.
1 – Detrital / Clastic Sedimentary RockRocks that consists of preexisting solid particles compacted and cemented together, depending upon the particles size they are:
Mudstone: Rocks with grain size less than 1/256 mm to diameter lacking any fissilty in their structure.
Shale: Rock with grain size less than 1/256 mm in diameter with fissile structure, which is their ability to split into very thin parallel surfaces.
Siltstone: Detrital sedimentary rock with grain size ranging from 1/256-1/16 mm in diameter.
Sandstone: Detrital sedimentary rocks with grain size range from 1/16 mm to 2 mm in diameter.
Conglomerate: Detrital sedimentary rock with grain size larger than 2 mm in diameter, the larger grains are rounded and embedded in fine grained ground mass.
Breccia: Detrital sedimentary rock with grain size larger than 2 mm in diameter, the larger grains are angular and embedded in fine grained matrix.
2 – Chemical Sedimentary Rock
Rock formed when dissolved minerals either precipitate or evaporated from solution.
Limestone: Composed largely of calcium carbonate, forms by the process of precipitation.
Evaporite (Rock Salt, Gypsum): Formed when salty water evaporates.
Chert: Consist largely of silicate, formed by the process of precipitation.
Organic Sedimentary Rock
Formed by accumulation of plant remains.
Calcareous Rock
Limestone
Coal Steam