**3.4. Hidaka Basin**

**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‐

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

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

ness.

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 progressively younger to the west (Figure 10).

**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 for legend of the index map.

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), 1000 m thick and 10 km width, is locally developed (the eastern part of the Abetsu and the Ukekoi Formations). The gravelly deposits pass rapidly down-dip to the south into the basinal turbidites [30].

In the central zone of the outcrops, middle to late Miocene turbiditic deposits (3000 m thick) are distributed (the western part of the Abetsu and the Ukekoi Formations, and the Azamizawa Formation). Basinal turbidites developed in the middle Miocene age show a slightly south‐ ward-fining megatrend. Paleocurrent directions show dominant southwestward flows in the northern area (Figure 10), although the basin-wide tendency is undefined. The basinal turbidites are overlain by late Miocene siliceous sandy mudstones and coarse-grained slopeapron turbidites [30, 31], but the detailed sedimentary style of the late Miocene deposits

Foreland Basins at the Miocene Arc-Arc Junction, Central Hokkaido, Northern Japan

http://dx.doi.org/10.5772/56748

143

In the westernmost zone of the outcrops, Pliocene sandy to gravelly deposits interfinger with and/or are prograded onto shelfal sandy mudstones [32]. The sand and gravelly beds often show large-scale cross bedding. The distribution area of the Pliocene deposits is roughly separated from the Miocene turbidites by a thrust fault parallel to the basin axis. The total

The detrital composition of the Middle Miocene to Pliocene succession in the Hidaka Basin records an exhumation history of deep-seated lower crustal rocks of the Chishima Arc (the

A sedimentary succession within the primary inner foredeep can be observed in the eastern Tenpoku Basin (Figure 4). The sedimentary fill typically includes syn-orogenic packages of chaotic MTDs. The MTDs rest directly on the pre-orogenic shallow marine sandstones and often bear abundant intrabasinal blocks. Thus, the primary inner foredeep setting was quite unstable because of rapid syn-depositional subsidence and thrusting. The laterally discontin‐ uous distribution of MTDs and sandy/gravelly turbiditic deposits with locally presented sand/ mud interbeds suggests the ponding of frequently generated dense flows in small accommo‐ dation spaces of the irregular basin floor. As discussed in the next subsection, a large collapse event also resulted in a thick accumulation of cohesive debris flow deposits in the axial

In the Hidaka Basin, a thick gravelly body is locally present in the boundary zone between the inner and the axial foredeep. Its basin-axial paleoflow direction suggests the development of a sediment conduit controlled by the thrust fault itself (cf. [34]). The gravelly body rapidly passes down-dip into the basinal turbidites of the axial foredeep and is interpreted as slope-

The sedimentary features of primary wedge-top basin (i.e., basins on thrust sheets and located at proximal side of inner foredeep) during the early syn-orogenic phase of foreland basin evolution are not evident. Non-marine to shallow marine deposits of middle to late Miocene age occur adjacent to the Tenpoku and Haboro Basins, but most of the deposits were formed

remains unknown. The sandy mudstones yield fossils of shallow marine shells [30].

thickness of the Pliocene deposits reaches 1500–2000 m.

Hidaka metamorphic rocks) [9, 15, 30].

**4.1. Primary wedge-top to inner foredeep**

fan deposits fed by a northern point source [30].

during the post-orogenic phase in the northern collision zone.

**4. Sedimentary processes**

foredeep setting.


**Table 1.** Summary of the features of the depressions formed in the Miocene foreland basin area, Hokkaido, northern Japan.

In the central zone of the outcrops, middle to late Miocene turbiditic deposits (3000 m thick) are distributed (the western part of the Abetsu and the Ukekoi Formations, and the Azamizawa Formation). Basinal turbidites developed in the middle Miocene age show a slightly south‐ ward-fining megatrend. Paleocurrent directions show dominant southwestward flows in the northern area (Figure 10), although the basin-wide tendency is undefined. The basinal turbidites are overlain by late Miocene siliceous sandy mudstones and coarse-grained slopeapron turbidites [30, 31], but the detailed sedimentary style of the late Miocene deposits remains unknown. The sandy mudstones yield fossils of shallow marine shells [30].

In the westernmost zone of the outcrops, Pliocene sandy to gravelly deposits interfinger with and/or are prograded onto shelfal sandy mudstones [32]. The sand and gravelly beds often show large-scale cross bedding. The distribution area of the Pliocene deposits is roughly separated from the Miocene turbidites by a thrust fault parallel to the basin axis. The total thickness of the Pliocene deposits reaches 1500–2000 m.

The detrital composition of the Middle Miocene to Pliocene succession in the Hidaka Basin records an exhumation history of deep-seated lower crustal rocks of the Chishima Arc (the Hidaka metamorphic rocks) [9, 15, 30].
