Post on 31-May-2020
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Primarilycomposedofcarbonateminerals,carbonaterocksdemonstrate2maintypes1. Limestone-composedofcalciteoraragonite2. Dolostone-composedofthemineraldolomite(CaMg(CO3)2).CarbonatesareresponsibleforKarstlandscapesandcavesenvironmentsduetotheirsolubilityinslightlyacidicwater.
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Limestonesaremostlycomposedofcalciumcarbonateintheformofeithercalciteandaragonite.Limestonesarecommonly(butnotexclusively)composedoftheskeletalfragmentsoforganisms.Ofallsedimentaryrockslimestonemakesupabout10%.Aragoniteisintheorthorhombicsystem.Almostallmolluskshellsandcorals(ScleracLnia)usearagonitefortheirhardparts.AragoniteisthermodynamicallyunstableandaltersintocalciteoverLme.At380-470°Caragonitechangesintocalcite.
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Calciteisinthetrigonal-rhombohedralsystembutrhombohedraaregenerallyrare.Thetwocommonformsofcalcitearelowmagnesiumcalciteandhighmagnesiumcalcite.OrganismsthatsecreteLowMgCalciteincludebrachiopods,trilobitesandechinoderms(seaurchins,starfish,crinoidsetc).Inorderofstabilitylowmagnesiumcalciteisthemoststableformofcalciumcarbonateinlimestonesfollowedbyhighmagnesiumcalciteandthenaragonite.OverLmemostlimestoneswillbealteredtoLowMgCalcite.
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CaCO3canalsobeprecipitatedinorganicallywiththethetypeofcalcitebeingdepositedintheoceansvaryingoverLme.In“calciteseas”low-magnesiumcalciteinthemainprecipitate.In“Aragoniteseas”aragoniteandhigh-magnesiumcalciteisprecipitated.CalciteseaswerecommonduringtheEarlyPaleozoicandtheMiddletoLateMesozoic.MiddlePaleozoicandCenozoicLmes(includingtoday)aregenerallyAragoniteseas.CalciteseasgenerallyformduringLmeswhentheEarthwaswarmerthantodaywithAragoniteseasmorecommonduringcoolerperiods.
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CalciteseastendtobecommonwhenseafloorspreadingisacLveandglobalgreenhouseclimatecondiLonsprevail.Thereason:increasedcyclingofseawaterthroughhydrothermalvents……AstheseawaterpassesthroughthehotbasalLcoceancrustcalciumrichmineralsareconvertedintomagnesium-richclaysviahydrothermalmetamorphism.ThisreducestheMg/CaraLooftheseawaterreturningtotheoceansfavoringtheproducLonofcalciteoveraragonite.TheassociaLonofcalciteseaswithgreenhousecondiLonsalsorelatestoincreasedspreadingrateswhichwouldalsoberelatedtoincreasedvolcanismandincreasedCO2deliverytotheatmosphere.WarmcondiLonsandwarmeroceansalsofavourtheprecipitaLonofcalciteoveraragonite.
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ThesearethebasicchemicalstepsoftheprecipitaLonofcalcite–animportantpartofthecarboncycle.
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InaddiLontothecatalyzingacLvityoforganisms,theconcentraLonofdissolvedCO2isaveryimportantfactorincalciumcarbonateprecipitaLon.ReducedCO2concentraLonmeansyouhavelowerproducLonofcarbonicacid(whichdissolvescalcite)andhigherCaCO3precipitaLon.CO2concentraLoniseffectedin3principleways:1. AtlowerpressuresCO2isreleasedfromwater.Asaresultlowerpressure
(shallowercondiLons)willbemoreconducivetotheprecipitaLonofcalcite.
2. CO2haslowerconcentraLoninwarmwaters.
3. IncreasedwateragitaLonleadstodiffusionofCO2totheatmosphere,makingcondiLonsmorehospitabletoCaCO3deposiLon
Overalltherefore,hightemperatures,lowerpressure&breakingwavesfavorcarbonateprecipitaLon
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AccumulaLonandprecipitaLonofCaCO3below5000metersisuncommon.ThedepthbelowwhichitcannotaccumulateisreferredtoastheCarbonateCompensaLonDepth(CCD),whichvariesbetween3000-5000mdependingonwatertemperature.AtdepthmoreCO2isdissolvedinthewatermakingtheoceanmoreacidic(carbonicacid–seeequaLononfigureabove)whichdissolvestheCaCO3.TheposiLonoftheseabedrelaLvetotheCCDthereforewilldeterminewhichtypeofsedimentcanbedeposited.IfabovetheCCDcalcareousoozescanformwhichwhenlithifiedformatypeoffinegrainedlimestoneweknowaschalk.ItispossibletofindcalcareousoozesinoceansdeeperthantheCCDifalayerofclayorsiliceousoozeisdepositedontopofcalcareousoozesthatformonaridgethatisabovetheCCD.AstheoceanfloormovesawayfromtheoceanridgeitcoolsandfallsbelowtheCCD(seediagram)–anysedimentthataccumulatedattheridge(abovetheCCD)andcappedbytheclayorsiliceousoozeswillbeprotectedinthedeeperocean.
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Modernlimestonesaremostlydepositedinreefs,carbonatesandbodiesclosetoreefandinlagoons.Inthepast(especiallyduringwarmgreenhouseperiods)limestonesweremorecommonduetowarmercondiLonsandgreaterareaofshallowshelfseas.
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AllochemsareequivalentsofgrainsinclasLcsediments.MaterialbetweentheallochemsisequivalenttothematrixorcementsinclasLcs.
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Skeletalgrainsareanybiologicalcomponentofalimestone.ThecharacteroftheskeletalgrainswithvarydependingontheenvironmentANDgeologicalperiodofdeposiLonofarock.
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Forexamplecrinoids(stalkedechinoderms)wereveryimportantcomponentsofDevonianandCarboniferousshallowmarinesegngsandsignificantlimestoneproducers.TheyaremuchlessimportanttodayandgenerallyrestrictedtodeepmarinesegngsinrelaLvelylownumbers.Youcanseesomeoftheimportantreefproducersinthisfigure-no,youdon’thavetolearnallofthisfigurefortheexam.
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a)Ooids=<2mmindiameterandspheroidal.Theyarereferredtoas"coated”sedimentarygrainsmeaningtheyareconcentricallylayered.Theyaremostcommonlycomposedofcalciumcarbonate,butcanalsobeformedbyironorphosphate-basedminerals.EnvironmentofdeposiLon-usuallyinshallowtropicalseas(egBahamasorPersianGulf).Pisoidsareooids>2mmindiameter.
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Ooidformasconcentriclayersofmaterialareprecipitatedaroundanucleussuchasashellfragmentorgrainofsand.Today,mostooidsarecomposedofaragonite.MostancientooidsarecomposedofcalciteformingeitherbydirectprecipitaLonduringperiodsofcalciteseasorviathealteraLonofooidsthatwereoriginallyaragonite.TheformaLonofanooidisthoughttorequiretheelevaLonofthesaturaLonofcalciumcarbonateincreases…thiscanoccurwhenwateriswarmand/oragitatedwhichwilldriveoffCO2andhelpprecipitatecalcite.
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b)Oncoids-sphericalconcentricallylayeredcoatedgrainsformedbyphotosyntheLccyanobacteriathatgrowaroundanucleussuchasashellfragment.Oncoids,likestromatolitesarelayeredassociaLonsofcyanobacteriabutformsphericalbodiesratherthancolumnsordomesofstromatolites.AsoncolitesformfromphotosyntheLccyanobacteriatheyformwithinthephoLczone(depthtowhichsunlightcanpenetratetotheoceanfloor).Oncolitesrarelyexceed10cmindiameter.
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c)Peloidsarethoughttobethefecalpellets(poop)oforganisms.IftheyoccurinhighconcentraLonsandbecomealiplesquashedtheycangivetheimpressionofarockcomposedenLrelyofmicrite(carbonatemud).
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d)Intraclastsareirregularly-shapedallochems.FormbyerosionofparLallylithifiedsedimentwhichisthenre-depositedasirregularlyshapedgrains(syndeposiLonal).e)Limestonesmayalsocontainnoncarbonaterockfragments,quartzandclaysbutcollecLvelythesemustbelessthan50%oftherockifitsLlltobeclassifiedasalimestone.
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Micrite=”CaCO3mud”composedofclaysizedcrystals.Intodaysoceansmicritecommonlyformsascalcareousalgaeskeletonsbreakdown.TheexactmodeofformaLonofancientmicriteisuncertain.
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Wewilldealwithlimestonecementsinmoredetailduringthelectureondiagensis.
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DevelopedbyRobertL.Folk–usesadescripLonofthethreemaincomponentsofcarbonaterocks:allochems(grains),matrix(mostlymicrite),andcement(sparite).Atwopartnamingsystemisused–thefirstnamereferstothegrainsandthesecondreferstothematrixorcement.ThefolkschemeiscommonlyusedwhendescribingthinsecLonsofcarbonaterocks.YoucanhavemorethanonetypeofallochemnamedintheFolkclassificaLon.EG:• Oolites+Fossils+Sparmatrix=Oobiosparite=Oobiosparite.• Pellets+Oolites+Fossils+Micritematrix=peloobiomicrite=Peloobiomicrite.
ThesystemcanalsobeusedifthereisbothmatrixandcementEG:Fossils+Sparmatrix+Micritematrix=biosparmicrite=Biosparmicrite
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TheDunhamsystemclassifiesusingdeposiLonaltextures.RocksaredividedintofourmaingroupsbasedproporLonsofallochemsandwhetherornottheywereoriginallyincontactwitheachother.Ifnotoriginallyincontact=mud-supported.Iforiginallyincontact=grainsupported.AstheDunhamschemedealswithtextureratherthangraincomposiLonitismoreusefulwhendescribinghandsamples.- Mudstone:<10%grainssupportedbyalimemud.- Wackestone:>10%grains,supportedbyalimemud.- Packstone:containslimemud,butisgrainsupported.- Rudstone:coarselimestonessupportedbygrains>2mm.- Grainstone:lacksmud,andisgrainsupported.- Boundstones:originalcomponentshavebeenboundtogetherarerdeposiLon
suchasinacoralreef
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LimestonedeposiLonhasacomplexandvariedhistory.WillhereprovideaVERYbriefoverviewofsomeoftheenvironmentsofdeposiLonoflimestoneandsomeofthemainlimestonetypesthatcharacterizethem.
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Althoughcarbonatescanforminanumberofterrestrialenvironments,wewillmostlyconcentrateonthelacustrineenvironmentshere.i. Inorganiccarbonates:limestones(micrites)precipitatedbylossofCO2dueto
evaporaLon(waterwarms)inashallowenvironment(lowpressure).Freshwaterenteringintoasalinelakecanalsohelpprecipitatecarbonates.Ifthewaterisagitated,ooidsmayalsoform.
ii. Algalcarbonates.Canformwhenyougetbloomsofcalcareousalgaeinalakebutalsomayforminstromatolitesasyoucanseeintheimageabove
iii. Ooidsandskeletalsands.FragmentsofalgaeorlargercarbonatesecreLngorganismslikemollusks.
AcommonfaciespapernincarboanteprecipitaLnglakeswouldbestromatolite“reefs”inshallowareas(withooidsandskeletalsandsinthemoreagitatedarea)passingintofinermicritesinthedeeper/quietersegngs
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Ancientmarinecarbonateenvironmentsweremuchmoreextensivethantheyaretoday.Ingeneralthoughcarbonatefaciespapernscanbedividedintoabroadpapernofshore(Ldalflatintheabove)–platormorshelf(bothdeepandshallow)–basin.
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IntermipentlycoveredwithwaterwithweekcurrentsandwaveacLon.Sedimentsaremicritedominatedandmaycontainpellets.Thesedimentsintheseenvironmentsmayexhibitfenestrae,gapswithinthesedimentthatmayrepresentdesiccaLonandshrinkageofmaterialwiththesedimentorthetrappingofgasbubbles.Thesespacesarethenfilledwitherwithsedimentorcement.Mudcracksmayalsobeafeatureofthesesediments.ThefossilsintheseenvironmentswillgenerallybeofrestricteddiversityasLdalenvironmentsareastressfulenvironmenttoinhabitgivendailychangesinwaterlevelandpotenLallysalinity.Inpartduetothis(astherewillbefewergrazingorganisms)stromatolitesandalgalmatsmaybecommon.Inveryaridenvironments,intenseevaporaLonmaypermitthedevelopmentofevaporitesinthisenvironment.InaddiLon,thisevaporaLonmaydrawwaterthroughthesedimentarypileandcausethedolomiLzaLonofsomeofthecalciumcarbonate(Seelater).
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Similarlithologicallytothepreviousenvironmentbutwithoutevidenceofexposuretotheair.Greencalcareousalgaearethemainsedimentcontributorsinthisenvironment.Suchlagoonallimestonesarecommoninthegeologicalrecord.
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OccurinareasofhighLdalacLvityandcurrent/waveacLon.DepthofdeposiLongenerallylessthan10mformingbeachesandshoalscomposedofooidsandroundedskeletalfragments.
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Theseareasaregenerallypoorlydevelopedtodaywhencomparedtothegeologicalpastwhenextensiveshallow(epeiric)seaswheremorecommon.ModernexampleswouldincludetheYucatanshelfandtheeasternGulfofMexico.Carbonatesinthisenvironmentaredepositedinstandardsalinity/welloxygenatedcondiLons.Thesedimentschangeincharacterfrommoregrainstonetowardstheshorelinetomoremicriterichocean-ward.Thesesedimentsarecommonlyheavilybioturbated.Theskeletalcomponentsarediverse.Patchreefsmaybecommon,parLcularlytowardstheoutershelfarea.
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Reefisatermthatisusedinmanycontexts.Foruswewillconsiderareeftobeatopographicwaveresistantstructure.Reefrocksarecommonlyclassedasbiolithotes,framestonesandbindstonesandmaycontainconsiderableprimaryporosityintheformoflargecaviLesrepresenLngthecomplexstructureoftheframeworkofareef.ThesecaviLesmaybefilledwithdebrisandcement.ThispotenLalporosityandpermeabilitymakesthemgreatpetroleumreservoirs.Infrontofthis,bioclasLcmaterialbrokenofffromthereefwillbedepositeddownthereefastalus.Inthisarearudstonesandfloatstonesmaybefoundassociatedwithslumpstructures.Itisalsointhisareathatcarbonateturbiditesmaybegeneratedthatpassdownthefore-reefandintothebasin.
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Todaycoralsandcorallinealgaearethemostcommonreefproducingorganisms.Inthepastthemainreefproducingorganismshavebeenverydifferent.DuringthePrecambrianandCambrianstromatoliteswerethemainreefproducers.Stromatoporoids(akindofsponge),rugoseandtabulatecoralsarecharacterisLcoflowerpaleozoicreefs…seethefigureearlierinthislectureformoreinformaLon.
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Reefscanroughlybebrokendowninto3zones:1. ForeReefandreefcrest:ThesteepslopeoftheforereefcanbealmostverLcal
withorganismsconstrucLngthereefatthereefcrest.TalluswillfallfromthereefandbedepositeddownslopesomeLmeintheformofturbidites.
2. ReefFlat:Nomorethan1–2mofwaterdepth,Majorareaofgrowthofreeforganisms
3. BackReef:Willreceivesomedebrisfromthereefflatandpassintolagoondeposits
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Thefollowingtakenfrom:hpp://www.nps.gov/gumo/naturescience/geologicformaLons.htm“ThroughmuchoftheearlyandmiddlePermianmuchofmodern-dayNewMexicoandTexasoccupiedthewesternedgeofPangaeaneartheequator.AvastoceansurroundedPangea,butanarrowinlet,theHoveyChannel,connectedtheoceanwiththePermianBasin,aninlandseawhichcoveredpartsofwhatisnownorthernMexicoandthesouthwesternUnitedStates.ThePermianBasinhadthreearms:theMarfa,Delaware,andMidlandBasins.Themiddlearm(theDelawareBasin)containedtheDelawareSeawhichcoveredandarea150mileslongand75mileswideoverwhatisnowWesternTexasandSoutheasternNewMexico.ForseveralmillionyearstheCapitanReefexpandedandthrivedalongtherimoftheDelawareBasinunLleventsalteredtheenvironmentcriLcaltoitsgrowthapproximately260millionyearsago.TheoutletconnecLngthePermianBasintotheoceanbecamerestrictedandtheDelawareSeabegantoevaporatefasterthanitcouldbereplenished.Evaporitesbegantoprecipitateoutofthevanishingwatersanddrirtotheseafloor,formingthin,alternaLngbandsofmineralsaltsandmud.”
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PelagicsedimentaLontakesoverinthesesystems.Toodeepforsignificantbenthic(bopomdwelling)organismstothrive.Maygetsometallusandtrubiditesclosertothereeffront.LowerlimitofdeposiLonofsedimentsetbytheCCD.
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Moderncarbonatepelagicorganismsarepteropods,coccolithsandformainifera.
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GoodexampleofextensivepelagiclimestoneproducLon:Cretaceouschalkseas.Perfectforpelagiccarbonates:warmclimaLccondiLons,shallowseasfloodedconLnentsprovidingextensiveshelfenvironmentsfarfromanyclasLcsources.
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Dolomite:calciummagnesiumcarbonateCaMg(CO3)2.Rockscomposedofdolomite=dolostones.TheproporLonofdolomiteinasedimentwilldetermineitsoverallclassificaLon:Limestone:0–10%dolomiteintherockDolomiLclimestone:10–50%dolomiteCalciLcDolomite:50-90%dolomiteDolomite:90–100%dolomite
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AlthoughthepossibilityofprimarysedimentaLonofdolomitehasbeenproposeditismostlyregardedasformingasthealternaLonoflime(CaCO3)sedimentsduringearlydiagenesisorthealteraLonoflimestonesindeepburialsegng.Inthisimage,diamond-shapedcrystalsofdolomitehavegrownarerdeposiLon,duringdiagenesis.Theyreplacethefinecalciumcarbonatemud(darkmaterialinthephoto)thatmakesuptherestoftherock.Becauseofthecoarsenatureofdolomitecrystalsmuchoftheoriginaltexturesoflimestonesarelostduring‘dolomiLzaLon.’
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