Lithofacies-paleogeography of middle-late Ordovician Daping stage-Aijiashan stage on the western margin of the Ordos Basin

Abstract: The western Ordos Basin and its adjacent areas, which belong to the west of north China plate, experienced a transformation process from passive continental margin to active continental margin. The transformation of tectonic attributes also led to the changes of the basin’s nature: the passive continental margin basin in the western Ordos was converted into back-arc basin during the period of Daping stage-Aijiashan stage. According to the study of lithofacies-palaeogeography in west Ordos Basin, 8 depositional facies can be recognized in the middle-late Ordovician period: shore, tidal flat, gentle slope, evaporative platform, open platform, shelf, slope and basin. The shelf could be classified into 3 sub-facies，i.e., carbonate shelf, clastic shelf and mixed carbonate-clastic shelf. A set of shelf carbonate rocks of clastic rocks was deposited in the western Ordos Basin during Daping stage-Darriwilian stage. Nevertheless, in the Aijiashan stage, Helanshan region subsided intensively and turned into a closed, black shale basin, with the influence of the Caledonian tectonic movement and the gradual seawater withdrawal from the Ordos Basin. It is therefore held that the black shale basin might have been an important depositional area of hydrocarbon source rocks in this stage.