Late Oligocene Formation of the Pearl River Triggered by the Opening of the South China Sea

The Pearl River is one of the largest rivers entering the South China Sea, yet its initiation time remains debated, a topic we address using Pb isotopes in detrital K‐feldspar. Based on these Pb data, Eocene and Early Oligocene sandstones from the northern South China Sea are interpreted to have been supplied with sediment by proximal rivers draining the Cathaysia Block. In contrast, the Late Oligocene and Miocene sandstones are mainly derived from the western Pearl tributaries (e.g., Hongshui River), suggesting that the Pearl River had formed by the Late Oligocene. Detrital zircon data from the Beibuwan Basin previously suggested that the western tributaries flowed into this basin before being captured by the paleo‐Pearl River. These lines of evidence suggest that progressive headward erosion of the eastern Pearl River and late Oligocene integration of this large fluvial system can be linked to contemporaneous sea‐floor spreading of the South China Sea.


10.1029/2023GL103049
2 of 11 to early Miocene. However, the mechanism of this late Oligocene drainage expansion of the paleo-Pearl River remains poorly constrained. Several investigations attributed this event to the uplift of the Tibetan Plateau (Cao et al., 2018;He et al., 2020;Jin et al., 2022), as similar Oligocene drainage network reorganization has been recognized in the other large rivers originating from SE Tibet, including the Red and Yangtze Rivers (Clift  (Cao et al., 2018) showing the locations and sample names of modern sand samples. The dashed line represents the possible boundary between the Yangtze Craton and Cathaysia Block (G. Zhao & Cawood, 2012). 10.1029/2023GL103049 3 of 11 et al., 2006, 2020Zheng et al., 2013). However, other studies have indicated that the opening of the South China Sea also influenced the progressive headward erosion of the major rivers in southeastern China (Lan et al., 2016;Z. Zhang, Daly, Yan, et al., 2022). Although many provenance studies have been conducted on the eastern catchments of the Pearl River (Chen et al., 2021;, little attention has been paid to the western tributaries, including the Hongshui and Yu rivers ( Figure 1). Thus, a comprehensive understanding of the drainage reorganization of the western tributaries is key to revealing the mechanism of the formation of the Pearl River.
Until now, detrital zircon U-Pb geochronology has been the most widely-used sedimentary provenance technique to investigate the drainage evolution of the Pearl River (Cao et al., 2018;Chen et al., 2021;He et al., 2020;C. Liu et al., 2017W. Wang et al., 2017W. Wang et al., , 2019C. Wang et al., 2018;Yan, Yao, Tian, Huang, Dilek, et al., 2018). However, several investigations have shown that inaccurately estimated mineral abundances in the sourcelands (i.e., zircon fertility) can lead to major provenance bias (Caracciolo, 2020;Malusà et al., 2016;Vezzoli et al., 2016). This is particularly true for the Pearl River as its catchment encompasses great lithological heterogeneity: the western part is mainly composed of carbonate and siliciclastic rocks, while the eastern tributaries drain the Cathaysia block, which is characterized by Mesozoic granites (Figure 1b). However, the effects of variable zircon fertility in the source regions have rarely been considered in previous detrital zircon provenance studies. In the Pearl River sand, K-feldspar is a major framework component (see below). Hence, K-feldspar is likely to be more representative of the source terranes, and could potentially provide a less biased constraint on the sediment routing system. In addition, Pb isotope mapping showed that the Yangtze Craton and Cathaysia Block have distinctive Pb isotopic characters (L. Zhang, 1995;Zhu, 1995), implying that the arrival of sediment from the Yangtze Craton due to drainage expansion westwards should be detectable.
In this study, Pb isotopic compositions of detrital K-feldspar from the Pearl River and Cenozoic deposits from the northern South China Sea, including drillholes from the Pearl River Mouth Basin (PRMB) and International Ocean Discovery Program (IODP) Site U1501, have been analyzed to constrain the initiation of the Pearl River. The Pb signal carried in detrital K-feldspar is now a well-established provenance method that has been widely applied to the evolution of major river systems (Alizai et al., 2011;Blowick et al., 2019;Clift et al., 2002Clift et al., , 2008Z. Zhang et al., 2016Z. Zhang et al., , 2017Z. Zhang et al., , 2021Z. Zhang, Daly, Yan, et al., 2022). Moreover, we have compared these Pb data with published detrital zircon U-Pb age datasets from offshore basins in South China to permit a more comprehensive discussion of the drainage evolution of the Pearl River. Our study highlights that the opening of the South China Sea exercised an important influence on the formation of the Pearl River.

Pearl River
As one of the longest rivers in China, the Pearl River is fed by a series of tributaries, including the Yu, Liu, Gui, Bei, and Dong rivers ( Figure 1). Originating from the southeastern Tibetan Plateau, the westernmost part of the Pearl River is known as the Hongshui. Downstream of the confluence with the Gui, it takes the name of Xi River. Farther east, the Xi is joined by the Bei and Dong rivers and then finally drains into the South China Sea (Figure 1). The Hongshui and Yu rivers drain the Yangtze Craton where upper Paleozoic carbonates, Emeishan basalts, Triassic dolomites, and continental red beds are widely exposed (Figure 1). The Liu River originates in the Neoproterozoic Jiangnan Orogen (Figure 1; G. Zhao, 2015), which contains significant volumes of post-tectonic Neoproterozoic granite (G. Zhao & Cawood, 2012). Tributaries in the eastern catchment mainly drain the Cathaysia Block, characterized by Mesozoic granites and volcanic rocks ( Figure 1).

Pearl River Mouth Basin
The

Sampling and Analytical Methods
Eight sand samples were collected from the major tributaries as well as the trunk stream of the Pearl River ( Figure 1), to determine their Pb isotopic composition. All modern river samples, fine to medium sands, were collected after the summer monsoon season in 2018 and were sieved with a size window from 63 to 500 μm. Sands from the Hongshui River contain a small number of basalt, siliciclastic and limestone clasts (up to 350 μm across), with an abundance of 5%-6%. In contrast, the eastern tributaries such as the Dong and Bei rivers contain a few angular granite clasts (c. 2%, ∼300 μm). In addition, seven sandstone samples were collected from two industrial boreholes (WC-14 and PY-33) in the PRMB ( Figure 2). Another two samples (IODP-13 and 14) of Eocene and pre-Eocene age were sampled from IODP Site U1501. Detailed sample information is given in supporting information Table S1. The Pearl River samples and cuttings from boreholes were mounted in epoxy resin and polished into thin sections. K-feldspar grains were identified by field-emission scanning electron microscopy (FEG-SEM; FEI Scios) at the Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China.
Detrital K-feldspar Pb isotope data from the modern Pearl River sands and sandstones from borehole PY-33 were measured in situ by laser-ablation attached to a Thermo Scientific Neptune Plus multiple collector inductively-coupled-plasma-mass spectrometer (MC-ICP-MS) at China University of Geosciences, Wuhan,  (Larsen et al., 2018). Stars represent sandstone samples collected for Pb isotopic analysis.

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following the procedures of W. Zhang et al. (2015). In situ K-feldspar Pb analyses from borehole WC-14 and IODP Site U1501 were targeted using a Teledyne Cetac G2 laser ablation system and analyzed on a Ther-moScientific Neptune MC-ICP-MS instrument at University College Dublin using methods described by Tyrrell et al. (2012).

Results
In this study, a total of 704 detrital K-feldspar grains were analyzed from 17 samples-eight modern sands and nine sandstone samples (Figure 3). Pb isotopic results are presented in Table S1. Plots of 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb are used to distinguish possible populations and for comparison with potential sources.

K-Feldspar Pb Isotopic Analyses of the Pearl River Sands
Three sand samples from the eastern tributaries (BJ-QY, DJ-DG, and GJ-WZ) have high modal abundances of K-feldspar (23%-26%, Table S1). This is not surprising as Mesozoic granites are widely exposed in Cathaysia Block and represent a third to 45% of the Bei and Dong catchments (Garzanti et al., 2021). The western tributaries' samples (HSH, YJ-GP, and LJ-LZ) have much lower K-feldspar abundances (2%-4%). Since there are rather few K-feldspar-bearing crystalline rocks exposed in the Hongshui River catchment (Figure 1b), detrital K-feldspar grains in this river are likely recycled from siliciclastic rocks which are widely distributed in its catchment (Garzanti et al., 2021).
K-feldspar grains from the eastern tributaries such as the Dong, Bei, and Gui Rivers form a relatively radiogenic population ( 206 Pb/ 204 Pb from 18.40 to 18.90; Figure 3a). Moreover, these K-feldspars mainly plot within the Pb field of the Cathaysia Block (Figure 3a).

Sedimentary Provenance of Cenozoic Deposits in PRMB and IODP Site U1501
Pb isotopic data from sandstone samples from the PRMB and IODP Site U1501 boreholes have been compared with those from the Pearl River and Cathaysia Block to determine their provenance (Figure 1b). Moreover,  Zhang, 1995) and potential sources. 7 of 11 multidimensional scaling (MDS) analysis (Vermeesch, 2018) has been used to assess similarities between the modern river sand and borehole samples ( Figure S1 in Supporting Information S1).
K-feldspar grains from the pre-Eocene and Eocene sandstones mainly fall within the Cathaysia Block field (Figure 4a). The MDS plots also indicate a close relationship between them ( Figure S1 in Supporting Information S1). Although the Eocene samples plot within the Cathaysia Block field, they have different Pb isotopic compositions (Figure 3). Compared to two samples from IODP Site U1501 (IODP-13 and 14), K-feldspar grains from borehole WC-14 (SCS-5 and 6) have higher 207 Pb/ 204 Pb values, implying that they have distinctive sources. This is consistent with previous 3D seismic data as well as detrital zircon U-Pb geochronology data, which indicate that the PRMB had experienced intense rifting during Eocene, with sediment supply mainly from nearby sources (Shao, Cao, et al., 2016;Zeng et al., 2019). Another potential sources is the Palawan Continental Terrane (Figure 1a). Unfortunately, we are unaware of any feldspar Pb isotopic data from the Palawan Continental Terrane, which precludes identifying a sediment contribution from this region. Several studies indicate that the Palawan Continental Terrane used to be attached to the South China margin from the Cretaceous to the Eocene (C. Liu et al., 2022;Shao et al., 2017;Yan, Yao, Tian, Huang, Chen, et al., 2018), indicating that these two terranes likely share a similar Pb isotopic character.
In addition, K-feldspar grains from the Lower Oligocene samples plot within the Pb field of the eastern tributaries such as the Dong and Bei Rivers (Figures 3e and 3f), indicating that these rivers were supplying sediment to the PRMB at this time. This suggestion agrees with sedimentary provenance inferences from detrital zircon U-Pb ages. The early Oligocene deposits share a similar zircon U-Pb age population with that of the Bei and Dong Rivers (Cao et al., 2018;He et al., 2020;W. Wang et al., 2017W. Wang et al., , 2019Yan, Yao, Tian, Huang, Dilek, et al., 2018), suggesting that the eastern tributaries of the Pearl River had formed by that time.
Twelve out of 39 K-feldspar grains from the Upper Oligocene sample SCS-7, with 206 Pb/ 204 Pb > 18.4, plot within the eastern tributaries' Pb field, suggesting that they still contributed sediment to PRMB (Figure 3h) at this time. Moreover, sample SCS-7 has lots of un-radiogenic K-feldspar grains ( 206 Pb/ 204 Pb < 18.3), which can only be sourced from the western tributaries such as the Hongshui River (Figure 3h). MDS results show that sample SCS-7 plots close to the Hongshui River as well as the Xi (i.e., trunk Pearl River), but is far away from the eastern tributaries ( Figure S1 in Supporting Information S1). Therefore, we suggest that the western tributaries have served as a sediment supplier to the PRMB since the late Oligocene. Moreover, we notice that the Baiyun Sag (Figure 1) is still fed by proximal sources as detrital K-feldspar from drill hole PY-33 are characterized by high 206 Pb/ 204 Pb values (Figure 3f). Detrital zircon U-Pb geochronology results of borehole BY-7 (Figure 1) is characterized by age population younger than 500 Ma, implying that this region is still fed by local sources (Yan, Yao, Tian, Huang, Chen, et al., 2018;Yan, Yao, Tian, Huang, Dilek, et al., 2018).
The Middle Miocene sample PY33-11 has a similar Pb isotopic character to that of the Xi River (Figure 3h; Figure S1 in Supporting Information S1), indicating that this river has become a dominant sediment supplier to the PRMB at that time.

Drainage Evolution of the Pearl River
The available sedimentary provenance proxies (U-Pb zircon ages and K-feldspar Pb isotopes) indicate that the PRMB, and perhaps much of the northern South China Sea was mainly fed by local sources during the Eocene ( Figure 4a). By the early Oligocene, the eastern tributaries of the Pearl River, such as the Bei and Dong rivers, had formed and were transporting sediments to the PRMB (Figure 4b).
Pb isotopic data suggest that the Hongshui River served as an important sand supplier to the PRMB since the Late Oligocene ( Figure 4c). As the Hongshui River is one of the westernmost tributaries (Figure 1), therefore, we propose that the Pearl River had achieved its present-day drainage character by that time. This drainage expansion model is consistent with that derived from detrital zircon U-Pb dating studies (Cao et al., 2018;He et al., 2020;Yan, Yao, Tian, Huang, Dilek, et al., 2018). Detrital zircon U-Pb geochronology datasets of Eocene-early Oligocene deposits are dominated by two peaks at 100-200 and 400-500 Ma, which were likely fed by the tributaries originating from the Cathaysia Block including the Bei and Dong rivers (Cao et al., 2018;He et al., 2020). In comparison to the older samples, detrital zircon from the overlying late Oligocene-Miocene sandstones exhibits five major populations: 100-200, 400-500, 700-1,000, 1,800-2,000, and 2,440-2,520 Ma (Cao et al., 2018;Yan, Yao, Tian, Huang, Dilek, et al., 2018), which are also present in the modern Pearl River (Cao et al., 2018).

Mechanism of the Westward Drainage Expansion of the Pearl River
As discussed above, both our new Pb isotopic data and previous zircon provenance investigations suggest that the paleo-Pearl River had achieved its present drainage character by the late Oligocene. However, the mechanism of the late Oligocene drainage expansion of the Pearl River remains poorly understood. Surface uplift of the SE Tibetan Plateau has been suggested as the primary factor influencing the initiation of the Pearl River (Cao et al., 2018;He et al., 2020;Jin et al., 2022). However, a late Oligocene initiation of the Pearl River is not synchronous with large-scale fluvial capture events in the eastern Tibetan Plateau. The large rivers draining SE Tibet have experienced dramatic reorganization at ∼35 Ma, including the break-up of the paleo-Red River (Chen et al., 2017;Zheng et al., 2020). Low-temperature thermochronological (F. Liu et al., 2020;Liu-Zeng et al., 2018;Shen et al., 2016) and paleo-altimetry (Hoke et al., 2014;S. Li et al., 2015;Wu et al., 2022) investigations indicate that the SE Tibetan Plateau had experienced sharp uplift and likely achieved its present elevation by the early Oligocene. This evidence suggests that the western tributaries of the Pearl River, that is, the Yu and Hongshui rivers, likely had started their journey no later than that time. However, the characteristic provenance signals of the Yu and Hongshui rivers were still absent in the PRMB (Cao et al., 2018;He et al., 2020), implying that they were not connected to the paleo-Pearl River by the early Oligocene.
Early Oligocene deposits in the Beibuwan Basin share a similar detrital zircon U-Pb provenance signal to that of the western tributaries such as the Yu, Hongshui, and Liu rivers (Gong et al., 2021), indicating that the western part of the Pearl River likely flowed into the Beibuwan Basin before being captured by the eastern paleo-Pearl River (Figure 4b). C. Liu et al. (2022) proposed that the paleo-Pearl River migrated from proximal rivers to the north and east during the late Eocene and attributed this to the fall in base level caused by the rifting of the PRMB. Similarly, westward expansion of the Pearl River during the late Oligocene is in accordance with the initiation of the South China Sea (Briais et al., 1993;C.-F. Li et al., 2014). The PRMB entered the post-rift stage during the Late Oligocene triggered by deep mantle activity related to seafloor spreading in the South China Sea (Pang et al., 2009). Similar drainage expansion events have also been identified from other rivers in southeastern China, for example, the Min River, which is unlikely affected by the Tibet uplift as the Min River only drains the coastal region in southeastern China (Figure 1a). Pb isotopic data from Cenozoic deposits in Taiwan suggested that the Min River migrated westwards during the late Oligocene (Zhang, Daly, Yan, et al., 2022). These various phenomena suggest that the formation of the Pearl as well as the Min rivers were likely driven by the opening of the South China Sea.

Conclusions
In this study, the Pb isotope compositions of detrital K-feldspar have been used to constrain the sedimentary provenance of the Pearl River as well as the PRMB. Our Pb isotopic data show that three stages of sediment transportation to the northern South China Sea can be identified since the early Cenozoic: (a) during the Eocene, the northern South China Sea was primary supplied by nearby highlands; (b) the eastern tributaries of the Pearl River such as the Dong and Bei rivers started to deliver sediment since the early Oligocene; and (c) during the late Oligocene, the western tributaries such as the Hongshui River have been contributing substantially to the PRMB. Capture of the western tributaries by the paleo-Pearl River during the late Oligocene can be attributed to the opening of the South China Sea. This study highlights the important role that the thermal response of sub-continental mantle to sea-floor spreading can play in the organization of adjacent continental drainage.

Data Availability Statement
Pb isotopic data analyzed in this study are stored in the Mendeley Data (available at https://doi.org/10.17632/ hp8j8mtdmx.1).