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Smith ScholarWorks Smith ScholarWorks
Geosciences: Faculty Publications Geosciences
7-15-2017
Trilobite Extinctions, Facies Changes and the ROECE Carbon Trilobite Extinctions, Facies Changes and the ROECE Carbon
Isotope Excursion at the Cambrian Series 2–3 Boundary, Great Isotope Excursion at the Cambrian Series 2–3 Boundary, Great
Basin, Western USA Basin, Western USA
Luke E. Faggetter University of Leeds
Paul B. Wignall University of Leeds
Sara B. Pruss Smith College, [email protected]
Robert J. Newton University of Leeds
Yadong Sun Friedrich-Alexander-Universität Erlangen-Nürnberg
See next page for additional authors
Follow this and additional works at: https://scholarworks.smith.edu/geo_facpubs
Part of the Geology Commons
Recommended Citation Recommended Citation Faggetter, Luke E.; Wignall, Paul B.; Pruss, Sara B.; Newton, Robert J.; Sun, Yadong; and Crowley, Stephen F., "Trilobite Extinctions, Facies Changes and the ROECE Carbon Isotope Excursion at the Cambrian Series 2–3 Boundary, Great Basin, Western USA" (2017). Geosciences: Faculty Publications, Smith College, Northampton, MA. https://scholarworks.smith.edu/geo_facpubs/121
This Article has been accepted for inclusion in Geosciences: Faculty Publications by an authorized administrator of Smith ScholarWorks. For more information, please contact [email protected]
Authors Authors Luke E. Faggetter, Paul B. Wignall, Sara B. Pruss, Robert J. Newton, Yadong Sun, and Stephen F. Crowley
This article is available at Smith ScholarWorks: https://scholarworks.smith.edu/geo_facpubs/121
1
Trilobiteextinctions,facieschangesandtheROECEcarbonisotopeexcursionatthe1
CambrianSeries2-3boundary,GreatBasin,westernUSA.2
Faggetter,LukeE.a;Wignall,PaulB.
a;Pruss,SaraB.
b;Newton,RobertJ.
a;Sun,Yadong
c;Crowley,3
Stephend;4
aTheSchoolofEarthandEnvironment,theUniversityofLeeds,Leeds,WestYorkshire,LS29JT,5
[email protected];[email protected];[email protected];b6
DepartmentofGeosciences,SmithCollege,Northampton,MA01063,UnitedStates.7
[email protected];cSchoolofGeographyandEarthSciences,Friedrich-AlexanderUniversity8
Erlangen-Nürnberg,Schloßgarten5,91054Erlangen,[email protected];dSchoolof9
EnvironmentalSciences,UniversityofLiverpool,JaneHerdmanBuilding,LiverpoolL693GP,United10
Correspondingauthor:LukeFaggetter,[email protected]
13
Abstract14
ThemassextinctionoftheolenellidtrilobitesoccurredaroundtheCambrianSeries2-Series315
boundary.Likemanyothercrises,itcoincidedwithanegativecarbonisotopeexcursionbutthe16
associatedpalaeoenvironmentalchangesremainunclear.Toinvestigatethecausalmechanismfor17
thisevent,wereportfacieschanges,pyriteframboidpetrographyandcarbonisotopevaluesfrom18
CambrianSeries2-Series3(traditionallyEarly-MiddleCambrian)boundarystrataoftheCarrara19
Formation(DeathValleyregion,California)andPiocheFormation(Nevada).Thesedatareveal20
regionallychangingwaterdepthsfromhigh-energy,nearshorefacies(ooliticgrainstone)tomore21
offshoresiltymarlandfiner-grainedcarbonatemudstone.IntheCarraraFormation,theseries22
boundaryoccurswithinadeepeningsuccession,transitioningfromhigh-energy,nearshorefacies23
(ooliticgrainstoneandoncoliticpackstone)tooffshoremarl,thelatterofwhichcontainspyrite24
2
framboidpopulationsindicativeoflow-oxygen(dysoxic)depositionalconditions.Intermittent25
dysoxiapersistedbelowsub-wavebasesettingsthroughouttheearlyandmiddleCambrian,butdid26
notintensifyatthetimeofextinction,arguingagainstanoxiaasaprimarycauseintheolenellid27
trilobiteextinction.Withinbothfieldareas,theextinctionintervalcoincidedwithaminimumin28
d13Ccarbvalues,whichweinterpretastheregionalmanifestationoftheRedlichiid-OlenellidExtinction29
CarbonisotopeExcursion(ROECE).TheSeries2-Series3boundaryisreportedtocloselycoincide30
withalarge-amplitudesea-levelfallthatproducedtheSaukI/IIsequenceboundary,butthe31
placementoftheSeries2-Series3boundarywithinatransgressiveintervaloftheCarrara32
Formationshowsthatthisisnotthecase.Themainsequenceboundaryinthesuccessionoccurs33
muchlowerinthesuccession(atthetopoftheZabriskieQuartzite)andthereforeprecedesthe34
extinctionoftheolenellidsandROECE.35
Keywords:Olenellidextinction,CarraraFormation,PyramidShaleMember,PiocheFormation,C-36
ShaleMember37
38
1.! Introduction39
ThefirstmajorbioticcrisisofthePhanerozoicoccurredduringtheCambrianSeries2,an40
intervalthatsawthecollapseofarchaeocyathanreefs(Newell,1972;Boucot,1990;Debrenne,1991;41
ZhuravlevandWood,1996).Thiswasfollowed,attheSeries2-Series3boundary,bysevere42
generic-levellossesofolenellidandredlichiidtrilobites(Palmer,1998;Zhuetal.,2004;Zhuetal.,43
2006;Fanetal.,2011;Wangetal.,2011;Zhangetal.,2013).Thistrilobiteextinctionhasbeenused44
todelineatetheSeries2-Series3boundary;however,theboundaryremainsunratifiedas45
internationalcorrelationisconfoundedbyalackofglobally-distributedtaxaatthistime(Sundberg46
etal.,2016).Thetrilobiteextinctioncoincideswithamajornegativeδ13Cexcursionthathasbeen47
termedtheRedlichiid-OlenellidExtinctionCarbonIsotopeExcursionorROECE(Zhuetal.2004,48
2006).49
3
InthewesternGreatBasinoftheUnitedStates,aCambriansedimentarysuccession50
developedonarapidlysubsidingpassivemargin(Prave,1999;Stewart,1972;FedoandCooper,51
2001;Hoganetal.,2011;Kelleretal.,2012;Morgan,2012).SectionsinthesouthernNopahRanges52
(Kelleretal.,2012)exposestratafromCambrianSaukIandSaukIIsupersequencesthatareof53
importancetothisstudy(Prave,1991).Thesearewidespread,large-scaleLaurentiansequencesthat54
providearegionalstratigraphicframework.ThetransitionfromSaukItoSaukIIrecordsamajor55
lithologicalchangethatsawthesiliciclasticdepositionoftheZabriskieQuartzitereplacedby56
carbonatedepositionoftheCarraraFormation(Kelleretal.,2012;Morgan,2012).Thecontact57
betweenthesetwounitsisconsideredtobetheSaukI/IIsequenceboundary(Kelleretal.,2012;58
Morgan,2012).59
ThroughoutthePhanerozoictherelationshipbetweenenvironmentalperturbationand60
extinctionisacommonfocusofstudies,includingthoseintheCambrian(HallamandWignall,1997;61
Wignall,2015).Inparticular,sea-levelchange,marineanoxia,carbonisotopeexcursionsand62
eruptionsofLIPs(largeigneousprovinces)oftencoincidewithmassextinctions(Zhuravlevand63
Wood,1996;Wignall,2001;GlassandPhillips,2006;Jourdanetal.,2014).Thus,Zhuravlevand64
Wood(1996)notedthetemporallinkbetweenwidespreaddepositionofblackshalesandthe65
disappearanceofthearchaeocyathansintheCambrianofSiberia,andtrilobiteextinctionsat66
“biomere”boundariesarealsoascribedtodysoxia(Palmer,1984).However,theroleofanoxiain67
Cambrianextinctionshastobeviewedinthecontextofpersistentlyoxygen-restrictedoceansatthis68
time(e.g.Montañezetal.,2000;Hurtgenetal.,2009;Prussetal.,2010;Gilletal.,2011;Saltzmanet69
al.,2015;Tarhanetal.,2015).70
VolcanismmayalsohaveplayedaroleinROECE(GlassandPhillips,2006).TheKalkarindji71
LIPisaCambrianfloodbasaltprovinceofnorthernandcentralAustraliawithanestimatedoriginal72
surfaceareaof~2.1x106km
2(GlassandPhillips,2006;Jourdanetal.,2014;Marshalletal.,2016).73
4
Latestdatingeffortsyieldazirconageof510.7±0.6Ma,whichisclosetothatoftheCambrian74
Series2-Series3boundary(Jourdanetal.,2014).75
InordertoimproveourunderstandingoftheeventsassociatedwithROECE,thisstudy76
examinessectionsspanningtheCambrianSeries2-Series3boundaryintervalinthewesternGreat77
Basin.TheolenellidextinctionhorizonhasbeenlocatedwithinthePiocheFormationinNevada78
(Palmer,1998).WehaveexaminedthislevelandthecorrelativelevelsintheCarraraFormationin79
Californiainordertheexaminechangesoflithofacies,carbonisotopevariabilityandpyrite80
petrography.81
82
2.! Geologicalbackgroundandbiostratigraphy83
FollowingbreakupoftheRodiniasupercontinentinthelateNeoproterozoic,the84
northwesternmarginofLaurentiasubsidedrapidly(BondandKominz,1984;LevyandChristie-Blick,85
1991;Prave,1999;Howleyetal.,2006).BytheearlyCambrian,thewesternGreatBasin(USA)was86
positionedalongthewesternmarginofLaurentiawhereawide,clastic-dominatedshelfdeveloped87
inanequatorialsetting(PalmerandHalley,1979;MacNiocaillandSmethurst,1994;Fig.1).Clastic88
inputdecreasedinlateSeries2andwasreplacedbycarbonatedeposition(ErdtmannandMiller,89
1981;Howleyetal.,2006;Landing,2012).90
WehaveassessedenvironmentalconditionsacrosstheSeries2-Series3boundaryfrom91
sectionsintheGreatBasinincludingEmigrantPass(NopahRange,DeathValley,easternCalifornia),92
andOakSpringsSummit(BurntSpringsRange,easternNevada;Fig.1).Theregionallithostratigraphy93
ofthePiocheFormation(atOakSpringsSummit)wasdescribedindetailbyMerriamandPalmer94
(1964),andtheCarraraFormation(atEmigrantPass)byPalmerandHalley(1979).Inaddition,we95
examinedasectionofthePiocheFormationatRuinWashineasternNevada.96
97
5
2.1.EmigrantPass98
AtEmigrantPasstheZabriskieQuartziteandCarraraFormationareeasilyaccessibleand99
wellexposed.ThestrataareseenonthenorthsideofOldSpanishTrailHighwayasacontinuous100
sectionofquartzareniteandshaleformingslopesandmoderatelysteephillsideswithlimestone101
formingprominentledges(Figs.1and2).TheZabriskieQuartziteisdominatedbyburrowedand102
hummockycross-beddedquartzarenitebeds(Prave,1991;Kelleretal.,2012).Itlacksage-diagnostic103
fossils,butrocksimmediatelyaboveandbelowhaveyieldedfaunafromtheBonnia-Olenellus104
trilobitezoneofSeries2(Diehl,1974;PalmerandHalley,1979;Prave,1991;Pengetal.,2012).105
TheCarraraFormationcomprisescyclesofsiltymarlandlimestonewithatrilobitefauna106
spanningtheBonnia-OlenellustotheGlossopleurazones,andthustheSeries2-Series3boundary107
(Adams,1995;PalmerandHalley,1979;SundbergandMcCollum,2000;Babcocketal.,2012;Keller108
etal.,2012;Fig.3).TheunithasbeendividedintoninemembersinthewesternGreatBasin:Eagle109
MountainShale,ThimbleLimestone,EchoShale,GoldAceLimestone,PyramidShale,RedPass110
Limestone,PahrumpHillsShale,JangleLimestoneandtheDesertRangeLimestone(Palmerand111
Halley,1979).However,thereissignificantlateralvariationwithintheCarraraFormationandat112
EmigrantPass,theThimbleLimestoneisnotpresent(PalmerandHalley,1979;Adamsand113
Grotzinger,1996).OverallthecarbonatecontentoftheCarraraFormationdecreasesinmore114
basinwardsettingstothewest(Hoganetal.,2011;Kelleretal.,2012;Foster,2014).Withinthe115
CarraraFormationatEmigrantPass,fivemembersareimportanttothisstudy.116
TheEagleMountainShaleMemberconsistsofgreentogrey-brownsiltyshalewith117
interbeddedlensesandbedsofbioclasticlimestonedevelopedtowardsthetop(PalmerandHalley,118
1979).ThisisoverlainbytheEchoShaleMemberwhichconsistsofgreen,platyshaleandbrown-119
orangelimestone.TheEchoShaleiscorrelatedwiththebasalCombinedMetalsMemberofthe120
PiocheFormationineasternNevada(PalmerandHalley,1979).ThesucceedingGoldAceLimestone121
isaprominent,cliff-forminglimestone(CornwallandKleinhampl,1961).Thestrataincludethinto122
6
medium-beddedlimemudstone,withdolomiticbedsandoncoliticlimestone(PalmerandHalley,123
1979).Basedonsharedtrilobitezones,theGoldAceLimestonecorrelateswiththeCombined124
MetalsMemberofthePiocheFormationatOakSpringsSummit(MerriamandPalmer,1964).125
TheoverlyingPyramidShaleMemberisagreenshalewithinterbedsofbrownandmaroon126
siltymarlandlensesofoncoliticandbioclasticlimestone.Trilobitebiostratigraphyindicatesthe127
PyramidShaleisequivalenttotwomembersofthePiocheFormationineasternNevada:theC-Shale128
MemberandtheSusanDusterLimestoneMember(PalmerandHalley,1979;Palmer,1998).TheRed129
PassLimestoneMemberistheyoungestunitexaminedinDeathValley.Itformsprominentcliffsof130
oncoliticandbioclasticlimestone,laminatedlimemudstoneandfenestrallimemudstone(Palmer131
andHalley,1979).ThereisnoequivalentlimestoneunitinthePiocheFormationofNevada(Palmer132
andHalley,1979).133
134
2.2.OakSpringsSummit135
AtOakSpringsSummit,thePiocheFormationcropsouttothewestofaparkingareainadry136
riverbed.Limestoneformsmoreprominentledgesandplatformswhilstshaleformsrecessively137
weatheredoutcrop(Figs.1and2).TheCombinedMetalsMemberiscomposedofsilty,oncolite-138
bearingdarklimestonewithOlenellus(Palmer,1998;SundbergandMcCollum,2000;Hollingsworth139
etal.,2011).ItisoverlainbytheC-ShaleMember(formerlytheCometShale),aseriesofshaleand140
thin-beddedlimestonebedswithpinch-and-swellbedboundaries(Palmer,1998;Sundbergand141
McCollum,2000).TheSeries2-Series3boundaryisplacedatthebaseoftheC-Shaleduetothe142
suddendisappearanceoftheOlenellidae,andtheirreplacementbyafaunadominatedby143
Eoptychopariapiochensisatthislevel(Palmer,1998;SundbergandMcCollum,2000).The144
succeedingSusanDusterLimestoneMemberisawell-bedded,greymarlwithoccasionalargillaceous145
andbioclasticlimestonebedscomposedoftrilobitefragments(MerriamandPalmer,1964).146
147
7
2.3.Biostratigraphy148
TrilobiteassemblagesfromtheCarraraandPiocheformationsbelongtotheOlenellus,149
Eokochaspisnodosa,AmecephalusarrojosensisandthePlagiura-Poliellazonesthatprovidea150
frameworkforregionalcorrelation(MerriamandPalmer,1964;PalmerandHalley,1979;Fig.3).The151
OlenellusZonerangesfromtheZabriskieQuartzitetothebasalportionofthePyramidShale152
MemberwithinDeathValley(PalmerandHalley,1979;Fig.2).InNevada,thiszonespansthe153
DelamarMembertothebaseoftheC-Shale(MerriamandPalmer,1964;SundbergandMcCollum,154
2000).Allolenellidtrilobitesdisappearovera~2cmintervalatthetopofthezone,forminga155
distinctextinctionhorizon(Palmer,1998;Fig.2).Thisisimmediatelyfollowedbyfirstappearanceof156
theptychopariidtrilobiteEokochaspisnodosa,whichdefinesboththebaseoftheE.nodosaZone,157
andtheSeries2-Series3boundary(SundbergandMcCollum,2000;Fig.2).E.nodosaZonefaunas158
alsooccurinthePyramidShaleMemberinDeathValley(SundbergandMcCollum,2000).159
ThesucceedingAmocephalusarrojosensisZonecontainsA.arrojosensis,Mexicellarobusta160
andKochina?walcotti.ThezoneisbestdefinedatHiddenValley,Nevada,whereitsbaseis30m161
abovethebaseoftheC-ShaleMember(MerriamandPalmer,1964),butithasnotbeenrecordedin162
theCarraraFormationduetoapaucityoffossilsabovetheE.nodosoaZoneinthePyramidShale163
(PalmerandHalley,1979).However,trilobites,fromthePlagiura-PoliellaZone,reappearinthe164
uppermostPyramidShaleandlowerRedPassLimestone(PalmerandHalley,1979).165
166
3.Methods167
SedimentaryloggingoftheCarraraandPiocheformationswasundertaken.AtEmigrant168
Pass,170moftheCarraraFormationwasloggedfromthebaseoftheformation(atthecontactwith169
theZabriskieQuartzite)uptotheRedPassLimestoneMember.AtOakSpringsSummita53m-thick170
sectionofthePiocheFormationwaslogged,rangingfromthebasalCombinedMetalsMemberto171
SusanDusterLimestoneMember,anintervalcorrelativewiththeEmigrantPasssectionbasedon172
8
trilobitebiostratigraphy(MerriamandPalmer,1964;PalmerandHalley,1979;Fig.2).Fromthese173
logs,fourfaciesweredefined(discussedbelow,Table1).Atthetwostudysections,30samplesfrom174
thePiocheFormationand57samplesfromtheCarraraFormationwereanalysedforδ13Ccarb(Table175
2).InLincolnCounty,Nevada,wealsosampledtheRuinWashlocation(Palmer,1998;Lieberman,176
2003)foradditionalfaciesandframboidanalysis.RuinWashprovidedasecondsection(afterOak177
SpringsSummit)wheretheextinctionhorizonoftheolenellidsisclearlyseen(seeSupplementary178
Material,Fig.S1).Faciesanalysiswasundertakeninthefieldandcomplementedbypetrographic179
examinationof49thinsections.Inordertoevaluateredoxconditions,pyriteframboidsize180
distributionwasalsoassessedon21samplesusingascanningelectronmicroscope(FEIQuanta650181
FEG-ESEM)inbackscattermode(seeBondandWignall(2010)forprocedure).182
Thecalcitecarbon(13C/
12C)andoxygen(
18O/
16O)isotopevaluesofpowderedbulk183
sedimentsamplesweremeasuredonatotalof98samplesattheGeoZentrumNordbayern,FAU184
Erlangen-Nurnberg,Germany(27samples)andtheSchoolofEnvironmentalSciences,Universityof185
Liverpool,UK(71samples).Carbondioxidewaspreparedbyreactionwithphosphoricacideitherat186
70oCusingaGasbenchIIpreparationsystem(FAU)orat25
oCusingtheclassical,‘sealedvessel’187
method(UoL).MassratiosoftheresultantpurifiedgasesweremeasuredwithaThermoFisherDelta188
Vplusmassspectrometeroperatingincontinuousflowmode(FAU)oraVGSIRA10dual-inletmass189
spectrometer(UoL).Rawgasdatawerecorrectedfor17OeffectsandcalibratedtotheVPDBscale190
usingacombinationofinternationalreferencematerials(δ13Cvaluesareassignedas+1.95‰to191
NBS19and–46.6‰toLSVECandδ18Ovaluesof–2.20‰toNBS19and–23.2‰toNBS18)and192
laboratoryqualitycontrolmaterialsandreportedasconventionaldelta(d)values.Analytical193
precision(1σ)isestimatedtobebetterthan0.1‰forbothisotoperatiosbasedonreplicate194
analysisofstandards.Somenotabledifferencesinoxygenisotopevalueswerereportedwhere195
specificsampleswereduplicatedbybothlaboratories.Thereasonforthesedifferencesisuncertain.196
Althoughsomediscrepancieswerefoundtobesignificant,theydonotimpactoneitherthe197
palaeoenvironmentalorchemostratigraphicinterpretationofthecarbonisotopedata.198
9
Thetopmost8samplesoftheCarraraFormationwereanalysedattheUniversityof199
Massachusetts,Amherst.Powdered,homogenizedsampleswereanalysedforδ13Ccarbandδ
18Ocarb200
valuesusingaFinniganDeltaXL+isotoperatiomassspectrometerwithanautomatedcarbonate201
prepsystem(KielIII).WereportresultsasthepermilledifferencebetweensampleandtheVPDB202
standardindeltanotationwhereδ18Oorδ
13C=(Rsample/Rstandard−1)!1000,andRistheratioofthe203
minortothemajorisotope.Resultswerecalibratedusingahousestandard(crushed,washedand204
sievedmarble)withVPDBvaluesof+1.28forδ13C‰and–8.48‰forδ
18O.Reproducibilityof205
standardmaterialsis0.1‰forδ18Oand0.05‰forδ
13C.206
Totalcarbon(TC)andtotalorganiccarbon(TOC),followingremovalofcalcitebyacid207
decompositionofbulksedimentsamples,wasmeasuredusingaLECOSC-144DRDualRangecarbon208
andsulphuranalyserattheUniversityofLeeds.Totalinorganiccarbon(TIC)wassubsequently209
calculatedbydifference(TIC=TC-TOC).Anestimateofthecalcitecontentforeachsamplewas210
madebyassumingthatallTICishostedbycalcite(wt%calcite=TICx8.333).211
212
4.Results213
4.1.FaciesAnalysis214
Fourfaciesandninesub-facieswereidentified(Table1):grainstone,packstone,siltymarl215
andmarlandtheyhavebeengroupedintoanonshore-offshoretrendspanningshallowsubtidalto216
outershelfenvironments(Table1).Theshalloweststrataconsistofgrainstonefacieswithcommon217
shellhashthatisoftenabraded.Bedsaretypicallydecimetresthickandcanshowahummockytop218
surfaceandsharp,erosivebases.Inclinedstacksofflat-pebbleintraclastswithherringbone-likecross219
stratificationarepresent,suggestingstormwaveprocesses(Fig.4D).Bioturbatedpackstone,with220
sub-faciesofoncolitic,bioclasticandsiltypackstonevarieties(Fig.5B),areinterpretedtobea221
deeperfaciesbasedonthepresenceofamicriticmudmatrix.Deeperwatersiltymarlincludefissile,222
homogenousandthoroughlybioturbatedvariants.Deepest-water,mostoffshoresuccessionsare223
10
dominatedbyfinegrainedmarl,includingsub-faciesoflaminated,pyriticdolomicriteand224
bioturbatedmarlwithichnofabricindex(II)valuesof2-3(II2andII3)intheschemeofDroserand225
Bottjer(1986).!226
Thefaciesdistributionrevealsconsistenttrendsinthetwoprincipalstudysections.Thebaseof227
theCarraraFormation,seenatEmigrantPass,isdominatedbydeeperwaterfacies(Facies4)ofthe228
EagleMountainShale.Commonly,themarlhasagrey-greencolourproducedbytheabundanceof229
chloriteandclinochloreinthematrix(Figs.4F,5Aand6E).Laminatedintervalsarecommon,230
althoughtheseoccurinterbeddedwithburrowedstratasuggestingtherewerefrequentfluctuations231
ofredoxconditions.232
Theexceptiontothegenerallyquiet,low-energydepositionoftheEagleMountainShaleis233
recordedbyasharp,erosive-basedbedofFacies1developedjustover30mabovethebaseofthe234
section.Thisshelly,oncoliticpackstonecontainsrip-upclastsoftheunderlyingmarlandsolemarks235
onitsbase(Fig.7A).Internally,thinintraclastsandshellsdisplayachevron-stackingpattern(Fig.236
4D).Amajorstormeventseemslikelytohaveproducedthishorizonwiththeshell-stacking237
producedbypowerfulbi-directionalcurrents.ThesucceedingEchoShaleandGoldAcemembers238
recordshallowing.Grainstoneandpackstonedominatethis20-m-thickintervalwhichincludes239
erosive-basedoncoliticpackstonebeds(Fig.7B).AbovethisinthePyramidShaleMembermarl240
faciesdominate,takentobeindicativeofasustaineddeepening.Grainstoneandpackstonefacies241
developedinthelower~15mofthememberareinterpretedtohavebeentransportedduring242
stormevents.ItiswithinthistransgressivephasethattheSeries2-Series3boundaryisrecorded,243
alongwiththeolenellidextinction(Foster,2014).Deep-watersedimentationisabruptlyterminated244
bythedevelopmentofshallow-watergrainstoneatthebaseoftheRedPassLimestoneMember245
(Fig.7).Ooidsandabradedfossilmaterial(Table1,Sub-Facies1.1)suggestanearshoresetting.246
ThePiocheFormationatOakSpringsSummitrecordsamoredistalversionofthesuccession247
seenwithintheCarraraFormationwithrelativelydeep-waterFacies3and4dominating(Fig.2),248
11
thoughthesameoveralldeepening-upwardstrendisseen.Thus,thelowerhalfoftheCombined249
MetalsMemberconsistsofalternatingsiltymarlandpackstone.Abovethis,theremainderofthe250
sectionisdominatedbydeeper-waterfacies(Fig.2).TheuppermostCombinedMetalsMemberand251
themajorityoftheC-ShaleMemberrecordasimilartransgressivedeepeningseenwithinthe252
PyramidShaleMemberoftheCarraraFormation.Theolenellidextinctionleveloccurswithinthis253
transgressivesuccessionbetweenamarlandasiltymarlinthebaseoftheC-ShaleMember(Fig.4E).254
Thisminorfaciesshiftdoesnotrepresentasignificantlydifferentenvironmentandassuch255
extinctionisnotthoughttobeafunctionoffacieschange.Immediatelyabovetheextinction256
horizon,chloriteintheformofbothroundedgrainsandcementbecomescommon(Fig.5A).The257
remainderoftheC-ShaleMemberisathickpackageofmarlthattransitionstosiltymarlatthebase258
oftheSusanDusterLimestone.259
260
4.2.PyriteFramboidAnalysis261
FramboidsizeanalysiswasperformedontheSeries2-Series3boundarystrata(andthus262
theextinctionhorizon)fromthePiocheFormationatOakSpringsSummit,where11sampleswere263
collectedina7mintervalspanning3.5meithersideoftheextinctionhorizon.Allsamplescontained264
abundantscatteredcrystalsofpyriteranginginsizefrom1-10µm,oftenfoundagglomeratedin265
clusteredpatches.Fivesamplesyieldedframboidspreservedasironoxyhydroxidesdueto266
weathering,withonlyminoramountsoforiginalpyritepreservedintheircore.Theframboids267
showedasizedistributionspanningananoxic-dysoxicrange(Fig.8).Themostdysoxicsample268
(smallestmeanframboiddiametersizeandsizerange,loweststandarddeviation)occurredinamarl269
approximately1mbelowtheextinctionhorizon.Dysoxicframboidpopulationsalsooccurredinthe270
1mofstrataoverlyingtheextinctionlevel.However,asamplefrom20cmbelowtheextinction271
leveldidnotyieldanypyriteframboidssuggestingfullyoxygenatedconditions.Thisvariabledegree272
12
ofoxygen-restrictionsuggestedbytheframboidanalysisisalsoseeninthevariabilityofthe273
associatedsedimentaryfabrics,whichvariesfromlaminatedtoslightlyburrowed(II2).!274
SevensampleswerealsoanalysedfromthePiocheFormationatRuinWashwherethe275
olenellidextinctionhorizonhasbeenlocatedwithinasuccessionofmarls(Palmer,1998;Lieberman,276
2003;Fig.S1).Generally,framboidalpyritewasabsentatthislocationwiththeexceptionoftwo277
samplesfrom10and15cmbelowtheextinctionhorizonwheretheyhadsizerangesthatplotinthe278
anoxicfield(Fig.8).AnadditionalfoursamplesfromaroundtheextinctionhorizonatEmigrantPass279
werealsoanalysed.Inthiscase,allsamplesonlyyieldedscatteredpyritecrystalsbutnotframboids,280
suggestingbetteroxygenatedconditionsinthisshallower-watersection.281
!282
4.3.Chemostratigraphy283
Inthebasal20moftheCarraraFormationδ13Cvaluesarehighlyvariableanddonotshowa284
cleartrend(Fig.7),buttheythenbegintostabilisearound–2‰beforeaconsistentpositivetrend285
develops.InthePyramidShaleMemberthebaseofthenegativeexcursionbeginswithat–0.1‰,286
abovethisδ13Ccarbvaluesbeginadeclinetoalowpointat105mof–3.5‰(anegativeshiftof3.4287
‰).Intheoverlying25m,nodatawasobtainedbecausecarbonatevaluesweretoolowfor288
analysis.Abovethisgap,δ13Ccarbvaluesshowapositivetrend,returningtovaluesaround–0.1‰,289
similartothosefromthebaseofthesection.290
Barringoneoutlieratthebaseofthesection,δ13Ccarbvaluesfromthefirst13mofthe291
PiocheFormationremainaround–2.5‰beforethereisasharp,positiveshiftto–1.0‰overthe292
next15m(Fig.7).Fromthisvalueof–1.0‰anegativeshiftoccurs,resultinginpeaknegative293
valuesof–4.8‰.ThenadiratthebaseoftheC-ShaleMembermarksanoverallshiftof–3.8‰,a294
similarsizetothatfoundatEmigrantPass.Atthetopofthesectionvaluesreturntoaround0‰.295
296
13
5.Discussion297
5.1.Carbonisotopesanddiagenesis298
Inordertoevaluatethereliabilityofourisotopedataweassessthepreservationofa299
primarycarbonisotopesignalinoursamples.InboththeCarraraandPiocheformationstheisotope300
analysesarederivedfromsampleswithawiderangeofcarbonatevalues(Table2).ThehighTIC301
samplesarelikelytorecordprimarycarbonisotopesignaturessincetheyarebufferedfromexternal302
changebyalargecarbonate-carbonreservoir(SaltzmanandThomas,2012).LowerTICsamplesare303
moresusceptibletopostdepositionalisotopicalterationoradditionofcarbonatewithanon-304
primarycarbonisotopecomposition(BrandandVeizer,1981;BannerandHanson,1990;Marshall,305
1992).Inbothsections,theexcursiontothelowestδ13Ccarbvaluesoccursatthelevelofthetrilobite306
extinction(mid-PyramidShale,CarraraFormationandbasalC-Shale,PiocheFormation)whereTICis307
<2wt.%.Hereweassessthepreservationofaprimarycarbonisotopecomposition,particularlyin308
sampleswithlowTIC.309
Twodiageneticprocessescanaltertheprimaryisotopiccomposition:recrystallizationof310
carbonateorprecipitationofadditionalauthigeniccarbonatewithadistinctisotopecomposition311
(Marshall,1992).Bothmarineporefluidsandmeteoricwaterscanhavedissolvedinorganiccarbon312
(DIC)enrichedin12Cfromtheoxidationoforganicmatterandthesemechanismshavediffering313
predictionsoftheδ13Ccarbandδ
18Ocarbvaluespreserved(Marshall,1992).BoththeCarraraand314
Piocheformationsdisplaycommonalitiesintheirrelationshipsbetweentheirδ13Ccarbandδ
18Ocarb315
ratiosandtheirTICandTOCconcentrations.Firstly(point1),neitherformationshowsaclear316
relationshipbetweenδ13Ccarbandδ
18Ocarb(Figs.S2andS3).Secondly(point2),sampleswiththemost317
negativeδ13Ccarbandthemostpositiveδ
18OcarbaremostlycharacterisedbylowTIC(definedas<2318
wt.%).BoththeCarraraandPiocheformationsexhibitgenerallylowTOC(point3).IntheCarrara319
FormationTOCconcentrationsrangefrom5.17to0.0wt.%TOCwithameanconcentrationof0.14320
wt.%TOC.InthePiocheFormationconcentrationsrangefrom2.69to0.0wt.%TOC,withameanof321
14
0.12wt.%TOC.Finally(point4),highTOCsamplesarecharacterisedbymorepositiveδ13Ccarb.The322
majordifferencebetweenthesectionsfortheseparametersisamuchclearerpositiverelationship323
betweenTICandδ13CcarbwithinthePiocheFormation.324
Theseobservationsruleoutwholesalerecrystallizationinameteoricfluidsinceneither325
sectiondisplaysapositivecorrelationbetweenδ18Ocarbandδ
13Ccarb(point1,Figs.S2andS3).The326
generallylowTOCconcentrationsandtherelationshipbetweenTOCandδ13Ccarb(point3and4)also327
makeslocalisedprecipitationoforganic-carbonderivedDICdoubtful.Fromtherelationships328
betweenδ18OcarbandTIC(point2)itislikelythataproportionofthelowTICsamples(<2wt.%)329
haveundergonevariableresettingoftheirδ18Ocarbtowardsmorepositivevalues.Thisobservationis330
notconsistentwithprecipitationofadditionalcarbonatefromunmodifiedmeteoricormarineearly331
diageneticporefluids,wheretheexpectationwouldbeachangetowardsmorenegativeδ18Ocarb332
values(Marshall,1992;KnauthandKennedy,2009;Cochranetal.,2010;SaltzmanandThomas,333
2012).Theremainingpossibilitytoexplaintheoxygenisotoperelationshipsisvariableexchange334
with,orprecipitationofcarbonatefrom,ahypotheticalhighδ18Ofluid(GlumacandWalker,1998).335
Sincetheclimateatbothsitesiscurrentlyarid,onepossibilityisthatthefluidinquestionisderived336
fromevaporatedmodernmeteoricwater,butotherpossibilitiesexist(SaltzmanandThomas,2012).337
Therelationshipsbetweenδ13CcarbandTICdiffersomewhatfromthosebetweenδ
18Ocarband338
TIC:fromtheCarraraFormation,therangeofδ13Ccarbinthe<2wt.%TICsamplesoverlapsstrongly339
withtherangefoundinnearpurelimestonesamplessuggestingthattheinfluenceofdiagenetic340
processonδ13Ccarbatthissiteislikelytobeminimal(Fig.S4).Incontrast,samplesfromthePioche341
Formationdisplayamuchclearerdivisionbetweenthesetwogroups(TICgroupsannotatedinFig.342
S5).Thissuggeststhattheinfluenceofpost-depositionalprocessonδ13Ccarbmayhavebeenmore343
pronouncedatthissite.However,theδ18OcarbrangesofbothhighandlowTICsamplesofthePioche344
Formationoverlap(Fig.S6),indicatingthatatleastsomeofthecarbonisotopevalueshave345
undergoneminimalresetting.346
15
Insummary,thereisclearevidenceforavariabledegreeofoxygenisotoperesetting347
towardsmorepositivevalues,whichisparticularlypronouncedinsampleswithlowTIC(<2wt.%).348
Thereisalsosomeevidenceofconcurrentvariableresettingofcarbonisotopestomorenegative349
valuesinlowTICsamples,withthisbeingsomewhatmorepronouncedinthePiocheFormation.350
Nonetheless,thepresenceofthenegativeδ13Ccarbvaluesandtheconsistencyofthemagnitudeof351
theexcursionatleveloftheSeries2-Series3boundary(e.g.,Zhuetal.,2006;Faggetteretal.,352
2016),correlatedindependentlybybiostratigraphybetweenthetwosectionssuggestthatthese353
samplesrecordapredominantlyprimarysignal.Assuch,weconcludethatthenegativeδ13Ccarb354
excursionwithintheCarraraandPiocheformationspreservesaprimaryrecord,givenitsco-355
occurrencewiththeolenellidextinctionhorizon,weinterpretittobeROECE.356
357
5.2.Extinctionandpalaeoenvironmentalchange358
IdentificationoftheROECEinthePiocheandCarraraformations(Fig.7)confirmstheclose359
temporalrelationshipbetweentrilobiteextinctionsandcarbonisotopeexcursions(Zhuetal.,2006;360
Faggetteretal.,2016).Italsoallowsexaminationoftheassociatedfaciesandrelativesea-level361
changesatthistime.InitiallytheSaukI/IIsupersequenceboundarywasplacedaroundtheSeries2-362
Series3boundary(Sloss1963).However,morerecentlythishasbeenplacedlowerinthesuccession363
atthetopoftheZabriskieQuartzite,underlyingtheCarraraFormation(Prave,1991).Thusthe364
CarraraFormationfallsentirelywithinSaukII(Kelleretal.,2003,2012;Morgan,2012).Nonetheless,365
therearealternativeregressionsurfacesintheCarraraFormation.Acandidateforasequence366
boundaryoccursatthebaseoftheRedPassLimestonewherethereisasharptransitionfromdeep-367
watertoshallow-water.Thislevelliesaround45maboveROECEintheCarraraFormation.368
Ratherthanregression,theolenellidextinctionoccurswithinadeepeningsuccession.369
Transgressionandshelfanoxiaoftengohand-in-hand,andoxygenstresshasbeenimplicatedin370
ROECEextinction(Montañezetal.,2000).However,atOakSpringsSummit,pyriteframboidanalysis371
16
suggestsdysoxicbutnoteuxinicconditionsintheextinctioninterval,andtheshallowerstudy372
locationsshownoevidenceforoxygenrestriction.Theevidenceforintensifiedoxygen-restricted373
depositionatthetrilobiteextinctionlevelisthereforeweak.Italsonoteworthythatlow-oxygen374
conditionswerecommoninCambrianoceans(Hurtgenetal.,2009;Prussetal.,2010;Gilletal.,375
2011),andthereisnosuggestionthatanoxiawasintensifiedatthelevelofROECE.376
TheSeries2-Series3boundaryintervalsawtheeruptionoftheKalkarindjifloodbasalt377
province(GlassandPhillips,2006;Jourdanetal.,2014;Marshalletal.,2016).Inyoungerintervalsof378
thePhanerozoic,theformationoflargeigneousprovincesfrequentlycoincideswithmass379
extinctions(Wignall,2015;BondandGrasby,2016)andtheeruptionoflargevolumesofvolcanic380
volatilesprovidesacausalmechanismfordrivingbiologiccrises.Thecontemporaneousnegative381
δ13CsignalofROECEisoftenseenattimesofLIPeruptionsandmayrecordtheinfluxofisotopically-382
lightvolcanicCO2(e.g.,Payneetal.2004).Thus,inmanyregardstheROECEhasthehallmarksof383
laterPhanerozoicLIP-relatedmassextinctionsalthoughevidenceforthecommonlyassociated384
environmentalchangessuchasthespreadofanoxia(Wignall,2015),isnotclearlyestablishedfor385
thisCambrianexample.386
387
6.Conclusions388
InthewesternGreatBasin,USA,theextinctionofthedominantolenellidtrilobitesoccurs389
withinadeepening-upwardshelfsuccession.Amajor–3.5‰negativecarbonisotopeexcursion390
(ROECE)occursatthesamelevel.Thisextinction/isotopeeventoccursaroundtheCambrianSeries2391
-Series3boundaryinterval.Pyriteframboidsizedistributiondataandlaminatedfaciessuggest392
periodicdysoxiaoccurredinthefaciesimmediatelysurroundingtheextinctionhorizon.However,393
theseconditionswereneitherwidespread(shallower-waterboundarysectionsinDeathValleydo394
notrecordoxygenstarvation)norespeciallyunusual(laminatedstrataaresporadicallydeveloped395
throughouttheoffshoreunitsoftheCarraraFormation)suggestingdysoxiadidnotplayamajorrole396
17
intheextinction.TheenvironmentaleffectsofthecontemporaneousKalkarindjifloodbasalt397
provinceofAustraliaprovideabetterpotentialcausalfortheextinctionattheSeries2-Series3398
boundary,althoughdetailedcorrelationwiththesectionsinNorthAmericaisrequired.399
400
Acknowledgments401
WethankEmilySmith,KristinBergmann,andJessicaCrevelingforvaluablediscussionsinthefield.402
WealsothankTessaBrowne,alongwithHelenaTatgenhorstforallowingtheuseoftheirCisotope403
datainthetop25mofEmigrantPass,andStephenBurnsinthestableisotopelaboratoryatthe404
UniversityofMassachusettsatAmherstforprovidingthesecarbonisotopeanalyses.Wethankan405
anonymousreviewerfortheirconstructivefeedback,andJessicaCrevelingforherextensiveand406
thoroughreview.ThisresearchwasfundedbyaNERCpostgraduatestudentshiptoLEF.407
408
Figurecaptions409
Figure1.LocationmapshowingstudysectionsatEmigrantPass,DeathValleyregion,California(35°410
53’29.24”N,116°04’39.08”W)andOakSpringsSummit,BurntSpringRange,LincolnCounty,411
Nevada(37°37’04.32”N114°43’17.20”W).Starindicatesapproximatelocationoffieldareaduring412
theCambrianSeries2(afterMcKerrowetal.,1992).413
Figure2.BiostratigraphiccorrelationofthetrilobitezonesoftheCarraraandPiocheformations414
(PalmerandHalley,1979;SundbergandMcCollum,2000).Faciescolumnisbasedonfieldand415
petrographicobservations,andnumbersrelatetofaciesdetailedinTable1.Ageneralised416
stratigraphiccolumnofPrecambrianandCambrianformationsinDeathValleyisgiven(fromCorsetti417
andHagadorn,2000).418
Figure3.CorrelationoftrilobitebiozoneswithintheCarraraandPiocheformations(Merriamand419
Palmer,1965;PalmerandHalley,1979;SundbergandMcCollum,2000).O.isOlenellus,P.isPoliella.420
18
Figure4.Fieldphotographs.421
A.Trilobitedebris(spinesandcarapacesandhyoliths)inabioclastichashonbeddingplanesof422
ooliticgrainstone,CarraraFormation.B.OncoliticpackstonefaciesatEmigrantPass.C.Bifurcating423
burrowsinwellbioturbatedsiltymarlatEmigrantPass,notebookforscale.D.Ooliticgrainstone424
faciesshowinginclinedchevron-stylepackingofthinintraclastandbioclasts(hyolith,ooidandother425
detritalfragments).E.OlenellidextinctionlevelatthebaseoftheC-ShaleMemberatOakSprings426
Summit.RedlineindicatesextinctionhorizonfromPalmer(1998).F.Fissile,laminatedmarlandsilty427
marlinthelowerEagleMountainShaleatEmigrantPass.G.Thalassinoidesinfine-grained,siltymarl428
oftheCarraraFormation.H.Verticalburrows(atthehammertip)insiltymarlbedsoftheCarrara429
Formation.430
Figure5.Scansofthinsectionsandphotomicrographs.431
A.Photomicrographofarangeofchloriteinthesiltymarlfaciesimmediatelyabovetheextinction432
horizonatOakSpringsSummit.Chloriteoccursaselongategrainsandalsoascement.B:433
PhotomicrographofasiltybioclasticpackstoneintheupperEagleMountainShale.C:Scanofslideof434
oncoliticpackstone(EagleMountainShale)showingoncoidswithbioclasticnucleusofechinoderm435
platesamongstamatrixofshelldetritusandmicrite.D:Scanofslideofbioclasticgrainstone.436
Elongate,trilobitefragmentsdominatethisfaciesalongsidehyolithremainsandechinodermplates.437
Darkbrownmineralgrowthshowsironoxidepreferentiallyreplacingshellmaterial.438
Figure6.Scansofthinsectionsandphotomicrographs.439
A:Scanofsiltymarlshowingquartzgrainsanddetritalchloritegrains(green).B:Photomicrographof440
marlfaciesintheCombinedMetalsMember,PiocheFormation.Trilobitecarapaceexhibitsbrown441
needlelikeironoxidereplacementofthecalciteshell.C:Photomicrographofpeloidalgrainstone442
faciesintheCombinedMetalsMember,PiocheFormation.Wellroundedmicritepelletsalongside443
roundedquartzgrainsamongstafinemicritematrix.D:Photomicrographofoolitic,bioclastic444
grainstonewithironoxidespartiallyreplacingooids.E:Photomicrographofsiltychloriticlimestone445
19
showingroundedchloritegrains(whitedashedlines).F:Photomicrographofchloriticsiltymarl446
faciesshowingsub-angulartoangularquartzsandgrainsalongsidehyolithandtrilobitedebris.447
Figure7.CarbonisotopechemostratigraphyoftheCarraraFormationatEmigrantPassandPioche448
FormationatOakSpringsSummit.A.Insetlogshowscontactbetweensiltymicriteandanerosive-449
basedoncoliticpackstonewithripupclastsoftheunderlyingsiltymicrite.Thishorizongrades450
laterallyintoanooliticgrainstone.B.Insetlogofcontactbetweensiltybioclasticpackstoneandan451
erosive-basedoncoliticpackstone.Botherosionalsurfacesmarkthetransportofshallowwater452
bioclasticmaterialduringstormevents.453
Figure8.SizeversusstandarddeviationforframboidsfromSeries2-Series3boundarystrataof454
CaliforniaandNevadashowingthepresenceofoxygen-restrictedfacies.Thethresholdseparating455
euxinic/anoxicanddysoxic/oxicsizerangesinmodernenvironmentsisfromWilkinetal.(1996).456
Table1:FaciesoftheCarraraandPiocheformations.457
Table2.GeochemicalandframboidmeasurementsfortheCarraraandPiocheformationsat458
EmigrantPass(EP)andOakSpringsSummit(OSS)andframboiddatafromthePiocheFormationat459
RuinWash(RW).460
461
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