Sedimentary and burial environment of black shales of Sinian to Early Palaeozoic in Upper Yangtze region

Black shales within the Sinian (Ediacaran) to Early Paleozoic strata from Upper Yangtze region are the three high quality source rocks of Sinian to Early Paleozoic marine Lower Assemblage in South China, which are generally thought to be the result of deposition under stagnant and suboxic conditions. According to the information of sedimentary structure and biological, ecological patterns, the sedimentary environment and burial condition of these black shales are discussed. The benthonic organisms and abundant natant organisms indicate that there were, in the sedimentary environment of black shales, an oxic upper-water in where abundant organisms lived and repropagated to provide organic-rich matters for deposits, and a suboxic or dysoxic bottom-water in where the decomposition of organic-matters were weakened and /or baffled for the preservation of organic-rich matters. In addition, the sedimentary structures formed by water-energy (e. g. the ranking orientation of fossils and their fragments, the oblique bedding in non-black-shale rocks intercalated in black shales) show that there were measure of water-energy or an periodic and intermittent hydroenergy in the bottom-water. The water-energy was able to promote the oxygenic diffusion in bottom-water for the benthonic organisms. With the increase of the water-rich muddy deposits, the oxygen-exchanging efficiency between the deposited water and sea-water was weakening to intensify the stagnation and reduction of the deposited water inner deposits. Simultaneously, the organic-rich matter (e. g. organismic corpses and excreta) buried by deposits were decaying and decomposing to advance the reduction around them, so that the organic-rich matter were preserved to form the black shales.