**4.3. Re-deposition experiment and the origin of AMS**

cally alternating facies in channel-levee systems. Thus, the sedimentological context of muddy sediments' AMS fabric can be interpreted in the light of sandy sediments' facies analysis.

248 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

**Figure 16.** AMS paleocurrent indicators of the Kawabata Formation. Directions of K1 (gray arrows) are shown as acute angles from the dotted baseline of K3 axis imbrication. Vertical positions of the data are based on the T parameter. Samples with negative T values are excluded from the diagram because such cases have a large scatter in the K3 direc‐

Azimuths of AMS maxima in natural sediments vary significantly, reflecting the size or shape of magnetic grains and changes in current velocities (e.g., [18]). Figure 16 presents the rela‐ tionship between paleocurrent proxies estimated from the imbrication of the AMS minimum axis (K3) and the K1 trend. Tarling and Hrouda [19] stated that the angle between K3 and K1 changes as a function of current velocity and the slope of the sedimentary surface. Our result suggests that the orientation between those AMS sedimentary indicators can vary, regardless of the level of hydraulic forcing, based on the shape parameter (*T*), which implies development

tions

In order to consider the origin of the AMS in the Kawabata samples, we organized a redeposition experiment. A silty sandstone (SP1C-1) and a mudstone (SP2F-1) samples were crushed and sieved into coarse, medium and fine fractions. The fine fraction (< 63 μm) was then separated into magnetic and non-magnetic fractions with an isodynamic separator. The 'magnetic' fraction actually contained no ferromagnetic opaque minerals such as magnetite, but had abundant biotite and common hornblende. It also contained garnet, probably derived from metamorphic rocks exposed around the hinterlands during the rapid deposition of the Miocene turbidite.

A suspension of the fine fraction was poured into a vertically settled plastic tube 1 m in length and 2.5 cm in diameter, filled with water. This deposit of artificial sediment was dehydrated at room temperature. After being soaked in an adhesive resin, the samples were trimmed into standard-sized specimens for rock-magnetic measurements. The AMS was measured with an AGICO KappaBridge KLY-3 S magnetic susceptibility meter. The AMS parameters for the artificial samples are summarized in Table 4.

Figure 17 presents the magnitudes of magnetic fabrics in natural sedimentary rocks and the re-deposited sediments of the Kawabata Formation. Obviously, the magnetic separation results in remarkable decrease of both the bulk susceptibility and the degree of anisotropy (PJ ). It is also noteworthy that the shape parameter (*T*) of the artificial sediments is almost null, suggesting a neutral magnetic fabric. The directions of the principal AMS axes (see Table 4) are not bound to the horizontal plane or to geomagnetic north. Thus, the detrital particles, free from paramagnetic minerals having shape anisotropy, like platy biotite, are deposited without any gravitational or geomagnetic forcing, creating an isotropic sediment.


N is the number of specimens. Directions of principal axes of AMS are shown in *in situ* coordinates.

**Table 4.** AMS parameters of re-deposited non-magnetic fine fraction of the Kawabata Formation

bata Formation indicated a westward current direction with minor southward flow contribu‐ tions, consistent with a sedimentary model that envisions burial of the Miocene N-S foreland basin by clastics derived from the eastern collision front. The intensity of alignment forcing of sedimentary particles inferred from the shape parameter (*T*) of the AMS data was closely related to sedimentary facies observed in the field. In investigating the origin of the AMS fabrics of turbidite deposits of the Kawabata Formation, we conducted a re-deposition experiment of fine detrital particles with no magnetic fraction including paramagnetic minerals with relatively high magnetic susceptibility, which demonstrated the significance of

Rock Magnetic Properties of Sedimentary Rocks in Central Hokkaido — Insights into Sedimentary and Tectonic…

http://dx.doi.org/10.5772/56650

251

The authors are grateful to N. Ishikawa for the use of the rock-magnetic laboratory at Kyoto University and for thoughtful suggestions in the course of the magnetic analyses. We thank S. Oshimbe and S. Nishizaki for their help with field work. Thanks are also due to N. Yamashita and Y. Danhara for their support in mineral separation. Constructive review comments by G.

the alignment of paramagnetic minerals having shape anisotropy.

Kawakami greatly helped to improve early version of the manuscript.

1 Graduate School of Science, Osaka Prefecture University, Osaka, Japan

Japan, Part 1: Hokkaido. Tokyo: Kyoritsu Shuppan; 1990.

3 JAPEX Research Center, Japan Petroleum Exploration Co. Ltd., Chiba, Japan

[1] Editorial Committee of Hokkaido, Regional Geology of Japan. Regional Geology of

[2] Tamaki M, Itoh Y. Tectonic implications of paleomagnetic data from upper Creta‐ ceous sediments in the Oyubari area, central Hokkaido, Japan. Island Arc 2008; 17:

\*Address all correspondence to: itoh@p.s.osakafu-u.ac.jp

2 Japan Oil Engineering Co. Ltd., Tokyo, Japan

and Osamu Takano3

**Acknowledgements**

**Author details**

**References**

270-284.

Yasuto Itoh1\*, Machiko Tamaki2

**Figure 17.** Magnitudes of magnetic fabrics in natural samples and re-deposited non-magnetic fine particles of the Ka‐ wabata Formation
