Provenance and Geological Significance of Cenozoic Sandstones in the Nankang Basin, Southern Cathaysia Block, China

Abstract

The Cenozoic Nankang Basin in China records a complex series of tectonic, magmatic, metamorphic, and sedimentary events associated with the surrounding Shiwanshan, Liuwanshan, and Yunkaishan orogenic systems. The Nankang Basin is a critical location for studying the Cenozoic tectono–sedimentary evolution and strategic mineral resources of the southern Cathaysia Block. We used core samples from multiple boreholes and regional geological survey data to analyze the rock assemblages, sediment types, and sedimentary facies of the Nankang Basin. In addition, we analyzed the detrital zircon U–Pb geochronology, sandstone detrital compositions, heavy mineral assemblages, and major element geochemistry. The detrital zircon grains from Cenozoic sandstones in the Nankang Basin have age peaks at 2500–2000, 1100–900, 500–400, and 300–200 Ma, with most grains having ages of 500–400 or 300–200 Ma. The provenance analysis indicates that the 300–200 Ma zircon grains originated mainly from the Liuwanshan pluton; the 500–400 Ma zircon grains originated from the Ningtan pluton; and the 2500–2000 and 1100–900 Ma zircon grains originated from the Lower Silurian Liantan Formation and Middle Devonian Xindu Formation. This indicates that the provenance of Cenozoic sandstones in the Nankang Basin primarily originates from Paleozoic–Early Mesozoic igneous in the surrounding area, while the regional old sedimentary rocks possibly serve as intermediate sedimentary reservoirs. The detrital compositions of the sandstones and heavy mineral assemblages indicate a change in the tectonic setting during the deposition of the Nankang and Zhanjiang Formations, with a change in the source of the sediments due to the uplift of the Shizishan. During the deposition of the Nankang Formation, the sediment transport direction was to the NNW, whereas during the deposition of the Zhanjiang Formation, it was to the NNE. The uplift of the Shizishan most probably occurred during the late Neogene and early Quaternary, separating the Hepu and Nankang Basins.

1. Introduction

The Cathaysia Block in southeastern China is an important metallogenic belt that has attracted attention from numerous geologists [1,2,3]. Its complex tectonic evolution has had a significant impact on the geomorphology and environmental evolution of surrounding regions. The Shiwanshan, Liuwanshan, and Yunkaishan orogenic systems are major structural units in the southern Cathaysia Block and key areas for studying the tectonic evolution of the block. These systems record complex tectonic, magmatic, and metamorphic events from the Paleozoic to Mesozoic [4,5,6].

Since the Cenozoic, important basins rich in coal, oil and gas, and natural quartz sand resources have formed in the southern part of the Cathaysia Block under the superimposed stress fields of the Pacific, Philippine, Eurasian, and Indian plates [7]. The stratigraphic sequences, sedimentary filling processes, and tectonic movements record the evolution of these Cenozoic Basins. Numerous studies have examined the stratigraphic sequences, sedimentary facies, provenance, paleocurrent directions, and tectono–thermal chronology of Cenozoic Basins, including the Maoming, Baise, Nanning, Hepu, and Nankang Basins, revealing the tectonic influence on sedimentation in the southern Cathaysia Block [8,9]. However, there are still uncertainties regarding the relative uplift and subsidence, and the mechanisms of basin–mountain coupling in the southern Cathaysia Block. These issues limit our understanding of the tectonic characteristics and evolution of the southern Cathaysia Block, thereby hindering the identification and evaluation of strategic mineral resources in the region.

The Cenozoic Nankang Basin is located in the southern part of the Cathaysia Block, situated at a critical juncture between the overlapping Liuwanshan, Shiwanshan, and Yunkaishan orogenic belts (Figure 1a,b). The Nankang Basin records complex tectonic, magmatic, metamorphic, and sedimentary events, making it a key area for studying the tectono–sedimentary evolution of the southern part of the Cathaysia Block [10]. Previous studies have shown that the basement of the Nankang Basin contains abundant shallowly buried high-temperature dry heat rocks [11]. In addition, the Beihai in China is a region where natural quartz sand resources are concentrated, most probably composed of sediments originating from the Nankang Basin. The ore bodies in the Nankang Basin are characterized by a wide distribution, shallow burial, large thickness, high quality, and ease of extraction [12], and after purification and processing, they can serve as raw materials for high-purity silica products [13]. Consequently, the Nankang Basin has become a hotspot for studying energy and non-metallic strategic mineral resources in China. In recent years, numerous studies have been conducted on the Cathaysia Block’s stratigraphic sequences, provenance characteristics, and basin type [7,8,9,10,14,15,16,17,18]. Wang and Yu [14] analyzed the geochemistry of Neoproterozoic–Cambrian sedimentary rocks in the Cathaysia Block and found high maturity and sedimentary recycling features. Hu and Cawood [15] used detrital zircon U–Pb dating, a whole-rock geochemical analysis, and paleocurrent analysis to study the Permian–Triassic sedimentary rocks in the southern Cathaysia Block, revealing the collisional orogeny process between the Cathaysia and Indochina Blocks. However, most of these studies have focused on Paleozoic to Mesozoic sedimentary rocks, while systematic research on Cenozoic sedimentary rocks in the Nankang Basin remains lacking.

In the last decades, U–Pb dating of detrital zircon in sandstones has become a precise tool for provenance analysis as detrital zircon is highly stable against weathering [19,20]. In addition, a heavy mineral and major element geochemistry analysis have been widely used to analyze the source–sink system of a basin and has been successfully applied in sedimentological studies [21,22,23].

In this study, multiple boreholes in the Nankang Basin were used to conduct systematic stratigraphic correlation, sedimentary petrology, detrital zircon LA–ICP–MS U–Pb geochronology, sandstone detrital component, and heavy mineral and major element geochemistry analyses. By integrating these results with regional geological survey data, this study discusses the Cenozoic sedimentary characteristics, provenance, and tectonic evolution of the Nankang Basin. The aim is to provide a geological framework for studying the tectono–sedimentary evolution, coupling between basins and mountains, and to assess the strategic mineral resources within the Cenozoic Basins of the southern Cathaysia Block.

Figure 1. (a) Sketch tectonic map of South China [24]. (b) Tectonic map of the Cathaysia Block. (c) Geotectonic map of the Cathaysia Block [25].

Figure 1. (a) Sketch tectonic map of South China [24]. (b) Tectonic map of the Cathaysia Block. (c) Geotectonic map of the Cathaysia Block [25].

2. Geological Setting

The Cathaysia Block is located in southeastern China, extending southward to coastal regions in Haikou, Beihai, Hongkong, and Fujian (Figure 1a). The study area is located in the Nankang Basin, Beihai, situated in the southern Cathaysia Block. The basin, formed as a Cenozoic intermontane faulted basin, trends northeast–southwest and has a fan-like shape. Its northern boundary is slightly convex, curving southward. The basin is ~50 km long and <20 km wide. The elevation within the basin ranges from 20 to 60 m above sea level. The eastern and northern boundaries of the basin are marked by the Shizishan Uplift and the Bobai–Hepu Fault, respectively, with the latter separating the Nankang Basin from the Hepu Basin. To the west, the basin borders the Liuwanshan orogenic belt and extends southward into the Beibu Bay. The study area is situated at the junction of the Shiwanshan, Liuwanshan, and Yunkaishan orogenic belts (Figure 1b). Its formation and evolution are primarily controlled by several northeast-trending regional-scale faults, such as the Luchuan–Cenxi Fault, Bobai–Hepu Fault, Lingshan–Tengxian Fault, and Nanning–Pingxiang Fault (Figure 1c). The basement is composed of Paleozoic Siletian and Devonian sandstones and carbonate rocks, along with late Paleozoic–early Mesozoic magmatic rocks and early Paleozoic magmatic rocks. The sedimentary cover of the Nankang Basin includes the Eocene, Neogene, and Quaternary systems (Figure 2).

3. Materials and Methods

A total of 23 sandstone samples were collected from the cores of boreholes ZKNK01 and ZKNK02 in the Nankang Basin. Specifically, these samples were derived from three formations: the Beihai, Zhanjiang, and Nankang Formations, with 6, 9, and 8 samples from each formation, respectively. Representative sandstone samples were carefully selected based on differences in color and grain size from each formation. A detailed investigation and description of the rock cores were carried out to facilitate the facies analysis.

The Gazzi–Dickinson method was employed to statistically analyze the detrital components of the 23 sandstone samples. Thin sections were examined under a microscope to measure the content of quartz, feldspar, and rock fragments. Specifically, 400–600 grains were counted per thin section through the point-counting method.

A heavy mineral analysis was performed on 23 representative sandstone samples, focusing on grain sizes between 1 and 0.063 mm. For this analysis, 20 g of each sample underwent gravity separation using bromoform (CHBr₃) with a density of 2.89 g/cm³ at 20 °C, following the procedure outlined by [26]. Subsequently, the point-counting method [27] was adopted to calculate the content of heavy minerals under the polarization microscope.

The collected sandstone samples were crushed, and the heavy minerals were separated using flotation and electromagnetic separation. Under a microscope, detrital zircon grains with well-formed crystals, no cracks, and good transparency were carefully picked out. After being made into targets, the zircons were ground and polished until their centers were exposed. Then, reflective and transmissive light photographs were taken of the zircon targets to observe their crystal form, cracks, and other micro-characteristics. Cathodoluminescence (CL) imaging was also conducted on the zircons. Finally, zircons with a relatively intact crystal form, and without cracks or inclusions, were selected for the LA–ICP–MS U–Pb dating analysis based on their morphology. The operating conditions of the LA system and ICP–MS instrument, as well as the data reduction procedures, have been described in detail by [28]. The analyses used a laser spot diameter of 32 μm and shot frequency of 6 Hz. The NIST 610 glass standard was used as an external standard for the U–Pb dating and trace element analyses. The zircon standard GJ–1 (603 ± 4Ma) was used as a secondary standard to assess data quality. Isotopic ratios and element contents were calculated using GLITTER version 4.4.1. Concordia ages and diagrams were generated using Isoplot/Ex version 3.0. Additionally, ²⁰⁷Pb/²⁰⁶Pb ages were reported for zircons older than 1000 Ma, while ²⁰⁶Pb/²³⁸U ages were reported for zircons younger than 1000 Ma [29].

A whole-rock geochemical analysis of 23 sandstone samples was conducted using X–ray fluorescence (XRF) at the Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, Guilin University of Technology, China. Fresh samples were collected and crushed, then chips were soaked in 4 N hydrochloric acid for half an hour to remove any altered material. The rock chips were then powdered using an alumina ceramic shatter box. Prior to major element analyses, loss-on-ignition (LOI) values were measured using a muffe furnace at a constant temperature of 1000 °C. The baked samples were then made into glass disks with Na₂B₄O₇·10H₂O at ~1150 °C. An ZSX Primus II X–ray fluorescence (XRF) was used to determine the major element compositions.

4. Results

4.1. Sedimentary Facies Analysis

The correlation between the boreholes in the Nankang Basin (Figure 3) [30] shows that the main sedimentary formations above the basement are the Neogene Nankang Formation, the Quaternary Zhanjiang Formation, and Beihai Formation. The Nankang Formation is composed mainly of thick green-gray horizontal and wavy siltstone, and it contains oil shales and coal seams, indicating a deltaic depositional environment (Figure 3). Peat and oil-rich sapropelic clay deposited in delta plain swamps forms the main coal deposits in the Nankang Basin. The Zhanjiang Formation, comprising pale yellow and pale grey sandstone, exhibits a bimodal alluvial pattern with graded bedding (Figure 4a), indicative of a fluvial origin (Figure 3). Sample ZKNK01–5, collected from the borehole core of Zhanjiang Formation, is a greyish white medium–grained sandstone, and contains quartz, plagioclase feldspar, and sedimentary lithic grains (Figure 4c). These rock fragments are moderately to poorly rounded, subrounded–subangular, argillaciously cemented, poorly sorted, and range from 0.11 to 0.3 mm in size. The Beihai Formation unconformably overlies the Zhanjiang Formation and is extensively exposed across the basin’s surface (Figure 4b), predominantly composed of red sandstone. Near the northern part of Beibu Bay, marine shelf deposits with foraminifera and bivalves can be found, indicating a littoral depositional environment. Sample ZKNK01–1, collected from the borehole core of Beihai Formation, is an inequigranular sandstone (Figure 4d). The debris in this sample includes gravel and silt. The gravels, which are mostly 1–3 mm in size and subrounded, are primarily composed of sedimentary lithic grains, quartz, and plagioclase feldspar, and exhibit poor sorting. The silt fraction consists of feldspar and quartz, with most grains in the 0.02–0.08 mm silt size range and some in the 0.08–0.3 mm fine sand size range.

4.2. Petrography

The statistical results of the detrital components of the sandstones in the Nankang Basin are shown in Table S1. The sandstone classification proposed by [31,32] is widely accepted. According to the projection of the data in the QFL diagram (Figure 5), all the examined sandstones are primarily feldspathic litharenite.

4.3. Zircon U–Pb Geochronology

Sample ZKNK01–1 is the inequigranular sandstone from the Beihai Formation (borehole ZKNK01). A total of 47 zircon analyses yielded Th/U ratios of 0.09–1.00 and formation ages of 1250–79 Ma, concentrated mainly in two age groups: 294–225 Ma and 497–421 Ma (Figure 6a). The zircon grains are euhedral–subhedral prisms and rounded anhedral grains that are 40–110 μm long (Figure 7).

Sample ZKNK02–1 is the inequigranular sandstone from the Beihai Formation (borehole ZKNK02). A total of 54 zircon analyses yielded Th/U ratios of 0.04–1.18 and formation ages of 1562–244 Ma, concentrated mainly in two age groups: 272–244 Ma and 491–423 Ma (Figure 6b). The zircon grains are euhedral–subhedral prisms and rounded anhedral grains that are 30–100 μm long (Figure 7).

Sample ZKNK01–5 is the medium-grained sandstone from the Zhanjiang Formation (borehole ZKNK01). A total of 76 zircon analyses yielded Th/U ratios of 0.01–2.07 and formation ages of 2775–240 Ma, concentrated mainly in three age groups: 498–425, 1090–890, and 2775–2039 Ma (Figure 6c). Zircon grains in the 498–425 and 1090–890 Ma groups are euhedral–subhedral prisms and rounded anhedral grains that are 30–120 μm long (Figure 7). Zircon grains in the 2775–2039 Ma age group are anhedral and nearly spherical, have no clear zoning, and are 30–80 μm long (Figure 7).

Sample ZKNK02–8 is the coarse-grained sandstone from the Zhanjiang Formation (borehole ZKNK02). A total of 63 zircon analyses yielded Th/U ratios of 0.09–1.53 and formation ages of 2554–258 Ma, concentrated mainly in three age groups: 481–407, 1218–901, and 2554–1999 Ma (Figure 6d). Zircon grains in the 498–425 and 1090–890 Ma groups are euhedral–subhedral prisms and rounded anhedral grains that are 30–120 μm long (Figure 6d). Zircon grains in the 2554–1999 Ma group are anhedral and nearly spherical with no clear zoning, and are 30–80 μm in diameter (Figure 7)

Sample ZKNK01–9 is the pelite from the Nankang Formation (borehole ZKNK01). A total of 27 zircon analyses yielded Th/U ratios of 0.03–1.35 and formation ages of 1811–113 Ma, concentrated mainly between 289 and 229 Ma (Figure 6e). Most zircon grains are euhedral–subhedral prisms that are 50–180 μm long (Figure 7).

Sample ZKNK02–12 is the pelite from the Nankang Formation (borehole ZKNK02). A total of 50 zircon analyses yielded Th/U ratios of 0.05–1.56 and formation ages of 1105–89 Ma, concentrated mainly between 260 and 233 Ma (Figure 6f). Most zircon grains are euhedral–subhedral prisms that are 40–190 μm long (Figure 7).

More than 95% of the zircon grains from all samples have Th/U > 0.1, indicating a magmatic origin (Table S2). The proportion of zircon particles with Th/U < 0.1 is less than 5%. It is noteworthy that zircons with a Th/U < 0.1 are relatively concentrated in the 500–400 Ma age range (Figure 8).

4.4. Heavy Mineral Contents

The percentages are 9% and 22.7%, respectively. Zircon, usually colorless and with subangular–subrounded to prismatic shapes, averages 15.9% (Figure 9a). Leucoxene is also a major mineral phase, making up about 7.6%. Furthermore, apatite, anatase, rutile, tourmaline, and pyrite serve as subordinate components. The ZTR, stability, and GZi indexes range from 14.3 to 25.2, 0.9 to 1.6, and 0 to 6.4, with an average of 19.15, 1.3, and 2.2, respectively.

The heavy mineral assemblages of sandstone samples from the Zhanjiang Formation are magnetite and zircon, with averages of 25.4% and 17.6%, respectively. Tourmaline makes up 12.9% of the heavy fraction, generally occurring in prismatic form with pale brown to pale yellow coloration (Figure 9c). Ilmenite is iron black, occurring as irregular granular and tabular forms, and constitutes 10.4% of the total heavy minerals (Figure 9c). Pyrite and leucoxene share similar abundances, with averages of 7.7% and 6.9%. Epidote is present at a concentration of 4.7%, characterized by pale green or pale brown coloration and irregular morphology (Figure 9c). Garnet is typically colorless, exhibits anhedral forms, and averages 1.6% (Figure 9c). Andalusite, with an average concentration of 1.6%, occurs as irregular granular forms and is pink (Figure 9b). The ZTR, stability, and GZi indexes range from 14.0 to 44.5, 0.8 to 7.9, and 5.5 to 28.2, with an average of 31.6, 3.2, and 10.7, respectively.

The heavy mineral assemblages of sandstone samples from the Beihai Formation are dominated by magnetite, zircon, and ilmenite, with averages of 28.7%, 18.4%, and 13.1%, respectively. In addition, moderate constituents are comprised of tourmaline, hematite, leucoxene, epidote, garnet, and andalusite, with averages between 1% and 10%. The ZTR, stability, and GZi indexes range from 20.0 to 39.3, 1.7 to 4.2, and 0 to 8.7, with an average of 28.9, 2.4, and 5.4, respectively.

The average heavy mineral assemblages of sandstones in each formation of the Nankang Basin are shown in Figure 10. The heavy mineral assemblages of the Zhanjiang and Beihai Formations differ from those of the Nankang Formation.

4.5. Major Element Geochemistry

The Beihai Formation samples have SiO₂ content of 81.80%–87.43% (average 84.14%), Al₂O₃ of 7.03%–12.15% (average 9.56%), and K₂O of 0.88%–2.62% (average 1.33%). Other oxides like Fe₂O₃ (0.95% on average), CaO (0.35%), MgO (0.12%), Na₂O (0.05%), and P₂O₅ (0.04%) are present in trace amounts. The SiO₂/Al₂O₃ and K₂O/Na₂O ratios are 4.73%–12.44% and 19.44%–27.32%, averaging 9.08% and 27.32%, respectively.

The Zhanjiang Formation samples have SiO₂ content of 79.76%–85.81% (average 83.81%), Al₂O₃ of 7.16%–14.76% (average 9.55%), and K₂O of 1.03%–2.24% (average 1.57%). Other oxides like Fe₂O₃ (0.98% on average), CaO (0.36%), MgO (0.18%), Na₂O (0.05%), and P₂O₅ (0.04%) are present in trace amounts. The SiO₂/Al₂O₃ and K₂O/Na₂O ratios are 5.40%–11.97% and 13.88%–49.64%, averaging 9.23% and 31.04%, respectively.

The Beihai Formation samples have SiO₂ content of 72.68%–84.72% (average 77.52%), Al₂O₃ of 9.17%–17.53 wt.% (average 14.43%), and K₂O of 0.99%–2.51% (average 1.55%). Other oxides like Fe₂O₃ (1.41% on average), CaO (0.36%), MgO (0.16%), Na₂O (0.07%), and P₂O₅ (0.08%) are present in trace amounts. The SiO₂/Al₂O₃ and K₂O/Na₂O ratios are 4.25%–9.24% and 18.06%–26.32%, averaging 5.65% and 21.41%, respectively.

As shown in Table S4, the Beihai and Zhanjiang Formation samples show higher SiO₂ content but lower Al₂O₃ and Fe₂O₃ content than the Nankang Formation samples.

5. Discussion

5.1. Tectono-Thermal Events Recorded by Detrital Zircon Grains

The detrital components of sedimentary strata are mixtures of materials from different source areas, preserving crucial information about the source regions. Detrital zircon grains preserved in sedimentary layers have a wide range of origins and are resistant to erosion [33]. By analyzing detrital zircon grains, it is possible to constrain the maximum depositional age of stratigraphic units, determine the provenance of sedimentary basins, conduct regional stratigraphic correlations, reconstruct paleogeographic patterns, and provide information on tectono-magmatic activities and orogenic evolution [34]. The detrital zircon geochronology for Cenozoic detrital rocks of the Nankang Basin has four age peaks: 2500–2000, 1100–900, 500–400, and 300–200 Ma. This indicates a complex provenance and frequent tectono-thermal events (Figure 11). The zircon ages are concentrated at 500–400 and 300–200 Ma, suggesting that late Paleozoic–early Mesozoic and early Paleozoic igneous rocks are the primary sources of the sediments. During the late Paleozoic–early Mesozoic (300–200 Ma), under the influence of the Indosinian Orogeny, composite granitic complexes, including the Dachongshan, Taima, and Jiuzhou Formations in the Liuwanshan orogenic belt, extended northeastward, forming a large granite belt [35]. These granites are characterized by cordierite, sillimanite, and andesine with zircon ages of mostly 260–220 Ma, and are a suite of Al–rich S-type granites that formed in a continental collision setting [36,37]. During the early Paleozoic (500–400 Ma), the Caledonian Orogeny led to widespread folding and deformation of Neoproterozoic–early Paleozoic strata in Guangxi and the surrounding area. Between 467 and 409 Ma [38], a series of post-collisional S-type granitic magmas were emplaced. The Ningtan pluton, which was intruded along the Baibai–Hepu fault zone, is characterized by granitic, porphyritic, and gneissic textures. In addition, previous studies have obtained U–Pb ages of 444–441 Ma for metamorphosed intermediate–mafic volcanic rocks near the Baibai–Hepu fault zone [39]. These ages match the magmatic ages constrained by our detrital zircon grains. During the Neoproterozoic–Mesoproterozoic (1100–900 Ma), a large volume of mafic volcanic rocks associated with island arcs was emplaced in the Yunkai region of the Cathaysia Block, corresponding to the Grenville Orogeny [40]. The Paleoproterozoic–Neoarchean (2500–2000 Ma) peaks in the detrital zircon ages record the early stages of the metamorphic basement of the Cathaysia Block, corresponding to global continental crust formation and accretion during the Archean [41].

5.2. Provenance Inference from Zircon U–Pb Studies

The recycling of zircons in the Nankang Basin and the surrounding area suggests that the detrital zircons originated from magmatic or ancient recycled rocks in the surrounding orogenic belts. The main rock associations in the Neogene Nankang Basin and its periphery are quartz–mica schists, metamorphosed sandstones, and mica–quartz beds of the Sheguang Gourp in the Shizishan; sandstone and shale interlayers of the Silurian Liantan Formation; mottled sandstone of the Devonian Xindu Formation; and the Ningtan and Liuwanshan plutons, which are potential sources of the Cenozoic sediments [24,25,26].

As shown in the U–Pb dating spectrum of detrital zircons from the Cenozoic sedimentary rocks in the Nankang Basin, the first peak ranging from 300 Ma to 200 Ma mainly originates from the zircon grains of the Nankang Formation. These grains are euhedral with clear oscillatory zoning, typical of magmatic zircon [43]. Many high-precision zircon U–Pb studies have been conducted on the Indosinian plutons in southeastern Guangxi, obtaining crystallization ages of 260–224 Ma for the Liuwanshan pluton [33,34,35]. From a geomorphological perspective, the Nanliu River and its tributaries are the primary river systems in southeastern Guangxi, converging into the Beibu Bay to form the Hepu–Nankang Delta. The weathered debris of the Liuwanshan pluton carried by the Nanliu River and its tributaries enters the Hepu and Nankang Basins and the Beibu Bay, becoming the main source of zircon grains with ages of 300–200 Ma.

The second peak ranging from 500 Ma to 400 Ma mainly originates from the zircon grains of the Beihai and Zhanjiang Formations. These grains are euhedral with clear oscillatory zoning, typical of magmatic zircon [43]. The Ningtan pluton (467–409 Ma) is exposed widely, including in Luchuan, Bobai, and Nabu [38]. Therefore, the weathered debris of the Ningtan pluton in southeastern Guangxi provides the main source of 500–400 Ma zircon grains through the Jiuzhou River and its tributaries. Among the 500–400 Ma age group, there are 10 grains with Th/U ratios of <0.1 (3.7% of the overall dataset), possibly originating from metamorphic rocks in the Shizishan. The detrital zircon grains from metamorphosed sandstone in the Shizishan have U–Pb ages of 456–423 Ma, and some grains have Th/U ratios of <0.1 [39].

Zircon grains with ages of 2500–2000 and 1100–900 Ma occur mainly from the Beihai and Zhanjiang Formations. These grains are mostly subrounded subhedral prisms, with a few subrounded and anhedral grains, similar to the detrital zircon grains in Paleozoic sedimentary rocks in the Cathaysia Block [43]. This indicates that the source of the 2500–2000 and 1100–900 Ma zircon grains is sedimentary rocks from the Liantan and Xindu Formations exposed around the Nankang Basin, as the Paleozoic sedimentary rocks of the Cathaysia Block are derived mainly from the Liantan and Xindu Formations [44].

5.3. Provenance Inference from Geochemical Studies

The geochemical composition of sandstone is widely used as a tool to infer the provenance of a sedimentary basin [21,22,23]. Most samples plot in the retrograde orogenic belt and mixed fields on Qm–F–Lt diagrams (Figure 12a), suggesting that the Neogene sediments in the Nankang Basin formed under tectonic conditions associated with retrograde orogenesis, rich in quartz but relatively poor in feldspar and lithics [31,32]. The source area is composed mainly of sedimentary or igneous rocks, possibly thrust sheets or overthrust terrains in collisional orogenic belts, where previous basin sediments, part of the basin basement, and intrusive igneous rocks had undergone intense uplift, thus providing abundant material for basin deposition [45]. The rock fragments in the Cenozoic sandstone in the Nankang Basin are mainly volcanic and sedimentary rock fragments, with metamorphic rock fragments being less common (Figure 12b). Metamorphic rock fragments are rare or non-existent in the Nankang Formation, but more common in the Zhanjiang and Beihai Formations, indicating a change in sediment sources during the period between the deposition of the Nankang and Zhanjiang Formations in the Nankang Basin.

Roser and Korsch [46] used major element concentrations in sandstones as the variables’ indiscriminant functions (F1 and F2). The Al₂O₃, TiO₂, Fe₂O₃, MgO, CaO, Na₂O, and K₂O contents were considered to discriminate among four sedimentary provenances: mafie, intermediate, felsic, and quartzose sediments. In the F1–F2 provenance discrimination diagram (Figure 13a), the Nankang Formation samples fall almost completely in the intermediate igneous, suggesting that their sediments are mainly composed of intermediate igneous rocks. However, the Zhanjiang and Beihai Formations’ samples are mostly plotted in the intermediate igneous provenance, and a few in the quartzose sedimentary provenance. These characteristics are similar to the Neoproterozoic–Cambrian sedimentary rocks in the Cathaysia Block [15], suggesting that the sediments in the Zhanjiang and Beihai Formations are mainly composed of intermediate igneous rocks with recycling sedimentary rocks. The SiO₂/A₂O₃ and K₂O/Na₂O ratios are often used to reflect the maturity and chemical weathering intensity of sedimentary rocks [47,48]. In the SiO₂/Al₂O₃–K₂O/Na₂O diagram (Figure 13b), the higher and more variable SiO₂/Al₂O₃ and K₂O/Na₂O ratios of the Beihai and Zhanjiang Formation samples are similar to the Neoproterozoic–Cambrian sedimentary rocks in the Cathaysia Block [15], and it can be interpreted that the data signify progressive recycling with advanced weathering and sediment maturation.

The ZIR and stability index could reflect the compositional maturity of the sandstone samples, which can help to infer the transportation distance of the clastic deposits [21,22,23]. As there is an increase in the transportation distance, the relative contents of the stable heavy minerals will rise, which will lead to a high ZTR and stability index [49,50]. The heavy minerals in the Nankang Basin’s Neogene Nankang Formation are primarily magnetite, ilmenite, and zircon, with a low ZTR and stability index. This suggests a primarily intermediate–silicic igneous rock source and is similar to the heavy mineral characteristics of Cenozoic Basins around the Liuwanshan orogenic belt [51], suggesting that they were sourced from the Liuwanshan pluton. The heavy mineral assemblage of the Quaternary Zhanjiang and Beihai Formations primarily consists of minerals from intermediate–silicic igneous and sedimentary rocks, such as apatite, magnetite, zircon, tourmaline, ilmenite, and hematite, along with minor metamorphic minerals like garnet, epidote, and andalusite. These minerals are characterized by a relatively high ZTR and stability index. When combined with previous research on the heavy mineral characteristics of different source rock types [52,53], it is suggested that the parent rocks are mainly the distant Ningtan pluton and ancient recycled rocks from the Yunkai orogenic belt. In addition, there are some unstable minerals in the Zhanjiang Formation and Beihai Formation, including garnet, epidote, and andalusite (red arrows in Figure 14a, b), suggesting that their parent rocks are metamorphic rocks of the Shiguang group in the Shizishan. The Beihai and Zhanjiang Formations have a higher GZi index than the Nankang Formation, also reflecting the participation of metamorphic rocks. This is because the GZi index correlates positively with garnet content [23], so the higher the proportion of metamorphic rocks in the source rock, the higher the GZi index.

5.4. Cenozoic Tectono–Sedimentary Evolution of the Nankang Basin and Its Periphery

During the early Paleogene, faults formed due to the subsidence of the South China Sea, including the Wushi–Quanshui, Pingxiang–Shangsi, and Xiangtang–Chenping Faults [54]. The Youjiang Fault underwent a left–lateral strike–slip movement, forming three pull-apart basins: the Beihai, Shangsi, and Nanning Basins [55]. At the late Paleogene and early Neogene, regional compression led to widespread block uplift. The Beihai Basin was one of the earliest basins to receive sediments that formed the Nankang Formation. Due to the higher elevation in the northern part of the basin, the main sediment source for the Nankang Formation was to the north; therefore, fluvial deposition was dominant in the northern part of the Beihai Basin, particularly in Hepu, with alluvial fans possibly developing locally. In contrast, in the southern part of the Beihai Basin around Nankang, deltaic deposition was dominant. The climate was warm and humid during this period, with abundant tropical and subtropical vegetation, leading to the formation of oil-bearing brown coal and oil-rich sapropelic clay in the swampy areas of the delta plain in Nankang.

During the late Neogene and early Quaternary, influenced by the intense uplift of the Yunnan–Guizhou Plateau and the Nanling Mountains, the southern part of the Cathaysia Block experienced large-scale intermittent uplift and subsidence of intermountain basins [56]. Previous studies suggested that the Hepu–Bobai Fault was reactivated at the end of the Neogene, and the uplift of the Shizishan occurred either at the end of the Neogene or the beginning of the Quaternary [57,58]. Consequently, the uplift of the Shizishan separated the Nankang Basin from the Hepu Basin, allowing them to evolve independently. Due to further uplift, most parts of the Hepu Basin experienced erosion and denudation, resulting in the absence of Quaternary deposits, while the Nankang Basin primarily received the fluvial deposits that formed the Zhanjiang Formation. The Zhanjiang Formation, with its fluvial conglomerates and sandstones, documents a late Neogene transition from deltaic to fluvial facies in the Nankang Basin. Detrital zircon U–Pb ages indicate a significant shift in provenance from the Liushan pluton to the Ningtan pluton, reflecting major changes in the palaeo-river system and basin–mountain coupling during the late Neogene. According to the Beihai City Regional Integrated Geological Survey Report [59], previous statistical analyses of the gravel flat–plane attitude direction in the Zhanjiang Formation have shown that the palaeo-flow direction of its fluvial deposits was from NNE to SSW. This also indicates that the sediment provenance during the late Neogene and early Quaternary was located to the NNE of the Nankang Basin. During the middle Pleistocene, the Nankang Basin was submerged by seawater and was characterized by relatively flat terrain with coastal plains, with marine coastal deposition forming the Beihai Formation.

A provenance analysis suggests a shift in the sediment transport direction in the Nankang Basin during the late Neogene, from NNW to NNE. This change in the sediment source might be related to the uplift of the Shizishan during this period. These findings suggest that the uplift of the Shizishan during the late Neogene and early Quaternary most probably resulted in the separation of the Nankang and Hepu Basins.

6. Conclusions

Based on field and petrological observations, sandstone composition, heavy mineral analysis, and detrital zircon U–Pb data, we have reached the following conclusions:

• The lithic fragments and heavy mineral assemblages suggest that the Cenozoic sandstones in the Nankang Basin were derived primarily from igneous and sedimentary rocks.
• The detrital zircon grains deposited in the Nankang Basin during the Neogene have four age peaks: 2500–2000, 1100–900, 500–400, and 300–200 Ma. The majority of the zircon grains belong to the 500–400 and 300–200 Ma groups. The complexity of the sediment sources indicates at least four tectono–thermal events in the zircon provenance area. A further provenance analysis indicates that the 300–200 Ma zircon grains originated mainly from the Liuwanshan pluton; the 500–400 Ma zircon grains originated from the Ningtan pluton, with a small portion originating from metamorphic rocks around the Shizishan; and the 2500–2000 and 1100–900 Ma zircon grains originated from the Silurian Liantan Formation and Devonian Xindu Formation.
• During the deposition of the Nankang Formation in the Nankang Basin, the sediment transport direction was to the NNW, whereas during the deposition of the Zhanjiang Formation, the direction changed to the NNE. The uplift of the Shizishan most probably occurred during the late Neogene and early Quaternary, separating the Nankang and Hepu Basins.