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Sedimentarystructuresformbyphysicalprocesses,before,duringanda9ersedimenta:on,whileothersresultfromorganicandchemicalprocesses.Sedimentarystructureshaveavarietyofuses:forinterpre:ngdeposi:onalenvironmentsintermsofwaterdepth,wind/currentstrengthetc;fordeterminingtheway-upofarocksequenceinanareaofcomplexfolding,andfordeducingpaleocurrentpaEernsandpaleogeography.

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Abedisthesmallestdivisionofageologicforma:onorstra:graphicrockseriesmarkedbywell-defineddivisionalplanes(beddingplanes)separa:ngitfromlayersaboveandbelow.Abedisthesmallestlithostra:graphicunit,usuallyranginginthicknessfromacen:metertoseveralmetersanddis:nguishablefrombedsaboveandbelowit.Bedscanbedifferen:atedinvariousways,includingrockormineraltypeandpar:clesize.Thetermisgenerallyappliedtosedimentarystrata,butmayalsobeusedforvolcanicflowsorashlayers.Alamina:onisasmallscalesequenceoffinelayers(socalledlaminae)thatoccursinsedimentaryrocks.Lamina:oniso9enregardedaseveryplanarstructuresmallerthan1cm,.Asinglesedimentaryrockcanhavebothlaminaeandbeds.Lamina:onconsistsofsmalldifferencesinthetypeofsedimentthatoccurthroughouttherock.Theyarecausedbycyclicchangesinthesupplyofsediment.Thesechangescanoccuringrainsize,claypercentage,microfossilcontent,organicmaterialcontentormineralcontentando9enresultinpronounceddifferencesincolour.Lamina:oncanoccurasparallelstructures(parallellamina:on)orindifferentsetsthatmakeananglewitheachother(cross-lamina:on–seelater).Itcanoccurinmanydifferenttypesofsedimentaryrock,fromcoarsesandstonetofineshales,mudstonesoreveninevaporites.Becauselamina:onisafinestructure,itiseasilydestroyedbybioturba:on(theac:vityofburrowingorganisms)shortlya9erdeposi:on.Lamina:onthereforesurvivesbeEerunderanoxiccircumstances,orwhenthesedimenta:onratewashighandthesedimentwasburiedbeforebioturba:oncouldoccur.

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Currentripplemarks,unidirec:onalripples,orasymmetricalripplemarksareasymmetricalinprofile,withagentleup-currentslope(stossslope)andasteeperdown-currentslope(leeslope).Thedown-currentslopeistheangleofrepose,whichmayvarydependingonsedimenttype.Currentripplescommonlyforminfluvialandaeolian(land)deposi:onalenvironmentsbutarealsoafeatureofunderwateravalancheevents(turbidites).Asymmetricripplesandareasignifierofthe“lowerpartoftheLowerFlowRegime.”Thismeanstheyarediagnos:cofapar:cularvelocityofafluid(eitherwindorwater).Atslowerandmorerapidcurrentveloci:esdifferenttypesof“ripple”structureswillformbutthisisoutofthescopeofthiscourse.Formoreinforma:onseeEOSC320.

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Ripplecrossbedding,stra:fica:onorlaminaeformwhendeposi:ontakesplaceduringmigra:onofcurrentorwaveripples.Aseriesofcross-laminaeareproducedbysuperimposingmigra:ngripples.Thisprocessresultsincross-beddedunitsthathavethegeneralappearanceofwavesinoutcropsec:onscutnormaltothewavecrests.Ripplecross-laminaewillalwayshaveasteeperdipdownstream,andwillalwaysbeperpendiculartopaleoflowmeaningtheorienta:onoftherippleswillbeinadirec:onthatisninetydegreestothedirec:onthatcurrentifflowing.

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Wave-formedripplesorwave-formedripplemarksareafeatureofsediments(sandstones,limestones,siltstones)anddunes.Theseripplemarksareo9encharacterised(andthusdis:nguishedfromcurrentripples)bysymmetriccrosssec:onsandlongrela:velystraightcrests,whichmaycommonlybifurcate.Commonly,thesecrestscanbetruncatedbysubsequentflows.Theirwavelength(periodicity)dependsonthesedimentgrainsize,waterdepthandwater-par:cleorbitsinthewaves.On:dalflatsthepaEernofwave-formedripplesmaybecomplicated,asaproductofchangingdepthandwindand:dalrunoffdirec:ons.Symmetricalripplesarecommonlyfoundinshallowwaters.Beachesareagoodenvironmentinwhichtolocatetheseripples.WaverippleswillforminaqueousenvironmentswhenthesedimentwaterinterfaceisABOVEwavebase.Thatistheleveltowhichsurfaceoscilla:onscandisturbthesediment.

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Differen:a:ngbetweenwaveandcurrentripplescanbedifficult.Asnoted,currentripplestendtobemoreasymmetrical.Theripplecrestscanalsobeusedasadiagnos:cfeature.WaveripplesMAYshowwell-developedbifurca:onofcrests,andcrestsareratherregular.Theymayalsohavemorepointedcrestincrosssec:on.Crestsofcurrentripples,ontheotherhand,arenotsoregular.Bifurca:onisabsent;however,creststerminateandarereplacebyothercrests.–A9erReineck&Singh,1975.

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Agradedbedisonecharacterizedbyasystema:cchangeingrainorclastsizefromthebaseofthebedtothetop.Mostcommonlythistakestheformofnormalgrading,withcoarsersedimentsatthebase,whichgradeupwardintoprogressivelyfinerones.Normallygradedbedsgenerallyrepresentdeposi:onalenvironmentswhichdecreaseintransportenergyas:mepasses(awaningcurrent).Theyareperhapsbestrepresentedinturbiditestrata,wheretheyindicateasuddenstrongcurrentthatdepositsheavy,coarsesedimentsfirst,withfineronesfollowingasthecurrentweakens.Theycanalsoforminterrestrialstreamandriverdeposits.

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Turbiditesaredepositedbydecelera:ng(waning)turbiditycurrents.Eachturbidite“event”rangesfromafewcmtoametreormorethick.Typicallyformsequencesmany100’sor1000’sofmetresthick.Asthecurrentwanesprogressivelyfinersedimentandspecificvelocityrelatedsedimentarystructuresaredeposited.Boumasequencesaretypicallyboundedbyerosionalsedimentarystructuresattheirtopandbase.Turbiditescanforminanyenvironmentwherethereisaslopethatsedimentcanflowdown.

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Averagefrom5–10cmacrossand10–20c,mlong.Occuringroupsofsiilarsizeandorienta:on.Forma:onisaEributedtolocalizederosionbysand-ladencurrentspassingoveracohesivemudsurface.Measuringthelongaxisoftheflutecastgivesthedirec:onofflow,withthetaperedendpoin:ngtowardtheflowandthesteependupcurrent.Theconcavityoftheflutecastalsopointsstra:graphicallyup.Flutecastsarecommoninturbiditeflow(sedimentavalanche).

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Toolmarksareatypeoferosionalfeatureformedbydebrisbeingdraggedalongbyacurrent.Theaveragedirec:onofthesecanbeassumedtobetheflowdirec:on,thoughitisbimodal,soitcouldbeeitherwayalongthemark.Toolmarksalsohaveamorespecificbreakdown.Therearegroovesandstria:ons,skiporprodmarks,androllmarks.Grooveorstria:onmarksresultfromthecon:nuouscontactwiththemuddybed.Skiporprodmarkscomefromobjectsthatbouncealongthesurfaceofthemuddybed.Androllmarksresultfromobjectsrollingalongthemuddybed.

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Channelstructuresoccuronthescaleofmetres,insomecaseskilometres.Smallerchannelsaresome:mescalledscours.Botharerecognizedbythewaytheycutintosedimentarystrata/lamina:onsbelowthem.Theyaregenerallyconcaveupinsec:onando9enfilledwithacoarsersedimentthanthethosethattheyareerodinginto.Channelsaregenerallylongerlivedfeaturesthansoursandmaybeverylongpathwaysforthemovementofwaterandsedimentintheformofriversorasunderwaterchannelsinsubmarinefeaturescalledsubmarinefans(kindoflikeanunderwaterdelta).

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Formswhencomplexfoldingandcrumplingofbedsorlamina:onsoccur.Thistypeofdeforma:oniscommonlyfoundinfineorsiltysands,andisusuallyconfinedtoonerocklayer.Possiblyrepresentstheslumpingofsedimentthatiss:llfairly‘wet’andnotyetlithified(turnedtorock)AFTERithasbeendeposited–therapid‘de-watering’ofthesedimentmayalsocontributetotheconvolu:ons

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Theweightofanoverlyingbed(o9enasand)forcesanunderlyingbed(o9enamud)topushupthroughtheoverlyingbed,generallywhenbothstrataaresaturatedwithwater.Theresul:ngpaEern(incrosssec:on)mayresembleflames.Thestructuresoccurunderpressure;theyareo9entriggeredbyearthquakesofmagnitude>6,whichcanini:ateflaminginlarge(hundredstothousandsofsquarekilometres)areas,butcanalsobecausedbytherepeatedpoundingofwaves.Usefulasaway-upstructure.Theareapushingdownintotheunderlyingbediscalledaloadcast.

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Naturallyformingmudcracksstartaswet,muddysedimentdesiccates,causingcontrac:on.Individualcracksjoinupformingapolygonal,interconnectednetwork.Thesecracksmaylaterbefilledwithsedimentandformcastsonthebaseoftheoverlyingbed.Mudcracksaregenerallypolygonalinplanviewandv-shapedincrosssec:on.The"v"openstowardsthetopofthebedandthecracktapersdownward.

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Saltpseudomorphsformasabodyofwaterevaporates.Halitemaybeprecipitatedwhichmaygrowonthethetopofthesediment.Laterinfluxofwaterortheraincandissolvethehalite“cube”leavingbehindasmallcubicdepressionwhichcanbefilledinwithlatersediment.

Fossils(fromLa:nfossus,literally"havingbeendugup")arethepreservedremainsortracesofanimals,plants,andotherorganismsfromtheremotepast.Thetotalityoffossils,bothdiscoveredandundiscovered,andtheirplacementinfossiliferous(fossil-containing)rockforma:onsandsedimentarylayers(strata)isknownasthefossilrecord.

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Microfossilsarefossilsgenerallynotlargerthanfourmillimeters(althoughthereisnofixeddefini:on),andcommonlysmallerthanonemillimeter,thestudyofwhichrequirestheuseoflightorelectronmicroscopy.Microfossilso9enrepresentthebaseofancientfoodchains.Theycanbecomposedofsilicaandcalcitebutalsoorganicmaterial.Itistheorganicwalledmicrofossilsthatcommonlyaccumulatetoformorganicrichsedimentsthatmaybe“cooked”atdepthtogenerateoil.

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Tracefossils,alsocalledichnofossils,aregeologicalrecordsofbiologicalac:vityandbehavior.Tracefossilsmaybeimpressionsmadeonthesubstratebyanorganism:forexample,burrows,borings,footprintsandfeedingmarks,androotcavi:es.Theterminitsbroadestsensealsoincludestheremainsofotherorganicmaterialproducedbyanorganism—forexamplecoprolites(fossilizeddroppings)orchemicalmarkers—orsedimentologicalstructuresproducedbybiologicalmeans-forexample,stromatolites.Tracefossilscontrastwithbodyfossils,whicharethefossilizedremainsofpartsoforganisms'bodies,usuallyalteredbylaterchemicalac:vityormineraliza:on.

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Stromatolitesarelayeredaccre:onarystructuresformedinshallowwaterbythetrapping,bindingandcementa:onofsedimentarygrainsbybiofilmsofmicroorganisms,especiallycyanobacteria(commonlyknownasblue-greenalgae).StromatolitesprovidesomeofthemostancientrecordsoflifeonEarth.Avarietyofstromatolitemorphologiesexistincludingconical,stra:form,branching,domal,andcolumnartypes.Theformsthathavethemostpillarsanddomestendtoforminquietwaters.Asstromatolitesfromtheac:vityofphotosynthe:ccyanobacteria,whenfoundtheyindicatethatwaterdepth(paleobathymetry)musthavebeenwithinthepho:czone(seeearlier).

StromatolitesoccurwidelyinthefossilrecordofthePrecambrian,butareraretodayduetohighnumberofgrazingorganismssuchassnails.Veryfewancientstromatolitescontainfossilizedmicrobes–onlythelayereddomestructurestheyproducedremain.

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Anoduleinpetrologyormineralogyisasecondarystructure,generallysphericalorirregularlyroundedinshape.Nodulesaretypicallysolidreplacementbodiesofchertorironoxidesformedduringdiagenesisofasedimentaryrock.NodulesmaybehollowasgeodesorvugsorfilledwithcrystalsandintricategeometricshrinkagepaEernsasinseptariannodules.Thereisanimportantdis:nc:ontodrawbetweennodulesandconcre:ons;thelaEerbeingformedfrommineralprecipita:onaroundsomekindofnucleuswhiletheformerisareplacementbody.

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