滇黔桂盆地及邻区二叠纪层序地层格架及古地理演化

滇黔桂盆地及邻区的二叠系包括船山统、阳新统和乐平统。船山统与阳新统之交的黔桂运动主幕不整合面、阳新统和乐平统之交的东吴运动主幕不整合面的不整合特征明显,同时因其代表了研究区沉积盆地性质发生较大变化的界面而成为较为典型的I型层序界面。滇黔桂盆地及邻区的二叠系以其明显的沉积相分异而引人注目：在连陆台地上发育含煤地层,而在连陆台地边缘及孤立台地上发育海绵生物礁,形成一个典型的“台-盆-丘-槽”的古地理格局。伴随着强烈的东吴运动第一幕,研究区西北部发生强烈的火山活动,形成分布范围较广的峨嵋山玄武岩;东南部的“钦防残留海槽”关闭,使研究区的古地理格局发生了较大变化。二叠系与三叠系之交发生的较为典型的台地淹没事件形成了一个较为典型的淹没不整合面。根据沉积物的时间演化序列和沉积相的空间展布形式所指示的规律,研究区的二叠系可以划分为以下三级层序：船山统与下伏石炭系顶部的地层构成一个三级层序(SQ¹⁹),阳新统包含四个三级层序(SQ²⁰到SQ²³),乐平统包括两个三级层序(SQ²⁴和SQ²⁵)。在层序地层框架内,系列古地理图和系列层序地层格架栅状图较为系统地反映了研究区二叠纪复杂而有序的时间和空间相变特征。主要的时间相变面即形成层序界面,具有明显的“跳相”和地层间断现象,由于在不同地点间断的时限不一致而造成“间断面穿时”;沉积环境的空间变化,以及伴随着时间变化中的相带迁移所造成的空间相变,其相应的相变面总是斜交时间面而造成“相变面穿时”;不同沉积阶段的层序具有不同的相序组构,并在空间上形成有序的变化,从而代表了不同的古地理背景。因此,层序地层研究的核心内容是“沉积物的时间演变序列和沉积相的空间展布形式所代表的规律性”,结合生物地层特征,这种规律性本身也可以作为地层划分和对比的标志。

Sequence stratigraphic framework and palaeogeographical evolution of the Permian of Dian-Qian-Gui Basin and its adjacent areas

The Permian in the Dian Qian Gui Basin and its adjacent areas can be divided into three series,the Chuanshanian, the Yangxinian and the Lepingian SeriesTwo obvious unconformity surfaces, one resulting from the main episode of the Qian Gui structural movement at the turn from Chuanshanian Epoch to Yangxinian Epoch, and another resulting from the main episode of Dongwu structural movement at theturn from Yangxinian Epoch to Lepingian Epoch, not only become the evidentsequence boundaries of type I but also represent the important changing surfaces of palaeogeographical patterns and the characters of the sedimentary basinin the study areaThe Permian System in the Dian Qian Ｇui Basin and its adjacent areas is marked by the clear sedimentary facies differentiationThe development of coal bearing strata on the attached platform as well as the formation ofsponge reefs and grain banks in the margin of attached platform and on the isolated platforms led to a particular palaeogeographical pattern thatis characterized by “platform basin hill trough”.Concomitantwith the first episode of the Dongwu structural movement,a strong volcanic eruption occurredin the north western part of the study area, which led to the formation of the widely distributed “Emeishan Basalt”.At the same time,the “Qinzhou Fangcheng relict trough" was closedThese important events resulted in the great changes of the palaeogeographical patternsThe typical drowning event of the carbonate platforms at the turn from the Permian to the Triassic formed a typical drowned unconformityAccording to the regularities represented by the evolution succession of sediments as well as the distribution patterns of sedimentary facies, the Permian in the study area could be divided into several third order sequencesThe top part of the Carboniferous and the Chuanshanian Series of the Permian form one sequence(SQ¹⁹),the Yangxinian Series includes four sequences  from SQ²⁰ to SQ²³, and the Lepingian Series could be divided into two sequences including SQ²⁴and SQ²⁵Ultimately,the complex but regular facies changes in time and in space can be clearly illustrated by the palaeogeographical maps as well as the panel diagrams of sequence stratigraphic frameworksFirst, the main temporal facies changing surfaces marked by the abrupt changes of sedimentary facies and the stratigraphic hiatus form the sequence boundariesDue to the different duration of stratigraphic hiatus at different locations,the diachrononism sequence boundaries of “punctuated surfaces" are formedSecond, the spatial changes of sedimentary environment as well as the migration of sedimentary facies during the temporal changing process always result in the spatial changing surfaces of sedimentary facies marking the obliqueness with chronostratigraphic surfaces and in the end the diachronism of “facies changing surfaces" is formedThird, various facies succession fabric of sequences is formed in different depositional periods, and is regularly changed in space, which means different palaeogeographical backgroundsTherefore, it could be concluded that the regularity represented by “the evolutionary succession of sediments and the distributional pattern of sedimentary facies" is the key factor on the study of sequence stratigraphyOn the basis of biostratigraphic materials, this kind of regularity could be used as an indicator in the division and correlation of strata