Ceol.Soc.Malaysia,Bulletin6,July1973;pp.87- 11 6. APreliminarySynthesisoftheGeologicalEvolutionof BurmawithReferencetotheTectonicDevelopmentof SoutheastAsia MAUNGTHEINI Farbetteranapproximateanswertotherightquestion,whichisoften)lague, thananexactanswertothewrongquestionwhichcanalwaysbemadeprecise. John W.Tukey(1962)* Abstract:Apreliminary synthesis is attempted with a view to trace the geological deve-lopment of the Burmeseregionintime and space. It isbasedpartly onregionalfieldexperi-encesa ndpartl yonthepubliheda ndunpubli shedinformation that havebecome available inthe last decade. The model approachis found to be quite usefulinthi s synthesis. It ishoped thatthispaperwouldstimul atelivelydiscuss ions,furtherresearchandsounder syntheses. The synthesisismadeintermof seventopics : tectonicfr amework;areas of deposition andnon-deposition;regi onal depositionalpattern; tectonic and depositional environments; structuraldeformation a ndrel atedmetamorphism; igneous activity and associated minerali -zation;Burmeseregioninthegeneralframeworkof continentaldrift.Thetectonicframe-work isdescribedwiththe aidof agenera li zedtectonic mapnewl y compiledbythe author; and four geo-tectonic domains are recognized andformallynamed. Theseq uenceofmajorgeologicaleventsintheBurmeseregionisencompassedina summarychart,andfullera ndmorerefi nedaccountsaregiveninthetextbymeansof time-lengthdiagramsandotherkindsof modelexpressions.Time-strat igra phyof thePre-cambrian, Paleozoic and Mesozoic rockunits is schemat ically shownby means of arepresen-tative time-length cross-sect ion a long the Shan-Tenasserim regionwhich has become a stable block sincethe close of theMesozoic. The regi ona l depositionalpattern of the Tertiary sedi-mentsintheCentra l CenozoicBeltisshownbymeansoftwotime-lengthcross-sections, one along the general depositi onal strike a ndthe other across it.The genera l pattern isinter-preted asone of cyclical sed imentation. Contrary to some recent conceptions of the Central Belt as ageosyncline,avail a ble evidences seemto support an old idea- thatthe Central Belt mostprobablyconstitutesagraben. The geological developmentof Southeast Asiain generaland of the Burmese regionin particular inthe general framework of continental driftisconsideredinthe last section.On thebasis of fitting of thepre-Terti a ryoutlines,presence of the Ninety East Ridge,andsimi-la rity of certainrock series,itisspeculatedthat the Bay of Bengalmay actually be a spheno-chasm,thusimpl ying that Southeast Asia andthe Indian Peninsula were probably once joi n-edtogetherinthedistantgeologicalpast. INTRODUCTION Tosomeitmayseemrather prematuretoattempt asynthesisof thegeological evolutionof Burma.It istruethat wedonotknowenoughyet,andthat there isnot muchinformation in the fieldof regionalstratigraphic andtectonicanalyses made for the Burmese region.However, there already are available new data and recent findings enoughtojustifyapreliminary synthesis(see"ReferencesCited").Totheauthorit always seems worthwhile to strivefor a uni fiedandintegrated picture of the geological eventsspreadover awiderangeof timeand space,provided,of course,that such an attempt isnot mere speculation. This stillholds even if one has to stick one's neck out, soto speak.Besides, suchan attempt wouldstimulate discussion, further research and 1Departmentof Geology,Arts&ScienceUniversity,Rangoon. * requotedfrom" StatisticalModelsinGeology"byW.e.KrumbeinandF.W.Graybill(1964). 88 MAUNGTHEIN sounder syntheses which are badly neededforabetter understanding of theBurmese geology.There may be theusualpitfall of over-simplification and over-generalization on the author's part. His excuse in this regardistwo-fold:(i) that they are desirable at a preliminary stage and may serve asbasisfor fullerand more refined accounts later; (ii) that they are unavoidable in the faceof incomplete information available at present. Thispaper maybe regardedasasequeltoan earlier paper by theauthor anda colleague that wasfirst written in 1966 andlater published with minor revision in 1970 (Maung TheinandBa Than Haq,1970).Many works on the subject,both published andunpublished,that have appearedsince then provide valuable information forthis synthesis.Thesearedulyacknowledgedinappropriate places. TopicsConsidered The modelapproachisconsideredindispensableinawork of this nature,andis thereforeemployedratherextensively.Thesynthesiswillbemadeintermsof the followingtopics: I .tectonicframeworkof sedimentation. 2.areasof depositionandnon-deposition(paleogeography). 3.regionalpatternof sedimentation,includingsedimentaryfacies . 4.typeoftectonicanddepositionalenvironmentsonthebasisof lithologic associations. 5.structuraldeformationandrelatedmetamorphism. 6.igneousactivitywithorwithoutassociatedmineralization. 7.Burmeseregioninthegeneralframeworkof continentaldrift. Thesetopicsobviouslyareinter-relatedandmoreorlessconsequentialsothat they cannot beconsideredseparately.Mostof thetopicsare encompassedin Fig.(3) which isa general summary chart of the geological events spread out in time and space fortheBurmeseregion.Fuller accountsaregiveninFigs. (4)-(7). One of the right and pertinent questions to be considered isin connection with the theoryof continentaldriftwhichhasbeenconfirmedlately(e.g.,seeHurley,1968 ; Wilson,1963 ; Heirtzler,1968).The questionis:How doesthe Burmese region fitin the general framework of continental drift? This isdealt with in the last section which, of course,containssomespeCUlations. TECTONICFRAMEWORKANDDOMAINS As is wellknown, the Burmese region may besubdividedinto four linear geomor-phic belts,informally named as (from east to west):the Eastern Highlands, the Central Belt, the Arakan-Chin Ranges, the Arakan Coastal Plain.These beltsare mere reflect-ionsof the underlying stratigraphy andstructuresin that each contains lithologically andstructurallyrelatedrockunits.Thustheyalsomayberegardedasdistinctgeo-tectonic domains.In order to convey the tectonic framework of the region, the author wouldproposeherethefollowingnamesforthesedomains:theShan-Tenasserim Massif, the Central Cenozoic Belt, the Arakan-ChinFolded Belt,theArakan Coastal Plain(seeFig.1). APRELIMINARYSYNTHESISOFTHEGEOLOGICALEVOLUTION89 o I .... /------ -:..., ....1========1 1--------......'" ~ - - - - - . - . - ,300KM I ___---,.J ..... ' .. . _,,, Fig.1.TheGeotectonicdomainsofBurma(1. Shan-TenasserimMassif;2.CentralCeno-zoicBelt;3. Arakan-Chin FoldedBelt;4.ArakanCoastalPlain). The Shan-Tenasserim massifisconstituted of Precambrian, Paleozoic and Meso-zoic rocks, formed partly in mobile beltsand partly on shelves, which were later highly deformed andmetamorphosed. The Cenozoic sedimentsare almost completely absent in this part except for smalloccurrences of oilshalesin the Tenasserim area and lacu-strinedepositsintheSouthern Shan States.There probably wereat least fiveperiods 90 .. " '4 II .. Ii II" " ,. ,0 i MAUNGTHEIN .." .." EXPLANA nON J{QENt:RALSTRUCTURALTREND MAJORFAULTS,DOWN-THPfOWNSIDE SHOWNANOLINEA/ItMOVEMENTS INDICATEDBYARROWS oSTRUCTURALBASIN oUPLIFTS - DOMESI!o ARCHES o CENOZOICSEDIMENTSDEPOSITED INNON - OROGENICBELT5 GRANITICROCKSaGRANITEGNEISS INTERMEDIATEaBASICVOLCANICS GULTRABASICINTRUSI VES .. ..... 10 .I;"" to '. .!. .. ,,"Ii \ _ . \ ~ i r Fig2.GeneralizedTectonicMapof Burma. 100' ,i ,i ... Ii Ii ,. Ii Ki 100 APRELIMlNARYSYNTHESISOFTHEGEOLOGICALEVOLUTION91 of major structural disturbances and upheavals, as manifestedby distinct stratigraphic breaks,up tillthe endof the Mesozoic when the areabecameastablelandmass (see alsoFig.3). As the name implies, the Central Cenozoic Belt containsalmost exclusively Ceno-zoic sediments. Available evidencesseem to indicate that the sediments were laid down in alinear, down-faulted trough, or, a graben (seeFig. 2).The rocks were later subject-ed to gentleandbroadfoldingrelating tothe Alpineorogenyin theBurmeseregion. The Arakan Coastal Plainisthe narrow southern part of the Assam Trough, and the lithologicaltypesdepositedtherearesimilartothoseoftheCentralBelt.Thetwo troughs,theAssamandtheCentralBelt,mayhaveactuallyformedatwintrough (or,atwingulfaswasfirstenvisionedbyStamp,1922)that wasdividedbyamedian positivebelt-the Arakan-ChinFoldedBelt.Thetwotroughsapparently werenon-orogenicinsetting,asisevidentfromthemolassiccharacterof theirlithologyand absenceof tight foldingandlarge-scaleover-thrusting.TheArakan-Chin belt on the other hand, was a mobile belt whichhasundergone rather intense orogenic movements manifestedbylarge-scaleoverthrustingandtightfoldinginthegreywacke-flysch sequencespresent. In short, the above-named geotectonic domains constitute the tectonic framework of Burma whichisbest seenin Fig.2,ageneralizedtectonic map compiled primarily fromthe followingsources:Tectonic Map of Burma andGeologicalMapof Burma (both compiledbyM.D.C.in1963?); Aung Khin and Kyaw Win (1969).It should be pointed out here that thepresent map isquite differentfrom the M.D.C. map in terms ofapproachandscope. NoteinFig.2thatthefourdomainsmaybefurtherdifferentiatedintosub-domains or geotectonic elements.Thus, there are at least three structural basins and at least three structural upliftsin theCentral Cenozoic belt whichmaybe regardedasa graben asitmost probably isbounded bymajor faultsor fault systems.In the absence of evidencestoindicatethe existenceof intervening tectonic upliftsand arches,these basinsshouldberegardedasstructural,ratherthandepositional,basins. In the Shan-Tenasserim massif, the differentiation into sub-domains isnot clearly seenpartlybecauseof thecomplexityof thestructuresinvolvedandpartlybecause theregionhasnot been properly mappedyet. It isinterestingtonotethatigneousbodiesformtheboundariesof somesub-domains.Thealignmentof thelinearbodiesof granitesandrelatedrockswiththe regionalstructuraltrendintheEasternHighlandsishighlysuggestiveof thesyn-tectonic or late-tectonic mode of emplacement formost of these rock bodies. EVENTSINTHEGEOLOGICALEVOLUTION Fig.3 illustrates,in summary form,thesequenceof major geologicaleventsthat are believed to have taken place through time in the Burmese region. The stratigraphi-calunitsarelistedinorder forthreemajorgeotectonicdomainsunderthegeneral heading, "Stratigraphy, Lithology and Thickness". The events, relating to alldomains aregroupedunder three generalheadingsasfollows:' 1.tectonicanddepositionalenvironment 2.structuraldeformationand metamorphism 3.igneousactivityand/ormineralization. 92MAUNGTHEIN STRATIGRAPHY,LITHOLOGYaTHICKNESSTECTONICa f-------,--------,----------' DEPOSITIONAL STRUCTURAL IGNEOUSACTIVY8 DEFORMATION aIIORMINERALZATION METAMORPHISM GEOLOGICALAGE I.Arokon-Chinr lQion2 . CentrolBe lt3 . EalternHiQhlondl .ENVIRONMENT >-I ZHOLOCENE a: New.,AlluviumN I JJj II _-' l____ploleougrovels, 5ForRegion2

a: '"PLEISTOCEN u:::>_ 1-' '+----1 0PLIOCENE OlderAlluvium ( thin) IrrawaddySer ies Land- arlO N 0 >-z a: -u a: .., >-MIOCENE OLIGOCENE

""'1 rock( thi ck) PeguSeries lubQreyw.aSh.fms ail- bearing (virYthick ) EOCENEAIc:1a lSeries subgreyw. a Sll.fms coal bearing ____ ,h":"(v"e", rY:-..;.th...;;c"k;...l aILimestonelimestone(thin) 1-- ____.....,.. uCRETACEOUS OtherUnits f-------j . 1\,greywockea r-oJURASSICmetasediments lacustrinea cavedeposils fluviol \brOOdfoldinga - - - - , - - locolthrust ing fluvlola deltaIc;upliftofcentr, belt continuedfilling volcanismolano Central V-lconicBelt porphyryCumineral!! InMonywoor eo of grabenorooenicmovementsintrusionofyounger Land- area______(LoteAlpine)-Qronite01000Mogok 1 r;.eofPeguyomoBelt poralica neritic; graben filling;orogenic movements moinlymolossic(EarlyAlpine) facies,iseofArokan yoma:!int rusionof oronite5; intrusionof ult rabasic L- f-F; 3" -::::;.alongArokanyomos claw RedBedsa Cr_Ni.PImineral!! conolomasilts! orooenic movementsemplace-men!of ___ _____ (:::Kimmeridoian)graniticrocksalong flyschl ower ;coolporoHca neritic;

V)(mostlyflysch6 ..,f-------j t urbiditefocies)?? (Verythick)'"TRIASSIC r11f (thin)orogenicmovements (:::: Appalachion) Limitedmetom.PERMIAN u 'Clastlcs(thin) I,..,____ ___ PlateauLImestone Limestonea dolomite unstableshelf _CAR90NIFEROU Land-ono? of-------j NDEVONIAN 0---'-----1 lond-oreo? III??

r-'"epicontinental; uppershallowworm seowithhigh IIS orogenicmovements k:.Coledonion) regionalmetam in Kayoh _Tenasserim extensivePb- Zn mineralizationin ShanStotesjgranifes WSILURIAN --'f------I i J1j J ,_,1 PRECAMBRIAN? Kanpetlet Schists pf:Basemen!? clasticsaminor Ls (mediumth.)C\ .__ -- ? --- --- ? -- Ls, sh,5S.!! (med;umthlif clastics8volcaniC! III (mediumth.)--? -- -?--ChaungMogyiSr. metasedim. aC\...:Ec; ?MogokGneiss EXPLANATION []]]] Breakinthegeoloqlcolrecord;couldreperesent non - depos.itionorerosionalvacuity Unconfor mity;brokenwhereinferredorapproximate I shownwith?wherepositIOnisuncer!aln _____Approximateboundarybetweentheoeologicolunits orevenls . mainlyneritic, m0Q8osyncl inol eUQeosynclinol l imitedvolcanism in S. ShanStates. acidvolcanisma Pb-Znmineralizat ion orogenicmovementsinN. ShonStates (:::::"Charnion) Ipeorogeniesa -metom. , loleorogenic granites(?) Thicknessclassification verythick> 10,000II . thick5000'10,000 It. mediumtho thin 1000- 5000 It. < 1000It. Fig.3.Chart illustratingtheGeologicalEvolutionof Burma. In addition, Figs.4-7 are prepared soasto give fuller accounts.Figs.5,6(a)and 7(b)areessentiallymodelexpressionswhicharegenerallyreferredtoastime-length diagrams.Such figureshave been usedas powerful tools in stratigraphic analysis (e.g., seeSloss,1962). Figs.3-7aretheoutcomeof interpretationsandcontemplationsof theauthor partly on hisown regional fieldexperiencesand partlyon the information contained APRELIMlNARYSYNTHESISOFTHEGEOLOGICALEVOLUTION93 (.) H 0 N 0 z Ilr:l U u H 0 N 0 (/) Ul ): u H 0 N 0 IZl til p.. GEOLOGICALAGE z &j HOLOCENE E-< p PLEISTOCENE 0' PLIOCENE >-< MIOCENE H E-< &; OLIGOCENE E-< EOCENE CRETACEOUS JURASSIC TRIASSIC PERMIAN CARBONIFEROUS DEVONIAN SILURIAN ORDOVICIAN CAMBRIAN PRECAMBRIAN DIASTROPHISMIGNEOUSACTIVITY Relat iveIilt ens ityRelativerntensity ? riseof Peguyomas rise ofShan-Tenasserim basic volcanics granites acid volcanics granites? Fig.4.Relativeintensitiesofdiastrophismand igneousactivitythroughtimeinthe Burmeseregion. 94MAUNGTHEIN in the followingworks:La Touche (1913);Chhibber (1934);NyiNyi(1964);Brunn-sweiler(1966);HildeandCeleste(1967);AungKhinandKyawWin(1968,1969); Maung Thein and Ba Than Haq (1970); Ba Than Haq (1970);Myint Lwin Thein et at (1971).Certainspecificinformationswouldbementionedin appropriate places. In order tosimplifytheauthor hasdeliberatelyomittedthestratigraphicandli-thologicaldetails.Onlysome well-known time-stratigraphic-unit namesare included. The sequence of the geological events would now be followedin the chronological order. Precambrian Stratigraphyand Age Asinother parts of the world,the Precambrian rockunitsin Burma aremainly of low-grade metasediments and gneisses (probably orthogneisses). The metasediment-aryunits,occurring rather widespreadly, were firstgroupedunder the Chaung Magyi Series, and the gneisses, of lesser areal extent,under the Mogok Gneiss by La Touche (1913).It isnowknownthat theChaungMagyiSeries*maybedesignatedasuper-group, being made up of at least two groups, each consisting of a number of formations (Ba ThanHaq et at.,1970;Maung Thein et aI. , 1971). The seriesobviously formsa basement of probably Precambrian age in the Shan States, as it is found to be always overlain unconformably by the lower Paleozoic units, whichinmanyplacesareOrdovicianinage(La Touche,1913;BrownandSondhi, 1933),but insome placesareCambrian(Myint Lwin Thein et ai.,1971).Thestrati-graphicequivalentsof theseriesintheKayahState(partsof theMawchiSeriesof Hobson(1941))andinTenasserim(partsof theMerguiSeriesof Rau(1930))also occupytheloweststratigraphic positionin therespectiveareas.In otherwords,the Chaung MagyiSeries and its stratigraphic equivalents,occurring as large inliers,pro-bably form the basement in the Shan-Tenasserim region.However, as shown inFig.5, thisbasementmayoccupyahigherstratigraphicpositionintheKayah-Tenasserim area.This isbecause the Mergui-MawchiSeriesinthe latter area isessentially a com-posite unit whose age may range from Precambrian up to Silurian (M. Thein and B.T. Haq,1970). The presence of astrong negativeanomalyalongthemiddleof theCentralBelt (Evans andCrompton,1945)suggeststhat the Precambrian sialic basement probably ispresent underthethick Cenozoic coverof theCentral Belt.Asforthe existenceor non-existenceof thePrecambrian rocksintheArakan-Chin region,the problem has notbeensolved.ThepresentconsensusisthattheKanpetletSchists,lithologically rathersimilartotheChaungMagyiSeries,maybePrecambrianinage(seeKyaw Win,1969). La Touche(1913)firstcorrelatedtheChaungMagyiSerieswithShillongSeries of Assam (Dharwarian?) on lithological similarity. This is not as far-fetchedasit first seems;as will be seen later it isquite possible. * The termsSeries, Bedsusedinthispaper are merely oldnames,andassuchthey generally do not conveyanytime-stratigraphic connotationsasunderstoodinthemodernstratigraphic nomencla-ture. Although they are obviously improper terms,their use cannot be avoidedat this stage asmost Burmese rock-units have not been reclassifiedinaccordance with the modern stratigraphic practice. 0 Cenozoic 109 CretaceousAmherstRedBedsk Jurassic 200 Triassic IOyalO/ Beds MortabanSeries Permian UpperPlateaulimestoneMoulmeinLimestone p 300 Carboni ferous UpperPlateauLs , C c 'EDevonian o 400 Silurian5 Ordovician o 500 .." Cambrian MaIO/chiSeries MerguiSeries 600 --------------Precambr ianCheungMagyiSeries PC N0 I NomTu 100 I 200300 14 001500milesI S I I MaymyoMyogyiPindayaKolawPinlaLllgMaIO/chiPaponE. ofMoulmeinE.ofYo Fig.5.Generalizednorth-southtime-lengthcross-sectionalongtheeasternhighlands(verticallyruledareasdenotebreaksin thestratigraphicrecords(lacunas),mostlyrepresentingnon-deposition(hiatus)inthiscase) 96MAUNGTHEIN Thestratigraphic positionof theMogok Gneississtilluncertain,nor isitsrela-tionshipwiththeChaungMagyiSeriesknown.Thelithologicalcharactersof these gneissesand migmatites-orthogneissic,streakyandbandedover fairlywideareas-are somewhat similar tothose of the Precambrian gneissesin some parts of the world. In fact,La Touche(1913),Pascoe(1950)and Iyer(1953)heldthat thesegneissesare similar to the Dharwars (Archaean) of Mysore, India. It isprobable that these gneisses andmigmatitesarePrecambrianinageandpossiblyolderthantheChaungMagyi Series. Lithology and Environment The Precambrianmetasedimentaryunits,particularlytheChaungMagyiSeries, contain (in order of abundance) phyllites, slates (some pyritiferous),metagreywackes, psammitic phyllites,mica schists,quartzitesand graphite schists.Thislithologic asso-ciation issuggestiveof depositionundereugeosynclinalconditions.If correlationof theChaungMagyiSeries/MerguiSerieswiththePhuketSeriesinThailand(Koba-yashi,1960) and with the Kao-liang Series in Yunnan (Loveman, 1919) holds,it means thatthisPrecambriangeosynclineprobablyextendsnorthwardsintoYunnanand southwards into Malaya. This geosyncline probably did not extend westwards into the siteof the present Central Belt.One possibility isthat thelatter was non-existent then (seeFig.8(a)). StructuralDeformationand Metamorphism It appearsthattheYunnan-Shangeosynclineenvisionedabovewasabeltof intense tectonic activity asissuggestedbythe highly deformedcharacter of the meta-sedimentsof theChaungMagyiSeriesanditsstratigraphicequivalents.Themeta-sedimentsaretightlyfoldedandisoclinalfoldingisnot uncommon.It isnot known how many orogenic episodesoccurredduring thePrecambrian.It isconceivablethat theregionalmetamorphism took placeduring theseepisodes.The episodes probably culminated into the Charnian orogeny towards the endof the Precambrian.This oro-genyaccountsforthe pronouncedstratigraphic break that existsbetweenthetightly foldedandmetamorphosedChaungMagyiSeriesandthefossiliferousOrdovician units in the Shan States.This break probably is absent in the southern area (seeFig.5). It wasonce believed that the so-calledMogok Series wasmetamorphosed during the Precambrian (La Touche, 1913; Iyer, 1953).However, new information obtained in recent yearssuggest that the metamorphismof part of thisseries,particularly the so-calledMogok CrystallineLimestones, occurredmuch later, probably during Tertiary in relation to the Alpine-Himalayan orogeny (Searle and B.T. Haq, 1964). The original rockunitsmaybePlateauLimestoneorevenCretaceouslimestones(Clegg,1941). In fact,Maung Thein and Soe Win (1969)have shown conclusively that the metamor-phism in parts of the Mogok Belt isdefinitely post Plateau Limestone. IgneousActivity and Mineralization It ispossiblethat emplacementof largebodiesof graniteaccompaniedthe Pre-cambrianorogenies.Up to nowonlysmallgranitestockshavebeenfoundtobein-trusiveintotheChaung MagyiSeries'(e.g.,Ba Than Haqet a!.,1970).It ispossible that older granites may have been later metamorphosed into granite gneisses.Even if radiometric dating is done, the problem is likely to remain because dates obtained may merelyrepresentmetamorphismof thepre-existinggranites.Mineralizationrelating tothe Precambrian igneousactivitieshave not been noted yet. APRELIMINARYSYNTHESISOFTHEGEOLOGICALEVOLUTION97 Paleozoic Stratigraphyand Age In Fig. 5 an attempt is made to explain thestratigraphic relationships of the Paleo-zoic andMesozoic rocksin the Eastern Highlands by meansof arepresentative time-length cross-section. It will be noted that the Shan States are the only area in the whole Burmese region where a reasonably complete Paleozoic sequence exists. The Cambrian unitswerenotrecognizedintheShanStatesupto1969.In1970afieldpartyfrom RangoonArts&ScienceUniversityfirstrecognizedtheexistenceof rocksbearing Cambrian trilobites in the core of the Pindaya Range in the Southern Shan States, and thelithostratigraphicunitsthusrecognizedwereformallygroupedunderthename Moelone-heinGroupwhichisunconformablewiththeunderlyingChaungMagyi Series (Myint Lwin Thein et al.,197 J).In many parts of the Shan States it appears that theCambrian unitsare missing,andastratigraphic break isgenerally recognized.In such areas the Middle and Upper Ordovician units lie unconformably on the Chaung Magyi Series.The Ordovician and Silurian units essentiallyare either conformable or para-conformable in most parts of the Shan States (seeLa Touche,1913; Myint Lwin Thein et al. , 1971).The correlation of the Ordovician and Silurian units of the North-ern Shan States with thoseof theSouthernShanStates hasnot been properly made. The tentative relationships of the major units are shown in Fig.5. Recent investigations bythe fieldpartiesof RangoonArts&ScienceUniversityindicate that theso-called OrthocerasBedsof theSouthernShanStatesmayactuallybeSilurian,ratherthan Ordovician,inage(Myint Lwin Thein et al.,1971). In theKayah-TenasserimareatheLowerPaleozoicisrepresentedbythecom-positeMergui-MawchiSerieswhoseagehasbeenshowntorangefromprobably Precambrian to Silurian (Maung Thein &Ba Than Haq, 1970).Saito's (1959a) attempt to sub-divide the Mergui Series into two groups-the upper of Paleozoic and the lower of Precambrianages-obviouslyisprematureandmisleadingashisobservationon PatawandPatitisletsnearMerguiisareallyverylimited.TheTaungnyoSeriesof Leicester(1930)intheKayahStatewasregardedby Pascoe(1959)to be equivalent totheMerguiSeries.Thestratigraphic relationshipbetweentheOrdovician-Silurian unitsof theShan StatesandtheMergui-MawchiSeriesisnot known. Exceptforsomepartsof theNorthernShanStateswhereZebingyiLimestone beds (Upper Silurian) are conformable with the Lower Plateau Limestone of Devonian age(LaTouche,1913),thereisgenerallyamajorstratigraphicbreakbetweenthe Lower Paleozoic unitsandthePlateauLimestone (Carboniferous-Permian in most parts)throughouttheShan-Tenasserimregion.Thisbreakisinterpretedtobethe widest in theKayah-Tenasserim area where the presence of Lower Plateau Limestone has not been noted(Fig.5).Its presence israther doubtful even in the Southern Shan States. That thisbreak becomeswidersouthwardsmaybeduetothesouthwardtrans-gressionof the warm,shallowUpper Paleozoic sea.Aspointedout byMaung Thein and Ba Than Haq (1970) , this isevident in the fact that in the Shan-Yunnan region the thickcarbonatesectionpresentisDevoniantoPermain in age(PlateauLimestone), whileinUpperTenasserimthecarbonatesection,whichisconsiderablythinner,is CarboniferoustoPermianinage(MoulmeinLimestone).Furthertothesouthin Thailand there isan extensive occurrence of athick limestone sequence of Permian age (RatburiLimestone). 98MAUNGTHEIN The lithostratigraphy of the thick and widespread Plateau Limestone, firstnamed anddescribedbyLa Touche(1907,1913),isstill under systematicinvestigation.It is now known that the Plateau Limestone, in the Southern Shan States at least,isa large group consisting of at least six rock units of formation rank (Maung Thein et al.,1971). It wasrecently noted byFay Lain (personal communication,1971),on thebasis of fossildiscoveries,thatthe PlateauLimestoneinsome partsof the SouthernShan States probably extends up to lower Triassic in age.Such Triassiccomponentscould bethelateralequivalentsof theKamawkalaLimestoneintheKawthoolei-upper Tenasserimarea.TheauthorconsiderstheYinyawBeds(Hobson,1941)andthe Martaban Series asrelatively thin clastic wedges in the upper part of the thick carbon-ate sequence of primarily Upper Paleozoicage(seeFig.5). The presence of the Paleozoic units has not yet been noted in both the Central Belt and the Arakan-Chin region. This probably meansthat these areasformed alandmass during the Paleozoic, possibly supplying detritus to the Paleozoic seas in the east. There isalso another possibility that the twoareas werenon-existent asyet then (Fig.8(a)). Lithology and Environment TheLowerPaleozoicconsistsofbothclasticandcarbonatesediments-lime-stones,silty,blotchy,alsoblueandbedded;marls;buff siltstones;graptolite shales; calcareoussandstones ;etc.Theselithictypestogetherwiththeirfaunalcontents suggestdepositionunderprimarily aneriticenvironment;theabsenceof greywacke-flyschtypes andthe very limited occurrence of dolomitized limestonessuggest that the tectonic set-up was very likely a miogeosyncline.On the other hand, the various lime-stone and dolomiteunitsof Upper Paleozoic (PlateauLimestone),bearing abundant coralremains,wereobviouslydepostiedinawarm,shallowseawithhigh(HC03) activity.The Permian clasticunitsof theKayah-upper Tenasserim area maybe con-sideredas clastic wedges formedinthe environment of an unstable shelf. StructuralDeformationand Metamorphism Thereoccurredat leasttwomajordiastrophicepisodesintheShan-Tenasserim regionduring thePaleozoic(Fig.4).The firstthatoccurredneartheendof Silurian (hence Caledonian) may beregarded asthe last phase of the development of the lower Paleozoic miogeosyncline of the Shan-Tenasserim region.It accounts for the structur-aldeformationof theLowerPaleozoicunits,andprobablyisthecauseof thebig, widespreadstratigraphicbreak that existsbetweenLowerPaleozoicandthePlateau Limestone. It appears that the orogenic activitywas feltmore strongly intheKayah-Tenasserim area than in the Shan States asthe beds of the Mergui-MawchiSeries are more highlydeformedandmetamorphosed. Thesecondthatoccurredneartheendof thePaleozoic(henceAppalachian) accounts for the broadand gentle folding in the thick carbonate sequence. Perhaps the movements were partly epeirogenic in nature, and the initial uplift of the Shan-Tenas-serimregionmusthavebegun.Thisepisodegaverisetothepost-Paleozoic,strati-graphic break whichisrather extensive.Regional metamorphic effectsrelating to this episode arerarelyseen.However,thesemovementscouldwellberesponsible forthe intensebrecciationcommonlyseenintheincompetentPlateauLimestonebeds. IgneousActivity and Mineralization There waslimitedacidvolcanism, representedbytheBawdwinVolcanicSeries, probably during Cambrian, in the Northern Shan States.Mineralization ofthe famous APRELIMINARYSYNTHESISOFTHEGEOLOGICALEVOLUTION99 lead-zinc deposits of the Bawdwin Mines was probably related to thisigneousactivity. Aratherli mitedcontemporaneousvolcanismoccurredduringOrdovicianinthe SouthernShanStates. The existence of igneous activity, of considerable intensity, relating to the postula-tedCaledonianandAppalachianorogeniesintheBurmeseregionseemsnolonger questionable.Granite bodieshaving(i)pre-Carboniferousandpost-Silurian,and(ii) pre-upper Triassicandpost-Permianages,havebeenshown(byradiometricdating) tooccurinnorthernThailand(BaumetaI.,1970).Thefirst-mentionedgranites, whichmaybeconsideredastheCaledoniangranites,arefoundtobecontinuous, across the borders, with the granties in southeastern Shan States (Fig. 2, 20 N/ IOOE). On the basisof the radiometric dates the presence of Permian and early Triassic gran-ites has also been established in Malaysia (Anonymous,1966). Thus it isreasonable to saythattheHercyniangraniteswereprobablyemplacedin theBurmeseregiontoo. Mesozoic Stratigraphyand Age Unlike the Paleozoic units the Mesozoic units are not welldeveloped in the Shan-Tenasserimregion. In fact,somerockunits,suchastheNapeng Beds(Rhaetic),are verylimitedinoccurrence.TheMesozoicrocksarefoundtobemostlyrestrictedto linear outcropbeltswhich arebelievedtorepresentdepositionalbasins.At least two suchbasins,tentatively namedastheKalaw andLashiobasins,arebelievedtohave existedduringtheJurassic(Fig.2).RegionalstratigraphicanalysisbyFayLain (personalcommunication,1971)indicatesthattheKalawBasin isasynclinalbasin. The Loian Series,depositedin this elongated basin, isagroup consisting of a number of formations.Recentfieldstudies(bytheauthor)alongthewesternmarginof the Shan Plateau have revealedthat another long belt of Jurassic units isalso present west of thisbasin.TheNamyauSeries(Jurassic)of theNorthernShanStatesaremore limitedinoccurrence. Thereexistsomemisconceptionsonthenatureof stratigraphicrelationshipbe-tweentheLoianSeriesandtheKalawRedBeds.Forexample,SaitoandUozumi (1959,p.23- 24)statedthat thetwoseriesare merelytwodifferent faciesdepositedin the same basin during the Cretaceous. This statement isincorrect, because there isan apparent stratigraphic break between the tightly foldedLoian unitsand theoverlying Kalaw Red Beds which are broadly folded.As willbe discussed later, this break marks the time when fl uvial anddeltaic conditions first prevailedin the Shan area. The age of the Kalaw Red Beds isstillproblematical asauthentic fossils have not been discovered sofar.(SaitoandUozumi(1959)erroneouslymentionedthattheKalawRedBeds contain Turrites and Baculites sp.).The present consensus isthat it probably isUpper Jurassic toCretaceousinage.Itsstratigraphic equivalentscouldbetheRedBedsof Amherst in Tenasserim (Chhibber,1934),andpart of theKhorat GroupinThailand (Sato,1971). The Mesozoic rockunitsare practically absentinthe Central Belt.Onthe other hand, they are now known to be present in the Arakan-Chin region. Myint Lwin Thein (1958)reportedtheoccurrenceof Doanellasp.inaslateunitoccurringinnorthern Chin Hills. Thus it isevident that at least part of the lithic sequence ofthisfolded belt isasold asthe Triassic.Kyaw Win (I969)concluded that alarge part of the Arakan-Chin region is Eocene or older in age. Thus it is reasonable to assume that the so-called Axial Series and relatedrock units are probably TriassictoEocene. 100MAUNGTHEIN An interesting point about the Axial Series isthe occurrence of large exotic blocks of Cretaceous limestoneswithin the shales(e.g.,along thePadaung-TaunggupPass). Thesourceof theirderivationisstillproblematical. Lithology and Environment The Mesozoic sediments asa whole may be considered to havebeen deposited in synclinal basins and linear troughs,the latter being probably down-faulted,occupying the still submerged pal tsof the rising Shan Plateau. Theoccurrenceof evaporites(PanngoEvaporiteof Brunnsweiler,1962)inthe Triassicof theNorthernShanStatessuggestsarestrictedmarineenvironment.The lithologyof lowerJurassicunitsintheSouthernShanStates,consistingmostlyof flysch-likesediments(containingshallow-marineammonites)andsometurbidites, indicates that a neritic, geosynclinal environment probably prevailed within the basin. Asdepositionproceededthebasinbecamegraduallyshallowersothataparalic environmentensued.Thischangeprobablyaccountsforthedepositionofsub grey-wackesandcoalmeasuresinupperJurassic.Recentfieldinvestigationsalongthe westernmarginof theShan Plateaushowthat athickunitof greywackes,slateand quartzitemaybeJurassicinage.Heretoo,geosynclinalconditionswereprevailing. TheKalawRed Bedsconsistof unitsof redsiltstonesandconglomerateswhich are generally polymictic.Such lithological typesareindicativeof an oxidising,mainly fluvialto deltaic environment.In termsof tectonic facies,the red beds-conglomerates (in places fanglomerates) association may be saidto represent amolassic faciesderived fromtherising highlandscomprisingmainlyof LoianSeriesandthePlateauLime-stones. The Axialand relatedunits intheArakan-Chinregion are made up primarily of greywackes,shales,slatesandintruded serpentine.The general sedimentary type may bedescribedasflysch.Sucha lithologicalassociation indicatesadeep-watergeosyn-clinalenvironmentof deposition.Thesedimentaryrecordshowsthatthiscondition continuedintoEocene(seeKyaw Win,1969). StructuralDeformationand Metamorphism Thereweretwomajorepisodesof diastrophismduringtheMesozoic.Thefirst which took place near the end of the Jurassic (hence Kimmeridgian) probably accounts for the intricate and tight folding and complex faulting undergone by the incompetent Loian beds. Brunnsweiler (1962),however,contendedthatthesecomplexstructures resultedfromgravitationaldown-sliding.Assmall-scaleover-thrusting hasnot been observed in association with these structures, thiscontention cannot be substantiated. It must be pointed out that tight folding may alsohave resulted from lateral compres-sion due to vertical block movements within the Shan massif.Regional metamorphism of theJurassic rocksalong the westernmarginof theShan Plateaumighthave been relatedtotheKimmeridgianorogenicmovements. Theotherepisodeprobablybeganwhi letheKalawRedBedswerestillbeing deposited.The movements involved are considered to be primarily epeirogenic as they causedthefinalupliftof theShan-Tenasserimregionalmosttotheirpresentlevels. (Thisuplift, of course, wouldbecoupledby theinitialsubsidence of the CentralBelt that lay immediately west of the faulted junction). In thisconnection, Sato's (1971,p. 53)statement that theso-calledBurmese-Malayangeosyncline became aland before APRELIMINARYSYNTHESISOFTHEGEOLOGICALEVOLUTION101 the endof the Triassic seemsincorrect.On theother hand,recentinformations (e.g., Kyaw Win,1969) suggest that the Arakan-Chin region became alandmassonly at the endof Eocene.This isdifferent fromChhibber's(1934)contentionthattheArakan-Chin region wasuplifted at the endof Cretaceous. Saito(1959b)contentedthatthecrustaldeformationof thecomplexMesozoic structuresalongthewesternbordersof theSouthernShanStatestookplaceduring MiddleTertiary.Thereisnosupportingevidenceforthisstatement,however.As mentionedabove,thedeformationcharactersandsequenceseemto suggestthat the LoianbedswerecomplexlydeformedduringtheKimmeridgianorogenyandthe overlyingKalawRedbedslesscomplexlydeformedattheendoftheMesozoic. TheMesozoictectonicandmagmaticeventsenvisionedinthisandsucceeding sections (seealsoFig.4)are in generalagreement withthosedated by thermolumine-sceceintheIndosinian-Thai-MalayanOrogen(Hutchison,1968a,b).Therearenot any discernible regionalmetamorphic effects relatingtothemovements attheendof theMesozoic. IgneousActivity and Mineralization The Mesozoic was a time of intense igneous activity as the bulk of the granites and granite gneissesin Tenasserim andalongthe westernmargin of the ShanPlateau (see Fig.2)were probably emplaced consequent to the postulatedKimmeridgianorogeny. Evidences,thoughlargelyindirect,arenotlackingfortIllSstatement.In viewof the factthat the tin-bearing granites of WestMalaysia have beenshown to be mainly late JurassicandpartlylateCretaceousinage*(Anonymous,1966;HildeandCeleste, 1967;Alexander,1968),itisreasonabletoassumesimilaragesforthetin-tungsten-bearinggranitesof Tenasserim,whichafterallarethenortherncontinuationof the WestMalaysiangranites.Furthermore,recentfieldinvestigationsintheYinmabin area have shown that the granite bodiesinsome places are intrusive into the metasedi-mentsof Jurassic age. These granites could well be the northern continuation of the Tenasserim granites sinceboth liein thesamegranite belt.Inthisregard,Chhibber' s conclusion that the Tenasserimgraniteswereintrudedduring lateCretaceousandearlyEocenetimesis onlypartly true. Granitegneissesarequitecommonalongthisbelt,especiallyinthemiddleand northernparts.Inmanycasestheyoccurcloselytogetherwithgranites,andtheir mutual contacts are mostly gradational. It is, therefore, clear that they are orthogneis-seswhich werederived fromthe associatedgranites by later regional metamorphism. It isnotknownwhetherornotsomeofthemwerederivedfromthepre-existing granitesemplacedduringtheearlierorogenies. TheKimmeridgianigneousactivitymentionedaboveisperhapsthelargestre-corded fortheBurmeseregion(Fig. 4). In thisconnection,radiometric dating of the Tenasserimgraniteswouldbeamost fruitfulundertaking. It is,of course, common knowledge that tin-tungsten mineralization accompanied thehydrothermalphaseof thelarge-scaleemplacementof lateMesozoicgranites. * Hutchison (1968b,Fig. 6)mentionssome data whichdisagreewiththisstatement.Onthebasisof 41age detemlinations compiled by him,he purportstoshow that thelargerpartof granitesinthe Indosinjan-Thai-MalayanOrogenwere emplacedinthe late Triassic-early Jurassic times. 102 MAUNGTHEIN Igneous activity relating tothe lateCretaceous-early Eocene movementsappears tobequite pronounced in the Arakan-Chin region (seeChhibber,1934).In that part, smallultrabasicbodies(mostlyserpentine)wereintrudedinto the AxialSeriesalong theeasternsideof thefoldedbelt.Nickel-chromiummineralizationwasassociated with these intrusions. Cenozoic Stratigraphyand Age TheCenozoicstratigraphyof Burmaisbetterknownbecauseofthefollowing facts:(i)relativelyeasieraccessibility;(ii)lesscomplexstratigraphyandstructures; (iii)OCCUlrenceof mineral fuels-oil , natural gas, andcoal- within the Tertiary forma-t ions.Nevertheless,therestill remain anumber of problems,e.g.,the problem of the general pattern of sedimentation and thatof regionalcorrelation whichismadediffi -cultbylateralfacieschanges,bothinlithologicalandfaunalaspects. TheoldestsedimentaryunitintheCentralBeltappearstoberepresentedby patchesof Cretaceous rocks in theKachin State whichmaybethewestern extension of the Cretaceous limestones exposedat the First and Second Defiles of the Irrawaddy (seeClegg,1941).These patches formsmallinliersinthe Tertiary belt . The lithostratigraphyandtime-stratigraphyof theTertiaryunitsof theCentral Belt andArakan Coastal Plain havelong beenasubjectof voluminousstudiessince Theobald(1874)firstpublishedhisclassicmemoironthe"Geologyof Pegu"(Here, only some major works will be mentioned: Cotter (1912); Stuart (1912); Stamp (1922); Lepper (1933) ; Tainsh (1949);Aung Khin and Kyaw Win (1969);Kyaw Win and Thit Wai (1971).In these works, many rock unitshave been recognized,but the status and correlation of these are stillin a state offlux. A review on the tertiary stratigraphy will not be attempted here. Onlysomeessentialpoints willbementioned. The Tertiary rock unitsmay begroupedunderthreebroadunitsof grouprank, namely(inascendingorder),theEoceneSeries,PeguSeries(Oligo-Miogene),and Irrawaddy Series (Pontian-Pliocene), which are separated from one another by distinct unconformities.These unitsare very thick sequences, having total accumulated thick-nessesof about 40,000ft. , 25,000ft. , and10,000ft. , respectively, intheMinbuBasin wheretheTertiarysuccessionisthemostcompleteandtheirstratigraphythebest known (data from Aung Khin and Kyaw Win, 1969). This also is the general type area fortheTertiarysuccessioninBurma.In thisgeneralarea,theEoceneSeriesconsist of at least fiverock unitsof formationrank,andthePeguSeriesof at least sixsuch units. AnunconformityislocallyrecognizedwithinthePeguSeries.Asforthe Irra-waddy Series, differentiation into formation has not been properly done because of the scarcity of indexfossilsandtheratheruniformlithologic character vertically. In the Arakan-Chin region, recent investigations (Kyaw Win,1969, p. 1) show that "the main bulk of the Southern Chin Hills isEocene with inliers of older rocks". Rock units younger than Eocene have not been recognized, and it is, therefore, very probable that theregion has been aland-area sincethe closeof Eocene. There areonlyminoroccurrencesof Tertiary sedimentsin theShan-Tenasserim region.Thebestknownoccurrencesaretheoilshalesof lateTertiaryageinthe Amherst and Mergui districts, and lacustrine deposits, probably of the same age, in the SouthernShanStates(Chhibber,1934). APRELIMINARYSYNTHESISOFTHEGEOLOGICALEVOLUTION103 It is not known what liesbeneath the Eocene beds in the Central Belt.As noted in an earlier section,the Paleozoic andMesozoic rocks may probably be absent beneath thethick coverof theCenozoic sediments. Lithology and Environment TheEoceneandPeguSeriesconsistof alternatingunitsof subgreywackesand-stonesandshales.Thesandstonesarefeldspathicandglauconitic,andmostlycal-careous, containing a large proportion of primary calcite cement.Shales in the Eocene seriesaremostlygreenish(henceprobablyglauconitic),but areblackandcarbona-ceousinsomeareas(e.g.,YawShales).ShalesinthePeguSeriesaremostlygrey-greenishor bluish.Allof theseunitsgenerallycontainbrackishandshallowmarine faunainthecentralandsouthernparts,andfresh-waterandbrackishfaunainthe northernpart.Moreover,theEoceneSeriesiscoal-bearingintheLowerChindwin area.Such lithological and faunalcharacters suggest that the generalenvironment of deposition is primarily neritic in the central and southern areas, and fluvialand deltaic inthenorthernarea.Thegenerallithologicalassociationismorelikemolassethan anythingelse.Thisviewisinagreementwiththatof Brunnsweiler(1966)whoput forwardaflysch-molassemodelfortheArakanY om as-CentralBelt.Aswillbedis-cussedinthenext section,thedepositionalframeworkmost probablywasagraben, andtheborderinghighlandsof theShanPlateau(sincetheendof Cretaceous)and theArakanYomas(sincetheendof Eocene)apparentlywerethesourceareas. There wasa change in the depositional environment asthe sediments of the Irra-waddySeries werelaiddown.Thisseriesconsistsprimarilyof buff or reddishsand-rocks*andsandstoneswithminorshalesandshalysandstones.Thesandstones generally are poorly cemented, and contain fossilwoodaswellasterrestrial vertebrate remains. Large-scale trough-type cross-bedding is commonly seen. All these characters indicate that the Irrawaddy sedimentsasawholeweredepositedunder fluvialtodel-taicconditionsduringcontinuedfillingof thegraben. Depositional Framework Contrary tosome recent descriptionsof theCentralBelt asthe"Burma Tertiary Geosyncline"(e.g.,Aung KhinandKyaw Win,1969), the author wouldrathersub-scribe, with somt- modifications, to the old view of Cotter \..1918) and Stamp ()922) who consideredtheCentralBeltessentiallyasariftvalkyorgraben.Totheauthorthis conceptual model isnot only simple, but it fitswith many observational facts (see also Fig.2). Availableinformationandevidencesinsupportof thisviewmaybelistedas follows: 1.The Central Belt isbounded by major faultsor fault systems whose existence is certain in the east, but isless certain inthe west (seeAung Khin and Kyaw Win,1969,GeologicalMap of Burma; Dey,1968). 2.EnormousthicknessesoftheTertiarysediments(inexcessof75,000ft.) deposited, under conditions generally not deeper than neritic, within a relati-vely short interval of geological time,imply a large amount of continual and rapid subsidence of the Central Belt during the Tertiary. This seems possible only if the depositional site isadown-faulted trough. * The term "sand-rocks" is used for very loosely cemented sandstones, following the usage of Chhibber (1934). 0 f ... z ... 20u 2 'l1s:II 0Z .. u ~ 0 E c ::; -4C 0 .. Z .. u 0 '" QO (b) S f 6 f __ Dept!. (nG!toIlea.te) 100 .... ,l,aTin 1440 Cha.uk 50. I n.ayetmJO "'0 m,L 8 Fig.6(a).Schematicnorth-southtime-lengthcross-sectionalongthecentralbelt(verticallyruledareasdenotebreaksinthe stratigraphicrecords(lacunas),mostly representing non-deposition(hiatus)inthis case). Fig.6(b).Hypothesizedtime-depthcurvefor6(a)(f,fastersubsidenceresultingintransgression;s,slowersubsidenceandsilting-up resultinginregression). APRELIMINARYSYNTHESISOFTHEGEOLOGICALEVOLUTION105 3.Thepresenceof azoneof strongnegati vegravityanomalyrunningnorth-south along the middle of the Central Belt (Evans and Compton, 1946).This finding alsoimpliesa large amount of subsidenceof the belt with respect to thebordering highlands. It isnot out of place todiscuss here the useof the term "geosyncline" to describe the tectonic framework of the Central Belt.The useis inappropriate and misleading in the sense used by most stratigraphers (e.g., Krumbein and Sloss,1963).For one thing, theTertiarysedimentsdonotexhibitthelithological,structuralanddeformational characteristics normally foundingeosynclinalareas(e.g., seeWeller,1960,p. 250-2). Also, asshown abovethe Central Belt most probably has beenalinear down-faulted trough.In any case, the author would propose asimple and non-committal name for theCentralBeltasadepositionalunit,namely,the"BurmaCenozoicTrough". Depositional Patterns The general depositional pattern of the Tertiary sediments asinterpreted and con-ceivedbytheauthor areshowninFigs.6 and7.Figs.6(a)and7(a)aretheauthor's interpretations based partly on the information giveninStamp (1922) and Aung Khin and Kyaw Win (1968), respecti vel y. ArakanYomasPeguYomasShanPlateau PPliocen MMiocene 1ioocene w- -- -- - ---z- ===== ==_-_ w "=2u - - - - ---g- ----=-=---= -= -E:J

w Z - deeperner i ticabat hyalfocies o w 60 50100 Arakanyomas 1502yOmiles MinbuPeguyomasYametrj n 2f Inlelake Fig.7(a).Diagrammatic stratigraphic cross-sectionalonglat.200Ntoshow (I)infilling of the raben, (II) off-lappingof the Tertiary formationsonboth sidesof theArakanYomas. Fig.7(b) .SchematicEast-WestTime-Lengthcross-sectionacrosstheCentralCenozoicBelt (along lat.20N)(vertically ruled areasdenote lacunas,mostlyrepresenting hiatusinthiscase). 106MAUNGTHEIN It shouldbe noted herethat Chhibber' s generaldescription11934,p.210)of the Tertiary history of the Central Belt as"the infilling of the Burmese gulf by river-borne sediments from the north and bymarine sediments in the south" seems still essentially correct. After all, it iseasier to visualize deposition at the present Irrawaddy Delta and off-shore as merely the continuation of the Tertiarysedimentation in the Central Belt. What theauthorattemptstopresenthere,bymeansof time-lengthdiagrams,isthe interpreted cyclical pattern of sedimentation during the Tertiary. Asshown in Figs.6 and 7,the author considers the various Tertiary formations, present in different structural domains, as genetically related. This consideration isthe logicaloutcomeof regardingtheCentralCenozoicBeltasagrabensubsidingcon-tinually,though not necessarilyat thesameratethroughouttheCenozoic.Inother words,arealvariationsseeninthe Tertiary formationsof theCentralBeltare inter-preted as merely lateral facies,shifting north and south across the time lines as a result of marine transgressions and regressions which, in turn, were probably brought about by the alternating phasesof slow and rapid subsidences (Fig.6(b)). In drawing Fig.6 (drawn across the depositional strike) the following factsabout the Central Beltare taken into account: 1.alternation of sandstone and shale formationsin many areas, particularly in the Minbu Basin. 2.predominantly marine sedimentation in the south, and alternation of marine and continental sedimentation in thenorth forthe Eocene and PeguSeries; fluvialanddeltaicsedimentation fortheIrrawaddySeries. 3.apparentwedgingoutof someunitsonaregionalscale(e.g.,seeStamp, 1922). Explanation for(1)and (3):Asshown inFigs. 6(a)and(b) alternation of shale and sandstone formationswithin theMinbu Basin,forexample,are interpreted asdue to sudden deepening of the trough (resulting in transgression) followed by slower subsid-ence, hence the gradual silting up (resulting in regression). In this context, a thick shale unit, such as Pyawbwe Clay,overlaps upon Okhmintaung Sandstone during the trans-gressive phase and isoff-lapped byKyaukkok Sandstone in the regressive phase.The wedge-shapedunitspresumably wereformedinthisway. Explanationfor (2):Although there probably wereat leastsix cyclesof major trans-gression-regression,thegeneraltrendof theCenozoicsedimentationintheCentral Beltappearstohave beengradualregressiontowardssouth(Fig.6(a).Thisexplains why,asChhibber (1934,p.212)put it,"fluvial and deltaic sediments kept continually advancing towards the south". Thus, the Irrawaddy Series constitutes a predominantly continental facies. Fig.7 attemptstoshowthegeneraldepositionalpatternmoreor lessalongthe depositional strike of the CentralBelt.Infilling of the graben and sideway off-lapping of the Tertiary formationson both sidesof the Arakan Yomasare depicted by means of a stratigraphic cross section and a time-length cross-section. In Fig.7(b) the follow-ing points areworthyof note: (1)regressionbygradualfilling-uponthe twoflanks of the Arakan Yomassince the closeof Eocene;(2)gradualtransgressionof thesea eastwards fromthesiteof the present PeguY omastowards the Shan Plateau. Minor reversalswhichmust haveoccurredduring majortransgressionsandregressionsare diagramatically shownby meansof "kinks" . APRELIMINARYSYNTHESISOFTHEGEOLOGICALEVOLUTION107 StructuralDeformationand Metamorphism There probably were at least three major orogenic episodesduringthe Cenozoic. The firstoccurredat the end of Eocene,and theseearlyAlpinemovementsprobably caused the folding as well as rise of the Arakan-Chin region. This wouldbe sobecause, as noted above, sediments younger than Eocene have not yet been found in that region. These movements evidently were also felt in the Central Belt as there isawell-marked unconformity betweenthe Eocene and the youngerunitsin most areas. There isnoevidencetoindicatethattheEocenesedimentswereregionallyme-tamorphosed during the early Alpine movements in the Central Belt proper. However, asbelievedbySearleandBa ThanHaq(1964),it isquitepossible thatregionalme-tamorphism along the marginal zone with theShan Plateau (the Mogok Belt of Searle andB.T.Haq)tookplaceduringthesemovements.Thisbecameazoneof intense structural deformation and metamorphism probably because the rocks of the marginal partswerestronglypushedagainstthestableblockof theShan-Tenasserimregion. It is conceivable that parts of the lateMesozoic granites were transformed intogranite gneissesduring thisdeformation.Up tonowthere isno evidencetoindicatethat the Pegu sediments, present just west of this zone,were ever metamorphosed. This,there-fore,seemsto ruleout thepossibility that metamorphism along theMogok-Beltwas post-Miocene. The second episode probably occurred near the end of Miocene, and this obvious-lyaccountsfortheunconformitybetweenthePeguandIrrawaddySeries.ThePegu Yomas must have been upliftedduring this time as there are no known occurrences of the Irrawaddy Serieson the PeguYomas.The movements involvedseemonly strong enough tocause gentle foldingand warping in manypartsof theCentralBelt. The third episode must have occurred after the deposition of the Irrawaddy Series, because in many parts of the Central Belt, the Irrawaddy Series and the underlying Pegu units are found to have been structurally deformed together.Local thrusting obviously accompaniedthefoldingmovements,e.g.,atChaukwherethrustingisdefinitely post-Irrawaddian.The Central Belt was probably uplifted at the end of this episode. IgneousActivity and Mineralization Igneous activity was pronounced during the Cenozoic. The intrusion of "younger" granitesalong parts of thenarrow Mogok belt wererelated tothe late Alpinemove-ments. A good evidence for this statement isthe late Miocene agesobtained (by radio-metricdating)fortwogranitesandapegmatitesamplefromtheMogokarea(see Maung Thein& Ba Than Haq, 1970, p.278).Radioactive minerals are associated with some pegmatites intrudedduring thisactivity. The volcanic activity that isbelieved to have occurred primarily during the Quar-ternaryiswellmanifestedalongtheCentralVolcanicLineof Chhibber(1934)that passesthroughtheWunthomass,LowerChindwinvolcanoesandMt.Popa.The principal rock types are basalts andandesites,but rhyolites and related typesare also present in lesseramounts.Asiswell-known,porphyry copper mineralization isasso-ciated with the acid volcanicsintheLower Chindwinarea. Summaryof theGeologicalEvolution Although it would be repetitious with respect toFig.3,it isthought desirable to givean outline summary of the geological evolution of Burma inamore readable and sequentialformasfollows: 108MAUNGTHEIN 1.Precambrian (a)depositionof theChaungMagyiSeriesand lowerpart of theMergui-Mawchi Series under eugeosynclinalconditions in the Shan-Tenasserim region. (b)Charnianorogenyandrelatedregionalmetamorphism. 2.Lower Paleozoic (a)depositionof theMoelone-heinclasticsintheSouthernShanStates, andextrusionof theBawdwinvolcanicsintheNorthernShan States, togetherwithassociatedlead-zincmineralization. (b)depositionofOrdovicianandSiluriancarbonatesandclasticsunder predominantlyneritic,miogeosynclinalconditionsintheShanStates, together with limited contemporaneous volcanism in the Southern Shan States. (c)Caledonianorogenyandrelated regional metamorphism in theKayah-Tenasserimarea,probably followedbyintrusionof granites,especially in the eastern part; rather extensive lead-zinc mineralization in the Ordo-viciancarbonates. 3.UpperPaleozoic (a)deposition of a thick carbonate sequence (Plateau Limestone) in a warm, shallow sea (covering most of the Shan-Tenasserim region) which trans-gressedsouthwardsduringtheUpperPaleozoic ;formationofsome clasticwedgesduringupperPermianin theKayah-Tenasserimarea. (b)epeirogenicmovementsandinitialemergenceof theShan-Tenasserim region at the end of the Paleozoic. 4.Triassic-Jurassic (a)depositionof clasticsandevaporitesunderrestrictedmarineenviron-ment in smallenclosed basinsduring the Triassic in the Northern Shan States;depositionof flyschandDaonellafaciesprobablybeganinthe Arakan-Chin region. (b)depositionof theLoianclasticsandcoalmeasuresinsynclinalbasins anddown-faultedtroughswithinthealreadyrisingShanmassif under geosynclinal conditions during the earlier part and under paralic condi-tions during the later part of the Jurassic. (c)Kimmeridgianorogenyandtightfoldingof theJurassicbeds; limited metamorphismalongthewesternmarginalzoneof theShan Plateau. (d)large-scale emplacement of the bulk of granitic rocks in Tenasserim and along the westernmarginof theShan Plateauwith associatedtin-tung-sten mineralization. 5.Cretaceous (a)depositionof theKalawRedBedsessentiallyasa molassic faciesinan oxidising,fluvial to deltaic environment; continued sedimentation inthe Arakan-Chin region. (b)epeirogenicmovementsandfinalupliftof theShan-Tenasserim region tobecomealandarea. Theuplift wasprobablycoupledbytheinitial subsidenceof the Central Belt lying in the west. APRELIMINARYSYNTHESISOFTHEGEOLOGICALEVOLUTION109 (c)intrusionofsomemoregranitesalongtheShan-Tenasserimregion; intrusionof smallultrabasicbodieswithassociatedchromium-nickel mineralizationalongtheArakan-ChinrangesduringlateCretaceous-earlyEocenetimes. 6.Eocene (a)continued sedimentation under eugeosynclinal conditions in the Arakan-Chin region;deposition of molassic faciesin the alreadysubsiding Cen-tral Belt graben-mainly under continental and deltaic conditions in the south. A cyclical patern of sudden deepening followed by gradual silting-upmighthaveprevailed. t b)Early Alpine movements resulting in the folding and rise of the Arakan-Chin region;metamorphism along the Mogok Belt. 7.Oligo-Miocene (a)continued fillingof thestillsubsidinggraben(PeguSeries)in thesame depositionalpatternasin6(a). (b)Late Alpine movements and uplift of the Pegu Yomas. (c)intrusionof "younger"granitesalongtheMogokBelt. 8.Pliocene-Pleistocene (a)depositionof the Irrawaddiansedimentsunder fluvialand deltaiccon-ditionsastheCentralBeltgrabenwasfinallyfilledup. (b)upliftof theCentralBeltwhiletheTertiarybedswerebeingbroadly foldedand locallythrusted. (c)volcanism along the Central Volcanic Line; porphyry copper mineraliza-tionintheacidvolcanicsof theLowerChindwinarea. 9.Holocene denudationandcontinentalsedimentationgoingonsidebyside,andthe developmentof the presentconfigurationof theBurmese region. SOMESPECULATIONS As mentioned in the introduction,oneof the right and pertinent questions to be considered is:How doesthe Burmese region fitin thegeneralframeworkof the con-tinental drift? The solution of this problem isobviously not easy, and speculations are bound toariseinanyattempt tosolveit.Neverthelessitoughtto beattemptedin viewof thefactthatthetheoryof continentaldrifthasbeenconfirmedlately.Such anattempt,evenif alittlespeculative,wouldatleastserveasfoodforgeological thought. Theresultsof variousattemptstoreconstructGondwanalandbyfivedifferent authorities(du Toit,Lester King,S.W. Carey,J.T.Wilson,and Arthur Holmes)are quitedifferent(seeHolmes,1964,figs,874and875),butallareverysimilar inone aspect.Thisistheomissionof theSoutheastAsiaregioninmakingtherespective "reasonable" fits.This may be due to the fact that firstit is difficult to fitthe region in the general framework.Secondly, to the above authors the region may seem a-little in-significantintheworld-widepicture.Tobesureonecannot simplydismissthat the Southeast Asia mass was non-existent before continental drift first took place probably 110MAUNGTHEIN during lateMesozoic,because thereexists alargearea of Precambrian andPaleozoic rocks in the region.The author thinks it isabout time toconsider thedevelopment of Southeast Asiainrelationto theContinentalDirft. It israther well-knownthat theSoutheast Asiamassisacontinentalarea,pro-bably underlain by the sial (see "Indian Ocean Floor" prepared by National Geographic magazine, 1967). Whether Southeast Asia has drifted from the west isopen to question. In any case,theauthor believesthat the geologicaldevelopmentof Southeast Asia in general, and of the Burmese region in particular, is closely connected with the develop-ment of the Bay of Bengal that separates Southeast Asia fromthe Indian Peninsula, In thisrespect,therearethreepossibleexplanationsforthedevelopmentof theBayof Bengal : 1.It hasalwaysbeenan oceanicbasin,probably underlainbythesimaunder-neath the GangesCone. 2.It represents the "scar" left by the drifting of Australia away fromthe Indian Peninsula aswas postulated by Ahmad (1961). 3.It isasphenochasm*formedbythewedgingapartof theIndianPeninsula westwardsandSoutheast Asiaeastwardsawayfromthe Ninety East Ridge. Asfor(1)there are not evidencesto indicate either foror against.However, this idea cannot explain the formationof the Ninety East Ridge and the Java Trench. Only by the spreading of the sea-floorcan their formationbeexplained. As for (2) Ahmad (1961) claimed"a particularly well-established Permian fit" for IndiaandwesternAustralia(seeHolmes,1964,Fig.871),andinCarey' s(1958)re-construction of the Gondwanaland thisIndo-Australia link is also shown (see Holmes, 1964, Fig.874).There are, however, at least three objections to this view: 1.Aspointedout by Smith (1971,p.227),perhaps the main generalobjection against fittingIndia and Australia together isthat theUpper Carboniferous tillite pattern does not matchacross the join. (seealsoFig.15.14of Smith's paper). 2.The regional trends of eastern Austral i a and of Southeast Asia are not fittable. 3.The postulateddirectionof driftof Australiadoesnotcorrespond withthe directionof sea-floorspreading knownat present (e.g.,seeHeirtzler,1968). Asfor(3)itwasrecentlydrawntotheauthor'sattentionthatBurton(1970) already made this kindof proposal formally inprint*. It must alsobe mentioned that the following statementbyBurton(1969)cameto theauthor'sattentiononlylate-ly.Referring to wrench faulting inMalaya,Burton states (p.13):"The present writer considersthat thisfaultingisan expressionof theseparationof theYunnan-Malaya orogen from the Indian Shield". There are at least four points that seem to support the sphenochasm speculation: 1.Presenceof theNinetyEastRidgewhichrunsnearlynorth-south,withits projected northern end apparently passing through the postulated hingearea near theShillong Plateau.Thisridgecouldwellmark theaxisof spreading * inthe senseasfirstusedbyCarey(1955) * inPaleogeography/Paleoclimatology/paleoecology,v.7.Ihavenot seenthispaper yet. APRELIMINARYSYNTHESISOFTHEGEOLOGICALEVOLUTION111 sea-floor fromwhich theopposing landsof Indian PeninsulaandSoutheast Asiahavedrifted. 2.Rather remarkable fitting of the pre-Tertiary outline ofthe Shan-Tenasserim-Malayaregionwiththeeasterncoastline(ofPrecambrianandPaleozoic rocks)of IndianPeninsula,except forCeylon. 3.Similarity between the Shillong Series of Assam and the Chaung Magyi Series of the Shan States (La Touche,1913) which can be fittedas a continuous belt beforethedrift (Fig. 8(a)). 4Spreading of thesea-floor northeastward fromthe mid-Indian Ocean Ridge, andthe floorisfinallyunderthrustedat thecontinentalmargin of Southeast Asia to formthe Java Trench. Whetherornotthesea-floorisspreadingeastwardsandwestwardsawayfrom theNinetyEast Ridgeisnot yetknown.It presence,oncedetected,wouldprovide a more conclusiveevidenceforthe presentspeculation.In thisconnection,fourpoints thatseemtodiscardthisspeculationshouldalsobementioned. 1.TheMerguiSeriesof Tenasserim arenot similar to thePrecambrianKhon-dalitesandunclassifiedcrystallinesof theVizagapatamarea. 2.AssummedupbyChhibber(1934,p.144,159)theLowerPaleozoic fauna of the Northern Shan States are more closelyrelated to that of the Northern Europe rather than tothat of theHimalayas.Thisthereforeseemsto reject theideathattheShan-TenasserimregionandeasternIndia were joined to-getherduringthePaleozoicaspostulatedinFig.8(a).However,thismay notbe aseriousobjectionbecause,asChhibber(p.174)alsosummedup, thereisastrikingresemblancebetweenthePlateauLimestonefaunaand the Permo-Carboniferous faunaof the Salt Range and CentralHimalayas. 3.ThePermianSystemofMalayaapparentlyaredissimilartotheLower Gondwana System of Peninsular India in terms of both lithology and paleon-tology.(seeGobbett,1968;Wadi a,1957). 4.The Upper Carboniferous glaciation in India has no counterpart in Malaysia. To sum up,it istheauthor's contention that the Indian Peninsula and the Shan-Tenasserim-Malayaregionwereoncejoinedtogetherinthedistantgeologicalpast. It shouldbepointedout that thiscontention isnot indisagreement withthe present-day plate tectonicsof Northeastern Indian Ocean, because it ismerely referring to the pre-Mesozoic tectonic framework of Southeast Asia to which the plate-tectonics model seems not applicable. It shouldbementioned inthisconnection that there are reasons to believe that only the post-Paleozoic geology of Burma can perhaps befittedinthe general frameworkof the plate tectonicsof Northeastern Indian Ocean.(Some of the reasonsare:existenceof blueschistterranesinthenorthernpart of theCentralBelt; alpinotype peridotite intrusions and thick flyschsequences on the trench-side (Arakan Yomas);molasseonthearc-side(CentralBelt).Allof thesearelocatedoutsidethe Shan-Tenasserimmassif of PrecambrianandPaleozoicrocks.) Thisspeculationisnot necessarilycontradictedbythepossiblesouth-southeast-wardmovement of theSoutheast AsianblocksuggestedbyRodolfo(1969),because thisisamorerecentevent occurring probably duringLateMioceneto Recent times accordingto Rodolfo(p.1203). 112MAUNGTHEIN The author's speculations regarding the development of the Burmese region in the general framework of the continentaldrift are diagrammatically shown in Fig.8.Fig. 8(c) and (d) correspond to the so-called Kuenen model as cited and named by Rodolfo (1969a). Thesespeculationsaresomewhatindisagreementwiththeinterpretationmade byRodolfo(1969a,b),namely,that theArakanYomasandtheAndaman-Nicobar ( d.) (c) Md.9yi SI!!1"les (b) ..;> CI I: 2: '1>Aenocho..s,"" ? , Cd) Fig.8.Speculated stagesintheGeologicalDevelopmentoftheIndo-Burmese Regionbased on the possibility of the Bay of Bengal being a Sphenochasm. a, at the end of the Paleozoic; b,at the end of the Mesozoic;c,duringtheTertiary-developmentof the Arakan-Andaman island are, and infilling of the Burmese andAssamtroughswhich were border-ed by landmasses; d, during the fillingof the Andaman Sea anddevelopment of thepresentlandconfiguration. APRELIMINARYSYNTHESISOFTHEGEOLOGICALEVOLUTION113 Ridge have been structurally and lithologically not continuous, and that there existed a landmass as far west asthe present AndamanBasin during the Paleogene.The present author considers that the continuity of topographic trends and of serpentine belt,and lithologicsimilarityof theflysch-typesediments(eugeo-synclinalfacies)of primarily Paleogeneageare good enough evidencestoindicate that the Arakan Yomas and the Andaman-Nicobar Ridge constitute a single geotectonic element, an island arc system, possiblywithanadjoiningsubductionzoneinthewest,firstdevelopedprobably during early Tertiary.Admittedly there isthe problem of source areas, particularly for theAndaman-Nicobar area; however,island arcs are widely accepted as major sources forgeosynclinalfills(e.g.,seeKrumbeinandSloss,1963,p.402),and in thiscontext the eugeosynclinal sediments of the regionmight havebeenderived from the ancestral ArakanYomas-Andaman-NicobarRidge,probablyageanticlinethen. Asforthepossiblepresenceof alandmassasfarwestasthepresentAndaman Basin,there isnoevidence to indicate that the 250kmwideMalay shelf (except inits easternmost part-the MerguiArchipelago)isunderlainbythePaleozoic andMeso-zoicrocks. ThusRodolfo'sassumptionaboutthebevelingof theshelf isratherim-pro bable. Also, in the absence of a proper paleocurrent study, inferences regarding the source areasonthebasisof some random observationsmaybequitemisleading. Further discussionon certain points contained inRodolfo's twopapers maynot beout of placeasit hasbearing onthespeculationspresented here. Rodolfo concludes that the Andaman Basin has not existed since Late Cretaceous, becausethe post-Cretaceoussediment-cover of thebasinistoothin(about1,500m). It can be argued that the relatively thin cover is apparently due to the basin being at the distalendof theBurmese Cenozoic Trough,thusreceiving onlya lowsupplyof pro-deltaicandhemi-pelagic sediments.Thus,theauthor'ssuggestionthat theAndaman Basinhasexistedsinceearly Tertiary isequally likely (seeFig.8(c)). Rodolfo suggests that the entire Southeast Asian block has moved ina net south-southeastdirection.Thissuggestionisquitepossibleinviewof somerecent findings that seemtoindicatethat thespreadingsea-floorfromthesouthwest isfinallybeing underthrustedbelow the Southeast Asian block along the Java Trench (seeHeirtzler, 1968,Fig. on p.64).Asmentionedearlier,thismovement probably isamore recent event. It must alsobementioned herethat there arecasesof significant reversemove-ments within the block, e.g., dextral strike-slip movements noted along the big Sagaing Fault (Hninze Fault of Dey) situated just west of the Shan Scarp. (see Win Swe,1970; Dey,1968). 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