Slumping in the Upper Jurassic Baisakhi Formation of the Jaisalmer Basin, western India : sign of synsedimentary tectonics?

JournalofPalaeogeography,2017,6(4):321e332(00129) Availableonlineatwww.sciencedirect.com

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Multi-origino f s o f t - s e d i m e n t d e f o r m a t i ons t r u c t u r e s

Slumpingin t h e U p p e r J u rassicB a i s a k h i FormationoftheJaisalmerBasin,westernIn dia:Signofsynsedimentarytectonics?

MatthiasAlberti

,DhirendraK.Pandey

,

JitendraK.Sharma

,NarendraK.Swami

,AlfredUchman

aInstitut

fürGeowissenschaften,Christian-Albrechts-Universita€tzuKiel,Ludewig-Meyn-Straße10,24118Kiel,Germany

bDepartment

ofGeology,UniversityofRajasthan,Jaipur302004,India

cEarth

&PlanetarySciencesGroup/Palaeontology,ManipalCenterforNaturalSciences,ManipalUnivers ity,Manipal576104,India

dInstitute

ofGeologicalSciences,JagiellonianUniversity,Oleandry2a,30-063Krako,w,Poland

AbstractA spectacularlyexposedslumpisdescribedfroma120-m-

longroadcutbetweenthevillagesofKanoda n d D e v a i n t h e n o r t h e a s t e r n J a i s a l m e r B a s i n o f R a j a s t h a n , I n d i a . T h e U p p e r J u r a s s i c p a r t o f t h e sedimentsattheoutcropwasformed in anear-

shoresetting and belongstotheLudharwaMember oftheBaisakhiFormation.The3-m-

thickunitshowsanumberofasymmetricfoldsandthrustfaultsleadingtoanimbricationofp a r t l y lithifieds a n d s t o n e beds.Thedeformationstructuresallowther e c o n s t r u c t i o n ofamovementtowardsthenorth west.Thisagreeswellwiththebasinconfigurationthatshowsadeepeningintothisdirection.Althoughthedeter minationofaspecifictriggermechanismisdifficultforsoft-

sedimentdeformations t r u c t u r e s , a n e a r t h q u a k e c a u s e d b y s y n s e d i m e n t a r y t e c t o n i c s i n t h e b a s i n s e e m s t o b e t h e mostl i k e l y e x p l a n a t i o n .

KeywordsS e d i m e n t o l o g y,Massmovements,Slumping,Soft- sedimentdeformationstructures,JaisalmerBasin,Jurassic

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M.Albertietal.

322

1. Introduction

Soft-sedimentdeformationstructuresareubiqui- tousf e atureso c c u r r i n g w i t h i n m a rine,l a c u strine,fluvial,orterrestrialenvironments.Slumps(orslum psheets)areconnectedwiththedown-

slopemovementofa s e d i m entm a s s a n d c h a r a c ter izedb yi n t er n aldeformation(e.g.,reversefaultsorfoldedb eds;Allen,1982;Owenetal.,2011).Suchmovementsofpoor lyconsolidatedsedimentshavebeenfrequently de scribedf r o m t h e g e o logicalr ecord( e.g.,A l s o p e ta l . ,2 0 1 6 ; D e b ackere t a l . ,2 0 0 1 ; K a r l i n e t a l . ,2004;Mastrogiacomoetal.,2012;OrtnerandKilian,2 016;S h a n m ugame t a l . ,1 9 94)a n d c a n a l s o b e observedinthepresent(e.g.,NiemiandBen-

Avraham,1994).Theirformationismostlyattributedtoaloss ofshearstrengthofwater-

saturated,softsedimentsduetosuddensedimentaryoverl oad,oversteepeningoftheslopegradient,suddenchange sinwaveaction,orallogenictriggermechanismssuchasea rthquakesorlesscommonlytsunamisandmeteoriteimpact s(Allen,1982;C h iarellae t a l . ,2 0 1 6 ; G a r c ia- Tortosae t a l . ,2011;G l a d k o v e t a l . ,2 0 1 6 ; M o rettie t a l . ,2 0 16;Obermeier,1 9 9 6 ; O w e n a n d M o r e t ti,2 0 1 1 ; O w e n etal.,2011;Shanmugam,2016).Ot herpossiblepro-

cessesleadingtoslumpsareoftenconfinedtospecialcircumsta nces(e.g.,glacial load ing,s altt ec t onics,andv o l c an i c a c t i v i t y;S h an mugam,2 0 1 6).S l um ps gener atedbyearthquakesarealsocommonlynamedseismites(Seil acher,1969),althoughthevalidityofthetermh a s b e e n q u e stionedr e cently(Shanmugam,2016).Whileslump shaveoftenbeenconnectedtocomparativelysteepslop es(e.g.,Kenyonetal.,1978;Lewis,1971),itisknownthatthe ycanalsooccuratveryweakgradients(<1^o;Allen,1982;Garcia -

Tortosaetal.,2011;Gibertetal.,2005;Wellsetal.,19 80).Oncetheirmodeofformationcanbereconstructed,s lumpscanaddvaluableinformationtothedeposi- tionalenvironment(e.g.,StrachanandAlsop,2006).

Thepresentstudyfocusesona slumpspectac- ularlyexposedalonganapproximately120-m-

longroadcutinthenortheasternpartoftheJaisalmerBasi nofwesternIndia.ThisoutcropofUpperJurassicrockswa smentionedbyPandeyetal.

(2014:p.101),butthedetailedmorphologyandgenesisof theslumparedescribedhereforthefirsttime.Itsanalysisa ndinterpretationcanserveasa casestudyfordeformatio nalstructuresinslumpsaswellasfurtherourknowledgeo ndepositionalconditionsintheJaisalmerBasinduringitsf ormation.

2. Geologicaloverview

TheJaisalmerBasininwesternmostRajasthan(F ig.1)wasformedduetothebreakupofGondwana(Pareek,1 981).Itisapericratonicbasinlocatedatthenorthwesternbou ndaryoftheIndianSubcontinent.DuringtheJuras s ic,itformedpartofthesouthernTethyanmargincloset otheMalagasyGulfseparatingAfricafromIndia(seePandeye tal.,2014:fig.14B).Geologicalresearchintheareastartedal readymorethanacenturyago(e.g.,Blanford,1877;Carter,1 861;Oldham,1886)andsincethencontinuedincessantly(f oracompleteandrecentreviewoftheliteratureseePandeye tal.,2014).Today,theJurassicsuccessionissubdividedi n t o f o u r f o r mations( Lathi,J aisalmer,Baisakhi,andBh adasar)and14members(Fig.2;DasGupta,1975;Garga ndSingh,1983;KachharaandJodhawat,1981;Pandey andKrishna,1996;Pandeyetal.,2012,2014;Sharma,2016).

IncomparisontotheneighbouringKachchhBasin (Fürsichetal.,2013),theJurassicsuccessionoftheJaisal merBasinisthinner,butalsorepresentsavari-

etyofdepositionalenvironments.Thelandscapeisch aracterizedbyflatplainswithfewlowhillsatthefringe oftheTharDesert.Consequently,theMesozoicrocksareofte ncoveredbydesertsandandoutcropsares c a t t e r ed.I n g e n e r a l,t he J u ra s sics u c c e s s i onstartswithfluvial, deltaic,lacustrine,andmarginal-

marinesedimentsoftheLathiFormation(EarlyJurass ict oB a j o c i an;B o n d e , 2 0 1 0 ; L u kose,1 9 72 ; Pan deye t a l . ,2 0 0 6 ; Pien,kowskie t a l . ,2 0 1 5 ; Sriva stava,1 9 6 6).M a rines h a l l o w -

waterd e p ositsdominatethefollowingJaisalmer,Baisakhi ,andBha-

dasarFormations(MiddletoLateJurassic),butrepeat edphasesofsubaerialexposurearerecordedinthesediment s(e.g.,byfluvialconglomerates,calichenodules,rootleth orizons,and potentialinsectbur-

rows;F ü r s i che t a l . ,1 99 2 ; M ah e n d r a a n d B a n er ji,1989;Pandeyetal.,2005,2010,2014).Formostofthet i m e , t h e waterd e p t h w a s v e r y l o w a n d e v e n sli ghtfluctuationsinsealevelledtodistinctfacies cha nges.M a n y o f t hel i t h ostratigraphicm e m b ersc orrespondtosuchsmall-

scalevariations.Inaddition,theycommonlyarepresentand/

orpreservedonlyinpartso f t h e b a s i n a n d t h eirb o u ndariesa r e d i a -

chronous(PandeyandPooniya,2015;Pooniya,2013).Th esecharacteristics,togetherwiththeoccasionallypoorou tcropconditions,makeintrabasinalcorrela-

tionsverydifficult(Pandeyetal.,2009,2014).Bed-by- bedcollectingofammoniteshashelpedtoclarifyproblem s( f o r e x a m p l e t hed a t i n g o f c e r t a i n l i t h o -

Fig.1AeSketchmapofRajasthaninwesternIndiashowingthemajorcitiesandthelocationoftheJaisalmerDistrict;BeGeologicalsketchmapoftheJuras sicoftheJaisalmerBasinindicatingthepositionofthestudylocalityalongtheKanod-

DevaRoadthroughtheKalaDungarHills(modifiedafterDasGupta,1975;Pandeyetal.,2014).

Fig.2 L i t h o s t r a t i g r a phicf r a m e w o r k o f t h e J u r a s s i c s t r a t a o f t h e JaisalmerBasin(modifiedafterSharma,2016).

stratigraphicunitsortheidentificationofdiachronousb oundaries;e.g.,SharmaandPandey,2016),butmanyo p e n q u e s t i o n s e x i s t e s peciallyr egardingu n - fossiliferousu n i t s.

TheBaisakhiFormationisLateJurassicinageandcons istso ff ourl i thostratigraphicm e mbers(Fig.2).Theira g e,r e lationship,a nds t ratigraphyh a veb e enrecently reviewedbyPandeyandPooniya(2015).Therocksareexp osedalonganarc,approximately8e10kmwide,stretchingf romtheKanoiFaultinthesouthwesttot h e a r ean o rthea sto fK a nod(Fig.1).T h e B a salMemberi sc h aracteriz edb yc arbonaceous,g r eyt oblack,siltyclay.Thesedim entcontainsammonitesandbelemnites,b u t n o b enthi cfossils.T h e d i versityo f tracefossilsisverylow.Bas edonthefossilcephalo-

pods,itsagehasbeengivenasMiddletoLateOxfordian(Chatt erjee,1990;PandeyandPooniya,2015;Prasad,2006).Th efollowingRupsiMemberconsistsmainlyofoccasionally- bioturbateds i ltyc layw i thi n tercalationsofpartlycross- bedded,gypsiferoussiltstonesandfine-

grainedsandstones.Characteristicisthecommonoccu rrenceo f f erruginousc o ncretions.T h e r o cks

containammonitesandbelemnites,aswellasfewtracefoss ils(e.g.,Zoophycus,Planolites,Thalassinoides;Pandey andPooniya,2015).AKimmeridgiantoEarlyTithoniana

gehasbeenassignedtotheunit(PandeyandKrishna,2000;Pan deyandPooniya,2015).TheLanelaMemberc o nsistso f t h inly-bedded,s i ltyfine-

grainedsandstoneinterbeddedwiththinfine-

grainedsand-

stonebedsandisEarlyTithonianinage(PandeyandPoo niya,2 0 15).W h ilet h e B asal,Rupsi,a n d L a nelamem bersaredominatedbyfine-

grainedsediments,theLudharwaMe mberi sc h aracteriz edb yb r own,c r oss-bedded,commonly-

bioturbated,mo derately-towe ll-cemented,fine- t o m e dium-

graineds a ndstonesa n d occasionallyc onglomerates.

T h e u n iti sf o s siliferouswitha mmonites,p l antf r agm ents,a n d d i verset racefossils(e.g.,Ancorichnus,Aste rosoma,Gyrochorte,Gyrolithes,Palaeophycus,Phycosi phon,Polykladichnus,Taenidium,Thalassinoides,Skoli thos;PandeyandPooniya,2015).Itsagehasbeengivenas Oxfordiant o E a rlyTithonian(Pandeya n d Pooniya,20 15).

Itshouldbenotedthatthe lowerboundaryoftheBai sakhiFormationaswellasboundariesbetweenitsmemb ersa red i achronous(Fig.2;e . g.,p a rtso f t h e BasalMe mberandthelowermostLudharwaMemberwered e po siteda t t h e s amet i mea st h e r u d-

a n d packstonesoftheupperJajiyaMemberoftheJaisalme rFormation).Thefine-

grainedBasal,Rupsi,andLanelamembersareinterprete dtorepresentconditionsinthebasincentre,whiletheLudh arwaMemberformedundershallow-

waterconditionsclosertotheshoreline.

3. Studylocalityanddepositionalsetting

Thepresentlydescribedslumpisexposedalonganapprox imately1 2 0 - m - longr o a d c u t b e t w eent h e v i l - lagesKanodandDevaintheKalaDungarHills,approxi mately3 0 k m n o r t h e asto f J a i salmer( N 27^o08⁰09 .0⁰⁰,E071^o05⁰49.2⁰⁰;Fig.1).Thesuccessionmeasureda crossthehillsidecomprisesaround39mofsiliciclastics e dime ntsb e l ongingt o t heL u d h arwaMember(Fig.3).Itconsi stsmainlyoffine-tomedium-

grainedsandstoneswithcommonferruginousconcreti onsinthe lo we r ha lfo f these c t io n . Towardsthetopcr oss-

beddedsandstonebedsoccuraswellashorizonswithcalic henodulesandrootlets.Inthelowerpart,thebedsarech aracterizedbytracefossilsincludingT halassinoides,G y r o l ithes,a n d u n i d e n tifi-

ableb u r r o ws.Towardst h e t op,t h e t r a c e f o s s i l Ophiomorphaappears.Duetoastrongferruginizationoft h e s e dimentso thers e dimentarys t ructures( e.g.,loador

flamestructures)arenotvisible. Fig.3S e c t i o n throughtheLudharwaMemberoftheBaisakhiFor- mationattheKanod-

DevaRoad(N27^o08⁰09.0⁰⁰,E071^o05⁰49.2⁰⁰;modifieda f t e rPandeye t a l .,2 0 1 4).c l = c l a y ; s i = s i l t ; f s = fine-grainedsand;ms=medium- grainedsand;cs=coarse-

grainedsand;g=gravel;m=mudstone;w/f=wacke-/floatstone;p=pac kstone;g=grainstone;r=rudstone.

Ingeneral,the Ja isa lm erBasinw a s en clo s edo nthrees i d e s a n d o p e n i n g t o wardst h e n o r t h west(Pandeye t a l . ,2 0 1 4).M o s t l i k e l y,t h e L u d h arw aMemberattheKanod-

DevaRoadsectionrepresentsamarginal-

marinetoterrestrialsetting.ThetracefossilGyrolithesi ndicatesve rysha l lowandb ra ckish wa t e r condition s(Fig.4;Gernant,1972;Nettoetal.,2007;Uchmana n d H a n k e n , 2 0 1 3).Towardst h e t o p , t h e succ essionshowsseveralbedswithcalichenodulesandrootletsw h i c h c e rtainlyf o r m e d a b o v e t h e s e a l e v e l . Theoverall coarsening-

upwardtrendintheLudharwaMemberreflectsashallowi ng,whichissuperimposedbysmall-

scalefluctuationsofrelativesea-

levelleadingtoanalternationofsedimentsofaterrestrial,b rackish,andmarineorigin.Whilefewammonitesareprese ntinthelowerpartoftheunit,thehigherenergyconditionsd u r i n g t h e f o r m a tiono f t h e c r o s s -

b e ddedsandstonesto wards th et o p ledt o thedestr uctionofhardpartsandtheabsenceofbodyfossils.Thereby,th esuccessionattheKanod-

DevaRoadcloselyresemblesthatof the b e t t e rkn o wn Ka h la V i ll a g e se ct ion w h i ch hasb e e n s t u d ieda n d i n t e r p retedr e centlyi n d e t a i l (PandeyandPooniy a,2015;Pandeyetal.,2014;Pooniya,2013).TheLudha rwaMemberatthislocal-

itydisplaysaseriesofdepositionalenvironmentsfromfully m a r i n e t o m a r g i n al-

marine,b ra c k i s h,d e lt aic,coastal-

plain,andfluvialtowardsthetop(Pandeyetal.,2014).

Mostofthebasalbedsweredepositedinasettingbelo wthefair-

weatherwavebase(possiblylowershoreface;Pandeyetal., 2014)asevidentbytheoccurrenceo f fi ne-

graineds e d i m ent,m a r i n e o r g a n - isms(e.g.,ammonites),andadiverse,well- preservedichnofauna.I n t e r v alswithl o w - a n g l e c r o s s - stratifica-

tionandve rt icalburrows(e.g.,S ko lit hos)havebeeni nterpretedtorepresenttheuppertolowerforeshore(

Pandeye t a l . ,2 0 1 4).T h e u p p e r p a r t s h o w s t r a c e

fossilsi n d i c ativeo f b r a c k ishwaterc o n d i t i o ns(

e.g.,Polykladichnus;F ü r s i c h a n d Werner,1 9 8 6)a n d h a s beeninterpretedasaprotectedbayenvironm entshowingfluvialinfluence(Pandeyetal.,2014).Thetopof theKahlaVillagesectionisthenformedbyaconglomera teoffluvialorigin(Pandeyetal.,2014).

Onamoreregionalscale,theLudharwaMemberrepres entsnear-

shoredepositsprogradingintothebasin.Depositionof thesedimentsstartedalreadyintheOxfordian,whileinothe rpartsofthebasinsedi-

mentsoftheJajiyaMember(JaisalmerFormation)fo rmed.D u e t o t h e p rogradationo f t h e L u d h arwaM ember,itcommonlyformsthetopofsuccessionsandcanbefo undoverlyingtheJajiya,Rupsi,andLanelamembersfurth ertowardsthebasincentre.

4. Slumpm o r p h o l o g y

Theu n i t e x h i b i tingt hed e f o r mationf e a t u r e s c o n - sistsofa lmost3 mo fp oo rlyce me n ted,fine- grainedsandw i t h i n t e r c alatedm oderately- cemented,fine-tomedium-

grainedsandstonebeds(Figs.5and6).Fewthinhorizonssho whigherclayandsiltcontents.Thebaseof the un it is not exp o sed.Thedef ormedsedi-

mentarylayersrepeatedlychangetheirstrikeandangle o f d i p . T heye x h i b i t a n a n g u l a r d i s c o n formit ywiththeu nd e r-

and ove rlyin g st ra t a (Fig.6A).Espe-

ciallyt h e h a r d e r s a n d s t oneb e d s s h o w e v idenc eo f foldinga n d f a u l t i n g (Fig.6BeG).I m b r i c ationo f b e d s canbeseenrepeatedly(Fig.6C,F).Theplanesoftheth rustfaultsdiptowardssoutheast.Thefoldaxesaredirect edn o r t h e ast-

southwesta n d t h e l i m b s d i r e c tedsoutheasta r e g e n e r allyl o n g era n d m oreg e n tlyi n -

clined,whilet hen o rthwesternflanksa re s t e e p e r (Figs.

5 a n d 6D).T h e s o f t e r,m o r e fi ne-grained

Fig.4T h e tracefossilGyrolithesisp.intheLudharwaMemberoftheBaisakhiFormationattheKanod-DevaRoadsection(modifiedafter Pandeyetal.,2014).

32 6

M.

Al b er ti et al Fig.5S k e tchoftheapproximately3-m-thickslumpalongthe120-m-

longroadcutbetweenKanodandDeva.TheroadhasadirectionfromSEEtoNWW(100^o)

withtheoutcropatitsnorthernside.TheboxesindicatethepositionofthephotographsinFig.6.

327

SlumpingintheUpperJurassicBaisakhiFormationoftheJaisalmerBasin,westernIndia

Fig.6F i eldphotographsoftheslumpexposedalongtheKanod-

DevaRoad.TheexactpositionsofthephotographedpartsoftheoutcropareillustratedinFig.5.AeDisconformity(shownasreddashedl ine)betweentil tedlayersw ithintheslumpandhorizontally-

bedded,overlyingsandstonebeds;B,CeSandstonebedsshowingimbricationduetothrustfaults.ThefaultplanesaredippingtowardsSEE(right)andthesedi mentsmovedtowardsNWW(left);DeFoldedsandstonebedwithasteeperflanktowardsNWW(left)andamoregentlyinclinedlimbtowardsSEE(right);E,F eImbricatedsandstonebeds;G,HePhotographandinterpretativedrawingofthree-dimensionallyexposed,foldedsandstonebeds.

Fig.7 AePartlyl e n t i c u l a r s a n d s t o n e s w i t h i r r e g u l a r u p p e r a n d l o w e r s u r f a c e s e x p o s e d 2 0 0 m t o w a r d s t h e s o u t h e a s t o f t h e s l u m p ( N 27^o0

8⁰09.3⁰⁰, E071^o0

5⁰56.

1⁰⁰;

33-cm-longhammerforscale);BeRegularlybeddedsandstoneswithsharp,evenboundariesoverlyingtheslump(33-cm- longhammerforscale).

sedimentisoccasionallymorestronglydeformedandlos ta llt racesofp ri ma ry l a m i n a t ion.

Followingtheroad200mtowardsthesoutheast, theu n iti sfirstc o v e r e d b y s c r e e , b u t a g a i n e x p o sed

beyondacurve(N27^o08⁰09.3⁰⁰,E071^o05⁰56.1⁰⁰).Atthisoutcr op,t h e b e d s a r e l e s s d e f o r meda n d s h o w a s u b - horizontalbedding(Fig.7A),butareoftenlenticular,showing sharp a n d ir r e gularupp er a n d low er s ur f a c e s.

Fig.8S k e tc h summarizingtheformationoftheslump.Forfurtherexplanationseetext.

Fig.9 A eBlockd i a g r a m i l l u s t r a t i n g t h e d i s t r i b u t iono f t h e d i f f e r e n t m e m b e r s o f t h e B a i s a k h i F o r m a t i o n i n t h e n o r t h e a s t e r n h a l f o f t h e JaisalmerBasin;BeSchematicmodelofaslumpshowingthezoneofextensionup-slopeandthezoneofcompressiondown-

slope(modifiedafterG i b e r t e t a l .,2 0 0 5).

Incontrast,thestrataoverlyingthedeformedunitarever yr egularlyb e d d eds a n d s t onesw i t h s h a r p , e v e n boundaries(Fig.7B).

5. Interpretationoftheslump

Ther e g u l arb e d d ingo f t heu n d e r -

a n d o v e r l yingstratap o i ntst o a s y n s e d imentaryn a t u r e o f t h e deformation.D u r ingn o r malb a c k g r o unds edimenta-tion,e v e n l y-

beddeds h e e t s o f s a n d wered e p o s i t e d (Fig.8A).

Theseunderwentdifferentialdiagenesiswithsomebe dscementedmorethanothers(Fig.8B).Duringt h e sl u m p in g e v e n t , t h e b e d s w e r e d e f o r medaboveadetac hmentplane,whichisnotvisibleintheoutcrop.Whilethe soft,notyetcementedfine-

grainedsandreactedductile,theharderbedswerefoldeda ndfaulted.A s y mmetricalf o l d s f o r m edw i tht h eirs t e e p sidepointingtowardstheflowdirection.Similar ly,thrustfaultsledtotheimbricationofsandstonebed s(Fig.8C;compareAllen,1982;Alsopetal.,2016).It canbeassumedthattheseafloorwasveryunevenandirregulard i r e c tlya f t e r t h e s l u m p i n g e v e nt( c o m p a r eGi bertetal.,2005).However,waveactionmostlikely

ledtoaplaningofthesurface.Backgroundsedimen- tationc o n t i nuedl e adingt o a d i s c o nformitywitht h e irregularlyinclinedlayersoftheslump(Fig.8D).Event ually,thesandatthetopofthesuccessionwasdeposited ,whichisnowstronglylithified(Fig.8E).

Ase v i d e n t b yt h e d e f o r mations t ru ct ures,t h e se dimentm ovedd o w n - s l opei n t o a n o r t h westernd i - rection.Thisfitsverywellwiththegeneralbasinconfigur ationwhichw a s d e e p e ninga n d o p e n i n g t o - wardsthenorthwest(Fig.9A;e.g.,DasGupta,1975;Pa reek,1 9 8 4).I n g e n e r al,a s l u m p i s c h a r a c t erized byazoneofextensionup-

slopeandazoneofcompressiondown-

slope(Fig.9B;Alsopetal.,2016;Gibertetal.,2005;Ortn erandKilian,2016).Thefirstischaracterizedbyextension altectonicsincludingnormalfaults.Thiszoneisnotwellexp osedalongtheKanod-

DevaRoad.Possibly,thesoutheasternoutcropsexhibitingsu bhorizontalsandstonebedswithirregularboundarieso f t e n f o r mingl e n s e s r e presentt h i s a r e a (Fig.7A).A l t h o u ghn o r malf a u l t s h a ven o t b e e n recorded,continu oussandstonebedsmighthavebeendividedintoindividual sandlensesbyextensionalforces( c o m p areA l s o p e t a l . ,2 0 1 6 ; M a r t í n-

Chiveleteta l . ,2 0 1 1).I n c o n t r a st,m o s t o f t h e d e f ormational

330 M.Albertietal.

structuresvisibleintheroadcut(Fig.5)canbeattributed tocompressionalforces.

Thedeterminationofthetriggermechanismfor soft-

sedimentdeformationstructures(ormoreparticularly slumps)isgenerallyachallengeandre-

quiresatleas ta detai ledknowledgeon theslump m orphologyaswellasthedepositionalenvironment(e.g.,M artín-

Chiveletetal.,2011;Owenetal.,2011;Shanmugam,2016 ).Alossofshearstrengthofthewater-

saturatedsedimentscanbecausedbyanin-

creaseofporefluidpressuregeneratedduringrapiddepo sitionorviaseismicwaves(Alsopetal.,2016).TheJ aisal merB a s i n f o r m edd u r i n g t h e r i f t i n g o f Go ndwanawhichcontinuedthroughouttheJurassic(Paree k,1981).Itisborderedbymajorfaultsystemswhichcanbe expectedtohaverepeatedlycausedstrongearthq uakesintheregion.Thisisimportantbecauseearthq uakesbelowacertainmagnitudewillnottriggerslumpingeve nts(e.g.,dependingonsedi-

mentconditionsandpotentialseismicwaveamplifi- cationd u e t o s p e c ificg e o l o g i c als t r u c tures;A l s o p etal.,2016;Obermeier,1996).SincetheJaisalmer Basinw a s a t e c t onicallya c t i v e r e g i o n d u ringt h e Jurassic,astrongearthquakecanbeconsideredasalikel ytriggermechanismoftheobservedslump.Incontra st,thereisnoevidenceforasuddensedimen-

taryo v e r load( e.g.,fl uid-

escapes t ructures)o r a n oversteepeningoftheslope gradient(alsocomparereasoningbyGladkovetal.,2016).

Thesedimentationrateseemstohavebeenrelativelyconst antwiththeunder-

andoverlyingbedsshowinggenerallysimilargrainsizesa ndsedimentarystructures.Endogenictriggermechani sms,however,oftenleadtoarepeatedoccurrenceofdeformati onstructuresinseveralbedsofthesamefaciestypesincontrastt othepresentcase(compareOwenetal.,2011).Similarly,t herearenosignsofsuddenchangesinwaveactionortidalrang einfluencingthesediments.Thebedsdirectlyunder- neathorabovetheslumpsheetdonotshowevidenceofstrong currentsorstormwaves,althoughthegen-

eraldepositionalsettingmighthavebeenmarginal- marine.Inaddition,soft-

sedimentdeformationstructurescausedbywavesorc urrentsreportedinliteraturearealsocommonlysmall erinscale(e.g.,Chiarellaetal.,2016).Anoversteepeningoft heslopegradientcouldhavebeencausedbyatectonictiltoft hebeds.Thiswouldhaveresultedinachangeinthedipoftheothe rwiseundisturbedbedsunderlyingtheslump.Incontrast,t hesedimentdepositedaftertheformationoftheslumpwo uldbehorizontallybeddedagain.Sincearesultingdifferen ceindipbetweenthebedsunder-

andoverlyingtheslumpunitcannotbeobservedinthestu diedsuccession,thisprocesscanalsobeexcludedasapo tentialtriggermechanism.

Eveniftheoreticallypossible,itseemsunlikelythatarareev ent,suchasameteoriteimpactortsunami,causedth edescribedslump(compareShanmugam,2016)w i t h o u t l e avinga n y o t h e r t r a c e withint h e sedimen taryrecord.

6. Conclusions

Duringt h e L a teJ u r assic,t h e J a i s a l m e r B a s i n i n westernIndiaexperiencedanoverallshallowingfrom marinet o p a r tlyt e r r estrialo r fl uvialc o n d i tions.T h e LudharwaMemberoftheBaisakhiFormationconsistsof c o m p arativelyc o a r s e -

g r aineds e d i mentsw h i c h wered e p ositeda l o n g t h e m a rgino f t heb a s in.T h e continuouss h a l l o w in gc a u s e d a c o a r s e n i n g -

upwardtrendw i t h i n t h e u n ita sw e l l a s t h e a p p earanceo f tracefossilspointingtohigh-

energy(e.g.,Ophiomor-pha)orma rg in al-

marinetob ra ck is h c o n d i t i o ns(e.g.,Gyrolithes).Tow ardsthetop,evidenceofterrestrialorfluviali n fluencec a n b e f o u nd( e.g.,r o o t l etsa n d calichenodules).

ThedescribedoutcropintheLudharwaMember oftheBaisakhiFormationrepresentsanimpressiveexam pleofa slumpinthegeologicalrecord.Sinceitsformationt ookplaceinabasinborderedbyactivefaultsystems,syns edimentarytectonicscausingstrongearthquakesinther egioncanbeconsideredasa likelytriggermechanism.Th easymmetricfoldsandorientationofthrustfaultswithinth eunitindicateamovementofthedisturbedsedimentstow ardsthenorthwest,inthedirectionofthebasincentre.

Acknowledgements

Thisworkwass upportedb y t heAlexandervonHumb oldtFoundation,Germany[MatthiasAlberti],theDepart mentofScienceandTechnology,India[DhirendraK.Pan dey],a ndt heJagiellonianU niver-

sity,Poland[ AlfredUchman].T hea uthorst hankt hejour nale ditorsa ndr eviewersf ort heirc onstructivecriticis m.

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