The Magnetic Properties of Lake Sediments from Dongying Depression and Their Relation to the Change of Environment
摘要: 本文应用环境磁学方法，测量了东营凹陷牛３８井中第三纪湖相沉积物（以泥、页岩为主）的磁化率值，并初步研究了造成沉积物磁化率值变化的沉积环境因素。研究结果表明：沉积物磁化率值的变化，与水体的含氧量有直接关系，是沉积物早期成岩过程中活化铁稳定性的反映。一般地，当水体富氧时，沉积物中活化铁稳定，激化率值高。反之，则活化铁易向黄铁矿转化，造成磁化率值的降低。Abstract: There are lot of Tertiary lake sediments along the east part of China. Many of them are composed of muds and shales and have high oil-bearing potentialities. To understand the genesis of those sediments, a new magnetic method drawn from environmental magnetism has been introduced. Also, feasibility in applying it to research Tertiary lake sediments has been evaluated. We chose the Niu 38 core composed of red and grey muds and black shales from dingying Depression as research material. Susceptibility (Sus.) of those rocks has been measured. Heavy mineral analysis and total iron, reactive iron and total sulfur analyses of the controlling samples have been done to explain the causes of the Sus. properties in those sediments.From the profile of Sus., we can get three interesting facts: the first is that sediments with different colors, shes have their own special Sus. ; the second is that the change rythem of the Sus. is very clear and it can be divided into three main cycles from the bottom to the top; the third is that the Sus. profile is more sensible to the change of muds and shales than resistivity logs.Heavy mineral analysis shows that red muds with high Sus. are composed chiefly of haematites; grey muds with low Sus. contains less haematites and a bit of pyrites ; black shale with low Sus. mainly pyrites.DOP index suggests that the red muds formed under oxygen water and black shell in euxinic environment.Maeanwhile the grey muds form in a condition of high rate of sedimentation. So we can conclude that the shift of reactive iron to pyrites in the sediments under different redox conditions is the main controlling factor of change of Sus. Then the link between the change of Sus. in Tertiary sediments and their environmental significance can be set up. It is possible for us to predict sedimentary conditions through analyses of Sus. logs.
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