Based on the differences in lithology, components and microscopic textures, Wu et al. (2019) recognized six facies in the neighbouring core XK-1, including coral reef, coral-algal reef, reef cap, inner bank, outer bank, and aeolianite. These facies types are also discovered in this newly described core (Fig. 3). The graphic log of the measured core is shown as Fig. 3.
Figure 3. Integrated data of late Quaternary strata of the Core SSZK1: (1) Chronostratigraphy based on ages come from U-Th dating results of the Well SSZK1 which propose the uppermost aeolianite facies formed after MIS 5; (2) facies; (3) variations in abundance of coral, red algae, green algae, benthic foraminifera and planktonic foraminifera; (4) vertical variations of carbonate and oxygen isotope; (5) correlation coefficients of carbonate and oxygen isotope vary in shallow-facies-dominated and deep-facies-dominated interlayer; and (6) mineralogical compositions of samples are concluded from analytical result of six samples and from the previous work in the nearby wells XK-1 and XC-1 in the Xisha Archipelago (Liu et al., 1997, 2019). ARAG-aragonite, HMC- high-Mg calcite, LMC- low-Mg calcite.
The coral facies are commonly observed in the Core SSZK1 profile (Fig. 3). It consists of coral framestone together with minor packstone and wackstone. The coral colonies exhibit massive morphologies, containing Porites, Turbinaria, Favia (Figs 4a and b). Generally, the wackstone and packstone matrix appear in the interstices between coral colonies and the pores within the coral individuals. The wackstone and packstone matrix are mainly composed of bioclastic debris, including abundant coral fragments and frequent to rare benthic foraminifera, red algae, rare green algae, echinoids and bivalves (Fig. 4c). Benthic foraminifera presented here are shallowest-living Amphistegina lobifera of the genus Amphistegina, which indicates a shallow and euphotic environment.
Figure 4. Coral reef facies. a. Core photo of coral framestone. 1. Favia, 22.56–22.64 m; b. Plane-polarized photomicrograph of coral framestone, 23.70 m; and c. Plane-polarized photomicrograph of coral-debris dominated packstone, formed in the interstices between coral colonies. 1. Coral fragments, 2. red algal fragments, 3. Amphistegina lobifera, and 4. micritic envelopes, 34.15 m.
This facie mostly occurs in three intervals (47.40 m to 47.00 m, 30.82 m to 29.70 m and 20.80 m to 20.43 m), comprising red algae and coral frame- to bindstone, red algae bindstone, floatstone and wackstone. Porites with coralline algae crusts are observed in red algal and coral frame- to bindstone (Fig. 5). Common to abundant coral debris is found, indicating a shallow environment. Red algae are mainly represented by specimens of corallinales such as Lithophyllum sp. Accessory components are debris of some foraminifera and echinoid (Fig. 5b). Foraminifera are dominated by Amphistegina lessonii which belong to the species that generally live in upper-to-intermediate photic environment, slightly deeper than Amphistegina lobifera occurring in coral reef facies. Therefore, the facies are thought to develop at a relatively shallow condition, but deeper than coral reef facies.
This facie is featured with evident iron-oxide staining and karst cave at 20.00 m to 18.39 m, 29.70 m to 27.35 m and 37.80 m to 33.89 m. A number of karst features are observed in these carbonate layers of reef cap facies. Small caves with a range from 1 cm to 2 cm in width and 5 cm to 10 cm in length were found perpendicular to the karst layers. The fillings in those karst spaces are dominated by carbonaceous mud, along with other mixed seepage materials, such as carbonate sand and calcite. The small- and medium-sized caves are mostly horizontal (Fig. 6a). This facie consists of framestone, grainstone, packstone and minor wackstone. The biological debris contains abundant to frequent coral and frequent to rare red algae and benthic foraminifera. Reef cap facie is formed in the subaerial settings during the sea-level lowstands.
Figure 6. Reef cap facies. a. Core photo of iron oxide-stained coral framestone; 1. Karst cave, 27.40–27.57 m. b. Plane-polarized photomicrograph of coral framestone showing intense leaching of bioclasts, at least leached coral fragments are distinguished from their cortical coatings, formed by original intergranular porosity occlusion; 1. Porites, and 2. coral moldic pores, 28.93 m. c. Plane-polarized photomicrograph of coral framestone with coral moldic pores, 37.05 m.
This facie is mainly found at depths between 54.60 m and 50.18 m, 46.90 m and 42.5 m, 32.80 m and 30.82 m, as well as between 24.90 m and 23.10 m. It comprises grainstone, packstone and wackstone. Biological fragments contain abundant to frequent red algae, abundant to frequent green algae, abundant to rare benthic foraminifera, frequent to rare coral, rare planktonic foraminifera, some echinoids, bryozoans and bivalves. Bioclastic debris is angular to subangular and mostly poorly-sorted (Figs 7a and b). Detrital material is frequently observed in this facie (Fig. 7c). Benthic foraminifera mainly consisting of Amphistegina lessonii and Cibicidoides subhaidingerii and rare planktonic foraminifera are observed in this facies. The observed specialties of benthic foraminifera Cibicidoides subhaidingerii generally live in deep-water settings, such as lagoon and inner shelf at warm temperature (Lei and Li, 2016; Weinmann et al., 2019). Red algae are mainly represented by specimens of Hapalidiales. The Hapalidiales thrives more in deep water while the corallinales mostly live in slightly shallower water (Coletti et al., 2018; Kroeger et al., 2006). The green algae are represented by specimens of Halimeda which is the most abundant organic constituent of the lagoon (Perry et al., 2016; Webster et al., 2009). According to the different preferred habitats of both benthic foraminifera and algae assemblages, the facies are thus considered to be formed in a relatively deep environment.
Figure 7. Inner bank facies, outer bank facies and aeolinite facies. a. Plane-polarized photomicrograph of grainstone in inner bank facies, exhibiting subangular and poorly-sored bioclasts; 1. Amphistegina lessonii, 2. Globorotalia truncatulinoides, and 3. Halimeda, 23.98 m. b. Plane-polarized photomicrograph of packstone in inner bank facies, exhibiting subangular and poorly-sored bioclasts; 1. Cibicidoides subhaidingerii (Parr) and 2. red algae, 32.70 m. c. Cross-polarized photomicrograph of packstone in inner facies, showing the occurrence of quartz, 46.35 m. d. Plane-polarized photomicrograph of grainstone in outer bank facies, showing planktonic foraminifera-rich bioclastic fragments; 1. planktonic foraminifera, 20.20 m.
The outer bank facie only occurs below the uppermost reef cap facies in the whole section. Outer bank facie is also observed below the uppermost reef cap facies in the carbonate succession of the core XK-1 (Wu et al., 2019). It consists of bioclastic grainstone and packstone. More planktonic foraminifera are found in this facie (Fig. 7d), suggesting that this facie was formed in deeper water than the inner bank facies. Furthermore, more Hapalidiales but no Corallinales are observed and indicate deep-water setting (Coletti et al., 2018; Kroeger et al., 2006). A generally better sorting in biological fragments than inner bank facies is observed due to the longer transportation distance that the sediment has experienced to reach the outer bank facies (Wu et al., 2019).
This facie was encountered in the interval of uppermost 18.39 m of the core (Fig. 3), which corresponds to the uppermost 21.00 m aeolianite facies encountered by the well XK-1 (Wu et al., 2019). The aeolianite compositions are mainly composed of loose bioclastic grainstone, with fragments mostly of abundant to rare benthic foraminifera, common to rare red algae, planktonic foraminifera, which were found in the core XK-1 (Li et al., 2018; Wu et al., 2019).
The shallow-water carbonates show evidence of having undergone various diagenetic processes such as neomorphism, micritization, cementation and dissolution (Melim et al., 2002). The term “neomorphism” refers to all transformations of minerals into either polymorphs or crystalline structures which are structurally identical to the original ones (Boggs, 2009). Neomorphism mainly occurs in reef association-facies. The fragments of coral, gastropod, mollusk shells, consisting of aragonite, and algae, echinoderms and some foraminifera, composing of HMC, have been altered or recrystallized to LMC. The size of the calcite crystals is determined by the degree of neomorphism. The central part of coral skeleton contains coarser calcite crystals than the fringe, suggesting a higher degree of neomorphism of central part of coral skeleton (Fig. 8a). Micritization of crustose coralline algal cellular skeletons is obvious. The micritization is mostly found in coral-algae facies and inner bank facies. Repeated algal microborings and subsequent filling of the microborings by micritic precipitates contribute to the 2–10 μm thick micrite envelopes (Figs 8b and c). Aragonite or calcite cements, are extensively observed in the Core SSZK1. There is a wide variety of cement morphologies, including meniscus, pendant, isopachous fibrous and isopachous dogtooth cements (Figs 8d–f). Dissolution features are well developed, especially in reef cap facies. Some biological grains have been considerably dissolved, leaving molds determined by calcite cements (Fig. 8c). Dissolution also occurs on these early generated cements (Fig. 8d).
Figure 8. Plane-polarized photomicrographs of products of meteoric diagenesis. a. Neomorphosed coral skeleton showing sparry calcite enclosed by microcrystalline calcite, 34.55 m. b. Micrite envelopes(yellow arrow), 33.20 m. c. Dissolved bioclasts, leaving molds with micrite envelope (yellow arrow), 42.5 m. d. Grains are connected by meniscus calcite cements causing round pore (yellow arrow), 32.9 m. e. Isopachous fibrous calcite fringe around grains; dissolution also occur on these early generated fibrous cements (yellow arrow), 25.14 m. f. Isopachous dogtooth calcite cements (yellow arrow), surrounding bioclasts, 47 m.
The δ18O values of bulk rock range from –10‰ to –1.9‰, with an average of –6.47‰. The δ13C values change from –7.2‰ to 5.2‰, and the average is 1.15‰. The mineralogical data for the six samples from the Core SSZK1 shows the vertical variation of the aragonite, HMC and LMC concentrations (Table 1). Based on variation in the distribution of aragonite, HMC and LMC, the 100 m shallow carbonate succession in the cores XC-1 and XK-1 has been divided into three units, namely Unit I (generally larger than 30 m), Unit II and Unit III (uppermost aeolianite facies)(Fig. 3). The rocks in Unit I display high amount of LMC, almost lacking aragonite; The rocks from Unit II contain high amount of LMC and more aragonite than Unit I. The rocks of Unit III in uppermost aeolianite facies are mainly composed of aragonite, HMC and minor LMC (Liu et al., 1997, 1998, 2019). The results of X-ray diffraction analysis on six samples show that the boundaries of the Unit I, Unit II and Unit III are 31.02 m and 18.39 m, respectively (Table 1). The X-ray diffraction pattern of the six samples is shown in Fig. S1. The showing curves of δ18O and δ13C display different characteristics of the three units (Fig. 3). The δ18O and δ13C values from Unit I are both low and fluctuate wildly while the values of both δ18O and δ13C from Unit II have very high-frequency and high-amplitude fluctuations. The δ18O and δ13C values at the depth interval between 7.2 m and 11.5 m are more negative than the underlying strata. The values of δ18O and δ13C from Unit III have relatively low-amplitude fluctuations and are comparatively high.
Depth/m Aragonite/% High-Mg calcite/% Low-Mg calcite/% 2.00 54.7 23.5 21.8 18.39 76.4 0 23.6 27.80 43.6 0 56.4 31.20 0 0 100.0 43.80 0 0 100.0 53.65 0 0 100.0
Table 1. Results of X-ray diffraction (XRD) analyses in samples
Positive correlations exist between the δ18O and δ13C for the bulk rock (Fig. 3). The correlation coefficients of δ18O and δ13C in the Unit I are lower than 0.5, highest in the Unit II and relatively high in Unit III. Further, in Unit II, the correlation coefficients are relatively small in the interval of inner bank facies and large in the interval of reef facies-association. From Unit II to Unit III, correlation coefficients decrease in the overall trend with the increase of depth. The correlation coefficients are slightly smaller in the interval of inner bank facies than the lowmost reef facies-association in Unit III.
The contents of major elements in different facies are listed in Table 2. The content of CaO in the section of Core SSZK1 is significantly higher than the average of continental crust (Taylor and McClenna, 1985). Correlation-coefficient matrices between element pairs for the entire dataset were determined in order to examine potential relationships and any controls on the chemical composition of the sediments. Table 3 shows the correlation coefficients among the elements’ concentrations. According to the correlation coefficients, these major elements can be divided into four clusters (Fig. 9).
Al2O3 CaO Fe2O3 MgO K2O SiO2 Na2O TiO2 MnO P2O5 S Reef facies-association Average 0.019 54.850 0.016 0.522 0.009 0.355 0.151 0.002 0.003 0.044 0.224 Max 0.144 55.561 0.102 0.980 0.034 2.115 0.512 0.006 0.005 0.183 0.006 Min 0.001 53.795 0.003 0.227 0.004 0.116 0.031 0.001 0.001 0.008 0.053 Inner bank facies Average 0.050 54.164 0.039 0.561 0.019 1.183 0.187 0.003 0.003 0.063 0.342 Max 0.300 55.380 0.166 0.889 0.094 9.736 0.667 0.008 0.005 0.183 0.014 Min 0.010 49.054 0.009 0.298 0.007 0.139 0.075 0.001 0.001 0.008 0.063 Aeolianite facies Average 0.046 51.798 0.039 1.973 0.015 0.258 0.494 0.002 0.003 0.109 0.340 Max 0.400 52.897 0.219 2.634 0.057 1.764 0.633 0.013 0.006 1.770 0.165 Min 0.016 49.901 0.009 0.725 0.010 0.081 0.363 0.001 0.002 0.035 0.266 Continental crust 15.040 5.390 6.170 3.670 2.580 61.710 3.180 0.670 0.090 0.170 –
Table 2. Descriptive statistics of major elements analysis results in reef facies-association, inner bank facies and aeolianite facies in comparison with the continental crust
Al2O3 CaO Fe2O3 MgO K2O SiO2 Na2O TiO2 MnO P2O5 S Sr Al2O3 1 CaO –0.34 1 Fe2O3 0.57 –0.43 1 MgO 0.08 –0.88 0.17 1 K2O 0.79 –0.48 0.68 0.09 1 SiO2 0.52 –0.21 0.53 0.15 0.82 1 Na2O 0.15 –0.88 0.26 0.75 0.26 –0.07 1 TiO2 0.90 –0.24 0.52 0.02 0.76 0.50 0.06 1 MnO 0.15 –0.21 0.30 0.23 0.23 0.14 0.09 0.13 1 P2O5 0.63 –0.30 0.22 0.19 0.42 0.16 0.16 0.58 0.02 1 S 0.07 –0.87 0.20 0.82 0.14 –0.19 0.98 –0.01 0.09 0.11 1 Sr 0.09 –0.75 0.21 0.61 0.15 –0.17 0.93 0.02 –0.02 0.10 0.92 1 Note: Correlation coefficients larger than 0.5 (P<0.01, n=138) are marked by bold numbers.
Table 3. The correlations of main elements in SSZK1 carbonate profile
Figure 9. Cluster diagram for variables using correlation coefficient. The coefficients larger than 0.5 among these major elements means a link among the major components.
(1) The first group is CaO, whose content is highest among the major components. CaO shows a negative correlation with the other major components and hence is regarded as one isolated type of major components. The concentrations of CaO from the Unit I are relatively lower than those of the Units II and III (Fig. 10). CaO shows an obvious positive anomaly in all of the facies (Fig. 11).
Figure 10. Vertical profile of facies and major elements variations, for symbols see legend in Fig. 3. The different colors of line titles mean different clusters of major elements.
Figure 11. Continental crust normalized major element composition (average value) for the reef facies-association, inner facies and aeolianite facies. Normalized values are from Taylor and McLennan (1985).
(2) The second group includes MgO, S and Na2O. The contents of MgO, S and Na2O are highest in the Unit III, medium in the Unit II and lowest in the Unit I (Fig. 10). Na2O and S content is relatively higher in inner bank facies especially in Unit II (Fig. 10). As the second most abundant composition, high-value MgO and Na2O anomalies occur in the observed carbonate facies (Fig. 11).
(3) The third group contains Al2O3, TiO2, SiO2, K2O, Fe2O3 and P2O5. Some sharp increases in major components of this type are identified in 42.5–46.9 m, 30.82–32.80 m, 18.39–18.41 m and in the top of the core. Continental crust normalized SiO2 value show a more positive anomaly in inner facies (Fig. 11). Low-value Al2O3, TiO2, K2O and Fe2O3 anomalies occur in the observed carbonate facies (Fig. 11). P2O5 values show an obvious increase in the top of the core.
(4) The fourth group is MnO. MnO shows poor or negative correlation with the other major elements. The distribution of this component fluctuates greatly in the vertical profile (Fig. 10).
For trace elements, Sr values show positive correlation with the MgO, S and Na2O, which belong to the second group of the major elements (Table 3).
The uranium content of the fossil corals that were dated is between 0.816 ppm and 2.946 ppm (Column 2, Table 4), which is generally lower than that of modern scleractinian corals collected from the Pacific Ocean and Atlantic Ocean (Robinson et al., 2004; Swart and Hubbard, 1982; Wienberg et al., 2010). Four corals have initial 234U/238U ratios that exceeded the expected seawater concentration of 1.14±0.03 (Coyne et al., 2007; Ku et al., 1977), and were therefore considered unreliable. Sample with 230Th/232Th ratios less than 20 was deemed unreliable (Coyne et al., 2007; Ku et al., 1977). Only the four shallow corals complied with these criteria. The U-series dating result is (8114±190) a BP at the depth of 13.43 m, indicating the loose sediments was mainly deposited during the Holocene. The core interval of depth between 18.39 m and 24.3 m was deposited during the late substages of the Marine Isotope Stage 5 (MIS 5a and MIS b). The U-series dating for the Well XK-1 also shows the reef limestones (25.2–27.8 m) underlying the aeolianite sediments were deposited during the early substage of the Marine Isotope Stage 5 (MIS 5e) (Li et al., 2018). Dates obtained from the four deep-buried corals fail to meet these criteria are considered unreliable, though the dated ages are within the interglacial periods (MIS 7, 9, 11 and 13) (Fig. 3 and Table 4).
Depth/m [U]/ppm 234U/238U 234U/238U (initial) 230Th/238U 230Th/232Th Age (a BP) Error (2σ) 7.21 0.816 1.147 1.149 0.047 3 59.4 4 239 246 13.47 2.450 1.141 1.144 0.082 1 7 090.3 8 114 190 18.39 2.490 1.126 1.153 0.5390 1 178.8 69 795 274 24.30 2.946 1.121 1.155 0.6300 2 886.6 88 094 349 30.11 1.028 1.110 1.214 1.0100 709.8 236 524 2 131 42.52 0.891 1.104 1.260 1.0830 710.0 329 184 5 601 44.89 1.279 1.097 1.294 1.1020 1 456.1 392 344 9 658 55.92 1.000 1.087 1.337 1.1070 1 227.7 479 636 19 388 Note: The unreliable dating results are marked by bold numbers.
Table 4. 230Th dating results for corals from core SSZK1, Yongxing Island
We thank the Institute of Hainan Marine Geological Survey for their assistance gathering this data set throughout two cruises in 2015. Yanyan Zhao is thanked for helping during sample preparation and XRD analysis. Christian Betzler is acknowledged for his precious advices.
Figure A1. X-ray diffraction pattern of the samples. Ar means aragonite; LH means low-Mg and high-Mg calcite; and L means low-Mg calcite.
Element Analytical method Detection limit CaO XRF 0.032 2 (10–2) MgO XRF 0.003 (10–2) Al2O3 XRF 0.002 7 (10–2) Fe2O3 XRF 0.002 7 (10–2) Na2O XRF 0.040 2 (10–2) SiO2 XRF 0.004 (10–2) K2O XRF 0.002 2 (10–2) P2O5 XRF 0.000 9 (10–2) MnO XRF 0.002 3 (10–4) TiO2 XRF 0.001 (10–2) S XRF 0.000 6 (10–2) Sr ICP-MS 1.0000 (10–6)
Table A1. Detection limit of the analytical method
Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession, Xisha Islands, South China Sea
- Received Date: 2020-03-30
- Accepted Date: 2020-05-07
- shallow-water carbonates /
- meteoric diagenesis /
- elemental concentration /
- facies cycles /
- Xisha Islands /
- late Quaternary
Abstract: The late Quaternary shallow-water carbonates have been altered by a variety of diagenetic processes, and further influenced by high-amplitude global and regional sea level changes. This study utilizes a new borehole drilled on the Yongxing Island, Xisha Islands to investigate meteoric diagenetic alteration in the late Quaternary shallow-water carbonates. Petrographic, mineralogical, stable isotopic and elemental data provide new insights into the meteoric diagenetic processes of the reef limestone. The results show the variation in the distribution of aragonite, high-Mg calcite (HMC) and low-Mg calcite (LMC) divides the shallow-water carbonates in Core SSZK1 into three intervals, which are Unit I (31.20–55.92 m, LMC), Unit II (18.39–31.20 m, aragonite and LMC) and Unit III (upper 18.39 m of core, aragonite, LMC and HMC). Various degrees of meteoric diagenesis exist in the identified three units. The lowermost Unit I has suffered almost complete freshwater diagenesis, whereas the overlying Units II and III have undergone incompletely meteoric diagenesis. The amount of time that limestone has been in the freshwater diagenetic environment has the largest impact on the degree of meteoric diagenesis. Approximately four intact facies/water depth cycles are recognized. The cumulative depletion of elements such as strontium (Sr), sodium (Na) and sulphur (S) caused by duplicated meteoric diagenesis in the older reef sequences are distinguished from the younger reef sequences. This study provides a new record of meteoric diagenesis, which is well reflected by whole-rock mineralogy and geochemistry.
|Citation:||Wanli Chen, Xiaoxia Huang, Shiguo Wu, Gang Liu, Haotian Wei, Jiaqing Wu. Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession, Xisha Islands, South China Sea[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-021-1713-6|