Sedimentary rocks are types of
rock that are formed by the accumulation or deposition of
mineral or
organic
Organic may refer to:
* Organic, of or relating to an organism, a living entity
* Organic, of or relating to an anatomical organ
Chemistry
* Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
particles at
Earth's surface, followed by
cementation.
Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called
sediment, and may be composed of
geological detritus (minerals) or
biological detritus (organic matter). The geological detritus originated from
weathering and
erosion 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 Mass movement may refer to:
* Mass movement (geology), the movement of rock and soil down slopes due to gravity
* Mass movement (politics)
A mass movement denotes a political party or movement which is supported by large segments of a population. P ...
, which are called agents of
denudation. 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). Sedimentation may also occur as dissolved minerals precipitate from
water solution
An aqueous solution is a 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), in water would be re ...
.
The sedimentary rock cover of the continents of the
Earth's crust
Earth's crust is Earth's thin outer shell of 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 and the upper part of the mantle. 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 and
metamorphic rocks. Sedimentary rocks are deposited in layers as
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
, forming a structure called
bedding. Sedimentary rocks are often deposited in large structures called
sedimentary basins. Sedimentary rocks have also been found on
Mars.
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,
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 condi ...
s,
tunnels,
canals or other structures. Sedimentary rocks are also important sources of
natural resources including
coal,
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 burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels m ...
s,
drinking water and
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 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 understanding of mo ...
. Sedimentology is part of both
geology and
physical geography and overlaps partly with other disciplines in the
Earth science
Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four spheres ...
s, such as
pedology,
geomorphology
Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or n ...
,
geochemistry and
structural geology
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informatio ...
.
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
Clastic sedimentary rocks are composed of rock fragments (''clasts'') that have been cemented together. The clasts are commonly individual grains of
quartz,
feldspar,
clay minerals
Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Clay mineral ...
, 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 basal cleavage. Mica is ...
. 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 (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 video game, multiplayer Time-keeping systems in games#Real-time, real-time virtual world, usually Text-based game, text-bas ...
(<1/16 mm diameter). Mud is further divided into
silt (1/16 to 1/256 mm diameter) and
clay (<1/256 mm diameter). The classification of clastic sedimentary rocks parallels this scheme;
conglomerates and
breccias are made mostly of gravel,
sandstones are made mostly of
sand, and
mudrock
Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too ...
s are made mostly of mud. 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), 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,
arkose
Arkose () or arkosic sandstone is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar.
Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose.
Quartz is c ...
, 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 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
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, especial ...
" 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
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
diatom
A diatom (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 comprising sev ...
s.
Chemical sedimentary rocks
Chemical sedimentary rock forms when mineral constituents in
solution become
supersaturated
In physical chemistry, supersaturation occurs with a solution when the concentration of a solute exceeds the concentration specified by the value of solubility at equilibrium. Most commonly the term is applied to a solution of a solid in a liqu ...
and inorganically
precipitate. Common chemical sedimentary rocks include
oolitic limestone and rocks composed of
evaporite minerals, such as
halite (rock salt),
sylvite
Sylvite, or sylvine, is potassium chloride (KCl) in natural mineral form. It forms crystals in the isometric system very similar to normal rock salt, halite ( NaCl). The two are, in fact, isomorphous. Sylvite is colorless to white with shades of ...
,
baryte
Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...
and
gypsum.
Other sedimentary rocks
This fourth miscellaneous category includes volcanic
tuff 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. The sediment that makes up these rocks was transported as
bed load,
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
breccias,
sandstone, and
mudrock
Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too ...
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 and the rock
dolomite.
*
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 ),
chlorides (halite and others built on ), and
sulfates (
gypsum and others built on ). Evaporite rocks commonly include abundant halite (rock salt),
gypsum, and
anhydrite.
*
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 natural gas.
*
Siliceous sedimentary rocks are almost entirely composed of silica (), typically as
chert,
opal,
chalcedony or other microcrystalline forms.
*
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
Sedimentary
rocks
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 composition, and the way in which it is formed. Rocks form the Earth's ...
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.
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
Compaction may refer to:
* Soil compaction, for mechanically induced compaction near the ground surface
* Compaction of ceramic powders
* Compaction (geology), part of the process of lithification involving mechanical dewatering of a sediment by ...
and
lithification 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 (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
that gives
red bed
Red beds (or redbeds) are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain th ...
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 an ...
processes, like the activity of
bacteria, 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 basal cleavage. Mica is ...
) are deformed, and pore space is reduced. Sediments are typically saturated with
groundwater 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 ions in solution.
As roc ...
are expelled. In addition to this physical compaction, chemical compaction may take place via
pressure solution. 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 becomes
lignite
Lignite, often referred to as brown coal, is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35%, and is considered the lowest rank of coal due to its relatively low heat ...
or coal.
Lithification follows closely on compaction, as increased temperatures at depth hasten the
precipitation 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
Competence may refer to:
*Competence (geology), the resistance of a rock against deformation or plastic flow.
*Competence (human resources), a standardized requirement for an individual to properly perform a specific job
*Competence (law), the me ...
.
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 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, the process that forms
metamorphic rock.
Properties
Color
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(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 ...
. Iron(II) oxide (FeO) only forms under low oxygen (
anoxic) circumstances and gives the rock a grey or greenish colour. Iron(III) oxide (Fe
2O
3) in a richer oxygen environment is often found in the form of the mineral
hematite
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
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, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain ...
. 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, 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. 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, since a rock is composed of clasts with different sizes. The
statistical distribution
In statistics, an empirical distribution function (commonly also called an empirical Cumulative Distribution Function, eCDF) is the distribution function associated with the empirical measure of a sample. This cumulative distribution functio ...
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.
# ordering: arranging items in a sequence ordered by some criterion;
# categorizing: grouping items with similar pro ...
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 shells of mollusks, trilobites, brachiopods, or other invertebrates. The term ''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 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'' describes the general smoothness of the shape of a grain.
* ''
Sphericity'' describes the degree to which the grain approaches a
sphere.
* ''
Grain form
A grain is a small, hard, dry fruit (caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legume ...
'' 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 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 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 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, using a
petrographic microscope.
Carbonate rocks predominantly 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 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. In this series, quartz is the most stable, followed by
feldspar,
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 basal cleavage. Mica is ...
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 hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Clay mineral ...
like
kaolinite
Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...
,
illite or
smectite.
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 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 a reduction in siz ...
.
Dead organisms in nature are usually quickly removed by
scavenger
Scavengers are animals that consume 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 herbivorous feeding b ...
s,
bacteria,
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 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 fossils, examples of which are
burrow
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 locomotion. Burrows provide a form of sh ...
s,
footprints, 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 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 various forms of
amorphous silica (
chalcedony,
flint,
chert),
carbonates (especially calcite), and
pyrite.
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
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 burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels m ...
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 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
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
. A bed is defined as a layer of rock that has a uniform
lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
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 called
cross-bedding. 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
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 currents.
The surface of a particular bed, called the
bedform, 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, f ...
s and
ripple marks. 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.
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
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
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 locomotion. Burrows provide a form of sh ...
s 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
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 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 (Greek: ''stylos'', pillar; ''lithos'', stone) are serrated surfaces within a rock mass at which mineral material has been removed by pressure dissolution, in a deformation process that decreases the total volume of rock. Minerals whi ...
. 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 rocks such as limestone or
chalk,
chert or
flint 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 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 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 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
Benjamin Scott Folds (born September 12, 1966) is an American singer-songwriter, musician, and composer, who is the first artistic advisor to the National Symphony Orchestra at the John F. Kennedy Center for the Performing Arts, Kennedy Center in ...
and faults, which can be difficult to distinguish from folds and faults formed by
tectonic forces acting on lithified rocks.
Depositional environments
The setting in which a sedimentary rock forms is called the
depositional 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 (
provenance
Provenance (from the 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 similar senses i ...
), but also on the environment itself.
A
marine
Marine is an adjective meaning of or pertaining to the sea or ocean.
Marine or marines may refer to:
Ocean
* Maritime (disambiguation)
* Marine art
* Marine biology
* Marine debris
* Marine habitats
* Marine life
* Marine pollution
Military
* ...
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 (including the
abyssal plain). 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,
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 are still deposited. 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 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.
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
shoal
In oceanography, geomorphology, and geoscience, a shoal is a natural submerged ridge, bank, or bar that consists of, or is covered by, sand or other unconsolidated material and rises from the bed of a body of water to near the surface. It ...
s 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 (rather than chemical) sediment.
A continental sedimentary environment is an environment in the interior of a continent. Examples of continental environments are
lagoons, 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,
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 be transported by wind or glaciers. Sediment transported by wind is called
aeolian
Aeolian commonly refers to things related to either of two Greek mythological figures:
* Aeolus (son of Hippotes), ruler of the winds
* Aeolus (son of Hellen), son of Hellen and eponym of the Aeolians
* Aeolians, an ancient Greek tribe thought to ...
and is almost 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, 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. 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, f ...
s (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. 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 unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
(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 content.
Coral, 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
Lithofacies may refer to:
* A mappable subdivision of a designated stratigraphic unit, distinguished from adjacent subdivisions on the basis of lithology; a facies characterized by particular lithologic features
* The rock record of any particular ...
; facies determined by fossils are
biofacies
In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...
.
Sedimentary environments can shift their geographical positions through time. Coastlines can shift in the direction of the sea when the
sea level drops (
regression
Regression or regressions may refer to:
Science
* Marine regression, coastal advance due to falling sea level, the opposite of marine transgression
* Regression (medicine), a characteristic of diseases to express lighter symptoms or less extent ( ...
), when the surface rises (
transgression
Transgression may refer to:
Legal, religious and social
*Sin, a violation of God's Ten Commandments or other elements of God's moral law
*Crime, legal transgression, usually created by a violation of social or economic boundary
**In civil law ju ...
) 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
Transgression may refer to:
Legal, religious and social
*Sin, a violation of God's Ten Commandments or other elements of God's moral law
*Crime, legal transgression, usually created by a violation of social or economic boundary
**In civil law ju ...
. In the case of transgression, deeper marine facies are deposited over shallower facies, a succession called
onlap.
Regression
Regression or regressions may refer to:
Science
* Marine regression, coastal advance due to falling sea level, the opposite of marine transgression
* Regression (medicine), a characteristic of diseases to express lighter symptoms or less extent ( ...
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 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 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
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), land eventually rises above sea level and the area becomes a source for new sediment as
erosion removes material. Where the lithosphere moves downward (
tectonic subsidence), 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. Rift basins are elongated, narrow and deep basins. Due to divergent movement, the lithosphere is
stretched
Stretching is a form of physical exercise in which a specific muscle or tendon (or muscle group) is deliberately flexed or stretched in order to improve the muscle's felt elasticity and achieve comfortable muscle tone. The result is a feeling ...
and thinned, so that the hot
asthenosphere
The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not ...
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 deposits.
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
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 equilibrium (social, economic, ecological, etc.), at least in the short r ...
. 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 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
In geology, continental collision is a phenomenon of plate tectonics that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains prod ...
, 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 belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...
. 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 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 basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of ...
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
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, ...
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 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 periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
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.
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 avalanc ...
s 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 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 on ...
, 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 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. 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.
Provenance
Provenance is the reconstruction of the origin of sediments. All rock exposed at Earth's surface is subjected to physical or chemical
weathering and broken down into finer grained sediment. All three types of rocks (
igneous, sedimentary and
metamorphic 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