Xuelin Li

Isolated drowned carbonate platforms in the Xisha Area represent critical research archives for deciphering the evolutionary mechanisms of carbonate systems and paleoenvironmental changes within the marginal sea of the northern South China Sea. Nevertheless, existing academic research lacks comprehensive investigations into the sedimentary processes and episodic demise signatures of typical drowned platforms in this region. This study targets the Ganquan Isolated Drowned Carbonate Platform—the largest and best-preserved drowned carbonate platform in the Xisha area. Based on high-resolution 2D multi-channel seismic data and multibeam bathymetric data, this research systematically characterizes the stratigraphic architecture and sedimentary differentiation of the Ganquan Carbonate Platform, and reconstructs its complete Cenozoic evolutionary history via interval velocity inversion, seismic sequence stratigraphic interpretation, sedimentary facies identification, and geomorphological feature analysis. The results reveal that the carbonate succession of the Ganquan Carbonate Platform exhibits prominent vertically zoned interval velocity variations, with two sharp velocity discontinuities identified at the seabed surface and the contact boundary between carbonate rocks and metamorphic basement. Four stratigraphic units are distinguished from top to bottom: seawater column, loose Upper Miocene carbonate deposits, compact Lower–Middle Miocene carbonate strata, and metamorphic basement. Five key Cenozoic seismic boundaries are recognized within the study area, which divide the evolutionary history of the Ganquan Carbonate Platform into four successive episodes: incipient initiation during the Early Miocene, vigorous expansion in the Early–Middle Miocene, transitional decline in the late Middle Miocene, and drowning reworking since the Late Miocene. This study establishes a comprehensive evolutionary model for isolated drowned carbonate platforms in the Xisha region, and advances the theoretical framework governing carbonate platform evolution in marginal seas.

Submarine pockmarks are typical seafloor micro-geomorphic landforms formed by deep fluid seepage and sediment erosional processes. Based on high-resolution multibeam bathymetric data, multi-channel seismic sections and sediment core data, the present study systematically investigates 64 pockmarks in the northwestern Xisha Uplift, focusing on their distribution, morphology and genetic mechanisms. These pockmarks exhibit a NE–SW zonal distribution, concentrated in the 1200–1600 m central slope transition zone, and are classified into circular–elliptical, crescentic and elongated types with distinct morphometric variability. Vertically, the T40 unconformity defines the stratified geological architecture: underlying carbonate uplifts and karst-fracture systems act as fluid reservoirs and migration conduits, while overlying Late Miocene–Quaternary fine-grained hemipelagic sediments form a low-permeability caprock. Fluid overpressure accumulation and hydraulic fracturing of the caprock trigger initial pockmark formation, while spatial heterogeneity of surficial sediments and bottom-current reworking control morphological differentiation. The present study clarifies the coupled controls of deep tectono-fluid activities and shallow sedimentary and hydrodynamic processes on pockmark evolution, establishing a refined dynamic model to address the research gap regarding pockmark group genesis in the study area.

The northwestern slope of the Dongdao Platform in the Xisha Sea exhibits a complex geomorphological structure. Utilizing high-resolution multibeam bathymetric data and 2D seismic profiles, this study systematically reconstructs the slope morphology and its evolutionary processes. The study area displays a distinct threefold zonation: the upper slope (160–700 m water depth) has a steep gradient of 15°–25°, characterized by deeply incised V-shaped channels and slump deposits, primarily shaped by gravity-driven erosion; the middle slope (700–1200 m water depth) features a gentler gradient of 10°–15°, where channels stabilize, adopting U-shaped cross-sections with the development of lateral accretion deposits; the lower slope (1200–1500 m water depth) exhibits a milder gradient of 5°–10°, dominated by a mixture of fine-grained carbonate sediments and hemipelagic mud–marine sediments originating partly from the open ocean and partly from the nearby continental margin. The slope extends from 160 m to 1500 m water depth, hosting the C1–C4 channel system. Seismic facies analysis reveals mass-transport deposits, channel-fill facies, and facies modified by bottom currents—currents near the seafloor that redistribute sediments laterally—highlighting the interplay between fluid activity and gravity-driven processes. The slope evolution follows a four-stage model: (1) the pockmark formation stage, where overpressured gas migrates vertically through chimneys, inducing localized sediment instability and forming discrete pockmarks; (2) the initial channel development stage, during which gravity flows exploit the pockmark chains as preferential erosional pathways, establishing nascent incised channels; (3) the channel expansion and maturation stage, marked by intensified erosion from high-density debris flows, resulting in a stepped longitudinal profile, while bottom-current reworking enhances lateral sediment differentiation; (4) the stable transport stage, wherein the channels fully integrate with the Sansha Canyon, forming a well-connected “platform-to-canyon” sediment transport system.

The Ganquan Platform, located in the South China Sea, exhibits distinctive topographic and geomorphological features shaped by complex geological processes. Utilizing high-resolution multibeam bathymetry and multi-channel seismic data, this study provides a comprehensive investigation into the sedimentary evolution and stratigraphic framework of the platform. Morphologically, the platform is identified as an elongated seamount, with water depths ranging from 530 m to 800 m and a maximum elevation of 538.115 m. Seismic facies analysis reveals seven distinct facies, reflecting a dynamic and intricate history of carbonate deposition. The stratigraphic framework delineates three primary evolutionary stages: the growth phase (Early Miocene), characterized by the initial accumulation of carbonate sediments; the flourishing phase (Middle Miocene), marked by extensive deposition and platform expansion; and the submergence phase (Late Miocene), defined by progressive submersion and erosion of carbonate features. This evolutionary trajectory was driven by key factors such as terrigenous clastic input, tectonic activity, sea-level fluctuations, and paleoenvironmental dynamics. The findings contribute to a deeper understanding of the Ganquan Platform’s geological history and its role in the broader context of submarine geology in the Xisha region.

Deep sea water (DSW) is a globally utilized source of renewable energy and other resources. To understand the characteristics of DSW resources in the South China Sea, in July 2022, the Guangzhou Marine Geological Survey (GMGS) investigated temperature, salinity, pH, dissolved oxygen (DO), inorganic salts (DIN, PO₄³⁻-P, and SiO3-Si), heavy metals (Hg, Pb, As, and Cd), trace elements (Cu, Zn, Fe, Mn, Ni, Se, and Mo), and other related indicators. The results of this investigation elucidate the horizontal and vertical changes in the physical and chemical properties of deep sea water in the Xisha Sea. The surface seawater quality in Xisha was found to be excellent and to meet first-class seawater survey standards. However, the concentrations of various nutrient salts in the surface layer were relatively low. As the seawater depth increased, different trace elements and heavy metals exhibited variations, and the concentrations of various nutrients also gradually increased.

Submarine pockmarks are typical indicators of submarine gas escape activity. The deep strata of the Xisha Uplift are rich in biogenic and thermogenic gas, accompanied by strong bottom current activity. Investigating the effects of controlling submarine gas escape and bottom current activity on the formation and development of pockmarks in the Xisha Uplift is significant for understanding the evolution of submarine topography and geomorphology. This study utilized high-resolution multibeam data to identify 261 submarine pockmarks in the northwest of the Xisha Uplift. These pockmarks were categorized based on their morphology into circular, elliptical, elongated, crescent-shaped, and irregular types. The diameters of pockmarks in the study area range from 0.21 to 4.96 km, with maximum depths reaching 30.88 m. Using high-resolution multi-channel seismic data, we conducted a detailed analysis of the subsurface strata characteristics of the pockmarks, identifying chaotic weak reflections, bright spots, and high-angle reflectors. We believe that deep gas in the northwest of the Xisha Uplift escapes to the seafloor through migration pathways, such as faults, fractures, and gas chimneys, resulting in the formation of submarine pockmarks. Bottom current activity has a significant impact on already-formed pockmarks. Crescent-shaped and elongated pockmarks in the Xisha Uplift are largely the result of bottom current modifications of pre-existing pockmarks.