Fig. 1

Bathymetric map of southern South China Sea showing the distribution of carbonates during the Miocene and the location of Well Nanke-1. The distribution of Miocene carbonates is compiled from Schlüter et al (1996), Li et al (2014), Steuer et al (2014), and Wang et al (2016)"

Fig. 2

Lithographic logs of Well Nanke-1 and its main mineralogical and geochemical characteristics. (a) Lithology and corals identified using core photos and thin sections; (b) mineral compositions; (c) Al, Fe and Mn concentrations; (d) δ18O and δ13C isotopes; (e) 87Sr/86Sr ratios (excluding 23 variegated samples) showing a monotonic increasing trend; (f) numerical Sr isotope ages were derived using the LOWESS 6 curve of McArthur et al (2020), with total uncertainties combining analytical errors (NBS987 and samples) and calibration errors of the curve. Four laser ablation U-Pb carbonate ages from the Nanwan, Meiji and Liyue formations are generally consistent with 87Sr/86Sr isotope ages, showing two hiatuses (indicated by red and blue wavy lines)"

Fig. 3

Photos of core slabs and thin sections showing petrographic features associated with subaerial exposure and weathering in the Meiji and Liyue formations. (a) Pores filled with terra rossa mudstones in the upper Meiji Formation (at 541.0 m); (b, c) most infillings are calcareous mudstones with scarce bio-shells, and tiny ferromanganese concretions concentrate along the boundary between red terra rossa and bioclastic limestones; (d) dark brown flowstones in the upper Liyue Formation consist of well-preserved sparry calcites with clear mm-scale layering structures (at 619.7 m); (e, f) thin-section photographs of flowstones in (d). Abbreviations: MJF = Meiji Formation; LYF = Liyue Formation; RTR = red terra rossa; OTR = olive terra rossa; CM = calcareous mudstone; FMC = ferromanganese concretions; FS = flowstone"

Fig. 4

PAAS-normalized REY distribution patterns of bio-carbonates (average values) and selected variegated samples in Well Nanke-1. Reference REY data include surface seawater from the southern SCS, fine-grained terrestrial sediments (Wan et al, 2017) and aragonites in Tutum Bay vent waters in Papua New Guinea (Pichler and Veizer, 2004)"

Fig. 5

Thin-section photomicrographs illustrating diagenetic features from Well Nanke-1 under cross-polarized light. (a) Fabric-retentive dolostone, with limpid dolomite cement lining the molds of bivalves and benthic foraminifera; (b) fabric-retentive bioclastic dolostone, with benthic foraminifera and euhedral to subhedral dolomite crystals filling intergranular pores; (c) dolomitized red algae fragment; (d) heavily dissolved limestone possibly from meteoric vadose zones, with pores partly filled with microspar calcite cement; (e) two-stage cementation in intergranular pores, with radial bladed-shaped calcite overgrowths on isopachous fibrous cement; (f) two-stage cementation in skeletal pores, with isopachous fibrous cement altered to bladed-shaped calcite crystals"

Fig. 6

Tera-Wasserburg concordia diagrams of U-Pb ages for four samples from Well Nanke-1"

Fig. 7

(a) Original and (b) NWW-trending seismic profile M1 across the northeastern slope of Meiji Atoll, and (c) simplified core log of Well Nanke-1. The inset shows the locations of profile M1 and Well NK1. Seismic facies associations are separated by four unconformities designated as R (Red), B (Blue, coincident with the Red line in this profile), B* (Blue*), and V (Violet), following Hinz and Schlüter (1985)"

Fig. 8

87Sr/86Sr ratios and Al concentrations in carbonate phases between 500 m and 800 m in Well Nanke-1. The red open circle represents the 87Sr/86Sr ratio of insoluble residue measured at 721.85 m. Black dots represent variegated samples. Yellow bands highlight samples with elevated Al concentrations in carbonate phases. Contamination by terrigenous Sr is marked by red arrows. The red wavy line (R/B) and dark blue wavy line (B*) denote unconformities. Measurement errors of NBS987 and analyzed samples are included"

Fig. 9

87Sr/86Sr ages of diagenetically altered limestones and dolostones. (a) The transition zone from limestone to dolostone at 120.5 m; (b) lower part of Nanwan Formation, where limestones formed in meteoric environments are interlayered with dolostone beds; (c) limestones in the Meiji and upper Liyue formations. Mineral compositions and carbon/oxygen isotopes are plotted to highlight meteoric and marine phreatic environments of limestones"

Fig. 10

Cenozoic stratigraphic correlations on the Borneo-Palawan Islands and the southern SCS. Revised stratigraphy in Miri zone and in the north Sabah is from Breitfeld et al (2020), Burley et al (2021), Hennig-Breitfeld et al (2019), and van Hattum et al (2013) and sources therein. Stratigraphical divisions in deep-water Sarawak and on the South Palawan Shelf are based on Krebs (2011), Madon et al (2013), and Steuer et al (2013). The global sea-level curve is from Miller et al (2020) and main tectonic events during the miocene are also marked. Principal unconformities named in previous studies include shallow regional unconformity (SRU), middle miocene unconformity (MMU), unconformity red and blue (Red and Blue), top crocker unconformity (TCU), nyalau unconformity (NU), unconformity blue* and violet (blue* and violet), and the base miocene unconformity (BMU)"

Fig. 11

Schematic diagrams showing the tectonic evolution of the southern SCS margin since the middle Miocene, modified from Hall (2013), Lai et al (2021), Madon et al (2013) and Tsikouras et al (2021). (a) During the spreading of the SCS, the underthrusting of the Dangerous Grounds terrane initiated the Miocene collisional mountain belt in northern Borneo, the flexural bending of attenuated continental lithosphere and the uplifting of some tectonic highs such as Meji Atoll; (b) after the cessation of the SCS spreading, a compressional regime remained at its southern margin due to the collision with the Philippine mobile belt and the rotation of Borneo during the late middle miocene; (c) after widespread uplift in the dangerous grounds during the middle miocene, the compressional regime ended at the southern margin of SCS due to the northward movement of Luzon Island"