Examination of grain fabric inferred from the anisotropy of magnetic susceptibility using the X-ray CT method and the fabric map method

砂岩沉积物的磁化率各向异性(AMS)被认为指示了沉积物的优先颗粒取向。然而，还没有研究证实这一假设。本项目旨在利用X射线CT方法和组构图法研究砂级沉积物AMS的产生原因。首先，采用上述三种方法对平行分层的颗粒组构进行检测。同时考察了床层淤积速率对平行层的形成和颗粒组构的影响。提出了一种新的分析X射线CT数据的方法--基于密度的聚类分析方法，该方法处理颗粒形状和取向的主要组合。结果表明，床层淤积速率对平行层理的特征以及颗粒的形状和取向组合有影响。此外，三种方法得到的颗粒平均掺入角在误差范围内一致。其次，我们检测了水槽实验中常用的“托尤亚石英砂”的矿物组成、化学成分和磁性特征，以确定对AMS有贡献的矿物。结果表明，托尤阿石英砂由石英、斜长石、钾长石、角闪石、石榴石、金红石、黑云母、钛铁矿和磁铁矿组成。钛铁矿颗粒主要贡献了该材料的磁化率。用CT值划分磁性矿物和非磁性矿物，CT值对应于每种矿物的X射线线性衰减系数理论值。三种方法的沉降实验结果表明，AMS显示了磁性矿物的择优取向，而不是块状沉积物的择优取向。颗粒流动实验的样品显示出三种方法不同的分布。为了解决这个问题，需要进行更多的分析，比如对更广泛的X射线CT数据进行分析。

The anisotropy of magnetic susceptibility (AMS) of sand-sized sediments has been believed to indicate the preferred grain orientation of the sediments. No study, however, has been verified this assumption. This project aimed to examine what generate the AMS of sand-sized sediments, using X-ray CT method and the fabric map method.At first, grain fabric of parallel-lamination is examined by three methods mentioned above. At the same time, we examined the effect of bed-aggradation rate on the formation and grain fabric of parallel-lamination. A new method, density-based cluster analysis that deal with the dominant combinations of shapes and orientation of grains, was developed to analyze the X-ray CT data. It is revealed that the bed-aggradation rate effects on the features of the parallel-laminations and the combination of the shapes and orientation of the grains. In addition, averaged angles of grain imblication obtained from the three methods agreed within their error ranges.Next, we examined the mineral and chemical compositions and the magnetic features of the "Toyoura quartz sand", which is commonly used for the flume experiments; to determine the mineral that contribute the AMS. As the results, it is revealed that the "Toyoura quartz sand" consists of quartz, plagioclase, potassium feldspar, amphibole, garnet, rutile, biotite, ilmenite, and magnetite. Ilmenite grains mainly contribute the magnetic susceptibility in this material. Magnetic minerals and non-magnetic minerals were divided by the CT values which correspond to the theoretical values of X-ray linear attenuation coefficient for each mineral. The results obtained by three methods for settling experiments showed that AMS showed the preferred grain orientations of magnetic minerals rather than bulk sediments. Samples those of grain flow experiments show different distribution for the three methods. More analyses such as for wider range of X-ray CT data will be needed to solve the problem.