Middle-Upper Ordovician (Darriwilian-Early Katian) Positive Carbon Isotope Excursions in the Northern Tarim Basin, Northwest China: Implications for Stratigraphic Correlation and Paleoclimate

0. INTRODUCTION

The stable carbon isotope in marine carbonate (δ¹³C_carb) reflects the initial isotope composition in the original seawater (Munnecke et al., 2011; Ainsaar et al., 2010; Saltzman, 2005), and a certain degree of diagenesis will not change the isotope fluctuation trend (Saltzman, 2005). In addition, δ¹³C_carb positive excursions generally occur in icehouse (Saltzman, 2005) and δ¹³C_carb fluctuation is used as an environmental change indicator (Ainsaar et al., 2010). Over the past decades, stable carbon isotope data have been applied widely in the study of regional and global correlation of carbonates and climate change (Ainsaar et al., 2010; Zhang et al., 2010).

The Ordovician is a special period in geological history, encompassing several significant events, such as the Great Ordovician Biodiversification Event (GOBE), Late Ordovician glaciation, and biotic mass extinction (Thompson et al., 2012; Munnecke et al., 2010; Trotter et al., 2008; Cocks, 2007). At present, three short-lived δ¹³C_carb positive excursions have been identified in the Ordovician globally: (1) the Middle Darriwilian Isotope Carbon Excursion (MDICE); (2) the Guttenberg Carbon Isotope Excursion (GICE); and (3) the Hirnantian Carbon Isotope Excursion (HICE). It is generally considered that the MDICE and GICE events were related to burial of abundant organic matter (Wang et al., 2015; Pancost et al., 2013; Sial et al., 2013; Rosenau et al., 2012; Saltzman and Young, 2005; Ludvigson et al., 2004), and that the Hirnantian glaciation and biotic extinction events were closely related to the genesis of HICE (Munnecke et al., 2011; Zhang et al., 2010).

In 1990s, the negative δ¹³C_carb excursions found in the Kalpin outcrop in the Tarim Basin have been used to study Cambrian-Ordovician sea-level fluctuations (Wang and Yang, 1994). Wang (2000) used δ¹³C_carb positive excursions as proxies for development of marine source rocks in the Middle–Upper Ordovician, but did not name the positive excursions. In recent years, some scholars have performed a δ¹³C_carb correlation on the boundary of Cambrian-Ordovician in the Kalpin outcrops (Wang et al., 2011; Hu et al., 2010; Jing et al., 2008). According to Ordovician δ¹³C_carb curves based on data from well cores and well cuttings in the Bachu and Tazhong uplifts (Fig. 1a), Zhang et al. (2014) indicated the existence of two δ¹³C_carb positive excursions in the Dapingian–Darriwilian and Sandbian–Early Katian.

Figure  1.  (a) Structrural units in Tarim basin; (b) geologic map of Middle–Upper Ordovician in Tahe oil-gas field.

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In this study, analyses of δ¹³C_carb and δ¹⁸O_carb and conodonts have been conducted on samples from two well cores from the Tahe oil-gas field in the northern Tarim (Fig. 1b). On the basis of previous research on conodont biostratigraphy, the identified δ¹³C_carb positive excursions have been compared with those in the Tarim Basin and in other regions of the world, and the inherent relationships of δ¹³C_carb positive excursion with source rock and paleoclimate have been discussed.

1. GEOLOGICAL SETTING

The Tarim Plate drifted near the equator from south to north in the Middle Ordovician as an independent plate (Cocks and Torsvik, 2013; Wang et al., 2013a), and close to the northwestern margin of Gondwana. Large carbonate platform was developed in the Cambrian-Middle Ordovician on the western part of the plate (Wu et al., 2012; Zhang et al., 2006). The Tarim Plate continued to move northward in the Late Ordovician, and two small carbonate platforms formed in the Tabei and Bachu-Tazhong uplifts in the Early Katian (Liu et al., 2012).

The Tahe oil-gas field is located in the Akelule uplift of the northern Tarim Basin (Fig. 1b). According to the lithology and well logging features, the Lower–Middle Ordovician contained the Penglaiba Fm., Yingshan Fm. and Yijianfang Fm. The Upper Ordovician is composed of the Qiaerbak Fm., Lianglitag Fm. and Sangtamu Fm.. The Lower–Middle Ordovician exhibit platform facies, and the platform margin is located on the east side of the Tahe oil-gas field (Fig. 1b).

The Penglaiba Fm. includes gray dolomite sandwiched with limy dolomite, suggesting a restricted platform facies. The Yingshan Fm. is composed of open platform deposits, the lithology of the upper part is yellowish-gray limestone sandwiched with thin-bedded calcarenites (Fig. 2a), which is changed into dolomitic limestones and limy dolomites with enhanced dolomitization downwards. The Yijianfang Fm. is composed of shoal facies deposits, and the lithology is yellowish-gray calcarenite sandwiched with thin bedded limestone (Fig. 2b). Deng et al. (2007) viewed this formation as belonging to Highstand System Tract (HST). The conodont fauna from the Penglaiba Fm. to and the Yijianfang Fm. is of the North American Midcontinent type (Li et al., 2009; Zhao et al., 2006)(Figs. 3 and 4), which is characterized by species of warm water, low abundance and slow evolution. It is difficult to carry out stratigraphic correlation using conodonts in those strata (Wang et al., 2007). Conodont zone of Pygodus serra first occurred in the Upper Yijianfang Fm. and extended to Qiaerbak Fm. of Upper Ordovician (Wang et al., 2013b; Zhao et al., 2006), which was the latest conodont zone of Middle Ordovician (Albanesi et al., 2013).

Figure  2.  (a) Yellowish-gray limestone of Yingshan Fm. in well S114; (b) yellowish-gray limestone of Yijianfang Fm. in well S114; (c) yellowish-gray limestone of Yijianfang Fm. and grayish-green muddy limestone of Qiaerbak Fm. in well S112-1; (d) red-brown nodular marlite of Qiaerbak Fm. in well S112-1; (e) gray limestone and marlite of Lianglitag Fm. in well S112-1; (f) grayish-black mudstone of Sangtamu Fm. in well S112-1; (g) micrite of Yingshan Fm. in well S114, 6 424.6 m. Dolomite crystals distributed along the stylolities, and the whole rock have dull cathodoluminescence; (h) calcarenite of Yijiangfang Fm. in well S114 with dull cathodoluminescence, 6 346.25 m; (i) bioclastic limestone of Yijianfang Fm. in well S112-1, 6 339.9 m. The limestone have weak cathodoluminescence, but the calcites of tectonic fissure have brown cathodoluminescence.

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Figure  3.  Stratigraphic diagram showing conodont zone relations in the study area. Conodont zone column modified from Zhao et al. (2006).

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Figure  4.  Stratigraphic correlation and conodonts distribution for well S112-1 and well S114 in Tahe oil-gas field.

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The Qiaerbak Fm. is composed of deep-water shelf deposits (Wang et al., 2009; Liu et al., 2010). The lower part is composed of grayish-green muddy limestone (Fig. 2c), and the upper part is red-brown nodular marlite (Fig. 2d), which are regarded to represent Transgressive System Tract (TST) and HST respectively (Wu et al., 2012). The conodont fauna of this formation is of the North Atlantic type, characterized by abundant species of cold (Li et al., 2009; Wang et al., 2009; Zhao et al., 2006). Conodont zone of Pygodus anserinus was developed in the Lower Qiaerbak Fm. (Fig. 4). The base of the Nemagraptusgracilis graptolite zone is taken as the boundary of the Middle–Upper Ordovician globally, but this boundary is slightly higher than that of the Pygodus anserinus conodont zone (Wang et al., 2013b; Chen and Wang, 2003). Therefore, grayish-green muddy limestones under the finger peak of gamma ray curve may belong partly to the Middle Ordovician (Figs. 2c and 4).

The Lianglitag Fm. is composed of carbonate ramp deposits, and consists of limestones, calcarenites and marlites (Fig. 2e), and contains both TST and HST sequences (Liu et al., 2012; Wu et al., 2012). The ramp margin is curved (Fig. 1b), and the stratigraphic thickness becomes thinner eastwards and southwards. The conodont faunais of the North American Midcontinent type. Typical conodont zones are lacking in the top of Qiaerbak Fm. and the bottom of Lianglitag Fm., which are possibly of Sandbian and Early Katian age respectively as indicated by conodont zones and the change of depositional environment (Li et al., 2009; Wang et al., 2009; Wang et al., 2007; Xiong et al., 2006; Zhao et al., 2006). The Belodina compressa conodont zone generally recognized from the upper most Qiaerbak Fm. to Lianglitag Fm. (Zhao et al., 2006).

The Sangtamu Fm. contains deep-water shelf deposits. The lower part is composed of grayish-black mudstone (Fig. 2f), and the upper part contains grayish-black mudstone sandwiched with thin-bedded marlite. The conodont Yaoxianognathus neimengguensis zone lies appear at the top of the Lianglitag Fm. and mostly in the basal Sangtamu Fm. (Jing et al., 2007).

A short-term erosional hiatus occurred near the tops of the Yijianfang Fm. and Lianglitag Fm. in the Akekule Uplift respectively. Exposed area of the Yijianfang Fm. is mainly in the east and north of the Akekule uplift, and the southwestern regions are successive deposition (Fig. 2c) (Liu et al., 2008a, 2010). Subaerial area of the Lianglitag Fm. are mainly in the north of ramp margin (Liu et al., 2008b), and the southern regions are successive deposition.

The Akekule uplift suffered from NW–SE extension stress in the Early Hercynian tectonic movement, as a result, a large nose uplift plunged to the southwest and extended in the northeastern direction, and the Ordovician strata were seriously denuded in the northern region of the study area (Liu et al., 2010). The Akekule uplift was further transformed later by Hercynian and Himalayan tectonic movements.

2. METHODS

2.1 Samples Collection and Analysis

Samples for this study came from drill cores of the wells S112-1 and S114 in the southern Tahe oil-gas field (Fig. 1b). There were no cores at the 11.01 m interval in the well S112-1 and the 5 m interval in the well S114 on the middle Qiaerbak Fm., respectively (Fig. 4). Samples were collected from micrites by avoiding drill cores with strong diagenesis, high argillaceous content and many organisms in the carbonate strata, and from conglomerations or bands with high limy content in muddy limestones or marlites.

Twenty-five samples were collected from well S114 in 2006, with sampling densities from 2.2 m to 16.3 m, mainly in 2.2 m–7 m accounting for 84% of the total samples. Based on the observation of petrographic thin sections and cathode luminescence, dolomitization occurred in the lower part of Yingshan Fm. In well S114, and dolomite crystals distributed along the stylolites (Fig. 2g). Consequently, samples were collected avoiding the stylolites and dolomite bands. δ¹³C_carb and δ¹⁸O_carb were measured by MAT251 mass spectrograph in the Isotope Laboratory of Ministry of Land and Resources.

Fifty-nine samples of the well S112-1 were collected in 2010, and the sampling intervals are from 0.2 m to 5.12 m, mainly in 0.2 m–2 m accounting for 87.7% of the total samples. Drill cores were not dolomitized as observed in thin sections and cathode luminescence. δ¹³C_carb and δ¹⁸O_carb were measured by MAT253 mass spectrograph in the Analysis and Test Research Center of the Beijing Research Institute of Uranium Geology.

The phosphate-continuous flow test method was adopted for testing, and errors were less than ±0.1‰ (VPDB) for δ¹³C_carb and less than ±0.2‰ (VPDB) for δ¹⁸O_carb. The analytical process followed Liu et al. (2013). Analytical data are shown in Table 1, where the sample depths are adjusted.

Table  1.  Data of carbon and oxygen isotope for well S112-1 and well S114 (relative to VPDB standard)

Well	Formation	Depth (m)	Core Number	Lithology	δ13C/‰	δ18O/‰
S112-1	Sangtamu	6 259.05	3 1/15	Mudstone	0.8	-6.1
		6 262.17	3 7/20	Mudstone	0.9	-6.1
		6 263.28	3 11/20	Mudstone	1	-5.9
		6 264.52	3 13/20	Mudstone	1	-6.2
		6 265.41	3 47/60	Mudstone	0.9	-5.9
		6 266.51	3 19/20	Mudstone	0.9	-6.3
		6 266.81	4 1/36	Mudstone	1	-6
		6 269.93	4 1/4	Mudstone	1	-5.9
		6 270.32	4 11/36	Mudstone	1	-6.1
		6 271.73	4 17/36	Mudstone	1	-5.6
		6 273.2	4 13/24	Mudstone	0.9	-6.3
		6 273.4	4 7/12	Mudstone	0.9	-6.2
		6 276.06	4 7/9	Mudstone	1	-6.3
		6 276.26	4 59/72	Mudstone	0.9	-6.3
		6 278.27	4 67/72	Mudstone	0.9	-6.2
		6 280.27	5 3/8	Mudstone	1	-5.9
		6 281	5 3/4	Mudstone	0.8	-6.5
		6 281.7	6 1/15	Mudstone	0.6	-6.2
		6 283.09	6 7/53	Mudstone	0.6	-6.5
		6 284.49	6 14/55	Mudstone	0.9	-6.2
		6 285.49	6 22/63	Mudstone	1.1	-6.1
	Lianglitag	6 286.6	6 23/53	Limestone	0.4	-5.5
		6 287.57	6 1/2	Limestone	0.7	-4.8
		6 288.52	6 42/73	Limestone	0.9	-5.1
		6 289.7	6 35/53	Limestone	0.9	-4.5
		6 290.48	6 39/53	Limestone	1.8	-3.9
		6 290.97	6 55/67	Limestone	1.7	-4.2
		6 291.93	6 50/57	Limestone	1.5	-4.5
		6 293.04	6 69/71	Muddy Limestone	1.6	-5
		6 294.28	7 1/19	Muddy Limestone	2	-3.9
		6 295.41	7 11/57	Muddy Limestone	2.5	-4.3
		6 295.84	7 14/57	Muddy Limestone	2	-4.8
		6 297.11	7 8/19	Muddy Limestone	2.5	-3.8
	Qiaerbak	6 298.95	7 12/19	Nodular Marlite	1.9	-3.8
		6 300.77	7 40/57	Nodular Marlite	1.8	-4.5
		6 300.97	7 43/57	Nodular Marlite	1.8	-4
		6 301.37	7 16/19	Nodular Marlite	1.6	-4.8
		6 303.77	7 56/57	Nodular Marlite	1	-5.8
		6 315.57	8 3/40	Muddy Limestone	1.3	-4.8
		6 317.45	8 11/80	Muddy Limestone	1.1	-5.7
		6 317.85	8 13/80	Muddy Limestone	1.2	-4.4
		6 319.47	8 1/4	Muddy Limestone	0.9	-4.4
		6 319.93	8 27/80	Muddy Limestone	1	-4.6
		6 320.12	8 1/2	Muddy Limestone	1.3	-6.5
		6 320.75	8 43/80	Muddy Limestone	1.5	-4.4
		6 322.62	8 9/16	Muddy Limestone	0.9	-6.1
		6 323.42	8 51/80	Muddy Limestone	0.8	-5.6
		6 324.69	8 59/80	Muddy Limestone	1	-5.4
		6 326.21	8 67/80	Muddy Limestone	0.8	-6
		6 327.28	8 73/80	Muddy Limestone	0.5	-6.5
		6 328.5	9	Muddy Limestone	0.6	-5.5
		6 329.1	9 6/47	Muddy Limestone	0.6	-5.9
		6 330.71	9 12/47	Muddy Limestone	0.4	-6.9
		6 331.39	9 16/47	Muddy Limestone	0.4	-6
		6 331.64	9 19/47	Muddy Limestone	0.4	-6.2
	Yijianfang	6 334.28	9 34/47	Limestone	0.5	-5.6
		6 334.78	9 38/47	Limestone	0.2	-6
		6 339.9	9 41/47	Limestone	0.3	-6
		6 340.1	9 43/47	Limestone	0.1	-6.4
	S114	Qiaerbak	6 320.35	6 1/35	Muddy Limestone	1.4	-4
6 325			7 7/32	Muddy Limestone	1.8	-4.7
6 329			7 47/64	Muddy Limestone	0.8	-5.3
Yijianfang		6 331.2	8 13/47	Limestone	1.4	-2.6
		6 337.35	8 44/47	Limestone	0.6	-5.4
		6 339.9	10 1/23	Limestone	0.8	-5.8
		6 346.25	10 61/69	Limestone	1.2	-6.4
		6 349.55	11 21/80	Limestone	1.4	-6.5
		6 354.6	11 9/10	Limestone	0.9	-6
		6 358.9	12 29/39	Limestone	0.8	-6.2
		6 365.8	13 2/3	Limestone	0.6	-6.5
		6 369.86	14 26/57	Limestone	0.4	-6.2
		6 374.7	15 16/25	Limestone	0.2	-6.7
Yingshan		6 385.1	18 37/69	Limestone	-0.2	-6.6
		6 391.15	19 20/23	Limestone	-0.2	-6.5
		6 397.1	20 52/73	Limestone	-0.4	-6.5
		6 403.5	21 14/19	Limestone	-0.3	-6.4
		6 414.85	23 7/26	Limestone	-0.3	-6.3
		6 419.9	24 13/15	Limestone	-0.4	-6.5
		6 424.6	26 8/43	Limestone	-0.4	-6.8
		6 433.15	28 1/18	Limestone	-0.6	-6.2
		6 437.9	28 11/18	Limestone	-0.7	-6
		6 442.4	29 13/75	Limestone	-0.8	-6.5
		6 446.5	29 2/3	Limestone	-0.5	-6.3
		6 462.8	31 64/65	Limestone	-0.8	-5.7

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2.2 Analysis of Diagenetic Alteration

Atmospheric freshwater diagenesis have strong impacts on δ¹³C_carb and δ¹⁸O_carb of carbonate rocks (Gradstein et al., 2012), and the values of δ¹³C_carb and δ¹⁸O_carb significantly decreased. The top surfaces of the Yijianfang and Lianglitag formations in the Tahe oil-gas field were exposed to be subaerial for a short time and underwent atmospheric freshwater karstification, and the exposure areas are located in the northeastern and northern parts of the study area. The wells S112-1 and S114 cores exhibit successive deposition (Fig. 2c), indicating that these samples were unlikely affected by atmospheric fresh water (Figs. 2h and 2i).

Compared with the δ¹³C_carb and δ¹⁸O_carb is more susceptible to the influence of the diagenesis (Metzger et al., 2014; Gradstein et al., 2012). The initial composition of δ¹³C_carb may be obviously changed when δ¹⁸O_carb value is less than -10‰ (Derry et al., 1992). As shown in Fig. 5 and Table 1, the δ¹⁸O_carb values of wells S112-1 and S114 are more than -7‰, reflecting the δ¹³C_carb value of samples underwent relatively weak influence of diagenesis.

Figure  5.  Cross plot of δ¹³C_carb versus δ¹⁸O_carb for well S112-1 and well S114 in Tahe oil-gas field.

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Positive covariation between δ¹³C_carb and δ¹⁸O_carb is also a method for judging diagenetic alteration of carbonate rock samples (Sial et al., 2013; Jing et al., 2008). Figure 5 indicates that the δ¹³C_carb and δ¹⁸O_carb of some samples have obvious linear relationships, but no correlation with depths (as shown in Fig. 6), and are closely related to strata. Therefore, this linear relationship may reflect a coordinated relationship between δ¹³C_carband δ¹⁸O_carb in a specific sedimentary background, and indicate the fluctuations of original seawater.

Figure  6.  The curves of δ¹³C_carb and δ¹⁸O_carb in Middle-Upper Ordovician for well S112-1 and well S114 in Tahe oil-gas field.

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3. RESULTS

3.1 Carbon Isotope Curve in Well S114

The δ¹³C_carb curve of the Yingshan and Qiaerbak fms. In well S114 show generally positive excursion features (Fig. 6), ranging from −0.8‰ to 1.8‰. The excursion process can be divided into five intervals, from a to e.

δ¹³C_carb values in interval-a shifted from -0.8‰ to -0.5‰, and fluctuation range is 0.3‰. The negative excursion in interval-b appears at depth near 6442.4 m in the Yingshan Fm., and the δ¹³C_carb values are shifted −0.3‰ compared with the value below. According to Ordovician δ¹³C_carb curves (Albanesi et al., 2013; Munnecke et al., 2010), the boundary of Dw 1 and Dw 2 in the Darriwilian corresponds to a small negative excursion. Whether the negative excursion seen in the interval-b (Fig. 6) is the same as that eeds to be verified. The positive excursion in interval-c spanning about 100 meter strata fluctuated from -0.8‰ to 1.4‰, and the fluctuation range is 2.2‰. The negative excursion in interval-d appears at depth near 6 337.35 m in the Yijianfang Fm., and the δ¹³C_carb values are shifted −0.8‰ compared with the values below. The positive excursion ininterval-e fluctuated from 0.6‰ to 1.8‰. It is noteworthy that uppermost δ¹³C_carb value of interval-e shifted -0.4‰, and there may be a negative fluctuation trend.

According to Figure 6, positive excursion-A of well S114 occurred between interval-c and d in Yijiangfang Fm., and another positive excursion B occurred in lower part of Qiaerbak Fm..

3.2 Carbon Isotope Curve in Well S112-1

The δ¹³C_carbcurve from the top of the Yijianfang Fm. to bottom of the Sangtamu Fm. in well S112-1 fluctuate sharply from a minimum value of 0.1‰ at the top of the Yijianfang Fm. to a peak value of 2.5‰ in the Lianglitag Fm.. The excursion process in this well also can be divided into five intervals, from a to e.

Interval-a represents the rising limb of a positive excursion (Fig. 6), where the δ¹³C_carb values shift from 0.1‰ to 1.3‰. Interval-b represents a negative excursion, where values fluctuate at least -0.3‰ in the middle of the Qiaerbak Fm.. This shift is estimated from the difference between upper and lower values, because there are no drill cores available from this interval. Interval-c is a positive excursion, and the values rise fluctuate up to 1.4‰ from the top of the Qiaerbak Fm. to the bottom of the Lianglitag Fm.. Interval-d is a negative excursion, where values decrease rapidly in the upper part of the Lianglitag Fm. and fluctuate up to 2.1‰. Interval-e corresponds to the lower part of the Sangtamu Fm., where the δ¹³C_carb values quickly shift to near 1‰, and then fluctuate in a narrow range around 1‰.

According to Fig. 6, positive excursion-B of well S112-1 can contrast with that of well S114, and interval-c and d composed positive excursion-C.

4. DISCUSSION

Wells S112-1 and S114 are relatively close to each other and their lithologies show similar sedimentary environments. Therefore, taking the top of the Yijianfang Fm. as the boundary, the δ¹³C_carb curvesof the two wells were merged, and contrasted with Ordovician δ¹³C_carb curve of Kalpin outcrop (Hu et al., 2010) and the generalized curve in the world (Albanesi et al., 2013) (Fig. 7).

Figure  7.  The δ¹³C_carb curve of Darriwilian-Early Katian in the Tahe oil-gas field contrast with Ordovician δ¹³C_carb curves of Kalpin outcrop and the world. g. graptolite zonation; c. conodont zonation; D. Dawangou Fm.; S. Saergan Fm.; K. Kanling Fm.

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4.1 Positive Carbon Isotope Excursions and Sea Level Change

A δ¹³C_carb excursion in a stratigraphic succession may simply record the lateral movement of an isotopically unique water body during a sea level change, and the help provided by carbon isotope excursion formed by sea level change to global comparison is limited in case of considering the asynchronization of sea level change (Edwards and Saltzman, 2014). Therefore, the relationships of the three δ¹³C_carb positive excursions shown in Fig. 6 with sea level change are very important.

According to Fig. 4 and researches on Ordovician sequence stratigraphy in the Tarim Basin or Tahe oil-gas field (Liu et al., 2012; Wu et al., 2012; Deng et al., 2007), positive excursion-Aoccurs in the HST of the Yijianfang Fm., positive excursion-B in the TST of the Qiaerbak Fm., and positive excursion-C in the HST of the Qiaerbak Fm. and the TST of the Lianglitag Fm., which indicated that these positive excursions have no direct relationships with sea level fluctuation, and can be correlated regionally or globally.

4.2 Correlation of Positive Carbon Isotope Excursions in the Tarim Basin and Globally

The Pygodus serra conodont zone in the Darriwilian, Pygodus anserinus conodont zone in the Sandbian, and Belodina compressa conodont zone in the Early Katian (shown in Figs. 3 and 4) of the Tahe oil-gas field in the northern Tarim Basin can be compared with the corresponding conodont biozones in Argentine Precordillera (Edwards and Saltzman, 2014; Albanesi et al., 2013; Sial et al., 2013; ), Oklahoma and Virginia, USA (Young et al., 2008), Cincinnati region, USA (Bergström et al., 2010), and Baltoscandia (Ainsaar et al., 2010), thereby providing a rough temporary calibration on δ¹³C_carb positive excursions.

According to Fig. 7, δ¹³C_carb positive excursion-A of Middle Ordovician Yijianfang Fm. in the Tahe oil-gas field corresponds with MDICE of generalized curve in the world, and peak values are generally less than 2‰ in South China, North America, South America and Europe. Positive excursion-C at the uppermost Qiaerbak Fm. and the bottom of the Lianglitag Fm. correspond with GICE. Maximum values of GICE are generally less than 3‰, although slightly greater than 3‰ in Pennsylvania (Pancost et al., 2013) and Virginia (Young et al., 2008), USA. Evidence of GICE from the uppermost Sandbian is also found in Oklahoma, USA (Rosenau et al., 2012).

Positive excursion-B between MDICE and GICE in the Tahe oil-gas field occur in the Pygodus anserinus conodont zone of Early Sandbian. The duration of this excursion in the generalized global δ¹³C_carb curve of Ordovician is small (Fig. 7), previous studies paid little attention to it. But the fluctuation range of it is no less than MDICE in Tahe oil-gas field. This excursion are currently not officially named, thereby it named Early Sandbian Isotope Carbon Excursion (ESICE) in this paper. In some areas of Middle-Upper Ordovician successive deposition, such as in the Honghuayuan and Huangnitang in South China (Munnecke et al., 2011), small positive excursions occurred in the bottom of Sandbian. However, this Sandbian excursion is not found in drill cores and outcrops of Baltoscandia (Ainsaar et al., 2010). The global correlation ability of ESICE still need further study. SAICE, also called Spechts Ferry Excursion, in Late Sandbian closed to GICE was not found in Tahe oilfield (Figs 6 and 7). Whether it can be globally correlated or notis controversial (Sial et al, 2013).

Chen et al. (2012) considered the black shale of Saergan Fm. in Kalpin Dawangou outcrop shown Fig. 1a spans Middle Darriwilian to Early Sandbian based on graptolite. According to Fig. 7, MDICE in Tahe oil-gas field is corresponding to the δ¹³C_carb positive excursion in the bottom of Saergan Fm. But ESICE is not clearly presented in the Saergan Fm., and the reason may be inadequate sampling. ESICE was also found in drill cores from western area of Tabei Uplift (Bao et al., 2006). The positive excursions of Qilang Fm. in Kalpin outcrop correspond to GICE (Fig. 7).

According to δ¹³C_carb curves from drill cores and cuttings (Zhang et al., 2014), the positive excursion in the Lianglitag Fm. of Bachu uplift and Tazhong uplift is corresponding to GICE.

4.3 Positive Carbon Isotope Excursions and Source Rocks

According to outcrops and drilling data from the Tarim Basin, source rocks consist of the Middle–Lower Ordovician Heituwa Fm. in eastern Tarim Basin (Fig. 1a), Middle–Upper Ordovician Saergan Fm. in the Kalpin outcrop, and Upper Ordovician Lianglitag Fm. in the Tazhong and Tabei uplifts (Zhao et al., 2008; Gao et al, 2007; Zhang et al., 2006; Wang, 2000).

In Kalpin area, the sedimentary facies in the Middle Darriwilian changed from slope to basin (Chen et al., 2012), and black shales of Lower–Middle Saergan Fm. were deposited in deep water, anoxic and starved sedimentary environment (Wang et al., 2008). With the global transgression event of the end of Darriwilian-Early Sandbian (Thompson et al., 2012; Hu et al., 2010; Munnecke et al., 2010; Wang et al., 2008), the black shales of Upper Saergan Fm. were deposited in basin. Saergan Fm. black shales have good corresponding relationships of chronology with MDICE and ESICE.

The Upper Ordovician Lianglitag Fm. source rocks distributed in the algal mound and depression sedimentary units have been confirmed by drillings in the Tazhong and Tabei uplifts (Zhao et al., 2008; Gao et al., 2007), and correspond to the GICE, but these source rocks are lower in organic matter abundance than Saergan source rocks (Wang et al., 2008; Zhao et al., 2008). The Shuntuoguole low uplift, between the Tabei and Tazhong uplifts, and the Awat fault depression were basin during sedimentation period of Lianglitag Fm. (Fig. 1a) and may be favorable source rock regions.

There are no published carbon isotope data for outcrops and drill cores in the Heituwa Fm. source rocks, eastern Tarim Basin, and well cuttings data showed negative excursions (Zhang et al., 2006). Chen et al. (2012) believes proposed that the geologic age of the Heituwa Fm. in Kuruktag outcrops are from Middle Tremadocian or Later Floian to Early Darriwilian.

4.4 Positive Carbon Isotope Excursions and Paleoclimate

At present, most scholars have affirmed the development of the disastrous Hirnantian glaciation in the end of Ordovician. But regarding the turnover from greenhouse of Early Ordovician to the End-Ordovician glaciation, there are debates and various hypotheses about the timing period the paleoclimate began to cooling obviously (Rosenau et al., 2012; Munnecke et al., 2010; Trotter et al., 2008; Cocks, 2007; Saltzman and Young, 2005).

The Ordovician oxygen isotope curves constructed on 182 brachiopods and 179 conodont apatites samples respectively (Trotter et al., 2008; Shields et al., 2003), there are some differences between them, but the overall trends gradually became heavier reflecting the climate began to cool from the Early Ordovician. Paleoclimate remained stable during Darriwilian-Katian, and global glaciations occurred in the End of Ordovician. Moreover, in addition to the remarkable positive excursion on HICE, there were lack abrupt fluctuations in GICE and MDICE (Trotter et al., 2008; Shields et al., 2003). Obvious drawdown of pCO₂ in GICE, as verified by paired δ¹³C_carb and δ¹⁸O_org (Young et al., 2008), reflects that the global cooling began in the Early Katian. Rosenau et al. (2012) documented the positive isotope oxygen excursion in the lower GICE from conodont apatites, considered a short-lived cooling event within the lower GICE, and returned to greenhouse in the upper GICE. According to the relationships of MDICE, GOBE and meteorite shower, Ainsaar et al. (2010) considered the onset of cooler climate coincides with MDICE.

In the uppermost Middle Ordovician, the large carbonate platform of Cambrian–Middle Ordovician was drawned in the transgressive event worldwide, and conodont fauna of Tarim changed from the North American Midcontinent type to the North Atlantic type, indicating paleoclimate have changed (Wang et al., 2009). The conodont total diversity of Tarim Basin reached peaks in the Late Darriwilian (Dw3) and the Late Sandbian (Sa2)(Wu et al., 2014), and decreased in the Early Sandbian (Sa1), and decreased more in Katian (Ka1, Ka2 and Ka3). The decrease of conodont total diversity in Sa1 and Ka1 correspond to ESICE and GICE respectively, and may be caused by icehouse.

Reconstruction of paleoclimate evolution using δ¹⁸O_carb is a debatable point (Sial et al., 2013; Munnecke et al., 2010). GICE is commonly interpreted to reflect enhanced burial of organic carbon on a global scale, pCO₂ decreased below a critical threshold for ice sheet grow, indicating GICE is icehouse period (Sial et al., 2013; Rosenau et al., 2012; Zhang et al., 2010; Young et al., 2008; Saltzman, 2005; Saltzman and Young, 2005). Larger δ¹⁸O positive excursion on organic matter and carbonate usually occurred in GICE. Therefore, ESICE and GICE in Tahe oil-gas field may indicate cooling climate, accompanied with obvious δ¹⁸O_carb positive excursion compared with MDICE. Paleoclimate dramatically changed in Sandbian (Sial et al., 2013), ESICE may be the first small-scale glaciation event evolved from greenhouse of Early–Middle Ordovician to Hirnantian glaciation.

5. CONCLUSIONS

1) Three δ¹³C_carb positive excursions without direct relationship to sea level changes occurred from Darriwilian to Early Katian in Tahe oil-gas field, northern Tarim Basin, and MDICE and GICE can be globally correlated. Positive excursion within Pygodus anserinus conodont zone is developed in Early Sandbian, and named Early Sandbian Isotope Carbon Excursion (ESICE) in this paper. Because the range of this excursion in the generalized δ¹³C_carb curve of the world is small, previous studies paid little attention to it, and the global correlation ability of ESICE still need further study.

2) The Saergan Fm. source rocks of Middle–Upper Ordovician in Kalpin Dawangou outcrop are in accordance with the MDICE and ESICE, whereas GICE corresponds to the source rocks of Lianglitag Fm. in basin, indicating abundant organic carbon burial is an important factor in genesis of δ¹³C_carb positive excursions.

3) Change of sedimentary facies, δ¹⁸O_carb positive excursion, conodont fauna turnover and conodont total diversity decrease indicated that the obvious icehouse of Ordovician may first occur at the time of ESICE.