Depositional Environment and Its Relationship with the Life and Preservation of Graptolites of the Silurian Wuxiahe Formation, Southern Shaanxi

The present paper focus on the depositional enviroment and its relationship with the life and the preservation of graptolites in the Silurian Wuxihe Formation, southern Shaanxi. First, The authors collected the fossils layer by layer in about one square metre and got the statistics of the fossils on the surface of each layer Then drawn the dead curves of some spcial species and analysed the characteristics and the content of some elements in country rocks, such as Fe⁺⁺. Fe⁺⁺⁺, S, C and pH, and so on, main conclusions obtained as follows: 1 .The depositional enviroment was a cutoff basin, the bottom water current was confined and the condition was reducing, so there no benthonic arganism could survive. 2.The rock types of Wuxiahe Formation is turbite, consisting of siltstone, silty shale, shale and black mudstone, correspond to the B、 C、 D and E divisions of the Bouma sequence, respectively, while A-divi sion(some time B-division) is lacuna, 3.The graptolites is richest in the siltyshale and shale, less in the mudstone and none in the siltstone. 4.The change of the graptolites abundance is mainly resulted from its sedimentary environment and the invasion of the turbidity current. When the current reached to the basin from the Yangtze Platform, croase sediments (sands) will deposite first, the dropping graptolites carcasses might be tear to shreds by the current so, none fossils was found in sandstone.while with the diffusion of the fine-grained particles to the upper water body, associate H₂S which agitated up by turbidity current form the bottom of basin, with most of the graptolites would be killed and their carcasses can be preserved at the bottom, of basinso a great deal of graptotite fossiles can be found in silty shale and shale(corespond to D₁ and D₂ division of Bouma sequenco).