Seismic architecture of Yongle isolated carbonate platform in Xisha Archipelago, South China Sea

Abstract

This study presented recently reprocessed multi-channel seismic data and multi-beam bathymetric map to reveal the geomorphology and stratigraphic architecture of the Yongle isolated carbonate platform in the Xisha Archipelago, northwestern South China Sea. Our results show that the upper slope angles of Yongle carbonate platform exceed 10° and even reach to ∼32.5° whereas the lower slope angles vary from .5° to 5.3°. The variations of slope angles show that margins of Yongle Atoll belong to escarpment (bypass) margins to erosional (escarpment) margins. The interior of carbonate platform is characterized by sub-parallel to parallel, semi-continuous to continuous reflectors with medium-to high-amplitude and low-to medium-frequency. The platform shows a sub-flat to flat-topped shape in its geometry with aggradation and backstepping occurring on the platform margins. According to our seismic-well correlation, the isolated carbonate platform started forming in Early Miocene, grew during Early to Middle Miocene, and subsequently underwent drowning in Late Miocene, Pliocene and Quaternary. Large-scale submarine mass transport deposits are observed in the southeastern and southern slopes of Yongle Atoll to reshape the slopes since Late Miocene. The magmatism and hydrothermal fluid flow pipes around the Yongle Atoll have been active during 10.5–2.6 Ma. Their activity might intensify dolomitization of the Xisha isolated carbonate platforms during Late Miocene to Pliocene. Our results further suggest that the Yongle carbonate platform is situated upon a pre-existing fault-bounded block with a flat pre-Cenozoic basement rather than a large scale volcano as previously known and the depth of the basement likely reached to 1400 m, which is deeper than the well CK-2 suggested.

Stratigraphic evolution and drowning steps of a submerged isolated carbonate platform in the northern South China Sea

“…Previous studies have documented that magmatic and hydrothermal activities were intense at 5.5 and 2.6 Ma in the flanks of modern carbonate platforms in the northern SCS, resulting in upturned reflectors extending upward to the contemporary sea bottom (Xia et al, 2018;Gao et al, 2019a;Gao et al, 2019b;Sun et al, 2020;Zhao et al, 2021;Gao et al, 2022;Liu G .et al, 2023). Submarine volcanoes, magmatic intrusions, and related upward strata extending to the modern seafloor have also been identified by submersible observations and seismic profiles on the flank of the GP (Figures 4,11), suggesting that contemporaneous Schematic representation of the drowning mechanism at the GP, as proposed here.…”

“…Aside from these coral islands, bathymetric grids show that several drowned ICPs crop out in waters hundreds of meters deep. Previous research has focused only on the architecture and evolution of modern carbonate platforms using seismic data and drilling wells on islands (Wu S. et al, 2020;Qin et al, 2022;Wu et al, 2022;Liu G. et al, 2023;. Several recent studies presented preliminary data on the lithostratigraphy and geochemistry of such wells (Wang et al, 2013;Shao et al, 2017;Chen et al, 2021;Wang R. et al, 2018;Wu et al, 2019;Wu et al, 2021a;Xie et al, 2008;Yang et al, 2022).…”