苏丹构造地质.docx
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苏丹构造地质.docx
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苏丹构造地质
StructuralgeologyofSudan
Contents
Introduction
Basementcomplexstructures
SouthernSudan
CentralSudan
JubalelBahrelAhmer
NorthernProvince
Ringcomplexesand“NewerGranites”
Post-basementcomplexstructures
Sudan-Ethiopianfrontierzone
Sudanplainlands
Riftstructures
Nimule-JubaandtheSudan-Uganda-Congofrontierzone
Sudan-Kenyafrontierzone
JubalelBahrelAhmer,RedSeaLittoral,andoffshorebeltoftheRedSea
Introduction
TheDemocraticRepublicoftheSudanoccupies967,498squaremilesinnorth-easternAfrica.Littlehasbeenpublishedpriorto1960aboutitsregionalstratigraphyandstructure,althoughnotinconsiderablelocaldetailexistsinofficialfilesandreports.
Datapresentedinthispaperarebasedonsomeofthesesources;thefewpublishedworksavailable(e.g.Andrew,1948;Delany,1954,etc.);newdatacollectedbythewriterandhiscolleaguessince1960;andanaccountofthegeologyofSudan(Whiteman,19656).
Themaingroupsofstructuresrecognizedanddescribedare:
(a)basementcomplexstructures;(b)post-basementcomplexstructures;and(c)riftstructures.
TheformationsandgroupsrecognizedinSudanarelistedinTable1;andthedistributionofmajorstructures,togetherwithotherbasicgeologicalinformation,ispresentedinFigure1.
TABLE1.Generalgeologicalcolumn
Nameofunit(SudanGeologicalSurveyMaps,2ndand3rded.Scale1:
4,000,000)
Nameofunit(Whiteman,1965d)
Age(Whiteman,1965d)
Percentagesurfacearea
Superficialdeposits
Superficialdeposits
Quaternary-Tertiary
Notdifferentiated
Coastaldeposits(RedSeamargin)
Continental-marineformationsRedSealittoral
Quaternary-Tertiary-
Mesozoic
0.57
UmmRuwabaseries
Gezira,UmmRuwabaandElAtshanformations
Quaternary-Tertiary
19.12
Lavas,Tertiary
Volcanicsandintrusives,mainlylavas(undifferentiated)
Quaternary-Tertiary
2.41
HudiChert,MiddleTertiary
HudiChertformation
Tertiary(?
)Oligocene(?
)
0.02
Nubianseries,Mesozoic
Nubiansandstoneformation
LateCretaceous(?
)
28.13
Yirolbeds
Yirolformation;equivalenttoNubianformation(?
)
LateCretaceous(?
)
0.05
Notdifferentiated
Gedarefsandstoneforma-tion
Mainly(?
)Jurassic(Pre-Oxfordian)
IncludedinNubianestimate
Nawaseries
Nawaformation
Palaeozoic(?
)
0.09
Palaeozoicsandstones
Palaeozoicformations
(undifferentiated)
Palaeozoic(?
)
0.23
Basementcomplex
Basementcomplexgroup
Pre-Cambrianmainly
49.51
Basementcomplexstructures
Thename“basementcomplex”isusedbelowtoincludethoseigneous,metamorphic,andsedimentaryrocks,whichareoverlainbyhorizontalandsub-horizontalPalaeozoicorMesozoicsedimentaryorigneousrocks.TheyareassumedtobemainlyofPre-Cambrianage(Whiteman,19656).
Littleisknownaboutthestructureofthebasementcomplexgroup.Fewobservationsareavailableconcerningdirectionanddipoffoliationetc.andlittleisknownofregionaltrends.TheofficialSudanGeologicalSurveyMapisat1:
4,000,000scaleanddoesnotshowstructuraldata.General
trendsandphotolinearsareshowninFigure1.
FIG.1GeneralgeologicalmapbasedonSudanGeologicalSurveyMap,1949and1963,scale1:
4,000,000;SouthernSudanGeologicalMap,scale1:
2,000,000;andWhiteman(1965b).
SOUTHERNSUDAN
General.ThebasementcomplexofsouthernSudanconsistsinpartofagroupofgranoblastic,foliatedbasictoacidgneisseswithhyperstheneandfeldspar,intrudedintofoliatedparaschistsandparagneisses.Thehypersthenebearingrocks,whicharerelatedtothecharnockiticgroupmappedinUganda(Fig.2),arecutbynon-charnockiticintrusions.Felspathoidalsodasyenites,thoughttobePost-Palaeo-zoic(?
)byAndrew(1948),alsooccur(Fig.2).TrendsaresaidtobeconstantoverwideareasofEquatoriaProvincevaryingfrom180-337degrees.
ThestructureoftheNile-CongodivideislargelyunknownbetweenJubaandHofratenNahas(Fig.2).TheregionisshownasaswellseparatingtheChad,Congo,SuddbasinsbyHolmes(1965,Fig.763);andontheProvisionalStructuralMapofAfrica(Furonetal.,1958)northerlystructuraltrendshavebeensketchedforthe“PI”basementcomplex,apparentlyasprojectionsfromtheCongoorsimplydrawninastrendsparallelingdrainagelines.Andrew,Auden,andWright(1955,GeologicalMapofSouthernSudan,scale1:
2,000,000)whomappedthearea,didnotplotsuchtrends,andthetrendsshownbyFuronetal.(ProvisionalStructuralGeologicalMapofAfrica,1958)differinplacesbyasmuchas15degreesfromtheknownonesintheHofratenNahascopperarea,forexample.
AssuaMylonitezone.ThismylonitezoneentersSudannearNimulebuthasnotbeenmappedindetail(Figs.1and2).Almond(1962)describedthezoneintheKitgumfrontierdistrictofUgandaascomplexandshowingvariabledegreeofmylonization.Originallydescribedasdextral,Almond(personalcommunication)nowthinksthefaultmovementwassinistral.Theamountoflateralmovementisnotknown;markedlydifferentrocktypesandstructuresoccuronthetwosidesofthezonehowever.
TheMaadiQuartzitefoldsinSudanparalleltheAssuatrendandtheseprobablycontinueasfarasKidi(Fig.2).TheBahrelJebe1followstheAssuatrendasfarasKajoKaji.ThemylonitezoneandrelatedstructureswereprobablyformedinLatePre-CambriantimesanditisagainstthesestructuresthattheMiocene-RecentriftfaultsendnearNimule(Whiteman,1965).
InthecentralSudan,especiallyeastoftheWhiteandMainNiles,aroughfivefolddivisionofthebasementcomplexmayberecognized(Table2,column5).
OutcropsarefewandscatteredintheGeziraandadjacentregions,howeveratJ.MoyaandJ.SaqadinearSennarmetasedimentsdipeastat70degreesandtrendeast-north-east.JebelBereima,southofJebelAuliaDamontheWhiteNile,iscomposedofhighlybrecciatedfelsitetrendingnorth-southanddippingsteeplyeast.
ThepredominantstrikeoftheBasalschistsandphyllitesintheGedaref-SennarandtheIngessanaHillsregionsis22degrees.
InKordofantothewestthestrikeofthemetamorphicrocksispredominantlynorthtonorth-east(MansourandSamuel,1957;HuntingTechnicalSurveys1:
250,000LandUseandGeologicalMaps,Kordofan,1964;Whiteman1965b).
Quartzo-felspathicgarnetiferousgneissesandamphibolites,whichformthecountryrockintowhichtheSabalokaringcomplexisintruded(Fig.1),trendbetween292and365degrees,andfoliationdips±65degrees(Whiteman,19656).TheSabalokaringcomplexisdiscussedbelow.
JUBALELBAHRELAHMER
IntheDerudebarea,southernJubalelBahreiAhmer,Delany(1955)describedthesequenceshowninTable2,column6.TheOdiSchistsheretrendENE.andNE.andthegreygneissesofKassalatrendroughlyN.-S.(Fig.1).
InthenorthernandcentralJubalelBahrelAhmerthesequenceshowninTable2,columns7and8wasdeterminedbyRuxton(1956),Gabert,RuxtonandVenzlaff(1960),Kabesh(1964)andWhiteman(19656,1968).
OntheMohammedQôlsheet,northernJubalelBahrelAhmer,foliationandlineationarewelldeveloped;theregionalstrikedirectioninthe“PrimitiveSystem”orKashebibgrouprangesfromeast-westinthecentreofthesheettoNE.-SE.toN.-S.inthenorthernpart(Fig.1).Therocksarefoldedinbroadnorthwardplunginganticlinesandsynclines.Aneast-westdykeswarmoccursintheMashebibgroup(Ruxton,1956).
TheNafirdeibgroupisfoldednearGebeitMine,andinthesouthoftheMohammedQôlarea,inopenanticlinesandsynclineswithfoldaxesplungingatlowanglesandbearingroughly80degreesand57degreesrespectively.Smallthrustfaultsarecommoninthearea(Ruxton,1956).
Acid,intermediate,andbasicdykescutthebasementcomplexformationsinmanypartsoftheJubalelBahrelAhmer.TheyformastrikingreticulatesystemofLatePre-Cambrianage(740m.y.±48m.y.K-40toAr-40Date,JebelHamashaweib,Whiteman,1965ú).Directionsvarybutarepredominantly70degreesand340degrees.ThedykeswereprobablyformedduringanearlytensionphaseinthedevelopmentoftheRedSeadepression(Whiteman,1968).
Ruxton(1956,p.328)statedthatallthebasementcomplexformationsarefaultedandthattheregionsouth-westofJebelErbais“amosaicoftiltedfaultblockswithprominentescarpmentsanddipslopes”.Faultbreccias,upto200feethavebeenrecordedalongsomewadis.InRuxton’sopinionthesefaultsaretensionalandareofuncertainage.Sestini(1965)mappedanumberofphotolinearswithsimilartrendsinthisarea.
ThedominantstrikeoftheOyoformation(equivalenttotheNafirdeib)ontheDungunabsheetis
north-eastswingingtonorthinthenorth-eastpartofthesheet.Tearfaultsaffectingbasementcomplexrocksonlyoccurinthesouth-eastpartofthearea(Gass,1955).
NORTHERNPROVINCE
InformationhereisofageneralkindandismainlyrestrictedtooutcropsalongtheNile,toJebelUweinatandtoisolatedareasinbetween.Thebasementcomplexincludesgneisses,schists,quartzitesandmarblestogetherwithlargebodiesofgranite,syeniteanddiorite(Hottingeretal.,1959).
JebelUweinat,originallyclassedasPre-Cambrianbasement(Sandford,1935etc.),formspartofalargeringcomplexofPost-Carboniferousageintrudedintothemetamorphicbasement.Thedominantstructuralbasementtrendisnorth-easterly.NostructuraldataisavailablefortheisolatedbutnumerousoutcropsbetweenUweinatandtheNile.
Marbles,interbeddedwithhighlycontortedgneisses,outcropinSudanPortlandCementCompany’squarryonthewestbankoftheNilenearAtbara.Regionalstrikeis70degreesanddipsareapproximately35degreestothesouth-south-east.Foldingiscomplicated,themarbleshavingactedi
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