in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
Black Shales, Isotope, Organic Geochemistry and the Early Toarcian
Oceanic Anoxic Event (OAE) in Northern Tibet
362
Minquan Xia 1* , Song Xiao 1 , Lan Chen 1 , Haisheng Yi 2
1. Chongqing University of Science and Technology, Chongqing 401331, China (E-mail: Xiamq304@163.com)
2. Chengdu University of Technology, Chengdu 610059, China
Jurassic black shales occur in northern Tibet,
which are the subjects for many geologists. We carried
out detailed field investigations of the Shuanghu oil
shales section in the southeastern Bilong Co area,
which is about 45 km away from the Shuanghu special
district. The whole succession is divided into three
intervals characterized by different lithological types.
From lower to upper they are gypsums, oil shales
interbedded with limestones and mudstones, and marls
interbedded with mudstones. Yi et al. (2003) surveyed
the section again and recognized ammonites at the top
of the section and suggested that these ammonites are
principally of early Toarcian age, roughly within the
Harpoceras falciferum Zone in the Paris Basin and may
be associated with the Posidonian Shale of SW
Germany, as well as with the Early Toarcian black
shales occurring extensively in Europe.
Organic geochemistry supplies powerful tools for
the analysis of organic-rich sedimentary rocks. Rock
Eval pyrolysis is a method to evaluate the thermal
maturity of rocks, quality and origin of organic matter.
Hydrogen index (HI) values provide information about
the type and preservation of organic matter. HI values
higher than 277 mg HC/g TOC generally indicate
preservation of marine organic matter (kerogen type II)
dominated by bacteria and phytoplankton with only a
minor contribution of terrestrial organic compounds.
Sedimentary organic matter of laminated shale anomal-
ously rich in organic carbon across the Shuanghu area is
characterized by high organic carbon contents, ranging
between 1.8% and 26.1%. The carbon isotope curve
displays the δ 13 C values of the kerogen (δ 13 Ckerogen)
fluctuating from –26.22‰ to –23.53‰ PDB with a
positive excursion close to 2.17‰, which, albeit
significantly smaller, may also have been associated
with other Early Toarcian Oceanic Anoxic Events
(OAEs) in Europe. The organic atomic C/N ratios range
between 6 and 43, and the curve of C/N ratios is
consistent with that of theδ 13 Ckerogen values. Abundant
biomarkers, including n-alkanes, isoprenoid, steranes
and terpanes from the black shales, have been detected
by GC and GC-MS. The n-alkanes are composed of
nC15-nC31 with nC15, nC16 and/or nC17 as the main
peaks, and characteristic of single peak distribution.
The nC17/nC31 ratios of 5.51 to 30.00 show the
predominance of light hydrocarbons in the n-alkanes.
OEP values vary from 0.90 to 1.01, and CPI from 0.93
to 1.13, close to equilibrium value of 1.0, indicating no
obvious odd-even carbon number predominance. Pr/Ph
values range between 0.77 and 1.59, and they fluctuate
in the vertical section. In the m/z191 GC-MS, C30
hopane is the most abundant in the terpanes. According
to the relative abundance, pentacyclotriterpanes are
characterized by highest abundances, tricyclic terpanes
by the second, and then tetracyclic terpanes. Minor
gammacerane is presented. In the m/z217 GC-MS, the
regular steranes C27-C28-C29 distributed with “V” shape,
and ∑(C27+C28)>∑C29 with varying ratios between 1.25
and 1.99, whereas steranes ∑C27/∑C29 from 0.51 to
3.65 and rearranged steranes C27/regular steranes C27
from 0.21 to 0.47. 4-methyl sterane have been detected,
but its abundance is relatively lower. Series of C27 and
C29-C35 hopanes, maximizing at C30 and C31, are
present in all samples. The 22S/(22S+22R) ratios of the
17α,21β(H)-hopanes are at near equilibrium values
(0.58-0.60), while the C29ααα20S/(20S+20R) ratios and
C29αββ/(αββ+ααα) of the steranes are by 0.35-0.43 and
0.43-0.54, respectively, which indicate maturity or over
maturity conditions during deposition of the rocks.
The biological assemblage was characterized by
scarcity of benthic organisms and bloom of calcareous
nannofossils (coccoliths) by means of optical polarizing
microscope and scanning electron microscope. Wat-
znaueriaceae family was firstly discovered pertaining to
watznaueria fossacincta with a central pore and
watznaueria britannica with a bar, which reveals high
biological productivity in the surface water and an
unfavorable environment for the benthic fauna in the
bottom water during the Oceanic Anoxic Event (Chen
et al., 2003).
During the lower Toarcian, coincident with the
peak of the Lower Jurassic transgression (De Gra-
ciansky et al., 1998), highly bituminous shales were
deposited extensively in the European epicontinental
seas and the Tethyan continental margins (Farrimond et
al., 1994; Jenkyns and Clayton, 1997). In addition,
contemporaneous black shales occur in western Canada,
Japan, Madagascar and Argentina, and on the Arctic
slope (Schouten et al., 2000). The most organic carbon-
rich facies of the Lower Toarcian in Germany, the
Posidonian Shale, has long been known for its excellent
preserved fossils, the high amount of organic matter (up
to 16%) and positive excursion of carbon isotope (Röhl
et al., 2001). Jenkyns (1988) demonstrated that the
global occurrence of the synchronous black shale facies
occurring in the upper part of the tenuicostatum Zone