julie lefever - department of mineral resources · to the plant kingdom as fossil algae,...

Julie LeFeverIntroductionThestudyoffossilizedtracks,trails,tubesandburrowsassociatedwithorganismsthatburrowintoandlivein,ornearthebottomdeposits of an ocean is called Ichnology. First studies of tracefossilsdatefrom1823to1881whentheywereoriginallyassignedto the plant kingdom as fossil algae, “fucoids” (Osgood, 1975).During those early examinations, the gross morphology of the“fucoids” was thought to resemble plants thus supporting theassignment.In1880,evaluationofpreviousworkraisedquestionsof validity. Many “fucoids” were determined to be traces ofinvertebrate activity and others were sedimentary structures.Discussions continued to 1927,when “fucoids”weredisprovedandthestudyoftracefossilsallbutceased.Littleprogresswasmade in the study of trace fossils from 1927 to 1960. Threereasons are cited for this lack of progress: 1) problems relatedto taxonomy (the classification of organisms), 2) there was nostatementastothevalueoftracefossilstudiestopaleontology,zoology, and sedimentology, and 3) there was limited interestby sedimentologists and paleontologists in paleoecology (theinteraction between ancient organisms and their environment)andtheuseoftracefossilstohelpdeterminepastenvironments.Sincethattimeaclassificationoftracefossilshasbeendevelopedusingsedimentologicalandbehaviormethods.Tracefossilshaveprovenusefulinmanyaspectsofgeologyincludingfaciesanalysis,animalbehavior,andmorphology.Thispaperisintendedtobeanintroductiontotracefossils,notanextensivereportwithallthedetails.

Trace Fossils versus Body FossilsTrace fossils represent behavior or activity by organisms ratherthan the actual body parts of the organism itself (Frey, 1975).Trace fossils (“facies fossils”) are significantly different frombody fossils in that they extend over longer periods of timeand have more restricted geographical distribution. Particularassemblages of trace fossils reflect particular environments ofdeposition thus making them very useful for environmentalreconstruction and extending those environments to strata ofsignificantlydifferentages.Thetracesarenotnecessarilyformedpenecontemporaneously with sedimentation, their presenceindicating the sediment had become more stable allowing formobile feeders. The abundance of different types of tracefossilsappearstobelinkedwithenergyconditionsatthetimeofsedimentation(largerdiversityisassociatedwithquieterwaters).

Body fossils comprise theprimary recordof ancient life. Tracefossilsrecordtheexpressionofthat life. Soft-bodiedorganisms areprimarilyrepresentedbytraces.Tracesinclude:

1) Tracks—impressionsleftbyamovingindividual’sappendage(walkingorrunning);

2) Trails — continuous groove produced during locomotion(slidingorslithering);

3) Burrows —more or less permanent structures excavatedwithinthesediment;and,

4) Bioerosion structures — organisms mechanically orbiochemically boring into a rigid substrate (Angulo andBuatois,2009).

Thereisagreatdealofdifficultyinrelatingthetracesbacktothetracemaker.Thetracemakerisrarely,ifever,foundinornearthetrace.Understandingisgainedbyinterpretingthebehaviorthatledtotheformationofthetrace,wasitresting,crawling,grazing,feeding,dwelling,orescaping.Thesebehavioralresponsesmayalsoberelatedtoenvironmentalchangesinsalinity,oxygenation,temperatureandwaterturbidity(Gingrasetal.2009).Featuresthatindicatetheenvironmentalchangeslistedaboveinclude:

1) Unburrowed–indicatoroffreshwater2) Distributionofthetypesoftraces3) Size of the trace – for example, the size of the trace in a

marineenvironmentwouldbeanindicatorofoxygencontentand food supplies; small traces might indicate a stressenvironment;andhighdiversitywouldresultfromabundantfood,oxygen,andnormalsalinities.

This leads to a simple, practical classification based on themorphologyoftheirtrace(fig.1).Thereisnotadirectcorrelationbetweenichnofaciesanddepositionalenvironments;insteadtheyrepresent a set of environmental factors (Angulo and Buatois,2009).Thesecanbesplitinto4maincategories(fig.2):

1) Softgroundmarine-Psilonichus,Skolithos,Cruziana,Zoophycos,Nereities

2) Substrate-controlled–Glossifungites,Trypanites,Teredollites3) Continentalinvertebrate4) Vertebrate

Subsurface studies in theWilliston Basin focus on the top twocategories, thesoftgroundmarineandthesubstratecontrolled.These are the traces that are encounteredwhenworkingwithcoresfromthebasin(fig.3).

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Figure 2. Ichnofaunamodel of the soft-ground and substratecontrolled trace fossilassemblages and how they relate to marine environment. Each iscomprised of a seriesof traces that reflectenvironmental factorswhich, in associationwith sedimentary and hard body fossils, as-sist in the determina-tion of the sedimen-tary environment.Diagram from AnguloandBuatois(2011).

Figure 1.Threedimensionaldiagramdisplayingmorphologicalclassifi-cationofburrowsandtheirassociatednames(fromGingrasetal.2009)

Ichnofabric AnalysisIchnofaciesanalysisstartsbydeterminingthediscreteandthepoorlydefinedtracesincross-sectionview.Emphasisisplacedoncross-cuttingrelationships.Thisviewallowsfortiering,wherethereisaverticalpartitioningofthehabitatduetochangesinthe environment including physical, chemical, and biological

parameters(AnguloandBuatois,2009).Therearethreefactorsthat are the most important in controlling tiering. The firstchangestheconsistencyofthesedimentbycompactionduetovertical accretion and progressive burial and dewatering. Thesecondreliesontheabundanceoforganicmatter.Thegreatestamount of organic matter is at the sediment-water interface.Thenumberoforganismspresentcanbedirectlyrelatedtotheabundanceoffood.Thenumberoffeedersdecreasesawayfromthefoodsource.Thethirdfactorshowsasimilartrendregardingthe oxygen content of the sediment. Sorting all of the tracefossils intoatieringstructurecanbedifficultespecially inareasofintensebioturbation.

Toassistintheanalysisoftracefossils,thedegreeofbioturbationis assigned a number (bioturbation index) that represents thepercentofthesedimentthatisdisturbed(table1,fig.4).Thiscanbeplottedvisuallyalongsidethetieringdiagramandotherpartsof the analysis to aid in the determination of the sedimentaryenvironments(fig.5).

Acollectionoftheinformationcanthenbeusedtodeterminethesedimentaryenvironment.Gingrasetal.(2009)developedaflowchart(fig.6)thataidsintheinterpretationofcommonbioturbatetextures.

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C.

Figure 4. ExamplesofvariationsinBioturbationIndexwithintheMiddleMemberoftheBakkenFormationinNorthDakota.AandCarefromtheWhitingOilandGasCorp.–Braaflat#11-11H(NWNWSec.11,T153N,R91W;API#33-061-00641-0000;depths–9,881ftand9,901ft).ThecoresamplethatisshowninBisfromtheEOGResources,Inc.–Fertile#1-12H(SESESec.12,T151N,R90W;API#33-061-00557-0000;depth–9,382ft).CoresampleAhaslimitedburrowingwithlaminationsthatarevirtuallyundisturbed.CoresampleBismoderatelybioturbatedwhilestillretainingsomeoftheoriginallaminations.CoresampleCisextensivelybioturbated.Allofthesedimenthasbeendisplacedbytheburrows.Scalebarinwhiteisoneinch.

A. B.

B.I.-0-1

B.I.3

B.I.5

Figure 3. Example of Thalassinoides burrowsfromtheOrdovicianRedRiverFormationfromtheTerraResources,Inc.–BNRR#1-17(NWSWSec. 17, T145N., R103W; API: 33-053-00955-0000;depth12,963.1ft)wellwithandiagram-matic3Drepresentationoftheboxworkoftheburrows.Preferentialdolomitizationofthebur-rowovertherockmatrixcausestheburrowstostandout.Italsocomprisesthereservoirfortheformation. DiagramfromGerardandBromley(2008).

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Figure 5. Example of a core log fromBakken well 15-31-03-11W2 showing theresultofapplyingtheichnofabricapproach.Illustration on the left represents thedescribed coredisplaying theburrowsandsedimentarystructures.Thisisfollowedbytheage,nomenclature,depth,bioturbationindex,andultimatelythesedimentaryfaciesand environments. Diagram from AnguloandBuatois,2012.

Table 1. Criteria for the determination oftheBioturbationIndex.SeeFigure4forthevisualdisplayofthistypeofdata.

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Figure 6. FlowchartdevelopedbyGingrasetal.(2009)demonstratingtheprocessofinterpretingichnologicalfabrics.

ConclusionsTrace fossils are an additional tool that assists the geologist indetermining the environment of deposition. Multiple factorscontribute to theabsenceorabundanceof trace fossils in therocks. Attention to detail, in addition with hard body fossilsand sedimentary structures, add to our knowledge of pastenvironments.

References CitedAngulo, S., and Buatois, L., 2009, Environmental setting of the Upper

Devonian-Lower Mississippian Bakken Formation of SubsurfaceSaskatchewan: IntegratingSedimentologicand Ichnologicdata,ACoreWorkshop:in,17thWillistonBasinPetroleumConferenceandProspectExpoCoreWorkshop:Regina,SK,p.66-163.

Angulo, S., and Buatois, L., 2012, Ichnology of a Late Devonian-EarlyCarboniferous low-energy seaway: The Bakken Formation ofsubsurface Saskatchewan, Canada: Assessing paleoenvironmentalcontrols and biotic responses: Paleogeography, Paleoclimatology,Paleoecology,v.315-316,p.46-60.

Buatois,L.A.,andMángano,M.G.,2011,Ichnology–Organism-substrateinteractionsinspaceandtime:CambridgeUniversityPress,358p.

Frey,R.W.,1975,Realmof Ichnology, in, Frey,R.W.,ed., TheStudyofTraceFossils:ASynthesisofPrinciples,Problems,andProceduresinIchnology:NewYork,Springer-Verlag,p.13-38.

Gerard,J.R.F.andBromley,R.G.,2008,Ichnofabricsinclasticsediments:Applicationstosedimentologicalcorestudies:Madrid,J.R.F.GerardPublishing,97p.

Gingras,M.K.,Bann,K.L.,MacEachern,J.A.,Waldron,J.,andPemberton,S. George, 2009, A Conceptual Framework for the Application ofTrace Fossils: in,MacEachern, J.A., Bann, K.L., Gingras,M.K., andPemberton,S.George,eds,AppliedIchnology:SEPMShortCourseNo.52.,p.1-26.

Osgood,R.G.,Jr.,1975,TheHistoryof InvertebrateIchnology, in,Frey,R.W., ed., The Study of Trace Fossils: A Synthesis of Principles,Problems,andProceduresinIchnology:NewYork,Springer-Verlag,p.3-10.

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