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,
Sedimentation,Stratification,and
Bedding
? 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,
Sediments Types And Characteristics
? Three broad classes:
? Clastic sediment is loose fragments of rock
debris produced by physical weathering.
? Sand and clay.
? Chemical sediment precipitates from solution
in water.
? Calcium carbonate and salt.
? Biogenic sediment is composed of the
fossilized remains of plants or animals.
? Coal,oil,and natural gas.
Characteristics of Clastic Sediment (1)
? Clastic sediment is divided into four main size
classes:
? Gravel >2mm
? Sand 0.0625 to 2mm
? Silt 0.0625 to 0.0039mm
? Clay <0.0039mm
Characteristics of Clastic Sediment (2)
? Gravel is subdivided into:
? Boulder gravel >256mm
? Cobble gravel 64 to 256mm
? Pebble gravel 64 to 2mm
Sorting (1)
? Sorting identifies sediment in terms of the
variability in the size of its particles.
? 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.
Sorting in sediments
Size-Frequency Curve
Sorting,Rounding and Sphericity
Sorting (2)
? 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.
Placer Nuggets
Sorting (3)
? Nonsorted sediment is a mixture of
different sizes arranged chaotically.
? Till is a nonsorted sediment of glacial origin.
? Smooth and rounded particles are
transported by water or air.
Till,nonsorted till deposited by a lobe of the Cordileran Ice Sheet in western Washington
Varves
? Rhythmic layering occurs in varves.
? 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.
Cyclical sediment,Varves deposited in a glacial age lake in easten Canada
Rhythmic Lamination and Cross Bedding
? 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.
? Cross bedding is the work of turbulent flow
in the transporting agency,
Graded Bedding
? 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.
? High content of quartz and potassium
feldspar.
Graded Bed,Mudflow sediment
Characteristics of Chemical Sediments
? Chemical sediments are formed by
precipitation of minerals from solution in
water.
? They form in two principal ways:
1,Through biochemical reactions resulting from
the activity of plants and animals in the water.
2,Inorganic reactions in the water.
? When water from a hot spring cools,it may precipitate
opal (a hydrated silicate) or calcite (calcium carbonate).
Salts (1)
? Evaporation of sea water or lake water
forms salts.
? Lake waters precipitate sodium
carbonate (Na2CO3),sodium sulfate
Na2SO4),borax硼砂 (Na2B4O7.10H2O),and
trona天然碱 (Na2CO3.NaHCO3.2H2O),
Salts (2)
? Lake water precipitates are used in the
production of:
? Paper.
? Soap.
? Detergents,
? Antiseptics.
? Tanning and dyeing.
Salts (3)
? 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)
? 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)
? Siliceous ooze resembles calcareous ooze,but is
composed mainly of the siliceous remains of tiny
floating protozoa (radiolarians) and algae
(diatoms).
? 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
? Vertical changes in strata reflect the passage of
time.
? Horizontal strata change over distance,but the
changes occur within the same stratum.
? 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.
Depositional Environments from the crest of a mountain range
to the adjaacent margin of a nearby ocean bacin
Sedimentary Facies
Nonmarine Depositional Environments
------ Streams
? Stream sediments are widespread.
? Stream deposited sediment differs from
place to place depends on the type of
stream
1,The energy available for doing work.
2,The nature of the sedimentary load.
? Lake sediments
? 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
------ Lake
? 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
------ Glacier
? Eolian (windblown) sediments.
? 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.
Nonmarine Depositional Environments
------ Wind
Shoreline and Continental Shelf
Environments (1)
? Deltaic sediments.
? Lakes and marine deltas,
? Estuarine sediments
? Sediments trapped in an estuary.
? When the stream water meets seawater,the clay
particles tend to aggregate into clumps.
? Beach sediments.
? Quartz is typical of beach sediments.
Shoreline and Continental Shelf
Environments (2)
? Offshore sediments.
? 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.
? Carbonate sediments accumulate mainly on broad,
flat carbonate shelves that border a continent.
Carbonate shelf surrounding the numerous islands of the Bahamas
Shoreline and Continental Shelf
Environments (3)
? Marine evaporate basins.
? Ocean water occupying a basin with restricted
circulation that lies in a region of very warm,dry
climate will evaporate.
? Deep-sea fans.
? Some large submarine canyons on the continental
slopes are aligned with the mouths of major rivers,
like the Amazon,Congo,Ganges,and Indus.
? At the base of many such canyons is a huge deep-sea fan.
Currents in the Mediterrannean Sea
Turbidity Current
Deep sea fans
Shoreline and Continental Shelf
Environments (4)
? Sediments drifts
? 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)
? Deep-sea oozes
? 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.
? Calcareous ooze is rare where the
water is deeper than 3-4 km.
Deep Sea Sediment Distribution
Deep-Sea Depositional Environments (2)
? Cold deep-ocean waters are under high
pressure and contain more dissolved
carbon dioxide than shallower waters.
? At a level called the carbonate
compensation depth,the shells are
dissolved,before they can reach the
seafloor.
Deep-Sea Depositional Environments (3)
? Land-derived sediment.
? Deep-sea strata include sediments carried to
the ocean by:
? Rivers
? Wave action
? Wind (fine desert dust and volcanic ash)
? Floating ice
Diagenesis:
How Sediment Becomes Rock (1)
? 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)
? Processes involved include:
? Compaction.
? As the weight of an accumulating sediment
forces the grains together,The pore space is
reduced.
? Cementation.
? Substances dissolved in water precipitate to
form a cement that binds the sediment grains
together.
A quartz sandstone cemented by anhydrite
? Recrystalization
? As sediment accumulates,less stable minerals
recrystallize into more stable forms,
? The mineral aragonite in the skeleton of living corals
recrystalizes to its polymorph calcite.
? Chemical alteration
? 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.
Diagenesis:
How Sediment Becomes Rock (3)
Formation of Peat:
Diagenesis Under Anaerobic Conditions
? 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
? The four basic classes of clastic
sedimentary rocks are:
? 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,
Conglomerate
Breccia
Sandstone
Shale
Chemical Sedimentary Rocks (1)
? Chemical sedimentary rocks result from
lithification of organic or inorganic chemical
precipitates.
? Evaporate deposits,salt domes.
? Banded iron deposits.
? 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.
Banded Iron Formation
? Phosphorus deposits
? A primary source of fertilizer
? Form through the precipitation of apatite from
seawater
? Phosphorus rich sediments are forming today
off the western coast of Africa and South
Americ
? Chert 燧石 is a hard,very compact sedimentary
rock composed almost entirely of very fine-
grained,interlocking quartz crystal
Chemical Sedimentary Rocks
? 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.
Biogenic Sedimentary Rocks (1)
Fossiliferous limestone
Bioclastic limestone
Coquina
Biogenic Sedimentary Rocks- Coal (2)
? 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 褐煤,
? 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
－ Oil shale (3)
? 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？
Environmental clues in sedimentary
rocks (1)
? Sedimentary rocks reflect the environment in
which they were formed.
? Clues from bedding planes.
? Ripples marks preserved in sandstones and
siltstones.
? Footprints,trails,and raindrop impressions.
? Clues from fossils.
? Some animals and plants are restricted to warm,
moist climates,whereas others are associated only
with cold,dry climates,
Modern Ripples
Mud Cracks
Environmental clues in sedimentary
rocks (2)
? Clues from rock color.
? 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)
? Clues from isotopes and magnetic properties.
? 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)
? 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
180-90 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)
? 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.
Carbon Dioxide in the Atmosphere
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,
Sedimentation,Stratification,and
Bedding
? 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,
Sediments Types And Characteristics
? Three broad classes:
? Clastic sediment is loose fragments of rock
debris produced by physical weathering.
? Sand and clay.
? Chemical sediment precipitates from solution
in water.
? Calcium carbonate and salt.
? Biogenic sediment is composed of the
fossilized remains of plants or animals.
? Coal,oil,and natural gas.
Characteristics of Clastic Sediment (1)
? Clastic sediment is divided into four main size
classes:
? Gravel >2mm
? Sand 0.0625 to 2mm
? Silt 0.0625 to 0.0039mm
? Clay <0.0039mm
Characteristics of Clastic Sediment (2)
? Gravel is subdivided into:
? Boulder gravel >256mm
? Cobble gravel 64 to 256mm
? Pebble gravel 64 to 2mm
Sorting (1)
? Sorting identifies sediment in terms of the
variability in the size of its particles.
? 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.
Sorting in sediments
Size-Frequency Curve
Sorting,Rounding and Sphericity
Sorting (2)
? 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.
Placer Nuggets
Sorting (3)
? Nonsorted sediment is a mixture of
different sizes arranged chaotically.
? Till is a nonsorted sediment of glacial origin.
? Smooth and rounded particles are
transported by water or air.
Till,nonsorted till deposited by a lobe of the Cordileran Ice Sheet in western Washington
Varves
? Rhythmic layering occurs in varves.
? 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.
Cyclical sediment,Varves deposited in a glacial age lake in easten Canada
Rhythmic Lamination and Cross Bedding
? 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.
? Cross bedding is the work of turbulent flow
in the transporting agency,
Graded Bedding
? 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.
? High content of quartz and potassium
feldspar.
Graded Bed,Mudflow sediment
Characteristics of Chemical Sediments
? Chemical sediments are formed by
precipitation of minerals from solution in
water.
? They form in two principal ways:
1,Through biochemical reactions resulting from
the activity of plants and animals in the water.
2,Inorganic reactions in the water.
? When water from a hot spring cools,it may precipitate
opal (a hydrated silicate) or calcite (calcium carbonate).
Salts (1)
? Evaporation of sea water or lake water
forms salts.
? Lake waters precipitate sodium
carbonate (Na2CO3),sodium sulfate
Na2SO4),borax硼砂 (Na2B4O7.10H2O),and
trona天然碱 (Na2CO3.NaHCO3.2H2O),
Salts (2)
? Lake water precipitates are used in the
production of:
? Paper.
? Soap.
? Detergents,
? Antiseptics.
? Tanning and dyeing.
Salts (3)
? 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)
? 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)
? Siliceous ooze resembles calcareous ooze,but is
composed mainly of the siliceous remains of tiny
floating protozoa (radiolarians) and algae
(diatoms).
? 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
? Vertical changes in strata reflect the passage of
time.
? Horizontal strata change over distance,but the
changes occur within the same stratum.
? 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.
Depositional Environments from the crest of a mountain range
to the adjaacent margin of a nearby ocean bacin
Sedimentary Facies
Nonmarine Depositional Environments
------ Streams
? Stream sediments are widespread.
? Stream deposited sediment differs from
place to place depends on the type of
stream
1,The energy available for doing work.
2,The nature of the sedimentary load.
? Lake sediments
? 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
------ Lake
? 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
------ Glacier
? Eolian (windblown) sediments.
? 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.
Nonmarine Depositional Environments
------ Wind
Shoreline and Continental Shelf
Environments (1)
? Deltaic sediments.
? Lakes and marine deltas,
? Estuarine sediments
? Sediments trapped in an estuary.
? When the stream water meets seawater,the clay
particles tend to aggregate into clumps.
? Beach sediments.
? Quartz is typical of beach sediments.
Shoreline and Continental Shelf
Environments (2)
? Offshore sediments.
? 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.
? Carbonate sediments accumulate mainly on broad,
flat carbonate shelves that border a continent.
Carbonate shelf surrounding the numerous islands of the Bahamas
Shoreline and Continental Shelf
Environments (3)
? Marine evaporate basins.
? Ocean water occupying a basin with restricted
circulation that lies in a region of very warm,dry
climate will evaporate.
? Deep-sea fans.
? Some large submarine canyons on the continental
slopes are aligned with the mouths of major rivers,
like the Amazon,Congo,Ganges,and Indus.
? At the base of many such canyons is a huge deep-sea fan.
Currents in the Mediterrannean Sea
Turbidity Current
Deep sea fans
Shoreline and Continental Shelf
Environments (4)
? Sediments drifts
? 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)
? Deep-sea oozes
? 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.
? Calcareous ooze is rare where the
water is deeper than 3-4 km.
Deep Sea Sediment Distribution
Deep-Sea Depositional Environments (2)
? Cold deep-ocean waters are under high
pressure and contain more dissolved
carbon dioxide than shallower waters.
? At a level called the carbonate
compensation depth,the shells are
dissolved,before they can reach the
seafloor.
Deep-Sea Depositional Environments (3)
? Land-derived sediment.
? Deep-sea strata include sediments carried to
the ocean by:
? Rivers
? Wave action
? Wind (fine desert dust and volcanic ash)
? Floating ice
Diagenesis:
How Sediment Becomes Rock (1)
? 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)
? Processes involved include:
? Compaction.
? As the weight of an accumulating sediment
forces the grains together,The pore space is
reduced.
? Cementation.
? Substances dissolved in water precipitate to
form a cement that binds the sediment grains
together.
A quartz sandstone cemented by anhydrite
? Recrystalization
? As sediment accumulates,less stable minerals
recrystallize into more stable forms,
? The mineral aragonite in the skeleton of living corals
recrystalizes to its polymorph calcite.
? Chemical alteration
? 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.
Diagenesis:
How Sediment Becomes Rock (3)
Formation of Peat:
Diagenesis Under Anaerobic Conditions
? 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
? The four basic classes of clastic
sedimentary rocks are:
? 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,
Conglomerate
Breccia
Sandstone
Shale
Chemical Sedimentary Rocks (1)
? Chemical sedimentary rocks result from
lithification of organic or inorganic chemical
precipitates.
? Evaporate deposits,salt domes.
? Banded iron deposits.
? 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.
Banded Iron Formation
? Phosphorus deposits
? A primary source of fertilizer
? Form through the precipitation of apatite from
seawater
? Phosphorus rich sediments are forming today
off the western coast of Africa and South
Americ
? Chert 燧石 is a hard,very compact sedimentary
rock composed almost entirely of very fine-
grained,interlocking quartz crystal
Chemical Sedimentary Rocks
? 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.
Biogenic Sedimentary Rocks (1)
Fossiliferous limestone
Bioclastic limestone
Coquina
Biogenic Sedimentary Rocks- Coal (2)
? 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 褐煤,
? 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
－ Oil shale (3)
? 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？
Environmental clues in sedimentary
rocks (1)
? Sedimentary rocks reflect the environment in
which they were formed.
? Clues from bedding planes.
? Ripples marks preserved in sandstones and
siltstones.
? Footprints,trails,and raindrop impressions.
? Clues from fossils.
? Some animals and plants are restricted to warm,
moist climates,whereas others are associated only
with cold,dry climates,
Modern Ripples
Mud Cracks
Environmental clues in sedimentary
rocks (2)
? Clues from rock color.
? 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)
? Clues from isotopes and magnetic properties.
? 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)
? 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
180-90 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)
? 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.
Carbon Dioxide in the Atmosphere
