**4.2. Axial foredeep**

In the Tenpoku Basin, much of the sandy to gravelly deposits were trapped in the accommo‐ dation space at the inner foredeep. As a result, the axial foredeep was filled mainly by basinplain muddy deposits more than 2000 m thick. A 200-m-thick chaotic interval, containing exceptionally of sandy to granular grains in muddy matrices, occurs in the basin-plain muddy succession, suggesting large mass failure events in the proximal setting (i.e., the inner fore‐ deep).

Except for the Tenpoku Basin, the axial foredeep is filled with a 1000–3000 m thick turbiditic succession mainly consisting of basinal turbidites (Figure 11a). The basinal turbidites are underlain by shelfal, slope, and/or basin-plain, muddy sediments with a condensed interval of blackish mud at their top, showing gradual but larger amount subsidence compared to the primary inner foredeep setting.

The basinal turbidites consists of monotonous interbeds of parallel-sided sand/mud beds. Spatiotemporally, they show no apparent coarsening or fining facies trends, other than a sandier upward trend in their basal section. The monotonous interbeds incorporate with isolated beds and packages of thick-bedded pebbly sandstone several tens of meters thick. These package-forming beds are more or less graded and unchannellized, and rarely show bed amalgamation. The concentration of paleoflows along the basin axis suggests that flows entering from lateral entry point(s) were deflected and transformed to basin-axial flows, which may explain the lack of an ordered facies trend. The basinal turbidites are transported by efficient turbidity currents in the confined basin plain (*sense* [35]).

Slumped MTDs intervene in the basinal turbidites of the axial foredeep and are particularly well developed in the Ishikari Basin. The MTDs mainly consist of debris flow deposits (pebbly mudstone) that contain many intrabasinal blocks and extrabasinal cobbles-boulders outsized for the succession (Figure 11b). These debris flow deposits are traceable basin-wide, suggesting large collapse events at the basin margin settings (i.e., the inner foredeep and wedge-top). MTDs of a similar scale do not exist in the slope-apron turbidites of the upper stratigraphic level.

## **4.3. Transition from axial to inner foredeep**

Temporally, an axial foredeep setting evolves into an inner foredeep setting owing to thrust propagation. Other than in the Tenpoku Basin, the basinal turbidites of the axial foredeep are overlain by a turbiditic succession consisting of poorly sorted and coarser-grained sand to gravel beds interfingered with monotonously interbedded sandstone and sandy mudstone. As a whole, these coarse-grained deposits form a gravelly wedge prograded on the basin floor of the inner to the axial foredeep settings. In the Haboro Basin, the gravelly wedge prograded foreland-ward, while the wedge in the narrow Ishikari Basin shows axial progradation southward.

show variable facies such as disorganized, massive, cross-stratified, crudely laminated to well laminated, or crudely graded to graded bedding. Some beds consist of gravel, sand, and minor thin mud layers partitioned by surfaces with abrupt grain-size reduction (tripartite beds). The sand beds frequently contain plant debris and the associated mudstones are sandy and bioturbated. Thus, these deposits were transported by poorly-efficient dense flows, and filled multiple chutes developed at relatively proximal and shallower settings than the former basinplain setting (e.g., the slope-apron setting: [36]). The paucity of muddy deposits can probably be attributed to the finer-grained dilute portions of the flows being stripped off and bypassing the area because of consistent infilling of the confined basin (e.g., [37]). In addition, as discussed

multiple chutes.

**Figure 11.** Field occurrence of the Miocene turbidites in the Ishikari Basin. (A) Monotonous, mud-prone interbeds of basinal turbidites, (B) extrabasinal-cobble bearing pebbly mudstone in the basinal turbidite succession, (C) sand-prone interbeds and an amalgamated sand bed more than 5 m thick in the slope-apron turbidites, arrow: interval of aligned mudclasts (D) amalgamated stack of disorganized conglomerates and coarse-grained sandstones interpreted as fills of

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The thickly bedded sandstones and disorganized cobble to boulder conglomerates show an erosive base and frequent bed amalgamation (Figure 11c, 11d). These coarser-grained beds

**4.2. Axial foredeep**

primary inner foredeep setting.

deep).

level.

southward.

In the Tenpoku Basin, much of the sandy to gravelly deposits were trapped in the accommo‐ dation space at the inner foredeep. As a result, the axial foredeep was filled mainly by basinplain muddy deposits more than 2000 m thick. A 200-m-thick chaotic interval, containing exceptionally of sandy to granular grains in muddy matrices, occurs in the basin-plain muddy succession, suggesting large mass failure events in the proximal setting (i.e., the inner fore‐

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

Except for the Tenpoku Basin, the axial foredeep is filled with a 1000–3000 m thick turbiditic succession mainly consisting of basinal turbidites (Figure 11a). The basinal turbidites are underlain by shelfal, slope, and/or basin-plain, muddy sediments with a condensed interval of blackish mud at their top, showing gradual but larger amount subsidence compared to the

The basinal turbidites consists of monotonous interbeds of parallel-sided sand/mud beds. Spatiotemporally, they show no apparent coarsening or fining facies trends, other than a sandier upward trend in their basal section. The monotonous interbeds incorporate with isolated beds and packages of thick-bedded pebbly sandstone several tens of meters thick. These package-forming beds are more or less graded and unchannellized, and rarely show bed amalgamation. The concentration of paleoflows along the basin axis suggests that flows entering from lateral entry point(s) were deflected and transformed to basin-axial flows, which may explain the lack of an ordered facies trend. The basinal turbidites are transported by

Slumped MTDs intervene in the basinal turbidites of the axial foredeep and are particularly well developed in the Ishikari Basin. The MTDs mainly consist of debris flow deposits (pebbly mudstone) that contain many intrabasinal blocks and extrabasinal cobbles-boulders outsized for the succession (Figure 11b). These debris flow deposits are traceable basin-wide, suggesting large collapse events at the basin margin settings (i.e., the inner foredeep and wedge-top). MTDs of a similar scale do not exist in the slope-apron turbidites of the upper stratigraphic

Temporally, an axial foredeep setting evolves into an inner foredeep setting owing to thrust propagation. Other than in the Tenpoku Basin, the basinal turbidites of the axial foredeep are overlain by a turbiditic succession consisting of poorly sorted and coarser-grained sand to gravel beds interfingered with monotonously interbedded sandstone and sandy mudstone. As a whole, these coarse-grained deposits form a gravelly wedge prograded on the basin floor of the inner to the axial foredeep settings. In the Haboro Basin, the gravelly wedge prograded foreland-ward, while the wedge in the narrow Ishikari Basin shows axial progradation

The thickly bedded sandstones and disorganized cobble to boulder conglomerates show an erosive base and frequent bed amalgamation (Figure 11c, 11d). These coarser-grained beds

efficient turbidity currents in the confined basin plain (*sense* [35]).

**4.3. Transition from axial to inner foredeep**

**Figure 11.** Field occurrence of the Miocene turbidites in the Ishikari Basin. (A) Monotonous, mud-prone interbeds of basinal turbidites, (B) extrabasinal-cobble bearing pebbly mudstone in the basinal turbidite succession, (C) sand-prone interbeds and an amalgamated sand bed more than 5 m thick in the slope-apron turbidites, arrow: interval of aligned mudclasts (D) amalgamated stack of disorganized conglomerates and coarse-grained sandstones interpreted as fills of multiple chutes.

show variable facies such as disorganized, massive, cross-stratified, crudely laminated to well laminated, or crudely graded to graded bedding. Some beds consist of gravel, sand, and minor thin mud layers partitioned by surfaces with abrupt grain-size reduction (tripartite beds). The sand beds frequently contain plant debris and the associated mudstones are sandy and bioturbated. Thus, these deposits were transported by poorly-efficient dense flows, and filled multiple chutes developed at relatively proximal and shallower settings than the former basinplain setting (e.g., the slope-apron setting: [36]). The paucity of muddy deposits can probably be attributed to the finer-grained dilute portions of the flows being stripped off and bypassing the area because of consistent infilling of the confined basin (e.g., [37]). In addition, as discussed in the Chapter 5, changes in proximity of sediment source and/or in lithologic compositions in the orogen also caused a decrease in the fine-grained fraction.

stage of basin evolution. Conversely, the thick accumulation of turbiditic deposits in the axial foredeep requires persistently high-relief basin physiography from the hinterland to

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Consistent basin infilling and/or thrust propagation results in shallowing of the foredeep and a transition in sedimentary style [37]. In addition, synchronous changes in the detrital composition in the Ishikari and Hidaka Basins suggest a close relationship between the sedimentary style in the foredeep and structural deformation in the hinterland (see discussion of [41]). In the Ishikari Basin, the compositional change in the detritus sug‐ gests lateral growth of the orogen (extension from the Hidaka Belt to the Sorachi-Yezo Belt). The proximity between the depocenter and the newly emergent source area resulted in an increase in the coarser-grained fractions and generation of relatively poorly-efficient flows. In contrast, the evolution of the detritus composition in the Hidaka Basin implies exhuma‐ tion of deep-seated crustal rocks. Thus, the increase in coarse-grained deposits through poorly-efficient dense flows is attributed to an increase in the distributional area of crystalline rocks in the Hidaka Belt (Hidaka metamorphic rocks). Although the detrital compositional signal is unclear in the Haboro Basin, subaerial erosion of the basin fills in

the eastern area suggests syn-depositional thrusting near the basin margin.

A similar succession, consisting of lower basinal turbidites and upper coarser-grained turbiditic deposits, is well documented from the Miocene foredeep turbidites in the Northern Apennines [e.g., 42]. In that area, the change in sedimentary style was control‐ led by the narrowing and closure (shallowing) of the foredeep due to thrust propagation [42]. In Hokkaido, in contrast, the stratigraphic architecture does not show obvious narrowing of the foredeep depressions. The coarse-grained slope-apron turbidites occur basin-wide and their thickness is approximately the same as that of the basinal turbidites. In addition, they are covered with relatively thick siliceous/diatomaceous muddy depos‐ its. Nevertheless, the depressions appear to shallow gradually upward, as indicated by the dominance of bioturbation, plant debris, and shell fossils in the Late Miocene basin fills. Despite the migration of the depocenter in the Hidaka Basin, the depth of the depocenter gradually decreases foreland-ward until the Pliocene. Initial regional shallowing occurred around 13–14 Ma, beginning in the northern foreland area. An eustatic sea-level fall [43] is not sufficient to explain such a long-term gradual shallowing of the basin; however, a flexural rebound of underlying lithosphere [44] can explain such shallowing. The re‐ bound was probably caused by isostatic readjustment for a thinning orogen or decreased horizontal compressional stress corresponding to a gradual or stepwise decline of thrust

This paper provides an introduction to a tectonically controlled foreland basin stratigra‐ phy at the arc-arc collision zone of Miocene age in Hokkaido, northern Japan. Spatial

the basin plain and high-volume sediment input.

activities in central Hokkaido [45, 46].

**6. Conclusions**
