7. Sedimentary rocks
Sediments and sedimentary rocks cover 90% of Earth’s surface. They are essential to ourunderstanding of the origin and evolution of life because they are the rocks containing fossils. They
are the rocks from which coal, oil, and natural gas can be extracted. Limestones are sedimentaryrocks made of CaCO3, a major component of cement and many other industrial products.
Moreover, salt deposits (evaporites) are a primary source of NaCl (table salt) and other chemicalcompounds. Sedimentary rocks are also often associated with reservoirs of groundwater.

7.1. What are sediments?
Examples of sediments include beach sand, gravels of riverbeds, and fine particles of desert
dunes. These are sediments that have not yet been transformed into sedimentary rocks. They arestill soft, unlithified. We will see in section 7.3 how soft sediments become a solid lithified rock.
Sediments can be classified into two broad categories: solid matter and ions in solution.
Examples of solid sedimentary particles: (1) fragments of minerals and rocks, (2) hard parts oforganisms (e.g. mollusk shell, coral skeleton), and (3) organic matter (e.g. plant leaves, wood).
Examples of ions in solution: Na+, K+, Cl-, Ca2+, CO32-…

7.2. How do sediments form?
7.2.1. Weathering
Sediments form by destruction of preexisting rocks. The destruction process is called rockweathering. Two types of weathering can be distinguished: (1) physical weathering and (2)chemical weathering.
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Physical weathering

Major physical weathering agents are wind (eolian weathering), water, and ice (glacialweathering). The dust carried by the wind abrades the surface of rocks. The polished surface ofstones in arid regions such as the Moroccan desert or Mars illustrates well the process of eolianweathering. Water is a powerful weathering agent. The destructive action of waves along
shorelines is a good example. Receding shorelines due to wave action are common around theworld. Given enough time, rivers can cut through hard rocks and form V-shaped valleys withsteep flanks (e.g. Grand Canyon, USA). Ice is also a powerful agent of rock destruction. Glaciers
are like rivers of ice which can carve large U-shaped valleys. During the last ice age, the northern
polar ice cap was much more extensive than today. Regions of North America and northern
Europe that are now ice-free still bear the marks of glacial weathering. Ice can also destroy rocks
through frost wedging. This weathering process results from the fact that the volume of water
increases when it freezes. If liquid water fills the fractures in a rock and then freezes, the ice
1expands and widens the fractures, which can in turn break the rock.
Plants may also participate in rock physical weathering through root wedging. Plant roots in thefractures of a rock force them to open further as the plant grows, which may lead to rockdislocation (biophysical weathering).
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Chemical weathering

Rainwater and CO2 combine in soils to form the weak carbonic acid H2CO3. In contact withcarbonate rocks, the carbonic acid dissolves CaCO3. The products of this reaction, Ca2+, HCO3and CO32-, are transported by rivers to the ocean. In the ocean, the ions Ca2+ and CO32- can beused by marine organisms to build their calcareous hard parts (see related slide). Dissolution ofCaCO3 is responsible for the formation of caves and other dissolution features (karst).
The process of CaCO3 dissolution does not necessarily require the action of living organisms.
However, the active dissolution of CaCO3 by organisms happens as well. For example, somebivalves are capable of dissolving CaCO3 and bore holes in which they live (biochemicalweathering).
Like carbonate rocks, silicate minerals can react with acidic groundwater. The products of thisweathering reaction include dissolved ions (e.g. Ca2+, Si4+, Fe2+, HCO3-, CO32-, and H+) and clayminerals. Ions are transported by rivers to the ocean and can be used by marine organisms tobuild hard parts made of CaCO3 and SiO2.
The weathering of silicate rocks results in a net removal of CO 2 from the atmosphere over millionsof years. This slow removal of CO2 is balanced by an equally slow input of CO2 from volcanoes
and hot springs. This is an important mechanism for regulating Earth’s climate at geologicaltimescales －over millions of years－ (see related slides for further explanation).
7.2.2. Erosion and transport
The removal of sediments from their source area is called erosion. Erosion is carried out by thesame flowing agents causing rock physical weathering: wind, water, and ice. Sediments are thentransported by these same agents to the site of their deposition. If there is no active removal,sediments may simply move away from their source area by gravity. The downslope movement ofsediments by gravity is called mass wasting.
7.2.3. Sediment deposition
Solid sedimentary particles carried either by wind (eolian transport), water or ice (glacialtransport) will at some point be deposited. Deposition of a sedimentary particle transported by windor water occurs when the wind/water speed decreases and is no longer capable of carrying the2particle in question.
In water, strong currents with a speed of >50 cm/s can carry all sizes of particles up to boulder-size
sediments (>25.6 cm). Moderately strong currents with a speed of 20-50 cm/s can carry sand (62.5µm-2 mm) and smaller particles. Weak currents (<20 cm/s) can carry silts and clays (<62.5 µm).
Note that we consider here only sediments consisting of fragments of rocks and minerals which
generally have roughly the same density. We don’t consider material with a very low density likewood.
Sediments carried by ice are deposited when the ice melts. The nature and morphology of these
deposits is typical of glacial deposits and can be recognized in the field. Glacial deposits are usefulindicators of past climate as they are indicative of great accumulations of ice.
In the case of mass wasting, deposition is controlled by the nature of sediments (e.g. size, shape)
and the slope angle.
For ions in solution, deposition occurs either abiotically or biotically. An example of abiotic
deposition is the precipitation of minerals in a saline lake by evaporation to form a type of rockcalled evaporite (see also chapter 4). Biotic deposition of ions in solution is the process ofbiomineralization. Corals, for example, use Ca2+ and CO32- present in the ocean to produce theirskeleton made of CaCO3.

7.3. How do sedimentary rocks form?
How are soft sediments, say a beach sand for example, transformed into hard sedimentary rocks,
in this case sandstone?
As soft sediments accumulate, they are progressively buried under younger layers of sediments.
The set of physical and chemical changes which happen to soft sediments after their burial iscalled diagenesis. The result of diagenesis includes the transformation of soft sediments into hardrock. This transformation is called lithification. Lithification results from two main diagenetic
processes:(1) Compaction: as sediments are buried deeper and deeper under younger sedimentary layers,
the pressure and temperature increase. Sediments are squeezed, compacted. The space
between sediment grains gets smaller. Dissolution also occurs where sediment grains are incontact as a consequence of increased pressure.
(2) Cementation: dissolved ions precipitate in the open space left between sediment grains.
Common mineral cements include calcite (CaCO3), hematite (Fe2O3), and quartz (SiO2).
Compaction and cementation contribute to reduce the rock porosity. Porosity is a measure of theamount of open space (or pores) in a rock.

37.4. Properties of sediments and sedimentary rocks
The characteristics of sediment grains can provide information about the environment in which
they have formed. Fundamental properties of sediments and sedimentary rocks include:
Grain size: Since mineral and rock fragments have generally roughly similar densities, grain size
gives a good indication of the strength of the current that has transported them. For example, arock composed of sand-sized sedimentary grains is indicative of a stronger current relative to arock composed of clay particles.
Sorting: Sorting is a measure of the variation in grain size within a sediment or sedimentary rock.
If all sediment grains have approximately the same size, the sediment is well sorted. If thesediment is made of a mixture of small grains and large grains, the sediment is poorly sorted. The
level of sorting provides information about the type of current which transported the sediment. If
the strength of the water current is constant over time, the size of sediment grains deposited tends
to be uniform. If the strength of the water current is variable, the size of grains deposited varies
accordingly.
Grain morphology: During transport sediment grains are abraded and become rounded.
Therefore, the roundness of sediment grains provides information about the distance over which
sediments were transported. Greater distances of transport result in greater roundness.

7.5. Sedimentary basins and depositional environments
Large amounts of sediments are deposited in depressions of the Earth’s crust. Large depressions
in which abundant sediments accumulate are called sedimentary basins. The largest basins are
the ocean basins. Their size and morphology is controlled by plate tectonics. An example of a large
continental sedimentary basin is the East African Rift which corresponds to a divergent boundary
where two plates are pulled apart. The rift contains many lakes in which sediments accumulate. In
the future, if the plates keep moving in opposite directions, the continental lithosphere will stretch
and get thinner; the rift will grow larger and deeper, leaving more space for sediments toaccumulate. The downward movement of the lithosphere as it is stretched and thinned is calledsubsidence. Subsidence is accentuated by the weight of sediments pushing down the lithosphericplate. Seawater will ultimately flood the rift and seafloor spreading will initiate, transforming thecontinental rift into an ocean basin. The two continental margins that are moving apartprogressively cool as they are moving away from the source of heat, i.e., the mid-ocean ridge. As it
cools, the lithosphere becomes denser and progressively sinks, creating space for sediments toaccumulate. This is a particular type of subsidence called thermal subsidence (seecorresponding slide). It enables the accumulation of thick piles of sediments along continental
margins and is a key process in the formation of continental shelves.
There are many different kinds of depositional environments (e.g. deserts, lakes, rivers, deltas,
beaches, organic reefs, continental shelves, deep sea…) which belong to either one of thefollowing three categories: (1) continental, (2) shoreline, and (3) marine environments. Each4

depositional environment is characterized by a unique set of physical, chemical, and biologicalprocesses. The nature of sediments deposited in these environments is determined by theseprocesses. For example, the deep sea is not affected by strong, sustained currents likeshallow-water environments. The deep sea is thus generally characterized by the deposition offine-grained sediments (mostly mud). Deep-sea organisms are also different from those living inshallow waters. Let’s take another example: coral reefs. Coral reefs are associated with tropicaland subtropical climates. They grow in shallow water and their rigid framework consists mainly ofcoral skeletons and crustose coralline algae.
Useful indicators of past depositional environments are sedimentary structures. When one looks
at an outcrop of sedimentary rocks, the most conspicuous sedimentary structure is often thebedding (see related slides for illustrations). Sedimentary beds can be a few cm thick to several mthick. Bedding results from changes in sedimentation affecting grain size and/or sedimentcomposition. The equivalent of bedding at the mm scale is called lamination. Bedding is useful tostudy tectonic deformation (folds and faults) because sediment layers are usually depositedoriginally flat and horizontal. Other remarkable sedimentary structures are ripple marks (seerelated slides for illustrations). Ripple marks are a common feature of sandy beaches. The
geometry of ripple marks preserved in sedimentary rocks provides information about the transport
agent (wave vs. wind, unidirectional vs. bidirectional, current direction). Burrows are sedimentary
structures too. They represent one example of sedimentary structures resulting from the activity ofliving organisms. Other biological sedimentary structures include traces left by marine organisms
moving on the seafloor.
In conclusion, geologists are able to reconstruct past environments based on the characteristics ofsedimentary rocks (characteristics of mineral grains and cements, fossil content, and sedimentarystructures). By studying successions of sedimentary rocks of different ages exposed on Earth’s
surface or uncovered by drilling, it becomes possible to study environmental changes overgeological time scales. The fossil content of the rocks can also be used to gain an understandingof the nature and mechanisms of biological evolution.

7.6. Types of sedimentary rocks
7.6.1. Siliciclastic sedimentary rocks
Siliciclastic sedimentary rocks are composed of fragments of rocks (lithic fragments) andminerals. The composition of these rocks is dominated by silicate minerals, like quartz or feldspar.
Siliciclastic rocks are classified according to either grain size (coarse = conglomerate, medium =sandstone, fine = shale) or grain composition (e.g. arkose = sandstone containing at least 25% ofa mineral called feldspar).
7.6.2. Biochemical sedimentary rocks

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Biochemical sedimentary rocks are usually formed by the accumulation of the hard parts ofmarine organisms (e.g. bivalve shells, coral skeletons…). A beach deposit consisting of fragmentsof bivalve shells (bioclasts) is an example of biochemical sediments. Coral reef framework is
another example. Bivalve shells and coral skeletons are made of CaCO 3 but another major
biomineral is silica (SiO2). Marine microorganisms, such as diatoms and radiolarians, producing
tiny shells made of silica can accumulate on the deep seafloor and form siliceous rocks called
diatomite and radiolarite.
The constituents of biochemical sedimentary rocks are generally formed by the direct biologicalprecipitation of CaCO3. But biological activity can also induce precipitation of CaCO 3 indirectly.
Photosynthetic microbial mats called stromatolites are a good example. The equilibrium between
dissolved calcium and carbonate ions in the sea and the mineral calcium carbonate can be written
as follows: Ca2+ + 2HCO3- ↔ CaCO3 + CO2 + H2O. The reaction of photosynthesis is 6H2O +6CO2 + sunlight → C6H12O6 + 6O2. Photosynthesis consumes CO2 whereas calcification releases
CO2 in the environment. Consequently, the photosynthetic activity of stromatolites tends topromote calcification by shifting the equilibrium state of the carbonate system to the right (Ca2+ +2HCO3- → CaCO3 + CO2 + H2O). Stromatolites can form rigid pinnacles and these structures owe
their rigidity to the early cementation induced by photosynthesis (see related slide for illustration).
7.6.3. Chemical sedimentary rocks
Chemical sedimentary rocks are formed by mineral precipitation which does not involve
biological activity. For example, minerals precipitating at the bottom of a saline lake subject tointense evaporation. The accumulation of minerals in this kind of setting produces a rock called
evaporite. One common constituent of evaporites is NaCl (halite or table salt).
7.6.4. Organic sedimentary rocks
Organic sedimentary rocks are formed by accumulation of organic matter (e.g. plants). Usually,organic matter is quickly decomposed by bacterial activity. However, in particular settings which
are characterized by a high rate of accumulation of organic matter and low levels of oxygen,organic matter can be preserved and form thick accumulations. An example of such accumulationis peat. Peat forms in wetlands by accumulation of plants. Low oxygen concentrations prevent thecomplete decomposition of organic matter. As peat is buried under younger sediments, theincrease in pressure transforms peat into lignite (compressed peat). At greater depth, pressure
and temperature increases further and lignite is converted into coal. Coal is a complex mixture ofmineral matter and organic carbon compounds. Increasing further the pressure and temperaturetransforms coal into anthracite. Anthracite has a higher carbon content than coal. Coal andanthracite have a different chemical composition than peat and lignite. Heat and pressure havemodified their composition and texture. These rocks have undergone some degree ofmetamorphism (see next chapter).
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