Introduction to the Landforms and Geology of Japan


Tei Mélange and Muroto
  -- Shimanto accretionary complex

Cape Gyodo (turbidite)

Alternating beds of sand and mud (turbidite) are well exposed at Cape Gyodo 2 km to the south of Kuromi. The strata (Muroto formation) on the beach are tilted almost perpendicularly and some of them were folded. Sand dikes intruded the beds like in Kuromi.

View Tei mélange and Muroto in a larger map

Cape Gyodo
Photo 10 Cape Gyodo [Another window]
Alternating beds of sand and mud

Turbidite is deposits laid down by turbidity currents which are flows of mixtures of sediment and water. These mixtures that are denser than sediment-free water flow down along the slope of sea or lake by gravity. Clastic materials produced by erosion on land move to the sea mainly through rivers and deposit in basins and trenches. Fine particles (mud) constantly deposit on the sea floor distant from the land (e.g. in a trench) to be mud beds. An event such as an earthquake causes a turbidity current containing sand that deposits on the mud bed. After the sedimentation by the turbidity current is completed, mud settles on the sand bed. The repetition of the sedimentation process creates alternating beds of sand and mud. The beds of turbidite in Muroto, which were originally horizontal, were deformed and tilted, and then were exposed on the land. The folded or broken strata resulted from which unsolidified or semi-solid sediments were deformed or destroyed by submarine sliding. Such structure is called a slump structure.

Slump structure
Photo 11 Folded beds (slump structure) [Another window]

The beach of the alternating beds of sand and mud is rugged. Because the mudstone is easier to erosion than the sandstone, the mud beds became grooves. The surface of sand beds exhibits a sedimentary structure. As turbidity current slows, largest (heavy) particles settle followed by smaller particles to form a graded bed. Since the current affects the way of particle deposition, sedimentary structures such as ripple laminae and parallel laminae are created. The sedimentary structure formed by turbidity current is referred to as “Bauma’s sequence”. Current ripples are also seen on the upper surfaces of sand beds.

Bauma's sequence
Fig. 5 Bauma's sequence model
Some parts in this model are often absent from actual sand beds depending on depositional conditions.

Side of sand bed
Photo 12 Sedimentary structure of a sand bed [Another window]
This bed consists of lower parallel lamination, cross lamination, and upper parallel lamination parts, lacking a graded bedding part. Probably, large particles in a turbidity current, which could compose a graded bedding part,  had deposited somewhere before this sand bed was created. Black beds on and under the bed are mud (shale).

Linguoid current ripples
Photo 13 Linguoid current ripples [Another window]
A white line shows the length of about 50 cm.

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