DSpace at VNU: Silurian and Devonian in Vietnam—Stratigraphy and facies

DSpace at VNU: Silurian and Devonian in Vietnam—Stratigraphy and facies tài liệu, giáo án, bài giảng , luận văn, luận án...

jo u r n al ho me p a g e :h t t p : / / w w w e l s e v i e r c o m / l o c a t e / j o g

T ´ˆong Duy Thanha,∗, Ta Hoàng Phuonga, Philippe Janvierb, Nguy˜ˆen H ˜ uu Hùngc,

Nguy˜ˆen Thi Thu Cúca, Nguy˜ˆen Thùy Duonga

a Laboratory of Historical Geology, Vietnam National University, 334 Nguyen Trai Str., Thanh Xuan District, Hanoi, Viet Nam

b UMR7207 du CNRS, Muséum National d’Histoire Naturelle, 8 rue Buffon, 75005 Paris, France

c Institute of Geosciences and Mineral Resources, Thanh Xuan District, Hanoi, Viet Nam

inallthesezonesarebrieflydescribed

TheSilurianconsistsofdeep-waterdepositsoftheupperpartsoftheCoToandTanMaiFormationsintheQuangNinhZone,theupperpartsofthePhuNguFormationintheEastBacBoZoneandtheupperpartsoftheLongDaiandSongCaFormationsintheViet-LaoRegion.ShallowwaterfaciesSilurianunitscontainingbenthicfaunasaremorewidelydistributed,includingtheupperpartoftheSinhVinhandBoHiengFormationsintheWestBacBoZone,theKienAnFormationintheQuangNinhZone,and,intheViet-LaoRegion,theDaiGiangFormationandtheupperpartoftheTayTrangFormation

NoLowerandMiddleDevoniandepositsindicatedeepwaterfacies,buttheyarecharacterizedbydifferentshallowwaterfacies.Continentaltonearshore,deltaicfaciescharacterizetheLowerDevonianSongCauGroupintheEastBacBoZone,theVanCanhFormationintheQuangNinhZone,andtheAChocFormationintheBinhTriThienZone.SimilarfaciesalsooccurintheGivetianDoSonFormationoftheQuangNinhZone,andtheTanLapFormationintheEastBacBoZone,andconsistofcoarseterrigenousdeposits—cross-beddedconglomerates,sandstone,etc.MostDevonianunitsarecharacterizedbyshallowmarineshelffacies.Carbonateandterrigenous-carbonatefaciesdominate,andterrigenousfaciesoccur

intheLowerandMiddleDevoniansectionsinsomeareasonly

Thedeep-water-likefaciesischaracterizticforsomeUpperDevonianformationsintheBacBo(Bang

CaandTocTatFormations)andViet-LaoRegions(ThienNhanandXomNhaFormations).Thesemationscontainchertyshaleorsiliceouslimestone,andfossilsconsistofconodonts,buttherearealsobrachiopodsandotherbenthos.Theywerepossiblydepositedinadeepwaterenvironmentontheslope

for-ofthecontinentalshelf

MostDevonianunitsdistributedintheNorthandtheCentralVietNamconsistofselfshallowwatersediments,andapparentlytheyweredepositedinapassivemarginalmarineenvironment.ThecoarseclasticcontinentalorsubcontinentaldepositsaredistributedonlyinsomeareasoftheEastBacBoand

oftheQuangNinhzonesoftheBacBoRegion,andinthesouthoftheBinhTriThienZone.ThissituationsuggeststheinfluenceoftheCaledonianmovementattheendoftheSilurianperiodthatcalledtheGuangximovementinSouthChina

© 2011 Elsevier Ltd All rights reserved

1 Introduction

The Silurian and Devonian deposits of Viet Nam were first

discoveredanddescribedbyFrenchgeologistsoftheGeological

SurveyofIndochina(Saurin,1956).Later,theyhavebeen

system-aticallystudiedbyDovjikovetal.(1965),Tong-DzuyThanh(1993),

∗ Corresponding author Tel.: +84 1696456546; fax: +84 438583061.

E-mail address: tongdzuythanh@gmail.com (T.D Thanh).

Tong-DzuyThanhetal.(1988),VuKhucandBuiPhuMy(1990),andTong-DzuyThanhandVuKhuc(2006).MajorcontributionsinthedescriptionofSilurianandDevonianunitshavebeenmadebygeologistsfromtheGeologicalMappingDivision,duringthelastdecadesofthe20thcentury(1:200,000geologicalmaps).Strati-graphicunitshavebeenwellestablishedandtheircorrelationsarebasedonabundantfaunas Theirstudyshows thatSilurian andDevonian depositsare widespreadin theNorth andtheCentralpartsofthecountry,whereasintheSouththereareonlyterrige-nousdepositsofsuspectedDevonianage.NewdataontheSilurian

0264-3707/$ – see front matter © 2011 Elsevier Ltd All rights reserved.

166 T.D Thanh et al / Journal of Geodynamics69 (2013) 165– 185

Fig 1.Zonal distribution of the Silurian and Devonian deposits of Viet Nam 1 Bac Bo Region: 1a Quang Ninh Zone; 1b East Bac Bo Zone; 1c West Bac Bo Zone 2 Viet-Lao Region: 2a Dien Bien-Nghe An Zone; 2b Binh Tri Thien Zone 3 South Viet Nam Region: 3a Trung Bo Zone; 3b West Nam Bo Zone Regional and zonal boundaries: I Song Chay Fault; II Dien Bien-Lai Chau Fault; III Song Ma Fault; IV Rao Nay Fault; V Tam Ky-Phuoc Son Fault.

andDevonianstratigraphyhavealsobeenobtainedrecently,and

contributetoprocessingthesequenceofsomesectionsand

com-pletingtheirdistributionareasbasedonnewdiscoveries

TheSilurianischaracterizedbymarinedepositsonly,withdeep

waterfaciesinsomeregionsandareasandshallow-waterfaciesin

others.Incontrast,theDevonianmainlycomprisescontinentaland

shallow-watermarinedeposits,anddeep-waterlikefaciesofthe

UpperDevonianinsomeareas.Thedifferencebetweenthevarious

sedimentarycomponentsandfaciesofthedepositsdependsonthe

distributionareasthatreflectthepaleoenvironmentconditionsof

theirsedimentarydepositionprocesses.Onthebasisofthe

litho-logicalcomponentsandfacies,thedistributionoftheSilurianand

Devoniandepositscanbedifferentiatedintoregionsandzonesas

follows(Fig.1):

1.TheBacBoRegion(NorthernRegion)extendsoveralmostallthe

Northernpartofthecountry(theTonkinofolderpublications)

ItislimitedintheWestbytheSongMaandDienBien-LaiChau

faultsandincludesthreezones:

1a.TheQuangNinhZoneincludesthenortheastcoastalareaof

BacBo,includingtheislandsoftheHaLongBay;

1b TheEasternBacBoZoneextendseastwardfromtheSong

ChayfaultandislimitedbytheQuangNinhZoneintheEast;

1c.TheWesternBacBoZoneislocatedbetweentheSongChay

FaultintheEast,theDienBien-LaiChauFaultintheWest,

andSongMaFaultintheSouthwest

2.TheViet-LaoRegion,whichislocatedbetweentheSongMaandDienBien–LaiChauFaultintheNorthwestandtheTamKy–PhuocSonFaultintheSouth,isdividedintotwozones–theDienBien–NgheAnandBinhTriThienzones,respectively

2a.TheDienBien-NgheAnZoneextendsfromtheDienBien-LaiChauandSongMaFaultsintheNorthernandEasternsidesandtheRaoNayFaultintheSouth

2b.TheBinhTriThienZoneislocatedbetweentheRaoNayFault

intheNorthandtheTamKy-PhuocSononeintheSouth

3.TheSouthVietNamRegion,whichextendssouthwardfromtheTamKy-Phuoc.SonFault,includestwozonesthatareseparated

byapresumedboundary

3a.TheSouthTrungBoZone(SouthernpartoftheCentralVietNam)extendsfromtheTamKy-PhuocSonFaulttotheEastNamBoarea

3b.TheWestern NamBoZone includestheterritoryofKienGiang Province and the Vietnamese part of the Gulf ofThailand

Thestratigraphicunitswillbepresentedbelowforeachregionanditsrespectivezone

2 Stratigraphic units

2.1 SilurianSee(Fig.2)

Fig 2.The distribution in regions and zones of the Silurian and Devonian Units of Viet Nam.

2.1.1 BacBoregion

2.1.1.1 Quang Ninh Zone In the Quang Ninh Zone, the most

importantSiluriandepositsaredistinguishedbydeep-waterfacies

containing graptolite faunas and form the upper part of the

Ordovician-Silurianformationsinthenortheasterncoastalareaand

intheCoToarchipelago.Theshallow-waterdepositscontaining

benthicfaunaareexposedinasmallareaoftheKienAnDistrict,in

thevicinityofHaiPhongCity

TheTanMaiFormation(O-Stm)ischaracterizedbyrhythmic

andbandedstructures,includingterrigenoussediments.Itisabout

7kmbroad anextendsdiscontinuouslyforhundredsof

kilome-tersalongthesoutheastsideoftheNE-SWtrendingYenTu–Tien

Yen–TanMaiFault,andcropsoutintheCaiChienandVinhThuc

islands

Thestratotypeoftheformationextendsalongtheroadfrom

MongCaiTownthroughTanMaitoThanPunvillages.Ithasbeen

subdividedintotwoparts:theloweroneis1000–1200mthickand

comprisesgrey,greenish-grey,fine-tomedium-grainedquartzitic

micaceous sandstone with flattened pebbles interbedded with

micaschist,micaceoussiltstone,andphyllite.Thelattergradually

increasesupwardsinthesection.Intheupperpart(about1000m

thick) occur green-grey mica schist, micaceous phyllite with

someinterbedsofmicaceoussandstone.Graptolites

(Bohemograp-tus tenuis, Lobograptus cf crinitus, Neoculograptus inexpectatus,

Pristiograptuspseudodubius)occurinthesiltstoneoftheupperpart

alongwithsporomorphs(Acanthotriletessp.,Lophozonotriletessp.,

Simozonotriletessp.,Stenozonotriletessp.,andPunctatisporitessp.),

notablyintheNamHa–TruongThusection.Silurianfossilshave

alsobeenfoundintheupperpartoftheformation.Yetthe

Ordovi-cianageofthis partisonlyinferredfromthethicknessofbeds

thatunderlietheSilurianfossiliferouslayers(NguyenCongLuong,

2001a,b)

TheCoToFormation(O-Sct)ismainlycharacterizedby

polymic-tic,cross-beddedsandstonebearingfragmentsoffelsiceffusives,

claystone withbanded and rhythmic structure, and tuffaceousinterbedsandgritstone.ItisthereforereadilydistinguishedfromtheTanMaiandPhuNguformationsofthesameage.ThetotalthicknessoftheCoToFormationreachesover1000m(Fig.3).TheCoToFormationoccursonover40islandsandisletsofCoToArchipelago,situatedintheextremenortheastofBacBo(Tonkin)Gulf.ItextendsalongaNE–SWtrend,fromLoChucSan,throughThanhLantoCoToIslands

The710mthickThanhLanSectioniscomposedoftwoparts:thelowerpartiscomposedofmixeddeepmarinesedimentsinter-calatedwithshallowmarinefacies,andtheupperpart,madeupbyfine-grained,bandedsedimentsthatindicatedeepmarinefacies.EarlySiluriangraptolites(Streptograptusexiguus,Monograptuspri-odon)havebeencollectedfromthesiltyclaystoneoftheuppermostlevelsofthelowerpart.Apartfromthesespecies,othergraptoliteswerealsoreportedfromCoToIsland(Spirograptuscf.minor,S.cf.turriculatus,S.cf.regularis,Oktavitesaff.O.planus,Campograptuscommunis,Demirastitessp.,Climacograptussp.,Monograptusexgr.pandus,Streptograptussp.(TranVanTrietal.,1972,1975,1977).ConNguaIslandalsoyieldedPristiograptuscyphus,Pristiograptussp.,Pseudoclimacograptussp.(inNuiNhonIslet),andDemirastritestriangularis(NguyenHuyMacandPhamTheHien,1972),andLoChucSanIslandyieldedStreptograptusexiguousandMonoclimacislinarsoni.AllthesegraptolitesindicateanEarlySilurianageandarelocatedinthelowerbedsoftheupperpartoftheformation.Theunfossiliferouslowerpartisratherthick,consistingofdeepmarinesedimentsand possiblycorresponds toarelativelylongperiod;therefore,theCoToFormationisapproximatelyconsid-eredasOrdovician–EarlySilurianinage(Tong-DzuyThanhandVuKhuc(2006)

TheKienAnFormation(S4kn)cropsoutinseveralhillsintheKien

AnDistrict,nearHaiPhongCity(Fig.4).AccordingtoTong-DzuyThanhetal.(2001)thetripartitesequenceoftheKienAnForma-tionconsistofa120mthicklowerpartofblue-greymarl,mudstone

168 T.D Thanh et al / Journal of Geodynamics69 (2013) 165– 185

Fig 3.Correlation of the Silurian deposits in the Bac Bo Region (A) West Bac Bo Zone: 1–3 Sinh Vinh Formation (O 3 -S sv); 4 Bo Hieng Formation (S 4 bh); (B) East Bac Bo Zone: 1–5 Phu Ngu Formation (O-S pn); (C) Quang Ninh Zone: 1–4 Co To Formation (O-S ct) (The legends in this figure are for Fig 6 and Fig 9 also).

andyellowish-greysandstonecontainingNikiforovaenacf

fergane-nsisandindeterminaterugosans,anda320m.thickmiddlepart

composedof grey, quartzitic sandstone withdark purplish-red

mudstoneinterbeds whichyields Retziellaweberi, Nikiforovaena

ferganensisandHowellella sp.Theupperpartof thesection,on

thenorthslopeofXuanSonMountain,consistsofabout100mof

thick-beddeddarkgreylimestonewithshaleandmarlinterbeds

intheuppermostlayers,whichyieldanabundantRetziellaweberi

assemblage.Atthesamelevel,butonthenorthslopeofTienHoi

Mountain,thebrachiopodsRetziellaweberi,Nikiforovaena

fergane-nsis,Howellellabragensis,Howellellasp.(VuKhucandBuiPhuMy,

1990)wererecorded,alongwithabundantcorals,notablyFavosites

admirabilis,Xiphelasmasp.,Nipponophyllumsp.,andMesofavosites

sp

Nolowerboundaryof theKienAnFormationhaseverbeen

observed,anditsrelationswiththeCoToandTanMaiFormation

areunknown.ItpossiblyunconformablyunderliestheLower

Devo-nianVanCanhFormation.TheKienAnFormationisreferredtothe

LateSilurian,(LateLudloworPridolian)(Tong-DzuyThanhetal.,

2001)

2.1.1.2 EasternBacBoZone InEasternBacBoZone,theSilurian

depositsarerepresentedbydeep-waterfaciescontaininggraptolite

faunaswhichoccurinthecentralareaofthezone.Theymakeup

theupperpartoftheOrdovician-SilurianPhuNguFormation

The PhuNgu Formation(O-S pn)is characterizedby banded,

mainlyfine-grainedrhythmicterrigenoussedimentsinterbedded

witheffusivesofmafictofelsicandalkalinecomposition(Fig.3).TheFormationoccursintwobands,thefirstofwhichextendsfromDaiTu(ThaiNguyenProvince)toChoRa(BacCanProvince).ThesecondbandextendsfromLuongMtthroughNgoiNac,VinhTuy

toHuongSonareas(HaGiangProvince)

Withathicknessof 2300–2400m, thePhu Ngustratotypeiscomposed of two distinct parts The 1100m thick lower partincludesargillaceousshale,cherty-argillaceousshalerhythmicallyinterbedded with silty sandstone, locally with lenses of lime-stoneandmaficeffusives.The1200mthickupperpartcomprisessilty sandstoneinterbeddedwithargillaceous shaleand cherty-argillaceous shale, locally with lenses of limestone and felsiceffusives Graptolites(e.g.,Climacograptus sp.,Glyptograptussp.,Monoclimacissp.) havebeen collectedfromtheuppermostlev-elsofthelowerpart,along withpoorlypreservedtrilobitesandbrachiopods(Fig.3)

InNaRiarea(BacCanProvince),theformationoccursinsmallexposureswitha thicknessofabout250m,includinggrey siltysandstone, greenish-grey marl, black, thin-banded argillaceousshalecontainingthegraptolitesDiplograptussp.,Climacograptuslatus,Climacograptuscf.C.scolaris,Ptilograptussp.,Glyptograptussp.,thetrilobitesAgnostusperrugatus,Remopleuridesaff.R.salteri,andthebryozoanFenestellasp

AllthesefossilsareofOrdoviciantoEarlySilurianage,butoccur

inafairlylowpositioninthesections,butthepartthesectionthatoverliesthefossil-bearingbedsisratherthickandcharacterized

bydeepmarinefacies,thereforetheformationcanreasonablybe

Fig 4. Distributed area (A) and stratigraphic sequence (B) of the Upper Silurian Kien An Formation (S 4 kn) in Kien An District (Hai Phong City).

datedasOrdovician-Silurian(PhamDinhLong,2001;NguyenKinh

Quoc,2001;Tong-DzuyThanhandVuKhuc,2006)

2.1.1.3 WesternBacBoZone IntheWesternBacBoZonethe

Silu-riandepositsincludetheupperpartsoftheOrdovician-Silurian

SinhVinhFormation,andtheUpperSilurianBoHiengFormations

Theymainlyconsistofshallow-waterfacies deposits,and

deep-waterdepositsarelimitedtothewestofSonLaProvince(KetHay

Formation)

TheSinhVinhFormation(O3—Ssv)(Fig.3)ismainly

character-izedbylight grey,thicklybedded dolomiticlimestone,exposed

inthelowersectionoftheSongDaRiverindiscontinuous

nar-rowbands.IntheSinhVinhSection,alongtheSongDaRiver,the

Formationis clearlydistinguishedby thethree following parts:

(1)The40mthicklowerpart,madeupbyconglomerate,whose

quartzitepebblesarederivedfromtheunderlyingBenKhe

For-mation;(2)The400mthickmiddlepart,madeupbycalcareous

sandstone and mudstone, dolomite, sandy limestone and marl

bearingthecoralsReuschiasp.,Plasmoporellakiaeri,Favositella

alve-olata;(3)The350mthickupperpart,whichconsistsoflightgrey,

thicklybeddeddolomiticlimestoneandcalcareousshaleinterbeds

bearingthecoralsFavositessp.,F.aff.F.forsbesi,F.exgr.F

goth-landica,F.cf.F.hisingeri,F.cf.F.coreaniformis,Mesofavositessp.,

Squameofavositessp.,andParastriatoporasp.Thetotalthicknessof

thissectionis800m,butinothersectionsitsthicknessreaches

1000–1200m

Apartfromtheabovementionedfauna,someotherfossilshavebeencollected,suchasthecoralsReuschiasp.,Favositellaalveolata,andFavositessp.,F.aff.F.forbesivar.similis,F.exgr.F.gothlandicus,

F.cf.F.hisingeri,F.cf.F.coreaniformis,Squameofavositessp.,favositessp.,Parastriatoporasp.,Multisoleniaexgr.M.tortuosa,andthecrinoidBistrowicrinusquinquelobatus

Meso-TheBoHiengFormation(S4bh)isdistributedinthelimitedarea

ofthelowersectionoftheSongDaRiverbasin,anditconformablyrestsupontheSinhVinhFormationandundertheLowerDevonianSongMuaFormation(Fig.3)

The 400m thick Bo Hieng Formation, mainly consists ofdark green marly shale interbedded withblack-grey limestoneand, inits lowerpart,somebandedargillaceous shalecontain-ing brachiopods (Retziella weberi, Orbiculoidea tonkinensis), andbivalves(Modiomorphabrevis,M.sp.,Cymatonotacf.C.yunnanen-sis,Dysodontasp.,Actinopteriasp.).Theupperpartiscomposedofblack,thin-tomedium-beddedlimestoneandmarlintheupperpart,containing Retziella weberi, Tadschikiaxuanbaoi, Howellellasp.,Scolioporasp.,andFavositeskunjakensis(TaHoaPhuongand

LeVanManh,1996)

In theNui Tau MtSection (Yen Lap,Phu ThoProvince), theblack-greylimestoneinterbeddedwithargillaceousshaleyieldedcorals (Palaeofavositesaff balticus, Favosites kernihoensis, Aphyl-lumaff.sociale)and brachiopods(Retziellaweberi,Ferganellaaff.Turkestanica)(NguyenHuuHungetal.,1988),whichindicateaLateSilurianageoftheformation(Tong-DzuyThanhetal.,2001).The

170 T.D Thanh et al / Journal of Geodynamics69 (2013) 165– 185

Fig 5. Distributed area and stratigraphic sequence of the Silurian Dai Giang Formation in Quang Tri Province.

lithologyandfaunaoftheBoHiengFormationissimilartothatof

theKienAnFormationdescribedabove

TheKetHayFormation(S1-2kh)occursinanarrowbandinthe

SouthwesternareaofSonLaProvinceanddiscontinuousexposures

northwestwardfromSonLatoTuanGiao,allofthembeing

proba-blyincloserelationtotheSongCaFormation(seebelow—Viet-Lao

Region) The 230m thick Ket Hay Formation consists of

thin-beddedsiliceousshale,interbeddedwithsiliceoussiltstone,shale,

sericite-chloriteschist,andcalcareous-siliceousschist.Mostofthe

graptolitesithasyielded(Demirastritestriangulates,Hedrograptus

cf.H.rectangulatus,Diplograptuscf.D.modestus)areEarlySilurian

inage(LeThanhHuuandVuXuanLuc,2005;TranVanTriandV ˜u

Khuc,2009)

2.1.2 Viet-LaoRegion

TheSiluriandepositsintheViet-Laoregionincludetheupper

partoftheOrdovician-SilurianSongCa Formation,and theTay

TrangFormationofLateSilurian-EarlyDevonianageintheDien

Bien-NgheAnZone.IntheBinhTriThienZone,theyarecomposed

oftheupperpartoftheOrdovician-SilurianLongDaiFormation,andtheUpperSilurianDaiGiangFormation

2.1.2.1 DienBien-NgheAnZone TheSongCaFormation(O3-S2sc)cropsoutalongtwomainstrips,thefirstofwhichextendsfromMuong Xen andTuong DuongtoQuy Chau,NgheAn Province,south ofthe SongCa River.The secondstrip extends fromtheVietnam-LaosborderthroughNgheAntoKyAnh,HaTinhProvince.Theformation is about2100–2300mthick andmainly consists

of argillaceous shale, sandstone, and some beds of black, andbedded limestone The graptolites Monoclimacis vomerinus andPristiograptuskweichihensisoccurintheargillaceousshaleofitsupperpart,andindicateanEarlySilurianage.Besides,someSilu-riansporesalsooccurinthisformation,suchasZonosphaeridium

sp.andLeiotrichoidessp.SincetheEarlySiluriangraptolitesoccur

intheupperpartoftheformation,itslowerpartmaythereforetentitavelybereferredtotheUpperOrdovician(Tong-DzuyThanhandVuKhuc,2006)

TheTayTrangFormation(S3-D2ett)isdistributedinLaiChauandDienBienareas,NorthwestBacBo,extendingtotheWestofNghe

oftheTayTrangFormationintroducedbyDovjikovetal.(1965)

andtheHuoiNhiFormation erectedbyNguyenVan Hoanh(Le

DuyBachandNguyenVanHoanh,1995);therefore,theTayTrang

namehaspriority

The800mthickTayTrangFormationconsistsof:(1)540mthick

grey,fine-tomiddle-grainedsandstone,interbeddedwithashgrey

todarkgrey,thin-beddedsiltstone,anddarkgreyclayshale;(2)

260mthick,darkgrey,thinbeddedclayshale,darkgreysiltstone

withsomedarkgrey,andfine-grained,thinbeddedsandstone.The

formationyieldsfossilsthatindicateanagerangingfromthe

Lud-low(Silurian)totheEifelian(MiddleDevonian).Theyinclude,in

theWestofNgheAnProvince,graptolites(Bohemograptus

bohemi-cus,Monograptusthomasi,M.yukonensis,M.uniformis)(NguyenVan

Phuc,1998),anddacryoconarids(Nowakiacancellata,N.holynensis,

Homoctenussp.),and,intheTayTrangarea(DienBienProvince,

WesternBacBoZone),cephalopods(Erbenocerassp.,Anetoceras

sp.),thedacryoconaridNowakiasp.(cf.N.acuaria),andthe

bra-chiopodChonetesexgr.tenuicostata.TheTayTrangFormation(=

HuoiNhiFm.)restsunconformablyupontheSongCaFormation

(O-Ssc)andconformablyundertheHuoiLoiFormation(D1-2hl)

(Tong-DzuyThanhandVuKhuc,2006)

2.1.2.2 BinhTriThienZone TheLongDaiFormation(O2-S2ld)

com-prisesterrigenoussedimentsofrhythmicstructure,interbedded

withacidictointermediateeffusive,exposedinthesouthoftheRao

NayFault.IntheBanHo–VitThuLustratotype,theformationisup

to1250mthick,anditslowerpart(about1000mthick)consists

ofrhythmicintercalationofsandstone,siltstoneandargillaceous

shale,clay-chertyshalewithsomeinterbedsofandesite, which

yieldtheLateOrdoviciantrilobites Cyclopygesp.,Microparia (?)

sagaviaformis,Ogygiocarissp.,Nileussp.,andthegraptolite

Diplo-graptussp.Itsupperpartiscomposedofblackargillaceousshale,

clay-chloriteshalewithsomeinterbedsofsandstoneandgritstone

containingtheLateSiluriangraptolitesNeodiversograptusnilssoni,

Monograptussp.,and Pristiograptussp.TheLeKysection,which

reaches1930minthickness,displaysthesamelithological

com-ponents, butbegins with90mthick conglomeratebeds,and is

distinguishedbythetuffaceoussandstoneinitsmiddlepart,and

thelackofandesiteinterbeds.Llandoverygraptolites(Demirastrites

convolutus,Monograptushalli,Oktavitesspiralis)havebeencollected

fromthemiddlepartofthesection,whereasitsupperpartyields

Bohemograptusbohemicus,Monoclimacissp.(NguyenXuanDuong,

1996;Tong-DzuyThanhandVuKhuc,2006)

The Dai Giang Formation (S3-4 dg) is exposed in the Quang

Binh and Quang TriProvinces, with thicknessof about 1100m

(Fig.5 anditssequencebeginswithgritstoneandcoarse-grained

sandstone,unconformablyoverlyingtheLongDaiFormation.The

formationconsistsessentiallyofsandstone,siltstoneand

calcare-oussandstone,withlimestoneand marlbedsinitsupper part

Abenthicfaunahasbeencollectedmainlyfromthemiddleand

the upper parts of the formation, and includes trilobites

(Cro-musbeaumonti,Metacalymenesp.,Praedechenellaaff.vietnamica,

Encrinurus cf sinicus), brachiopods (Retziella weberi, R altaica,

Nikiforovaenavietnamica,“Howellella”cf.lynxoides),corals

(Multi-soleniacf.formosa,Nipponophyllumanmaense),bivalves(Schizodus?

myducensis,Sphenotusantecedens,Modiomorphaparacrypta,

Gonio-phora dianensi), and fish remains referred to the placoderm

Myducosteusanmaensisandanindeterminateacanthodian(Nguyen

XuanDuong,1996;Tong-DzuyThanhandVuKhuc,2006)

2.2 Devonian

See(Fig.2;Fig.6)

DevoniandepositsarewidespreadinVietNam,buttheir

lithol-ogyandsequences,especiallyintheLowerDevonian,aredifferent

intheBacBo,Viet-Lao,andSouthVietNamregions.ThemostpleteDevoniansectionsareobservedintheBacBoregion,whereasseparate,discontinuoussectionsaremetwithonlyinsomeareasoftheSouthVietNamregion(Tong-DzuyThanhandVuKhuc,2006)

com-IntheBacBoregion,theresemblancesbetweentheDevoniansectionsisconspicuousandwellcharacterized,bothlithologicallyandfaunally,therebyindicatingthattheywereformedinasingle,largebasin.IntheQuangNinh,EasternBacBoandWesternBac

Bozones,theDevoniansectionsarecomposedoftwoconspicuousparts,theloweronemainlyconsistingofterrigenoussediments,and the upper one of carbonate.However, detailed differencesbetweenthesesectionscanbenoticed.Notably,intheEasterntheBacBoZone,theLowerDevonianischaracterizedbyRedBedsthatunconformablyoverlytheLowerPaleozoic,whileintheWesternBacBothereisaconformitybetweentheUpperSilurianBoHiengFormationandtheLowerDevonianSongMuaFormation(Fig.6).2.2.1 BacBoRegion

2.2.1.1 QuangNinhZone IntheQuangNinhZone,theDevoniandepositsconsistoftheVanCanh,DuongDong,DoSon,andTrangKenhFormations, successively.The uppermostof theDevoniansequencesinthiszonedisplaysthelowermostpartoftheUpperDevonian-LowerCarboniferousConVoiandPhoHanformations.TheVanCanhFormation(D1vc)isthelowermostformationoftheDevoniansequenceintheQuangNinhZone,andcropsoutintheVanCanhislet,TraBan,NgocVung,andotherislandsintheBai

TuLong(FaiSiLung)Bay,andontheDoSonPeninsula(NguyenHuuHungetal.,2007).IntheVanCanhtypesection,theformationcon-sistsofa200mthickseriesofalternatingsandstoneandmudstonebeds,whichcontainsplantremains(Tainiocrada?sp.cf.T.decheni-ana,andPsilophytites?sp.).Inthissection,theVanCanhFormationconformablyunderliestheDuongDongFormation, whichyieldsthebrachiopodTulynetesnonpoensis

On the Do Son Peninsula, the Van Canh Formation formably underlies the Do Son Formation In previous works,the NgocXuyen mudstoneand sandstone that has yielded theeurypteridsRhinocarcinosomadosonensisandHughmilleriasp.,iso-latedplacodermfishplates(referredtoaprimitiveantiarchanda

uncon-“Wangolepis”-likeplacoderm)(Bradyetal.,2002),andLowerniansporomorphs,wereconsideredasthelowerpartoftheDoSonFormation(Tong-DzuyThanhandVuKhuc,2006).TheyarenowreferredtotheVanCanhFormation(NguyenHuuHungetal.,

Devo-2007),buttheireurypteridandvertebratefaunaseemsmoregestiveoftheUppermostSilurian(Bradyetal.,2002;Janvieretal.,

sug-2003)

TheDuongDongFormation(D1–D2edd)isexposedinKinhMon(HaiDuongProvince),DuongDong(TrangKenh,HaiPhongCity)andDongTrieu(QuangNinhProvince),andinChauDop,TraBan,andNgocVungislands(HaLongBay)

ThenewdataprovidedbyNguyenHuuHungetal.(2007)showthatthecrossbeddedquartziticsandstone,siltstoneandshaleoftheupperpartoftheDuongDongFormationdescribedinthepreviousworks(Tong-DzuyThanhandVuKhuc,2006)shouldbereferred

totheoverlyingDoSonFormation,andthatthebrachiopod-andcoral-bearinglimestone lensesof itsuppermost bedsbelonginfact tothe TrangKenh Formation Thisaccords with theorigi-naldescriptionof theformationby NguyenQuang Hap (1967).AccordingtoNguyenHuuHungetal.(2007),theDuongDongFor-mationintheLoSonareaconsistsof:(1) 50mthickclayshaleandmudstonecontainingbrachiopods;(2)70mthickfine-grainedsandstoneandinterbedsofshalecontainingbrachiopods,and(3)

80mthickfine-grained sandstone,mudstoneinterbedsing poorly preserved brachiopods The fossils from the DuongDongFormationaremainlybrachiopods,andhavebeencollectedfrommanylocalities,suchastheLoSon,DuongDongandHiepSon(KinhMon)areas,andonNgocVungand VanCanhislands

contain-172 T.D Thanh et al / Journal of Geodynamics69 (2013) 165– 185

Fig 6. Correlation of the Devonian deposits in the Bac Bo Region (A) West Bac Bo Region: 1 Upper Silurian Bo Hieng Formation (S 4 bh); 2 Song Mua Formation (D 1 sm); 3 Ban Nguon Formation (D 1 bn); 4 Ban Pap Formation (D 1 p-D 3 fr bp); 5 Bang Ca Formation (D 2 gv-D 3 fr bc); 6 Toc Tat Formation (D 3 fm-C 1 t tt) (B) East Bac Bo Zone: 1 Ordovician Lutxia Formation unconformably is underlying the Si Ka Formation; 2 Si Ka Formation (D 1 sk)—Red beds containing vertebrate remains; 3 Bac Bun Formation (D 1 bb); 4 Mia

Le Formation (D 1 ml) containing abundant Pragian benthos of the Euryspirifer tonkinensis assemblage; 5 Ban Pap Formation (D 1 p-D 3 fr bp); 6 Bang Ca Formation (D 2 gv-D 3 fr bc); 7 Toc Tat Formation (D 3 fm-C 1 t tt) (C) Quang Ninh Zone: 1 Upper Silurian Kien An Formation (S 4 kn); 2 Van Canh Formation (D 1 vc); 3 Duong Dong Formation (D 1 -D 2 e dd);

4 Do Son Formation (D 2 gv-?D 3 fr ds); 5 Trang Kenh Formation (D 2 gv-D 3 tk.

(BaiTuLongBay).ThebrachiopodsarereferredtoEuryspirifercf

tonkinensis,Indospiriferkwangsiensis,Schellwienellacf.S.lantenoisi,

Stropheodonta cf.S interstrialis,Athyris concentric,Atrypa ex gr

reticularis,Bacbochonetessp.,Retichonetessp.,Bacbochonetes

jan-vieri, Perichonetes mutabilis, and occur along with the bivalves

Pterinopectensp.Carinatina cf.C.arimaspa,Desquamatia sp.The

DuongDongFormation conformablyoverliestheVan Canh

For-mation,andunconformablyunderliestheDoSonFormation.Itis

referredtotheLowerDevonian—Eifelian

TheDoSonFormation(D2gv-?D3frds)cropsoutintheDoSon

PeninsulaandsomeislandsofBaiTuLongBay.Onthemainlandof

theQuangNinhZone,itisusuallyexposedalongwiththeDuong

DongFormationandhassometimesbeenincorrectlyreferredto

partof thelatter.It was firstdescribed by Lantenois(1907) as

the“DoSonSandstone”(GrèsdeDo-Son),whichoccursalongthe

beachesofDoSonPeninsula(HaiPhongCity).Later,ithasbeen

describedasatripartiteformation(Tong-DzuyThanhandVuKhuc,

2006).Ournewinvestigationsshowthat theDoSon Formation

includestwopartswhichcorrespondtoLantenois’s“DoSon

Sand-stone”,andalowerpartofthe“tripartiteformation”,whichisnow

referredtotheVanCanhFormation.Thesequenceoftheformation

isthusasfollows:

-150–200m thick, thick-bedded, locally cross-bedded quartz

gravelstoneatthebase,overlainbyquartziticsandstone,

coarse-grained sandstone, sometimes with interbeds of

chocolate-colouredsiltstone.Thismemberismostclearlyexposedatthe

backoftheDoSonMarketandalong theroadtowardsBeach

2,particularlyaroundtheVanHuongQuay.Somebivalves,such

as:Schizodus(?)sp.,Ptychopteria (Actinopteria)hunanensisand

Goniophorasp.werecollectedonthemountainslopesituatedin

thebackofthehamletlyingbetweentheDoSonMarketandthe

mainofficeoftheDoSonTownlet.Vertebrateremainsmaspistrii,Briagalepissp.,Bothriolepiscf.B.gigantea)havebeenfoundalongwiththelycophyteBergeriaorKnorria(cf.Lepidoden-dropsissp.)ontheshoreoftheVanHuongQuayarea(JanvierandTong-DzuyThanh,1998;Longetal.,1990;Tong-DzuyThanhandCaiChong-yang,1995)

(Vietna 200mthick,brown,medium-bedded,locallycross-beddedstone,darkchocolate-colouredsiltstoneinterbeddedwithgrey,greenish-grey,yellow-brownweatheringclayshale,.ThisbarrenmemberisclearlyvisibleontheroadtowardstheCasinobuild-ing,fromnearthe“BaoDaiVilla”tothesouthernmostendofthepeninsula

sand-ThesedimentologyandfossilsoftheDoSonFormationindicate

adeltaictocoastalfacies.However,theoutcropsofthisformationarescatteredoveralargeareaanddisplayagreatdiversityoffaciesthatischaracterizticfordeltaicenvironments.Thereforeitdeservesinvestigation

The unconformity between the Do Son Formation and theunderlyingDuongDongFormationisonlysuggestedbythepres-enceofgravelstonebedsatthebaseoftheformerformation,butisconformablyunderlyingthecarbonateTrangKenhFormation.TheTrangKenhFormation(D2gv-D3tk)islargelydistributedintheQuangNinhZone,includingislandsofHaLongBay,andcon-sistsoflimestonecontainingabundantcorals,stromatoporoidsandsomebrachiopods.Inpreviousworks,theTrangKenhFormationwasdescribedasatripartiteunit,whichiscomposedoftwolime-stone parts,and a middlechertypart.Its total thicknessvariesfrom250mto300m.ThenewstudybyNguyenHuuHungetal.(2007)showsthatthisformationonlyconsistsoflimestone,thechertypartbelonginginfacttothePhoHanFormationofUpperDevonian-Lower Carboniferous age.The Trang Kenh Formation

denticulata,Caliaporabattersbyi,Alveolitesadmirabilis,andthe

bra-chiopodStringocephalusburtini,whichallsuggestaGivetianage

However,theupperpartoftheformationyieldsstromatoporoids

and corals(Amphipora laxeperforata,A pervesiculata,Stachyodes

costulata,S.fasciculate,whichindicateaGivetian-Frasnianage.In

thePhucSon–NuiHanarea, Famennianconodonts(Palmatolepis

subperlobata,P.triangularis,P.glabraglabra,P.subrecta,P.perlobata)

havebeenrecordedfromtheuppermostbedsoftheTrangKenh

Formation (Tong-Dzuy Thanhand VuKhuc, 2006;Nguyen Huu

Hungetal.,2004).TheTrangKenhFormationconformablyrests

betweentheunderlyingDoSonFormation,andtheoverlyingCon

VoiandPhoHanFormations,whicharedescribedbelow

TheConVoiFormation(D3fm-C1cv)mainlyconsistsof

lime-stoneandsomethinbedsofsiliceouslimestonethatoccurintheMt

NuiConVoiKienAnandAnLaoDistricts,HaiPhongCity.ThePho

HanFormation(D3fm-C1 ph)includeslimestone,siliceous

lime-stoneandchertinCatBaIsland(HaLongBay),andisthecoeval

withtheConVoiFormation.Thesetwoformationsarethus

distin-guishedbysiliceouscomponents:somethinbedsintheKienAn

Formation,andmorediffuseonesinthePhoHanFormation.The

faunaoftheConVoiFormationmainlyconsistsofbrachiopods,

corals,andsomeforaminifera,whereasthatofthePhoHan

For-mationincludesmanyconodontsandarichbenthicfauna,suchas

corals(Fuchungoporasp.,Syrigoporadistans,Tetraporinussp.)and

manybrachiopodsinthelowerbedsoftheformation.Both

forma-tionsweredatedasUpperFamennian-LowerCarboniferous,but

mostofthemareLowerCarboniferousinage,theirFamennianpart

beingnegligibleinthesectionsandmaybereferredtotheTrang

KenhFormation.ItispossiblethattheConVoiandPhoHan

Forma-tionsrepresentdifferentsectionsofasingleformationthatdisplays

avariablenumberofsiliceouscomponents(Tong-DzuyThanhand

VuKhuc,2006)

2.2.1.2 EasternBacBoZone TheSongCauGroup(D1sc)includesthe

SiKaFormation(red,subcontinentalfacies)andtheBacBun

For-mation(deltaicfaciesgradingupwardsintoshallowmarinefacies)

TheSiKaFormation(D1 sk)isexposedintheVietNam–China

frontier areas of Ha Giang Province and in Lang Son Province,

whereasintheCaoBangandThaiNguyenProvinces,itiscomposed

ofpartofthesequencesoftheSongCauGroup.Initsstratotype

(Fig 3 theSi KaFormation consistsof greenish-greysiltstone

interbeddedwithclayshale,greenish marlyshale,thin-bedded,

fine-grainedlimestone,andgreysiltstonewithinterbedsof

vio-letchocolatemarlyshale.Thetotalthicknessoftheformationis

about650m.AccordingtoHoangXuanTinh,thebasal

conglom-eratebedsarelocallyupto5–10mandmetwithintheframeof

theGeologicalMap(1:200,000)oftheBaoLacMapsheet(Hoang

XuanTinh,2001).ItssectionsarealsoobservedinDongMo(Lang

SonProvince) andTrangXa(Thai NguyenProvince), wherethe

redsandstoneisanimportantcomponentinthesequence.Early

publicationsonthisformationbrieflymentioned(e.g.,thefishes

“Asterolepis”sp.,“Homosteus”sp.,andtheplant“Bythotrephisaff

antiquate”(Saurin,1956).Duringthelasttwodecadesthe

verte-bratesfromtheSiKaFormationhave beenshown tobelongto

thetwo major fishgroups that also populatethe Lower

Davo-nianofsouthernChina,namelytheGaleaspidaandtheantiarchan

Placodermi,towhichcanbeaddedvariousSarcopterygii.Neither

Asterolepis,norHomosteushavebeenconfirmedinthefauna.To

date,theupperbedsoftheSiKaFormationconsistingofsandstone

inThaiNguyenProvincehasyieldedthegaleaspidBannhuanaspis

vukhuciandfragmentaryremainsofvariousPolybranchiaspis-like

species,alongwithvariousyunnanolepiformand

procondylolepi-formantiarchs(Procondylolepis sp.,Yunnanolepisspinulosa), and

youngolepididsarcopterygianremains(Janvieretal.,1993;

Tong-DzuyThanhetal.,1994;JanvierandTaHoaPhuong,1999).TheSiKa

Fig 7.Lower Devonian Si Ka Formation unconformably rests upon the Ordovician Lutxia Formation (Left cliff on the road Dong Van-Lung Cu (Photo: Ta Hoa Phuong).

FormationunconformablyrestsupondifferentlevelsoftheLowerPaleozoicdeposits(Fig.7)andconformablyunderliestheBacBunFormation

TheBacBunFormation(D1bb)(Fig.4)consistsofgrey,chocolate,variegatedweathered clayshaleandmarlyshale,siltstonecon-tainingvertebratesinthestratotype(DongVanDistrict,HaGiangProvince).WestofDongMo(LangSonProvince),itcomprisesamemberofgreycarbonatephosphatecontainingwell-preservedvertebrates,andtheoverlyingclayshaleandsiltstonemembercon-tainsabundantbrachiopods.InTrangXa(ThaiNguyenProvince),itconsistsofgreylimestoneandmarlyshalecontainingcorals,bra-chiopodsandvertebrates.Thethicknessoftheformationvariesfromsometensto500metres.Thedistributionareasofthisfor-mationaremuchthesameasfortheSiKaFormation,i.e.intheHaGiang,CaoBang,LangSon,andThaiNguyenProvinces

Vertebrate-bearing faunas of the lower member of theBac Bun Formation include, in the stratotype, the galeaspidsPolybranchiaspis sp cf P liaojaoshanensis, numerous antiarchs(Yunnanolepis deprati, Y bacboensis, Chuchinolepis dongmoensis,Vanchienolepislangsonensis),thepetalichthyidTongdzuylepisviet-namensisandthesarcopterygianYoungolepiscf.Y.praecursor.TheDong Mosection yields much thesame vertebrate fauna with,

inaddition,astillindeterminedlungfish(Tong-DzuyThanhandJanvier,1990,1994;Tong-DzuyThanhand VuKhuc,2006).Theupper member of the formation yields brachiopods (Howittiawangi,Cyrtochonetesindosinensis,Parachoneteszeili)(RacheboeufandTong-DzuyThanh,2000).Thelithologyandfaunalcomposi-tionoftheBacBunFormationindicatealittoraltodeltaicfaciesgradingupwardsintoashallowmarinefacies

TheMiaLeFormation(D1ml)iswidespreadintheEasternzone

ofBacBoregionandconsistsofgreenish-greysiltstoneinterbeddedwithclayshaleandmarl,withsomethininterbedsofargillaceouslimestone,andgreylimestoneinitsuppermostpart(Figs.6and8).ThethicknessoftheformationintheLungCo–MiaLestratotype(DongVanDistrict,HaGiangProvince)reaches500m,butinothersections,itcandecreasetoafewhundredmetres,asintheHaLangarea(CaoBangProvince),whereitisonlyabout200m.Thisfor-mationisthemostfossiliferousunitinthePaleozoicofVietNam,containinghundredsofspeciesofcorals,brachiopods, trilobites,crinoids,andothergroups,allindicatingaPragianage.Theyarecomponentsofthewell-knownEuryspirifertonkinensisAssemblage(Tong-Dzuy Thanh and Ta Hoa Phuong, 1994), which is domi-natedbybrachiopods(e.g.,Euryspirifertonkinensis,Undispiriferaff.pseudoaculiatus,Elymospiriferkwangsiensis,Dicoelostrophiaanna-mitica,Atrypaaff.reticularis,Schellwienelladouvillei,S.lantenoisi,Athyris?tiaomachiensis,Parachoneteszeili)andcorals(e.g.,Favosites

174 T.D Thanh et al / Journal of Geodynamics69 (2013) 165– 185

Fig 8.Stratigraphic sequence of the Lower Devonian Lung Cu-Ma Le section (Dong Van District, Ha Giang Province).

goldfussi,F.styriacus,F.pencolei,F.preplacenta, Squameofavosites

cechicus,Sqf.giganteus,Sqf.sokolovi,Sqf.delicatus,Emmonsia

yen-lacensis,Squameopora vukhuci,Echyropora grandiporosa, Coenites

bulvankerae,Heliolitespraeporosus),butalsoincludesthetrilobite

Proetussp.andbivalves(e.g.,Pterineamieleensis).TheMiaLe

For-mationliesconformablybetweentheBacBun(D1bb)andBanPap

(D1-2bp)formations(Tong-DzuyThanhandVuKhuc,2006)

TheKhaoLoc Formation(D1-2 kl) ischaracterizedby its

car-bonate composition and is limited to the Khao Loc Structure

WestwardtoHaGiangProvince.Itconsistsof15mthickdarkgrey,

yellow-grey,red-brownweatheringmarlandclayshale

contain-ingabundantvertebrates,and800mthick,black-greylimestone

interbedded with argillaceous limestone that yield corals and

Amphiporasp.Thevertebrate faunanotably includesgaleaspids

(Polybranchiaspis liaojaoshanensis, Laxaspis yulongsus), antiarchs

(Yunnanolepissp.,Minicranialissa,Heteroyunnanolepisqujingensis),

theacanthothoracid placodermHagiangella goujeti,and the

sar-copterygianYoungolepispraecursor(JanvierandTa HoaPhuong,

1999; Racheboeufet al., 2005) The lowermostlimestone beds

oftheformationyieldcorals(e.g.,Favositesstellaris,F.vilvaensis,

Squameofavositesenormis, S.vanchieni,S baolacensis,S

spongio-sus,Thamnoporabeliakovi,Cladoporacf.C.yavorskyi,Parastriatopora

champungensis, Caliapora cf C primitive) and the brachiopods

HowittiawangiandHowellellaexgr.crispa.Themiddlepartofthe

sectionyieldsStriatoporasp.,Thamnoporakolodaensis,T.cf.T.siavis,

Alveolitellacf.A polenowi,Coenitessp.,andCoroliteshaoi(DzuyThanhetal.,1988;Tong-DzuyThanhandVuKhuc,2006).Thecorals,brachiopods,andvertebratesfromthebaseoftheformationaretobecorrelatedwiththeBacBunandMiaLeforma-tions(LowerDevonian),whereasthecoralsfromitsmiddlepartindicateaMiddleDevonianage

Tong-TheBanPapFormation(D1p-D3frbp)ischaracterizedmainlybygrey,thintomediumbeddedlimestone,widelyextendinginbothEasternand WesternBacBoZones,andwillbepresentedhere-inafterforallBacBoRegion.Thetotalthicknessoftheformationreaches1200m,butincertainsections,itmayonlybe300–400mthick

The lower partof the Ban Pap Formation mainly consistsofdarkgrey,thin-tomedium-bedded,bituminouslimestonecontain-ingEmsianstromatoporoidsandcorals,suchasAmphiporaacerbaalaiskiensis,A.agrestevojachia,Favositesstyriacus,F.stellaris,F.reg-ularissimus,Emmonsiayenlacensis,Cladoporayavorskyi

The middle part of the Ban Pap Formation includes darkgrey, medium-bedded, fine-grained limestone, and its upperpart grey to light grey, thick-bedded to massive fine-grainedlimestone Although the boundary between the lower and theupperparts isdifficulttoidentifyonthebasis ofthelithology,theirrespectivefaunasarereadilydistinguished.Itsmiddlepartyieldscharacteriztic Eifelian fossils,such asthe dacryoconaridsNowakia sp., Viriatellinairregularis, V dalejensis, and thecorals