*2.2.1. Stratigraphic framework*

The Sorachi and Yubari subbasins are located in central Hokkaido (Figures 4), and situated near the northern end of the Eocene Ishikari–Sanriku-oki forearc basins (Figure 2B). The Sorachi and Yubari subbasins developed and started sedimentation at the early Middle Eocene time, and continued until the Early Oligocene time with some short breaks by unconformities [12] (Figure 3). This section focuses on the Middle Eocene Ishikari Group (Figure 3), which constitutes the major part of the Sorachi and Yubari subbasin fill.

**Figure 3.** Generalized stratigraphic framework of the Paleocene, Eocene and Lower Oligocene in the Sorachi, Yubari and Sanriku-oki subbasins of the Ishikari–Sanriku-oki forearc zone. Colored columns beside the stratigraphic unit names denote the major depositional systems. This chapter mainly targets the Early to Middle Eocene basin fills. Com‐ piled after [10–13, 15].

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.

This section picks up the Sorachi, Yubari and Sanriku-oki subbasins for examining the basin

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

The Sorachi and Yubari subbasins are located in central Hokkaido (Figures 4), and situated near the northern end of the Eocene Ishikari–Sanriku-oki forearc basins (Figure 2B). The Sorachi and Yubari subbasins developed and started sedimentation at the early Middle Eocene time, and continued until the Early Oligocene time with some short breaks by unconformities [12] (Figure 3). This section focuses on the Middle Eocene Ishikari Group (Figure 3), which

**Figure 3.** Generalized stratigraphic framework of the Paleocene, Eocene and Lower Oligocene in the Sorachi, Yubari and Sanriku-oki subbasins of the Ishikari–Sanriku-oki forearc zone. Colored columns beside the stratigraphic unit names denote the major depositional systems. This chapter mainly targets the Early to Middle Eocene basin fills. Com‐

filling condition and basin configuration.

*2.2.1. Stratigraphic framework*

piled after [10–13, 15].

**2.2. Sorachi and Yubari subbasins (Ishikari Group)**

constitutes the major part of the Sorachi and Yubari subbasin fill.

**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].

## *2.2.2. Depositional systems*

Sedimentary facies analysis reveals that the Ishikari Group in the Sorachi and Yubari subbasins is composed of 24 sedimentary facies. These sedimentary facies are further assembled into five facies associations: braided fluvial facies association (BF), meandering fluvial facies association (MF), lacustrine facies association (LA), bay margin–estuarine facies association (ES) and bay center facies association (BA), as groups of sedimentary facies based on genetically related sedimentary environments and succession patterns [11, 13] (Figure 5). These five facies associations indicate that the Ishikari Group consists of five depositional systems: braided fluvial system, meandering fluvial system, lacustrine system, bay margin–estuarine system and bay system. Figure 5 depicts the schematic cross section showing the temporal and spatial distributions of depositional systems within the sequence stratigraphic framework in the Sorachi subbasin. As Figure 5 shows, the Ishikari Group mainly consists of a meandering fluvial system with some developments of a braided fluvial system. Lacustrine, bay margin / estuarine and bay center systems cyclically occur as marine incursion beds at around the maximum flooding surface of each 3rd-order depositional sequence.

braided and meandering fluvial environments were dominant, whereas at the maximum flooding phase, bay center and bay margin environments were dominant. These facies maps indicate that marine influence became strong northeastward, whereas terrestrial environments such as braided / meandering river environments were dominant in the southern or south‐

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One of the notable characteristics of the depositional systems in the Ishikari Group in the Sorachi subbasin can be predominance of tidal deposits in a bay margin–estuarine system. Even though shallow marine condition periodically occurred during deposition, there are no wave-influenced sandy shallow marine facies such as foreshore and shoreface sandstone facies in the Ishikari Group. These facts indicate that the basin setting of the Sorachi subbasin was

**Figure 6.** Spatial distribution maps of depositional systems in the Sorachi subbasin, showing paleogeographical changes for systems tracts (TST: transgressive interval; mfs: maximum transgression; HST: regressive interval) of 3rd-or‐ der Sequences Isk-1 to -3. Maps were created on the basis of facies association plots at the survey transect position. MF: meandering fluvial, BF: braided fluvial, LA: lacustrine, ES: bay margin–estuarine, BA: bay center. Blue contours de‐

protected by wave action, and was not directly facing an open sea.

western area of the Sorachi subbasin.

note isopach (iso-thickness) lines. Modified after [11].

**Figure 5.** Schematic cross section of the Middle Eocene Ishikari Group in the Sorachi subbasin, showing sequence stratigraphic division and temporal and spatial distributions of depositional systems. Numbers above the cross section denote transect numbers of geologic survey shown in Figure 4B. Modified after [11, 13].

Figure 6 depicts facies maps showing spatial distributions of depositional systems for each systems tract of Sequences Isk-1 to -3. It is estimated that in response to relative sea level changes, the sedimentary environments in the Sorachi subbasin changed by cyclic transgres‐ sion and regression. At the early phase of transgression and the late phase of regression, braided and meandering fluvial environments were dominant, whereas at the maximum flooding phase, bay center and bay margin environments were dominant. These facies maps indicate that marine influence became strong northeastward, whereas terrestrial environments such as braided / meandering river environments were dominant in the southern or south‐ western area of the Sorachi subbasin.

*2.2.2. Depositional systems*

Sedimentary facies analysis reveals that the Ishikari Group in the Sorachi and Yubari subbasins is composed of 24 sedimentary facies. These sedimentary facies are further assembled into five facies associations: braided fluvial facies association (BF), meandering fluvial facies association (MF), lacustrine facies association (LA), bay margin–estuarine facies association (ES) and bay center facies association (BA), as groups of sedimentary facies based on genetically related sedimentary environments and succession patterns [11, 13] (Figure 5). These five facies associations indicate that the Ishikari Group consists of five depositional systems: braided fluvial system, meandering fluvial system, lacustrine system, bay margin–estuarine system and bay system. Figure 5 depicts the schematic cross section showing the temporal and spatial distributions of depositional systems within the sequence stratigraphic framework in the Sorachi subbasin. As Figure 5 shows, the Ishikari Group mainly consists of a meandering fluvial system with some developments of a braided fluvial system. Lacustrine, bay margin / estuarine and bay center systems cyclically occur as marine incursion beds at around the

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

**Figure 5.** Schematic cross section of the Middle Eocene Ishikari Group in the Sorachi subbasin, showing sequence stratigraphic division and temporal and spatial distributions of depositional systems. Numbers above the cross section

Figure 6 depicts facies maps showing spatial distributions of depositional systems for each systems tract of Sequences Isk-1 to -3. It is estimated that in response to relative sea level changes, the sedimentary environments in the Sorachi subbasin changed by cyclic transgres‐ sion and regression. At the early phase of transgression and the late phase of regression,

denote transect numbers of geologic survey shown in Figure 4B. Modified after [11, 13].

maximum flooding surface of each 3rd-order depositional sequence.

One of the notable characteristics of the depositional systems in the Ishikari Group in the Sorachi subbasin can be predominance of tidal deposits in a bay margin–estuarine system. Even though shallow marine condition periodically occurred during deposition, there are no wave-influenced sandy shallow marine facies such as foreshore and shoreface sandstone facies in the Ishikari Group. These facts indicate that the basin setting of the Sorachi subbasin was protected by wave action, and was not directly facing an open sea.

**Figure 6.** Spatial distribution maps of depositional systems in the Sorachi subbasin, showing paleogeographical changes for systems tracts (TST: transgressive interval; mfs: maximum transgression; HST: regressive interval) of 3rd-or‐ der Sequences Isk-1 to -3. Maps were created on the basis of facies association plots at the survey transect position. MF: meandering fluvial, BF: braided fluvial, LA: lacustrine, ES: bay margin–estuarine, BA: bay center. Blue contours de‐ note isopach (iso-thickness) lines. Modified after [11].

#### *2.2.3. Subsidence history*

Figure 6 also depicts isopach contours for each systems tract of depositional sequences in the Sorachi subbasin. These isopach maps suggest that the thickness trend, indicating the depocenter, changed intermittently during deposition of the Ishikari Group. Since the depositional environments (altitude of deposition) in the Sorachi subbasin were more or less equivalent to a relative sea level or base level, it is regarded that the thickness trend indicates a spatial trend of total basin subsidence. Figure 7A demonstrates total subsi‐ dence curves of three different positions of the Sorachi subbasin, which were created on the basis of the thickness information. These isopach maps and total subsidence curves indicate that the western part of the subbasin rapidly subsided first. Subsequently during deposition of Sequence Isk-3 and -4, the northeastern part selectively subsided at a drastically rapid rate, and finally accumulated 3000 m-thick tidal-dominant deposits. Thus the Sorachi subbasin is characterized by a differential subsidence especially in the later half of the Ishikari Group deposition [11].

**2.3. Sanriku-oki subbasin**

*2.3.1. Stratigraphic framework*

the depositional condition and basin configuration.

strongly controlled by the trench slope development.

*2.3.2. Depositional systems and basin setting*

The Sanriku-oki subbasin is located in northeastern offshore of the Honshu Island, and situated near the southern end of the Eocene Ishikari–Sanriku-oki forearc basins (Figure 2B). After the K/T gap unconformity, the Sanriku-oki subbasin started basin-filling sedimentation in Late Paleocene time, and continued until the large-scale Oligocene unconformity (Ounc [10]) was formed (Figure 3). This section focuses on the Lower to Middle Eocene forearc basin-filling sediments, which are divided into the B2, C1, C2, C3 and C4 units [15] (Figure 3), for examining

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According to the MITI Sanriku-oki well report [15], the Lower to Middle Eocene succes‐ sions in the Sanriku-oki subbasin are mainly composed of mudstone, sandstone and coalbearing alternating beds of sandstone and mudstone, which were deposited in terrestrial, brackish and neritic marine environments. Figure 8 demonstrates interpreted seismic facies maps of the lower and upper intervals of the Lower Eocene B2 unit in the 3D seismic surveyed area, including the MITI Sanriku-oki well location, in the central part of the Sanriku-oki subbasin (Figure 2B). These seismic facies maps, which were displayed by different colors assigned for each "seismic trace shape" class, show intricate meandering, braided or partly networked fluvial channel zones and floodplain–back mash zones.

Based on the sedimentary environment information from the MITI Sanriku-oki well and the seismic facies maps, it is interpreted that the B2 and C3 units consist mainly of a coalbearing meandering fluvial system with minor bay center to bay margin systems as marine incursion beds, and the C1, C2 and C4 units consist mainly of bay to muddy shelf systems (Figure 3). Since all these component depositional systems resemble those of the Sorachi/ Yubari subbasins, it is regarded that the Eocene Sanriku-oki subbasin was in a confined forearc setting, which was not directly facing an open sea and was protected by wave action. This basin setting during the Eocene time is supported by the basin configuration shown on a long 2D seismic section transecting the Sanriku-oki subbasin (Figure 9), in which the trench slope break prominently uplifted and eroded by Ounc (Oligocene Unconformity [10]), and the Cretaceous to Eocene basin-filling succession seems to be confined within the arcward side of the uplifted trench slope break. This confined forearc setting was terminated by the Ounc event, accompanied with seaward migration and large subsidence of the trench slope break, which finally caused transformation of the forearc basin setting from a fluvial to deep-marine slope condition as shown in the cross section in Figure 9. Consequently, it is regarded that the Sanriku-oki forearc basin setting was

In addition to a differential subsidence within a subbasin, the subsidence patterns between subbasins show a notable difference. Figure 7B depicts the schematic cross section across the Sorachi and Yubari subbasins, showing a large thickness difference [14], possibly related to the difference in subsidence pattern as shown in Figure 7A. Accordingly, it is suggested that the segmented forearc basins in the Ishikari–Sanriku-oki forearc zone show highly variable subsidence patterns within and between subbasins.

**Figure 7.** A) Diagram showing total subsidence histories along the selected transects during deposition of the Ishikari Group in the Sorachi and Yubari subbasins. Modified after [11]. B) Schematic sectional diagram showing thickness change of the Ishikari Group between the Sorachi and Yubari subbasins. Numbers above the Ishikari Group on the section denote transect numbers of surveys shown in Figure 4B. SB: sequence boundary, mfs: maximum flooding sur‐ face. Modified after [14].

#### **2.3. Sanriku-oki subbasin**

*2.2.3. Subsidence history*

of the Ishikari Group deposition [11].

face. Modified after [14].

subsidence patterns within and between subbasins.

Figure 6 also depicts isopach contours for each systems tract of depositional sequences in the Sorachi subbasin. These isopach maps suggest that the thickness trend, indicating the depocenter, changed intermittently during deposition of the Ishikari Group. Since the depositional environments (altitude of deposition) in the Sorachi subbasin were more or less equivalent to a relative sea level or base level, it is regarded that the thickness trend indicates a spatial trend of total basin subsidence. Figure 7A demonstrates total subsi‐ dence curves of three different positions of the Sorachi subbasin, which were created on the basis of the thickness information. These isopach maps and total subsidence curves indicate that the western part of the subbasin rapidly subsided first. Subsequently during deposition of Sequence Isk-3 and -4, the northeastern part selectively subsided at a drastically rapid rate, and finally accumulated 3000 m-thick tidal-dominant deposits. Thus the Sorachi subbasin is characterized by a differential subsidence especially in the later half

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

In addition to a differential subsidence within a subbasin, the subsidence patterns between subbasins show a notable difference. Figure 7B depicts the schematic cross section across the Sorachi and Yubari subbasins, showing a large thickness difference [14], possibly related to the difference in subsidence pattern as shown in Figure 7A. Accordingly, it is suggested that the segmented forearc basins in the Ishikari–Sanriku-oki forearc zone show highly variable

**Figure 7.** A) Diagram showing total subsidence histories along the selected transects during deposition of the Ishikari Group in the Sorachi and Yubari subbasins. Modified after [11]. B) Schematic sectional diagram showing thickness change of the Ishikari Group between the Sorachi and Yubari subbasins. Numbers above the Ishikari Group on the section denote transect numbers of surveys shown in Figure 4B. SB: sequence boundary, mfs: maximum flooding sur‐

#### *2.3.1. Stratigraphic framework*

The Sanriku-oki subbasin is located in northeastern offshore of the Honshu Island, and situated near the southern end of the Eocene Ishikari–Sanriku-oki forearc basins (Figure 2B). After the K/T gap unconformity, the Sanriku-oki subbasin started basin-filling sedimentation in Late Paleocene time, and continued until the large-scale Oligocene unconformity (Ounc [10]) was formed (Figure 3). This section focuses on the Lower to Middle Eocene forearc basin-filling sediments, which are divided into the B2, C1, C2, C3 and C4 units [15] (Figure 3), for examining the depositional condition and basin configuration.

#### *2.3.2. Depositional systems and basin setting*

According to the MITI Sanriku-oki well report [15], the Lower to Middle Eocene succes‐ sions in the Sanriku-oki subbasin are mainly composed of mudstone, sandstone and coalbearing alternating beds of sandstone and mudstone, which were deposited in terrestrial, brackish and neritic marine environments. Figure 8 demonstrates interpreted seismic facies maps of the lower and upper intervals of the Lower Eocene B2 unit in the 3D seismic surveyed area, including the MITI Sanriku-oki well location, in the central part of the Sanriku-oki subbasin (Figure 2B). These seismic facies maps, which were displayed by different colors assigned for each "seismic trace shape" class, show intricate meandering, braided or partly networked fluvial channel zones and floodplain–back mash zones.

Based on the sedimentary environment information from the MITI Sanriku-oki well and the seismic facies maps, it is interpreted that the B2 and C3 units consist mainly of a coalbearing meandering fluvial system with minor bay center to bay margin systems as marine incursion beds, and the C1, C2 and C4 units consist mainly of bay to muddy shelf systems (Figure 3). Since all these component depositional systems resemble those of the Sorachi/ Yubari subbasins, it is regarded that the Eocene Sanriku-oki subbasin was in a confined forearc setting, which was not directly facing an open sea and was protected by wave action. This basin setting during the Eocene time is supported by the basin configuration shown on a long 2D seismic section transecting the Sanriku-oki subbasin (Figure 9), in which the trench slope break prominently uplifted and eroded by Ounc (Oligocene Unconformity [10]), and the Cretaceous to Eocene basin-filling succession seems to be confined within the arcward side of the uplifted trench slope break. This confined forearc setting was terminated by the Ounc event, accompanied with seaward migration and large subsidence of the trench slope break, which finally caused transformation of the forearc basin setting from a fluvial to deep-marine slope condition as shown in the cross section in Figure 9. Consequently, it is regarded that the Sanriku-oki forearc basin setting was strongly controlled by the trench slope development.

**2.4. Forearc setting model**

Based on the characteristics of depositional systems and basin configurations of the Sorachi, Yubari and Sanriku-oki subbasins, a forearc setting model of the Eocene Ishikari–Sanriku-oki forearc basins can be proposed as shown in Figure 10. The trench slope break ridge is estimated to have emerged above the sea along the eastern margin (subduction zone side) of the forearc basins, and formed a barrier to the open sea condition in the trench side of the trench slope break. This uplifted trench slope break condition is supported by previous petrography studies [17–19], which reveal that sandstones of the forearc basin fill (Ishikari Group) contain chrom‐ spinels derived from an emerged ridge of the Kamuikotan Belt. The N-S trending Kamuikotan Belt is distributed along the eastern margin of the forearc zone in Hokkaido (Sorachi–Yezo Belt), and mainly consists of serpentinite and various kinds of high pressure-type metamorphic

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**Figure 10.** Schematic and conceptual forearc setting model for the Eocene Ishikari–Sanriku-oki forearc basins, includ‐ ing the Sorachi, Yubari and Sanriku-oki subbasins. Small rectangles inside the basin denote approximate positions of

the mapped areas for the Sorachi subbasin (Figure 6) and the Sanriku-oki subbasin (Figure 8).

rocks with tectonic mélanges, formed in an accretional prism [3, 20].

**Figure 8.** Seismic facies maps showing the distributions of a fluvial channel zone and a floodplain–back marsh zone in a meandering fluvial system in the central part of the Sanriku-oki subbasin. Map colors were assigned for each differ‐ ent seismic trace shape, which can indicate difference in sedimentary environment. A) Case of a lower horizon of the B2 unit. Bluish colors are interpreted as a channel zone on the basis of the seismic trace shape and distribution pat‐ tern. B) Case of an upper horizon of the B2 unit. Reddish colors are interpreted as a channel zone. The map location is shown in Figure 2B.

**Figure 9.** An E-W long 2D seismic section transecting the Sanriku-oki subbasin, showing trench slope break uplift and subbasin confinement. Although the present status of the Cretaceous to Eocene forearc basin fill and trench slope break seems to be inclined seaward, it is estimated that the trench slope break was more or less uplifted and emerged as a barrier because of the leaping-up morphology and fluvial-dominated environments in the Cretaceous to Eocene forearc basin-fill successions. The 2D seismic data were acquired in a MITI survey [16]. The seismic survey line location is shown in Figure 2B.

#### **2.4. Forearc setting model**

**Figure 8.** Seismic facies maps showing the distributions of a fluvial channel zone and a floodplain–back marsh zone in a meandering fluvial system in the central part of the Sanriku-oki subbasin. Map colors were assigned for each differ‐ ent seismic trace shape, which can indicate difference in sedimentary environment. A) Case of a lower horizon of the B2 unit. Bluish colors are interpreted as a channel zone on the basis of the seismic trace shape and distribution pat‐ tern. B) Case of an upper horizon of the B2 unit. Reddish colors are interpreted as a channel zone. The map location is

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

**Figure 9.** An E-W long 2D seismic section transecting the Sanriku-oki subbasin, showing trench slope break uplift and subbasin confinement. Although the present status of the Cretaceous to Eocene forearc basin fill and trench slope break seems to be inclined seaward, it is estimated that the trench slope break was more or less uplifted and emerged as a barrier because of the leaping-up morphology and fluvial-dominated environments in the Cretaceous to Eocene forearc basin-fill successions. The 2D seismic data were acquired in a MITI survey [16]. The seismic survey line location

shown in Figure 2B.

is shown in Figure 2B.

Based on the characteristics of depositional systems and basin configurations of the Sorachi, Yubari and Sanriku-oki subbasins, a forearc setting model of the Eocene Ishikari–Sanriku-oki forearc basins can be proposed as shown in Figure 10. The trench slope break ridge is estimated to have emerged above the sea along the eastern margin (subduction zone side) of the forearc basins, and formed a barrier to the open sea condition in the trench side of the trench slope break. This uplifted trench slope break condition is supported by previous petrography studies [17–19], which reveal that sandstones of the forearc basin fill (Ishikari Group) contain chrom‐ spinels derived from an emerged ridge of the Kamuikotan Belt. The N-S trending Kamuikotan Belt is distributed along the eastern margin of the forearc zone in Hokkaido (Sorachi–Yezo Belt), and mainly consists of serpentinite and various kinds of high pressure-type metamorphic rocks with tectonic mélanges, formed in an accretional prism [3, 20].

**Figure 10.** Schematic and conceptual forearc setting model for the Eocene Ishikari–Sanriku-oki forearc basins, includ‐ ing the Sorachi, Yubari and Sanriku-oki subbasins. Small rectangles inside the basin denote approximate positions of the mapped areas for the Sorachi subbasin (Figure 6) and the Sanriku-oki subbasin (Figure 8).

Inside the forearc basin, major depositional systems were bay to fluvial systems without any wave-influenced facies. In response to relative sea level changes, transgression and regression repeated, and the major depositional system alternated between a fluvial system-dominated condition and a bay system-dominated condition. Because of the existence of marine sedi‐ ments, it is estimated that there were an inlet interconnecting between the open sea and the inside of the forearc basin, through which the seawater came into the inside of the forearc basin.

Our forearc setting model also demonstrates forearc basin segmentation, reflecting the fact that the Eocene Ishikari–Sanriku-oki forearc basins were segmented into 50 to 150 km long subbasins aligned along the forearc extension (Figure 2B). As described above, the segmented subbasins show a different subsidence pattern and different sediment thickness for each subbasin.
