Sedimentology encompasses the study of modern
sedimentSediment is any particulate matter that can be transported by fluid flow, and which eventually is deposited.Sediments are most often transported by water transported by wind and glaciers...
s such as sand, mud (silt), and clay, and the processes that result in their deposition. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and
sedimentary structuresSedimentary structures are those structures formed during sediment deposition.Sedimentary structures such as cross bedding, graded bedding and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terranes.-Flow structures:There are two...
.
Sedimentary rocks cover most of the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's surface, record much of the
Earth's historyThe history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation to the present day during the last 4.54 billion years. Nearly all branches of natural science have contributed to the understanding of the main events of the...
, and harbor the fossil record. Sedimentology is closely linked to
stratigraphyStratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
, the study of the physical and temporal relationships between rock layers or
strataIn geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. They may extend over...
.
The premise that the processes affecting the earth today are the same as in the past is the basis for determining how sedimentary features in the rock record were formed.
Sedimentology encompasses the study of modern
sedimentSediment is any particulate matter that can be transported by fluid flow, and which eventually is deposited.Sediments are most often transported by water transported by wind and glaciers...
s such as sand, mud (silt), and clay, and the processes that result in their deposition. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and
sedimentary structuresSedimentary structures are those structures formed during sediment deposition.Sedimentary structures such as cross bedding, graded bedding and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terranes.-Flow structures:There are two...
.
Sedimentary rocks cover most of the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's surface, record much of the
Earth's historyThe history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation to the present day during the last 4.54 billion years. Nearly all branches of natural science have contributed to the understanding of the main events of the...
, and harbor the fossil record. Sedimentology is closely linked to
stratigraphyStratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
, the study of the physical and temporal relationships between rock layers or
strataIn geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. They may extend over...
.
The premise that the processes affecting the earth today are the same as in the past is the basis for determining how sedimentary features in the rock record were formed. By comparing similar features today to features in the rock record—for example, by comparing modern sand dunes to dunes preserved in ancient aeolian sandstones—geologists reconstruct past environments.
Sedimentary rock types
There are four primary types of sedimentary rocks: clastics, carbonates, evaporites, and chemical.
- Clastic rocks
Clastic rocks are composed of fragments, or clasts, of pre-existing rock. The term is most commonly, but not uniquely, applied to sedimentary rocks. - Clastic metamorphic and igneous rocks :...
are composed of particles derived from the weatheringWeathering is the decomposition of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice,...
and erosionErosion is a gravity driven process that moves solids in the natural environment or their source and deposits them elsewhere...
of precursor rocks and consist primarily of fragmental material. Clastic rocks are classified according to their predominant grain size and their composition. In the past, the term "Clastic Sedimentary Rocks" were used to describe silica-rich clastic sedimentary rocks, however there have been cases of clastic carbonate rocks. The more appropriate term is siliciclasticSiliciclastic rocks are clastic noncarbonate sedimentary rocks that are almost exclusively silica-bearing, either as forms of quartz or other silicate minerals. All siliciclastic rocks are formed by inorganic processes, or deposited through some mechanical process, such as stream deposits that are...
sedimentary rocks.
- Organic sedimentary rocks are important deposits formed from the accumulation of biological detritus, and form coal
Coal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
and oil shaleOil shale, an organic-rich fine-grained sedimentary rock, contains significant amounts of kerogen from which technology can extract liquid hydrocarbons. The name oil shale represents a double misnomer, as geologists would not necessarily classify the rock as a shale, and its kerogen differs from...
deposits, and are typically found within basinsA structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat lying strata. Structural basins are geological depressions, and are the inverse of domes. Some elongated structural basins are also known as synclines...
of clastic sedimentary rocks
- Carbonates are composed of various carbonate minerals
Carbonate minerals are those minerals containing the carbonate ion: CO32-.-Anhydrous carbonates:*Calcite group: Trigonal**Calcite CaCO3**Gaspeite CO3**Magnesite MgCO3...
(most often calcium carbonateCalcium carbonate is a chemical compound with the chemical formula CaCO
3. It is a common substance found in rock in all parts of the world, and is the main component of shells of marine organisms, snails, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural...
(CaCO3)) precipitated by a variety of organic and inorganic processes. Typically, the majority of carbonate rocks are composed of reefIn nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
material .
- Evaporite
Evaporites are water-soluble mineral sediments that result from the evaporation of bodies of surficial water. Evaporites are considered sedimentary rocks.- Formation of evaporite rocks :...
s are formed through the evaporation of water at the Earth's surface and are composed of one or more salt minerals, such as haliteHalite is the mineral form of sodium chloride, NaCl, commonly known as rock salt. Halite forms isometric crystals. The mineral is typically colorless to yellow, but may also be light blue, dark blue, and pink depending on the amount and type of impurities...
or gypsumGypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO
4·2H
2O.-Crystal varieties:...
.
- Chemical sedimentary rocks, including some carbonates, are deposited by precipitation of minerals from aqueous solution. These include jaspilite and chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
.
Importance of sedimentary rocks
Sedimentary rocks provide a multitude of products which modern and ancient society has come to utilise.
- Art
Art is the process or product of deliberately arranging elements in a way that appeals to the senses or emotions. It encompasses a diverse range of human activities, creations, and modes of expression, including music, literature, film, sculpture, and paintings...
: marbleMarble is a non foliated metamorphic rock resulting from the metamorphism of limestone, composed mostly of calcite . It is extensively used for sculpture, as a building material, and in many other applications...
, although a metamorphosedMetamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
, is an example of the use of sedimentary rocks in the pursuit of aesthetics and art
- Architectural uses: stone derived from sedimentary rocks is used for dimension stone
Dimension stone is natural stone or rock that has been selected and fabricated to specific sizes or shapes. Color, texture and pattern, and surface finish of the stone are also normal requirements...
and in architectureFor a topical guide to this subject, see Outline of architecture. Architecture is the art and science of designing and constructing buildings and other physical structures for human shelter or use....
, notably slateSlate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary...
, a meta-shaleShale is a fine-grained, clastic sedimentary rock composed of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable...
, for roofing, sandstoneSandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow,...
for load-bearing buttressA buttress is an architectural structure built against or projecting from a wall which serves to support or reinforce the wall...
es
- Ceramic
A ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s and industrial materials: clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
for potteryPottery is the ceramic ware made by potters. Major types of pottery include earthenware, stoneware, and porcelain. The places where such wares are made are called potteries. Pottery is one of the oldest human technologies and art-forms, and remains a major industry today...
and ceramicA ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s including brickA brick is a block of ceramic material used in masonry construction, usually laid using mortar.-History:The oldest shaped bricks found date back to 7,500 B.C. They have been found in Çayönü, in the upper Tigris region, and in south east Anatolia close to Diyarbakir. Other more recent findings,...
s; cementIn the most general sense of the word, a cement is a binder, a substance which sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term "opus caementicium" to describe masonry which resembled concrete and was made from crushed...
and limeLime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
derived from limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
.
- Economic geology
Economic geology is concerned with earth materials that can be used for economic and/or industrial purposes. These materials include precious and base metals, nonmetallic minerals, construction-grade stone, petroleum minerals, coal, and water. The term commonly refers to metallic mineral deposits...
: sedimentary rocks host large deposits of SEDEXSedimentary exhalative deposits are ore deposits which are interpreted to have been formed by release of ore-bearing hydrothermal fluids into a water reservoir , resulting in the precipitation of stratiform ore.SEDEX deposits are the most important source of lead, zinc and barite, a major...
ore deposits of leadLead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
-zincZinc , also known as spelter, is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
-silverSilver is a chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, large deposits of copperCopper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
, deposits of goldGold is a chemical element with the symbol Au and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is...
, tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A steel-gray metal, tungsten is found in several ores, including wolframite and scheelite...
and many other precious minerals, gemstonesGemstones is the third solo album by Adam Green, released in 2005. The album is characterised by the heavy presence of Wurlitzer piano, whereas its predecessor relied on a string section in its instrumentation.-Track listing:#Gemstones – 2:24...
and industrial minerals including heavy mineral sands ore depositsHeavy mineral sands are a class of ore deposit which is an important source of zirconium, titanium, thorium, tungsten, rare earth elements, the industrial minerals diamond, sapphire, garnet, and occasionally precious metals or gemstones....
- Energy: petroleum geology
Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons .-Sedimentary basin analysis:...
relies on the capacity of sedimentary rocks to generate deposits of petroleumPetroleum or crude oil is a naturally occurring, flammable liquid found in rock formations in the Earth consisting of a complex mixture of hydrocarbons of various molecular weights, plus other organic compounds.The term "petroleum" was first used in the treatise De Natura Fossilium, published in...
oilAn oil is any substance that is liquid at ambient temperatures and is hydrophobic but soluble in organic solvents. Oils have a high carbon and hydrogen content and are nonpolar substances. The general definition above includes compound classes with otherwise unrelated chemical structures,...
s. CoalCoal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
and oil shaleOil shale, an organic-rich fine-grained sedimentary rock, contains significant amounts of kerogen from which technology can extract liquid hydrocarbons. The name oil shale represents a double misnomer, as geologists would not necessarily classify the rock as a shale, and its kerogen differs from...
are found in sedimentary rocks. A large proportion of the world's uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
energy resources are hosted within sedimentary successions.
- Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in...
: sedimentary rocks contain a large proportion of the Earth's groundwater aquiferAn aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
s. Our understanding of the extent of these aquifers and how much water can be withdrawn from them depends critically on our knowledge of the rocks that hold them (the reservoir).
Basic principles
The aim of sedimentology, studying sediments, is to derive information on the depositional conditions which acted to deposit the rock unit, and the relation of the individual rock units in a basin into a coherent understanding of the evolution of the sedimentary sequences and basins, and thus, the Earth's geological history as a whole.
The scientific basis of this is the principle of uniformitarianism, which states that the sediments within ancient sedimentary rocks were deposited in the same way as sediments which are being deposited at the Earth's surface today.
Sedimentological conditions are recorded within the sediments as they are laid down; the form of the sediments at present reflects the events of the past and all events which affect the sediments, from the source of the sedimentary material to the stresses enacted upon them after
diagenesisIn geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
are available for study.
The
principle of superpositionThe law of superposition is a key axiom based on observations of natural history that is a foundational principle of sedimentary stratigraphy and so of other geology dependent natural sciences:...
is critical to the interpretation of sedimentary sequences, and in older metamorphic terrains or fold and thrust belts where sediments are often intensely
foldedThe term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in size...
or deformed, recognising younging indicators or fining up sequences is critical to interpretation of the sedimentary section and often the deformation and metamorphic structure of the region.
Folding in sediments is analysed with the
principle of original horizontalityPrinciple of Original Horizontality was proposed by the Danish geological pioneer Nicholas Steno . This principle states that layers of sediment are originally deposited horizontally...
, which states that sediments are deposited at their angle of repose which, for most types of sediment, is essentially horizontal. Thus, when the younging direction is known, the rocks can be "unfolded" and interpreted according to the contained sedimentary information.
The
principle of lateral continuityThe principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous...
states that layers of sediment initially extend laterally in all directions unless obstructed by a physical object or topography.
The principle of cross-cutting relationships states that whatever cuts across or intrudes into the layers of strata is younger than the layers of strata.
Methodology of sedimentology
The methods employed by sedimentologists to gather data and evidence on the nature and depositional conditions of sedimentary rocks include;
- Measuring and describing the outcrop and distribution of the rock unit;
- Describing the rock formation
This is a List of rock formations, meaning isolated, scenic, or spectacular surface rock outcrops. These are usually the result of weathering and erosion sculpting the existing rock...
, a formal process of documenting thickness, lithology, outcrop, distribution, contact relationships to other formations
- Mapping the distribution of the rock unit, or units
- Descriptions of rock core (drilled and extracted from wells during hydrocarbon exploration)
- Sequence stratigraphy
Sequence stratigraphy is a relatively new branch of geology that attempts to link subdivide sedimentary deposits into unconformity bound units on a variety of scales and explain these stratal units in terms of control by relative sea-level changes and variations in sediment supply...
- Describes the progression of rock units within a basin
- Describing the lithology of the rock;
- Petrology
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form...
and petrographyPetrography is a branch of petrology which focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. Petrographic descriptions start with the field notes at the...
; particularly measurement of textureRock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...
, grain size, grain shape (sphericity, rounding, etc), sorting and composition of the sediment
- Analysing the geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and their...
of the rock
- Isotope geochemistry
Isotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Variations in the abundance of these isotopes, typically measured with an isotope ratio mass spectrometer or an accelerator mass spectrometer,...
, including use of radiometric datingRadiometric dating is a technique used to date materials, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
, to determine the age of the rock, and its affinity to source regions
Recent developments in sedimentology
The longstanding understanding of how some
mudstoneMudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
s form has been challenged by geologists at Indiana University (Bloomington) and the
Massachusetts Institute of TechnologyThe Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological research...
. The research, which appears in the December 14th, 2007, edition of
ScienceScience is the academic journal of the American Association for the Advancement of Science and is considered one of the world's most prestigious scientific journals. The peer-reviewed journal, first published in 1880 is circulated weekly and has a print subscriber base of around 130,000...
, counters the prevailing view of geologists that mud only settles when water is slow-moving or still, instead showing that "muds will accumulate even when currents move swiftly." The research shows that some mudstones may have formed in fast-moving waters: "Mudstones can be deposited under more energetic conditions than widely assumed, requiring a reappraisal of many geologic records."
Macquaker and Bohacs, in reviewing the research of Schieber et al., state that "these results call for critical reappraisal of all mudstones previously interpreted as having been continuously deposited under still waters. Such rocks are widely used to infer past climates, ocean conditions, and orbital variations."
See also
- Important publications in sedimentology
- Sequence stratigraphy
Sequence stratigraphy is a relatively new branch of geology that attempts to link subdivide sedimentary deposits into unconformity bound units on a variety of scales and explain these stratal units in terms of control by relative sea-level changes and variations in sediment supply...
- Rock formation
This is a List of rock formations, meaning isolated, scenic, or spectacular surface rock outcrops. These are usually the result of weathering and erosion sculpting the existing rock...
- Coal
Coal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
- Oil shale
Oil shale, an organic-rich fine-grained sedimentary rock, contains significant amounts of kerogen from which technology can extract liquid hydrocarbons. The name oil shale represents a double misnomer, as geologists would not necessarily classify the rock as a shale, and its kerogen differs from...
- Ore genesis
The various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust. Ore genesis theories are very dependent on the mineral or commodity....
- Clastic rocks
Clastic rocks are composed of fragments, or clasts, of pre-existing rock. The term is most commonly, but not uniquely, applied to sedimentary rocks. - Clastic metamorphic and igneous rocks :...