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Evaporite-formsbyconcentra5on/crystalliza5onofwatersolublemineralsvia
evapora5onofanaqueoussolu5on.Evaporitescanbemarine,andnon-marine.In
addi5ontoprecipita5nginanopenbodyofwatertheymayalsoprecipitatefrom
porewatersinsedimentsclosetothesurface.Mostevaporitesarederivedfrom
evapora5ngseawater.Chlorineions(Cl-)makeup94.5%ofseawaterandsulfate
(SO42-)makesup4.9%.Thisprobablyexplainswhythefollowingarethemost
evaporiteminerals:
Halite(NaCl)
Gypsum(CaSO4.H2O)
Anhydrite(CaSO4)–effec5velydehydratedgypsum
Evaporiteminerlasareextremelyvaluabletoindustry.Forexample,tablesalt
(Halite),Sylvite(KCL“potash”),gypsumandmanyotherevaporitemineralsare
extremelyvaluabletothechemicalindustry.Inaddi5on,structurescreatedby“salt
tectonics”areo[enassociatedwithoilandgasplays.
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Thisfantas5cimageisNOTfromamovieset.Thesearegiantgypsumcrystals(some
11mlong)inMexico’sCuevadelosCristales.Thecrystalsgrewinthecavewhenit
wasfilledwithmineral-richwateratabout58°C.Miningopera5onsexposedthe
crystalswhenthewaterwaspumpedoutofthecave.
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OfallthesedimentaryrocksdepositedsincethebaseoftheCambrian,evaporites
onlymakeuparound1%.Evenso,theycanformextensivedepositspar5cularly
whencon5nentsaregroupedaroundtheequatorresul5nginelevatedratesof
evapora5oninshallowmarineseangs.
Asevaporitemineralsareverysolubletheyareusuallyfairlyrareinoutcrop.Most
evaporitedepositsareknownformsubsurfacesec5ons,o[enrevealedbydrilling.
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Wehavealreadyseenthisenvironmentinrela5ontotheforma5onofdolomites.In
supra5dalenvironmentsunderhotaridcondi5onstheevapora5onofwaterfromthe
surfaceoftheexposedsedimentwilldrawsalinefluidsthroughthesedimentpores
viaEVAPORATIVEPUMPING.Asthewaterisevaporatedgypsumisprecipitated
directlyinthesediments.Halitemayalsoformonthetopofthesedimentforminga
“mudcracked”surface.Thenetdeposi5onofmaterialwillcausethesabkhato
prograde“buildout”intotheshallowsea.Thiswillcreatesedimentarysuccessions
withaverydis5nc5vesetoffaciesinaver5calsense.
GoodexamplesofmodernSabkhaenvironmentscanbefoundalongthecoastofthe
UnitedArabEmiratesandBajaCalifornia.
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Gypsumandanhydritewillformanumberofpossiblesedimentarystructures:
1.  Discretegypsumnodules
2.  Chickenwiregypsum:nodulesbecomecloselypackedandthesedimentbetween
themrestrictedtothinstringers
3.  Enterolithictextures:layersofnodulesthatgrowtogetherandcontort/foldas
theydevelop
Inmorelandwardareaswheretemperaturesarehigheryoumaygetthedirect
precipita5onofanhydriteratherthangypsum.
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Theseareprobablythemostimportantsourceofevaporitesthoughinmodern5mes
wehavenomodernanalogue.Theyformwhenseawaterisevaporatedfromadeep
basin.Ingeneralmarineevaporiteswillprecipitateinthefollowingorder:
When50%ofmarinewaterisevaporateditispossibleforaminoramountof
carbonatestobeprecipitated.
When84%ofmarinewaterisevaporated:Gypsum
When90%ofmarinewaterisevaporated:Halite
When96%ofmarinewaterisevaporated:PotassiumsaltslikeSylvite
Thesebasinswillcanhaveaveryrestrictedconnec5ontotheopenocean,possibly
restrictedbyafault,carbonatebank,reeforsimilarstructure.Thesestructuresmay
starttorestrictbasinsduringperiodsofloweringsealevel.Themayalsoonlyreceive
wateroccasionally,perhapsduringstormswhichwillbeevaporatedformingsalt
depositsandatalaterreceivingmoreseawater.
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Thereisaproblemthough.A1000mcolumnofseawaterwillonlyproduce15.9mof
evaporite(themajorityofithalite).Sohowdoyouaccountforsomeevaporite
depositsthatareover1000mthickwithavariableevaporitecomposi5on?
ANSWER:Reflux:Arefluxmodelsuggestsrepeatedinfluxesofseawaterintothe
basinreplenishingwaterthathasbeenevaporatedtoallowtheforma5onofthicker
evaporites.INADDITIONtheconcentratedbrinesthatareformedduringevapora5on
aredenseandwilleitherseepintotheunderlyingsedimentsormoveoutBELOWthe
influxoffreshseawaterinthecaseofbarredbasins.Inthiswayevaporiteswillhave
amineralogicalsequencethatmightnotreflectasimpleconcentra5onofseawater
andprecipita5onofmineralsatvarioussolubilitypoints.Indeedtherefluxofbrines
canmaintainasteadyseawaterchemistryanddepositjustonetypeofevaporite
mineralorifratesofrefluxaltercanproduceavariablesequenceofevaporite
minerals.
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Theevaporitesthatareprecipitatedinthesedeepbasinso[enformlaminaeand
bedsthathaveaverywidelateralpersistence.AVERYgeneralsequenceoffacies
thatmightberepresentedinaver5calsequenceofevaporitesdepositedinbasin
seangcouldbe:
1.  Organicrichsedimentsformedastheoceanbasinstartstostagnate.The
circula5oninthebasinwouldbereducedandthebopomwatersrapidlybecome
anoxic.Inthesecondi5onsanoxicorganicrichshaleswouldform.
2.  Laminatedcarbonate/anhydriteasthewaterstartstoevaporate
3.  Laminatedanhydrite
4.  Laminatedanhydrite/halite
HaliteprecipitatedduringLOWperiodsofreflux(highersalinity),anhydriteduring
HIGHERperiodsofreflux(lowersalinity)
5.  Thickhalitedepositswouldformasevapora5onbecomeswelladvanced
6.  Finalevapora5onofbrinesproducingpotassiumandmagnesiumsalts(egSylvite)
inisolatedsalinelakesandsabkha
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Apartfromhalite,anhydriteandgypsumlakeevaporitescandemonstratesome
uniquefreshwaterprecipitatedminerals.Themineralogyoflacustrineevaporites
mayvarygreatlydependingonthelakechemistrywhichwillbeheavilyinfluencedby
thelocalenvironment.Twobroadcategoriesofevaporiteforminglakeare:
1.  Saltlakes.Dominatedbyhalitedeposi5on.GoodexamplesaretheDeadSeaand
theGreatSaltLake,Utah.
2.  Biperlakes.Dominatedbysodiumcarbonateandsulfatedeposi5on.Good
example:MonoLakeinCaliforniaandCarsonLakeinNevada.
Lakeevaporitedepositscommonlyexhibitazona5onwiththeleastsolublearound
theedgesofthelakeandthemoresolubletowardsthecentre.
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Makenotes:page315-318
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Unfortunatelywedon’thaveenough5metogointoalotofdetailwiththese
sedimentsandIhavehadtoomitsomeveryimportantsedimentslikephosphorites.
BandedIronForma,ons(BIF’s)
ABIFiscomposedofrepeatedlayersofironoxides(magne5te(Fe3O4)orhema5te
(Fe2O3))alterna5ngwithlayersofiron-poorshaleandchert.BIF’saremostcommon
between1.8–2.5billionyearsagobuthavenotbeendepositedforaroundthelast
500millionyears.Theconven5onalconceptregardingtheirforma5onisthattheiron
layersinBIFformedasoxygenwasreleasedbyphotosynthe5ccyanobacteria(blue
greenalgae).Thisincreasedrateofoxygenproduc5onbythebiospherecorrelates
wellwiththemajorityofBIFforma5on(andstructurescalledstromatolitesproduced
bythecyanobacteria)andiscalledtheGreatOxygena5onEvent.Theoxygen
producedcausedtheprecipita5onofironinsolu5onintheEarth'soceanstoform
insolubleironoxides(rust),formingthethinlayersofironrichsedimentinBIFs.
Somehavesuggestedthateachbandmayresultfromcyclicvaria5onsoxygen-
possiblyrelatedtoincreasedphotosynthesis(andthereforeoxygenrelease)during
summermonths.BandedIronForma5onsareaveryimportantsourceofironore.
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Chert
Fine-grainedsilica-richo[enmicrocrystallinesedimentaryrock.Demonstratesgreat
varia5onincolorbutismosto[engray/brown.Cherto[enpossessesaconchoidal
fracture.Theexactmechanismofchertforma5onispoorlyunderstood.Somecherts
canformwhenthesiliceousskeletonsofmarineplanktonaredissolvedandthesilica
reprecipitatedduringdiagenesis.
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Coal
Black–brownsedimentaryrockcomposedprimarilyofcarbonalongwithvariable
quan55esofhydrogen,sulfur,oxygen,nitrogen.Commonlyfoundinbedsorcoal
seams.Someformsofcoalsuchasanthracitemayberegardedasametamorphic
rockduetothehightemperaturesandpressuresithasbeensubjectedto.Coalforms
whenorganicvegeta5oncollectsandisnotoxidized.Ini5allytheplantmaterialforms
peatbutincreasingburialtemperaturesandpressuresdriveoffmuchofthevola5le
material,increasingthecarboncontentandeventuallyformingacoal.
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Volcaniclas,cDeposits:Clas5crockscomposedmostlyofvolcanicmaterialsthat
havebeentransportedbywindorwater.
Pyroclas5crocks:generatedbyexplosivevolcanicac5vity.Depositsincludeash,
lapilliandbombsorblocksandmayincludeshaperedcountryrock.Ignimbritesarea
typeofpyroclas5cdepositassociatedwithhigh-temperaturegasandashflows.
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Hyaloclas5tes
Volcanicbrecciacontainingahighpropor5onofblackvolcanicglassfragments.
Hyaloclas5tes
formwhenvolcanicerup5onsinteractwithwater.Fragmenta5oniscausedbyforce
ofthevolcanicexplosion,orbythermalshockwhenthevolcanicmaterialiscooled
rapidlybywater.
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Lahars
Mudflowordebrisflowcontainingahighpropor5onofvolcanicdebris.They
resemblesedimentarybrecciasandconglomeratesinoutcrop.Laharscanreach
speedsof100kphandflowforover300km.LaharsfromtheNevadodelRuiz
erup5on(Colombia1985)killedanes5mated23,000whenthecityofArmerowas
buriedunder5metresoflahar.
Laharsdonothavetobeassociatedwithvolcanicerup5onandcanbegenerated
duringwetcondi5ons(suchasmightbecausedbymel5ngsnowandglaciersorby
highrainfall)whenpartofanexis5ngvolcaniclandscapecollapses.Earthquakes
couldactasatriggeringmechanism.
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Sedimentaryfacies–aunitofsedimentaryrockwithspecificcharacteris5cs.
Generallythesecharacteris5csreflectapar5cularprocessorenvironment.For
example,thewhitecliffsofDoverintheimageaboveformedinaspecificwarm
shallowmarineseangthatallowedforthedevelopmentofchalkfacies.Thedune
crossbeddedsandstonefaciesintheotherphotographformedinahotariddesert
environment.
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Faciesaredefinedby:sedimenttype,typeofsedimentarystructures(example:
ripples)andsome5mestheirfossilcontent.Forexampleyoucoulduse“rippled
sandstonefacies”a“fossilferousmudstonefacies”a“crossbeddednearshorefacies”
ora“conglomera5cfacies”todescribetheimagesabove.
Sincedeposi5onalenvironmentsgradelaterallyintootherenvironments-facies
changesareo[enGRADUAL.Considerthenextslide…….
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Thisimageshowsriverdeposi5ngsedimentintoashallowoceanthatpassesintoa
deepermarineenvironment.NOTE:theslopeonthisdiagramisGREATLY
exaggerated.Astheriverbringssedimentintothissystemitdepositsitscoarsest/
heaviestloadfirst,inthiscaseaconglomera5cfacies…possiblyrepresen5ngabeach
environment.Inthisshallow/turbulentenvironmentfinesedimentwilltendto
remaininsuspensionandwillnotbedeposited.Thefinercomponentwillgradually
bedepositedinquieterwatercondi5onsgivingaprogressionoffaciesfromcoarse
clas5csclosetotherivermouthwiththefinestmudsdepositedinthedeepest/
quietestcondi5ons.Eventuallyalltheclas5cmaterialwillhavebeendepositedandin
thishypothe5calmodelitispostulatedthatcalcareoussedimenta5onof
coccolithophoretestswillbecomeimportantproducingacarbonatefaciesinthe
quietestcondi5ons.
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Thepreviousmodelisnotdifficulttounderstandbutthingsgetaliplemorecomplex
whenyouconsiderthatfaciesMIGRATEgeographically.REMEMBERfaciesarethe
sedimentological“fingerprint”oftheENVIRONMENTinwhichtheyarebeing
deposited.Theloca5onofpar5cularenvironmentalcondi5onsCHANGESover5me…
justconsiderthecurrentadvanceoftheSaharadesertorhowfullyglaciated
condi5onsaregivingwaytoperiglacial(placesontheedgeoffullyglacialareas)in
current5mes.Ascondi5onsinspecificareaschange,thesedimentstheyrecordover
TIMEwillchangeasenvironmentsANDthefaciesthatforminthemMIGRATE.This
changeinTIMEwillberepresentedbychangeissedimenttypeVERTICALLY.
Oneofthemajordrivingforcesinthemigra5onofenvironmentsandfaciesissea
levelchange.Sealevelchangecanbelocal(forexampleifanareaisexperiencinga
generaldown-warpingofthecrustorifanareaisbeingraisedtectonically)orglobal
(eusta5c)astheresult(forexample)ofmel5ngofpolaricecaps.
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Inthefollowingslideswearegoingtoconsiderthemodelwelookedatearlierofa
riverdeposi5ngsedimentintoanoceanandseewhathappensifsealevelwereto
rise(transgression).WhatIwantyoutorecognizeisthemannerinwhichfacies
MIGRATEOVERONEANOTHERoverTIME.ALSONOTE:thereisadifferencebetween
theFACIESBOUNDARIESandtheTIMEBOUNDARES.Facieso[enCROSSCUT5me
horizonsarereferredtoasDIACHRONOUS.
At5me1wecanseewehaveconglomeratesdepositedclosetoshoreandsands
depositedfurtheroffshore.
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At5meT2sealevelhasrisenandasaresultshorelinehasmovedlandwardASHAVE
THEBOUNDARIESBETWEENTHEFACIES.
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AtTimeT3sealevelisevenhigherandthefaciesandshorelinehavecon5nuedto
migratetotheright.
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LocatelineA-A’onthefigurerepresen5ngonefixedloca5oninspace.Notehow,as
sealevelhaschanged,DIFFERENTFACIEShavebeendepositedatthatloca5on.This
hasoccurredbecauseenvironmentalcondi5onshavebeenchangingassealevel
increased.
ALSONOTE:The5meplanesarerepresentedbytheredlines.Ifyougroupthefacies
together(theyellowlines)youcanseethatthefaciesCROSSCUTthe5mehorizons…
thisiswhatwemeanbydiachronous.
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Migra5onwillalsooccurin5mesofsealevelfall(regression).NotethatatA-A’the
reversedver5calsuccessionoffacieswillberecorded.
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We know the following facts:
1.  Facies reflect changing conditions of deposition across our planet. In our
example, shallow high energy, near shore conditions are characterized by
sand. Deeper, quieter conditions are characterized by finer grained
sediments.
2. 
As conditions change (such as sea level) facies will appear to MIGRATE,
“following” their particular environmental conditions.
3. 
Over time this will lead to patterns of facies as one facies MIGRATES
OVER ANOTHER.
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4. This means that when we recover a vertical succession of rock (like the one
we used in our example) we can PREDICT the lateral equivalence of facies
……
As we go from coarse, near-shore facies at the bottom of the core to offshore, deeper water carbonate sediments at the top of the core, sea level (in
this example) must have been RISING over time.
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This is summed up in WALTHER’S LAW
“Facies that occur in a CONFORMABLE VERTICAL SUCCESSION of strata,
were deposited in laterally adjacent depositional environments.”
and put another way……
adjacent sedimentary environments (facies) will end up overlapping one
another over time.
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PLEASE NOTE: the concept of facies changes and Walther’s law does not
JUST apply to near shore / marine environments. Facies can migrate over
each other in ALL Sedimentary environments. Consider a meandering river. As
the river migrates across it’s flood plain it both erodes (on the outside of the
meander loop) and deposits (on the inside of the meander loop) sediments. As
the meander migrates it deposits sediments as various sedimentary facies
characteristic of particular “environments of deposition” within the river channel
system. As the meander migrates these facies are deposited OVER each
other in accordance with Walther’s Law. NO - You don’t have to learn this
figure.
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