**2. Eocene Ishikari–Sanriku-oki forearc basins**

#### **2.1. Geologic setting**

The Eocene Ishikari–Sanriku-oki forearc basins were developed in the forearc zone along the NE Japan Arc (Figure 2A), which corresponds to the N-S trending narrow zone extending from the "Sorachi–Yezo Belt [3]" in central Hokkaido to the Pacific side offshore of northeast Honshu Island (Figure 2B). Although paleogeography around the NE Japan Arc was quite different from the present because the backarc basins of the NE Japan and Kuril Arcs had not opened [e.g., 4, 5], the tectonic history along the forearc zone during the Cretaceous to Paleogene can be summarized using the geologic evidences as follows. During the Cretaceous time, the eastern plates, which were regarded as the Izanagi and Kula Plates [4, 6], subducted under‐ neath the western volcanic arc, and the forearc basin fully developed along this zone (Yezo Forearc Basin; Figure 2B [6, 7, 8]). During the Early Paleocene time, it is believed that a ridge between the Kula and Pacific Plates passed by along this forearc zone [5], causing total extinction of the forearc basins once. This tectonic event was widely recorded as "KT gap

unconformity [6, 8] (Figure 3)" seen in sedimentary successions along the Sorachi-Yezo Belt and the Ishikari–Sanriku-oki forearc zone with a minor time transgressive trend of the unconformity development possibly related to the ridge passage [8, 9]. After this tectonic event, fragmented small basins sporadically developed along the Ishikari–Sanriku-oki forearc zone. The Eocene was a relatively widespread phase of forearc basins, extending from Sanriku-oki to central Hokkaido (Figures 2B, 3). These Eocene forearc basins were segmented into several subbasins: Sanriku-oki, Yufutsu-oki, Yubari, Sorachi and Uryu subbasins [10, 11] (Figure 2B).

[23, 24].

**Figure 2.** A) Index map showing the locations of two example forearc basins: Early to Middle Eocene Ishikari–Sanrikuoki forearc basins (ISFB) and Pleistocene Tokai-oki–kumano-nada forearc basins (TKFB). B) Close-up map showing the distribution of the Early to Middle Eocene Ishikari–Sanriku-oki forearc basins (orange lines) and the Cretaceous Yezo forearc basins (sky blue lines). The Eocene Ishikari–Sanriku-oki forearc basins are segmented into several subbasins (blue dashed lines). Compiled after [6, 8, 10, 11]. C) Close-up map of the Pleistocene Tokai-oki–Kumano-nada forearc basins, showing the mapping area and 2D seismic survey line positions used for seismic facies analysis. Modified after

Variation in Forearc Basin Configuration and Basin-filling Depositional Systems…

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Variation in Forearc Basin Configuration and Basin-filling Depositional Systems… http://dx.doi.org/10.5772/56751 5

This chapter attempts to examine these two major factors: basin filling condition and basin configuration, by diagnosing two contrasting actual forearc basin packages around Japan: the Eocene Ishikari–Sanriku-oki forearc basins along the NE Japan Arc and the Pleistocene Tokaioki–Kumano-nada forearc basins along the SW Japan Arc (Figure 2). To delineate the basin filling condition, we examine sedimentological characteristics including depositional systems, sequence stratigraphic contexts and related controlling factors. Regional seismic survey sections are used to manifest the basin configuration and to discuss controlling factors on forearc sedimentation for the two example forearc basins. In addition to these major factors, we discuss the role of strike-slip tectonics on the forearc basins, as it is reported in a former literature that strike-slip movement related to oblique plate subduction may affect the forearc

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

**Figure 1.** Schematic cross section of a forearc zone including a forearc basin, showing the basic terms used in this

The Eocene Ishikari–Sanriku-oki forearc basins were developed in the forearc zone along the NE Japan Arc (Figure 2A), which corresponds to the N-S trending narrow zone extending from the "Sorachi–Yezo Belt [3]" in central Hokkaido to the Pacific side offshore of northeast Honshu Island (Figure 2B). Although paleogeography around the NE Japan Arc was quite different from the present because the backarc basins of the NE Japan and Kuril Arcs had not opened [e.g., 4, 5], the tectonic history along the forearc zone during the Cretaceous to Paleogene can be summarized using the geologic evidences as follows. During the Cretaceous time, the eastern plates, which were regarded as the Izanagi and Kula Plates [4, 6], subducted under‐ neath the western volcanic arc, and the forearc basin fully developed along this zone (Yezo Forearc Basin; Figure 2B [6, 7, 8]). During the Early Paleocene time, it is believed that a ridge between the Kula and Pacific Plates passed by along this forearc zone [5], causing total extinction of the forearc basins once. This tectonic event was widely recorded as "KT gap

basin tectonics and sedimentation.

chapter. Modified after [1].

**2.1. Geologic setting**

**2. Eocene Ishikari–Sanriku-oki forearc basins**

**Figure 2.** A) Index map showing the locations of two example forearc basins: Early to Middle Eocene Ishikari–Sanrikuoki forearc basins (ISFB) and Pleistocene Tokai-oki–kumano-nada forearc basins (TKFB). B) Close-up map showing the distribution of the Early to Middle Eocene Ishikari–Sanriku-oki forearc basins (orange lines) and the Cretaceous Yezo forearc basins (sky blue lines). The Eocene Ishikari–Sanriku-oki forearc basins are segmented into several subbasins (blue dashed lines). Compiled after [6, 8, 10, 11]. C) Close-up map of the Pleistocene Tokai-oki–Kumano-nada forearc basins, showing the mapping area and 2D seismic survey line positions used for seismic facies analysis. Modified after [23, 24].

unconformity [6, 8] (Figure 3)" seen in sedimentary successions along the Sorachi-Yezo Belt and the Ishikari–Sanriku-oki forearc zone with a minor time transgressive trend of the unconformity development possibly related to the ridge passage [8, 9]. After this tectonic event, fragmented small basins sporadically developed along the Ishikari–Sanriku-oki forearc zone. The Eocene was a relatively widespread phase of forearc basins, extending from Sanriku-oki to central Hokkaido (Figures 2B, 3). These Eocene forearc basins were segmented into several subbasins: Sanriku-oki, Yufutsu-oki, Yubari, Sorachi and Uryu subbasins [10, 11] (Figure 2B). This section picks up the Sorachi, Yubari and Sanriku-oki subbasins for examining the basin filling condition and basin configuration.

The Ishikari Group is divided into nine lithostratigraphic units: the Noborikawa, Horokabetsu, Yubari, Wakkanabe, Bibai, Akabira, Ikushunbetsu, Hiragishi and Ashibetsu Formations. From the standpoint of sequence stratigraphy, the Ishikari Group can be divided into four 3rd-order depositional sequences: Sequence Isk-1 to -4 in ascending order, and each depositional sequence is further divided into TST (transgressive systems tract) and HST (highstand systems tract), based on transgressive/regressive trends and marine incursion beds (Figures 3, 5) [11, 13]. In the Sorachi subbasin, the nine lithostratigraphic units are all developed, whereas in the Yubari basin, the Bibai, Akabira, Hiragishi and Ashibetsu Formations are absent, suggesting that the basin filling sedimentation was not continuous but episodic in the Yubari subbasin.

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**Figure 4.** A) Geologic map showing the distributions of the Eocene forearc basin sediments in central Hokkaido, near the northern end of the Ishikari–Sanriku-oki forearc basins. B) Close-up geologic map showing the surface distribu‐ tions of the Middle Eocene Ishikari Group in central Hokkaido. The Middle Eocene forearc basin in this area was seg‐ mented into the Sorachi subbasin on the north and the Yubari subbasin on the south. Numbers shown along rivers denote transect numbers of geologic survey, which correspond to numbers on the geologic cross section in Figures 5

and 7B. Modified after [11].
