Sedimentary rocks are types of rock that are formed by the deposition
and subsequent cementation of that material at the Earth's surface and
within bodies of water.
Sedimentation is the collective name for
processes that cause mineral or organic particles (detritus) to settle
in place. The particles that form a sedimentary rock by accumulating
are called sediment. Before being deposited, the sediment was formed
by weathering and erosion from the source area, and then transported
to the place of deposition by water, wind, ice, mass movement or
glaciers, which are called agents of denudation.
also occur as minerals precipitate from water solution or shells of
aquatic creatures settle out of suspension.
The sedimentary rock cover of the continents of the Earth's crust is
extensive (73% of the Earth's current land surface), but the total
contribution of sedimentary rocks is estimated to be only 8% of the
total volume of the crust. Sedimentary rocks are only a thin veneer
over a crust consisting mainly of igneous and metamorphic rocks.
Sedimentary rocks are deposited in layers as strata, forming a
structure called bedding. The study of sedimentary rocks and rock
strata provides information about the subsurface that is useful for
civil engineering, for example in the construction of roads, houses,
tunnels, canals or other structures. Sedimentary rocks are also
important sources of natural resources like coal, fossil fuels,
drinking water or ores.
The study of the sequence of sedimentary rock strata is the main
source for an understanding of the Earth's history, including
palaeogeography, paleoclimatology and the history of life. The
scientific discipline that studies the properties and origin of
sedimentary rocks is called sedimentology.
Sedimentology is part of
both geology and physical geography and overlaps partly with other
disciplines in the Earth sciences, such as pedology, geomorphology,
geochemistry and structural geology. Sedimentary rocks have also been
found on Mars.
1 Classification based on origin
1.1 Clastic sedimentary rocks
1.1.1 Conglomerates and breccias
1.2 Biochemical sedimentary rocks
1.3 Chemical sedimentary rocks
1.4 "Other" sedimentary rocks
2 Compositional classification schemes
3 Deposition and transformation
Sediment transport and deposition
3.2 Transformation (Diagenesis)
4.5 Primary sedimentary structures
4.6 Secondary sedimentary structures
5 Sedimentary environments
5.1 Sedimentary facies
6 Sedimentary basins
6.1 Influence of astronomical cycles
9 See also
11 External links
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, and other minor processes.
Clastic sedimentary rocks
Main article: Clastic rock
Claystone deposited in Glacial
Lake Missoula, Montana, United States.
Note the very fine and flat bedding, common for distal lacustrine
Clastic sedimentary rocks are composed of other rock fragments that
were cemented by silicate minerals. Clastic rocks are composed largely
of quartz, feldspar, rock (lithic) fragments, clay minerals, and mica;
any type of mineral may be present, but they in general represent the
minerals that exist locally.
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
(>2 mm diameter), sand (1/16 to 2 mm diameter), and mud
(clay is <1/256 mm and silt is between 1/16 and
1/256 mm). The classification of clastic sedimentary rocks
parallels this scheme; conglomerates and breccias are made mostly of
gravel, sandstones are made mostly of sand, and mudrocks are made
mostly of the finest material. This tripartite subdivision is mirrored
by the broad categories of rudites, arenites, and lutites,
respectively, in older literature.
The subdivision of these three broad categories is based on
differences in clast shape (conglomerates and breccias), composition
(sandstones), grain size or texture (mudrocks).
Conglomerates and breccias
Conglomerates are dominantly composed of rounded gravel, while
breccias are composed of dominantly angular gravel.
Sedimentary rock with sandstone in Malta
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
"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
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, arkose, and quartz
sandstone are still widely used by non-specialists and in popular
Antelope Canyon was carved out of the surrounding sandstone by
both mechanical weathering and chemical weathering. Wind, sand, and
water from flash flooding are the primary weathering agents.
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 water or air, and deposited
as the flow calms and the particles settle out of suspension.
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" as a term for a fissile mudrock (regardless of
grain size) although some older literature uses the term "shale" as a
synonym for mudrock.
Biochemical sedimentary rocks
Ordovician oil shale (kukersite), northern Estonia
Biochemical sedimentary rocks are created when organisms use materials
dissolved in air or water to build their tissue. Examples include:
Most types of limestone are formed from the calcareous skeletons of
organisms such as corals, mollusks, and foraminifera.
Coal, formed from plants that have removed carbon from the atmosphere
and combined it with other elements to build their tissue.
Deposits of chert formed from the accumulation of siliceous skeletons
of microscopic organisms such as radiolaria and diatoms.
Chemical sedimentary rocks
Chemical sedimentary rock forms when mineral constituents in solution
become supersaturated and inorganically precipitate. Common chemical
sedimentary rocks include oolitic limestone and rocks composed of
evaporite minerals, such as halite (rock salt), sylvite, barite and
"Other" sedimentary rocks
This fourth miscellaneous category includes rocks formed by
Pyroclastic flows, impact breccias, volcanic breccias, and other
relatively uncommon processes.
Compositional classification schemes
Alternatively, sedimentary rocks can be subdivided into compositional
groups based on their mineralogy:
Siliciclastic sedimentary rocks, are dominantly composed of silicate
minerals. The sediment that makes up these rocks was transported as
bed load, suspended load, or by sediment gravity flows. Siliciclastic
sedimentary rocks are subdivided into conglomerates and breccias,
sandstone, and mudrocks.
Carbonate sedimentary rocks are composed of calcite (rhombohedral CaCO
3), aragonite (orthorhombic CaCO
3), dolomite (CaMg(CO
2), and other carbonate minerals based on the CO2−
3 ion. Common examples include limestone and dolostone.
Evaporite sedimentary rocks are composed of minerals formed from the
evaporation of water. The most common evaporite minerals are
carbonates (calcite and others based on CO2−
3), chlorides (halite and others built on Cl−), and sulfates (gypsum
and others built on SO2−
Evaporite rocks commonly include abundant halite (rock salt),
gypsum, and anhydrite.
Organic-rich sedimentary rocks
Organic-rich sedimentary rocks have significant amounts of organic
material, generally in excess of 3% total organic carbon. Common
examples include coal, oil shale as well as source rocks for oil and
Siliceous sedimentary rocks are almost entirely composed of silica
2), typically as chert, opal, chalcedony or other microcrystalline
Iron-rich sedimentary rocks
Iron-rich sedimentary rocks are composed of >15% iron; the most
common forms are banded iron formations and ironstones.
Phosphatic sedimentary rocks are composed of phosphate minerals and
contain more than 6.5% phosphorus; examples include deposits of
phosphate nodules, bone beds, and phosphatic mudrocks.
Deposition and transformation
Sediment transport and deposition
Cross-bedding and scour in a fine sandstone; the Logan Formation
(Mississippian) of Jackson County, Ohio
Sedimentary rocks are formed when sediment is deposited out of air,
ice, wind, gravity, or water flows carrying the particles in
suspension. This sediment is often formed when weathering and erosion
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
(the source area of the sediment). However, some sedimentary rocks,
such as evaporites, 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 which it formed.
Pressure solution at work in a clastic rock. While material dissolves
at places where grains are in contact, that material may recrystallize
from the solution and act as cement in open pore spaces. As a result,
there is a net flow of material from areas under high stress to those
under low stress, producing a sedimentary rock that is more compact
and harder. Loose sand can become sandstone in this way.
Main article: diagenesis
The term diagenesis is used to describe all the chemical, physical,
and biological changes, exclusive of surface weathering, undergone by
a sediment after its initial deposition. Some of those processes cause
the sediment to consolidate into a compact, solid substance from the
originally loose material. Young sedimentary rocks, especially those
Quaternary age (the most recent period of the geologic time scale)
are often still unconsolidated. As sediment deposition builds up, the
overburden (lithostatic) pressure rises, and a process known as
lithification takes place.
Sedimentary rocks are often saturated with seawater or groundwater, in
which minerals can dissolve, or from which minerals can precipitate.
Precipitating minerals reduce the pore space in a rock, a process
called cementation. Due to the decrease in pore space, the original
connate fluids are expelled. The precipitated minerals form a cement
and make the rock more compact and competent. In this way, loose
clasts in a sedimentary rock can become "glued" together.
When sedimentation continues, an older rock layer becomes buried
deeper as a result. The lithostatic pressure in the rock increases due
to the weight of the overlying sediment. This causes compaction, a
process in which grains mechanically reorganize. Compaction is, for
example, an important diagenetic process in clay, which can initially
consist of 60% water. During compaction, this interstitial water is
pressed out of pore spaces. Compaction can also be the result of
dissolution of grains by pressure solution. The dissolved material
precipitates again in open pore spaces, which means there is a net
flow of material into the pores. However, in some cases, a certain
mineral dissolves and does not precipitate again. This process, called
leaching, increases pore space in the rock.
Some biochemical processes, like the activity of bacteria, can affect
minerals in a rock and are therefore seen as part of diagenesis. Fungi
and plants (by their roots) and various other organisms that live
beneath the surface can also influence diagenesis.
Burial of rocks due to ongoing sedimentation leads to increased
pressure and temperature, which stimulates certain chemical reactions.
An example is the reactions by which organic material becomes lignite
or coal. When temperature and pressure increase still further, the
realm of diagenesis makes way for metamorphism, the process that forms
A piece of a banded iron formation, a type of rock that consists of
alternating layers with iron(III) oxide (red) and iron(II) oxide
(grey). BIFs were mostly formed during the Precambrian, when the
atmosphere was not yet rich in oxygen. Moories Group, Barberton
Greenstone Belt, South Africa
The color of a sedimentary rock is often mostly determined by iron, an
element with two major oxides: iron(II) oxide and iron(III) oxide.
Iron(II) oxide (FeO) only forms under low oxygen (anoxic)
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 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. 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.
Diagram showing well-sorted (left) and poorly sorted (right) grains
The size, 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, porosity or permeability.
The 3D orientation of the clasts is called the fabric of the rock.
Between the clasts, the rock can be composed of a matrix (a cement)
that consists of crystals of one or more precipitated minerals. The
size and form of clasts can be used to determine the velocity and
direction of current 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 – a rock is composed of clasts with different
sizes. The statistical distribution of grain sizes is different for
different rock types and is described in a property called the sorting
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'.
Diagram showing the rounding and sphericity of grains
The form of the clasts can reflect the origin of the rock.
Coquina, 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
Surface texture describes the amount of small-scale relief of the
surface of a grain that is too small to influence the general shape.
rounding describes the general smoothness of the shape of a grain.
'Sphericity' describes the degree to which the grain approaches a
'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.
Most sedimentary rocks contain either quartz (especially siliciclastic
rocks) or calcite (especially carbonate rocks). 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 can have formed by precipitation during
sedimentation or by diagenesis. In the second case, the mineral
precipitate can have grown over an older generation of cement. A
complex diagenetic history can be studied by optical mineralogy, using
a petrographic microscope.
Carbonate rocks dominantly consist of carbonate minerals such as
calcite, aragonite or dolomite. Both the cement and the clasts
(including fossils and ooids) of a carbonate sedimentary rock can
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 Bowen's reaction series. In this series,
quartz is the most stable, followed by feldspar, micas, 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 reduced to clay minerals like kaolinite, illite or
Fossil-rich layers in a sedimentary rock, Año Nuevo State Reserve,
Main articles: fossil and fossilisation
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
Dead organisms in nature are usually quickly removed by scavengers,
bacteria, rotting and erosion, but sedimentation can contribute to
exceptional circumstances where these natural processes are unable to
work, causing fossilisation. The chance of fossilisation is higher
when the sedimentation rate is high (so that a carcass is quickly
buried), in anoxic environments (where little bacterial activity
occurs) or when the organism had a particularly hard skeleton. Larger,
well-preserved fossils are relatively rare.
Burrows in a turbidite, made by crustaceans, San Vincente Formation
(early Eocene) of the Ainsa Basin, southern foreland of the Pyrenees
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 fossils, examples of which are burrows, footprints,
As a part of a sedimentary or metamorphic rock, fossils undergo the
same diagenetic processes as does the containing rock. A shell
consisting of calcite can, for example, dissolve while a cement of
silica then fills the cavity. In the same way, precipitating minerals
can fill cavities formerly occupied by blood vessels, vascular tissue
or other soft tissues. This preserves the form of the organism but
changes the chemical composition, a process called
permineralization. The most common minerals involved in
permineralization are cements of carbonates (especially calcite),
forms of amorphous silica (chalcedony, flint, chert) and pyrite. In
the case of silica cements, the process is called lithification.
At high pressure and temperature, the organic material of a dead
organism undergoes chemical reactions in which volatiles such as water
and carbon dioxide are expulsed. The fossil, in the end, consists of a
thin layer of pure carbon or its mineralized form, graphite. This form
of fossilisation is called carbonisation. It is particularly important
for plant fossils. The same process is responsible for the
formation of fossil fuels like lignite or coal.
Primary sedimentary structures
Cross-bedding in a fluviatile sandstone, Middle Old Red Sandstone
(Devonian) on Bressay, Shetland Islands
A flute cast, a type of sole marking, from the
Book Cliffs of Utah
Ripple marks formed by a current in a sandstone that was later tilted
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 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 or strata. A bed
is defined as a layer of rock that has a uniform lithology 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. 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. 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
Cross-bedding forms when small-scale erosion
occurs during deposition, cutting off part of the beds. Newer beds
then form at an angle to older ones.
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 varves. 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 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
The surface of a particular bed, called the bedform, can be indicative
of a particular sedimentary environment, too. Examples of bed forms
include dunes and ripple marks. Sole markings, such as tool marks and
flute casts, are groves dug into a sedimentary layer that are
preserved. 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 are two types of
ripples: symmetric and asymmetric. Environments where the current is
in one direction, such as rivers, produce asymmetric ripples. 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.
Mudcracks 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 bars along rivers.
Secondary sedimentary structures
Halite crystal mold in dolomite, Paadla Formation (Silurian),
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 burrows are examples of trace fossils
(also called ichnofossils). Such traces are relatively rare. Most
trace fossils are burrows of molluscs or arthropods. This burrowing is
called bioturbation 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.
Chert concretions in chalk, Middle Lefkara Formation (upper Paleocene
to middle Eocene), Cyprus
Secondary structures can also form by diagenesis or the formation of a
soil (pedogenesis) when a sediment is exposed above the water level.
An example of a diagenetic structure common in carbonate rocks is a
stylolite. 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 concretions. 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 based rocks such as limestone or chalk, chert or flint
concretions are common, while terrestrial sandstones can have iron
Calcite concretions in clay are called septarian
After deposition, physical processes can deform the sediment,
producing a third class of secondary structures.
between different sedimentary layers, such as between sand and clay,
can result in flame structures or load casts, 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 can result or grow when one of the lithologies
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 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 volcanoes 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, 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. 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 forces acting on lithified rocks.
The setting in which a sedimentary rock forms is called the
sedimentary environment. 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, but also on the environment itself.
A marine environment means that the rock was formed in a sea or ocean.
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. Shallow marine environments exist
adjacent to coastlines and can extend to the boundaries of the
continental shelf. 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, clay and silt 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 reefs, 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 (such as
radiolarians) are not as soluble and still deposit. An example of a
rock formed of silica skeletons is radiolarite. When the bottom of the
sea has a small inclination, for example at the continental slopes,
the sedimentary cover can become unstable, causing turbidity currents.
Turbidity currents are sudden disturbances of the normally quite deep
marine environment and can cause the geologically speaking
instantaneous deposition of large amounts of sediment, such as sand
and silt. The rock sequence formed by a turbidity current is called a
The coast is an environment dominated by wave action. At a beach,
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 flats and shoals are places
that sometimes dry because of the tide. They are often cross-cut by
gullies, 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 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 sediment (in contrast to chemical).
A sedimentary rock formed on land has a continental sedimentary
environment. Examples of continental environments are lagoons, lakes,
swamps, floodplains and alluvial fans. 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 in continental environments
also be transported by wind or glaciers.
Sediment transported by wind
is called aeolian and is always very well sorted, while sediment
transported by a glacier is called glacial till and is characterized
by very poor sorting.
Aeolian deposits can be quite striking. The depositional environment
of the Touchet Formation, located in the Northwestern United States,
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. The infilled
sections formed vertical inclusions in the horizontally deposited
layers of the Touchet Formation, and thus provided evidence of the
events that intervened over time among the forty-one layers that were
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 dunes (where the dominant deposition is well
sorted sand) or a lagoon (where fine clay and organic material is
deposited). Every sedimentary environment has its own characteristic
deposits. The typical rock formed in a certain environment is called
its sedimentary facies. 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 type of rock)
and facies eventually change.
Shifting sedimentary facies in the case of transgression (above) and
regression of the sea (below)
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 content. Coral, for example, only lives in warm and shallow
marine environments and fossils of coral are thus typical for shallow
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 drops, when the surface rises 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
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. A sequence of maps for
different ages can give an insight in the development of the regional
Main article: sedimentary basin
Places where large-scale sedimentation takes place are called
sedimentary basins. 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,
movements within the Earth's lithosphere. Where the lithosphere moves
upward (tectonic uplift), land eventually rises above sea level, so
that and erosion removes material, and the area becomes a source for
new sediment. Where the lithosphere moves downward (tectonic
subsidence), a basin forms and sedimentation can take place. When the
lithosphere keeps subsiding, new accommodation space keeps being
A type of basin formed by the moving apart of two pieces of a
continent is called a rift basin.
Rift basins are elongated, narrow
and deep basins. Due to divergent movement, the lithosphere is
stretched and thinned, so that the hot asthenosphere 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 grows and the sea can enter, forming marine
When a piece of lithosphere that was heated and stretched cools again,
its density 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 continental margins, 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. The total thickness of the
sedimentary infill in a sag basins can thus exceed 10 km.
A third type of basin exists along convergent plate boundaries –
places where one tectonic plate moves under another into the
asthenosphere. The subducting plate bends and forms a fore-arc basin
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. When the
convergent movement of the two plates results in continental
collision, the basin becomes shallower and develops into a foreland
basin. 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 and has either a shallow marine or a
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 and is usually filled by shallow
marine deposits and molasse.
Cyclic alternation of competent and less competent beds in the Blue
Lias at Lyme Regis, southern England
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 every two weeks. On a larger time-scale,
cyclic changes in climate and sea level are caused by Milankovitch
cycles: cyclic changes in the orientation and/or position of the
Earth's rotational axis and orbit around the Sun. There are a number
Milankovitch cycles known, lasting between 10,000 and 200,000
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 period), 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.
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 slides or
flooding. 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
In many cases, sedimentation occurs slowly. In a desert, for example,
the wind deposits siliciclastic material (sand or silt) in some spots,
or catastrophic flooding of a wadi 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
Erosion removes most deposited sediment shortly
Jurassic stratigraphy of the
Colorado Plateau area
Utah that makes up much of the famous prominent rock
formations in protected areas such as
Capitol Reef National Park
Capitol Reef National Park and
Canyonlands National Park. From top to bottom: Rounded tan domes of
the Navajo Sandstone, layered red Kayenta Formation, cliff-forming,
vertically jointed, red Wingate Sandstone, slope-forming, purplish
Chinle Formation, layered, lighter-red Moenkopi Formation, and white,
Cutler Formation sandstone. Picture from Glen Canyon National
Recreation Area, Utah.
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. 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. They contain fossils, the preserved remains of ancient
plants and animals.
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.
List of minerals
List of rock types
^ Wilkinson, Bruce H.; McElroy, Brandon J.; Kesler, Stephen E.;
Peters, Shanan E.; Rothman, Edward D. (2008). "Global geologic maps
are tectonic speedometers—Rates of rock cycling from area-age
frequencies". Geological Society of America Bulletin. 121: 760–779.
^ Buchner & Grapes (2011), p. 24
^ a b Dott (1964)
^ a b Blatt et al. (1980), p. 782
^ a b c Prothero & Schwab (2004)
^ a b Boggs (2006)
^ Stow (2005)
^ a b Levin (1987), p. 57
^ Tarbuck & Lutgens (1999), pp. 145–146
^ Boggs (1987), p. 105
^ Tarbuck & Lutgens (1999), pp. 156–157
^ Levin (1987), p. 58
^ Boggs (1987), pp. 112–115
^ Blatt et al. (1980), pp. 55–58
^ Levin (1987), p. 60
^ Blatt et al. (1980), pp. 75–80
^ Folk (1965), p. 62
^ For an overview of major minerals in siliciclastic rocks and their
relative stabilities, see Folk (1965), pp. 62–64.
^ Stanley (1999), pp. 60–61
^ Levin (1987), p. 92
^ Stanley (1999), p. 61
^ Levin (1987), pp. 92–93
^ Tarbuck & Lutgens (1999), pp. 160–161
^ Press et al. (2003), p. 171
^ Boggs (1987), p. 138
^ For descriptions of cross-bedding, see Blatt et al. (1980), p. 128,
pp. 135–136; Press et al. (2003), pp. 171–172.
^ Blatt et al. (1980), pp. 133–135
^ For an explanation about graded bedding, see Boggs (1987), pp.
143–144; Tarbuck & Lutgens (1999), p. 161; Press et al. (2003),
^ Collinson et al. (2006), pp. 46–52
^ Blatt et al. (1980), pp. 155–157
^ Tarbuck & Lutgens (1999), p. 162
^ Levin (1987), p. 62
^ Blatt et al. (1980), pp. 136–154
^ For a short description of trace fossils, see Stanley (1999), p. 62;
Levin (1987), pp. 93–95; and Collinson et al. (2006), pp. 216–232.
^ Collinson et al. (2006), p. 215
^ For concretions, see Collinson et al. (2006), pp. 206–215.
^ Collinson et al. (2006), pp. 183–185
^ Collinson et al. (2006), pp. 193–194
^ Collinson et al. (2006), pp. 202–203
^ For an overview of different sedimentary environments, see Press et
al. (2003) or Einsele (2000), part II.
^ For a definition of shallow marine environments, see Levin (2003),
^ Tarbuck & Lutgens (1999), pp. 452–453
^ For an overview of continental environments, see Levin (2003), pp.
^ Baker, Victor R.; Nummedal, Dag, eds. (1978). The Channeled
Scabland: A Guide to the
Geomorphology of the Columbia Basin,
Washington. Washington, D.C.: Planetary
Geology Program, Office of
Space Science, National Aeoronautics and Space Administration.
pp. 173–177. ISBN 0-88192-590-X.
^ Tarbuck & Lutgens (1999), pp. 158–160
^ Reading (1996), pp. 19–20
^ Reading (1996), pp. 20–21
^ For an overview over facies shifts and the relations in the
sedimentary rock record by which they can be recognized, see Reading
(1996), pp. 22–33.
^ For an overview of sedimentary basin types, see Press et al. (2003),
pp. 187–189; Einsele (2000), pp. 3–9.
^ For a short explanation of Milankovitch cycles, see Tarbuck &
Lutgens (1999), pp. 322–323; Reading (1996), pp. 14–15.
^ Stanley (1999), p. 536
^ Andersen & Borns (1994), pp. 29–32
^ a b Reading (1996), p. 17
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Basic Sedimentary Rock Classification, by Lynn S. Fichter, James
Madison University, Harrisonburg.VI ;
Sedimentary Rocks Tour, introduction to sedimentary rocks, by Bruce
Perry, Department of Geological Sciences,
California State University