Introduction to Landforms and Geology of Japan

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

Load cast

Load casts are irregular or roll-like protrusions at the base of a bed. These are produced by fluidization of sand or mud in soft deposits due to uneven load on the bedding plane in the condition of reverse density gradient between two beds adjoining each other. For instance, load casts are often seen in the boundary between beds where the upper sand bed is denser than the lower mud bed. The sand in the upper bed hangs into the mud bed and the mud rises into the sand bed like flame; it is called flame structure. When the sand in the upper bed flows over a little after the flame structure formed, the flames may turn in the same orientation.

Flame structure (Jogashima)

Photo 16: Flame structure (Jogashima) [Click to enlarge]
A flame-shaped white bed is tuff underlying scoriaceous sand. This tuff is also called "Odori tuff (dancing tuff)" in Japanese. A minor reverse fault cut the beds. The height of flames is 7 cm or so.

Flame structure (Arasaki)

Photo 17: Flame structure and lamina (Arasaki) [Click to enlarge]

Flame structure 2 (Arasaki)

Photo 18: Deformed bed with flame structure (Arasaki) [Click to enlarge]
A bed probably flowed and deformed after the formation of flame structure.  The structure is upside-down in the photo because black flames are sand. The dipping beds give the upside-down appearance of the structure at the photo location.

Others

Other structures found on wave-cut benches include dish structures produced when water escapes from deposits, ripple marks created by water flow, and trace fossils such as borings.

Ripple mark  

Photo 19: Ripple marks (Hamamoroiso) [Click to enlarge]

Trace fossils

Photo 20: Trace fossils (Hamamoroiso) [Click to enlarge]

Although the Misaki Formation comprises deposits of accretionary prism as mentioned above, all sedimentary structures introduced in this topic are not unique to the deposits, found in deposits formed in other settings. However, those are the keys to understand accretionary prisms and the geologic history of this area.

Access and References

Access

Photos in this topic were taken at Arasaki, Hamamoroiso, and Jogashima. Visiting these spots at low tide is better. Jogashima, an island, is a major scenic site in the Miura Peninsula.

Locations: see Figure 2 or Google Map

Arasaki [荒崎]: Take a bus for Arasaki at Misakiguchi [三崎口] Station (Keikyu line) and get off at Arasaki.

Hamamoroiso [浜諸磯]: Take a bus for Hamamoroiso at Misakiguchi Station and get off at Hamamoroiso. Also, take a bus for Misaki Higashioka [三崎東岡]/Misakiko [三崎港]/Jogashima [城ヶ島], get off at Tenjinmachi [天神町], and walk westward (right to the travelling direction of the bus; about 15 minutes).

Jogashima [城ヶ島]: Take a bus for Jogashima at Misakiguchi Station and get off at Jogashima.

References

References referred in this topic are in the reference page.

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