, 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.
Moodies Group,
Barberton Greenstone Belt,
South Africa 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(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.
Texture (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. 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 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'. and
sphericity of grains The form of the clasts can reflect the origin of the rock. For example,
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 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 File:00065 sand collage.jpg|thumb|Global collage of sand samples. There is one square centimeter of sand on every sample photo. Sand samples row by row from left to right: 1. Glass sand from Kauai, Hawaii 2. Dune sand from the Gobi Desert 3. Quartz sand with green glauconite from Estonia 4. Volcanic sand with reddish weathered basalt from Maui, Hawaii 5. Biogenic coral sand from Molokai, Hawaii 6. Coral pink sand dunes from Utah 7. Volcanic glass sand from California 8. Garnet sand from Emerald Creek, Idaho 9. Olivine sand from Papakolea, Hawaii. Most sedimentary rocks contain either quartz (
siliciclastic rocks) or
calcite (
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,
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 weathered to
clay minerals like
kaolinite,
illite or
smectite.
Fossils ,
California 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. Dead organisms in nature are usually quickly removed by
scavengers,
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. s 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, 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. 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 in a
fluviatile sandstone,
Middle Old Red Sandstone (
Devonian) on
Bressay,
Shetland Islands s, a type of
sole marking on the base of a vertical layer of Triassic
sandstone in Spain formed by a current in a sandstone that was later tilted (
Haßberge,
Bavaria) 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 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
dunes 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 crystal mold in dolomite, Paadla Formation (
Silurian),
Saaremaa, Estonia 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. 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 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 and faults, which can be difficult to distinguish from folds and faults formed by
tectonic forces acting on lithified rocks. ==Depositional environments==