Geochemical characteristics and organic matter enrichment mechanism of black shale in the Upper Triassic Xujiahe Formation in the Sichuan basin: Implications for paleoweathering, provenance and tectonic setting

Marine and Petroleum Geology 109 (2019) 698–716

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Research paper

Geochemical characteristics and organic matter enrichment mechanism of black shale in the Upper Triassic Xujiahe Formation in the Sichuan basin: Implications for paleoweathering, provenance and tectonic setting

T

Tao Denga, Yong Lia,∗, Zhengjiang Wangb, Qian Yub, Shunli Donga, Liang Yana, Wenchao Hua, Bin Chena a b

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China Chengdu Center, China Geological Survey, Chengdu 610081, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Shale gas Source rock enrichment Chemical weathering Depositional environment Provenance Xujiahe formation black shale Sichuan basin

The first, third and fifth members (henceforth referred to as T3x1, T3x3 and T3x5, respectively) of the Upper Triassic coal-bearing Xujiahe Formation black shale are among the most significant hydrocarbon source rocks in the Sichuan basin. Here, we present geochemical data for the Upper Triassic black shales from core from Well LD-1 to determine their paleoenvironmental conditions, paleoweathering, provenance transitions and tectonic setting. The V/(V + Ni) vs. U/Th, V/Cr vs. U/Th and Ni/Co vs. U/Th bivariate plots and the TFe-TOC-TS (total Fe-total organic carbon-total Sulphur) ternary diagram indicate that the synsedimentary redox regime of almost all the shale samples was oxidizing. Ba/Al, Sr/Al and P/Ti data combined with quantitative biogenic Ba data indicate that moderate-high primary paleoproductivity levels prevailed during deposition. The Sr/Cu ratio and C-value combined with the sedimentary features are indicative of warm-humid climate conditions. The Ti/Al deposition rate proxy and decompacted sedimentation rate are positively correlated with the total organic carbon (TOC) content. According to our multiproxy approach, black shale development in the Xujiahe Formation was mainly controlled by the primary productivity level, foreland basin setting, high tectonic subsidence and sedimentation rate and exhibited a limited correlation with water column redox conditions. Through the compilation and calculation of various weathering indices for the Xujiahe black shale, we suggest that caution must be taken when inferring paleoclimate characteristics based on the chemical index of alteration (CIA) because of the influence of multiple nonweathering factors. Provenance-sensitive elemental ratios (Th/Sc vs. Zr/Sc and Co/Th vs. La/Sc) indicate that the clastic contribution to Xujiahe black shales was of a predominantly felsic character. The black shales exhibit a transition from T3x1 to T3x3 to T3x5, whereby T3x1 was primarily sourced from the Proterozoic to early Paleozoic strata of the Qinling orogeny, T3x3 was primarily sourced from the Neoproterozoic complex in the Longmen Shan, and T3x5 was primarily sourced from the Songpan-Ganzi flysch strata, which contributed recycled material because of folding and strong southeastward thrusting.

1. Introduction Exploration for shale gas, which is a significant unconventional energy source, has become a primary focus of exploration and production companies in the last decade or so (Bohacs et al., 2005; Zou et al., 2016; Crombez et al., 2017). The Upper Triassic Xujiahe Formation in the Sichuan basin is enriched in both tight sandstone and shale gas (Yue et al., 2018). Previous work has focused on study of the tight sandstone gas, though the coal-bearing Xujiahe Formation black shale is known to consist of hydrocarbon source layers (Qin et al., 2018; Yue et al., 2018). Organic-rich sediments of the Upper Triassic Xujiahe ∗

Formation in the Sichuan basin are mainly present in the first member, the third member and the fifth member and include black mudstone, shale and thin coal seams of shallow marine facies, lacustrine facies, and delta facies (Dai et al., 2012). Guo et al. (2015) studied the shale gas within the fifth member of the Xujiahe Formation (T3x5) in the Western Sichuan basin, which is currently regarded as the most prolific emerging unconventional gas play in China. Huang et al. (2015) combined drilling data and the test results of a large number of shale samples collected from the Upper Triassic Xujiahe Formation and concluded that members T3x1, T3x3 and T3x5 satisfy the basic conditions for the generation of shale gas. However, previous studies have

Corresponding author. Chengdu University of Technology, No. 1, East 3 Road, ErXian Bridge, Chenghua District, Chengdu, Sichuan Province, China. . E-mail address: [email protected] (Y. Li).

https://doi.org/10.1016/j.marpetgeo.2019.06.057 Received 9 November 2018; Received in revised form 27 June 2019; Accepted 28 June 2019 Available online 29 June 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

Marine and Petroleum Geology 109 (2019) 698–716

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Shan. The Qiyao Mountain fault is the eastern boundary, the Emei-Wa Shan fault is the southern boundary, and the Chengkou fault is the northern boundary (Liu et al., 2018). The western Sichuan low uplift basin is a foreland basin that formed in the Late Indosinian and features an Upper Triassic series up to 2000–4000 m thick. The basal boundary of the Upper Triassic series shows a change from the passive basin margin in the West Yangtze Craton to the foreland basin (Li et al., 2014b) (Fig. 1c). These sediments overlie the Jurassic and Cenozoic sedimentary layers and were buried deepest in the Sichuan basin (Li et al., 2013, 2014a). The Upper Triassic stratigraphy in the Longmen Shan foreland basin has been divided into the basal Maantang Formation (T3m), the Xiaotangzi Formation (T3xt) above, and the Xujiahe Formation (T3x) on top (Fig. 1c); furthermore, the Xujiahe Formation consists of five members, labeled T3x2, T3x3, T3x4, T3x5 and T3x6 (which are basically completely missing in the western Sichuan depression). Note that in Chinese lithostratigraphy, the Xiaotangzi Formation (T3xt) is the equivalent to the first member of the Xujiahe Formation (T3x1) (see Fig. 1c). These members consist of interlayered sandstone- and mudstone-dominated deposits of fluvial, lacustrine and swamp facies (Li et al., 2003). Sediments from the first (T3x1), third (T3x3) and fifth (T3x5) members are dominated by black shale and mudstone interbedded with coal, the sediments and organic material of which were deposited in a marine or shallow lacustrine setting (Hu et al., 2012; Yue et al., 2018). Well LD-1 (Fig. 1b) is a key shale gas survey well in the southern segment of the Longmen Shan foreland basin. The western Sichuan low

not performed systematic evaluations of the formation mechanism of high-quality black shale in the Xujiahe Formation including estimation of fine-grained sediment provenance, tectonic setting, paleoweathering or depositional environment. Well LD-1 is a key shale gas survey well in the southern segment of the Sichuan basin and has the advantage of being the only well with a full core of the Upper Triassic Xujiahe Formation in the southwest Sichuan basin at present. Using this core, we integrate variations in multiple geochemical proxies related to paleo-redox conditions, paleoproductivity, paleoclimatic conditions, and clastic sediment flux with a stratigraphic framework to identify the principal mechanisms of organic matter accumulation in different shale members of the Xujiahe Formation. In addition, we utilize major and trace element contents to investigate sedimentary provenance shifts, tectonic setting, paleoweathering and multiple influential nonpaleoclimatic factors. The results of this study provide insight into the shale reservoir formation mechanisms and features for the Xujiahe Formation in the Sichuan basin.

2. Geological setting The Sichuan basin is a large rhombic NE—SW-striking tectono-sedimentary basin located in the northwestern Yangtze Platform (Fig. 1a; Fig. 1b) (Li et al., 2003). In the west, it is overthrusted by the nappe belt of the Longmen Shan and the Songpan-Ganzi fold belt, and in the north, it is adjacent to the overthrust nappe belt of the Micang Shan-Daba

Fig. 1. Regional tectonics (a), the generalized geological map of the surface in the area of the studied well (b) and the generalized stratigraphy (not to scale) of the Upper Triassic Xujiahe Formation in the Sichuan basin (c). F1: Wenchuan-Maoxian Fault; F2: Yingxiu-Beichuan Fault; F3: Pengxian-Guanxian Fault; F4: Dayi Fault; F5: Pujiang-Xinjing Fault; F6: Longquan Shan Fault; A1: Wuzhongshan Anticline; A2: Pingluoba Anticline; A3: Zhangjiaping Anticline; A4: Lianhuashan Anticline; A5: Hanwangchang Anticline; A6: Daxinchang Anticline; A7: Xiongpo Anticline; A8: Suamtou Anticline; A9: Longquan Shan Anticline. 699

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Fused glass disks were produced by adding Spectromelt® and LiF to the sample powder and subsequent melting in platinum crucibles. Loss on ignition (LOI) was determined gravimetrically by stepwise heating to 1000 °C. Analyses were conducted on a PANalytical Axios X-ray fluorescence spectrometer at ALS Chemex (Guangzhou, China). For all major elements, the analytical accuracy is better than 5% and uncertainty is less than 5%. The detection limit is 0.01% for the major element oxides. Solution inductively coupled plasma mass spectrometry (ICP–MS) was performed for the trace element geochemical analysis of the 36 samples. The sample powder (∼50 mg per sample) was dissolved in a PicoTrace® acid digestion system. The analytical procedure involved an initial prereaction with 2 ml HNO3 at 50 °C overnight. After cooling to room temperature, the samples were treated with 3 ml HF and 3 ml HClO4 and then heated to 150 °C for 8 h during the first pressure phase. For evaporation, the digestion vessels were heated to 180 °C for 16 h. After cooling, 10 ml H2O (double deionized), 2 ml HNO3 and 0.5 ml HCl were added to the samples for the final pressure phase, and the solution was reheated to 150 °C for 4 h. An internal standard (100 μl) for ICP–MS analysis was added to the solution after final cooling. The detection limit is 0.01% for the trace elements and rare earth elements (REEs). HREE depletion/enrichment was calculated using the LaN/YbN ratio. The subscript N denotes PAAS-normalized values (Taylor and McLennan, 1985). Also, we performed subsidence analysis on the Upper Triassic to Quaternary succession in accordance with drilling stratum data, biostratigraphy and approximate paleowater depth data (see supplement spreadsheet). Our approach to backstripping utilizes the one-dimensional local isostatic method of Steckler and Watts (1980).

Table 1 Sample ID, stratigraphic position, lithology, depth, TOC values and two trace elements compositions (in ppm). Sample ID

Strata

Lithology

Depth (m)

TOC (wt.%)

B (ppm)

Ga （ppm）

LD001 LD002 LD004 LD010 LD018 LD025 LD030 LD032 LD037 LD038 LD048 LD060

T3x1 T3x1 T3x1 T3x1 T3x1 T3x1 T3x1 T3x1 T3x1 T3x1 T3x1 T3x1

1716 1709.9 1694.2 1688.1 1681.3 1663.8 1654.3 1649.43 1636.28 1633.9 1604.75 1549.47

1.19 1.01 0.63 0.19 0.91 1.04 0.64 0.56 2.72 3.01 1.14 27.4

72 83 79 52 63 63 116 81 146 63 120 34

19.6 26.3 20.3 10.6 22.0 21.8 31.0 24.4 33.4 28.3 28.9 42.2

LD090 LD102 LD103 LD116 LD118 LD120

T3x3 T3x3 T3x3 T3x3 T3x3 T3x3

1457.18 1449.55 1447.65 1432.03 1423 1420.81

1.11 1.30 1.03 1.03 0.90 0.91

71 55 101 58 91 65

28.4 27.4 28.3 20.7 27.6 21.2

LD129 LD137 LD139 LD188 LD201 LD199 LD207 LD208 LD209 LD210 LD211 LD212 LD213 LD223

T3x3 T3x3 T3x3 T3x3 T3x3 T3x3 T3x5 T3x5 T3x5 T3x5 T3x5 T3x5 T3x5 T3x5

1412.16 1405.5 1401.58 1373.5 1354.48 1352.46 1300.86 1294 1290 1280 1268.02 1255.4 1247.60 1239.5

1.10 1.15 1.70 1.30 5.40 0.57 1.39 0.49 0.38 0.68 0.39 1.17 0.83 12.90

83 56 71 110 89 75 105 101 118 88 52 74 58 87

22.1 27.3 26.0 28.3 28.4 28.2 25.7 20.6 28.4 26.7 28.1 25.0 21.7 30.5

LD224 LD227 LD228 LD233

T3x5 T3x5 T3x5 T3x5

Black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Coal seambearing black shale Black shale Black shale Black shale Black shale Black shale Coal seambearing black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Black shale Coal seambearing black shale Thin coal seam Thin coal seam Black shale Black shale

1235.76 1233.05 1207.15 1182

8.84 35.4 1.02 0.90

89 70 68 85

28.4 25.3 20.9 30.1

4. Results 4.1. Whole-rock geochemistry Immobile trace element compositions and in particular, rare earth element (REE) abundances of sediments are considered good indicators of source rock source area chemistry, since these elements are littlefractionated by sedimentary processes, diagenesis and low-grade metamorphism, and because the REE abundances of source rock source areas are little affected by alteration, as most REEs remain in more resistant accessory phases (Taylor and McLennan, 1985, 1995; McLennan, 1989). The chondrite-normalized REEs of the analyzed shale samples in different members of the Xujiahe Formation show light REE enrichment, flat heavy REE distributions, and weak negative Eu anomalies, all of which are similar to the characteristics of the upper continental crust (UCC) and Post-Archean Australian Shale (PAAS) (Taylor and McLennan, 1985; Hu and Gao, 2008) (Table 2; Fig. 2a), indicating that they originated from a weakly differentiated silicic source. The uniformity of the REE patterns among most of the samples also reflects the recycled nature of the source rocks. Slight differences among the REE patterns (Fig. 2a) of the different shale members probably reflect changes in the source-area composition as well as variations in the depositional process, such as hydrodynamic mineral sorting, chemical weathering and/or sediment recycling (McLennan, 1989). In the primitive mantle-normalized diagram, the selected trace element concentrations of the samples are depleted in the high field strength elements (HFSEs) Ti, Nb and P (Table 3; Fig. 2b). Because HFSEs usually exhibit slight positive anomalies, these results suggest felsic sediment sources prevailed over mafic sediment sources (Taylor and McLennan, 1995). Furthermore, the enrichment of large-ion lithophile elements (LILEs) Rb, Th and U (Table 3; Fig. 2b) resembles the characteristics of the UCC (Taylor and McLennan, 1985).

uplift tectonic belt is the region with the greatest thickness of source rocks in the Xujiahe Formation, the highest hydrocarbon generation potential and the highest quantities of hydrocarbon generation throughout the basin, thus providing a rich foundation for shale gas reservoir formation. 3. Samples and methods For this study, a set of 36 samples was collected along the 534-mlong core, with 12 samples chosen to represent each of the three different shale members of the Xujiahe Formation (Table 1). Our multiproxy assessment of these samples includes an analysis of the preserved organic matter via Carbon Sulphur Determinator Analyzer (an Elementar® Vario MACRO CHNS elemental analyzer) and an analysis of the major and trace elements. The total organic carbon (TOC) content by weight was measured by an organic element analyzer (Carbo-Erba model EA1110, Italy) after leaching the bulk samples with 1 N HCl acid to remove calcium carbonate at ALS Chemex (Guangzhou, China). Replicate analyses of standards of crystine and sulfanilamide and unknown samples yielded a mean precision of approximately 0.5% for organic carbon. For the elemental oxide concentration analysis, sample powders were mixed with dry lithium tetraborate and borate and fused into glass beads. Whole-rock geochemical analyses were carried out using a PANalytical Axios Advanced sequential X-ray spectrometer.

4.2. Paleoredox conditions Redox conditions can be deciphered based onTh/U, V/Cr, V/ (V + Ni) and Ni/Co ratios (e.g., Jones and Manning, 1994; Rimmer, 700

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Table 2 Rare earth element (REE) compositions (in ppm) for core samples from Well LD-1. ID

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

LD001 LD002 LD004 LD010 LD018 LD025 LD030 LD032 LD037 LD038 LD048 LD060 LD090 LD102 LD103 LD116 LD118 LD120 LD129 LD137 LD139 LD188 LD201 LD199 LD207 LD208 LD209 LD210 LD211 LD212 LD213 LD223 LD224 LD227 LD228 LD233

35.9 46.5 41.8 25.7 43.5 43.1 44.3 47.8 62.6 49.0 52.7 120.5 52.6 48.5 49.1 47.7 46.2 46.7 46.0 49.6 49.6 49.4 50.6 51.7 58.6 46.6 47.2 37.2 60.9 45.4 37.7 52.2 41.1 31.1 44.9 43.5

71.4 91.4 82.3 51.9 85.9 86.5 88.5 93.1 122.0 97.4 104.0 236 103.5 96.2 96.5 93.0 89.4 92.2 90.7 96.8 96.2 95.5 98.9 101.5 116.0 89.0 91.3 74.3 117.0 88.7 73.2 102.5 76.1 57.9 90.2 87.0

9.23 11.45 10.50 7.08 10.45 10.50 10.90 11.05 14.95 11.35 12.40 27.0 12.05 11.05 11.20 10.70 10.30 10.45 10.35 10.90 10.85 10.90 11.25 11.35 13.65 10.55 10.80 8.71 13.15 9.81 8.18 11.10 7.60 6.09 9.19 8.97

33.0 40.2 36.1 26.9 36.0 37.2 38.9 37.2 54.3 39.2 43.7 90.5 42.1 38.8 39.4 37.3 36.3 36.5 36.6 38.2 37.9 38.8 39.7 39.3 46.4 36.4 37.6 31.0 44.5 33.9 30.1 37.2 22.7 22.0 33.8 32.9

6.77 7.87 6.92 6.97 7.18 7.79 7.81 6.59 11.00 7.26 8.63 17.90 8.26 7.81 7.99 7.19 7.03 7.23 7.42 7.51 7.52 8.02 7.57 7.22 8.70 6.93 7.13 6.22 7.33 6.14 6.89 6.38 2.95 3.93 6.39 6.12

1.39 1.58 1.33 1.61 1.45 1.63 1.45 1.21 2.03 1.34 1.67 0.91 1.71 1.45 1.63 1.48 1.44 1.45 1.46 1.54 1.58 1.59 1.43 1.25 1.59 1.37 1.39 1.15 1.19 1.16 1.63 1.16 0.43 0.79 1.28 1.12

5.93 6.78 5.40 9.63 5.92 6.36 6.25 5.16 7.88 5.17 6.98 13.40 7.10 6.47 6.79 5.87 5.90 5.93 6.12 6.14 6.30 6.65 5.74 5.55 6.42 5.44 5.56 5.26 4.41 4.89 7.28 5.03 2.09 2.77 5.58 4.95

0.95 1.06 0.87 1.55 0.95 1.00 1.02 0.78 1.12 0.83 1.09 2.29 1.11 1.04 1.10 0.94 0.96 0.93 0.98 0.98 1.04 1.04 0.91 0.87 0.97 0.87 0.88 0.88 0.72 0.79 1.11 0.87 0.40 0.52 0.80 0.71

5.17 6.01 5.04 7.51 5.39 5.63 5.98 4.47 5.93 5.30 6.14 13.90 6.26 5.75 6.02 5.37 5.69 5.13 5.76 5.83 6.01 6.04 5.41 5.43 5.46 5.02 5.27 5.09 4.49 4.72 6.13 5.38 2.92 3.28 5.01 4.79

1.00 1.18 1.05 1.23 1.08 1.10 1.22 0.90 1.21 1.12 1.19 2.88 1.25 1.19 1.26 1.09 1.15 1.08 1.15 1.14 1.20 1.23 1.13 1.12 1.07 1.04 1.09 1.02 0.98 0.98 1.20 1.12 0.69 0.69 1.04 0.97

2.97 3.34 3.02 3.05 3.11 3.07 3.72 2.73 3.44 3.43 3.45 8.38 3.45 3.49 3.57 3.25 3.38 3.03 3.33 3.48 3.52 3.48 3.44 3.35 3.26 3.01 3.19 2.95 3.01 2.91 3.51 3.38 2.43 1.92 3.04 2.75

0.42 0.51 0.46 0.37 0.46 0.48 0.57 0.41 0.54 0.54 0.54 1.31 0.51 0.52 0.55 0.48 0.51 0.45 0.50 0.52 0.54 0.54 0.52 0.51 0.51 0.44 0.47 0.45 0.47 0.45 0.50 0.50 0.40 0.28 0.44 0.40

2.61 3.23 2.81 2.05 2.85 3.00 3.55 2.66 3.36 3.40 3.40 7.87 3.21 3.32 3.43 3.01 3.18 2.96 3.19 3.30 3.38 3.43 3.31 3.37 3.18 2.83 3.07 2.88 3.15 2.87 3.24 3.23 2.74 1.86 2.91 2.71

0.40 0.47 0.45 0.28 0.48 0.47 0.52 0.41 0.51 0.52 0.51 1.18 0.50 0.50 0.54 0.47 0.49 0.46 0.51 0.53 0.52 0.53 0.51 0.51 0.48 0.43 0.47 0.43 0.49 0.45 0.51 0.51 0.47 0.29 0.45 0.42

Additionally, we used paleontological and sedimentological evidence to constrain the oxygen level in the bottom waters during deposition of the Xujiahe Formation. T3x3 contains a mass of benthic bivalve fossils concentrated in a black shale bedding plane (Fig. 4a), abundant vertical biological burrows (Fig. 4b), bioturbation structures (Fig. 4c) and plant leaf fossils (Fig. 4d). The same features are also common in T3x1 (Fig. 4e, g, h) and T3x5 (Fig. 4f). These sedimentary features are consistent with the results of geochemical characterization

2004; Rimmer et al., 2004; Tribovillard et al., 2006; Racka et al., 2010; Wójcik-Tabol, 2015). We used U/Th ratio in combination with other trace metal proxies for redox conditions, such as V/(V + Ni), V/Cr and Ni/Co ratios, to constrain the redox state of the Xujiahe Formation (Hatch and Leventhal, 1992; Mouro et al., 2017). In Fig. 3a–c,n all but a few samples plot in the oxic area. On the ternary diagram of TFe-TOCTS, all samples plot in the extreme oxic area, which is consistent with the redox-sensitive trace metal proxy results.

Fig. 2. Chondrite-normalized REE patterns (a) and primitive mantle-normalized spider diagrams (b) for Xujiahe Formation samples. 701

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Table 3 Selected trace element compositions (in ppm) for core samples from Well LD-1. ID

Rb

Ba

Th

U

Ta

Nb

Sr

Zr

Hf

Y

Zn

Sc

Co

Cu

Mo

LD001 LD002 LD004 LD010 LD018 LD025 LD030 LD032 LD037 LD038 LD048 LD060 LD090 LD102 LD103 LD116 LD118 LD120 LD129 LD137 LD139 LD188 LD201 LD199 LD207 LD208 LD209 LD210 LD211 LD212 LD213 LD223 LD224 LD227 LD228 LD233

125.5 179.5 126.0 72.9 133.0 133.5 201 167.0 235 182.0 209 14.9 206 198.5 189.5 113.0 181.5 110.5 120.5 173.0 184.0 178.5 182.5 177.5 234 171.0 172.0 140.0 175.5 196.5 149.0 196.5 138.0 49.1 187.5 143.5

1125 1125 375 295 390 411 634 499 651 533 592 125.5 705 734 767 545 844 571 594 760 649 783 664 753 732 723 740 608 764 665 636 567 389 183.0 589 448

12.90 17.00 15.65 7.18 15.40 15.35 19.40 17.05 19.30 21.6 19.75 61.6 19.15 19.55 20.1 15.40 16.75 14.70 16.25 18.25 18.90 18.55 19.40 19.35 20.9 18.95 17.05 13.65 19.15 17.40 13.25 18.50 17.25 26.5 15.85 14.85

2.96 3.65 3.44 1.98 3.45 3.73 4.29 3.36 4.87 4.78 3.90 12.40 3.94 4.30 4.17 3.58 3.68 3.27 3.62 3.89 3.81 3.98 7.05 4.48 4.52 3.62 3.48 3.07 4.18 3.57 3.27 5.48 4.78 5.64 3.55 3.21

0.74 1.01 1.02 0.35 0.97 0.99 1.14 1.03 1.18 1.22 1.22 3.42 1.12 1.07 1.11 0.93 1.06 0.97 1.01 1.13 1.14 1.12 1.15 1.17 1.24 1.14 1.15 0.93 1.25 0.96 0.67 1.21 1.69 2.70 1.00 0.90

12.0 15.8 16.4 6.9 15.8 15.8 16.5 15.3 17.0 18.3 17.3 42.0 17.4 17.1 17.3 17.0 16.5 17.3 18.2 17.9 18.0 17.4 16.0 18.9 17.9 16.1 17.0 14.0 20.2 15.1 11.9 16.0 23.2 36.6 16.2 15.1

99.6 116.5 132.0 76.9 114.5 134.5 157.0 193.5 207 172.0 188.0 127.5 181.5 176.0 195.5 153.5 180.5 166.0 160.5 179.0 164.0 187.0 180.5 162.0 211 202 226 262 177.0 151.5 165.0 119.5 108.5 89.9 120.0 99.8

123 163 227 73 218 192 174 153 182 256 197 491 171 166 174 241 176 252 241 182 201 176 171 208 168 162 189 165 244 171 125 161 262 277 234 244

3.8 4.7 6.5 2.0 6.1 5.4 5.2 4.3 5.1 6.8 5.7 14.2 5.1 4.8 4.9 6.7 5.0 6.5 6.7 5.4 5.7 5.1 4.9 5.9 5.0 4.8 5.2 4.6 6.8 4.8 3.3 4.8 7.3 7.4 5.8 6.3

25.8 30.0 25.4 34.3 27.9 29.3 31.4 24.4 30.6 29.1 30.9 75.5 32.7 32.3 33.5 30.4 30.3 29.3 32.2 32.3 34.3 32.7 30.6 29.8 28.9 27.6 29.5 27.7 25.3 26.3 35.5 29.8 17.1 18.4 28.1 25.6

60 147 68 44 125 64 68 100 123 104 134 35 107 112 115 74 90 86 112 98 131 110 101 90 90 78 91 78 85 128 60 64 14 12 87 140

18.8 17.5 12.8 15.7 16.8 14.5 18.2 14.7 20.4 16.6 18.3 12.1 19.3 19.3 19.0 14.0 20.3 14.4 16.3 17.9 17.8 20.2 20.4 19.4 18.0 15.7 17.4 15.0 19.3 16.9 20.8 18.3 14.6 13.1 16.0 15.1

17.0 19.3 13.7 18.9 14.7 9.6 19.1 10.4 27.3 22.8 14.2 4.8 17.4 19.6 21.1 13.4 18.0 16.2 21.5 26.8 18.5 19.7 22.6 24.7 20.4 14.1 14.3 13.9 19.5 21.2 17.2 13.1 4.3 2.2 21.6 21.5

29.4 30.9 22.7 17.3 35.3 29.9 43.3 31.3 47.8 45.9 33.8 42.1 35.7 47.1 44.6 25.4 42.6 28.6 35.9 38.0 41.0 39.6 58.8 39.4 32.7 39.8 39.0 31.7 44.8 39.2 33.8 27.6 25.7 9.9 37.8 31.1

0.85 0.54 0.65 0.32 1.16 0.40 0.85 0.34 1.66 0.80 0.57 1.18 0.90 1.18 0.80 0.83 1.03 0.92 0.98 0.90 1.21 0.91 1.01 1.13 1.02 0.50 0.71 1.33 0.75 0.51 0.84 1.08 0.74 1.21 0.90 0.77

Fig. 3. Bivariate plots of trace element ratios used as proxies to infer paleoredox conditions: (a) U/Th vs. V/(V + Ni); (b) U/Th vs. V/Cr; and (c) U/Th vs. Ni/Co. The ternary diagram of TFe-TOC-TS (d) also represents the paleoredox condition (base map sourced from (Ross and Bustin, 2009)).

702

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Fig. 4. Representative photographs of the core of the Xujiahe Formation from Well LD-1 in the Sichuan basin.

generally considered to range between 1000 and 5000 μg/g in high productivity regions of the modern equatorial Pacific (Murray and Leinen, 1993; Algeo et al., 2011). The three ratios examined in this study, i.e., the Ba/Al, Sr/Al and P/Ti ratios, all covary with TOC (Fig. 5), indicating an enhanced organic productivity-model enrichment mechanism. In T3x1, the mean Babio value is 562 μg/g (the bottom two samples exceed 1000 μg/g and reach a high productivity level) (Table 4, Fig. 5); in T3x3, the mean Babio is 697 μg/g (the highest observed value); and in T3x5, the mean Babio value is 587 μg/g. These values are indicative of a moderate level of paleoproductivity (200–1000 μg/g). The strong positive correlation between moderate productivity and TOC suggests that the enrichment of organic matter was mainly controlled by the primary productivity of surface water.

and our approximate water depth estimation, and both suggest oxic conditions and proximal-source characteristics in all the black shales of the different members of the Xujiahe Formation. All facies contain varying degrees of bioturbation and benthic bivalve shell fragments, indicating that long-lived bottom water anoxia was not the main factor controlling the organic richness of this unit. 4.3. Paleoproductivity Many geochemical proxies have been used to evaluate the role of productivity in the accumulation of organic-rich rocks (Brumsack, 2006; Shen et al., 2015). However, due to redox conditions, detritus input and diagenetic alteration effects, we cannot use biolimiting nutrient elements in a formal manner to evaluate the primary productivity level. Because no single productivity proxy is completely reliable, we examined multiple proxies in this study, including Ba/Al (Dean et al., 1997), Sr/Al and P/Ti ratios (Ti normalization was used to remove the effects of terrestrial detritus) (Reolid et al., 2012). In addition, we calculated the primary productivity level quantitatively based on the biogenic Ba (Babio) content, which is estimated by subtracting the detrital Ba fraction from the total Ba concentration as in the following equation (Dymond et al., 1992; Algeo et al., 2011; Dong et al., 2018):

4.4. Paleosalinity Salinity affects the water column in a lacustrine or marine basin and has a marked influence on the development of source rocks. With high salinity, salinity stratification is easily formed, water body convection is restricted and a reducing bottom environment is formed; these changes are beneficial to the preservation of organic matter. Studies have shown that Ca/(Ca + Fe) ratio can be used to indicate paleosalinity qualitatively (Nelson, 1967). Another commonly used geochemical index to reconstruct paleosalinity is the Sr/Ba value, with higher values corresponding to higher paleosalinity in the corresponding sedimentary water body. Usually, a Sr/Ba value greater than 1 represents a marine environment, and a value less than 0.8 represents a freshwater

Babio = Batot − [Al × (Ba/Al)detrital] where Batot and Al are the total amounts of Ba and Al, respectively, and (Ba/Al) detrital represents the content of terrestrial detrital Ba proportion, which is equal to 0.0039 (Dong et al., 2018). The Babio content is 703

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Fig. 5. The distribution of geochemical indicators in the shale members of the Xujiahe Formation.

Alternatively, the indices may be affected by the low-salinity continental runoff sourced from the eastern Yangtze region, thereby resulting in low surface water salinities and high oxygen levels. The paleosalinity proxy of Sr/Ba ratio for T3x1 are highly correlated with the TOC (data for the Ca/(Ca + Fe) vs. TOC and Sr/Ba vs. TOC plots are not shown here), indicating that the stratified water body with high salinity was conducive to the preservation of organic matter (Liang et al., 2018), whereas the TOC in T3x3 and T3x5 is almost unrelated to the salinity of the ancient water body.

environment (Wei et al., 2018). Marine sedimentary rocks have higher B contents than those derived from freshwater environments, in contrast to Ga. Hence, higher B/Ga ratio corresponding to higher salinity. Usually, B/Ga > 5 indicates saltwater deposition, 2.5 ≤ B/Ga ≤ 5 indicates brackish water deposition, and B/Ga < 2.5 indicates freshwater deposition (Wei et al., 2018). We have not exhaustively listed the variety of paleosalinity indices in this study; instead, we use only the Ca/(Ca + Fe), Sr/Ba and B/Ga ratios to determine the paleosalinity characteristics of the first, third and fifth members of the Xujiahe Formation. The Ca/(Ca + Fe) and Sr/ Ba ratio results reveal that these units are almost entirely freshwater deposits. Only three abnormal points feature brackish/marine water characteristics (Fig. 5); however, previous research has clearly shown that the first member of the Xujiahe Formation contains marine fossils (Zheng et al., 2009). B/Ga ratio have more reliable and higher-resolution results (Fig. 5) because B and Ga are both immobile trace element. Accordingly, we interpret T3x1 to represent marine facies, while T3x3 and T3x5 represent continental freshwater lakes with local intermittent transgressions. The above two indicators of Ca/(Ca + Fe) and Sr/Ba ratios may be controlled by authigenic carbonate deposits and may therefore poorly reflect the actual conditions (Turner et al., 2019).

4.5. Paleoclimate The Sr/Cu ratio is the most commonly used geochemical indicator in paleoclimate analyses. Generally, Sr/Cu ratios between 1 and 10 indicate a warm and humid climate, and ratios greater than 10 indicate a dry and hot climate (Lerman, 1978; Jia et al., 2013). Alternatively, Zhao et al. (2007) and Cao et al. (2012) applied the C-value as an indicator of paleoclimate. The C-value is defined as follows:

C− value =

∑

(Fe + Mn + Cr + Ni + V + Co) /∑ (Ca + Mg

+ Sr + Ba + K + Na) 704

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Table 4 Concentrations of major element oxides (units in %) and biogenic Ba concentrations (units in ppm) in black shale samples from the Xujiahe Formation. ID

SiO2

Al2O3

TFe2O3

CaO

MgO

Na2O

K2O

TiO2

MnO

P2O5

LOI

Total

Babio

LD001 LD002 LD004 LD010 LD018 LD025 LD030 LD032 LD037 LD038 LD048 LD060 LD090 LD102 LD103 LD116 LD118 LD120 LD129 LD137 LD139 LD188 LD201 LD199 LD207 LD208 LD209 LD210 LD211 LD212 LD213 LD223 LD224 LD227 LD228 LD233

42.13 55.11 60.97 23.77 61.32 61.06 53.62 51.70 56.14 63.05 60.90 36.16 57.03 55.53 55.36 59.22 56.61 60.61 60.03 56.17 60.68 55.98 55.98 61.35 57.90 56.32 54.17 51.69 60.89 58.40 39.01 49.43 63.46 36.60 62.33 66.92

13.74 18.31 13.97 7.64 15.58 15.62 21.24 17.14 22.36 19.13 19.99 23.46 19.21 18.84 19.69 14.40 19.42 14.30 15.58 18.48 17.61 19.37 19.42 19.74 21.26 19.86 17.18 14.50 19.76 18.15 13.86 19.18 16.24 16.61 18.17 15.90

20.20 8.37 4.95 40.53 6.35 5.42 5.73 5.89 4.94 2.98 4.71 0.54 7.61 8.73 7.93 6.64 6.58 5.78 6.26 6.98 6.37 6.42 5.37 5.82 5.30 4.55 5.66 5.60 5.58 6.26 20.14 4.66 1.01 0.42 5.49 5.33

1.76 1.58 3.95 1.81 1.92 2.78 2.11 6.40 0.24 0.16 0.30 0.08 0.58 0.46 0.60 2.69 1.30 3.48 2.36 1.40 0.52 1.51 0.21 0.14 0.20 2.55 5.07 8.11 0.11 1.58 4.19 0.14 0.08 0.07 0.46 0.20

2.44 2.50 2.80 0.86 2.28 2.27 2.26 3.07 2.04 1.37 1.92 0.07 2.45 2.38 2.38 3.01 2.90 2.91 3.11 2.87 2.26 2.75 1.96 2.11 2.17 2.35 2.89 2.91 2.22 2.46 2.51 1.68 0.62 0.18 2.05 1.81

0.45 0.53 0.71 0.20 0.73 0.77 0.75 0.55 0.42 0.50 0.80 0.22 0.73 0.70 0.82 1.14 0.82 1.20 1.12 0.70 0.74 0.73 0.47 0.72 0.56 0.79 0.57 0.69 1.14 0.57 0.28 0.21 0.27 0.13 0.67 0.34

2.77 3.76 2.70 1.40 2.80 2.85 4.34 3.54 4.60 3.32 4.04 0.28 4.14 4.04 3.99 2.49 4.01 2.48 2.71 3.59 3.67 3.78 3.57 3.56 4.50 3.50 3.58 2.94 3.62 4.02 3.08 3.50 1.97 0.70 3.56 2.87

0.56 0.74 0.72 0.31 0.78 0.74 0.84 0.72 0.80 0.83 0.82 0.47 0.79 0.78 0.80 0.78 0.79 0.77 0.84 0.83 0.83 0.81 0.78 0.89 0.81 0.73 0.76 0.67 0.95 0.74 0.58 0.75 1.07 1.20 0.79 0.72

0.49 0.14 0.08 0.61 0.10 0.08 0.08 0.11 0.02 0.01 0.02 < 0.01 0.15 0.28 0.15 0.13 0.09 0.10 0.07 0.09 0.06 0.07 0.03 0.03 0.02 0.07 0.09 0.12 0.03 0.09 0.46 0.01 < 0.01 < 0.01 0.03 0.02

0.18 0.17 0.15 0.32 0.14 0.17 0.10 0.14 0.13 0.09 0.19 0.03 0.19 0.18 0.17 0.14 0.15 0.18 0.18 0.17 0.19 0.18 0.09 0.09 0.11 0.11 0.14 0.13 0.04 0.15 0.25 0.06 0.02 0.01 0.14 0.14

14.75 8.52 8.44 22.38 7.21 7.12 7.67 10.96 7.80 7.65 5.40 38.08 6.48 7.35 7.26 8.56 6.84 8.31 7.85 7.60 6.29 7.98 11.33 5.32 5.88 7.97 9.35 12.00 4.64 6.92 15.28 19.48 14.42 43.72 5.50 4.94

99.79 99.99 99.54 100.05 99.33 99.03 98.98 100.36 99.76 99.29 99.26 99.4 99.55 99.49 99.33 99.36 99.72 100.31 100.35 99.07 99.43 99.78 99.78 100.26 98.97 99.01 99.67 99.59 99.21 99.53 99.83 99.15 99.33 99.65 99.36 99.39

1124.96711 1124.95655 374.95831 294.98427 389.95732 410.95809 633.9494 498.9571 650.95193 532.94973 591.95094 125.46931 704.95402 733.95523 766.95237 544.95666 843.95435 570.95776 593.95303 759.95039 648.95303 782.94962 663.95193 752.94973 731.95204 722.95259 739.95281 607.95875 763.94599 664.95886 635.967 566.95446 388.93741 182.92696 588.95413 447.95985

5. Discussion

where elemental concentrations are presented in ppm. This value may be used to infer the paleoclimate from warm and humid to hot and arid based on generally accepted hypotheses suggesting that the numerators (Fe, Mn, Cr, Ni, V and Co) are enriched under moist conditions while the denominators (Ca, Mg, Sr, Ba, K and Na) are concentrated under arid conditions because saline minerals are more likely to precipitate under arid conditions (Zhao et al., 2007; Cao et al., 2012; Moradi et al., 2016). In this study, Sr/Cu ratio results indicate that T3x1, T3x3 and T3x5 were all deposited during a period of warm and humid climatic conditions with little fluctuation (Fig. 5). Based on our Sr/Cu data, representative core photographs (Fig. 4), and current literature on sporopollen analysis, clay mineral content analysis and characteristic elemental analysis (Xu et al., 2010), we suggest that the T3x1 and T3x3 depositional period were hot and humid and featured the formation of marshy deposits and massive coal seams with plant leaf fossils (Fig. 4d and g). The T3x5 temperature and humidity values were the highest throughout the entire Xujiahe Formation (Fig. 5, one and only sample plotted in hot-arid area). Based on the C-value results, most samples of the Xujiahe Formation were situated in the transitional climate area (semi-humid), and the remaining samples were situated in the humid climate area (Fig. 5). Importantly, warm, humid climates such as those indicated here likely lead to increased terrestrial run-off and delivery of nutrients to the basin, thereby favoring increased primary biologic production of organic matter.

5.1. Black shale formation mechanism in T3x1, T3x3 and T3x5 When shale is characterized by higher organic matter content relative to that of average marine shale (ca. 0.5% TOC, Arthur, 1979), it is referred to as an organic matter-rich shale or, more commonly, a black shale. Black shale is the general term used for any dark-colored, finegrained, organic matter-rich clastic sedimentary rock. According to these two definitions, all of our samples except four are organic matterrich, black shales (Table 1). Three basic models can account for the accumulation of organic matter in organic-rich sediments, i.e., black shales: (1) increases in the organic productivity (Pedersen and Calvert, 1990; Sageman et al., 2003; Tyson, 2001, 2005; Wei et al., 2012); (2) enhancement of organic matter preservation, which may be related to reducing conditions (Demaison and Moore, 1980; Arthur and Sageman, 1994; Mort et al., 2007); and (3) low sedimentation rates, which are also referred to as “preservation-type” deposits (involving minimum clastic dilution; Creaney and Passey, 1993; Tyson, 2005). The three possible mechanisms all feature complicated, nonlinear interactions (Bohacs et al., 2005; Tyson, 2005). By thoroughly synthesizing the geochemical results to quantify the paleoproductivity, paleosalinity, paleoclimate and detrital input flux, we propose depositional models for the T3x1, T3x3 and T3x5 black shales. Late Triassic megamonsoon conditions prevailed in western equatorial Pangea and the entire circular Paleo-Tethys Ocean (Parrish, 1993; Nordt et al., 2015). During this period, the Sichuan basin was situated in the eastern part of the Paleo-Tethys, and maximum monsoon strength occurred in the basin (Shi et al., 2017). Therefore, in the T3x1 705

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Fig. 6. (a), Depositional model for the T3x1 black shale as an underfilled (restricted connections?) sea containing an episodic/fluctuating anoxia in photic zone, a thermocline, and salty water. Primary productivity was enhanced occasionally by offshore winddriven upwelling of cold, nutrient-rich bottom waters linked to the South China Block mid-latitude coastal margin region. (b), Depositional model for the T3x3 black shale, featuring a filled, weak-silled and semi-restricted transitional foreland basin. The brackish water column represents frequent deepwater renewal and intermittent transgression. (c), Depositional model for the T3x5 black shale, featuring an overfilled, strong-silled and strongly restricted terrestrial foreland basin with rare deepwater renewal. Moderate primary productivity promoted organic matter deposition and a high sedimentation rate promoted organic matter preservation by isolating organic matter from an oxic water column.

basin (Xie and Heller, 2009). Additionally, in the T3x1, T3x3 and T3x5 depositional phases, the tectonic subsidence magnitude and sedimentation rate are higher than those in the coarser clastic phases (Fig. 7). Therefore, we adopt the perspective of Heller et al. (1988): the onset of tectonic activity is reflected by rapid subsidence in the foreland basin and by widespread flooding and fine-grained sediments (Chen and Sharma, 2017). Hence, the T3x3 and T3x5 depositional stages corresponded to a filled and overfilled foreland basin, respectively. Importantly, for almost all the shale samples was oxidizing in this study, the burial efficiency and the dilution is therefore one of the main controls on organic matter enrichment in our case study. Tyson (2001) and Ding et al. (2015) proposed that sedimentation rate (SR) threshold is 5 cm/ka for organic matter enrichment in marine sediments and lacustrine basin, respectively. Noteworthily, they don't conduct compaction correction of thickness data, because compaction would appear to have no significant effect on the relation between TOC and SR (Ibach, 1982) and Tyson (2001)'s all material are modern marine sediment data. For this study, all of the compacted members in the Xujiahe Formation have a sedimentary rate ＜5 cm/ky, only T3x5 similar to that threshold value (original compacted sedimentary rate plots are not shown in Fig. 7). In sum, moderate primary paleoproductivity combined with a high sedimentation rate enhanced organic matter burial efficiency by isolating it from the oxic water column (Tyson, 2001; Ding et al., 2015) (Fig. 5; Fig. 6b; Fig. 6c). Meanwhile, the long-term uplift of the Longmen Shan thrust belt eventually isolated the Paleo-Tethys to the west. Furthermore, almost all of the total terrigenous macerals (vitrinite + desmocollinite) in the source rock of the Xujiahe Formation (Dai et al., 2014) imply that a major transverse foreland basin drainage

depositional stage, intense megamonsoon-related high surface productivity in the eastern margin of the Paleo-Tethys (Kiessling et al., 1999), which maybe was connected with the Sichuan basin episodically by submarine shallow sill of Longmen Shan. Meanwhile, the monsoon provided the driving force in the development of upwelling (Flügel, 2002; Zonneveld et al., 2007) and the connection with intermediate water (Parrish, 1993) (Fig. 6a). In addition, rain-triggered periodic weathering of subaerial highlands provided substantial river-runoff materials, which in turn increased the surface primary productivity. Thus, the isolation of the Sichuan basin from the Paleo-Tethys realm and the influx of freshwater favor haline stratification and the preservation of organic matter (Fig. 6a). Traditionally, coarse-grained deposits in tectonic cyclothems are an indicator of renewed tectonic activity (Davis, 1973). However, several controversies have emerged in foreland basin stratigraphy since the mid-1980s, and one key question is at the heart of these debates: Does coarse-grained detritus reflect intensified tectonic activity in the adjacent fold-thrust belt, or is it a signature of postthrusting isostatic adjustment (e.g., Burbank et al., 1988; Heller et al., 1988; GarciaCastellanos, 2002; Horton et al., 2004)? Because foreland basin strata are controlled by multiple factors, such as the size of the antecedent drainage and the prevalence of resistant rocks in the source area (Burbank et al., 1988), this issue has had no satisfactory solution. However, here we propose a possible solution. In this study, we performed a subsidence analysis on the Upper Triassic to Quaternary succession according to the stratum data, geochronological data and approximate paleowater depth data. The results are very interesting—the Late Triassic subsidence curves have a typical convex-up shape (Fig. 7) with frequent episodic subsidence events in the foreland 706

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Fig. 7. Subsidence analysis of the Well LD-1. Note that the initial increase in tectonic subsidence began at ∼237 Ma, followed by a rapid increase at approximately 227 Ma (corresponding the initiation of T3x1 deposition).

system was established. As the foreland basin filled with sediment, occasionally possible upwelling became progressively blocked, and strong continental runoff contributed moderate or substantial amounts of terrestrial organic matter (Fig. 6b; Fig. 6c). The combination of varying degrees of water body restriction and moderate-high productivity sustained by the high riverine terrestrial nutrient flux delivered by the large exorheic drainage systems originating from the Tibetan Plateau hinterland generated an episodic fluctuating low O2 concentrations in the lake basin. Active orogenesis affects organic carbon oxidation, which is limited under conditions of low oxygen availability (Galy et al., 2007). In conclusion, the Xujiahe Formation black shale development was mainly controlled by the primary productivity level, the foreland basin setting, rapid tectonic subsidence and a comparatively high sedimentation rate (Fig. 7) that buried and preserved but did not dilute the organic matter. Black shale development had little perceptible relation to water redox conditions. The formation of this unit was characterized by rapid subsidence, moderate-high primary productivity levels were perhaps controlled by occasional upwelling (T3x1) or continental runoff (T3x3 and T3x5) nutrients, the latter of which may have been influenced by warm, humid climatic conditions. Our results suggest that organic

matter enrichment trends in the Xujiahe Formation shale can be directly linked with the Indosinian Orogeny.

5.2. Chemical weathering Chemical weathering changes secularly in surficial processes, and its products are closely linked to plate tectonics, atmospheric composition, solar evolution and climate (Young, 2013). Chemical weathering of silicate minerals, such as feldspar or chain silicates, leads to the exchange of cations of alkaline and alkaline earth elements for H+ via hydrolysis (e.g., Bahlburg and Dobrzinski, 2011). Mineralogically, this process is characterized by the transformation of original minerals into clay minerals, with kaolinite commonly reflecting intense weathering and complete removal of mobile elements, including potassium (Nesbitt and Young, 1982). Various chemical weathering indices were determined to quantify the intensity of subaerial chemical weathering. Here, we compiled available and widely used chemical weathering indices (Table 5). For example, the weathering index values (Fig. 8) for our samples vary in the first member (CIA: 72–96, PIA: 82–97, and WIP: 4–45), in the third member (CIA: 69–80, PIA: 74–93, and WIP: 35–43) and in the 707

708

Molar Molar Molar Molar Mass Mass Mass Mass Mass Mass Mass Mass Molar Molar Molar Mass Mass Mass Molar

PIA = 100 × (Al2O3 – K2O)/(Al2O3 + CaO* + Na2O – K2O)

CPA = 100 × (Al2O3)/(Al2O3 + Na2O)

CIX = 100 × (Al2O3)/(Al2O3 + Na2O + K2O)

Rb/Sr; Rb/K αNa = [Sm/Na]sediment/[Sm/Na]source rock αCa = [Ti/Ca]sediment/[Ti/Ca]source rock αMg = [Al/Mg]sediment/[Al/Mg]source rock αK = [Th/K]sediment/[Th/K]source rock αSr = [Nd/Sr]sediment/[Nd/Sr]source rock αBa = [Th/Ba]sediment/[Th/Ba]source rock αRb = [Th/Rb]sediment/[Th/Rb]source rock αAl E = [Al/E]sediment/[ Al/E]source rock R3+/(R3++ R2+ +M+)=(Fe3++Al3+)/[(Fe3++Al3+)+(Fe2++Mg2++Mn2+)+(Na++K++2*Ca2+) 4Si = Si/4; 4Si% = [Si/4]/([Si/4] + M+ + R2+) △4Si% = [(4Si%sample-4Si%source rock)*100]/(100-4Si%source rock) MIA-O = 100 × (Al2O3 + Fe2O3T)/(Al2O3+ Fe2O3T + MgO + CaO* + Na2O + K2O)

LCWP = (CaO* + Na2O + MgO)/TiO2

τNa=(Na/Zr)sample/(Na/Zr)protolith-1 0.203 × ln(SiO2)+0.191 × ln(TiO2)+0.296 × ln(Al2O3)+0.215 × ln(Fe2O3)−0.002 × ln(MgO)−0.448 × ln(CaO*)−0.464 × ln(Na2O)+0.008 × ln(K2O)-1.374 PWI = 100 × [(4.20 × Na)+(1.66 × Mg)+(5.54 × K)+(2.05 × Ca)]

CaO* denotes the corrected CaO in silicate minerals following the method of Mclennan et al. (1993).

Mafic index of alteration for oxidative condition Loss chemical weathering proxy (LCWP) Chemical weathering depletion W index (MFW trinary plot) Paleosol weathering index (PWI)

Weathering intensity scale

Molar

Molar

CIA = 100 × Al2O3/(Al2O3 + CaO* + Na2O + K2O)

CIW = 100 × Al2O3/(Al2O3 + CaO* + Na2O)

Molar Molar

WIP = 100 × [(2Na2O/0.35) + (MgO/0.9) + (2K2O/0.25) + (CaO*/0.7)] MWPI = [(Na2O + K2O + CaO* + MgO)/(Na2O + K2O + CaO* + MgO + SiO2+Al2O3+Fe2O3)*100]

Weathering index of Parker Modified weathering potential index, MWPI Chemical index of alteration, CIA

Chemical index of weathering, CIW Plagioclase index of alteration, PIA Chemical proxy of alteration, CPA Modified chemical index of alteration Rb-type indices Elemental weathering indices, αE andαAl E

Note

Equation for index calculation

Weathering index

Table 5 Compilation of chemical weathering indices and their computational formula.

Rasmussen et al. (2011) Ohta and Arai (2007) Gallagher and Sheldon (2013)

Yang et al. (2006)

Babechuk et al. (2014)

Garzanti et al. (2013) Meunier et al. (2013)

Mclennan et al. (1993) Gaillardet et al. (1999)

Cullers (2000); Buggle et al. (2011) Garzanti et al. (2014)

Fedo et al. (1995)

Nesbitt and Young (1982) Harnois (1988)

Parker (1970) Vogel (1975)

References/Sources

T. Deng, et al.

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Fig. 8. Stratigraphic lithology variations and values of representative weathering indices. CIA (Chemical index of alteration) (Nesbitt and Young, 1982), CIW (Chemical index of weathering) (Harnois, 1988), WIP (Weathering index of Parker) (Parker, 1970), LCWP (Loss chemical weathering proxy) (Yang et al., 2006), τNa (Chemical weathering depletion of Na) (Rasmussen et al., 2011), αK (Elemental weathering indices) (Gaillardet et al., 1999) and αAl K (Garzanti et al., 2013), PWI (Paleosol weathering index) (Gallagher and Sheldon, 2013). Sedimentary recycling or first-cycle are based on the CIA/WIP plot following the method of Garzanti et al. (2013). Al Al Al Al Al Al sequences as follows: αAl Ca > αNa > αMg > αSr > αBa ≥ αK > αRb (Fig. 10). In addition, a principal component analysis (PCA) was performed using 1) the values of the weathering-sensitive major and trace elements K2O, NaO, CaO, Rb, Sr, and Ba and the immobile oxides Al2O3 and SiO2 (Belov and Belova, 1979) and 2) provenance control based on the different αAl values. To accomplish this analysis, we used the free software package PAST and treated all variables with normalization by dividing their standard deviations by the correlation matrix function (variance-covariance) and transforming them into centered log-ratio (clr) coordinates (Aitchison and Egozcue, 2005). The data points are illustrated in a scatter plot (biplot) showing the first two principal components (Fig. 11a). Fig. 11a shows that different members of the Xujiahe Formation cannot be discriminated by their Al2O3 concentrations, which reflect mixtures of weathering intensities and/or degrees of recycling and quartz dilution (Schneider et al., 2016) as well as a minor diversity of feldspar types. The three groups of αAl values are positively correlated with each other (Fig. 11b), documenting weathering rather than provenance control. Na, Ca, Sr, Mg, K, Rb and Ba are enriched in all members of the Xujiahe Formation (Fig. 11b), indicating that parent rocks are rich in carbonate, K-feldspar and illite.

fifth member (CIA: 72–98, PIA: 83–97, and WIP: 6–45). However, because lithology has a major controlling effect on the composition of sediments (Johnsson, 1993; Schneider et al., 2016), chemical indices of weathering can be markedly affected by tectonic effects, transport effects, hydraulic sorting, grain size, recycling and provenance effects (Fedo et al., 1995; Garzanti and Resentini, 2016). On the A-CN-K ternary diagram (Fig. 9a) proposed by Fedo et al. (1995), most samples are shown to have undergone a moderate degree of chemical weathering (CIA < ∼80) and are parallel to the A-CN line, indicating sustained weathering of granitoid parent rocks (between granodiorite and granite) (Fig. 9a). In the M+-4Si-R2+ ternary diagram (Meunier et al., 2013), almost all samples are situated in the felsic area (Fig. 9b). These results are also supported by the A-CN-K plot (Fig. 9a). In addition, we observe a gradual shift from T3x1 to T3x3 to T3x5 towards the 4Si apex (representing the maximum intensity of alteration of parent rock; Meunier et al., 2013) (Fig. 9b). Therefore, the samples indicate increased chemical weathering from deep to shallow in general. Moreover, on the AF-CNK-M ternary diagram proposed by Babechuk et al. (2014; Fig. 9c) and the R3+/(R3+ + R2+ + M+) - Δ4Si% diagram (Meunier et al., 2013, Fig. 9d), the samples exhibit a general weathering trend for a source region with an intermediate felsic rock composition similar to that of the average upper continental crust. Notably, a small degree of mafic rock participation is also observed (Fig. 9b and d) and the samples display a quite advanced weathering trend according to the AF-CNK-M ternary of Babechuk et al. (2014; Fig. 9c). To avoid a hydraulic sorting bias (Garzanti et al., 2013), a comparison of the calculated αAl values reveals different elemental mobility

5.2.1. Evaluation of sedimentary recycling The application of chemical weathering indices to sedimentary rocks suffers from a variety of problems (Garzanti and Resentini, 2016), including diverse parent lithologies (Garzanti and Resentini, 2016), grain-size effects (von Eynatten et al., 2012); recycling and inheritance from previous sedimentary cycles (Borges et al., 2008; Li and Yang, 2010); hydraulic effects, such as suspension sorting and selective 709

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Fig. 9. Plots of (a) A-CN-K ternary diagram with CIA values on the vertical axis, (b) M+-4Si-R2+, (C) AF-CNK-M ternary diagram with MIA-O values on the vertical axis, and (D) R3+/(R3++R2++M+)-Δ4Si% for black shale from cores LD-1 showing a chemical weathering trend and possible diagenetic influences. Abbreviations: A = Al2O3, CN= CaO*+ Na2O, K = K2O, CIA = chemical index of alteration, M+ = Na++K++2Ca2+, 4Si = Si/4, R2+ = Fe2++Mg2+, MIA-O = mafic index of alteration for oxidative condition, △4Si% = [(4Si%sample-4Si%source rock)*100]/(100-4Si%source rock), R3+/(R3++ R2+ +M+)=(Fe3++Al3+)/[(Fe3++Al3+) +(Fe2++Mg2++Mn2+)+(Na++K++2*Ca2+).

with the first-cycle and multiple-cycle sediments indicating moderatehigh recycling effects. However, in T3x1, T3x3 and T3x5, different sedimentary facies are generally observed, with T3x1 featuring purely neritic marine facies, T3x3 featuring marine-terrestrial transitional facies and T3x5 featuring purely fluvial-lacustrine facies. Moreover, multiple-cycle characteristics increase. The index of compositional variability (ICV) (Cox et al., 1995) of the studied samples is generally less than 1 but one-third of samples actually are 1 or greater than 1 (Fig. 12b), suggesting that the samples are compositionally mature and experienced intense weathering (CIA) mostly; thus, they were likely dominated by recycling effects and had recycled sedimentary sources. Recycling processes are also shown by the Th/Sc and Zr/Sc ratios (after Mclennan et al., 1993, Fig. 15a in latter section 5.4), indicating zircon addition and, thus, recycling processes. The results show that sedimentary facies maybe exert partial control because of the repeated recycling of marine sediments and that the samples exhibit high compositional maturity, as reflected by low abundances of nonclay silicates (low ICV; Fig. 12b) and enrichment of heavy minerals (high Zr; Fig. 15a in latter section 5.4).

Fig. 10. Mobility of alkali/alkaline earth elements during chemical weathering. αAl E = [Al/E]sediment/[ Al/E]source rock (Garzanti et al., 2013), E represent element of Na, Ca, Sr, Mg, K, Ba and Rb.

5.2.2. Hydrodynamic sorting and grain-size effects The chemical and mineralogical compositions of sediments and sedimentary rocks are strongly controlled by grain size (Ohta, 2008; von Eynatten et al., 2012, 2016), which also has a significant effect on the degree of chemical weathering. Lupker et al. (2013) used the Al/Si ratio as a proxy for grain size to study the sediment sorting effects in detrital records. As in Lupker et al.

entrainment (Garzanti et al., 2010, 2011); difficulties in separating CaO in silicates from CaO in carbonates and/or phosphates (Buggle et al., 2011); and various diagenetic processes (Fedo et al., 1995; Goldberg and Humayun, 2010; Price and Velbel, 2003; Yang et al., 2016). In Fig. 12a, the shale samples all show mixed weathering conditions, 710

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Fig. 11. Biplots for the shale (a) to discriminate between strongly weathered sediments and less altered sediments in the Xujiahe Formation. (b) α values (calculated with reference to the UCC standard for all samples) used to discriminate between dominantly provenance-controlled vs weathering-controlled compositional variations.

that the Al/Si ratio correlates positively with the major elements Fe2O3, MgO and K2O but not with Na2O (plots not shown in this paper). These correlations reflect grain-size control and are consistent with modern Yellow River sediment results (Pang et al., 2018). In the present study, routine chemical weathering indices (e.g., CIA, WIP, PIA, CPA, CIX, and αAl) are subjected to the combined effects of bedrock physical denudation rates, the weathering regime of the source area, the relief and variable gradients in the source area, sedimentary recycling, hydrodynamic sorting, grain size, provenance changes and diagenetic alteration. Notably, in cases such as our shale samples in which the sediments are derived from multiple heterogeneous sources, the CIA can be interpreted in terms of the relative inputs of materials

(2013), our samples show a strong sorting-induced positive correlation (R2 = 0.84) between the Al/Si and K/Si ratios, except for two extreme outliers (Fig. 13a). The chemical weathering indices of these sediments are substantially controlled by sediment sorting effects that occurred during transport and deposition; hence, the CIA and other indices may not faithfully reflect temperature and precipitation changes. In our study, chemical weathering indices appear to be somewhat influenced by grain size (Fig. 13b). Members T3x1 and T3x3 show CIA vs. Al/Si (grain size proxy) correlation coefficients of 0.79 and 0.62 (though the latter illustrates considerable clustering), respectively (and not including one outlier in T3x1), while member T3x5 shows a correlation coefficient of 0.58 (and not including one outlier). In addition, we find

Fig. 12. Discriminating first-cycle and polycyclic sediments with chemical indices. Because quartz dilution strongly affects WIP but not CIA, the CIA/WIP plot readily reveals quartz enrichment in sediments (Garzanti et al., 2013). CIA and WIP are linearly inversely correlated for first-cycle muds; when recycling increases, quartz enrichment causes a marked decrease in WIP (a). (b), The index of compositional variability (ICV) ([Fe2O3+K2O + Na2O + CaO + MgO + MnO + TiO2]/Al2O3, mass proportion) decreases due to input of recycling of sedimentary material and CIA increasing due to intense weathering (Cox et al., 1995). 711

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Fig. 13. (a) Hydraulic sorting effect on the Si–Al–K chemical composition of sediments (adapted from Lupker et al., 2013). (b) Grain size effect on CIA.

sedimentary recycling during the formation of clastic rocks and the sedimentary source area rock type (Mclennan et al., 1993; Yang et al., 2012; Zhu et al., 2017). In the Th/Sc-Zr/Sc plot, certain samples of the T3x3 and T3x5 shales are associated with intermediate degrees of sedimentary recycling (Fig. 15a). A compilation of the potential source area geochemistry data in the southwest Sichuan basin (Fig. 15a) shows that all samples plot in the field defined by the Qinling orogeny Proterozoicearly Paleozoic strata but do not exhibit Triassic synorogenic granitoid characteristics (Dong et al., 2011, 2012). Furthermore, Songpan-Ganzi Triassic strata began to provide recycled detrital material to the younger strata (Wang et al., 2012). In addition, the shale of the Xujiahe Formation has an intermediate to acidic rock origin (e.g., tonalitetrondhjemite-granodiorite (TTG) rocks on the northern margin of the Yangtze block), although all samples show some limited input from mafic sources, which is consistent with the chemical weathering trend results (Fig. 9). Based on Fig. 15b and the regional geological context, we suggest that source rocks of the T3x1, T3x3 and T3x5 shales exhibit the following transition: T3x1 was primarily sourced from the Proterozoic to early Paleozoic strata of the Qinling orogeny, with no input of synorogenic Qinling Triassic granites or Longmen Shan sources, which implies that longitudinal/axial river systems developed in the early foreland basin system. The Neoproterozoic complex in the Longmen Shan was the main source of the T3x3 shale. Because of folding and strong southeastward thrusting, the Songpan-Ganzi flysch strata contributed recycled material to T3x5, which also retained the input of the Neoproterozoic complex and Paleozoic material of the Longmen Shan (Fig. 15b).

that have been weathered to different degrees; therefore, the CIA is not directly a product of paleoclimate as is traditionally conceived. The black shale compositional maturity itself is an inherited feature from previous sedimentary cycle(s) and hence is in some sense a provenance indicator (Borges et al., 2008). In any case, when linking chemical weathering with deep-time paleoclimate, we must exclude the influence of the abovementioned factors (Yang and Du, 2017). 5.3. Geochemistry and tectonic setting Whole-rock geochemical data on sedimentary rocks can provide information for the interpretation of the tectonic setting of depositional basins. The conventional discrimination diagrams of Bhatia (1985), Roser and Korsch (1986) and Bhatia and Crook (1986) have long been used to discriminate between active and passive continental margin settings; however, recent re-evaluations by Verma and ArmstrongAltrin (2013, 2016) and Basu et al. (2016) have questioned the efficiency of existing plots. Armstrong-Altrin and Verma (2005) used Neogene sediments of known tectonic settings to evaluate six commonly used tectonic setting discrimination diagrams and found that the discrimination success rate of these diagrams varies from 0% to ∼62%, illustrating these diagrams must be used with caution. Recently, Verma and Armstrong-Altrin (2013, 2016) used statistical tools and proposed new multidimensional diagrams based on a linear discriminant analysis and log-ratio transformations of major elements and combinations of major and trace elements. To determine more accurate constraints of tectonic settings, this study used these new plots and did not use the previous conventional approach of Roser and Korsch (1986) in deciphering tectonic setting using sedimentary whole-rock geochemistry (Fig. 14). Figs. 14a and b were calculated using revised equations published in the corrigendum to Verma and Armstrong-Altrin (2016) (personal communication). These equations are not dependent on the major elements or major and trace element plots. Most of the data points fall in the passive margin setting (Fig. 14a and b). However, in the highsilica and low-silica multidimensional diagrams (Fig. 14c, d), T3x1, T3x3 and T3x5 all plotted in the collision field, suggesting an active margin, which is consistent with the interpretation of a foreland basin tectonic setting (Cawood et al., 2012) as well as with the geologic evolutionary history of the Sichuan basin. In summary, we believe that the T3x1 marine shale exhibits the geochemical characteristics of a passive continental margin because of the effects of strong elutriation caused by high compositional maturity and multiphase recycling on material in the foreland basin background (Fig. 14a and b).

6. Conclusions The core observations and geochemical data presented in our study provide the first constraints on the Xujiahe Formation black shale formation mechanism, various chemical weathering indices, multiple nonclimatic disruptive factors, provenance and depositional tectonic setting of sediments from the southwestern Sichuan basin. Our findings can be summarized as follows: (1) The redox-sensitive element ratio biplots (V/(V + Ni)-U/Th, V/CrU/Th and Ni/Co-U/Th) and the ternary diagram of TFe-TOC-TS reveal that the synsedimentary redox regime of almost all the shale samples was oxidizing. (2) The black shales in the Xujiahe Formation were deposited under warm-humid climate conditions, and moderate to high primary paleoproductivity prevailed during deposition of this formation. Generally, from T3x1 to T3x3 to T3x5, the paleosalinity gradually decreases. The development of the black shales in the Xujiahe Formation was mainly controlled by the photic zone primary productivity level, a foreland basin setting, tectonic subsidence and a high sedimentation rate, and limited correlations were observed with the water redox level. The formation of these shales was likely

5.4. Provenance of the Upper Triassic black shales The inactive elements Th, Sc, Co, and Zr and their ratios Co/Th, La/ Sc, Zr/Sc, and Th/Sc can reflect provenance and supply useful information regarding the effects of chemical weathering, the extent of 712

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Fig. 14. Tectonic discrimination for samples from Well LD-1. (a, b) Multidimensional discriminant function diagrams for the discrimination of active and passive margin settings after Verma and Armstrong-Altrin (2016). (c, d) Multidimensional diagram after Verma and Armstrong-Altrin (2013) for discrimination of low-silica (35% ≤ SiO2＜63%) and high-silica (63% ≤ SiO2 ≥95%) tectonic settings, respectively. See more details in Verma and Armstrong-Altrin (2016) and Verma and Armstrong-Altrin (2013).

depositional period based on the CIA because of the influence of multiple nonweathering factors. (4) Multidimensional diagrams based on the isometric logarithmic ratio (ilr) are used to discriminate tectonic setting and suggest a collisional setting for these shales, which is entirely consistent with the interpretation of the setting as a foreland basin. (5) Provenance-sensitive element ratios (Th/Sc-Zr/Sc and Co/Th-La/ Sc) indicated a predominantly felsic character of the source lithologies, which resemble TTG rocks, as well as a small amount of mafic components in select samples. (6) The inferred provenance exhibits the following transition from T3x1

controlled by rapid subsidence, a warm-humid climate and high productivity. (3) Through the compilation and calculation of a variety of chemical weathering indices, we can state for the first time that chemical weathering indices (e.g., CIA) of the Xujiahe Formation shale are controlled by sedimentary recycling (as evidenced by CIA-WIP); hydrodynamic sorting, including suspension separation and selective enrichment (as evidenced by K/Si-Al/Si); diagenetic alteration (especially illitization); and small changes in the source area lithology. In general, we encourage caution when inferring the paleoclimate (air temperature, precipitation) characteristics of the

Fig. 15. Th/Sc–Zr/Sc diagram (after Mclennan et al., 1993) for shale samples from Well LD-1. The average upper continental crust (UCC) (Taylor and McLennan, 1985) is plotted in (a) and (b) for comparison. Data from Longmen Shan (Zhou et al., 2006; Chen et al., 2015; Meng et al., 2015; Li et al., 2017, 2018), Emeishan highTi basalt (Xu et al., 2001; Xiao et al., 2004; Fan et al., 2008; Zi et al., 2012), Qinling orogen (Zhou Lian et al., 2007b; Zhou et al., 2007a; Zhao and Zhou, 2009a, 2009b; Dong et al., 2011, 2012; Shi et al., 2013), Songpan Ganzi basin (Liu Fei, 2006; Wang et al., 2012), Yidun arc complex (Wang et al., 2011, 2013), and Jiangnan Xuefeng thrust belt (Wang et al., 2012). 713

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to T3x3 to T3x5: T3x1 was primarily sourced from the Proterozoic to early Paleozoic strata of the Qinling orogeny, with no input of synorogenic Qinling Triassic granites or Longmen Shan sources; T3x3 was primarily sourced from the Neoproterozoic complex in the Longmen Shan; and T3x5 was primarily sourced from recycled material from the Songpan-Ganzi flysch strata because of folding and strong southeastward thrusting, and also retained input from the Neoproterozoic complex and Paleozoic material of the Longmen Shan.

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Acknowledgments Deng Tao would like to thank Professor Verma for providing the correction formula for the tectonic settings, associate professor Yang Jianghai (China University of Geosciences, Wuhan) for providing advice on the recycling effects analysis, and Dr. Yan Zhaokun for engaging in valuable discussions. We thank the journal associated editor Friedemann Baur and four anonymous reviewers for constructive comments, which considerably improved the manuscript. We especially thank reviewers #1 and reviewers #4 for them provided very thoughtful reviews and considerably constructive refinements to the manuscript. This study was funded by the National Key Research and Development Project (2018YFC1504702), China National Natural Science Foundation of China (No: 41372114), basic geological survey project of oil and gas of the Ministry of Land and Resources of the People's Republic of China (No: DD20160193), and the key project of the Sichuan Province Science and Technology Office (No: 2017JY0140; No: 17ZB0043). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.marpetgeo.2019.06.057. Chondrite data and primitive mantle data are from Sun and Mcdonough (1989); average upper continental crust (UCC) and postArchean Australian shale (PAAS) (Taylor and McLennan, 1985) are plotted on (a) for comparison. (a) Abundant benthic bivalve fossils in T3x3. (b) Vertical biological burrow in T3x3. (c) Bioturbation structure in T3x3. (d) Plant leaf fossil in T3x3. (e) A small trunk fossil in the marine facies setting of T3x1, indicating a depositional site close to shore. (f) Thin coal seam in T3x5. (g) Plant leaf fossil in T3x1, with wave ripples developed in a shallow water zone affecting the distribution of fossil fragments. (h) Vertical to highangle burrow in T3x1. References Aitchison, J., Egozcue, J.J., 2005. Compositional data analysis, where are we and where should we be heading? Math. Geol. 37, 829–850. Algeo, T.J., Kuwahara, K., Sano, H., Bates, S., Lyons, T., Elswick, E., Hinnov, L., Ellwood, B., Moser, Jessa, M., Maynard, J.B., 2011. Spatial variation in sediment fluxes, redox conditions, and productivity in the Permian–Triassic Panthalassic Ocean. Palaeogeogr. Palaeoclimatol. Palaeoecol. 308 (1), 65–83. Armstrong-Altrin, J.S., Verma, S.P., 2005. Critical evaluation of six tectonic setting discrimination diagrams using geochemical data of Neogene sediments from known tectonic settings. Sediment. Geol. 177, 115–129. Arthur, M.A., 1979. North Atlantic Cretaceous black shales, the record at site 398 and a brief comparison with other occurrences. In: Sibuet, J.C., Ryan, W.F. (Eds.), Initial Reports of the Deep Sea Drilling Project. U.S. Government Printing Office, Washington, DC, pp. 719–751. Arthur, M.A., Sageman, B.B., 1994. Marine black shales, depositional mechanisms and environments of ancient deposits. Annu. Rev. Earth Planet Sci. 22, 499–551. Babechuk, M.G., Widdowson, M., Kamber, B.S., 2014. Quantifying chemical weathering intensity and trace element release from two contrasting basalt profiles, Deccan Traps, India. Chem. Geol. 363, 56–75. Bahlburg, H., Dobrzinski, N., 2011. A review of the chemical index of alteration (CIA) and its application to the study of Neoproterozoic glacial deposits and climate transitions. In: Arnaud, E., Halverson, G.P., Shields-Zhou, G. (Eds.), The Geological Record of Neoproterozoic Glaciations. vol. 36. Geological Society, London, Memoirs, pp. 81–92. Basu, A., Bickford, M.E., Deasy, R., 2016. Inferring tectonic provenance of siliciclastic

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