上扬子区寒武系娄山关群白云岩层序地层格架及其古地理背景

上扬子区特别是贵州及其邻区的寒武系娄山关群发育一套化石贫乏的浅水台地相白云岩，横跨中、上寒武统，厚度上千米，自北西向南东随着沉积环境水体的加深逐渐相变为开阔海台地相灰岩、陆棚相泥页岩等。由于化石贫乏难以进行详尽的年代地层划分，但是发育石盐假晶泥晶白云岩所反映的沉积环境水体变浅以及发育颗粒白云岩所反映的水体加深，表明可以应用从岩相序列到沉积相序列的方法对该套白云岩进行层序地层划分，成为在“海平面变化的框架内探讨沉积相迁移”的典型代表。巨厚的贫乏化石的娄山关群白云岩与下伏厚度较小、产三叶虫化石的陡坡寺组白云质细粒混合岩系形成了鲜明的对照，它们组成了1个二级层序，并可以划分为7个三级层序。陡坡寺组细粒混合岩系代表了二级海侵作用所造成的水体加深，而娄山关群白云岩自北西向南东进积尖灭则代表了二级海退过程所造成的水体变浅。该二级层序本身即是1个总体向上变浅的沉积相序列，其相序组构与三级层序相似，三级层序的相序组构与那些由岩相序列所组成的若干环潮坪型米级旋回也具有相似性。因此研究区娄山关群白云岩构成了1个复杂而有规律的、独特的层序地层格架，并由此反映了沉积物的时间演变序列和沉积相的空间展布特征具有一定的规律性，代表了其古地理演变过程既受制于海侵加深和海退变浅的时间过程，同时又受控于沉积环境水体自北西向南东由浅变深的空间变化过程。娄山关群白云岩的层序地层研究表明：层序地层学研究的核心是沉积物的时间演变序列以及沉积相的空间展布形式所代表的规律性，并根据这种规律性进行地层划分和对比。

Sequence stratigraphic framework and its palaeogeographic setting for the Loushanguan Group dolostones of Cambrian in Upper Yangtze Region

In the Upper Yangtze Region, especially in Guizhou Province and its adjacent are as, the dolostones of the Loushanguan Group of Cambrian were deposited in a shal low-water setting from Middle to Late Cambrian with a thickness of more than 1 000 meters. From northwest to southeast, they gradually changed into open-platf orm limestones and shelf micrites with water gradually deepening. Because of the poorly developed fossils, it is difficult to make a detailed chronostratigraphi c subdivision. The development of pseudomorphic halite crystal dolostones reflec ts that the water of the sedimentary environment was shallowing and the developm ent of grain dolostones reflects that the water was deepening. These features sh ow that sequence stratigraphic division can be done on this set of dolostones by the method from lithofacies succession to sedimentary facies succession, which represents the "sedimentary facies shifting under the framework of sea level ch anging". Thickly-bedded and poorly fossiliferous dolostones of the Loushanguan Group contrast sharply with the thinly-bedded fine-grained mixed deposits wit h trilobite fossils of the Douposi Formation, and they make up a second-order s equence that can further be divided into seven third-order sequences in the stu dy area. The fine-grained deposits of the Douposi Formation represent the water deepening by the second order transgression, while the poor fossiliferous dolos tones of the Loushanguan Group progradated from northwest to southeast and repre sented a water shoaling process by the second order regression. The second order sequence itself is an upward shoaling sedimentary facies succession. The facies succession fabric of the second order sequence is similar to that of the third order sequence, and the fabric of the latter is similar to those of the meter-s cale cyclic sequences of zonal tidal flat which are composed by lithofacies succ essions. The Loushanguan Group dolostones in the study area constitute a complex , regular and distinct stratigraphic framework. It reflects that there is some r egularity in the evolutionary succession of the sediments and their distribution characters. Its palaeogeographic evolution is controlled by both the deepening of transgression and the shoaling of regression, and at the same time, is contro lled by the deepening of water depth from northwest to southeast. The study resu lts of the Loushanguan Group dolostones show that the core of sequence stratigra phy is the regularity represented by the evolutionary succession and the distrib ution pattern of the sediments, and making a stratigraphic division and correlat ion according to the regularity.