MAGNETIC SUSCEPTIBILITY AS INDICATORS OF SEDIMENTARY ENVIRONMENT DURING UPPER JURASSIC AND LOWER CRETACEOUS IN JIANMENGUAN, SICHUAN PROVINCE
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摘要: 通过对四川剑门关地区上侏罗统—下白垩统磁化率的研究, 发现磁化率的大小与沉积物颗粒的粗细有着紧密的关系.中粗粒砂岩的磁化率通常偏小, 而细砂岩、粉砂岩及泥岩的磁化率明显增大.由于沉积物颗粒的粗细在一定程度上反映了不同的沉积速率, 因此, 磁化率大小的变化可作为沉积速率变化的定量指标.而沉积速率的变化与沉积环境及沉积相之间有一定的联系, 所以, 可认为磁化率的大小可作为沉积相及沉积环境分析的一种参考.另外, 通过剑门关地区上侏罗统—下白垩统的磁化率各向异性研究发现, 本区磁化率最大主轴方向可分为北东—南西向及北西—南东两组.北西—南东方向的磁化率主轴与当时的古水流方向一致, 而北东—南西方向的磁化率主轴则可能与后期的构造挤压有关Abstract: It is found that the values of magnetic susceptibility have apparent relations with the size of sedimentary particles based on the study of magnetic susceptibility of Upper Jurassic and Lower Cretaceous in Jianmenguan, Sichuan Province. Sediments with medium or coarse grained particles usually have low values of magnetic susceptibility, while the fine grained sandstone, silt and mudstone often appear to have much higher values. As sedimentary rate has something to do with the size of sedimentary particles, this susceptibility can be used as a quantitative indicator for the variation of sedimentary rate. What's more, the sedimentary rate usually varies in different environments, so the magnetic susceptibility can also be used as reference of sedimentary facies. The study of magnetic susceptibility anisotropy in Jurassic and Cretaceous shows that there exit two maximal axis directions of magnetic susceptibility in the study area of Jianmenguan. The southeastward maximal axis direction corresponds to the direction of paleocurrent in the area. However, the southwestward maximal axis direction is probably caused by tectonism.
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Key words:
- magnetic susceptibility /
- Upper Jurassic /
- Lower Cretaceous /
- Sichuan Province
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图 1 四川剑门关剖面晚侏罗世至早白垩世红层磁化率与沉积物粒度之间的关系
Fig. 1. Relations between magnetic susceptibility and sedimentary particles in J3-K1 red clastic rocks in Jianmenguan section, Sichuan Province
表 1 剑门关地区上侏罗统-下白垩统平均磁化率(κ)
Table 1. Average magnetic susceptibility of Upper Jurassic and Lower Cretaceous in Jianmenguan, Sichuan Province
表 2 磁化率各向异性
Table 2. Magnetic susceptibility anisotropy
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