Transcript Chapter 5

Chapter 7: Sediments and
Sedimentary Rocks:
Archives of Earth’s History
Introduction: Sediments In The Rock
Cycle

Sediments are transported by:
Water.
– Ice.
– Wind.
– Gravity.
The transport and deposition of sediments are
initial phases of the rock cycle.
The ultimate fate of most sediment is burial and
conversion to sedimentary rock.
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Sedimentation, Stratification, and
Bedding
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Stratification, or bedding, is the arrangement of
sedimentary particles in layers.
Each stratum (plural = strata) or bed is a distinct
later of sediment.
The top or bottom surface of a bed is a bedding
plane.
Figure 7.1
Sediments Types And Characteristics
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Three broad classes:
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Clastic sediment is loose fragments of rock debris
produced by physical weathering.
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Chemical sediment precipitates from solution in
water.
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Sand and clay.
Calcium carbonate and salt.
Biogenic sediment is composed of the fossilized
remains of plants or animals.
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Coal, oil, and natural gas.
Figure B7.1
Characteristics of Clastic Sediment (1)
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Clastic sediment is divided into four main size
classes:
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Gravel.
Sand.
Silt.
Clay.
Characteristics of Clastic Sediment (2)
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Gravel is subdivided into:
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Boulder gravel.
Cobble gravel.
Pebble gravel.
Sorting (1)
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Sorting identifies sediment in terms of the
variability in the size of its particles.
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Poorly sorted (wide range of particle size),
Well sorted (range is small).
Changes of grain size typically result from
fluctuations in the velocity of the transporting
agent, such as water or wind.
Figure 7.2
Figure 7.3
Figure 7.4
Sorting (2)
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The greater the speed and energy involved, the
larger or heavier are the particles that can be
transported.
Unusually dense minerals (e.g. gold,
platinum,magnetite) are deposited first when
stream velocity slows.
Lighter particles are carried onward.
Figure 7.5
Sorting (3)
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Nonsorted sediment is a mixture of different
sizes arranged chaotically.
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Till is a nonsorted sediment of glacial origin.
Smooth and rounded particles are transported
by water or air.
Figure 7.6
Varves
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Rhythmic layering occurs in varves.
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A varve is a pair of sedimentary layers deposited over
the cycle of a single year.
In spring, the inflow of sediment laden water
increases and coarse sediment is then deposited
throughout the summer.
During winter, very fine sediment slowly settles to
form a thinner, darker layer.
Figure 7.7
Rhythmic Lamination and Cross
Bedding
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Rhythmic laminations may result from a
succession of large storms.
Cross bedding creates beds that are inclined with
respect to a thicker stratum within which they occur.
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Cross bedding is the work of turbulent flow in the
transporting agency.
Graded Bedding
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In graded bedding, the particles are sorted more or
less according to size, grading upward from coarser
to finer.
Most coarse clastic sediments consist of mineral
grains and rock least susceptible to chemical and
physical breakdown.
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High content of quartz and potassium feldspar.
Figure 7.9
Figure 7.10
Characteristics of Chemical Sediments
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Chemical sediments are formed by precipitation of
minerals from solution in water.
They form in two principal ways:
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Through biochemical reactions resulting from the activity
of plants and animals in the water.
Inorganic reactions in the water.
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When water from a hot spring cools, it may precipitate opal (a
hydrated silicate) or calcite (calcium carbonate).
Salts (1)
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Evaporation of sea water or lake water forms
salts.
Lake waters precipitate sodium carbonate
(Na2CO3), sodium sulfate Na2SO4), borax
(Na2B4O7.10H2O),and trona
(Na2CO3.NaHCO3.2H2O).
Salts (2)
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Lake water precipitates are used in the
production of:
 Paper.
 Soap.
 Detergents.
 Antiseptics.
 Tanning
and dyeing.
Salts (3)
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The most important salts that precipitate from
seawater are halite (rock salt: NaCI) and gypsum
(CaSO4.2H2O).
Much of the common table salt we use daily and
the gypsum used for plaster and construction
materials are recovered from marine evaporate
deposits.
Characteristics of Biogenic Sediments
(1)
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Biogenic sediments contain fossils.
If the remains are broken and scattered it is
bioclastic sediment.
When floating microscopic marine organisms die,
their remains settle and accumulate on the seafloor
to form a muddy sediment called deep-sea ooze.
Characteristics of Biogenic Sediments
(2)
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Siliceous ooze resembles calcareous ooze, but is
composed mainly of the siliceous remains of tiny
floating protozoa (radiolarians) and algae (diatoms).
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Calcareous ooze is a fine-grained, deep-sea deposit of
skeletal material containing more than 30% calcium
carbonate.
Siliceous ooze is made of skeletal material made mainly
of silica.
Sedimentary Environments And Facies
Changes
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Vertical changes in strata reflect the passage of time.
Horizontal strata change over distance, but the
changes occur within the same stratum.
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Such a lateral change from one depositional environment
to another is called a change of facies.
Each environment has distinctive physical,
chemical, and biological characteristics.
Figure 7.14
Figure 7.15
Nonmarine Depositional Environments:
Streams
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Stream sediments.
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Widespread.
The type depends on:
The energy available for doing work.
 The nature of the sedimentary load.
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Nonmarine Depositional Environments:
Lakes
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Lake sediments.
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Sediments deposited in a lake accumulate both on the
lakeshore and on the lake floor.
Sand and gravel form beaches, bars, and spits across
the mouths of bays.
Finer sediment forms a delta.
Nonmarine Depositional Environments:
Glaciers
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Glacial sediments.
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Sedimentary debris eroded and transported by a
glacier.
Random mixture of particles that range in size from
clay to boulders.
Stones in glacially deposited sediment often are
angular and some are striated;
Nonmarine Depositional Environments:
Wind
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Eolian (windblown) sediments.
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Sediments carried by the wind tends to be finer than
that moved by other erosional agents.
Sand may pile up to form dunes.
When powdery dust picked up and moved by the
wind is deposited, it becomes progressively thinner
and finer with increasing distance downwind.
Shoreline and Continental Shelf
Environments (1)
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Deltaic sediments.
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Estuarine sediments
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Lakes and marine deltas.
Sediments trapped in an estuary.
When the stream water meets seawater, the clay particles tend to
aggregate into clumps.
Beach sediments.
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Quartz is typical of beach sediments.
Shoreline and Continental Shelf
Environments (2)
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Offshore sediments.
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Fine grained sediment, carried in suspension, settle
slowly to the seafloor.
Only about 10 percent of the sediment reaching the
continental shelves remains in suspension long enough
to arrive in the deep sea.
Carbonate shelves.
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Carbonate sediments accumulate mainly on broad,
flat carbonate shelves that border a continent.
Figure 7.16
Shoreline and Continental Shelf
Environments (3)
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Marine evaporate basins.
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Ocean water occupying a basin with restricted
circulation that lies in a region of very warm, dry
climate will evaporate.
Deep-sea fans.
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Some large submarine canyons on the continental
slopes are aligned with the mouths of major rivers,
like the Amazon, Congo, Ganges, and Indus.
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At the base of many such canyons is a huge deep-sea fan.
Figure 7.19
Figure 7.17
Figure 7.18
Figure 7.20
Shoreline and Continental Shelf
Environments (4)
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Sediments drifts;
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Huge bodies of sediments up to hundreds of
kilometers long, tens of kilometers wide, and two
kilometers high have been discovered along the
continental margins bordering the North Atlantic
ocean and Antarctica.
An exceptional rate of deposition, 200 cm/1000 years,
has been measured for a sediment drift near
Bermuda.
Cores obtained from sediment drifts can provide a
record of environmental change measurable at a
decadal, rather than millennial, scale.
Deep-Sea Depositional Environments
(1)
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Deep-sea oozes.
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Calcareous ooze occurs over wide areas of ocean floor
at low to middle latitudes where warm sea surface
temperatures favor the growth of carbonate-secreting
organisms in the surface water.
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Calcareous ooze is rare where the water is deeper than
3-4 km.
Figure 7.21
Deep-Sea Depositional Environments
(2)
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Cold deep-ocean waters are under high pressure and
contain more dissolved carbon dioxide than
shallower waters.
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At a level called the carbonate compensation depth, the
shells are dissolved, before they can reach the seafloor.
Deep-Sea Depositional Environments
(3)
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Land-derived sediment.
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Deep-sea strata include sediments carried to the ocean by:
Rivers.
 Wave action.
 Wind (fine desert dust and volcanic ash),
 Floating ice.
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Diagenesis: How Sediment Becomes
Rock (1)
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Lithification is the overall process of creating
sedimentary rock.
Diagenesis is the collective term for all the
chemical, physical, and biological changes that
affect sediment as it goes from deposition through
lithification.
Diagenesis: How Sediment Becomes
Rock (2)
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Processes involved include:
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Compaction.
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As the weight of an accumulating sediment forces the
grains together. The pore space is reduced.
Cementation.
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Substances dissolved in water precipitate to form a cement
that binds the sediment grains together.
Figure 7.22
Diagenesis: How Sediment Becomes
Rock (3)
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Recrystalization.
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As sediment accumulates, less stable minerals recrystallize into
more stable forms.
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The mineral aragonite in the skeleton of living corals recrystalizes
to its polymorph calcite.
Chemical alteration.
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In the presence of oxygen, organic remains are quickly converted
to carbon dioxide and water.
If oxygen is lacking (reducing or anaerobic environment), the
organic matter does not completely decay but instead may be
slowly transformed to solid carbon.
Formation of Peat: Diagenesis Under
Anaerobic Conditions
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In stagnant swamp water, oxygen is used up and not
replenished, creating an anaerobic environment
(lacking air or free oxygen).
The oxygen and hydrogen (combined in gases, such
as methane) escape, and the carbon gradually
becomes concentrated in the residue.
In this manner, plant matter is gradually converted
to peat.
Clastic Sedimentary Rocks
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The four basic classes of clastic sedimentary rocks
are:
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Conglomerate, a lithified gravel.
Sandstone, consisting mainly of sand grains.
Siltstone, composed mainly of silt-size mineral
fragments, commonly quartz and feldspar.
Mudstone, still-finer grain size.
Figure 7.24A
Figure 7.24B
Figure 7.24C
Figure 7.24D
Figure 7.24E
Figure 7.24F
Chemical Sedimentary Rocks (1)
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Chemical sedimentary rocks result from lithification
of organic or inorganic chemical precipitates.
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Evaporate deposits: salt domes.
Banded iron deposits.
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Some of the Earth’s most important iron concentrations are in
sedimentary rocks that formed billions of years ago.
Every aspect of banded iron deposits indicates chemical
precipitation.
Figure 7.25
Chemical Sedimentary Rocks (2)
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Phosphorus deposits.
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A primary source of fertilizer.
Form through the precipitation of apatite from seawater.
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Phosphorus rich sediments are forming today off the western coast
of Africa and South Americ.,
Extensive marine phosphates are found in Florida (world’s highest
annual production).
Chert is a hard, very compact sedimentary rock
composed almost entirely of very fine-grained,
interlocking quartz crystal (useful to early cultures for
tool production).
Figure 7.26
Biogenic Sedimentary Rocks (1)
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Limestone.
 The
most important biogenic rock.
 One coarse-grained type, composed entirely of shelly
debris is called coquina.
 Reef limestone consists of cemented reef organisms.
 Chalk is composed of the compacted carbonate shells
of minute floating organisms.
 Limestone accounts for a major proportion of the
carbon dioxide stored in the Earth’ crust.
Figure 7.27A
Figure 7.27B
Figure 7.27C
Figure 7.27D
Biogenic Sedimentary Rocks (2)
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Coal.
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Plant remains accumulate to form peat (carbon
content of about 60 percent).
Peat is compressed (causing water loss and generation
of organic gases such as methane CH4).
The peat is thereby converted into lignite.
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The luxuriant plant growth needed to form thick, extensive
coal seams required tropical or subtropical climate.
Coal deposits that we find today in the frigid lands of
northern Alaska and Antarctica must have formed in warm,
relatively low-latitude environments.
Biogenic Sedimentary Rocks (3)
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Oil shale.
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An organic-rich clastic rock.
Dead organisms buried in marine or lake muds contain
organic oils and fats.
An alteration process occurs in which wax-like
substances are formed (oil shale).
Extensive oil shales occur in Alberta, Colorado, Utah,
and Wyoming.
Environmental clues in sedimentary
rocks (1)
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Sedimentary rocks reflect the environment in which
they were formed.
Clues from bedding planes.
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Ripples marks preserved in sandstones and siltstones.
Footprints, trails, and raindrop impressions.
Clues from fossils.
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Some animals and plants are restricted to warm, moist
climates, whereas others are associated only with cold,
dry climates.
Figure 7.30
Figure 7.31
Environmental clues in sedimentary
rocks (2)
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Clues from rock color.
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Most dark colors are the result of iron sulfides and
organic detritus buried with sediment.
Reddish and brownish colors result mainly from the
presence of iron oxides.
Environmental clues in sedimentary
rocks (3)
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Clues from isotopes and magnetic properties.
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The ratio of two isotopes of oxygen, oxygen-18 and
oxygen-16, can provide important information about
changing climate.
Magnetic measurements in sediments and
sedimentary rocks permit us to reconstruct changes in
the Earth’s magnetic field through time.
Limestone, CO2, And Global Climate
Change (1)
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Carbon dioxide is one of the most important
greenhouse gases.
Periods of high CO2, well above today’s value,
occurred in intervals from about 550-390 and 18090 million years ago.
These intervals, corresponding to much of the early
Paleozoic and late Mesozoic eras, are well known
for their extensive limestone formation.
Limestone, CO2, And Global Climate
Change (2)
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The earlier period followed an important
interval of cold climate about 600 million years
ago that coincided with low atmospheric CO2
values and important glaciation.
Figure 7.34