**4. Sedimentary processes**

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

**deformation Width and length at Remarks**

**Post-orogenic** (overlying deposits)

Siliceous / diatomaceous mudstone (1000–2000 m thick)

Shallow marine sandstone (<500 m thick) and subsequently aggraded diatomaceous mudstone (500–1000 m thick)

–

–

**Syn-orogenic** a: primary inner foredeep, b: axial foredeep, c: transition from axial to inner foredeep

a: MTD dominant, coarse-grained turbiditic deposits (1000–2000 m thick) b: Muddy turbidites and basin-plain muddy de-posits (2000–3000 m thick)

a: MTD dominant, coarse-grained turbiditic deposits, details unknown b: Basinal turbidites (1500–3000 m

c: Slope-apron turbidites (1000–1500

b: Slope to basin-plain mudstones (<500 m) and overlying basinal turbidites (300–2400 m thick) c: Slope-apron turbidites (2000–2800 m thick) locally accompanied with siliceous / diatomaceous mudstones

a: MTD dominant, coarse-grained turbiditic deposits (>500 m thick), locally developed slope-fan deposits

b: Basinal turbidites (middle Miocene, 1500 m thick), shelf-type fan-delta (Pliocene, 1000–1800 m thick) c: Slope-apron turbidites (1100–2600

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

thick)

m thick)

a: Not preserved

(1400 m thick)

m thick)

**Tectonic**

Slumped interval (200 m thick) in the basin-plain muddy deposits of axial foredeep

Initially segmented axial foredeep

Very narrow geometry Longitudinal sediment transport Pebbly mudstones in the basinal turbidite succession

Depocenter migration

Restrictive Declining thrust activity in early Late Miocene

Moderate The western margin of the basin is bordered by KBH Declining thrust activity during late Miocene ?

Moderate–high The western margin of the basin is bordered by KBH Continuing thrust activity to the present?

High Continuing thrust activity to the present

**Dimension Stratigraphy and sedimentary features**

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

**Pre-orogenic** (underlying deposits)

Shallow marine sand-

stone

turbidites [30].

**Locality**

**middle Miocene age**

**Haboro** >50 km, 90 km Shelfal mudstone

**Ishikari** >>15 km, 60 km Shelfal mudstone

**Hidaka** >30 km, >70 km Shelfal mudstone

Japan.

**Tenpoku** 80 km, >60 km
