西沙西科1井三亚组生物礁沉积的磁性地层及其环境意义

生物礁沉积是南海乃至西北太平洋重要的沉积类型之一。本文利用新近钻进取芯的南海西沙西科1井生物礁沉积,开展了详细的磁性地层学研究。结果显示,早中新世三亚组记录了跨度为C5Dn~C6An正极性时的生物礁沉积。在综合全孔年代学结果的基础上,划分出新近纪西沙地区生物礁生长、发育的3个阶段:约16.5 Ma和约13.5 Ma两个稳定堆积时期,以及16.5~13.5 Ma快速堆积的"中中新世跃迁事件"。此外,本文推测西沙地区生物礁初始发育的时间显著早于过去认为的约20 Ma,同时三阶段的演化模式可能与南海扩张-结束的构造环境密切关联。

Magnetostratigraphy of Biogenic Reef in the Sanya Formation of Borehole XK-1 From Xisha Islands and Its Environmental Significance

Biogenic reef is one of the important sediment types not only in the South Chi-na Sea (SCS),but also in the Western Pacific Ocean,and plays an important role in modulating global climate and carbon cycle through biological processes.In this paper, a detailed study of paleomagnetic chronostratigraphy is carried out for a Neogene biogen-ic reef sequence of the Sanya Formation in Borehole XK-1 drilled recently in the Xisha Islands in the northern SCS.The main results are as follows:(1 )The Early-Miocene Sanya Formation recorded C5Dn-C6An normal chrons.(2)Based on the age constraints of the whole borehole,the Neogene biogenic reefs in the Xisha region have three stages:two regular stages prior to about 1 6.5 Ma and after about 13.5 Ma,respectively and one rapid developing stage from 1 6.5 Ma to 13.5 Ma.The latter one is named'Mid-Miocene Shift Event'herein.By comparing the new findings with the results obtained previously, it is speculated that the origin of the biogenic reefs in the Xisha region would be notably earlier than ~20 Ma which was considered in the past,and that the three-stage evolu-tionary model for the biogenic reefs was possibly related to the tectonic environment during the termination of seafloor spreading of the SCS.