The Study Of Rock-magnetism And Natural Remanent Magnetization Of The Ordovician Sedimentary Rocks In The Southwest Ordos Basin,China

The southwestern margin of the Ordos Basin is one of the areas with the most developed,the most complete exposure and high continuity of the Ordovician in the north of China.The researchers have made great progress in the tectonic evolution and the prospects of oil and gas of Ordos basin.However,the study on the palaegeographic location of the Ordovician for Ordos is still blank,and the main reason is that the Paleozoic rocks have weak remanence,and a majority of the rocks have undergone late changes,making it difficult to obtain primary remanence.In this paper,the Qishan section and the Pingliang section of southwestern of the Ordos,with ideal sedimentation,continuous stratum and large sedimentary thickness of the Ordovician marine carbonate rocks and clastic rocks,are selected to carry out paleomagnetic and rock-magnetism studies.The purpose is to identify and discuss the remanence magnetism characteristics and magnetic carrying minerals of Ordovician rocks in the study section,to lay a foundation for further research of paleomagnetism and magnetostratigraphy.This paper draws the following results:(1)The magnetic bearing minerals of the Qishan section are mainly maghemite and magnetite;and magnetic bearing minerals of Pingliang section are maghemite,magnetite,goethite and hematite.The size of magnetic mineral in research section is dominated by single-domain particles and multi-domain particles,and a small amount of superparamagnetic particles.(2)The intensity of natural remanent magnetization for research sections is relatively low.The characteristics of remanence magnetization show that these are low temperature component(NRM-150 °C),medium temperature component(150-350 °C)and high temperature component(350-580/650 °C)in the Qishan section and the Pingliang section,carried by goethite,maghemite and magnetite respectively.Moreover,the high temperature components of rocks in Pingliang section are carried by magnetite and a small amount of hematite.(3)According to the remanent magnetic direction of different components in geographical and stratigraphic coordinates,and calculating the corresponding paleomagnetic pole position,compared with the paleomagnetic pole position of the North China block obtained by the predecessors,we obtain that the low temperature component is the viscous remanence of the modern geomagnetic field,and the medium temperature component is the secondary remanence.However,the high temperature component is more complicated,which may be remagnetized for the Majiagou Formation and the Pingliang Formation in two sections.The remagnetization may be related to the exhumation of Ordos Basin in the Middle Permian and the collision between the North China plate and the Yangtze plate in Triassic.The primary(characteristic)remanence may be existed in Yili formation and Liangjiashan formation,at the lower part of the section.