| The tectonic-sedimentary evolution of Ordos block in Mesoproterozoic has always been concerns of structural and petroleum geologists. On the basis of geochemical characteristics of rocks, researchers speculate about the tectonic background in this period, however this paper is mainly the use of geophysical methods and techniques to analyze the stratigraphic texture and structural characteristics of internal block in Mesoproterozoic. The conclusions are summarized as follows:(1) The Changcheng System can be divided into two parts in internal Ordos block from bottom to top. The lower, corresponding to lower submember of Gaoshanhe Group develop grabens and half-grabens, and sedimentary environments are continental delta, fan delta and underwater fan with limited sedimentary water. Whereas the upper corresponding to middle-upper submember with a wider range of water, sedimentary environments are mainly littoral, shallow sea and barrier island-coast plain.(2) Carbonate platforms develop in internal Ordos block during Jixian Period, and the residual thickness of Jixian System increases gradually from inside to edge of the block. The boundary of sea and land in Jixian Period may be located in well Yutan 1-well Qingshen 1-Yaoxian, and the event deposits in Qishan County may be indicative of platform margin environment. Tectonic landforms formed in Changcheng Period may have had an impact on the carbonate sediments, such as near well Xuntan 1, where the depth of water is great due to uncompletely filled in the Changcheng graben, which inhibit carbonate rocks development in Jixian Period.(3) The Mesoproterozoic gravity lower zones alternate with higher in southern margin of Ordos block. The gravity lower zones with the thick Middle Proterozoic, mainly located in Huangling-Xunyi, Dingbian-North Huanxian, South Suide; while the higher zones with smaller thicknesses of the Middle Proterozoic, mainly located in Wuqi-Jingbian-Yulin, Yanchuan-Yan’an-Qingyang, Heyang-Tongchuan and the western margin of Ordos basin. Because Jixian System distributes along the block margin, and has smaller thickness (between 0-1000 meters), the gravity anomaly mainly reflects variations in thickness of Changcheng System. Combined with seismic interpretation, we deem the gravity low indicate grabens in Changcheng Period, while the gravity high are on behalf of horsts.Through calculating the derivative of horizontal gradient of gravity anomaly on research area, there are 10 regional faults:Qianxian-Heyang, Linyou-North Tongchuan, North Lingtai-Dangjiayuan-Yanchuan, Ganquan-Qingjian, Huachi-Zhidan, Yuanshan-Yinmagou, Zhamuyinwusu-Yangxinzhuang-GaoJiazhuang, North Jingbian-North Yulin, Dashuikeng-East Pingliang and Bainijing-Hongqicun. The fault strikes mainly are NE and control sags and uplifts as synsedimentary faults.(4) The Mesoproterozoic strata texture and tectonic characteristics suggest interal and peripheral block can be affected by post-orogenic extension and form rifts which influence by pre-strcutures in 1.8-1.6Ga, such as Xunyi-Huangling rift zone coincides with the collision belt of microplates in late Archean (2.5Ga). This tectonic weak belt changes into large-scale graben, where filled delta facies clastic rock, littoral and neritic facies carbonates and clastic rocks, and the maximum thickness is more than 8000 meters in Changcheng Period. And then in 1.6-1.4Ga, Kuanping limited oceanic basin forms in the southern margin of Ordos block, and develops carbonate platforms and deposits mainly marine dolomites. At last 1.4-1.0Ga, as the Rodinia supercontinent formation period, Qinling collides with Ordos block resulting to Kuanping ocean close. |
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