西藏南部羊卓雍措地区晚侏罗世维美组沉积特征及环境演化

通过沉积岩石学、岩相学、粒度分析等研究方法,对维美组沉积特征、沉积环境及演化提出新的认识和探讨。晚侏罗世维美组主要为一套陆源碎屑物质沉积,沉积期共经历4个沉积旋回,每个沉积旋回的岩石组合大致为含砾石英砂岩-中、粗粒石英砂岩-细粒石英砂岩(长石石英砂岩)-粉砂岩(泥岩、页岩),由底到顶粒度变细,层厚减薄。识别出6种岩相类别:含砾石英砂岩、石英砂岩、长石石英砂岩、泥质粉砂岩、薄层页岩-泥岩互层;发现平行层理、水平层理、粒序层理和重荷模等沉积构造;识别出陆棚、滨岸和陆棚下切河道3类沉积相,其中陆棚相包括内陆棚和外陆棚亚相,滨岸相包括近滨上部和近滨下部亚相;陆棚下切河道充填结构大致为:下部河道砂砾岩层,中部不等粒长石砂岩、杂砂岩层,上部细粒石英砂岩、泥岩层。综合来看,维美组沉积环境为位于克拉通边缘上浅海环境,沉积物质及沉积序列受海平面升降变化影响显著,主要形成于全球海平面下降之后上升的阶段。

Sedimentary Characteristics and Environmental Evolution of the Jurassic Weimei Formation in Yamdrok Region of Southern Tibet

In this paper,the sedimentary characteristics,environment and evolution of Weimei Formation are studied by means of sedimentary petrology,petrography and particle size analysis.The Late Jurassic Weimei Formation (J3w) was mainly composed of a set of terrigenous clastic sediments.The sedimentary period has experienced four sedimentary cy- cles,the rock assemblage of each sedimentary cycle is pebbly sandstone (breccia)-medium,coarse quartz sandstone - fine grained quartz sandstone (feldspathic quartz sandstone)-siltstone (mudstone,shale),the size of the bottom to the top is fin- er and the thickness is thinner.There are 6 major types of lithofacies were identified:pebbly sandstone,quartz sandstone, feldspar quartz sandstone,clayey siltstone,shale-mudstone interlayer,shale.It is found that parallel bedding,horizontal bed- ding,grain sequence stratification structure,load casts deformation structure.Three types of sedimentary facies are identi- fied:shelf facies,shoreline and incised valley,among which shelf facies include inner shelf and outer shelf subfacies, and shoreline facies include backshore,foreshore and nearshore subfacies.The filling structure of the incised valley is roughly as follows:sand and gravel strata in the lower channel,arkose and miscellaneous sandstone layers of unequal grains in the middle,quartz sandstone and mudstone layers of fine grains in the upper part.On the whole,the sedimentary environment of Weimei Formation is located in the shallow sea environment on the edge of craton, and the sedimentary material and sequence are significantly affected by the change of sea level rise and fall, mainly forming the rising stage after the glob- al sea level drop.