Cretaceous and Paleogene boundary strata in southern Tibet and their implication for the...

Cretaceous and Paleogene boundary strata in southern Tibetand their implication for the India–Eurasia collision

XIA QIAO WAN, LUBA F. JANSA AND MASSIMO SARTI

Xia qiao Wan, Jansa, L.F. & Sarti, M. 2002 06 14. Cretaceous and Paleogene boundarystrata in southern Tibet and their implication for the India–Eurasia collision. Lethaia,Vol. 35, pp. 131–146. Oslo. ISSN 0024-1164.

Recent stratigraphic studies in southern Tibet provide new information about thetiming of the initial collision between the India and Eurasia continental blocks. Thestratigraphic and paleontological evidence document dramatic changes in sedimentaryfacies and microfauna content across the Cretaceous–Paleogene (K/Pg) boundary. Inthe Zhongba and Gamba areas in southern Tibet, the K/Pg boundary is marked by amajor disconformity, separating platform carbonates from overlying terrigenous con-glomerates and sandstones. The stratigraphy of the boundary sequences has recentlybeen improved with the recognition of three foraminiferal assemblages. They are:Maastrichtian Orbitoides–Omphalocyclus, Danian Rotalia–Smoutina–Lockhartia andThanetian Miscellanea–Daviesina microfaunal assemblages. The K/Pg boundary at theGamba area is placed between the Orbitoides–Omphalocyclus and Rotalia–Smoutina–Lockhartia faunas. In Tingri, Cretaceous Globotruncana and tertiary Globigerina–Globo-rotalia microfauna demark the position of the K/Pg boundary. The occurrence of terri-genous sandstones and boulder-size conglomerates in the early Paleocene is compellingevidence for tectonic uplift and emergence of the southern margin of the Lhasa blockand of the northern margin of the Indian plate. Therefore, supported by biostrati-graphic evidence, we argue that the uplift is caused by the onset of continental collisionduring the earliest Danian. Progressing plate collision resulted in deformation and frag-mentation of the Paleocene carbonate platform and deposition of limestone breccias,which we consider as further evidence for tectonic compression as a result of early con-tinental collision during the Thanetian, earlier than indicated by previous studies in theHimalayas. It is the change in the sedimentary facies and depositional environmentthat provides the earliest evidence and dating of the initiation of the collision process.From studies of sedimentary strata in southern Tibet, the collision of the India andLhasa continental blocks was initiated at ¹K/Pg boundary time (¹65Ma). If that is thecase, than the major lithofacies changes at the K/Pg boundary observed in the westernTethys, mostly referred to as intrinsic to the eustatic sea level change, has been drivenby continental convergence and collision of the Indian and Eurasian plates. & Collision,Cretaceous, foraminifera, India, K/Pg boundary, Paleogene, southern Tibet.

Xia qiao Wan [[email protected]], China University of Geosciences, Xueyuan Road 29,Beijing 100083, China; Luba F. Jansa [[email protected]], Geological Survey of Canada-Atlantic, P.O. Box 1006, Dartmouth N.S., Canada B2Y 4A2 and Department of EarthSciences, Dalhousie University, Halifax, N.S., Canada; Massimo Sarti [[email protected]],Istituto di Scienze Del Mare, Universita di Ancona, via Brecce Bianche 60131, Ancona,Italy; 25th April 2001; revised 4th March 2002.

It is � rmly established that the rise of the Himalayas isthe result of collision of the Indian and Eurasiancontinental plates. Prior to this collision, thicksedimentary sequences were deposited at the northernmargin of the Indian and southern margin of theLhasa continental blocks. The main problem re-occurring in studies of continental collision is howto de� ne it. Geologic studies of the late Cretaceous–early Paleogene stratigraphic record of southern Tibetreveal distinct episodes of collision and post-colli-

sional compression. The composition of sedimentsexposed along the Indus–Yarlung Zangbo suture,tertiary tectonic deformation, changes in the directionof oceanic spreading, paleomagnetism, magmaticactivity of the Gangdese Arc, high pressure meta-morphism and terrestrial � ora and fauna havevariously been used to diagnose the timing ofIndian–Eurasia continental plates collision (Searle etal. 1987; Gaetani & Garzanti 1991; Burtman 1994;Beck et al., 1995; Garzanti et al., 1996; Clift &

# 2002 Taylor & Francis

Robertson 1989; 1990; Liu & Einsele 1994; Dewey et al.1989; Rowley 1996, 1998; de Sigoyer et al. 2000). Forexample, terrestrial faunas at the Cretaceous–Paleo-gene boundary in India, which are similar to those inEurasia, have been used to imply that the collisionoccurred at 65 Ma (Jaeger et al. 1989; Rage et al. 1995).Sedimentary response and tectonic deformationresulting from a continental collision between Eurasiaand India have been considered as time indicators ofthe � nal closure of the Neo-Tethys Ocean (Searle et al.1987; Gaetani & Garzanti 1991; Burtman 1994;Garzanti et al. 1996; De Segoyer et al. 2000). The agespread of the dating by the above authors is varied. Anearly Cretaceous time of collision in the southernGreek Neo-Tethys has been favored by Clift &Robertson (1989); in the Himalayas the late Cretac-eous–Paleogene by Burtman (1994), Liu & Einsele(1994); the Paleogene by Clift and Robertson (1990),Paleocene–Eocene boundary by Garzanti et al. (1987);Eocene (50–40 Ma) by Searle et al. (1987), Gaetani &Garzanti (1991); and the middle Eocene by Dewey etal. (1989) and Rowley (1998). From stratigraphicevidence, Willems et al. (1996) suggested that the platecollision began in the early Maastrichtian, with the� nal closure of the remnant Tethys in the Tingri area

during the Lutetian. Evidently, a wide range of agesfor the collision has been proposed by different re-searchers and indirectly applied to the eastern Neo-Tethys ocean closure. Rowley (1996) reviewed differ-ent collision ages varying from 64 Ma to 40 Ma andsuggested diachronous collision, propagating from thewestern to the eastern Himalayas.

We studied in detail stratigraphic sections located inthree different regions of southern Tibet (Fig. 1) torestrict more closely the timing of the initiation of thecontinental collision between the Indian and Tibetancontinental plates. The studied sections outcrop alongboth sides of the Yarlung Zangbo Suture Zone. TheZhuolei Section is located in the Zhongba region andtectonically belongs to the southern margin of theLhasa block. The Gongzha Section in Tingri and theZongpu Section in Gamba belong to the northernmargin of the India plate. The tectonic setting of thesethree regions therefore provides an unparalled oppor-tunity to compare synchronous depositional processeson adjunct continental margins at the time ofcontinental collision. As in the literature, it is debatedhow to de� ne a continental collision (Searle 2001; deSigoyer et al. 2000, 2001); we brie� y comment on thisproblem in the light of the data presented here.

Fig. 1. Simpli� ed geologic map showing location of the studied areas in a broader context of the Himalayas. 1 GangdiseÃ Belt, 2 Forearc Basin, 3Ophiolites, 4 Tethys Himalayas, 5 High Himalayas, 6 Lesser Himalayas, 7 Sub-Himalayas.

132 Xia qiao Wan et al. LETHAIA 35 (2002)

Fig. 2. Columnar section of the Cretaceous–Paleogene boundary sequence in Zhongba. 1 Limestone, 2 Marl, 3 Shale, 4 Sandstone, 5Conglomerate.

LETHAIA 35 (2002) Cretaceous and Paleogene boundary strata in southern Tibet 133

Cretaceous and Paleogene boundarystrata in southern Tibet

Cretaceous and Paleogene boundary strataoutcrop in southern Tibet

The Zhuolei Section containing uppermost Cre-

taceous to Paleocene strata is a new section recentlystudied by the authors. Two other sections that alsoprovide complete stratigraphic sequences from upperCretaceous to Eocene have been studied by WenShixuan (1974), Hao Yichun & Wan Xiaoqiao (1985),Wan Xiaoqiao (1985), Willems (1993) and by WanXiaoqiao et al. (2000). The uppermost Cretaceous to

Fig. 3. Columnar section of the Cretaceous–Paleogene boundary sequence in Gamba. 1 Limestone, 2 Marl, 3 Shale, 4 Sandstone, 5Conglomerate, Brecciated limestone, 7 Nodular limestone, 8 Conglomeratic limestone.


Paleocene transition beds at those two sections werere-measured and re-studied as part of this project.

The Zhongba region – Zhuolei Section

Liu Chengjie et al. (1988) studied the Cretaceous andPaleogene stratigraphy of the Zhongba region, wherethe Cretaceous sedimentary strata overlie ophiolites.During our � eldwork, we focused on a detailed studyof a new section located at Zhuolei, northwest of theQuxia Valley, some 30 km north from Zhongba town.At Zhuolei the K/Pg boundary strata are well preservedand comprise the Qubeiya, Quxia and Jialazi Forma-tions (Fig. 2).

Qubeiya Formation

The upper part of the Qubeiya Formation (more than80 m thick) is grey, thick-bedded, foraminiferal lime-stone intercalated with marl beds. The strata yieldedforaminifera Lepidorbitoides minor (Schlumberger),L.blanfordi Rao, Sigmomorphia sp. and Ophthalmi-dium sp., indicating a Maastrichtian age. This forma-tion rests directly on the ophiolites.

Quxia Formation

The overlying Quxia Formation (107 m thick), incontrast, is formed by brownish-grey colored con-glomerate, sandstone and shale. The lithology isarranged in three sedimentary cycles, with massiveconglomerates at the base, grading upward into shaleat the top of each cycle. Most of the pebbles in theconglomerate are 3–10 cm in diameter, but in thelower part of the formation boulders up to 30 cm insize occur. The pebbles are mostly composed ofsandstone, granite and some siliceous rock. Fossilsare rare in the formation, only some plant debris hasbeen found. The stratigraphic position suggests aDanian age for this formation. It overlies with asedimentary hiatus the Qubeiya Formation.

Jialazi Formation

At the Zhuolei section, only the lower part of theJialazi Formation is exposed. It is over 140 m thick andconformably overlies the Quxia Formation. It iscomposed of several cycles of alternating conglomer-ate, sandstone and shale, with dark grey, massiveforaminiferal limestone beds at the top of each cycle.In the limestone, a rich foraminiferal fauna occursrepresented by Miscellanea miscella (d’Archiac &Haime), Daviesina intermedia Smout, D. persica

Rahaghi, D. khatiyahi Smout, Operculina canaliferad’Archiac, O. palalensis Davies, Ranikotalia nuttalli(Davies) and Operculina sp., indicating a Thanetianage for this formation.

The Gamba region – Zongpu Section

The Late Cretaceous to early tertiary boundarysequence in the Gamba region has been much studied(Wen Shixuan 1974; Zhang Binggao 1981; WanXiaoqiao 1988; Willems 1993). The sequence cropsout in the Zongpu valley 1 km north of the town ofGamba, where the strata were subdivided by WenShixuan (1974). We re-studied the sequence involvingthe Zongshan, Jidula and Zongpu Formations (Fig. 3).

Zongshan Formation

The name Zongshan Formation was � rst used in theGamba area by Wen Shixuan (1974). The upper bedsof the Zongshan Formation consist of dark greyforaminiferal limestone enclosing rudist biostromes.The limestone yields abundant larger foraminifera,such as Orbitoides media (d’Archiac), O. apiculataSchlumberger, Omphalocyclus macroporus (Lamarck)and rudist bivalves. The foraminiferal assemblageindicates a Maastrichtian age for the beds.

Jidula Formation

The name of the Jidula Formation was established byWen Shixuan (1974). The type locality is at JidulaShan in the Gamba area. The Jidula Formation overliesthe Zongshan Formation with a sharp contact and asedimentary hiatus is present at the top of theZongshan Formation. In sharp contrast to thelithologic composition of the Zongshan Formation,the Jidula Formation (177 m thick) is dominated byyellowish-white, indurated, homogeneous quartzsandstone, which in the middle and upper part ofthe formation is intercalated with sandy limestonebeds. The exposure surface at the base of the formationis overlain by a 2–5 cm thick yellowish grey clay bedincorporating dispersed gravel. Foraminifera such asRotalia hensoni Smout, R. dukharni Smout, Smoutinacruysi Drooger and Lockhartia haimei Davies werefound in this clay bed (Fig. 3). Ostracods and algaeoccur in the limestone intercalations in the middlepart of the formation. The microfauna indicates anearly Danian age for the formation.


Zongpu Formation

The overlying Zongpu Formation established byZhang Binggao & Geng Liangyu (1983) in the Gambaarea is 174 m thick. It consists of massive, in the lower

part partly dolomitized limestone, nodular limestoneand a small amount of calcareous marlstone and marl.From � eld examination we subdivided it into fourmembers. Member 1 is medium-bedded dark-greypartly dolomitic, brecciated limestone. It contains

Fig. 4. Columnar section of the Cretaceous–Paleogene boundary sequence in Tingri. 1 Limestone, 2 Marl, 3 Shale, 4 Sandstone, 5 Nodularlimestone, 6 Dolomitized limestone.


frequent dasycladacean algae, debris of Halimeda andforaminifera. Member 2 is dark grey to blackish thicklybedded limestone intercalated with thick, dark-colored beds of marlstone. Toward the top thelimestone becomes nodular. All beds are separatedby laminae of marls. Limestones in Members 1 and 2are intensively fractured (Figs 3, 6A) with a strongbituminous odor. The foraminifera in the limestoneand marl include Rotalia dukharni Smout, R. hensoniSmout, R. orientalis Cushman & Bermudez, R. sp.,Lockhartia altispira Smout, L. conditi (Nuttall), L.cushmani Applin & Jordan, L. diversa Smout, L. haimeiDavies and Smoutina cruysi Drooger, which indicate alate Danian age for the lower part of the formation.Member 3 is a massive dark-grey marly and conglom-eratic limestone with nummulitoids. It is indistinctlybedded. Member 4 is a thick-bedded conglomeraticlimestone incorporating several beds of limestonebreccia (Figs 3, 6B). The limestone clasts in the brecciaare rounded to sub-rounded, 5–20 cm, occasionally upto 30 cm in diameter. A larger foraminiferal faunaoccurs in Members 3 and 4 which includes Miscellaneamiscella (d’Archiac & Haime), M. minor Sheng &Zhang, M. multicolumnata Sheng & Zhang, M. stampi(Davies), Daviesina khatiyahi Smout, D. langhamiSmout, Operculina canalifera d’Archiac, O. patalensisDavies and O. sp. The same species of foraminifera inboth the limestone breccias and the marly matrixindicate a Thanetian age for the upper part of theformation.

A bed of a polymictic conglomeratic limestone(Fig. 6C) separates Member 4 from the overlying,thick-bedded Eocene limestone. The conglomeraticbeds are foraminiferal grainstones, documenting ahigh energy environment for these beds, which differsfrom the mostly wackestone and packstone composi-tion of the Paleocene limestone. The presence of thelimestone conglomerate at the top of Member 4indicates that a diastem separated the Thanetian andEocene strata (Fig. 6D). In contrast to the underlyingPaleocene carbonate platform, the lower part of theEocene limestone has not been strongly tectonicallyfractured.

The Tingri region – Gongzha Section

The Gongzha Section is located about 50 km west ofthe town of Tingri, and about 200 km northwest of theGamba section. Willems (1993), in a study of theCretaceous to tertiary sequence in this area, sub-divided the uppermost Cretaceous to Paleogene stratainto the Zongshan, Zhepure Shanpo, Jidula andZhepure Shan Formations. The lithology of theZhepure Shanpo Formation differs from the synchro-nous beds in the Gamba region, and therefore weretain this formation name to represent passage bedsbetween the Cretaceous and Paleogene in this area(Fig. 4). Other formations as recognized by Willems(1993) are correlative with those in the Gamba region.Table 1 gives regional correlation of the boundarysequences in the Gamba and Tingri areas.

Zongshan Formation

Hao Yichun & Wan Xiaoqiao (1985) � rst applied thisformation name to the strata in the Tingri area. TheZongshan Formation consists of bioclastic limestonethat is lithologically similar to correlative strata in theGamba region. In the Gamba area the formation isCampanian to late Maastrichtian in age. But in theTingri area the late Maastrichtian beds are of differentlithologic composition. They are composed of terri-genous clastics and are therefore referred to as theZhepure Shanpo Formation by Willems (1993). AnOrbitoides–Omphalocyclus fauna in association withplanktonic foraminifera such as Globotruncana sp.found in the Zongshan Formation point to aCampanian to early Maastrichtian age for the Zong-shan Formation in the Tingri area.

Zhepure Shanpo Formation

The Zhepure Shanpo Formation (225 m thick) in theTingri area is time-correlative with the upper Zong-shan and lower Jidula Formations of the Gamba area.It straddles the K/Pg boundary. Willems (1993)

Table 1. The Cretaceous–Paleogene boundary sequences in the Zhongba, Gamba and Tingri areas

Stage Zhongba Gamba Tingri Fauna

Paleogene/Paleocene Thanetian Jialazi Fm. Zongpu Fm. Zongpu Fm. Miscellanea–DaviesinaJidula Fm. Rotalia–Smoutina–Lockhartia

Danian Quxia Fm Jidula Fm. Globigerina (Tingri)Zhepure Shanpo Fm.

Cretaceous Maastricht. Qubeiya Fm. Zongshan Fm. Lepidorbitoides (Zhongba)Zongshan Fm. Orbitoides (Gamba, Tingri)


de� ned the Zhepure Shanpo Formation in Tingri,because a similar, correlative sequence is not present inthe Gamba area. The latter formation is composed ofsandstone intercalated with shale and limestone beds.It overlies well-bedded limestone of the ZongshanFormation and in turn is overlain by sandstones of theJidula Formation. In the course of the present study,many microfossils have been found in this formation,allowing a rede� nition of this formation as passagebeds from the late Cretaceous into the early tertiary. Inthe lower part of the formation, microfossils arecommon and include foraminifera species. Two fossilbeds were recognized. Lower bed yields foraminiferaGlobotruncana linneiana (d’Orbigny) and G. ventricosa(White) Verneuilina sp., Textularia sp., Oolina sp.,Spiroloculina sp., Nonion sp., and upper bed containsGlobotruncanita stuarti (de Lapparent), Heterohelixsp., Omphalocyclus marcroporus (Lamark). Ostra-codes, algae and gastropods are also common. Themicrofauna refers a middle Maastrichtian age for thelower bed and late Maastrichtian age for the upperbed.

A 3.7 m thick, grey, thin-bedded calcareous sand-stone layer marks the Cretaceous/Paleogene boundary.With the exception of a few bioclasts of gastropods,benthic foraminifera and ostracoda, no age diagnosticfossils were found in this sandy bed. The absence ofplanktonic foraminifera from this bed is due to facieschange. According to the age of underlying andoverlying strata, this bed can be referred to the passagebed between Cretaceous and Paleogene.

The beds above the K/Pg boundary transitioncontain early tertiary microfauna (Fig. 4). The tertiarymicrofauna is represented in the upper part of theformation by the planktonic foraminifera Globigerinasp. and Globorotalia sp. Other fossils present includethe benthic foraminifera Miliammina sp., Textulariasp., Cibicides sp., Coryphostoma sp., Textularia sp.Rotalia sp., Para� ssurina sp., the ostracodes: Zonocy-pris sp., Bairdia sp., Herpetocyprella sp., and Cyprideasp. The Zhepure Shampo Formation is overlainconformably by the Jidula Formation.

Jidula Formation

In Tingri, this formation is 97 m thick, which is onlyabout half of the thickness as measured in Gamba. It iscomposed of coarse-grained, calcareous and glauco-nitic sandstones. Few fossils were previously reportedfrom this formation. We have found additional speciessuch as the foraminifera: Lenticulina sp., Oolina sp.,Morozovella sp. and ostracods Bairdia sp in the strata.According to the stratigraphic position, this formationis most probably of early Danian age. The presence offoraminifera in the sandstone and their composition

provides evidence for their marine origin. The bedswere deposited on a shelf with a lower sedimentationrate than that in the Gamba region.

Zongpu Formation

The Zongpu Formation overlies the Jidula Formationwith a conformable contact. It is composed of massivelimestone enclosing abundant benthic foraminifera. Itgrades upward into oolitic and nodular limestone,with nodular, bioclastic limestone at the top of theformation. Only the lower part of this formation hasbeen included in this study. It was Hao Yichun & WanXiaoqiao (1985) who � rst referred to correlative bedsin Tingri as the Zongpu Formation, but Willems(1993) renamed them the Zhepure Shan Formation.However, we retain the original name – the ZongpuFormation to represent the sediments in both Gambaand Tingri because of the similarity of sedimentaryfacies in both areas. A rich Paleocene microfauna inthese strata is represented by the foraminifera Rotaliahensoni Smout, Lockhartia conditi (Nuttall), L. cush-mani Applin & Jordan, Smoutina cruysi Drooger,Miscellanea miscella (d’Archiac & Haime), M. stampi(Davies), Daviesina khatiyahi Smout, D. langhamiSmout, and Operculina canalifera d’Archiac, andindicate a late Danian–Thanetian age for the forma-tion.

Discussion

The Cretaceous/Paleogene Boundary inSouthern Tibet

The new, detailed � eld data obtained during the studyof Cretaceous–Paleogene boundary sequences insouthern Tibet revise some of the earlier stratigraphicinterpretations of the geology of the region.

The uppermost Cretaceous strata in the Zhongbaand Gamba areas are represented predominantly bycarbonate platform deposits. But in the Tingri areathey are composed of a sandstone-shale sequence thatwe interpret as pro-delta deposits. This lithofaciescrosses the K/Pg boundary and extends into the lowerPaleocene. Whatever the differences in sedimentaryfacies, larger foraminifera faunas are found in all threeareas discussed above, an Orbitoides–Omphalocyclus atGamba and Tingri areas, and a Lepidorbitoides fauna atthe same horizon in the Zhongba area. The occurrenceof foraminiferal species such as Orbitoides media, O.apiculata, Omphalocyclus macroporus, Lepidorbitoidesminor and the planktonic foraminifera Globotruncanaventricosa suggest that the strata in all three sections


are of Maastrichtian age. The K/Pg boundary is,therefore, located above this horizon (Fig. 5).

Coarse-grained sandstone and conglomerate weredeposited directly on top of a carbonate platform inboth the Zhongba and Gamba areas. The stratigraphicage of the Jidula and Quxia formations in the aboveareas provides key information about the location ofthe K/Pg boundary. Previously, Wen Shixuan (1974)referred to the Jidula Formation as of Maastrichtianage, and Liu Chengjie (1988) placed the K/Pgboundary at the top of the Quxia Formation. Otherauthors (Willems 1993; Zhou Zhicheng et al. 1996; XuYulin & Mao Shaozhi 1992) suggested that the JidulaFormation spans the K/Pg boundary. In this study, anexposure surface has been found separating theZongshan and Jidula Formations in Gamba, andbetween the Qubeiya and Quxia Formations in theZhongba area. Therefore, the tops of the formationswere most probably subaerially exposed for a shorttime followed by submergence in a shallow sea.

The Rotalia–Smoutina–Lockhartia foraminiferalfauna has been found in the bottom clay bed depositedat the top of the underlying exposure surface. There-fore, we place the K/Pg boundary at the base of theclayey bed, which underlies coeval sandstone and

conglomerate (Fig. 5). The Jidula and Quxia forma-tions are of Danian age. In Gamba, the Rotalia–Smoutina–Lockhartia fauna is present in the Jidula andin the lower part of Zongpu Formations. It is overlainby a Thanetian Miscellanea–Daviesina foraminiferalfauna (Table 1). Based on the lack of fossils at theCuojianding Section, Liu Chengjie et al. (1988)assumed that the Quxia Formation is of late Cre-taceous age. But, by correlation of the underlying bedsenclosing Lepidorbitoides faunas and the overlyingbeds with Miscellanea–Daviesina fauna, the strati-graphic position of the unfossiliferous part of thesequence can be restricted to correlative beds contain-ing a Rotalia–Smoutina–Lockhartia fauna horizon inthe Gamba area (Fig. 5). Thus, the presence offoraminiferal faunas of Orbitoides–Omphalocyclus (orLepidorbitoides) and Rotalia–Smoutina–Lockhartiamarks the K/Pg boundary in the Zhongba and Gambaareas.

The exposure surface and the overlying basalconglomerate, which are of early Danian age, delineatethe disconformity between the Cretaceous and Paleo-gene in the above-discussed regions, except for theTingri area. As documented for the Gongzha Sectionin the Tingri area, the depositional environment

Fig. 5. Correlation of the Cretaceous–Paleogene boundary strata in southern Tibet. 1 Limestone, 2 Marl, 3 Shale, 4 Sandstone, 5 Conglomerate,6 Nodular limestone, 7 Brecciated limestone, 8 Dolomitized limestone 9 Conglomeratic limestone.


during late Maastrichtian–early Danian time wasdifferent from that in the Zhongba and Gamba areas.The interbedded sandstone and shale of the ZhepureShanpo Formation deposited in a pro-delta environ-ment indicate that during the K/Pg boundary time thesurrounding area was subaerially exposed and terri-genous clastics were transported into the ‘Tingribasin’. Planktonic foraminifera found in the ZhepureShanpo Formation are a typical late CretaceousGlobotruncana microfauna and a tertiary Globigerina–Globorotalia microfauna, which allow location of theK/Pg boundary in the formation. Since a distinct facieschange at this stratigraphic boundary, or recognizablebiostratigraphic hiatus are obscure, therefore, moredetailed study is necessary to de� ne the deposition ofcontinuous or discontinuous in the Tingri area.

Implication for Early Collision of theIndia–Eurasia Continents

A wide range of ages has been proposed for thecollision of the Indian and Eurasian plates, with agesvarying from 64 Ma to 40 Ma, as discussed in theintroduction and as reviewed by Rowley (1996). Themain problem is the one emphasized by de Segoyer etal. (2000); namely, how to de� ne continental collision.One group of thoughts recently presented by Searle(2001) considers the end of the last marine sedimenta-tion during the Eocene to represent true continentalcollision. In contrast, de Sigoyer et al. (2000) suggestedthat formation of eclogites in northeastern Ladakhdated 55 § 6 Ma provides timing for the onset ofcontact between India and Eurasia. In contrast to deSigoyer et al. (2000) we argue that the continentalcollision does not manifest itself in several diachronicevents, but is a complex, continuing process which,when studied by different methods, provides differentresults. Our approach is to make a distinction aboutthe initial contact of the continental plates revealed byinitiation of compressional stresses and differentiate itfrom the progressing collision resulting in platethickening and associated with Barrovian metamorph-ism. As the continental plates do not have straightboundaries, and collision does not need to occur inexact perpendicularity, as is the case for the Indiaplate, which involves plate rotation during the colli-sion (Crouzet et al. 2000; Schill et al. 2000), thecompressional stresses resulting for initial platescontact will not be uniform along plate boundariesand will inevitably produce lateral diachroneity inprocesses related to the plate collision, as noted byRowley (1996) and Najman & Garzanti (2000). In thewestern Himalaya, India collided with the southern

margin of Eurasia in latest Cretaceous time (Royden &Burch� el 1997). In northwestern Pakistan, accretion-ary-prism and trench strata were thrust onto thenorthwest Indian passive margin after 66 Ma (Beck etal. 1995). The age of the Deccan Trap volcanism andsimilarities of the terrestrial faunas of India andEurasia also suggest that the India–Eurasia collisionhad to occur close to the K/T boundary (Jaeger et al.1989). However, high-pressure granulite data wereinterpreted to indicate that the initial collisional eventin the east Himalayan syntaxis and western Sichuanand Yunnan in China occurred during the Paleocene(60–55 Ma) (Zhong Dalai et al. 2000). Therefore,different methods studying different aspects of thecollision processes come up with different results anddifferent dates for the collision.

The stratigraphic data presented in this articleindicate that the initial collision of the plates occurredalmost 10 million years earlier than suggested fromstudies of crust processes. The development of LateCretaceous strata in southern Tibet indicates regres-sion after the Cenomanian/Turonian boundary time,when the sea level was at its highest in southern Tibet(Wan Xiaoqiao et al. 1997). During the Late Cre-taceous, the Xigaze forearc basin evolved into a shal-low marine basin as the consequence of convergenceand emplacement of ophiolites. Such a basin attainedcharacteristics of a foreland, epicontinental basinduring the Late Cretaceous, formed as a result ofdownward de� ection of the Indian plate in response toloading of the Asian margin. This interpretation issupported by widespread development of a Campa-nian and Maastrichtian shallow water carbonate plat-form, which in places encloses rudist reefs andbioclastic shoals with larger foraminifera, lack of anydeep-sea–oceanic deposits in this time and a lack ofoceanic crust. The presence of terrigenous conglom-erates and cross-bedded sandstones of Danian age inthe Gamba and Zhongba regions resting disconform-ably on Maastrichtian platform carbonates, providesevidence that at the K/Pg transition time both regionswere tectonically uplifted, culminating in a shorthiatus in deposition followed by deposition ofcoarse-grained clastics. The tectonic uplift of thehinterland occurred earlier, but less strongly in theTingri region, where the pro-delta environmentprevailed from late Maastrichtian to early Paleocene.The initial age of the pro-delta is indicated by thelarger foraminifer Omphalocyclus macroporus and theplanktonic foraminifer Globotruncana ventricosa, G.linneiana and Globotruncanita stuarti. As mentionedabove, the deposition across the K/Pg boundary waspossibly continuous in the Tingri area. The in� ux ofclastics at late Maastrichtian time deposited in a pro-delta on a shelf document that the hinterland behind


the delta was tectonically uplifted, with depositionalsag developing on the shelf, allowing uninterruptedmarine deposition from latest Maastrichtian to theearly Danian. A different paleogeographic setting isindicated by the coarse-grained conglomerates deposi-ted during the Danian in the Zhongba region, whichtectonically belong to the southern margin of theLhasa block. The occurrence of such deposits suggestsstrong tectonic uplift of the southern margin of theLhasa block, resulting in an increased slope gradientand in a large amount of debris transported into thesea from the rising area to the north.

The differences in the K/Pg boundary lithofaciesbetween conjugate India–Lhasa margins could beeither the result of differences in the transmission oftectonic stresses between plates, or of the differences inpaleogeographic setting. The terrigenous clastics inTingri most probably represent distal lithofaciesdeposited on the wider continental shelf of the Indiamargin in comparison to the Lhasa margin where all

the � eld information from the Zhongba area suggeststhat the shelf was narrower because the early Tertiaryconglomerate deposits are of proximal character. Thethird possibility is that the lithofacies difference mayre� ect differences in paleo-relief, with the relief on theLhasa block being higher, due to previous tectonicevents and suturing. Even though a high paleo-relief ofthe southern margin of the Lhasa block resulting fromsubduction is evident from deposition of Xigazeconglomerates at the slope of the fore-arc basin duringAlbian–Coniacian time (Durr 1996; Wan Xiaoqiao etal. 1998), the widespread carbonate platforms deposi-ted during the Campanian–Maastrichtian on bothmargins indicate that the relief was mostly erased.Therefore, the paleo-relief of the Lhasa block had to berejuvenated at the K/Pg boundary time.

The carbonate platform development recoveredtowards late Danian and Thanetian time in southernTibet. The upper Danian nodular limestone wasfragmented in situ (Fig. 6A). Its nodular texture was

Fig. 6. Outcrops of the Paleocene limestone strata in the Gamba area. &A. Medium-bedded dark-grey nodular limestone beds from member IIof the Zongpu Formation which are separated by laminae of marls and intensively fractured. &B. Massive brecciated limestone from member3 of the Zongpu Formation; the clasts in the breccia are rounded to sub-rounded and 5–20 cm in size, maximum is up to 30 cm. &C. Apolymictic conglomeratic limestone occurs at the top of the member 4 of the Zongpu Formation, the gravels are up to several tens ofcentimeters in size and cemented by marls. &D. The Paleocene and Eocene boundary from the Zongpu Section; lower part is Paleoceneconglomeratic limestone and upper the Eocene limestone that has not been strongly tectonically fractured. A diastem occurs at the boundary.


acquired during diagenesis, indicating that tectoniccompression affected the limestone shortly after itsdiagenetic consolidation. The overlying Thanetianstrata are also composed of nodular limestone, butthese include limestone breccia and conglomeratebeds (Fig. 6B, C). The clasts in the breccia beds arerounded and subrounded, up to several tens ofcentimeters in size, which indicates that they wereredeposited probably as debris � ows. The Thanetianlimestone breccia is overlain discomformably byEocene bedded shallow water limestone, which doesnot show strong fracturing, or the presence of debris� ows. This difference leads us to suggest that as thetectonic compression stresses increased, the carbonateplatform was tectonically deformed, and debris fromthe fragmented carbonate platform re-deposited downthe slope. The Miscellanea–Daviesina foraminiferalfauna occurs in the marly matrix and limestone clastsof the debris � ows as well as in the marlstone andnodules of the nodular limestone. Therefore, thediagenetic transformation, tectonic deformation andbrecciation all had to occur during the Thanetian. Ifthis suggestion is correct, it implies that the Gambaregion was affected by increased compressional stres-ses during the late Thanetian (¹58 Ma). However, weconsider the major lithologic change occurring at theCretaceous–Paleogene boundary (¹65 Ma) as indicat-ing the initial India–Eurasia contact.

Conclusions

Recent � eld studies in southern Tibet led to the� ndings of larger foraminifera in sedimentary strata(Figs 7, 8), which allowed us to de� ne the Cretaceous/Paleogene boundary. In the Gamba and Zhongba areasthe extinction of the Orbitoides–Omphalocyclus (Lepid-orbitoides in Zhongba) assemblage marks the top of theCretaceous, and the appearance of the Rotalia–Smou-tina–Lockhartia assemblage identi� es the base of thePaleocene. Occurrence of the above microfossil assem-blages indicates that the K/Pg boundary is locatedbetween the Qubeiya and Quxia Formations in theZhongba area, and between the Zongshan and JidulaFormations in Gamba. The third M iscellanea–Davie-

sian microfossil assemblage recognized identi� es theThanetian strata in this region. In the Tingri area, theCretaceous–Paleogene boundary is recognized by theoccurrence of Cretaceous Globotruncana and tertiaryGlobigerina–Globorotalia microfauna assemblages.

The implication of this new stratigraphic data fromsouthern Tibet is that it indicates that the collision ofthe India and Eurasia continental plates was initiatedat ¹K/Pg boundary time. Our conclusion supports theresults of Klootwijk et al. (1991), who from paleo-magnetic data from Ninetyeast Ridge concluded thatthe initial contact of the continental plates mightalready have been established at that time. The earlycompression evident in the Paleocene sedimentarydeposits in southern Tibet places the initial compres-sion earlier than suggested by many previous hypoth-eses. The lateral diachronicity of collisional processeswas noted by several authors, who concluded that thecollision was propagating from west to east (Rowley1996; Najman & Garzanti 2000). However, age datingof various processes related to the plate collision showthat the diachronism in collision processes is not onlylateral, but also a vertical across-crust entity.

The main problem in the study of continentalcollision processes is how to de� ne continentalcollision. Our approach is to make a distinctionbetween initial plates contact and progressing colli-sion, resulting in plate thickening re� ected in Barro-vian metamorphism, similar to that adopted by deSigoyer et al. (2000). In our view, the complexity in thereconstruction of earth processes involved in con-tinental plate collision is increased by the fact thatcontinental plate edges do not form straight lines, buthave numerous indentations, embayments and pro-montories as documented by geophysical surveys ofcontinental/ocean crust boundaries. Therefore, com-pressional stresses will not be uniform along such plateboundaries. Furthermore, collision may not haveoccurred exactly perpendicularly to the plate margin,as documented by the rotation of the India plateduring convergence (Crouzet et al. 2000; Schill et al.2000).

The � rst sign of plate collision (initial plate contact)can be noted by the change in the paleo-relief, asrecorded by changes in depositional environment andmajor changes in composition of deposited sediments.

Fig. 7. All � gured specimens are from the Lower Paleocene of Gamba, Tibet, lower part of the Zongpu Formation, and placed in the ChinaUniversity of Geosciences, Beijing. The repository number GB indicates the fossils locality Gamba. &A. Rotalia dukharni Smout, axial section,£30, GB2-38. &B. Rotalia hensoni Smout, axial section, £30, GB3-13. &C. Rotalia orientalis Cushman & Bermudez, axial section, £30, GB3-15. &D. Rotalia sp., axial section, £30, GB4-12. &E. Smoutina cruysi Drooger, axial section, £30, GB2-32. &F. Lockhartia haimei Davies, axialsection, £30, GB4-35. &G. Lockhartia cushmani Applin and Jordan, axial section, £30, GB4-24. &H. Lockhartia haimei Davies, axial section,£30, GB4-37. &I. Lockhartia diversa Smout, axial section, £30, GB4-30. &J. Miscellanea.minor Sheng & Zhang, axial section, £18, GB6-12.&K. Miscellanea multicolumnata Sheng & Zhang, axial section, £18, GB6-11. &L. Miscellanea miscella (d’Archiac & Haime), axial section,£18, GB6-7. &M. Lockhartia sp., axial section, £18, GB6-18. &N. Daviesina khatiyahi Smout, axial section, £30, GB7-1. &O. Daviesinalanghami Smout, axial section, £30, GB7-5.


Deep crustal processes related to plate collision, suchas formation of high pressure granulites and eclogites,provide a younger age (in the case of Himalayas, 10Ma younger, de Sigoyer et al. 2000) than the change insedimentary processes. However, not all changes insedimentation indicate plate collision. The closing ofmarine sedimentation in basins that developed abovethe suture zone was considered by Searle (2000) as thetrue representation of continental collision. However,in southern Tibet the end of marine deposition wasdated as Late Eocene (¹35 Ma) (Wan Xiaoqiao 1987),which is 20 Ma after the initial plate contact as de� nedin this article. The Paleogene basins in southern Tibetare not oceanic, but foreland basins, with all of thecharacteristics of epicontinental basins. The cessationof marine sedimentation in such basins is the result ofcontinuing plate collision, but does not identify theinitial stage of the plate collision. Earth processesinvolved in the collision of continental plates studiedby different methods provide different results; andgeneralization of such results from a single method, orfrom one area only, may be misleading.

In southern Tibet the stratigraphic evidence docu-ments initiation of the India–Eurasia continentalplates collision at K/Pg boundary–earliest Daniantime (¹65 Ma).

Acknowledgements. – This study was supported by the China Na-tional Project (G1998040800) and the National Natural ScienceFoundation (project no. 49872003) of China. We thank LiangDingyi, Li Guobiao, Pan Yusheng, Xu Ronghua, Ding Lin, LiJianguo, Cai Huawei for their assistance during � eld work in2000 and 2001. L. Jansa acknowledges the support of the Geo-logical Survey of Canada-Atlantic (Geological Survey of Canada,contribution 2001195). A. Hallam, M. Lamolda, P. Ascoli and F.Thomas reviewed the manuscript and provided helpful sugges-tions for its improvement.

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