Paleogene Clastic Deposits Of Sanshui Basin And Their Response To The Continental Rift Stage Of The South China Sea

Sanshui basin is the most important Mesozoic and Cenozoic basin in the northern margin of South China sea(SCS),and the early cretaceous-Eocene volcanic-sedimentary records are very complete and very developed.Many of the systematic research of Sanshui basin have been done,especially the magmatic activity,but the study of sediment characteristics of Sanshui basin is relatively less.In this paper,clastic rock samples in Sanshui basin were collected to be analyzed by lithology,sedimentary sequence and geochemistry.Combined with data of volcanic rocks,and compared with characteristics of typical rift basins to obtain the nature of the basin,so as to know there is a continental rift vally before the expansion of SCS.The main conclusions from this paper are as follows:The gravel in the Paleogene clastic rocks of Sanshui basin is mostly metamorphic rocks,sedimentary rocks and volcanic rocks,and clastic rocks formed by compressional uplifting in the late Mesozoic formation contain a large number of granite gravel.This contrast indicates that the region has entered into a regional stretch mode in the early Cenozoic,this large-scale regional extension may be a precursor to continental rift activity before the expansion of SCS.Sedimentary sequence characteristics show the rift valley environment marked by "binary structure".And the rift valley environment has a source of sediment similar to the passive margin,but because of the volcanic activity,the rift valley has its distinctive features.During the Palaeocene,volcanic eruptions dominated by the acid magma,suggesting that the magma was shallow.Since the Eocene,acid magma gradually reduced and the magmatic rocks are mainly basic rocks and alkaline rocks.Later,the eruption increased,indicating the depth of the magma deepened.On the basis of different geochemical indexes and the discriminating diagram of the structure,except for the individual samples,most of the samples of the Paleogene clastic rocks in Sanshui basin show that the structural characteristics of the passive continental margin.According to the phenomena observed under themicroscope,the particles and structural maturity of the samples are have high maturity,which is in contrast to the near-source samples of less maturity and rapid accumulation during the Mesozoic,but both of them reflected the great changes of the tectonic environment in the early Cenozoic.The characteristics of Sanshui Basin are similar to the typical rift basin around the world.Sanshui basin experienced a process of rift,contraction,rift,decay,and if the rift-constraction as a cycle,the basin experienced two tectonic cycles,as same as the Viking grout,and they have a similar tectonic evolution pattern.The trachyte in Sanshui Basin and the Red Sea show a good commonality in geochemistry,indicating that the tectonic environment of these two places is basically the same as the continental rift environment,But after the Eocene,because of the influence of regional tectonic activity,the development of Sanshui Basin was controlled and became a "aborted" continental rift basin.