上扬子区中、上寒武统的层序地层划分和层序地层格架的建立

From Sequence-Stratigraphic Division to the Establishment of Sequence-Stratigraphic Framework,the Middle to Upper Cambrian in the Upper-Yangtze Region

上扬子区特别是贵州及邻区的中、上寒武统发育完整,是一套从少量灰岩到大套白云岩所组成的地层序列;以娄山关群白云岩为代表的自北西而南东的进积作用穿时,代表了与二级海退作用相对应的碳酸盐台地进积作用的基本特征,从而形成一个复杂而有序的层序地层格架。在研究区域,特别是浅水台地背景之中,从厚度较小的含化石的陡坡寺组到厚度近千米的贫乏化石的娄山关群白云岩,给层序地层研究带来了许多困难;而在较深水背景之中的中、上寒武统,沉积相序列的有序性正好弥补浅水背景中的不足。中、上寒武统组成一个二级层序,其中可以进一步划分为七个三级层序,形成一个有序的海侵—海退旋回序列。伴随着上述时间变化的同时,从北西向南东,古地理背景由浅变深,中、上寒武统由大套白云岩地层相变为泥页岩夹灰岩地层。在不同古地理背景的典型剖面的层序划分的基础上,根据三级层序的两大属性———“空间上相序的有序性和时间上环境变化的同步性”,可以建立研究区域能反映出地层记录中“两种相变面和两种穿时性”的层序地层格架。研究区域的中、上寒武统层序地层格架的建立表明:虽然地层的沉积记录较为复杂,但是建立在以“沉积物的时间演化序列与沉积相的空间展布形式所代表的规律性”为基础的层序地层研究,可以从复杂的地层记录之中寻找出更多的规律性。

In the Upper-Yangtze region, especially in the Guizhou Province and its adjacent areas, the Middle to Upper Cambrian is well developed and is marked by a succession from limestone to dolomite. The progressive diachronism represented by the great-thick ＂Loushanguan Group dolomite＂ indicates a progression of carbonate platform resulted from the second-order regression. Thus, a complex and regular sequence-stratigraphic framework of the Middle to Upper Cambrian was formed in the Upper-Yangtze region. In the shallow-water background, the Middle to Upper Cambrian is made up of both the thin Douposi Formation with fossils a^nd the great-thick Loushanguan Group lacking fossils, in which there are some difficulties to discern third - order sequences. But the regularity of sedimentary-facies of the Middle to Upper Cambrian in the deeper background make it easy to discern the third - order sequence, and this regularity can be used to make up the shortage for the division of third-order sequences in the shallow-water background. The Middle to Upper Cambrian in the study area constitute a second-order sequence that can be further divid- ed into seven third-order sequences, which forms a regularly transgressive-regressive cyclic succession. Along with these temporal changes, the palaeogeographic background of the Middle to Late Cmbrian in the study area become deep toward southeast, and the Middle to Upper Cambrian marked by a great-thick dolomite in the shallow setting is changed into a set of muddy shale with limestone in the deep setting. According to the elementary features of the thirdorder sequence, i.e. the regularity of sedimentary-facies succession in space and the synchronism of sedimentary-environment changes, the detailed division of the third-order sequence at the main logged section in different palaeogeographical background become the basis to establish the sequence-stratigraphic framework that can demonstrate two types of facies-changing surface and two types of diachronism in the stratigraphic record. There is an important meaning for this sequence-stratigraphic framework, i.e. the sequence-stratagraphic study on the basis of ＂the regularity represented by the temporal succession of sediments and the spatial pattern of sedimentary-facies＂ is a useful tool to get more regularities from the complex stratigraphic records.