Introduction to the Landforms and Geology of Japan


Tei Mélange and Muroto
  -- Shimanto accretionary complex

Records of uplifts

The Muroto peninsula has uplifted and the records of uplifts in the Late Quaternary are well preserved in landforms. The most outstanding landforms are coastal terraces created by sea-level change and crustal movement. Hills with the same height of flat surface (old shore platforms) are arranged along the western coast of the Muroto peninsula. The broad platforms (referred as M1 surface) about 150 m high around Cape Muroto were formed at sea level 125 thousand years ago. Multiple old shore platforms are found in this area. Three platforms whose heights are different each other were formed in the Holocene at the foot of the hills. The elevations of the M1 surface become low northward, showing tilting. This tilting is also found in coastal terraces of the outer zone (southwest Japan outer zone). It is known that coseismic uplift caused by plate subduction at the Nankai Trough and the activity of branch faults from the trough has been active since about 300 thousand years ago. The 1946 Nankai earthquake (8.0 on Richter Scale) uplifted Cape Muroto by 1.27 meters. Other topographic records of uplift are raised notches and sea caves (e.g. Mikurodo). They were produced at a sea level and then raised.

Coastal terraces
Photo 19 Coastal terraces [Another window]
View from the top of a hill (near a lighthouse)

Photo 20 Raised notch (arrow) near Spot 1 [Another window]
Notch is formed on the lower part a sea cliff by wave erosion or corrosion of seawater. The level of a pool in the front of the notch is higher than the sea level.

Photo 21 Front of Mikurodo [Another window]
A sea cave, Mikurodo, is at the back of a parking space with a parked car, 100 to 120 m away from the sea (see Fig. 6 on Page 5). The car park was a shore platform and the cliff was a sea cliff. The road is Route 55.

Some fossils are useful to know past uplifts. Assemblages of small tubes of calcium carbonate adhering to gabbroic and other rocks are found at levels several meters higher than the sea level. They are fossil tubes of annelid worms (Pomatoleios kraussii, Yakko-kanzashi [Japanese common name]). The worms build their calcareous tubes on rocks in the middle part of an intertidal zone. When the area rises above the sea level, the tubes emerge and then fossilize. So, the fossil tubes tell us old shore lines.

Photo 22 Fossil tubes on gabbroic rock [Another window]
Assemblages of fossil tubes adhered to gabbroic rock at different heights, indicating past sea levels (red circles). The height above the sea level of tubes at the highest position is about 6 meters. (Near Spot 1)

Fossil tubes
Photo 23 Assemblage of fossil tubes [Another window]
Annelid worms (Pomatoleios kraussii, Yakko-kanzashi [Japanese common name]) built the tubes with the diameter of about 2 mm.    

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