lecture 14 feb - dr. hoyle

SedimentaryRocks

Therockcycleandsedimentaryrocks• Weathering• Erosion• Transportation• Deposition• Burial• Compaction• Cementation• Lithification

PhysicalGeologybyStevenEarleusedunderaCC-BY4.0

RockOutcrop- source

• Theoutcropofrockcanbe• Igneous• Metamorphic• Sedimentary

•Allcangiveanewsedimentaryrockgoingthroughthismultistepprocess

Nowsedimentisformedbyweathering,erosion,transportationanddeposition…..

WeatheringI• Whenrocksareexposedatthesurface

• Temperaturesvarywidely• Pressureisreduced• Theyareexposedto20%oxygen(andothergases)• Usuallywaterisabundant

• Weatheringincludestwomainprocesses• Mechanicalweathering- breakdownofrockintosmallerfragments(freeze/thawcycles)• Chemicalweathering- changeofmineralstonew,morestableforms

• Mechanicalweatheringgivesnewsurfacesforattackbychemicalprocesses• Chemicalweatheringweakensrock– moreeasilyweatheredmechanically• Weatheringcreates:

• Soil- maybemorelaterinsemester

• Sedimentaryrock– fromclastsandionsinsolution

WeatheringII- mechanicalweathering

• Decreasedpressurewhenoverlyingrockisremoved• Decreasedconfiningpressurecausescrackingorexfoliationofrock

• Freezing/thawingofwaterrockcracks• Formationof‘salt’crystalsinrockcracks• Crackingcausedbyplantroots• Exposurebyburrowinganimals(removessoilandaffectsweathering)

Severalprocesses….

WeatheringIII- chemicalweathering1

• Chemicalchangestomineralswhenexposedtosurfaceconditions• water(intheairandonthegroundsurface)• abundanceofoxygen• presenceofcarbondioxide(formscarbonicacidwhencombinedwithwater)

• Veryspecifictoboththemineralandtheenvironmentalconditions• e.g.1– Quartzisvirtuallyunaffectedbychemicalweathering• e.g.2– Feldspariseasilyaltered.

• Generallychemicalweatheringis:• greaterinwarm/wetclimates• muchlessincold/dryclimates

• Theprocessthatismostimportanttoalmostallchemicalweathering is:• H2O+CO2 àH2CO3 followedbyH2CO3 à H+ +HCO3

-

WeatheringIV- chemicalweathering2

• 2maintypesofchemicalweathering• Hydrolysiswheresomemineralsarealteredtootherminerals• Completedissolution

• Forexample,feldsparundergoeshydrolysis—tokaoliniteviz.• CaAl2Si2O8 +H2CO3 +🁬O2 à Al2Si2O5(OH)4 +Ca2++CO3

2- J

• andCalcite(e.g.limestone)completelydissolvesviz.• CaCO3(s) +H+ +HCO3

- à Ca2+(aq)+2HCO3- (aq)

WeatheringV– productsofweathering


ErosionandTransportation• Mechanicalweatheringisgreatlyfacilitatedbyerosion• Erosion istheremovalofweatheringproductscausingtheexposureofmorerock• Transportation isthemovementofsediment/dissolvedionsfromerosiontodeposition.Itcanoccurby:• Wind• Flowingwater• Glacialice• Massmovementdownaslope

• Fasterflowrates(windorH2O)causelargerpiecestobetransported

Deposition• Occursasconditionschangeandtransportnolongeroccurse.g.watercurrentslowsdown/stops

• SeeTable6.3intext–• Environment• Transport• Sedimentformed

• Sedimentaryrockisthenformedbyburialandlithification…..


Principlesofsedimentaryaccumulation• Principlesof…..1. Superposition - sedimentarylayersaredepositedinorder,andthat

unlesstheentiresequencehasbeenoverturned,thelowerlayersareolderthantheupperones

2. Originalhorizontality- sedimentsaccumulateinhorizontallayers,bygravity.• Thisimpliesthatobservedtiltedsedimentarylayersmusthavebeenmoved(plates?)

3. Inclusions - allrockfragmentsinasedimentarylayermustbeolderthanthelayer(fromLyell- 1830s)

4. Faunalsuccession- thereisaspecificorderthatwidelydistributedorganismshaveevolvedthroughtime,• thustheIDofspecificfossilscanbeusedtodeterminearock’sage

(possiblythemostcontroversial)

Burial,cementationandlithification• Lithification isthetermusedtodescribetheprocessesthattakeplacewithinasedimenttoturnitintosolidrock(atdepthsof100-1000sm)• Burial byothersedimentsleadstocompactionofthematerialandremovalofsomeoftheinterveningwaterandair.• Afterthisstage,theindividualclastsarealltouchingoneanother.• Cementation istheprocessofcrystallizationofmineralsinporesbetweenthepieces,andatpointsofcontactbetweenlargerclasts(sandsize+)• DependingonP,T&chemicalconditions - cementcrystalsmaybe:• calcite,hematite,quartz,clayminerals,thereareothers

Nowontosedimentarystructures…..

Sedimentarystructures- bedding

• Bedding istheseparationofsedimentsintolayers• Layersdifferfromoneanotherby• texture,composition,colour,orweatheringcharacteristics

• Bedsseparatedbypartings—narrowgapsbetweenadjacentbeds• Beddingshowschangesindepositionthatarerelatedto• seasonaldifferences,climatechange,locationschangeofrivers/deltas,tectonicchanges.

• Partingsmayrepresentperiodsofnon-depositionfor10-100sy• Beddingcanforminalmostanydepositionalenvironment

Sedimentarystructures– graded&cross-bedding

• Gradedbeddingisseenasarangeofgrainsizefrombottomtotopwithinasinglebed• Normally- coarseatthebottomandfinertowardthetop.• Somearereversed

• Mostgradedbedsforminasubmarine-fanenvironment(fig6.17intext)• sediment-richflowsdescendperiodicallyfromshallowshelfandontoseafloor.

• Cross-bedding isbeddingthatcontainsangledlayersandformswhensedimentsaredepositedbyflowingwaterorwind.(seefigs6.1&6.20intext)• Cross-bedssizes…

• fromwatertendtobe1-10cm• fromwinddeposits(aeolian)<5m

• Cross-bedsformbydepositonleadingedgeofadvancingripple/dune• ‘Eachlayerisrelatedtoadifferentripplethatadvancesintheflowdirection,andispartiallyerodedbythefollowingripple’(fig6.21intext,nextslide)


Cross-beds(fig6.21)PhysicalGeologybyStevenEarleusedunderaCC-BY4.0

Cross-beddingYouTubevideo

Sedimentarystructures- ripplesandmudcracks

• Ripples associatedwithformationofcross-beddingmaybepreservedonthesurfacesofsedimentarybeds.• Clastsinstreamstendtotiltwiththeirupperendspointingdownstreambecausethisisthemoststablepositionwithrespecttothestreamflow

• Mudcracksformwhenmuddysedimentaredepositedinashallowwaterthendriesup(e.g.atidalflatsorpond)• Thishappensbecausetheclayintheuppermudlayertendstoshrinkondrying,andsoitcracksbecauseitoccupieslessspacewhenitisdry.

Andnowsedimentaryrockclassification….

Classificationofsedimentaryrock• Thereareseveralpossiblewaystosubdividesedimentaryrocks• Usuallytheclassificationsisinto:

• Clastic• Nonclastic (chemicalandbioclastic/fossiliferous)

• Clasticrefersto- solidfragmentsthataretransportedtotheburialsite• Nonclastic/chemicalrefersto– transportedmaterialisinsolution• Thereissomeoverlapbetweenthetwoas

• almostallclasticsedimentaryrockscontaincementformedfromdissolvedions• manychemicalsedimentaryrocksincludesomeclasts

• Mostsedimentaryrocksareoneofthese2typeswithabitoftheotherJ

ClasticRocks– features/characteristics


Clasts…….

• Smallerclaststendtobesinglemineralcrystals• Largerclastsaretypicallypiecesofrock• Bysize…..• Mostclasts<0.063mmareactuallycomposedofclayminerals• Mostsand-sizedclasts (0.063-2mm)arequartz• Commonasquartzresistsweatheringmorethanothercommonminerals

• Mostclasts>2mmarerockfragments• fine-grainedrocklikebasaltorandesite• coarse-grainedrocklikegraniteorgneiss.

Udden-Wentworthgrainsandsizescale

Unitsinmm Unitsinmmµm


Namingsandstone

• Sandstoneisthemostimportantclasticrock• Sandstoneiseither• <15%siltandclay- Arenite - cleansandstoneOR• >15%silt/clay- Greywacke(“wacka”buttextsays“wackie”!)

• Namesforarenites:consideringthesand-sizedgrainsonly• >90%quartzthencalledquartzarenites• >10%feldsparand feldspar>rockfragments,thencalled:

• feldspathicarenitesorarkosic arenitesorjustarkose• >10%rockfragments,and rockfragments>feldspararelithicarenites

Chemicalsedimentaryrocks

• Chemicalsedimentaryrocksaremadefrommaterialtransportedasionsinsolution• Notpossibletorelatechemicalsedimentsbacktosourcerocks• Themostcommonchemicalsedimentaryrock,byfar,islimestone• Butotherrocksarechert,evaporites andbandedironformation(BIF)• Biologicalprocesses- importantinformationofsomenonclasticrocks,e.g.• Limestone

• limestoneismostlyfragmentsofmarineorganismsthatmakecalcitefortheirshells• Chert

• mostchert includesatleastsomeofthesilicatests(shells)ofdiatomsandothermicroscopicorganisms

Chert andBIFDalesGoge byGraemeChurchard underCCBY2.0.

Howevaporites formHassanZ.Harrazh [email protected]

Precipitationsequencefrom(sea)waterwithincreasingevaporation:

1.CalciteCaCO32.GypsumCaSO43.HaliteNaCl4.Sylvite KCl +otherK&Mgsalts

(leastsolublefirst)

Evaporite – [email protected]

Evaporite - bigdepositinCanada

• World’slargestreservesof‘potash’occurinSaskatchewan• Oreisexceptionallyhighgrade(25-30%K2O)• atdepthsof950-1100mincreasingto>3500m• Uniformmineralizationandthickness(2.4-3m)• nostructuraldeformations• MostlySylvinite (NaCl+KCl)• carnallite - hydratedKMgCl)• andclay

Publicdomain

• NowaYouTubevideoonSedimentaryRockIdentification(7mins)

• Reading forafterstudybreak182-194,202-210

•TEST• Iwillsendyousomequestionsbytheendoftheday.

• Thesewillbequestionsontheactualtest.• ThankyoutothosewhosubmittedaquestionforconsiderationJ• THEEND

lecture 14 feb - dr. hoyle

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