Sedimentological and geochemical imprint of environmental changes in late Pleistocene palaeodelta-hosting deposits, southwest of the Hainan Island (South China Sea)
Introduction
The vast shelf of the South China Sea (SCS) was, during the Quaternary, an area of multiple marine transgressions and regressions associated with glaci-eustatic changes of the global sea level (Molengraaff and Weber, 1921, Voris, 2000, Yao et al., 2009, Hanebuth et al., 2009, Hanebuth et al., 2011, Wang et al., 2014). A particularly interesting SCS area is the Beibu Gulf (Gulf of Tonkin), especially its part confined by the Hainan Island (HI) and the Vietnamese coast, known as the so-called Yinggehai-Song Hong Depression. The area is one of the world’s pull-apart basin with a strong subsidence in progress throughout almost the entire Palaeogene, Neogene, and Quaternary until the present time (Clift and Sun, 2006, Yan et al., 2011, Lei et al., 2015). Within the last 1 Ma, the magnitude of subsidence in the central part of the basin is estimated at about 127 m (Lei et al., 2015).
The Yinggehai - Song Hong Depression has been in the past, and is at present, abundantly supplied with fluvial sediments, particularly from the north-west and west by rivers descending from the Yunnan Plateau and the Annam Mountains (Truong Son); in the east, the sediment supply is provided by the HI rivers. Doubtless, the highest amount of sediment is at present supplied by the Red River (Song Hong). As shown by Milliman et al., 1995, Liu et al., 2014, the annual sediment supply to the Beibu Gulf with the Red River runoff is within the range of 110–130 million tonnes. Based on 1960–2008 monitoring data, Dang et al. (2010) determined the annual average suspended particulate supply at 90 million tonnes; they showed the supply to change from 24 to 200 million tonnes, depending on the year. The Red River was also in the Later Quaternary the major source of terrigenous sediments for the Gulf (Mathers and Zalasiewicz, 1999, Tanabe et al., 2003). However, as shown by seismic data from the central-eastern part of the Yinggehai-Song Hong basin, an important source of fluvial sediments deposited during the last glacial cycle was also HI, the foreground of which showing the presence of a palaeodelta (Chen et al., 2015, Chen et al., 2018, Feng et al., 2018, Miluch et al., 2018).
This work is aimed at exploring the lithological and geochemical characteristics of shelf formations deposited during the Late Quaternary on the eastern slope of the Yinggehai-Song Hong, associated with the development of the Hainan palaeodelta and its under- and overlying sediments. This aim poses the following research questions addressed here:
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How big is the variability of lithological and geochemical characteristics of SCS shelf sediments in an area subjected to Pleistocene variations in the extent and changes of the sea level?
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What is the relationship between lithological and geochemical characteristics of sediments and the seismic profiling record?
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What are the characteristics of sediments associated with the seismic reflector surfaces?
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What type of sediments were accumulated within the series interpreted – based on seismic profiling – as deltaic forms?
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How did the sea level change when the Hainan delta sediments were accumulated?
Section snippets
Materials and methods
Sedimentological and geochemical analyses were carried out on sediments obtained from full-core marine drillings (ϕ 72 mm) in the western part of the SCS (Fig. 1):
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core LDW of a total length of 70.5 m, acquired in 2014 from a depth of 74 m, (18°21.5851′ N, 108°17.4845′ E);
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core ZBW of a total length of 100.75 m, acquired in 2015 from a depth of 105 m, (17°10.44′ N, 109°01.68′ E).
The drilling sites were selected based on previous seismic profiling made by the Guangzhou Marine Geological Survey
Grain size distribution
The two cores differed substantially in their grain size composition (Table 1). The LDW sediments are characteristic in showing the per cent contribution of sands with admixture of gravel to be twice that found in core ZBW, the silt–clay fraction being negligible. The mean phi-based grain size (Mz) was much higher in the ZBW sediment, indicating it being generally finer-grained. The LDW sediments showed a poorer sorting, their averaged grain size distributions being closer to normal than those
Geochemical facies
The sediment facies variation was explored in all the samples from both profiles in which the chemical composition was determined using XRF and ICP-MS. The cluster analysis involved selected geochemical variables (concentrations of SiO2, Na2O, K2O, Fe2O3, CaO, MnO, TiO2, Cu, Pb, V, Li, Rb) which were mostly poorly correlated. As a result, the analysis identified 12 geochemical facies denoted A to L (Fig. 17).
The shelf sediments analysed were found to support four groups of geochemical facies.
Conclusions
The study shows the SCS shelf sediments to be highly lithologically variable. They are dominated by silts and sandy silts, with local accumulation of sands, frequently containing shell debris. Erosional surfaces, visible in the seismic record as distinct reflective horizons, are present as well. Where they occur, distinct lithological boundaries are visible, usually accentuated by increased contributions of shell debris and even grains of gravel.
The reflection horizons, associated with
CRediT authorship contribution statement
Ryszard K. Borówka: Conceptualization, Methodology, Investigation, Writing - original draft. Andrzej Osadczuk: Investigation, Visualization, Writing - original draft, Writing - review & editing. Zhao Li: Formal analysis, Resources. Jakub Miluch: Investigation, Resources. Krystyna Osadczuk: Investigation, Supervision. Bartosz Bieniek: Formal analysis. Łukasz Maciąg: Writing - review & editing. Julita Tomkowiak: Resources, Formal analysis. Hongjun Chen: Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The research was carried out as part of the project “Evolution of the Hainan Delta as a response to the palaeoenvironment since the late Pleistocene at the SCS north-western shelf” (ERES)”, co-supported by the Polish National Science Centre (NCN project No. UMO-2016/21/2016/21/B/ST10/02939), China Geological Survey (No. GZH201500207), National Program on Key Basic Research Project of China (No. 2013CB956104) and National Natural Science Foundation of China (No. 41376063).
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2021, Journal of Asian Earth SciencesCitation Excerpt :The core showed a distinct coarse-grained layer representing R2 at a depth of 75.39–76.0 m, and consisting of fine gravel, coarse sand and shell debris. The grain size profile (Borówka et al., 2020) indicates a substantial difference in the mean grain size between the sedimentary series below and above R2. The deltaic series in core ZBW is 49.26 m thick and lithologically represents homogenous silt and sandy silt fractions, composed mostly of quartz, clay minerals and shell debris.