**2. Geology**

#### **2.1. Background**

The Yezo Supergroup deposited on the Cretaceous forearc and consists of monotonous mudstone intercalated by coarse clastics and ash layers. After a stagnant subsidence stage at the beginning of the Cenozoic, fluvial sediments of the Ishikari Group and its correlative units

Rock Magnetic Properties of Sedimentary Rocks in Central Hokkaido — Insights into Sedimentary and Tectonic… http://dx.doi.org/10.5772/56650 235

**Figure 2.** Cenozoic tectonic context of Hokkaido, geology of the study area of the Neogene strata (simplified from Kawakami et al. [7]), and locations of rock magnetic samples

began to bury depressions on the forearc. As a result of strong deformation and continued sedimentation on the active margin, surface distribution of the Eocene Ishikari Group is rather restricted. However, numerous exploration drilling clarified that voluminous Paleogene units are concealed under the alluvial plain (Figure 1). Paleogene depositional sequence and facies classification were described by Takano et al. [5]. They are shown in Figure 3 using abbrevia‐ tions.

The study area of the Kawabata Formation is located in the southern part of the middle Miocene basins of the Ishikari-Teshio belt. Folded sedimentary units are distributed with a NNW-SSE trend, and are cut by numerous faults (Figure 2). The area is divided into the following formations in ascending order [7]: the Takinoue Formation, the Kawabata Formation, the Karumai Formation, and the Nina Formation (Figure 4). They represent the sequence by which an elongate N-S foreland basin was filled. The middle Miocene Kawabata Formation comprises mainly turbidites and associated coarse clastic rocks derived from the eastern hinterland [7].

#### **2.2. Sedimentary facies of the Miocene unit**

rocks related to the tectonic regime and sedimentation processes in the mobile zone. This study

**Figure 1.** Index map of the study area of the Cretaceous and Paleogene strata. Geologic map is after Editorial Commit‐

The Yezo Supergroup deposited on the Cretaceous forearc and consists of monotonous mudstone intercalated by coarse clastics and ash layers. After a stagnant subsidence stage at the beginning of the Cenozoic, fluvial sediments of the Ishikari Group and its correlative units

is an attempt to apply magnetic properties to tectono-sedimentology.

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

**2. Geology**

**2.1. Background**

tee of Hokkaido, Regional Geology of Japan [1]

This study conducted sedimentary facies analysis for the Kawabata Formation along the Rubeshibe River (Figure 2). The analysis revealed that the turbidites of the Kawabata Forma‐ tion mainly consisted of sheet-flow turbidite facies association and channel-levee facies association (Figure 5). The sheet-flow turbidite facies association comprises aggradational stacking of rhythmic alternating beds of turbidite sandstone and mudstone with rare upward

turbidites of the Kawabata Formation commonly contain sedimentary structures indicating paleocurrent directions; e.g., sole marks (mostly flute marks) at the bottom of individual

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

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

237

turbidite bed, and current ripples in Bouma Tc division [11].

**Figure 5.** Facies association classification of the Kawabata Formation along the Rubeshibe River

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

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

**3. Rock magnetism**

**3.1. Basic measurements**

subsequent demagnetization tests.

related to paleocurrent directions (e.g., [12]).

**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]


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

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 turbidites of the Kawabata Formation commonly contain sedimentary structures indicating paleocurrent directions; e.g., sole marks (mostly flute marks) at the bottom of individual turbidite bed, and current ripples in Bouma Tc division [11].

**Figure 5.** Facies association classification of the Kawabata Formation along the Rubeshibe River
