SCIENCE CHINA Earth Sciences • RESEARCH PAPER •
August 2012 Vol.55 No.8: 1230–1237 doi: 10.1007/s11430-012-4448-6
Biostratigraphic correlation of the Ordovician black shales in Tarim Basin and its peripheral regions CHEN Xu1,2*, ZHANG YuanDong1,2, LI Yue1, FAN JunXuan1,2, TANG Peng1,2, CHEN Qing1,3 & ZHANG YuanYuan1,3 1
Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; 2 State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing 210008, China; 3 Graduate University of Chinese Academy of Sciences, Beijing 100049, China Received April 29, 2011; accepted August 15, 2011
Ordovician black shales are widely distributed in the Tarim Basin and its peripheral regions, and some of them may serve as potential hydrocarbon source rocks. The present study of the Ordovician graptolite fauna from these shales, together with the yielded conodonts and chitinozoans etc., permits a refined correlation of the rocks. Based mainly on a new collecting of the graptolites and a study of the faunas in Kalpin and Kuruktag regions, and the successful identification of the new graptolite material from a few drill cores within the basin, we are able to update our knowledge of the Ordovician in the regions, and draw some conclusions: (1) The most widespread distribution of the black shales in the Tarim Basin and its peripheral regions, which correspond to the Nemagraptus gracilis Zone, may be related to a global sea-level rise during this time interval. (2) Black shales of Ordovician occur most frequently and extensively in the Kuruktag (also spelled as Quruq Tagh) region in eastern Tianshan Mountains, spanning Tremadocian to mid-Katian (D. spiniferus Zone) temporally and extending southeastwards into Manjiaer Depression. The black shales of the D. spiniferus Zone may even extend into the central Tarim Basin. (3) Three different bio- and litho-facies belts (Bachu: carbonate platform and reef belt; Kalpin-Aksu: marginal platform and upper slope belt; Wushi: slope belt) are recognized in the northwestern Tarim Basin. (4) The internationally well-correlated Saergan black shale, which has been considered to possess high potential for hydrocarbon source rock, may possess a restricted distribution in the Kalpin and Aksu areas. Tarim Basin, Ordovician, black shale, biostratigraphic correlation, graptolite Citation:
Chen X, Zhang Y D, Li Y, et al. Biostratigraphic correlation of the Ordovician black shales in Tarim Basin and its peripheral regions. Sci China Earth Sci, 2012, 55: 1230–1237, doi: 10.1007/s11430-012-4448-6
Ordovician rocks of the Tarim Basin and its peripheral regions have been investigated since 1940. Petroleum exploration has promoted the research on the Ordovician of the Tarim Basin, and the publications and geological reports dealing with the Ordovician have increased significantly in recent years. Four conclusive studies of the Ordovician of the Tarim Basin have been conducted and the results pub-
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lished [1–4]. Recently, we investigated the northern peripheral regions of the Tarim Basin, i.e. the Kalpin and Kuruktag regions, and conducted a new collecting of fossils and a further study. In addition, we identified some recently discovered graptolites from a few critical wells in the Tarim Basin. Based on these data, a finer correlation of the Ordovician black shales across the Tarim Basin and its peripheral regions is now available, and this is the subject of the present study (Figure 1). We believe that the present correlation chart based on the vertical distribution of graptolites earth.scichina.com
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Figure 1
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Stratigraphic regionalization of the Ordovician in Tarim Basin and its peripheral regions [4].
provides a greater precision than those based on seismic results and other log data of the wells in the Tarim Basin. Here we employ the graptolite biozonation of the Dawangou section in the Kalpin area, the global auxiliary section for the base of the Upper Ordovician, as the standard for the correlation of the entire region (Figure 2). The global chronostratigraphic chart has been accepted as the standard for stratigraphic correlation by both SinoPec and PetroChina companies. Thus, the results and conclusions of the present study may be easily adopted by the petroleum industry in China.
1 The Ordovician black shales in the Kalpin area Ordovician black shales in the Kalpin-Asku belt are assigned mainly to the Saergan and Yingan formations. Graptolite zones of these two formations were studied by Zhan [6], Qiao [7], Chen and Ni (see ref. [1]), and Chen et al. [8]. Very recently, Chen and his colleagues revised the graptolite biozonation based on a systematic study of the Saergan graptolite fauna. The graptolite zones are revised as in ascending order: the Pterograptus elegans, the Didymograptus murchisoni, the Dicellograptus vagus, and the lower Nemagraptus gracilis zones [9]. Thus, the graptolite zones in the Dawangou section of Kalpin range from the upper Darriwilian to the basal Sandbian. The Darriwilian Saergan Formation decreases in thickness southeastwards from Dawangou to Subashigou near the Kalpin County Town,
where the graptolites are seriously deformed, and only the N. gracilis Zone is well represented. The Saergan Formation extends northeastwards from Dawangou to Sishichang near Aksu, where there are a thick P. elegans Zone and a thin lower N. gracilis Zone. At Sishichang the top of the formation is replaced by shelly beds with a few bivalves. Zhou et al. [1] first illustrated two different Ordovician facies belts from Bachu to Kalpin. An identical carbonate facies (the Qiulitag and Dawangou formations) is developed in both the Bachu and Kalpin areas below the P. elegans Zone, although coeval trilobite belts are differentiated between these two areas. This suggests that even though the Himalayan Shajinzhi-Sanchakou fault belt transected the connection between Bachu and Kalpin, only a relatively small segment of the carbonate strata between the two areas had been removed by faulting. Above the P. elegans Zone there is a continuous succession of platform reef deposits in the Bachu area, whereas black graptolitic shales of the Saergan Formation (P. elegans to Lower N. gracilis zones) representing deeper water slope were deposited in the Kalpin area. The Saergan Formation is overlain by a nautiloid-bearing limestone (Kanling Formation, Upper N. gracilis to Lower Diplcanthograptus lanceolatus zones) and successively the graptolitic shale of the Yingan Formation (Upper D. lanceolatus to Diplacanthograptus spiniferus zones). On the north side of Yinshan, near the Dawangou section, graptolites of D. spiniferus Zone (Yingan Formation) have been recorded by Zhang Shiben. At the top of the Ordovician sequence at Dawangou, upper Katian and Hirnantian strata are all absent. There is a significant break
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Figure 2 Division and correlation of the Ordovician strata in Tarim Basin and its peripheral region. (a) A composite Ordovician graptolite sequence of China; (b) division and correlation of the graptolite zones in Tarim Basin and its peripheral regions. The lithological logs are generalized from the concerned formation. We regard the Kalpintag Formation as of early Silurian age [5]. YBS Fm.–Yuanbaoshan Formation.
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between the Yingan Formation and the Llandovery Kalpintag Formation at Dawangou (Figure 2). In recent years, Jing et al. [10] investigated the Ordovician at Yakeruike near the Wushi County town, north of Kalpin, and documented a conodont sequence from the Lower Ordovician to middle Katian. Above the conodont sequence, a graptolitic turbidite has been found by Zhang Shiben and us as well. The graptolite fauna includes Dicellograptus cf. D. complanatus (Lapworth), Amplexograptus latus (Elles et Wood) and Anticostia macgregorae Stewart and Mitchell, indicative of the D. complanatus Zone of upper Katian. The former two species are common forms in the D. complanatus Zone of South China and the latter was previously described from the Amplexograptus prominens Zone of the Vaureal Formation on Anticosti Island [11] and the upper part of the D. complanatus Zone in Scotland [12]. As noted above, Ordovician strata in Bachu, Kalpin and Wushi span three lithofacies and biofacies belts, i.e., the Bachu carbonate reef-bank facies belt, the Kalpin upper slope belt (II1), and the Wushi slope belt (II2). They are separated by later fault systems (possibly Himalayan) and are thus distributed discontinuously across the region. For many years, the formations of the Dawangou section have been employed for the Ordovician of Wushi area. Based on the new data in the present paper, the rock units in Wushi area should be re-named. At the Mondaleke section near the Wushi County town, from the top of the so-called Kalpintag Formation, which is overlain by a Carboniferous basal conglomerate, we found a new brachiopod fauna. The fauna has been identified by Rong Jiayu as middle-late Katian age, including Ovalospira, Hesperorthis, Dolerorthis, Leptellina, Christiania, Rhynchotrema, and Eospirigerina. The new brachiopod fauna confirms that the Ordovician facies of Wushi is different from that of the Kalpin-Aksu area. The black graptolitic shale of the Saergan Formation has been considered by many petroleum specialists as a potential hydrocarbon source rock. It is deposited within a depression formed on the Dawangou dolomitic limestone. The depositional pattern is similar to that of the Miaopo Formation deposited in several carbonate depressions on the Yangtze Platform of South China. Gao et al. [13] recently thought that the Saergan Formation and Yingan Formation may extend into the Awat Depression. However, the graptolite data available at present indicate that the Saergan Formation is distributed only within the Kalpin-Aksu facies belt (II1) and does not extend into the Tarim Basin. Thus, the potential for the Saergan Formation to be the hydrocarbon source rocks in Tarim Basin may be lower than previously regarded.
2 Ordovician black shales in the Kuruktag region (also spelled as Quruq Tagh), eastern Tianshan Mountains Dr. Erik Norin and the Sino-Swedish scientific expedition
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team are the pioneer workers who investigated the Kuruktag region during 1927–1933. The Ordovician Charchaq Formation was named (after the Queerqueke Mountain) and described by Norin [14]. A Sandbian N. gracilis graptolite fauna was later described by Bulman [15] based on their collections, including Didymograptus cf. D. superstes (Lapworth) (=Xiphograptus cf X. superstes (Lapworth)), Climacograptus cf. C. uniformis Hsu (=Normalograptus cf. N. uniformis (Hsu)), Glyptograptus teretiusculus cf. euglyphus (Lapworth) (=Hustedograptus teretiusculus (Hisinger)), Amplexograptus sp. (possibly Archiclimacograptus caelatus (Lapworth)), and Cryptograptus tricornis (Carruthers). During the past 50 years, Ordovician rocks of the Kuruktag region were re-divided for many times. Among them, the most important division was by Zhong and Hao [2] based on their field work in the Queerqueke Mountain, Yuanbao Mountain, and South Yaerdang Mountain Their graptolite collections have been identified and partially described by Chen Xu and Ni Yunan, on which the following graptolite zones were identified in descending order: (1) Yinpingshan Formation: Feldspathic sandstone and mudstone, containing graptolites of the Dicellograptus cf. D. complanatus Zone. (2) Yuanbaoshan Formation: Dark gray mudstone interbedded with conglomerate-bearing sandstone, containing graptolites of the Orthograptus quadrimucronatus Zone. (3) Zatupo Formation: Dark gray mudstone and siltstone, containing graptolites of the Dicranograptus ramosus longicaulis Zone. (4) Charchaq Formation: Dark sandstone, shale and turbidites, containing graptolites of the Nemagraptus gracilis Zone. (5) Heituao Formation: Black shale and siliceous beds, containing graptolites of the Undulograptus austrodentatus Zone (upper part) and Tetragraptus pendens-T. quadribrachiatus Zone (lower part). (6) Baiyungang Formation: Carbonates, yielding graptolites of Adelograptus-Clonograptus Zone at the top. Unfortunately, all of the illustrated graptolite specimens as well as the specimens of other fossil group have been lost by the sponsor institutions of PetroChina. Thus, we re-sampled the sections in 2007. Together with a graptolite collection from Wang Pu who kindly provided to us when he learned we were working on the graptolite fauna, the lithological divisions and graptolite zones in the Kuruktag region are revised herein as below: (1) Yinpingshan Formation: Dicellograptus ornatus Zone (upper) and Dicellograptus cf. D. complanatus Zone (lower). (2) Yuanbaoshan Formation: Geniculograptus pygmaeus Zone. (3) Zatupo Formation: Amplexograptus praetypicalis Zone (corresponding to the Diplacanthograptus spiniferus Zone). (4) Charchaq Formation: Nemagraptus gracilis Zone
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(upper) and Archiclimacograptus? confertus Zone (lower). (5) Heituao Formation (in desending order): Undulograptus austrodentatus Zone, Exigraptus clavus Zone, Isograptus victoriae Zone, Expansograptus abnormis Zone, and Kiaerograptus kiaeri-Aorograptus victoriae Zone (South Yaerdang Mountain). Based on the graptolite zonation, a correlation between the two main hydrocarbon source rocks units, the Saergan and the Heituao formations from northwestern and northeastern sides of Tarim Basin respectively, is demonstrated as in Figure 3 [16]. The top of the Saergan Formation is biostratigaphically higher than that of the Heituao Formation. On the other hand, the base of the Heituao Formation is clearly diachronous, as it is of late Tremadocian age in the South Yaerdang Mountain, but early Dapingian in the Queerqueke Mountain. Correlated to carbon isotope development, the Heituao Formation corresponds to the interval of Dapingian typified by minor fluctuation, whereas the Saergan Formation is correlated to a positive excursion called mid-Darriwilian carbon excursion (MDICE). The black shale of the Heituao Formation may possess higher potential to be a major hydrocarbon source rock unit than the Saergan Formation, in view of its much wider distribu-
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tion––it is developed not only in the Kuruktag region and the Konqi (Peacock) River, but also in the Manjiaer Depression of eastern Tarim Basin.
3 Ordovician black shales in the Altun Mountains The Ordovician black shale in the Huanxing Mountain of Altun Mts. area was named the Huanxingshan Formation [17]. It contains black mudstone and siliceous beds with graptolites occurring in the middle part of the formation. The graptolite fauna, as identified by Chen and Ni, includes Hustedograptus cf. H. teretiusculus (Hisinger), Normalograptus? euglyphus (Lapworth), Amplexograptus cf. A. maxwelli Decker, Glossograptus sp., and Dicellograptus sp. The graptolite fauna of the Huanxingshan Formation was thus regarded representing the H. teretiusculus Zone [18]. A latest study of H. teretiusculus (Hisinger) itself indicates that the species is a long-ranging species spanning the P. elegans Zone to the lower Sandbian Stage. Thus, the species should no longer be employed as a zonal species, and in this case it is likely the graptolite fauna indicates N. gracilis
Figure 3 Stratigraphic correlation of the Saergan and Heituao formations. The base boundary of the Heituao Formation is drawn according to our latest study of the sections in the South Yaerdang Mountain and the Queerqueke Mountain. The base map of this figure is from ref. [16]. g–graptolite zonation; c–conodont zonation. HICE–Hirnantian carbon isotope excursion; GICE–Guttenberg carbon isotope excursion.
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Zone of lower Sandbian.
4 Ordovician black shales in Tarim Basin Since the 1990s, one of us (Chen Xu) has identified graptolites from many wells in the Tarim Basin, which were provided by colleagues working for Tarim projects. Some other published graptolite data and their correlation are also discussed herein with new interpretations (Figure 2). (1) Southwest Kuqa: Obtained graptolites from the Daxikumu Formation include Hustedograptus cf. H. teretiusculus (Hisinger), Amplexograptus praetypicalis Riva, Rectograptus cf. R. amplexicaulis (Hall), etc., presumably suggesting a Rectograptus amplexicaulis Zone [18]. However, the present paper considers this fauna to be diagnostic of Diplacanthograptus spiniferus Zone, and is comparable to that of the Yingan Formation in Kalpin [8]. A nearby well produces graptolites Diplacanthograptus spiniferus (Ruedemann) and Amplexograptus praetypicalis Riva, which are from the so-called Yingmaili Formation, indicating clearly the same horizon. Viewing that the two formations adopted by different authors for the two separate wells are clearly identical, we propose one of them should be avoided. (2) South Korla: One of us (Tang Peng) recently identified two graptolite-bearing horizons in black siltstone and shale of different well depths in south Korla. The graptolites identified by Chen Xu indicate an identical level of strata, which is likely to be a repetition in different depths within the drill core. The graptolite fauna includes Amplexograptus latus (Elles et Wood), Rectograptus abbreviatus (Elles et Wood), R. songtaoensis Li, Anticostia fastigata (Davies), Pararetiograptus magnus Mu and Dicellograptus sp. It represents the highest Ordovician strata in Tarim Basin and may correspond to the Dicellograptus complexus Zone of the Wufeng Formation in the Yangtze region of South China. (3) South of the Konqi (Peacock) River: A graptolite fauna of Nemagraptus gracilis Zone was obtained also by Tang Peng recently from the black shale in the south of the Konqi River, including Dicellograptus sextans (Hall), D. sextans exilis Elles and Wood, Pseudazygograptus incurvus (Ekström), Acrograptus sp., Hustedograptus sp., Cryptograptus tricornis (Carruthers), Archiclimacograptus angulatus (Bulman), A. riddellensis (Harris), and Normalograptus? euglyphus (Lapworth). Planktonic phyllocarids Caryocaris occurs in association with the graptolite fauna, indicating a typical black graptolitic facies. The above-mentioned graptolite-bearing, black shale in the south of the Konqi River was lithologically identified to that of Heituao Formation. This is somewhat puzzling and
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there could be two possibilities of their correlation. One is that the top of the Heituao Formation is diachronous southwards from the U. austrodentatus Zone in the Kuruktag region up to the N. gracilis Zone in the south of the Konqi River. In this case, it means that the Heituao Formation thickens significantly southwards. The other is that the black shale in the south of the Konqi River represents a different lithological unit from the Heituao Formation, which probably corresponds to the Saergan Formation of Kalpin area. From the point of view of the hydrocarbon source rock, either of these possibilities is significant for the evaluation of the potentials in this new area. (4) Eastern Tarim: Two graptolitic layers of Ordovician were recognized1). The lower layer contains Pseudoclimacograptus aff. P. scharenbergi minor Mu, Lee et Ge (=P. aff. P. scharenbergi (Lapworth)), and Pseudoclimacograptus cf. P. angulatus Bulman (=Archiclimacograptus cf. A. angulatus (Bulman)) in dark siltstone. The upper layer contains Orthograptus cf. O. apiculatus Elles et Wood in black siltstone. Both the layers are within the N. gracilis Zone. (5) Central Tarim: Zhang et al. [19] reported some graptolites from two different layers of black shales in the drill cores of this area, including Glyptograptus ex gr. G. teretiusculus (Hisinger) (= Hustedograptus ex gr. H. teretiusculus (Hisinger)), Pseudoclimacograptus cf. P. angulatus (Bulman) (= Archiclimacograptus cf. A. angulatus (Bulman)), and P. cf. P. scharenbergi minor Mu, Lee et Ge (= Pseudoclimacograptus cf. P, scharenbergi (Lapworth)). They assigned this fauna to the Pseudoclimacograptus angulatus Zone of Darriwilian. It may, however, represent a part of the N. gracilis Zone. Another graptolite fauna assigned by Zhang and others to the Dicranograptus clingani resicis Zone includes Dicranograptus cf. D. clingani resicis Williams and Bruton, Pseudoclimacograptus scharenbergi stenostoma (Bulman), and Cryptograptus tricornis (Carruthers). However, the horizon of Dicranograptus clingani resicis Williams and Bruton is not clear as shown in a recent study by Zalasiewicz [20]. In the light of their occurrences, the above-mentioned graptolites seem more likely to belong to the N. gracilis Zone. In a higher layer, Zhang et al. [19] recorded graptolites Dicellograptus ex gr. D. pumilus Lapworth, Normalograptus cf. N. brevis (Elles et Wood), and Orthograptus quadrimucronatus timidus (Ruedemann). D. pumilus is a common form in the Diplacanthograptus caudatus Zone of northwest China. N. brevis commonly occurs in the N. gracilis and C. bicornis zones in the same region. The biostratigraphic level of O. quadrimucronatus timidus is not clear in either China or North America, where it occurs. We assume that the assemblage may represent a slightly higher horizon than Sandbian.
1) Geng L Y, Zhao Z X, Wang Z H. Stratigraphic subdivision and correlation of the Tarim Coverage area. Report of the “Eight-Five” National Key Project “Petroleum and Gas Resources of the Tarim Basin”, 1994. 1–81
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A critical species, Amplexograptus praetypicalis Riva, was obtained recently by one of us (Li Yue) from dark gray siltstones of drill cores in central Tarim area. The occurrence of this species, which was recorded elsewhere from the Diplacanthograptus spiniferus Zone in the Yingan Formation at Dawangou, together with those species recorded
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by Zhang et al. [19] as mentioned above, indicate that the black or dark-gray graptolitic layers in central Tarim may span a continuous interval from the N. gracilis Zone to the D. spiniferus Zone. Some biostratigraphically significant graptolites are illustrated in Figure 4, for readers’ convenient reference.
Figure 4 Some biostratigraphically significant graptolites of Ordovician from Tarim Basin and its peripheral region. All the specimens are stored in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, with the registration numbers prefixed by NIGP. Scale bars = 1 mm. (a) Archiclimacograptu riddellensis (Harris), NIGP152517, Nemagraptus gracilis Zone, South of the Konqi River; (b) Rectograptus songtaoensis Li, NIGP152518, Dicellograptus complanatus Zone, South of Korla; (c) Pararetiograptus magnus Mu, NIGP152519, Dicellograptus complanatus Zone, South of Korla; (d) Amplexograptus praetypicalis Riva, NIGP152520, Diplacanthograptus spiniferus Zone, central Tarim; (e) Amplexograptus latus (Elles & Wood), NIGP152521, Wyk-HB-26-10, Dicellograptus complanatus Zone, Yakeruike, Wushi; (f) Geniculograptus pygmaeus (Ruedemann), NIGP152522, AFT-X139, Yuanbaoshan Fm., Queerqueke Mt, Kuruktag; (g) Undulograptus austrodentatus (Harris & Keble), NIGP152523, AFT-X86-1, Heituao Fm., Queerqueke Mt, Kuruktag; (h) Anticostia macgregove Stewart & Mitchell, NIGP152524, Wyk-HB-26-10, Dicellograptus complanatus Zone, Yakeruike, Wushi; (i) Dicellograptus vagus Hadding, NIGP152525, Saergan Fm., Dawangou, Kalpin; (j) Acrograptus ellesae (Ruedemann), NIGP152526, AFT-X89-14, Charchaq Fm., Queerqueke Mt, Kuruktag; (k) Isograptus victoriae Harris, NIGP152527, AFT-X67-34, Heituao Fm., Queerqueke Mt, Kuruktag; (l) Archiclimacograptus angulatus (Bulman), NIGP152528, South of the Konqi River, Nemagraptus gracilis Zone; (m) Expansograptus abnormis (Hsu), NIGP152529, AFT-X65-15, Heituao Fm., Queerqueke Mt, Kuruktag; (n) Dicellograptus sextans (Hall), NIGP152530, Nemagraptus gracilis Zone, South of the Konqi River; (o) Pterograptus elegans Holm, NIGP152531, Saergan Fm., Dawangou, Kalpin; (p) Dicellograptus elegans Carruthers, NIGP152532, AFT-X145, Yinpingshan Fm., Queerqueke Mt, Kuruktag; (q) Nemagraptus gracilis (Hall), NIGP152533, Saergan Fm., Dawangou, Kalpin; (r) Dicellograptus cf. complanatus Lapworth, NIGP152534, AFT-X148a, Yinpingshan Fm., Queerqueke Mt, Kuruktag; (s) Didymograptus murchisoni (Beck), NIGP152535, Saergan Fm., Dawangou, Kalpin.
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5 Ordovician black shales at the northern foot of Kunlun Mountains
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The Ordovician System in the western Kunlun Mountains was first studied by Ma et al. [21]. Wang [22] later found Late Ordovician graptolites from the Kandilike area and consequently erected the Qiujiuboyinake Formation. The graptolite fauna of the formation includes Dicranograptus nicholsoni Hopkinson, D. brevicaulis Elles et Wood, Amplexograptus maxwelli Decker, Orthograptus calcaratus Lapworth, Dicellograptus divaricatus (Hall), D. sextans exilis Elles et Wood, and Xiphograptus sagiticaulis (Gurley) [22]. Based on this fauna, we propose the Qiujiuboyinake Formation correspond to the Sandbian Stage and possibly the basal part of the Katian Stage.
6 Conclusions Ordovician black shales are widely distributed in the Tarim Basin and its peripheral regions and some of them may be highly potential hydrocarbon source rocks. Our study of the Ordovician graptolite faunas that occur in many different layers, together with that of conodonts and chitinozoans by others, permits a finer correlation of these deposits in the block. The following conclusions are drawn. (1) The most widespread distribution of the Ordovician black shales in the Tarim Basin and its peripheral regions occurs in early Sandbian (corresponding to N. gracilis Zone), and may be related to a contemporary global sea level rise event, i.e. the “Late Llanvirn-Caradoc Highstand Interval” [23]. (2) In the Kuruktag region (Quruq Tagh), eastern Tienshan, thick Ordovician black shales occur repeatedly through up, spanning an interval from Tremadocian to middle Katian (D. spiniferus Zone) and extending southeastwards into the Manjiaer Depression. The black shale of the D. spiniferus Zone may even extend into the central Tarim Basin. (3) Three different facies belts are recognized in northwestern Tarim Basin: carbonate platform and reef belt (Bachu), marginal platform to upper slope belt (Kalpin-Aksu), and slope belt (Wushi). Although the Saergan black shale has been considered as a highly potential hydrocarbon source rock unit, its restricted distribution in the Kalpin-Aksu belt, as shown in this paper, limits its potential. We thank Profs. Rong Jiayu, Jin Zhijun, Zhang Shiben and Zhao Zhixin for reading the manuscript, and Prof. Melchin, M. J. for reading the English manuscript. The present study was supported by Chinese Academy of Sciences (Grant Nos. KZCX2-EW-111 and KZCX2-YW-Q05-01) and China Geological Survey (Grant No. 1212011120116). This is a contribution to the Geobiodiversity Database project (www.geobiodiversity.com).
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