Introduction to Landforms and Geology of Japan

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Structures Found on Wave-cut Benches in the Southern Miura Peninsula

Chaotic rock

Rock in Photo 10 chaotically contains blocks in various shapes. Such rock is called chaotic rock, indicating disordered sedimentation. The matrix of chaotic rock in the Misaki Formation is sand and gravel and the blocks are mud. The matrix grains are coarser than those of the blocks. This feature is contrary to the character of Mesozoic and Paleozoic chaotic rocks in accretionary complexes such as the Shimanto Group; generally, the matrix is mud (e.g., Chaotic rock [mélange] in Kuromi). The blocks are intruded or surrounded by the matrix. Some borders between the blocks and matrix are vague. There are also chaotic rocks injected into between beds. The formation of chaotic rocks of the Misaki Formation is attributed to liquefaction; coarse-grained sediments including sand and gravel liquefied due to tremors (e.g., earthquakes) and intruded into mud layers with pressure to break mudstone into blocks. Pressured liquefied sediment moves not only downward by gravity but also toward the directions easy to move. However, chaotic rocks may also be formed by submarine sliding. (Ogawa, 2007a)

Chaotic rock

Photo 10: Chaotic rock (Hamamoroiso) [Click to enlarge]

Chaotic rock

Photo 11: Fine-grained mud blocks within matrix containing scoria (Hamamoroiso) [Click to enlarge]

Neptunian dike

Photo 12: Neptunian dike (Hamamoroiso) [Click to enlarge]
Liquefied sediment entered into a crack of the silt bed under the chaotic layer. The dike formed under the seafloor. The scale is 53 cm long.

Melange

Photo 13: Mélange (Hamamoroiso) [Click to enlarge]
This outcrop is chaotic rock but the blocks are sand beds. The strata of cliff are considered as mélange (Murata, 2000).

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