The Ordovician Lithofacies Paleogeographical Research Of Qishan Region In Southwest Ordos

Qishan region lies on the south edge of Ordos. Teconically it situated on the connexion of Helan aulacogen and Qingqi geosyncline, west section of Weibei uplift. During the Ordovician, the region deposited marine carbonate rocks. However, the former research of this place are faultiness, the main existing problems are like:The reason why lithology of Majiagou formation and Pinglaing formation between Qishan region and its adjacent still have greater differences is not confirmed; From the data of overseas and domestic, it can be ensured that the stratum of Majiagou formation in Qishan region is not divided into sect and lithofacies paleogeography in this place needs more particular and systematic research and expatiation.Aiming at these problems, under the guidance of sedimentology, tectonics, sequence-stratigraphy and geophysics, on the basic of former investigation and combined with exploration of field sections, sampling, slice identification and data collection of paleontology and well logging, the paper did the research of rock combination, sedimentary construction, lithofacies paleogeography evolvement and deposit mode in Ordovician.In Ordovician period, no matter transgression or regression, Qishan region was always under the sea-level, belonged to epicontinental sea deposition. In Early Ordovician Yeli-Liangjiashan period, the region was in restricted marine platform facies which was mainly of dolomite deposit. In Middle Ordovician Majiagou period, here was a suit of limestone belonged to open-sea platform to platform marine slope zone. The lithology in this place didn't compose sedimentary cycles like other region of base and its sorth edge. Therefore, the paper divided Majiagou formation into lower Majiagou formation, upper Majiagou formation and Fengfeng formation. In Late Ordovician Pingliang period, Huabei mesa was wholly uplifted because of Caledonian movement.From this period Ordos ancient continent became much larger, terrigenous matter mainly of calcareous mudstone and shale were manifolded in Qishan region. So in this period, research area was in platform margin slope zone. In Beiguoshan period, Qishan region mainly deposited breccia dolostone, pebbly mudstone, conglomerate and some sandstone, developed debris flow and turbidity current which belonged to foreslope facies of platform deposit. Till this period, Qishan region mainly experienced three sedimentary environment like restricted sea platform—open sea platform—foreslope facies of platform, two regression period like Yeli-Liangjiashsan, Pingliang-Beiguoshan period and Majiagou period transgression. Synthesizing above all outcomes, the paper finally built carbonate sedimentary mode of Qishan region and its adjacent area.