Audio-magnetotelluric surveys to constrain the origin of a network of narrow synclines in Eocene limestone, Western Desert, Egypt

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Audio-magnetotelluric surveys to constrain the origin of a network of narrow synclines in Eocene limestone, Western Desert, Egypt

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Audio-magnetotelluric surveys to constrain the origin of a network of narrow synclines in Eocene limestone, Western Desert, Egypt
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Audio-magnetotelluricsurveystoconstraintheoriginofanetworkof narrowsynclinesinEocenelimestone,WesternDesert,EgyptElhamyA.Tarabeesa,**,BarbaraJ.Tewksburyb,*,CharlotteJ.Mehrtensc, AbdellatifYounisdaDepartmentofGeology,DamanhourUniversity,Damanhour,22516,EgyptbDepartmentofGeosciences,HamiltonCollege,Clinton,NY,13323,USAcDepartmentofGeology,UniversityofVermont,Burlington,VT,05405,USAdNationalResearchInstituteofAstronomyandGeophysics,Cairo,11421,EgyptarticleinfoArticlehistory: Received30November2016 Receivedinrevisedform 17February2017 Accepted3March2017 Availableonlinexxx Keywords: Geophysicalsurvey Satelliteimageanalysis Karst WesternDesert EgyptabstractRecentworkwithhighresolutionsatelliteimageryhasrevealedanetworkofnarrowsynclinesdevelopedduringtheOligoceneorMioceneovertensofthousandsofsquarekilometersinEocenelimestone oftheThebesGroupintheWesternDesertofEgypt.Thesynclinesarenon-tectonic,andtheirscaleand geometrystronglyresemblesagsynclinesinQatarthatwereproducedbydissolutionofsubsurface evaporitesandresultingsagofoverlyinglayers.EvaporitedissolutioncannotexplaintheEgyptsynclines, becausesubsurfaceevaporitesofanysigni cancehaveneverbeenreportedinthispartofEgypt.Inthis study,weuseaudio-magnetotelluricsurveystoilluminatethesubsurfaceunderthesynclinesinorderto constrainpossiblemodelsfortheirformation.Wesuspectedkarstdissolutionatdepth,and,givena modernwatertabledepthofover400m,weexpectedthatdryfracturenetworksandvoidspacesunder thesynclinesmightresultinhigherelectricalresistivitiesthansurroundingcoherentlimestone.Wealso anticipatedasigni cantchangefromhightolowresistivityatthecontactbetweentheThebesGroupand theunderlyingEsnaShaleatdepthsof400mormore.Instead,wefoundlocalizedlowresistivityzones extendingfromabout50-100mbelowthesurfacetodepthsofmorethan400mthatarestrongly correlatedwithsynclines.Wesuggestthattheselocalizedlowresistivityzonesare lledwithartesian groundwaterthathasinsuf cienthydraulicheadtorisetothemoderntopographicsurfaceandthatis localizedinsubsurfacevoidsandcollapsebrecciasproducedbydissolution.Sagofoverlyinglimestone layersisareasonablemodelforsynclineformationbut,giventheOligocene/Mioceneageofthesynclines,dissolutionandsagwouldbeunrelatedtoyounggroundwaterprocesses. 2017TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-ND license( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).1.Introduction SatelliteimageryoftheareawestoftheNileincentralEgypt displaysstrikingcolorpatterning( Fig.1 A).Recentworkcombining both eldworkandanalysisofhighresolutionsatelliteimagery ( Tewksburyetal.,inthisissue )hasestablishedthatmuchofthis patterningisduetoanextensivenetworkofsynclinesinEocene limestone( Fig.1 B e D). Tewksburyetal.(inthisissue) haveargued thatsynclinegeometriesandscalesarenottypicaloftectonicfold systemsandhaveproposedanon-tectonicsagoriginforthesynclinenetwork. ThesynclinesoftheWesternDesertbearastrikingresemblance tostructuresthataredevelopedintheEoceneDammamand MioceneDamFormationsinQatarandthatarealsoeasilyvisiblein highresolutionsatelliteimagery( Fig.1 E).LiketheWesternDesert synclines,thestructuresinQatararesynclinalandhaveshallow limbdips,branchingandcuspategeometries,andshallow,but doublyplunging,hingesthatde nemultiplebasinclosures.Also liketheWesternDesertsynclines,theQatarstructuresaresynclinal structuresinotherwise at-lyinglimestonelayers.Previous workershaveestablishedthattheQatarstructuresarenon-tectonic synclinesformedasaresultofdissolutionofunderlyingEocene evaporitesoftheRusFormation,withaccompanyingsaginthe Correspondingauthor. ** Correspondingauthor. E-mailaddresses: etarabees@yahoo.com (E.A.Tarabees), btewksbu@hamilton. edu (B.J.Tewksbury), Charlotte.Mehrtens@uvm.edu (C.J.Mehrtens), abdougeoman@yahoo.com (A.Younis). Contentslistsavailableat ScienceDirectJournalofAfricanEarthSciencesjournalhomepage: www.elsevier.com/l ocate/jafrearsci http://dx.doi.org/10.1016/j.jafrearsci.2017.03.001 1464-343X/ 2017TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). JournalofAfricanEarthSciencesxxx(2017)1 e 8 Pleasecitethisarticleinpressas:Tarabees,E.A.,etal.,Audio-magnetotelluricsurveystoconstraintheoriginofanetworkofnarrowsynclinesi n Eocenelimestone,WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.03.001

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overlyinglimestonelayers( Prost,2014;SadiqandNasir,2002; Cavelier,1970 ). Stewart(2015) describessimilarstructuresin easternSaudiArabiaresultingfromsubsurfacedissolutionofRus Formationevaporitesandsaginoverlyinglayers.Structuresof similargeometryandscalealsooccurinthePecosValleyofNew MexicoandWestTexas,USA,andwereformedbydissolutionof subsurfaceevaporitesintheCastileandSaladoFormations(e.g., Staffordetal.,2008;LandandLove,2006;Motts,1962 ). DespitesimilaritiesinsurfaceexpressionbetweenthesestructuresandtheWesternDesertsynclines,evaporitesofanysigni cancehaveneverbeenreportedinthesubsurfaceintheeastcentral WesternDesertofEgypt(e.g., BarakatandAsaad,1965;Issawietal., 2009 ).Knowledgeofwhatliesinthesubsurfacebeneaththe WesternDesertsynclinesiscriticalforevaluatingwhethersagisa reasonablemodelfortheoriginofthesynclinesand,ifso,what mechanismmighthavecausedthesag.Weconductedaseriesof Fig.1. A)SatelliteimageoftheeastcentralWesternDesertofEgypt;insetmapshowslocationinEgypt.B & C)ObliqueviewsinGoogleEarthof200 e 400mwidesynclinesin otherwise at-lyingEocenelimestonethatformanextensivenetworkresponsibleforprominentcolorpatterninginthesatelliteimagery.D)Highresolutionsa telliteimageof synclinestructuresinourstudyarea(locationshownwithasteriskinFig.1A).E)StructureswithsimilargeometryandscaleinQatarandinterprete dtobetheresultofsubsurface dissolutionofevaporites.Imagecenters:B)26.560788,30.794290;C)26.165253,30.964619;D)26.844229,31.094569;E)24.843343,50.893781.Im agecredit:A)Satelliteimage fromESRI'sArc2Earth(imagecreditEsri,DigitalGlobe,GeoEye,EarthstarGeographics,CNES/AirbusDS,USDA,USGS,AEX,Getmapping,Aerogrid,IG N,IGP,swisstopo,andtheGIS UserCommunity);B-E)GoogleEarth.(Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) E.A.Tarabeesetal./JournalofAfricanEarthSciencesxxx(2017)1 e 8 2 Pleasecitethisarticleinpressas:Tarabees,E.A.,etal.,Audio-magnetotelluricsurveystoconstraintheoriginofanetworkofnarrowsynclinesi n Eocenelimestone,WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.03.001

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audio-magnetotelluricsurveysdesignedtoilluminatethebedrock downtoabout500mdepth.Inthispaper,wepresentevidencefor thepresenceofalocallyelevatedwatertablebeneaththesynclines, andweaddresstheconstraintsthisplacesonpossiblemodelsfor originofthesynclines. 2.Geologicsetting Theeast-centralportionoftheWesternDesertispartofwhatis knownastheLimestonePlateau,agentlynorth-slopingandlittledissectedregionthatsits200 e 250mabovetheNileValleytothe eastandnearly300mabovetheKhargaValleytothesouthwest ( Figs.1Aand2 A).ThePlateauhaslittletopographicreliefexcept wherewadisdissecttheareaimmediatelyalongtheNileescarpment.Theareaistraversedbyonlytworoads,theAssiut-ElKharga RoadandtheWesternDesertRoad( Figs.1Aand2 A),andmuchof theLimestonePlateauisessentiallyinaccessibleandhasbeenlittle studiedexceptinsatelliteimagery. ThePlateauiscappedbyEocenelimestone,withunderlyingless resistantCretaceousthroughearliestEoceneshales,marls,and chalkexposedintheescarpmentsborderingthePlateaualongthe NileandKhargaValleys( Figs.2Band3 ).UpperJurassicand Cretaceous uvialandshallowmarineclasticsedimentaryrockslie unconformablyonPrecambrianbasementatthebottomofthe sectionandareexposedintheKhargaValley.Althoughthisgeneral stratigraphyiswell-established,thicknessesofindividualunitsas reportedintheliteraturevarywidely,andstratigraphicnomenclature,particularlyfortheEocenelimestones,hasnotbeenstandardized.Becauserocktypesinthestratigraphicsequence,aswell asthicknessesofspeci cunits,arecriticalforinterpretingour geophysicalpro les,weaddresseachoftheseissues. Determiningareasonablepictureofthestratigraphyand thicknessesofunitsbeneathourstudyareaisnotastraightforward task.BecausethePlateauisessentiallyundissected,thereareno placeswherearepresentativestratigraphiccolumncanbe measuredinthe eld.Wehaveuseddatafrommeasuredsections Fig.2. A)HillshadeddigitalelevationmodelshowinglocationofstudyareaontheLimestonePlateau(elevationsshowninyellowsandbrownsare250 e 350m.a.s.l.) ankedbylow elevationsintheNileandKhargaValleys(elevationsshowningreensare~50m.a.s.l.);elevationdatasource:ShuttleRadarTopographyMission.B) Generalizedgeologicmapforthe areashowninFig.2Amodi edfromgeologicmapsby Klitzschetal.(1987) and Riadetal.(2005) andincorporatingadditionaldatafrom Tewksburyetal.(thisissue) .C)Syncline networkinourstudyareasuperimposedonhighresolutionsatelliteimage;locationsofthethreeareaswherewecollectedAMTdata.ImagecenterforC :26.829860,31.142241; satelliteimageryfromESRI'sArc2Earth(imagecreditEsri,DigitalGlobe,GeoEye,EarthstarGeographics,CNES/AirbusDS,USDA,USGS,AEX,Getmap ping,Aerogrid,IGN,IGP, swisstopo,andtheGISUserCommunity.(Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) E.A.Tarabeesetal./JournalofAfricanEarthSciencesxxx(2017)1 e 8 3 Pleasecitethisarticleinpressas:Tarabees,E.A.,etal.,Audio-magnetotelluricsurveystoconstraintheoriginofanetworkofnarrowsynclinesi n Eocenelimestone,WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.03.001

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tothesouthwestintheKhargaescarpmentby ElAzabiandFarouk (2011) andtothesoutheastatGebelGurnah( Fig.2 B)intheNile escarpmentby Kingetal.(inthisissue) inadditiontodatafrom Said(1960,1962,1990),ElHinnawietal.(1978),Issawi(1972), Issawietal.(2009),KeheilaandEl-Ayyat(1990),KhalilandElYounsy(2003),Keheilaetal.(1990) ,and ElHinnawietal.(2005) Awelldrilledinthe1960sapproximatelyhalfwayalongthedesert roadbetweenAssiutandEl-Kharga( Fig.2 B)providestheonlyinformationaboutlithologiesandunitthicknessesbeneaththe Plateauitself( BarakatandAsaad,1965 ),althoughcorrelatingthe welldatawithmorerecentmeasuredstratigraphiccolumnsis complicatedbythefactthatstratigraphicnomenclaturehas changed. Wehavecompiledastratigraphiccolumnin Fig.3 thatwe believeisasrepresentativeaspossibleofthesequenceunderlying ourstudyarea.Awordisinorderaboutourchoiceofstratigraphic terminology.EocenelimestonesinEgyptwereoriginallydivided intoanumberofformationsexposedindifferentareas,andthe term “ Thebes ” wasoriginallyappliedtooneoftheseformations. Klitzschetal.(1987) elevatedthetermThebestoGroupstatus whentheycreatedthe1:500,000scalegeologicmapsofEgyptand assignedanewformationname,theSerai,towhathadbeenthe ThebesFormation.Inourarea,theElRufuf,Drunka,andSeraion thesemapsarepartoftheThebesGroup.The20051:250,000scale geologicmaps( Riadetal.,2005 )returntheThebestoFormation statusanddeprecatetheterm “ Serai ” Kingetal.(inthisissue) advocatea “ ThebesLimestoneFormation ” containingallofthe variousfaciesofEocenecarbonatestrataintheWesternDesert. Wehavechosentouse “ ThebesGroup ” terminologyinpart becauseofuncertaintyaboutspeci cformationsandthicknessesin thesubsurfaceunderthePlateauandinpartbecausewehave chosentobasethegeologicmapin Fig.2 Bprimarilyon Klitzsch etal.(1987) becausethe2005maps( Riadetal.,2005 )donot extendeastoftheNilenortothenorthernedgeofourstudyarea. Furthermore,althoughtheEocenecarbonatesequenceiswelldocumentedtoconsistofinterlayeredlimestonesofvariouscharacters,marlylimestones,andsilici ed/chertylimestone(e.g., Khalifaetal.,2004,2014;KhalifaandZaghloul,1990;Kingetal.,in thisissue ),theselithologiesshouldallhavehighelectricalresistivities,andgroupingthemtogetherinthestratigraphiccolumn isreasonableforthisstudy.Themainchangeinresistivitywith depthwouldbeexpectedatthetransitiontotheunderlyingEsna Shale. Wehaveindicatedathicknessofapproximately400mforthe ThebesGroupinourstudyarea.Thisisaconservativeestimate basedonapproximately430mmeasuredintheAssiut-Khargawell ( BarakatandAsaad,1965 ),350mmeasuredby Kingetal.(inthis issue) intheNileescarpmentatGebelGurnahnearThebes,and thinnersectionsreportedelsewhereintheliterature.TheThebes Groupiscappedbyasigni cantunconformityandislocallyoverlainbygravelsoftheKatkutFormation( Issawietal.,2009;ElHinnawietal.,2005;AbuSeif,2015 ).Theoriginalthicknessofthe ThebesGroupisuncertain,although Macgregor(2012) hassuggestedthatasmuchas200moflimestonemayhavebeenremoved bypost-Eoceneerosion. Themodernhydrologicsettingisalsorelevantforinterpretation oftheresistivitydata.Theenvironmentishyperarid,rainfallis negligible,andthemodernwatertablelies400 e 500mbelowthe Plateausurface( Salim,2012 ). 3.Objectives ThepurposeofthisstudyistodeterminewhatunderliessynclinesintheEocenelimestonesinordertoconstrainmodelsfor originofthesynclinenetwork.Sagoflayerscausedbycollapseof subsurfacevoidspacecreatedbyepigenicorhypogenickarst dissolutioninlimestoneisareasonablemodeltotest.Giventhat themodernwatertableliesatdepthsofatleast400mbeneaththe LimestonePlateau,anysuchcollapsefeatureswouldberelictofan Fig.3. RepresentativestratigraphiccolumnfortheareawherewecollectedourAMT data.Colorscorrelatewithgeologicmapin Fig.2 .Depthstospeci cformationsbased mainlyontheAssiut-Khargawell( BarakatandAsaad,1965 )locatedapproximately 50kmSWofourgeophysicallines.Thewelldidnotreachbasement.Additionaldata sourcesaredetailedinthetext.(Forinterpretationofthereferencestocolourinthis gurelegend,thereaderisreferredtothewebversionofthisarticle.) E.A.Tarabeesetal./JournalofAfricanEarthSciencesxxx(2017)1 e 8 4 Pleasecitethisarticleinpressas:Tarabees,E.A.,etal.,Audio-magnetotelluricsurveystoconstraintheoriginofanetworkofnarrowsynclinesi n Eocenelimestone,WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.03.001

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earliertimeperiod,andwewouldexpectanyvoidspacesand collapsezonestobedryinthetopseveralhundredmetersofthe limestone.WechosetheAMTmethodforoursurveysbecauseit shouldbeabletodetectdryvoids,extensivefractures,andcollapse breccias,whichshouldhavehigherresistivitythanthesolidhost limestone. OurstudyareaislocatedalongtheWesternDesertRoad approximately40kmSSWofAssiut( Fig.2 AandB).Wechose thisareainpartforaccessibilityandinpartbecausesynclinesin avarietyoforientationsarewell-exposedandhadbeenmapped indetailinhighresolutionsatelliteimagery( Fig.2 C)aspartof ourmainsynclinemappingproject( Tewksburyetal.,inthis issue ). 4.Methodology 4.1.Audio-magnetotelluric(AMT)method Theaudio-magnetotelluricmethod(AMT)isanimportanthigh resolution,non-seismicgeophysicaltechniquethatmeasuresvariationsintheEarth'snaturalelectromagnetic eldstodetectelectricalresistivityvariationsinthesubsurfaceatshallowto intermediatedepths.Resistivityvaluesarecontrolledmainlyby porosity,fractures,watercontent,concentrationofdissolvedsolids, andpermeability.TheAMTmethodprovidesdetailedinformation aboutvariationsinsubsurfaceelectricalresistivityvaluesthatcan beusedforinterpretationoflithologicaland/orstructuraldifferencesalongthepro leline.Themethodiscapableofimagingthe subsurfacewithresolutionsgoodenoughtodetectfeaturesafew metersacross.Theshallowsubsurfaceisimagedfromhighfrequencymeasurements,andthedeepersubsurfaceisimagedfrom lowfrequencymeasurements.Thedepthofinvestigationcan extendto1km,althoughthedepthofinvestigationisreducedby thepresenceofnear-surfaceconductivesediments. Ouranticipateddepthofinvestigationof400 e 500miswell withintherangeofthecapabilityoftheAMTmethod,especially giventhatthehyperaridsettingandlimestonebedrockareunlikelytoresultinhighlyconductivenear-surfacematerials.Additionally,theAMTmethodislowcost,lightweight,accurate,and requiresaminimumof eldpersonnelincomparisontoseismic methods. 4.2.Fieldsurveyanddataacquisition Figs.2C,4,5,and6 showthelocationsofouraudiomagnetotelluric(AMT)surveylines.Allbutonelinecrossessynclinesatahighangleinordertoinvestigatedifferencesbetween whatunderliesthenarrowsynclinesandwhatunderliesadjacent inter-synclineareas.Theremaininglineisorientedparalleltothe axialsurfacetraceofasynclineandprovidesdataalong,ratherthan across,thetrendofasyncline.Wecollecteddataat25stations alongthesefourpro les. WeusedaStratagemhybridAMTinstrumentthatbothmeasuresnaturalmagnetotelluricwavesinalowfrequencyrangeand usescontrolled-sourceelectromagneticsignalsinahighfrequency rangetoobtainacontinuouselectricsoundingbeneaththemeasurementsite.Theinstrumentuseselectrodestakeswithdipole lengthof50mformeasuringelectric elds,andhighlysensitive magneticcoilsformeasuringmagnetic elds.Werecordednatural magnetotelluric(MT)wavesusingtheStratagemlowfrequency antennaandthenrepeatedthemeasurementusinganarti cial signalsourceforControlled-SourceAMT(CSAMT)inthehighfrequencyrange.TheCSAMTmethoddiffersfromtheAMTmethodin thatagroundeddipoletransmitterisusedtogeneratethesource elds.Weusedadual-loopantenna,locatedfrom250to400m awayfromthemeasuredsite,toprovideanon-polarizedelectromagnetic eldsource. Wecollecteddatainbothfrequencyrangesbecauseitpermits investigationofstructurestodepthsof500m,andweareinterestedinpotentialkarstvoidsandcollapsebrecciaslyinginthetop fewhundredmeters.ThedistancebetweentheMTstationsranged from100mto250mdependingonthewidthofthesurface structures,withcloserspacingforthenarrowsynclinesandlarger spacingforinter-synclineareas. Fig.4. A)StructuremapofSubarea1showingaxialsurfacetracesofnarrowsynclines andattitudesofbeddinginferredfromsatelliteimagery.Line1crossesthecoreofanEWsynclineatstation6andaNNW-SSEsynclineatstation10.Stations5,7,9,and11lie inbroadlyanticlinalinter-synclineareas.B)Two-dimensioninversionresistivity(r) modelfortheAMTdatameasuredalongLine1usingREBOCC2-Dinversioncode. Imagecenter:26.847791,31.098670.Imagesource:GoogleEarth. E.A.Tarabeesetal./JournalofAfricanEarthSciencesxxx(2017)1 e 8 5 Pleasecitethisarticleinpressas:Tarabees,E.A.,etal.,Audio-magnetotelluricsurveystoconstraintheoriginofanetworkofnarrowsynclinesi n Eocenelimestone,WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.03.001

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4.3.AMTdataanalysis WecarriedoutdataprocessingandinterpretationoftheAMT datausingWinGLinksoftware(GeosystemSrl).Forpreliminary dataprocessing,wemergedAMTdataatdifferentfrequenciesfrom thesameAMTsiteinordertoobtaininformationatdifferentdepths andtodeterminethedimensionalityofthegeologicalstructures. Wethenusedthecombineddatatoestimatetheelementsofthe MTimpedancetensor,whichweusedtodeterminetheapparent resistivitiesandphasesforbothtransverseelectric(TE)and transversemagnetic(TM)modes.Becauseourstudyareahasvery littletopographicrelief,wedidnotperformaterraincorrectionon ourdata. Weusedstandarddataprocessingtechniquesfordetermining theappropriatedimensionalityofthedata,testingthedatafor internalconsistency(inparticularthemagnetotellurictensor),and modelingthegeoelectricstructures.BeforeconvertingtheAMT dataintoaresistivitymodelofthesubsurface,itisessentialto understandthedimensionalityoftheAMTdata,becauseAMT tensorshavesigni cantlydifferentformsdependingonwhether thesubsurfacestructureisapproximately1D,2D,or3D.In2D,the AMTtensorisdecoupledintotwoindependentmodes:theTE mode,wheretheelectricaxisisparalleltothestrike,andtheTM mode,wheretheelectricaxisisperpendiculartothestrike ( SimpsonandBahr,2005 ).Wecarriedout1DinversionofourAMT datausingtwotypesofalgorithms,BostickandOccam,bothof whicharebasedoninformationfromstratigraphicwelldata ( Bostick,1977 ).WethenusedaD smoothingtechniquetosmooth resistivityandphasecurves.Thistechniquerelatestheapparent resistivityandphaseofthesamecomponentthroughaD function ( BeamishandTravassos,1992 ).Wethencarriedout2Dinversionof Fig.5. A)StructuremapofSubarea2showingaxialsurfacetracesofnarrowsynclines andattitudesofbeddinginferredfromsatelliteimagery.Line2crossesthecoreofa NW-SEsynclineatstation13andaWNW-ESEsynclinebetweenstations16and17. Stations14and15lieinaninter-synclineareathatisanticlinalnearthenoseinthe westbutthatbecomes at-lyingtotheeast.B)Two-dimensioninversionresistivity(r) modelfortheAMTdatameasuredalongLine2usingREBOCC2-Dinversioncode. Imagecenter:26.855595,31.137815.Imagesource:GoogleEarth. Fig.6. A)StructuremapofSubarea3showingaxialsurfacetracesofnarrowsynclines andattitudesofbeddinginferredfromsatelliteimagery.Line3liesalongtheaxial surfacetraceofasmallelongatebasinfromstation18to21.Line4crossesthecoreofa smallelongatebasininthenorthatstation24,crossesthecoreofacomplexelongate cuspatebasinbetweenstations27and29,andcutsLine3orthogonallybetween stations30and31.BandC)Two-dimensioninversionresistivity(r)modelsforthe AMTdatameasuredalongLines3and4respectivelyusingREBOCC2-Dinversioncode. Imagecenter:26.853938,31.183317.Imagesource:GoogleEarth. E.A.Tarabeesetal./JournalofAfricanEarthSciencesxxx(2017)1 e 8 6 Pleasecitethisarticleinpressas:Tarabees,E.A.,etal.,Audio-magnetotelluricsurveystoconstraintheoriginofanetworkofnarrowsynclinesi n Eocenelimestone,WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.03.001

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AMTdatabyinvertingthedeterminantaverageoftheimpedance tensorfromthebestMTsitesbythereduced-basisOccam'sinversion( REBOCC )2Dinversioncode.Thegoaloftheinversionisto nd theminimumstructuremodelsubjecttoadesiredmis tlevel( de Groot-HedlinandConstable,1990 ).Thedeterminantdataprovides ausefulaverageoftheimpedancetensorforallcurrentdirections ( PedersenandEngels,2005;Younisetal.,2015 ). 5.Results WecollectedAMTdataalongfourpro lesinthreesubareasof ourstudyarea( Fig.2 C).Inthissection,wereportbothonthe structuralcontextofthefourpro lesandonthe2Dinversion modelscalculatedfromtheAMTdataalongeachpro leline. 5.1.Subarea1 Fig.4 Ashowsourstructuralinterpretationbasedonsatellite imageanalysisforSubarea1.Ourmethodologyfordeterminingdip directionsisdescribedin Tewksburyetal.(inthisissue and 2012) Weshowdipdirectionsin Fig.4 Abutnotdipamounts,becausethe resolutionoftheavailabledigitalelevationdataistoolowtoallow ustocalculatedipamounts. InSubarea1,synclinesare200 e 300macross,havemultiple basinclosuresalongtheirlengths,andarecommonlycuspatein planview.NNW-SSEandE-Wtrendsdominate,althoughsynclines branch,merge,andchangeorientationalongtheirlengths.Synclinesareseparatedbygenerallybroaderinter-synclineareasthat de nedomesanddoublyplunginganticlineswheresynclinesare closetogether.Wheresynclinesaremorewidelyspaced,intersynclineareasdisplayhorizontalbedding(fordiscussion,see Tewksburyetal.,inthisissue ). Line1isadog-leglineinordertocrossbothasynclineoriented E-WandoneorientedNNW-SSEandtocutthesynclinesatahigh angletotrend( Fig.4 A). Fig.4 Bshowsthe2Dinversionmodelfor AMTdatarecordedatstations5to11.Themodelshowslowresistivityzonesbeneathstations6and10,whichlieinsyncline cores.Themodelshowshighresistivityvaluesbeneathstations7,9, and11,whichlieinthecentersofthebroadlyanticlinalintersynclineareas. 5.2.Subarea2 Fig.5 Ashowsourstructuralinterpretationbasedonsatellite imageanalysisforSubarea2.AsinSubarea1,synclinesare 200 e 300macrossandhavemultiplebasinclosuresalongtheir lengths.ThetwosynclinesinSubarea2areseparatedbyabroad inter-synclineareathathasagentlyplunginganticlinalnoseinthe westbutischaracterizedby at-lyingbeddingwherethesynclines aremorewidelyspacedinthecentralandeasternpartofthe subarea.Line2crossesonesynclineorientedNW-SEandoneorientedWNW-ESE( Fig.5 B). Fig.5 Bshowsthe2DinversionmodelforAMTdatarecorded fromstations13to17.Themodelshowsmoderatetohighresistivitiesdowntodepthsofabout50m.Themodelshowslow resistivityvaluesatdepthsbelow100mbeneathstation13and belowabout75mbeneathstations16 e 17.Thesestationsare locatedinthecoresofsynclines.Highresistivityvaluesatstations 14and15arelocatedbeneaththebroadlyanticlinalinter-syncline area. 5.3.Subarea3 Fig.6 Ashowsourstructuralinterpretationbasedonsatellite imageanalysisforSubarea3,wherewerantwoAMTlines.Line3providesdataalong,ratherthanacross,thetrendofasyncline.Line 4isadog-leglinethat,fromNNEtoS,crossesasmallelongatebasin orientedWNW-ESE,acomplexelongatecuspatebasinoriented approximatelyE-Wbutwitharmsinmultipleorientations,anda smallelongatebasinorientedWNW-ESE.Line4crossesallthree synclinesatahighangletosynclinetrendandcrossesLine3 orthogonallyinthecoreofasmallelongatebasin. Fig.6 Bshowsthe2DinversionmodelsforAMTdatarecorded alongsyncline-parallelLine3(stations18 e 21)andalongsynclinecrossingLine4(stations23 e 32).Inbothlines,themodelsshow moderatetohighresistivitiesdowntodepthsofabout50m.The modelforLine3showsanextensivezoneoflowresistivityat depthsbelowabout150mallalongthetrendofthesynclineand belowabout100mbeneaththesynclinecore.ThemodelforLine4 showsazoneoflowresistivityatdepthsbelowabout150munder stations23 e 25,whichcoincidewithabasinclosurealongasyncline.Station30isinthesmallsyncline,andthehighresistivity between31and32liesoutsidethesynclineandintheadjacent inter-synclineregion. Thecomplexcuspatesyncline,whichliesbetweenstations27 and29,hasalessclearcorrelationbetweensurfacestructureand thesubsurfaceresistivitymodel.Unliketheothersynclinecores, whichareunderlainbyzonesoflowresistivity,thecuspatesynclineishalfunderlainbylowresistivityatshallowdepthsandhigh resistivitybelowabout100m(betweenstations27and28)andthe otherhalfofthesynclineisunderlainbytheopposite(between stations28and29). 6.Interpretations PriortocollectingtheAMTdata,wehypothesizedthatcollapse ofdissolutionfeaturesbyeitherepigenicorhypogenickarstprocessesmighthavecausedsaginoverlyinglimestonelayers.Given thatthewatertableiscurrentlydeeperthanabout400 e 500m,we anticipatedthatanyvoidsandcollapsebrecciascreatedduringpast humidintervalswouldbedryandhaveresistivitiesevenhigher thanthoseintheadjacentcoherentlimestone.Weexpectedtosee adownwardtransitionfromhighresistivityEocenelimestoneto lowresistivityEsnaShaleatdepthsofatleast400m,andpossibly asdeepas450mbasedonthicknessoftheThebesGroupinthe Assiut-Khargawell( BarakatandAsaad,1965 ). OurAMTdata,however,showamorecomplicatedpicture. Althoughthepro lesshowstrikinglateralchangesinresistivityat depthsasshallowas100mthatcorrelatespatiallywithsyncline cores,synclinesareunderlaindominantlybyvery low resistivity materials,ratherthanveryhighresistivitymaterials.Theselow resistivityzonesareunlikelytobeshale e theEsnaShaleismuch toodeep,theThebesconsistsalmostentirelyoflimestone,and signi cantlateralchangesinresistivityduetomajorchangesin lithologyarenotconsistentwiththeoverall at-lyingcharacterof thesedimentarysequence. Waterinfracturesandvoidspacescouldaccountforlow resistivityvalues,butthewatertableismuchtoodeepforthistobe relatedtotheregionalwatertable.Weproposethatthelowresistivityzonesbeneaththesynclinesareduetoartesiangroundwaterthathasrisenfromadeep,con nedaquifer(perhapsthe Nubian)andstalledout100mormorebelowthesurfacedueto insuf cienthydraulichead.Extensivetravertinedepositsalongthe KhargaandNileescarpments( Fig.2 B)suggestthatthisisnotan unreasonableinterpretation. 7.Conclusions ResultsofourAMTsurveyssuggestthatartesiangroundwater hasrisenfromdepthintotheThebeslimestonealongzonesthatE.A.Tarabeesetal./JournalofAfricanEarthSciencesxxx(2017)1 e 8 7 Pleasecitethisarticleinpressas:Tarabees,E.A.,etal.,Audio-magnetotelluricsurveystoconstraintheoriginofanetworkofnarrowsynclinesi n Eocenelimestone,WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.03.001

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underliethesynclines.Giventhespatialcorrelationbetweensynclinekeelsandartesiangroundwater,sagofoverlyinglimestone layerscausedbycollapseofsubsurfacekarstvoidspaceisa reasonablemodeltoproposefortheoriginofthesynclines. Awrinklecomesinwhenweconsidertiming,however.The obviousinferencethatthesynclinesareyoungfeaturesrelatedto relativelyrecentlimestonedissolutioncannotbecorrect.Satellite imageanalysis( Tewksburyetal.,inthisissue )revealsthateroded synclinesareunconformablyoverlainbytheKatkutFormation, whichpre-datesdevelopmentofathrough-goingNileandhasbeen interpretedasOligocene( Issawietal.,2009 )orperhapsearly Mioceneinage( AbuSeif,2015 ).Satelliteimageanalysis ( Tewksburyetal.,inthisissue )alsoclearlyshowsthatthesynclines arecutbyfaultsrelatedtoinitiationofRedSearifting,which Bosworthetal.(2015) placesattheOligocene/Mioceneboundary. Theserelationshipsshowclearlythatthesynclinesarenotyoung features.Wespeculatethatanetworkoffracturesandfaults controlledmovementofaggressive uidsthroughtheThebes duringtheOligoceneorearlyMiocene,causingdissolutionalong thefracturesystem.Collapseofvoidspacecreatedbydissolution couldhavecausedsaginoverlyinglayers,aswellascreatingfeaturessuchascollapsebrecciasthatnowhostyoungartesianwater. Acknowledgements WegratefullyacknowledgeWaelGawishandMohamedAbdElSaborfromNRIAGandMahmoudHanafyandAhmedMokhtarfor eldobservationsmadealongthegeophysicallines.Wearealso gratefulforcarefulreviewsbyDr.LewisLandandoneanonymous reviewer;theirsuggestionsmadethisamuchbettermanuscript. ThisworkispartofthelargerDesertEyesProjectfundedbyU.S. NationalScienceFoundationIRESgrant1030224andHamilton College.DamanhourUniversityprovidedin-kindsupportfor geophysicaldatacollection. 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Originofanextensivenetworkofnon-tectonicsynclinesinEocenelimestones oftheWesternDesert,Egypt.J.Afr.EarthSci. Younis,Abdellatif,El-Qady,Gad,AbdAlla,Mohamed,AbdelZaher,Mohamed, Khalil,Amin,AlIbiary,Mohamed,Saraev,Alexander,2015.AMTandCSAMT methodsforhydrocarbonexplorationatNileDelta,Egypt.Arab.J.Geosci.8, 1965 e 1975 .E.A.Tarabeesetal./JournalofAfricanEarthSciencesxxx(2017)1 e 8 8 Pleasecitethisarticleinpressas:Tarabees,E.A.,etal.,Audio-magnetotelluricsurveystoconstraintheoriginofanetworkofnarrowsynclinesi n Eocenelimestone,WesternDesert,Egypt,JournalofAfricanEarthSciences(2017),http://dx.doi.org/10.1016/j.jafrearsci.2017.03.001


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