New interpretation on the provenance changes of the upper Pinghu–lower Huagang Formation within Xihu Depression, East China Sea Shelf Basin

1. Introduction

The East China Sea Shelf Basin (ECSSB) is an extensional basin lying on the southeastern edge of the Eurasia Plate. It is deposited with thick Cenozoic successions bearing plentiful hydrocarbon resources (Liu, 1988; Peng, 2001; Lee et al., 2006; Su et al., 2015; Zhang et al., 2013a, 2016). Significant progress has been achieved on the ECSSB petroleum exploration undertakings since the 1970s, represented by the Xihu Depression situated at its northeastern region (Zhang et al., 2011; Su et al., 2014a; Qin et al., 2017; Wu et al., 2017; Jiang, 2019; Zhou et al., 2019). Numerous studies were carried out on the sedimentary archive of the Xihu Depression including but not limited to the mineral composition maturity, thickness and percentage of sandstones, heavy mineral assemblages, bulk geochemical features, seismic profiles, etc. (Chen, 2001; Xu et al., 2010; Hao et al., 2011; Gao et al., 2013; Qin et al., 2017; Wu et al., 2017; Zhang et al., 2018; Jiang, 2019; Zhao et al., 2020). However, constrained by the lack of certain geological records, or high dependence on a singular analytical technique and provenance indicator, identification of source and provenance evolution of the lower Cenozoic Xihu deposition, especially regarding the sediments of Pinghu and Huagang formations are still in debates.

Many researchers correlated the recognition of abundant Precambrian detrital zircons and garnet mineral within the upper Eocene drill-hole sediments, to the Hupijiao Uplift north of the Xihu Depression (Jiang, 2019; Zhao et al., 2020; Liu et al., 2022). The Diaoyu Island Fold-Thrust-Uplift belt and the Haijiao-Yushan Uplift, on the other hand, were suggested to have played a more important role as nearby source regions (Wang et al., 2011; Gao et al., 2013; Li et al., 2017; Wu et al., 2017; Hou et al., 2019). It was generally implied that the Diaoyu Island and the Haijiao Uplift developed Paleozoic or Proterozoic metamorphic complexes, and upper Jurassic–middle Cretaceous basements beneath the Cenozoic cover, respectively (Chen et al., 2002; Yang et al., 2006; Wang et al., 2011; Tang et al., 2018). However, most studies were solely generated from petrographic observation, gravity-magnetic data processing and other qualitative analyses. In addition, west ward sedimentary transport derived from the Diaoyu Island region was also arguable due to the possible geotectonic and geomorphological hindrance during this time (Zhao et al., 2020).

Regarding the overlain lower Oligocene Huagang sediments, no consensus has been reached either if they were delivered from the similar provenance terranes to that of Pinghu Formation (Zhao et al., 2018). Some scholars considered that the Xihu Depression was dominated by an eastward transport of clastic materials from the Central Uplift Belt during the early Oligocene. There might have been an additional but relatively minor contribution from the Diaoyu Islands to the east (Wang et al., 2002; Xu et al., 2010; Hao et al., 2011). Although further investigation remains required, those short-distance contributors seemingly made more contributions to yielding an overall sedimentary pattern with spatial distinction. By contrast, some researchers preferentially addressed the long-lasting significance of the Hupijiao Uplift over the entire Xihu Depression (Qin et al., 2017; Jiang, 2019).

In any case, most of these interpretations failed to accurately delineate the nature of surrounding provenances and quantify their separate contribution proportion. Whether a provenance change occurred between the Pinghu and Huagang formations is also required to be supported by more reliable geological evidence. This paper integrated heavy mineral composition, geochemical analyses and detrital zircon U-Pb spectra to reveal the sedimentary features of the Pinghu and Huagang formations quantitatively. A source-to-sink comparative restoration was performed by compiling both published and unpublished data from the Haijiao, Yushan Low, Hupijiao Uplifts and the Diaoyu Island Fold-Thrust-Uplift belt. This paper aims to clarify the possible change within the Paleogene Xihu successions, and to provide an improved interpretation of the sedimentary transport processes in this region.

2. Geological background

The ECSSB is located on the eastward extension of the South China continental basement (Fig. 1). There are three major first-order subdivisions for this rifting basin. From the west to east, there are the Western Depression Belt, the Central Uplift Belt and the Eastern Depression Belt. Geographically, the ECSSB is flanked by several NNE-to-SSW trending structural units, including the northeastern South China Zhejiang-Fujian paleo-uplift to the west, the Okinawa Trough, the Ryukyu Magmatic Arc and Ryukyu Trench to the east (Fig. 1).With the ongoing convergence between the Pacific, Eurasia and the Philippine Sea Plate, the ECSSB had experienced complicated geotectonic and sedimentary processes since at least the late Mesozoic (Feng et al., 2003; Zheng et al., 2005; Su et al., 2014b; Tang et al., 2018; Tian et al., 2021; Hou et al., 2022). Due to the NNE-directed oblique subduction of the Pacific Plate, the ECSSB was initiated with half-grabens and grabens within its Western Depression Belt during the late Cretaceous–Paleocene (Zhang et al., 2013b; Dai et al., 2014). This activity was followed by the back-arc basin extension occurring within the Eastern Depression Belt and the first-episode inversion of the Central Uplift Belt during the Eocene (ca. 56–35 Ma) (Fig. 2). Then most of the ECSSB was dominated by a post-rift stage of thermal subsidence, interspersed with several compressional events, resulting insecondary inversion thrust faults. Another episode of inversion took place during the late Miocene, especially in the Xihu region of the Eastern Depression Belt, and led to folding, uplifting and a large scale erosion processes.

Figure  1.  Tectonic units of the East China Sea Shelf Basin (modified after Zhou et al. (1989) and Zhang et al. (2018)).

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Figure  2.  Regional stratigraphic framework of the Xihu Depression (modified after Zhu (2010) and Su et al. (2014b)).

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The Xihu Depression lied within the northeastern section of the ECSSB, and forms a part of the NNE-SSW trending sedimentary basin system (Fig. 1). It stretches for 75–130 km in width and up to 460 km in length, and makes up an extensive area of ca. 5.0×10⁴ km². The Xihu Depression stretched in the middle between the Fujiang and Diaobei Depressions. It was further subdivided as several units, including the western slope belt, the western sub-depression, the central inversion zone, the eastern sub-depression and the eastern fault terrace belt (Fig. 2). Although reliable evidence is still required, its surrounding paleo-orogenesis or uplifts were previously postulated as potential source suppliers, including the Haijiao, Yushan Low, Hupijiao Uplifts and the Diaoyu Islands Fold-Thrust-Uplift Belt (Fig. 1). The Xihu Depression was generated under the overall back-arc extensional setting by developing three main evolutionary phases, including the rifting and faulting episode from the late Cretaceous to the Eocene, the tectonic inversion and depression during the Oligocene–Miocene, and the tectonic quiescence and regional subsidence after the early Pliocene (Fig. 2). To be noted, the Xihu Depression was featured by an absence of Paleocene rifting successions compared to those deposited within the Western Depression Belt (Fig. 2).

As one of the largest reservoirs with huge petroleum and gas potential along the South China continental margin (Lee et al., 2006; Wang et al., 2011; Su et al., 2015; Tang et al., 2018; Zhang et al., 2021a; Zhang and Feng, 2021; Zhao et al., 2021a, 2021b; Zeng et al., 2021; Shao et al., 2022), the Xihu Depression developed an approximate 10 km thickness of Cenozoic sediments, and was sequentially classified into Oujiang, Pinghu, Huagang, Longjing, Yuquan, Liulang, Santan and Donghai formations (Fig. 2). The Pinghu Formation was deposited in a tidal, swamp and fluvial delta environment during the Eocene with the thickness of 2000–3000 m. It mainly consisted of siltstone, silty mudstone and carbonaceous mudstone inter layers, with minor recognition of coal seams (Fig. 2). The Pinghu deposition was terminated by a regional tectonic activity and termed as the Yuquan Movement at the boundary of the late Eocene and the early Oligocene (Shen et al., 2021). The overlying Huagang Formation was considered to yield the main reservoir rocks, which was separated by an angular unconformity from the older sequences. Comprised of 1000–2000 m of sandstone, siltstone and mud stone alternation, the Huagang Formation was dominated by lacustrine-, fluvial- and deltaic-facies sediments. Furthermore, a trend of upward fining-grained was identified within its sandstone layers (Fig. 2). The Huagang deposition was terminated by the Huagang Movement with another angular unconformity during the late Oligocene–early Miocene.

3. Methods and sampling

Detrital zircon U-Pb dating tests were conducted on the Pinghu and Huagang drill-hole sandstone samples at the Shanghai Branch of China National Offshore Oil Corporation (CNOOC) Co., Ltd. (Table 1). The assigned stratigraphicages were adopted based on unpublished seismic and paleontological data from CNOOC. Zircon grain collection, mounting, Cathodoluminescence (CL) imaging and LA-ICP-MS isotopic dating were conducted at the State Key Laboratory of Marine Geology, Tongji University using Thermo Elemental X-Series ICP-MS coupled to a New Wave 213 nm laser ablation system. In order to obtain better sensitivity, helium gas was used as carrier gas, and argon gas is was used as compensation gas during the laser ablation process. 10 Hz ablation frequency with a 30 μm spot size was used in the laser beam setting with a background signal of 30 s and a sample signal of 70 s. The international standard zircon 91500 (1065.4±0.3 Ma) was used as an external standard, together with the zircon standard Plešovice (337.1±0.4 Ma) to monitor the accuracy of the tests. U-Pb isotopic ratio and age calculations were performed using ICPMS DataCal software, and common Pb correction was performed based on Andersen (2002) method. According to the suggestion of Compston et al. (1992), calculated ²⁰⁶Pb/²³⁸U ages were used for zircons with the age younger than 1000 Ma, while ²⁰⁷Pb/²⁰⁶Pb ages were used for zircon with the age older than 1000 Ma. The finalized ages were selected from a subset of both ≤10% discordance and ≤10% uncertainty (1 σ). The detailed analytical test procedures were described in previous studies (e.g., Shao et al., 2019; Cui et al., 2021a; Zhang et al., 2021b).

Table  1.  The information of drill-core sediments, Pinghu-Huagang Formation, Xihu Depression

Drill-core No.	Sample No.	Strata	Depth/m	Lithology	Method
T-1	T-1A	Pinghu Fm.	3744	sandstone	heavy mineral/elemental geochemistry
	T-1B	Huagang Fm.	3209	sandstone	heavy mineral
	T-1	Pinghu Fm.	3743	sandstone	zircon U-Pb dating
T-12	T-12A	Pinghu Fm.	3439	sandstone	heavy mineral
	T-12B	Huagang Fm.	3684.5	sandstone	heavy mineral
D-1	D-1A	Pinghu Fm.	3853	sandstone	heavy mineral
	D-1B	Huagang Fm.	3045	sandstone	heavy mineral
T-6	T-6A	Pinghu Fm.	3898	sandstone	heavy mineral
	T-6B	Huagang Fm.	3366	sandstone	heavy mineral
B-3	B-3	Pinghu Fm.	3408−4126	sandstone	elemental geochemistry
Q-1	Q-1A	Pinghu Fm.	4414−4516	sandstone	elemental geochemistry
	Q-1B	Huagang Fm.	4354−4404	sandstone	elemental geochemistry
H-29	H-29A	Pinghu Fm.	3175−3204	sandstone	zircon U-Pb dating
	H-29B	Huagang Fm.	3030−3060	sandstone	zircon U-Pb dating
G-1	G-1	Huagang Fm.	3552	sandstone	zircon U-Pb dating
N-13	N-13	Baoshi Fm.	4136−4182	sandstone	zircon U-Pb dating
B-1	B-1	Lowermost Pinghu Fm.	3341.68	sandstone	zircon U-Pb dating

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Geochemical analyses were also performed on the clastic samples of the Pinghu and Huagang formations based on the recommendation of Chen et al. (2018). The samples preparation and geochemical tests were carried out at the State Key Laboratory of Marine Geology, Tongji University. The bulk sediments were washed with deionized distilled water, and dried in oven at 50℃ for 48 h to prevent external contamination. Approximately 3 g of each crushed sample was then heated to 600℃ for 2 h to remove organic matter and inter layer water. After dissolved in a HF-HNO₃ mixture, the major and trace element concentration of the samples were determined by ICP-AES (Thermo ICP-IRIS Intrepid II) and ICP-MS (Thermo Elemental X-Series), respectively. Three certified materials (i.e., GSR-5, GSR-6, and GSD-9 from the Institute of Geophysical and Geochemical Exploration, China) were used as unknown samples and repeatedly analyzed to assess the precision of the test. The deviation of external precision (1 σ) was less than 5%. The major and trace element concentrations from the tests were in agreement with the recommended data of these reference materials. Seven sediment samples from this study were measured in duplicate, and the relative differences in concentrations between duplicates were usually lower than 5%.

Heavy mineral analyses were conducted on the Pinghu and Huagang Formation sediments at the laboratory of the Institute of Regional Geology and Mineral Resources, Hebei. Samples were first dried, gently disaggregated and sieved through a 420 μm mesh. Detrital heavy mineral components were separated from the bulk sediments by a centrifugal elutriation. Further mineral separation was achieved by magnetic and electrostatic filters and heavy liquids. A total of about 1000 non-opaquegrains were identified under the binocular microscope (Cui et al., 2022).

4. Results

4.1 Heavy mineral assemblages

The majority of heavy mineral compositions in percentages for eight Pinghu and Huagang samples were illustrated in Fig. 3. The major types of heavy minerals were basically identical among different samples. The major heavy minerals of this study were garnet, zircon, tourmaline, magnetite and rutile, with subordinate quantities in apatite, monazite, leucoxene and chrome spinel. Minerals of hornblende, epidote and staurolite were in extremely limited concentration. In general, the samples were dominated by stable heavy mineral assemblages, and relatively absent from less stable or unstable minerals (lower than 2% in concentration).

Figure  3.  Heavy mineral assemblages of upper Pinghu (left) and lower Huagang (right) samples.

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Except for Sample T-6A (21.43%), the upper Pinghu Samples T-1A, T-12A and D-1A the respective predominance of zircon mineral (over 50% in abundance of the total non-opaque heavy minerals) were 71.07%, 63.71% and 54.20%. Besides, 5.39% tourmaline, 16.20% rutile and 7.27% chrome spinel of the total non-opaque heavy minerals were found in Sample T-1A. Sample T-12A contained 13.35% tourmaline and 15.36% rutile. 11.13% tourmaline, 15.36% rutile and 5.63% chrome spinel were identified in Sample D-1A. All the samples were also comprised <5% abundance of leucoxene, apatite, monazite and hornblende minerals. Notably, the mode of garnet mineral (7.58% and 13.29% respectively) for Samples T-12A and D-1A were smaller than Sample T-6A (76.01%) (Fig. 3).

The composition and percentage of the lower Oligocene samples (lower Huagang Formation) comprised extremely significant abundance in garnet mineral, which were exceeding 70% (73.43% for T-1B, 85.51% for T-12B, 90.29% for D-1B and 77.49% for T-6B). The other stable heavy minerals were decreased largely in proportion. Sample T-1B comprised of 13.17% zircon, 7.16% tourmaline and 5.53% rutile. Sample T-12B comprised 8.62% zircon, 2.39% tourmaline and 3.49% rutile. Sample D-1B comprised 3.31% zircon, 3.37% tourmaline and 2.59% rutile. In addition, the compositions of other minerals such as apatite, monazite, chrome spinel and hornblende were below 0.1%.

4.2 Geochemical variations and discrimination results

Elemental geochemical results were shown in Fig. 4a. The geochemical discrimination of major oxide ratios for drill-holes B-3 and T-1A were varied among different depositional stages of the Pinghu Formation. Drill-holes B-3 and T-1A were located in north and south separately (Fig. 1). The Na₂O/SiO₂ ratios of the northern drill-hole B-3 was tended to increase from the lower to upper Pinghu deposition and with constant values of Na₂O over K₂O. Both the Na₂O/SiO₂ and Na₂O/K₂O values of southern drill-hole T-1A were remained constant. In general, the discrimination plots based on Na₂O/SiO₂ versus Na₂O/K₂O visually showed a larger overlap between Samples B-3 and T-1 in their Eocene depositions (Fig. 4a).

Figure  4.  Geochemical discrimination plots of different depositional stages of Pinghu Formation samples (a) and elemental changes between Pinghu and Huagang depositions of drill-hole Q-1 (b).

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Major and trace elemental indicators and discrimination diagrams of the samples drill-hole Q-1 were also illustrated in Fig. 4. A modest degree of overlapping was observed between the Pinghu and Huagang groups. However, most of the Pinghu and Huagang results were fallen in different ranges. The phosphorous concentration and the ratio of Na₂O/SiO₂×100 of Pinghu samples were ranged from 0.08 to 0.14 and 1.56 to 2.36 respectively. However, the respective phosphorous oxide content and Na₂O/SiO₂×100 of Huagang samples were ranged from 0.05 to 0.16 and 1.47 to 1.75. Similarly, discrimination diagram of Sr/Ba versus Zr/Sr also demonstrated correlation between Pinghu and Huagang sediments but the differences were clearly observed based on the values of Zr/Sr (7.8 to 16.5, and 6.3 to 21.8). Furthermore, the ratio of Sr/Ba of Pinghu sediments were identified of a wider range between 0.03 and 0.19, if compared to Huagang values (0.08–0.14). For the plot of Ni/Co versus U/Th, it was observed that the Ni/Co values (2.52–2.92) of Pinghu sediments were larger than Huagang samples (2.31–2.64). The compositions of ferrous and magnesium compounds were mostly ranged between 5.79% and 9.03%. The relatively positive relationship between the Pinghu and Huagang samples was observed from the diagram of MgO/Al₂O₃ versus Fe₂O₃+MgO. However, the MgO/Al₂O₃ values of the Pinghu group (0.11–0.14) were different with those samples from Huagang (0.10–0.11) (Fig. 4).

4.3 Spectra of detrital zircon U-Pb age

Samples T-1, G-1, H-29A and H-29B were obtained from borehole-penetrated sedimentary cutting of the Xihu Depression (Fig. 5). Boreholes T-1 and G-1 were located within the southern region, while boreholes H-29 was drilled farther north (Fig. 1). Sample T-1 was collected at the depth of 3743 m from the upper Pinghu Formation. Zircon grains of T-1 were mostly euhedral to subhedral in shape with clear core-rim oscillatory structures. Sample T-1 comprised of relatively variable Th (1.61–1584 ppm) and U (50.8–5300 ppm) concentrations with yielding Th/U ratios of 0.11 and 1.81. Most of zircons of Sample T-1 were identified as magmatic origin excepted two zircons samples. The Th/U values of these two zircons were ranged from 0.01 and 0.06 and indicated the presence of metamorphic grains. From the total 71 test results, 41 test results indicated that ²⁰⁶Pb/²³⁸U ages were ranged between 982 Ma and 102 Ma, and 30 test results indicated ²⁰⁶Pb/²⁰⁷Pb ages were ranged between 2671 Ma and 1278 Ma. In general, Sample T-1 demonstrated a typical multi-modal pattern featured by the predominance of an Indosinian peak centered at ca. 249 Ma. Other secondary age peaks mainly included the Yanshanian (ca. 127 Ma), Caledonian (ca. 404 Ma) and large groups of Paleoproterozoic (ca. 1828 Ma) and Archean (ca. 2551 Ma) zircons. However, the clusters of Phanerozoic ages were much higher than those Precambrian peaks (Fig. 5).

Figure  5.  Detrital zircon U-Pb age spectra of Pinghu and Huagang samples.

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Sample G-1 was collected at the depth of 3552 m from the lower Huagang Formation (Fig. 1). Their zircon crystals were subhedral, subrounded and rounded in shape with clear concentric zoning. The Th and U concentrations were varied within 26.65–2161.6 ppm and 28.63–3390.10 ppm, respectively. Most of Th/U ratios were ranging between 0.12 and 2.03 and indicating magmatic origin. Metamorphic origin was suggested for the samples with Th/U of 0.07. From the 97 test results, 38 test results indicated that ²⁰⁶Pb/²³⁸U ages were ranged from 923–30.8 Ma, and 59 test results indicated that ²⁰⁶Pb/²⁰⁷Pb ages were ranged from 2598–1204 Ma. The youngest grain (30.8±0.8 Ma) represented the maximum depositional age. These 97 test results demonstrated a broad range with multiple age clusters. In contrast to Sample T-1, Sample G-1 was dominated by a significant Paleoproterozoic population at ca. 1846 Ma together with subordinate peaks of an early Indosinian (ca. 295 Ma) and Archean ages (Fig. 5).

Drill-hole H-29 was located at the most northern of the Xihu Depression, and is extracted together with Pinghu (H-29A) and Huagang (H-29B) samples at the depth of 3175–3204 m, 3030–3060 m respectively (Fig. 1). Zircon crystals from sample H-29A mostly retain their subrounded to rounded shapes with clear zonation. A total of 138 effective tests were conducted. Twenty spots of the total generate Th/U ratios were ranging between 0.01 and 0.10, while the remainders were varied largely from 0.11 to 2.71. Sixty-two test results indicated that the ²⁰⁶Pb/²³⁸U age were ranged between 908 Ma and 32.6 Ma, while seventy-six test results indicated that the ²⁰⁶Pb/²⁰⁷Pb ages were varied extensively from 1098 Ma to 2705 Ma. The youngest age of 32.6 Ma was suggested as maximum depositional age. Sample H-29A yields a dominant Paleoproterozoic group at ca. 1837 Ma in addition to several minor age peaks, including Yanshanian (ca. 127 Ma), Indosinian (ca. 213 Ma), Caledonian (ca. 403 Ma), Jinningian (ca. 750 Ma) periods and scattered Archean grains. The zircon grains of overlying Huagang sample H-29B were identified with subrounded to rounded shapes. From the total of 151 effective test results, 14 samples yielded with Th/U ratios lower than 0.1, and were possibly reflecting a metamorphic origin. However, the others implied magmatic origin with a wide range Th/U ratios between 0.11 and 2.71. Fifty-two test results indicated that the ²⁰⁶Pb/²³⁸U ages were ranged from 984–96.7 Ma, and ninety-nine test results indicated that the ²⁰⁶Pb/²⁰⁷Pb ages were ranged from 2820–1019 Ma. The U-Pb age distribution pattern from sample H-29B resembled with the overlain Pinghu sample, exhibiting a prominent Paleoproterozoic cluster at ca. 1846 Ma. In addition, other minor populations of the Indosinian, Jinningian and Archean ages were in significantly lower proportion.

5. Discussion

5.1 Potential source areas

From north to south, the Central Uplift Belt of the ECSSB consists of three structure units including the Hupijiao, Haijiao and Yushan Low Uplifts. A large angular unconformity was revealed between their Mesozoic or even older basement and the direct deposition of the Neogene sedimentary cover. No presence of the Paleogene strata was documented beneath the 2000–4000 m of the Neogene thick successions. Due to the limitation of the ECSSB basement-penetrated drillings and low precision of the dating techniques, these possible provenances were roughly investigated via magnetic anomaly data, indirect postulations from the continental outcrops, etc. (Chen et al., 2002; Liu et al., 2003; Liang et al., 2006).

WZ6-1-1 was an early-extracted borehole sample with biotite plagioclase gneiss rocks, yielding a wide Paleoproterozoic Rb-Sr age span of 1970–1806 Ma. Occurrence of these late Paleoproterozoic zircons also seems to be consistent with the similar gneiss rocks from the farther northern drill-holes JDZ-V-2 and KV-1 near the Korean Peninsula. Since zircon age spectra of river sediments bear quite ample information of the basement geology of their catchments, surface sediments near the Hupijiao Uplift were recompiled in this study (Fig. 6). Featured by a predominant late Paleoproterozoic peak centered at ca. 1854 Ma, the Hupijiao Uplift showed great consistence with most of modern rivers across the northern Korea (Choi et al., 2013, 2016) (Fig. 6). Adjacent to the Haijiao Uplift margin, Sample NB-13 was extracted from the Baoshi Formation deposited directly after the initial rifting stage (Fig. 6). It was deposited in a fan delta setting and mainly comprised of conglomerates with mudstone, siltstone and volcanic interlayers. Therefore, this sample was used to represent the provenance feature of the nearby Haijiao Uplift (Tang et al., 2018). It developed the Indosinian (ca. 216 Ma) and Paleoproterozoic (ca. 1831 Ma) populations of almost identical percentages, in addition to scattered groups of the Jinningian and Archean–aged zircons. Proximal to the Yushan Low Uplift, another sample B-1 of lower most Pinghu Formation otherwise was confirmed with greater overprinting influence by the Mesozoic arc magmatism. Its overall U-Pb combination pattern was featured by overwhelming Yanshanian zircons if compared to the trail of scattered Indosinian, Jinningian and late Paleoproterozoic zircons (Fig. 6).

Figure  6.  Sketch-up and location map of the Xihu Depression and its potential surrounding source areas (a); detrital zircon U-Pb age combination patterns of the potential source end-members (b). Hupijiao Uplift data was recompiled from Choi et al. (2013, 2016).

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There were many hot debates regarding the originality of ancient Archaen and Paleoproterozoic detrital zircons due to the complex interactions between the North China Craton, the South China Block, the Sino-Korean Block and the Pacific Plate. In general, the Precambrian basement rocks in the South China Block were believed to be widely distributed in the Yangtze Craton, with minor in the southwestern Zhejiang and northwestern Fujian Province of the eastern Cathaysia Block, such as the Paleoproterozoic Badu granite and metamorphic complex, the Mesoproterozoic Wanquan Group, etc.Plentiful zircon U-Pb dating analyses (ca. 1900–1800 Ma and ca. 1800–1760 Ma) of granitoids and meta-mafic rocks revealed a complete tectonic cycle from syn- to post-orogenic, ending up with a rifting setting in this area during the assembly of the Nuna/Columbia supercontinent (e.g., Yu et al., 2009). The South China continental margin was previously considered to largely extend eastward to form the Haijiao and Yushan Low Uplift basements (Liu et al., 2003). Scholars used to imply those offshore Paleoproterozoic metamorphic suites derived from the scattered outcrops of the northeastern Cathaysia Block. The overlying strata was classified as Jurassic–Cretaceous sedimentary and volcaniclastic layers based on previously-processed seismic profiles. Additionally, the northeastern Cathaysia was linked by some researchers to the southern Korean Peninsula, namely the Yeongnam Massif, to form an integrated structure from the Paleoproterozoic to early Mesozoic (Yu et al., 2009). This hypothesized paleo-structure was destroyed and modified into rugged basement units during subsequent geotectonic activities. The remnant of the Hupijiao Uplift situated within the South Yellow Sea Basin area, was envisaged by its large population of the late Paleoproterozoic zircons. In particular, the northern part of the Zhejiang-Fujian orogenic belt experienced significant tectonothermal activation during ca. 250–225 Ma, yielding medium to high grade of metamorphism (Yu et al., 2009). Xu et al. (2007) preferred extremely extensive reworking on the Paleoproterozoic complexes, serious Pb loss and overgrowth of new zircons during Jurassic–Cretaceous (ca. 155–100 Ma). In either case, the Paleo-Pacific subduction initiating since at least the late Permian casted greater influence on the additional South China continental growth by drastic Mesozoic magmatic arc activities (Fig. 6) (Li and Li, 2007; Cui et al., 2021b).

Some researchers considered that the 1900–1800 Ma thermal event was more common in the North China Craton, mainly caused by the collision and assembly of the Western and Eastern blocks along the N-S oriented Trans-North China Orogen during Paleoproterozoic era (e.g. Zhao et al., 2005; Wu et al., 2007). Some scholars linked the synchronism of magmatic and metamorphic events across both North China Craton and the entire Korean Peninsula during ca. 1900–1800 Ma to a certain basement consanguinity between these two blocks (Wu et al., 2007; Lee et al., 2014; Zhang et al., 2017). It is apparently controversial to the aforementioned view that the Yeongnam Massif of the southern Korean Peninsula was affiliated with the northeastern Cathaysia Block (Yu et al., 2009; Oh et al., 2019). In any case, from the initial cratonization to Paleozoic, the North China Craton did rarely undergo intensive magmatism or deformation, and its stability or integrity was not seriously affected. Several Phanerozoic magmatism subsequently took place, such as the late Triassic, Jurassic and early Cretaceous stages of magmatic activities (Zhu et al., 2012). Notably, the crustal modification and destruction were comparatively not homogeneous within the eastern North China Craton. On the one hand, there was deep subduction of the Yangtze Block beneath the North China Craton and resulting in the formation of the Dabie-Sulu Orogen during the late Triassic (Yang et al., 2005). On the other hand, the drastic paleo-Pacificsubduction was also being identified within the eastern North China Craton since at least the late Triassic, and became weakened towards the interior (Zhu and Zheng, 2009). The southern Korean modern river sediments (geographically closer to the present-day Hupijiao Uplift) are distinct from the north by displaying relatively higher Mesozoic clusters (Choi et al., 2016). This is likely to be caused by the frequent shifting in the subduction direction and moving speed of the Pacific Plate. NW-to-SE-oriented strike-slips and large-scale offsets were yielded along the continental margin (Fig. 1). Due to the ongoing westward subduction, the prevailing development of the late Mesozoic magmatism, extensional structures and continental crustal growth commonly indicate the North China Craton destruction during this time. Prior to the Quaternary transgression, the wide Yellow Sea region between the eastern China and Korean Peninsula was non-marine facies (Shinn et al., 2010). It is suggested that the South Yellow Sea Basin was deposited with sediments from multiple sources, with the North China Craton being one of the most significant contributors (Zhu et al., 2020). Therefore, it was also reasonable to postulate that certain paleo-uplifts, such as the Hupijiao Uplift, were actually denuded and supplied detritus with a North China Craton affinity.

5.2 Paleo-environmental changes implied by geochemical compositions

A series of parameters and correlation diagrams are involved in Fig. 4 in order to investigate potential controls on geochemical compositions. Paleo-water depth, paleosalinity and redox environment are often studied by certain elements sensitive to the sedimentary environment, such as major oxides MgO, CaO, Na₂O, K₂O, P₂O₅, various trace elements Sr, Ba, Rb, V, Cr, Ni, Co, etc. Our results show that the contents of major and trace elements as well as related ratios fluctuated with time, implying certain instability of paleo-environment during the Pinghu and Huagang deposition intervals. To avoid misinterpretation, ratios and discrimination functions are preferred instead of judging from a single elemental variation.

In general, K₂O/Na₂O or Al₂O₃/SiO₂ ratios are often linked to sedimentary recycling and are used as proxies of terrestrial detrital input. A modest elevation of both K₂O/Na₂O andNa₂O/SiO₂ values is observed for different stages of B-3Pinghudepositions.This up-going trend might suggest a potential addition of clastic material transport. T-1 to the further south, on the other hand, shows a relatively stability on both indicators (Fig. 4a). Our correlation plots reveal a larger overlap between these two end-members (geochemical analyses of B-3 and T-1, respectively), and possibly reflect increasing homogeneity of the depositional condition.

Trace elements of Ni, Co, Th and U are sensitive to the fluctuation of redox conditions. Namely, their depletion and enrichment can record changes of the oxygenation state in a depositional environment (Jones and Manning, 1994). In addition, Ni/Co lower than 5 and higher than 5 generally indicate oxic and anoxic conditions, separately (Jones and Manning, 1994). Similarly, U/Th is also a good paleo-redox proxy, with ratios <0.75 and >1.25 indicating an oxic and anoxic environment, respectively. Therefore, it is obvious to note that drilling Q-1 documented a typical oxic condition during Pinghu-Huagang deposition (Fig. 4b). As a sensitive paleosalinity indicator, the Sr/Ba ratios have recorded the long-lasting existence of a freshwater sedimentary environment in the studied region. This shows certain consistence with the possible detrital addition monitored by the Zr/Sr, Na₂O/SiO₂ and MgO/Al₂O₃ indicators (Fig. 4b). Basically, Q-1 analyses were also divided into two end-members of the Pinghu and Huagang deposition, separately, for geochemical comparison (Fig. 4b). Major and trace elemental proxy pairs show apparently higher degree of overlapping, and imply a paleo-environmental shifting with time.

5.3 Source-to-sink pathway reconstruction of the upper Pinghu Formation – lower Huagang Formation

The temporal and spatial variations of the zircon U-Pb age spectra indicated multiple transport pathways in the Xihu Depression during the late Eocene–early Oligocene. At least the northern, western and southwestern provenances have been clarified for the Xihu depositions, but exerted influences of different proportion during various stages (Fig. 7).

Figure  7.  Source-to-sink reconstruction of the Pinghu Formation (a) and Huagang Formation (b), Xihu Depression.

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The U-Pb age populations from both Samples H-29A and T-1 revealed that a provenance distinction within different part of the Xihu Depression during the late Eocene (Fig. 5). Sample H-29A was typically featured by a predominant Paleoproterozoic age population (ca. 1850 Ma) together with much lower abundance of Mesozoic and Neoproterozoic zircons. The contemporaneous sample T-1 to the farther south, on the other hand, exhibited extremely high peak of Yanshanian–Indosinian zircons with relatively less abundant Paleoproterozoic zircons (ca. 1830 Ma). Due to the limitation of drilling records, understandings on the Hupijiao Uplift had long been confined by a lack of precise geochronological dating. However, zircon age distributions of river sediments usually provide comprehensive information for the basement geology. Choi et al. (2013) collected several surface sandy sediments proximal to the Hupijiao Uplift, and the consistence between their U-Pb age combination patterns revealed a unique northern provenance to the Xihu Depression (Fig. 6). In addition, their U-Pb age spectrum also shows greater compatibility with the fluvial sediments across the northern Korea Peninsula, probably with tectonic affinity to the North China Craton (Choi et al., 2016). As a possible eastward extension of the northeastern Cathaysia basement remnant, the Haijiao Uplift might also generate a non-neglectable Paleoproterozoic population when compared to the Yushan Low Uplift. However, further evidence was still required for judging the originality of those Paleoproterozoic-aged zircons, which was not the main scope of this study. In addition, the Hupijiao Uplift was more likely to form heavy mineral assemblages of metamorphic origin, such as garnets, to the northern Xihu area. Correspondingly, the tested northern-most sample T-6A displayed higher abundance of garnets relative to other samples to the south (Fig. 3). Both Haijiao and Yushan Low Uplifts might be more related to the high concentration of Mesozoic zircons as two nearby sources (Fig. 6). Noteworthy was that the Haijiao Uplift possibly resulted in a modest Paleoproterozoic group which was also identified within Sample T-1 (Fig. 5).

Both Samples H-29B and G-1 of Huagang depositions exhibited almost identical U-Pb spectra with dominance of Paleoproterozoic clusters and fewer Mesozoic zircons (Fig. 5). Correspondingly, increasing proportion of garnets was widely recognized in the center sag of the southern Xihu Depression, while the compositions of zircons and tourmalines were sharply decreased (Fig. 3). This obviously implies an additional input from a potential source with higher-degree metamorphism. This is largely caused by the northern provenance since the nearby Haijiao and Yushan Low Uplifts are featured by more heavy mineral assemblages of igneous parent rocks. On the one hand, Sample H-29B suggested that the northern Xihu Depression remained influenced from Hupijiao Uplift as a stable provenance, or from the even farther northern region. Sample G-1 suggested that the unique northern contributor might have largely extended its impact and delivered plentiful clastic sediments to the southern part of the Xihu Depression (Fig. 5).

6. Conclusions

This study integrated heavy mineral assemblages, geochemical analyses and detrital zircon U-Pb dating to restore the multiple source-to-sink pathways for the upper Pinghu–lower Huagang depositions of the Xihu Depression. In general, at least three major provenances were identified for their separate compositions, including the Hupijiao Uplift or even farther northern area, Haijiao Uplift to the west and Yushan Low Uplift to the southwest. The overall sedimentary distribution pattern of the Xihu Depression showed significant distinction on both spatial and temporal scale. During the late Eocene, influence of the northern source was relatively limited whereas the southern depression was transported by large abundance of materials from the nearby Haijiao and Yushan Low Uplifts. The northern source expanded its impact substantially to the farther south during the early Oligocene by delivering plentiful sediments of higher-degree metamorphic parent rocks. Combined with the western and southwestern suppliers, the overall Xihu Depression was under control from both long-distance and nearby provenances.