Sedimentary Rocks
Sedimentary Rocks
Sedimentary rocks account for about 5 to 10 percent of the Earth’s
outer 10 miles of crust. Because they are concentrated at the Earth’s
surface, the importance of this group is much greater than this
percentage implies.
Sedimentary rocks contain much of the basic information needed to
reconstruct Earth history. In addition, this group is associated with
many important energy and mineral resources.
Examples of
Sedimentary Rocks
Sandstone
Shale
Limestone
Bituminous Coal
Rock Salt
Breccia
Origins of Sedimentary Rock
Weathering begins the process. It involves the physical disintegration
and chemical deposition of pre-existing rocks (igneous, metamorphic
and sedimentary), which are the raw materials for sedimentary rocks.
Origins of Sedimentary Rock
Soluble materials are carried away by runoff and water. Solid particles
are moved down slope by gravity (called mass wasting). Transportation
by wind, water, glacial ice, etc., moves the materials from the sites
where they originated to the sites where they accumulate.
Origins of Sedimentary Rock
Deposition of solid particles occurs when wind and water currents slow
down and as glacial ice melts. The word sedimentary is derived from
the Latin sedimentum, which means “to settle.” Examples include the
mud on the floor of a lake, a gravel bar in a stream bed, particles in a
desert sand dune – even household dust in a corner of a room.
Origins of Sedimentary Rock
As deposition continues, older sediments are buried beneath younger
layers and gradually converted to sedimentary rock (lithified) by
compaction and cementation. This and other changes are referred to as
diagenesis, a collective term for all of the changes (short of
metamorphism) that take place in texture, composition, and other
physical properties after sediments are deposited.
Classification of Sedimentary Rocks
Three main categories of sedimentary rocks are recognized.
-Sediment has two principal sources: First, it may be an accumulation of
material that originates and is transported as solid particles derived from
both mechanical and chemical weathering. These deposits are termed
detrital, and the rocks they form are called detrital sedimentary rocks.
-The second major source of sediment is soluble material produced
largely by chemical weathering. When these ions in solution are
precipitated either by inorganic or biologic processes, the material is
called chemical sediment, which makes chemical sedimentary rocks.
-The third category is organic sedimentary rocks. The primary example
is coal, which is made up of organic carbon from the remains of plants
that died and accumulated on the floor of a swamp.
Detrital Sedimentary Rocks
Though a wide variety of minerals and rock fragments (clasts) may be
found in detrital rocks, clay minerals and quartz are the most common.
Remember that clay materials are the most abundant product of the
chemical weathering of silicate materials, especially the feldspars. Clays
are fine-grained materials with sheetlike crystalline structures similar to
the micas. The other common mineral, quartz, is abundant because it is
extremely durable and very resistant to chemical weathering.
Detrital Sedimentary Rock Examples:
Sandstone
Shale
Breccia
Detrital Sedimentary Rocks
Other common solid particles found in detrital rocks include pieces of
feldspars and micas. Because chemical weathering rapidly transforms
these minerals, their presence in sedimentary rocks indicates that erosion
and deposition were fast enough to preserve some of the primary
minerals from the source rock before they could be decomposed.
- No matter what minerals come together to form a detrital rock, particle
size is the primary basis for classifying detrital sedimentary rocks.
Detrital Sedimentary Rocks
The sizes of grains within detrital rocks may provide useful information
about environments of deposition. Currents of water or air sort the
particles by size – the stronger the current, the larger the particle size
carried. Gravels are moved by swiftly flowing rivers as well as by
landslides and glaciers. Less energy is required to transport sand; thus it
is found on windblown dunes and beaches. Even less energy is required
to move clay, so it settles very slowly in water environments.
Detrital Sedimentary Rocks
After determining particle size, the next way to classify a detrital
sedimentary rock is by describing how well the particle sizes are sorted
within a rock. Then, classification depends on a particle’s shape.
Detrital Sedimentary Rocks
By studying the degree of sorting, we can learn much about the
depositing current. Deposits of windblown sand are usually better sorted
than deposits sorted by wave activity. Particles washed by waves are
commonly better sorted than materials deposited by streams.
By studying particle shape, we
can see if grains have rounded
edges or sharp ones, which help
us learn about the distance and
area a rock was formed in. For
example, when streams, winds, or
waves move sand, the quartz
grains lose their sharp edges and
corners and become more
rounded as they collide with other
particles during transport.
Detrital Sedimentary Rocks
Common detrital rocks, in order of increasing particle size, are shale,
sandstone, and conglomerate (breccia).
Shale
Sandstone
Breccia
Shale consists of silt- and clay-size particles. These fine-grained detritals
account for more than half of all sedimentary rocks. The tiny grains in
shale tell us deposition occurs as the result of gradual settling from quiet,
nonturbulent currents such as lakes, river floodplains, lagoons, or deepocean basins. True shale has the ability to break into thin layers along
well-developed, closely spaced planes (fissility). If the rock breaks into
chunks (lacks fissility), it is called mudstone or siltstone.
Detrital Sedimentary Rocks
Common detrital rocks, in order of increasing particle size, are shale,
sandstone, and conglomerate (breccia).
Shale
Sandstone
Breccia
Sandstone rocks have more sand-sized grains than any other. These
detritals account for 20 percent of all sedimentary rocks. Owing to its
durability, quartz is the most common mineral in most sandstones. If is
made of mostly quartz, it may be called quartz sandstone. If it is made
up of 25 percent or more of feldspar, it is called arkose sandstone. If
more than 15 percent of the rock’s composition is silt and clay particles
along with large particles (called matrix), the rock is called graywacke.
Sandstone Types
Arkose
Sandstone
Quartz
Sandstone
Graywacke
Detrital Sedimentary Rocks
Common detrital rocks, in order of increasing particle size, are shale,
sandstone, and conglomerate (breccia).
Shale
Sandstone
Breccia
Conglomerates (like breccia) consist largely of gravel and are made of
poorly sorted particles. If the large particles are more angular, the rock is
called breccia. These large, angular particles indicate the material did not
travel far from its source area before being deposited. The particle size of
conglomerates tells us the strength of the current that transported them,
while the degree of rounding tells us how far the particles traveled. The
fragments in a rock help us identify the source rock that formed them.
Chemical Sedimentary Rocks
In contrast to detrital rocks, which form from solid materials, chemical
sediments come from ions that are carried in solution to lakes and seas.
This material does not remain dissolved in water indefinitely, however.
Some of it precipitates to form chemical sediments, which later become
chemical sedimentary rocks such as limestone, chert, and rock salt.
Precipitation of material occurs in two ways: Inorganic (like evaporation
and chemical activity) and organic (biochemical life processes from
water-dwelling organisms).
Chemical Sedimentary Rock Examples:
Limestone
Chert
Rock Salt
Chemical Sedimentary Rocks
Limestone
Chert
Rock Salt
Representing about 10 percent of the total volume of all sedimentary
rocks, limestone is the most abundant chemical sedimentary rock. It is
composed chiefly of the mineral calcite (CaCO3) and forms both
inorganically and organically.
Organically, limestone is found in forms of coral reefs, coquina (a rock
composed of shell fragments), and chalk. Inorganic limestone includes
travertine (commonly seen in caves), oolitic limestone (a rock made up
of spherical grains), and dolostone (calcium and magnesium-rich rocks).
Limestone Examples
Travertine
Coquina
Coral Reefs
Chalk
Chemical Sedimentary Rocks
Limestone
Chert
Rock Salt
Chert is a name used for a number of very compact and hard rocks
made of microcrystalline quartz (SiO2). One well-known form is flint,
whose dark color results from the organic matter it contains. Jasper, a
red variety, gets its color from iron oxide. The banded form is usually
referred to as agate.
Like glass, most chert has a conchodial fracture. Its hardness, ease of
chipping, and ability to hold a sharp edge made chert a favorite of
Native Americans for making points for spears and arrows.
Chert Examples
Jasper
Flint
Agate
Chemical Sedimentary Rocks
Limestone
Chert
Rock Salt
Very often, evaporation is the mechanism triggering deposition of
chemical precipitates. Minerals commonly created in this fashion
include halite (the chief component of rock salt), and gypsum.
When a body of seawater evaporates, less soluble minerals precipitate
first, and more soluble minerals precipitate later as salinity increases.
Gypsum precipitates when 80 percent of seawater has evaporated, while
halite settles out when 90 percent of the water has been removed. During
the last stages, potassium (potash) and magnesium salts precipitate out.
Examples of Evaporites
Sylvite:
Mined for
potassium
(potash)
fertilizer
Rock Salt
Bonneville Salt Flats
Gypsum
Coal – An Organic Sedimentary Rock
Coal is quite different from other sedimentary rocks. Unlike limestone
and chert, which are calcite- or silica-rich, coal is made mostly of
organic matter.
Organic Sedimentary Rock
Coal forms as decaying plant matter
accumulates in swamp environments.
There are different stages of coal, and it
isn’t until the material is compacted to a
specific amount that it becomes a
sedimentary rock called lignite and
bituminous coal.
Bituminous Coal
When the bituminous coal is heated or put under further stress, it
eventually creates anthracite coal, which is technically a
metamorphic rock.
Formation of Coal
Partial decomposition of plants in an
oxygen-poor swamp creates a layer of
peat, a soft brown material in which plant
structures are still easily recognized.
With shallow burial, peat slowly changes
to lignite, a soft brown coal. Burial
increases the temperature of the sediments
as well as the pressure on them, eventually
creating a harder, more compacted
sedimentary rock called bituminous coal.
More compaction, and thus a higher
concentration of fixed carbon, creates
anthracite – a very hard, shiny, black
metamorphic rock.
Classification of Sedimentary Rocks
Sediments and
Earth History
Sedimentary
environments are
those places where
sediment
accumulates. They
are grouped into
continental, marine,
and transitional
(marine)
environments.
Sediments and
Earth History
Because sediment
contains clues about
the environment in
which it was
deposited,
sedimentary rocks are
important in the
interpretation of
Earth’s history.
Sedimentary Structures
Ripple marks are produced by currents of water or wind. Mud cracks
form when wet mud or clay dries out and shrinks. Fossils, the remains
or traces of prehistoric life, are important inclusions in sedimentary
rock. They are important tools for interpreting the geologic past.
Sedimentary
Rocks and
Resources
- Sedimentary rocks provide
humans with nonmetallic
and energy resources, as
well as help make metallic
resources for every day use.
- Producing one ton of steel
requires about 1/3 ton of
limestone and between 2
and 20 pounds of fluorite.
Sedimentary Rocks and Energy Use
Coal, petroleum, and
natural gas are the
primary fuels of our
modern industrial
economy. About 85
percent of the energy
consumed in the
United States today
comes from these
basic fossil fuels.
Today, coal provides 22
percent of the energy needs
used in the United States.
Coal Fields of the United States
The Carbon Cycle