primarily composed of carbonate minerals, carbonate rocks ...€¦ · sedimentary grains meaning...

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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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