Analysis on Ordovician Period Stratigraphic Framework in Xiangshan-Qinglongshan Area, Ningxia

Regional data indicate that Xiangshan-Qinglongshan area was a shallow open mesa in the early Ordovician Period,and they accepted the platform facies carbonate deposition.To early Ordovician later period,because the Helan aulacogen crack fell again,the ancient geography appearance and the environment of deposition in this area had the remarkable change,the western Xiangshan region turned into continental slope-times of deep-sea,deep-sea basin,and the water depth of the eastern (Qinglongshan region) had more significantly increased than previous period,but it still inherited nature of the early plat form.By comparating and analyzing the charact eristics of Ordovician sediment ary strata,sedimentary facies and biot a period of Xiangshan,Miboshan,Niushou Hill,Luoshan and Qing long shan etc.,and based on stratig raphic structure and identification of key interface,we divide the Ordovician period stratumint of our depo sitional system domains: the early highstand systems tract (HST₁),low stand wedge (LSW),the transg ressive sytemstract (TST),and the late highstand systems tract (HST₂),of which,HST₁ and LSW are separated by Ⅰ type unconformity (SB1); TST and HST₂ are separated by hunger parag raph (SS).