The Sedimentary Characteristic And Model Of Sinian Sediments In The Eastern Guizhou And Western Hunan Province

The Neoproterozoic Sinian strata which is studied in this paper is between overlying Nantuo Tillite and underlying Cambrian dark shale of Niutitang Formation.They are mainly found in eastern Guizhou Province,in the northwest and north of Hunan Province,and dispersed in the center and north of Guizhou Province.Base on the Sinian research of Yangtze Block in Huanan,Twelve Sinian sections in the eastern Guizhou and western Hunan Province are systematically collected and studied,which included one self-measured section and three sections of repetition measurement,the other eight surveyed sections.By the way of analysis of sequence strata,sedimentary facies,isotope and geochemistry,The principal knowledges such as sedimentology,petrology,isotope and geochemistry are applied to study the characteristic of Sinian sediments,and discuss sedimentary models of Sinian sediments in the region.1,By the analyse of sequence strata in the region,it is believed that the Sinian strata together with its underlying Nantuo Tillite consists of a complete sedimentary circle.The Nantuo Formation represents a set of sediments of lowstand system tract;the Doushantuo Formation of the early Sinian reveals a set of its transgressive system tract and developes the Condensed Section which indicates extremely slow precipitation rates in its top;the Dengying Formation of the late Sinian characters a set of highstand system tract. Moreover,the regional phosphorite intervel of the Doushantuo Formation in the region represents a set of transgressive circle.2,Like coeval Neoproterozoic cap carbonates worldwide,the Nantuo cap dolostone overlies glaciogenic diamictite without evidence of reworking or hiatus.the lithological contact between the cap carbonates and overlying sedimentary rocks varies among sedimentary facies.In the siliciclastic successions of the deep water basinal environment,the Nantuo cap carbonate is composed of thin-bedded dolostone(1.7-4.5m)conformably overlain by organic-rich black shales of the Doushantuo Formation.In the shelf-slope successions,the cap dolostones are overlain by organic-rich black shales with variable thickness that pass upward into a sequence of carbonates and phosphorite.In the carbonate platform and shelf facies.the Nantuo cap consists of fine-laminated and usually pinkish dolostone.It is overlain by the thick-bedded carbonates as well as phosphates where the earliest animal fossils were discovered.3,Like these"Cap Carbonate"found in global continents elsewhere,A set of"Cap Dolostones"is found in the lower of the Doushantuo Formation in the region.After the isotope and geochemistry being analysed,it is found that:(1)their carbon isotope of these Cap Dolostone immediately overlying the Nantuo Tillite has a large negative excursion,correlative with the result of global Cap Carbonates found elsewhere;(2)these Cap Dolostones formed in different sedimentary environments,the negative excursion of their carbon isotope is also different,i.e.carbon isotopic stratification,there is a trend which carbon isotopic values is more negative,when the water column of sedimentary environment varies deeper.Shen et al(2005)attribute the carbon isotopic gradient to the effect of biological pumping,the author thinks it is possible to cause by increase of biological productivity in surface waters along with enhance of greenhouse effect and the deeper sediment seawater after deglaciation.4,through the analysis of sedimentary facies in the region,together with the characters of sediment in different environments,it is considered that sedimentary model of the Doushantuo Period in the region is the tidal flat—shelf environment and those of the Dengying Period is the Carbonate platform—shelf environment.