上扬子区下寒武统的层序地层划分和层序地层格架的建立

Sequence stratigraphic division and framework of the Lower Cambrian in the Upper Yangtze region

上扬子区特别是贵州及邻区的下寒武统发育完整,是一套从黑色页岩系到碳酸盐岩的地层序列。寒武系与震旦系之交的台地淹没事件之后,在研究区域普遍形成一套以牛蹄塘组为代表的缺氧盆地相黑色岩系,随着沉积环境的逐渐变浅,在下寒武统顶部发育一套以清虚洞组为代表的碳酸盐岩地层。因此,下寒武统组成一个二级层序,其中可以进一步划分为5个三级层序,形成一个有序的海侵—海退旋回序列。在二级层序之中,三级层序形成一个有序的垂直叠加形式,自下而上三级层序由"凝缩段(CS)+高水位体系域(HST)"序列演变为"海侵体系域(TST)+凝缩段(CS)+高水位体系域(HST)",层序界面类型由淹没不整合型层序界面演变为明显的暴露间断面,意味着二级层序的相序组构与三级层序存在相似性。上述时间变化在不同的空间古地理背景之中形成了一些差异,从北西向南东,古地理背景由浅变深,下寒武统厚度变薄,向东南变为一个难以进行三级层序划分的凝缩序列。以不同古地理背景的典型剖面的层序划分为基础,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。层序地层格架的建立表明,研究区域的下寒武统是一个复杂而有序的地层序列,代表以凝缩序列为基底的碳酸盐台地生长与形成过程:伴随着寒武纪早期的快速海侵作用事件所造成的生态空间的拓展,为寒武纪生物大爆发奠定了古地理基础,同时意味着沉积环境变化与生物多样性事件之间的较为密切的关系以及若干有待进一步研究的问题。

The Lower Cambrian in the Upper Yangtze region, especially m Guizhou and its adjacent completely developed and marked by a succession from black shales to carbonate rocks. The drowning areas, is event of the platform that occurred at the turn from the Sinian to the Cambrian resulted in the formation of a series of black shales of anoxic basin facies represented by the Niutitang Formation in the study region. With shallowing of the sedimentary environment a sequence of carbonate rocks represented by the Qingxudong Formation formed at the top of the Lower Cambrian. Thus the Lower Cambrian forms a second-order sequence, which may be subdivided into five third-order sequences, forming a regular cyclic succession of transgression-regression. The third-order sequences in the second-order sequence form a regular vertical the top, the third-order sequences change from the succession of ＂CS stacking pattern. From the bottom to （condensed section） ＋HST （highstand systems tract）＂to the succession of ＂TST （transgressive systems tract）＋CS ＋HST＂, and correspondingly the drowning-type sequence boundary changes to the pronounced exposed hiatus-type one, implying the siraihrity between the second-order sequence and the third-order sequence in respect to lithofacies organization. This temporal change produced some differences in different paleogeographic settings. From northwest to southeast, the sedimentary environment deepened and the Lower Cambrian became thin and changed into a condensed succession that cannot be subdivided into third-order sequences. On the basis of the sequence stratigraphic divisions at typical sections with different paleogeographic environments and according to the elementary features of the third-order sequence, i.e. the regularity of sedimentary facies succession in space and the synchroneity of environmental changes in time, we may establish a sequence stratigraphic framework that can demonstrate ＂two types of facies change surface and two types of diachroneity＂ in the stratigraphic record. This sequence stratigraphic framework shows that the Lower Cambrian is a complex and regular stratigraphic succession, representing the growth process of the carbonate platform with the condensed succession as the base. Concomitantly with the rapid transgression in the Early Cambrian, the ecological space expanded, which formed the antecedent conditions of paleogeographic setting for the ＂Cambrian explosion of life＂. In addition, the very complex, close relationship between the depositional events and the biological diversity events means many problems that remain to be further studied in the future.