Science in China Series D: Earth Sciences © 2008
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Carbon isotope composition and correlation across the Cambrian-Ordovician boundary in Kalpin Region of the Tarim Basin, China JING XiuChun1,2†, DENG ShengHui2, ZHAO ZongJu2, LU YuanZheng2 & ZHANG ShiBen2 1 2
School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China; Research Institute of Petroleum Exploration and Development, Beijing 100083, China
Biostratigraphically constrained sequences at the Wushi Yingshan and Kalpin Cement Plant sections (Kalpin Region; Tarim Basin) were densely sampled for geochemical studies. Carbonates across the Cambrian-Ordovician boundary of both sections mainly record negative carbon isotope values. Stable isotope curves show four negative and four positive excursions appearing alternately at the Wushi Yingshan section and five negative alternating with five positive excursions at the Kalpin Cement Plant section. The carbon isotope logs of these two sections are correlated with the international Cambrian-Ordovician boundary key sections: (1) Dayangcha section in China, (2) Green Point section in Canada, (3) Black mountain section in Australia and (4) Lowson Cove section in USA. These correlations suggest that the Cambrian-Ordovician boundary of the Wushi Yingshan section and the Kalpin Cement Plant section can be placed within a particular horizon that also corresponds to the observed biostratigraphic units. Cambrian-Ordovician boundary, carbon isotopes, conodonts, Tarim Basin
The Green Point section at Newfoundland in Western Canada was decided as the Global Stratotype Section and Point (GSSP) for the Cambrian-Ordovician boundary in 2000[1]. The base of the Ordovician is defined by the First Appearance Datum (FAD) of Iapetognathus fluctivagus. However, this index conodont is not widespread and only reported from Canada, USA and a few ― areas of Kazakhstan and China[1 3]. In Kalpin Region of the Tarim Basin, the interval across the CambrianOrdovician boundary chiefly consists of dolomite, intercalated with limestone, which represents a restricted to semirestricted carbonate platform. The entire sequence has a poor fossil record. The index conodont I. fluctivagus has neither been found in Kalpin Region nor in the whole Tarim Basin. Therefore a high-resolution biostratigraphy could not be applied for this region until now. Since the 1990s, research of stable carbon isotope
profiles combined with conodont biostratigraphy has ― made remarkable progress[4 10]. At several international key sections of the Cambrian-Ordovician boundary well-marked relations have been established for large intercontinental areas. Nevertheless, in China, research on systematic and high-resolution carbon isotope stratigraphy was carried out only at the Dayangcha section[5,8,11,12] located in Hunjiang (Baishan today) of the Jilin Province, as one of the GSSP candidate sections for the Cambrian-Ordovician boundary. But because of the several depositional breaks within the boundary interval, correlation of carbon isotope curve of the Dayangcha section[5] with sections outside of China is not very satReceived January 2, 2008; accepted April 3, 2008 doi: 10.1007/s11430-008-0093-5 † Corresponding author (email:
[email protected]) Supported by the Tarim Oilfield Company, PetroChina, the Innovating Foundation of Research Institute of Petroleum Exploration & Development, PetroChina and the Outstanding Doctoral Dissertation Support Foundation of China University of Geosciences (Beijing)
Sci China Ser D-Earth Sci | Sep. 2008 | vol. 51 | no. 9 | 1317-1329
―
isfactory. Despite some previous results[13 15] on stable carbon isotope stratigraphy of the Lower Paleozoic in the Tarim Basin,, the studied strata are too long-ranging, and the carbon isotope curve is too general, to be correlated clearly with the international key sections. Wushi Yingshan section and Kalpin Cement Plant section (Figure 1) are two of the most important Cambrian-Ordovician boundary sections in Kalpin Region. Boundary intervals at both sections are exposed, and seem to be continuous without any obvious sedimentary gaps. To place the Cambrian-Ordovician boundary of Kalpin Region by the method of carbon isotope stratigraphy and to provide dense carbon isotope reference profiles in China, sampling works of the two sections took place during September 2004 and June 2007.
1 Geological setting and sampling principle The Upper Cambrian to the Lower Ordovician of the study area includes Lower Qiulitag Formation, Penglaiba Formation and Yingshan Formation (Lower Member). The Lower Qiulitag Formation consists of dolomite and algae dolomite that indicates restricted platform facies. The Penglaiba Formation to the Lower Member of Yingshan Formation are made up of dolomite, limy dolomite, limestone and dolomitic limestone which is indicative for semirestricted to open platform facies[16]. Since the systematically biostratigraphical studies in the Tarim Basin have been undertaken from a later period of last century, conodont data have constrained the Cambrian-Ordovician boundary to the lower part of the
Figure 1
Penglaiba Formation. However, it is difficult to confirm the definite horizon of the boundary by the use of biostratigraphy as the only tool; without additional methods the boundary problem remains unsolved. For the needs of petroleum exploration in the Tarim Basin, paleontologists placed the Cambrian-Ordovician boundary on ― the FAD of Monocostodus sevierensis[17 19] or the FAD of Variabiloconus aff. bassleri[20, 21] successively, both of them are roughly correlative of the FAD of I. fluctivagus. But in the petroleum exploration practice, the boundary is usually placed at the base of the lowest continuous limestone bed in Penglaiba Formation, namely the base of Penglaiba Formation; such division scheme is obviously lower than the limited scope by biostratigraphy[16]1). Apparently, the current division scheme has become increasingly unsuitable for the need of petroleum exploration. Based on the current biostratigraphic research, one assemblage and two conodont zones have been established in the lower part of the Penglaiba Formation1): Teridontus nakamurai-T. huanghuachangensis-T. gracilis assemblage, Variabiloconus aff. bassleri zone and Chosonodina herfurthi-Rossodus manitouensis zone. The T. nakamurai-T. huanghuachangensis-T. gracilis assemblage is characterized by the coexistence of the three assemblage namesake species. The occurrence of V. aff. bassleri indicates the top of the assemblage, which is assigned to the top of the Upper Cambrian. The V. aff. bassleri zone is characterized by the first appearance of the species V. aff. bassleri. The top of this zone is marked by the occurrence of C. herfurthi and R. mani-
Location of sampled sections. 1, Wushi Yingshan section; 2, Kalpin Cement Plant section.
1) Deng S H, Zhang S B, Lu Y Z, et al. A study on classification and correlation of the Ordovician in the Tarim Basin. A Research Report of Tarim Oilfield Company, PetroChina. 2007 1318
JING XiuChun et al. Sci China Ser D-Earth Sci | Sep. 2008 | vol. 51 | no. 9 | 1317-1329
touensis. The FAD of the zone indexing species is roughly referred to the base of the Ordovician, but the exact Cambrian-Ordovician boundary remains ambiguous. The C. herfurthi-R. manitouensis zone is characterized by the occurrence of one of two index conodonts. The upper boundary of the biozone is defined by the first appearance of Glyptoconus quadraplicatus. This zone is identified as the lower part of the Lower Ordovician, compared to the middle part of the Xinchangian (=Tremadocian) Stage. Based on the existing conodont sequence, continuous and dense carbonate isotope sampling from the uppermost part of the Lower Qiulitag Formation to the lower part of the Penglaiba Formation was carried out in Wushi Yingshan section and Kalpin Cement Plant section. To reduce the effect of diagenetic alteration on the primary carbon isotope composition, only bulk samples without calcite veins, traces of significant recrystallization or weathering were used for analyses.
2 Analytical methods and data reliability 2.1 Analytical methods and data A total of 132 carbonate samples were analyzed at PetroChina Oil Geologic Lab Center, Research Institute of
Diagenetic process can alter primary paleoceanographic signals, thus complicating the interpretations of the C and O isotope data. Carbon and oxygen isotope compositions of marine carbonates would exchange with isotope compositions in pore water circulating in carbonate rocks during diagenesis. Carbon isotope compositions are relatively insensitive to post-depositional alteration, which is one of the advantages of carbon isotope strati-
Data of carbon and oxygen isotope, Wushi Yingshan section (relative to PDB standard)a)
Sample Thickness (m) WYS-00-02 5 a) L.Q. WYS-00-01 0.5 WYS-01-01 0 WYS-01-02 3 WYS-01-03 6 WYS-01-04 8 WYS-01-05 11 WYS-01-06 12 WYS-02-01 15 WYS-02-02 17 WYS-02-03 19 WYS-03-01 20.6 WYS-03-02 21.4 WYS-03-03 22.6 WYS-03-04 24.4 WYS-03-05 26.6 WYS-04-01 29.4 WYS-04-02 32.2 WYS-05-01 33.5 WYS-05-02 34.7 WYS-05-03 35.7 WYS-05-04 36.7 WYS-05-05 37.7 WYS-05-06 38.7 a) “L.Q.”: Lower Qiulitag. Penglaiba
Fm.
2.2 Analysis of diagenetic alteration
Lithology dolomite dolomite limestone limestone limestone limestone limestone limestone limestone limestone limestone dolomite dolomite dolomite dolomite limestone dolomite limestone limestone limestone limestone dolomite dolomite limestone
δ 13C (‰) δ 18O (‰) −0.4 −0.5 −1.2 −0.7 −0.8 −0.5 −0.5 −0.4 −0.2 0.1 −0.2 −0.4 −0.3 −0.5 0.0 −0.3 −0.3 0.0 −0.5 −0.4 −0.6 −0.8 −0.8 −0.9
−7.3 −7.9 −9.7 −9.5 −9.3 −9.9 −9.3 −9.3 −9.3 −9.6 −9.6 −7.8 −9.6 −9.6 −5.7 −7.2 −9.8 −9.5 −9.5 −7.2 −7.4 −7.2 −7.1 −6.9
Fm.
Penglaiba
Table 1
Petroleum Exploration & Development. For analysis the McCrea orthophosphoric acid method is used. Ten mg of powder was taken from each sample for carbon and oxygen isotope analysis. Powders to be analyzed were roasted to remove volatile contaminants and then reacted with anhydrous phosphoric acid in vacuum condition at 25℃ for 24 hours; the CO2 gases formed in the reaction were purified by liquid nitrogen. All samples were conducted with a Finnigan-MAT 252 mass spectrometer. Data are reported in per mil (‰) relative to the PDB standard; a national standard was first calibrated by using GBW04405. Analytical precision for C isotope composition is better than ±0.1‰, and for O isotope is better than ±0.2‰. The data are tabulated in Tables 1 and 2.
Sample WYS-05-07 WYS-05-08 WYS-06-01 WYS-06-02 WYS-06-03 WYS-07-01 WYS-07-02 WYS-07-03 WYS-07-04 WYS-07-05 WYS-07-06 WYS-07-07 WYS-07-08 WYS-07-09 WYS-07-10 WYS-07-11 WYS-07-12 WYS-08-01 WYS-08-02 WYS-08-03 WYS-08-04 WYS-08-05 WYS-08-06
Thickness (m) 40.7 42 42.2 43 43.2 43.8 45.9 47.5 48.5 49.8 51.3 52.3 53.3 54.3 56 57.5 60 63.4 64.9 68.9 72.9 75.4 77.3
Lithology limestone limestone limestone limestone limestone limestone limestone limestone limestone limestone limestone limestone limestone dolomite dolomite limestone limestone dolomite dolomite dolomite dolomite dolomite dolomite
JING XiuChun et al. Sci China Ser D-Earth Sci | Sep. 2008 | vol. 51 | no. 9 | 1317-1329
δ 13C (‰) δ18O (‰) −0.8 −0.7 −0.6 −0.5 −0.7 −1.0 −1.5 −1.4 −0.9 −0.8 −1.3 −1.1 −1.2 −1.0 −1.2 −1.0 −1.1 −1.1 −1.0 −0.9 −0.9 −0.8 −1.1
−5.9 −9.6 −9.1 −9.0 −9.0 −9.4 −9.1 −9.9 −7.8 −6.5 −7.3 −8.8 −9.7 −9.4 −7.2 −6.5 −7.5 −7.9 −8.2 −7.1 −7.2 −8.0 −9.3
1319
Penglaiba
Lower Qiulitag
Fm.
Carbon and oxygen isotope data, Kalpin Cement Plant section (relative to PDB standard) Sample K-T-01-01 KS-01-01 K-T-01-02 KS-01-02 K-T-01-03 K-T-02-01 KS-02-01 K-T-02-02 KS-02-02 K-T-03-01 K-T-03-02 KS-03-01 K-T-03-03 K-T-04-01 KS-04-01 K-T-04-02 K-T-05-01 K-T-06-01 K-T-06-02 K-T-07-01 K-T-08-01 K-T-09-01 KS-09-01 K-T-09-02 KS-09-02 KS-09-03 K-T-09-03 K-T-10-01 K-T-11-01 K-T-12-01 K-T-13-01 KS-13-01 K-T-13-02 KS-13-02 K-T-14-01 K-T-15-01 K-T-16-01 K-T-17-01 K-T-18-01 K-T-19-01 KS-19-01 KS-20-01 K-T-21-01
Thickness (m) 0.1 1 2.5 4.5 5.2 6.5 7.8 8.3 8.6 8.7 10.7 13.7 14.2 15.2 18.8 20 21.3 22 24 25.3 26.2 28.9 29.9 36.9 41.7 44.7 45.2 45.7 46.9 48.2 49.5 50.4 53.5 55.1 57.1 59 59.8 61.2 62.2 63.2 63.8 67.8 68.1
Lithology dolomite dolomite dolomite dolomite dolomite dolomite dolomite limestone lime-dolo limestone limestone marlstone limestone limestone dolomite limestone limestone dololime dolomite limestone dolomite limestone limestone limestone limestone limestone limestone dolomite dolomite dolomite dolomite dolomite limestone marlstone dolomite dolomite dolomite dololime dolomite dolomite dolomite dolomite limestone
δ13C (‰) δ18O (‰) −0.6 −1.4 −1.3 −1.5 −0.9 −0.8 −0.9 −1.3 −1.0 −1.4 −1.2 −0.4 −0.6 −0.9 −0.5 −0.5 −0.6 −0.8 −0.9 −0.4 −0.4 −0.2 −0.1 −0.2 −0.1 0.1 0.0 −0.8 −0.7 −0.7 −0.5 −0.4 −0.6 −0.3 −0.8 −0.6 0.0 0.2 0.0 −0.2 −0.2 −0.4 −0.2
−7.3 −8.3 −4.8 −7.7 −4.7 −6.5 −9.5 −10.0 −9.9 −9.6 −9.7 −8.8 −9.0 −10.1 −9.2 −9.8 −10.4 −8.7 −6.1 −9.1 −9.0 −9.2 −8.7 −8.5 −8.7 −8.1 −8.2 −8.1 −7.0 −6.2 −7.4 −7.3 −6.5 −10.2 −7.3 −6.3 −5.7 −9.2 −8.7 −6.7 −5.6 −8.2 −8.7
graphy. But, oxygen isotope compositions are more sensitive to post-depositional alternation, so that a remarkable decline in δ18O values may be caused. The carbonate samples with δ18O values between −10‰ and −5‰ are generally believed that their carbon isotope values may be slightly affected but not enough to change the primary carbon isotope compositions. Samples with intense negative δ18O values (
