上扬子台地西缘峨眉地区三叠纪高频层序与海平面振荡研究

HIGH-FREQUENCY SEQUENCES AND SEA-LEVEL OSCILLATIONS IN THE EMEI AREA ON THE WESTERN MARGIN OF THE UPPER YANGTZE PLATFORM

关键词:  上扬子台地  层序旋回等级  高频层序  海平面振荡  Fischer图解

Key words:  Upper Yangtze platform  cycle order  high-frequency sequence  sea-level oscillation  Fischer plot

摘要:

通过精细露头层序地层学的研究,将三叠纪标定了二级层序2个,三级层序9个。提出并厘定了层序旋回的等级系列:一级(200-400Ma),二级(10一100Ma),三级(1-10Ma),四级(0.1-0.2Ma),五级(0.2一0.05Ma)和六级(±0.02Ma)层序旋回。将三叠纪盆地划分为被动边缘(T₁+2)和前陆盆地(T₃)阶段。进一步标定了古构造沉降对有效容纳空间及层序形成的贡献。提出并规范了高频层序的内涵本质及其划分方案。建立并标定了不同物质背景之下的高频层序四种鉴定类型;三角洲河口坝或滨岸旋回序列型;波痕指数垂向有序变化型;厚层灰岩或白云岩与薄层灰岩或白云岩有序叠置偶合型和砂屑灰岩与砂、泥岩层偶型。高频层序行为堆叠样式及其所携带的信息可深刻地揭示三级层序的内部构架,提高地层对比精度。为建立高分辨率层序地层学和定量编制全球海平面变化曲线奠定了基础。

Abstract:

The present paper has emphasized that the accommodation space and sediment supply are a dual control on sequence stratigraphic framework.The Triassic strata in the Emei area on the western margin of the Upper Yangtze platform are divided into two second-order cyclic sequences and nine third-order cyclic sequences on the basis of the detailed study of outcrop sequence stratigraphy and seismic stratigraphy.The following cyclic hierarchy are ascertained;1st order(200-400 Ma),2nd order(10-100 Ma),3rd order (1-10 Ma). 4th order(0.1-0.2 Ma),5th order(0.2-0.5 Ma)and 6th order(±0.02 Ma).
The geodynamical model for basin subsidence during Triassic time has been constructed for the study area.The model has revealed that the Triassic strata in this region have undergone two evolutionary stages;the passive continental margin during the Early and Middle Triassic,and foreland basin during the Late Triassic.Morever the classification scheme for the high-frequency sequences are also suggested in this paper,including the following four types produced in different environment;deltaic mouth bar or shoreline cyclic sequence type;vertical ordering variation type in ripple index;the superimposed dual type of thick-bedded limestones (dolostones) and thin-bedded limestones (dolostones),and the dual type of calcarenite,sandstones and mudstones.
The inner framework of the 3rd-order sequences may be indicated by the accummulation styles and other information about the high-frequency sequences.These results represent the refinement of the high-resloution sequence stratigraphy,and lay the foundations for the quatitative delineation of the global sea-level curves.