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Abstract

Wufeng Formation shale is an important source rock of unconventional hydrocarbons in the Lower Paleozoic shales of Sichuan Basin. However, the study on its provenance and paleoclimate is still relatively limited. In this study, mineralogical and geochemical data of the shales from the Upper Ordovician Wufeng Formation in southwestern China has been used to interpret the provenance and conditions of weathering and paleoclimate. The Wufeng shales have intermediate to high SiO₂ (57.72–82.38 wt. %, av. = 68.84 wt. %) and Al₂O₃ (5.26–16.17 wt. %, av. = 10.62 wt. %), are rich in transition metal elements (i.e. V, Ni, Cu, Co and Cr) and Y as well as moderate depletion in Na₂O and Sr, relative to the concentrations of the upper continental crust (UCC). In the chondrite-normalized (CN) rare earth elements (REE) distributions, these rocks display light REE (LREE) enrichment (La/Yb_CN= 6.69–12.63, av. = 9.28), ﬂat heavy REE (HREE) (Gd/Yb_CN = 1.35–2.41, av. = 1.70), and clearly negative Eu anomalies (Eu_an= 0.50–0.66, av. = 0.58), showing similar characteristics with the CN post-Archean Australian Average Shales (PAAS). Wufeng Formation shales are immature composition without evident recycling sediments, and they are originated from an intermediate-felsic igneous source composed of tonalite–trondhjemite–granodiorite (TTG), granitic and andesitic igneous rocks. The chemical weathering conditions of studied shales decreased from moderate to low in the provenance region, suggesting a gradual cooling trend of the climate at Late Ordovician Thus, this article will be helpful to discern the provenance and variations of chemical weathering conditions and paleoclimate of Wufeng Formation shales.

Keywords

Paleoclimate provenance weathering Wufeng Formation Ordovician

Introduction

The Middle-Late Ordovician was an important stage in geological history because of the significant change that occurred in the biosphere and environment (Gibbs et al., 1997; Sheehan, 2001; Schmitz et al., 2008; Trotter et al., 2008; Servais et al., 2010; Zhang et al., 2010; Luo et al., 2016). The Great Ordovician Bio-diversiﬁcation Event (GOBE) in the Middle Ordovician was part of wider Cambrian-Ordovician radiation and was regarded as one of the two most important biological radiation events of marine life in history (Figure 1(a)) (Servais et al., 2010). On the other hand, a major mass extinction that occurred at the end of the Ordovician period (Hirnantian) was the second largest and oldest of the five mass extinctions, which has been causally linked to the Late Ordovician glaciations (Sheehan, 2001). The Upper Ordovician Wufeng Formation is mainly composed of black shales which may preserve important information about the palaeoenvironment and paleoclimate of that period (Figure 1(b)).

Figure 1.

(a) Global biodiversity change at family level through Early Paleozoic Era (after Sepkoski, 1995; Schmitz et al., 2008). Trem.: Tremadocian (international) and Tremadoc (British); Caradoc: Caradocian; Ash.: Ashgillian; Llanvirn: Llanvirnian; Lland.: Llandoverian; We.: Wenlockian; Lud.: Ludlowian; F.: Floian; Dap.: Dapingian; Sand.: Sandbian; H.: Hirnantian; GOBE: Great Ordovician Biodiversiﬁcation Event. (b) The stratigraphic column of the Wufeng and Guangyinqiao (GYQ) Formations in well Y1, paleoclimatic change (Chen et al., 2004) and the sample position.

The composition of siliciclastic sediments was controlled by a lot of active factors during sedimentary process, such as weathering, source rock composition, abrasion, and sorting during transport and diagenesis during burial (McLennan, 1989; Johnsson, 1993). Major and trace elements and mineralogy in clastic sediments were useful tools to study provenance weathering conditions and/or provenance (Ronov and Migdisov, 1971; Nesbitt et al., 1980, 1996; McLennan et al., 1993; Cox and Lowe, 1995; Cox et al., 1995; Bauluz et al., 2000; Das and Kaur, 2008; Sun et al., 2008; Gabo et al., 2009; Jian et al., 2013).

Previous studies of Wufeng shales mainly focus on its organic petrology (Luo et al., 2017; Luo et al., 2018; Luo et al., 2020), paleogeographic environment (Chen et al., 2004), paleontology (Chen et al., 2000), and petroleum geochemistry (Liang et al., 2008; Chen et al., 2011; Liu et al., 2011; Zou et al., 2012; Guo and Zhang, 2014; Gao et al., 2022), but until now the study on its provenance and the variations of weathering and paleoclimate is still insufficient. Chen et al. (2004) proposed that the palaeoclimate changed from Greenhouse effect to icehouse effect at Ashgillian (Figure 1(b)). Yan et al. (2009) concluded a warmer climate and a colder climate at Katian and Hirnantian, respectively, based on the isotopic compositions of sulfur in iron sulﬁde (δ³⁴S_sulﬁde) and of carbon in organic matter (δ¹³C_org) in the sediments from the Yangtze Platform, South China. However, the episodic promoted organic matter burial rates during the Early to Late Ordovician, as suggested by the change in high-resolution δ¹³C_carb and δ¹³C_org records from worldwide sediments, might have contributed to a long-term cooling in climate, which peaked during the Hirnantian glaciation (Brenchley et al., 2003; Yan et al., 2009; Zhang et al., 2010; Zhou et al., 2015; Algeo et al., 2016 and references therein). Cooling in climatic due to prompted organic carbon burial might be an important trigger for the end of Ordovician mass extinction (Sheehan, 2001; Zhang et al., 2010; Zhou et al., 2015; Algeo et al., 2016; Liu et al., 2021; Qiao et al., 2022). Enhanced organic carbon burial associated with paleoclimate changes may have been considered as an important factor of the end-Ordovician mass extinction.

The geochemical features of shales are considered to maintain more information of original geochemical signatures of provenance (McCulloch and Wasserburg, 1978; Bhat and Ghosh, 2001; Qiao et al., 2022). This time, all the samples came from shale rather than sandstone, and we only discussed the shale. The major and trace element concentrations and mineralogy from the Wufeng shales from southwestern China were examined in this study to (1) determine the provenance of the Wufeng shales; (2) evaluate the chemical weathering conditions; and (3) reconstruct the Late Ordovician paleoclimate.

Geological setting

The studied region is located in southeastern Chongqing, close to Sichuan Basin (Figure 2). The structural pattern is partition-style folds in southeast and trough-like folds in northwest (Zhai, 1987). From the Early to Middle Ordovician, the investigated region was submerged by a broad epeiric sea and was isolated by the uplift (Chen et al., 2004; Luo et al., 2016; 2017; Mu et al., 2011). During the Late Ordovician, there was a global transgression in Yangtze Block, which led to the development of Wufeng Formation (Mu et al., 2011). The Wufeng Formation mainly comprises black graptolitic shales including Dicellograptus complanatus–Paraorthograptus paciﬁcus graptolite zones (Chen et al., 2000; 2005), indicating the transgressive systems tracts (TST) in a typical sea-level sequence. The Guanyinqiao Formation, which includes typical Hirnantia Fauna (Rong et al., 2002) and is primarily composed of argillaceous limestones (Figure 1(b)), lie conformably on the Wufeng Formation, indicating a shelf-margin systems tract (SMST) at the margin of Yangtze Platform. The Guanyinqiao Formation was characterized by a drop in eustatic sea level and cool/cold water (Rong et al., 2002; Yan et al., 2009, 2010), which is time-equivalent to the Hirnantian period (Sheehan, 2001). Plenty of K-bentonite beds were found in the Ordovician and Silurian sediments on the Yangtze Block. The geochemical data of the K-bentonites indicated a parent magma that was sourced in and within-plate tectonic, volcanic-arc and syn-collisional settings (Su et al., 2006, 2009).

Figure 2.

Location and paleogeographic map of the study area in South China.

Sampling and methods

Thirty-seven core shales of the Wufeng Formation were collected from Y1 well, with sampling interval of 10–50 cm (Figures 1(b) and 2 and Table 1). Major elements contents were measured by using the X-ray fluorescence spectrometer (AB-104L, PW2404) at the laboratory of China National Nuclear Corporation (CNNC) Beijing Research Institute of Uranium Geology. The slice melting method was applied for determining concentrations of major elements. The process of measurement was described by Hu et al. (2014) in detail. The standard materials dolomite, andesite, and shale were used as standard samples to check the analysis accuracy with the analytical precision and accuracy of <2% for all major elements.

Table 1.

Mineralogical composition (wt.%) of the Wufeng shales.

Depth (m)	Quartz	Clay ^a	K-feldspar	plagioclase	Calcite	Dolomite	Pyrite	Anhydrite	Anatase	Marcasite	Kaolinite	Chlorite	Illite	I/S	C/S
1749.65	45	19.3	4.2	12.2	5.1	7	4.6	1.2	1.4	\	0.6	1	3.1	12	2.7
1749.88	49.1	19.4	3.7	10.9	2.9	7.2	4.6	0.8	1.4	\	0.4	0.8	3.5	12.8	1.9
1749.98	54.5	17	5.2	9.7	2.5	5.4	4.6	\^b	1.1	\	0.7	0.9	2.4	10.7	2.4
1750.08	56.7	15.2	3.7	11.2	2.5	6.5	4.2	\	\	\	0.3	0.8	2.1	10.6	1.4
1750.18	53.1	15.7	4.7	11.7	2.7	6.4	4.4	\	1.3	\	0.2	0.6	2	11.9	0.9
1750.33	48.7	18.3	5.3	12.1	2.2	5.9	6	\	1.5	\	0.5	0.7	2.6	12.4	2
1750.43	57.3	17.8	4.4	8.8	2.5	5	2.3	0.8	1.1	\	0.4	0.9	3.2	11.6	1.8
1750.73	65.3	12.4	2.1	4.6	1.6	11.8	1.5	0.7	\	\	0.4	0.5	2	8.2	1.4
1750.85	36.5	38.6	1.2	8.2	2.3	6.9	3.2	0.9	2.2	\	0.8	1.9	4.2	29	2.7
1751.08	66.3	14.9	2.5	7	1.3	3.2	3.9	\	0.9	\	0.3	0.4	2.1	10.9	1.2
1751.25	46.5	23.9	3.4	9.2	1.1	5.8	7.9	0.9	1.3	\	0.5	1.2	2.9	17	2.4
1751.95	60.8	17.1	2.1	7.8	1.1	5.3	5	\	0.8	\	0.3	0.5	2.4	12.8	1
1752.65	67.9	12.4	2.8	6.7	1.9	4.4	3.9	\	\	\	0.2	0.5	1.9	8.7	1.1
1753.15	40.5	24.8	3.3	9.1	1.3	7.6	10.2	2.1	1.1	\	0.2	0.7	2.7	20.1	1
1753.65	60.5	18.4	2	8	1.7	5.5	3.9	\	\	\	0.4	0.9	2	13.4	1.7
1754.07	61.3	17	3	7	2	4.4	4.1	\	1.2	\	0.7	0.7	2.4	11.6	1.7
1754.55	68.6	14	1.7	4.7	1.9	7	2.1	\	\	\	0.4	0.7	3.2	8.4	1.3
1755.05	63.9	15.1	2.1	5.8	2.2	5.3	2.9	2.7	\	\	0.3	0.8	2.7	9.2	2.1
1755.65	62.4	16.2	3.2	7.6	0.9	6.7	3	\	\	\	0.5	1	2.4	10.9	1.5
1756.05	61.3	25.6	2.8	6.4	0.2	3.1	\	0.6	\	\	0.5	1	3.1	19.7	1.3
1756.55	69.8	14.3	1.9	4.5	5.3	3.3	0.9	\	\	\	0.3	0.6	2.7	9	1.7
1757.05	85.8	1.5	1.6	4.9	0.4	4.1	1.7	\	\	\	0.1	0.1	0.1	1	0.3
1757.55	71	18.4	1.4	5.1	\	3.1	1	\	\	\	0.6	0.9	2.6	12.3	2
1758.25	40.4	26.1	6.2	6	0.5	16.9	2.5	\	\	1.4	0.8	1	3.7	19.1	1.6
1758.75	36.6	33.5	5.9	7	\	4.6	6.9	1.8	2.6	1.1	0.7	1	1.7	28.8	1.3
1759.25	45.8	26.5	5.4	11.4	1.7	5.4	2	\	\	1.8	1.1	1.6	3.4	17.8	2.7
1759.75	62.9	18.7	3.1	7.4	0.7	5.9	\	\	1.3	\	0.9	1.1	0.9	12.7	3
1760.13	45.2	31.4	3	11	1.3	5.2	\	1.2	1.7	\	0.9	1.6	3.5	23.9	1.6
1760.73	46.9	34	3	8.2	0.7	2.9	0.9	1.7	1.7	\	1	1.7	3.7	24.5	3.1
1761.23	39.6	29.9	6.6	8.8	1.4	6.7	1.9	2.7	2.4	\	0.9	1.5	5.1	19.1	3.3
1761.86	57.8	23.8	3	7.5	1.2	2.3	2.1	0.9	1.4	\	1	1.9	3.3	13.1	4.5
1762.36	42.6	22.5	3.4	8.1	0.3	8.9	10.5	1.9	1.8	\	1.4	2.9	5.6	7.9	4.7
1762.86	35.2	31.7	6.5	7	1.1	13.1	1.8	2.2	1.4	\	0.6	1.3	4.8	23.5	1.6
1763.36	42.1	19.9	4.7	9.1	1.5	15.9	1.9	2.7	2.2	\	0.4	0.8	3.6	13.5	1.6
1763.86	52.8	26.9	3.3	8.7	1.9	4.3	\	\	2.1	\	1.6	2.7	4	15.3	3.2
1764.4	57.6	27.1	3	6.9	1.6	2.9	\	0.9	\	\	1.1	2.2	3	16.3	4.6
1765.05	52.9	29.7	3.2	5.4	2	2.7	\	2.4	1.7	\	0.6	2.1	4.8	18.4	3.9
Average	54.4	21.3	3.5	8	1.7	6.2	3.1	0.8	1	0.1	0.6	1.1	3	14.5	2.1

Clay = Kaolinite + Chlorite + Illite + Illite–smectite mixed layers (I/S) + Chlorite–smectite mixed layers (C/S).

“\” represents that the minerals are absent in the samples.

The concentrations of trace element were performed by using the inductively coupled plasma-mass spectrometry (ICP-MS) as described by Qi et al. (2000) in detail. The powdered sample of about 0.05 g was combusted in a muffle furnace before put into a PTFE bomb and then adding a mixture solution (HNO₃: HF = 2:1) (Zhang et al., 2018). Then sealed bomb was put in an electric oven and heated to 185°C for 24 h (Zhang et al., 2018). After cooling, about 500 ng of Rh (standard material), and mixed liquid (water: HNO₃= 5:2) were added into the bomb, and the sealed bomb was put into an electric oven again at 135°C for heating 5 h to dissolve the residue materials (Zhang et al., 2018). The final dilute factor of trace elements measurement was about 3000 after cooled (Zhang et al., 2018). The analysis accuracy was calibrated by the standard materials of plagiogneiss, andesite and slate (OU-6) (Zhang et al., 2018). The precision of most elemental determination was ±5–10% (relative) (Zhang et al., 2018). The Eu_an (Eu anomaly) is calculated as Eu_CN/(Sm_CN ×Gd_CN)^0.5 (McLennan, 1989), in which CN is chondrite-normalized concentrations (Boynton, 1983).

A Bruker D8 advanced Phaser diffractometer was used to measure X-ray diffraction (XRD), using a 40 mA current and 40 kV voltage, CuKα radiation (k = 1.54 Å), and the detector is LynxEye. Powder samples were randomly oriented, and were taken with 2θ from 3° to 45° (step size of 0.02°). The isolation of clay mineral fraction of < 2 μm was performed by sedimentation based on Stoke's Law after removal of organic matter and carbonate by dissolution with 30% hydrogen peroxide and 0.3 N EDTA. The < 2 μm oriented smear slide was measured three times: (1) after air drying at room temperature (scanning from 2.5° to 15° 2θ with a step size of 0.02°); (2) after ethylene-glycol solvation for 8 h (2.5–30° 2θ with 0.02° steps); and (3) after heating at 460 °C for 2.5 h (2.5–15° 2θ with 0.02° steps).

Results

Mineralogical composition

Results of mineral compositions in Wufeng shales using XRD are listed in Table 1. The major minerals in the studied samples are quartz (35.2–85.8 wt. %, av. = 54.4 wt. %) and clay minerals (1.5–38.6 wt. %, av. = 21.3 wt. %). K-feldspar shows low concentrations, with the highest of 6.6 wt. %, whereas plagioclase ranges from 4.5 to 12.2 wt. %, with an average of 8.0 wt. %. Dolomite falls in the range of 2.3–16.9 wt. % (av. = 6.2 wt. %), while calcite is present in lower concentrations (0–5.3 wt. %, av. = 1.7 wt. %). Pyrite displays values up to 10.5 wt. % (av. = 3.1 wt. %), whereas anhydrite, anatase and marcasite are absent or present in trace amounts.

Clay minerals composition

The grain-size fraction with size of <2 µm (i.e. phyllosilicate) is dominated by illite-smectite mixed layers (I/S) (1.0–28.8 wt. %, av. = 29.0 wt. %). The illite, chlorite, and chlorite-smectite mixed layers (C/S) are shown in variable amounts of 0.1–5.6 wt. % (av. = 3.0 wt. %), 0.1–2.9 wt. % (av. = 1.2 wt. %), and 0.3–4.7 wt. % (av. = 2.1 wt. %), respectively. The kaolinite is present up to a few percent, mostly lower than 1% (Table 1).

Geochemistry

Concentrations of major and trace element are shown in Tables 2 and 3. The elemental composition was normalized with upper continental crust (UCC) to determine the geochemical characteristics of the studied samples (Taylor and McLennan, 1985) (Figure 3). The Wufeng shales contain moderate to high SiO₂ (57.72–82.38 wt. %, av. = 68.84 wt. %) and Al₂O₃ (5.26–16.17 wt. %, av. = 10.62 wt. %) compared to the UCC. These sediments display slightly higher average SiO₂ and TiO₂ concentrations, and lower average Al₂O₃, K₂O, Fe₂O₃, MgO, CaO, MnO, and especially Na₂O concentrations compared to the concentrations in the UCC (Figure 3).

Figure 3.

UCC-normalized major and trace elements for the Wufeng shales (Taylor and McLennan, 1985). Note the strong depletion in Na and Sr, and the enrichment in transition metals for most samples. The red dotted line represents the average of the studied shales.

Table 2.

Major oxides (wt.%) of the Wufeng shales.

Depth (m)	SiO₂	Al₂O₃	K₂O	Fe₂O₃	MgO	CaO	MnO	Na₂O	P₂O₅	TiO₂	LOI^a	Total
1749.65	61.79	12.43	3.42	4.36	2.01	3.62	0.04	1.48	0.12	0.65	9.63	99.55
1749.88	64.92	11.72	3.25	4.48	1.91	2.43	0.03	1.36	0.12	0.75	8.56	99.53
1749.98	67.92	10.33	2.99	3.72	1.72	2.69	0.03	1.22	0.11	0.56	8.4	99.69
1750.08	67.97	9.83	2.86	4.11	1.75	2.86	0.03	1.23	0.11	0.55	8.29	99.59
1750.18	66.54	10.86	3.27	4.19	1.83	2.61	0.03	1.27	0.12	0.61	8.07	99.4
1750.33	65.52	11.31	3.15	4.81	1.73	2.1	0.03	1.21	0.15	0.79	8.68	99.48
1750.43	70.79	8.9	2.56	3.48	1.72	2.71	0.03	1.02	0.09	0.5	7.91	99.71
1750.73	75.03	6.05	1.68	2.73	1.77	3.34	0.05	0.63	0.08	0.33	7.78	99.47
1750.85	58.79	16.17	4.33	6.38	3.07	2.04	0.04	1.09	0.12	0.73	6.85	99.61
1751.08	74.71	8.98	2.59	2.8	1.43	1.45	0.02	0.82	0.08	0.51	6.12	99.51
1751.25	66.3	11.45	3.19	5.45	1.72	1.33	0.02	1.15	0.19	0.71	8.01	99.52
1751.95	72.62	8.92	2.47	3.96	1.45	1.33	0.02	0.99	0.09	0.49	7.1	99.44
1752.65	75.28	7.7	2.08	3.21	1.28	1.62	0.02	0.82	0.09	0.41	7.02	99.53
1753.15	62.83	12.12	3.49	7.33	1.75	1.03	0.02	1.29	0.12	0.64	8.83	99.45
1753.65	74.64	8.42	2.42	3.41	1.4	1.55	0.02	0.95	0.09	0.48	6.28	99.66
1754.07	72.44	8.62	2.37	3.58	1.68	2.26	0.04	0.89	0.09	0.48	7.07	99.52
1754.55	79.11	6.25	1.7	2.56	1.31	1.88	0.03	0.6	0.07	0.34	5.57	99.42
1755.05	75.82	7.75	2.1	2.87	1.4	1.7	0.03	0.82	0.09	0.42	6.46	99.46
1755.65	71.03	9.79	2.71	3.56	1.99	1.96	0.04	0.89	0.12	0.52	6.91	99.52
1756.05	73.35	10.16	2.88	2.96	1.66	1.02	0.02	0.79	0.09	0.48	6.1	99.51
1756.55	79.59	7.07	1.99	2.57	1.28	0.77	0.02	0.65	0.07	0.37	5.02	99.4
1757.05	82.38	5.26	1.46	2.17	1.15	0.97	0.02	0.58	0.08	0.3	5.03	99.4
1757.55	79.04	7.49	2.05	2.56	1.39	0.79	0.02	0.69	0.08	0.39	4.98	99.48
1758.25	61.75	13.15	3.73	5.44	3.09	2.44	0.08	0.96	0.12	0.7	7.97	99.43
1758.75	61.45	15.18	4.35	6.95	2.53	0.62	0.04	1.02	0.12	0.75	6.44	99.45
1759.25	66.1	12.34	3.48	4.95	2.34	1.43	0.05	1.07	0.12	0.66	6.9	99.44
1759.75	70.36	10.02	2.77	3.63	2.15	1.73	0.05	0.87	0.16	0.54	7.17	99.45
1760.13	62.93	14.15	3.86	4.74	2.79	2.14	0.07	1.33	0.1	0.75	6.63	99.49
1760.73	65.83	13.37	3.91	5.36	2.56	1.21	0.05	1.15	0.09	0.72	5.4	99.65
1761.23	63.09	13.4	3.85	5.39	2.74	1.87	0.06	1.17	0.11	0.74	7.27	99.69
1761.86	69.02	10.14	2.72	4.19	2.01	2.38	0.04	0.83	0.8	0.52	6.78	99.43
1762.36	57.72	12.51	3.56	10.65	2.41	1.18	0.06	1.05	0.1	0.68	9.71	99.63
1762.86	59.83	13.56	3.79	6.35	3.24	3.22	0.16	0.91	0.08	0.7	7.63	99.47
1763.36	59.94	12.9	3.63	6.43	3.41	3.55	0.19	0.87	0.07	0.66	7.76	99.41
1763.86	69.52	11.79	3.1	4.87	2.46	1.52	0.06	0.85	0.07	0.62	4.63	99.49
1764.4	71.52	11.13	2.93	4.14	2.16	1.32	0.04	0.83	0.07	0.59	4.79	99.52
1765.05	69.57	11.88	3.18	4.41	2.29	1.8	0.06	0.83	0.06	0.6	4.76	99.44
Average	68.84	10.62	2.97	4.45	2.02	1.9	0.04	0.98	0.12	0.57	6.99	99.51

LOI: loss on ignition.

Table 3.

Trace elements (in ppm) of the Wufeng shales.

Depth (m)		Rb	Ba	Sr	Th	U	Ni	V	Cr	Co	Cu	Sc	Zr	Nb	Hf	Y	La	Ce	Pr	Nd	Sm
1749.65		145.5	836.5	147	16.21	18.86	124.5	541	104.85	17.1	79.25	10.6	212	17.1	5.63	35.8	40.4	79.55	9.29	34.2	6.68
1749.88		134	813	115	15.75	18.27	115	488	80.3	15.7	70.3	10.7	201	16.6	5.32	35	41.4	82.4	9.5	34.9	6.8
1749.98		123	754	109	14.16	15.22	109	460	134	14.2	68.2	9.1	186	15.2	4.59	33.4	38.1	72.8	8.39	30.9	6.11
1750.08		114	735	107	12.98	15.5	114	476	81.6	13.9	69.2	8.69	173	14.6	4.21	34.4	36.3	67.5	7.88	29.5	6.12
1750.18		129	798	107	14.02	16.21	109	480	77.4	13.2	62.3	9.12	181	14.5	5.31	35.1	38	72.1	8.5	31.9	6.32
1750.33		132	826	107	14.46	21.81	143	472	132	19.1	91.2	9.75	183	18.6	4.88	35.8	38.9	78.6	9.13	33.9	6.6
1750.43		102	692	96.9	11.51	12.5	103	411	81	13	58.6	8.04	140	11	3.57	26.2	31.9	57.9	6.7	24.9	5
1750.73		72.2	598	89.1	7.82	8.4	70	243	64.5	8.74	43.3	5.72	110	9.38	2.44	19	23.7	42.9	4.85	18.4	3.65
1750.85		155	951	97.4	16.86	4.75	50.2	140	88.5	17.2	43.6	13.1	122	15.4	3.31	24.1	41.4	80.9	9.12	32	5.77
1751.08		105	767	78.1	10.6	8.73	67.1	314	55.5	9.96	38.9	6.75	217	13.4	4.13	29.8	45.8	89.4	9.72	33.9	6.06
1751.25		131.5	817.5	85.15	15.62	20.28	131	592	73.25	19.2	80.8	9.52	182	17.1	4.58	34.75	40.9	76.85	9.58	35.8	7.13
1751.95		106	679	76.5	11.79	13.13	104	467	62.1	15	62.9	8.01	148	10.5	3.78	30.8	30	56.1	6.9	25.7	5.31
1752.65		87.6	627	75.1	9.66	9.22	91.5	417	62.8	10.5	68.2	6.95	123	8.82	3.07	24.9	25.3	44.3	5.44	20.5	4.03
1753.15		141	831	85.4	18.3	18.61	125	531	71.6	19.3	98.7	9.91	187	14.6	4.89	36	40.4	71.8	9	32.7	6.56
1753.65		94.2	618	86.4	10.76	9.74	86	484	67.6	10.6	68.8	7.65	136	19.3	3.26	21.8	29.2	50.3	6.07	22.1	4.25
1754.07		98.9	666	86.8	11.42	10	83.6	479	68	10.6	57.9	7.74	129	17	3.24	21.3	28.5	47.5	5.9	21.2	4.16
1754.55		76	567	71.7	8.21	6.49	61.1	420	101	7.61	57.6	6.02	90.8	6.06	2.39	17.9	20.8	37.7	4.54	16.9	3.39
1755.05		90	649	75.5	10.05	7.95	79.1	610	117	9.08	64.5	7.36	112	8.13	2.85	22	25.2	44.3	5.49	19.8	4
1755.65		117	726	84.1	12.35	9.68	84.8	627	102	10.2	73.4	9.01	133	11.2	3.36	30.2	32.7	54	6.92	24.5	4.97
1756.05		119.5	848	71.2	13.83	8.24	63.1	566.5	69.7	8.41	59.4	8.91	163.5	14.1	3.96	27.25	36.35	66.9	8.23	29.85	5.67
1756.55		86.7	623	54.4	9.73	5.75	47.7	491	58.5	6.83	63.9	6.97	102	15.3	2.61	14.9	27.2	45.4	5.13	18.3	3.37
1757.05		63	509	49.2	7.11	4.9	50.4	403	107	6.41	42.7	5.34	75.6	5.98	1.95	17.2	18.8	34.9	3.98	15.1	3.14
1757.55		88.9	628	56.8	9.8	6.19	47.6	418	85.8	6.85	59.3	7.18	114	10.3	2.65	21.1	27.8	48.6	5.91	22.4	4.48
1758.25		163	778	95.6	16.5	4	53.7	175	90.2	12	77.4	12.6	166	13.9	4.36	33.2	41.1	76.3	9.29	34	6.58
1758.75		171	907	84.5	17.52	3.91	48.3	117	67	18.1	71.3	13.9	221	17.4	5.98	37.3	59.1	114	14.3	52.8	10.3
1759.25		151	740	89.6	16.88	4.2	44.2	147	139	9.22	85.4	12.2	181	15.3	4.34	37	43.2	79.2	9.8	36.4	7.26
1759.75		120	673	77.1	13.43	5.56	46.4	587	98.8	5.99	83.3	10.1	131	11.4	3.45	31.8	32.5	56	7.26	27.4	5.46
1760.13		179	851	104	19.59	3.68	54.9	108	82.3	14.2	83.8	14	192	18.4	5.02	30.7	46.6	96.9	10.7	38.7	7.36
1760.73		173	830	90.9	20.6	3.67	53.9	104	79.2	15.1	81.3	14.5	169	16.3	4.54	32.6	45.7	96.8	10.6	38.9	7.36
1761.23		159	823	95.2	17.54	3.91	67.45	147.5	95	16.9	114.5	14.1	168.5	15.7	4.38	37.5	44.45	87	10.05	36.2	7.11
1761.86		118	678	96.4	12.4	4.01	52	148	82.3	10.3	102	9.35	129	11.6	3.24	57.3	37	73.2	9.89	39.2	9.71
1762.36		151	1130	82	17.72	3.88	102	199	105	29.4	140	13.1	147	14.1	3.78	34.5	42.6	81.8	9.4	34.4	6.74
1762.86		167	881	119	19.8	3.18	54.2	84.9	93.8	15.4	53.7	16.1	147	17.2	3.95	29.6	49.3	99.1	10.8	37.8	7.02
1763.36		163	793	109	16.78	2.4	53.8	75.3	64.9	19.2	51.9	15.3	147	15.2	3.84	29.1	39.6	82.6	9.11	32.7	6.41
1763.86		138	696	83.6	15.15	2.4	49.7	75.5	64.4	20.3	97.6	12.4	139	14.3	3.74	24.9	35.9	71.3	8.04	28.6	5.56
1764.4		131	686	76.6	14.39	2.98	68.5	88.5	75.4	30.7	88.8	10.4	124	13	3.25	24.9	37.2	76.2	8.45	30.7	5.85
1765.05		143	712	83.1	14.99	2.39	36.7	69	92.9	10.1	40.4	13.4	137	15.6	3.53	20.6	39.9	76.5	8.03	27.5	4.93
Average		125.38	749.65	89.14	13.95	8.67	76.9	342.06	85.84	13.77	71.74	10.1	151.88	13.88	3.87	29.45	36.57	69.45	8.16	29.86	5.87
Depth (m)	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	CIA	CIW	PIA	ICV	∑REE	LREE	HREE	∑LREE/∑HREE	La/Yb	La/Yb_CN	Gd/Yb_CN	Eu_an
1749.65	1.17	6.56	1	5.78	1.19	3.55	0.51	3.42	0.51	59.18	71.85	64.18	1.25	193.8	163.44	9.17	17.82	11.83	7.98	1.55	0.54
1749.88	1.19	6.67	1.02	5.88	1.2	3.42	0.48	3.16	0.46	59.45	72.37	64.7	1.21	198.48	168.2	8.72	19.29	13.1	8.83	1.7	0.54
1749.98	1.04	6.18	0.95	5.65	1.12	3.23	0.47	3.05	0.44	58.75	72.02	63.86	1.25	178.44	150.19	8.31	18.07	12.49	8.42	1.64	0.52
1750.08	1.09	6.44	0.99	5.73	1.13	3.26	0.45	2.96	0.44	57.91	70.84	62.46	1.36	169.79	141.18	8.24	17.13	12.26	8.27	1.76	0.53
1750.18	1.14	6.21	0.94	5.59	1.15	3.38	0.51	3.39	0.51	58.45	72.21	63.66	1.27	179.63	150.5	8.94	16.83	11.21	7.56	1.48	0.55
1750.33	1.29	6.56	1.02	5.72	1.16	3.37	0.5	3.21	0.47	60.47	73.96	66.49	1.22	190.43	160.53	8.71	18.43	12.12	8.17	1.65	0.6
1750.43	0.95	4.9	0.76	4.32	0.88	2.52	0.35	2.35	0.35	59.22	72.62	64.61	1.35	143.78	121.4	6.45	18.82	13.57	9.15	1.68	0.59
1750.73	0.72	3.77	0.55	3.25	0.64	1.83	0.25	1.64	0.24	60.81	74.42	67.05	1.74	106.39	89.85	4.6	19.53	14.45	9.74	1.85	0.59
1750.85	1.14	5.33	0.77	4.27	0.85	2.47	0.36	2.35	0.34	66.15	81.85	76.2	1.09	187.07	163.42	6.37	25.65	17.62	11.88	1.83	0.63
1751.08	1.16	5.61	0.82	4.72	0.97	2.85	0.42	2.64	0.39	61.99	76.87	69.57	1.07	204.46	178.82	7.27	24.6	17.35	11.7	1.71	0.61
1751.25	1.34	6.98	1.04	5.99	1.19	3.51	0.49	3.23	0.48	61.27	75.16	67.88	1.19	194.48	163.13	8.88	18.37	12.68	8.55	1.75	0.58
1751.95	0.99	5.12	0.83	4.83	0.97	2.87	0.42	2.75	0.4	60.1	73.31	65.79	1.2	143.19	118.7	7.41	16.02	10.91	7.35	1.5	0.58
1752.65	0.79	3.99	0.64	3.68	0.76	2.23	0.32	2.14	0.33	60.87	74.05	66.88	1.23	114.46	95.54	5.78	16.53	11.82	7.97	1.51	0.6
1753.15	1.23	6.55	1.02	5.92	1.21	3.45	0.5	3.26	0.49	61.18	75.59	68.07	1.28	184.09	153.9	8.91	17.27	12.39	8.36	1.62	0.57
1753.65	0.76	4.02	0.62	3.5	0.73	2.14	0.3	2.08	0.32	59.49	73.01	65.07	1.21	126.39	107.67	5.57	19.33	14.04	9.46	1.56	0.56
1754.07	0.8	4.25	0.61	3.48	0.68	1.98	0.29	1.92	0.29	61.11	74.71	67.47	1.31	121.56	103.1	5.16	19.98	14.84	10.01	1.79	0.58
1754.55	0.65	3.37	0.47	2.7	0.56	1.62	0.24	1.57	0.23	62.06	75.93	69	1.35	94.75	79.94	4.22	18.94	13.25	8.93	1.73	0.59
1755.05	0.76	3.88	0.58	3.3	0.7	1.98	0.28	1.92	0.29	60.86	74.08	66.89	1.21	112.48	94.79	5.17	18.33	13.13	8.85	1.63	0.59
1755.65	0.93	5.01	0.74	4.36	0.89	2.64	0.38	2.53	0.39	62.49	76.9	69.98	1.19	140.97	118.12	6.83	17.29	12.92	8.71	1.6	0.57
1756.05	1.04	5.17	0.77	4.41	0.93	2.78	0.41	2.78	0.43	63.94	79.55	72.94	0.97	165.71	141.33	7.33	19.28	13.08	8.82	1.5	0.58
1756.55	0.53	3.14	0.47	2.57	0.54	1.55	0.24	1.58	0.25	62.2	76.75	69.65	1.08	110.28	96.03	4.16	23.08	17.22	11.61	1.61	0.5
1757.05	0.57	3.09	0.48	2.73	0.58	1.6	0.23	1.45	0.22	60.07	73.31	65.77	1.26	86.87	72.78	4.08	17.84	12.97	8.74	1.72	0.55
1757.55	0.77	4.04	0.62	3.45	0.72	2.02	0.29	1.89	0.29	62.52	76.74	69.89	1.05	123.28	104.71	5.21	20.1	14.71	9.92	1.73	0.55
1758.25	1.18	6.37	1.02	5.67	1.16	3.3	0.49	3.25	0.51	64.66	80.68	74.32	1.25	190.21	160.69	8.71	18.45	12.65	8.53	1.58	0.56
1758.75	1.92	8.86	1.27	6.73	1.35	3.78	0.54	3.46	0.53	67.14	84.81	79.38	1.07	278.94	240.2	9.66	24.87	17.08	11.52	2.07	0.61
1759.25	1.36	7.31	1.1	6.15	1.26	3.48	0.52	3.23	0.49	62.87	77.8	70.89	1.13	200.76	168.6	8.98	18.78	13.37	9.02	1.83	0.57
1759.75	1.06	5.4	0.84	4.91	1.07	3.09	0.45	3	0.46	63.08	77.76	71.01	1.17	148.89	123.16	8.07	15.26	10.83	7.3	1.45	0.59
1760.13	1.37	6.88	1	5.45	1.12	3.21	0.48	3.21	0.48	62.32	76.38	69.5	1.11	223.46	192.9	8.5	22.69	14.52	9.79	1.73	0.59
1760.73	1.37	7	1.04	5.74	1.18	3.32	0.49	3.29	0.49	62.51	77.94	70.71	1.12	223.28	192	8.77	21.89	13.89	9.36	1.72	0.58
1761.23	1.35	7.02	1.1	6.47	1.27	3.65	0.53	3.42	0.51	62.56	77.68	70.57	1.18	210.11	177.7	9.37	18.96	13	8.76	1.66	0.58
1761.86	2.25	11.16	1.63	9.4	1.82	4.67	0.62	3.73	0.53	64.18	78.88	72.61	1.25	204.81	159.29	11.37	14.01	9.92	6.69	2.41	0.66
1762.36	1.25	6.56	0.98	5.94	1.16	3.4	0.49	3.24	0.48	63.12	78.36	71.47	1.57	198.44	168.2	8.77	19.18	13.15	8.86	1.63	0.58
1762.86	1.3	6.48	0.95	5.23	1.03	2.91	0.42	2.78	0.42	65.67	81.96	76.01	1.35	225.54	197	7.56	26.06	17.73	11.96	1.88	0.59
1763.36	1.18	5.85	0.89	5.2	1.03	3.01	0.43	2.91	0.41	65.49	81.82	75.79	1.45	191.33	164.01	7.79	21.05	13.61	9.17	1.62	0.59
1763.86	1	4.99	0.74	4.21	0.86	2.47	0.37	2.42	0.35	65.67	80.77	75.03	1.14	166.81	143.84	6.47	22.23	14.83	10	1.66	0.58
1764.4	1.07	5.64	0.78	4.41	0.86	2.47	0.36	2.32	0.34	65.32	80.26	74.4	1.08	176.65	152.55	6.35	24.02	16.03	10.81	1.96	0.57
1765.05	0.95	4.89	0.69	3.8	0.72	2.08	0.32	2.13	0.32	65.78	81.27	75.5	1.11	172.76	151.93	5.57	27.28	18.73	12.63	1.85	0.59
Average	1.1	5.71	0.86	4.9	0.99	2.84	0.41	2.69	0.4	62.19	76.61	69.6	1.23	169.8	144.04	7.34	19.82	13.77	9.28	1.71	0.58

For large ion lithophile elements (LILE; i.e. Rb, Ba, Sr, U, and Th, the average concentrations of Rb, Ba and Th are similar to those in UCC. U are slightly higher than that of UCC in the studied shales, whereas Sr contents are obviously depleted relative to UCC. The transition trace elements, such as Co, Cr, Cu, V, and Ni) are enriched compared to UCC, whereas Sc display slightly depleted. The distributions of high field strength elements (HFSE) are different; Nb, Zr and Hf display slightly depletion, whereas Y is slightly enriched in the UCC-normalized diagram (Figure 3).

Generally, the rare earth elements (REE) concentrations are uniform for the studied samples. Total REE (∑REE) contents fall between 86.67 and 278.94 ppm in these sediments, with an average of 169.80 ppm, while the concentrations of HREE (from Ho to Lu), LREE (from La to Nd), and MREE (from Sm to Dy) range from 4.08 to 11.37 ppm, 72.78 to 240.20 ppm, and 10.00 to 34.15 ppm, respectively. Our samples display clear LREE (La–Nd) enrichment as indicated by high ratios of La/Yb = 9.92–18.73 (av. = 13.77), La/Yb_CN= 6.69–12.63 (av. = 9.28) and ∑LREE/∑HREE = 14.01–27.28 (av. = 19.82). The obvious negative Eu anomalies (Eu_an= 0.50–0.66, av. = 0.58) have been observed in these samples (Figure 4). The HREE display a relatively ﬂat distribution, with Gd/Yb_CN ratios falling between 1.35 and 2.41 (av. = 1.70). The PAAS-like shape of the CN REE distributions indicates homogenization of these samples (Figure 4).

Figure 4.

CN REE distributions of the wufeng sediments. PAAS values (after Taylor and McLennan 1985) are shown as a red line. Values of. Note negative Eu anomaly, fractionated LREEs, and flat HREEs, similar to the CN (chondrite data after Boynton, 1983) PAAS.

Discussion

Sorting and recycling of sediments

The Index of Compositional Variation (ICV), calculated as equation (1), is very useful to determine the clastic sediment composition and maturity (Cox et al., 1995). Oxides are expressed as weight percentages in this formula. Immature minerals, such as detrital ferromagnesian minerals and feldspars, contain ICV values of >1, whereas ICV values are smaller than 1 in the weathering products (muscovite, kaolinite, and illite) (Cox et al., 1995; Cullers and Podkovyrov, 2000). The studied samples display high ICV values, ranging between 0.97 and 1.74 (1.23 on average), suggesting that these samples were compositionally immature. These results indicate that the studied sediments in study area were likely dominated by first cycle sediments and were related to tectonically active settings (Cox et al., 1995; Cullers and Podkovyrov, 2000), according to the geological setting of South China (Su et al., 2006, 2009). $ICV = (F e_{2} O_{3} + K_{2} O + N a_{2} O + CaO + MgO + MnO + Ti O_{2}) / A l_{2} O_{3}$ (1)The Zr/Sc versus Th/Sc diagram was useful tool for the determination of the presence of sorting-related fractionations (McLennan et al., 1993). The data of the samples concerned in this study are varied around the “compositional variations” line and are clustered together (Figure 5(a)), indicating that they are first-cycle deposits without recycling processes, consistent with ICV values.

Figure 5.

(a) Th/Sc versus Zr/Sc plot for the Wufeng sediments (after McLennan et al. 1993). Note that the samples plot along the “compositional variations” line and cluster together, suggesting no obvious inﬂuence of recycling and heavy mineral sorting. (b–d) Major oxides versus SiO₂ for the Wufeng shales. Note the strong negative correlations for SiO₂ versus Al₂O₃, Fe₂O₃, and TiO₂.

Provenance of Wufeng shales

Before using immobile elements to discriminate the provenance, element mobility in the samples concerned in this study during the processes, such as chemical weathering, sorting, and/or others, should be evaluated (Singh, 2009). Chemically mobile elements display a wider scatter, while chemical immobile elements will keep constant and show a linear array extending from the origin along radians. (Fralick and Kronberg, 1997). A linear arrangement of samples illustrates the immobility of the elements in the studied samples (Fralick and Kronberg, 1997). In the plots of SiO₂-Al₂O₃, -Fe₂O₃, and -TiO₂ (Figure 5(b)–(d)), the linear arrays of points along lines extending toward 100% SiO₂ indicate that these elements were immobile in chemistry. These immobile elements can be used to discriminate the source rocks because TiO₂/Al₂O₃ ratios vary in different igneous rocks (acidic > intermediate > mafic > ultramafic) and Ti and Fe are enriched in mafic minerals (Nesbitt and Wilson, 1992; Sugitani et al., 1996). In the plots of TiO₂/Al₂O₃ versus TiO₂/Fe₂O₃ and TiO₂/Al₂O₃ versus Fe₂O₃/Al₂O₃, the elements from the most studied samples distribute close to the average compositions of andesites (Figure 6), indicating that the mainly source was andesites or rocks with similar composition.

Figure 6.

(a) 100TiO₂/Fe₂O₃–100TiO₂/Al₂O₃; (b) 100Fe₂O₃/Al₂O₃–100TiO₂/Al₂O₃, note that the samples plot close to the andesites. The average compositions of Phanerozoic granites, TTG, andesites, and basalts are cited from Condie (1993).

The Eu anomalies and the CN REE distributions can be used to discriminate the provenances of sediments (McLennan et al., 1993; Fedo et al., 1996). Felsic rocks generally show negative Eu anomalies and higher ratios of LREE/HREE, however, basic rocks usually display no Eu anomalies with low ratios of LREE/HREE (Cullers and Graf, 1983; Taylor and McLennan, 1985; Luo et al., 2015; Qiao et al., 2021; Radwan, 2022). The consistent CN REE distributions illustrate that they were likely to be mainly sourced from felsic source rocks, as suggested by negative Eu anomalies, enriched LREE, and relatively ﬂat distributions of HREE (Figure 4).

Hayashi et al. (1997) proposed that the relationship between TiO₂ and Zr could be used to discriminate sediment provenance. The strongly positive correlation between TiO₂ and Zr suggests a homogeneous source, and the samples are plotted in the felsic igneous rock field (Figure 7(a)), showing a predominantly felsic source for the studied shales. The studied samples contain low and constant ratios of La/Th, ranging from 2.21 to 4.32. The Hf contents span between 1.95 and 5.98 ppm (3.87 ppm on average; Table 3). Ti/Zr and Co/Y ratios concerned in this study are 0.18–1.23 and 13.95–36.01, respectively. In the plots of La/Th versus Hf (Floyd and Leveridge, 1987) and Ti/Zr versus Co/Y, most samples distribute in the felsic materials field, suggesting a mixed provenance composed of intermediate igneous to felsic and andesitic to granitic components (Figure 7(b) and (c)). The shales mainly display relatively low and stable Co/Th ratios but more decentralized La/Sc ratios, also indicating an intermediate to felsic source components (Figure 7(d)). In the diagrams of Sc–La–Th and Co–Th–Hf, the studied samples concerned in this study distribute around the average composition of TTG and granites (Figure 8).

Figure 7.

(a) TiO₂ versus Zr (after Hayashi et al., 1997), note that the studied samples mainly distribute in the felsic igneous field; (b) La/Th versus Hf (after Floyd and Leveridge, 1987), note that all samples distribute close to the TTG and felsic volcanic rocks; (c) Co/Y–Ti/Zr, note that the samples mainly plot around TTG and andesites; (d) La/Sc versus Co/Th (Gu et al., 2002), note that these samples distribute around the TTG and granites. The average compositions of Phanerozoic granites, TTG, andesites and basalts are cited from Condie (1993).

Figure 8.

La–Th–Sc and Th–Hf–Co ternary plots. Note the studied samples plot around to TTG and granites. The average compositions of Phanerozoic granites, TTG, andesites and basalts are cited from Condie (1993).

In conclusion, the source rocks of the Wufeng shales were dominated by TTG-like, andesitic and granitic igneous rocks, which were only present in the older Proterozoic basement in this area (Chen et al., 1995; Yan et al., 2008; He et al., 2010), indicating that the Wufeng shales were derived from the Proterozoic basement.

Paleoclimate reconstruction

The chemical index of alteration (CIA; Nesbitt and Young 1982) (equation (2)) to reconstruct the paleoweathering conditions of the source terrain, in which CaO* is CaO present in silicate component only (see McLennan, 1993, for corrections). The content of CaO* is the lower values between Na₂O and CaO-P₂O₅ after corrected phosphate by P₂O₅ (McLennan, 1993). In equation (2), all components are used in molar proportions (the same as in Figure 9(a) and equations of (3) and (4)). The CIA value is one of the useful chemical weathering indexes (Fedo et al., 1995; Bauluz et al., 2000; Mongelli et al., 2006; Absar et al., 2009; Schoenborn and Fedo, 2011; Jian et al., 2013; Liu et al., 2020, Qiao et al., 2023; Wu et al., 2022). CIA values for the studied samples are 57.91–67.14 (62.19 on average; Table 3), illustrating low to intermediate intensity of weathering conditions in the provenance area (Fedo et al., 1995; Selvaraj and Chen, 2006). $CIA = {A l_{2} O_{3} / (A l_{2} O_{3} + CaO * + N a_{2} O + K_{2} O)} \times 100$ (2)

Figure 9.

(a) Ternary plot of Al₂O₃–(CaO*+Na₂O)–K₂O (A–CN–K) (Nesbitt and Young, 1984). Note that all studied samples distribute along the line parallel to the line A–CN; (b) Ternary plot of (Al₂O₃–K₂O)–CaO*–Na₂O ((A–K)–C–N) (Fedo et al., 1995). Note that the low to moderate values of CIA and PIA, indicating low to moderate chemical weathering of the source area.

The data of Wufeng Formation is plotted in the A(Al₂O₃)–CN (CaO* + Na₂O)–K (K₂O) diagram (Figure 9(a)). This ternary can be used to evaluate the degree of K-metasomatic effects and chemical weathering conditions (Nesbitt and Young, 1984; 1989; Fedo et al., 1995), indicating that the removal of Na and Ca was intermediate extent because of plagioclase destruction with the data plotting intermediate between plagioclase–K feldspar and A–K lines. The plots do not deviate from the predicted weathering trend line and exhibit no trends toward the K apex, indicating that K-metasomatism did not happen in these sediments during diagenesis. This trend may be the nature of non-steady state weathering related to the different conditions and different provenance of balance between physical (such as erosion, tectonism, uplift) and chemical processes (Nesbitt et al., 1997). The vertical dimension (percent Al₂O₃) in A–CN–K diagram is in agreement with CIA values, also suggestive of low to moderate chemical weathering conditions.

Harnois (1988) proposed chemical index of weathering (CIW) which can be calculated as equation (3). Like CIA, CIW also can be used to monitor the degree of conversion from feldspars to clay minerals (Fedo et al., 1995; Maynard et al., 1995). This parameter is similar to CIA, even though it eliminates K₂O. CaO*, in the formula of CIW and PIA, is it equivalent to that in the formula of CIA. The CIW values in these studied samples are 70.84–84.81 (mean value = 76.6), indicating moderate chemical weathering. $CIW = {A l_{2} O_{3} / (A l_{2} O_{3} + CaO * + N a_{2} O)} \times 100$ (3)The weathering of plagioclase can also be described by (Al₂O₃–K₂O)–CaO*–Na₂O ((A–K)–C–N) diagram, following equation (4) for plagioclase index of alteration (PIA) proposed by Fedo et al. (1995). $PIA = {(A l_{2} O_{3} – K_{2} O) / ((A l_{2} O_{3} – K_{2} O) + CaO * + N a_{2} O)} \times 100$ (4)In the (A–K)–C–N triangle diagram, all studied samples distributing intermediate between (A–K)–N and (A–K)–C lines indicate moderate feldspars alteration with PIA values ranging from 62.46 to 79.38 (69.60 on average; Table 3 and Figure 9(b)), indicating chemical weathering degree was low to moderate in the provenance region. Such conclusion is consistent with the results of CIA and CIW values. CIA values are positively related to PIA and CIW values (r²= 1.00 and 0.99, respectively). In the UCC normalized multielement diagram, the lowly to moderately depleted CaO, Na₂O and Sr and slightly enriched Ba and Rb (Figure 3) also indicate low to moderate chemical weathering conditions of the provenance area (Nesbitt and Wilson, 1992; Fedo et al., 1996). The dominant clay minerals are illites and mixed-layer clays in the studied sediments, which also supports the low to moderate chemical weathering conditions (Nesbitt and Young, 1982).

The intensity of chemical weathering conditions is primarily determined by precipitation and temperature, and a warm and wet climate condition is stimulative for chemical weathering condition (Nesbitt and Young, 1982; Riebe et al., 2004). It is worth noting that CIA, CIW, and PIA gradually decrease from the bottom toward the top part of the Wufeng sediments (Figure 10). On the other hand, the intense paleoweathering conditions are characterized by the abundance of kaolinite and chlorite, and the paucity of particularly plagioclase, detrital feldspars (Nesbitt and Young, 1982; Schoenborn and Fedo, 2011; Jian et al., 2013). For the studied samples, the kaolinites and chlorites display decreasing trends with decreasing depth, similar to the stratigraphic variations of CIA and PIA, however, the plagioclase contents display the reverse trend (Figure 10), suggesting that the chemical weathering is gradually decreasing at the Late Ordovician and humidity seems to have decreased from the basal part of the Wufeng Formation.

Figure 10.

The stratigraphic variations of CIA, PIA, CIW, kaolinite, chlorite, plagioclase, Rb/Sr, Sr/Cu, Na₂O/Al₂O₃, and CaO/Al₂O₃. Note the CIA, PIA, kaolinite, chlorite, and Rb/Sr decrease as the depth decreases, whereas the plagioclase, Sr/Cu, Na₂O/Al₂O₃, and CaO/Al₂O₃ display the opposite trends.

The intensity of the chemical weathering controlled the mobility of elements in the provenance region. Compared to immobile element (Rb, Cu, and Al), sequestered in the residual phases K, Na, Ca, Mg, and Sr are regarded as mobile elements during the process of weathering (Nesbitt and Young, 1984, 1989). Thus, Rb/Sr, Sr/Cu, Na₂O/Al₂O₃, and CaO/Al₂O₃ are useful for the evaluation of the variations of chemical weathering. The higher Rb/Sr, and lower Sr/Cu, Na₂O/Al₂O₃, and CaO/Al₂O₃ values generally imply a stronger chemical weathering. In the studied samples, Rb/Sr values show a decreasing trend with decreasing depth, whereas the Sr/Cu, Na₂O/Al₂O₃, and CaO/Al₂O₃ ratios display the opposite trends (Figure 10). These results also indicated that the weathering intensity in the provenance region decreased gradually. The stratigraphic variations of CIA, PIA, elements ratios and mineralogy indicate that the chemical weathering degree decreased from moderate to low in the provenance region at Katian, suggesting a gradual cooling trend of the climate at Late Ordovician.

Conclusions

The Wufeng shales contain intermediate to high SiO₂ and Al₂O₃, show low to moderate depleted in Sr, Na₂O, and, CaO, and are rich in transition metal elements and Y compared to their concentrations in the UCC. The Wufeng shales might be the product of first-cycle deposits and are related to the tectonically active settings, as indicated by ICV values and the diagram of Zr/Sc versus Th/Sc. The Wufeng shales were derived from a mixture of felsic-intermediate provenances including TTG-like, granitic and andesitic igneous rocks. The source area experienced low to moderate chemical weathering. The paleoclimate gradually cooled during Late Ordovician.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This work was supported by Joint Fund Project of National Natural Science Foundation (No. U22B6002);CNPC Scientific Research and Technology Development Project (No. 2021DJ0605).

ORCID iD

Caiyuan Dong

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Geochemistry and mineralogy of the Upper Ordovician Wufeng formation,Southwestern China: Implications for Paleoclimate construction

Abstract

Keywords

Introduction

Geological setting

Sampling and methods

Results

Mineralogical composition

Clay minerals composition

Geochemistry

Discussion

Sorting and recycling of sediments

Provenance of Wufeng shales

Paleoclimate reconstruction

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References