isotope analysis, and also thank Dr. H. Van. Iten for proofreading the manuscript. ... Strangelove ocean before the Cambrian explosion. Nature 316(29): 809-811.
CARBON ISOTOPE PROFILES AND THEIR CORRELATION ACROSS THE NEOPROTEROZOICCAMBRIAN BOUNDARY INTERVAL ON THE YANGTZE PLATFORM, CHINA JUNMING
ZHANG, GUOXIANG
LI, CHUANMING
ZHOU, MAOYAN ZHU and ZIYE YU
Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing 210008, China
ABSTRACT
-Analysis
and Tianzhushan
of the stable carbon isotope profiles
sections
through
of the Xiaotan,
the Neoproterozoic-Cambrian
there are two sharp drops and two rises in
boundary
Ol3C values between
Meishucun, interval
Maidiping
indicates
that
the top of the Upper Sinian
Dengying Formation and the Lower Cambrian Yu'anshan Formation. Listed in ascending order, the four carbon isotope excursions are: 1. negative shift in Ol3C values at the base of the Daibu Member;
2. rapid increase
Member;
3. sharp decrease
Formation; pronounced respectively
in ol3C values from the upper Zhongyicun
and 4. positive excursions
in Ol3C (from positive Ol3C excursion
respectively,
in the Neoproterozoic-Cambrian from the upper Zhongyicun
excursion
through
excursion
KEY WORDS:
the Yu'anshan
Stable carbon isotope Platform.
to the Dahai These four
markers A, B, C and D
section at Meishucun.
or pre- Tommotian
predates
Formation.
to the Dahai Member
event suggests that the Meishucun
and Dahai Members)
Member
at the base of the Shiyantou
with the biostratigraphical boundary
Member
the Nemarkit-Daldynian
The Dahai positive Zhongyicun,
through
correlate
excursion
to negative)
The positive
is equivalent
Ol3C rise on the Siberian Formation
ol3C
to the positive
(including
Platform. the Daibu,
the Tommotian.
stratigraphy,
Neoproterozoic-Cambrian
INTRODUCTION
boundary,
Yangtze
sequences in some regions, and by differences in sedimentary facies between regions. Therefore, additional stratigraphical information is needed in order to overcome
The Neoproterozoic-Cambrian transition interval is the most important period in the histories of Earth and life. The rapid emergence of skeletonized faunas and composite trace fossils has traditionally been regarded as an important marker for the Neoproterozoic-Cambrian boundary. Intercontinental stratigraphical correlation based on this event has been hindered, however, by a lack of continuous fossiliferous
the limitation of the palaeontological data. Stable carbon isotope stratigraphy offers an important new means of surmounting these difficulties. Secular variations in stable carbon isotope composition of NeoproterozoicCambrian carbonates have been analyzed in detail in Siberian Platform (Magaritz et aI., 1986, 1991; Knoll et aI., 1995), Morocco
Bulletin of National Museum of Natural Science, No.10, PP. 107-116,1997 107
108
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of National
Museum
(Tucker, 1986; Magaritz et aI., 1991), Iran (Brasier et aI., 1990), Lesser Himalaya, India (Aharon et aI., 1987), South Australia (Tucker, 1991), and South Newfoundland/Avalonia (Brasier, 1992), etc. The fact that stable carbon isotope profiles from these localities are maturally similar demonstrates the reliability of secular variations in carbon isotope for global correlation of carbonate succession through the NeoproterozoicCambrian boundary interval. In China on the Yangtze Platform, stable carbon isotope profiles from the Meishucun section at Jinning, Yunnan (Hsu, et aI., 1986; Brasier et aI., 1990; Chen Jinshi et aI., 1992), the Maidiping section in Emei, Sichuan (Brasier et aI., 1990) and the Liantuo section in Yichang, Hubei (Hsii et aI., 1985; Lambert et aI., 1987), have been studied in detail. However these sections contain several major stratigraphic breaks. Consequently, the stable carbon isotope records of these sections can not be precisely correlated with the Ol3C global reference profile. Fortunately, the stratigraphical record in the NeoproterozoicCambrian boundary section at Xiaotan, Yongshan, Yunnan, is complete, and also contains abundant skeletal fossils. This section in thus very suitable for detailed analysis of stable carbon isotope evolution through the Neoproterozoic-Cambrian boundary interval. In 1994, we analyzed ol3C profiles of micritic dolo stones and limestones extending from the top of the Baiyanshao Member to the Yu'anshan Formation. The stable carbon isotope profile from Xiaotan ( Zhou et aI., 1996) shows a positive ol3C excursion within the Dahai Member (zone II assemblages), with maximum value of +7%0,and a sharp drop of ol3C to negative values at the base of the Shiyantou Formation (Marker C). The increase in ol3C through the Dahai Member correlated with the pre- Tommotian rise (PTR) on the Siberian Platform (Ripperdan, 1994). The present paper is based on the
of Natural Science,
No.10,
1997
lithostratigraphy and stable carbon isotope profile across the Neoproterozoic-Cambrian boundary interval in the Xiaotan section, and on correlations with the ol3C profiles from Meishucun, Maidiping and Tianzhushan, Liantuo, Our results should prove useful for correlations between the Yangtze Platform and Neoproterozoic-Cambrian boundary section in other parts of the world.
STRATIGRAPHICAL CORRELATIONS The sections at Xiaotan, Meishucun, Maidiping, and Tianzhushan are crucial to studies of Neoproterozoic-Cambrian strata on the Yangtze Platform (fig. 1). The Meishucun section has been chosen as the standard for the subdivision and correlation of Neoproterozoic-Cambrian boundary sequences in China. The litho- and biostratigraphical sequences of Meishucun have been studied in detail by Luo et ai. (1982, 1984) and Qian & Bengston (1989). Three small shelly fossil assemblages and a trilobite zone have been established. List in ascending order, these are: Zone I, the Anabarites-Protohertzina assemblages (Xiaowaitoushan Member to the lower Zhongyicun Member); Zone II, the Paragloborilus-Siphogonuchites assemblages (the upper Zhongyicun Member to the Dahai Member); Zone III, the SinosachitesLapworthella assemblage (the Shiyantou Formation); and Zone IV, trilobite Parabadiella zone (base of the Yu'anshan Formation). The sedimentary sequences and contact relationships of the above members demonstrate that sedimentation at Meishucun was discontinuous. There are erosional hiatus between the Xiaowaitoushan Member and the Zhongyicun Member, and also between the Dahai Member and the Shiyantou Formation. The stratigraphical sequence of the Xiaotan section, located at Yongshan, northeastern
Zhang et al. : Isotpes
of the Pre- Cambriam
/ Cambrian
boundary,
Yangtze Plateform
109
laminated silicalite in its lower part. The base of the member consists of grey sandstones intercalated with lenticular micritic Sichuan
dolostones, and small shelly fossils. The middle and upper parts of the member consist of dark grey, horizontally-laminated silicalite interbedded with grey, thin-bedded phosphatic siliceous dolo stones (5-10cm thick per layer). The Daibu Member disconfor-mably overlies the Baiyanshao Member.
1
Province
Xichang 0 Yanyuan 0
Province
,r-,
-,-1
/ ,t', t. r-'
\ ~'.~. .
Yunnan
''''''''
I.
r-./' '
J
Guangxi Province 0
Figure
1
Geographic isotope
Meishucun Emei
map showing
profiles
of linning
through
boundary
localities
from Xiaotan County
of carbon
of Yongshan and Maidiping
County, of
The Zhongyicun Member is 76.7m thick and consists of three parts. the lower part, 34.9m thick, consists of black silicalite interbedded with lenticular grey phosphatic dolomites, and siliceous pelletal phosphorites. The middle part is 2.17m thick black carbonaceous phosphatic silty mudstone; the 41.8m thick upper part consists of black thinbedded siliceous phosphorites intercalated with dark grey micrites and black thin-bedded phosphatic silicalite, and is rich in the small shelly fossils of Zone II PalagloborilusSiphogonuchites Assemblages.
the Neoproterozoic-Cambrian
interval.
Yunnan, is very similar to that of the Meishucun section. However, the lithology and thickness of the succession through the Neoproterozoic-Cambrian transition interval are obviously different from those of the Meishucun section. The Xiaotan section consists of the following stratigraphical units, briefly described in ascending order: The upper part of Baiyanshao Member consists of grey, thick-bedded micritic dolostones intercalated with light grey siliceous strips. The Daibu Member, 43.7m thick, is comprised of grey, thick-bedded siliceous dolostone intercalated with greyish black
The Dahai Member, 61.3m thick, consists of dark grey, thick-bedded pelmicrites with phosphatic pellets and abundant small shelly fossils of Zone II assemblages. The base part of the unit consists of nodular micritic limestones. The Shiyantou Formation is 286.6m thick and is comprised of four parts. The lowest part, 6.1m thick consists of dark grey thickbedded phosphatic micrites and glauconitebearing phosphatic siltstones intercalated with thin-bedded phosphorites. The lower part, 38m thick, consists of black carbonaceous silty mudstones intercalated with lenticular micrite. The middle part is 138.6m thick and consists of greenish grey, horizontally laminated, calcareous quartz siltstone. The upper part 103. 9m thick and consists of grey, thick-bedded calcareous siltstones intercalated
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with grey lenticular silty micrites. The Shiyantou Member conformably overlies the Dahai Member. The Yu'anshan Formation is 239.4m thick and consists of four parts. The lowest part is 60.2m thick and consists of black carbonaceous siltstones intercalated with grey lenticular micrites. The part above it is 61m thick and consists of black carbonaceous mudstones and dark grey horizontallylaminated, calcareous mudstones intercalated with grey lenticular and thin-bedded micrites. The middle part is 46.3m thick and consists of dark grey silty mudstones interbedded with grey, thin-bedded silty micrites. The upper part is 71.9m thick and consists of grey, thickbedded siltstones intercalated with grey, thinbedded silty mudstone, silty oolitic grainstones and bioclastic grainstone. The Maidiping section is located 3km west of Gaojiao, Emei, Sichuan. Here the Neoproterozoic-Cambrian boundary interval includes the upper part of the Maoergang Member of the upper Sinian Dengying Formation (Hongchunping Formation), the lower Cambrian Maidiping Formation and the Jiulaodong Formation. Based on the biostratigraphic zonation of the small shelly fossils, the Maidiping Formation and the lower Member of the Jiulaodong Formation may be correlated with the lower Cambrian Meishucun Formation and the Shiyantou Formation respectively. The undulating erosional surface between the Jiulaodong Formation and the Maidiping Formation indicates that the Maidiping section is incomplete compared with the Dahai Member at Xiaotan. The Tianzhushan section, located near Liantuo, Yichang, Hubei, is the type section for the Neoproterozoic-Cambrian boundary in the east Gorges area. The Tianzhushan Formation is 2-3m thick. The topmost O.3m consist of brownish grey phosphatic
of Natural Science,
No.IO,
1997
Table 1 Stratigraphic correlation of major NeoproterozoicCambrian boundary sections in Yangtze Platform "
"
'E
Vj
VJ
,
t:: ro '2 := u :=
1
:E [/)
0 ~
