华北地台中元古代串岭沟组页岩中的砂脉构造：17亿年前甲烷气逃逸的沉积标识？  

Sand Veins and Microbially Induced Sedimentary Structures from the Black Shale of the Mesoproterozoic Chuanlinggou Formation (ca. 1.7Ga) in North China:Implications for Methane Degassing from Microbial Mats

作　　者：史晓颖[1,2] 蒋干清[3] 张传恒[1] 刘娟[1] 高林志[4] 

机构地区：[1]中国地质大学地球科学与资源学院,北京100083 [2]中国地质大学地质过程与矿产资源国家重点实验室,北京100083 [3]美国内华达大学地球科学系,Las Vegas,NV89154-4010 [4]中国地质科学院地质研究所,北京100037

出　　处：《地球科学（中国地质大学学报）》2008年第5期577-590,共14页Earth Science-Journal of China University of Geosciences

基　　金：国家创新研究群体基金(No40621002);教育部创新团队发展计划基金(NoIRT00546);中国石油化工股份有限公司海相油气勘探前瞻性项目(G0800-06-ZS-319);高校引智计划(B07011)的支持

摘　　要：华北地台中元古界串岭沟组暗色页岩中发育一种特殊的砂质脉状构造;层面上表现为不规则密集分布的细砂脊,垂向上由不连续薄砂层和近于直立的"肠状"砂脉体相交互组成.浅色砂脉由较纯的石英粉砂-细砂组成,含微量自生白云石、菱铁矿及微晶碳酸盐岩斑块,与黑色泥质围岩边界截然.研究表明,砂脉构造可能是在早期成岩阶段前由来自薄砂层的细-粉砂灌入甲烷缓慢逃逸通道而成,并由于压实缩短而褶皱成"肠状".薄砂层与黑色页岩形成的能量条件完全不同,可能系由风暴将海岸带或砂坝细砂带入低能环境而形成;甲烷源自沉积中埋藏的微生物席腐烂分解.围岩层面有微生物席成因微皱痕和气体逃逸形成的气泡构造,围岩中发现有细菌化石和草霉状黄铁矿.串岭沟组中密集发育的砂脉构造是目前地层中识别的最古老的甲烷排放证据,并有可能作为指示地质时期陆源碎屑沉积环境甲烷逃逸的沉积标识.大量高效温室气体进入大气圈可能是导致元古宙地球表层高温室气候和无冰川发育的重要原因.The Mesoproterozoic Chuanlinggou Formation （ca. 1.7 Ga） consists of up to 900 m thick, dark-gray to black shale and fine-grained sandstone that are widespread in the North China platform. Abundant centimeter-scale sand veins are present within the black shale layers of this unit, particularly in the central part. Sand veins display ptygmatic shapes, perpen- dicular or with a high angle to sedimentary bedding. They penetrate the black shale layers but are vertically discontinuous and often terminated by thin, lenticular sandstone beds, forming small-scale ＇tepee-like＇ structures. On bedding planes, sand veins are expressed as small sand ridges with 1--3 mm positive relief. Lack of polygonal shapes and their occurrence in thinly laminated, deep-water shale prevent an origin from sand-filled desiccation cracks. Instead, their close association with microbially induced sedimentary structures （MISS）, such as micro-wrinkles and gas blisters, and putative bacteria fossils （possibly coccoidal cyanobacteria） and framboidai pyrites, suggests that they were formed by degassing of methane from microbial mat decay. Methane gas disrupted overlying sedimentary layers, creating fractures open to seawater. Fine-grained quartz sands, which were transported into the depositional environment by storm events, filled the open fractures. Sandfilled fractures were shortened and deformed during burial compaction, forming ptygmatic shapes, The presence of dispersed dolomite and siderite in these sand veins suggests authigenie carbonate precipitation from anaerobic oxidation of methane （AOM）. Sand veins are mainly distributed within the Chuanlinggou Formation and are spatially widespread in the North China platform. If their methane origin is confirmed, they may have important implications for the Mesoproterozoic plaeoclimate. With a low seawater sulfate concentration during the Mesoproterozoic, methane release from microbial mat decay and/ or microbial methanogenesis during shallow burial may have been proportionally h

关 键 词：砂脉构造 甲烷逃逸 微生物席 串岭沟组 中元古界 华北地台. 

分 类 号：P736[天文地球—海洋地质]