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Recognizing and Utilizing Vertebrate Tracks in Cross Section: Cenozoic Hoofprints from

NebraskaAuthor(s): David B. LoopeSource: PALAIOS, Vol. 1, No. 2 (Apr., 1986), pp. 141-151Published by: SEPM Society for Sedimentary GeologyStable URL: http://www.jstor.org/stable/3514507

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RESEARCHEPORTS 141

Recognizing a n d Uti l iz ing Vertebrate T r a c k s inC r o s s S e c t i o n :Cenozo ic Hoofprints f r o m Nebraska

DAVIDB. LOOPEDepartmentf Geology,University fNebraska,Lincoln,NE 68588-0340

PALAIOS, 986, V. 1, p. 141-151Because vertebrateracksexposedn verticaloutcropsarefre-quently verlooked rmisinterpreteds physicallynduceddefor-mationstructures, mportant aleoecologic nd sedimentologicinformationemainsuntapped.Laminationswithineolian dunesands of theNebraskaSand Hills (Holocene)and ephemeral-streamdeposits f the lowerArikareeGroup lateOligocene) recommonlybrokenor sharplydownwarpedoform isolatedorpaired, concave-up tructures hat varyfrom 4 to 22 cm indiameter.A centralridge divides the lowerportionof somestructuresnto two distinct obes.Althoughbedding-planexpo-suresare rare in bothdeposits,extensive earch revealed omestructuresn linearalignment,confirming biogenic rigin orthedeformation.The Holocene racks,probablymadebybison,wererepeatedlyroduced uring hemigration f the argeeolianbedforms, uggestingthat food and water were available ininterduneareas. Smectitegrain coatingsmade surfacesandscohesive, hereby tronglynfluencing rackmorphologynd en-hancingpreservation otential.Oligocenerackswereproducedin very ine sand byseveralspeciesof hoofedvertebrates.Closevertical pacingof track-bearingedsreveals hatmostsedimentaccumulatedn relativelyhinpackages.Thegeneralabsenceofheavily rampled orizons as wouldbeexpectedlongdiastems)may be the resultof rapidconsolidation f sands by evaporitecementation.Tracksmayhavebeenproduced uringbrief nter-vals of time immediatelyollowing depositionand precedingcementation.

INTRODUCTIONVertebrate rackwaysare a well-known ourceof paleonto-logicandsedimentologicnformationSarjeant,1975).Perhapsbecausevertebrate racksare so easy to recognizeonbedding-planeexposures,untilrecently ittleattentionhas beengiventotheirappearancenverticalsection.Photographs nddrawingspublished y Van der LingenandAndrews 1969), McKee andBigarella 1972), Lewis andTitheridge 1978), Laury(1980),and Hunter et al. (1984) have, however, documentedthe

Copyright 1986,TheSocietyof EconomicPaleontologists nd Mineralogists

deformationhat takes placewhen largeanimalsmove acrosssoft, laminatedediment.On the basisof theirobservationsofQuaternaryeposits nEastAfrica,LaporteandBehrensmeyer(1980) have recentlyarguedthat large vertebrateshave thepotential o rework terrestrialsedimentsto the same extentthat benthic nvertebrates eworkmarinestrata.Preservationof tracksrequirescompactibleubstrates hatareaccessibletovertebrates; rates of tramplingand burial control whethersedimentsrecord individualracks or are totallybioturbated(LaporteandBehrensmeyer,1980, fig. 4a).The Holocenetracks describedhere are fromexposuresofduneand nterdunedepositswithin he NebraskaSandHillsandwere probablyproducedby bison. Oligocenehoofprintsarewell exposed in outcropsof fluvial heet-flooddepositswithinthe GeringFormationArikareeGroup)at Scotts BluffNationalMonumentnwesternmostNebraska; hese trackswere madeby several differentspecies of cloven-hoofedmammals.Themorphologyof both Holoceneand Oligocenehoofprints ndi-cates that they were producedin cohesive sand. Much ofsurfacesandin the Sand Hills s cohesive even whendry, dueto claycoatingson grains;suchcoatingsgreatlyenhanced hepreservationpotentialof the buried racks.Individualracksorpairsof tracksin the Holoceneand Oligocenesedimentsaretypicallywidely spacedlaterally,but closely spacedvertically.The verticalspacingof track-bearingaminae llowsdivisionofstrata nto discretedepositional ackages.Closeverticalspac-ing suggests that sand-drivingwindsdepositedrelatively hinsedimentpackages n the NebraskaSand Hillsandthattrackswere producedby resident,rather hanmigratory opulations.Much of the GeringFormations composedof similarly hinpackages hataccumulatedwithinanephemeral treamsystemdominated y sheet-floodevents. The lackof heavily rampledzones (whichmightbe expected to markdiastems)suggeststhat the Holocenesedimentaccumulatedteadily; n contrast,the samepattern n the Oligocenerockscouldbe the resultofrapidcementationof newlydepositedsandby evaporites.Theabundanceof vertebratetracks in these strata shows that,under certainconditions,eolian dune fields and ephemeral-streamfloodplainsanbe very favorable ites forpreservationof vertebratetracks.0883-1351/86/0001-0141/$03.00

141ESEARCH EPORTS

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142 LOOPE

430

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102?FIGURE1-Map showing outline of the Nebraska Sand Hills andlocation of Burwell (B), Thedford (T), Valentine (V), and Scotts,BluffNational Monument (SB).

The mainpurposesof this paperare: 1) to documentsedi-mentarystructuresproducedby the hooves of largemammalsin nonmarine ands and sandstones of central and westernNebraska;2) to providecriteriaby which hese featurescan bedifferentiatedromnonbiogenic eformationtructures;and3)to furtherdemonstrate the utility of vertebrate tracks forsedimentologicalndpaleoecologicalnterpretations.HOLOCENEOF THE NEBRASKASAND HILLS

LocationandGeologicSettingOccupyingan area of 57,000 squarekilometers, the Ne-braska Sand Hills (Fig. 1) are the largest dune field in theWestern Hemisphere(Smith, 1965). This dune field, nowstabilizedby prairievegetation, is composed of simple andcompound, ransverse to obliquebedforms hatreachheights

upto 100m (AhlbrandtndFryberger,1980).Carbon-14 atesreported by Ahlbrandt t al. (1983) from sedimentsdirectlybeneath the dunes suggest that the dune field is primarilyHolocene nage. The sedimentary tructureswithin he dunesare visible na largenumberof exposures throughouthe SandHills (AhlbrandtndFryberger,1980). Most of the observa-tions for this paperwere made near the easternmarginof theSand Hills at excavationsfor the CalamusRiver dam nearBurwell, Nebraska, and at blowouts and stream cuts nearThedford n the central SandHills(Fig. 1). Structures nter-preted here as vertebratetracks are present throughout heSand Hills;many are recognizable n publishedphotographsfrom the northern and central Sand Hills (AhlbrandtandFryberger, 1980, figs. 9, 10, 11, and 13). At all verticallyextensive exposures, it is clear that the tracks are not re-strictedto the upperportionsof the dunes,but are distributedthroughouthe thicknessof the eoliandeposit (Fig. 2).Alldeformationtructures nterpretedas hoofprints re de-veloped nthin, nverselygradedaminae ipping etween 0 and24 degrees. These laminaewere clearlydepositedbymigratingwindripplesand, in the terminology f Hunter(1977), can beclassifiedas subcritically limbing ranslatent trata. Laminaewithvery low dipsaccumulatedn interduneareas or as sandsheets (Frybergeret al., 1979); more steeply dipping tratawere depositedon leewardslopes of bedforms,where side

~L- 'd r

?rrS;r*e .:~ :" I----- 7- 4~,-.----~;

FIGURE -Concave-up deformation structures in large-scale cross-stratified Holocene dune deposits at Calamus River dam site nearBurwell, Nebraska. Note that deformation occurs throughout verticalextent of exposure.

windsswept ripplesacross surfaces with dipswell below theangleof repose.Descriptionof Tracks

The featuresinterpretedhere as vertebrate tracks are con-cave-updeformationtructuresthat are circular o oval inplanandrangefrom7 to 16 cm indiameter Figs. 3, 4, 5). Near thetops of structures,laminaeare abruptly runcatedor sharplyfolded(Figs. 3, 4a). As in the structuresreportedby VanderLingenandAndrews 1969),concavitydies outgradually own-ward.Althoughdeformation xtends verticallyas much as 25cm, individualaminae redisplacednomore than 15cm. Withinsome individualtructures, a centralridge dividesthe lowerportionntotwo distinct obes, whichare visible nboth cross-sectionalandplanviews (Fig. 4). The interiorsof manystruc-

FIGURE3-Tracks in horizontal wind-ripple laminae. Interdune orsand-sheet deposit near Burwell. Pen is 13 cm in length.

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CENOZOICOOFPRINTS 143

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2 4 6 8 10 12 14 16TRACK DIAMETER CENTIMETERS)

,,Hs*'""~nN! FIGURESize-frequency data for Holocenetracks measuredfromAFIGURE......~~~~verti cal exposures,NebraskaSand Hills.Some variation n diameterAtI"Ar...-:e?o: ~ .. .. is result of oval plan of structures(Fig. 4b).l' ~ '-i:;:':"!",.- -~,! b,,."/.the Sand Hills, fortuitousexposures and small excavationsprovidedglimpsesof short trackways(Fig. 6) that alongwiththe blgobedmorphologyof some structures, confirmedthe.." '. ,: biogenichypothesis. After the structuresare recognizedastracks,the distributionf individualeformation tructuresandotheraspectsof theirmorphology rovide mportant edimen-tologicalandpaleoecologicalnsights.

ContrastwithNonbiogenicStructuresThe size-frequency istributionf thedeformation tructures(Fig. 5) stronglysupports he biogenic nterpretation: one ofappearing32the structuresare too largeor too smallto be tracks. Whenclosely spaced,however,the SandHillsstructuressuperficiallyresemble convolute bedding, a structure that Allen (1982)definesas a "laterayxterallyxtensive series of more or less regularfoldsdeveloped throughoutor confined o the upper partof asingle sedimentationunit." Convolutebedding, however, issFIGURE(d-A)17 Trackith verticalalls dand ilobedst B ENoaowerortion.o rootedimntatonuit.etres iFIGo~~~~uRE-AuTacthvetiypicallyee le ir th atdue to rapidedimentation,r c dSurface oncavity as infilledby coarse-grainedagdeposit.Note typicalydeveloped in stratathat, due to rapidsedientationraised "rim" t upper margins.Subhorizontalines crossing struc- were originally very loosely packed (Allen, 1982, p. 343).turesarepost-depositionalaccumulationsof silt andclay (soil lame- Accordingly, this structure has been used as evidence for rapidIlae or "dissipation tructures" f Ahlbrandtnd Fryberger,980). B) deposition (Collinson and Thompson, 1982, p. 145). Studies ofPlan view of bilobedtracknear the middleof a three-meter-thickcrossbed set. ?'*': '. ' ,'.i: R,_tures are composedof material hat is texturallydistinct rom t . .the underlyingand surroundingdeformed sediment. Com-monly, this central portioncontainsvery coarse sand and- ' -granules Fig. 4a); inrarecases, a thin(about5 mm)layerof .:! -* ... Ssilt is preservedon the floorof the concave-up tructure.:Alongindividual tratigraphicevels, concave-up tructures _, -are laterallysolatedor inpairs.The structuresare commonlyclosely spacedvertically Fig. 3) andmaydominate he aspect -"of outcrops exposing several meters of strata (Fig. 2); laterally -' -' 'adjacentbeds maylacksuch deformation. F

InterpretationIn contrastto the easily recognizedvertebratetrackwaysappearingon beddingplanes (Sarjeant,1975), most tracksappearingncross sectionhaveprobably eeneither gnoredor FIGURE-Part of shortrackwaynlarge-scaleross-stratifiedand,misinterpreted (Lewis and Titheridge, 1978). Tracks in vertical Calamusdam site, Burwell,NE.Note lack of root traces in laminaeoutcropscan closely resembleother types of deformation. n below racks.Machete andle s 15 cmlong.

I

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144 LOOPE

moder wind-ripple epositsindicate hat the stratacontainingthe SandHillsdeformationtructureshad ow initialporositiesand were deposited relatively slowly. Because of strongdifferences n grainpackingandporosity,the differenteolianstratificationypes vary in their susceptibilityo deformation(Bagnold,1941; Hunter,1981). Avalancheor grainflowtrataare ooselypacked; helarge-scaledeformationommonlyoundin ancient eolian sandstones is commonlyconcentratedindeposits of this type, because their high porosity allowsliquefaction. n contrast, stratadominatedby climbing-rippledeposits-apparently due to their low initialporosities-arerarelynvolvednthis kindof deformationDoeandDott, 1980).Inlightof the evidence frommodem andancienteoliansands,the abundance f relatively arge-scaledeformationtructuresin the wind-ripple eposits of the SandHillsinitially eemedincongruous.t was thisparadoxhat ed to the hypothesis hatthe deformationtructureswithin he typically tablestrataarebiogenic.Ahlbrandt ndFryberger 1980)explained omedeformationstructureswithin eolianstrata of the Sand Hills (whichareidenticalo the ones interpretedhere as tracks)as the result ofcompressionat the base of slipfacedeposits.Several workershavereportedobservationsof this type of deformation.Fromthe Coorongregionof southernAustralia,Brown(1969) de-scribedarcuate oldsupto 200 m longand 0.5 m in amplitudethatdevelopedwithin agoonalmuds that were overriddenbydunes 15-30 m high. McKee et al. (1971) experimentallyproduced mall-scalewarps,folds,andoverthrustsnavalanch-ing sand, but noted that "no contorted structurescausedbytensionalor compressionaltresses normally ccur n saltateddeposits.... " There are, furthermore, undamental iffer-ences informbetween the productsof lateralcompression ndthe SandHillsdeformationtructures.Thecircular o ovalplan,lack of directional symmetry,andsteep marginsof the SandHills structuresgive them a "punched-in"ppearance, learlyindicating hat they were formedby vertically,rather thanlaterallydirected stress.

TheSubstrateTrackmorphologyndicates hat trackswere made in cohe-sive sand. Coarse-grainededimentsoverlying runcated am-inae (Fig. 4a) are lag deposits that filled verticallywalleddepressions.Siltspreserved nsimilar ositionsrepresentdustthat was trapped ndprotectedby concavities.Tracksmade nmoist, well-sorted sand(wetted sand of McKee et al., 1971)havesteep walls,whichcanbe maintained s longas the sandremainsmoist(LewisandTitheridge,1978, fig. ld). WeretheSand Hills tracks formed in moist sand and buried beforedrying?Observations f moder cattle tracks n the SandHillssuggest an alternativehypothesis. Unlikebeach sand, dunesand fromthe Sand Hills containsas much as 4%silt andclay(AhlbrandtndFryberger,1980, p. 21). The clay fraction ssmectite, occurringas thin, detritalcoatingson sand grains(Fig. 7; Ahlbrandt ndFryberger,1982, fig. 21d). Suchgraincoatingsare depositedby water movingthroughthe vadosezone;theycan surviveconsiderableolian ransportWalker tal., 1978;Walker,1979).In the Sand HillsduringAugust, 1985, vertical-sided attletrackswere abundantn wind-rippled,ohesive surfacesands

FIGURE-SEM imageof sand grainswithdetritalclay coatings andbridges. Sample is a "crumb" f recently deposited, but cohesivesurfacesand collectedfrom he side of a vertical-sidedcow trackinthe centralSand Hills.

with a moisturecontent less than1%."Crumbs" f this cohe-sive sand collected in the field were found to retain theirstructure even after 24 hours of oven dryingat 40?C.Usingsamplesof loose sand fromthe SandHills anddistilledwater,cohesivesand"crumbs"dentical o those collected n the fieldcanbe producedn the laboratorywith a singlewetting/dryingcycle (Fig. 8).Inthe field,cohesivesurfacesands arerelativelyresistanttowinderosion. Tracks made withinsuch sands have a muchhigherpreservationpotential hantracksproduced n mobile,cohesionlesssandsthathave notbeen moistened(Figs. 9, 10).This observationmay explainthe prevalenceof tracks withvertical ides inHolocenestrata of the SandHillsandsuggeststhatclay-coateddune sand s anespeciallysuitablemedium orthe preservation f tracks. Intheirstudyof the PermianLyonsSandstone,Walkerand Harms(1972) hypothesizedthat thinlayers of clay deposited duringcalm periods between sand-drivingwindsmayhaveallowed hepreservation f small racksandraindropmprintsmade in dry sand. Walker(1979) latershowed the importance f graincoatingscomposedof detritalclays to the reddeningof eolian dune sands. With further

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CENOZOICOOFPRINTS

FIGURE-Crumbs of cohesive sand (left) produced nthe laboratorywhenloose surficialsand from he Sand Hillswas placed ina papercup, saturated with distilled water, and dried in an oven at 40?C.Cohesionless sample on rightwas ultrasonically leaned and wetsieved to removeclays prioro identical reatment.Originalample isa moderatelysorted, nearly symmetrical ine sand (Mz = 2.21;ua=0.662; Skl=.088) containing1.3% clay, 10 YR7/3 (MunsellColorChart).petrographicwork and aboratory xperiments, t maybe pos-sible to assess the role (ifany)of detritalclay coatings n thepreservationof the delicate surfacetraces found n Paleozoiceoliansandstones.

Track-MakersndPaleoenvironmentsThe size andmorphology f the tracks,the probableage ofthe SandHills, the knownmammalianossil record, andpa-leoecologicalarguments stronglysuggest that bison are re-sponsible or the tracks. The tracksof modembison(Fig. 10)arenearly denticaln size andformto those preservedwithinthe Sand Hills.Untilrecently, studentsof the SandHillshadplacedmajorduneformation n either the earlyor late Wisconsin Pleisto-cene). Fluvialdepositsfrom the east-centralSandHills,whichare now known o underlie s muchas 40 mof dunesand, have,however, yielded10 radiocarbon ates ranging rom 8410 to3000 yrs. B.P., indicatinghat the dune field is primarily fHoloceneage (Ahlbrandtt al., 1983). This view has beenchallengedby Wright t al. (1985), on the basisof radiocarbondates and pollen from five interdune lake deposits in thenorthernand western SandHills.At the Calamusdamsite, alltracks ie above a peat depositthat has been radiocarbon-datedat 7260 ? 90 yrs. B.P. (J. Swinehart,unpublishedata;Beta-11621);most tracks are above an organic-richnterdunede-posit dated at 3450 + 110 yrs. B.P. (ibid,Beta-11622). Adiverse assemblage of large herbivores, includingcamels,mammoths,horses, and sloths roamed he GreatPlainsduringthe Pleistocene;of these, onlycamelsproduce racks that areat all similar n size and shape to those described here. Theyoungest radiocarbon ate for a North American ossil sitecontaining amel bones is 8240 + 960 yrs. B.P. (MeadandMeltzer, 1984). All track observationsare consistent with a

FIGURE-Bison tracks in clay-coatedeolian sand containinglessthan 1%water (byweight), FortNiobraraNationalWildlifeRefuge,nearValentine.Knife s 15 cm long.Holoceneage forthe deposits;the largestcloven-hoofedmam-malsof the Holocene-bison-are capableof producing llthetracksso farobserved.Paleoecologicalonsiderations lsosuggest thatthese trackswere made by bison. Archaeologicas well as paleontologicevidence romseveralGreatPlainsandMiddleRockyMountainsites links bisonto semiaridgrasslandenvironmentsn whichsandy sediments have been subjectedto episodes of eoliantransport.Not onlyhave bisonbones been recovered fromtheSand Hills (Ahlbrandt nd Fryberger, 1980) and from dunesands at many other localities, but at the 10,000-year-oldCaspersite in east-centralWyoming,paleo-Indians sed para-

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bolic eoliandunes as natural rapsforbisonprocurement Fri-son, 1974).The abundance f tracksin these sediments seems difficultto reconcile with the lack of root traces below track-bearinglaminae. Were the tracks produced duringtransient move-mentsof nonresidentherds?The wide spacingof tracksalongbeddingplanesand the close verticalspacingof track-bearinglaminae argue against infrequent mass-migrationevents.McKee and Bigarella 1972) observed that small roots pene-tratingdune sand ollow amination,hereby eaving ew traces.Trackabundance nd distributionuggest thatsemipermanentwater-perhaps located in interduneareas-and vegetationwere available o residentindividualsr herdswhile the duneswere activelymigrating.

OLIGOCENEOF SCOTTSBLUFF NATIONALMONUMENTStratigraphicndSedimentologic etting

Nonmarine ocks of mid-Tertiaryge are well exposed andeasily accessible at Scotts BluffNationalMonument n west-ernmost Nebraska (Fig. 1). These rocks contain abundantvolcaniclasticmaterial,which, together with epiclasticdebrisshed eastward romthe RockyMountains,blanketed he sur-faceof the GreatPlains Stanley,1976;Swinehart t al., 1985).At Scotts Bluff,siltstones of the Brule FormationWhiteRiverGroup) are unconformably verlainby 27 m of very-fine-grained, horizontallytratified andstonesof the GeringFor-mation ArikareeGroup) Fig. 11). Structuresnterpretedhereas vertebratetracks are restrictedto the lower two-thirdsofthe GeringFormation.The Gering s overlainby about65 m oflarge-scalecrossbedded o massive sandstones of similar ex-ture(MonroeCreek-Harrisonnit,Fig. 11).Avolcanic sh bednear the base of the Geringat Scotts Bluff has yielded aradiometric ate of 25.6 m.y. (Everndenet al., 1964), placingthese basal Arikaree rocks within the Oligocene Series(Harlandt al., 1982).Figure12 shows the locationof featuresdescribedandillustrated n this paper.At Scotts Bluff,the Gering s horizontally edded;the onlychannelsobserved during his study are a few tens of centi-meters deep. Small-scale, steeply dippingcross-lamination(Fig. 13), andplanaraminationxhibiting artingineation,arewidespread hroughouthis stratigraphicntervalandare clearevidenceof fluvialdeposition StanleyandFagerstrom,1974).Earlydiagenetic eatures nfluvial edimentscommonlypro-videimportantlues to depositional rocesses andpaleoclimate(Collinson,1978). Rosettes of calcite-cementedsand com-posed of discoidsup to 8 cm in diameterare present at fourseparate stratigraphicntervals withinthe Gering(Figs. 11,14). The morphology f the discoidsandrosettes is identicalothat of modem gypsum sand crystals described by Cody(1979).Afterburial, s the gypsumwasdissolvedbyless salinegroundwater, eplacementof gypsumby calcite was probablyfacilitatedby the commonness of the calcium on. The thickvolcanic sh bednear the middleof the Gering Fig. 11)and he

C-rv Silicic ashFIGURE1-Stratigraphic section and interpretationf depositionalprocesses; Scotts Bluff National Monument(fromSwinehartandLoope,in press). g=evidence of gypsum crystallization.

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FIGURE2-Approximate map patternof contact between BruleFor-mation(WhiteRiverGroup) nd GeringFormationArikareeGroup);from Scottsbluff South Quadrangle,4400-foot contour. Numbersshow locations of text figures.

uppermostone meter of the Brule Formationcontainabun-dantdiscoids,0.4 to 8 cm in diameter,that are composedofcalcitespar.Within his finer-grainedmaterial,gypsumcrystalgrowth pushedthe matrixaside to formrelativelypure crys-tals; after dissolutionof gypsum,voids were filledby calcite.Gypsumcrystalsof similar ize andmorphologyo those re-cordedin the strata of the study area are todayconfined oinlandand coastalsabkhas,where the groundwaterableap-proachesthe land surface and undergoesevaporation Wat-son, 1983). Accordingto Watson, such materials are re-

FIGURE3-Subaqueous climbing-ripple eposits and concave-updeformation tructures,near middle of GeringFormation.Head ofhammer s 18.5 cm long.

FIGURE4-Calcite-cemented sand, pseudomorphous ftergypsum"desertroses,"middlepartof GeringFormation.Knifehandle is 9 cmlong.stricted to environmentswhere annual ainfalls less than 200mm/yrand where there is a monthlyexcess of evaporationover precipitationhroughouthe year. Evidence of growthofevaporiteswithin he relativelypermeable luvial ands of thestudy area stronglysuggests that the strata were depositedby ephemeral treams.Several lines of evidence indicate that a portion of theArikareeGroup n eastern Wyomingand western Nebraskaaccumulatedwithinan eolian dunefield (Bart, 1977; Stanley,1980; Hunt,1985;SwinehartandLoope, inpress). Within heMonroe Creek-Harrisonnterval at Scotts Bluff, startingatabout 10 m above the highest occurrence of tracks, inversegradingandwind-rippleoresets (Hunter,1977) occurin hor-izontallybeddedsandsand withinwedge planar rossbed setsup to 1.7 m thick. Where the crossbed sets are overlainbyhorizontalwind-ripple eposits, the interveningbounding ur-faces commonlyhave an irregular,"corrugated" ppearancewithlocalreliefup to 10 cm (Fig. 15). Analogous urfacesarecommon n moder interduneareas where wind erosion hasetchedmoist, wet, or evaporite-cemented, rossbeddeddunesands into strong relief (McKee, 1966, pl. VII, c and d;Fryberger et al. 1983, p. 298). Trenches dug in moderinterdunesrevealcrossbeds withwavyor "corrugated"pperbounding urfaces,overlainby flat-bedded nterdunedeposits(Frybergeret al., 1983, fig. 23a; SimpsonandLoope, 1985).Stanley(1980)has noted thatphysicalandbiogenicstructuresof the mid-Tertiaryolianstrata of the GreatPlainsbearmanysimilaritieso those within he Holocenesands inthe NebraskaSandHills. Traces of invertebratesandplantroots are espe-ciallycloselyanalogous, uggestingto Stanley hathabitatsandclimatic onditionswere very similar.The absence, however,of vertebrate tracks in the eolian strata of the ArikareeGroup-both at Scotts Bluffand at the Bear Creek localitydescribedby Bart (1977)-indicates that, unlike he Holocene

147

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FIGURE5-Eolian interduneepositsoverlyingrregularontactwithcross-stratifiedunesands, MonroeCreek-Harrisonnterval,summit o museum rail. Differential inderosionof lightly e-mented rdamp ross-strataookplace n nterdunereaadjacentostoss side of dune.No rackswereobservednthese eolian trata.

SandHills,the mid-Tertiary unefieldseither did not harborapopulation f largevertebrates,or lacked conditons avorablefor the preservationof their tracks.DescriptionandInterpretationf Tracks

Concave-updeformation tructuresthat closely resemblethe Sand Hills (Holocene)bison tracks are commonin theGeringFormationFigs.13, 16, 19).DeformationsntheGeringvaryinapparentdiameter rom4 to 22 cm (Fig. 17). Becausethese rocksarepoorly ndurated, edding-planexposuresaresmall. On the undersidesof overhangingedges, however,thecircular o oval planof the deformation tructures is easilyobserved. An extensive search of such exposures revealedseveral distincttrackwayscomposedof three to five alignedtracks (Fig. 18). In verticaloutcrops, isolated, paired, and

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N=104

2 4TRACK DIAMETER (CENTIMETERS)

FIGURE7-Size-frequency data for deformationstructures inter-pretedas tracks in GeringFormation, cotts Bluff.Notethat largesttracksare larger hanthose of Holocenebison (Fig. 5).

closely spaced deformation tructures are visible. Some ofthese structureshave verticallywalled infillingsand bilobedlower portions(Fig. 19). As in the Sand Hills, tracks wereproducedn relatively irmbut compactible and. None of thetracksso farobservedsuggest that the substrate was "quick"at the time of deformation.Accordingo Simons et al. (1961),horizontallyaminated ands deposited duringupper-flow-re-gime conditionsare firmrelative to sediments deposited byavalanchingnthe lee side ofripplesordunes. Formosttracks,due to uniformity f grainsize and to disruptioncaused byburrowingand evaporitediagenesis, it is difficult o discernwhether the parentsedimentwas originallyparallelor cross-laminated.Therefore,no attempthas been made to comparetrackdepthin the two types of strata.Incontrast o the Holocenehoofprints, he Oligocene racksappearnsediments hatalso showsome evidence of physicallyinduceddeformation: onvolute laminationappears in someexposures(Fig. 20). Innearlyallcases, however,biogenicand

FIGURE 6-Large, closely spaced tracks at the tops of numerous, thin sediment packages, near base of Gering Formation.

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FIGURE8-Overhanging ledge revealing four aligned, bilobedtracks,middlepartof GeringFormation.

physicallynducedstructurescan be confidentlydifferentiatedon the basis of scale, continuity,or three-dimensionaleome-try.A diverseassemblageof largemammals s known rom ateOligocenerocks of the Great Plains. The distinctpolymodalnatureofthe size-frequency istributionFig. 17)suggests thatthe tracksof severalspeciesarepreserved ntheGering ands.The moder ungulateorders,Perissodactyla ndArtiodactyla,first appearedin the Eocene (Romer, 1966). By the lateOligocene,diversification f artiodactyl ubgroupswas wellunder way. Representativetaxa of six artiodactyland fourperissodactylamilies re known nGering edimentsofWildcatRidge, southeast of Scotts Bluff (Swisher, 1982). A singlefortuitousbedding-plane xposure (Fig. 21a) indicates hat atleast some of the largesttracks were producedby a two-toedanimal.Morphologicallyimilartracks from Oligocene andMiocene rocks have been described by Robertson andSternberg(1942), Chaffee(1943), Bjork(1976), and Dema-thieu et al. (1984). Of the two-toed membersof the Geringfaunalassemblage, only entelodonts were large enough tomake hese tracks(R.M. Hunt,personal ommunication).hesmall,bilobed racks(Figs. 18, 19)were probably roducedbysmallerartiodactylsuch as camels.It is difficult o divide a horizontally eddedsequencecom-posed of very-well-sortedsediment into distinctdepositionalpackages. Traces of invertebratesprovide clues in marinesequences(Howard,1978);tracks of terrestrialvertebrates nthe Geringcan be utilizedn a similarway. The trackdistribu-tion at Scotts Bluffmirrors that of the Sand Hills: laterallyscattered tracks are typicallyseen at closely spaced verticalintervals.If tracks are assumed to markboundariesbetweendepositional vents, the distribution f tracks indicates thatstreamfloodsdepositedpackagesof stratabetween a few cen-timeters to over three meters in thickness.Thoroughlyram-pledhorizons Fig. 16), as mightbe expected alongdiastems,are relativelyrare. Whyare the tracks thatmark the tops ofsedimentpackagesso widely spaced?One possibility s thattime intervals between depositionalevents were very brief

FIGURE9-Cross-sectional view of small, bilobed track made incohesive sand bysmall entelodont or camel; middlepartof GeringFormation.Coin is 1.9 cm in diameter.(Laporte ndBehrensmeyer,1980, fig.4a). Another s thatthepopulation ensityof largevertebrates was very low. A thirdpossibility s that evaporiticsurfacecrusts formed after eachpulse of sedimentation.Earlyevaporitediagenesismay havequicklymade the sandy substrateless compactibleand thusunsuitable orpreservationof tracks.Ifthis was the case, onlya verysmallpercentageof thevertebrateactivity hatoccurredheremaybe recordedbythe tracks. Thepreservationpotentialof tracks or other surficial races in sandyfluvialsequenceswouldseem to be quite low because of their vulnerabilityoscour. Earlyevaporitecementationmaypartially xplainboththe preservation f tracksand,conversely,the rarityand smallscale of channels n these ancient luvial trata.

CONCLUSIONS1. HoloceneeolianandOligocene luvial ediments ofNebraskacontainabundant racks of large vertebrates. The most

FIGURE0-Convolute lamination nonbiogenic),middlepartof Ger-ing Formation.Note consistent asymmetry f folds.

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4I-? . .

FIGURE1-Tracks of large entelodonts. A)Small, overhanging ledgerevealing two large tracks (12 and 16 cm in length). Both tracks arecloven in front; large track was made by front foot. B) Cross sectionof asymmetric pair of tracks. Deeper track on right probably made byfront foot. Middle part of Gering Formation.

obviousmanifestation f the tracksis the downwarpingflaminae een in verticalsection.2. Althoughtracks may superficially esemble nonbiogenicsoft-sedimentdeformationtructures,their isolationalongbedding planes, circular o oval plan, and restricted sizedistribution id in their recognition.Convolutebeddingofphysicaloriginis generallyrestricted to initiallyporous,rapidlydepositedsediments.3. TrackswithinHolocenestrataof the SandHillswere madein clay-coatedsands depositedby migratingwind ripples.Residentbisonthat were sustainedby food andwater re-sourcesmadethe tracks whilethe dune field was active.4. Oligocenerocks at ScottsBluffarepreservedat the tops ofthin packages of fluvialdeposits. Abundantevidence ofevaporiteprecipitation uggests depositionby ephemeralstreams. Size-frequencyplots indicate that tracks weremadeby several different ypes of ungulates;entelodontsproducedhelargesttracks. Cementation y evaporitesmay

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haveenhanced hepreservationpotentialof individualracksby preventingscour andintense trampling.ACKNOWLEDGMENTS

J. B. Swinehartprovided imelyfieldtripsandlivelydiscus-sions. R. M. Hunt and M. R. Voorhiesfreely shared theirknowledgeof Cenozoic aunasandenvironments. thank hesethree Universityof Nebraskacolleaguesfor their enthusiasmand editorialassistance. DonaldBairdof PrincetonUniversityreviewed the manuscript ndprovidedadditionaleferencesonmammaltracks. Larry Cast of the Bureau of Reclamationhelped with access to the Calamus dam site. Mike Leite'sdarkroom kills improvedmanyof the photographs;Kit Leehelpedwith the SEM. Acknowledgments madeto the Donorsof the PetroleumResearchFund,administeredby the Ameri-canChemicalSociety, for supportof this research.

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