sedimentary rocks
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Sedimentary rocks are types of
rock Rock most often refers to: * Rock (geology) In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical compos ...
that are formed by the accumulation or deposition of
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
or organic particles at
Earth's surface Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be found throughout the Solar System, only water distributi ...
, followed by cementation.
Sedimentation Sedimentation is the deposition of sediments. It takes place when particle (ecology), particles in Suspension (chemistry), suspension settle out of the fluid in which they are Entrainment (engineering), entrained and come to rest against a barr ...
is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...
, and may be composed of geological detritus (minerals) or biological detritus (organic matter). The geological detritus originated from
weathering Weathering is the deterioration of Rock (geology), rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little o ...
and
erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust, and then sediment transport, tra ...
of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of
denudation Denudation is the geological Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they ch ...
. Biological detritus was formed by bodies and parts (mainly shells) of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies (
marine snow In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to ...
). Sedimentation may also occur as dissolved minerals precipitate from
water solution An aqueous solution is a Solution (chemistry), solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), ...
. The sedimentary rock cover of the continents of the
Earth's crust Earth's crust is Earth's thin outer shell of Rock (geology), rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the Crust (geology), crust and the ...
is extensive (73% of the Earth's current land surface), but sedimentary rock is estimated to be only 8% of the volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of
igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
and
metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, cau ...
s. Sedimentary rocks are deposited in layers as
strata In geology and related fields, a stratum (plural, : strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separate ...
, forming a structure called
bedding Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environme ...
. Sedimentary rocks are often deposited in large structures called
sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...
s. Sedimentary rocks have also been found on
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin atmosph ...
. The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for
civil engineering Civil engineering is a Regulation and licensure in engineering, professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads ...
, for example in the construction of
road A road is a linear way for the conveyance of traffic that mostly has an road surface, improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are ro ...
s,
house A house is a single-unit residential building. It may range in complexity from a rudimentary hut to a complex structure of wood, masonry, concrete or other material, outfitted with plumbing, electrical, and heating, ventilation, and air condit ...
s,
tunnel A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A Pipeline transport, pipeline is not a tunnel, though some recent tunnels have used ...
s,
canal Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface flo ...
s or other structures. Sedimentary rocks are also important sources of
natural resource Natural resources are resources that are drawn from nature and used with few modifications. This includes the sources of valued characteristics such as commercial and industrial use, aesthetic value, scientific interest and cultural value. O ...
s including
coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen ...
,
fossil fuel A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and combustion, burned as a fuel. The main fossil fuels are coal, petroleum, oil, and natura ...
s,
drinking water Drinking water is water that is used in drinking, drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity ...
and
ore Ore is natural Rock (geology), rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Ret ...
s. The study of the sequence of sedimentary rock strata is the main source for an understanding of the
Earth's history The history of Earth concerns the development of planet A planet is a large, rounded Astronomical object, astronomical body that is neither a star nor its Stellar remnant, remnant. The best available theory of planet formation is the nebu ...
, including
palaeogeography Palaeogeography (or paleogeography) is the study of historical geography Historical geography is the branch of geography that studies the ways in which geographic phenomena have changed over time. It is a synthesizing discipline which share ...
,
paleoclimatology Paleoclimatology (American and British English spelling differences, British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the ...
and the
history of life The history of life on Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be found throughout the ...
. The
scientific discipline The branches of science, also referred to as Science, sciences, scientific fields or scientific disciplines, are commonly divided into three major groups: *Formal sciences: the study of formal systems, such as those under the branches of logic ...
that studies the properties and origin of sedimentary rocks is called
sedimentology Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation ( erosion and weathering), transport, deposition and diagenesis. Sedimentologists apply their understandi ...
. Sedimentology is part of both
geology Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. Modern geology ...
and
physical geography Physical geography (also known as physiography) is one of the three main branches of geography Geography (from Ancient Greek, Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘Graphien’ (to describe), ...
and overlaps partly with other disciplines in the
Earth science Earth science or geoscience includes all fields of natural science Natural science is one of the branches of science concerned with the description, understanding and prediction of Nature, natural Phenomenon, phenomena, based on empirical evid ...
s, such as
pedology Pedology (from Greek: πέδον, ''pedon'', "soil"; and λόγος, ''logos'', "study") is a discipline within soil science which focuses on understanding and characterizing soil formation, evolution, and the theoretical frameworks for modeling ...
,
geomorphology Geomorphology (from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following ...
,
geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
and
structural geology Structural geology is the study of the three-dimensional distribution of Rock (geology), rock units with respect to their Deformation (geology), deformational histories. The primary goal of structural geology is to use measurements of present-day ...
.


Classification based on origin

Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical (biogenic) sedimentary rocks, chemical sedimentary rocks, and a fourth category for "other" sedimentary rocks formed by impacts,
volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evi ...
, and other minor processes.


Clastic sedimentary rocks

Clastic sedimentary rocks are composed of rock fragments (''clasts'') that have been cemented together. The clasts are commonly individual grains of
quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra ...
,
feldspar Feldspars are a group of rock-forming aluminium Aluminium (aluminum in American and Canadian English) is a chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, ...
,
clay minerals Clay minerals are Hydrate, hydrous aluminium phyllosilicates (e.g. kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, Alkaline earth metal, alkaline earths, and other ...
, or
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect Cleavage (crystal), basa ...
. However, any type of mineral may be present. Clasts may also be ''lithic fragments'' composed of more than one mineral. Clastic sedimentary rocks are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions:
gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...
(>2 mm diameter),
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...
(1/16 to 2 mm diameter), and
mud A MUD (; originally multi-user dungeon, with later variants multi-user dimension and multi-user domain) is a multiplayer A multiplayer video game is a video game in which more than one person can play in the same game environment at the same ...
(<1/16 mm diameter). Mud is further divided into
silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...
(1/16 to 1/256 mm diameter) and
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4). Clays develop plasticity (physics), plasticity when wet, du ...
(<1/256 mm diameter). The classification of clastic sedimentary rocks parallels this scheme; conglomerates and
breccia Breccia () is a rock composed of large angular broken fragments of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a speci ...
s are made mostly of gravel,
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. ...
s are made mostly of
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...
, and
mudrock Mudrocks are a class of fine-grained Sedimentary rock#Clastic sedimentary rocks, siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone ...
s are made mostly of mud. This tripartite subdivision is mirrored by the broad categories of
rudite Rudite is a general name used for a sedimentary rock composed of rounded or angular Detritus, detrital grains, i.e. Granule (geology), granules, pebbles, Cobble_(geology), cobbles, and boulders, which are coarser than sand in Particle size (grain si ...
s,
arenite Arenite (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day ...
s, and
lutite Lutite is old terminology, which is not widely used, by Earth scientists in field descriptions for fine-grained, Sedimentary rock, sedimentary rocks, which are composed of silt-size sediment, clay-size sediment, or a mixture of both. When mixed wi ...
s, respectively, in older literature. The subdivision of these three broad categories is based on differences in clast shape (conglomerates and breccias), composition (sandstones), or grain size or texture (mudrocks).


Conglomerates and breccias

Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel.


Sandstones

Sandstone classification schemes vary widely, but most geologists have adopted the Dott scheme, which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of a muddy matrix between the larger grains. :''Composition of framework grains'' :The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. ::* Quartz sandstones have >90% quartz grains ::* Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains ::* Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grains :''Abundance of muddy matrix material between sand grains'' ::When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud (silt and/or clay sized particle). ::* "Clean" sandstones with open pore space (that may later be filled with matrix material) are called arenites. ::* Muddy sandstones with abundant (>10%) muddy matrix are called wackes. Six sandstone names are possible using the descriptors for grain composition (quartz-, feldspathic-, and lithic-) and the amount of matrix (wacke or arenite). For example, a quartz arenite would be composed of mostly (>90%) quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc. Although the Dott classification scheme is widely used by sedimentologists, common names like
greywacke Greywacke or graywacke (German language, German ''grauwacke'', signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock ...
,
arkose Arkose () or arkosic sandstone is a detrital sedimentary rock Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's su ...
, and quartz sandstone are still widely used by non-specialists and in popular literature.


Mudrocks

Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles. These relatively fine-grained particles are commonly transported by
turbulent flow In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by Chaos theory, chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disrup ...
in water or air, and deposited as the flow calms and the particles settle out of
suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology) In topology, a branch of mathematics, the suspension of a topological space ''X'' is intuitively obtained by stretching ''X'' into a cylinder (geometry), cylind ...
. Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud. Mudrocks can be divided into siltstones, composed dominantly of silt-sized particles; mudstones with subequal mixture of silt- and clay-sized particles; and claystones, composed mostly of clay-sized particles. Most authors use "
shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments ( silt-sized particles) of other minerals, espec ...
" as a term for a
fissile In nuclear engineering, fissile material is material capable of sustaining a nuclear fission Nuclear fission is a nuclear reaction, reaction in which the atomic nucleus, nucleus of an atom splits into two or more smaller atomic nucleu ...
mudrock (regardless of grain size) although some older literature uses the term "shale" as a synonym for mudrock.


Biochemical sedimentary rocks

Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include: * Most types of
limestone Limestone (calcium carbonate ) is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime_(material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphis ...
are formed from the calcareous skeletons of organisms such as
coral Corals are marine invertebrates within the class (biology), class Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important C ...
s,
mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...
s, and
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class (biology), class of Amoeba, amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm ...
. *
Coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen ...
, formed from plants that have removed
carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—its atom making four electrons available to form covalent bond, covalent chemical bonds. It belongs to gro ...
from the atmosphere and combined it with other elements to build their tissue. * Deposits of
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of S ...
formed from the accumulation of siliceous skeletons of microscopic organisms such as
radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell (biology), cell into the inner and outer portions of endoplasm and Ecto ...
and
diatom A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...
s.


Chemical sedimentary rocks

Chemical sedimentary rock forms when mineral constituents in solution become
supersaturated In physical chemistry, supersaturation occurs with a solution (chemistry), solution when the concentration of a solute exceeds the concentration specified by the value of solubility at Solubility equilibrium, equilibrium. Most commonly the term ...
and inorganically
precipitate In an aqueous solution, precipitation is the process of transforming a dissolved chemical substance, substance into an insoluble solid from a Supersaturated solution, super-saturated solution. The solid formed is called the precipitate. In cas ...
. Common chemical sedimentary rocks include
oolitic limestone Oolite or oölite (''egg stone'') is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Ancient Greek word for egg (biology), egg (ᾠόν). Strictly, oolites consist of ooids of dia ...
and rocks composed of
evaporite An evaporite () is a water-solubility, soluble sedimentation, sedimentary mineral deposition (geology), deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposit ...
minerals, such as
halite Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride (sodium, Nachlorine, Cl). Halite forms Cubic (crystal system), isometric crystals. The mineral is typically colorless or white, but may al ...
(rock salt),
sylvite Sylvite, or sylvine, is potassium chloride (KCl) in natural mineral form. It forms crystals in the Cubic (crystal system), isometric system very similar to normal rock salt, halite (sodium, NaCl). The two are, in fact, Isomorphism (crystallograph ...
,
baryte Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate (barium, Basulfur, Soxygen, O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (m ...
and
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and ...
.


Other sedimentary rocks

This fourth miscellaneous category includes volcanic
tuff Tuff is a type of Rock (geology), rock made of volcanic ash ejected from a Volcano, vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is conside ...
and volcanic breccias formed by deposition and later cementation of lava fragments erupted by volcanoes, and impact breccias formed after impact events. File:PIA17603 Erosion by Scarp Retreat in Gale Crater, Annotated Version.jpg, Sedimentary rocks on Mars, investigated by NASA's Curiosity Mars rover File:Chalus road - ninara 02.jpg, Steeply dipping sedimentary rock strata along the Chalous Road in northern Iran File:Entropy in Nature.jpg, Stratified remains of Puʻu Mahana cinder cone.


Classification based on composition

Alternatively, sedimentary rocks can be subdivided into compositional groups based on their mineralogy: * Siliciclastic sedimentary rocks, are dominantly composed of
silicate minerals Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...
. The sediment that makes up these rocks was transported as
bed load The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the stream bed. Bed load is complementary to suspended load and wash load. Bed load moves by rolling, sliding, and/or Saltation (geolo ...
,
suspended load The suspended load of a flow of fluid, such as a river, is the portion of its sediment uplifted by the fluid's flow in the process of sediment transportation. It is kept suspended by the fluid's turbulence. The suspended load generally consists of ...
, or by sediment gravity flows. Siliciclastic sedimentary rocks are subdivided into conglomerates and
breccia Breccia () is a rock composed of large angular broken fragments of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a speci ...
s,
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. ...
, and
mudrock Mudrocks are a class of fine-grained Sedimentary rock#Clastic sedimentary rocks, siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone ...
s. * Carbonate sedimentary rocks are composed of
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
(rhombohedral ), aragonite (orthorhombic ), dolomite (), and other carbonate minerals based on the ion. Common examples include
limestone Limestone (calcium carbonate ) is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime_(material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphis ...
and the rock
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
. * Evaporite sedimentary rocks are composed of minerals formed from the evaporation of water. The most common evaporite minerals are
carbonates A carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in the form of a natural crystallinity, crystalline mineral is kn ...
(calcite and others based on ),
chlorides The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride ...
(halite and others built on ), and
sulfates The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...
(
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and ...
and others built on ). Evaporite rocks commonly include abundant halite (rock salt),
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and ...
, and
anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect Cleavage (crystal), cleavage parallel to the three planes of symmetry. It is not Is ...
. * Organic-rich sedimentary rocks have significant amounts of organic material, generally in excess of 3%
total organic carbon Total organic carbon (TOC) is the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organic c ...
. Common examples include
coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen ...
,
oil shale Oil shale is an organic-rich Granularity, fine-grained sedimentary rock containing kerogen (a solid mixture of Organic compound, organic chemical compounds) from which liquid hydrocarbons can be produced. In addition to kerogen, general compos ...
as well as
source rock In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been deposi ...
s for oil and natural gas. * Siliceous sedimentary rocks are almost entirely composed of silica (), typically as
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of S ...
,
opal Opal is a hydrated amorphous form of silicon dioxide, silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike cry ...
,
chalcedony Chalcedony ( , or ) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. ...
or other microcrystalline forms. *
Iron-rich sedimentary rocks Iron-rich sedimentary rocks are sedimentary rock Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followe ...
are composed of >15% iron; the most common forms are
banded iron formation Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness a ...
s and
ironstone Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement, that contains a substantial proportion of an iron ore compound from which iron (Fe) can be smelted commercially. Not to be con ...
s. * Phosphatic sedimentary rocks are composed of phosphate minerals and contain more than 6.5%
phosphorus Phosphorus is a chemical element with the Symbol (chemistry), symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly Reactivity (chemistry), reactive, phosphor ...
; examples include deposits of phosphate nodules, bone beds, and phosphatic mudrocks.


Deposition and transformation


Sediment transport and deposition

Sedimentary
rocks In geology Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over tim ...
are formed when
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...
is deposited out of air, ice, wind, gravity, or water flows carrying the particles in
suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology) In topology, a branch of mathematics, the suspension of a topological space ''X'' is intuitively obtained by stretching ''X'' into a cylinder (geometry), cylind ...
. This sediment is often formed when
weathering Weathering is the deterioration of Rock (geology), rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little o ...
and
erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust, and then sediment transport, tra ...
break down a rock into loose material in a source area. The material is then transported from the source area to the deposition area. The type of sediment transported depends on the geology of the
hinterland Hinterland is a German word meaning "the land behind" (a city, a port, or similar). Its use in English was first documented by the geographer George Chisholm (geographer), George Chisholm in his ''Handbook of Commercial Geography'' (1888). Origin ...
(the source area of the sediment). However, some sedimentary rocks, such as
evaporite An evaporite () is a water-solubility, soluble sedimentation, sedimentary mineral deposition (geology), deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposit ...
s, are composed of material that form at the place of deposition. The nature of a sedimentary rock, therefore, not only depends on the sediment supply, but also on the
sedimentary depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition (geology), deposition of a particular type of sediment and, therefore, the se ...
in which it formed.


Transformation (Diagenesis)

As sediments accumulate in a depositional environment, older sediments are buried by younger sediments, and they undergo diagenesis. Diagenesis includes all the chemical, physical, and biological changes, exclusive of surface weathering, undergone by a sediment after its initial deposition. This includes compaction and
lithification Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificatio ...
of the sediments. Early stages of diagenesis, described as ''eogenesis'', take place at shallow depths (a few tens of meters) and is characterized by
bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...
and mineralogical changes in the sediments, with only slight compaction. The red
hematite Hematite (), American and British English spelling differences#ae and oe, also spelled as haematite, is a common iron oxide compound with the formula, Iron(III) oxide, Fe2O3 and is widely found in Rock (geology), rocks and soils. Hematite crysta ...
that gives
red bed Red beds (or redbeds) are sedimentary rocks Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed ...
sandstones their color is likely formed during eogenesis. Some
biochemical Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
processes, like the activity of
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
, can affect minerals in a rock and are therefore seen as part of diagenesis. Deeper burial is accompanied by ''mesogenesis'', during which most of the compaction and lithification takes place. Compaction takes place as the sediments come under increasing overburden (lithostatic) pressure from overlying sediments. Sediment grains move into more compact arrangements, grains of ductile minerals (such as
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect Cleavage (crystal), basa ...
) are deformed, and pore space is reduced. Sediments are typically saturated with
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...
or seawater when originally deposited, and as pore space is reduced, much of these
connate fluids In geology and sedimentology, connate fluids are liquids that were trapped in the pores of sedimentary rocks as they were deposited. These liquids are largely composed of water, but also contain many mineral components as Ion, ions in Solution (ch ...
are expelled. In addition to this physical compaction, chemical compaction may take place via
pressure solution In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the Solvation, dissolution of minerals at Terzaghi's principle, grain-to-grain contacts into an Aqueous solution, aqueous Por ...
. Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. The increased pressure and temperature stimulate further chemical reactions, such as the reactions by which
organic material Organic matter, organic material, or natural organic matter refers to the large source of Carbon compounds, carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic comp ...
becomes
lignite Lignite, often referred to as brown coal, is a soft, brown, combustion, combustible, sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35%, and is considered the coal rank, lowest rank of coal due to its ...
or coal. Lithification follows closely on compaction, as increased temperatures at depth hasten the
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, Rain and snow mixed, sleet, snow, ice pellets, ...
of cement that binds the grains together. Pressure solution contributes to this process of cementation, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. This further reduces porosity and makes the rock more compact and competent. Unroofing of buried sedimentary rock is accompanied by ''telogenesis'', the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to
meteoric water Meteoric water is the water derived from precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizz ...
produces additional changes to the sedimentary rock, such as leaching of some of the cement to produce secondary porosity. At sufficiently high temperature and pressure, the realm of diagenesis makes way for
metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ...
, the process that forms
metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, cau ...
.


Properties


Color

The color of a sedimentary rock is often mostly determined by
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...
, an element with two major oxides:
iron(II) oxide Iron(II) oxide or ferrous oxide is the inorganic compound with the formula FeO. Its mineral form is known as wüstite. One of several iron oxides, it is a black-colored powder that is sometimes confused with rust, the latter of which consists of ...
and
iron(III) oxide Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally a ...
. Iron(II) oxide (FeO) only forms under low oxygen (
anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...
) circumstances and gives the rock a grey or greenish colour. Iron(III) oxide (Fe2O3) in a richer oxygen environment is often found in the form of the mineral
hematite Hematite (), American and British English spelling differences#ae and oe, also spelled as haematite, is a common iron oxide compound with the formula, Iron(III) oxide, Fe2O3 and is widely found in Rock (geology), rocks and soils. Hematite crysta ...
and gives the rock a reddish to brownish colour. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an important process, giving the rock a red or orange colour. Thick sequences of red sedimentary rocks formed in arid climates are called
red beds Red beds (or redbeds) are sedimentary rocks Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed ...
. However, a red colour does not necessarily mean the rock formed in a continental environment or arid climate. The presence of organic material can colour a rock black or grey. Organic material is formed from dead organisms, mostly plants. Normally, such material eventually decays by oxidation or bacterial activity. Under anoxic circumstances, however, organic material cannot decay and leaves a dark sediment, rich in organic material. This can, for example, occur at the bottom of deep seas and lakes. There is little water mixing in such environments; as a result, oxygen from surface water is not brought down, and the deposited sediment is normally a fine dark clay. Dark rocks, rich in organic material, are therefore often shales.


Texture

The
size Size in general is the magnitude or dimensions of a thing. More specifically, ''geometrical size'' (or ''spatial size'') can refer to linear dimensions (length, width, height, diameter, perimeter), area, or volume. Size can also be me ...
, form and orientation of clasts (the original pieces of rock) in a sediment is called its texture. The texture is a small-scale property of a rock, but determines many of its large-scale properties, such as the
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek language, Greek letter Rho (letter), rho), although the Latin letter ''D'' ca ...
,
porosity Porosity or void fraction is a measure of the Void (composites), void (i.e. "empty") spaces in a material, and is a volume fraction, fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Str ...
or permeability. The 3D orientation of the clasts is called the
fabric Textile is an Hyponymy and hypernymy, umbrella term that includes various Fiber, fiber-based materials, including fibers, yarns, Staple (textiles)#Filament fiber, filaments, Thread (yarn), threads, different #Fabric, fabric types, etc. At f ...
of the rock. The size and form of clasts can be used to determine the velocity and direction of
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
in the sedimentary environment that moved the clasts from their origin; fine, calcareous mud only settles in quiet water while gravel and larger clasts are moved only by rapidly moving water. The grain size of a rock is usually expressed with the Wentworth scale, though alternative scales are sometimes used. The grain size can be expressed as a diameter or a volume, and is always an average value, since a rock is composed of clasts with different sizes. The
statistical distribution In statistics Statistics (from German language, German: ''wikt:Statistik#German, Statistik'', "description of a State (polity), state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, ...
of grain sizes is different for different rock types and is described in a property called the
sorting Sorting refers to ordering data in an increasing or decreasing manner according to some linear relationship among the data items. # Collating order, ordering: arranging items in a sequence ordered by some criterion; # categorization, categorizing ...
of the rock. When all clasts are more or less of the same size, the rock is called 'well-sorted', and when there is a large spread in grain size, the rock is called 'poorly sorted'. The form of the clasts can reflect the origin of the rock. For example,
coquina Coquina () is a sedimentary rock that is composed either wholly or almost entirely of the transported, abraded, and mechanically sorted fragments of the Seashell, shells of Mollusca, mollusks, trilobites, brachiopods, or other invertebrates. T ...
, a rock composed of clasts of broken shells, can only form in energetic water. The form of a clast can be described by using four parameters: * ''Surface texture'' describes the amount of small-scale relief of the surface of a grain that is too small to influence the general shape. For example, ''frosted grains'', which are covered with small-scale fractures, are characteristic of eolian sandstones. * ''
Rounding Rounding means replacing a number with an approximation, approximate value that has a shorter, simpler, or more explicit representation. For example, replacing $ with $, the fraction 312/937 with 1/3, or the expression with . Rounding is oft ...
'' describes the general smoothness of the shape of a grain. * ''
Sphericity Sphericity is a measure of how closely the shape of an object resembles that of a perfect sphere. For example, the sphericity of the ball (bearing), balls inside a ball bearing determines the quality (business), quality of the bearing, such as t ...
'' describes the degree to which the grain approaches a
sphere A sphere () is a Geometry, geometrical object that is a solid geometry, three-dimensional analogue to a two-dimensional circle. A sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...
. * '' Grain form'' describes the three-dimensional shape of the grain. Chemical sedimentary rocks have a non-clastic texture, consisting entirely of crystals. To describe such a texture, only the average size of the crystals and the fabric are necessary.


Mineralogy

Most sedimentary rocks contain either quartz (
siliciclastic Siliciclastic (or ''siliclastic'') rocks are clastic noncarbonate sedimentary rocks that are composed primarily of silicate minerals, such as quartz or clay minerals. Siliciclasic rock types include mudrock, sandstone, and Conglomerate (geology), ...
rocks) or
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
(
carbonate rock Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and Dolomite (rock), dolomite rock (also k ...
s). In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. However, the origin of the minerals in a sedimentary rock is often more complex than in an igneous rock. Minerals in a sedimentary rock may have been present in the original sediments or may formed by precipitation during diagenesis. In the second case, a mineral precipitate may have grown over an older generation of cement. A complex diagenetic history can be established by
optical mineralogy Optical mineralogy is the study of minerals and Rock (geology), rocks by measuring their optics, optical properties. Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrog ...
, using a
petrographic microscope A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify Rock (geology), rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of ...
. Carbonate rocks predominantly consist of
carbonate A carbonate is a salt (chemistry), salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containin ...
minerals such as calcite,
aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring polymorphism (materials science), crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and ph ...
or
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
. Both the cement and the clasts (including fossils and
ooid Ooids are small (commonly ≤2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or Phosphate mineral, phosphate-based minerals. Ooids usually form on ...
s) of a carbonate sedimentary rock usually consist of carbonate minerals. The mineralogy of a clastic rock is determined by the material supplied by the source area, the manner of its transport to the place of deposition and the stability of that particular mineral. The resistance of rock-forming minerals to weathering is expressed by the
Goldich dissolution series The Goldich dissolution series is a method of predicting the relative stability or weathering Weathering is the deterioration of Rock (geology), rocks, soils and minerals as well as wood and artificial materials through contact with water, atmo ...
. In this series, quartz is the most stable, followed by
feldspar Feldspars are a group of rock-forming aluminium Aluminium (aluminum in American and Canadian English) is a chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, ...
,
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect Cleavage (crystal), basa ...
s, and finally other less stable minerals that are only present when little weathering has occurred. The amount of weathering depends mainly on the distance to the source area, the local climate and the time it took for the sediment to be transported to the point where it is deposited. In most sedimentary rocks, mica, feldspar and less stable minerals have been weathered to
clay minerals Clay minerals are Hydrate, hydrous aluminium phyllosilicates (e.g. kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, Alkaline earth metal, alkaline earths, and other ...
like
kaolinite Kaolinite ( ) is a clay mineral, with the chemical composition aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedron, tetrahedral sheet of silica () ...
,
illite Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a Silicate minerals, phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedro ...
or
smectite A smectite (from ancient Greek ''σμηκτός'' smektos 'lubricated'; ''σμηκτρίς'' smektris 'walker's earth', 'fuller's earth'; rubbing earth; earth that has the property of cleaning) is a mineral mixtures of various swelling sheet sil ...
.


Fossils

Among the three major types of rock, fossils are most commonly found in sedimentary rock. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. Often these fossils may only be visible under
magnification Magnification is the process of enlarging the angular diameter, apparent size, not physical size, of something. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to ...
. Dead organisms in nature are usually quickly removed by
scavenger Scavengers are animals that consume Corpse decomposition, dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a h ...
s,
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
, rotting and erosion, but under exceptional circumstances, these natural processes are unable to take place, leading to fossilisation. The chance of fossilisation is higher when the sedimentation rate is high (so that a carcass is quickly buried), in
anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...
environments (where little bacterial activity occurs) or when the organism had a particularly hard skeleton. Larger, well-preserved fossils are relatively rare. Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and the woody tissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while they were still alive are called
trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of Earliest known life forms, biological activity but not the preserved remains of the plant or animal itself. Trace fossils ...
s, examples of which are
burrow file:Chipmunk-burrow (exits).jpg, An Eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of Animal lo ...
s,
footprint Footprints are the impressions or images left behind by a person walking or running. Hoofprints and pawprints are those left by animals with hooves or paws rather than feet, while "shoeprints" is the specific term for prints made by shoe ...
s, etc. As a part of a sedimentary rock, fossils undergo the same diagenetic processes as does the host rock. For example, a shell consisting of calcite can dissolve while a cement of silica then fills the cavity. In the same way, precipitating minerals can fill cavities formerly occupied by
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide aw ...
s,
vascular tissue Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of embryophyte, land plants ( a ...
or other soft tissues. This preserves the form of the organism but changes the chemical composition, a process called
permineralization Permineralization is a process of fossilization of bones and tissues in which mineral deposits form internal Casting, casts of organisms. Carried by water, these minerals fill the spaces within organic tissue. Because of the nature of the casts, p ...
. The most common minerals involved in permineralization are various forms of
amorphous silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other chemical element, element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (mo ...
(
chalcedony Chalcedony ( , or ) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. ...
,
flint Flint, occasionally flintstone, is a sedimentary rock, sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Flint was widely used historically to make stone tool ...
,
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of S ...
),
carbonate A carbonate is a salt (chemistry), salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containin ...
s (especially calcite), and
pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...
. At high pressure and temperature, the
organic material Organic matter, organic material, or natural organic matter refers to the large source of Carbon compounds, carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic comp ...
of a dead organism undergoes chemical reactions in which
volatiles Volatiles are the group of chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Un ...
such as
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
and
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
are expulsed. The fossil, in the end, consists of a thin layer of pure carbon or its mineralized form,
graphite Graphite () is a crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in al ...
. This form of fossilisation is called
carbonisation Carbonization is the conversion of organic matters like plants and dead animal remains into carbon through destructive distillation. Complexity in carbonization Carbonization is a pyrolytic reaction, therefore, is considered a complex process ...
. It is particularly important for plant fossils. The same process is responsible for the formation of
fossil fuel A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and combustion, burned as a fuel. The main fossil fuels are coal, petroleum, oil, and natura ...
s like lignite or coal.


Primary sedimentary structures

Structures in sedimentary rocks can be divided into ''primary'' structures (formed during deposition) and ''secondary'' structures (formed after deposition). Unlike textures, structures are always large-scale features that can easily be studied in the field.
Sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition (geology), deposition. Sediments and sedimentary rocks are characterized by bed (geology), bedding, which occurs when layer ...
can indicate something about the sedimentary environment or can serve to tell which side originally faced up where tectonics have tilted or overturned sedimentary layers. Sedimentary rocks are laid down in layers called
beds A bed is an item of furniture that is used as a place to sleep, Lying (position), rest, and Relaxation (psychology), relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid bed base, ...
or
strata In geology and related fields, a stratum (plural, : strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separate ...
. A bed is defined as a layer of rock that has a uniform
lithology The lithology of a Rock (geology), rock unit is a description of its physical characteristics visible at outcrop, in hand or core sample, core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain ...
and texture. Beds form by the deposition of layers of sediment on top of each other. The sequence of beds that characterizes sedimentary rocks is called
bedding Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environme ...
. Single beds can be a couple of centimetres to several meters thick. Finer, less pronounced layers are called laminae, and the structure a lamina forms in a rock is called
lamination Lamination is the technique/process of manufacturing a material Material is a matter, substance or mixture of substances that constitutes an Physical object, object. Materials can be pure or impure, living or non-living matter. Materials ...
. Laminae are usually less than a few centimetres thick. Though bedding and lamination are often originally horizontal in nature, this is not always the case. In some environments, beds are deposited at a (usually small) angle. Sometimes multiple sets of layers with different orientations exist in the same rock, a structure called
cross-bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The Principle ...
. Cross-bedding is characteristic of deposition by a flowing medium (wind or water). The opposite of cross-bedding is parallel lamination, where all sedimentary layering is parallel. Differences in laminations are generally caused by cyclic changes in the sediment supply, caused, for example, by seasonal changes in rainfall, temperature or biochemical activity. Laminae that represent seasonal changes (similar to tree rings) are called
varve A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish language, Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''H ...
s. Any sedimentary rock composed of millimeter or finer scale layers can be named with the general term ''laminite''. When sedimentary rocks have no lamination at all, their structural character is called massive bedding.
Graded bedding In geology, a graded bed is one characterized by a systematic change in Crystallite, grain or Clastic rock, clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, ...
is a structure where beds with a smaller grain size occur on top of beds with larger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clasts in suspension settle first, then smaller clasts. Although graded bedding can form in many different environments, it is a characteristic of
turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...
s. The surface of a particular bed, called the
bedform A bedform is a geological feature that develops at the interface of fluid In physics, a fluid is a liquid, gas, or other material that continuously Deformation (physics), deforms (''flows'') under an applied shear stress, or external force. ...
, can also be indicative of a particular sedimentary environment. Examples of bed forms include
dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, fl ...
s and
ripple marks In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water (Ocean current, current or wind wave, waves) or wind. Defining ripple cross-laminae and asymmetric ripples * ''Curre ...
. Sole markings, such as tool marks and flute casts, are grooves eroded on a surface that are preserved by renewed sedimentation. These are often elongated structures and can be used to establish the direction of the flow during deposition. Ripple marks also form in flowing water. There can be symmetric or asymmetric. Asymmetric ripples form in environments where the current is in one direction, such as rivers. The longer flank of such ripples is on the upstream side of the current. Symmetric wave ripples occur in environments where currents reverse directions, such as tidal flats.
Mudcrack Mudcracks (also known as mud cracks, desiccation cracks or cracked mud) are sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition (geology), deposition. ...
s are a bed form caused by the dehydration of sediment that occasionally comes above the water surface. Such structures are commonly found at tidal flats or
point bar A point bar is a depositional feature made of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on ...
s along rivers.


Secondary sedimentary structures

Secondary sedimentary structures are those which formed after deposition. Such structures form by chemical, physical and biological processes within the sediment. They can be indicators of circumstances after deposition. Some can be used as way up criteria. Organic materials in a sediment can leave more traces than just fossils. Preserved tracks and
burrow file:Chipmunk-burrow (exits).jpg, An Eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of Animal lo ...
s are examples of
trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of Earliest known life forms, biological activity but not the preserved remains of the plant or animal itself. Trace fossils ...
s (also called ichnofossils). Such traces are relatively rare. Most trace fossils are burrows of
molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...
or
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s. This burrowing is called
bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...
by sedimentologists. It can be a valuable indicator of the biological and ecological environment that existed after the sediment was deposited. On the other hand, the burrowing activity of organisms can destroy other (primary) structures in the sediment, making a reconstruction more difficult. Secondary structures can also form by
diagenesis Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play ...
or the formation of a
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organism In biology, an organism () is any life, living system that functions as an individual entity. All organi ...
(
pedogenesis Soil formation, also known as pedogenesis, is the process of soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organism In biology, an organism () is any l ...
) when a sediment is exposed above the water level. An example of a diagenetic structure common in carbonate rocks is a
stylolite Stylolites (Greek: ''stylos'', pillar; ''lithos'', stone) are serrated surfaces within a rock Rock most often refers to: * Rock (geology) In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mine ...
. Stylolites are irregular planes where material was dissolved into the pore fluids in the rock. This can result in the precipitation of a certain chemical species producing colouring and staining of the rock, or the formation of
concretion A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...
s. Concretions are roughly concentric bodies with a different composition from the host rock. Their formation can be the result of localized precipitation due to small differences in composition or porosity of the host rock, such as around fossils, inside burrows or around plant roots. In carbonate rocks such as limestone or
chalk Chalk is a soft, white, permeability (Earth sciences), porous, sedimentary rock, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic pl ...
,
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of S ...
or
flint Flint, occasionally flintstone, is a sedimentary rock, sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Flint was widely used historically to make stone tool ...
concretions are common, while terrestrial sandstones sometimes contain iron concretions. Calcite concretions in clay containing angular cavities or cracks are called septarian concretions. After deposition, physical processes can deform the sediment, producing a third class of secondary structures. Density contrasts between different sedimentary layers, such as between sand and clay, can result in
flame structure A flame structure is a type of sedimentary structure, soft-sediment deformation that forms in unconsolidated sediments. The weight of an overlying stratum, bed forces an underlying bed to push up through the overlying bed, generally when both stra ...
s or
load cast Load casts are bulges, lumps, and lobes that can form on the bedding planes that separate the layers of sedimentary rocks. The lumps "hang down" from the upper layer into the lower layer, and typically form with fairly equal spacing. These features ...
s, formed by inverted diapirism. While the clastic bed is still fluid, diapirism can cause a denser upper layer to sink into a lower layer. Sometimes, density contrasts occur or are enhanced when one of the lithologies dehydrates. Clay can be easily compressed as a result of dehydration, while sand retains the same volume and becomes relatively less dense. On the other hand, when the
pore fluid pressure Pore water pressure (sometimes abbreviated to pwp) refers to the pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (al ...
in a sand layer surpasses a critical point, the sand can break through overlying clay layers and flow through, forming discordant bodies of sedimentary rock called sedimentary dykes. The same process can form
mud volcano A mud volcano or mud dome is a landform A landform is a natural or anthropogenic land Land, also known as dry land, ground, or earth, is the solid terrestrial surface of the planet Earth that is not submerged by the ocean or other bod ...
es on the surface where they broke through upper layers. Sedimentary dykes can also be formed in a cold climate where the soil is permanently frozen during a large part of the year. Frost weathering can form cracks in the soil that fill with rubble from above. Such structures can be used as climate indicators as well as way up structures. Density contrasts can also cause small-scale
faulting In geology, a fault is a Fracture (geology), planar fracture or discontinuity in a volume of Rock (geology), rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust (geolo ...
, even while sedimentation progresses (synchronous-sedimentary faulting). Such faulting can also occur when large masses of non-lithified sediment are deposited on a slope, such as at the front side of a delta or the
continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being oceanic basin, deep-ocean basins and mid-ocean ridges. The conti ...
. Instabilities in such sediments can result in the deposited material to slump, producing fissures and folding. The resulting structures in the rock are syn-sedimentary folds and faults, which can be difficult to distinguish from folds and faults formed by
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of orogeny, mountain building, the growth and behavior of the strong, old cores of con ...
forces acting on lithified rocks.


Depositional environments

The setting in which a sedimentary rock forms is called the
depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition (geology), deposition of a particular type of sediment and, therefore, the se ...
. Every environment has a characteristic combination of geologic processes, and circumstances. The type of sediment that is deposited is not only dependent on the sediment that is transported to a place (
provenance Provenance (from the French language, French ''provenir'', 'to come from/forth') is the chronology of the ownership, custody or location of a historical object. The term was originally mostly used in relation to works of art but is now used in ...
), but also on the environment itself. A marine environment means that the rock was formed in a
sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of Saline water, salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Water distributio ...
or
ocean The ocean (also the sea or the world ocean) is the body of Saline water, salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Water distribution on Earth, Earth's water. An ocean can also refer to any of the ...
. Often, a distinction is made between deep and shallow marine environments. Deep marine usually refers to environments more than 200 m below the water surface (including the
abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a Continental shelf#Topography, continental rise and a mid-ocean ridge, abyssal plains cover more than ...
). Shallow marine environments exist adjacent to coastlines and can extend to the boundaries of the
continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
. The water movements in such environments have a generally higher energy than that in deep environments, as wave activity diminishes with depth. This means that coarser sediment particles can be transported and the deposited sediment can be coarser than in deeper environments. When the sediment is transported from the continent, an alternation of
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...
,
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4). Clays develop plasticity (physics), plasticity when wet, du ...
and
silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...
is deposited. When the continent is far away, the amount of such sediment deposited may be small, and biochemical processes dominate the type of rock that forms. Especially in warm climates, shallow marine environments far offshore mainly see deposition of carbonate rocks. The shallow, warm water is an ideal habitat for many small organisms that build carbonate skeletons. When these organisms die, their skeletons sink to the bottom, forming a thick layer of calcareous mud that may lithify into limestone. Warm shallow marine environments also are ideal environments for
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...
s, where the sediment consists mainly of the calcareous skeletons of larger organisms. In deep marine environments, the water current working the sea bottom is small. Only fine particles can be transported to such places. Typically sediments depositing on the ocean floor are fine clay or small skeletons of micro-organisms. At 4 km depth, the solubility of carbonates increases dramatically (the depth zone where this happens is called the lysocline). Calcareous sediment that sinks below the lysocline dissolves; as a result, no limestone can be formed below this depth. Skeletons of micro-organisms formed of
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
(such as
radiolarian The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell (biology), cell into the inner and outer portions of endoplasm and Ecto ...
s) are not as soluble and are still deposited. An example of a rock formed of silica skeletons is
radiolarite Radiolarite is a siliceous, comparatively hard, fine-grained, chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz Quartz is a hard, crystalline mineral composed of sil ...
. When the bottom of the sea has a small inclination, for example, at the
continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being oceanic basin, deep-ocean basins and mid-ocean ridges. The conti ...
s, the sedimentary cover can become unstable, causing
turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...
s. Turbidity currents are sudden disturbances of the normally quiet deep marine environment and can cause the near-instantaneous deposition of large amounts of sediment, such as sand and silt. The rock sequence formed by a turbidity current is called a
turbidite A turbidite is the geologic Deposition (geology), deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing ...
. The coast is an environment dominated by wave action. At a
beach A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from Rock (geology), rock, such as sand, gravel, shingle beach, shingle, pebbles, etc., or biological so ...
, dominantly denser sediment such as sand or gravel, often mingled with shell fragments, is deposited, while the silt and clay sized material is kept in mechanical suspension.
Tidal flat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal fl ...
s and
shoal In oceanography, geomorphology, and Earth science, geoscience, a shoal is a natural submerged ridge, bank (geography), bank, or bar that consists of, or is covered by, sand or other unconsolidated material and rises from the bed of a body o ...
s are places that sometimes dry because of the
tide Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide t ...
. They are often cross-cut by
gullies A gully is a landform A landform is a natural or anthropogenic land Land, also known as dry land, ground, or earth, is the solid terrestrial surface of the planet Earth that is not submerged by the ocean or other body of water, bodies ...
, where the current is strong and the grain size of the deposited sediment is larger. Where rivers enter the body of water, either on a sea or lake coast,
deltas A river delta is a landform A landform is a natural or anthropogenic land Land, also known as dry land, ground, or earth, is the solid terrestrial surface of the planet Earth that is not submerged by the ocean or other body of water, bo ...
can form. These are large accumulations of sediment transported from the continent to places in front of the mouth of the river. Deltas are dominantly composed of clastic (rather than chemical) sediment. A continental sedimentary environment is an environment in the interior of a continent. Examples of continental environments are
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...
s, lakes,
swamp A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...
s,
floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...
s and
alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but ...
s. In the quiet water of swamps, lakes and lagoons, fine sediment is deposited, mingled with organic material from dead plants and animals. In rivers, the energy of the water is much greater and can transport heavier clastic material. Besides transport by water, sediment can be transported by wind or glaciers. Sediment transported by wind is called aeolian and is almost always very well sorted, while sediment transported by a glacier is called
glacial till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...
and is characterized by very poor sorting. Aeolian deposits can be quite striking. The depositional environment of the
Touchet Formation The Touchet Formation or Touchet beds consist of large quantities of gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also ...
, located in the
Northwestern United States The Northwestern United States, also known as the American Northwest or simply the Northwest, is an informal geographic region of the United States. The region consistently includes the states of Oregon, Washington (state), Washington, Idaho, ...
, had intervening periods of aridity which resulted in a series of rhythmite layers. Erosional cracks were later infilled with layers of soil material, especially from
aeolian processes Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may erosion, erode, transport, and deposit ...
. The infilled sections formed vertical inclusions in the horizontally deposited layers, and thus provided evidence of the sequence of events during deposition of the forty-one layers of the formation.


Sedimentary facies

The kind of rock formed in a particular depositional environment is called its
sedimentary facies In geology Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. ...
. Sedimentary environments usually exist alongside each other in certain natural successions. A beach, where sand and gravel is deposited, is usually bounded by a deeper marine environment a little offshore, where finer sediments are deposited at the same time. Behind the beach, there can be
dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, fl ...
s (where the dominant deposition is well sorted sand) or a
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...
(where fine clay and organic material is deposited). Every sedimentary environment has its own characteristic deposits. When sedimentary strata accumulate through time, the environment can shift, forming a change in facies in the subsurface at one location. On the other hand, when a rock layer with a certain age is followed laterally, the
lithology The lithology of a Rock (geology), rock unit is a description of its physical characteristics visible at outcrop, in hand or core sample, core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain ...
(the type of rock) and facies eventually change. Facies can be distinguished in a number of ways: the most common are by the lithology (for example: limestone, siltstone or sandstone) or by
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
content.
Coral Corals are marine invertebrates within the class (biology), class Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important C ...
, for example, only lives in warm and shallow marine environments and fossils of coral are thus typical for shallow marine facies. Facies determined by lithology are called lithofacies; facies determined by fossils are biofacies. Sedimentary environments can shift their geographical positions through time. Coastlines can shift in the direction of the sea when the
sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...
drops ( regression), when the surface rises ( transgression) due to tectonic forces in the Earth's crust or when a river forms a large delta. In the subsurface, such geographic shifts of sedimentary environments of the past are recorded in shifts in sedimentary facies. This means that sedimentary facies can change either parallel or perpendicular to an imaginary layer of rock with a fixed age, a phenomenon described by Walther's Law. The situation in which coastlines move in the direction of the continent is called transgression. In the case of transgression, deeper marine facies are deposited over shallower facies, a succession called onlap. Regression is the situation in which a coastline moves in the direction of the sea. With regression, shallower facies are deposited on top of deeper facies, a situation called offlap. The facies of all rocks of a certain age can be plotted on a map to give an overview of the
palaeogeography Palaeogeography (or paleogeography) is the study of historical geography Historical geography is the branch of geography that studies the ways in which geographic phenomena have changed over time. It is a synthesizing discipline which share ...
. A sequence of maps for different ages can give an insight in the development of the regional geography.


Gallery of sedimentary facies

File:Facies_migration2.jpg , A regressive facies shown on a stratigraphic column


Sedimentary basins

Places where large-scale sedimentation takes place are called
sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...
s. The amount of sediment that can be deposited in a basin depends on the depth of the basin, the so-called '' accommodation space''. The depth, shape and size of a basin depend on
tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of orogeny, mountain building, the growth and behavior of the strong, old cores of con ...
, movements within the Earth's
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
. Where the lithosphere moves upward (
tectonic uplift Tectonic uplift is the orogeny, geologic uplift of Earth#Surface, Earth's surface that is attributed to plate tectonics. While Isostasy, isostatic response is important, an increase in the mean elevation of a region can only occur in response to te ...
), land eventually rises above sea level and the area becomes a source for new sediment as
erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust, and then sediment transport, tra ...
removes material. Where the lithosphere moves downward (
tectonic subsidence Tectonic subsidence is the Subsidence, sinking of the Earth's Crust (geology), crust on a large scale, relative to crustal-scale features or the geoid. The movement of Plate tectonics, crustal plates and accommodation spaces created by Fault (geolo ...
), a basin forms and sediments are deposited. A type of basin formed by the moving apart of two pieces of a continent is called a
rift basin In geology Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time ...
. Rift basins are elongated, narrow and deep basins. Due to divergent movement, the lithosphere is stretched and thinned, so that the hot
asthenosphere The asthenosphere () is the mechanically weak and ductility (Earth science), ductile region of the upper mantle (Earth), upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . Howeve ...
rises and heats the overlying rift basin. Apart from continental sediments, rift basins normally also have part of their infill consisting of volcanic deposits. When the basin grows due to continued stretching of the lithosphere, the
rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grab ...
grows and the sea can enter, forming marine deposits. When a piece of lithosphere that was heated and stretched cools again, its
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek language, Greek letter Rho (letter), rho), although the Latin letter ''D'' ca ...
rises, causing isostatic subsidence. If this subsidence continues long enough, the basin is called a sag basin. Examples of sag basins are the regions along
passive Passive may refer to: * Passive voice, a grammatical voice common in many languages, see also Pseudopassive (disambiguation), Pseudopassive * Passive language, a language from which an interpreter works * Passivity (behavior), the condition of su ...
continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being oceanic basin, deep-ocean basins and mid-ocean ridges. The conti ...
s, but sag basins can also be found in the interior of continents. In sag basins, the extra weight of the newly deposited sediments is enough to keep the subsidence going in a
vicious circle A vicious circle (or cycle) is a complex chain of events that reinforces itself through a feedback loop, with detrimental results. It is a system with no tendency toward List of types of equilibrium, equilibrium (social equilibrium, social, Eco ...
. The total thickness of the sedimentary infill in a sag basin can thus exceed 10 km. A third type of basin exists along convergent plate boundaries – places where one
tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
moves under another into the asthenosphere. The subducting plate bends and forms a
fore-arc basin Forearc is a plate tectonic term referring to a region between an oceanic trench Oceanic trenches are prominent long, narrow topography, topographic depressions of the ocean floor. They are typically wide and below the level of the surroundin ...
in front of the overriding plate – an elongated, deep asymmetric basin. Fore-arc basins are filled with deep marine deposits and thick sequences of turbidites. Such infill is called
flysch Flysch () is a sequence of sedimentary rock layers that progress from deep-water and turbidity current, turbidity flow deposits to shallow-water shales and sandstones. It is deposited when a deep basin forms rapidly on the continental side of a mo ...
. When the convergent movement of the two plates results in
continental collision In geology, continental collision is a phenomenon of plate tectonics that occurs at Convergent boundary, convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destro ...
, the basin becomes shallower and develops into a
foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain range, mountain belt. Foreland basins form because the immense mass created by crust (geology), crustal thickening associated with the evolution of a mount ...
. At the same time, tectonic uplift forms a mountain belt in the overriding plate, from which large amounts of material are eroded and transported to the basin. Such erosional material of a growing mountain chain is called
molasse __NOTOC__ The term "molasse" () refers to sandstones, shales and conglomerate (geology), conglomerates that form as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse deposits accumulate in a foreland basin, es ...
and has either a shallow marine or a continental facies. At the same time, the growing weight of the mountain belt can cause isostatic subsidence in the area of the overriding plate on the other side to the mountain belt. The basin type resulting from this subsidence is called a
back-arc basin A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found ...
and is usually filled by shallow marine deposits and molasse.


Influence of astronomical cycles

In many cases facies changes and other lithological features in sequences of sedimentary rock have a cyclic nature. This cyclic nature was caused by cyclic changes in sediment supply and the sedimentary environment. Most of these cyclic changes are caused by astronomic cycles. Short astronomic cycles can be the difference between the tides or the
spring tide Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide t ...
every two weeks. On a larger time-scale, cyclic changes in climate and sea level are caused by
Milankovitch cycles Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term was coined and named after Serbian geophysics, geophysicist and astronomer Milutin Milanković. In the 1920 ...
: cyclic changes in the orientation and/or position of the Earth's rotational axis and orbit around the Sun. There are a number of Milankovitch cycles known, lasting between 10,000 and 200,000 years.For a short explanation of Milankovitch cycles, see ; . Relatively small changes in the orientation of the Earth's axis or length of the seasons can be a major influence on the Earth's climate. An example are the ice ages of the past 2.6 million years (the
Quaternary The Quaternary ( ) is the current and most recent of the three period (geology), periods of the Cenozoic era (geology), Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spa ...
period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...
), which are assumed to have been caused by astronomic cycles. Climate change can influence the global sea level (and thus the amount of accommodation space in sedimentary basins) and sediment supply from a certain region. Eventually, small changes in astronomic parameters can cause large changes in sedimentary environment and sedimentation.


Sedimentation rates

The rate at which sediment is deposited differs depending on the location. A channel in a tidal flat can see the deposition of a few metres of sediment in one day, while on the deep ocean floor each year only a few millimetres of sediment accumulate. A distinction can be made between normal sedimentation and sedimentation caused by catastrophic processes. The latter category includes all kinds of sudden exceptional processes like mass movements,
rock slide A rockslide is a type of landslide caused by rock failure in which part of the bedding plane of failure passes through compacted rock and material collapses ''en masse'' and not in individual blocks. Note that a rockslide is similar to an avala ...
s or
flood A flood is an overflow of water (list of non-water floods, or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study o ...
ing. Catastrophic processes can see the sudden deposition of a large amount of sediment at once. In some sedimentary environments, most of the total column of sedimentary rock was formed by catastrophic processes, even though the environment is usually a quiet place. Other sedimentary environments are dominated by normal, ongoing sedimentation. In many cases, sedimentation occurs slowly. In a
desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one ...
, for example, the wind deposits siliciclastic material (sand or silt) in some spots, or catastrophic flooding of a
wadi Wadi ( ar, وَادِي, wādī), alternatively ''wād'' ( ar, وَاد), North African Arabic Oued, is the Arabic term traditionally referring to a valley. In some instances, it may refer to a wet ( ephemeral) riverbed that contains water ...
may cause sudden deposits of large quantities of detrital material, but in most places eolian erosion dominates. The amount of sedimentary rock that forms is not only dependent on the amount of supplied material, but also on how well the material consolidates. Erosion removes most deposited sediment shortly after deposition.


Stratigraphy

That new rock layers are above older rock layers is stated in the principle of superposition. There are usually some gaps in the sequence called
unconformities An unconformity is a buried erosion surface, erosional or non-depositional surface separating two Rock (geology), rock masses or Stratum, strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer ...
. These represent periods where no new sediments were laid down, or when earlier sedimentary layers were raised above sea level and eroded away. Sedimentary rocks contain important information about the
history of the Earth The history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geology, ...
. They contain fossils, the preserved remains of ancient
plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
s and
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage i ...
s. Coal is considered a type of sedimentary rock. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment 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 remains.


Provenance

Provenance is the reconstruction of the origin of sediments. All rock exposed at Earth's surface is subjected to physical or chemical
weathering Weathering is the deterioration of Rock (geology), rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little o ...
and broken down into finer grained sediment. All three types of rocks (
igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
, sedimentary and
metamorphic Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of ...
rocks) can be the source of sedimentary detritus. The purpose of sedimentary provenance studies is to reconstruct and interpret the history of sediment from the initial parent rocks at a source area to final detritus at a burial place.


See also

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References


Citations


General and cited references

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External links


Basic Sedimentary Rock Classification
by Lynn S. Fichter, James Madison University, Harrisonburg.VI;

by Bruce Perry, Department of Geological Sciences, California State University at Long Beach . {{DEFAULTSORT:Sedimentary Rock Petrology Geology la:Sedimentum