上扬子区北缘宝塔组龟裂纹灰岩结构分异特征及发育模式

Structural differentiation and development model of cracked limestone of Pagoda Formation in northern margin of Upper Yangtze Region

龟裂纹灰岩在扬子板块上奥陶统宝塔组普遍存在,前人针对龟裂纹的成因进行了一定的探讨,但看似单一结构,然而是否存在石灰岩的成分、岩石和岩相类型构成、结构构造、古生物等的时空分异性等问题还尚待探讨。为此,在前人研究的基础上,以沉积学、古生物学和地球化学理论为指导,通过层面和垂直层面形态结构分析,结果表明:①明确“龟裂纹”可进一步划分为脑纹状、似波浪状、鱼鳞状、网格状及透镜状等五种结构类型,并归纳出6类岩相类型,不同的龟裂构造在岩性、古生物学和沉积环境等方面存在差异;②认为研究区龟裂纹灰岩不同结构类型主要受控于沉积环境、泥质组分含量的影响,其裂纹的形成则受控于成岩作用、构造作用、沉积速率等等多种因素;③在此基础上,建立了缓坡背景下基于不同岩石类型、古生物组合、水动力条件、泥质含量、氧化还原条件、古盐度等龟裂纹综合发育模式,认为从浅水区到深水区具有脑纹状、似波浪状、鱼鳞状、网格状、透镜状等的主体龟裂纹结构类型发育序列。

Cracked limestones are common in the Upper Ordovician Pagoda Formation of the Yangtze Plate.Previous studies have fo-cused on the origin of the cracks.However,in the seemingly single structure,the existence of limestone,the temporal-spatial differen-tiations in rock and lithofacies types,structure and texture,and palaeobios remain uncertain.Therefore,based on the previous studies,and guided by the theories of sedimentology,paleontology and geochemistry,the morphological structure of plane and vertical plane were analyzed.The results show that(1)the cracks can be divided into five shapes:brain striate,wave,fish scale,grid and lens;and six types of lithofacies are concluded;the various crack structures are different in lithology,paleontology and sedimentary environ-ment;(2)the cracked limestones with different types are mainly controlled by sedimentary environment and shale content;the forma-tion of the cracks is controlled by diagenesis,tectonism,and deposition rate,etc.;and(3)on this basis,a comprehensive development model of cracks under gentle slope setting is established depending on different rock types,paleontological assemblages,hydrodynam-ic conditions,shale content,redox conditions and palaeosalinity;it is indicated that,from shallow water to deep water,there develops crack shapes in brain striate,wave,fish scale,grid,and lens.