上扬子地区震旦系-早古生界黑色页岩的沉积和埋藏环境

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

上扬子地区震旦(伊迪卡拉)系-早古生界黑色页岩是我国南方震旦纪-早古生代"下组合"海相地层中的"三大优质烃源岩",并普遍认为系缺氧、滞流环境下的沉积。本文从"下组合"黑色页岩的沉积构造以及生物生态的角度,探讨其沉积环境和埋藏条件,认为其中不仅产出有丰富的浮游和游泳类型的生物,也产出有底栖类型的生物,表明黑色页岩沉积环境中的上层水体为充氧的(oxic)环境,而下层水体中含有一定量的氧,为贫氧(suboxic)或稀氧(dysoxic)环境。上层水体的充氧环境有利于丰富生物的生长和繁殖,以提供丰富的沉积有机质;而下层水体的贫氧或稀氧阻碍着沉积有机质的分解,有利于有机质的保存。黑色页岩中的生物化石及其碎片均具有定向性排列等沉积构造,表明其沉积环境的下层水体中存在一定能量或周期性和间歇性的水动力条件,从而有助于水体中氧的扩散,维持底栖生物对氧的基本需求。黑色页岩沉积时为富含沉积水的泥质沉积物,随着富水泥质沉积物厚度的迅速加大,沉积水与水体中的氧交换减弱,沉积物内逐渐形成滞流还原的环境;而被埋藏的丰富生物遗体和排泄物等有机物质在腐烂和分解过程中促进了周围环境的还原性,导致有机物质得以保存而形成黑色的页岩。

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 nonblack-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.