河流相沉积中的准层序——以四川中部须家河组为例

Parasequence of fluvial deposit:a case study of the Xujiahe Formation in Central Sichuan

高志勇 韩国猛 张丽华

Gao Zhiyong Han Guomeng Zhang Lihua

在详细分析了曲流河、辫状河沉积动力学特点的基础上,在四川中部须家河组河流相沉积中初步建立了3种河流相沉积的准层序模式。其中,在曲流河沉积中发育2种准层序模式,其一为粒级向上变细准层序,表现为与河道水体逐渐向上变浅相对应的水下河道(滞留沉积)-边滩-水面之上的天然堤、泛滥平原的相序组合;其二为粒级向上变粗的泛滥平原、决口扇交替沉积相序组合的准层序。辫状河准层序大多表现为粒级向上变细、与河道水体向上变浅相对应的水面之下河道(滞留沉积)-心滩-水面之上的漫滩沉积相序组合的准层序,而粒级向上变粗的准层序不发育。河流相准层序界面是河道砂体底部的冲刷侵蚀面,也是与湖(海)盆准层序边界湖泛面或者海泛面在陆上所相对应的界面,但河流相准层序界面更与海相沉积中海侵滞留沉积物而非暗色泥岩的准层序界面特征相近似。在粒级向上变粗的泛滥平原-决口扇或决口河道沉积中,准层序界面为暴露过泥岩与上部洪水期的暗色泥岩间的界面。在建立了河流相准层序模式的基础上,针对四川中部上三叠统须家河组河流相沉积地层,进行了准层序在垂向相序叠加样式与时空演化关系的分析,并探讨了河流相准层序中非均质性特征。

Based on a detailed analysis of the dynamics characteristics of meandering stream and braided river deposits,3 parasequence models of fluvial deposit are preliminarily recognized in the Xujiahe Formation in central Sichuan basin.Two parasequences,i.e.the graded fining-upward parasequence and the graded coarsening-upward parasequence,occur in meandering river deposits.The graded fining-upward parasequence represents a water shallowing-upward facies sequence combination of subaqueous fluvial channel(lag deposits),marginal bank,natural levee and flood plain deposits.Whereas the graded coarsening-upward parasequences represents a facies sequence combination of alternative flood plain-crevasse-splay deposits.The braided river parasequence mostly appear as a graded fining-upward and water shallowing-upward parasequence of facies sequence combination of subaqueous fluvial channel(lag deposits),channel bar and overbank deposits,while the graded coarsening-upward braided river parasequences are not developed.The fluvial parasequence boundary is the erosional surface at the bottom of channel sandbodies and also is the onshore equivalent interface of the lake water or sea water flooding surface on the parasequence boundaries in lake/marine basins.However,it is more similar with the parasequence boundary of transgressive lag deposits rather than dark mudstone.In the graded coarsening-upward flood plain,crevasse splay or channel deposits,the parasequence boundary is the interface between exposed mudstone and upper dark flood mudstone.Based on the proposed fluvial parasequence models,the vertical supe-rimposition pattern of fluvial parasequence and its time-space evolution are analyzed,and the heterogeneity of fluvial parasequence are studied for the Upper Triassic Xujiahe Formation in central Sichuan basin.

10.11743/ogg20070108