鄂尔多斯地区奥陶纪构造古地理演化与沉积充填特征

鄂尔多斯地区在奥陶纪处于华北克拉通板块的西南边缘地区。由于受秦岭—祁连—贺兰三叉裂谷系开裂—聚合作用的影响,奥陶纪鄂尔多斯地区与华北克拉通已开始出现明显构造与沉积作用的分异,突出表现为奥陶纪沉积期鄂尔多斯东部大规模膏盐岩沉积层的发育。古构造分析表明鄂尔多斯地区奥陶纪总体呈现为“三隆—两坳—一古陆”的古构造分布格局;奥陶纪经历了冶里—亮甲山期的早期边部海侵、马家沟期振荡性的整体沉降海侵以及平凉—背锅山期的西南边缘快速沉陷的古地理演化过程,表现出较强的阶段性演化特征。鄂尔多斯地区奥陶纪整体的沉积充填作用具有以下重要特征:一是受中央古隆起控制、东西向沉积分异明显;二是随时代演进,早中期以内源沉积为主,晚期则以混源或陆源碎屑沉积为主;三是西南边缘沉积巨厚,是奥陶纪最为活跃的构造沉降区。

Tectonic palaeogeography evolution and sedimentary filling characteristics of the Ordovician in the Ordos area

The Ordos region was located in the southwestern margin of the North China Craton plate in the Ordovician. Due to the dehiscing-converging effects of the Qinling-Qilian-Helan three-armed rift system,obvious tectonic sedimentary differentiation between the Ordos area and the North China Craton had begun to occur in Ordovician. The prominent expression is the large-scale development of evaporite deposits in the eastern part of Ordos area during the Ordovician depositional period. Palaeotectonics analysis shows that the Ordos area was generally characterized by the palaeotectonics distribution pattern of “three uplifts-two depressions-one archicontinent” in the Ordovician. During the Ordovician,it experienced sevaral palaeogeography evolution processes, respectively the marginal transgression in the early Yeli-Liangjiashan period,the oscillating whole-subsidence transgression in the Majiagou period,and the southwestern marginal fast subsidence in the Pingliang-Beiguoshan period, which is characterized by a strong evolution. The sedimentary-filling effect of the Ordos area in the Ordovician has the following important features: Firstly,being controlled by the central palaeo-uplift,the east-west sedimentary differentiation is obvious; secondly,with the evolution of the times,the inner-source sedimentation is dominant in the early and mid-term,while the mixed source or terrigenous clastic deposition is dominant in the late stage; thirdly,the deposits in the southwestern margins are hugely thick,where the structural subsidence is the most active during the Ordovician.