鄂尔多斯盆地西缘北部奥陶纪盆地原型：来自贺兰山和桌子山地区奥陶系的沉积响应

The Ordovician Basin Prototype in the Northwest Ordos Basin:Constraint from the Ordovician Sedimentary Respond in the Helan—Zhuozi Mountains

鄂尔多斯盆地西北缘贺兰山—桌子山地区奥陶纪盆地原型一直存在争议。因此,作为连接盆地与造山带的纽带,鄂尔多斯盆地西北缘奥陶系的沉积响应特点理应引起重视。贺兰山—桌子山地区奥陶系沉积组合普遍存在二元结构,即下部以发育台地相碳酸盐岩为特征,上部发育一套复理石沉积,代表了盆地原型从伸展向敛合发展的不同阶段。本文选取贺兰山—桌子山盆地东、西缘奥陶系两个代表性的剖面作为研究对象,结合当时的古地理背景和区域构造演化特征,通过野外露头的详细描述和岩石薄片矿物组分的鉴定,揭示了桌子山地区奥陶纪沉积环境的演变序列和贺兰山地区樱桃沟组的事件沉积特征。借助本文部分作者在桌子山地区同步采样获取的牙形石化石的最新成果,确定了桌子山地区奥陶系二元沉积组合界线处的精确时限为牙形石生物带Pygodus anserinus结束的位置,时间上为晚奥陶世桑比期最早期,接近中、上奥陶统界线的年龄(458.4±0.9 Ma,ICG 2014),这与前人对鄂尔多斯盆地西缘奥陶纪构造转换面(或是盆山转换面)的时限一致。通过对比研究认为樱桃沟组与其下伏地层构成的被动大陆边缘—复理石沉积序列结构实为前陆盆地序列,代表了贺兰山地区于中奥陶世已进入前陆盆地发育阶段,樱桃沟组复理石沉积序列中多套滑塌角砾岩夹层的存在和显微研究表明阿拉善陆块(?)逐渐靠近华北陆块过程中可能从阿拉善陆块(?)东缘(被动大陆边缘)垮塌的产物。而桌子山剖面奥陶系相同的二元沉积结构,同样对应了Sinclair前陆盆地地层模型的前两个阶段,暗示当时前陆盆地业已影响到了桌子山地区。综合分析表明,鄂尔多斯盆地西北缘奥陶纪构造体制由伸展转换为敛合的时限可能代表了阿拉善陆块与华北板块发生了初始碰撞。

Basin prototype in the Helan—Zhuozi Mountains located in the northwestern margin of Ordos Basin has long been controversial. Therefore, as a link connecting basin and orogen, the characteristics of the Ordovician sedimentary response in the northwestern margin of Ordos Basin should be brought to the forefront. The dual structure of the Ordovician sedimentary assemblages in the northwestern margin of Ordos Basin is ubiquitous——the lithology of the lower part is carbonate rocks of platform facies, while the upper is characterized by a set of flysch deposits, which represents different stages, from extension to aggregation, in the evolutionary process of the basin prototype. Methods： Combined with the Ordovician geographic background and regional tectonic evolution, this paper studies two representative sections in the Zhuozi Mountain and Helan Mountain area, which are located in the northwestern margin of Ordos Basin. The evolution of the Ordovician sedimentary environment in Zhuozi Mountain section and the depositional characteristics of event deposits in Zhuozi Mountain section were revealed through describing the field outcrop in detail, fine depicting the petrology features. Results： This paper ascertains the precise time of the dual structure boundary of the Ordovician sedimentary assemblages by the lights of the simultaneous sampling and getting the latest conodonts biostratigraphy of Zhuozi Mountain section, which corresponds to the location of the end of conodont zone Pygodus Anserinus, earliest Sandbian Stage, approaching the boundary of Middle—Upper Ordovician age （458.4 ± 0.9 Ma, ICG2014）. This happens to have the same view on the time of prototype conversion of Ordos Basin. Comparative study indicates that the Yingtaogou Formation and its underlying stratum constitute a passive continental margin—flysch sequence structure. As a typical foreland basin sequence, it represents the corresponding strata during the formation of foreland basin during the Middle Ordovician time. Several sets of slump breccia in the flysch sequence of the Yingtaogou Formation and microscopic studies show that these slump breccias may be the result of the collapse from the eastern margin of Alxa Block （passive continental margin） during the Alxa Block（？） gradually close to the North China Craton. And the Ordovician dual structure of Zhuozi Mountain section corresponds to the the first two stages of Sinclair foreland basin formation model, which suggests Helan Mountain area was affected by the foreland basin at that time. Comprehensive analysis showed that the Ordovician tectonic regime of the northwest margin of the Ordos Basin changed from extension to aggregation, and this time limit of the transform may represent the initial collision between the Alxa Block（？） and the North China Block.Conclusions：The northeastern margin of the Ordos Basin probably constitute a foreland basin system formed during the collision between the Alxa Block（？） and the North China Block during the Ordovician. And this foreland basin system occurred at the early Sandbian Stage, Middle Ordovician, which may represent the initial collision between the Alxa Block（？） and the North China Block.