Previous   |    Next

doi:10.2204/iodp.proc.338.103.2014

Paleomagnetism

Holes C0002K and C0002L

Remanent magnetization of archive-half sections from Holes C0002H and C0002J–C0002L were measured at demagnetization levels of 0, 5, 10, 15, and 20 mT peak fields to identify characteristic remanent magnetization. Profiles of declination, inclination, and intensity after demagnetization at 20 mT with depth (mbsf) are shown in Figure F111.

Inclinations of archive sections in the interval of Holes C0002K and C0002L (200–500 mbsf), which were mostly cored using the ESCS (only the top 5.5 m and the interval from 205.5 to 239 mbsf of Hole C0002K were cored using the HPCS and EPCS, respectively), are significantly biased toward the positive side. However, because the results of Holes C0002B and C0002D during Expedition 315 (Expedition 315 Scientists, 2009a, 2009b) revealed that the interval of 160–490 mbsf at Site C0002 ranges from 1.078 to 1.24 Ma, all the interval of Holes C0002K and C0002L should correspond to the middle part of the Matuyama reversed polarity interval, and the inclinations are expected to be in the negative side. Thus, we suspected that the predominant positive magnetization of the archive sections is due to a modification of the initial paleomagnetic record.

In order to examine the magnetic nature of the interval, discrete samples were carefully collected from consolidated biscuit pieces, not from the softer sediment, which is probably a mixture of sediment and drilling slurry. Magnetic grain fabric of the interval was measured by the anisotropy of magnetic susceptibility (AMS) apparatus to detect any indication of coring disturbance. AMS results show clear magnetic foliations parallel to the horizontal plane (Fig. F112). It is interpreted that grain fabrics were formed by a natural vertical compaction and have no evidence for coring disturbance. The discrete samples were demagnetized with a higher level than those on the archive-half sections. Demagnetization experiments on the discrete samples reveal that magnetizations of samples are stable (Fig. F113), and a substantial number of discrete samples show negative inclination (Fig. F111). This fact indicates that the original magnetization remains in biscuit pieces and suggests that a significant amount of softer sediment was magnetized strongly with positive inclination during coring. Although this situation makes it difficult to interpret magnetic data of archive sections, magnetic polarity interpretation based on data from discrete samples is still valid. According to the results of Expedition 315, the interval of Holes C0002K and C0002L should correspond to the middle part of the Matuyama Reversed Chron. The normal polarity interval observed between 240.72 and 299.37 mbsf seems to be assigned to the “Cobb Mountain” Subchron (1.173–1.185 Ma). However, the nannofossil event of 1.04 Ma is found in the interval at ~250 mbsf in Hole C0002K (see “Biostratigraphy”). It is, therefore, more reasonable that this normal polarity interval is correlated to the Jaramillo Subchron, although the horizon at 119.58 mbsf in Hole C0002D is interpreted to be the top of the Jaramillo Subchron (Expedition 315 Scientists, 2009b).

Magnetic fabric of Hole C0002J

Paleomagnetic inclinations of Hole C0002J are mostly positive but widely scattered. AMS, an index of sediment grain fabric, shows that sediment in Holes C0002K and C0002L are compacted subvertically (Fig. F112). On the other hand, AMS of Hole C0002J appears more prolate (Fig. F114A), suggesting that grain fabrics in this interval did not form by vertical compaction alone. Restored AMS directions of the interval with paleomagnetic declinations reveal that magnetic foliations gently dip southeastward (Fig. F114B). Preliminary interpretation of those data indicates bedding planes in this interval gently dip southeastward.

Top of page   |    Previous   |    Next