Sedimentary Rock Formation Classification Clastic Sedimentary

Sedimentary rock is one of the three main rock groups (along with igneous and metamorphic rocks) and is formed in three main ways—by the deposition of the weathered remains of other rocks (known as clastic sedimentary rocks); by the deposition of the results of biogenic activity; and by precipitation from solution.

1 Formation

Sedimentary rocks are formed from overburden pressure as particles of sediment are deposited out of air, ice, or water flows carrying the particles in suspension. As sediment deposition builds up, the overburden (or lithostatic) pressure squeezes the sediment into layered solids in a process known as lithification ("rock formation") and the original connate fluids are expelled. The term diagenesisIn geology, diagenesis refers to all the chemical, physical, and biological changes undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering). These changes happen at relative is used to describe all the chemical, physical, and biological changes undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering).

Sedimentary rocks contain important information about the history of the EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla. They contain fossilFor other uses of the term, see Fossil (disambiguation Fossils are the mineralized remains of animals or plants or other artifacts such as footprints. The totality of fossils and their placement in rock formations and sedimentary layers is known as the fos, the preserved remains of ancient plantGreen algae land plants (embryophytes non-vascular embryophytes Hepatophyta liverworts Anthocerophyta hornworts Bryophyta mosses vascular plants (tracheophytes seedless vascular plants Lycopodiophyta clubmosses Equisetophyta horsetails Pteridophyta "true"s and animalSubkingdom Parazoa Porifera (sponges) Subkingdom " Agnotozoa" Placozoa Orthonectida Rhombozoa Subkingdom Metazoa "Radiata" Cnidaria Ctenophora (comb jellies) Bilateria Protostomia Acoelomorpha Platyhelminthes (flatworms) Nemertina (ribbon worms) Gastrotris. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment which have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remnants.

The sedimentary rock cover of the continents of the Earth's crustIn geology, a crust is the outer layer of a planet, part of its lithosphere. Planetary crusts are generally composed of a less dense material than that of its deeper layers. On partially-molten planets, such as Earth, the lithosphere is floating on fluid is extensive, but the total contribution of sedimentary rocks is estimated to be only five percent of the total. As such, the sedimentary sequences we see represent only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.

2 Classification

2.1 Clastic sedimentary rocks

Clastic sedimentary rocks are composed of discrete clast s of materials derived from other rocks. They are composed largely of silica (i.e. quartzFor other uses of this word, see Quartz (disambiguation). Milk quartz rock Quartz is the most abundant mineral on Earth (about 12% vol. It has a hexagonal crystal structure made of trigonal-crystallized silica (silicon dioxide, SiO), with a hardness of 7), with other common minerals including feldspars, amphiboles, clay minerals and sometimes more exotic igneous minerals.

Clastic sedimentary rocks may be regarded as falling along a scale of grain size, with shale being the finest, siltstone being intermediate, and sandstone being coarser still, and conglomerates being the coarsest.

The classification of clastic sedimentary rocks is complex because there are many variables involved. Particle size (both the average size and range of sizes of the particles), composition of the particles, the cement, and the matrix (the name given to the smaller particles present in the spaces between larger grains) must all be taken into consideration.

Shales, which consist of very fine grains of quartz and feldspar, are generally further classified on the basis of composition and bedding.

Courser clastic sedimentary rocks are classified according to their particle size and composition. Orthoquartzite is a very pure quartz sandstone; arkose is a sandstone with quartz and abundant feldspar; greywacke is a sandstone with quartz, clay, feldspar, and metamorphic rock fragments present.

All rock disintegrate slowly as a result of mechanical weathering and chemical weathering.

Mechanical weathering is the breakdown of rock into particles without producing changes in the chemical composition of the minerals in the rock. Ice is the most important agent of mechanical weathering. Water percolates into cracks and fissures within the rock, freezes, and expands. The force exerted by the expansion is sufficient to widen cracks and break off pieces of rock. Heating and cooling of the rock, and the resulting expansion and contraction, also aids the process. Mechanical weathering contributes further to the breakdown of rock by increasing the surface area exposed to chemical agents.

Chemical weathering is the breakdown of rock by chemical reaction. In this process the minerals within the rock are changed into particles that can be easily carried away. Air and water are both involved in many complex chemical reactions. The minerals in igneous rocks may be unstable under normal atmospheric conditions, those formed at higher temperatures being more readily attacked than those which formed at lower temperatures. Igneous rocks are commonly attacked by water, particularly acid or alkaline solutions, and all of the common igneous rock forming minerals (with the exception of quartz which is very resistant) are changed in this way into clay minerals and chemicals in solution.

Rock particles in the form of clay, silt, sand, and gravel, are transported by the agents of erosion (usually water, and less frequently by ice and wind) to new locations and redeposited in layers, generally at a lower elevation.

These agents reduce the size of the particles and then deposit them in new locations. The sediments dropped by streams and rivers form alluvial fans, flood plains, and deltas, as well as contributing to the deposits on the bottom of lakes and the sea floor. The wind may move large amounts of sand and other smaller particles. Glaciers transport and deposit great quantities of rock material.

These deposited particles eventually become compacted and cemented together, forming clastic sedimentary rocks. Such rocks contain inert minerals which are resistant to mechanical and chemical breakdown such as quartz, zircon, rutile, and magnetite. Quartz is one of the most mechanically and chemically resistant minerals.

2.2 Biogenic sedimentary rocks

Biogenic sedimentary rocks contain materials generated by living organisms, and include carbonate minerals created by organisms, such as corals, molluscs, and foraminifera, which cover the ocean floor with layers of calcite which can later form limestone. Other examples include stromatolites, and the flint nodules found in chalk (which is itself a biogenic sedimentary rock, a form of limestone).

2.3 Precipitate sedimentary rocks

Precipitate sedimentary rocks form when mineral solutions, such as sea water, evaporate. Examples include the evaporite minerals halite and gypsum.

3 Other information

Sedimentary rocks are economically important in that they can be used as construction material. In addition, sedimentary rocks often form porous and permeable reservoirs in sedimentary basins in which petroleum and other hydrocarbons can be found.

It is believed that the relatively low levels of carbon dioxide in the Earth's atmosphere, in comparison to that of Venus, is due to large amounts of carbon being trapped in limestone and dolomite sedimentary layers. The flux of carbon from eroded sediments to marine deposits is known as the carbon cycle.

The shape of the particles in sedimentary rocks has an important effect on the ability of micro-organisms to colonize them. This interaction is studied in the science of geomicrobiology. One measure of the shape of these particles is the roundness factor, also known as the Krumbein number after the geologist W. C. Krumbein .

4 See also

Petrology Sedimentary rocks

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