川西北杨坝剖面埃迪卡拉系灯影组核形石组构特征

Fabric Characteristics of Oncoids from Yangba Section,Ediacaran Dengying Formation,Northwestern Sichuan

埃迪卡拉系灯影组核形石研究对揭示该期古环境特征及演化具有重要意义。在杨坝剖面埃迪卡拉系灯影组2段核形石发育段的宏观及微观描述(564块薄片)的基础上,分析了核形石组构、类型及垂向分布。该剖面核形石发育段共分为上、下两个亚段,厚度分别为109.53 m、126.2 m,核形石累积厚度分别为22.6 m、49.5 m。核形石核心及壳层的成分、组构、形态多样;核形石类型包括不规则状核形石、椭圆状核形石、次圆状—圆状核形石、帽状核形石等4种,每种类型的粒径、形态、层位分布、形成环境不同;粒度在垂向上表现为10个反旋回。整体上核形石发育不够完善,以薄皮核形石、弥散粒为主,代表着核形石发育的最初阶段。不规则状、次圆状、帽状核形石形成于弱搅动的浅水低能环境,包括潮下低能带、潮间带等;椭球状、圆状核形石形成于连续搅动的潮下高能带。核形石粒度差异大、形态多样、垂向多旋回变化的特征反映了埃迪卡拉纪灯影期海水受限、水体相对深浅及能量频繁动荡,同时受一定物源影响的特征。

The study of oncoids in the Neoproterozoic Ediacaran Dengying Formation is of great significance in revealing the characteristics and evolution of the paleoenvironment in this period.the fabric,types and vertical distribution of the oncoids were analyzed based on macroscopic and microscopic descriptions of 564 oncoid thin sections from member 2 of the Ediacaran Dengying Formation in the Yangba section.Member 2 was divided into upper and lower submembers(thicknesses 109.53 m and 126.2 m,respectively),reflecting the degree of development of the oncoids.The thicknesses of the oncoid-bearing levels are 22.6 m and 45.9 m,respectively.The variations in oncoid composition,fabric and morphology of the nucleus and laminae are described as irregular,ellipsoidal,circular/subcircular and dome-like types,with each type having different grain diameters and shape,and from different distribution horizon and formation environments.In the vertical direction,10 sequences occur in which the grain diameter increases upward.The study shows that the oncoids were formed in a tidal flat environment that was affected by the provenance,such that the oncoid development was not perfect overall.Thin-skinned oncoids and dispersal grain are the main types.Irregular,subcircular and dome-like oncoids were formed in a low-energy shallow-water sedimentary environment,including the subtidal low-energy zone and intertidal zone with weak agitation.The ellipsoidal and circular oncoids were formed in the continuously agitated subtidal high-energy zone.In the vertical direction,the characteristics of large differences in grain size,variable morphology and vertical polycyclic change evident in the oncoids imply limited sea water,relatively shallow water,frequent energy turbulence and the influence of particular terrestrial sources during the Dengying period of the Ediacaran.