湘西震旦—寒武纪交替时期古海洋环境的恢复

RECOVERY OF THE PALEOOCEAN ENVIRONMENT IN THE ALTERNATING EPOCH OF LATE SINIAN AND EARLY CAMBRIAN IN THE WEST HU'NAN

前寒武纪至寒武纪的交替时期是地质史和生命史的重要转折。中国南方地区广泛发育了上震旦统、下寒武统的黑色岩系。文中运用沉积学和沉积地球化学的理论和方法对湘西黑色岩系的岩石类型、矿物组成、元素地球化学特征、干酪根的结构以及碳同位素的组成等方面进行了研究 ,讨论了黑色岩系的形成条件 ,恢复了震旦纪 /寒武纪地史转折期的古环境。研究认为 ,湘西地区在晚震旦世和早寒武世地史转折期的生物爆发和高有机质的产率是形成缺氧环境的重要因素之一。

During the Upper Sinian and Lower Cambrian the Yangtze area was a typical passive continental margin basin. Moreover,the black shale series are the best\|developed sedimentary rocks in this kind of basins. They are usually the hosted rock of the Ni, Mo, P, Ba and Mn deposits. Hence, it is very important to study the black shales. Black shales are also the indicators of the oceanic anoxic event. The global evolution of oxygen content can be determined by the variation of the biomarkers in the black shales. There are two series of black shales, one is in the Upper Sinian Doushantuo Formation which is the hosted rock of the phosphorite, and the other is in the Lower Cambrian Niutitang Formation, which is the hosted rock of the barite and V, Ni, Mo deposits. The sequence of Upper Sinian and Lower Cambrian is as follows (in ascending order): Upper Sinian\|dolostone and black slate and siliceous slate, lenticular barite, black slate, phosphorite, black siliceous slate; Lower Cambrian\|black slate, concretionary phosphorites, barite, stone coal, V\|Ni\|Mo deposits, black slate, black sandstone, K\|shale and limestone.The origin of the black shale series, the source of organic matter, sedimentary environment, diagenesis and mineralization are discussed on the base of the detailed study of sedimentary geology, mineral composition, element association and geochemistry,as well as stable isotope and PGE cosmochemistry. The analysis methods include the ICP\|MS, C stable isotope, main element analysis, and micropetrology. Finally, the author concludes that the ore\|bearing characteristics controlled by the abundant source of organic matter from algae and fungi and the preserved and transverse conditions of the organic matter in the anoxic environment.