07.03 Sedimentary Rocks

7/31/2019 07.03 Sedimentary Rocks

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Sedimentary Rocks

From sediments to rocks

Dr Marcus Matthews


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Sedimentary Rocks

Sedimentary rocks and the rock cycle

Sedimentary Environments

Facies

Diagenesis

Classification of Sedimentary rocks


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Sedimentary Rocks and theRock Cycle

Weathering parent rocks decomposed and orfragmented

Erosion carries particles away

Transportation moves particles to new locations

Deposition

particles settle, minerals areprecipitated

Burial layers of sediment accumulate

Diagenesis physical & chemical changes that

transform a sediment into a rock

Sedimentary rocks are the product of severaloverlapping processes


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Weathering and Erosion Yieldthe Raw Materials

Particles and dissolved substances

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Clastic sediments Chemical depositsCementing materials


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Transported solid fragments

Boulders to clay

Often called siliclasticStable minerals e.g. quartz found unaltered

Less stable minerals e.g. feldspar often absent or partiallyaltered

New minerals e.g. clay minerals

Mineralogy of sediment reflects intensity of weathering as wellas length and type of transport

Clastic sediments generally accumulate more rapidly than othertypes of sediments

Clastic sediments


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Dissolved products of weathering

Chemical sediments are formed at or near place of deposition

(e.g. rock salt, limestone)Biochemical sediments contain un-dissolved remains oforganisms (e.g. chalk)

In practice many chemical and biochemical sediments overlap

Chemical and Biochemical Sediments


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Transportation and Deposition

Most particles are transported by currents of water or air (wind).Rivers annually carry a solid and dissolved sediment load of morethan 20 billion tonnes. Currents in air move material globally but inmuch smaller quantities


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Transportation and DepositionCurrents sort sediments into different size groups

L.S. Fichter (1993, 2000)http://csmres.jmu.edu/geollab/fichter/sedrx/simpbasclas.html

Clastic rocksChemical & biochemical rocks


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Transportation and DepositionTransportation changes the character of the particles.The key factors are time, distance and mode of transport.

Transportation results in reduced particle angularity and size


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Transportation and Deposition

Example: Clastic particles eroded at the headwaters of

the Missouri River in the mountains of western Montanatake hundreds of years to travel the 2000 miles to theGulf of Mexico. These particles may be affected byphysical weathering (rounding) as well as intermittent

chemical weathering.

Transportation changes the character of the particles.The key factors are time, distance and mode of transport.

Transport by glaciers does not result in rounding ofparticles. The particles are reduced in size.Transport by wind rounds and polishes particles.


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Type and amount of water (ocean, lake, river, aridland)

Topography (lowland, mountains, coastal plain,shallow ocean, deep ocean)

Biological activity (coral reefs, swamps)

Asedimentary environment is a geographic locationcharacterised by a particular combination of geologicalprocesses.


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Continental EnvironmentsAlluvialDesertLakeGlacial

Shoreline EnvironmentsDeltaic

Tidal flatBeachMarine EnvironmentsContinental shelfContinental slope

Organic reefsDeep-sea


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Facies

Facies are combinations of sediments orsedimentary rocks characteristic of a particularsedimentary environment.

Examples: Alluvial facies and deltaic facies


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Sedimentary Facies:Alluvial

The sediments deposited by a meandering river form a sequencewhere each bed forms in a different part of the channel. As thechannel migrates they are deposited in sequence on top of eachother. The sequence is cyclic with each cycle ending in over-bankflooding.

Fining upwardsequence


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Sedimentary Facies: DeltaicThe river delta is a complex environmentas it involves the combination of river,tides and wave action. Channel switchingby river causes delta lobes to beperiodically abandoned resulting in a cyclicsequence of sediments.

Progradingdelta lobe

Marine shale

Coal (abandonment)

Onedeltacycle

Onedeltacycle

Coarseningupwardssequence

Active delta lobe

Abandoned delta lobe


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DiagenesisPhysical and chemical changes thattransform a sediment into a rock

Mud (clay) Mudstone & shale

Sand Sandstone

Gravel Conglomerate/breccia

Carbonate Limestone & marlrich muds


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Diagenetic Processes:Compaction

10-20% water

50-60% water

Compaction (primarily of muds)

Pressure due to overburden

(burial) squeezes water outof pore space.

Particles forced closer

together as pore volumereduces

NoteGeologists say compactionGeotechnical Engineers say consolidation

To a Geotechnical Engineer compaction means removal of air throughapplication of energy


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Diagenetic Processes:Cementation

Common cements

Iron oxideCalcium CarbonateClay minerals

Silica

Precipitation of new minerals

Loose sand Cemented sandstone


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Diagenetic Processes: Re-Crystallisation

New crystal faces precipitated on existing mineral grains

Loose sand

Example: development of a strong quartzite throughprecipitation of silica on original quartz grains

Cemented sandstone= same mineral


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Diagenetic Processes:Dissolution


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Classification of Clastic Sedimentary Rocks

Cobbles 60-200mmm, Boulders >200mm

ClaySiltSandGravel

0.002mm0.06mm2mm60mm0.

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Breccia(angular fragments)

Conglomerate(rounded fragments)

Sandstone SiltstoneClaystoneShale

Grains visiblewith naked eye

Grains visible withnaked eye (course),with aid of x10 handlens (fine)

Grains not visible with aid ofx10 hand lens

Mudrocks


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Classification of ClasticSedimentary RocksMajor types of sandstone based on mineral/rock content

Quartzite Arkose Lithic Sandstone Greywacke


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Classification of SedimentaryRocks

Classification of chemical and biochemical sedimentaryrocks

Sediment Rock Composition Minerals

Sand & mud Limestone CaCO3 Calcite

Siliceous sediment Chert SiO2 Opal,

ChalcedonyQuartz

Peat, organic matter Lignite, Coal Carbon (Coal)Organics compounds (Oil)

(Gas)

Biochemical


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Evaporite Evaporite Sodium chloride HaliteCalcium sulphate Anhydrite

Gypsum

Iron oxide sediments Iron formation Iron oxide HematiteLimoniteIron carbonate Siderite

Formed by diagenesis Dolomite Calcium-magnesium Dolomite(Dolostone) carbonatePhosphorite Calcium phosphate Apatite


Chemical

o


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Cl ifi ti f S di t


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Sand & mud Limestone CaCO3 Calcite

Siliceous sediment Chert SiO2 Opal,

ChalcedonyQuartz

Peat, organic matter Lignite, Coal Carbon (Coal)Organics compounds (Oil)

(Gas)

Biochemical

o

Cl ifi ti f S di t


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Evaporite Evaporite Sodium chloride HaliteCalcium sulphate Anhydrite

Gypsum

Iron oxide sediments Iron formation Iron oxide HematiteLimoniteIron carbonate Siderite

Formed by diagenesis Dolomite Calcium-magnesium Dolomite(Dolostone) carbonatePhosphorite Calcium phosphate Apatite


Chemical

o

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07.03 Sedimentary Rocks