What are Sedimentary Rocks?
Sedimentary rocks are formed from the accumulation of sediments by water, ice or wind. A wide variety of live forms exist in sedimentary environments however, sedimentary rocks contain evidence of life in form of fossil. Fossil and fragments of fossil can build up to form limestone. Sedimentary rocks occur in layers or strata, they are said to be stratified rocks only when they become harder in nature by compressional forces.
All sedimentary rocks are non-crystalline and they seldom contain fossil. All rocks are due to weathering processes, these rocks are transported by water wind or ice, small fragments of rocks are easier to transport than large fragments and these fragments can be deposited on the surface to form sediments.
Sediments can form for instance on a beach, in a river, in the sea, in connection with an ice age etc. there are two main types environment in which sediments are formed: terrestrial (land) environment and under the sea or marine environment. Sediment formed at the surface of the earth occurs in layers, with the younger layers at the top, this is often seen in cliffs.
Layers of Sedimentary Rocks
Layering is also called bedding. The layers are separated from each other by bedding planes, several layers are jointly referred to as strata an alternative term for layering or stratification. Different layers shows changes in the type of material deposited and or in the condition during deposition. Depositioanl conditions change rapidly in a stream, resulting in many, thin, laterally discontinous layers of different types of clastic sediments. For example, marine condition can be stable over long period of time, giving thick seequences with constant composition like thick layers of chalk for example, the White Cliffs Of Dover.
However, sedimentary rocks cover 75% of the Earth but contribute less than 1% to its mass. Sediments are economically important to man as they contain major sources of energy such as oil, gas and coal.
Sedimentary Rocks Formation
Sedimentary rocks are types of rocks formed by weathering of rocks followed by subsequent erosion and deposition of material, other types of rocks are Igneous and metamorphic rocks. The deposited sediments can accumulate and eventually build up before being compacted and hardened over long periods by the weight and pressure of sediments above and internal chemical changes.
Rocks such as sandstone, siltstone or shale are good examples of these rocks. These rocks often contain a record of the physical conditions present when the rocks were deposited, including fossils. However, some rocks such as chalk or coal are almost entirely made from the remains of animals and plants. During the weathering processes, many of the original minerals are broken down to form clay minerals but quartz does not breakdown during this process. As they become buried, loosed sediments like sand clay and silt are consolidated and form compact rock which is sedimentary rocks. Shale is one of the examples of this rock; other sedimentary rocks are formed due to precipitation of minerals from water for example rock salt (halite). Some types of rock are more difficult to break down than other types of rock and so weathering can result in interesting landforms such as headlands or areas of protruding rock in an otherwise flat landscape. Example of this is the Ayers Rock now known by its Aborigine name Uluru in central Australia where one type of rock wears down more quickly than the other in the vicinity.
Types of Sedimentary Rocks
There are four types of sedimentary rocks
- Clastic sedimentary rocks (detrital sedimentary rocks)
- Organic sedimentary rocks
- Biochemical sedimentary rocks
- Chemical sedimentary rocks
Clastic sedimentary Rocks (detrital sedimentary rocks)
As rock and mineral fragment are being transported from their original source, they become smaller and change shape. The sediments that are deposited near the source will be different from those deposited afar off after transportation. Clastic sedimentary rocks that are deposited near to its source will have angular, large fragments, this large fragment (clasts) will occur in ground mass or matrix, having smaller fragments. Example of this rock is breccia. During the transport process, the angular fragments becomes rounded in shape The fragment may consist of Igneous rock, sedimentary rock and metamorphic rockor a combination of all depending on the nature of rock type that was eroded. A clastic sedimentary rock with large rounded fragments in a finer grain matrix is called conglomerate for instance in breccia, the nature of large particles in a conglomerates show the type of rock that was eroded. Conglomeratesare dominantly composed of roundedgravel, whilebreccia is composed of dominantly angulargravel.) However, a loosed detritus becomes sedimentary rocks due to different factors such as weathering, transportation, deposition and lithification or consolidation.
Biochemically Sedimentary rocks
In biochemically formed sedimentary rock, organisms play an important role; many organisms have shells of calcite or it polymorph aragonites (CaCO3) while others have shells of silica (SiO2). When these organisms die, their shells are then accumulated to form biochemical sedimentary rocks, the soft part of the organism rots away or turns into oil. Some of the notable rocks form from the remains of animals include chalk and corals, however, plant can also contribute as an organic component to sediments, those that result from plants include peat, coal and lignite. The environment around coral reef is mostly rich with organisms such as algae, oysters, clams, snails and corals and all of these have shells of calcite or aragonite. When they die, the skeletons remain on the same position or are transported away. During the transport, the shells may break into smaller fragments. When this material is deposited, it forms a calcium carbonate, a rich sediment (limestone) or a biochemical sediment since it is majorly composed of remains of organisms. Limestones are sometimes formed in environment where clay is deposited simultaneously which gives rise to a type of rock called Marl (a mixture of limestone and clay).
Chemically sedimentary rock
These are rocks which are form as a result of chemical composition. There are three types of chemical sediments namely evaporites, travertine, dolomite and chert.
- Evaporite- this is formed by the evaporation of salt water, the evaporation of salt water leaves a residue behind. For salt water to evaporates, it needs a warm climate closed system for example a salt lake with no outlet. Salt water contains many other ions in solution and not just Ca+ and Na– There are different minerals formed in a regular sequence during the evaporation of salt water. The first mineral to form is gypsum (CaSO4 2H2O)) when about 80% of the water has evaporated, followed by halite (NaCL), this is when about 90% has evaporated. After this a sequence of relatively rare evaporate minerals may form including the potassium equivalent of slat, Sylvite (KCl).
- Travertine- limestone can be formed by the accumulation of biochemical materials, it can also be formed by direct precipitation from water without organism being involved. This chemical variety of limestone is called travertine. Water most especailly acidic water can dissolve calcite in limestone, the carbonate material is however, commonly precippited again often in limestone caves ( as stalagamites and stalacites etc.) or around hot springs travertine which is usually baded and beige in colour is widely used as facing stone.
- Dolomite– the term dolomite is used for both the mineral (CaMgCO2)3 and the rock that is formed. dolomite is a carbonate mineral in which half the calcium in calcite replaced magnesium. Dolomite is formed as a result of reaction between calcite and magnesium bearing groundwater. Calcite can become pertially replaced by dolomite, the replacement can take place after the formation of the limestone. However, chert is an extremely fine grained (crytocrystalline) variety of quartz. Black chert is called flint. Chert or flint are very fine-grained and almost glassy with a choncoidal fracture.
Organic sedimentary rocks
Coal is an example of organically formed sedimentary rocks, it is formed from the remain of plants that grew in the forests or swamps, the plants remains are buried in the soil after being subjected under harsh temperature and pressure, they are converted to a black, combustible rock known as coal which consists of >50% of carbon. However, plankton float in the water, but the soft part could mix with mud on the sea floor to form shale, this organic material is gradually converted in oil, changes the shale to black color. These types of shale are called oil shale.
Sedimentary rocks examples
We are going to look at the examples of sedimentary rocks based on the types of sedimentary rockss
Examples of Clastic sedimentary rocks
- Sand stone
Conglomerate- thisis a clastic sedimentary rock made of rounded pebbles greater than two millimeters in diameter cemented together. The space between the pebbles is generally filled with smaller particles and/or chemical cement that bind the rock together.
Breccia-is a clastic sedimentary rock that is made of large angular fragments, this isformed when grains are transported over short distances and then are deposit in valleys or streams. They may be transported by mudslides, avalanches or water. Breccia is characterized by the shape of the fragments of boulders, pebbles and other rocks, which are angular as opposed to rounded. The fragments are also typically greater than 2mm and may vary in color; the spaces between the large fragments can be filled with a matrix of smaller particles or mineral cement which binds the rock together.
Sandstone- sandstone is a clastic sedimentary rock that contains mainly of sand-size (1/6 – 2 millimeter in diameter) and weathering debris. Sand can accumulate in various environments like beaches, deserts, flood plains, and deltas.
Siltstone– Siltstoneis a clastic sedimentary rock that forms from silt-size (between 1/256 and 1/16 millimeter in diameter) weathering debris. It is made from silt particles cemented together. Pure siltstone is made of very fine dusty grains, but it may contain significant amount of sand and clay. Loess for example is a variety of wind siltstone.
Shale- is a clastic sedimentary rock that is made up of clay-size (less than 1/256 millimeter in diameter) weathering debris. It typically breaks into thin flat pieces.
Examples of chemical sedimentary rocks
- Iron Ore
- Halite or rock salt
Chert– Chert is an extremely fine grained (crytocrystalline) variety of quartz. Black chert is called flint, the color may also be dark gray or tan as well. Chert or flint are very fine-grained and almost glassy with a choncoidal fracture. It is made during the chemical changes that occur during the formation of sedimentary rock. It can be found as nodules in chalk or some marine limestone.
Limestone- Limestone is typically found in waters that are shallow and warm. Common areas where it can be found include the Persian Gulf, the Caribbean Sea, Gulf of Mexico, the Indian archipelago and the Pacific Ocean Islands. It can be chemical or organic. When organic, it has formed from organic materials such as coral, shell and fecal debris. When limestone is a chemical sedimentary rock, it is formed from the carbonate in the lake or ocean. It is mostly used in the production of cement.
Selenite- This is a common chemical sedimentary mineral made of calcium sulfate. Seleniteis an evaporate rock which is one of the three types of chemical sedimentary rocks.Satin spar and alabaster are also common forms. The other two are calledCarbonate rocks, andsiliceous rocks, it has many forms and colors.
Iron Ore- iron Ore is a chemical sedimentary rock formed when iron and oxygen (and sometimes-other substances) are combined together in the marine and fresh water and deposit as sediment. The important iron oxides in these deposits are hematite and magnetite. These are ores from where iron is extracted. Iron ores vary when it comes to colors, some of them are dark gray while others may be rust colored, purple, or yellow.
Halite or rock salt– is a chemical sedimentary rock; it is formed when a body of seawater becomes closed off and evaporates. As the water evaporates, the remaining water can no longer hold the same amount of salt. The salt precipitates out and then it is deposited as crystallized halite or rock salt. Some halite are processed for use as a seasoning for food and it is mostly colorless, but sometimes appears blue, gray, or even an orange like in color with a very salty taste.
Dolomite– dolomite is also known as “dolostone” and “dolomite rock”). Dolomite is a chemical sedimentary rock that is very similar tolimestone. It is formed when limestone or lime mud is modified by magnesium-rich ground water. Dolomiteis a carbonate of calcium and magnesium. It has fine crystalline structure with high brightness. Dolomite is mostly used steel plant, ceramic tiles, plastics and paper. It has very good exterior durability and so it has good application in paints.
Examples of organic sedimentary rocks
- Oil shale
- Fossiliferous limestone
Oil shale- oil shale is anorganic-richfine-grained sedimentary rock containing kerogen (a solid mixture of organic chemical compounds) from whichliquid hydrocarbons, called shale oil (but not same as tight oilcrude oil occurring naturally in shales).
Coal- coal is an organic sedimentary rock that forms mainly from plantdebris or organic matter. The plantdebrisusually settles in a swampy environment. Coal iscombustibleand is often mined for use as a fuel.
Fossiliferous limestone – this is a type of limestone, made mostly of calcium carbonate in the form of the minerals calcite or aragonite, which contains an abundance of fossils or fossil traces. The fossils in these rocks may be of macroscopic or microscopic size.
Characteristics of Sedimentary Rocks
- Sedimentary rocks are resistant to erosion.
- They occur in layer or strata.
- They contain fossils of plants and animals.
- Sedimentary rocks are not found in massive forms such as batholiths, laccoliths, dykesas in the case of igneous rock.
- They are non-crystalline in nature.
- The deposition of sediments of various types and sizes to form sedimentary rocks takes place in certain sequence and system.
- Sedimentary rocks cover over 75% of the earth surface, while the remaining 25% are taken over by igneous rocks and metamorphic rocks.
- Sedimentary rocks may be well consolidated, poorly consolidated and even unconsolidated. The composition of the rocks depends on the nature of cementing elements and rock forming minerals.
- Most of the sedimentary rocks are permeable and porous but a few of them are also non-porous and impermeable. The porosity of the rocks depends upon the ratio between the voids and the volume of a given rocks mass.
- Sedimentary rocks are characterized by different sizes of joints. These are generally perpendicular to the bedding planes.
Where are Sedimentary Rocks Found?
Sedimentary rocks can be found in almost every area of the world and almost any climate and continent, from the bottom of the ocean to the desert. Some of the common place you can find sedimentary rocks in the world are:
- Terrestrial environments
- Glacial environment
- Mountain stream environment
- Mountain front environments
- Desert environment
- Fluvial environment
- Delta environment
- Deep ocean environment
- Marine environment
- Coastal environment