Proceedings of the Geologists’ Association 127 (2016) 247–265 Contents lists available at ScienceDirect Proceedings of the Geologists’ Association jo urnal homepage: www.elsevier.com/locate/pgeola Bathonian–Callovian (Middle Jurassic) ammonites from northwestern Qiangtang Block, Tibet, and the revised age of the Suowa Formation Jiarun Yin * China University of Geosciences (Beijing), 29 Xueyuan Lu, Beijing 100083, PR China*Tel.: ++86 1084825070. A R T I C L E I N F O A B S T R A C T Article history: The age of the Suowa Formation, the latest Jurassic marine deposit in the Qiangtang Block, is important Received 22 August 2015 not only because of its great hydrocarbon potential, but also because it defines the termination of the Received in revised form 6 February 2016 Jurassic ocean in the area. New ammonite evidence from the upper part of the Suowa Formation in the Accepted 10 February 2016 northwestern Qiangtang region allows us to revise the age of the formation previously regarded as late Available online 19 March 2016 Jurassic due to the absence of time-diagnostic fossils. Newly collected ammonites include Gracili- sphinctes suprapanatinus (Arkell), Oxycerites orbis (Giebel), Macrocephalites (Macrocephalites) madagas- Keywords: cariensis Lemoine, Macrocephalites compressus (Quenstedt), Nothocephalites semilaevis (Waagen) [M], Qiangtang Block (northern Tibet) Nothocephalites asaphus Spath [m], Eucycloceras cf. cogginbrowni Spath, Choffatia (Subgrossouvria) Bathonian–Callovian Ammonite recuperoi (Gemmellaro) and Choffatia cf. sakutala Spath. Correlation with the biostratigraphical Biostratigraphy framework based on ammonites from Kachchh, western India, places the age of the Suowa Formation as Suowa Formation Middle Bathonian to Early Middle Callovian. An unconformity and depositional break between the Suowa Formation and the overlying Lower Cretaceous Xueshan Formation is proposed. Characteristic ammonite-taxa of Kachchh appearing in the northwestern Qiangtang area document significant dispersal from the Indo-Malgach Province to the northern margin of eastern Tethys region for the first time. ß 2016 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. 1. Introduction sedimentary environments (Tan et al., 2002, 2003, 2004; Jin et al., 2006; Zeng et al., 2012a,b). The Qing-Zang (Qinghai-Xizang) Plateau consists of three The Jurassic lithostratigraphical units of the Yanshiping section tectonic terrains, from north to south they are the Qiangtang, comprise the Quemocuo, Buqu, Xiali, Suowa, and Xueshan Lhasa and Himalayan blocks. The Qiangtang Block including formations in ascending order (Yin, 1986; Yang and Yin, 1988; northern Tibet, parts of southern Qinghai and western Yunnan Bai, 1989). Depositional environments were either tide-dominated provinces was subjected to continental collision with the southern shelf or inner carbonate shelves throughout Jurassic times, and margin of the Eurasian continent by the late Triassic and were bivalve fossils are quite diverse and abundant, being represented separated from the Lhasa Block by the Bangonghu-Nujiang Ocean by marine, brackish-water, and freshwater forms (Yin, 1989a,b, during the Jurassic (Wang, 1985). The ammonite specimens 1990; Yin and Fu¨ rsich, 1991; Yin et al., 1993). Therefore the studied here are from northern Tibet (Fig. 1). stratigraphical units above have been dated only by their bivalve Jurassic strata are well developed and widely distributed in the assemblages, and the Suowa Formation has been regarded as Late Qiangtang region, an area of approximately 650 km from east to Jurassic in age (Yin, 1986, 1987; Yang and Yin, 1988; Bai, 1989). west and 300 km from north to south. Different depositional basins This stratigraphical scheme has been applied in both geological existed in the north and south, separated by an uplift zone around a survey mapping and petroleum oil exploration in the Qiangtang latitude of 338 N (Fig. 1, right). As a result, different Jurassic region since the early 1990s (Fig. 2). sedimentary facies developed (Wang et al., 2001, 2004, 2009; Zhao Jurassic ammonite assemblages were not reported from et al., 2001). The northern Qiangtang region can be subdivided into northern Qiangtang until the 1988, particularly in the area from the eastern Yanshiping-Cuoriju area and the western Bailong- Yanshiping to Cuoriju where ammonites are extremely rare due to binghe-Bandaohu (Changhonghe) area based on differences in the very shallow-water facies. The only ammonite previously known from the area is Collotia sp. (Wang et al., 1979). The first report of Late Jurassic ammonites in the Bailongbinghe section of E-mail address: [email protected]. northwestern Qiangtang (Fig. 1, right) was made by Fan et al. (1988) http://dx.doi.org/10.1016/j.pgeola.2016.02.005 0016-7878/ß 2016 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. 248 J. Yin / Proceedings of the Geologists’ Association 127 (2016) 247–265 Tectonic suture zones of the Qinghai-Xizang (Tibet) Plateau (left) and Middle Jurassic ammonite localities known in the Qiangtang Block (right). Fig. 1. J. Yin / Proceedings of the Geologists’ Association 127 (2016) 247–265 249 Fig. 2. Middle Jurassic, Upper Jurassic and Lower Cretaceous lithostratigraphical schemes in northern Qiangtang. and these ammonites were neither figured or described, but many the Spiti Shale depositional environments, by containing shallow- taxa were listed, such as Virgatosphinctes sp., V. cf. subfrequens, V. water carbonate platforms, but also because the fossil assemblage minusculus, V. cf. pompeckji, V. cf. kraffti, V. cf. frequens, Spiticeras, lists have been compiled by researchers inexperienced in the Paraboliceras, and Perisphinctes. Most of these ammonites are determination of ammonites. It is highly unlikely that so many characteristic forms of the South-Tethyan margin, being known Spiti Shale ammonite species were able to survive in such mainly from the Spiti area of the Himalaya (Uhlig, 1903–1910). In shallow-water environments, and the so-called Late Jurassic fact, all ammonite names that Fan et al. (1988) listed are from a book, ammonites recorded from above are likely to have been in which the ammonites from the Agri area, Himalayan Tibet, misidentified, as highlighted by Yin (2005), Yin et al. (2005) originally studied by Uhlig (1903–1910) have been re-figured and and Yin (2010a,c). re-described in a Chinese translation by Zhao (1965). A recent study on ammonites from Qiantang has made it Yi et al. (2003) provided a further list of ammonites from the possible to date the age of the Suowa, Xiali, and Buqu formations as Suowa Formation in the Bandaohu (Changhonghe) area: Aulaco- Middle Jurassic. In northern Qiangtang three ammonite levels were 0 sphinctoides sp., A. infundibulum, A. hollandi, A. rareplicatus, A. recognized in the Buqu Formation of the Cuoriju section (N 33840 , 0 moerikeanus, A. spitiensis, Virgatosphinctes dacquei, V. haydeni, V. E 91840 ) (see Fig. 13). The specimens have been referred to multifasciatus, V. frequens, V. lemoinei, V. broilli, V. kutianus, V. Procerites sp., Siemiradzkia cf. matisconensis (Lissajous, 1923), indistinctus, V. subfrequens, V. holdhausi, Haplophylloceras, Blanfor- Neuqueniceras cf. yokoyamai Kobayashi and Fukada, 1947, and diceras cricki, B. celebrant, Berriasella, Paraboliceras spitiense, Choffatia (Choffatia) cf. vicenti Mangold, 1970, indicating a Middle Dhosaites sp., Uhligites griesbachi, Substeueroceras sp., and Kossma- to Late Bathonian age (Yin, 2005). About 120 km south of the tia sp. These are well known taxa of the Spiti Shale fauna described Yanshiping section an isolated specimen from the Suowa Forma- by Uhlig (1903–1910) and are mainly Tithonian elements. tion has been referred to Reineckeia (Yin, 2005). Westermann and Unfortunately, as in the case of ammonites, reported by Fan Wang (1988) described Macrocephalites cf. macrocephalus from the et al. (1988), it is impossible to examine these ‘so-called’ Tithonian neighbourhood of Dingqing. specimens, because their whereabouts is unknown. Since that time It is pertinent that Lu and Xu (2001) provided an ammonite ammonite names listed by Fan et al. (1988) and Yi et al. (2003) have list with figures from the Xiali Formation in the Changhonghe repeatedly been cited to document the Late Jurassic (mainly section, including Oxycerites sp., Choffatia cf. madani Spath, C. cf. Tithonian) age of the Suowa Formation (Sun and Xu, 1998; Fang baliensis (Neumayr, 1875), and Macrocephalites cf. macrocephalus and Liang, 2000; Fang et al., 2000, 2002; Li et al., 2005; Lu and Xu, (Schlotheim, 1813). These authors therefore regarded the Xiali 2001; Wang et al., 2001, 2009; Yi et al., 2005; Tan et al., 2004; Zeng Formation as Callovian in age. However, the species cited by Lu and et al., 2012a,b) (Fig. 2). Xu (2001) has now been revised as Oxycerites orbis (Giebel, 1852), It is known that the Late Jurassic ammonite faunas of the so- Macrocephalites madagascariensis Lemoine, 1911, respectively (see called Spiti Shales are characterized by a high level of endemicity below). Thus, the Xiali Formation has been proven on the basis of at the generic level within the southwestern Pacific region (Page, these ammonites to contemporary with the Suowa Formation 1996; Enay and Cariou, 1997; Enay and Cariou, 1999; Hallam, (Bathonian–early Middle Callovian) in the Chonghonghe section, 2001; Yin and Enay, 2004; Enay, 2009). In Himalayan Tibet, a sea- this is further demonstrated below. level rise in the latest