1 INTRODUCTION

The tectonic evolution of the South China Continental Margin (SCCM) since the Late Mesozoic has been a hot topic for many researchers. During the Cretaceous, the SCCM experienced a transition from an Andean-type continental margin to a Western Pacific-type margin in response to the slab roll-back and the increase of subduction dip of paleo-Pacific Plate (Charvet et al., 1994; Lapierre et al., 1997; Zhou and Li, 2000; Shu and Zhou, 2002; Li et al., 2012, 2014). Subsequently, the Cenozoic rifting and the opening of South China Sea (SCS) denote the passive extensional environment in SCCM since the Cenozoic (Shi and Li, 2012; Sun et al., 2016; Wu et al., 2018; Ye et al., 2018; Suo et al., 2019; Jiao et al., 2020). However, it's still controversial about when the continental margin shifted from active subduction to passive extension (e.g., Holloway, 1982; Charvet et al., 1994; Lapierre et al., 1997; Shu et al., 2004; Shi and Li, 2012; Ye et al., 2018). Two main hypotheses were proposed. Hypothesis 1 suggested that the SCCM had turned into passive extensional continental margin before the Late Cretaceous (Holloway, 1982; Lapierre et al., 1997; Mao et al., 1997; Zou, 2001; Shu et al., 2004; Shi and Li, 2012; Zhang et al., 2015). Hypothesis 2 believed that the transition of SCCM from active continental margin to passive extensional continental margin should have happened around the Late Cretaceous to Paleocene boundary (~66 Ma). Before the Paleocene, the northern continental margin of the South China was still under subduction (Zhu et al., 2001; Shen, 2014; Fang, 2016; Ye et al., 2018). Lacking direct sampling and geochronological evidence makes this controversy yet unresolved.

The northern SCS continental margin, located in the southernmost part of the South China Block and near the subduction zone during the Cretaceous, is an ideal place to study the tectonic transition during this period. However, previous studies on the northern SCS continental margin mainly focus on the hydrocarbon reservoirs and the Cenozoic tectonic evolution (Xie et al., 2011; Sun et al., 2019; Li et al., 2021), and comparatively few studies have focused on pre-Cenozoic tectonic evolution of this area. Although gravity-magnetic inversion data, seismic reflection data, and geochronological and geochemical data of igneous and metamorphic rocks from pre-Cenozoic basement have been used to analyze tectonic setting during the Cretaceous (Li et al., 2008; Shi and Li, 2012; Ye et al., 2018; Sun et al., 2021), few pieces of sedimentary evidence (detrital zircon ages, sedimentary geochemical data, and clay mineral assemblages) from the northern SCS continental margin area have been provided. Clay minerals, major and trace elements, rare earth elements (REE) (e.g., La, Ce, Eu), and detrital zircon data from sedimentary rocks could preserve a relatively complete record of provenance and tectonic setting, which have been considered as important indicators for provenance discrimination and tectonic analyses in many studies (Bhatia, 1983; Bhatia and Crook, 1986; Cullers et al., 1988; McLennan et al., 1990; Cawood and Nemchin, 2000). However, less attention on sedimentary evidence prevents us from getting comprehensive understanding of the tectonic transition in the northern SCS area during the Late Cretaceous.

Located in the northernmost of the SCS continental margin (Fig. 1a–b), Sanshui Basin has developed a continuous stratigraphic sequence from the Lower Cretaceous to Eocene with clear sequence boundaries and abundant fossils (Yu et al., 1981; Zhang, 1984; Hou et al., 2007a), which could provide precious outcrops and sedimentary rocks to study sediment sources and tectonic setting of the northern SCS area during the Cretaceous. The Sanshui Formation (Fm) is the stratigraphic unit of Sanshui Basin developed in the Late Cretaceous, which are at the critical period when many scholars have proposed that tectonic transition from subduction to extension had happened. If this tectonic transition occurred in the Late Cretaceous, it would be reflected in the sedimentary records of the Sanshui Fm. Therefore, through petrological, clay mineral, geochemical analyses, and U-Pb dating from Sanshui Fm, we hope to understand the tectonic-sedimentary environment of the Sanshui Basin in the Late Cretaceous, and discuss its implications for the tectonic evolution of the South China continental margin. The improved knowledge of the sedimentary evidence of the northern SCS continental margin may provide a new perspective for us to better understand the tectonic transition from subduction to passive extension during the Late Mesozoic in the northern SCS area.

2 GEOLOGICAL SETTING 2.1 Northern SCS continental margin and Sanshui Basin

The modern northern continental margin of the SCS is located between the South China Fold Belt and the continent-ocean transition of SCS. A series of the Late Mesozoic-Cenozoic basins (e.g., Sanshui, Maoming, Zhujiang (Pearl) River Mouth, and Qiongdongnan Basins; Xie et al., 2011; Sun et al., 2014; Li et al., 2018) lies along the northern SCS margin. In the Mesozoic, the paleo-Pacific Plate was subducting northwestward beneath the southeast Eurasian Plate, and consequently a series of NE trending Mesozoic volcanism developed in the SCCM (e.g., Zou, 2001; Shu et al., 2004; Zhou et al., 2006a; Li and Li, 2007; Shao et al., 2007). The large-scale taphrogenic and magmatic activities generating a huge Basin and Range province in South China. The subduction-related igneous suites in the SCCM had changed due to the transition of subduction from Andean-type to western Pacific-type during the Cretaceous. Previous studies suggest that the widespread I-type granitoids and Nb-Ta depleted and large ion lithophile element enriched basalts in the southeastern South China area during the Jurassic to Early Cretaceous represented the continental arc magmatism caused by Andean-type subduction (Zhou and Li, 2000; Zhou et al., 2006b; Li et al., 2012). In comparison, the development of igneous suites related to the extension (bimodal volcanic suites (rhyolite/basalt) and A-type granitoids) and syn-extensional basin in the southeastern South China area during the Late Cretaceous indicate the extensional environment (Charvet et al., 1994; Zhou et al., 2006b; Li et al., 2014). After the Late Cretaceous, with the termination of the paleo-Pacific subduction, the development of rifting basin, and the opening of the SCS, the northern SCS continental margin was in passive continental margin environment (Zhou et al., 1995; Lapierre et al., 1997; Li and Li, 2007; Savva et al., 2014; Li et al., 2018). Consequently, the northern SCS margin area encompassed a series of tectonic processes from subduction to rifting and seafloor spreading since the Late Mesozoic.

The Sanshui Basin is located on the northern continental margin of SCS. It is a diamond-shaped Meso-Cenozoic continental basin controlled by roughly NS- and NW-trending faults. Sanshui Basin can be divided into nine units in fault structures (Fig. 1b). The major fault-controlled sequences include the Lower Cretaceous to the Lower Tertiary (Fig. 1c). From bottom to top, eight stratigraphic formations were discriminated (Fig. 1d), they are: Lower Cretaceous Baizushan Fm (K₁b) and Baihedong Fm (K₁bh), Upper Cretaceous Sanshui Fm (K₂s) and Dalangshan Fm (K₂d), Paleocene Xinzhuangcun Fm (E₁x), Eocene Buxin Fm (E₂bx), Baoyue Fm (E₂by), and Huayong Fm (E₂h) (Hou et al., 2007a, b). The basement of the basin is composed of carbonate rocks, coal-bearing sandstone and shale of Carboniferous and Permian, as well as inland clastic rocks of Triassic or light gray medium-grained biotite granite of Yanshanian. The basement has undergone multi-stages of tectonic deformations and appear angular unconformity with the above-lying Cretaceous sequences, suggesting that the Late Yanshanian tectonic movement generated the prototype of the basin (Hou et al., 2007a). The development of Sanshui Basin is strongly affected by the tectonic evolution of the northern SCS margin. Therefore, sedimentary rocks from the Sanshui Basin may record some information that could have implications for the tectonic processes of this area.

2.2 Sanshui Fm

Our main study formation in this paper, the Sanshui Fm, is the lower Upper Cretaceous stratum of the Sanshui Basin. The Sanshui Fm is in parallel or angular unconformity with the underlying Lower Cretaceous Baihedong Fm, and in conformity with the overlying Upper Cretaceous Dalangshan Fm. The Sanshui Fm is mainly distributed in the northwest and southeast basin with a maximum thickness of over 600 meters. Its horizontal distribution is significantly larger than that of the Baihedong Fm (Fig. 2). The lithology of the Sanshui Fm is mainly composed of brown reddish glutenite, pebbly sandstone, siltstone and marlstone, with silty mudstone interlayers. There are abundant fossils of animals and plants in Sanshui Fm, for example, Oolithes elongatus and Latochara curtula (Zhang, 1984; Zhang et al., 2008). Oolithes elongatus is the typical dinosaur egg fossil of the Late Cretaceous. Latochara curtula is the common charophyte in the Upper Cretaceous strata of South China. According to the paleomagnetic analysis, Yuan et al. (1994) suggested that the Sanshui Fm belongs to the Cretaceous magnetostatic period with geological age of about 93–80 Ma. Both the fossil and paleomagnetic data confirmed the Sanshui Fm to be deposited in the early Late Cretaceous. Hence, the Sanshui Fm may have recorded the tectonic evolution of Sanshui Basin during the Late Cretaceous, which could provide a new perspective for us to understand the tectonic transition of northern SCS continental margin.

3 MATERIAL AND METHOD

Samples were collected from Gaozui-Yangmeiya in the northern basin (23°33′56.47″N, 113°0′10.99″E), near Guangzhou Iron and Steel Work in the east (23°04′12.36″N, 113°14′24.79″E), and around Cuikeng in the west (23°08′43.92″N, 112°50′00.20″E) (see Fig. 1b for sample localities). The outcrops in the southern basin were poorly preserved and no samples were collected. The details of samples (such as lithology) can be found in the Data Availability Statement.

For clay mineral analyses, 5 samples from the east (sample ID: ssc1–ssc5), 3 samples from the west (sample ID: ssc6–ssc8), and 10 samples from the north (sample ID: ssc9–ssc18) were first dried at 50 ℃, and then crashed into powders by agate mortar. Carbonate and organic matter of samples were removed by acetic acid and hydrogen peroxide (10%), respectively. After rinsed with deionized water for 3 times, the clay-sized fraction (< 2 μm) was separated from the rest of sample by application of Stokes' Law through settling. Detailed experimental procedures can be seen in Zhang (2018). The analysis was identified by X-Ray Diffraction (XRD) using a D2 Phaser diffractometer in the laboratory of School of Ocean Sciences, China University of Geosciences (Beijing). Semi-quantitative estimates of smectite, illite, kaolinite, and chlorite content were carried out on glycolated samples using Jade software with the empirical factor based on Biscaye (1965).

For geochemical analyses, a total of 11 sandstone samples (sample ID: ssg1–ssg11) from the Gaozui-Yangmeiya (the northern Sanshui Fm) were selected for major and trace elements analysis. After heat-dried and grounded, major and trace elements of samples were measured in the laboratory of the Institute of Regional Geology and Mineral Resources, Langfang, Hebei Province, China, using Axios max X-ray spectrometer and X Serise2 ICP-MS, respectively. GSR-1 and JSD-1 were used as the certified materials to assess test precision and accuracy. The analytical precision is generally better than 5% for major elements and 10% for trace elements. Based on Cox et al. (1995), the index of compositional variability (ICV) is calculated as:

ICV=(Fe₂O₃^T+K₂O+Na₂O+CaO+MgO+TiO₂)/Al₂O₃. The Fe₂O₃^T value is all-iron content.

For detrital zircon U-Pb dating analyses, the zircons from matrix of two granitic gravelly sandstone samples (named sample A and sample B) from the northern Sanshui Fm (in Gaozui-Yangmeiya) were collected. Samples were washed and sieved through a 63-μm mesh with deionized water, then dried at 60 ℃. Zircons in the > 63-μm fractions were selecting and making target in the laboratory of the Institute of Regional Geology and Mineral Resources, Langfang, Hebei Province, China, using conventional heavy liquid and magnetic separation techniques. A total of 91 zircon grains from sample A and 89 grains from sample B were extracted. Zircon U-Pb dating was determined on the Finnigan Neptune MC-ICP-MS and the matching New wave UP 213 laser ablation system in the Institute of Mineral Resources, Chinese Academy of Geological Sciences. The zircons SRM 610, GJ-1, and 91500 were used as external standard, and were measured every ten analyses. The testing accuracy of homogeneous zircon particles ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁶Pb/²³⁸U, and ²⁰⁷Pb/²³⁵U is about 2%, and the dating accuracy of zircon standard is about 1%. Detailed experimental process is referred to that in Hou et al. (2009). The best ages were selected from a 90% concordant subset. The ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁶Pb/²³⁸U ages were adopted for zircons older and younger than 1 000 Ma, respectively (Compston et al., 1992). The age data are reported at 1 sigma level of uncertainty (Data Availability Statement). The age results and relevant data are visualized as kernel density estimate (KDE) plots and histograms.

4 RESULT 4.1 Petrography 4.1.1 Petrological features of northern Sanshui Fm

The outcrop of northern Sanshui Fm was observed and sampled in Gaozui-Yangmeiya, the northern part of the basin. An angular unconformity between Sanshui Fm and the underlying Baihedong Fm can be observed in this area (Fig. 3). Below the angular unconformity, Baihedong Fm is composed of interbedded reddish-brownish siltstones and fine to pebbly sandstone. The occurrence of siltstone is 260°∠26°. Horizontal bedding is well developed in this stratum, suggesting a weak hydrodynamic depositional environment. Above the angular unconformity, the occurrence of the Sanshui Fm is 265°∠12°. It is a set of coarse clastic deposits with a thickness of more than 100 m (Fig. 4a). It is mainly composed of reddish to brownish gravelly sandstone and coarse sandstone with pebbles and cobbles. Almost all of the gravels are granitoids, suggesting that the provenance of northern Sanshui Fm was dominated by granitic rocks. Most of gravels show angular and subangular shapes, indicating a short transport process. The gravels are poorly sorted, with size varying from 2 cm to 10 cm (Fig. 4b–d). Under microscope (Fig. 4g), the grains of sandstone from the northern Sanshui Fm show poor roundness and sorting, mostly are angular and subangular. The maturity of texture and composition are low. The grain size ranges from 0.03 mm to 0.5 mm. The clastic particles are supported by matrix. Rock fragments mainly consist of granitoid rock, marlstone and siliceous rock. These petrological evidences suggest that the northern Sanshui Fm shows typical debris flow deposition characteristics. The northern Sanshui Fm was deposited in a proximal sedimentary environment.

4.1.2 Petrological features of western Sanshui Fm

In Cuikeng, the western part of Sanshui basin, the western Sanshui Fm has obviously smaller grain size than that in the north. It is a set of interbedded sandstone and mudstone. Some layers contain carbonate gravel interlayers (Fig. 4e). The carbonate gravels are not widespread in the western Sanshui Fm, but occasionally appear in a certain layer. Thin sections (Fig. 4h) show that the sandstone from the western Sanshui Fm is characterized by a low compositional and textural maturity. Clastic grains have poor roundness and subangular shape. Rock fragments mainly consist of siliceous rock and carbonate rock.

4.1.3 Petrological features of eastern Sanshui Fm

In the Guangzhou Iron and Steel Work, the eastern Sanshui Fm is mainly composed of reddish, brownish coarse to fine sandstone and pebbly sandstone. Some gravel interlayers can be found in the outcrops (Fig. 4f). Theses gravels show multi provenances, including low-medium grade metamorphic rock, granitoid rock, and siliceous rock. The tabular and trough shaped cross bedding are developed in this profile, indicating a strong hydrodynamic environment. Microscopic observations of sandstone from the eastern Sanshui Fm (Fig. 4i) show that clastic particles are supported by grains and cemented by silicate. The clastic grains are poorly to moderately rounded and sorted. The texture and composition maturity are intermediate. Metamorphic and granite fragments can be found under the microscope. Some quartz grains show undulose extinction, suggesting that the grains are likely to be sourced from metamorphic or igneous rocks.

4.2 Clay mineralogy

The clay mineral assemblages of the Sanshui Fm mainly consist of illite and less amount of kaolinite and chlorite. Relative content of smectite is almost zero. Samples from the northern, western, and eastern Sanshui Fm show differences in clay mineral XRD patterns and relative contents (Fig. 5). The clay mineral assemblages of the northern and eastern Sanshui Fm are dominated by illite+chlorite, while the western Sanshui Fm is dominated by illite+kaolinite. In comparison, clay mineral content of Sanshui Fm has the highest chlorite in the north, the highest illite in the east, and the highest kaolinite in the west.

4.3 Geochemistry

The major and trace elements results of samples from the northern Sanshui Fm can be found in Data Availability Statement. As shown in Table 1, compared with major elements of sandstone in different tectonic settings from Bhatia(1983, 1985) and Taylor and McLennan (1985), the SiO₂ of Sanshui Fm is between 56% and 86%, with an average of 73.60%, which is higher than the upper continental crust and close to the active continental margin; TiO₂ (0.10%–0.54%, average 0.26%), Al₂O₃ (7.80%–16.15%, average 9.97%), Fe₂O₃ (0.61%–3.6%, average 1.60%), MgO (0.22%–1.47%, average 0.58%), Na₂O (0.48%–2.34%, average 1.54%), K₂O (1.94%–4.50%, average 3.15%), and P₂O₅ (0.02%– 0.16%, average 0.04%) are lower than the average of the upper continental crust. Compared with the major elements of Paleogene samples in Sanshui Basin (Jia, 2016), SiO₂ and TiO₂ of Sanshui Fm are lower, and the other major elements are higher (Table 1). The contents of major elements in Sanshui Fm samples are more prone to active continental margin setting, while the Paleogene samples from the Sanshui Basin are more prone to passive continental margin setting (Table 1).

In terms of trace elements, as shown in Fig. 6a, chondrite-normalized curves of Sanshui Fm samples basically display the characteristics of two peaks and one valley. On the whole, the large ion lithophile elements Sr and Ba are relatively depleted, while Rb is relatively enriched. The high field-strength elements Th and Hf show relatively enriched, while Nb, Ta, and Zr display relatively depleted. The enrichment of Th suggests that the felsic provenance may have a significant effect on the northern Sanshui Fm. The appearance of large amount of granitic gravels in the northern Sanshui Fm outcrops supports this interpretation (Fig. 4). The ratios of trace elements Sc/Cr (0.26), Ti/Zr (13.81), Zr/Th (11.55), K/Th (1 940.94), and Ba/Sr (4.29) in the northern Sanshui Fm samples are more prone to the active continental margin setting (Table 1).

In terms of rare earth elements, as shown in Table 1, the total amount of rare earth elements is 157.83×10^-6 on average, higher than that of the upper continental crust. La_N/Yb_N value is 6.5, showing obvious enrichment of light rare earth elements. The value of Eu/Eu* (average 0.60) shows obvious Eu negative anomaly, suggesting that the source rocks of the northern Sanshui Fm are mainly felsic rocks. The value of Ce/Ce* (average 0.72) is below 0.95, indicating a relatively oxidized depositional environment. In the Fig. 6b, the rare earth elements chondrite-normalized distribution patterns of northern Sanshui Fm samples are similar to that of active continental margin sandstone according to Bhatia's standard (Bhatia, 1985), but far away from those of oceanic island arc, continental island arc, and passive continental margin sandstone.

4.4 Detrital zircon dating of the northern Sanshui Fm granitic gravelly sandstone

Cathodoluminescence (CL) images of most zircons show the characteristic of euhedral to subhedral shape, columnar to long columnar shape with a large length/width ratio and magmatic oscillatory zones (Supplementary Fig.S1). Most zircon grains are colorless or pale brown, 50–200 μm in size, suggesting limited transport distance or near proximal accumulation. Only several zircon grains Th/U ratio are below 0.1, falling in the metamorphic origin area (Fig. 7).

The results of detrital zircon dating can be found in Data Availability Statement. The age distribution and Kernel density estimation plots of zircon ages are shown in Fig. 8. The Sanshui Fm is characterized by a high density of ages at 300–100 Ma (with peak at ~200 Ma), with lesser densities at ~500 Ma, 1 000 Ma, and 2 500 Ma. The youngest age is 86 Ma, which is quite close to the sedimentary age of Sanshui Fm.

5 DISCUSSION 5.1 The tectonic compressional event at the Early to Late Cretaceous boundary

In regional geological survey and previous studies, angular unconformity between the Lower Cretaceous Baihedong Fm (129–93 Ma) and the Upper Cretaceous Sanshui Fm (93–80 Ma) has been mentioned for many times, but it has not been described in detail (Hou et al., 2006). In Gaozui-Yangmeiya, we find the unconformity interface between the Baihedong Fm and the Sanshui Fm, and conduct detailed measurements and sampling (Fig. 3). Below the angular unconformity, the interbedded siltstones and mudstones of Baihedong Fm, with development of horizontal bedding, suggest a lacustrine depositional environment with weak hydrodynamic condition. However, above the angular unconformity, Sanshui Fm is a set of coarse clastic deposit with massive bedding in hundreds of meters thickness, containing a large number of poor sorted and rounded granitic pebbles and cobbles. The low maturity of texture and composition, extremely poor roundness and sorting, and the existence of pebbles and cobbles suggest the typical debris flow depositional environment in the northern Sanshui Fm. Chen (2018) and Hou et al. (2007a) also identified typical channel, sheetflood, and sieves sedimentary in the northern Sanshui Fm, based on the field observation and well facies analysis. The occurrences of these sedimentary features in the northern Sanshui Fm indicate that the northern Sanshui Fm was formed and deposited in the context of alluvial fan sedimentary environment. The appearance of angular unconformity and sedimentary condition transition from the interbedded lacustrine facies in Baihedong Fm to the alluvial fan facies in Sanshui Fm, indicate that a strong tectonic movement occurred in this area at the boundary of these two strata. This tectonic event in the northern SCS continental margin was also reported by different researchers, such as Ye et al. (2018) identified the WNW-striking thrust system which took place later than 101.7 Ma and earlier than the Late Cretaceous regional extension. Shu and Zhou (2002) and Wang and Shu (2012) suggest that a regional angular unconformity exists between the Early and Late Cretaceous strata in most South China coastal basin. Shi and Li (2012) also described a sharp paleogeographic change at SCS margins between the Early and Late Cretaceous (~100 Ma). Hence, we draw a conclusion that this tectonic event took place at nearly 100 Ma, at the boundary of the Early and Late Cretaceous.

There are two scenarios for the tectonic movement in the northern SCS area around 100 Ma in previous studies. Scenario 1 is that this movement is related to the transition from an active to a passive continental margin (Charvet et al., 1994; Lapierre et al., 1997). Geochemical studies on volcanic rocks show that potassium-rich orogenic suites generated in the Early Cretaceous changed to bimodal volcanism (rhyolite/basalt) in the Late Cretaceous (Charvet et al., 1994; Lapierre et al., 1997). Similarly, palaeolithofacies indicate that the sedimentary environment changed from abyssal to bathypelagic in Early Cretaceous and then to offshore clastic in Late Cretaceous, which further confirmed the results of geochemical studies (Zhou et al., 2006b; Shi and Li, 2012). However, Scenario 2 interprets this tectonic event at the Early-Late Cretaceous boundary as a short-term compressive uplifting before the onset of the Late Cretaceous extensive back-arc extension, which was likely related to intense oblique convergence between the Paleo-Pacific Ocean Plate and the Eurasia Plate (Shu and Zhou, 2002; Wang and Shu, 2012; Li et al., 2014; Ye et al., 2018). The regional angular unconformity is common between the widespread folded Lower Cretaceous strata and the Upper Cretaceous sediments in most of the South China continental marginal basins (Shu and Zhou, 2002; Wang and Shu, 2012). Furthermore, based on newly acquired 3D seismic reflection data, Ye et al. (2018) identified the WNW-striking thrust system in the northern SCS margin, and they related it to the large-scale left-lateral transpression. In addition, based on geochronological and geochemical analyses of the adakite granodiorite from the interior Cathaysia Block, Sun et al. (2017) found a significant thickening of crust at the Early-Late Cretaceous boundary, which reflects a significant compressive event.

Our petrology and detrital zircon chronology studies on granitic gravelly sandstone from the Sanshui Fm support the Scenario 2 of a rapid and intense compressive uplift at the Early-Late Cretaceous boundary. In the northern Sanshui Fm, hundreds of meters granitic gravelly sandstone is widely exposed. However, it has not been paid enough attention in previous studies. Granite, as a plutonic intrusion, is an important benchmark for studying tectonic movements, such as crustal uplift and compressional orogeny. It is generally considered to be uplifted to the surface by tectonism after formed in the deep crust, and appear in the stratum after weathered, denudated, transported, and deposited. The suddenly appearance and wide distribution of granitic gravelly sandstone in the northern Sanshui Fm support that there is a tectonic uplift in the northern continental margin of the South China Sea at the turn of Early and Late Cretaceous.

Understanding the crystallization age of detrital zircon from granitic gravelly sandstone is also of great significance for explaining this tectonic event. The subhedral to euhedral, columnar to long columnar shape and a characteristic magmatic oscillatory growth ring or zoning of most zircon indicate the magmatic origin of these grains (Supplementary Fig. S1; Corfu et al., 2003). In addition, Th and U in metamorphic zircon often register low values, with Th/U ratio mostly < 0.1, whereas higher Th/U ratio (> 0.1) in magmatic zircon (Möller et al., 2003). Most of zircon grains from the Sanshui Fm have a higher value of Th/U ratio (above 0.1), supporting that a large amount of zircon was originating from igneous rocks. Therefore, the U-Pb zircon dating age results could be used to constrain crystallization age of these grains. Considering that the gravels in the northern Sanshui Fm is almost entirely composed of granitoid rocks, and the igneous rock fragments found under the microscope are mainly granitic fragments, we assume that the most of detrital zircons in the Sanshui Fm is from the granitoid parent rock. The dating results show that there are mainly four peaks, which are 100–300-Ma, around 500 Ma, around 1 100 Ma, and around 2 500 Ma, and the peak of 100–300 Ma is significantly stronger than the other three (Fig. 8). The 100–300 Ma zircons may reflect the strong magmatic activities in the northern SCS area during the Late Mesozoic due to the subduction of paleo-Pacific Pate beneath the Eurasian plate (Shi et al., 2011). The magmatism of Hercynian movement and Indosinian movement could also be reflected in the Sanshui Fm detrital zircon ages. The detrital zircons are likely to be the products of multiple magmatic intrusions. Therefore, the source of zircon grains in Sanshui Fm is complex but mainly Yanshanian igneous rock, which is consistent with the widespread Triassic-Cretaceous granite parent rocks in and around the basin in modern days. Eleven zircon grains ages (118–86 Ma) are around the boundary of Early and Late Cretaceous, which is quite close to the sedimentary age of the Sanshui Fm. It means that granite was uplifted to surface through denudation and then deposited in a very short time after it's formed in the deep crust, which indicates that the tectonic uplift took place very rapidly at that time.

The occurrence of granitic gravelly sandstone is not an isolated case in most of the South China continental margin basins during this period. Previous studies have found that the Aiwu Fm in the lower part of the Upper Cretaceous in Maoming Basin and the Changba Fm in the upper part of the Lower Cretaceous in the Nanxiong Basin contain several layers of sediment composing mainly of coarse granitic clasts (Tang, 2014; Jia, 2016). The widespread occurrence of granitic gravelly sandstone from the end of the Early Cretaceous to the beginning of the Late Cretaceous suggest a strong regional compressive uplift event. Therefore, we conjecture that a large-scale orogenic movement may have occurred in the continental margin of the South China at the turn of early and late Cretaceous, which caused deep granite bodies to rise to the surface and provide materials for adjacent basins.

Based on the above studies on the lithofacies and detrital zircon chronology of the Sanshui Basin, we suggest that the tectonic event that occurred at the boundary of the Early and Late Cretaceous in the northern SCS was a strong and rapid compressive uplifting event. This event not only resulted in the development of the WNW-thrust system in the northern SCS (Ye et al., 2018), but also the widespread angular unconformity between the folded Early Cretaceous and the Late Cretaceous sediments in most of the South China continental margin (Shu and Zhou, 2002; Wang and Shu, 2012). The granite provenance formed by this compressional orogeny had become an important source region of the adjacent basin (e.g., Sanshui Basin, Maoming Basin, Nanxiong Basin).

5.2 The back-arc extension of the northern SCS area in the Late Cretaceous

It is generally believed that the northern SCS area has entered into extensional environment during the Late Cretaceous (Charvet et al., 1994; Lapierre et al., 1997; Shu et al., 2004, 2009; Zhou et al., 2006b; Li et al., 2012; Morley, 2012; Ye et al., 2018). Bimodal volcanic and A-type granitoids associated with a post-collisional crustal extension were identified. The Late Cretaceous extensional basins in the South China continental margin are mainly infilled by terrestrial red beds (Shu et al., 2004, 2009; Zhou et al., 2006b; Wang and Shu, 2012). The key question, however, is whether the Late Cretaceous extension was caused by back arc extension or driven by far field passive extension. According to our studies on the petrology, clay mineralogy, geochemistry, and detrital zircon chronology of samples from Sanshui Fm, we prefer the back arc extension scenario due to the following evidences.

5.2.1 The extensional environment of Sanshui Basin in the Late Cretaceous

Compared with the Lower Cretaceous, the expansion of the sediment horizontal distribution and the increase of sediment thickness in the Upper Cretaceous Sanshui Fm indicate the extensional environment of the Sanshui Basin in the Late Cretaceous (Hou et al., 2006; 2007a). The structural sequence of Sanshui Basin had evolved from the basal coarse clastic progradation in the Early Cretaceous to the initial flooding in the Late Cretaceous (Hou et al., 2007a). Our studies on the petrology and sediment sources of the Sanshui Fm support this result of extensional environment. Petrological results show that the Sanshui Fm is characterized as a set of coarse clastic deposits with the development of massive, tabular and trough shaped cross bedding. Most of clastic grains are poor sorted and rounded, indicating the limited transport distance and rapid proximal accumulation. Such petrological characteristics may reflect the strong hydrodynamic environment at the beginning of basin extension stage, with the basin drop becoming larger and water body deepening.

In terms of sediment source, the different lithology of gravels from different part of the basin (widespread granitic gravels in the north, carbonate gravel interlayers in the west, and complex gravels including sedimentary, metamorphic, and granitoid rocks in the east) indicate the multiple provenances of the Sanshui Fm. Such provenance differences in different parts of the basin are also reflected in clay mineral assemblages of Sanshui Fm. The clay mineral XRD patterns and relative contents in Sanshui Fm are different in the north (illite+chlorite, with highest chlorite content), west (illite+kaolinite, with highest kaolinite), and east (illite+chlorite, with highest illite) (Fig. 5). Considering that the provenance has a significant influence on the clay mineral assemblages and content (Chamley, 1989), we suggest that this difference in Sanshui Fm clay mineral may also reflect the multiple sources of the basin in the Late Cretaceous. Roser and Korsch(1986, 1988) suggest that the major elements of sandstone can be used to discriminate the sandstone provenance. In this provenance discrimination diagram (Fig. 9), the Sanshui Fm samples mainly distribute in the area of felsic igneous provenance, with partial intermediate igneous and quartzose sediment, also indicating the multiple provenances of the basin. Therefore, the results of petrology, clay mineralogy, and geochemistry suggest the diversity of provenances in the basin during the Late Cretaceous. Furthermore, provenances vary greatly in different regions of the basin. We suggest that such characteristic of basin provenance may reflect that the basin was divided into many small intermontane basins in the Late Cretaceous due to the regional extension. These small basins have different provenances and are poorly connected to each other.

5.2.2 The active continental margin environment of Sanshui Basin in the Late Cretaceous

Above discussion suggests that the Sanshui Basin was in an extensional environment in the Late Cretaceous. Thus, whether the Late Cretaceous extension was caused by back arc extension or driven by far field passive extension?

Major and trace elements of sedimentary rock have been used to understand tectonic setting in various studies (Bhatia, 1983; Bhatia and Crook, 1986; Bahlburg, 1998). Four main types of tectonic (oceanic island arc, continental island arc, active continental margin, and passive continental margin) had been distinguished by Bhatia (1983), Bhatia (1985), and Bhatia and Crook (1986) based on the major elements, trace elements and REE of sandstone (Table 1, Figs. 6b–9). In the tectonic environment discrimination diagram of sandstone proposed by Bhatia (1983), the samples of Sanshui Fm fall into the range of active continental margin (Fig. 9). In the figure of REE chondrite-normalized curve (Fig. 6b), the curve of Sanshui Fm is closer to the active continental margin curve based on the Bhatia (1985). As show in Table 1, the Late Cretaceous Sanshui Fm samples are rich in Ca, poor in Al, K, Na, and Mg compared with the upper continental crust, indicating that the composition maturity is low and belongs to the rapid accumulation of near source with short transportation distance. The major, trace, and rare earth elements parameters (Sc/ Cr, Ti/Zr, Zr/Th, K/Th, Ba/Sr, La, Ce, total REE, Eu/ Eu*, La/Yb, and La_N/Yb_N) of the Sanshui Fm are closer to the active continental margin setting, while the Paleogene samples from the Sanshui Basin closer to the passive continental margin (Table 1). Cox et al. (1995) proposed to use the ICV to reflect sediment recycling process and changes to indicate compositional maturity as well as the tectonic evolution. The ICV value greater than 1 reflects the first cycle deposition under active tectonic environment; otherwise, it reflects the first cycle deposition controlled by intense chemical weathering or recycling deposition under passive tectonic environment. The average ICV value of Sanshui Fm is 1.37, while Paleogene samples are 0.87. It suggests that the sediment in Sanshui Fm is likely to be the first cycle deposition under active tectonic environment, while the Paleogene sediment probably deposited in intense chemical weathering or passive tectonic environment.

Detrital zircon ages could also reflect the tectonic setting of the basin in which they are deposited (Cawood and Nemchin, 2000; Fedo et al., 2003; Cawood et al., 2012). Cawood et al. (2012) distinguish three types of tectonic setting (convergent, collisional, and extensional setting) based on the difference between detrital zircon crystallization age and the succession deposition age. Compared with the extensional setting, the crystallization age of most detrital zircon from convergent and collisional setting are closer to the deposition age of succession (Cawood et al., 2012). In the above discussion, most of detrital zircon grains of the Sanshui Fm are origin from magmatism, therefore the crystallization age of the Sanshui Fm detrital zircon can be used to indicate tectonic setting of the basin. As shown in the Fig. 10, the Sanshui Fm curve lies in the area of convergent and collisional setting, thus pointing to an active continental margin rather than passive continental margin.

Both the geochemistry and detrital zircon chronology results suggest the active continental margin setting of the northern SCS margin in the Late Cretaceous, while the Paleogene samples show the geochemical characteristic of passive continental margin setting. Hence, we conjecture that the extension in the northern SCS area in the Late Cretaceous is most plausibly caused by the back-arc extension (Fig. 11a), instead of far field passive extension after Paleocene (Fig. 11b). Li et al. (2012) and Ye et al. (2018) suggest this back-arc extension is in response to the rollback and retreat of the earlier low-angle subducting slab of the paleo-Pacific Plate.

According to the above discussion and previous studies, the multi-phase tectonic evolution model of the northern SCS margin during the Late Cretaceous can be proposed based on the sedimentary evidences from the Sanshui Fm. At the Early-Late Cretaceous boundary (~100 Ma), a tectonic compression uplift event occurred in the northern SCS margin due to the intense oblique convergence between the Paleo-Pacific Ocean Plate and the Eurasia Plate (Shu and Zhou, 2002; Ye et al., 2018). During the early Late Cretaceous (100–80 Ma), the northern SCS margin was in a back-arc extension environment related to the rollback and retreat of the paleo-Pacific Plate subducting slab (Li et al., 2012; Ye et al., 2018). The transition from active subduction to passive extension in the northern SCS margin is likely to occur during the late Late Cretaceous to early Paleogene. During the Paleocene-Eocene, the northern continental margin of SCS experienced multi-stage extensional rifting movements (with bimodal magmatism), and developed a series of NE-NEE rift basins (Zhou and Li, 2000; Li et al., 2012). The continuous lithospheric thinning of the continental margin eventual led to the formation of oceanic crust and the seafloor spreading of the SCS during the Early Oligocene (~32 Ma) (Briais et al., 1993; Franke et al., 2014). The development of rifting and the opening of SCS indicate a passive continental margin setting in the northern SCS margin during the Cenozoic (Li, 2011; Sun et al., 2016; Suo et al., 2019).

6 CONCLUSION

Based on outcrops observation, petrology, clay mineralogy, geochemistry, and detrital zircon U-Pb chronology analyses of sedimentary samples from Sanshui Fm, Sanshui Basin, the following conclusions are drawn:

(1) The Upper Cretaceous Sanshui Fm is in angular unconformity with the underlying Lower Cretaceous Baihedong Fm. The sedimentary facies of northern basin changed from lacustrine sedimentary environment in the late Early Cretaceous to alluvial fan facies in the early Late Cretaceous. These evidences suggest a regional tectonic event in the northern SCS margin at the Early and Late Cretaceous boundary. The widely exposed granitic gravelly sandstone in northern Sanshui Fm have a dominating detrital zircon age distribution from 100 Ma to 300 Ma, supporting that a strong and rapid compressive uplifting event took place in the northern SCS area around 100 Ma.

(2) The Sanshui Fm is characterized as a set of coarse clastic deposits with poor rounded and sorted, suggesting a strong hydrodynamics and near source accumulation environment in the Late Cretaceous. Petrology, clay mineralogy, and geochemistry results show that there are multiple provenances in different parts of the basin in the Late Cretaceous. These suggest that the basin is likely to be divided into many small intermontane basins due to regional extension in the Late Cretaceous (~100–~80 Ma).

(3) Geochemistry results from the Sanshui Fm show the characteristics of active continental margin. The dominating crystallization age of detrital zircon from the Sanshui Fm is quite close to the deposition age of the formation, indicating a convergent or collisional setting rather than passive extension setting in the northern SCS margin during the early Late Cretaceous. Hence, we conjecture that the regional extension in the Late Cretaceous occurred at the environment of the back-arc, which is different from the Cenozoic passive rifting. The northern SCS margin may transit from active subduction to passive extensional during the late Late Cretaceous to the Paleogene.

7 DATA AVAILABILITY STATEMENT

The data used in this manuscript are available at https://doi.org/10.6084/m9.figshare.19093373.v7.

8 ACKNOWLEDGMENT

We thank Xianqiu ZHANG from China New Star (Guangzhou) Petroleum Corporation for the help in the fieldwork. We acknowledge the analytical assistance of Lei JIA, Xiaobo YUAN, and Yang LIU from China University of Geosciences (Beijing). We are grateful to Jiaju WEI, Heping SUN, and Helang HUANG from China University of Geosciences (Beijing) for laboratory work. Prof. Chiyue HUANG from Tongji University, and Prof. Di ZHOU from South China Sea Institute of Oceanology, Chinese Academy of Sciences, are greatly appreciated for their constructive suggestions.

Electronic supplementary material

Supplementary material (Supplementary Fig.S1) is available in the online version of this article at https://doi.org/10.1007/s00343-022-2055-8.