Early Oligocene gravity-flow deposits at the base of the Meiji Reef system in the South China Sea, and their palaeoclimate significance

In this paper, we describe gravity flow facies that form the base of the Meiji Reef system in the South China Sea, and discuss their palaeoclimate significance. These Early Oligocene (26.1–23.5 Myr) deposits occur in Well NK1 and comprise a 110.6 m thick succession developed on top of volcanic basement. The deposits can be divided into two intervals, both of which are dominated by debris flow facies. The lower 17.6 m thick interval has a gravel content of 50–65% with parent rocks including dacite, basalt, tuff, and coral reef fragments. From base to top, these units coarsen upward and show increasing gravel contents. Gravel clasts are well-rounded but poorly sorted (diameter 10–350 mm), which, together with their complex composition, indicate transportation by the gravity flows from the near-shore volcanic rock basement to the slope. The upper 93 m thick interval comprises abundant bioclastic materials, including benthic foraminifera, crustal red algae, and red algae detritus, with a few volcanic gravel clasts. The content of volcanic rock gravels decreases towards the top, which may be attributed to the reduction or disappearance of volcanic rocks in the provenance area due to the establishment of coral reef facies that cap the succession and dominate the overlying interval from a core depth of 885.6 m to the seafloor. Based on U-Pb dating, the cap-shaped red algae laminae at the facies transition were formed at 24.6 ± 1.1 Ma, dating the earliest initiation of reef building to end-Oligocene times. The emergence of gravity flow sediments during the early Oligocene may be attributed to the onset of the Antarctic glaciation and concurrent regional tectonic activities that led to a shallowing of the sea. The origin of debris with high roundness characteristics is believed to be associated with transportation along the slope triggered by tectonic and climatic events, resulting in the formation of debris flow deposits. Studying the gravity flow facies at the Meiji Reef system is crucial for determining the onset of reef-building and helps establish the relationship between gravity flow deposition, paleoclimate, and regional tectonic evolution in the South China Sea.