boulder New types accumulations of in the hyper-arid Atacama Desert Geomorphology

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ContentslistsavailableatScienceDirect

Geomorphology

j o ur na l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / g e o m o r p h

New types of boulder accumulations in the hyper-arid Atacama Desert

Christof Sager

^a,∗

, Alessandro Airo

^a

, Felix L. Arens

^a

, Carolin Rabethge

^b

, Dirk Schulze-Makuch

^a,c,d,e

aZentrumfürAstronomieandAstrophysik,TechnischeUniversitätBerlin,Germany

bInstituteofGeologicalSciences,FreieUniversitätBerlin,Germany

cLeibniz-InstituteofFreshwaterEcologyandInlandFisheries(IGB),DepartmentofExperimentalLimnology,Stechlin,Germany

dGFZGermanResearchCentreforGeoscience,SectionGeomicrobiology,Potsdam,Germany

eSchooloftheEnvironment,WashingtonStateUniversity,Pullman,Washington,USA

a r t i c l e i n f o

Articlehistory:

Received30April2019

Receivedinrevisedform7October2019 Accepted7October2019

Availableonline28October2019

Keywords:

Hyper-aridAtacamaDesert Boulder

Seismic Mars

a b s t r a c t

Theaccumulationofthousandsofboulder-sizedclastsintoboulderfieldsintheAtacamaDeserthas beenlinkedtoseismic-drivendownslopetransport,araresedimentaryprocesscorroboratedbythis study.WesurveyedboulderarrangementsoccurringintheAtacamaDesertandidentifiedthreeaccu- mulationtypesforfurtherinvestigation:asmallcircularbouldercluster(BC),alongchannelizedboulder stream(BS),andawideconvex-shapedboulderfield(BF).Drone-basedphotogrammetrictechniques andfieldobservationswereusedtogeneratehigh-qualitydigitalelevationmodelsandorthophotosto determinebouldercount,size,coverage,orientation,lithologyandlocaltopography.Ourdatashows thatthearrangementofboulderaccumulationscorrespondswiththeshapeoftheaccommodationspace andtheboulderinput,whereBCsoccuratthecenterofcompletelyconfinedtopographicdepressions, BSsoccuralonglaterallyconfinedandincisedhillslopeswithbouldersstackedaboveeachother,and BFsoccuronlargelyunconfinedshallowandlow-reliefslopeswithadistinctboulderfront.Ageneral downslopeincreaseofaveragebouldersizeandcoveragewasmeasuredinallboulderaccumulationsand along-axisorientationofbouldersparalleltothetransportdirectionwasobservedfortheBS.Basedon theseresultsandthelackoffluvialtransportindicators,weconcludethattransportandarrangementof boulderaccumulationsarelargelycontrolledbytheinterplayoftopographyandseismic-drivenboulder transport,resultinginuniquelandscapefeaturespresentinthehyper-aridAtacamaDesert.Suchsedi- mentarytransportprocessesarerareonEarthbutpotentiallyplayagreaterroleonotheraridplanetary surfacesthatarecoveredbybouldersandsubjecttosufficientseismicactivity.

1. Introduction

Terrestrialboulderaccumulationsareknowntoformbymul- tipleprocessessuchasweatheringandfrostwedgingofbedrock forming autochthonous blockﬁelds (Ballantyne, 2010), tsunami events(Maoucheetal.,2009;Biolchietal.,2016;Coxetal.,2018), largelandslides(Dai etal., 2018)or throughglacier movement (Rose, 1992). Extraterrestrial boulder accumulations have been observedonthehyper-aridandcoldMars,however,theirforma- tionprocesses,suchasbouldersortingalongthermalcontraction crackpolygons,remain highlydebated(Orloffetal., 2013).The little-studiedboulder accumulations in the hyper-arid Atacama Desert, are located in one of the driest and oldest deserts on

∗Correspondingauthor.

E-mailaddress:christof.sager@tu-berlin.de(C.Sager).

Earth (Quade et al.,2012; Matmon et al., 2015). Besidesinfre- quentrainfall,thehighabundancesofraresalts(e.g.,nitratesand perchlorates)andthehighseismicactivitybelongtothedesert’s characteristicfeatures.Previousworkhasshownthatlargeseis- miceventstriggerthedownslopetransportoflargeclastsaround theElBuitreareaandtheYungayarea(SocompaRoad)intheAta- camaDesert,alsoeffectingthelandscapeevolution,withregardsto bedrockweatheringanderosion(Quadeetal.,2012;Matmonetal., 2015).Granitoidbouldersemergeasresistantcorestonesfromthe bedrock oncrests or hillslopesand slide or bouncedownslope throughdiscretemovementeventsduringseismicshaking.Boul- dersaccumulateandbundleintheplainsandatthefootofthehills, formingfrequentlydistinctpolishedmidsectionsbymutualrub- bingduringseismicevents.Apartfromseismicrubbing,boulders seemtoexperienceonlyminorerosion,makingthemlong-exposed surfacefeatures,whichissupportedbyindividualboulderexpo- sureagesof>12Ma(Matmonetal.,2015).Previousstudiesalso

https://doi.org/10.1016/j.geomorph.2019.106897

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consideredaeolian,ﬂuvial,glacialandsaltheaveprocessesbeing responsiblefor boulder movement and polishing (Quadeet al., 2012).However,wind asatransport mechanismwasexcluded, sincegustingwindsofatleast100–170m/sarenecessarytomove bouldersweighing0.5–0.8tons(Quadeetal.,2012).Takenintocon- siderationthescarcityofsurface water,thedesert’s millionsof yearsexistenceandboulderpolishingbeingrestrictedtotheirmid- sections,ﬂuvialorglacialtransportappearsunlikely.Althoughsalt heaveasabouldertransportmechanismcannotbeexcluded,the occurrenceofsmoothandunbrokensoilsurfacesbetweenboulders contradictsactivesalterosion(Quadeetal.,2012).Weherepresent acharacterizationofavarietyofboulderaccumulationtypesinthe Yungayarea,∼90kmsouthwestoftheElBuitrearea,advancingour understandingoftheirformationprocesses,inthecontextofthe localtopographyandthus,landscapeevolutioninthehyper-arid AtacamaDesert.

2. Regionalsetting

Thestudysiteselectionwasprecededbyfieldreconnaissance andsatelliteimagerysurveyofboulderaccumulationsinthenorth- ernmountainsoftheYungayarea,∼60kmsoutheastofAntofagasta (Fig.1).TheYungayareareceiveslessthan1mm/yrprecipitation (Rundeletal.,1991;McKayetal.,2003),resultinginerosionrates thatareclosetozeroandbouldershavingexposureagesofmultiple millionsofyears(Placzeketal.,2014).Weidentifiedthreedistinct typesofboulderaccumulationsasstudysites:multiplesmallcircu- larboulderclusters(BCs),alongchannelizedboulderstream(BS), andabroadconvex-shapedboulderfield(BF).Amongtherangeof

boulderaccumulationmorphologiesobservedintheYungayarea, wechoseforthisstudythoseexamplesthatbestexemplifyapartic- ularmorphology,althoughadditionalboulderaccumulationtypes couldbedefined.Theareaischaracterizedbysmoothroundedhills withlocallyoutcroppingquartzmonzodioriticbedrock,intersected bypartlyexposeddioriticdikes,aswellasadjoiningalluvialfans passingoverintothevalleyfloors.Smallrillsandgulliesarevis- ibleonhillslopes,indicatingminimalsurfacerunoff.Theregion lacksvascularplantsbutiscoveredbyvarnishedpebbles,cobbles and boulders.Adenseand coherentdesertpavement islargely absent.Playasandsalarsdevelopedinthelowestpartsofthevalley containingfine-grainedsedimentsandsalts.

3. Methods

For depicting morphologic characteristics of five boulder accumulations (three BCs, BF and BS) we used the“Structure- from-Motion” photogrammetry technique to reconstruct the 3D-geometryofaterrainusingmatchingfeaturesonoverlapping drone-basedaerial2D-images(Westobyetal.,2012).Lowaltitude images(3–60mabovegroundlevel)wereacquiredusingthecam- eraofaDJIPhantom4withadown-facingcameraorientation(pitch 0^◦)andparallelflightpathsfortheBFandtheBS,whileBCswere mappedin 360^◦ flight patharound theobject withaninclined camera(pitch∼50^◦).ForscalingtheBC1andtheBFcodedmark- erswereevenlydistributed withinthestudysite.Thedistances andrelativeheightdifferencesbetweenthemarkersweremea- suredusingalaserrangefinderwithlevelingfunctionmountedon atripod. Besidesthemarkers thatfunctionasscalebars for3D

Fig.1.Geomorphiclocationmapofthestudysites.TheyellowdotandframeoutlinethesurveyedareainwhichtheinvestigatedBC(bluedot),BF(reddot),BS(greendot) arelocated,includingtheadditionalBCs(lightbluedots)thatwereobserved.LocationmapisbasedonAsterdigitalelevationmodel(DEM)fromU.S.GeologicalSurveyand satelliteimagefromBingmaps,2019Microsoft.

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reconstruction,uptosix1mplastictubeshavebeenlaidoutfor theBC1andtheBFtransectascheckscalebars.TheBSandthe twoadditionalBCswerescaledbasedoncamerapositionsandno markerswereused.AgisoftMetashapewasusedforphotogrammet- ricreconstructionsand3D-spatialdatageneration(Agisoft,2019).

Atotalofseven3Dmodelsweregenerated.Onemodelwascre- atedforeach,theBC1andtheBS.FortheBF,threemodelswere generated,oneoverviewlarge-scalemodel,whiletheboulderfront andthetransectwerereconstructedatsmallerscales.Modelsfor twoadditionalBCs(BC10andBC11)weregenerated.Processing reportsoftherespective3Dmodels,containingrelevantparam- eters(numberof photos,resolutionsoforthophotosandDEMs) anderrors(e.g.,reprojectionerror)areavailableasSupplementary Data(seeSupplementarymaterial#2a-#2g).Inordertoconvert themeasuredrelativeheightdifferencesbetweenthemarkersinto absoluteheightsfortherespective3Dmodels,theestimatedele- vationofonemarkerwasdefinedtobecorrectandthemeasured heightdifferencesinthefield werethensubtractedoraddedto theremainingmarkersand thereby,improve thetiltandverti- calaccuracyofthemodel.Thealtitudeestimationisbasedonthe drone’sGPScoordinates,storedintheimagedataduringflightor inahandheldGPSdevicefortheBS.High-resolutiondigitalele- vationmodels(DEMs)andgeometricallycorrectedaerialimages (orthophotos)of theBCs, theBFand theBS weregenerated in AgisoftMetashape.The1mlongcheckscalebarsshowedlengths rangingfrom99 to101cmintherespectiveorthophotosofthe BC1andtheBFtransect.Theorthophotosexportedas.tiffileswere mappedbyhandinAdobePhotoshopCS5.1.Thereby,theentire visibleboulderareaintheorthographicviewwasmapped,which oftencontrastsdarkfromthebrightground.TheGISapplication QGISwasusedtocreatemapsinwhichtheDEMsandorthopho- tosaredisplayed(QGISDevelopmentTeam,2019).Hand-mapped orthophotoswereconverted tobinaryimagesofsimilarresolu- tionwiththeimageanalysissoftwareFIJIandbouldersize,count andcoverageweremeasuredwiththeBiovoxxelplugin(Schindelin etal.,2012;Brocher,2015).Theorientationwasmeasuredusing theEllipseSplitplugin,whichappliesafittingellipsetoeachpar- ticleandmeasuresitslong-axisorientation(WagnerandEglinger, 2017).Theorientationofboulderswithanaspectratiolargerthan 1.2wasplottedwithGeoRose(YongTechnologyInc.,2014).Detailed investigationswerecarriedoutfortheshownBC1,twoadditional BCsandmultiplesubareasoftheBFaswellastheBS.Onlyclasts withavisiblesurfacearealargerthan0.067m²(correspondingto a26×26cmsquare)wereconsideredforboulderanalysis.Boul- dercoverageisdefinedasthepercentageofboulderscoveringa certainsurfacearea.Bouldercountreferstothenumberofboul- derswithinaspecifiedarea.Forcoverageanalysis,theBCswere dividedintothreeconcentricringswiththeircenterbeingbasedon thepixel-weightedcenterofmassofthemappedorthophoto.The BC11wasdividedintofourrings.Boulderslocatedwithintworings wereassignedtotheringcontainingthelargerfractionoftheboul- dersurfacearea.A180×15mtransectoftheBFwasdividedinto 12subareas(15×15m)andtwoadditionalsub-areasweredefined forboulderorientationalongtheboulderfront,whilefivesubar- eas(15×15m)alongtheBSweredefinedforanalysis.Forboulder size,countandorientationanalysis,boulderslocatedwithintwo subareaswereassignedtotheringor15×15msquarecontain- ingthelargerfractionofthebouldersurfaceresultinginadjusted ringsandadjusted15×15msubareas(Fig.2a,b).Toestimatethe averageelevationofasamplingarea,theaverageofthefourcorner pointsofasamplingareafortheBSandBFwasused.Theelevation estimationfortheBC1isbasedontheheightofthreepointslocated onboulderfreeareainthemiddleofthethreerings.Ahandmap- pingerroroffourpixelswasassumedfortheBCsandBF,aswellas 2pixelsfortheBS.Therebyeachparticlewasdilatedanderodedby oneortwopixels.Themeasurementsforbouldersize,medianand

Fig.2. Mappingmethodology.a)BinaryimageoftheBC1dividedintoconcentric rings(blue)andadjustedrings(red)(A-outer,B-middle,C-innerring).b)TheBF andtheBSweredividedinto15×15msubareas(blue)forcoverageanalysisand adjustedsubareas(red)forgrainsizeandorientationanalysis.

coveragewererepeatedandsubtractedfromtheoriginalvaluesto calculatetheerrorbarsincludedintheresults.

4. Results

Boulderslocatedwithinthestudyareaeitheroriginatefrom quartz monzodioriticbedrockorintersecting dioritic dikesout- croppingonslopesandhillcrests.Whilethequartzmonzodioritic bedrockcommonlyemergesasonion-weatheredsub-roundcore- stones, the dioritic dikes frequently protrude up to one meter abovethesoilsurfaceasangularblocks(Fig.3a,b).Outcropping rocksareintenselycoatedbydesertvarnishbutlackrubbingsur- faces(Fig.3a,b).Asageneraldownslopetrendforallinvestigated boulderaccumulationswemeasuredanincreaseofbouldercov- erageandaveragesizewhilethebouldershapesbecomeoverall moreroundedandsmoothened;frequentlyshowingmultiplegen- erationsofrubbingsurfacesalong theirmidsections(Fig.4and Supplementarymaterial#1).Similartothedownslopeincreaseof theaveragesize,thebouldermediansizeincreasesaswelldueto areductionofthesmallestbouldersandanincreaseoftherela- tiveamountofthelargestboulders.Additionally,incontrasttothe moreerosionresistantdiorites,thequartzmonzodioritesweather readilyintogrus.Withincreasingbouldercoveragethetopfewcen- timetersofsoilbecomedominatedbyloosesilttocoarsesandin whichthebouldersareembedded(Fig.3c,d).Wherebouldercov- erageislow(<4%),thesurfacesoilsurroundingscatteredboulders isidenticaltoboulder-freeareas.However,noneofthesurveyed boulders,rangingfromtheirsourcetotheboulderaccumulations, wereobservedtobeburiedandaregenerallyonlysubmergeda fewcentimeterswithinthesurfacesoil.Besidesoccasionalsmall rillsand gullies onhillslopes,nofurtherindications for ﬂuvial transport,suchasrecentlyactiveriverbeds,scourmarksaround boulders,norboulderimbricationhavebeenobserved.Allthree studiedboulderaccumulationshaveshapesthatcorrespondtothe topographyoftheiraccommodationspace(Fig.5andSupplemen- tary material#1). The circular BC1,containing 125bouldersof mainlydioriticlithology,islocatedat∼1208melevationaround thecenterofaveryshallowbowl-shapedandcirculardepression withaslopeof∼0.9^◦and∼12mindiameter(Fig.5a).Whileonly fewbouldersenterthisfullyconﬁnedaccommodationspacedue tothesmallcatchmentarea, theboulder coverageand average sizeincreaseabruptlytowardsthecenterofthedepressionreach- ing71.8%(Fig.4a).Thesedenselypackedbouldersshowmultiple extensivemidsectionrubbing-surfacesand frequentboulder-to- bouldercontactswithseveralbouldersthatimprintedthemselves intoeachother.ApreferredorientationofboulderswithinBC1is notobserved(Fig.5a’).Theloosegruslayerisbeddedhorizontally andpebble-to-cobble-sizedclastsarelargelyabsentbetweenthe

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Fig.3.Photographsofboulderaccumulationsinthestudysites.a)AngulardioriticbouldersoriginatingupslopeoftheBC1fromanoutcroppingdioriticdike.b)Corestones weatheringoutofthequartzmonzodioriticbedrock.c)RoundedandpolishedbouldermidsectionsoftheBC1withgrusbetweenthem.d)Theconvex-shapedboulderfront oftheBFwiththevisiblegruslayerpresentbetweenbouldersbeingabsentoutsidetheboulderfront.e)StackedbouldersintheBSshowingrubbingsurfacesonallsides.

Fig.4. Boulderaccumulationdownslopetrendsshowingbouldercount,transectdistance,coverage(blue),averagesize(black),mediansize(grey)andelevation(yellow, thelowestelevationintheDepressionofBC1wassetto0mm).a)ThedownslopetrendofBC1.b)ThedownslopetrendofBS.c)ThedownslopetrendofBFwiththeboulder ﬁeldmarginat150m(dottedline).

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Fig.5.Boulderaccumulationsareshownincombinedcolor-codedDEMandorthophotoviewwithlabeledcontourlines(black).SubareasoftheBFandBS(blackframes) correspondtothelabeledrosediagrams.Thebouldersourceareasarenotwithinthedepictedarea,however,thedirectiontomainsourceareasandthetransportpathsare showninyellow.a)TheBC1islocatedwithinafullyconfinedtopographicdepressionat∼1208melevation,butthelowestpointoftheDEMissetto0mb)TheBFislocated onanunconfinedlow-reliefslope.Thetransect(largedottedframe)is180mlong.c)TheBSislocatedwithinaconfinedmorphologyofadryriverbed.a’)b’)c’)Rosediagrams showingboulderorientationindicatedbyblackbars(N=north).Alargerbarradiuscorrespondstoahighernumberofbouldersorientatedintherespectivedirection.The whitenumbershowstheamountofsurveyedboulders.

boulders(Fig.3c).TheBF,containingtensofthousandsofboul- ders,originatingupslopefromextensiveoutcropsandterminating sharplywithaconvex-shapedfront,islocatedonanorthwest-to- southeastorientedlowgradientslopeof∼2.0^◦withnodownslope topographicconﬁnement(Fig.5b).WhereastheBFdescribedin thisstudyexhibitsahighlydistinctfront(Fig.3d),otherBFsinthe Yungayareawereobservedtoshowarangeofmorphologies,upto highlydefuseboulderfrontsgradingintoscatteredboulderﬁelds.

Thebouldercoveragealonga15×180mtransectshowsadowns- lopeincreasefrom8.9to31.0%anddroppingto1.6%beyondthe

boulderfront(Fig.4c).TheDEMshowsthattheBFislocatedona lowreliefmorphologywhiletheslopedecreasesto∼0.45^◦atthe boulderfront.AlthoughtheBFislocatedonalargelyﬂatterrain,the centeroftheconvexboulderfrontwhichismostadvanceddownhill isslightlylowered(areabetweensubarea1and3inFig.5b)inrela- tiontothemarginalareasoftheboulderfront.Thus,theboulders thataremostadvancedarecurrentlyatthelowestpointwithinthe occupiedmigrationpath.Fewbouldersareindirectcontactwith eachotherandrubbingsurfacesarelessfrequentandobservedto bemorevarnishedcomparedtothoseinBCs.Elongatedbouldersof

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thesubareas1and4showapreferredorientation,notobservedat subareas2and3(Fig.5b’).TheBS,containingthousandsofboulders, islocatedonawest-to-eastorientedslope,incisedbyanapparently inactiveﬂuvialchannelwithaslopeof∼1.0^◦inwhichthemajority ofbouldershaveaccumulated(Fig.5c).Theextensivesourcearea ofboulderstotheeastandnortheastcausingahighboulderinput incombinationwiththelaterallyconﬁnedaccommodationspace withchannelslopesgreaterthan5.0^◦resultsinveryhighcoverage alongtheformerthalwegexceeding100%forsomesubareaswith bouldersbeingstackedontopofeachother(Fig.4b).Onlyboulders intheBSshowpolishedrubbingsurfacesonallsides,especially thosethatarestacked (Fig.3e).Boulderslocatedinthethalweg showaclearorientationoftheirlong-axisparalleltothethalweg (Fig.5c’).

5. Discussionandconclusion

Ourﬁeldobservationsandmeasurementsshownoindication for ﬂuvially-driven boulder movement, supporting a seismic- drivendownslopetransportmechanismasproposedbyQuadeetal.

(2012);Matmonetal.(2012,2015).Besidesthelackofscourmarks aroundbouldersortheirimbrication,noneofthethousandsoflarge bouldersobserved wereeven partially submerged aswould be expectedforfluvialtransport,whereasseismic-inducedparticle- size segregationwould prevent boulder burial, as knownfrom kineticsievingmodelswherelargerparticlesareleveledupwards (Rosatoetal.,1987).Thecorrelationbetweentheincreaseofmid- sectionrubbingsurfacesandbouldercoveragesupportsaseismic shakingmechanisminwhichthebouldersrotatehorizontallyand shakelaterallywithoutrolling,asitwouldbeotherwisethecase for fluvialbed-load transport. Furthermore,thedownslope ori- entationoftheboulderlong-axissupports asliding mechanism andnotadownhillrollingmechanismwheretheboulder’slong- axiswouldbeexpectedtolargelyhaveaperpendicularorientation (Fig.5c’).Thesefindingsareinaccordancewithinsituexposuredat- ingresultsfromindividualbouldersoftheYungayarea(Socompa Road)andtheElBuitreboulderfieldindicatingnoboulderturnover formillionsofyears(Matmonetal.,2015).Furthermore,thedowns- lopeincreaseintheaverageboulderssizeandcoveragecorroborate previousfindingsbyQuadeetal.(2012),whichsuggestedthatthis trendisduetolargergranodioriticbouldersundergoingonlyminor erosionduringseismic-driventransport,whilethesmallergran- odioriticbouldersdisintegratedue tocollisionalspallationwith thelargerbouldersintogrus (Fig.4).We observesimilarboul- derdegradationforthequartzmonzodioriticboulderswhereaswe foundlittletonoindicationforthiserosionprocessforthedioritic boulders.Althoughwecannotexcludesuchaspallation-induced grain-sizesortingmechanism for thedioriticboulders, wepro- pose,that,inversetofluvialtransportbutanalogoustodrygrain flowincolluvialfans,largerboulderadvancemorerapidlydowns- lopeduringseismic-driventransport (Blikra and Nemec,1998).

Basedonourresultsweconcludethatthedownslopetransport pathsand theshape of boulder accumulations is controlledby thetopographyoftheaccommodationspace,wherecircularBCs occuratthecenteroffully-confinedtopographicdepressions,BSs occuralongincisedhillslopeswithlateralconfinement,andBFs occuronlargelyunconfinedlow-reliefslopes(Fig.5).Inaddition totheaccommodationspace,thedegreeofboulderinputcontrols theextentoftheboulderaccumulationanditsbouldercoverage, sincealargenumberofbouldersislikelytoresultinhighercov- erageandalargerextentthanasmallnumberofboulderswithin aparticularaccommodationspace,suchasanincisedhillslope.

IncontrasttomostotherBFsweobservedinthearea,whichlack

adiscretefrontandtransitioningintoscatteredboulderﬁeldsof lowercoverage,theBFinvestigatedhereexhibitsa sharpfront, whichweproposetooriginatefromthehighboulderinputand theslopereduction resultingina reduced downslopetransport andthusboulderdamming.Theconvexshapeofthefrontpresum- ablyresultsfromtheslightlylowerelevationatthecenterofthe boulderfront.Althoughweobserveadammingofboulders,their coveragedoesnotexceed27%,whichcouldbelimitedbyboul- dercollision-induceddispersionduringearthquakeevents(Fig.4c).

Sucha collision-driventransportofboulderscouldresultinthe migrationofbouldersintohorizontalterrainifacontinuousinﬂux ofupslopebouldersisprovided.If,however,theboulderinputis highandtheaccommodationspaceislaterallyconﬁned,asthecase fortheBS,boulder coveragecanevenexceed100% resultingin stackedboulders(Figs.3e,4b).Duringseismiceventsthesestacked bouldersweighingseveraltonsareassumedtorotate,whichisindi- catedbyextensiverubbingsurfacesoccurringonallsides(Fig.3e).

TheherenewlydescribedBCsshowintheircenterbouldercov- eragesexceeding50% (Fig.4a and Supplementarymaterial#1).

Weinterpretthishighcoveragetoresultfromtheirfullyconﬁned topography,whileboulderstackingdoesnotoccurduetotheshal- lowslope.IncontrasttotheBFandBS,whichweassumetostill propagatedownslopeandchangetheirshape,wesuggesttheBCsto bemorestableinshapeandpositionsincetheyarealreadylocated atthelowestpointofacirculardepression.Anincreasedboulder inputtotheBC1wouldpresumablyresultinahigherbouldercov- erage,butonlyinaminorBC-sizeincrease,becauseanongoing increaseinsizewouldentailbouldersbeingtransportedfurther downslopeatthesouth-easternmarginofthedepression,where onlyashallowtopographicbarrierislocated.Thisiscorroborated bytheobservationofpolishedboulderslocatedfurtherdownslope oftheBC1.Boulderaccumulationswiththeirrelativelylowtrans- portrates,locatedinahyper-aridenvironmentrepresentarare settingonEarth,whileatvariouslocationsonMarsnumerousboul- deraccumulationshavebeendiscovered(e.g.,Grantetal.,2006;

Levyetal.,2008;Küppersetal.,2014).Multipleformationscenarios havebeenproposedfor thedifferenttypesof boulderaccumu- lationsfoundonMars,suchas(1)‘Boulderhalos’beingcircular boulderarrangementsthatareassumedtobecreatedbyimpact ejecta(Levyetal.,2018),(2)‘Rubblepiles’thatareregularlyspaced boulderaccumulations(20–35m)sittingonlocalhighgroundsfor whichtheirformationhasbeenlinkedtothermalcontractionand rocksortingonpolygonatedgrounds(Mellonetal.,2008),or(3)

‘Boulderclustering’alongpolygonmargins,whichhasbeenpro- posedtoresult froma boulder transport mechanism drivenby seasonalCO₂frostformationandsublimationcycles(Orloffetal., 2013).SomeboulderﬁeldsonMarsshowmorphologicalsimilar- itiestotheherestudiedBF,suchasthedistincttransitionfrom thedenselycoveredterraintothesmoothterrain lackingboul- ders(Fig.6).However,fornoneoftheboulderaccumulationson Marsatopography-controlledandseismic-drivenformationmech- anismhasbeenproposedalthoughmanyanalogiesbetweenMars andtheAtacamaDesertexist,suchasthelong-termhyper-aridity (Hartleyetal.,2005;Amundsonetal.,2012).Hence,theAtacama DesertsoilsarefrequentlyusedasananalogtotheMartianregolith (Warren-Rhodesetal.,2019).Althoughasimilaritybetweensome morphologicalattributesofboulderaccumulationsintheAtacama DesertandonMarsexists,itcannotbeconcludedthattheyhave asimilarformationprocess.Hence,toourknowledge,theforma- tionofthestudiedboulderaccumulationsintheAtacamaDesert remainsuniquetoEarthbutcouldoccuronotherdryplanetary surfacesthatarecoveredbybouldersandaresubjecttosufﬁcient seismicactivity.

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Fig.6. Contactbetweenaboulderfieldwithsmoothandflatterraininthe(a)AtacamaDesertandon(b)Mars,HiRISEimageESP0241171185(latitude:-61.339^◦centered, longitude:100.730^◦east)(NASA/JPL/UniversityofArizona,2011).c)EnlargedareaoftheBF(yellowframeina).d)EnlargedareaofaMartianboulderfield(yellowframein b).

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompetingﬁnan- cialinterestsorpersonalrelationshipsthatcouldhaveappearedto inﬂuencetheworkreportedinthispaper.

Acknowledgements

WethankJ.Meyer,J.Hulin,L.Jentzsch,M.HomannandA.Bern- hardtfortheirsupportanddiscussionsduringtheproject.Wealso acknowledgesupportbytheERCAdvancedGrantHabitabilityof MartianEnvironments:ExploringthePhysiologicalandEnviron- mentalLimitsofLife(#339231).

AppendixA. Supplementarydata

Supplementarymaterial relatedto thisarticle canbe found, intheonline version,atdoi:https://doi.org/10.1016/j.geomorph.

2019.106897.

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