中国科学院南京地质古生物研究所机构知识库

The South China Sea (SCS) in West Pacific Ocean is the largest marginal sea in Asia. Due to the unique location, its Cenozoic evolution has been attracting great attentions during the past decades. As one of the most debated issues, geochronological frameworks of the SCS basin have been widely investigated mainly based on geophysical data or regional unconformities. As one of two major types of depositions (terrigenous and marine biogenic) in this critical region, numerous coral reefs have been developed on the Xisha-Zhongsha terrain since the early Miocene. Previous studies of the Xisha carbonate platforms have mainly focused on the analyses of sedimentology, tectonics, and modern ecology, but little geochronological work, which ought to be an important component of such researches, has been performed. This lack of geochronological information significantly hinders our understanding of the initiation and development of these biogenic reefs. Herein, multi-parameter rock magnetic measurements, including isothermal remanent magnetization (IRM) acquisition, hysteresis loops-and magnetic scanning, were performed on typical samples collected from Borehole XK-1 in the Xisha Islands. The results suggest the predominance of ferrimagnetic phases in the XK-1 biogenic reefs, and the average sizes of magnetic mineral grains are relatively coarse, falling within the pseudo-singledomain to multidomain-like grain size region. Further mathematical partitioning demonstrates the predominance of low-coercivity component (mainly magnetite). Considering that the study area is far away from the East Asian continent and the growing process of the biogenic reefs, it is inferred that the main magnetic mineral in biogenetic reefs is magnetite, which is from terrestrial detrital materials and absorbed or filtered into the reef with its growth. Further magnetostratigraphic study shows that the identified normal and reverse magnetozones can be correlated with geomagnetic polarity time scale (GPTS), providing several reliable age constraints for those reefs. It records C2An.3n and C3n.4n chrons in the Yinggehai Formation, and C3An.2n to C5n.2n chrons in the Huangliu Formation. Our new magnetostratigraphic findings have provided valuable geochronologic constraints for regional stratigraphic correlation and paleoenvironmental processes.