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

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.