扬子地体船山组碳酸盐岩中的核形石

ONCOIDS IN THE CARBONATE ROCKS OF CHUANSHAN FORMATION FROM YANGTZE TERRANE

扬子地体上石炭统船山组内核形石极为丰富,按照内部结构和构造可分五种纹层类型、二种壳层构造和四种核心结构。核形石并非过去所认为的单一的“葛万藻核形石”,而可以根据形态和壳层中的生物组合划分为十种基本类型,其沉积环境有滩、潮下低能带和泻湖等。 石炭纪晚期,由于全球的气候发生强烈分异,从而在低纬度地区产生大量分泌粘液的缠绕生物(藻类为主),核形石就是由这种缠绕生物捕集沉积质点围绕核心而成,其形成过程有四个阶段—核心形成阶段,壳层形成阶段、沉积搬运阶段和成岩后生阶段。 在成岩后生阶段,核形石岩可以产生大量的核间和核内溶孔,这对于油气、矿液的储集和运移是十分有利的。

Oncoids occur very abundantly in the upper Carboniferous of Chuan-shan formation from Yangtze terraue. Based on the study of 29 profiles and detailed identification of 803 thin sections,their characteristics,typcs,dcpositional environment and genetical mechanism of oncoids have been studied and discussed. The texture and structure of oncoids are very complex. They have five types of lamination, two types of coating structure and four types of nuclear textures. They are not a single 'Givanella oncoids', as were thought in the past. According to the combination of forms and biologic assemblage, they can be divided into ten basic types. The dcpositional environment of oncoids is by no means single 'beach fades', as was considered before. They arc widely developed in the many places of old carbonate platform, such as channel, intertidal zone, subtidal zone, beach or sanbar in Huanglong formation of Weining period and beach, lower subtidal zone and lagoon in Chuanshan formation of Mapin period. Oncolitic limestones are always plentiful in the late Carboniferous of Mapin period. They may be formed during the period of strong climatic diffentiation of the whole globe which caused universal decreasing of the sea-leavel and propagation of large amount of algca in the area of low latitueds during that time. Oncoids are formed in such way: binding microbes (mainly algae) secrete viscous substance and capture sedimentary particales round the nuclei. The formation of oncoids can be divided into four stages:formation of nuclei,formation of coating,sedimentation-transportion and diagenesis. Oncolites may form a lot of dissolved pores in the interoncoids and intraoncoids. These pores arc favorable to rcscrois and migration of oil, gas and mineral solution.