in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
sedimentary sequence was deposited rapidly and is
characterized by coarse and fine clastics after the
basaltic volcanism. Deposition was followed by the
rhyolite extrusion with columnar jointing and dissilient
fabrics of partial quartz phenocrysts. These features
indicate that volcanism in the Sangxiu Formation was a
short period, and the basaltic magmatism was only
slightly earlier than the eruption of rhyolite. Both the
rhyolite and the basalt in the Sangxiu Formation were
formed in an extensional tectonic setting. The zircon
SHRIMP age of rhyolite, therefore, is the age of
volcanism of the upper Sangxiu Formation during the
Valanginian Stage following deposition of the Calcicalathina
oblongata–Speetonia colligata assemblage.
Gyangze
98
K1
J 3
Hauterivian
-
Valanginian
Brriasian
Tithonian
Jiabula Fomation
Weimei F.
220
200
180
160
140
120
100
80
60
40
20
0m
Sandstone
7
6
5
4
3
2
1
Calcicalathina oblongata
Speetonia colligate
W. fossacincta
Cy. margerelii
Nannoconus steinmannii
Watznaueria barnesae
Inoceramus everesti
Spiticeras
Hi malayites
Corongoceras
Haploghylloceras strigile
Siltstone S hale Mu ddy Silty sh ale
T uff Dioritoid
san dsto ne
p orp hyrite
The Upper Jurassic and Lower Cretaceous
stratigraphy of southern Tibet has been reported by
many geologists since the first scientific exploration
was carried out in 1966. The ammonites found recently
by the present authors in their geological survey in
Gyangze provide some crucial links for the correlation
with other regions of the SW Pacific domain where
these ammonite genera are widely distributed. In the
Gyangze area the Haplohylloceras strigile–Coron-
goceras–Himalayites assemblage is of latest Jurassic
age, and the Spiticeras assemblage, including S.
spitiense, S. stanleyi, Sarasinella sp., and Cuyaniceras
sp. represents the earliest Cretaceous (Fig. 1) (Yin and
Enay, 2004).
Nagarze
K1
J 3
Brriasian Valanginian - Hauterivian
Tith on ian
Tuffaceous
sandstone
Sangxiu Formation
.
Weimei F
220
200
180
160
140
120
100
80
60
40
20
0m
14
13
12
11
10
9
8
7
6
5
4
3
2
1
XZ0506, 136 ±3.0 Ma
Calcicalathina oblongata
Speetonia colligata
Manivitella pemmatoidea
Watznaueria barnesae
Inoceramus everesti
Haploghylloceras
Basalt Rh yolite
Fig.1 Lower Cretaceous stratigraphic correlation of Gyangze and Nagarze sections, southern Tibet
Numbers at right of each column refer to stratigraphic units.
Ammonites of these two assemblages are also
found in shale and siltstone of the lower part of the
Nagarze section. In addition the Berriasian genera
Spiticeras and Berriasella have been found. More
indicative data are the nannofossils reported herein. The
globally distributed Nannoconus steinamnnii stein-
mannii–N. st. minor–Watznaueria barnesae Assem-
blage indicates Berriasian age, and the Calcicalathina
oblongata–Speetonia colligata Assemblage correlates
with the Valanginian. The first appearance of the
Berriasian nannofossil assemblage in the Gyangze-
Nagarze area is about 3 to 5 m above the base of the
Sangxiu and Jiabula formations (Fig. 1). Therefore the
J/K boundary is placed near the bottom of the Jiabula
Formation at Gyangze, and at the base part of the
Sangxiu Formation at Nagarze. The boundary is
marked by the first appearance of the ammonite Spi-
ticeras and the nannofossil Nannoconus st. stein-
mannii–N. st. minor–Watznaueria barnesae Assem-
blage.
The present SHRIMP U-Pb analysis on the rhyo-
lite sample dates the volcanic rocks in the upper unit of
the Sangxiu Formation at 136±3.0 Ma. Therefore, the
Berriasian-Valanginian fossil assemblages in the lower
unit of the Sangxiu Formation are older, which is
consistent with ages in Gradstein et al. (2004) and Ogg
et al. (2004). The rhyolite sample dated by the SHRIMP
U-Pb analyses is 190 m above the J/K biostratigraphic
boundary. As estimated above, the sedimentary ac-
cumulation rate of the lower Sangxiu Formation is 22.6
m/myr. By consideration of underlying sedimentary
deposits, the J/K boundary in the Gyangze-Nagarze
area is approximately 145 Ma as suggested by the new
International Stratigraphic Chart (2008).