贺兰山中段早古生代地层旋回层序研究

CYCLIC SEQUENCES OF THE EARLY PALAEOZOIC STRATA IN THE MIDDLE PART OF THE HELAN MOUNTAIN AREA

关键词:  贺兰山  沉积相  沉积体系  旋回层序

Key words:  Helan Mountain  sedimentary facies  depositional system  cyclic sequence

摘要:

研究区位于鄂尔多斯西缘凹陷带的贺兰山中段地区,寒武—奥陶系地层出露齐全。作者在综合了野外特征和室内分析的基础上,将该区沉积地层划分为三大沉积体系和十四种沉积相类型,并分述了每一沉积相类型的岩性组合及其古生物化石、沉积构造和沉积相序特征,进而对区内早古生代地层的旋回层序进行了分析,认为从早寒武世开始至早奥陶世结束为一个Ⅱ级旋回层序,其中包括四个Ⅲ级海平面变化周期;从中奥陶世开始至其结束。代表另一个独立的受古构造因素控制的Ⅱ级旋回层序,其中每一内源重力流单元与陆源重力流单元的规则变化则相当于一个Ⅲ级旋回层序。

Abstract:

The study area lies in the middle part of the Helan Mountain area in the depression zone on the western margin of the Ordos block,where the Cambro-Ordovician strata are well exposed. Three depositional systems and fourteen sedimentary facies types have been distinguished for the sedimentary strata in this region. The lithologic associations and fossils,sedimentary structures and facies sequences as well as cyclic sequences of the Early Palaeozoic strata are also discussed in the present paper. The study of the Palaeozoic strata shows that one second-order cyclic sequence began during the Early Cambrian and ended during the Early Ordovician, including four third-order sea-level fluctuation periods, whereas another second-order cyclic sequence controlled by palaeotectonic factors began during the Middle Ordovician and ended at the end of the Ordovician. Morever, the regular alternation of individual intrabasinal and terrigenous gravity-flow units corresponds to a third-order cyclic sequence.