**3. Rock magnetism**

We obtained samples for rock magnetic analyses exclusively from fine-grained parts of the target sedimentary units, since fine sedimentary rocks generally preserve stable detrital remanent magnetization (DRM). Few visible markers of the sedimentation process accompany such sediments, so we attempted to measure their microscopic magnetic fabric, which may be related to paleocurrent directions (e.g., [12]).

## **3.1. Basic measurements**

thickening or thinning successions, and is interpreted to be sheet-like turbidites with minor occurrences of depositional lobes, which occupied major part of the trough-like foreland basin fill [7,10]. The channel-levee facies association is composed of thick amalgamated sandstone facies with slump blocks and thinly bedded alternating beds of sandstone and mudstone. These two facies appearing coupled is indicative of an elongated channel-levee system made of the main channel with levees on both sides. These two facies associations are believed to have been deposited in an elongated trough-like foredeep in the foreland basin [7]. The

**Figure 3.** Sampling localities for rock magnetic analyses of the Cretaceous and Paleogene strata. The base maps are parts of the "Sunagawa", "Kamiashibetsu", "Okuashibetsu", "Ikushunbetsu" and "Bibaiyama" 1:25,000 topographic maps published by the Geographical Survey Institute. As for the Paleogene sites (a, c and d), geologic units (Yezo, Ye‐ zo Supergroup; Bibai, Bibai Formation; Akabira, Akabira Formation; Ikushunbetsu, Ikushunbetsu Formation), deposi‐

tional sequence and facies classification are shown in parentheses after Takano et al. [5]

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

**Figure 4.** Neogene stratigraphy of the study area of the Kawabata Formation

The Cretaceous and Eocene samples were taken from outcrops along the streambed in central Hokkaido (Figure 3) using an engine or electric drill at 21 sites. Samples of the Kawabata Formation were collected with a battery-powered electric drill at 21 sites along the Rubeshibe River (Figure 2). The bedding attitudes were measured on outcrops to allow us to compensate for tectonic tilting later. Between seven and sixteen independently oriented cores 25 mm in diameter were obtained at each site using a magnetic compass. Cylindrical specimens 22 mm in length were cut from each core and the natural remanent magnetization (NRM) of each specimen was measured using a cryogenic magnetometer (model 760-R SRM, 2-G Enterprises). Low-field magnetic susceptibility was measured on a Bartington MS2 susceptibility meter, and the anisotropy of magnetic susceptibility (AMS) was measured using an AGICO KappaBridge KLY-3 S magnetic susceptibility meter. After the basic measurements, pilot specimens with average NRM intensities, directions and susceptibility levels were selected from each site for subsequent demagnetization tests.

#### **3.2. Demagnetization tests**

In order to isolate stable components of the remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) tests were carried out on two pilot specimens per site that had average NRM directions. The PAFD test loading ranged from 0 to 80 mT using a three-axis tumbling system with specimens contained in a μ-metal envelope. The PThD test was performed using an electric furnace, with a residual magnetic field less than 10 nT, beginning at 100 ºC and continuing until the specimen was either fully demagnetized and a characteristic remanent magnetization (ChRM) component was isolated, or until the thermal treatment provoked erratic behavior of the magnetic direction. Specimens' low-field bulk magnetic susceptibilities were measured using a suscept‐ ibility meter after each PThD step in order to monitor chemical changes in ferromagnetic minerals.

Figure 6 presents typical PThD and PAFD results for the Yezo Supergroup and Ishikari Group. It is obvious that the ChRM direction was not isolated because of unstable behavior in thermal treatment (Figure 6a), overlapping spectra of primary and secondary magnetization (Figure 6b) and partial remagnetization within a site (Figure 6c,d). Therefore further analyses for magnetic granulometry were not applied on the Cretaceous and Eocene samples. On the other hand, PThD treatment was effective for isolating stable ChRM in the sedimentary rocks of the Kawabata Formation. Figure 7 shows typical results of the progressive demagnetization tests.

**Figure 7.** Results of progressive thermal demagnetization for samples of the Neogene Kawabata Formation with sta‐ ble (upper) and unstable (lower) magnetization. All coordinates are geographic (*in situ*). Units are bulk remanent in‐ tensity. The solid and open circles in the vector-demagnetization diagrams (left) are projections of vector end-points on the horizontal and north-south vertical planes, respectively. The solid and open circles in the equal-area Schmidt

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Hysteresis parameters were determined for the Kawabata samples with an alternating gradient magnetometer (Princeton Measurements Corporation, MicroMag 2900). Ten sample chips up to 1 mm in size were randomly selected from site RB16, where stable ChRM has been successfully isolated. Figure 8 displays typical hysteresis of the Kawabata mudstones. The raw diagram seems to suggest the absence of ferromagnetic material. After correcting the linear gradient of paramagnetism, a weak ferromagnetic behavior signature can be recognized. Saturation magnetization (Js), saturation remanence (Jrs) and coercive force (Hc) values were determined for all samples from their hysteresis loops. Their relatively low Hc (~ 100 mT) implies that magnetite is the dominant remanence carrier. After acquiring coercivity of remanence (Hcr) values through backfield demagnetization experiments, we constructed a correlation plot of Jrs/Js versus Hcr/Hc [13] as shown in Figure 9. All the data are plotted in

nets (right) are projections on the lower and upper hemispheres, respectively

the pseudo-single domain (PSD) region of magnetite.

**3.3. Hysteresis properties**

**Figure 6.** Typical results of progressive thermal demagnetization (PThD) and progressive alternating field demagneti‐ zation (PAFD) in geographic coordinates for the Paleogene Ishikari Group (a,b) and the Cretaceous Yezo Supergroup (c,d). On the vector-demagnetization diagrams, solid (open) circles are projection of vector end-points on horizontal (N-S vertical) plane. Equal-area projection and normalized intensity decay curve are shown on the right-side of each vector diagram. Solid (open) circles in equal-area nets are projections on the lower (upper) hemisphere. Numbers at‐ tached on data points are demagnetization levels in °C or mT

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**Figure 7.** Results of progressive thermal demagnetization for samples of the Neogene Kawabata Formation with sta‐ ble (upper) and unstable (lower) magnetization. All coordinates are geographic (*in situ*). Units are bulk remanent in‐ tensity. The solid and open circles in the vector-demagnetization diagrams (left) are projections of vector end-points on the horizontal and north-south vertical planes, respectively. The solid and open circles in the equal-area Schmidt nets (right) are projections on the lower and upper hemispheres, respectively

#### **3.3. Hysteresis properties**

**3.2. Demagnetization tests**

minerals.

In order to isolate stable components of the remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) tests were carried out on two pilot specimens per site that had average NRM directions. The PAFD test loading ranged from 0 to 80 mT using a three-axis tumbling system with specimens contained in a μ-metal envelope. The PThD test was performed using an electric furnace, with a residual magnetic field less than 10 nT, beginning at 100 ºC and continuing until the specimen was either fully demagnetized and a characteristic remanent magnetization (ChRM) component was isolated, or until the thermal treatment provoked erratic behavior of the magnetic direction. Specimens' low-field bulk magnetic susceptibilities were measured using a suscept‐ ibility meter after each PThD step in order to monitor chemical changes in ferromagnetic

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

Figure 6 presents typical PThD and PAFD results for the Yezo Supergroup and Ishikari Group. It is obvious that the ChRM direction was not isolated because of unstable behavior in thermal treatment (Figure 6a), overlapping spectra of primary and secondary magnetization (Figure 6b) and partial remagnetization within a site (Figure 6c,d). Therefore further analyses for magnetic granulometry were not applied on the Cretaceous and Eocene samples. On the other hand, PThD treatment was effective for isolating stable ChRM in the sedimentary rocks of the Kawabata Formation. Figure 7 shows typical results of the progressive demagnetization tests.

**Figure 6.** Typical results of progressive thermal demagnetization (PThD) and progressive alternating field demagneti‐ zation (PAFD) in geographic coordinates for the Paleogene Ishikari Group (a,b) and the Cretaceous Yezo Supergroup (c,d). On the vector-demagnetization diagrams, solid (open) circles are projection of vector end-points on horizontal (N-S vertical) plane. Equal-area projection and normalized intensity decay curve are shown on the right-side of each vector diagram. Solid (open) circles in equal-area nets are projections on the lower (upper) hemisphere. Numbers at‐

tached on data points are demagnetization levels in °C or mT

Hysteresis parameters were determined for the Kawabata samples with an alternating gradient magnetometer (Princeton Measurements Corporation, MicroMag 2900). Ten sample chips up to 1 mm in size were randomly selected from site RB16, where stable ChRM has been successfully isolated. Figure 8 displays typical hysteresis of the Kawabata mudstones. The raw diagram seems to suggest the absence of ferromagnetic material. After correcting the linear gradient of paramagnetism, a weak ferromagnetic behavior signature can be recognized. Saturation magnetization (Js), saturation remanence (Jrs) and coercive force (Hc) values were determined for all samples from their hysteresis loops. Their relatively low Hc (~ 100 mT) implies that magnetite is the dominant remanence carrier. After acquiring coercivity of remanence (Hcr) values through backfield demagnetization experiments, we constructed a correlation plot of Jrs/Js versus Hcr/Hc [13] as shown in Figure 9. All the data are plotted in the pseudo-single domain (PSD) region of magnetite.

**Figure 8.** An example of hysteresis loop for a sample of the Kawabata Formation from site RB16 (Left: raw data, Right: data corrected for slope of paramagnetism)

**Figure 10.** Site-mean ChRM directions of the Kawabata Formation in the study area. The solid and open circles in all the equal-area nets are projections on the lower and upper hemispheres, respectively. Dotted ovals show 95 % confi‐ dence limits. Lower diagrams are polarity-converted for calculating formation mean directions and Fisher's precision

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parameters as annotated in the diagrams (Shaded ovals depict 95 % confidence for the formation means)

**Site Latitude Longitude D I Dc Ic α<sup>95</sup> κ N ϕ λ** RB14 42.7361 142.1771 -167.1 -18.7 151.2 -47.0 21.9 13.1 5 62.5 29.5

RB16 42.7379 142.1793 11.4 2.8 -21.8 42.5 14.0 14.4 9 64.5 13.9

RB17 42.7381 142.1793 26.8 17.4 -17.4 52.7 6.8 66.5 8 73.3 23.1

D and I, *in situ* site-mean declination and inclination before tilt correction in degrees, respectively; Dc and Ic, site-mean declination and inclination after tilt correction in degrees, respectively; α95, radius of 95% confidence circle in degrees; κ, precision parameter; N, number of specimens; ϕ and λ, latitude (N) and longitude (E) of north-seeking virtual

A previous study [15] suggested a clockwise tectonic rotation around central Hokkaido based on a paleomagnetic study of the Kawabata Formation. Takeuchi et al. [16] proposed a coherent rotational model with 'domino-style' rigid crustal blocks. However, Tamaki et al. [17] criticized the block rotation scheme as being overly simplistic based on differential rotations inferred from Oligocene paleomagnetic data. They restored crustal deformation in central Hokkaido using dislocation modeling, and found complicated vertical-axis rotations around termina‐ tions of the faults that contributed to the formation of N-S elongate sedimentary basins. Figure

11 demonstrates differential rotation in central Hokkaido since the middle Miocene.

geomagnetic pole for untilted site-mean direction in degrees, respectively.

**Table 1.** Paleomagnetic directions of the Kawabata Formation

**Figure 9.** Logarithmic plot of hysteresis parameters [13] of ten samples of the Kawabata Formation from site RB16. Abbreviations: SD, single domain; PSD, pseudo-single domain; MD, multi-domain

### **4. Discussion**

#### **4.1. Rotational motions**

We found stable magnetic components at three sites of the Kawabata Formation. Their directions were determined with a three-dimensional least squares analysis technique [14]. Figure 10 and Table 1 present site-mean ChRM directions obtained from the Kawabata Formation. They exhibit antipodal directions, and precision parameter (κ) improves after tilt correction. Although the number of data points is minimal for tectonic discussion, we can interpret the site-mean directions as a record of the Earth's dipole magnetic field, acquired before the strata tilted. The declination of the formation mean exhibits a significant westerly deflection, which suggests counterclockwise rotation of the study area.

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**Figure 10.** Site-mean ChRM directions of the Kawabata Formation in the study area. The solid and open circles in all the equal-area nets are projections on the lower and upper hemispheres, respectively. Dotted ovals show 95 % confi‐ dence limits. Lower diagrams are polarity-converted for calculating formation mean directions and Fisher's precision parameters as annotated in the diagrams (Shaded ovals depict 95 % confidence for the formation means)


D and I, *in situ* site-mean declination and inclination before tilt correction in degrees, respectively; Dc and Ic, site-mean declination and inclination after tilt correction in degrees, respectively; α95, radius of 95% confidence circle in degrees; κ, precision parameter; N, number of specimens; ϕ and λ, latitude (N) and longitude (E) of north-seeking virtual geomagnetic pole for untilted site-mean direction in degrees, respectively.

**Table 1.** Paleomagnetic directions of the Kawabata Formation

**Figure 8.** An example of hysteresis loop for a sample of the Kawabata Formation from site RB16 (Left: raw data, Right:

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

**Figure 9.** Logarithmic plot of hysteresis parameters [13] of ten samples of the Kawabata Formation from site RB16.

We found stable magnetic components at three sites of the Kawabata Formation. Their directions were determined with a three-dimensional least squares analysis technique [14]. Figure 10 and Table 1 present site-mean ChRM directions obtained from the Kawabata Formation. They exhibit antipodal directions, and precision parameter (κ) improves after tilt correction. Although the number of data points is minimal for tectonic discussion, we can interpret the site-mean directions as a record of the Earth's dipole magnetic field, acquired before the strata tilted. The declination of the formation mean exhibits a significant westerly

Abbreviations: SD, single domain; PSD, pseudo-single domain; MD, multi-domain

deflection, which suggests counterclockwise rotation of the study area.

data corrected for slope of paramagnetism)

**4. Discussion**

**4.1. Rotational motions**

A previous study [15] suggested a clockwise tectonic rotation around central Hokkaido based on a paleomagnetic study of the Kawabata Formation. Takeuchi et al. [16] proposed a coherent rotational model with 'domino-style' rigid crustal blocks. However, Tamaki et al. [17] criticized the block rotation scheme as being overly simplistic based on differential rotations inferred from Oligocene paleomagnetic data. They restored crustal deformation in central Hokkaido using dislocation modeling, and found complicated vertical-axis rotations around termina‐ tions of the faults that contributed to the formation of N-S elongate sedimentary basins. Figure 11 demonstrates differential rotation in central Hokkaido since the middle Miocene.

**Site** *N K1 Str. K2 Str. K3 Str. L F P PJ T q* **Unit / Sequence**

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BP01 11 1.010 1.005 0.984 1.005 1.021 1.026 1.028 0.619 0.213 Bb / Isk-2HST

BP02 11 1.009 1.008 0.982 1.001 1.026 1.027 1.031 0.917 0.043 Bb / Isk-2HST

BP03 11 1.010 1.005 0.985 1.005 1.020 1.025 1.027 0.593 0.229 Ik / Isk-4TST

BP04 13 1.010 1.002 0.988 1.008 1.014 1.022 1.022 0.259 0.458 Ik / Isk-4TST

HP01 18 1.010 1.004 0.987 1.006 1.017 1.023 1.024 0.479 0.303 Ak / Isk-3HST

HP02 11 1.007 1.004 0.989 1.003 1.015 1.018 1.019 0.662 0.186 Ak / Isk-3HST

NP01 15 1.009 1.006 0.985 1.003 1.022 1.025 1.027 0.762 0.128 Bb / Isk-2HST

NP02 9 1.013 1.006 0.981 1.006 1.026 1.032 1.034 0.597 0.227 Bb / Isk-2HST

NP03 10 1.006 1.004 0.989 1.002 1.016 1.018 1.019 0.779 0.118 Bb / Isk-2HST

NP04 7 1.009 1.006 0.985 1.002 1.022 1.024 1.027 0.791 0.111 Bb / Isk-2HST

NP05 9 1.007 1.003 0.990 1.004 1.013 1.017 1.018 0.536 0.264 Bb / Isk-2HST

HC01 12 1.007 1.003 0.990 1.005 1.013 1.018 1.018 0.487 0.296

HC02 10 1.011 1.005 0.984 1.006 1.021 1.027 1.028 0.541 0.262

HC03 11 1.010 1.002 0.988 1.008 1.014 1.022 1.022 0.274 0.447

PA01 13 1.006 1.003 0.991 1.003 1.012 1.015 1.016 0.650 0.193

PA04 14 1.017 1.014 0.969 1.004 1.046 1.049 1.055 0.852 0.078

**Table 2.** Site-mean AMS parameters of the Paleogene and Cretaceous units in central Hokkaido

*N* denotes the number of specimens. Directions of AMS principal axes are in stratigraphic coordinates. Abbreviations for the Paleogene geologic units: Ak, Akabira Formation; Bb, Bibai Formation; Ik, Ikushunbetsu Formation. Depositional sequence is

**(D, I) (D, I) (D, I) (***K***1/***K***2) (***K***2/***K***3) (***K***1/***K***3)**

(207, 4) (297, 1) (35, 86)

(226, 3) (136, 4) (0, 85)

(233, 9) (141, 7) (16, 78)

(225, 6) (134, 3) (15, 83)

(271, 4) (180, 8) (29, 81)

(38, 6) (307, 4) (187, 83)

(264, 2) (174, 7) (11, 83)

(252, 10) (162, 2) (59, 80)

(253, 1) (163, 7) (349, 83)

(80, 10) (170, 4) (282, 79)

(39, 28) (141, 21) (261, 54)

(71, 5) (340, 11) (187, 78)

(66, 2) (156, 4) (315, 86)

(51, 4) (321, 11) (163, 78)

(336, 11) (70, 19) (217, 68)

(8, 9) (99, 11) (240, 76)

after Takano & Waseda [4] and Takano et al. [5].

*Paleogene*

*Cretaceous*

**Figure 11.** Comparison of the mean paleomagnetic directions of the Kawabata Formation in central Hokkaido be‐ tween this study and [15]. Data are plotted on the lower hemisphere of the equal-area projection. Dotted ovals repre‐ sent 95 % confidence limits

#### **4.2. Sedimentation process inferred from AMS fabric**

We found that the AMS fabric (orientation of principal axes) were precisely determined at all the sampled localities. Tables 2 and 3 show the AMS parameters for the Cretaceous/Eocene units and the Miocene unit, respectively. Figure 12 delineates typical AMS fabric obtained from the Ishikari (left) and Yezo (right) samples. After tilt-correction, the maximum (K1) and intermediate (K2) axes of AMS are bound to the horizontal plane with a subtle imbrication suggestive of hydrodynamic forcing.

**Figure 12.** Anisotropy of magnetic susceptibility (AMS) fabric (principal susceptibility axes) for all specimens of typical sites of the Ishikari Group (HP02) and Yezo Supergroup (HC01) plotted on the lower hemisphere of equal-area projec‐ tions. Data are shown in stratigraphic coordinates. Ovals surrounding mean directions of three axes (shown by larger symbols) are 95% confidence regions. See Table 2 for all the AMS parameters

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**Figure 11.** Comparison of the mean paleomagnetic directions of the Kawabata Formation in central Hokkaido be‐ tween this study and [15]. Data are plotted on the lower hemisphere of the equal-area projection. Dotted ovals repre‐

We found that the AMS fabric (orientation of principal axes) were precisely determined at all the sampled localities. Tables 2 and 3 show the AMS parameters for the Cretaceous/Eocene units and the Miocene unit, respectively. Figure 12 delineates typical AMS fabric obtained from the Ishikari (left) and Yezo (right) samples. After tilt-correction, the maximum (K1) and intermediate (K2) axes of AMS are bound to the horizontal plane with a subtle imbrication

**Figure 12.** Anisotropy of magnetic susceptibility (AMS) fabric (principal susceptibility axes) for all specimens of typical sites of the Ishikari Group (HP02) and Yezo Supergroup (HC01) plotted on the lower hemisphere of equal-area projec‐ tions. Data are shown in stratigraphic coordinates. Ovals surrounding mean directions of three axes (shown by larger

symbols) are 95% confidence regions. See Table 2 for all the AMS parameters

sent 95 % confidence limits

**4.2. Sedimentation process inferred from AMS fabric**

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

suggestive of hydrodynamic forcing.

*N* denotes the number of specimens. Directions of AMS principal axes are in stratigraphic coordinates. Abbreviations for the Paleogene geologic units: Ak, Akabira Formation; Bb, Bibai Formation; Ik, Ikushunbetsu Formation. Depositional sequence is after Takano & Waseda [4] and Takano et al. [5].

**Table 2.** Site-mean AMS parameters of the Paleogene and Cretaceous units in central Hokkaido

Figure 13 delineates typical AMS fabrics of the Kawabata Formation. Site RB08 typifies an elongate (prolate) fabric reflecting aligned detrital grains. Site RB14 has highly oblate fabric, as shown by a positive *T* parameter near unity. This fabric is essentially confined to the bedding plane under gravitational force. As the hysteresis study showed a negligible amount of ferromagnetic material in the Kawabata samples (Figure 8), we consider the AMS fabric as being governed simply by the shape anisotropy of paramagnetic minerals, i.e. alignments of elongate or platy grains such as amphibole or mica.

**Site N K1 K2 K3 L (K1/K2) F (K2/K3) P (K1/K3) PJ T q**

RB06 14 1.007 1.001 0.992 1.005 1.009 1.014 1.015 0.256 0.459

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RB07 12 1.005 0.999 0.996 1.007 1.002 1.009 1.009 -0.459 1.151

RB08 11 1.004 1.000 0.997 1.004 1.003 1.007 1.007 -0.207 0.866

RB09 17 1.005 1.001 0.994 1.004 1.007 1.011 1.011 0.313 0.416

RB10 10 1.010 1.007 0.983 1.003 1.024 1.027 1.030 0.777 0.120

RB11 12 1.002 0.999 0.998 1.003 1.001 1.004 1.004 -0.410 1.090

RB12 12 1.005 1.004 0.991 1.002 1.013 1.015 1.016 0.763 0.127

RB13 9 1.004 1.002 0.994 1.001 1.009 1.010 1.011 0.732 0.144

RB14 19 1.007 1.006 0.986 1.001 1.021 1.022 1.024 0.908 0.048

RB15 12 1.009 1.004 0.986 1.005 1.018 1.023 1.024 0.558 0.251

RB16 15 1.013 1.010 0.977 1.003 1.034 1.036 1.041 0.848 0.080

RB17 14 1.009 1.003 0.987 1.006 1.016 1.022 1.023 0.455 0.318

RB18 7 1.009 1.001 0.990 1.008 1.011 1.018 1.018 0.169 0.528

RB19 13 1.012 1.003 0.985 1.009 1.018 1.028 1.028 0.324 0.411

RB20 11 1.013 1.005 0.982 1.008 1.023 1.031 1.032 0.458 0.317

RB21 17 1.015 1.005 0.980 1.010 1.026 1.036 1.037 0.433 0.335

N is the number of specimens. Directions of AMS principal axes are in stratigraphic coordinates.

**(D, I) (D, I) (D, I)**

(336, 5) (66, 7) (208, 82)

(135, 24) (41, 9) (291, 64)

(152, 2) (61, 36) (245, 54)

(121, 1) (31, 7) (218, 83)

(343, 2) (252, 12) (84, 78)

(313, 42) (105, 45) (210, 15)

(3, 6) (93, 1) (192, 84)

(119, 26) (214, 11) (325, 61)

(78, 4) (348, 5) (206, 84)

(188, 10) (96, 10) (324, 76)

(281, 4) (11, 5) (152, 83)

(292, 6) (201, 6) (66, 81)

(120, 1) (30, 2) (234, 87)

(26, 15) (277, 50) (127, 36)

(300, 19) (205, 15) (78, 66)

(215, 75) (90, 9) (358, 12)

**Table 3.** Site-mean AMS parameters of the Kawabata Formation

**Figure 13.** Typical tilt-corrected AMS fabric for the Kawabata Formation muddy samples. Prolate (left) and oblate (right) fabrics are numerically described by negative and positive T parameters, respectively, posted on the equal-area diagrams. All the data are plotted on the lower hemisphere. Square, triangular and circular symbols represent orthog‐ onal maximum (K1), intermediate (K2), and minimum (K3) AMS principal axes, respectively, and larger symbols show their mean directions. Shaded areas are 95 % confidence limits based upon Bingham statistics


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N is the number of specimens. Directions of AMS principal axes are in stratigraphic coordinates.

**Table 3.** Site-mean AMS parameters of the Kawabata Formation

Figure 13 delineates typical AMS fabrics of the Kawabata Formation. Site RB08 typifies an elongate (prolate) fabric reflecting aligned detrital grains. Site RB14 has highly oblate fabric, as shown by a positive *T* parameter near unity. This fabric is essentially confined to the bedding plane under gravitational force. As the hysteresis study showed a negligible amount of ferromagnetic material in the Kawabata samples (Figure 8), we consider the AMS fabric as being governed simply by the shape anisotropy of paramagnetic minerals, i.e. alignments of

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

**Figure 13.** Typical tilt-corrected AMS fabric for the Kawabata Formation muddy samples. Prolate (left) and oblate (right) fabrics are numerically described by negative and positive T parameters, respectively, posted on the equal-area diagrams. All the data are plotted on the lower hemisphere. Square, triangular and circular symbols represent orthog‐ onal maximum (K1), intermediate (K2), and minimum (K3) AMS principal axes, respectively, and larger symbols show

**Site N K1 K2 K3 L (K1/K2) F (K2/K3) P (K1/K3) PJ T q**

RB01 11 1.013 1.006 0.981 1.007 1.025 1.033 1.034 0.546 0.260

RB02 11 1.007 1.003 0.991 1.004 1.012 1.016 1.017 0.508 0.282

RB03 12 1.006 1.002 0.992 1.004 1.010 1.014 1.015 0.475 0.304

RB04 13 1.008 1.002 0.990 1.005 1.013 1.018 1.019 0.406 0.352

RB05 15 1.005 1.001 0.993 1.004 1.008 1.012 1.012 0.330 0.404

their mean directions. Shaded areas are 95 % confidence limits based upon Bingham statistics

**(D, I) (D, I) (D, I)**

(16, 6) (107, 7) (242, 81)

(3, 20) (98, 11) (215, 66)

(326, 11) (58, 11) (190, 74)

(159, 2) (249, 6) (52, 83)

(4, 19) (101, 19) (232, 62)

elongate or platy grains such as amphibole or mica.

Sedimentological context of the AMS fabric is demonstrated in Figures 14 and 15. Paleocurrent directions inferred from the Eocene AMS data tend to align in N-S azimuth (Figure 14), and accord with development process of the forearc basin [4]. Takano and Waseda [4] demon‐ strated that the Eocene paleo-Ishikari basin experienced differential subsidence during deposition. Such deformation may be related to longstanding strike-slip faulting around central Hokkaido [17], and tectono- / sedimentological context of the AMS fabric will be better evaluated in the light of quantitative study of basin-forming processes described in this book. For reliable interpretation of AMS data, it is necessary to assess properties of ferromagnetic minerals, such as composition, grain size and contribution to bulk magnetic susceptibility, as shown in this paper.

Our field survey revealed indicators of paleocurrent directions in the Kawabata Formation along the Rubeshibe River as depicted in Figure 15. After correction for the counterclockwise rotation identified in our paleomagnetic study, most of the markers indicate a westward current direction with minor southward flow contributions. This is consistent with a tectonosedimentary model of rapid burial of the Miocene N-S foreland basin by clastics derived from the eastern collision front presented in such research as Kawakami et al. [7]. Notably, the imbrication of the oblate AMS fabric matches visible sedimentary structures. Although the transport direction of muddy detrital material spilled out of a levee is not necessarily parallel to the turbidity current within a channel, AMS data can serve to indicate paleocurrents after the contributors to the magnetic fabric have been identified. Also note that K1 of prolate samples (with negative *T* parameters) tend to align perpendicular to the paleocurrent direc‐

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**Figure 15.** Paleocurrent map of the Kawabata Formation around the Rubeshibe River route. Formation boundaries

Figure 16 delineates groups of microscopic fabrics identified in the Kawabata Formation as a function of the AMS shape parameter (*T*). The intensity of alignment forcing inferred from AMS data is closely related to sedimentary facies (shown on the right in the figure) determined by field observation. For example, weak hydrodynamic forcing corresponds to fine rhythmi‐

are after Kawakami et al. [7]

tion, implying that elongate grains roll on the sediment surface.

**Figure 14.** Paleocurrent directions inferred from AMS fabric of the Paleogene and Cretaceous samples. Geologic map is compiled from Editorial Committee of Hokkaido, Regional Geology of Japan [1] and Takano and Waseda [4]

Our field survey revealed indicators of paleocurrent directions in the Kawabata Formation along the Rubeshibe River as depicted in Figure 15. After correction for the counterclockwise rotation identified in our paleomagnetic study, most of the markers indicate a westward current direction with minor southward flow contributions. This is consistent with a tectonosedimentary model of rapid burial of the Miocene N-S foreland basin by clastics derived from the eastern collision front presented in such research as Kawakami et al. [7]. Notably, the imbrication of the oblate AMS fabric matches visible sedimentary structures. Although the transport direction of muddy detrital material spilled out of a levee is not necessarily parallel to the turbidity current within a channel, AMS data can serve to indicate paleocurrents after the contributors to the magnetic fabric have been identified. Also note that K1 of prolate samples (with negative *T* parameters) tend to align perpendicular to the paleocurrent direc‐ tion, implying that elongate grains roll on the sediment surface.

Sedimentological context of the AMS fabric is demonstrated in Figures 14 and 15. Paleocurrent directions inferred from the Eocene AMS data tend to align in N-S azimuth (Figure 14), and accord with development process of the forearc basin [4]. Takano and Waseda [4] demon‐ strated that the Eocene paleo-Ishikari basin experienced differential subsidence during deposition. Such deformation may be related to longstanding strike-slip faulting around central Hokkaido [17], and tectono- / sedimentological context of the AMS fabric will be better evaluated in the light of quantitative study of basin-forming processes described in this book. For reliable interpretation of AMS data, it is necessary to assess properties of ferromagnetic minerals, such as composition, grain size and contribution to bulk magnetic susceptibility, as

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

**Figure 14.** Paleocurrent directions inferred from AMS fabric of the Paleogene and Cretaceous samples. Geologic map is compiled from Editorial Committee of Hokkaido, Regional Geology of Japan [1] and Takano and Waseda [4]

shown in this paper.

**Figure 15.** Paleocurrent map of the Kawabata Formation around the Rubeshibe River route. Formation boundaries are after Kawakami et al. [7]

Figure 16 delineates groups of microscopic fabrics identified in the Kawabata Formation as a function of the AMS shape parameter (*T*). The intensity of alignment forcing inferred from AMS data is closely related to sedimentary facies (shown on the right in the figure) determined by field observation. For example, weak hydrodynamic forcing corresponds to fine rhythmi‐ 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.

of a preferred orientation. Although the AMS fabric is a diagnostic tool for patterns of sediment transportation, laboratory-based experiments that analyze natural sediments under conditions where a few of the prevailing factors are controlled, are essential to allow firm sedimentological

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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

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

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

). 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.

**Sample N K1 K2 K3 L (K1/K2) F (K2/K3) P (K1/K3) PJ T q**

SP1C-1 1 1.0009 1.0000 0.9992 1.001 1.001 1.002 1.002 -0.080 0.740

SP2F-1 1 1.0014 1.0002 0.9984 1.001 1.002 1.003 1.003 0.180 0.517

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

interpretation of formation processes.

artificial samples are summarized in Table 4.

**(D, I) (D, I) (D, I)**

(167, 75) (265, 2) (356, 15)

(250, 27) (343, 6) (84, 62)

Miocene turbidite.

(PJ

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

**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‐ tions

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 of a preferred orientation. Although the AMS fabric is a diagnostic tool for patterns of sediment transportation, laboratory-based experiments that analyze natural sediments under conditions where a few of the prevailing factors are controlled, are essential to allow firm sedimentological interpretation of formation processes.
