Cretaceous Research (2001) 22, 481–490 doi:10.1006/cres.2001.0271, available online at http://www.idealibrary.com on The Cenomanian–Turonian anoxic event in southern Tibet *C. S. Wang, *X. M. Hu, †L. Jansa, ‡X. Q. Wan and *R. Tao *Chengdu University of Technology, 610059, Chengdu, China; e-mail: [email protected] †Earth Science Department, Dalhousie University, Halifax, NS, Canada ‡Department of Geosciences, China University of Geosciences, 100083, Beijing, China Revised manuscript accepted 31 May 2001 The Cenomanian–Turonian black shales in southern Tibet record a global oceanic anoxic event (OAE). A combined sedimentological, geochemical and micropalaeontological study shows: (1) increased total organic carbon (TOC: 0.5–1.7%) with a peak accumulation across the Cenomanian/Turonian boundary (CTB); (2) sulfur/carbon ratios (S/C) and high degree of pyritization (DOP) indicating that the depositional environment in the Gyangze area was oxygen-depleted and H2S-rich, while synchronous, shallower deposits in the Gamba area accumulated in an oxic environment; (3) bulk-rock 13C analyses of carbonates indicating a positive 2‰ excursion across the CTB, similar to that observed in the western Tethys and Pacific; and (4) an extinction rate of planktic foraminifera across the CTB reaching 50–70%, while the extinction of benthic foraminifera was as high as 90%. A major extinction of benthic foraminifera indicates the development of an inhospitable environment associated with the presence of poorly oxygenated bottom waters. The CTB OAE in Tibet documents a fundamental change in Late Cretaceous palaeoceanography associated with increased dispersal of the southern continents. As the Indian plate moved northward, there was a change in ocean circulation which may have led to the development of a layer of poorly oxygenated bottom and intermediate waters within both the western and eastern Tethys. 2001 Academic Press K W: oceanic anoxic events; Cenomanian/Turonian boundary; southern Tibet. 1. Introduction Reyment & Bengtson, 1986). However, the circum- stances causing this event remain a matter of specu- Organic-rich black shales are widespread within lation. The objective of this paper is to describe the Berriasian–Turonian marine sedimentary deposits of occurrence of black shales of Cenomanian/Turonian- the Tethys. Their occurrence has been mostly boundary age in southern Tibet, and to consider the explained by deposition in a poorly oxygenated palaeoceanographic implications of this occurrence environment. Several of these organic-rich intervals and their potential significance as source rocks for have an ocean-wide distribution and are known as hydrocarbon generation in southern Tibet. ‘Oceanic Anoxic Events’ (OAEs) associated with positive 13C anomalies (Schlanger & Jenkyns, 1976; Jenkyns, 1980). One anomaly occurs at the 2. Geologic setting and biostratigraphy Cenomanian/Turonian boundary (CTB OAE2). This has been recorded in widespread locations (Bralower, The study area is located to the south of the Yarlung 1988), including southern Germany (Hilbrecht & Zangbo Suture Zone in the Tethyan Himalayas. Dahmer, 1994), southern England (Jarvis et al., 1988; During the mid Cretaceous this region was located at Leary et al., 1989), southern France (Crumie`re, a latitude of c. 21S(Patzelt et al., 1996), and was 1988), southeastern Poland (Peryt & Wyrwicka, surrounded by an ocean connected eastward to the 1991), northern Spain (Paul et al., 1994), northern Pacific Ocean and westward to the Mediterranean Japan (Hasegawa & Saito, 1993), India (Govindan & Tethys. Two Cretaceous localities near the towns of Ramesh, 1995), the Atlantic Ocean Basin (Herbin Gamba and Gyangze were studied in detail. Although et al., 1987, ODP Leg 103), and Brazil (Mello et al., both localities are not far apart, they represent 1989). The CTB OAE developed in deep ocean different depositional settings on a northern passive basins, on continental slopes, on submarine plateaux continental margin of the Indian plate (Yu & and in epicontinental seas (Schlanger et al., 1987; Wang, 1990)(Figure 1). East of Gamba, at Zongshan 0195–6671/01/040481+10 $35.00/0 2001 Academic Press 482 C. S. Wang et al. Lhasa Quxu Xigaze Lhaze Gyangze 2 Tingri Kangmar 1 Gamba Everest 02040 km 2 Gangdese magnetic Forearc basin Yarlung Zangbo Northern Subzone Southern Subzone Crystalline Himalayan Position of arc zone suture zone of Tethyan Himalayas of Tethyan Himalayas zone measured sections Figure 1. Generalized geological map of the central part of southern Tibet, showing the localities studied: 1, Zongshan east of Gamba; 2, Gyabula, east of Gyangze. (Figure 1), the Cretaceous sedimentary deposits con- 125.2 m (Figure 3). A significant change in the for- sist predominantly of intercalated glauconite-bearing, aminiferal assemblage occurs in the W. archaeocretacea fine-grained quartzose and calcareous sandstones, Zone. The diversity and abundance of the compo- fossiliferous mudstones and hemipelagic limestones. nents of the assemblage decrease continuously from The succession has been subdivided into the 93.7 m upwards through the section until foraminifera Dongshan, Chaqiela, Lengqingre, Xiawuchubo, are no longer present at 102.2 m. They reappear Jiubao and Zongshan formations (Wan, 1985; Xu at 103.2 m (Figure 3), and the first occurrence of et al., 1990), which have been described in detail by Helvetoglobotruncana praehelvetica is immediately Mu et al. (1973), Wen (1974), Wan (1985), Xu et al. above this level. There was an obvious extinction (1990), Willems & Zhang (1993) and Liu & Einsele bioevent during this depositional interval at 102.2– (1994) (Figure 2). They are considered to reflect 103.2 m (see below for details). The CTB at deposition on a deepening shelf to upper slope, Zongshan, as defined by the first occurrence of as discussed in more detail below. Helvetoglobotruncana praehelvetica, has been placed at A study of the microfauna of the Gamba succession 103.2 m within the Whiteinella archaeocretacea Zone shows that the top of the late Cenomanian Rotalipora (Wan et al., 1997). cushmani Zone is marked by the last occurrence of the In the Gyangze area, the Cretaceous strata planktic foraminifera Rotalipora greenhornensis and consist mainly of black shales. Recently, Li et al. Hedbergella trocoidea, and of the benthic foraminifera (1999) and Wang et al. (2000) redefined the Mesozoic Lenticulina franki, Gyroidina excerta and Gyroidinides strata in the Gyangze area and subdivided the primitiva (Figure 3; for authors of all taxa noted in this Cretaceous succession into the Gyabula, Chuangde, paper, see Appendix). The Whiteinella archaeocretacea and Zongzhuo formations (Figure 2), with boundaries Zone extends from the last occurrence of Rotalipora that are markedly different from those recognised by cushmani at the base to the first occurrence of previous authors (e.g., Wen, 1974; Wu, 1987). The Helvetoglobotruncana helvetica at the top (Figure 3). Gyabula Formation is composed of black shales with The first occurrence of H. helvetica is regarded as the frequent pyrite nodules, intercalated with turbiditic base of the early Turonian Helvetoglobotruncana hel- sandstones. A Berriasian–Santonian age for the for- vetica Zone (Figure 3). All three foraminiferal zones mation is supported by the occurrence of various are present within the upper Lengqingre and lower radiolarians including Eucyriticium sp., Hemicrypto- Xiawuchubo formations (Figure 2). The W. archaeo- capsa sp., Pseudoaulacophacus floresensis, and Theo- cretacea Zone corresponds to the strata from 90.6 to campe tina. The overlying Chuangde Formation The Cenomanian–Turonian anoxic event in southern Tibet 483 STAGE FORMATION THICKNESS LITHOLOGY STAGE FORMATION THICKNESS LITHOLOGY m m STAGE ZONE FORMATION THICKNESS LITHOLOGY SAMPLES 1400 180 17-2 17-1 m 280 Zongshan Fm 16-2 STAGE FORMATION BEDS THICKNESS LITHOLOGY 16-1 160 m 15-1 Maastrichtian 14-2 1200 Zonguzo Fm Coniacian–Maastrichtian 14-1 240 Jiubao Fm H. helvetica H. 140 13-3 50 Xiawuchubo Fm Xiawuchubo 13-2 Fm 13-1 12-60 Turonian Xiawuchubo 14-18 1000 LOWER TURONIAN 120 Turonian 12-40 200 12-40 40 12-35 100 12-30 12-25 Cenomanion 800 Chuangde Fm W. archaeocretacea W. Lengqingre Fm 160 12-22 Santonian–Campanian 30 80 12-21 12-20 10-13 Cenomanian Gyabula Fm Fe 12-19 12-18 Albian 600 12-17 12-16 Chaqiela Fm 60 12-15 120 12-14 20 12-13 12-12 12-11 Fe 12-10 Lengqingle Fm 12-9 40 12-8 12-7 400 12-6 12-5 80 Fe 1-9 Albian 10 UPPER CENOMANIAN 12-4 Rotalipora cushmani 20 Fe 12-3 Gyabula Fm 12-2 200 Berriasian–Coniacian 12-1 40 Dongshan Fm Si Berriasian–Aptian AB 0 LEGEND Shale Mudstone Sandstone Marlstone Micritic Bioclastic Forminifera Nodular Siliceous Olisto- limestone limestone marlstone limestone shale strome Figure 2. Lithostratigraphy of the Cretaceous strata and data for the CTB in southern Tibet. A, Zongshan; B, Gyabula. consists of violet-red marlstones and mudstones. Two dark grey to black shales enclosing various olistoliths foraminiferal zonal-marker species, Dicarinella asymet- of sandstone, limestone, and siliceous rocks, corre- rica and Globotruncana ventricosa, have been recovered sponding to the ‘Beijia Olistostrome’ (Liu & Einsele, from the red marls, indicating their Santonian–early 1994). Campanian age (Robaszynski & Caron, 1995). The At the Gyabula locality, east of Gyangze (Figure 1), overlying Zongzhuo Formation is of late Campanian– radiolarians and planktonic foraminifera both define Maastrichtian age and predominantly composed of the base and the top of the Cenomanian. Beds 8–9 484 C. S. Wang et al. Planktic foraminifera Benthic foraminifera STAGE ZONE FORMATION THICKNESS LITHOLOGY SAMPLES G. bentonensis G. appenninica R. deeckei R. greenhomensis R. montealvensis R. reicheli R. cushmani R. aumalensis P. gibba P. stephani P. delrioensis H. simplex H. algeriana D. planispira H. eaglefordensis G. baltica W. Heterohelix sp. hagri D. brittonensis W. archaeocretacea W. reussi H. pulchra H. aprica W. globulosa H. elata D. prachelvetica H. helvetica H. schneeganol M. renzi M. shaeroidalis D. conicula D. Anomalins solis Lenticulina franki primitiva G. sp. Textularis intermedia G. Dentalina sp. Dentalinoides sp. Tristix sp. Lenticulina sp.