Sedimentary Rocks
7
Sedimentary Rocks begins with a discussion of diagenesis and lithification of sediment followed by a detailed
examination of the various detrital and chemical sedimentary rocks, including shale, sandstone, conglomerate,
limestone, dolostone, chert, and coal, as well as several evaporites. Following a look at the classification of
sedimentary rocks and the various sedimentary environments, the chapter concludes with a discussion of
sedimentary structures.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
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Discuss the physical and chemical processes involved in diagenesis.
List the common minerals found in detrital sedimentary rocks.
Explain how detrital sedimentary rocks are formed.
List and briefly define the major types of detrital sedimentary rocks.
Explain how chemical sedimentary rocks are formed.
List and briefly define the major types of chemical sedimentary rocks.
Discuss the classification of sedimentary rocks.
List and briefly discuss the major sedimentary depositional environments.
Explain the concept of sedimentary facies.
List and discuss the importance of sedimentary structures.
Chapter Outline___________________________________________________________________
I. What is a sedimentary rock?
A. Products of mechanical and chemical
weathering
B. Account for about 5 percent (by volume)
of Earth’s outer 16 kilometers (10 miles)
C. Contain evidence of past environments
1. Provide information about sediment
transport
2. Often contain fossils
D. Economic importance
1. Coal
2. Petroleum and natural gas
3. Sources of iron, aluminum, and
manganese
B. Diagenesis – all chemical, physical, and
biological changes that take place after
sediments are deposited
1. Occurs within the upper few
kilometers of Earth’s crust
2. Includes:
a. Recrystallization – development
of more stable minerals from less
stable ones
b. Lithification – unconsolidated
sediments are transformed into
solid sedimentary rocks by
1. Compaction
2. Cementation by the materials
a. Calcite and/or
b. Silica
c. Iron oxide
II. Turning sediment into sedimentary rock
A. A great deal of change can occur to
sediment after it is deposited
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III. Types of sedimentary rocks
A. Material originates from mechanical
and/or chemical weathering
B. Rock types are based on the source of the
material
1. Detrital rocks – material is solid
particles
2. Chemical rocks – material that was
once in solution
IV. Detrital sedimentary rocks
A. Chief constituents
1. Clay minerals
2. Quartz
3. Others
a. Feldspars
b. Micas
B. Particle size is used to distinguish among
the various types of detrital rocks
C. Common detrital sedimentary rocks (in
order of increasing particle size)
1. Shale
a. Thin layers (lamina)
b. Most common sedimentary rock
2. Sandstone
a. Form in a variety of environments
b. Sorting, shape, and composition
of the grains can be used to
interpret the rock’s history
c. Quartz is the most predominant
mineral
3. Conglomerate and breccia
a. Conglomerate consists largely of
rounded gravels
b. Breccia composed mainly of large
angular particles
V. Chemical sedimentary rocks
A. Consist of precipitated material that was
once in solution
B. Precipitation of material occurs in two
ways
1. Inorganic processes
2. Organic processes (biochemical
origin)
C. Common chemical sedimentary rocks
1. Limestone
a. Most abundant chemical rock
b.
2.
3.
4.
5.
Composed chiefly of the
mineral calcite
c. Marine biochemical limestones
1. Coral reefs
2. Coquina
3. Chalk
d. Inorganic limestones
1. Travertine
2. Oolitic limestone
Dolostone
Chert
a. Made of microcrystalline
quartz
b. Forms
1. Flint
2. Jasper (banded form called
agate)
Evaporites
a. Evaporation triggers deposition
of chemical precipitates
b. Examples
1. Rock salt
2. Rock gypsum
Coal
a. Different from other rocks –
made of organic material
b. Stages in coal formation
1. Plant material
2. Peat
3. Lignite
4. Bituminous
VI. Classification of sedimentary rocks
A. Classified according to the type of
material
B. Two major groups
1. Detrital
2. Chemical
C. Two major textures used in the
classification of sedimentary rocks
1. Clastic
a. Discrete fragments and
particles
b. All detrital rocks have a clastic
texture
2. Nonclastic
a. Pattern of interlocking crystals
b. May resemble igneous rocks
Sedimentary Rocks
VII. Sedimentary environments
A. A geographic setting where sediment is
accumulating
B. Determines the nature of the sediments
that accumulate
C. Types of sedimentary environments
1. Continental
a. Dominated by erosion and
deposition associated with streams
b. Glacial
c. Wind (eolian)
2. Marine
a. Shallow (to about 200 meters)
b. Deep (seaward of continental
shelves)
3. Transitional (shoreline)
a. Tidal flats
b. Lagoons
c. Deltas
D. Sedimentary facies
1. Different sediments often accumulate
adjacent to one another at the same
time
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2. Each unit (called a facies) possesses a
distinctive set of characteristics
reflecting the conditions in a
particular environment
3. The merging of adjacent facies tends
to be a gradual transition
VIII. Sedimentary structures
A. Provide information useful in the
interpretation of Earth history
B. Types
1. Strata, or beds (most characteristic
feature of sedimentary rocks)
2. Bedding planes that separate strata
3. Cross-bedding
4. Graded beds
5. Ripple marks
6. Mud cracks
7. Fossils
Answers to the Review Questions
1. In Earth's crust, igneous rocks exceed sedimentary rocks in volume. Neither rock type is evenly
distributed. In the interiors of continents, sedimentary rocks occur as thin veneers covering over much
larger volumes of igneous and metamorphic rocks deeper in the crust. Ocean basin rocks are mainly
igneous with very thin covers of sediments. Sedimentary strata many kilometers in thickness accumulate
only in relatively restricted basins along the edges of continents or in deep rift basins where continental
blocks are splitting apart.
2. The three basic categories of sedimentary rocks are detrital, chemical, and organic. Detrital sedimentary
rocks originate from the weathering, transportation, and deposition of solid particles (preexisting earth
materials). Chemical sedimentary rocks are formed from soluble material that is produced mainly by
chemical weathering. The ions in solution are then precipitated by either inorganic or biologic processes
to form various chemical rocks. Organic sedimentary rocks owe their origin to the accumulation of
undecayed plant material that constitutes the “sediment” in such rocks. Coal, formed from the remains of
plants that accumulated in ancient terrestrial swamps, is the primary example of an organic sedimentary
rock.
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3 Detrital refers to mineral grains and rock fragments, such as sand grains or pebbles, which are produced
during the weathering process and transported to the site of deposition as particles. The most abundant
detrital minerals in sediments are quartz and clays. Quartz is an abundant mineral in many rocks. It resists
fracturing and mechanical weathering and is resistant to solution and decomposition from chemical
weathering. Thus quartz can be thought of as a residual mineral, left over and concentrated in detrital
sediments after the other rock-forming minerals, such as feldspars and ferromagnesians, have
decomposed. Clays are a major component of shales and mudstones, but they are rarely abundant in other
common rocks. Clays are the weathering products of feldspars and aluminum-bearing, ferromagnesian
minerals like hornblende and biotite. Being stable in the weathering environment, clays are major soil
components; thus they are readily eroded, transported, and deposited as detrital sediments.
4. This is done on the basis of clast size, as shown in the following table.
Detrital Fragment (Clast) Size
Dominant sediment clast
pebble, cobble, boulder; > 2 mm
sand; between 1/16 & 2 mm
silt; between 1 / 16 & 1 / 256 mm
mud; clay and silt mixture
clay; < 1/256 mm
Name of lithified rock
conglomerate
sandstone
siltstone
mudstone
shale
5. Shales are typically fissile with numerous, small cracks that allow water ready access during weathering,
and they contain clay minerals as their dominant component. Many clay minerals strongly absorb water
and swell, a process that greatly lowers the mechanical strength of the shale. This process generates
internal stresses that push open new cracks and extend old ones deeper into the unweathered portions of
the rock. Thus deeply weathered shales typically crumble; fresh, unweathered shales are brittle and
splintery, and the broken fragments can be very hard and tough. Black shales typically contain
disseminated pyrite; which readily decomposes in the presence of water and oxygen, generating acidic
waters that hasten disintegration of the rock.
6. A sand grain, regardless of initial shape, is gradually rounded during transport. Sharp corners and edges
are preferentially abraded, resulting in rounded but not necessarily spherical grains. Well-sorted sands are
deposited in aqueous and terrestrial environments characterized by vigorous current activity. Finer
particles are winnowed out and carried elsewhere; coarser, gravel-sized clasts have already been
deposited or abraded and broken into smaller particles. The currents are highly selective and although
finer particles may be available, only sand grains of roughly equal size are deposited, such as in a sand
dune or on a beach. Generally, well-sorted sands are also well-rounded, the grains having been
extensively transported prior to deposition.
7. Conglomerates are detrital sedimentary rocks dominated by pebbles or larger clasts. The clasts are
rounded, mainly by stream transport, and many conglomerates exhibit good stratification. Breccias are
rocks also dominated by pebbles or larger particles, but the clasts are angular rather than rounded. They
can form by explosive fragmentation during volcanism, by fracturing along faults and as products of mass
wasting. Breccias of sedimentary origin are closely related to mass-wasting processes, and the detrital
clasts have moved only a short distance from their bedrock source.
Sedimentary Rocks
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8. The dominant constituents of chemical sedimentary rocks were transported to the site of deposition in
solution. Two different categories are defined on the basis of the precipitation mechanism. If evaporation
caused precipitation, the rocks are termed evaporites. If living organisms were involved, such as with the
precipitation of calcium carbonate by algae or invertebrate animals, the rocks are biogenic (biochemical)
in origin.
9. Evaporites are chemical sedimentary rocks, such as bedded salts, precipitated from isolated bodies of
seawater or saline lakes undergoing intense evaporation. Gypsum, anhydrite (calcium sulfate), and halite
(sodium chloride) are all examples of evaporites.
10. Minerals precipitate from the evaporation of seawater in the order of their relative solubilities. Calcium
carbonate (calcite and aragonite) and dolomite are the least soluble and precipitate first. However, much
of the dolomite associated with evaporite deposits is known to originate secondarily by reaction of
primary calcium carbonate and very late stage, Mg-rich brines. The calcium sulfates (anhydrite and
gypsum) reach saturation next. Extensive, additional evaporation is required for sodium chloride to reach
saturation, and the highly soluble salts such as potassium chloride (sylvite) reach saturation only after
most of the sodium chloride has been removed from the brine by precipitation of halite.
11. (a) Gypsum (calcium sulfate) is a major ingredient of plasters. (b) Shale typically breaks into thin plates
or pen-shaped fragments, as contrasted to mudstone, which typically breaks into more equidimensional
pieces or blocks. (c) Coal, formed from the accumulation of land plants in ancient swamps, is the primary
example of an organic sedimentary rock (d) The most abundant chemical sedimentary rock is limestone,
which is composed of calcium carbonate (CaCO3). (e) Chert is a bedded deposit of very fine-grained
quartz (silica). (f) Oolitic limestone is composed of small, concentric grains of calcite called oolites.
12. Chemical sedimentary rocks are distinguished from one another by their mineralogy. Limestones are
calcite and dolostones are dolomite, a calcium magnesium carbonate mineral. Evaporites include bedded
halite (sodium chloride) and gypsum or anhydrite (calcium sulfates). Hardness, taste, luster, and response
to acids help in mineral identification; microscopic and x-ray analyses can be employed if necessary.
13. Diagenesis refers to the collective chemical, physical, and biological changes that take place following
deposition and during and after lithification of sediments. Typically, diagenesis occurs within the upper
few kilometers of Earth’s crust at temperatures below 200ºC. An example of diagenesis is
recrystallization, which involves the formation of more stable minerals from less stable ones.
14. Compaction is most important as a diagenetic process in fine-grained sedimentary rocks such as shales or
mudstones. Sand and other coarse sediments are much less compressible so that compaction is not as
significant as it is in finer sediments.
15. The three common cementing agents for sedimentary rocks are calcite, silica (quartz), and iron oxide.
They are relatively easy to distinguish from one another by their physical or chemical properties. Calcite
will readily effervesce with dilute hydrochloric acid, silica is much harder than the other two, and a
distinctive orange to dark red color normally identifies iron oxide.
16. Clastic means fragmental or broken and texture describes the shapes, sizes, and mutual packing
arrangements of the mineral grains in a rock. Since they form from transported mineral grains and/or rock
fragments, all detrital sediments and sedimentary rocks have clastic textures. Nonclastic textures include
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the crystalline textures of chemical rocks like chert and evaporites. Limestones may have crystalline or
clastic textures, depending on the nature of the original sediment and on the subsequent geologic history
of calcite solution and crystallization in the rock.
17. The similarities arise because many igneous and chemical sedimentary rocks have crystalline textures, in
which the mineral grains are tightly interlocked and bonded together. Most igneous rocks consist of two
or more dominant silicate minerals. All sedimentary rocks may show stratification, and those with
crystalline textures are typically composed of only one dominant mineral, such as calcite, dolomite, or
silica. The minerals (calcite, dolomite, and evaporites) of nonclastic sedimentary rocks are much softer
than the silicate minerals of igneous rocks. Carbonate minerals react with strong acids and the common
evaporite minerals such as halite are easily soluble and can be identified by taste. Fine-grained to
microcrystalline, siliceous, sedimentary rocks such as chert might be confused with aphanitic igneous
rocks. However, “bedded” chert is monomineralic and is commonly stratified.
18. The three main categories of sedimentary environments are continental, marine, and transitional.
Continental environments include streams, alluvial fans, glacial deposits, eolian (deposited by wind)
deposits, and playa lakes. Marine environments are subdivided into shallow marine deposits (depths of
200 meters or less) such as coral reefs and deep marine deposits (depths greater than 200 meters) like
those that accumulate on the bottom of deep ocean basins. Transitional environments, as the name
implies, are those areas in between continental and marine settings such as beaches, tidal flats, barrier
islands, and deltas.
19. Terrigenous – These sediments are composed of rock and mineral grains derived by subaerial and
submarine weathering, erosion, and mass wasting of preexisting rocks and minerals. They are detrital
sediments with clastic textures.
Biogenous – The sediment particles were precipitated by once-living organisms. Most biogenous or
biogenic sediments are oozes, consisting mainly of minute coccolithophores and foraminifera (calcareous;
calcium carbonate) or diatoms and radiolarians (siliceous; silica).
Hydrogenous – These are sediments chemically precipitated from seawater without biological
intervention. Examples would be the manganese-rich nodules on parts of the ocean bottom and metal-rich
sediments around seafloor, hot spring vents.
20. Most sediments and sedimentary rocks show an original layering (stratification) because they were
deposited in nearly horizontal sheets or layers. Numerous thin strata in shales and some sandstones are
easily visible. In other rocks such as graywacke and reef-deposited limestone, the deposit is a single,
massive bed or lens and therefore internal stratification may not be so evident.
21. Both are characteristic of sedimentary rocks, but they originate in quite different environments. Graded
bedding indicates that bottom currents were absent from the depositional environment. The sediment was
not reworked following deposition. A sediment-laden turbidity current, initiated by slumping of
sediments in shallower waters, loses energy and slows down as it moves along the bottom into deeper
waters. Unaffected by bottom currents, the particles settle out in the order of their grain sizes, the coarser
ones first and the fine silts and clays last. Thus the deposit exhibits an internal, vertical grading in particle
sizes but lacks internal stratification (bedding) surfaces.
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Cross-bedding describes the multiple sets of typically thin, non-parallel strata that develop internally as
sand dunes, ripples, and some fluvial beds accumulate. Some sets of strata are laid down parallel to subhorizontal, gently inclined, transport surfaces. Others are inclined at angles up to 35°, the angle of repose
for sand. These inclined strata are deposited on the steeper, leeward, slopes (usually a slip face) of
mound-like sand accumulations such as dunes, ripples, or sand waves in a stream channel. By definition,
deposits with graded bedding are poorly sorted whereas cross-bedded sands are commonly well-sorted.
22. Current ripples result from unidirectional flowing currents. They can be considered as small-scale dunes
that migrate up current in higher velocity regimes and down current in moderate to low velocity regimes.
The ripples are asymmetric with a gentler slope facing up current and the steeper slip face inclined
downstream. The different slope inclinations can be used to infer paleocurrent directions and the “dunes”
are usually cross-bedded, allowing the original stratigraphic top direction to be determined in most cases.
Current ripples are common features in fluvial and eolian sandstones and they can also form in other
environments, such as in a tidal channel.
Oscillation ripples form in response to the back and forth sloshing of bottom waters as shallow-water
waves pass overhead. The ripples are symmetrical with relatively narrow, peaked crests and broad,
shallow depressions. Slightly coarser “lag deposit” grains typically occupy the centers of the depressions.
These ripples form in shallow water environments subjected to a steady supply of incoming waves that
pass by in more or less a consistent direction. Because they require steady, consistent, incoming waves to
develop, symmetrical ripples are formed mainly in coastal marine environments.
Lecture outline, art-only, and animation PowerPoint presentations for each chapter of Earth,
9e are available on the Instructor’s Resource Center CD (0131566911).
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