Origin of an extensive network of non-tectonic synclines in Eocene limestones of the Western Desert, Egypt

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Origin of an extensive network of non-tectonic synclines in Eocene limestones of the Western Desert, Egypt

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Origin of an extensive network of non-tectonic synclines in Eocene limestones of the Western Desert, Egypt
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Journal of African earth sciences
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Tewksbury, Barbara J.
Tarabees, Elbamy
Mehrtens, Charlotte J.
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Satellite image maps ( lcsh )
Karst ( lcsh )
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serial ( sobekcm )
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Egypt

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Satellite images of the Western Desert of Egypt display conspicuous sinuous color patterning that previous workers have interpreted as erosional flutes formed by catastrophic flooding. Our work with high resolution satellite imagery shows that the patterning is not erosional but, rather, the result of a network of thousands of narrow synclines in the Eocene bedrock capping the Limestone Plateau. Synclines form as isolated, 200-400 meter-wide downwarps in otherwise flat-lying strata. Limb dips are shallow, and doubly plunging hinges form multiple basin closures along syncline lengths. Anticlines form “accidentally” in inter-syncline areas where two adjacent synclines lie close together. Synclines have two dominant orientations, WNW-ESE and NNW-SSE, parallel to two prominent joint and fault sets, and synclines branch, merge, and change orientation along their lengths. Synclines are all at the same scale with neither larger structures nor parasitic structures and are best described as non-tectonic sag synclines. An Egypt-wide inventory reveals that these synclines are both confined to Eocene limestones and developed, albeit it sporadically, over nearly 100,000 km2. The syncline network predates plateau gravels of the Katkut Formation, which have been interpreted as Oligocene or early Miocene in age, and the network is cut by faults related to Western Desert extension associated with Red Sea rifting. The mechanism that caused sag of overlying layers is not clear. Modern karst collapse, subsurface dissolution of evaporites, and collapse of paleokarst are all unlikely mechanisms given the timing of formation and the underlying stratigraphy. Silica diagenesis and downslope mobilization of underlying shales are possibilities, although uncertainty about the origin of silica in the limestones, plus the consistency of syncline orientations over large areas, make these models problematic. Hypogene karst, perhaps related to aggressive fluids associated with basaltic intrusions, may be the model most consistent with the admittedly limited data we currently have for the network.

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Originofanextensivenetworkofnon-tectonicsynclinesinEocene limestonesoftheWesternDesert,EgyptBarbaraJ.Tewksburya,*,ElhamyA.Tarabeesb,CharlotteJ.MehrtenscaDepartmentofGeosciences,HamiltonCollege,Clinton,NY13323,USAbDepartmentofGeology,DamanhourUniversity,Damanhour22516,EgyptcDepartmentofGeology,UniversityofVermont,Burlington,VT05405,USAarticleinfoArticlehistory: Received4December2016 Accepted10February2017 Availableonlinexxx Keywords: Non-tectonicfolds Satelliteimageanalysis Hypogenekarst WesternDesert EgyptabstractSatelliteimagesoftheWesternDesertofEgyptdisplayconspicuoussinuouscolorpatterningthatpreviousworkershaveinterpretedaserosional utesformedbycatastrophic ooding.Ourworkwithhigh resolutionsatelliteimageryshowsthatthepatterningisnoterosionalbut,rather,theresultofanetwork ofthousandsofnarrowsynclinesintheEocenebedrockcappingtheLimestonePlateau.Synclinesform asisolated,200-400meter-widedownwarpsinotherwise at-lyingstrata.Limbdipsareshallow,and doublyplunginghingesformmultiplebasinclosuresalongsynclinelengths.Anticlinesform “ accidentally ” ininter-synclineareaswheretwoadjacentsynclineslieclosetogether.Synclineshavetwo dominantorientations,WNW-ESEandNNW-SSE,paralleltotwoprominentjointandfaultsets,and synclinesbranch,merge,andchangeorientationalongtheirlengths.Synclinesareallatthesamescale withneitherlargerstructuresnorparasiticstructuresandarebestdescribedasnon-tectonicsagsynclines.AnEgypt-wideinventoryrevealsthatthesesynclinesarebothcon nedtoEocenelimestonesand developed,albeititsporadically,overnearly100,000km2.Thesynclinenetworkpredatesplateaugravels oftheKatkutFormation,whichhavebeeninterpretedasOligoceneorearlyMioceneinage,andthe networkiscutbyfaultsrelatedtoWesternDesertextensionassociatedwithRedSearifting.The mechanismthatcausedsagofoverlyinglayersisnotclear.Modernkarstcollapse,subsurfacedissolution ofevaporites,andcollapseofpaleokarstareallunlikelymechanismsgiventhetimingofformationand theunderlyingstratigraphy.Silicadiagenesisanddownslopemobilizationofunderlyingshalesare possibilities,althoughuncertaintyabouttheoriginofsilicainthelimestones,plustheconsistencyof synclineorientationsoverlargeareas,makethesemodelsproblematic.Hypogenekarst,perhapsrelated toaggressive uidsassociatedwithbasalticintrusions,maybethemodelmostconsistentwiththe admittedlylimiteddatawecurrentlyhaveforthenetwork. 2017TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-ND license( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).1.Introduction ThehyperaridSaharaofcentralEgyptdisplaysdistinctivecolor patterninginsatelliteimagery.Someofthepatterningisdueto prominentNNW-SSEgrabensthatarerelatedtoRedSearifting (arrowsin Fig.1 a).Thesegrabensalsohavecleartopographic expressionbotheastandwestoftheNile( Fig.1 b).Someofthe patterningisduetoaeolianfeatures,suchasthelinearGhardAbu Muharikdune eld( Fig.1 a).Muchofthepatterningisneitherof these-ithaslittletopographicexpressionandis “ wormy ” in character( Fig.1 aandb),enigmaticinorigin,andpervasively developedoveranareaofatleast20,000km2intheeast-central WesternDesert. Fewworkershaveaddressedthispervasivepatterning. Klitzsch etal.(1987) ,workingwithLandsatMSSimageryfromthe1970s, mappedsomeofthesefeatureswhendevelopingthe1:500,000 scalegeologicmapsofEgyptbutdesignatedthemsimplyas “ faults andfractures ” .The rststudytoaddresstheoriginofthe patterningspeci cally( Brookes,2001 )alsoworkedwithLandsat MSSimagesandsuggestedthatthewormypatternsaregiant erosional utesproducedbycatastrophic ooding.Hisinterpretationhasbeenreiteratedmanytimesintheliterature(e.g., Goudie, 2005;Mostafa,2013;AbuSeif,2015;AbdelkareemandEl-Baz, 2016 ).Revisitingthequestionandworkingmorerecentlywith Correspondingauthor. E-mailaddresses: btewksbu@hamilton.edu (B.J.Tewksbury), etarabees@yahoo. com (E.A.Tarabees), Charlotte.Mehrtens@uvm.edu (C.J.Mehrtens). Contentslistsavailableat ScienceDirectJournalofAfricanEarthSciencesjournalhomepage: www.elsevier.com/l ocate/jafrearsci http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017 1464-343X/ 2017TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). JournalofAfricanEarthSciencesxxx(2017)1 e 20 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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LandsatETM AbotalibandMohamed(2013) cametothesame conclusion. Previousstudieswerehampered,however,bytheresolutionof thedataexamined.AttheLandsatimageresolutionsusedbypreviousworkers(~70m/pixelforMSSand15 e 30m/pixelforETM ), imageryofanareawiththeproposedcatastrophic ooding utes doesshowintriguingpatterns( Fig.2 aandb).Zoomingin,however, revealsthattheresolutionistoolowtoresolvedetailsneededto criticallyevaluatethehypothesis( Fig.2 candd),eveninthehighest resolutionLandsatpanchromaticimagery.ImageryinGoogleEarth atresolutionsof1 e 2m/pixel,ontheotherhand,revealsstunning detailthatishiddenbelowtheresolutionofLandsatimagery ( Fig.2 eandf).Thisdetailmakesitpossibletostudysmall-scale featuresandevaluatewhatisresponsibleforthepatterningin thislargelyinaccessibleregion. Inthispaper,wepresentevidencethatanextensivelydeveloped networkofsagsynclinesinEocenelimestonebedrockisresponsibleforthepervasivewormypatterninginsatelliteimageryofthe WesternDesert.Wecombineinterpretationofhighresolution satelliteimagerywithgeophysicaldataandamodestamountof elddatatoconstrainmodelsforthecauseandtimingofsag. 2.Background 2.1.Regionalsetting Theeast-centralportionoftheWesternDesertisagently north-slopingplateauthatsits200 e 250mabovetheNileValley totheeastandnearly300mabovetheKhargaValleytothe southwest.KnownastheLimestonePlateau( Fig.1 b)foritsunderlyinglimestonebedrock,theareahaslittletopographicrelief andisessentiallyundissectedbydrainagenetworksexceptalong theNileandKhargaescarpments.Theareaistraversedbyonly tworoads,theAssiut-ElKhargaRoadandtheWesternDesertRoad ( Fig.1 aandb),andmuchoftheareaisessentiallyinaccessible.The environmentishyperarid,modernrainfallisnegligible,andthe watertablelieshundredsofmetersbelowthePlateausurface ( Salim,2012 ). TheLimestonePlateauiscappedbyEocenelimestone,with underlyinglessresistantCretaceousthroughearliestEoceneshales, marls,andchalkexposedintheescarpmentsborderingthePlateau alongtheNileandKhargaValleys( Fig.3 ).UpperJurassicand Cretaceous uvialandshallowmarineclasticsedimentaryrockslie unconformablyonPrecambrianbasementatthebottomofthe sectionandareexposedintheKhargaValley. Thestratigraphiccolumnthatwepresentforourstudyarea ( Fig.4 )isbasedinpartonmeasuredsectionsintheescarpments ankingthePlateau( Kingetal.,thisissue;Said,1990;Issawi etal.,2009;Khalifaetal.,2004 )andinpartononewelldrilled inthe1960sapproximatelyhalfwayalongtheAssiut-ElKharga Road( BarakatandAsaad,1965 ).Awordisinorderaboutour choiceofstratigraphicnomenclature.EocenelimestonesinEgypt wereoriginallydividedintoanumberofformationsexposedin differentareas,andtheterm “ Thebes ” wasoriginallyappliedto oneoftheseformations. Klitzschetal.(1987) elevatedtheterm ThebestoGroupstatuswhentheydevelopedthe1:500,000scale geologicmapsofEgyptandassignedanewformationname,the Serai,towhathadbeentheThebesFormation.Inourarea,theEl Fig.1. Contextmaps,eastcentralWesternDesert,Egypt;locationshownwithrectangleoninsetmap.a)Satelliteimageshowingenigmaticdark “ wormy ” patterning.Othercolor patterningisduetograbensassociatedwithRedSearifting(redarrows)andtoaeolianfeatures(e.g.GhardAbuMuharikdune eld).b)Colorizedelevationmodelofareain Fig.1 a. Redarrowsshowsamegrabensas Fig.1 a.ElevationsofWesternDesertLimestonePlateauare250 e 350m.a.s.l.(yellowsandbrowns);NileandKhargaValleyelevationsare ~50 e 60m.a.s.l.(greens).SatelliteimageryfromArc2Earth(imagecreditEsri,DigitalGlobe,GeoEye,EarthstarGeographics,CNES/AirbusDS,USDA,US GS,AEX,Getmapping,Aerogrid, IGN,IGP,swisstopo,andtheGISUserCommunity);colorizedhillshadedevelopedfromShuttleRadarTopographyMission(SRTM)elevationdata.(Fori nterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 2 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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Rufuf,Drunka,andSeraionthesemapsarepartoftheThebes Group( Fig.3 ).The20051:250,000scalegeologicmapsofthe southernWesternDesert( Riadetal.,2005 )returntheThebesto Formationstatusanddeprecatetheterm “ Serai ” Kingetal.(this issue) advocatea “ ThebesLimestoneFormation ” containingallof thevariousfaciesofEocenelimestonesintheWesternDesert. WehavechosentoreturntotheThebesGroupterminologyof Klitzschetal.(1987) inpartbecausethe2005geologicmapsdo notcompletelycoverourmainstudyarea( Fig.3 ),andwehave reliedonthemapsof Klitzschetal.(1987) forbedrockgeology. Furthermore, BarakatandAsaad(1965) reportedalimestone sequenceabovetheEsnaShaleintheAssiut-Khargawell. Althoughtheselimestoneswereassignedformationnamesthat arenolongerinuseintheWesternDesert,thelimestonesareall partofKlitzschetal. ’ sThebesGroup.Intherepresentativestratigraphiccolumnin Fig.4 ,wehaveshowntheEocenelimestones simplyas “ ThebesGroup ” .AlthoughtheDrunkaFormationofthe ThebesGroupcapsthesectionintheareaswherewehavedone mostofourmapping,formationsinanyspeci cstratigraphic columnvarywithlocationinourbroaderstudyarea( Fig.3 ). Furthermore,essentiallynothingisknownaboutdetailsofthe stratigraphiccolumnundertheLimestonePlateauawayfromthe NileandKhargaEscarpments,sopresentingamoredetailed representativestratigraphiccolumnisimpossible. Fromthestandpointofourworkinmappingstructures,the mostimportantaspectoftheEocenelimestonesistheinterlayeringofunitstypicallyranginginthicknessfromlessthana metertoseveralmetersthatcharacterizetheformationsthat makeuptheThebesGroup(e.g., Kingetal.,thisissue;Khalifa etal.,2014;Khalifaetal.,2004 ).Differencesinerosionalresistanceamonglimestone,siliceousandconcretionarylimestone, argillaceouslimestone,andmarlaccentuatebeddingandproduce lowscarpsanddipslopesthatarevisibleinhighresolutionsatelliteimagery,makingitpossibleforustointerpretdipdirections Fig.2. a & b)LandsatMSScolorandLandsatETMpanchromaticsatelliteimagesof “ wormy ” patterning,locationshownin Fig.1 a.c-e)Sameareaatdifferentresolutions.Landsat MSScolor,~70m/pixel;LandsatETMpanchromatic,15m/pixel;DigitalGlobefromGoogleEarth,1 e 2m/pixel.Letter “ X ” marksthesamelocationineachoftheimages.f)~40Xzoom relativetoaandbshowingextraordinarylevelofdetailavailableforanalysisinGoogleEarth.Imagecenters:a-e)26.800261,30.742611;f)26.801 575,30.747456.(Forinterpretation ofthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 3 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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andtomapstructures. 2.2.Previousworkonbedrockstructures InthecentralWesternDesert( Fig.1 ),NNW-SSEgrabensand relatednormalfaultsassociatedwithOligocene/MioceneRedSea riftinghavebeenaddressedinmanypreviousstudies(e.g., Bosworthetal.,2015;Issawietal.,2009 andreferencestherein; Omaraetal.,1975 )andhaveappearedonmapsformanyyears(e.g., EGSMA,1981 ).Bycontrast,aspectsofbedrockstructureinthis regionthatcouldberelatedtothepervasivepatterningseeninthe satelliteimagery( Fig.1 )havebeenlittlestudied.Asfaraswehave beenabletodetermine,theonlypreviousstudythathasrecognized foldnetworkssimilartothosethatwepresentinthispaperiswork by Youssefetal.(1998) ,whomappedasetofsynclinesandanticlinesinEocenelimestoneinanareaabout25 40kmeastofthe NilenearSohagandinterpretedthemashavingformedbyslip alongblindNW-SEstrike-slipshearzones. Tewksburyetal.(2012) carriedoutapilotstudytoinvestigate theoriginofthecolorpatterninginasmallareaoftheThebes GroupatthecontactbetweentheElRufufandDrunkaFormations alongtheAssiut e ElKhargaRoad(locationshownin Fig.3 ).We de nedseveralgeneralunitsthathavedistinctivecharacteristicsin satelliteimageryandareusefulformappingremotely.By combiningmappingonsatelliteimagerywith eldveri cation,we demonstratedthathighresolutionimagerycanbeusedtotrace thesemappableunitsinthelimestones,determinedipdirections, de nefoldstructures,andmapfaults. Fig.5 illustratesaspectsofthisearlierworkthatarecriticalto ourcurrentstudy.Weinterpretedtheovalfeatureinthesatellite imagery( Fig.5 a)tobeasmalldoublyplungingsyncline(anelongatestructuralbasin)withtheerosionalremnantofaresistantpale brownunitde ningthebasinshape.Inthe eld,wecon rmed boththeinwarddipsofbedding( Fig.5 b)andthefactthaterosion hasremovedthepalebrownunitfromtheunderlyingwhiterock (stars, Fig.5 )exceptinthebasincore. Our eldworkalsodocumentedthedifferencesbetweenthe whiteunitinthesatelliteimageryandthepalebrownunit.The whiteunitisapurewhitelimestonethatislessresistantanderodes intoprominentyardangs(stars, Fig.5 ).Thepalebrownunitisa partiallysilici edlimestonethatismoreresistantthanthewhite unitanddevelopsprominentscarpsafewmetershighrimmingthe basin,aswellasdipslopeswithinthebasinitself( Fig.5 b e d).The palebrownunitiswhiteonafreshsurfacebutpalebrownoverall inoutcropandsatelliteimagerybecauseofaccumulationofdesert varnishonchertysectionsandfragments.Thepalebrownunitis alsocharacterizedbylargeconcretionsuptoameterormorein diameter( Fig.5 bande). Thesepalebrownandwhitemappingunitsarerepeated throughoutboththeDrunkaandElRufufFormations.Werecognize thattheseverygeneralmappingunits,whicharedistinguishablein thesatelliteimagery,donothavea1:1correlationwithspeci c repeatingsubunitsde nedinstratigraphicsectionsmeasuredin the eldbyothers(e.g., Khalifaetal.,2004;Kingetal.,thisissue ). Theyarestillusefulformappinginthesatelliteimagery,however, becausetheyfundamentallyre ectthedifferencesinrocktypeand erosionalresistanceofthecycliclithologiesthatmakeupthe Eocenelimestones. Fig.5 alsoshowstwootherfeaturescommonin thesatelliteimagery,streaksanddriftsoftanaeoliansandanddark graypatchesoflagdepositsthataredark-coloredduetodesert varnishonchertfragmentsinthelag. 3.Thisstudy Thisstudyextendsourexperienceinmappingalongthe Drunka-ElRufufcontacttoevaluatewhetherthepatterningin satelliteimageryoverlargeareasoftheWesternDesertre ects bedrockstructuresand,ifso,whattheoriginofthosestructuresis. Ourstudyareainitiallyencompassedanareaofabout20,000km2westoftheNileintheDrunkaandElRufufFormationsofthe ThebesGroup( Figs.3and4 )butultimatelyincorporatedareconnaissancesurveyoftheentireStablePlatformofEgyptinhigh resolutionimagerytoassesstheextentofdevelopmentofthe featureswediscovered. Fig.3. a)Generalizedgeologicmapmodi edfrom Klitzschetal.(1987) and Riadetal.(2005) ,withadditionsfromourownmapping.Brownrectangleshowsmainmappingareaof thisstudy;brownellipseshowslocationofpriorworkby Tewksburyetal.(2012) .b & c)SatelliteimageryfromGoogleEarthandhillshadedDEMdevelopedfromSRTMelevation dataforsameareashownin Fig.3 a.(Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 4 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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4.Dataandmethodology Ourprimarydatasetishighresolution(~1 e 2m/pixel)DigitalGlobesatelliteimagerythatisfreelyavailableinGoogleEarth. CurrentimageryinGoogleEarthformostofourstudyareaisrecent andhighresolution,althoughcontrastistypicallylow,whichis dif cultformappingandanalysis( Fig.6 a).EarlierGoogleEarth imagerythatisequallyhighinresolutionbutpansharpenedwitha differentalgorithmiscommonlyhigherincontrast( Fig.6 b)andcan beaccessedusingthehistoricalimagerysliderinGoogleEarth.We usethehighestcontrastimageryavailabletodoourmapping. Althoughtheseimagesdonotre ectreal-worldcolors,thebasic dataarestillthesame.Someportionsofourstudyareastillhave onlyCNES-SPOTimageryat3 e 5m/pixel( Fig.6 c),andwe augmentedthisimagerywith0.5m/pixelWorldView1panchromaticimageryprovidedbythePolarGeospatialCenteratthe UniversityofMinnesota.Forasmallportionofourstudyarea,we have0.5mDEMsproducedatOhioStateUniversityfromstereo WorldView3imagesthatallowustocalculateapproximatedipsfor well-exposeddippinglayersandtomeasureapproximatelayer thicknesses. SunilluminationinalloftheDigitalGlobeimagesinourstudy areaisfromthesoutheast,althoughshadowlengthvarieswiththe dateofimageacquisition.Weusemicro-topographicpatternsto interpretdipdirections,inthesamewaythatdipslopes,scarps, atirons,andVsinoutcroptracestwoordersofmagnitudelarger canbeusedinerodedfoldandthrustbeltstodeterminedipdirections.Wehavedetailedthemethodologyin Tewksburyetal. (2012) andpointouttheremarkablecombinationoffactorsthat makesthispossibleintheWesternDesert:layersafewmeters thickwithdifferentresistancestoerosion,verylittletopographic reliefwithmicro-scarpsafewmetershigh,micro-wadiseroded acrossthescarps,andessentiallynoregionaldissectionbyvalley networks. 5.Results 5.1.Mappableunitsandtheirfeatures Bedrockfeaturesareexceptionallywell-exposed,easilyvisible inthesatelliteimagery,andconsistentwithwhatweobservedin ourpilotstudyalongtheDrunka-ElRufufcontact( Tewksburyetal., 2012 ).Throughoutourstudyarea,weseemultiplesequencesof resistantpalebrownunitsandlessresistantwhiteunitsinthe satelliteimagery( Fig.7 a). Palebrownunitsformprominentdipslopesandscarps,anddip slopesarecommonlylitteredwithlargeconcretionsthatarevisible intheimageryasdarkspecklesonbeddingsurfaces( Fig.7 ainset). Whiteunitserodemoreirregularlyand,inmanyareas,displayvast eldsofyardangsformedbywinderosionalongaregionally prominentNNW-SSEjointset(bluedashedlinesin Fig.7 b).The whiteunitsalsotypicallyexhibitoneormoreadditionaljointsets, themostcommonofwhichisorientedWNW-ESE(reddashedlines in Fig.7 b).Thebrownrockunitstypicallydonotdisplayjointsthat arevisibleinsatelliteimagery. Fig.7 bshowsa eldofyardangswhereerosionbetweenyardangshasscouredallthewaythroughahorizontalwhiteunitdown tothepalebrownbedrockunitunderneath(redstarsin Fig.7 b). Thetopoftheresistantpalebrownunitformsanareallyextensive beddingsurfacewithyardangsandpatchesofremainingwhite limestonesittingontop. Fig.4. Representativestratigraphiccolumnforstudyarea;colorscorrelatewiththe generalizedgeologicmapin Fig.3 a.Coloredhorizontallinesshowgeneralstratigraphic positionsofunconformitiesdescribedbyseveralauthors(e.g., Khalifaetal.,2004 ).Red linesshowunconformitieswheresigni canterosionmayhaveoccurred.Bluelines designateparaconformitieswhereadepositionalhiatusoccurredbutwithoutsubaerialerosion.HalflineattopoftheDuwishowsalocalparaconformity.Oligocene/ MioceneKatkutFormationisincludedinthestratigraphiccolumnbutnotthegeologic map( Fig.3 )becauseKatkutgravelshavenotbeenreliablymappedacrosstheLimestonePlateau.Depthstospeci cformationsbasedprimarilyonAssiut-Khargawell ( BarakatandAsaad,1965 ),locatedinthemiddleofourstudyarea.DatafromElAzabi andFarouk,(2011),Kingetal.(thisissue), Said(1960,1962,1990),ElHinnawietal. (1978),Issawi(1972),Issawietal.(2009),KeheilaandEl-Ayyat(1990),KhalilandElYounsy(2003),Keheilaetal.(1990) ,and ElHinnawietal.(2005) .(Forinterpretation ofthereferencestocolourinthis gurelegend,thereaderisreferredtotheweb versionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 5 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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Fig.7 ashowsclearlythattheDrunkaFormationconsistsof manyinterlayersofwhiterockandpalebrownrockunits.Differenceinpixelelevationdeterminedfromthe0.5mDEM( Fig.7 c) betweenpoints1and2in Fig.7 ais1.3mwithanaccuracyof 0.4m.Thisthicknessforoneofthepalebrownunitsisconsistent bothwithwhatwesawinthe eldinourpilotstudy( Tewksbury Fig.5. Priorworkby Tewksburyetal.(2012) thatestablished eldbasisforcurrentwork.Redandpurplestarsshowsamelocationsinseveralimages.a)Smalldoublyplunging syncline(elongatestructuralbasin)coredbythinlayerofsilici edlimestone.Arrowsshowlookdirectionsfor eldphotosinb,c,ande.b)Fieldphoto(lookdirectionshownin Fig.5 a)showingshallowinwarddipslopesonsouthwestlimbofbasin.c)Viewlookingeastacrosserodedwhitelimestonewithyardangs(redstar)towardsca rpformedby southwestlimb.d)Scarp2 e 3mhighinsilici edlimestoneformingsouthwestrimofbasin,withunderlyingwhitelimestone(foreground).e)Largeconcretionsweatheringoutof silici edlimestoneatnoseofbasinin Fig.5 a.Purplestarshowsyardangsintheunderlyingwhitelimestone.Imagesource:GoogleEarth.Imagecenter:a)26.298122,30.735671. (Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 6 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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etal.,2012 )andwithwhathasbeenreportedforgeneralsub-unit thicknessesintheDrunkaFormation( Khalifaetal.,2004 ). 5.2.Synclinesandthesynclinenetwork Ourmappingoftheseunitsinhighresolutionsatelliteimagery revealsthatthelimestonesintheDrunkaandElRufufFormations donotdipuniformlyandgentlytothenorth,ashasbeenpreviously describedbut,instead,displayanetworkofthousandsofnarrow synclines.Wewill rstdescribethegeneralcharacteristicsofindividualsynclinesandthendescribethesynclinenetworkinfour places. Fig.6. ThreeimagerysetsavailableinGoogleEarthforsamearea.a)DigitalGlobeimagewithhighresolution(1 e 2m/pixel);lowcontrastmakesstructuralmappingdif cult.b) EquallyhighresolutionDigitalGlobeimage,accessedusinghistoricalimageryslider;highcontrastisidealforstructuralmapping.c)OlderCNES /SPOTimage;lowerresolution(4 e 5 m/pixel)notadequateformapping.Imagecentersareallthesame:26.848120,31.204321.(Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferred tothewebversionofthisarticle.) Fig.7. a)Fine-scaleinterlayeringofmoreresistantsilici edlimestone(palebrown,borderedbybrightlylitscarps)andlessresistantlimestone(white).Imageresolutionisadequate toresolveubiquitousconcretionsweatheredoutofsilici edlimestone(darkspeckles,insetimage).b)Whitelimestoneunitpartiallyerodedfromunderlyingquasi-horizontalpale brownsilici edlimestoneunit(redstars).Overlyingwhitelimestoneunitpreservedaspatchesandyardangs.ProminentjointtrendsWNW-ESE(reddashes)andNNW -SSE(blue dashes,paralleltoyardangs).c)HighresolutionDEMofsameareain Fig.8 aindicatessubunitthicknessof1 e 3m,consistentwithscarpheightsmeasuredin eldbyTewksburyelal. (2012).Imagesources:a & b:GoogleEarth;c:DEMfromDigitalGlobeWorldView3stereoimagerycourtesyofPaulMorin,PolarGeospatialCenter,andIanHowat,OhioState University.Imagecenters:a & c:26.283963,30.938284;b:26.309653,30.903450.(Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothe webversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 7 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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5.2.1.Characteristicsofindividualsynclines Fig.8 ashowsatypicalsyncline.Scarps,inward-facingdip slopes,anderosionalpatternsinmicro-wadisintheresistantpale brownunitprovideclearevidenceofasynclinalstructure.Onthe northside,darkscarpsandbright,southfacingdipslopesindicate SSWdips.Onthesouthside,brightscarpsandpartlyshadowed, northfacingdipslopesindicateNNEdips.Bothareconsistentwith dipdirectionsindicatedbytheVsandscallopsformedbymicrowadis( Fig.8 b). Wecalculatedapproximatedipanglesusingdifferencesinpixel elevationsinthe0.5mDEM( Fig.8 c)betweenthetopsandbottoms ofdipslopes.Calculateddipanglesonthenorthandsouthlimbs areontheorderof3 e 6 ~1( Fig.8 a).Atthesoutheastnose, calculateddipangleisabout2 ~1.Thesedipvaluesare approximateandlikelytobeabitunderestimatedbecausewe measurederosionaldipslopesratherthanactualbedding.Nevertheless,theseshallowcalculateddipsareconsistentwiththe shallowdipsthatwemeasuredinthe eldinourpilotstudy. Shallowdipinthenoseindicatesveryshallowfoldplunge,and thehinge “ porpoises ” alongtrend,formingmultiplebasinclosures alongthelengthofthesyncline.Theaxialsurfacetraceofthe synclinein Fig.8 aalsochangesorientationalongthelengthofthe syncline,fromNNW-SSEatthesoutheastendtoWNW-ESEinthe west.Thesynclinerangesfromabout100to300minwidthandis narrowestbetweenbasinclosures.Aresistantpalebrownunitlies inthecoreofthemainbasinandistopographicallyprominent ( Fig.8 aandc). Despiteveryshallowlimbdips,theoutcroppatternofthedippingsynclinelimbsisstrikinglydifferentfromtheoutcroppattern onlyabout100mawayfromthesynclinewherethelimestoneis horizontal( Fig.8 a).Inobliqueview,itisquiteclearthatmostofthe surroundingbeddingis,infact,horizontalandthatthesynclineis actuallyanisolateddownwarpinotherwise at-lyinglimestone ( Fig.8 d). 5.2.2.Anetworkofsynclines SynclinesacrosstheElRufufandDrunkaFormationsarelinked togetherinanetwork,and Fig.9 illustratesthegeneralcharacteristicsandscaleofthenetwork.Synclinesare150 e 350macrossand havemultiplebasinclosuresalongtheirlengths.Twodominant synclinetrendsarecommoninmostareas,althoughsubsidiary trendsdooccur.Synclinesofalltrendsbranchandmergeintoone Fig.8. Characteristicsofindividualsynclinesinsynclinenetwork.a)Dipslopes,Vsinmicro-wadis( Fig.8 b),andscallopsinmicrohogbackridgesshowinwarddipsthatde nea narrowsyncline.Samesynclinesymbologyas Fig.5 a.DipscalculatedfromelevationsinhighresolutionDEM( Fig.8 c).Within100mofthesyncline,dipsinlimestonelayersare horizontal(H),asindicatedbyirregularoutcroptracesofjointedwhitesubunit.b)Colorizedelevationhillshadeshowslowrelief(lightgreen~3 07 e 310m.a.s.l.,darkgreen ~315 e 317m.a.s.l.)butclearlyre ectssynclinestructureandhorizontallayering(H)awayfromthesyncline.d)ObliqueviewinGoogleEarthshowssyncline(brownlineshowsaxial surfacetrace)asanisolateddownwarpinotherwise at-lyinglimestone.Imagesources:a,b, & d:GoogleEarth;c:DEMfromDigitalGlobeWorldView3stereoimagery,courtesyof PaulMorin,PolarGeospatialCenter,andIanHowat,OhioStateUniversity.Imagecenters:a & c:26.282978,30.954699;b:26.284189,30.952856.(Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 8 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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anotherandcurvealongtrendfromoneorientationtoanother ( Figs.8and9 ).Basinclosuresarecommonlylocatedwheretrends mergeorintersect. Fig.9 aalsoshowsthecorrelationbetweenthe orientationofthetwodominantjointsets(dashedlinesin Fig.9 a) andthedominantsynclinetrends. Synclinesarethemostwell-de nedinsatelliteimagerywhere atleastonepalebrownunitisinvolved.Thisresultsinpartfromthe colorcontrastwhereawhiteunitliesadjacenttoasynclineinmap viewandinpartfromthefactthatthepalebrownunitsaremore resistantandformscarpsanddipslopesinthesynclinesthatare easilyvisibleinthesatelliteimagery.Becausesynclineshave porpoisinghinges,synclineshavedeeperkeelsinsomeplacesthan others,andportionshavingmorelayersinthecorealsohave deeperkeels.Synclineswithveryshallowkeelscanbe “ ghosty ” and dif culttosee,especiallyiftheyinvolveonlywhiteunits. 5.3.Synclinemapping Wemappedtheaxialsurfacetracesofallsynclinesinthetwo areasshownin Fig.10 usinghighresolutionsatelliteimagery.The largerareacoversapproximately4000km2inlimestonesofthe ThebesGroupandextendsfromsouthoftheDrunka-ElRufuf contactnorthintotheDrunkaFormation.Thesecondareais smaller(~300km2)andliesabout40kmsouthofAssiutjustwestof theNileescarpment.Itisclearin Fig.10 thatthecharacterofthe synclinenetworkvariesacrosstheregion,andwewillfocusonfour areastopaintapictureofsimilaritiesanddifferences. 5.3.1.Area1(centralDrunkaFormation) Inthenorthernandcentralportionofourlargermappingarea (Area1, Fig.10 ),synclinesarespacedwidely,typically1 e 3kmapart ( Fig.11 a).Despitethefactthatthesesynclinescommonlycontain sur cialdepositsthatformthedark-colored “ worms ” insatellite imagery( Fig.11 ),zoominginonthehighresolutionimageryreveals thatthesesur cialdepositsareverythinandthattheinward dippingbedrocklayersthatde nethesynclinestructuresarestill easilyvisible( Fig.11 binset). Twosynclinetrendsaredominant,NNW-SSEandWNW-ESE, andsynclinesfromthetwotrendsbranchandmerge.Although synclinesdointerconnect,thenetworkis,infact,discontinuous, andindividualsynclinescommonlyterminateabruptly,asatthe northwesttipofthe “ herringbonesyncline ” in Fig.11 b.Syncline widthsrangefrom150to400m. Betweenthesynclines,thelimestonelayersarehorizontal. Fig.11 cshowsanextensivehorizontalbeddingsurfaceofoneofthe resistantpalebrownunits.Remnantsofanearlycompletelyeroded overlyingwhiteunitsitonthebeddingsurfaceaspatchesandsmall yardangs(whitearrowsin Fig.11 c),andthecontactwiththeunderlyinglessresistantwhiteunithasthetypicaldendriticoutcrop patternofahorizontalcontact(redarrowsin Fig.11 c).Zoomingin alsorevealshorizontalbeddinginthelargeryardangs( Fig.11 c inset).Theonlydippinglayersinthesceneareassociatedwithtwo smallisolatedbasinsandanE-Wsyncline(starsin Fig.11 c).In short,overlargeareasofthecentralandnorthernportionsofour mappingarea,synclinesaretheonlyfoldstructures e thereareno companionanticlines.Synclinesformisolateddownwarpsin otherwisehorizontallimestone. 5.3.2.Area2(Drunka-ElRufufcontactregion) Inthesouthernpartofthemaparea(Area2, Fig.10 ),synclines aremorecloselyspaced(typically200 e 500mapart)thantheyare inthenorthernandcentralportions,althoughindividualsynclines aresimilarinscale(150 e 300mwide)tothoseelsewhere( Fig.12 a andb).Asinthenorthernzone,beddingishorizontalbetween synclines( Fig.12 ainset).Wheresynclinesarecloselyspaced, adjacentsynclinelimbsdode nean “ anticline ” ( Fig.12 b), commonly at-topped,inbetweenthesynclines,butanticlinesare“ accidental ” andsimplyaconsequenceofclosesynclinespacing. Synclinesinthisareahavetwodominantorientations( Fig.12 a andc),WNW-ESEandNNW-SSE(nearlyN-Sinsomeplaces).The twodominantsynclineorientationsareparalleltotwoprominent jointsetsinthewhitelimestone(dashes, Fig.12 b).Aselsewhere, jointsarenotvisibleinsatelliteimagesofthepalebrownunit. ThecontactbetweentheDrunkaandElRufufFormationsliesin thiszoneofcloselyspacedsynclines,andweusedsatelliteimagery tomapthecontactindetail( Fig.12 c).Themappatternofthe contactiscomplex,withoutliersofDrunkaintheElRufufandinliersofElRufufintheDrunka( Fig.12 a e d).SouthoftheDrunka-El Rufufcontact,awhiteunitatthetopoftheElRufufdominates,but Fig.9. Interconnectedsynclinesformanetwork.a)Mainsynclinetrendsareparallelto twoprominentjointsets(whitedashes).Synclineshavemultipleelongatebasinclosuresalongtheirlengths;basinclosureswithdeeperkeelsarecommonatintersections.b)ObliqueviewinGoogleEarthlookingNEacross Fig.9 a.Brownlines showaxialsurfacetracesofsynclines.Imagecredit:GoogleEarth.Imagecenter:a) 26.177166,30.970082.(Forinterpretationofthereferencestocolourinthis gure legend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 9 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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thekeelsofnarrow,doublyplungingsynclinescreatediscontinuousoutliersofpalebrownDrunka( Fig.12 aandinset).Northofthe contact,thepalebrownunitatthebaseoftheDrunkadominates, butinliersofElRufufappearwhereerosionhasbreachedthepale brownDrunkaunitandexposedawhiteunitoftheElRufufinthe broad,accidentalanticlinesandblockydomesthatliebetween closelyspacedsynclines. Thisareadisplaystwodistinctlydifferentsynclinedomains ( Fig.12 c).DomainAiswideanddominatedbynarrowWNW-ESE synclines.DomainBisnarrowandhasthesameWNW-ESEsynclinetrendasdisplayedinDomainAbutalsodisplaysasetofN-Sto NNW-SSE-trendingnarrowsynclines.TheresultisachocolatetabletarrangementinDomainBofblocky,broad, at-topped domesseparatedbyinterconnectednarrowsynclines( Fig.12 d Fig.10. Axialsurfacetracesofallsynclinesinthetwooutlinedregions.Overalllocationshownin Figs.1aand3 a.DetailsofsynclinesinAreas1 e 4appearin Figs.11 e 13 and Fig.2 e e f respectively.SatelliteimageryfromArc2Earth(imagecreditEsri,DigitalGlobe,GeoEye,EarthstarGeographics,CNES/AirbusDS,USDA,USGS,AEX ,Getmapping,Aerogrid,IGN,IGP, swisstopo,andtheGISUserCommunity).(Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 10 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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ande).DarkerkeelsoflowerDrunkade nethesynclines,andthe whiterockoftheElRufufisexposedinthebreachedareasbetween thesynclines( Fig.12 dande).ClustersofNNW-SSE enechelon fault segmentsarearrayedalongroughlyN-SzonesinDomainB.Allbut theeasternmostoftheN-S “ B ” zonessteptheDrunka-ElRufuf contactdowntotheeastbyacombinationoffaultingandbroad monoclinal exure,accountingforthegenerallysouthward migrationfromwesttoeastoftheoutcroptraceofthecontactin the “ A ” blocks.Thefarthesteast “ nger ” isdownonbothsides. Fig.12 dandealsoshowthattheN-Ssynclinesinthe “ B ” zones connecttocloselyspacedWNW-ESEsynclinesina “ ladder ” geometry,withbasinclosuresattheintersections.WNW-ESEsynclinehingesplungetowardtheN-Ssynclinesandsomepeterout altogetherawayfromtheN-Ssynclines.TheN-Ssynclinesalso showmorelayersintheircoresthandotheWNW-ESEsynclines, indicatingdeeperkeelsalongtheN-Ssynclines. 5.3.3.Area3(northeasternDrunkaFormation) InthesmallmappingareasouthofAssiut,synclineshavethe samescaleandcharacterasthoseinAreas1and2,butNNW-SSE synclinesdominatethenetworkandareconnectedbysubsidiary WNW-ESE,E-W,andENE-WSWsynclines(Area3, Fig.10 ).NNWSSEsynclinesaretypicallyspacedonlyafewhundredmetersto lessthanakilometerapart,resultingininter-synclineareaswith broadlyanticlinalcharacter( Fig.13 a).Theobliqueviewsin Fig.13 b andcshowthattheseinter-synclineareasaredominatedbyhorizontalbedding,withdippingbeddingonlyimmediatelyadjacent tothenarrowsynclinesthemselves.Weranseveralaudiomagnetotelluricsurveysacrosssynclinesinthismappingarea anddeterminedthatzoneswithlowelectricalresistivityunderlie thesynclinesalongthesurveylinesatdepthsrangingfromabout 100mtomorethan400mbelowthesurface( Tarabeesetal.,this issue ). 5.3.4.Area4( “ catastrophic ooding utes ” ) Intheintroductiontothispaper,wementionedthatseveral authorshaveproposedthatthedarkwormypatternsinsatellite imageryacrossthispartoftheWesternDesertareerosional utes fromcatastrophic oodingacrosstheLimestonePlateau.High resolutionimageryreveals,however,thattheseproposedcatastrophic ooding utesareactuallyde nedbythesamekindand scaleofnarrowsynclineswithmultiplebasinclosuresthatcharacterizeallofourothermappingareas( Fig.2 eandinset).NNW-SSE trendsdominatehere,althoughsubsidiaryENE-WSWtrendsdo occur.Fig.2 falsoshowsfaultoffsetsoflimestonelayersinoneof thesynclines,con rmingthatthesearebedrock,noterosional, features. 5.4.Extentofsynclinedevelopment InadditiontodoingdetailedmappingintheDrunkaandupper ElRufufFormations,wecarriedoutacountry-widereconnaissance surveyusinghighresolutionsatelliteimagerytodeterminethe extentofsynclinedevelopmentacrossEgypt( Fig.14 ).Thesynclines andsynclinenetworksarenotalocalphenomenon e theyoccur, albeitsporadicallydeveloped,overanareaofnearly100,000km2. SynclinesaredevelopedpreferentiallyintheEocenelimestonesof Egypt(mediumpinkonthegeologicmapin Fig.14 ).Althoughnot equallywell-developedeverywhereintheEocenelimestones, synclinesaresimilarincharacterandscaleeverywheretheyoccur. Synclinesarethekeystructures e theyformisolateddownwarpsin otherwise at-lyinglimestonewithoutanticlinesinbetween. Althoughsynclinesoccurinavarietyoforientationsinallofthe networks,WNW-ESEandNW-SEorNNW-SSEorientationsdominateinvirtuallyalloftheregions. Fig.11. CharacteristicsofsynclinesinArea1(locationshownin Fig.10 ).a)Synclines arenarrowbutwidelyspacedanddiscontinuous;WNW-ESEandNNW-SSEtrends dominate.b)Darksynclinecoresshowdippingbedrocklayers(inset).Samesyncline symbologyas Fig.5 a.c)Limestonelayersarehorizontalbetweensynclines.Erosional remnantsofawhitelimestonelayer(whitearrows)lieonunderlyinghorizontalbrown unit,whichhasanirregularoutcroptrace(redarrows)withunderlyingwhiteunit. Yardangsalsoshowhorizontalbedding(inset).Theonlylocationswithdippinglayers aretwosmallisolatedbasinsandasmallsyncline(whitestars).Imagecredit:Google Earth.Imagecenters:a)26.596757,30.945977;b)26.629558,30.929505;c) 26.558700,30.886916.(Forinterpretationofthereferencestocolourinthis gure legend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 11 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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Fig.12. CharacteristicsofsynclinesinArea2alongcontactbetweenElRufufFormation(south)andDrunkaFormation(north).Locationshownin Fig.10 .a)Awhiteunitliesattop ofElRufuf(E)andabrownunitatbottomofDrunka(D).NarrowWNW-ESEandNNW-SSEbrownpatchesaresynclinaloutliersofyoungerDrunka(D)inthetopo ftheElRufuf(E). Layersarehorizontalbetweensynclines(H,inset,obliqueviewlookingSEinGoogleEarth)exceptimmediatelyadjacenttosynclinelimbs( Fig.1 b).b)Broadwhiteareasareinliersof olderElRufuf(E)whereerosionhasbreachedareasbetweennarrowsynclinescoredbyDrunka(D).Synclinesareparalleltotwoprominentjointsets(r eddashes).c)Syncline network,faults,andcontactsinArea2.ColorsforDrunkaandElRufufFormationssameasin Figs.3and4 .Mappedoutcroptraceofcontact(blue)iscomplexbecauseofnetworkof closelyspacedsynclines. “ A ” domainsaredominatedbyWNW-ESEsynclines; “ B ” domainsarenarrowerandhaveWNW-ESEsynclinescrossedbyNNW-SSEtoN-Ssynclines paralleltofaultsofsimilarorientation.d) “ B ” domainshaveblocky, at-topped “ accidental ” domesbetweensynclinesofthetwotrends.e)ObliqueviewlookingNWinGoogleEarth ofladder-likenetworkofsynclines.Imagecredit:GoogleEarth.Imagecenters:a)26.167041,30.855670;b)26.104511,30.924132;d:26.083510,30 .953724.(Forinterpretationofthe referencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 12 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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Fig.13. CharacteristicsofsynclinesinArea3(locationshownin Fig.10 ).a)Samesynclinesymbologyas Fig.5 a.Wheresynclinesarecloselyspaced,inter-synclineareasarebroadly domaloranticlinal(redstars)but “ anticlines ” areanaccidentofclosespacingofadjacentsynclinelimbs.b)Wheresynclinesaremorewidelyspaced,beddingishorizontal.Oblique viewinGoogleEarthlookingENE,showinghorizontalbeddinginyardangsinforeground.Redstarscorrelatewith Fig.13 a.c)ObliqueviewinGoogleEarthshowingthatbeddingis horizontal(H)exceptimmediatelyadjacenttosynclines.Imagecredit:GoogleEarth.Imagecenter:a)26.850487,31.096667.(Forinterpretationo fthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) Fig.14. LocationsofareasinEgypt(blueasterisks)withsynclinessimilartothoseinourmainmappedsynclinenetwork(brownellipse).Synclinesarelocal izedinEocenelimestone (mediumpink).GoogleEarthimagesshowthatsynclinesaresimilarincharacteristicsandorientationovertheentirearea.Synclinesshowninsouth ernEgypt(lowerright,three images)donotliealongthemajormappedE-WandN-Sfaultsoftheregion.Mapadaptedfrom EGSMA,1981 .Imagecenters,clockwisefromupperright:28.533771,31.248534; 27.708135,31.256808;26.962901;31.695877;24.536072,31.846536;24.283936,31.772569;24.333132,31.470419;26.700149,29.922060;27.7284 06,28.743558;28.480802, 28.639428;28.332968,26.834424.(Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 13 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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5.5.Timingofsynclineformation 5.5.1.AgeofsynclinesrelativetoPleistocene/Holocenedrainage networks AwadisystemdissectstheedgeoftheLimestonePlateauboth eastandwestoftheNile.Highresolutionimageryshowsclearly thatthesynclinenetworkisunrelatedtothehydrologicsystemthat producedthewadis. Fig.15 aandbshowsdeepwadiscuttingacross andparalleltothesynclines,andthewadisareconspicuouslynot coincidentwiththesynclines.Thelackofcorrelationindicatesthat thesynclinesareunrelatedtothePleistocene/Holocenehydrologic systemandwerepresentinthelimestoneswhenthewadis dissectedthePlateau. WherewadiseastofSohagcutdeeplyintothesynclinenetwork, highresolutionimageryplusthe3DterrainviewinGoogleEarth allowustoseeabitofwhatunderliesthesynclines.Wherethe coresofsynclinesarevisibleinwadicliffexposures,weconsistently seecontinuouslayersbeneaththesynclinecores( Fig.15 aandb). 5.5.2.Ageofsynclinesrelativetopre-Nilegraveldeposits Thegeologicmapin Fig.3 showsanumberofPlio/Pleistocene andHolocenesedimentaryunitsthatoccurintheNileValley,on theescarpment anks,andinthewadisthatdissecttheedgeofthe LimestonePlateau.Inadditiontothoseunitsshownonthemap, twoclasticsedimentaryunits,theKatkutFormationandtheAbu RetagFormation,lieovertwohundredmetersabovetheNileValley ontheLimestonePlateausurfaceitselfandsitunconformablyon limestoneoftheDrunkaFormation.TheKatkutFormationwas originallyde nedby Issawietal.(1999) andappliedto uvial graveldepositsdevelopedsporadicallyacrosstheLimestone Plateau.Mahranetal.( 2013 ,ascitedin AbuSeif,2015 )proposedthe nameAbuRetagFormationforasetof uvialgravelsthatare youngerthantheKatkut.Wehavenotincludedeitherunitonthe geologicmap e althoughtheyhavebeenwellmappedinplaces neartheNileValleywestofSohag( AbuSeif,2015 ),theyhavebeen poorlymappedelsewhereontheLimestonePlateau. SouthofSohag,highresolutionsatelliteimageryshowsvery clearlythatgravelsoftheKatkutFormationlieunconformablyon erodedsynclinesintheDrunkaFormation( Fig.15 c),indicatingthat thesynclineswerepresentintheDrunkaFormationandthatan erosionalsurfacehaddevelopedonthesynclinesbeforetheKatkut gravelsweredepositedonthem. TheabsoluteagesoftheKatkutandtheAbuRetagFormations arepoorlyconstrainedbecausetheylackindexfossils.Bothpredatedevelopmentofathrough-goingNileintheLateMiocene, and Issawietal.(1999) assignedanOligoceneagetotheKatkut. AbuSeif(2015) andMahranetal.( 2013 ,ascitedin AbuSeif,2015 ) suggestthattheKatkutmightbeasyoungasEarlyMiocene becausesomesectionssouthwestofSohagweredepositedinNWSEgrabens,whichwerepresumablyassociatedwithRedSearifting. AbuSeif(2015) notesthepresenceofPrecambrianbasementpebblesintheAbuRetagbutnottheKatkut,suggestingthattheKatkut predatesmajorupliftintheRedSeaHills.Onthisbasis, AbuSeif (2015) suggestsaLateMioceneagefortheAbuRetag. 5.5.3.AgeofsynclinesrelativetoRedSeaRift-relatedfaulting SynclinesarecutandoffsetbyNW-SEtoNNW-SSEnormal faultsassociatedwithextensionintheWesternDesertduring riftingoftheRedSea.DespitegenerallysimilartrendsbetweenRed SeaRift-relatedfaultsandoneofthemainsynclinetrends(NNWSSE), Fig.16 ashowsclearlythatsynclinesofallorientations, includingonesorientedNNW-SSE,werepresentinthelimestones beforethefaultsdeveloped. Bosworthetal.(2015) havesuggested thatrapidunzippingoftheRedSeaca.23MaattheOligocene-Mioceneboundarygeneratedonlyshort-livedregionalextension intheWesternDesertandthatfaultingceasedshortlythereafter. Wepointedoutearlierthatthelimestonesdisplayprominent WNW-ESEandNNW-SSEjointsetsandthatthetwomainsyncline trendsareparalleltothesejointsets.Wealsoobservemanysmall NNW-SSEmicrofaults( Fig.16 b).Theclosespacingandsmall,dip slipoffsetssuggestreactivationofanolderNNW-SSEjointset duringRedSeaRift-relatedextensionintheWesternDesert. 6.Interpretations Thegeometriesandscalesofsynclinesinourmappednetwork Fig.15. a & b)ObliqueviewsinGoogleEarthshowlackofcorrelationbetweenwadis andsynclines,indicatingthatsynclinesareunrelatedtoPleistocene/Holocenehydrologicsystem.Synclinecoresshowcontinuouslayerswherebreachedbywadierosion. c)KatkutFormationgravelslieunconformablyonerodedsynclines.Imagesource: GoogleEarth.Imagecenters:a)foregroundat26.955176,31.701103;b:foreground 26.645921,31.861146;c)26.121816,31.625994.(Forinterpretationofthereferencesto colourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 14 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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areverydifferentfromthoseintypicalregionaltectonicfoldsystems.Ineveryplacewehaveexaminedthesynclines,wehavebeen struckbyhowsimilarallthesynclinesare e widthsof150 e 400m, shallowlimbdips,noparasiticfolds,nolargerstructures.Anareaof 4000km2inafoldandthrustbeltsuchastheZagroswouldtypicallycontainadozenwhalebackanticlinesandrelatedsynclines, whereasourmainmappingareaintheDrunkaFormationcontains over1000synclines.Furthermore,overlargeareas,synclinesare “ theonlydealintown ” ,with at-lyinglimestonelayers,ratherthan anticlines,inbetweenthesynclines.Inter-synclineareasthatare broadlyanticlinalincharacterareduetoproximityofthelimbsof twoadjacentsynclines,ratherthantoanactiveanticline-forming mechanism. Thesynclinenetworkisalsounliketheprominentdomeand basinstructuresthatareco-locatedwithandgeneticallyrelatedto sliponthemajorlong-livedE-WandNNEfaultsthathavebeen previouslystudiedbymanyworkersfarthersouthinEgyptinthe southcentralWesternDesert(e.g., Issawi,1968;ElHinnawietal., 1978;Sehim,1993;Youssef,2003;Alfarhanetal.,2006; Tewksburyetal.,thisissue ).Ratherthanbeingco-locatedwith faults,thesynclinesthatwehavemappedintheThebesforman areallyextensivenetworkoflong,narrow,curving,andbranching synclines( Fig.10 )inlimestonebedrockthatlargelylacksthelonglived,through-goingfaultsthatcharacterizetheWesternDesertto thesouth.Thefactthatoursynclinesaredevelopedasanetwork oversuchalargeareaalsosuggeststhatitisnotareasonablemodel toproposethateachsynclineisrelatedtoablindstrike-slipshear zoneatdepth,whichwassuggestedby Youssefetal.(1998) fora smallareaintheDrunkaFormationeastoftheNile. ThesynclinesintheThebesarenottypicaltectonicstructures, andwesuggestthattheyarebestdescribedassagstructures.A viablemodelwouldneedtooperateinamannersimilartomine collapse,wherebroadsubsidenceresultsfromarelativelysmall amountofvolumereduction/removalofmaterialatdepth.Sag aboveminecollapsecanoccurinlayershundredsofmetersabove thehorizonofvolumereduction( R.Loucks,pers.comm.,2015 ),and thesurfacecommonlyshowslittlebutsagoflayers,although faultingcanoccuronshoulders.Werequireasimilar,butnatural processthat1)isnon-tectonic,2)operatedonaregionalscaleina fairlynarrowtimewindowbetweentheendoflimestonedepositionintheMiddleEoceneandOligoceneorMiocenedepositionof theKatkutFormationandextensionalfaultingrelatedtoRedSea rifting,3)developedstructureswithconsistentorientations controlledbyprominentjointsets,and4)isconsistentwithboth thenatureoftheunderlyingstratigraphicsequenceandthe observationthatsynclinesarecon nedtoEocenelimestones. Ourdataareadmittedlylimitedforevaluatingpossiblemodels fortheoriginofthesynclinenetwork.Consequently,wehave chosentopresentanumberofmodelsandtoevaluateeachin termsofhowlikelyitistobeaviablemechanism. 6.1.Epigenic/vadosezonekarst Epigenicspeleogenesisisthecave-andsinkhole-formingkarst processwithwhichgeologistsaremostfamiliar.Dissolutionfeaturesformbydescendingandlaterallymovinggroundwater,and aggressivenessofgroundwaterwaterwithrespecttocarbonate rocksisacquiredinthesoilzone.Processesaredirectlyrelatedto contemporarysurfacetopography,andcaves,passages,and collapseareallnear-surface. Collapseofepigenickarstisacommoncauseofsubsidencein limestoneterrains,andpotentialcollapseofknownPleistocene/ Holoceneepigenickarstfeaturesdoes,infact, gureprominentlyin moderngeotechnicalengineeringconsiderationsforexpansionof citiesinEgyptoutsidetheNile oodplain(e.g., Abdeltawab,2013; AbdelAatiandShabaan,2013;Ashraf,2012 ).Roadcutsandcliff exposuresintheNileEscarpmentshowabundantshallowdissolutionfeatures,smallcaves,grikes,terrarossa,andshaftsandsolutioncavitiesin lledwithconglomerate,sand,andbreccia ( Mostafa,2013 ).Oursynclinenetworkcannot,however,berelated totheseyoungepigenickarstfeatures,becausegravelsofthe Oligocene(Miocene?)KatkutFormationunconformablyoverlie erodedsynclinesofournetwork. WhataboutepigenickarstcollapseduringtheOligocene,when theclimateinEgyptwaswetterandriversmeanderedacrossa broad,lowlandsurface(e.g., BownandKraus,1988;Rasmussen etal.,2001;Holmesetal.,2010 )?Althoughwecannotrulethis outasamechanismforgeneratingoursynclinenetwork,wethink itisunlikelybecauseourevidencesuggeststhatwhatevercaused thesagwasnotanear-surfacephenomenon.Despiteaminimumof manytensofmetersoferosionofEocenelimestonesincetheearly Oligocene(andMacgregor,2012 ,suggests200mremovedby erosion),weseenoevidenceofsinkholecollapseintheplaces whereitisthemostlikelytohaveoccurred,namelyinthecoresof structuralbasinsalongthesynclines.Insteadofchaoticbrecciain thecoresofbasins,weseecoherentbeddedlimestonethat commonlysitsmanymetersabovethesurroundingerosionsurface ( Fig.8 ).Furthermore,inwadisexcavatedmanytensofmetersdeep acrossthecoresofsynclinesandbasins,weseecontinuous, coherentlimestonelayers,ratherthancollapsebreccias( Fig.15 ). Fig.16. a)NNW-SSE-strikingfaultsrelatedtoRedSeariftingoffsetsynclinesofall orientations.b)CloselyspacedNNW-SSEmicrofaultsmayhaveformedbyreactivation ofaprominentjointsetinthelimestonesduringRedSearifting.Imagesource:Google Earth.Imagecenters:a)26.792324,31.071276;b)26.901297,31.129621.(Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothe webversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 15 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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6.2.Deepdissolutionofevaporites QatarandeasternSaudiArabiadisplaysagsynclinenetworksin theEoceneDammamandMioceneDamFormationsthatare strikinglysimilartooursynclinenetworkinEgypt(forcomparison, see Tarabeesetal.,thisissue ).Previousworkershaveestablished thattheQatarstructuresformedasaresultofdissolutionofunderlyingEoceneevaporitesoftheRusFormation,withaccompanyingsagintheoverlyinglimestonelayers( Prost,2014;Sadiqand Nasir,2002;Cavelier,1970 ). Stewart(2015) describessimilar structuresineasternSaudiArabiaresultingfromsubsurface dissolutionofRusFormationevaporitesandsaginoverlyinglayers. StructuresofsimilargeometryandscalealsooccurinthePecos ValleyofNewMexicoandWestTexas,USA,andwereformedby dissolutionofsubsurfaceevaporitesintheCastileandSaladoFormations(e.g., Staffordetal.,2008;LandandLove,2006;Motts, 1962 ). Deepdissolutionofevaporitesisanunlikelymechanismfor formationofthesynclinenetworkinEgypt,however,despitethe similaritiesingeometryofbothsetsofstructures.Nolayered evaporiteshaveeverbeenreportedinthestratigraphicsection underlyingtheEocenecarbonatesinEgypt(e.g., BarakatandAsaad, 1965;Issawietal.,1999,2009 ),andproposingsuchamechanism wouldrequiresuggestingthatevaporiteswerepresentatonetime buthavebeencompletelyremovedbydissolution,whichistheoreticallypossiblebutdoesnotmakeaverysatisfyingargument. Itisintriguingtospeculate,however,whetheranyoftheshales underlyingtheThebesmighthavebeendepositedinanenvironmentconducivetothenucleationandgrowthofdisplacivehalite afterdeposition. Benisonetal.(2015) describeshalesthat,indrill cores,consistofupto80%displacivehalitebutthat,inoutcrop,look likerathernormalshaleswithabitofanhydrite,becausegroundwaterhaslongsincedissolvedthehaliteandcollapsedthesedimentbacktoitsoriginalvolume.Localizeddisplacivehalite dissolutionalongfaultsandfractureswarmsinshalesunderlying theThebes,withaccompanyingsaginoverlyinglimestonelayers,is worthentertainingasaninterestingspeculation,althoughitwould requiredrillcorestoinvestigatethepresenceofdisplacivehaliteat depth. 6.3.Collapsed,coalescedpaleokarst Whenanextensiveepigeniccavesystemformsduringlongtermexposureoflimestoneandsubsequentlyundergoessubsidenceandburial,theweightofnewsedimentlayerscancause collapseoftheoldburiedpaleokarstsystem( Loucks,1999,2007; McDonnelletal.,2007 ).Paleocavesandpassagescoalesceon collapse,creatingsagsynclinesthataremuchwiderintheyounger sedimentarycoverthanindividualcavesandpassagesintheoriginalpaleokarst.Thescale,features,andgeometriesofoursyncline networkarestrikinglysimilartofeaturesintheEllenburgerGroup ofWestTexas,USA,thathavebeeninterpretedby Loucksetal. (2004) and McDonnelletal.(2007) ascollapsed,coalesced paleokarst. CollapseofpaleokarstisunlikelyforourareainEgypt,however, becausethestratigraphicrecorddoesnotcontainunconformities thatre ectlong-termsubaerialexposureofcarbonatespriorto depositionofthelimestonesthatcontainthesagsynclines( Fig.4 ) ( ElAzabiandFarouk,2011;KeheilaandKassab,2001;Aubry,pers. comm.,2016 ).Although uctuatingsealevelsdidplacecarbonate horizonswithinthefreshwaterphreaticzone(e.g., Khalifaetal., 2004 ),thestratigraphiccolumncontainsneitherevidenceofsubstantialdissolutionduringtheseintervalsnorevidenceofareal extentlargeenoughtoaccountforoursynclinenetwork. 6.4.Silicadiagenesisabovepolygonalfaults Davies(2005) investigatedtheoriginofahummockyterrainin thesubsurfaceoftheNorthSeathatdisplaysanetworkofnarrow synclinesseparatingbroad at-toppeddomesakilometerorso across.Hesuggestedthatwarm uidsrosealongpolygonalfaults andintooverlyingunits,triggeringlocalizeddiagenesisofbiogenic silicawithsigni cantvolumelossandporespacecollapseaccompanyingconversionofOpalAtoOpalCT.Volumelosscausedsubsidencelocalizedabovethepolygonalfaults,formingapolygonal networkofnarrowsynclinesandaccidentaldomesinbetween. Oursynclinenetworkisstrikinglysimilarinbothscaleand synclinecharacteristics,althoughthenetworkpatternisnot beautifullypolygonal,asitisintheNorthSea.OurEocenelimestonesdo,infact,overliesequencesthatcontainabundantshale andchalk,theonlyrocktypesthathostpolygonalfaults ( Cartwright,2011 ).Furthermore,biogenicsilicacouldhavebeen thesilicasourcefortheabundantchertintheWesternDesert limestones,althoughnosiliceousmicrofossilshavebeenfoundin theDrunka,andthesourceofthesilicaisdebated(e.g., AbuElGhar andHussein,2005 ). Wethoughtthatasilicadiagenesismodelmightbeplausiblefor localizedvolumereductioninourlimestonesandsagofoverlying layersuntilweranouraudio-magnetotelluricsurveys,which indicatethatlowresistivityzonesunderliethesynclines( Tarabees etal.,thisissue ).Itisdif culttoseehowalowresistivityzone wouldbeconsistentwithlimestoneunderthesynclinesthatis denserandlessporousduetoanOpalAtoOpalCTtransition. 6.5.Downslopemobilizationofunderlyingshales BecauseourEocenelimestonesareunderlainbyseveralLate CretaceousandPaleogeneshaleunits( Figs.3and4 )thathavethe potentialforinstability,wehaveexploredthepossibilitythat mobilizationinunderlyingshalesmighthavecreatedsagstructures intheoverlyingEocenecarbonates. Lsethetal.(2011) describe suchstructuresintheNorthSeaformedbyhighPf-inducedgravity glidinginorganic-richblackshalesafterdepositionofanoverlying sequence.Theblackshalesexhibitstrata-boundnormalfaults,with drape/sagfoldsinoverlyingunits. Moscardellietal.(2012) have alsoreporteddown-slopemobilizationofshalesintheGulfof Mexicothatdistortedanexistingnetworkofstrata-boundpolygonalfaults,causinganetworkpatternofsaginoverlyinglayersas underlyingpolygonalfaultswerereactivated.Thelowresistivity zonesbeneaththesynclinesinouraudio-magnetotelluricsurveys ( Tarabeesetal.,thisissue )couldbeconsistentwithhighlyfaulted andfracturedzonesthatarenow lledwithmodernartesian groundwaterbutthatweregeneratedoriginallybymobilizationof underlyingshale. Anymobilizationmodelwillforma “ pile-up ” zonedownslope wherethrustfaults,folds,and/ordiapirsaccommodatethetranslatedmaterial.WewereintriguedbythefactthatthesouthernSinn el-KaddabPlateaudisplaysverylarge(10 e 40kmdiameter),low amplitudedomestructures( Fig.17 )thatareunderlainbyCretaceousshales.WeinitiallyspeculatedthatupliftofnorthEgyptin theSyrianArcattheBartonian/Priabonianboundary( Guiraudetal., 2001 )mighthavetriggeredsouthwardmobilizationofshales(with orwithoutpolygonalfaults),developmentofsagsynclinesin overlyinglimestones,anddiapiricriseofmobileshalesinthesouth toformthegiantlow-amplitudedomes.FromtheLateEocene onward,centralEgypthasbeenaterrestrialenvironment(e.g., Macgregor,2012 ),butitisunclearwhetherupliftinthenorthwas enoughtoreverseslopesneartheendoftheEoceneandtrigger southwardmobilization. ThisseemedtoustobeamodelthatwasworthpursuingforourB.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 16 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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synclinenetworkuntilwecompletedacountry-widesynclineinventoryandrealizednotonlytheextentofdevelopmentofthe featuresbutalsotheremarkableconsistencyindominantorientationsofthesagstructures.Bothoftheseobservationsmake downslopemobilizationalesslikelymodel,buttheydon'truleit outentirely. 6.6.Hypogenespeleogenesis Hypogenespeleogenesis,incontrasttoepigenicspeleogenesis, istheformationofdissolutionstructuresbywatersascending throughacave-formingzonefrombelow,drivenbyhydrostatic pressureorothersourcesofenergy( Klimchouk,2009 ).Hypogene karstfeaturesareindependentofrechargefromthesurface. Aggressivenessofthewaterwithrespecttocarbonaterockisacquiredatdepthandcanbethermalorchemical(e.g.,elevated temperature,dissolvedCO2,H2S)( FrumkinandGvirtzman,2006; Deckeretal.,2015;Kempeetal.,2009 ).Jointandfaultsystems commonlycontroltheverticalcomponentof owacrosslithologic boundaries,resultinginbroadlyrectilinearpatternsofdissolution ( Klimchouketal.,2012 ).Hypogenespeleogenesiscanproduce enormousfeatures,goonformillionstotensofmillionsofyears, andoperateatdepthsrangingfromtensofmeterstoseveralkilometers,anddeephypogenedissolutionmaycausenovisibleeffects atthesurface(e.g., Klimchouketal.,2016;Bayarietal.,2009 ). Hypogenespeleogenesisisanintriguingpossibilitytoconsider foroursynclinenetwork,becauseitcouldproducedeep-seated collapsewithsagofoverlyinglimestonelayerswithoutrequiring eithersubsurfaceevaporitesorthepresenceofburiedpaleokarst. Modernriseofartesiangroundwaterintothesevoidsandcollapse zoneswouldbeconsistentwiththelowresistivityzonesthatwe measuredbeneaththesynclinesinouraudio-magnetotelluric surveys( Tarabeesetal.,thisissue ).Below,weconsiderwherehypogenespeleogenesismayhaveoperated,whatrolestratigraphy mighthaveplayed,andwhatthesourceofaggressive uidsmight havebeen. 6.6.1.Structuralcontrol Inhypogenespeleogenesis,faultsandfracturenetworksserve asconduitsthatcarrydeeplyderived uidsupwardthroughaless permeablehorizonintoarocksequencewheredissolutionisnot onlyspatiallycorrelatedwiththefault/fracturenetworkbutalso commonlymostintensewherefaultsandfractureswarmsintersect oneanother( Klimchouketal.,2012 ).Synclineorientationsinour networkarestronglycorrelatedwithjointandfaultorientations thatpredateRedSeaRift-relatedextension,andsynclinesare con nedtolimestonesoftheThebesGroup,whichisunderlainby theEsnaShale.Furthermore,alongtheDrunka-ElRufufcontact, synclinesintheN-Sstrikingfaultzones(the “ B ” domainsof Fig.12 c)exhibitmoresagthansynclinesparalleltotheWNW-ESE strikingjointset,andWNW-ESEsynclinesconnecttotheN-S synclinesinaladder-typearrangement,withbasinclosuresatthe “ ladder ” intersectionsandwithsagdecreasingawayfromtheN-S synclinesalongtheWNW-ESE “ ladderrungs ” ( Fig.12 e). Thesefeaturesareconsistentwithmoredissolutionatdepth alongmorepermeableN-Sfaults,migrationof uidslaterallyaway fromtheN-SfaultsalongWNW-ESEfracturesswarms,and maximumdissolutionandsagaboveconduitswherethefaultsand fractureswarmsintersect.WealsonotethattheN-Sfaultsarebest developedinthesouthernpartofoursynclinemappingareaalong theDrunka-ElRufufcontactandthattheseN-Sfaultsdieouttothe north( Figs.3and12 c),whichcouldaccountforthecloserspacing ofsynclinesnearthecontactandmorewidelyspacedsynclines farthernorthintheDrunkaFormation. Onemightalsoexpectthatvariationsfromplacetoplaceinthe geometryofthesagsynclinenetworkshouldre ectthegeometry oftheconduitnetwork.Forexample,theoverallsigmoidalshapeof thenetwork( Figs.10and12 c)mightwellre ectwidelyspacedN-S faultzoneconduitsatdepth,withmigrationof uidsalongjoint setsWNWandESEawayfromthefaultzones. 6.6.2.Roleofstratigraphyandhydrostratigraphicunits Contrastinglithologiesand uidchemistriesplayanimportantroleinhypogenespeleogenesis.Ifasedimentarysequencedisplays afacies-controlledhydrostratigraphyconsistingofalternating layersofmoreandlesspermeablelithologies,diffuselateral owof formation uidsoccursalongthemorepermeableunits ( Cunninghametal.,2006 ),whereasrisingdeeplyderived uidsare localizedalongfaultandmasterfractureconduits( Klimchouketal., 2012 ).Mixingof uidswithdifferentchemistriesinthepermeable unitscanproducea uidthatisnewlyaggressivewithrespectto thehostrock( Palmer,1991 ),triggeringdissolutioninaseriesof stackednetworksseparatedbylesspermeablehorizons ( Klimchouketal.,2012 ).TheThebesGroupconsistsofless permeablehorizons(e.g.,chertyandsilici edlimestones)interlayeredwithmorepermeablelayers(e.g.,chalkyandporous limestones),makingitsusceptibletothiskindofintrastratalmixing owcorrosion. 6.6.3.Aggressive uids Extensivehypogenekarstdevelopmentrequireswidespread uidsthatareappreciablyaggressive( Klimchouk,2014;Bayari etal.,2009 ).Onecandidateisa uidrichinH2S.Thepresenceof pyriteinshalesunderlyingtheThebessuggestsapossiblesource forsulfurinrisingwaters. Anothergoodcandidateisathermal uidchargedwithCO2, whichisnotonlyaggressivebutbecomesmorecorrosiveasitrises Fig.17. ThesouthernSinnel-KaddabPlateaudisplayslargelow-amplitudedomesthat mayhaveformedbymobilizationofLateCretaceousand/orPaleoceneshales.Yellow starshowslocationofsouthernpartofourmainmappingarea.Imagecredits:Imagery copyright2012DigitalGlobe,Inc.;elevationmodelfromShuttleRadarTopography Missiondata.Imagecenters:a)24.052393,30.943229;b)23.708226,30.673595.(For interpretationofthereferencestocolourinthis gurelegend,thereaderisreferredto thewebversionofthisarticle.) B.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 17 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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andcoolsduetotheinverserelationshipbetweenwatertemperatureandcalcitesolubility. Bayarietal.(2009) describeenormous hypogenekarstfeaturesinTurkeythataredevelopedinaregionof morethan50,000km2by uidsthatwereaggressiveduetohigh mantleCO2 uxassociatedwithextensionalthinningofthelithosphereandrelatedbasalticvolcanism. Frumkinetal.(2015) has shownthathypogene uidsdon'tevenhavetobeparticularlyhot e evenafewdegreesCelsiusisenoughtopromotedissolution. Deckeratal.(2015) havealsorecentlyarguedthatsupercritical CO2(scCO2)isaparticularlypotentagentofdissolution.scCO2migrateseasilyupwardthroughfaultsandfracturesanddissolves veryreadilyinwaterencounteredinaquifers,makingthe uid highlyaggressive.scCO2-related uidaggressivenessisata maximumjustabovethesupercritical/subcriticaltransition.The pressureatwhichthetransitionbetweensupercriticaland subcriticalCO2occursistemperature-dependent,but,atnormalto somewhatelevatedgeothermalgradients,scCO2becomessubcriticalatdepthsof350 e 750m.Weareintriguedbythefactthatthe baseoftheThebeswouldhavebeenatdepthsof400 e 600m (dependingontheamountofpost-Eoceneerosion),atthetime whenhypogenespeleogenesismighthaveoccurredintheThebes. Hypogene uidwithmagmatogenicCO2isnotbeyondtherealm ofpossibilityinEgypt.ModernspringsinEgyptshowthepresence ofmantle-derived,magmatogenicCO2eventoday( Mohammed, 2015;Mohammedetal.,2014 ),andrecentworkby Leeetal. (2016) suggeststhatahigh uxofCO2fromthemantleisalonglivedfeatureinareasundergoingcrustalextension.Bothsuggest thatitisnotunreasonabletoproposethatrisinghypogene uids associatedwithpastbasalticvolcanisminEgyptmighthavehada high uxofmantle-derivedCO2. Whetherbasalticmagmatismcouldhaveplayedarolein possiblehypogenespeleogenesisintheThebeshingesonwhenthe synclinenetworkdeveloped.Thesynclinenetworkpredatesboth depositionofKatkutFormationgravelsandnormalfaultinginthe WesternDesert,whichwaspresumablyassociatedwithRedSea rifting. Bosworthetal.(2015) providecompellingevidencethatthe RedSeaunzippedrapidlyattheOligocene-Mioceneboundaryca. 24-22Ma,andtheysuggestthatnormalfaultingintheWestern Desertwascoevalandequallyshort-lived,althoughthereisno directevidenceforthis.UnfortunatelythereisalsonodirectevidencefortheageoftheKatkutFormation,sotheminimumageof formationofthesynclinenetworkisnotwell-constrainedbutis likelyaroundtheOligocene-Mioceneboundary.Themaximumage is,ofcourse,theageofthelimestonesthemselves,whichareas youngas49.6Ma( Kingetal.,thisissue ).Giventhestrongcorrelationbetweenfaultsandfractureswarmsandoursyncline network,wesuggestthatthesynclinesformedafterashort contractionaleventattheBartonian/Priabonianboundary(ca.38 Ma)thatprimarilyaffectedtheSyrianArcbutalsoreactivatedfaults anddevelopedjointsetsinthe “ StablePlatform ” ofEgypt( Guiraud etal.,2001;Youssef,2003;Tewksburyetal.,thisissue ). CenozoicbasaltsarewidespreadinEgypt( Meneisy,1990; Klitzschetal.,1987 ),butmostoccurrenceswitholderradiometric dateshavenotbeenre-datedwithmodernhighprecisionmethods. Itisnotclear,therefore,whetherthereareanybasaltsinEgyptthat datebetween38and24Ma.Recenthighprecisiondatesdoshowa narrowwindowoftimeca.22 e 24MawhentheCairobasaltsand relatedintrusionswereemplacedinconjunctionwithinitiationof RedSearifting( Bosworthetal.,2015 ).AlthoughtheThebesitselfis essentiallybasalt-freeatthesurface,twogeophysicalstudies ( Bakheit,2005;Bakheitetal.,2003 )proposesubsurfacebasaltsin EocenelimestoneeastandwestoftheNile,suggestingthatbasaltic intrusionsmightwellunderlieotherpartsoftheThebes.Theageof theseproposedbasaltsisunknown. Weknowthatthesynclinenetworkpredatesnormalfaultingin theWesternDesert,butwedon'tknowbyhowmuch.Eveniffuture highprecisiondatingrevealsadearthofbasaltsinthe38-24Maage range,itispossiblethatthesynclinenetworkformedbyhypogene speleogenesisassociatedwithbasalticintrusionsduringanearly phaseintheOligocene-Mioceneriftingeventandwascut,perhaps notmuchlater,bynormalfaultsassociatedwiththesameevent. 7.Conclusions Ofthesevenmodelswehaveconsideredfortheoriginofthe synclinenetwork,fourarehighlyunlikely.Regionalfoldingand faultingisnotconsistentwiththegeometryofthesyncline network,andrecentepigenickarst,dissolutionofevaporites,and collapsedcoalescedpaleokarst,arenotconsistentwiththeageof thenetworkortheknownstratigraphicsection.Silicadiagenesisis anintriguingpossibilitybutappearstobelesslikelygivenour geophysicaldata.Ofthemodelswehaveconsidered,subsurface mobilizationofshalesandhypogenespeleogenesisarethemost plausible,althoughconsistencyoforientationofsynclinesover largedistancesisdif culttoexplainwithdownslopemobilization. Giventhedatawehaveatpresent,hypogenespeleogenesismaybe themostviablemodel. Studiesofhypogenespeleogenesiselsewherehavebeenbased oninvestigationofaccessiblehypogenecavesystems.InEgypt,we arefacedwithevaluatingthepossibilityofhypogenespeleogenesis basedonnear-surfaceeffectsthatmighthaveresultedfromhypogenedissolutionatdepth.Ontheotherhand,wehaveanunusualcombinationoffactorsthatmakesitpossibletomapthese subtlefeaturesoverhugeareas.Theoutcropareaoflimestoneis extensive,dissectionisnegligible,vegetativecoverisabsent,and sur cialdepositsareminimal.Inmanyways,ourhighresolution satelliteimageryisabitlikeaseismichorizonmap.Wehopethat thefeatureswehavedocumentedintheThebeswillencourage othersworkinginterrainswherehypogenekarstisapossibilityto lookforsagsynclinesabovezonesofdeepdissolution. Ifoursynclinenetworkistheresultofhypogenespeleogenesis, theprocessoperatedoveralargearea.Mostareaswherehypogene speleogenesishasbeenpreviouslyproposedaremuchsmaller.If theconditionsarerightforaggressive uidstoriseintosoluble strata,though,wecanseenoreasoninprinciplewhyalargearea wouldbeunreasonable.Andonemightexpectdifferentialdevelopmentacrossalargearea,whichiswhatweseeintheThebes. Diachronousdevelopmentofthenetworkwouldalsobelikely,and itispossiblethatsomesagsynclinesinEgyptmightstillbeactive today.Wefullyadmit,though,thatwearecurrentlyintherealmof speculationbecauseweareconstrainedbyalackofdata,and furthercriticalevaluationofspeci cmodelsmustawaitmoredata, particularlysubsurfacedataandbetterageconstraints.And,of course,theremaybeyetanothermechanismouttherethatwe haven'tconsidered. Acknowledgements Wegratefullyacknowledgethecarefulmappingandsatellite imageanalysisofvariousareasintheWesternDesertoverthepast sixyearsbyKennethChristleandbyHamiltonstudentsHannah Allen,AubreyCoon,JosephCoons,LaurenDeGennaro,Gaela Dennison-Leonard,DevinFarkas,TonyHernandez,AlexHolmwood, DavidHyman,TuckerKeren,AlexanderKerman,PeterLaciano, TheodoreMcLean,ClaireSayler,andCliffordYu.Weespecially acknowledgeHamiltonstudentJoshuaWolpert,whocarriedouta detailedinventoryofsynclinefeaturesacrossEgypt.Wethank MahmoudHanafyandAhmedMokhtarfor eldobservationsmade alongthegeophysicallines.WealsothankDr.GaryProstforB.J.Tewksburyetal./JournalofAfricanEarthSciencesxxx(2017)1 e 20 18 Pleasecitethisarticleinpressas:Tewksbury,B.J.,etal.,Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestonesofthe WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.02.017

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pointingoutthefabulousevaporitedissolution-relatedsagsynclinesinQatar.Wearegratefultocolleaguesinthekarstcommunityforinvaluablediscussions,andweareespeciallygratefultoDr. LewisLandandDr.GeorgeVenioftheNationalCaveandKarst ResearchInstitute,aswellasDr.DavidDeckerandDr.KevinCunningham,forespeciallythoughtfulinsights.DaveTewksburywas instrumentalindevelopingGISdatasetsandmaps.Geospatial supportforthisworkprovidedbythePolarGeospatialCenterunderNSFPLRawards1043681 & 1559691.WethankDr.IanHowatat OhioStateUniversityforderivinghighresolutionDEMsfromDigitalGlobe,Inc.imagery.FundingforthisprojectwasprovidedbyU. S.NationalScienceFoundationIRESgrant1030224andHamilton College. 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