西沙礁相第四纪地层初步研究

本文根据西沙三个全取芯钻孔的纵向序列,结合部分年代学数据,开展了岩石地层学、生物地层学、碳、氧稳定同位素和地球化学地层学研究,对西沙礁相第四纪地层进行了划分。整个第四系称西沙群,下更新统称永兴组,中更新统称琛航组,上更新统仍沿用石岛组,全新统称为西沙洲组。这一新的系统划分,对海陆第四纪地层间的互相对比提供了一定的依据。

A STUDY ON QUATERNARY STRATIGRAPHY OF REEF DEPOSITS ON XISHA ISLANDS

The Quaternary stratigraphy of the reef deposits on the Xisha Islands is established based on the study of geochronology, lithology, biological stratigraphy remains, carbon and oxygen stable isotope and geochemical characters from the three cores from drilling holes. Lithostratigraphically, the Quaternary series named Xisha Group consists of Yongxing Formation (Lower Pleistocene), Shenhang Formation (Middle Pleistocene), Shidao Formation (Upper Pleistocene) and Xishazhou Formation (Holocene). The Quaternary reef carbonates were formed under the influence of sea level fluctuation, subsidence of basement and climatic changes. There are event deposits as well as normal deposits in the sequence. The bottom boundary of Quaternary is put on the position of the first occurrence of Globorotalia truncatulinoides just above the bed with strong dolomitization. An erosional surface is marked by a recemented packstone indicating a hiatus. The boundaries between Lower and Middle Pleistocene, Middle and Upper Pleistocene are defined with correlation to oxygen stable isotopic and other geochemical parameters. The boundary between Holocene and Pleistocene has not been found yet in this area. The Quaternary organic reef carbonates were mostly submerged under sea water at 218 to 256m in depth.