A Method for Core Reorientation Based on Rock Remanent Magnetization:Application to Hemipelagic Sedimentary Soft Rock

Abstract

In-situ orientations of drilled core samples are valuable for a broad area of geological and geophysical researches (e.g. the determination of in-situ principal stress directions or the analyses of geological structures). In many cases, however, the information on in-situ orientation of core samples is not available. In this paper, a method for restoring the in-situ orientation of drilled whole-round core samples and its application to hemipelagic ocean sedimentary rocks are presented. The method is based on natural remanent magnetization (NRM) of rocks. As a case study, we applied this method to the 15 oceanic sedimentary soft rock samples collected at the toe of the Nankai trough, SW Japan, during International Ocean Discovery Program (IODP) Expedition 370. We developed a new sample preparation procedure which enabled us to get more data and then to evaluate the NRM measurement results well. Some samples were not able to be reoriented due to magnetic overprints associated with the drilling operations. In order to evaluate the magnetic overprints caused by drilling operation and to assess the data quality of the core reorientation by this method, we propose an evaluating system including three ranks defined for the different levels in which NRM results were affected by drilling operations. This evaluating system could be useful to assess the data quality of core reorientation by NRM method in the other similar applications.