**3. Basin geometry and stratigraphy**

#### **3.1. Tenpoku Basin**

The northernmost depression is 80 km wide and at least 60 km long (wider than other depressions later described, see Table 1), and is known as the Tenpoku Basin. The western half and the northern part of the basin extend to the Japan Sea (Figure 3). Because the fold-thrust propagation and resultant basin deformation is restrictive, a Middle to Late Miocene basin fill, 1000–2000 m thick, crops out only in the easternmost area. The Miocene axial foredeep is located around the present-day coastline along the Japan Sea, where, according to well data, the basin fill reaches its maximum thickness (ca. 4000 m) [20].

and rough topography due to thrust propagation). The turbiditic deposits are covered with siliceous/diatomaceous and/or shelfal muddy deposits of the late Miocene to Pliocene ages

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

**Figure 2.** Stratigraphy of the middle Miocene to early Pliocene deposits in central Hokkaido. Depositional ages have been determined by diatom biostratigraphy and chronostratigraphic data. Diatom biostratigraphic zonation after [19]. Columns show the successions of inner (orogen-ward, proximal) and outer (foreland-ward, distal) areas of each

The northernmost depression is 80 km wide and at least 60 km long (wider than other depressions later described, see Table 1), and is known as the Tenpoku Basin. The western half and the northern part of the basin extend to the Japan Sea (Figure 3). Because the fold-thrust propagation and resultant basin deformation is restrictive, a Middle to Late Miocene basin fill, 1000–2000 m thick, crops out only in the easternmost area. The Miocene axial foredeep is located around the present-day coastline along the Japan Sea, where, according to well data,

(Figure 2).

depression.

**3.1. Tenpoku Basin**

**3. Basin geometry and stratigraphy**

the basin fill reaches its maximum thickness (ca. 4000 m) [20].

**Figure 3.** Cross section of the Tenpoku Basin (after [21]). Primary basin geometry is well preserved because of restrict‐ ed later tectonic disturbances. See Figure 1 for legend in the index map.

In the easternmost outcrop, a middle Miocene turbiditic succession (the Masuporo Formation) is characterized by abundant mass-transport deposits (MTDs), such as slumped sand/mud interbeds and chaotic sand to gravel beds, bearing many intrabasinal blocks [20, 22–25] (Figure 4). It is noteworthy that the MTDs at the base of the Masuporo Formation rest directly on the shallow marine sandy deposits of the early Middle Miocene (the Onishibetsu Formation), which settled prior to the foreland basin subsidence. In the uppermost horizon of the turbiditic succession, mud-prone basinal turbidites and basin-plain mudstones are predominant, and the succession thus shows an overall fining-upward trend.

The basin fill fines also foreland-ward drastically, and the axial foredeep is filled mainly with basin-plain muddy deposits (2000–3000 m thick) [26]. As an exception, a 200 m thick slumped interval occurs in the upper part of this muddy succession [26]. This interval is characterized by muddy chaotic deposits containing granule-grade grains, although no detailed sedimentary features are described.

The entire part of the basin was covered by basin-plain muddy deposits in the late-Middle Miocene, after which siliceous/diatomaceous muddy deposits (>1000 m thick) were accumu‐ lated basin-wide during the late Miocene.

Although the sediment dispersal pattern in the Tenpoku Basin is not clearly understood, a clastic composition of basin fill, which is rich in granite and hornfels clasts, indicates a sediment supply from the Hidaka Belt in the east.

basinal turbidites are invariably underlain by relatively thin (<100 m) shelfal muddy deposits of the upper part of the Chikubetsu Formation. The blackish muddy deposits at the top of the Chikubetsu Formation indicate a condensed horizon formed during rapid basin subsidence [27]. At the southwestern margin of the basin, the turbiditic deposits show lateral onlap onto the early Miocene shallow-marine/non-marine deposits fringing the KBH. On the other hand, the basin fill was exposed subaerially around 12 Ma at the eastern margin of the basin.

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**Figure 5.** Rose diagrams showing paleocurrent directions of the middle Miocene turbidites in the Haboro Basin (above: compiled from [2, 14]), and transverse profile of the basin fill (below: modified from [2]). The solid and open petals of rose diagram mean the direction measured from sole-marks and clast fabric (solid) and cross lamination (open). Pink-colored dashed lines are stratigraphic markers (ash turbidite beds). CF: Chikubetsu Fault. See Figure 1 for

legend of the index map.

**Figure 4.** Basin-axial profiles of the middle Miocene turbidites at the eastern margin of the Tenpoku Basin (after [22]) (above), and representative photo of the slumped MTDs (below). The cliff is about 12 m high.

#### **3.2. Haboro Basin**

The stratigraphic and sedimentary architecture of the Haboro basin fill, (at least 50 km wide and 90 km long), has been well reconstructed by many studies (e.g. [2, 3, 14, 27]). A large part of the accommodation space is filled with 2000–3000 m thick middle Miocene turbidites (the Kotanbetsu Formation) composed of lower basinal turbidites and upper coarse-grained immature turbiditic deposits (slope-apron turbidites, to be discussed later) (Figure 5). The basinal turbidites are invariably underlain by relatively thin (<100 m) shelfal muddy deposits of the upper part of the Chikubetsu Formation. The blackish muddy deposits at the top of the Chikubetsu Formation indicate a condensed horizon formed during rapid basin subsidence [27]. At the southwestern margin of the basin, the turbiditic deposits show lateral onlap onto the early Miocene shallow-marine/non-marine deposits fringing the KBH. On the other hand, the basin fill was exposed subaerially around 12 Ma at the eastern margin of the basin.

**Figure 4.** Basin-axial profiles of the middle Miocene turbidites at the eastern margin of the Tenpoku Basin (after [22])

The stratigraphic and sedimentary architecture of the Haboro basin fill, (at least 50 km wide and 90 km long), has been well reconstructed by many studies (e.g. [2, 3, 14, 27]). A large part of the accommodation space is filled with 2000–3000 m thick middle Miocene turbidites (the Kotanbetsu Formation) composed of lower basinal turbidites and upper coarse-grained immature turbiditic deposits (slope-apron turbidites, to be discussed later) (Figure 5). The

(above), and representative photo of the slumped MTDs (below). The cliff is about 12 m high.

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

**3.2. Haboro Basin**

**Figure 5.** Rose diagrams showing paleocurrent directions of the middle Miocene turbidites in the Haboro Basin (above: compiled from [2, 14]), and transverse profile of the basin fill (below: modified from [2]). The solid and open petals of rose diagram mean the direction measured from sole-marks and clast fabric (solid) and cross lamination (open). Pink-colored dashed lines are stratigraphic markers (ash turbidite beds). CF: Chikubetsu Fault. See Figure 1 for legend of the index map.

A north-south stretched topographic high lying parallel to the basin-axis separates an initial depression into two segments [2, 3, 14, 28]. Basinal turbidites buried the segments progres‐ sively from the inner (eastern orogen-ward segment) to the outer (western foreland-ward segment), and flattened the irregular bottom of the basin (Figure 5). Slumped MTDs are developed, especially in the inner segment [27]. Subsequently deposited coarse-grained turbidites characterized by amalgamated and channelized sandy/gravelly beds are prograded on the basinal turbidites [2, 3]. These turbiditc deposits contain abundant large granite clasts indicating a sediment supply from the Hidaka Belt in the east. Sole marks within the basinal turbidites in the central to southern outcrops reveal southwest-to-south directed flows. The coarse-grained turbidites found at the stratigraphically uppermost part of the northern outcrop reveal west-directed flow (Figure 5).

The basin is filled mainly with lower basinal turbidites and upper coarse-grained slope-apron turbidites of the middle to late Miocene Kawabata Formation, 3500 m thick. The basinal turbidites buried the irregular basin floor and they onlap to the slope in the western and southern margins of the basin. The slope-apron turbidites are longitudinally prograded

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**Figure 7.** Rose diagrams showing paleocurrent directions of the middle to late Miocene tubidites in the Ishikari Basin. The solid and open petals of diagram mean the direction measured from flute marks and clast fabric (solid) and from groove marks and parting lineation (open). Note the diagrams are drawn with square root scaling. See Figure 8 for

The basinal turbidites are underlain by shelfal, slope, and basin-plain muddy deposits (the early Middle Miocene Takinoue Formation). The muddy deposits are generally several hundreds of meters thick, but reach a thickness of 1000 m in local areas, where thick chaotic intervals (70–420 m thick), consisting of slump and debris flow deposits, occur [29]. The debris flow deposits notably contain abundant boulder-size serpentinite and sandstone blocks in

Three chaotic intervals (slumped MTDs) are also encased within the basinal turbidites, and these consist of slump and debris flow deposits similar to the intervals in the Takinoue Formation. They lack serpentinite blocks, but contain granite cobbles-boulders, indicating a sediment supply from the Hidaka Belt [9]. Subsequently, clastic compositions changed synchronous with the change in sedimentary style from basinal turbidites to coarse-grained slope-apron turbidites. Abundant granite and hornfels grains in the basinal turbidites decrease upward. This is counterbalanced by an increase of green rocks, chert, and coeval andesiticrhyolitic volcanic grains in the slope-apron turbidites (Figure 9). Such change in clastic composition indicates that a sediment provenance advanced from the Hidaka Belt to the

southward onto the basinal turbidites (Figure 8).

approximate localities of the measured data.

addition to slate and chert cobbles.

Sorachi-Yezo Belt.

#### **3.3. Ishikari basin**

The Ishikari Basin is characterized by its very narrow basin geometry (Figure 6). At present, the Middle Miocene basin fill reveals a north-south stretched elongated distribution (<15 km wide and 60 km long). Although the original dimension of the basin is uncertain because of the post-dated basin deformation, the strongly concentrated paleoflow data in the basin-axis direction indicates a confined basin floor (Figure 7). The primary western margin of the basin, where a thrust-front advanced during the Pliocene age [5], was bordered by horst structures formed from the basement rocks and the overlying Upper Oligocene to Lower Miocene strata.

**Figure 6.** Cross section of the Ishikari Basin (after [6]) suggesting very narrow basin geometry concordant with strong‐ ly concentrated paleoflow data shown in Figure 7. UAF: Umaoi active fault. See Figure 1 for legend of the index map.

The basin is filled mainly with lower basinal turbidites and upper coarse-grained slope-apron turbidites of the middle to late Miocene Kawabata Formation, 3500 m thick. The basinal turbidites buried the irregular basin floor and they onlap to the slope in the western and southern margins of the basin. The slope-apron turbidites are longitudinally prograded southward onto the basinal turbidites (Figure 8).

A north-south stretched topographic high lying parallel to the basin-axis separates an initial depression into two segments [2, 3, 14, 28]. Basinal turbidites buried the segments progres‐ sively from the inner (eastern orogen-ward segment) to the outer (western foreland-ward segment), and flattened the irregular bottom of the basin (Figure 5). Slumped MTDs are developed, especially in the inner segment [27]. Subsequently deposited coarse-grained turbidites characterized by amalgamated and channelized sandy/gravelly beds are prograded on the basinal turbidites [2, 3]. These turbiditc deposits contain abundant large granite clasts indicating a sediment supply from the Hidaka Belt in the east. Sole marks within the basinal turbidites in the central to southern outcrops reveal southwest-to-south directed flows. The coarse-grained turbidites found at the stratigraphically uppermost part of the northern outcrop

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

The Ishikari Basin is characterized by its very narrow basin geometry (Figure 6). At present, the Middle Miocene basin fill reveals a north-south stretched elongated distribution (<15 km wide and 60 km long). Although the original dimension of the basin is uncertain because of the post-dated basin deformation, the strongly concentrated paleoflow data in the basin-axis direction indicates a confined basin floor (Figure 7). The primary western margin of the basin, where a thrust-front advanced during the Pliocene age [5], was bordered by horst structures formed from the basement rocks and the overlying Upper Oligocene to Lower Miocene strata.

**Figure 6.** Cross section of the Ishikari Basin (after [6]) suggesting very narrow basin geometry concordant with strong‐ ly concentrated paleoflow data shown in Figure 7. UAF: Umaoi active fault. See Figure 1 for legend of the index map.

reveal west-directed flow (Figure 5).

**3.3. Ishikari basin**

**Figure 7.** Rose diagrams showing paleocurrent directions of the middle to late Miocene tubidites in the Ishikari Basin. The solid and open petals of diagram mean the direction measured from flute marks and clast fabric (solid) and from groove marks and parting lineation (open). Note the diagrams are drawn with square root scaling. See Figure 8 for approximate localities of the measured data.

The basinal turbidites are underlain by shelfal, slope, and basin-plain muddy deposits (the early Middle Miocene Takinoue Formation). The muddy deposits are generally several hundreds of meters thick, but reach a thickness of 1000 m in local areas, where thick chaotic intervals (70–420 m thick), consisting of slump and debris flow deposits, occur [29]. The debris flow deposits notably contain abundant boulder-size serpentinite and sandstone blocks in addition to slate and chert cobbles.

Three chaotic intervals (slumped MTDs) are also encased within the basinal turbidites, and these consist of slump and debris flow deposits similar to the intervals in the Takinoue Formation. They lack serpentinite blocks, but contain granite cobbles-boulders, indicating a sediment supply from the Hidaka Belt [9]. Subsequently, clastic compositions changed synchronous with the change in sedimentary style from basinal turbidites to coarse-grained slope-apron turbidites. Abundant granite and hornfels grains in the basinal turbidites decrease upward. This is counterbalanced by an increase of green rocks, chert, and coeval andesiticrhyolitic volcanic grains in the slope-apron turbidites (Figure 9). Such change in clastic composition indicates that a sediment provenance advanced from the Hidaka Belt to the Sorachi-Yezo Belt.

**3.4. Hidaka Basin**

for legend of the index map.

progressively younger to the west (Figure 10).

The Hidaka Basin, (>30 km wide and 70 km long), is fragmented by thrust propagation. The deformation is especially intensive in the eastern area. Depositional ages of the basin fill are

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**Figure 10.** Cross section of the Hidaka Basin (after [6]). Paleoflow data are measured from flute marks and AMS (ani‐ sotropy of magnetic susceptibility) fabrics of the Middle Miocene turbidites (compiled after [30, 31, 33]). See Figure 1

In the eastern margin of the basin, pre-orogenic shallow marine to shelfal muddy deposits of the early-Middle Miocene age are widely distributed (the lower part of the Niniu Formation and its correlatives, several hundred meters thick). These muddy deposits are locally overlain by turbiditic deposits accompanied with slumped MTDs (the upper part of the Niniu Forma‐ tion, >500 m thick). A lenticular gravelly body (slope-fan deposits, which are discussed later),

**Figure 8.** Basin-axial sedimentary profile of the Ishikari basin fill.

**Figure 9.** Modal evolutions of the lithic fragments in the coarse-grained sand to granule-grade turbiditic beds filling the Ishikari Basin, measured by point-counting method for thin section. White diagonal hatch indicates the horizon of the change in sedimentary style from basinal to slope-apron turbidite system. Stratigraphic level is based on the thick‐ ness.
