四川龙门山区中泥盆统金宝石组核形石岩石学特征及其成因

Petrologic characteristics and genesis analysis of oncoids from the Middle Devonian Jinbaoshi Formation in Longmenshan area,Sichuan Province

四川龙门山区甘溪剖面中泥盆统金宝石组顶部发育一套核形石灰岩。通过野外剖面实测和镜下详细观察,认为该套核形石灰岩垂向上可分为3个沉积旋回,每个旋回均由核形石灰岩和泥晶灰岩组成。葛万藻(Girvanella)丝状体在核形石的核心和包壳中均有发育,对核形石的形成具有重要的作用。结合核形石中的纹层类型、Girvanella丝状体形态以及渗滤砂与核形石纹层的关系,认为核形石是在浅滩和礁后潟湖环境中沉积形成的,核形石及其围岩中赋存的粒内渗滤砂和粒间渗滤砂,形成于核形石沉积之后的大气水淋滤作用。通过对核形石形成环境的分析可知,龙门山区甘溪剖面中泥盆统金宝石组顶部核形石灰岩形成的水动力条件从下到上逐渐减弱,反映了当时相对海平面由低到高的变化特点。

A set of oncoidal limestone was developed in the top of Jinbaoshi Formation of Middle Devonian in Longmenshan area,Sichuan Province. Based on field profile survey and detailed observation under the microscope,the oncoidal limestone could be divided into three sedimentary cycles vertically. Each cycle was composed of oncoidal limestone and micrite. Girvanella filaments can be seen in both the coreand cortex of oncoids,and they played an important role in the formation of oncoid. Combined with the types of laminae,morphology of Girvanella filaments,and the relationship between infiltration sand and laminae,it can be concluded that the oncoids were formed in bank and lagoon in the reef back. There are two types of infiltration sands,intragranular and intergranule sands,subjected to leaching of meteoric water during the exposure after the oncoids deposition. By analysis of the formation environment of oncoids,it can be concluded that the hydrodynamic condition gradually diminished from bottom to top,reflecting a gradual rise of the relative sea level at that time.