基于陆生植被对河流沉积作用的影响论上三叠统须家河组的冲积构架:以重庆永川普安剖面为例

Discussion of alluvial architecture for the Upper Triassic Xujiahe Formation in terms of sedimentological impact of terrestrial vegetation on fluvial sedimentation: A case study at Pu′an section in Yongchuan of Chongqing

尽管还存在一些不同的认识和争论,陆生植被对河流沉积作用的影响,得到了越来越多的地质事实的支持,主要表现在以煤或泥炭的形式保存下来的碳质物质对河堤的保护,从而促进了曲流河与网状河的发育。四川盆地上三叠统须家河组为一套含有煤系地层的冲积序列为主的地层,属于具有前陆盆地性质的磨拉石序列,成为研究植被发育条件下河流序列的典型代表。重庆永川普安剖面的须家河组是以河流相沉积为主的须家河组的典型代表,包括6个正式的岩性段,在其中一段、三段和五段为含煤细粒碎屑岩组成的低能河流相地层,二段、四段和六段由砂质底荷载高能河流相沉积构成,岩石地层划分及其所反映出的旋回性成为层序地层的识别和划分的基础,从而将须家河组划分成3个河流相沉积层序。两个主要的特征赋予了普安剖面须家河组河流相层序序列重要意义:(1)总体向上变粗的而且与现行的河流相层序模式形成巨大差异的河流相沉积序列组成的冲积构架;(2)明显受到植被发育影响的高能底荷载河道相砂岩。基于植被发育对河流沉积作用的影响,对普安剖面须家河组的河流相沉积序列的观察与研究,对长期以来须家河组沉积相分析存在的较大争议将提供重要的线索和思考途径。

Although there are some different recognitions or controversies,more and more geological facts support that terrestrial vegetation had made obvious impact on fluvial sedimentation,which is chiefly expressed through the protection to riverbank resulted from carbonaceous materials that are preserved in the form of coal or peat and this promoted the development of meandering and anastomosing rivers.The Xujiahe Formation of the Upper Triassic in the Sichuan Basin is marked by a set of alluvial succession with coal seams,which makes up a succession of the molasses with the nature of foreland basin and becomes an example to study the fluvial succession controlled by vegetation.The Xujiahe Formation at the Pu′an section in Yongchuan of Chongqing is mainly composed of a fluvial succession,which can be subdivided into 6 members,ie., the finegrained clastic deposits with coal seams belonging to the lowenergy fluvial facies constitute the Members 1,3 and 5,and the sandy bottomload deposits belonging to the highenergy fluvial facies make up the Members 2,4 and 6.The stratigraphic cyclicity reflected by lithostratigraphic divisions becomes the foundation to sequencestratigraphic discerning and division;thus,the Xujiahe Formation at the Pu′an section can be grouped into three fluvial thirdorder depositional sequences. The fluvial sequences of the Xujiahe Formation at the Pu′an section have great significance, which is mainly based on the following two main features: (1) a particular alluvial architecture made up a generally upwardcoarsing fluvial sequences that are different from the current model of the fluvial sequence;(2)highenergy channel sandstones that are obviously influenced by the development of vegetation.The observation and study on the fluvial depositional sequences of the Xujiahe Formation at the Pu′an section based on the sedimentological impact of the terrestrial vegetation to fluvial sedimentation will provide an important clue and a thinking approach for the further study on analysis of sedimentary facies of the Xujiahe Formation that is one of the more controversial issues in current stratigraphic research.