黔桂地区泥盆系层序地层格架及相对海平面变化

志留纪末期的加里东运动,使黔桂地区的古地理格局发生了重大变化,早古生代的滇黔桂古陆演变为晚古生代的滇黔桂海。泥盆纪初期,海水由南而北逐渐海侵超覆,在泥盆系的底部形成一套海侵碎屑岩系地层;早泥盆世晚期的大规模海侵事件造成了明显的相分异,在习称的滇黔桂盆地中发育了若干孤立碳酸盐台地,这种“台-盆-丘-槽”的格局一直延续到晚泥盆世早期;而泥盆纪末期的紫云运动所造成的地壳抬升,又使晚泥盆世地层产生海退尖灭。因此,该地区的泥盆系构成一个二级构造层序,其中包含13个三级沉积层序。在前人大量生物地层学研究的基础上,追索和对比不同沉积背景中的三级沉积层序的相序组构特征以及由其所代表的相对海平面变化,即可建立黔桂地区泥盆系的层序地层格架。层序地层格架较为充分地表现出三级沉积层序的两大特征--空间上相序的有序性和时间上环境变化的同步性,同时也明显地显示出地层记录中的两种相变面和两种穿时性。

Devonian Sequence Stratigraphy and Relative Sea-level Changes in Guizhou and Guangxi Area, South China

There is a great change of paleogeography setting resulted from the Caledonian orogeny at the end of Silurian in Guizhou and Guangxi area in south China, the Dianqiangui oldland in early Paleozoic evolved as the Dianqiangui sea in late Plaeozoic. In the initial stage of Devonian Period, seawater is onlaped gradually from the south to the north caused by transgression, so the bottom part of Devonian system is constituted by a set of transgressive clastic sedimentary rock. The great scale transgressive event at the end of the early Epoch of Devonian forms evident differentiation of depositional facies in Guizhou and Guangxi province, a lot of isolated carbonate platforms are developed in Dianqiangui Basin, the special paleogeographic setting marked by "platforms-basins-hills-trough" is formed and lasted to the early stage of the late Epoch of Devonian. The strata of the upper Epoch of Devonian become regressive thin out from paleosea to oldland resulted from the Ziyun epeirogeny that took place in the transitional period from Devonian Period to Carboniferous Period. On the basis of the study on the evolutional natures of sediments in time and the distribution characters of sedimentary facies in space, 13 third-order sedimentary sequences are discerned in Devonian strata in Guizhou and Guangxi province, which could be grouped into one second-order tectonic sequence. In terms of the correlation on biostratigraphy, the sea-level changes delegated by third-order sequences with different architecture natures of facies-succession also could be correlative in research region, in the end, the framework of sequence stratigraphy could be constructed. Many characters such as two elementary natures of third-order sequences--“the regularity of facies-succession in space and the synchronism of the changes of sedimentary environment in time”, two kinds of diachronism that are relative to two types of facies-changing surfaces--“the diachronism of facies-changing surfaces caused by static facies-changing surface and the diachronism of punctuated surfaces formed by dynamic facies-changing surfaces" in strtigraphic records, are clearly demonstrated in the framework of sequence stratigraphy.