西藏聂拉木地区中、下侏罗统化石碳酸盐岩微相研究及沉积环境分析

ANALYSIS OF LOWER MIDDLE JURASSIC FOSSIL CARBONATE MICROFACIES AND SEDIMENTARY ENVIRONMENTS IN NYALAM,TIBET

西藏聂拉木地区早、中侏罗世碳酸盐岩分布广泛，岩石类型多，海相化石丰富。在结构成因分类基础上，将本区碳酸盐岩分为３个大类、１２种化石碳酸盐岩微相类型，分析了每种微相的基本沉积条件和形成环境。根据微相的相邻和共生关系，建立了４个主要微相组合，解释了各种微相组合的沉积环境及其演化，建立了藏南早、中侏罗世的沉积模式。通过综合分析认为，该地区中、下侏罗统普普嘎组及聂聂雄拉组是大西洋型被动陆缘的产物。

The materials analyzed in this paper were mainly collected from the Niehnieh Hsiongla Section in southern Tibet in 1995.The Jurassic of this area was divided into the Lower Jurassic Pupuga and Middle Jurassic Niehnieh Hsiongla formations.They are composed of marine clastic and carbonate rocks.The carbonate deposits account for 60 percent of the sedimentary rocks.According to textural origin classification,types of grains,sedimentary structures and fossils,the carbonate rocks can be recognized as three types consisting of twelve kinds of microfacies:grainstone(MG1 to MG8 microfacies),wackstone(GM1 and GM2 microfacies)and mudstone(M1 and M2 microfacies).Each microfacies represents respectively different sedimentary conditions and environments.Four kinds of microfacies associations are classified on the basis of neighbouring and paragenetic relationship of microfacies,as well as characteristics of fossil assemblages. Each microfacies association corresponds a designated sedimentary environment and its evolution.Although many types of sedimentary environments have been discerned in the study area,the sedimentary environment of the Early Middle Jurassic is dominated by shoreface and shallow water sea.In this paper,the authors suggest that the Early Middle Jurassic sea level changed periodically,and this change shows the characteristics of the initial stage of passive continental margin.In general,the Early Middle Jurassic of the region was in a transgression period which is related to the expansion of the Himalayan ocean.