鄂尔多斯西缘中晚奥陶世大坪阶—艾家山阶岩相古地理

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

鄂尔多斯西缘地区位于华北板块的边缘,在中晚奥陶世经历了明显的构造转型,其大陆边缘的性质由被动大陆边缘转向活动大陆边缘。构造属性的改变也导致了盆地性质的转换,鄂尔多斯西缘沉积盆地的性质从大坪期—达瑞威尔期被动陆缘盆地向艾家山期的弧后盆地转换。岩相古地理研究表明,中晚奥陶世该地区的沉积类型可划分出滨岸相、潮坪相、缓坡相、蒸发台地相、开阔台地相、陆棚相、斜坡相和盆地相,其中的陆棚相还可以分出碳酸盐岩陆棚、碎屑岩陆棚以及混积陆棚相。大坪期—达瑞威尔期以稳定的陆表海沉积为主,沉积一套陆棚相的碳酸盐岩夹碎屑岩。进入晚奥陶世的艾家山期,在加里东期构造运动的不断挤压抬升影响下,海水逐渐退出鄂尔多斯盆地地区,贺兰地区发生了强烈的凹陷,形成了一个相对封闭的以黑色泥页岩为主深水盆地,为该时期的一个重要烃源岩沉积区。

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-are 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, elastic shelf and mixed carbonate- elastic 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.