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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Formation be assigned to early Early Jurassic. The Triassic and Jurassic boundary on the northern margin of the Tarim Basin is proposed to be placed in between the transition of the Tariqike Formation and Ahe Formation in this region. Acknowledgements: This work was jointly sup- ported by the Knowledge Innovation Program of CAS (KZCX-2-YW-154), the National Key Basic Research 128 Program of China (2006CB701401), National Natural Sciences Foundation of China (NSFC 40472004, 40972008). This is a contribution to UNESCO-IUGS project IGCP 506. Key words: Palynology; The Tariqike Formation; The Ahe Formation; Triassic and Jurassic boundary; The Tarim Basin; Xinjiang
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 An Oxfordian Ammonite Fauna from the Northern Margin of the Lhasa Block, and its Geological Significance Jiarun Yin, Ying Tian China University of Geosciences, Beijing 100083, China (E-mail:yjr@cugb.edu.cn) The Qinghai-Tibet Plateau today is composed of Mesozoic geological blocks, namely Himalayan, the Lhasa and Qiangtang blocks. It was suggested that the Qiangtang block was separated from the Lhasa block by the Bangonghu-Nujiang oceanic basin during the Jurassic times. However, because of few ammonite- biostratigraphic evidence available in both Qiangtang and Lhasa blocks, it is controversial about the closure time of the Bangonghu-Nujiang oceans. Table 1 Ammonite biostratigraphical evidence in north Europe, south Europe, and on the Lhasa block during the Middle and Upper Oxfordian With recent geological mapping work carrying out in Qiangtang, new ammonite specimens were collected there in the last decade, and ammonite fauna ranging from the late Bathonian to early Callovian was found in the Chinbuzhang area, north Qiangtang for the first time, including Homoeoplanulites sp., and Neuqueni- ceras cf. yokoyamai Kobayashi & Fukada. In the other hand, several ammonite localities in south Qiangtang indicating the age from the Toarcian to Early Callovian is a deeping stage for the south Qiangtang basin, including Harpoceras sp. Ptychophylloceras cf. tatricum (Pusch, 1837), Lytoceras cf. rasile Vacek, 1886, Lytoceras cf. penicillatum (Quenstedt, 1886), ?Pseudaptetoceras cf. amplectens (Buckman, 1920), Euhoploceras cf. marginata Buckman 1892 , Euhoploceras cf. modesta Buckman 1892, Haplo- pleuroceras mundum Buckman 1892, Haplo- pleuroceras cf. subspinatum (Buckman 1881), and Stephanoceras cf. telegdirothi Geczy 1967. In addition, Oxycerites orbis (Giebel), Oxycerites cf. subcotarius (Oppel), Homoeoplanulites cf. acuticosta (Roemer), Homoeoplanulites cf. homoeomorphus (Buckman), Homoeoplanulites cf. furculus (Neumayr), and Neuqueniceras cf. yokoyamai Kobayashi & Fukada, ranging from the Discus Zone of the Upper Bathonian to the Koenigi Zone of the Lower Callovian. It is noted that none of the late Jurassic ammonites has been confirmed in Qiangtang block up to now. However, Oxfordian fauna, represented most by Radulopecten and Gryphaea,was distributed widely over the Qiang- tang region, suggesting that shallow-water carbonate plateform was well established at that time. Both depositional and bivalve successions of the north Qiangtang exhibit a regression sequence with brackish and freshwater bivalve faunas ranging from Kim- meridgian to Tithonian. While the Himalayan ocean which separated Lhasa from Himalayan blocks was well established during the Late Jurassic times, an epimayayitid-fauna dispersal reached the Nyalam area, south Tibet, and the depth of Himalayan oceans might reached its maximum by the early Tithonian time with appearances of various forms of Virgatosphinctes and Hplaphylloceras. With regards to the Lhasa block, it is very poorly known about Jurassic ammonites before. This paper is to describe an Oxfordian ammonite collection which was sampled in the Biru area of the northern margin of the Lhasa Block. This fauna is dominated by aspi- doceratids, including Euaspidoceras hypselus (Oppel, 1863), Euaspidoceras cf. varicornatum (Dorn, 1931), exhibiting a close affinity with west Tethyan faunas, although two new forms appear which are Euaspi- doceras lhasense n. sp., and Ataxioceras biruense n. sp. The ammonite fauna, particularly the appearances of an 129
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