173
Chapter in Review
detrital material, which originates and is transported as solid
particles from both mechanical and chemical weathering, which,
when lithified, forms detrital sedimentary rocks; and (2) from
soluble material produced largely by chemical weathering,
which, when precipitated, forms chemical sedimentary rocks. Coal
is the primary example of a third group called organic sedimentary rocks, which consist of organic carbon from the remains of
partially altered plant material.
Particle size is the primary basis for distinguishing among
various detrital sedimentary rocks. The size of the particles in a
detrital rock indicates the energy of the medium that transported
them. For example, gravels are moved by swiftly flowing rivers,
whereas less energy is required to transport sand. Common
detrital sedimentary rocks include shale (silt- and clay-size particles), sandstone, and conglomerate (rounded gravel-size particles)
or breccia (angular gravel-size particles).
Precipitation of chemical sediments occurs in two ways: (1) by
inorganic processes, such as evaporation and chemical activity; or
(2) by organic processes of water-dwelling organisms that produce
sediments of biochemical origin. Limestone, the most abundant
chemical sedimentary rock, consists of the mineral calcite
(CaCO 3 ) and forms either by inorganic means or as the result of
biochemical processes. Inorganic limestones include travertine,
which is commonly seen in caves, and oolitic limestone, consisting
of small spherical grains of calcium carbonate. Other common
chemical sedimentary rocks include dolostone (composed of the
calcium-magnesium carbonate mineral dolomite), chert (made of
microcrystalline quartz), and evaporites (such as rock salt and
rock gypsum).
Diagenesis refers to all of the physical, chemical, and biological
changes that occur after sediments are deposited and during and
after the time they are turned into sedimentary rock. Burial
promotes diagenesis. Diagenesis includes lithification.
Lithification refers to the processes by which unconsolidated
sediments are transformed into solid sedimentary rock. Most
sedimentary rocks are lithified by means of compaction and/or
cementation. Compaction occurs when the weight of overlying
materials compresses the deeper sediments. Cementation, the
most important process by which sediments are converted to
sedimentary rock, occurs when soluble cementing materials,
such as calcite, silica, and iron oxide, are precipitated onto sediment grains, fill open spaces, and join the particles. Although
most sedimentary rocks are lithified by compaction or cementation, certain chemical rocks, such as the evaporites, initially form
as solid masses of intergrown crystals.
Sedimentary rocks are divided into three groups: detrital,
chemical, and organic. All detrital rocks have a clastic texture,
which consists of discrete fragments and particles that are
cemented and compacted together. The main criterion for subdividing the detrital rocks is particle size. Common detrital rocks
include conglomerate, sandstone, and shale. The primary basis for
distinguishing among different rocks in the chemical group is
their mineral composition. Some chemical rocks, such as those
deposited when seawater evaporates, have a nonclastic (crystalline)
texture in which the minerals form a pattern of interlocking crystals. However, in reality, many of the sedimentary rocks classified
into the chemical group also contain at least small quantities of
detrital sediment. Common chemical rocks include limestone,
chert, and rock gypsum. Coal is the primary example of an organic
sedimentary rock.
Sedimentary environments are those places where sediment
accumulates. They are grouped into continental, marine, and
transitional (shoreline) environments. Each is characterized by
certain physical, chemical, and biological conditions. Because
sediment contains clues about the environment in which it was
deposited, sedimentary rocks are important in the interpretation
of Earth’ s history.
Layers, called strata or beds, are probably the single most
characteristic feature of sedimentary rocks. Other features found
in some sedimentary rocks, such as ripple marks, mud cracks,
cross-bedding, graded bedding, and fossils, also give clues to past
environments.
Earth materials that are not used as fuels or processed for
the metals they contain are referred to as nonmetallic resources.
Many are sediments or sedimentary rocks. The two broad groups
of nonmetallic resources are building materials and industrial
minerals. Limestone, perhaps the most versatile and widely
used rock of all, is found in both groups.
Coal, petroleum, and natural gas, the fossil fuels of our modern
economy, are all associated with sedimentary rocks. Coal originates from large quantities of plant remains that accumulate in
an oxygen-deficient environment, such as a swamp. More than
80 percent of present-day coal usage is for the generation of
electricity. Air pollution from the sulfur-oxide gases that form
from burning most types of coal is a significant environmental
problem.
Oil and natural gas, which commonly occur together in the
pore spaces of some sedimentary rocks, consist of various
hydrocarbon compounds (compounds made of hydrogen and carbon) mixed together. Petroleum formation is associated with the
accumulation of sediment in ocean areas rich in plant and animal
remains that have become buried and isolated in an oxygendeficient environment. As the mobile petroleum and natural gas
form, they migrate and accumulate in adjacent permeable beds
such as sandstone. If the upward migration is halted by an
impermeable rock layer, referred to as a cap rock, a geologic
environment develops that allows for economically significant
amounts of oil and gas to accumulate underground in what is
termed an oil trap.
Chapter in Review
detrital material, which originates and is transported as solid
particles from both mechanical and chemical weathering, which,
when lithified, forms detrital sedimentary rocks; and (2) from
soluble material produced largely by chemical weathering,
which, when precipitated, forms chemical sedimentary rocks. Coal
is the primary example of a third group called organic sedimentary rocks, which consist of organic carbon from the remains of
partially altered plant material.
Particle size is the primary basis for distinguishing among
various detrital sedimentary rocks. The size of the particles in a
detrital rock indicates the energy of the medium that transported
them. For example, gravels are moved by swiftly flowing rivers,
whereas less energy is required to transport sand. Common
detrital sedimentary rocks include shale (silt- and clay-size particles), sandstone, and conglomerate (rounded gravel-size particles)
or breccia (angular gravel-size particles).
Precipitation of chemical sediments occurs in two ways: (1) by
inorganic processes, such as evaporation and chemical activity; or
(2) by organic processes of water-dwelling organisms that produce
sediments of biochemical origin. Limestone, the most abundant
chemical sedimentary rock, consists of the mineral calcite
(CaCO 3 ) and forms either by inorganic means or as the result of
biochemical processes. Inorganic limestones include travertine,
which is commonly seen in caves, and oolitic limestone, consisting
of small spherical grains of calcium carbonate. Other common
chemical sedimentary rocks include dolostone (composed of the
calcium-magnesium carbonate mineral dolomite), chert (made of
microcrystalline quartz), and evaporites (such as rock salt and
rock gypsum).
Diagenesis refers to all of the physical, chemical, and biological
changes that occur after sediments are deposited and during and
after the time they are turned into sedimentary rock. Burial
promotes diagenesis. Diagenesis includes lithification.
Lithification refers to the processes by which unconsolidated
sediments are transformed into solid sedimentary rock. Most
sedimentary rocks are lithified by means of compaction and/or
cementation. Compaction occurs when the weight of overlying
materials compresses the deeper sediments. Cementation, the
most important process by which sediments are converted to
sedimentary rock, occurs when soluble cementing materials,
such as calcite, silica, and iron oxide, are precipitated onto sediment grains, fill open spaces, and join the particles. Although
most sedimentary rocks are lithified by compaction or cementation, certain chemical rocks, such as the evaporites, initially form
as solid masses of intergrown crystals.
Sedimentary rocks are divided into three groups: detrital,
chemical, and organic. All detrital rocks have a clastic texture,
which consists of discrete fragments and particles that are
cemented and compacted together. The main criterion for subdividing the detrital rocks is particle size. Common detrital rocks
include conglomerate, sandstone, and shale. The primary basis for
distinguishing among different rocks in the chemical group is
their mineral composition. Some chemical rocks, such as those
deposited when seawater evaporates, have a nonclastic (crystalline)
texture in which the minerals form a pattern of interlocking crystals. However, in reality, many of the sedimentary rocks classified
into the chemical group also contain at least small quantities of
detrital sediment. Common chemical rocks include limestone,
chert, and rock gypsum. Coal is the primary example of an organic
sedimentary rock.
Sedimentary environments are those places where sediment
accumulates. They are grouped into continental, marine, and
transitional (shoreline) environments. Each is characterized by
certain physical, chemical, and biological conditions. Because
sediment contains clues about the environment in which it was
deposited, sedimentary rocks are important in the interpretation
of Earth’ s history.
Layers, called strata or beds, are probably the single most
characteristic feature of sedimentary rocks. Other features found
in some sedimentary rocks, such as ripple marks, mud cracks,
cross-bedding, graded bedding, and fossils, also give clues to past
environments.
Earth materials that are not used as fuels or processed for
the metals they contain are referred to as nonmetallic resources.
Many are sediments or sedimentary rocks. The two broad groups
of nonmetallic resources are building materials and industrial
minerals. Limestone, perhaps the most versatile and widely
used rock of all, is found in both groups.
Coal, petroleum, and natural gas, the fossil fuels of our modern
economy, are all associated with sedimentary rocks. Coal originates from large quantities of plant remains that accumulate in
an oxygen-deficient environment, such as a swamp. More than
80 percent of present-day coal usage is for the generation of
electricity. Air pollution from the sulfur-oxide gases that form
from burning most types of coal is a significant environmental
problem.
Oil and natural gas, which commonly occur together in the
pore spaces of some sedimentary rocks, consist of various
hydrocarbon compounds (compounds made of hydrogen and carbon) mixed together. Petroleum formation is associated with the
accumulation of sediment in ocean areas rich in plant and animal
remains that have become buried and isolated in an oxygendeficient environment. As the mobile petroleum and natural gas
form, they migrate and accumulate in adjacent permeable beds
such as sandstone. If the upward migration is halted by an
impermeable rock layer, referred to as a cap rock, a geologic
environment develops that allows for economically significant
amounts of oil and gas to accumulate underground in what is
termed an oil trap.