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ENGINEERING GEOLOGY
SPRING 2017
LECTURE FIVE: 1.SEDIMENTARY AND
METAMORPHIC ROCKS 2. GEOLOGIC TIME
PREPARED BY
DR. KHAYYUN A. RAHI
DEPARTMENT OF ENVIRONMENTAL ENGINEERING
Rock Cycle
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SEDIMENTARY ROCKS
What Is a Sedimentary Rock?
Sedimentary rocks are types of rock that are formed by
the deposition and subsequent cementation of that
material at the Earth's surface and within bodies of
water.
Sedimentary rocks are important for economic
considerations because they may contain:
• Coal
• Petroleum and natural gas
• Sources of Fe, Al, and Mn.
DETRITAL SEDIMENTARY ROCKS
Detrital or clastic sedimentary rocks are composed of rock
fragments. They are different than chemical sedimentary rocks,
which are composed of mineral crystals
Common detrital sedimentary rocks
• Shale
• Mud-sized particles in thin layers
• Most common sedimentary rock
• Sandstone
• Sand-sized particles
• Forms in a variety of environments
• Predominant mineral = quartz
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DETRITAL SEDIMENTARY ROCKS
• Conglomerate and breccia
• Both are composed of particles greater than
2 millimeters in diameter.
• Conglomerate consists largely of rounded
gravels.
• Breccia is composed mainly of large angular
particles.
Sediment
Process
Rock
Gravel > 2 mm
How sediments are
transformed into clastic
sedimentary rocks.
Conglomerate
Compaction/ Rounded
cementation clasts
Breccia
Angular
clasts
Sand 2 mm–.062 mm
Compaction/
cementation
Sandstone
Silt .062 mm–.004 mm
Compaction/
cementation
Siltstone
Compaction
Shale
Clay <.004 mm
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DETRITAL SEDIMENTARY ROCKS
Shale with Plant
Remains
Quartz Sandstone
Conglomerate
Breccia
Gravel Deposits, if Lithified Would Become
Conglomerate
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CHEMICAL SEDIMENTARY ROCKS
Chemical sedimentary rock forms when mineral constituents
in solution become supersaturated and inorganically
precipitate. Common chemical sedimentary rocks include
limestone and rocks composed of evaporite minerals, such as
halite (rock salt) and gypsum.
 Other chemical sedimentary rock produced by organic
processes (biochemical origin): biochemical sedimentary
rocks form from sediment derived by biological processes
COMMON CHEMICAL SEDIMENTARY ROCKS
• Limestone
• Most abundant chemical rock
• Composed chiefly of the mineral calcite
• Marine biochemical limestones: coquina (broken shells), and chalk
(microscopic organisms).
• Inorganic limestones include oolitic limestone.
• Dolostone (dolomite)
• Typically formed secondarily from limestone.
• Evaporites
• Evaporation triggers deposition of chemical precipitates.
• Examples include rock salt and rock gypsum.
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Samples of in organic
chemical sedimentary
rocks
Coquina
Gypsum
Limestone
Rock salt (halite)
BIOCHEMICAL SEDIMENTARY ROCKS
• Coal
• Different from other rocks because it is composed of
organic material.
• Stages in coal formation (in order):
1. Plant material
2. Peat
3. Lignite
4. Bituminous
5. Anthracite
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STAGES OF COAL FORMATION
METAMORPHIC ROCKS
• Changed in solid state from preexisting rocks by: heat,
pressure, and chemical processes
• New structures, textures, minerals
• Foliation: flattening and layering of minerals by nonuniform stresses
• Recrystallization: heat or uniform stresses create
larger, more perfect grains
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METAMORPHIC ROCKS
Metamorphic rocks are produced from:
 Igneous rocks
 Sedimentary rocks
 Other metamorphic rocks
 During metamorphism, the rock must remain
essentially solid
AGENTS OF METAMORPHISM
Heat
• Most important agent
• Recrystallization results in new, stable
minerals.
• Two sources of heat:
1. Contact metamorphism—heat from
magma
2. An increase in temperature with depth—
geothermal gradient
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AGENTS OF METAMORPHISM
Pressure and differential stress
• Increases with depth
• Confining pressure applies forces equally in
all directions.
• Rocks may also be subjected to differential
stress, which is unequal in different
directions.
PRESSURE IN METAMORPHISM
Recrystallization
Foliation
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COMMON METAMORPHIC ROCKS
• Foliated rocks
• Schist
• Gneiss
• Nonfoliated rocks
• Marble:
• Parent rock was limestone or dolomite
• Quartzite:
Formed from a parent rock of quartz-rich sandstone
COMMON
METAMORPHIC
ROCKS
Mica Schist
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Geologic Time
• Study of the dating and relationships of geologic events
• Relative-age dating determines sequences of events
• Absolute-age dating provides actual ages for rocks
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Geological time
scale.
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RELATIVE-AGE DATING
Structural relations of rocks
• Law of Superposition: In undeformed
sedimentary rocks, the top layer is youngest
• Law of Cross-Cutting Relationships: A fault is
younger than the youngest rocks it cuts
• Principle of Original Horizontality: Material
was originally deposited horizontally
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CROSS-CUTTING RELATIONSHIPS
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ABSOLUTE-AGE DATING
• Law of Fossil Succession: indicator fossils
• Radiometric dating:
• Radioactive element decays and releases particles;
can stay same element or become a new element
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(a) Alpha emission, whereby a heavy nucleus spontaneously
emits a helium atom and is reduced four atomic mass units
and two atomic numbers. (b) Emission of a nuclear electron
(b2 particle), which changes a neutron to a proton and
thereby forms a new element without a change of mass.
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ABSOLUTE-AGE DATING (CONTINUED)
• Half-life: time required for ½ of atoms to decay
• Different compounds have different half-lives and can be
used for different age ranges
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Figure 3.31 decay
of a radioactive
parent element with
time. Each time unit
is one half-life. Note
that after two halflives, one fourth of
the parent element
remains, and that
after three half-lives,
one-eighth remains.
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END OF LECTURE FIVE
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