湘西地区前寒武纪-寒武纪转折期碳酸盐-硅泥质沉积体系的截然转换：地层-沉积样式，形成机理及意义  

Sharp transition from carbonates to cherts across the platform margin in western Hunan,south China during Precambrian-Cambrian transition: Stratal-depositional patterns,mechanisms and implications

作　　者：汪建国[1] 陈代钊[1] 严德天[2] 韦恒叶[1,3] 遇昊[1,3] 

机构地区：[1]中国科学院地质与地球物理研究所油气资源重点实验室北京100029 [2]中国地质大学(武汉)构造与油气教育部重点实验室武汉430074 [3]中国科学院研究生院北京100049

出　　处：《地质科学》2011年第1期27-41,共15页Chinese Journal of Geology(Scientia Geologica Sinica)

基　　金：国家重点基础研究发展计划"973”项目(编号:2005CB422101);国家自然科学基金项目(编号:40839907)资助

摘　　要：前寒武纪—寒武纪转折期是地球历史演化的重要阶段之一,不仅纪录了后生动物的产生、灭绝以及加速分异的过程,同时也伴随着海洋地球化学明显的变化、长期全球性海洋缺氧等。为了更好地认识这一重要时期地球表层环境演变及动力机制,我们对湘西地区台地—盆地转换带不同沉积类型及空间变化进行了重点解剖,发现寒武纪最早期该地区在台—盆转换带上,碳酸盐岩与硅质岩地层呈现一种截然接触,显示硅岩的形成受张性同生断层控制;该带内硅质岩成分、结构独特,主要有富管、孔丘状硅质岩、漏斗状/楔状硅质岩和脉状硅质岩体,为深部富硅热液流体沿(同生)断裂向上运移到海底喷流后沉淀(硅烟囱)而形成的一套硅质沉积。此带向盆地方向,主要发育层状硅质岩,反映热液活动衰减而海水影响增强。考虑到扬子北缘及南缘台—盆转换带的硅质岩广泛分布,因此,热液活动很有可能沿着台地边缘发育,规模巨大。在这种情况下,大量热液来源的富金属和非金属元素的还原性流体和温室气体进入海洋或大气,加速气候变暖以及海洋缺氧和富营养化。热液活动后期,由于地壳热力衰减、盆地沉降导致大规模海平面上升,深部富营养盐随上升洋流被输送至浅部,促使生物产率得到极大地提高,进而形成牛蹄塘组富有机质黑色岩系沉积。The Preeambrian-Cambrian transition is a key period for the understanding of one of the most important intervals in Earth＇s history, as it records the advent, extinction and accelerated diversification of metazoans coeval with sharp shifts in ocean geochemistry, a prolonged global oceanic anoxia. In order to investigate the trigging mechanism for these co-occurring processes, detailed crossing-platform-strike facies tracing from the transitional zone of platform-basin in western Hunan, were carried out to unravel different sedimentary facies and their spatial variation across this critical interval. A sharp transition from carbonates to cherts across the platform margin in western Hunan was identified in the earliest Cambrian, indicating ~ syndepositional extensional faulting could have occurred along the transitional zone of platform- basin. Mounded chert, funnelized brecciated chert and vein chert which characterized by unique mineral compositions and internal structures in this transition zone, were formed by submarine hydrothermal venting on the fault-controlled terraces along the margin-to-slope of Yangtze carbonate platform where syndepositional fault/fracture zone could have acted as conduits channeling the hydrothermal fluids heated up by the deep-seated thermal source updip to the seafloor. The bedded chert, mainly distributed in the localities farther basinwards, implies enhanced influences of seawater away the vent fields during deposition. Considering widespread chert along the transitional zone of platform-basin in Yangtze area,intense hydrothermal venting might have occurred along the platform margin such that vast amounts of greenhouse gases, anoxic fluids rich in metal and nonmetal elements would have been released to the ocean/ atmosphere, leading to climate warming, oceanic anoxia and oceanic eutrophication. As the hydrothermal activity waned, sea level would rise due to basin subsidence induced by crust cooling, abundant nutrient materials in the deep ocean were then brough

关 键 词：前寒武纪-寒武纪转折期 碳酸盐岩 硅质岩 同生断层 热液活动 

分 类 号：P534[天文地球—古生物学与地层学]