Sedimentary Characteristics Of The Black Shales Of Lower Cambrian Niutitang Formation In Northwestern Guizhou Province

Northwestern Guizhou belongs to the tectonic unit of Yangtze Block, and is adjacent to the North China Block and South China orogenic belt. A set of organic-rich black shales developed at the lower part of Lower Cambrian Niutitang Formation. And the black shales belong to the instantaneous and catastrophic deposition, and are of great significance of the Global Oceanic Anoxic Event. A lot of previous researches have been made on certain typical Niutitang sections in northwestern Guizhou. But there is still a lack of systematic and in-depth studies on deposition characteristics. Notably there is no unified understanding of the depositional system and its temporal and spatial relations and the black shale deposition mode. Therefore, combined with paleontology and sequence stratigraphy and field work and interior analysis, this paper aims to make a study on the black shale deposition characteristics of the Lower Cambrian Niutitang in northwestern Guizhou.(1) Analysis of the Niutitang biological burial characteristics has been made combining with previous research on Niutitang fossils. And the significance to the depositional environment of Niutitang Biota has been discussed. Researches on the Songlin Heishapo field section shows that Niutitang fossils mainly distributed in three layers of Niutitang. According to the fossils’ enrichment characteristics, Niutitang Biota is divided into three fossil-rich layers, and they are Perspicaris layer at the bottom, sponges layer at the lower part, and Chengjiang Biota like biologies layer at the top. The sedimentary environment of Niutitang is overall hypoxia, and of multi-phase short oxygenation. The biota lived in the upper part of the oxide layer in the water, and deposition occurred in the hydrostatic reduction layer, which is autochthonous.(2) This paper has made the study of measured sections sequence stratigraphy. Niutitang formation has been divided into three fourth order sequences through the combination of biology, lithologic features and geochemical characteristics of the samples. From bottom to top Niutitang lithology is predominantly black phosphorite, black carbonaceous shale, black silty shale or mud shale, calcareous shale, celadon silty shale and light gray lime-shale. Formation thickness reduces from the central east region to northeastern and southwestern.(3) Through the microscope work of rock section, Niutitang petrological characteristics can be drawn. Then sedimentary facies signs have been studied through sedimentary structures flag, sample geochemical test and paleontology markers. Thus, the depositional environment of neritic shelf fades has been identified, which can be subdivided into deepwater shelf phase and shallow water shelf phase. Niutitang contains a transgressive system tract at the lower part and a highstand system tract at the upper part. Particle size gradually grows from bottom to top, which reflects the shallowing of the water. Shallow water shelf phase distributes in the northern part of the study area. In the central part deepwater shelf phase and shallow water shelf phase develop during different periods. Deepwater shelf phase mainly distributes in the southwestern belonging to the South Sichuan Deepwater Shelf and southeastern belonging to the Hunan and Guizhou Hot Deepwater Shelf. Deposition and subsidence center is located in the east-central region of the study area, and the source is mainly from the northwest. The depositional model of the black shale of Lower Cambrian Niutitang Formation in northwestern Guizhou is a reducing environment depositing of the deepwater shelf in the bay system, which can be named as a bay system-deepwater shelf-benthos mat deposition model.