weathering, soil and sedimentary rocks -...

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Chapter 6

Weathering, Soil

and Sedimentary Rocks

Sediments and Sediments and Sedimentary RocksSedimentary Rocks

Sediments, Soils & Sedimentary RocksSediments, Soils & Sedimentary RocksProcesses of the rock cycleProcesses of the rock cycle

• Weathering

(Soils)

• Erosion

• Transportation

• Deposition (sedimentation)

• Burial

• Diagenesis

IntroductionRocks and minerals are disintegrated and decomposed by the processes of mechanical and chemical weathering.

This breakdown occurs because the parent material reacts with its new physical and chemical environment transforming it into a new equilibrium state.

Geo-inSight 4., p. 136

Introduction

How does weathering differ from erosion?

Weathering is the mechanical and chemical alteration of Earth materials at or near the surfaceErosion involves removing weathered materials from their place of origin-by running water or wind, for example.

Fig. 6.2, p. 135

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How Are Earth Materials Altered?

The products of weathering include soluble salts, ions in solution, and solid particles

These products of weathering can be eroded and become sedimentary rock or modified in place to become soils.

Fig. 6.1, p. 134


Weathering and erosion take place at different rates

This can occur even on the same body of rock because rocks are not compositionally and structurally homogenous throughout, thereby producing uneven surfaces.



Mechanical Weathering

Frost actionPressure releaseThermal expansion and contractionCrystal growthActivities of organisms.

The products of mechanical weathering are chemically the same as their parent materials.

Fig. 6.9d, p. 142


Mechanical WeatheringFrost Action

When water freezes in cracks in rocks it expands and then it contracts when it thaws, thus exerting pressure and opening the cracks wider. Repeated freezing and thawing disaggregates rocks into angular pieces that may tumble downslope and accumulate as talus.

Fig. 6.3a, p. 138

4. Physical weathering: frost wedging4. Physical weathering: frost wedging

Frost wedging due the expansion of freezing water can turn small cracks into large ones


Mechanical WeatheringPressure Release and Sheet Joints

Sheet joints are fractures that more or less parallel exposed rock surfaces, especially rocks now at the surface that formed under great pressure at depth. These joints form in response to pressure release; that is, when the rocks formed, they contained energy that is released by outward expansion.

Fig. 6.4 a-b, p. 138

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Mechanical /Physical weathering: exfoliationMechanical /Physical weathering: exfoliation

Exfoliation occurs where large flat & curved sheetsof rock fracture and detach from outcrop

Mechanical / Physical weathering: joints in rocksMechanical / Physical weathering: joints in rocks

Breakage along natural bedding joints plus crackingfrom expansion due lowered pressure at surface

How Are Earth Materials Altered?Mechanical Weathering

How do organisms contribute to mechanical and chemical weathering?

Any organic activity such as tree roots growing in cracks contributes to mechanical weatheringOrganic acids and the tendrils of mosses and lichens aid in the chemical alteration of parent material.

Fig. 6.5b, p. 139

Mechanical / Physical weathering: tree rootsMechanical / Physical weathering: tree roots

The force of the growing roots pry the cracks apart


Chemical weathering

SolutionOxidationHydrolysis

Hot and wet environments accelerate chemical weathering.Chemical weathering occurs in all environments, except, possibly, permanently frozen polar regions.

Fig. 6.7, p. 141


Chemical WeatheringThese processes cause a change in the chemical

composition.

The parent material is transformed into products including ions in solution, soluble salts and clay minerals.

Fig. 6.6, p. 140

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Chemical WeatheringSolution – rocks dissolve

Carbonate RocksRocks such as limestone (CaCO³) are nearly insoluble in neutral or alkaline solutions, but they rapidly dissolve in acidic solutionsThe atoms making up the minerals dissociate, that is, they separate and the rock dissolves.

Chemical weathering: carbon dioxide

Chemical weathering: carbon dioxide


Chemical WeatheringOxidation – rocks rust

Rocks such as sandstone may contain iron minerals that will breakdown when exposed to the atmosphere

The atoms making up the minerals dissociate, that is, they separate as the rock rusts away.


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Chemical weathering Chemical weathering

●● Role of oxygen in weathering: Role of oxygen in weathering: from iron silicates to iron oxidesfrom iron silicates to iron oxides

●● ferric and ferrous ironferric and ferrous iron

●● hematite, a common mineralhematite, a common mineral

●● red and brown red and brown –– the colors ofthe colors ofoxidized ironoxidized iron

Pyroxene dissolves,Pyroxene dissolves,releasing silica andreleasing silica andferrous iron.ferrous iron.

Silica Ferrousiron

Ferric iron

Ferrous iron is oxidized,Ferrous iron is oxidized,forming ferric iron.forming ferric iron.

Iron oxide (hematite)Fe2O3

Ferric iron precipitatesFerric iron precipitatesa solid, iron oxide.a solid, iron oxide.

Pyroxene (FeSiO3)

Chemical weathering: iron and oxygen

Chemical weathering: red means ironChemical weathering: red means iron How Are Earth Materials Altered?

Chemical WeatheringHydrolysis – breakdown to clays

Potassium FeldsparDuring hydrolysis hydrogen ions react with and replace positive ions in potassium feldsparThe result is clay minerals and substances in solution such as potassium and silica.

Mr. Granite

Chemical weathering: the disintegration of granite

Feldspar

Magnetite

Biotite

Quartz

Granite is made up of several minerals that decay at different rates.


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Feldspar

Magnetite

Biotite

Quartz

Cracks form along crystal boundaries.



Feldspar

Magnetite

Biotite

Quartz

Cracks form along crystal boundaries.

The decay progresses, and the rock weakens and disintegrates.




Chemical WeatheringFactors That Control the Rate of Chemical Weathering

Mechanical weathering enhances chemical weathering by breaking material into smaller pieces, thereby increasing the surface area for chemical reactions.Because chemical weathering is a surface process, the more surface exposed, the faster the weathering.

Fig. 6.8 a-c, p. 141

2 cm

2 cm

Chemical weathering: the role of increasing surface area 24 sq cm

2 cm

2 cm

1 cm

1 cm

Chemical weathering: the role of increasing surface area 24 to 48 sq cm

2 cm

2 cm

1 cm

1 cm

Large rocks have less Large rocks have less surface area for chemical surface area for chemical weatheringweathering……

Chemical weathering: the role of increasing surface area 24 to 48 sq cm

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2 cm

2 cm

1 cm

1 cm

Large rocks have less Large rocks have less surface area for chemical surface area for chemical weatheringweathering……

……than small rocks do,than small rocks do,so smaller rocks weather so smaller rocks weather more quickly.more quickly.

Chemical stability: a speed control for Chemical stability: a speed control for weatheringweathering

•• Solubility (halite high, quartz low)Solubility (halite high, quartz low)•• rate of dissolution (feldspar higher than rate of dissolution (feldspar higher than

quartz)quartz)•• relative stability of common rockrelative stability of common rock--

forming minerals (halide to iron oxide)forming minerals (halide to iron oxide)

Chemical weatheringChemical weathering

A. duration of weathering

B. bedrock type -- stability of mineralsstability of minerals

C. climatei.i. water & temperature >>> chemical weathering; water & temperature >>> chemical weathering; ii.ii. lower temperature >>> mechanical weathering; lower temperature >>> mechanical weathering; iii.iii. more acidity >>> chemical weatheringmore acidity >>> chemical weathering

D. topography i. steep slopes >>> mechanical/physical weathering; ii. gentle slopes >>>chemical weathering

Weathering factorsWeathering factors

weatheringweatheringHow Does Soil Form and Deteriorate?

The Soil Profile

Soils consist of weathered materials, air, water, humus and also the plants which they support.

Fig. 6.10a, p. 143

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How Does Soil Form and Deteriorate?

The Soil Profile

Soil formation produces horizons that are known in descending order as O, A, B, and C. These horizons differ from one another in texture, structure, composition and color.

Fig. 6.10b, p. 143

How Does Soil Form and Deteriorate?Factors That Control Soil FormationClimate - Certainly climate is the most important factor because chemical processes operate faster where it is warm and wet.

Soils known as pedalfersdevelop in humid climates such as that of the eastern United States and much of Canada. Soils of arid and semiarid regions are known as pedocals, and may contain hard, irregular masses of caliche (calcium carbonate) in horizon B.

Fig. 6.11, 6.12, p. 144-145

How Does Soil Form and Deteriorate?Factors that Control Soil Formation

Laterite is a deep red soil typical of the tropics where chemical weathering is intense.

Laterites are made up of clays and the most insoluble compounds that were present in the parent material.

Fig. 6.12, p. 145


Other Factors That Control Soil Formation

Parent materialOrganic activityRelief and slopeTime

Fig. 6.7, p. 141


Soil Degradation - Any soil losses, physical changes, or chemical alteration is called soil degradation, and all lead to reduced soil productivity.

Causes include erosion, compaction, and any kind of chemical pollution that inhibits plant growth.

Fig. 6.14, p. 147


Soil DegradationSoil erosion is caused mostly by sheet and rill erosion.

It is a problem in some areas, especially where accelerated by human activities such as construction, agriculture, ranching, and deforestation.

Fig. 6.13, p. 146

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Geo-Focus Fig. 1 a-c, p. 149

The Dust Bowl – An American Tragedy


Soil DegradationNutrient depletion

Loss of nutrients is most prevalent in areas of land overuse. Improper disposal of chemicals and concentrations of insecticides can destroy soil.

Fig. 6.14, p. 147

Weathering and Resources

Intense chemical weathering causes the concentration of valuable mineral resources

Residual concentrations – bauxite and other valuable minerals are concentrated by selective removal of soluble substances during chemical weathering

Bauxite, which forms in lateritic soils in the tropics, occurs in areas where chemical weathering is so intense that only the most insoluble compounds accumulate in the soil. Aluminum is just such an insoluble compound. Laterites are the primary source of aluminum oxide, called bauxite. It is the main source of aluminum ore.

Gossans - hydrated iron oxides formed on the earth’s surface by oxidation of iron. Sulfide minerals leach out and concentrate as deposits of iron ore, copper ore, lead and zinc ore beneath the gossan.

Sedimentary rocks are produced by Sedimentary rocks are produced by surface processes in the rock cycle.surface processes in the rock cycle.

• Weathering processes break up rock to create sediment.• Physical - Mechanical breakage and disintegration.• Chemical - Decomposition by reaction with water. • Weathering processes occur at Earth’s surface.

- Rocks react with hydrosphere, atmosphere & biosphere. - Low temperature and pressure.

Weatheringto >>>>>>sediment

Sediment and Sedimentary RockThe two primary types of sediment are detrital and chemical. Sedimentary rock is simply rock made up of consolidated sediments.

Detrital sediment consists of solid particles, products of mechanical weathering.

Chemical sediments consist of minerals precipitated from solution by inorganic processes and by the activities of organisms thru chemical weathering.

Fig. 6.15, p. 150

Physical WeatheringPhysical Weathering• Mechanical breakup; doesn’t change mineral makeup.• Creates broken fragments or “detritus.”• Detrital fragments classified by size.

– Coarse grained – Boulders cobbles and pebbles.– Medium grained – Sand-sized.– Fine grained – Silt and clay (mud).

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Chemical WeatheringChemical Weathering• Weathering often forms stable from less stable minerals.

– Dissolution.– Hydrolysis.– Oxidation.– Hydration.

• Dissolution– halite, gypsum, &

calcite dissolve.• Hydrolysis

– Water breaks apart cations that hold silicates together.– Dissolved cations - Clay minerals.– Alteration residues - Iron oxides (rust).

SOURCE OF SEDIMENT

TRANSPORT

Fig. 6-15 (top), p. 150Stepped Art

Transport

Deposition(detrital sediments)

Lithification

Detrital sedimentary rocks(e.g.,sandstone)

Transport

Lithification

Chemical sedimentary rock(e.g., limestone)

Precipitationfrom solution

Used byorganisms

Deposition(chemical sediment)

(clay minerals and ions,compounds in solution)

CHEMICALWEATHERING

(gravel, sand, silt,clay–sized particles)

MECHANICALWEATHERING

Sediment and Sedimentary Rocks

Sediment Transport and Deposition

Sedimentary material weathers, undergoes erosion and transport to a new location.Transportation of sediment results in rounding and sorting.

Why are rounding and sorting important in sediments and sedimentary rocks?

Both are important in determining how fluids move through sediments and sedimentary rocksThe amount of rounding and sorting depends on particle size, distance of transportation, and depositional processes.


Sediment Transport and Deposition

Eventually the sediment comes to rest in a depositional environment.Depositional environments are areas of sediment deposition that can be defined by their physical characteristics (topography, climate, wave and current strength, salinity, etc.). They provide geologist with clues as to how the rock formed and what the geologic past was like.


Sediment Transport and DepositionMajor depositional settings are continental, transitional, and marine.

Each of these depositional settings includes several specific subenvironments.

Fig. 6.17, p. 151

Sedimentaryrocks

Metamorphicrocks

Plutons

DesertPlayalake

DeltaGlacier

Sedimentary environments

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Sedimentaryrocks

Metamorphicrocks

Plutons

DesertPlayalake

DeltaGlacier

Weatheringbreaks downrocks.

Processes forming sedimentary rock

Sedimentaryrocks

Metamorphicrocks

Plutons

DesertPlayalake

DeltaGlacier


Erosion carriesaway particles.

Processes -Weathering

thenErosion

Sedimentaryrocks

Metamorphicrocks

Plutons

DesertPlayalake

DeltaGlacier



Transportation moves particles downhill.

Process -transport

Sedimentaryrocks

Metamorphicrocks

Plutons

DesertPlayalake

DeltaGlacier




Deposition occurs when particles settle out or precipitate.

Process -Deposition

Sedimentaryrocks

Metamorphicrocks

Plutons

DesertPlayalake

DeltaGlacier





Burial occursas layers of sediment accumulate.

Process –Burial

Sedimentaryrocks

Metamorphicrocks

Plutons

DesertPlayalake

DeltaGlacier





Diagenesis causeslithification of the sediment, making sedimentary rocks.

Burial occursas layers of sediment accumulate.

Process –Diagensis

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Sediment ClassesSediment ClassesSediments are diverse, as are the rocks made from them. Sedimentary rocks divide to groups based on sediments type. 1) Siliciclastics – Made from weathered rock fragments

(clasts primarily of silicates). 2) Biological & Chemical (Bio/Chemical) - subdivided as

– Bioclastic seds.– Shells of organisms (reefs, clams, etc)– Chemical seds.– Minerals crystallized directly from water– Organic seds.– Carbon-rich remains of plants (coal).

ClasticClastic ChemicalChemicalOrganicOrganicBiochemicalBiochemical


Transport agents - oceans, wind (minor/yr), rivers (25 billion ton/yr), etc

Current strength distance affect: particle size• strong >50cm/s – gravel• weak <20cm/s - muds

Transport distance affect:• Size of clastic particles• Sorting of clastic particles• Rounding of clastic particles

Sorting examples : Well Sorting examples : Well vsvs Poor Poor

Sorting affected by strength, distance, time, agent

Size & rounding versus transport distance

More rounding with longer transport, stronger current, low rock hardness, clay minerals

Size & rounding versus transport distance

More rounding with longer transport, stronger current, low rock hardness, clay minerals


Chemical mixing vats:Chemical mixing vats:•• OceansOceans•• LakesLakes

Salinity varies with water input & Salinity varies with water input & evaporation. e.g. evaporation. e.g.

•• Great Salt Lake, Great Salt Lake, UtUt ((NaClNaCl))•• Tularosa Basin, NM (~65Tularosa Basin, NM (~65--50 ma, 50 ma,

white sands (CaSO4) precipitate)white sands (CaSO4) precipitate)

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Sedimentary basinsSedimentary basins

•• Sediments tend to accumulate in Sediments tend to accumulate in depressions in the Earthdepressions in the Earth’’s crust.s crust.

•• Depressions are formed by subsidence.Depressions are formed by subsidence.

•• Sedimentary basins are depressions filled Sedimentary basins are depressions filled with thick accumulations of sediment. They with thick accumulations of sediment. They are sinks for sediment.are sinks for sediment.

Sedimentary environmentsSedimentary environments

Types of environments:Types of environments:

1. Continental1. ContinentalLakeLakeRiver (alluvial)River (alluvial)DesertDesertGlacierGlacier

3.3. Sedimentary environmentsSedimentary environments


2. Shoreline2. ShorelineDeltaDeltaTidal flatTidal flatBeachBeach

3.3. Sedimentary environmentsSedimentary environments


3. Marine3. MarineContinental shelfContinental shelfOrganic reefOrganic reefContinental marginContinental marginContinental slopeContinental slopeDeep seaDeep sea

Sedimentaryenvironments 3. Sedimentary environmentsSedimentary environments

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Sedimentary environmentsSedimentary environments

Environments of siliciclastic Environments of siliciclastic sediments:sediments:

1.1. Continental (alluvial, desert, Continental (alluvial, desert, lake, and glacial)lake, and glacial)

2.2. Shoreline (deltas, beaches, Shoreline (deltas, beaches, and tidal flats)and tidal flats)

3.3. Marine (shelf, margin, slope, Marine (shelf, margin, slope, and deep sea)and deep sea)

Sedimentary environmentsedimentary environments

Environments of chemical and Environments of chemical and biological sediments:biological sediments:

1.1. Carbonate deposits (organic reefs, Carbonate deposits (organic reefs, beaches, shelves, and tidal flats)beaches, shelves, and tidal flats)

2.2. Siliceous environments (deep Siliceous environments (deep sea)sea)

3.3. Evaporite environments (lakes)Evaporite environments (lakes)

Sediment and Sedimentary RockHow Does Sediment Become Sedimentary Rock?

Thru the process of lithification of sediment is converted into sedimentary rock.

Lithification involves two processes1. Compaction - The volume of a deposit of sediment decreases as the weight of overlying sediment causes a reduction in pore space (open space) as particles pack more closely together. Compaction alone is sufficient for lithification of mud into shale.

Fig. 6.19c, p. 153

Sediment and Sedimentary Rock

How Does Sediment Become Sedimentary Rock?

Lithification involves two processes2. Cementation is a process that glues the sediments together.The most common cements are calcium carbonate and silica, but iron oxide and iron hydroxide are important in some rocks.Compaction alone will not form rocks from sand and gravel. Cementation is necessary to glue the particles together into rocks.

Fig. 6.18, p. 152

Fig. 6-18, p. 152Stepped Art

Sediment Process Rock

Gravel > 2 mm Conglomerate

Sedimentarybreccia

Rounded clasts

Angular clasts

Compaction/cementationSand 2 mm–1/16 mm

Compaction/cementation

Compaction

Quartz sandstone(mostly quartz)

Arkose(> 25% feldspars)

Mostly silt

Silt and clay

Mostly clay

Shale iffissile*

Claystone

Mudstone

Siltstone

Mud

rock

s

*Fissile refers to rocks capable of splitting along closely spaced planes.

Clay < 1/256 mm

Silt 1/16 mm–1/256 mm

Sandstone

Compaction/cementationTypes of Sedimentary Rock

Detrital Sedimentary Rocks are made of solid particles of pre-existing rocks.

Detrital sedimentary particles are classified according to grain (particle) sizes, in decreasing diameter:

Gravel (including boulders, cobbles and pebbles)

SandSiltClay (or mud).

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Detrital sedimentary rocks are classified on the basis of particle size.

Examples include conglomerate, breccia, sandstone, siltstone, mudstone, and shale.

How do conglomerate and sedimentary breccia differ?Both begin as detrital gravel. Conglomerate consists of rounded gravel, breccia consists of gravel with sharp edges.

Types of Sedimentary Rocks

Fig. 6.19 a and b , p. 153


Chemical and Biochemical Sedimentary Rocks

Chemical and biochemical sedimentary rocks are substances derived from solution by inorganic or biochemical processes. Some have a crystalline texture, meaning they are composed of a mosaic of interlocking crystalsOthers have a clastic texture, meaning that they are made of fragments, like shells that are glued together.


Chemical Sedimentary RocksChemical sedimentary rocks are classified on the basis of composition.

Carbonate rocks consist primarily of minerals containing the carbonate ion, such as limestone and dolostone. Dolostone forms when magnesium replaces calcium in limestone.

Fig. 6.20b-d, p. 154


Chemical Sedimentary Rocks

EvaporitesBedded rock salt (halite) and rock gypsum are chemical evaporite sediments formed by precipitation of minerals during the evaporation of water.

Fig. 6.21a-b, p. 155


Chemical Sedimentary Rocks

Bedded ChertMarin County, California

The origin of chert is highly debated.

Fig. 6.21c, p. 155


Biochemical Sedimentary Rocks

Coal is a biochemical sedimentary rock composed largely of altered land plant remains

Fig. 6.21d, p.155

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Sedimentary FaciesGeologists realize that if they trace a sedimentary layer far enough, it will undergo changes in composition and/or texture.

Bodies of sediment or sedimentary rocks which are recognizably different from adjacent sediment or sedimentary rocks and are deposited in a different depositional (sub) environment are known as sedimentary facies.Today we recognize modern facies changes when we go from an inland area with rivers to the beach.

Fig. 6.22, p. 156

Sedimentary Facies

A marine transgressionoccurs when sea level rises with respect to the land, resulting in offshore facies overlying nearshore facies.A marine regression,

caused when the land rises relative to sea level, results in nearshore facies overlying offshore facies

Note the difference in the vertical rock sequence that occurs in a transgression versus a regression.

Marine Transgression and Regression

Fig. 6-22, p. 156Stepped Art

Three Stages of Marine Regression

Old landsurface

Timelines

Timelines

Old land surface

Limestonefacies

Shalefacies

Sandstonefacies

Timeline

Three Stages of Marine TransgressionOffshore

Low-energyNear shore

High-energyLandsurface

Peter Kresan Fig. 7.6

Cross-bedded Sandstone

Sedimentary structuresSedimentary structures

Sedimentary structures Sedimentary structures –– all kinds all kinds of features in sediments formed at of features in sediments formed at the time of deposition.the time of deposition.

Bedding (stratification)Bedding (stratification)CrossCross--beddingbeddingGraded beddingGraded beddingRipplesRipplesBioturbation structuresBioturbation structures

Reading the Story in Sedimentary Rocks

Sedimentary StructuresSome sedimentary structures, such as ripple marks, bedding, cross-bedding, and mud cracks form shortly after deposition.

Sedimentary structures are useful in determining the types of environments in which the sediments were deposited. Sediments are most commonly deposited flat in water. One of the most common is strata or bedding.

Fig. 6.23 a, p. 158

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Sedimentary StructuresDepositional environments are also inferred by comparison of these

structures with present-day depositional environments.

Cross-bedding preserves layers deposited at an angle.They are common in depositional environments like sand dunes, shallow marine deposits and stream-channel deposits How is cross-bedding used to determine ancient current directions?Understanding how physical features like cross-beds form today can reveal important ancient climate information such as current directions.

Fig. 6.23b-c, p. 158

Formation of CrossFormation of Cross--bedsbeds

Fig. 7.7

RipplesRipples


Sedimentary StructuresCross-bedding

Depositional environment: streams or shallow marine?Streams have a current and leave behind asymmetric dunes.Shallow marine crossbeds exhibit a symmetrical shape from the rocking motion of the waves.

Fig. 6.25 a-d, p. 159

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Fig. 7.9

Bioturbation structuresReading the Story in Sedimentary Rocks

Sedimentary StructuresMud cracks

Depositional environment: Lagoons and mudflats

Fig. 6.26 a-b, p. 159


Sedimentary StructuresGraded Beds

Depositional environment: Submarine fans – tell us the location of the ancient shelf margin

Fig. 6.24a-b, p. 158


Fossils-Remains and Traces of Ancient Life

Fossils are the remains of past life and are usually found only in sediments and sedimentary rocks.

They provide the only record of prehistoric life, and are used by geologists to correlate strata, and to interpret depositionalenvironments.

Fig. 6.27 a-b, p. 160

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Burial and diagenesisBurial and diagenesis

Burial is the preservation of Burial is the preservation of sediments within a sedimentary sediments within a sedimentary basin. basin.

Diagenesis is the physical and Diagenesis is the physical and chemical change that converts chemical change that converts sediments to sedimentary rocks.sediments to sedimentary rocks.

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Burial and diagenesisBurial and diagenesis

Lithification includes:Lithification includes:

CompactionCompaction

CementationCementation

Classification of siliciclastic sedimentsClassification of siliciclastic sediments

and sedimentary rocksand sedimentary rocks

Classification of sediments by Classification of sediments by particle sizeparticle size

Classification of sedimentary Classification of sedimentary rocks by texture and compositionrocks by texture and composition

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7. Classification of chemical and 7. Classification of chemical and

biological sedimentary rocksbiological sedimentary rocks

LimestoneLimestoneChertChertEvaporiteEvaporite

OrganicsOrganicsPhosphoritePhosphorite

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Organic reef developmentOrganic reef development

Organic reef developmentOrganic reef development Organic reef rock

Foraminifer in the Eye of a Foraminifer in the Eye of a NeedleNeedle

Fig. 7.17Chevron Corporation

FossiliferousFossiliferous LimestoneLimestone

Peter Kresan

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Fig. 6-17, p. 109

One Model for theOne Model for theFormation of Formation of EvaporitesEvaporites


Determining the Environment of DepositionHow do we know that the Navajo Sandstone formed as a desert dune deposit?

Fig. 6.28 a, p. 161


Determining the Environment of DepositionSedimentary Rocks in the Grand Canyon

Fig. 6.28 b, p. 161

Important Resources in Sedimentary Rocks

Many important natural resources are sedimentary rock deposits. These include:

Sand and gravelCoalClayEvaporites (like salt)Banded-iron formations. Oil and gas


Petroleum and Natural GasMost oil and gas reserves are found within sedimentary rocks.

What are stratigraphic and structural traps? Both are areas where petroleum, natural gas, or both accumulate in economic quantities.Stratigraphic traps form because of facies changes in the rock layers (strata).

Fig. 6.29a p. 162

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Petroleum and Natural Gas

Structural traps form as the result of folding or fracturing (faulting) of rocks.

Fig. 6.29b, p. 162


Petroleum and Natural Gas

Oil shale is a fine-grained sedimentary rock that contains kerogen from which liquid oil and combustible gases can be derived.

None is mined at present in the United States because oil and gas from conventional sources are cheaper. Oil shale and tar sands are increasingly important petroleum reserves.

Fig. 6.29c p. 162


Uranium

Most uranium is used in nuclear reactors. The uranium comes from the minerals carnotite and uraninite. The richest ores are found in Wyoming, Utah, Arizona and New Mexico in ancient stream deposits.Large reserves of low grade ore is found in the Chattanooga Shale, which covers portions of several states.

Fig. 6.30 a-b, p. 163


Banded Iron Formation

Why is banded iron formation such animportant sedimentary rock?

Banded iron formation consists of alternating thin layers of chert and iron minerals, mostly iron oxides. Nearly all of Earth’s iron ore is mined from ancient banded iron formations.

Fig. 6.30b, p. 163

End of Chapter 6

weathering, soil and sedimentary rocks -...

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