湖南桑植二叠系茅口组硅泥岩段的有机白云岩环境  

Organogenic dolomitization within the Middle Permian siliceous-rock setin Maokou Formation of Sangzhi,Hunan Province

作　　者：曹长群[1,2] 章明圆 郑全锋[1] 丁奕[1] 

机构地区：[1]中国科学院南京地质古生物研究所、现代古生物和地层学国家重点实验室,南京210008 [2]WA Organic and Isotopic Geochemistry Centre,Curtin University,Perth 6845 [3]浙江省地质调查院,杭州311203

出　　处：《岩石学报》2013年第10期3649-3657,共9页Acta Petrologica Sinica

基　　金：国家科技部973项目(2011CB808905);国家基金委重大项目(41290260)联合资助

摘　　要：湖南仁村坪剖面二叠系茅口组中段发育一套富有机质硅泥质岩,夹含有多层白云岩沉积。其白云岩的岩石学特征显示,白云石颗粒具有簇状低有序度隐晶质晶体特征,不同于次生白云岩的自形-半自形菱形的晶体类型,应属于沉积孔隙水体埋藏有机质的硫酸盐氧化过程产生的有机自生白云石类型。该层段碳-氧同位素表现出特征的负相关关系,与硫酸盐还原菌作用(SRB)的埋藏有机质氧化强度有关。生屑颗粒白云岩所表现出的重氧同位素特征,代表浅水环境有机质氧化过程特殊的氧同位素分馏途径。4.5m和5.9m处生屑白云岩表现的碳同位素强烈负偏,应与甲烷的硫酸盐缺氧氧化(AOM)过程有关。硅质泥灰岩和钙质泥岩因具有较低的孔隙度和早期碳酸钙沉降供应,难以充分发生与硫酸盐还原作用相关的自生白云石化,因而表现出甲烷生成作用带碱性孔隙水特征的重碳同位素组成特征。仁村坪剖面茅口组中段硅泥质岩段的岩石学特征、草莓状黄铁矿特征以及碳-氧同位素负相关关系等诸多证据,均显示其沉积环境处于斜坡相的富有机质供应和埋藏环境。The Rencunping （RCP） section in Sangzhi of Hunan Province is a typical location developing a set of organic-rich siliceous deposits in the middle part of Maokou Formation, significantly, which is intermitted with several dolomite beds. Micromorphologic features of dolomitic crystallites present poorly-ordered and aphanitic textures in the form of anhedral clusters, which is distinctly different from the rhomboidal textures formed in diagenesis. These features might evidently indicate that organogenic dolomitization occurred in the environment of oxidation of burial organic matters by bacterial sulfate reduction in sedimentary pore water. In addition, negative relationship of δ^13C vs. δ^180 values has been detected in the entire siliceous-rock interval. We infer that should be the results also from the enhanced influences of SRB processes of burial organic matters. The occurrence with relatively high δ^18O values in the bioclastic dolomites might track the O-isotopic compositions back in the shallow-water environments indicating unique fractionating pathway on degradations of organic matters after reworking. The abnormalities with relatively low δ^13 C values occurred distinctively in the dolomite beds at sampling positions of 4. 5m and 5.9m, which should mark the dominance of anoxic oxidation of methane （AOM） releasing from the underlying layers. Due to poor conditions in terms of porosities and availabilities of precipitated carbonates in the rocks of marls and calcareous mudstone, inwhere it prevented the activities of SRB to form the organogenic dolomite, alternatively, to be replaced by methanogensis in deep burial depth to derive^ 13 C-enriched DIC in alkaline pore fluids. Therefore, the consistent evidence mentioned above inferred that the depositional environment of the siliceous-rock set in Rencunping section was preferably in the slope facies in cases of enhanced supplies and deposits of organic matters.

关 键 词：仁村坪剖面 茅口组 有机白云岩 硅泥质岩段 碳-氧同位素 斜坡相沉积环境 

分 类 号：P588.245[天文地球—岩石学] P593[天文地球—地质学]