Late Eocene–early Miocene provenance evolution of the Crocker Fan in the southern South China Sea

1. Introduction

Located at the southeastern margin of the Eurasian continent, the southern South China Sea (SCS) has experienced a series of subduction, collision, extrusion, extension, transtension and rotation due to the complex interaction of the Eurasian, Indo-Australian and Pacific plates. Since the early Cenozoic, demise of the Proto–SCS took place within the southern SCS area and was followed by the opening of the SCS oceanic crust. There are thirteen sedimentary basins, including the Zengmu, Beikang, Brunei-Sabah, Liyue, Wan’an, Mekong, Zhongjiannan, Malay, Natuna basins, etc. with a total stretching area of 629 100 km² (Zhang et al., 2015a,b, 2020, 2021a; Tian et al., 2021) (Fig. 1). The distribution of oil and gas in the southern SCS is not only vertically stratified, but also horizontally segmented (Zhang et al., 2018). According to previous calculations, the southern SCS might account for at least half of the entire SCS resources, and play an important role in the deep-water petroleum exploration undertakings (Zhang et al., 2007, 2010, 2013; Mi et al., 2018; Tang et al., 2021).

Figure  1.  Simplified geological map and sample location of the southern SCS (a) and stratigraphic framework of the northern Borneo based on Hall and Breitfeld (2017) (b) speculated geographic extent of the Crocker Fan.

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The Rajang Group forms as sequences of deep-water flysch sediments of the upper Cretaceous–Eocene ages, which are widely deposited in the northern Borneo, mainly including the central Sarawak, southern Sabah and northeastern Kalimantan (Haile, 1974; Bakar et al., 2007). With the continual southward subduction of the Proto-SCS, most of the Sarawak experienced collisional and orogenic processes during the late Eocene (ca. 45–37 Ma), resulting in the termination, compression, deformation, uplifting and denudation of the Rajang Group deep-marine turbidite successions (Hutchison, 1996, 2005). Subsequently, the prominent Crocker Fan initially generated in contrast with the underlying Rajang group by an angular unconformity. The Crocker Fan is the largest submarine fan of the southern SCS, with a total extension of 25000 km² stretching from the Zengmu Basin to the Brunei-Sabah Basin. In addition, its deep-water successions are also widely exposed in the onshore area of the northern Borneo. For instance, the Sabah has been identified with over 10 kilometers in thickness (Hamilton, 1973). Previous scholars carried out some studies on the sedimentary composition characteristics, potential source contributors and paleo-environmental evolution of the Crocker Fan (Lambiase et al., 2008; van Hattum et al., 2006, 2013). According to Hamilton (1973), the northern Borneo may have received a large amount of detrital materials from the eastern Himalaya or the Indo-China paleo-highlands, under the general background of the large-scale collision and extrusion between the Indian and Eurasian plates in the Eocene. To be more detailed, these sediments were possibly transported via long-distance fluvial networks across the Sundaland shelf, and finally formed the Crocker Fan in the deep-water basins of the southern SCS. Some scholars also believe that the Crocker Fan is an accretionary prism generated from the single-directed southward subduction of the Proto–SCS during the late Eocene–early Miocene (Tongkul, 1991). It has also been suggested that the Crocker Fan was largely sourced from local provenances within the Borneo terrane itself, such as the Schwaner Mountain in the Southwest Kalimantan, the metamorphic crystalline basement in the southwest Borneo, or the denuded and reworked materials from the Rajang Group in the central Sarawak (van Hattum et al., 2006, 2013). By comparing the color, shape and structural characteristics of zircons, some researchers proposed that the Malay Peninsula on the western side of the Sundaland shelf, might have also provided sediments to the northern Borneo (van Hattum et al., 2006; Hennig-Breitfeld et al., 2019). However, most of these earlier studies were based on qualitative descriptions of mineral composition, lithological assemblages and sedimentary facies patterns. Systematic and quantitative source-to-sink restoration is still lacking, which largely constrain a better understanding on the formation and development of the Crocker Fan.

In this paper, we conduct an integrated source-to-sink comparative analysis of the Crocker Fan sediments and their surrounding potential source areas by utilizing field observation, petrographic identification, heavy mineral assemblages, elemental geochemical variations and detrital zircon U-Pb age spectra. On the basis of these combined techniques, we aim to reveal the provenance distribution patterns and sedimentary infilling processes of the northern Borneo during the late Eocene–early Miocene, and to provide more corroborative geological evidence for the exploration undertakings in the deep-water areas of the southern SCS.

2. Geological background

Situated in the southern SCS, the Borneo is bordered by the Sundaland to the west, the Sumatra-Java Islands to the south, the Sulu Sea, Celebes Sea and Sulawesi Island to the east, and several micro-blocks with affinity to the Southeast Asia to the north (Fig. 1). Since the late Paleozoic, several micro-continents including the Southwest Borneo, East Java-West Sulawesi, and Northwest Sulawesi were separated from the Gondwana landmass and gradually drifted northward. The Borneo is interpreted as a complex collage of several continental slices after a series of collision and amalgamation, and basically formed its main crystalline basement during the late Cretaceous (Hennig et al., 2017). In the late Mesozoic (ca. 130–80 Ma), the Paleo-Pacific Plate gradually weakened its subduction beneath the Eurasia, consistent with the occurrence of the upper Cretaceous ophiolitic suites, volcanic arcs and accretionary wedges along the Lupar Line (Hall and Breitfeld, 2017). The Zengmu Block collided with the western Borneo first, leading to the wide deformation and orogenic events, namely the Sarawak Orogeny in the late Eocene (ca. 45–37 Ma). W–E-oriented thrust-fold-uplift belt were formed along the Lupar Line and ceased the early deep-marine sedimentation by a large-scale angular unconformity. With the continual southeastward subduction of the Proto-SCS, the Reed Bank-Dangerous Ground then docked with the northeastern Borneo in the middle Miocene (ca. 15.5 Ma), and resulted in the Sabah Orogeny of a regional scale. Meanwhile, the Proto-SCS oceanic crust disappeared in an overall “scissor” pattern from the west to east (Hutchison, 1996, 2005; Zhang et al., 2015a, b; Wang et al., 2016; Zhao et al., 2019).

Geographically, the Zengmu Basin is bounded to the Wan’an Basin by the Xiya Uplift, and to the Brunei-Sabah and Beikang Basins by the West Baram–Tinjar Line. It is the largest Cenozoic basin in the southern SCS, with a stretching area of 169000 km² and an average water depth exceeding 500 m. The entire Zengmu Basin is divided into several geological provinces, including the Balingian Province, Central Luconia Province, etc. commonly displaying pronounced NW-SE structural lineaments. The Zengmu Basin might have initially formed as a peripheral foreland basin before subsequently undergoing thermal subsidence on the passive continental margin (Yao et al., 2005, 2008). It mainly lies on the eastern edge of the Sundaland shelf but part of its strata was deformed and outcropped at the Borneo onshore regions. To be more detailed, its southern part was generated over the thrust-fold-uplift basement due to the collision between the Zengmu Block and northern Borneo terrane (Fig. 1). Adjacent to the Zengmu Basin, the Brunei-Sabah Basin is defined as a middle Miocene to Pliocene sedimentary basin, and has a relatively smaller area of ca. 94000 km² and a shallower water depth of <500 m, accumulating a maximum succession of 12.5 km in thickness during the Cenozoic. Stretching in a general NE–SW trend, it is separated from the Nansha Trough, and is bordered by the Balabac Line to the east (Fig. 1). With the previous stage of deep marine sedimentation being its deformed basement, the shelf deposition of the Brunei-Sabah Basin prograded northwestward off the NW Sabah area since the middle Miocene (Hinz et al., 1989).

The Crocker Fan is composed of upper Eocene–lower Miocene thick-bedded turbidite sandstone and shale alterations, which is widely distributed in the central Sarawak and Brunei-Sabah regions. Syn-sedimentary deformation and imbricate thrust-nappe structures are commonly observed within the Crocker submarine fan. The Crocker Fan in the central Sarawak mainly includes the Tatau, Buan, Nyalau and Setap Shale Formations, while the contemporaneous strata deposited in the Brunei-Sabah consist of Trusmadi, Crocker (including the West and East Crocker formations), Temburong, Kudat, Wariu, Meligan, Kulapis, and Labang formations, locally interbedded with thin layers of Gomantong and Melinau limestones (Fig. 1b) (Liechti et al., 1960; Hutchison, 1996, 2005). The Nyalau Formation is mainly exposed in the Sibu and Miri zones of Sarawak, and comprises thick sandstone successions featured by tidal-flat, coastal floodplain and littoral-neritic facies (Hutchison, 2005). The Nyalau Formation grades laterally into the Setap Formation of contemporaneous marine sediments, which stretches in both of the central Sarawak and southwestern Sabah. The Setap Shale is dominated by dark blackish clay and shale layers, interspersed with thin laminations of sandstones and siltstones. In particular, Skolithos fossils and algal limestone lenses have been identified within the Setap sediments (Wilson and Wong, 1964). The Crocker Formation and its equivalents, on the other hand, are widely distributed within the western, central, southwestern and northeastern Sabah, and form typical Bouma sequences composed of calcareous sandy and flysch shale sediments. Multiple submarine fans indicate the dominance of an open marine shelf environment. To the farther eastern Sabah, the Oligocene Labang Formation sandstones are of low compositional maturity interbedded with plentiful pyroclastic materials based on light and heavy mineral combination patterns (Hall and Breitfeld, 2017). However, early studies often assigned similar lithological characteristics to different segments of the northern Borneo based on limited field observations and paleontological assemblages. Therefore, the affinities and properties of separate parts of the Crocker Fan remain controversial. For example, the Trusmadi Formation is comprised of deep-marine shales intercalated with sandstones and siltstones, and was partially metamorphosed into phyllites during the early Eocene. Some researchers, on other hand, considered that the Trusmadi Formation was deposited as a part of the Rajang deep-marine turbidites during the late Cretaceous–Eocene, and was unconformably overlain by the Crocker Fan successions (Hutchison, 2005). Deposition of the Crocker Fan was not eventually terminated until the early Miocene when the Sabah Orogeny dominated this area and resulted in ubiquitous compression, deformation and uplifting. Most of the Sabah was subsequently deposited with neritic-, deltaic- and fluvial- facies sediments.

3. Materials and methodology

Combined with field observations, a total of fourteen samples of the Crocker Fan sediments were collected from the Sabah–Sarawak area for heavy mineral assemblage comparison, elemental geochemical analyses as well as detrital zircon U-Pb geochronology studies (Cui et al., 2022). In this paper, we also compiled previous datasets of the surrounding potential source terranes, including the Borneo complex, Malay Peninsula and the northern SCS area in order to conduct an integrated source-to-sink pathway restoration. Sampling geographic locations and related information are displayed in detail by Fig. 1 and Table 1.

Table  1.  Sample information

Sample No.	Age	Lithology	Method	Reference
EK14-1	Cretaceous	granodiorite	zircon U-Pb dating	Hennig et al., 2017
EK14-6	Cretaceous	quartz diorite	zircon U-Pb dating	Hennig et al., 2017
EK14-10	Cretaceous	diorite	zircon U-Pb dating	Hennig et al., 2017
TB-76	Cretaceous	granodiorite	zircon U-Pb dating	Hennig et al., 2017
TB71a	Cretaceous	granodiorite	zircon U-Pb dating	Hennig et al., 2017
TB54	upper Eocene/Rangsi	conglomerate	heavy mineral analysis/ Zircon U-Pb dating	Hennig-Breitfeld et al., 2019
TA04	upper Eocene /Rangsi	conglomerate	zircon U-Pb dating	Hennig-Breitfeld et al., 2019
TB199b	upper Eocene /Rangsi	conglomerate	zircon U-Pb dating	Hennig-Breitfeld et al., 2019
TB200a	lower Oligocene/Tatau	sandstone	heavy mineral analysis/Zircon U-Pb dating	Hennig-Breitfeld et al., 2019
TB250a	Triassic/Kuching	volcaniclastic rocks	zircon U-Pb dating	Breitfeld et al., 2017
713b	Triassic/Sadong	volcaniclastic rocks	zircon U-Pb dating	Breitfeld et al., 2017
712	Triassic/Sadong	volcaniclastic rocks	zircon U-Pb dating	Breitfeld et al., 2017
SA-54	boundary of Eocene and Oligocene strata	sandstone	heavy mineral analysis	this study
SA-51	upper Eocene−lower Oligocene/Tatau	sandstone	heavy mineral analysis	this study
SA-69	lower Miocene/Lambir	sandstone	heavy mineral analysis	this study
SA-61	Oligocene−lower Miocene/Nyalau	sandstone	heavy mineral analysis	this study
S87	Oligocene−lower Miocene/Setap	sandstone	heavy mineral analysis/elemental geochemistry	this study
S27	Paleocene−lower or middle Eocene/Trusmadi	sandstone	heavy mineral analysis/elemental geochemistry	this study
S17	upper Eocene-Oligocene/Crocker	sandstone	heavy mineral analysis/elemental geochemistry/zircon U-Pb dating	this study
S7	Oligocene/Kudat	sandstone	heavy mineral analysis/elemental geochemistry	this study
S6	Oligocene−Miocene/Wariu	sandstone	heavy mineral analysis/elemental geochemistry/zircon U-Pb dating	this study
K23	lower Oligocene/Tebidah	sandstone	zircon U-Pb dating	this study
M28	Triassic	sandstone	zircon U-Pb dating	this study
M23	Jurassic	sandstone	zircon U-Pb dating	this study
M16	Carboniferous	sandstone	zircon U-Pb dating	this study
M6	Carboniferous	sandstone	zircon U-Pb dating	this study

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Heavy mineral assemblage analyses were performed on nine sandstone samples at the laboratory of the Institute of Regional Geology and Mineral Resources, Hebei. Samples were first dried, gently disaggregated, and were subsequently processed by being sieved via a 420 μm mesh. Detrital heavy mineral components were separated by centrifugal elutriation from the bulk sediments. Further mineral separation was completed by heavy liquids, and magnetic and electrostatic filters. In general, each sample was identified for a total of about 1000 non-opaque grains under the binocular microscope.

Geochemical discrimination and analyses were also conducted on five clastic samples from the Crocker Fan sediments. All sample pretreatment and geochemical data processing were carried out at the State Key Laboratory of Marine Geology, Tongji University. Bulk sediments were first washed through deionized distilled water, followed by 48 h of 50℃ dryness in order to eliminate external contamination. About 3 grams of each crushed sample was then heated to 600℃ for 2 hours to remove interlayer water and organic matter. Immersed within a HF-HNO₃ mixture, the samples were examined by ICP-AES (Thermo ICP-IRIS Intrepid II) and ICP-MS (Thermo Elemental X-Series) for major and trace element concentration, respectively. Three certified materials were repeatedly analyzed as unknown samples to assess the precision (i.e., GSR-5, GSR-6, and GSD-9 from the Institute of Geophysical and Geochemical Exploration, China). The external precision (1 σ) was generally better than 5%, and the obtained concentrations were in consistent with the recommended data of these reference materials. Seven sedimentary samples from this study were measured as duplicates. The relative differences between duplicates were lower than 5%. Detailed analytical procedures are described in detail in Chen et al. (2018).

Detrital zircon U-Pb geochronological study was conducted upon seven sandstone outcrops. Zircon grain collection, mounting work, Cathodoluminescence (CL) imaging and LA-ICP-MS isotopic analysis were all completed at the State Key Laboratory of Marine Geology, Tongji University. The instrumentation is a Thermo Elemental X-Series ICP-MS coupled to a New Wave 213 nm laser ablation system. In the laser ablation process, we used helium gas as carrier gas, and argon gas acted as compensation gas to adjust sensitivity. The laser beam was set to a 10 Hz ablation frequency and a 30 μm spot size. Each analysis included a 30 s background signal in addition to a 70 s sample signal. The external standard was international standard zircon 91500 ((1065.4±0.3) Ma), with zircon standard Plešovice ((337.1±0.4) Ma) to adjust the accuracy of the analysis results. U-Pb isotopic ratio and age calculations were achieved on the software ICPMSDataCal. Common Pb correction was performed by using Andersen (2002) method. Based on Compston et al. (1992), calculated ²⁰⁶Pb/²³⁸U ages are adopted for zircons<1000 Ma, while ²⁰⁷Pb/²⁰⁶Pb ages are adopted for ones over 1000 Ma. The accepted ages were selected from less than or equal to 10% discordance and less than or equal to 10% uncertainty (1 σ) subsets. Detailed analytical procedures are described by Shao et al. (2019b).

4. Results

4.1 Sedimentary facies distribution patterns of the Crocker Fan

In general, the Crocker deposition lies in contrast with the underlain Rajang Group by a large-scale angular unconformity (Fig. 2a). The lowermost section of the Crocker Fan is featured by thick conglomerate layers and typical erosion boundaries (Figs 2a and b). Those conglomerates mostly range between 30 cm and 1 m in diameter, and display poor roundness and sorting with various lithological composition including sandstone, quartzite, siliceous rock, slate, tuff, pyroclastic rock and basalt (Fig. 2b). During this period, the Crocker Fan was possibly delivered with in-situ or nearby clastic materials. Tatau and Buan Formations developed directly over the conglomerate layers and are mainly comprised of sandstones and siltstones intercalated with mudstone layers (Figs 2c and d). The rapid shifting into the shallow-marine depositional environment largely suggests a long-distance transport under a littoral-neritic or neritic environment during the Crocker Fan Formation. To be noted, the shallow-marine Buan Formation has also been identified with apparent trace fossils. It was followed by the extensive generation of the Nyalau Formation in the Sarawak from the early Oligocene to the early Miocene. The Nyalau deposits are featured by thick sandstone sequences of relatively high maturity which are interbedded with lenticular mudstone layers. Wave marks and tidal-flat beddings are identified, along with small laminations and cross-beddings (Figs 2e and f). The Setap Shale is the finer-grained equivalent of the Nyalau Formation distributed in the western and northwestern Sabah, and mainly comprises of siltstones, shale stones and limestone lenses (Fig. 2g).

Figure  2.  The Rajang Unconformity between the underlain Rajang Group and the overlying upper Eocene Crocker Fan (a); Rangsi conglomerates within the lowermost layer of the Crocker Fan (b); shallow-marine sediments of the Tatau Formation (c); trace fossils perpendicular to the strata, Buan Formation (d); shallow-marine sediments of the Nyalau Formation (e); small cross-beddings in the Nyalau Formation, sandstones interbedded with mudstone laminations (f); limestone of the Setap Shale enriched with foraminiferal fossils, Sabah (g); erosion surface at the lowermost section of the Crocker Fan turbidtes (h); normal upward-graded beddings in the Bouma Sequence (i); wedge-shape sandstone layer, Kudat Formation (j); and thick sandstone layers, Kudat Formation (k).

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The Sabah area is widely exposed with the Crocker Formation strata, which are rhythmic flysch deposits under deep-marine environment mainly composed of sandstones, siltstones and shales in addition to minor tuff and limestones. The Crocker Formation develops typical Bouma Sequence (Fig. 2i) with obvious erosion surfaces or sharp contrasts at the bottom, and poorly-sorted, ungraded gravel layers within the Ta sub-division (Fig. 2h). These gravels mainly include volcaniclastic and metamorphic rocks and rarely display any directional arrangement structures. A normal upward-graded sequence has been observed for the Crocker Formation strata with parallel bedding, small ripple cross-bedding, large cross-bedding structures and massive mudstones at the top of the turbidites (Fig. 2i). Temburong, Kudat and Wariu Formations are all equivalents of the Crocker Formation indicating shallow marine and outer continental shelf depositional setting (Figs 2j and k), while they are relatively less exposed in the Sabah area. In any case, the Kudata Formation has generated thick sandstone successions with typical wedge shape.

4.2 Heavy mineral assemblages

In this study, the upper Eocene–lower Miocene sandstone samples (SA-54, SA-51, SA-69, SA-61, S27, S17, S7, S6 and S87) were collected from the Crocker Fan sediments (including the Crocker Formation and its equivalent strata in both Sarawak and Sabah) and were carried out with heavy mineral composition and assemblage analyses together with the compiled data from Hennig-Breitfeld et al. (2019) (Fig. 3). Located at the equatorial region, the study area is fairly susceptible from the influence of chemical weathering. Therefore, secondary heavy minerals have been ruled out in our provenance analysis, such as magnetite, anatase, and ilmenite generally accounting for more than 40% of the total minerals. Only original and transparent minerals were utilized in this study in order to eliminate the potential impact from the alteration processes.

Figure  3.  Heavy mineral assemblages of the Crocker Fan sediments; Samples TB200a and TB54 are compiled data from Hennig-Breitfeld et al. (2019).

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Consistent with samples TB54 and TB200a from Hennig-Breitfeld et al. (2019), our samples are dominated by ultra-stable mineral assemblages, most of which exceed 90% of the total (91.8%–99.5%). Sample SA-51 is the only sample with relatively lower ultra-stable mineral proportion of 86.0% (Fig. 3). To be more detailed, the ultra-stable heavy minerals are mainly zircons accounting for 66.5%–89.8% of the total, in addition to the subordinate minerals including rutile (0.6%–11.1%), tourmaline (0.9%–6.5%), chrome spinel (0.4%–24.4%) and monazites (0.2%–0.7%) in our study. As a comparison, samples TB200a and TB54 in Sarawak have lower zircon contents of 21.2%–45.8%, but display higher rutile and tourmaline contents in excess of 30% (Hennig-Breitfeld et al., 2019). Both compiled and analytical data of this study commonly indicate that relatively unstable and unstable mineral groups are rare, such as garnet, apatite, epidote, etc. Sample S87 collected from Sabah generates garnet (3.1%), epidote (0.1%) and trace quantities of hornblende (0.1%). Sample S6 contains apatite and garnet far less than 1% of the total minerals. A 3.4% proportion of garnet and apatite is detected in Sample S7. Sample S17 contains garnet (6.3%), apatite (1.9%) and significantly less hornblende. Similarly, the Sarawak Sample SA-61 contains garnet at 7.0% and subordinate amount of apatite. Sample SA-51 contains garnet at 14.0% in addition to extremely low abundance of hornblende.

4.3 Bulk rock geochemical analysis

Five Sabah outcrop samples of the Crocker Fan sequences (S6, S7, S17, S27 and S87) were geochemically analyzed in this study (Fig. 4). The chondrite-normalized rare earth elemental (REE) distribution curves commonly show a light REE enrichment and depleting in the heavy ones, which is consistent with that of Post-Archean Australian Shale (PAAS) (Fig. 4a). The total REE concentrations range between 42.9 ppm and 157.7 ppm. Values of La/Yb_CN and Gd/Yb_CN are 5.3–11.3 and 1.3–2.7, respectively. The overall REE patterns are featured by obvious negative Eu anomalies with Eu/Eu* varying from 0.5 to 0.9, indicating sources dominated by acidic or sedimentary parent rocks (Fig. 4a). As complementary confirmation, the TiO₂ vs. Zr discrimination plot further indicate that the majority of samples fall within the acidic-sedimentary rock region (Fig. 4b). In addition, most of our samples are concentrated at the junction of the “basalt–granite” and “granite–basalt” sections in the Al₂O₃ vs. TiO₂ diagrams (Fig. 4c).

Figure  4.  Rare earth elemental distribution pattern (PAAS from Taylor and McLennan (1985))(a); TiO₂ vs. Zr discrimination plot(b); and Al₂O₃ vs. TiO₂ discrimination plot (based on Hayashi et al. (1997)) (c).

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4.4 Detrital zircon U-Pb age spectra

Various methods, such as the aforementioned sedimentary facies identification, heavy mineral assemblages, bulk-rock elemental geochemical analysis, etc. have been utilized on both of river networks and their depositional end-members globally to study the sedimentary provenance, denudation-transport pathways and paleogeographic evolution. The compositional and geochronological characteristics of the whole-rock sediments are generally considered as useful indicators for the temporal and spatial changes in a source-to-sink system. Therein, the single grain techniques, such as the zircon U-Pb dating analysis, are more precise in quantifying the erosional detritus from mixed source areas compared to the other whole-rock methodology which can only provide an average value. In particular, zircon is a typical mineral of high abundance in many rock types and generates high uranium and thorium element concentration. Theoretically, the U-Th-Pb system is featured by a high closure temperature which is seldom biased by the high-grade metamorphism lower than 750℃. In addition, detrital zircons are also not affected by secondary geochemical weathering and diagenesis processes during sedimentary transportation. Their U-Pb age spectra and combination patterns can be utilized as a robust tool to investigate the properties of parent rocks in the provenance evolution researches (Shao et al., 2019a; Cui et al., 2021a, 2022; Meng et al., 2021; Zhang et al., 2021b; Zhao et al., 2021). In this study, seven sedimentary rock samples of the northern Borneo and Malay Peninsula (including M6, M16, M23, M28, K23, S6 and S17) were conducted by detrital zircon U-Pb dating together with the compilation and reinterpretation of previously published data (Galin et al., 2017; Hennig-Breitfeld et al., 2019) (Figs 5, 6).

Figure  5.  Detrital zircon U-Pb age spectra of the turbidite sediments within the northern Borneo (N represents the number of effective analyses; Samples TB200a, TB54, TA04, TB199b are compiled data from Hennig-Breitfeld et al. (2019); Samples Unit1, Unit2 and Unit3 are compiled data from Galin et al. (2017))

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Figure  6.  Detrital zircon U-Pb age spectra of the potential source terranes (N represents the number of effective analyses; samples “eastern”, “central” and “western” are compiled data from Sevastjanova et al. (2011); Samples TB250a, 713b and 712 are compiled data from Breitfeld et al. (2017); Samples EK14-1, EK14-6, EK14-10, TB-76, TB71a are compiled data from Hennig et al. (2017))

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Galin et al. (2017) revealed that the deep-sea turbidite of the Rajang Group was widely generated in the central Sarawak from the late Cretaceous to the early–middle Eocene, as being deposited as Unit1, Unit 2 and Unit 3 strata (Fig. 5). Its detrital zircon U-Pb age spectra collectively show a dominant peak of the late Cretaceous centralized at ca. 115 Ma, in addition to a subordinate Triassic population accumulated at ca. 245 Ma and extremely low contents of Paleozoic and Precambrian zircons (Fig. 5). Regarding the conglomerate layer at the lowermost section of the Crocker Fan (typically termed as “Rangsi Conglomerate”), it is featured by a prominent Mesozoic cluster while the Paleozoic or Archean zircons are rarely found (Fig. 5; Hennig-Breitfeld et al., 2019). The overall narrow U-Pb age distribution pattern shows certain distinction to the underlying Rajang Group sediments. Thick successions of the turbidite sandstones are deposited over the Rangsi gravel layer to form the main Crocker Fan body in the broad Sarawak and Sabah areas. Both samples S6 of the Wariu Formation and S17 of the Crocker Formation display typical multimodal patterns, implying an increased transport distance or higher degree of provenance mixing and enlargement. Sample S17, on the one hand, is dominated by a higher Indosinian peak (ca. 230 Ma) and a secondary Yanshanian peak (ca. 145 Ma) along with a modest Hercynian-Caledonian peak (ca. 385 Ma). A small amount of Archean and Proterozoic zircons are also identified in Sample S17, in contrast to the low abundance within the early-deposited Rajang Group sequences and the Rangsi conglomerates at the bottom segment of the Crocker Fan (Fig. 5). Comparatively, both Samples S6 and TB200a display similar detrital zircon U-Pb age combination, and commonly exhibit an even broader U-Pb age spectrum including a major Indosinian cluster of ca. 245 Ma and minor groups of Yanshanian (ca. 125 Ma), Caledonian (ca. 440 Ma), Jinning (ca. 1175–790 Ma), Paleoproterozoic (ca. 1850 Ma) and Archean ages (Hennig-Breitfeld et al., 2019). In particular, the predominant Indosinian peak of Sample S6 is much higher than the Yanshanian and Caledonian populations. Both Paleoproterozoic and Archean zircons also show obvious elevation in abundance compared to the Sample S17 (Fig. 5).

5. Discussion

5.1 Potential source terranes

Many micro-continental blocks including the SW Borneo, East Java-West Sulawesi and NW Sulawesi were continually drifting away from the northwestern margin of Australia during the late Paleozoic–middle/late Jurassic. They were not accreted to the Sundaland shelf to eventually form the Borneo basement until the late Cretaceous (Sevastjanova et al., 2011; Hennig et al., 2017). Kalimantan has long been confined with better understanding due to the limitation of geological evidence and systematic investigation. According to van Hattum et al. (2006), Kalimantan is widely exposed with ancient metamorphic crystalline basement which is separated from the Cenozoic Crocker Fan in the northern Borneo by the Lupar Line. Possibility was not fully excluded by previous researchers that Kalimantan might have been an important source area to the Crock Fan sediments (van Hattum et al., 2006). In this study, the lower Oligocene outcrop K23 of the Tebidah Formation is located in the interior of ​​Kalimantan which is distant away from the Sarawak-Sabah area of the northern Borneo. It has a fairly low degree of compositional maturity possibly reflective of an in-situ or nearby sedimentary transport. Therefore, it is reasonable for us to use this sample representing the provenance features of Kalimantan. Sample K23 shows a complex multimodal pattern with a prominent Triassic peak centered at ca. 220 Ma together with minor late Yanshanian (ca. 85 Ma), Hercynian-Caledonian (ca. 395 Ma) and Jinningian groups (ca. 945 Ma) of almost identical proportion and scattered Paleoproterozoic zircons (Fig. 6). A large difference is obviously observed between the Sample K23 of Kalimantan and those contemporaneous depositions of the wide Sarawak and Sabah regions on their separate detrital zircon U-Pb population patterns. In this case, Kalimantan seems to be less likely to provide the Sarawak and Sabah areas with abundant clastic materials.

The West Sarawak basement was previously presumed to mainly comprise of pre-Carboniferous metasedimentary rocks, which possibly bore close tectonic affinity to the Kalimantan crystalline basement. Quartz-mica schists and granodiorite were recently collected from the Western Sarawak basement for muscovite Ar-Ar and zircon U-Pb dating analyses, respectively (Breitfeld et al., 2017). It turned out that the West Sarawak basement experienced extensive Triassic magmatism during ca. 240–205 Ma with strong metamorphism at ca. 220–215 Ma. Occurrence of those volcanic, intrusive rocks and related ophiolitic suites was interpreted as a Triassic subduction margin in the West Sarawak, with sediments derived from the proximal magmatic arc into a forearc basin (Breitfeld et al., 2017). Volcaniclastic rocks of the Sadong and Kuching Formations also confirmed this magmatic activity by correspondingly displaying a modest zircon population of Permo–Triassic ages (clustered at ca. 245 Ma) (Fig. 6). In addition, the youngest zircon age also indicates a maximum depositional age of Triassic–Cretaceous for the West Sarawak basement. It should be noted that the West Sarawak sedimentary supergroup is dominated by a very high peak of 1.8 Ga compared to the less abundant Triassic zircons (Fig. 6). The Paleoproterozoic age peak of ca. 1.8 Ga in the Triassic sedimentary strata implies an addition of recycled ancient materials. Although further evidence was still required, Breitfeld et al. (2017) suggested that the extremely old detritus was possibly derived from the Sundaland Shelf or the Cathaysia Continent to the farther north.

With the ongoing northwestward or westward subduction of the Paleo-Pacific Plate during the Jurassic–Cretaceous, the eastern margin of the Sundaland Shelf, such as the SE Vietnam, western Sarawak, NW Kalimantan, etc. was extensively preserved with upperr Mesozoic magmatic records. In particular, in addition to the minor Triassic metatonalites, abundant Cretaceous U-Pb ages were obtained from the metapelites and I-type granites in the Schwaner Mountains situated in the SW Kalimantan. Therein, the upper Cretaceous magmatic rocks have been interpreted to mark the gradual cessation and retreat of the Paleo-Pacific subduction at ca. 90–85 Ma (Hennig et al., 2017; Cui et al., 2021b; Zhu et al., 2021) (Fig. 6). Correspondingly, the Pedawan Formation distributed in the Kuching Zone is likely to be part of a Cretaceous forearc basin with eroded materials from the nearby magmatic arc. The youngest U-Pb zircon ages from the uppermost Pedawan tuff layer possibly indicate that the volcanic activities did not stopp until ca. 90 Ma (Breitfeld et al., 2017).

Located on the western Sundaland Shelf, the Malay Peninsula is comprised of the Sibumasu and East Malaya Blocks which are intruded by two granitoid provinces of Main Range and Eastern Range. Carboniferous and Triassic–Cretaceous strata were widely deposited on the underlain Proterozoic basement, and also developed abundant Permo-Triassic and Cretaceous tin-bearing granitic intrusions (Sevastjanova et al., 2011). The Cenozoic strata, on the other hand, are rarely exposed on the Malay Peninsula. Both compiled published data and our studied samples altogether show that the Malay Peninsula is featured by a broad detrital zircon U-Pb age spectrum distinct from the other aforementioned provenances (Fig. 6). It displays a major Indosinian peak at ca. 230 Ma along with other subordinate groups of the late Yanshanian (ca. 80 Ma), Hercynian-Caledonian (ca. 360 Ma) and Neoproterozoic-Paleoproterozoic zircons.

5.2 Provenance evolution of the Crocker Fan deep-water sediments

From the late Cretaceous to the middle–late Eocene, the deep-sea turbidite sequences of the Rajang Group were widely deposited in the Central Sarawak. Their significant late Yanshanian age peak and relatively low abundance of Indosinian zircons are consistent with the combined U-Pb age patterns of the Indochina Block and northern SCS pre-Cenozoic basement (Li et al., 2022; Cui et al., 2022). Due to the drastic magmatic and uplifting activities during the late Mesozoic, the corresponding orogenesis might be prominent paleo-high structures being severely eroded and transported abundant detritus to the northern Borneo (Shao et al., 2020; Li et al., 2022). Previous scholars preferentially considered that both West Sarawak basement and the Schwaner Mountain were important provenances. However, our study has shown that the West Sarawak was deposited with Triassic metasedimentary and igneous rocks in significant abundance with large bodies of Paleoproterozoic detrital zircons (ca. 1.8 Ga) (Fig. 6). The Schwaner Mountain, on the other hand, developed late Mesozoic intrusive and extrusive rocks of ca. 100–85 Ma and resulted in the Cretaceous volcaniclastic Pedawan Formation. Indosinian peaks or even older zircons have rarely been identified within the Schwaner Mountain ranges. Considering the large extension and the zircon U-Pb population patterns of the Rajang Group turbidites, the influence of the Schwaner Mountain would be relatively limited.

With the continual subduction of the proto-SCS, the Zengmu Block initially collided with the western Borneo causing the Sarawak Orogeny during ca. 37 Ma. The Rajang Group was subsequently terminated its deposition, and formed a large-scale angular unconformity in contrast to the overlying Crocker Fan (Hennig-Breitfeld et al., 2019). The massive layers of the ungraded conglomerates accumulated at the lowermost section indicate that the Crocker Fan was initially sourced from the in-situ or adjacent volcanic arc or deformed-uplifted structures due to the proto-SCS subduction (Fig. 2a). Correspondingly, the U-Pb ages of those conglomerates develop an obviously singular Yanshanian peak of ca. 135 Ma without Paleozoic, Proterozoic or Archean grains (Fig. 5).

Both Sarawak and Sabah regions were subsequently deposited with thick layers of Crocker Fan turbidite sediments. In contrast to the Rajang Group and the gravel layers at the Crocker Fan lowermost segment, Samples S17, S6 and TB200a commonly display distinctive detrital zircon U-Pb age combination patterns. They are dominated by prominent Indosinian populations of ca. 245–230 Ma in addition to the subordinate Yanshanian peak and a modest group of Proterozoic–Archean analyses (Fig. 5). To be more detailed, Sample S17 is dominated by Indosinian–Yanshanian clusters, while Samples S6 and TB200a contain higher contents of Caledonian (ca. 440 Ma), Jinningian (1175–790 Ma), Paleoproterozoic (1.8 Ga) and Archean zircons. The increasing complexity of the U-Pb age distribution patterns reveals additional influences from other potential provenances. In general, the majority of the Crocker Fan sediments are in high consistence with the Malay Peninsula to the west on their U-Pb age combination (Fig. 6). Furthermore, both heavy mineral assemblages and elemental geochemical analyses also confirmed that the Crocker Fan was possibly delivered from acidic or sedimentary parent rocks of high maturity via a fairly long distance (Figs 3, 4). Hennig-Breitfeld et al. (2019) also suggested that the Natuna Arch might not have been initiated during the Oligocene–early Miocene as a geographic hindrance structure. Therefore, plentiful sediments were likely to be directly transported eastward from the Malay Peninsula to the Sarawak and Sabah Basins.

In addition to the high abundance of ultra-stable and stable minerals, certain samples including S87, S7, SA-61, SA-69 and SA-51 exhibit a modest number of chrome spinel and garnet minerals (Fig. 3). Therefore, some scholars speculated that the Crocker Fan sediments might also have been influenced from the ophiolitic suites of the Borneo complex and the metamorphic crystalline basement, such as the Schwaner Moutain, northwestern Kalimantan or the supergroup strata of the West Sarawak (van Hattum et al., 2006, 2013). However, as being described in the aforementioned section, the Schwaner Mountain is dominated by a singular age population centralized at ca. 90 Ma, which yielded the upper Cretaceous volcaniclastic sediments of the Pedawan Formation. The broad Kalimantan is also less likely to provide abundant sediments although it also generates complex and multimodal U-Pb age spectra. Its Yanshanian, Caledonian, Jinningian and Paleoproterozoic zircon proportions are almost of identical proportion, significantly different from the U-Pb combination of the Crocker Fan (Fig. 6).

In summary, the Cenozoic basins within the southern SCS areas were partially in the marine setting under the geological background of the proto-SCS continual consumption. On the one hand, the Sarawak Basin sediments might have been entirely deposited in a bay environment, except for limited, regional delta sediments. The Sarawak Basin mostly developed shore-to-coastal facies during the early–middle Eocene. The Beikang Basin, on the other hand was subjected to transgression from the southern and eastern areas. To be more detailed, the southern-eastern parts are comprised of littoral and neritic facies, while the western-northern regions mostly generate terrestrial facies of lacstrine and swamp depositions. It shows that the northern Borneo was widely deposited with Rajang Group turbidite sequences prior to the late interval of the middle Eocene. The southern SCS basins changed significantly on their sedimentary settings during the late Eocene. Flysch formations were initially originated in the Sarawak Basin, which were then covered by molasses structures. This succession showed a typical signature of foreland basin, and the previous pelagic-hemipelagic facies were replaced by a shallow shelf environment. At this time, most of the Beikang Basin gradually shifted into transitional-to-neritic facies. Only the northern and western regions of the Beikang Basin developed fan delta depositions. With the ongoing subduction of the proto-SCS oceanic crust, the Zengmu Block first collided with the west Sarawak resulting in a large-scale regional orogeny during ca. 37 Ma. The subsequent deposition of the Crocker Fan was in angular unconformity to the underlain highly deformed Rajang Group. Basal gravel layers and mélange complexes were developed at the lowermost section of the Crocker Fan, which were largely sourced from the in-situ or nearby paleo-uplifts. The Sarawak Basin was primarily consisted of coastal plain facies during the Oligocene. To be more detailed, the southern area was featured by fluvial and delta depositions while the northern part developed a littoral and neritic environment. The Beikang Basin, on the other hand, was dominated by a littoral setting. Correspondingly, the major body of the Crock Fan turbidites were generated in the northern Borneo with an extensive age range, roughly from the late Eocene to the early Miocene. Large abundance of highly mature clastic materials were delivered from the Malay Peninsula via a long distance into the Sarawak and Sabah Basins.

6. Conclusions

In this paper, a combination of field observation, heavy mineral assemblages, elemental geochemistry and detrital zircon U-Pb spectra were used to study the compositional features, provenance evolution and sedimentary infilling processes of the upper Eocene–lower Miocene Crocker Fan deep marine turbidites. It shows that the northern Borneo was widely deposited with Rajang Group turbidite sequences during the late Cretaceous–middle Eocene. Their detrital zircon U-Pb populations are dominated by an apparent Yanshanian peak and a subordinate Indosinian cluster. With the ongoing subduction of the proto-SCS, the Zengmu Block first collided with the west Sarawak resulting in a large-scale regional orogeny during ca. 37 Ma. The subsequent deposition of the Crocker Fan was in angular unconformity to the underlain highly deformed Rajang Group. Basal gravel layers and mélange complexes were developed at the lowermost section of the Crocker Fan, which were largely sourced from the in-situ or nearby paleo-uplifts. Their predominant Yanshanian U-Pb peak indicate erosion and transport from the Mesozoic magmatic arc and the associated sedimentary succesions. The majority of the Crock Fan turbidites were generated in the northern Borneo from the late Eocene to the early Miocene. Large abundance of highly mature clastic materials were delivered from the Malay Peninsula via a long distance into the Sarawak and Sabah Basins. The U-Pb age patterns turned out to be dominated by multiple populations exhibiting a major Indosinian cluster and several secondary Yanshanian, Hercynian-Caledonian, Jinningian, Paleoproterozoic and Archean peaks. During this time, the Sarawak Basin lying in the west mainly developed delta plain, littoral-neritic and neritic facies while the Brunei-Sabah Basin in the east were dominated by deep-water turbidite sequences. Due to their thick successions and modest grain-sizes, the Crocker Fan sandstones are promising to form high-quality reservoirs and are of great significance in the southern SCS petroleum exploration undertakings.