鄂尔多斯盆地中南部上古生界层序与岩相古地理演化*

为明确鄂尔多斯盆地中南部上古生界层序特点与岩相古地理演化规律,利用周缘野外露头和盆地钻井测井相特征,分析层序界面、体系域界面的岩性、古构造及海侵方向变化特征,总结层序发育特点与岩相古地理演化规律。结果表明: 不同风化序列的区域性不整合面及海侵方向转换面为二级层序界面,区域性海退面、下切冲刷面及陆上暴露面为三级层序界面; 潮间带砂坪及近岸相海侵含砾砂岩顶为海侵面,最大海侵面发育灰岩、泥页岩及煤层,是海侵体系域与高位体系域分界面; 上古生界包括二级层序2个: MSQ1、MSQ2,三级层序6个: SQ1、SQ2、SQ3、SQ4、SQ5、SQ6,其中SQ1—SQ2发育水进体系域与高位体系域,不发育低位体系域,SQ1为潟湖—障壁海岸沉积体系,SQ2为泥炭坪—泥坪相潮坪沉积;SQ3—SQ6发育完整的低位—海侵—高位体系域,SQ3发育区域性海退进积海陆过渡相三角洲沉积,SQ4早期为低位体系域下切冲蚀砂体,晚期沉积古环境由温暖湿润还原环境演变为炎热干燥的氧化环境,SQ5—SQ6早中期为氧化环境三角洲沉积,SQ6晚期为高位体系域具海侵夹层的潮坪相沉积。研究为鄂尔多斯盆地及其他盆地层序与岩相古地理演化提供理论依据。

Sequence stratigyaphy and lithofacies palaeogeography evolution of the Upper Paleozoic in central-southern Ordos Basin

In order to classify the stratigraphic sequence characteristics and lithofacies palaeogeography evolution of the Upper Paleozoic in the central-southern Ordos Basin,the development and variation characteristics of lithology,paleostructure and transgression direction of sequence boundary and system tract interface were analyzed by using field outcrops and drilling logging facies in the basin. The characteristics of sequence development and lithofacies palaeogeography evolution were summarized. The results showed that the regional unconformity surface and transgressive direction conversion surface of different weathering sequences were the second-order sequence boundary,and the regional regression surface, scouring surface and subaerial exposure surface are the third-order sequence boundary. The sand flat of intertidal zone and the top of coastal transgression gravelly sandstone were transgression surfaces,and the typical lithology of the maximum transgressive surface were limestone,shale and coal seam,which were the interface between transgressive systems tract and highstand systems tract. The Upper Paleozoic includes two second-order sequences MSQ1 and MSQ2,and six third-order sequences SQ1,SQ2,SQ3,SQ4,SQ5 and SQ6. Among them,SQ1-SQ2 did not develop lowstand systems tract,but only develop transgressive systems tract and highstand systems tract,and SQ3-SQ6 developed complete lowstand-transgression-highstand systems tract;The lagoon-barrier coastal sedimentary system developed in SQ1,the tidal flat sedimentary system developed in SQ2 and the transitional delta sedimentary system developed in SQ3. In the early stage of SQ4,the downcutting sand body developed in the lowstand systems tract. In the late stage of SQ4,the sedimentary environment evolved from warm and humid reduction to hot and dry oxidation environment. In the early and middle stages of SQ5-SQ6,the delta front developed in the oxidation environment,and in the late stage of SQ6,the tidal flat developed in the highstand systems tract with transgression interlayer. The investigation provides a theoretical basis for the study of sequence and lithofacies paleogeographic evolution in the Ordos Basin as well as in other basins.