Upper Jurassicelower Cretaceous Stratigraphy in South-Eastern Tibet: a Comparison with the Western Himalayas

Author's personal copy Cretaceous Research 29 (2008) 301e315 www.elsevier.com/locate/CretRes Upper JurassiceLower Cretaceous stratigraphy in south-eastern Tibet: a comparison with the western Himalayas Xiumian Hu a,*, Luba Jansa b, Chengshan Wang c a State Key Laboratory of Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, China b Department of Earth Sciences, Dalhousie University, Halifax B3H 3J5, Canada c Geological Centre for Tibetan Plateau, China University of Geosciences, Beijing 100083, P.R. China Received 24 January 2007; accepted in revised form 14 May 2007 Available online 28 November 2007 Abstract Lithostratigraphic studies of the Upper JurassiceLower Cretaceous sedimentary successions exposed in the TingrieGyangze area, south- eastern Tibet resulted in the establishment of a revised stratigraphic framework. A major crustal fault separates the southern Tibetan sedimentary successions into a Southern Zone and a Northern Zone. The Upper JurassiceLower Cretaceous strata of the Southern Zone are subdivided into the Menkadun Formation (Oxfordian, Kimmeridgian, up to lower Upper Tithonian), the Gucuo Formation (Upper Tithonian to Lower Albian) and the overlying Dongshan Formation (Upper Albian). The Gucuo Formation is further subdivided into a quartz arenite unit, which is overlain by a shale unit, in turn overlain by a volcaniclastic sandstone unit. The youngest cluster of detrital zircon absolute age data (127.7 Æ 1.8 Ma) from the lower part of the volcaniclastic unit of the Gucuo Formation place the volcanic event before the Late Barremian. In the Northern Zone, the Upper JurassiceLower Cretaceous strata have been subdivided into four formations: the Zhera Formation (Upper Jurassic), the Weimei Formation (Tithonian), the Rilang Formation (?Berriasian) and the Gyabula Formation (? post-Valanginian). The appearance of silica cemented quartz arenites in both the quartz arenite unit of the Gucuo Formation and the Weimei Formation in the uppermost Jurassic is considered to provide strong evidence for an eustatic sea level drop affecting the northern margin of Greater India. In contrast the appearance of volcaniclastic sandstones in the Gucuo and Rilang formations are interpreted to represent a rift related volcanic event. The MenkaduneZhera formations, the quartz arenite unit of the Gucuo FormationeWeimei Formation, the volcaniclastic sandstone unite Rilang Formation in south-eastern Tibet are lithologically similar and broadly correlative with the Spiti Shale, Takh and Pingdon La formations in Zanskar (India), and Spiti Shale, Dangardzong, Kagbeni and Dzong formations in the Thakkhola area (Nepal). The onset of quartzose and subsequently volcaniclastic deposition becomes progressively younger westward, from the Gucuo region toward Zanskar. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Stratigraphy; South-eastern Tibet; Tethys Himalaya; Lower Cretaceous; Upper Jurassic 1. Introduction 1964; Fuchs, 1982; Gaetani et al., 1986; Gaetani and Garzanti, 1991; Bhargava and Bassi, 1998; and references cited therein). Mesozoic sedimentary strata deposited on the northern Later, the interest has increasingly focused on the Mesozoic Greater India continental margin crop out in the Tethys Hima- sedimentary strata in the Thakkhola region of the central layas, along the whole length of the Himalayan mountain Himalayas (Bordet et al., 1967, 1971, 1975; Garzanti and chain, from Ladakh in the west to south-eastern Tibet Pagni Frette, 1991; Gradstein et al., 1989, 1991; Gibling (Fig. 1). Detailed studies of the Mesozoic sedimentary strata et al., 1994. Nagy et al., 1995), in an attempt to investigate were mostly carried out in the western Himalayas (Gansser, the tectonic evolution of the conjugated margins of northern Greater India (Powell et al., 1988; Gradstein and von Rad, 1991). It was found that Jurassic and Lower Cretaceous sedi- * Corresponding author. mentary successions in central Nepal have many similarities E-mail address: [email protected] (X. Hu). with the strata drilled by the Ocean Drilling Project (ODP) 0195-6671/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2007.05.005 Author's personal copy 302 X. Hu et al. / Cretaceous Research 29 (2008) 301e315 Fig. 1. Sketch geological map of the central Himalayas showing the tectonic setting and location of studied sections e the Gucuo, Chuangde and Gyabula sections; and the Wo¨long section investigated by Jadoul et al. (1998). off north-western Australia (Gradstein and von Rad, 1991; tectonically undisturbed sedimentary successions. Lithostratig- Gibling et al., 1994). Later, several studies were published raphy has been used to establish a stratigraphic framework for comparing the Mesozoic sedimentary strata of southern Tibet the studied areas and for inter-regional correlation. This ap- with the coeval strata cropping out in northern central Nepal proach revealed several disagreements between biostratigraphic (Garzanti et al., 1998; Jadoul et al., 1998). ages and ages derived from regional lithostratigraphic correla- In southern Tibet (Fig. 1) Upper JurassiceLower Creta- tions and isotopic absolute age determination. ceous strata were studied predominantly in the Gucuo area, The results of our stratigraphic studies of the Upper Juras- located west of Old Tingri and in the Gyabula area east of siceLower Cretaceous strata cropping out in south-eastern Gyangze. These studies were mainly biostratigraphic. The am- Tibet are presented. The correlation of Upper JurassiceLower monite fauna was studied by Liu (1983, 1988), Liu and Wang Cretaceous strata between the western and eastern Himalayas (1987), Yin and Enay (2004), the bivalves by Yao et al. (1991), is also discussed. Li and Grant-Mackie (1994), Gou (1985, 1997), and the belemnites by Yin (1975). No detailed lithostratigraphic and 2. Geologic setting sedimentological studies of the Upper JurassiceLower Creta- ceous strata were published from the above areas. During the Mesozoic sedimentary rocks are exposed in the Tibetan last ten years, we have made six field expeditions into the Tethyan Himalayas between the Greater Himalayas and the TingrieGyangze area to study the Mesozoic sedimentary InduseYarlung Zangbo Suture (Gansser, 1964; Wang et al., strata, their compositions and depositional evolutions. 1996; Yin and Harrison, 2000)(Fig. 1). They were deposited A major difficulty encountered during this study is the gen- along the Greater India continental margin, which was part eral scarcity of fossils, including microfossils, in the Lower Cre- of the Pangaea supercontinent (Powell et al., 1988; Patzelt taceous sedimentary strata, hindering precise biostratigraphic et al., 1996). dating. The other difficulty emanates from the intensive tectonic Deposition on the margin began during the Late Palaeozoic deformation and faulting of the region, so providing rarely (Gaetani and Garzanti, 1991; Garzanti, 1999) and continued Author's personal copy X. Hu et al. / Cretaceous Research 29 (2008) 301e315 303 through the Mesozoic, when a broad, passive, continental more precise 206Pb/238U ages for grains with 207Pb/206Pb margin was built along the northern edge of the Indian super- ages < 1000 Ma, and 207Pb/206Pb ages for older grains. Be- continent. During the mid-Cretaceous northward-directed sub- cause most grains were concordant as analyzed, no common- duction of the Neo-Tethyan oceanic crust beneath the southern lead corrections have been applied (e.g., Andersen, 2002). margin of Asia resulted in the development of a magmatic arc We have discarded grains that are discordant by more than and of an associated fore-arc basin along the southern margin 20% (i.e., where the 206Pb/238U age is less than 80% of the of the Lhasa continental block (Einsele et al., 1994; Du¨rr, 207Pb/206Pb age). 1996). With continued subduction, the India PlateeAsia Plate collided at about 65 Ma (Yin and Harrison, 2000; Wan et al., 4. Upper JurassiceLower Cretaceous 2002), even though some researchers place the collision as stratigraphy of the Southern Zone young as the Eocene (Fuchs, 1982; DeCelles et al., 2004; Najman et al., 2005). The stratigraphic framework of the Upper JurassiceLower Continental collision resulted in the onset of the Himalayan Cretaceous in southern Tibet was not well-established (Wang orogenesis when the strata experienced variable tectonic de- et al., 1980; Wan, 1985; Liu and Wang, 1987; Jadoul et al., formation. Mesozoic strata are variably exposed in thrust 1998), with numerous problems (Table 1). In this study, the Up- sheets in southern Tibet. The TingrieGamba thrust separates per JurassiceLower Cretaceous sedimentary strata of the South- the Mesozoic strata in southern Tibet into a Southern and ern Zone were studied in detail at Gucuo area (Figs. 2, 3). Mainly a Northern Tectonic Zone (Fig. 1). The Southern Zone is char- based on lithostratigraphic changes, the stratigraphic classifica- acterized by the presence of shallow-water calcareous and ter- tion is revised herein. The strata are subdivided into the Menka- rigenous rocks of Early Palaeozoic to Eocene age (Liu and dun Formation (Upper Jurassic), which is overlain by the Gucuo Einsele, 1994; Willems et al., 1996), whereas the Northern Formation (Upper Tithonian to Lower Albian), in turn overlain Zone of Mesozoic to Palaeogene age is dominated by deep- by the Dongshan Formation of the Gamba Group (Upper Albian) water outer shelf, continental slope and rise deposits (Yu and (Table 1). In the Gucuo area,

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