Geochemistry and zircon U-Pb ages of the Oligocene sediments in the Baiyun Sag, Zhujiang River Mouth Basin

1. Introduction

Previous studies on provenance in the South China Sea (SCS) area mainly focused on the Neogene (postrift units), whereas provenance evolution in the Paleogene (synrift units) was rarely investigated (Cao et al., 2015, 2018; Wang et al., 2019; Zeng et al., 2019). The SCS has sparse borehole coverage of the Paleogene strata because of high drilling costs together with a complex geological setting and deeply buried units (Shao et al., 2016a). For provenance analysis in the Zhujiang River Mouth Basin (ZRMB) of the northern SCS, the intrabasinal uplifts of the ZRMB also formed a series of important source regions during the Paleogene synrift stage (Wang et al., 2017, 2019; Liu et al., 2017; Cao et al., 2018; Zeng et al., 2019), but their ability to provide debris is still unclear. An important issue is determining the roles of the intrabasinal source system from the uplifts and the potential extrabasinal provenance from the South China Block.

Element geochemistry analysis is an efficient method to identify the weathering conditions, parent rock lithology and provenance of sedimentary rocks (Castillo et al., 2015; Amendola et al., 2016; Shu et al., 2016; Ma et al., 2019). The trace element contents of sedimentary rock are closely related to its environment of formation; thus, the contents and ratios of trace elements can be analyzed to infer the sedimentary environment and invert for the relevant geological conditions (Domini and Stanley, 1993). Rare earth elements (REEs), with good chemical stability, may not undergo unbalanced fractionation during the weathering, denudation, transport, deposition and early diagenesis of clastic particles. Changes in REE contents are believed to be highly relevant to the source composition, exchange reactions and diagenesis. As one of the key geochemical indicators, the REE contents and their changes can indicate the environment of formation and the sources and can imply the diagenesis of sedimentary rocks (Nesbitt and Young, 1982; Taylor and McClennan, 1985; McLennan, 1989; Condie, 1993; Fedo et al., 1995, 1997; Ma et al., 2019). The detrital zircon U-Pb chronology can define the maximum depositional age; thus, it has become a useful tool widely applied around the world to analyze basin provenance in recent decades (Cao et al., 2015; Jiang et al., 2015; Shao et al., 2016b; Liu et al., 2016; Zeng et al., 2019). Detrital zircons in sedimentary basins are stable and widespread, and can preserve the original source information well, so detrital zircon age dating is popular for analyzing provenance systems (Wu and Zheng, 2004; Wang et al., 2015; Fan et al., 2015; Benyon et al., 2016). In this study, major elements, trace elements and detrital zircon U-Pb ages were combined to identify the weathering conditions, parent rock lithology and provenance of the Oligocene sediments in the Baiyun Sag of the ZRMB.

2. Geological setting

The ZRMB is located in the northern SCS and the southern margin of the South China continent and near the intersection of the Eurasian, Pacific and Indian plates (Zhang et al., 2007; Zhang, 2010). As a large deep sag in the basin, the Baiyun Sag has a deep-water area of more than 1×10⁴ km², a maximum sedimentary thickness exceeding 10 000 m and the most complete Cenozoic strata in the ZRMB. It neighbors the Dongsha Uplift to the east, the Yunkai Low Uplift to the west, the Panyu Low Uplift to the north, and the southern uplift to the south (Fig. 1). The Baiyun Sag experienced three stages of tectonic evolution: a rifting stage, transitional stage, and depression stage. The environmental characteristics are as follows: the lacustrine facies is present in the Eocene Wenchang Formation; the marine-continental transitional facies, in the Eocene−lower Oligocene Enping Formation; the neritic facies, in the upper Oligocene Zhuhai Formation; and the bathyal to abyssal facies, in the Miocene Zhujiang Formation and the overlying strata (Shao et al., 2005; Sun et al., 2011; Zhang et al., 2015) (Fig. 2). In the late depositional period of the Enping Formation, the sedimentary environment gradually changed from lacustrine to marine-continental transitional conditions, and the wide and gentle paleogeographic setting on the northern slope facilitated the development of great deltas (Zhang et al., 2014; Zeng et al., 2017; Sun et al., 2020).

Figure  1.  Location and tectonic units of the Baiyun Sag and the locations of sampled boreholes. Major and trace element analyses were performed for samples from wells BY1–BY5, and detrital zircon dating was performed for samples from Well BY3.

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Figure  2.  Composite column of stratigraphy, tectonic evolution, and environmental characteristics in the Baiyun Sag (modified from Pang et al. (2008)).

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3. Samples and methods

The samples used in this paper were core and cutting samples, which were taken from gray to dark gray mudstones of the Oligocene Enping and Zhuhai Formations in the Baiyun Sag. In this study, major and trace element analyses and zircon U-Pb dating were carried out. The sample conditions and experimental methods are described as follows.

A total of 24 samples from the Enping Formation were used for major element analysis. Major element analysis was completed in two steps. (1) Powder sample pressing. Approximately 4 g of powder sample was weighed using a balance and then placed into the cylindrical mold of a semiautomatic presser. White boric acid solid powder was added around the mold. The mold was removed, the pressure cap was covered, and the sample was removed after 20 s at a pressure of 0.3 MPa. Thus, a sample cylinder of 4 cm in diameter and 8 mm in thickness was prepared. (2) X-ray fluorescence (XRF) testing. The testing was conducted using an E3080 XRF spectrometer (Rigaku, Japan). The accuracy of the XRF analysis was estimated to be better than 1% for all major oxides. All operations above were completed at the Key Laboratory of Petroleum Resources Research, Chinese Academy of Sciences.

In total, 24 mudstone samples from the Enping Formation were analyzed for trace elements using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (Thermo Fisher Scientific, USA) according to the following steps. (1) The sample was ground to approximately 200 mesh and placed in a drying oven at 55°C for 12 h. Approximately 20–30 mg of dried sample was weighed and placed in a Teflon container. A small amount of ultrapure water was added for wetting. (2) There are 1 mL nitric acid and 1 mL hydrofluoric acid were added to the Teflon container, and the container was vibrated in an ultrasonic oscillator for 20 min. Then, the plate was placed on a 150°C heating plate until it was nearly dry. (3) There are 1 mL nitric acid and 1 mL hydrofluoric acid were added to the Teflon container, and the container was vibrated in an ultrasonic oscillator for 20 min. The sample was inserted into a stainless steel tank and tightened. Then the tank was placed in a 190°C drying oven for at least 24 h. (4) The Teflon container was removed from the stainless steel tank after cooling. The Teflon container was placed on a 150°C heating plate until it was nearly dry. There is 1 mL nitric acid was added and dried. This operation was repeated twice. (5) There are 2 mL nitric acid and 3 mL ultrapure water were added to the Teflon container, which was placed in a stainless steel tank, tightened and left in a 150°C drying oven for at least 24 h. (6) The Teflon container was removed, and the volume was made approximately 2 000 times the sample weight. The minimum detection limit of the equipment was less than 1×10^–9. The errors for trace and REEs in this study were within ±6%. All operations above were completed at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences.

The samples used for zircon U-Pb age dating were taken from Well BY3, including 3 bags of samples from the upper Enping Formation and 1 bag of samples from the Zhuhai Formation. Forty zircons from each bag of samples were selected for dating analysis, and 160 zircons in total were obtained. The zircon grains for these samples were selected from the sandstones. The detrital zircon samples with high clarity and good crystal form under the microscope were selected randomly to make sample targets. After polishing, the sample target was subjected to cathodoluminescence (CL) imaging. Then, zircon U-Pb age dating was conducted using laser ablation (LA)-ICP-MS technology with a GeoLas 2005 laser denudation system (Coherent, Germany) and an Agilent 7500a ICP-MS (Agilent, Japan). Of the 160 detrital zircons analyzed, 134 results were considered adequate (concordance ≥90%) for evaluating provenance. Offline data processing, including the selection of sample and blank signals, drift correction of instrument sensitivity, and calculation of element contents, U-Th-Pb isotope ratios and ages, was completed using ICPMSDataCal (Liu et al., 2008, 2010). Because of the high content of radioactive Pb, ²⁰⁷Pb/²⁰⁶Pb ages were used for zircon particles older than 1 000 Ma, while for those younger than 1 000 Ma, the more reliable ²⁰⁶Pb/²³⁸U ages (Compston et al., 1992; Dickinson and Gehrels, 2003) were used. The dating ages were analyzed with 1σ absolute uncertainties. Zircon U-Pb dating was completed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan).

4. Analytical results

4.1 Major element analysis

The major element contents, chemical index of alteration (CIA) values and chemical index of weathering (CIW) values for mudstone samples from the Enping Formation are reported in Table 1. The data of post-Archean Australian shale (PAAS) come from Taylor and McClennan (1985), and the data of upper continental crust (UCC) come from Rudnick and Gao (2003). In comparison with UCC, the samples have lower contents of Na₂O. Most of the samples have higher Al₂O₃ contents than the UCC. The samples show obviously wide ranges of K₂O and CaO contents. In general, the CIA values vary from 62.61 to 81.68 (mean 73.88), and the CIW values range from 68.51 to 96.11 (mean 85.65) (Table 1).

Table  1.  Major element contents, chemical index of alteration (CIA) and chemical index of weathering (CIW) of the lower Oligocene Enping Formation mudstones in the Baiyun Sag

Well	Depth/m	Sample type	SiO2/%	TiO2/%	Al2O3/%	Fe2O3/%	MnO/%	MgO/%	CaO/%	Na2O/%	K2O/%	CIA	CIW
BY1	2 653.0	cutting	59.41	0.31	22.20	5.45	0.04	2.65	0.77	2.00	4.45	69.98	82.55
BY2	4 628.6	core	63.36	0.43	23.17	2.01	0.01	1.37	0.26	0.35	5.91	75.64	95.67
BY3	4 293.2	core	67.22	0.43	21.63	3.81	0.02	1.41	0.26	0.34	3.52	81.68	95.44
BY3	4 295.0	core	63.70	0.50	24.13	2.62	0.01	1.63	0.22	0.35	5.75	76.98	96.11
BY3	5 091.0	core	61.10	0.43	21.41	3.85	0.02	2.80	0.24	0.53	5.98	73.30	94.24
BY4	3 062.0	cutting	62.01	0.42	11.85	4.29	0.11	1.43	9.79	0.94	1.43	71.92	79.37
BY4	3 099.5	cutting	61.04	0.52	14.45	5.02	0.09	1.60	8.06	0.98	1.82	73.53	81.72
BY4	3 114.5	cutting	63.13	0.35	14.71	4.92	0.04	1.96	5.94	1.83	2.00	64.23	70.96
BY4	3 144.5	cutting	59.21	0.42	17.04	5.78	0.05	2.26	5.22	2.38	2.16	62.61	68.51
BY4	3 146.0	cutting	63.40	0.62	16.36	5.63	0.07	1.84	4.84	1.22	2.15	72.10	80.36
BY4	3 198.5	cutting	58.39	0.67	18.31	6.50	0.17	2.13	5.73	1.15	2.34	74.38	82.93
BY5	3 587.5	cutting	64.79	0.34	19.23	5.46	0.05	1.70	3.24	0.51	3.12	79.16	91.97
BY5	3 599.5	cutting	62.70	0.54	19.22	4.99	0.05	1.75	3.97	0.42	3.18	79.92	93.32
BY5	3 644.5	cutting	66.34	0.46	16.18	5.46	0.05	1.55	3.74	0.46	2.75	78.21	91.41
BY5	3 647.5	cutting	67.25	0.30	19.19	4.80	0.03	1.51	2.40	0.62	2.96	78.51	90.39
BY5	3 668.0	cutting	63.64	0.35	20.84	5.64	0.05	1.92	1.15	0.79	3.10	77.75	88.91
BY5	3 674.5	cutting	60.67	0.67	20.60	5.93	0.07	2.39	3.09	0.82	3.28	76.70	88.40
BY5	3 686.5	cutting	65.17	0.67	21.28	5.84	0.06	1.44	0.65	0.96	3.36	75.76	87.04
BY5	3 713.5	cutting	65.70	0.64	19.50	6.11	0.07	1.81	1.18	1.17	2.83	73.80	83.51
BY5	3 758.5	cutting	63.45	0.62	19.56	6.29	0.08	1.79	2.43	1.28	2.52	73.77	82.27
BY5	3 764.5	cutting	61.79	0.37	20.08	6.71	0.08	2.80	2.57	1.31	3.05	72.49	82.33
BY5	3 866.5	cutting	59.79	0.44	19.89	6.71	0.07	2.67	1.88	1.43	3.86	69.10	80.87
BY5	3 950.5	cutting	60.58	0.37	20.51	6.57	0.04	2.83	1.00	1.47	4.09	70.27	82.87
BY5	4 049.0	cutting	62.68	0.31	20.14	5.72	0.04	2.40	1.87	1.13	4.08	71.20	84.42
PAAS	–	–	62.80	1.00	18.90	7.22	0.11	2.20	1.30	1.20	3.70	–	–
UCC	–	–	66.60	0.64	15.40	5.04	0.10	2.48	3.59	3.27	2.80	–	–
Note: PAAS represents post-Archean Australian shale, UCC represents upper continental crust.

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4.2 Trace element analysis

The detailed REE contents are presented in Tables 2 and 3. The trace element compositions and the ratios of La/Th, La/Sc, and Co/Th for the mudstone samples in the Enping Formation are given in Table 4.

Table  2.  Rare earth element compositions (mass fractions) of the mudstone samples from the lower Oligocene Enping Formation

Well	Depth/m	La/10–6	Ce/10–6	Pr/10–6	Nd/10–6	Sm/10–6	Eu/10–6	Gd/10–6	Tb/10–6	Dy/10–6	Ho/10–6	Er/10–6	Tm/10–6	Yb/10–6	Lu/10–6
BY1	2 653.0	31.51	58.71	5.58	20.12	3.43	0.95	2.69	0.35	1.97	0.37	1.03	0.15	1.04	0.16
BY2	4 628.6	42.15	81.05	9.05	33.63	6.26	1.26	5.06	0.77	4.84	1.02	3.20	0.50	3.60	0.55
BY3	4 293.2	49.73	97.36	10.66	40.27	8.14	1.29	7.17	1.18	7.14	1.44	4.29	0.63	4.26	0.62
BY3	4 295.0	57.18	103.40	10.89	38.80	6.86	1.20	5.82	0.90	5.21	1.03	3.06	0.45	2.99	0.44
BY3	5 091.0	57.68	114.40	12.64	47.32	9.42	1.80	8.39	1.26	7.50	1.48	4.38	0.63	4.30	0.63
BY4	3 062.0	19.06	43.08	4.55	17.71	3.62	0.76	3.52	0.50	2.89	0.58	1.66	0.24	1.66	0.24
BY4	3 099.5	24.51	56.19	5.81	22.53	4.46	0.95	4.52	0.63	3.52	0.71	2.03	0.29	1.97	0.30
BY4	3 114.5	24.10	53.16	5.50	20.71	4.04	0.83	3.61	0.54	3.20	0.62	1.81	0.26	1.77	0.26
BY4	3 144.5	29.99	66.06	6.79	25.32	4.93	0.99	4.32	0.67	3.75	0.70	2.04	0.29	2.08	0.30
BY4	3 146.0	30.87	68.65	7.19	28.09	5.43	1.13	5.41	0.75	4.20	0.84	2.40	0.33	2.27	0.34
BY4	3 198.5	36.07	82.43	8.67	34.00	6.84	1.42	6.86	0.95	5.41	1.04	2.95	0.41	2.80	0.40
BY5	3 587.5	38.35	81.49	8.37	31.14	5.85	1.07	4.93	0.76	4.36	0.84	2.44	0.35	2.38	0.34
BY5	3 599.5	36.86	81.63	8.72	32.85	6.22	1.27	6.60	0.84	4.53	0.87	2.65	0.37	2.57	0.37
BY5	3 644.5	28.50	62.04	6.65	25.56	4.95	1.20	5.54	0.73	3.97	0.76	2.23	0.31	2.11	0.31
BY5	3 647.5	35.84	76.04	8.03	29.74	5.73	1.05	4.90	0.74	4.30	0.83	2.41	0.36	2.37	0.35
BY5	3 668.0	42.84	88.51	9.51	35.10	6.58	1.16	5.22	0.78	4.51	0.88	2.57	0.38	2.56	0.37
BY5	3 674.5	41.42	88.82	9.78	37.19	7.16	1.49	7.43	0.97	5.26	1.04	3.13	0.43	2.94	0.43
BY5	3 686.5	41.29	88.64	9.68	37.14	7.05	1.47	7.51	0.99	5.22	1.02	3.04	0.43	2.92	0.43
BY5	3 713.5	38.06	81.67	8.86	33.15	6.03	1.16	6.05	0.76	4.13	0.85	2.57	0.36	2.49	0.37
BY5	3 758.5	37.89	80.62	8.77	33.34	6.12	1.25	6.32	0.82	4.29	0.83	2.52	0.35	2.42	0.36
BY5	3 764.5	38.70	80.33	8.47	31.58	5.80	1.06	4.85	0.74	4.14	0.79	2.26	0.33	2.22	0.32
BY5	3 866.5	47.97	97.99	10.61	39.50	7.46	1.28	6.07	0.89	4.96	0.94	2.72	0.41	2.91	0.44
BY5	3 950.5	41.39	81.86	9.12	33.77	6.25	1.24	5.30	0.80	4.44	0.83	2.45	0.36	2.49	0.38
BY5	4 049.0	43.42	89.40	9.50	35.16	6.74	1.20	5.81	0.89	5.00	0.96	2.74	0.40	2.76	0.40
PAAS	–	38.20	79.60	8.83	33.90	5.55	1.08	4.66	0.77	4.68	0.99	2.85	0.41	2.82	0.43
UCC	–	31.00	63.00	7.10	27.00	4.70	1.00	4.00	0.70	3.90	0.83	2.30	0.30	2.00	0.31
Note: PAAS represents post-Archean Australian shale, UCC represents upper continental crust.

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Table  3.  Calculated results for rare earth elements

Well	Depth/m	LREE/10–6	HREE/10–6	LREE/HREE	ΣREE/10–6	(La/Yb)N	(La/Sm)N	(Gd/Yb)N	δEu	δCe
BY1	2 653.0	120.30	17.80	6.76	138.10	20.45	5.78	2.09	0.92	0.99
BY2	4 628.6	173.40	48.51	3.57	221.91	7.90	4.23	1.14	0.67	0.95
BY3	4 293.2	207.45	68.60	3.02	276.05	7.87	3.84	1.36	0.50	0.97
BY3	4 295.0	218.32	50.16	4.35	268.48	12.89	5.25	1.57	0.57	0.94
BY3	5 091.0	243.26	70.59	3.45	313.85	9.05	3.85	1.58	0.61	0.98
BY4	3 062.0	88.79	24.67	3.60	113.46	7.76	3.31	1.72	0.64	1.08
BY4	3 099.5	114.45	29.98	3.82	144.43	8.38	3.45	1.85	0.64	1.10
BY4	3 114.5	108.35	28.61	3.79	136.96	9.16	3.75	1.64	0.65	1.07
BY4	3 144.5	134.09	33.11	4.05	167.19	9.72	3.82	1.67	0.64	1.07
BY4	3 146.0	141.37	35.20	4.02	176.57	9.15	3.58	1.92	0.63	1.07
BY4	3 198.5	169.42	44.43	3.81	213.85	8.69	3.32	1.98	0.63	1.09
BY5	3 587.5	166.27	39.17	4.25	205.44	10.85	4.13	1.67	0.59	1.05
BY5	3 599.5	167.53	39.66	4.22	207.19	9.65	3.73	2.07	0.60	1.06
BY5	3 644.5	128.90	34.22	3.77	163.12	9.12	3.62	2.12	0.70	1.05
BY5	3 647.5	156.43	38.67	4.04	195.10	10.18	3.93	1.66	0.59	1.04
BY5	3 668.0	183.70	40.75	4.51	224.45	11.27	4.10	1.65	0.59	1.01
BY5	3 674.5	185.85	45.78	4.06	231.63	9.51	3.64	2.04	0.62	1.03
BY5	3 686.5	185.26	45.54	4.07	230.80	9.55	3.68	2.08	0.61	1.03
BY5	3 713.5	168.92	37.02	4.56	205.95	10.33	3.97	1.96	0.58	1.03
BY5	3 758.5	167.99	37.61	4.47	205.60	10.57	3.89	2.11	0.61	1.03
BY5	3 764.5	165.94	36.74	4.52	202.68	11.74	4.20	1.76	0.59	1.02
BY5	3 866.5	204.81	44.23	4.63	249.04	11.13	4.05	1.68	0.57	1.00
BY5	3 950.5	173.63	40.01	4.34	213.64	11.19	4.16	1.71	0.64	0.97
BY5	4 049.0	185.42	44.87	4.13	230.28	10.59	4.05	1.70	0.57	1.02
Note: ΣREE: total rare earth elements, LREE/HREE: light to heavy rare earth element ratio.

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Table  4.  Trace element compositions (mass fractions) and the ratios of La/Th, La/Sc and Co/Th for the mudstone samples from the Enping Formation

Well	Depth/m	Hf/10–6	La/10–6	Th/10–6	Sc/10–6	Co/10–6	La/Th	La/Sc	Co/Th
BY1	2 653.0	4.25	31.51	13.51	10.88	10.62	2.33	2.90	0.79
BY2	4 628.6	11.15	42.15	17.01	13.13	17.41	2.48	3.21	1.02
BY3	4 293.2	6.46	49.73	28.43	14.57	8.91	1.75	3.41	0.31
BY3	4 295.0	5.93	57.18	32.01	16.25	5.39	1.79	3.52	0.17
BY3	5 091.0	7.64	57.68	26.13	14.98	10.21	2.21	3.85	0.39
BY4	3 062.0	2.69	19.06	10.75	9.96	8.57	1.77	1.91	0.80
BY4	3 099.5	3.30	24.51	14.54	11.73	11.18	1.69	2.09	0.77
BY4	3 114.5	2.85	24.10	12.13	11.49	10.11	1.99	2.10	0.83
BY4	3 144.5	4.00	29.99	13.96	12.97	10.52	2.15	2.31	0.75
BY4	3 146.0	4.29	30.87	16.93	13.13	13.21	1.82	2.35	0.78
BY4	3 198.5	4.83	36.07	18.32	15.16	13.77	1.97	2.38	0.75
BY5	3 587.5	3.11	38.35	20.27	12.43	10.96	1.89	3.09	0.54
BY5	3 599.5	3.64	36.86	22.29	11.80	9.37	1.65	3.12	0.42
BY5	3 644.5	3.08	28.50	16.81	10.38	13.04	1.70	2.75	0.78
BY5	3 647.5	2.67	35.84	19.21	11.91	8.73	1.87	3.01	0.45
BY5	3 668.0	3.27	42.84	20.69	13.04	11.36	2.07	3.29	0.55
BY5	3 674.5	4.59	41.42	22.92	13.61	10.87	1.81	3.04	0.47
BY5	3 686.5	4.64	41.29	22.95	13.41	12.47	1.80	3.08	0.54
BY5	3 713.5	4.29	38.06	20.47	12.72	13.63	1.86	2.99	0.67
BY5	3 758.5	4.24	37.89	20.32	12.30	12.21	1.86	3.08	0.60
BY5	3 764.5	2.92	38.70	18.89	13.67	10.98	2.05	2.83	0.58
BY5	3 866.5	10.70	47.97	22.09	13.13	11.91	2.17	3.65	0.54
BY5	3 950.5	7.31	41.39	17.16	12.67	13.00	2.41	3.27	0.76
BY5	4 049.0	5.32	43.42	19.59	11.93	8.91	2.22	3.64	0.45

 | Show Table

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The mudstone samples generally have high REE contents, from 113.46×10^-6 to 313.85×10^-6 with an average of 205.66×10^-6 (Table 3). The light REEs (LREEs) are relatively enriched, with LREE/heavy REE (HREE) ratios of 3.02–6.76 (4.16 on average). The chondrite-normalized values of (La/Yb)_N are 7.76–20.45 (10.28 on average), and (La/Sm)_N ratios are 3.31–5.78 (3.97 on average). The HREEs are relatively flat, with (Gd/Yb)_N values of 1.14–2.12 (1.78 on average). Clear negative Eu anomalies are observed, and δEu (δEu = Eu/Eu* = 2(Eu)_N/[(Sm)_N+(Gd)_N], where N means chondrite-normalized) ranges from 0.50 to 0.92 with an average of 0.62. The Ce anomalies are not evident, and δCe (δCe = Ce/Ce* = 2(Ce)_N/[(La)_N+(Pr)_N]) ranges from 0.94 to 1.10 with an average of 1.03 (Table 3).

4.3 Zircon U-Pb ages

The kernel density estimation diagrams of U-Pb ages with relative ages are shown in Fig. 3. The CL images of representative zircons from the studied samples together with spot ages are also shown in Fig. 3. The Th/U ratios versus U-Pb ages of concordant zircons are plotted in Fig. 4. The upper Enping Formation in the Baiyun Sag has developed three stages of deltas: delta I, delta II and delta III. During the sedimentary period of the Zhuhai Formation, the deltas continued to develop in succession (Zhang et al., 2014).

Figure  3.  Results of the zircon U-Pb ages of the Oligocene sediments in the Baiyun Sag. The detrital zircon age concordance diagrams (a, d, g and j), and the kernel density estimations of the U-Pb age data with the relative ages (b, e, h and k), and the cathodoluminescence images, analyzed points, and geologic ages of detrital zircons (c, f, i and l).

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Figure  4.  Th/U ratios versus U-Pb ages of concordant zircons from the Oligocene sediments in the Baiyun Sag.

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For the samples collected from the upper Enping delta I, forty zircon grains were analyzed, and thirty-one usable ages were obtained (Figs 3j and k). The morphologies of these zircon grains show a wide range of shapes from prismatic crystals to oval grains with mostly subrounded corners (Fig. 3l), suggestive of middle distance grain transport prior to deposition. In addition, some zircon grains are incomplete, and they may have been damaged during transport. The zircon grains are stubby to elongate and variable in size, with the largest grains having lengths of around 230 μm, but most zircons range from 70 μm to 115 μm in length. Most grains show oscillatory growth zoning in CL (Fig. 3l) images and have high Th/U values (>0.4) (Fig. 4d), indicating that the majority of the analyzed zircons are of magmatic origin (Wu and Zheng, 2004). The measured ²⁰⁶Pb/²³⁸U (<1 000 Ma) and ²⁰⁷Pb/²⁰⁶Pb (>1 000 Ma) ages range from 1 933 Ma to 67.7 Ma. Seven grains show Precambrian ages from (1 933±35) Ma to (704±6.3) Ma, four grains have Ordovician ages from (483±5.5) Ma to (443±4) Ma, four grains show Triassic ages from (241±3.2) Ma to (236±2.3) Ma, eight grains display Jurassic ages from (169±2.1) Ma to (148±1.6) Ma, and seven grains show Cretaceous ages from (137±2.0) Ma to (67.7±0.9) Ma. Statistically, the overall age spectrum shows four major age peaks ca. 68 Ma, 136 Ma, 168 Ma and 239 Ma, along with several subordinate age peaks ca. 443 Ma, 705 Ma, 1 236 Ma and 1 949 Ma (Fig. 3k).

For the samples collected from the upper Enping delta II, forty zircon grains were analyzed, and thirty-two usable ages were obtained (Figs 3g and h). These zircon grains are colorless and transparent and show a wide range of morphologies from prismatic crystals to oval grains with mostly subrounded corners (Fig. 3i), suggestive of middle distance grain transport prior to deposition. The zircon grains are stubby to elongate and variable in size, with the largest grains having lengths of around 120 μm, but most of them range from 50 μm to 75 μm in length. Most grains show oscillatory growth zoning in CL images (Fig. 3i) and have high Th/U values (>0.4) (Fig. 4c), indicating that the majority of the analyzed zircons are of magmatic origin (Wu and Zheng, 2004). The measured ²⁰⁶Pb/²³⁸U (<1 000 Ma) and ²⁰⁷Pb/²⁰⁶Pb (>1 000 Ma) ages range from 2 427 Ma to 58.5 Ma. Three grains show Precambrian ages from (2 427±35.5) Ma to (1 007±9.2) Ma, four grains have Ordovician ages from (474±5.0) Ma to (460±3.8) Ma, three grains display Silurian ages from (440±5.6) Ma to (439±6.1) Ma, four grains show Permian ages from (292±2.7) Ma to (251±2.8) Ma, four grains exhibit Triassic ages from (242±2.5) Ma to (223±3.0) Ma, five grains have Jurassic ages from (169±2.1) Ma to (153±2.1) Ma, and four grains show Cretaceous ages from (128±2.5) Ma to (97.3±1.2) Ma. Statistically, the overall age spectrum shows four major age peaks ca. 59 Ma, 98 Ma, 153 Ma and 241 Ma, along with three subordinate age peaks ca. 441 Ma, 1 828 Ma and 2 452 Ma (Fig. 3h).

For the samples collected from the upper Enping delta III, forty zircon grains were analyzed, and thirty-eight usable ages were obtained (Figs 3d and e). The zircon grains are stubby to elongate and variable in size, with the largest grains having lengths of around 200 μm, but most of them range from 80 μm to 130 μm in length. Most grains show oscillatory growth zoning in CL images (Fig. 3f) and have moderate Th/U values (Fig. 4b), indicating that the grains are of magmatic origin (Wu and Zheng, 2004). The measured ²⁰⁶Pb/²³⁸U (<1 000 Ma) and ²⁰⁷Pb/²⁰⁶Pb (>1 000 Ma) ages range from 2 531 Ma to 116 Ma. Eight grains show Precambrian ages from (2 531±31.2) Ma to (769±6.5) Ma, six grains have Silurian ages from (479±5.1) Ma to (448±5.3) Ma, four grains exhibit Devonian ages from (438±3.9) Ma to (427±3.6) Ma, seven grains display Permian ages from (280±3.6) Ma to (252±3.7) Ma, four grains show Jurassic ages from (167±1.7) Ma to (143±2.1) Ma, and five grains have Cretaceous ages from (134±1.1) Ma to (116±1.1) Ma. Statistically, the overall age spectrum shows four major age peaks ca. 118 Ma, 167 Ma, 260 Ma and 437 Ma, along with several subordinate age peaks ca. 771 Ma, 830 Ma, 1 212 Ma, 1 837 Ma and 2 568 Ma (Fig. 3e).

For the samples collected from the Zhuhai Formation, forty zircon grains were analyzed, and thirty-two usable ages were obtained (Figs 3a and b). The morphologies of these zircon grains with mostly rounded corners (Fig. 3c) suggest long-distance grain transport prior to deposition. The zircon grains, with the largest grains having lengths of around 200 μm, are approximately 100 μm in length. Most grains show good roundness, ring structure, and secondary enlargement in CL images (Fig. 3c) and have low Th/U values (<0.4) (Fig. 4a), indicating that the majority of the analyzed zircons are of metamorphic origin (Wu and Zheng, 2004). The measured ²⁰⁶Pb/²³⁸U (<1 000 Ma) and ²⁰⁷Pb/²⁰⁶Pb (>1 000 Ma) ages range from 3 231 Ma to 141 Ma. Twenty grains show Precambrian ages from (3 231±32.1) Ma to (550±7.0) Ma, two grains have Carboniferous ages from (325±3.1) Ma to (308±3.9) Ma, two grains display Permian ages from (252±2.7) Ma to (251±2.7) Ma, three grains exhibit Jurassic ages from (176±2.6) Ma to (149±1.4) Ma, and two grains show Cretaceous ages from (143±1.7) Ma to (141±2.6) Ma. Statistically, the overall age spectrum shows three major age peaks ca. 149 Ma, 252 Ma and 380 Ma, along with numerous subordinate age peaks ca. 553 Ma, 765 Ma, 886 Ma, 1 160 Ma, 1 678 Ma, 1 923 Ma, 2 207 Ma, 2 522 Ma and 3 250 Ma (Fig. 3b).

5. Discussion

5.1 Weathering conditions

The chemical compositions of clastic sediments are mainly controlled by a number of geological factors, including the source rock composition, intensity of weathering, rate of sediment supply, sorting during transport and deposition, and finally postdepositional weathering (Cullers et al., 1997; McLennan, 1989; Roddaz et al., 2006; Armstrong-Altrin et al., 2013). Currently, the CIA proposed by Nesbitt and Young (1982) is used to evaluate the weathering degree in the provenance of clastic rocks. It is defined as CIA = [Al₂O₃/(Al₂O₃+CaO*+Na₂O+K₂O)]×100, where Al₂O₃, CaO*, Na₂O and K₂O are molar contents, and CaO* refers to only the calcium in silicates, excluding the calcium in calcite, dolomite and apatite. The weathering degree is considered low if CIA ranges from 50 to 60, moderate if CIA ranges from 60 to 80, and high if CIA>80. For the Enping Formation, the mudstone samples exhibit CIA values of 62.61–81.68 (73.88 on average), indicative of a moderate weathering degree (Table 1).

The CIA values can be projected on the triangular diagram of Al₂O₃-(CaO*+Na₂O)-K₂O (A-CN-K) to analyze the weathering history of the parent rock (Nesbitt and Young, 1982). The CIA distribution of the parent rock should be parallel to the A-CN trend (the solid line with an arrow in Fig. 5). However, the distribution may deviate from the theoretical line toward the K₂O end when the sample undergoes potassic metasomatism to some extent (the dotted line with an arrow in Fig. 5). In this case, a line is extended from the K₂O vertex through the sample points to intersect the A-CN theoretical weathering line, and the intersection point is deemed the CIA value before potassic metasomatism (Bhat and Ghosh, 2001). A great majority of mudstone sample points deviate from the theoretical weathering line, and the CIA values range from 62.78 to 85.79 with an average of 74.29 after correction using the mentioned method, indicating that the mudstone experienced moderate weathering.

Figure  5.  Al₂O₃-(CaO*+Na₂O)-K₂O (A-CN-K) triangular diagram showing the weathering degrees of the lower Oligocene Enping Formation mudstones in the Baiyun Sag (modified from Fedo et al. (1995)). The solid line with an arrow indicates the theoretical weathering line, and the dotted line with an arrow indicates the sample undergoes potassic metasomatism to some extent. CIA: chemical index of alteration.

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Some scholars have proposed the CIW to evaluate the weathering degree, with the expression CIW = [Al₂O₃/(Al₂O₃+CaO*+Na₂O)]×100 (molar contents), which removes the K₂O value in the CIA formula to avoid the increment resulting from potassic metasomatism (Condie et al., 1992). The CIW values are 80 for unweathered potassic granite and 100 for fresh potash feldspar (Fedo et al., 1995). Similar to the CIA, the CIW evaluates the extent to which feldspar converts to clay minerals during the weathering process (Price and Velbel, 2003). The CIW values are 68.51–96.11 with an average of 85.65 for mudstone samples (Table 1). The CIW values also indicate that sediments underwent a moderate degree of weathering.

5.2 Parent rock lithology

The chondrite-normalized REE patterns almost coincide (Fig. 6), showing a subparallel trend, and are similar to those of the PAAS and UCC. In summary, the lower Oligocene Enping Formation mudstone samples show enriched LREEs, flat HREEs, negative Eu anomalies, negligible Ce anomalies, and apparent right-inclined REEs after chondrite normalization. These characteristics are almost completely consistent with the PAAS and the UCC representing the average REE composition in the upper crust, indicating that the samples in the study area originated from the upper crust (Taylor and McClennan, 1985). In contrast, slight positive Eu anomalies are observed in samples from well BY5 (Fig. 6), showing potential provenances from intermediate to basic volcanic rocks, which indicates the possibility of other sources in local areas (Chen et al., 2018; Shao et al., 2019; Zhang et al., 2020).

Figure  6.  REE distribution of the lower Oligocene Enping Formation mudstones in the Baiyun Sag. PAAS: post-Archean Australian shale, UCC: upper continental crust.

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On the Hf-La/Th diagram (Fig. 7a), most mudstone samples plot in the field of acid sediments in the upper crust, and only a few mudstone samples plot in the field of ancient sediments from passive margin sources. On the La/Sc-Co/Th diagram (Fig. 7b), all samples plot in the field of acid volcanic rocks, indicating that the parent rocks were acid volcanic rocks. Based on the two diagrams, most mudstones in the Enping Formation are believed to have originated from acid volcanic rocks in the upper crust, with a few ancient sediments whose parent rocks were also acid volcanic rocks in the upper crust after multiple sedimentary cycles.

Figure  7.  The Hf-La/Th diagram (modified from Floyd and Leveridge (1987)) and the La/Sc-Co/Th diagram (modified from McLennan et al. (1993)) for the mudstone samples from the Enping Formation.

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5.3 Provenance evolution

The basement lithology within the ZRMB is very different from that outside the basin, which lays a good foundation for zircon U-Pb dating to identify the provenance of the Oligocene rocks in the Baiyun Sag. Based on the latest research on detrital zircon geochronology in the SCS (Xu et al., 2007; Zhao et al., 2015; Shao et al., 2016a, 2016b; Liu et al., 2017; Wang et al., 2015, 2017), the extrabasinal South China Block and the intrabasinal uplifts in the ZRMB represent the two most important source regions for the Paleogene synrift sequences on the northwestern margin of the Baiyun Sag (Zeng et al., 2019).

The South China fold zone in the northern part of the basin is a provenance system based on the superposition of Precambrian to Paleozoic magmatic episodes and metamorphism and corresponds to complex parent rock lithologies and geologic ages (Yu et al., 2006), while the low uplifts within the basin dominantly contain Mesozoic intermediate-acid igneous basement rocks (Liu and Wu, 2011; Shi et al., 2011). Therefore, the Precambrian to Paleozoic metamorphic and igneous parent rocks are considered an extrabasinal provenance system (Shao et al., 2016a, 2016b; Wang et al., 2017), while the Mesozoic igneous parent rocks in the basement uplifts on the margin of the Baiyun Sag are classified as an intrabasinal provenance system (Li, 2000; Zhou and Li, 2000; Zhou et al., 2008; Yan et al., 2014; Liu et al., 2017). The detrital zircons with metamorphic genesis differ greatly from those with magmatic genesis in morphology and internal structure. Thus, the CL image analyses and U-Pb ages of detrital zircons can be combined to analyze the relative contents of Precambrian to Paleozoic zircons or Mesozoic zircons to identify the supply patterns of the two provenance systems.

Well BY3 is located in the predominant source pathway on the northwestern margin of the Baiyun Sag, which involves three stages of deltas in the upper Enping Formation, i.e., deltas I–III. During the sedimentary period of the Zhuhai Formation, the deltas continued to develop in succession (Zhang et al., 2014). In this study, zircon U-Pb dating of this well is used to study the provenance evolution on the northwestern margin of the Baiyun Sag in the Oligocene.

The U-Pb dating results show a large age span for zircons, with many ages in both the Precambrian to Paleozoic and the Mesozoic. The Mesozoic zircons are the most abundant in the Jurassic, followed by the Cretaceous, and finally the Triassic (Fig. 8). According to the U-Pb ages of zircons in the lithic sandstones in this well, from bottom to top, the content of zircons from Mesozoic igneous rocks decreases from 62% to 50% and further from 29% to 22% in the Zhuhai Formation (Fig. 8), and the Precambrian to Paleozoic detrital zircon content increases gradually. These results indicate that the supply of sediments originated from two provenance systems: the Precambrian-Paleozoic metamorphic rocks in the extrabasinal South China fold zone and the Mesozoic volcanic rocks in the intrabasinal Panyu Low Uplift, and the former supply increased from the early to the late sedimentation period.

Figure  8.  Zircon U-Pb age distributions in the upper Enping Formation to the Zhuhai Formation from Well BY3 in the Baiyun Sag. GR: natural gamma ray.

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6. Conclusions

In this study, major elements, trace elements and detrital zircon U-Pb ages were combined to identify the weathering conditions, parent rock lithologies and provenances of the Oligocene sediments in the Baiyun Sag of the ZRMB. The following conclusions can be drawn:

(1) For the Enping Formation mudstone samples, the CIA values are 62.61–81.68 (73.88 on average), and the CIW values are 68.51–96.11 (85.65 on average), which indicate that the clastic particles in the study area experienced a moderate weathering degree during transport from the provenance to the sedimentary basin.

(2) The lower Oligocene mudstone samples feature enriched LREEs, depleted HREEs, “V”-shaped negative Eu anomalies, negligible Ce anomalies, and apparent inclinations toward the right, similar to the REE distributions and contents of PAAS and UCC and indicative of the upper crust as the provenance. Both the Hf-La/Th and La/Sc-Co/Th diagrams show that the parent rocks were mainly acid igneous rocks from the upper crust.

(3) The analysis of detrital zircon morphologies and U-Pb ages shows two provenance systems during the Oligocene epoch on the northwestern margin of the Baiyun Sag: the Precambrian-Paleozoic metamorphic rocks in the extrabasinal South China fold zone and the Mesozoic volcanic rocks in the intrabasinal Panyu Low Uplift, and the extrabasinal supply gradually became stronger from the early to late sedimentation period. The results indicate that the provenance of the Oligocene deltas underwent a transition from an early proximal source in the intrabasinal system to a late distal source in the extrabasinal system.

Acknowledgements

We thank Xianghua Yang from China University of Geosciences for his help. We specially thank to the anonymous reviewers for their constructive comments and corrections, which have greatly improved the quality of this manuscript.