in Jurassic and Cretaceous Stratigraphy

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
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