Introduction to the Landforms and Geology of Japan

GLGArcs

Tei Mélange and Muroto
  -- Shimanto accretionary complex

Tei mélange (Nishibun, Geisei Village)

A beach on the east by the Nishibun fishing port in Geisei Village, Kochi Prefecture is about 25 km from Kochi City on the way to Cape Muroto (Figure 1). This beach is in a mélange zone of the Cretaceous Shimanto belt. Mélange exposed here is called the Tei mélange or Yokonami-Tei mélange. The beach is a good observational point for rocks constituting oceanic plate stratigraphy. However, I have not made sure whether an outcrop of turbidite can be seen or not, although it is known that the turbidite beds (alternating beds of sand and mud) contact with the mélange near the beach. Please see “Cape Gyodo” for turbidite.


View Tei melange and Muroto in a larger map

Sumiyoshi beach
Photo 2 Outcrops on the beach (the Nishibun fishing port is on the left side)
Black rocks in the foreground are basaltic pillow lava

Mélange

The mélange contains exotic blocks of basalt (pillow lavas), limestone, chert, and varicolored shale in black muddy/sandy matrix. The tei mélange is characterized by which the proportion of matrix to the blocks is low and has fragments of sandstone (not as matrix) (Osozawa, 2006). The age of mudstone/sandstone of the matrix is about 70 million years, similar to that of the turbidite near the beach. The matrix consists mainly of granitic clastic materials, such as quartz, feldspar, and mica, derived from the land. The exotic blocks in the matrix were scraped off from the subducting oceanic plate.

Tei melange
Photo 3
Mélange [Another window]  (A closeup photo of the outcrop is in another figure page.)

Little melange
Photo 4
 Little mélange [Another window]

Pillow lava

The pillow lavas were produced near the equator. Although they were altered, most of the pillow lavas have the characteristics of mid-ocean-ridge basalt (MORB). Some cavities of the lavas contain reddish brown limestone with coccolith fossils (remains of algae, calcareous nannoplanktons) giving an age of 130 million years.

Pillow lava
Photo 5 Pillow lava (Closeup of a rock in Photo 2) [Another window]

Chert

The chert is red or green bedded rock, whose layers are several centimeters wide, mainly composed of radiolarian fossils 130 to 90 million years old (Early to Middle Cretaceous). Most of the red chert is of the Middle Cretaceous and is alternated with red shale that is fine clayish rock including quartz particles and few radiolarian fossils. The red bedded chert alternating with the shale is about 90 million years old.

Chert
Photo 6 Bedded chert [Another window]

Varicolored shale

The varicolored shale is green, red, or white bedded hemipelagic sedimentary rock 90 to 80 million years old. The red or green shale is mudstone containing radiolarian fossils and terrigenous silty clastic materials such as quartz and feldspar particles. The white shale has volcanic ash.

(Descriptions of rocks in this section are based on Taira, 2004.)

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