Introduction to Landforms and Geology of Japan

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

Tilted black and white strata

The Misaki Formation comprises alternating beds of silt, basaltic/scoria tuff and tuffaceous sand. The appearance of the Misaki Formation is characterized by black and white stripes (Photo 2). The white (bright gray) beds are siltstone and the black beds are scoria tuff.

 Arasaki strata

Photo 2: Strata of the Misaki Formation (Arasaki) [Click to enlarge]
A brown part of a cliff in the background is horizontal Quaternary beds overlying the Misaki Formation with unconformity.

The Misaki Formation is considered as deposits of the youngest accretionary prism (Miocene) that emerged from the ocean floor. Beds at Arasaki (Photo 2) are aged nine million to six million years. Japanese basement mainly comprises Mesozoic and Paleozoic accretionary complexes (see Basement Geologic Map). One of the significant differences between the Misaki Formation and other Mesozoic accretionary complexes is color of mudstone; it is white in the Misaki Formation and black in the Mesozoic accretionary complexes (e.g., Outcrop at Kuromi in the Shimanto zone). This color difference is related to the ancient abyssal environment. It is an amount of oxygen in deep ocean currents.

In the Mesozoic, the deep currents were poor in oxygen. Such environment prevented the decomposition of organic matter in sediments, which resulted in mud becoming black. The Mesozoic climate was much warmer than that of the present day, but it became cooler in the Cenozoic. The cold climate made glaciers in the Antarctic. Salinity level of the ocean in the polar regions increased because the sea ice does not contain salt. In addition, cold seawater is denser than warm seawater. As a result, seawater in the polar regions flowed down to the seafloor and global deep ocean currents developed, the characteristics of which was different from those of the Mesozoic. Frequent storms in the polar regions are responsible for which seawater takes in an abundant of oxygen. Therefore, the deep ocean currents have been rich in oxygen since the middle of Tertiary. This led to the oxidation of deposits on the seafloor. The color of oxidized iron is red for trivalent iron and green or blue for bivalent iron. The mud of the Misaki Formation includes bivalent iron. Although the silt beds are white, when the siltstone is broken, its fresh face is green. Weathering oxidized bivalent iron changes the color from green to white.
(Based on Ogawa, 2007a)

Black tuffaceous beds contains scoria and lapilli less than a few centimeters in size. Well-sorted scoria and lapilli layers in some beds are a good example of graded bedding. A few red scoria are found in scoria layers. The red scoria were ejected and oxidized on land, suggesting that there was a volcanic island or a part of marine volcano appeared above sea level near the peninsula.

Scoria

Photo 3: Well-sorted scoria (Hamamoroiso) [Click to enlarge]

These beds tilted at high angles: about 65 degree southeastward at Arasaki where Photo 2 was taken. The Misaki Formation strikes east on the whole, but the beds at Arasaki strike northeast or north. The difference of strike directions probably resulted from the rotation of the Miura and Boso area caused by the Izu-Bonin Arc colliding with the Honshu arc. Paleomagnetic data indicated about 80-degree rotation at Arasaki and 30-degree rotation at the southern end of the Boso Peninsula. Because Arasaki is closer to the Izu Peninsula, it is thought that the rotation angle is higher than those of the areas to the south of Arasaki in the Miura Peninsula and the Boso Peninsula. (Ogawa, 2007b)

Duplication

In the alternating beds of the Misaki Formation, some parallel beds look very similar to each other (Photo 4). Geological surveys for the Misaki Formation found duplicated strata (duplex structure, which is one of the features of accretionary prism) in places. Many small faults cutting beds and the duplication made the Misaki Formation extremely complicated and difficult to determine its actual distribution area and thickness. The thickness of the formation was estimated to be over 1300 meters (Editorial committee of Geology of Japan [Kanto Region], 1986), but there is another opinion that it should be thinner than the estimation (Kanie, 1999). An outcrop of small typical duplex is shown on Page 6.

Parallel beds

Photo 4: Look-alike beds (Arasaki) [Click to enlarge]

Differential erosion of wave-cut bench

 A wave-cut bench is a level or nearly level rocky platform extending outward from a sea cliff, created by wave erosion mainly in an intertidal zone. Wave-cut benches at Arasaki uplifted about 1.4 meters at the 1923 Great Kanto earthquake (magnitude 7.9). The surface of bench has various forms such as wave furrows, which are produced by erosion along joints and faults of rock, and depressions including tide pools and marine potholes.

Wave-cut bench

Photo 5: Differential erosion of wave-cut bench (Arasaki) [Click to enlarge]
This photo was taken at low tide.

Rugged surfaces of wave-cut benches are often seen in the Misaki Formation. The form looks like a washboard or its cross-section is like a comb. The ridges are the black beds (scoria tuff) and the furrows are the white beds (silt). Wave erosion created this relief; the white silt beds were eroded more easily than the black tuff beds. Such erosional form reflecting the properties of rock and geological structure is referred to as structural relief, and the unequal erosion is called differential erosion. It is generally thought that the harder rock is, the more it is resistant to erosion, but the main factor of differential erosion in the Misaki Formation appears not to be rock hardness. What is responsible for differential erosion?

Suzuki et al. (1970) investigated the properties of rocks constituting wave-cut benches at Arasaki to find the main factor of differential erosion. Their study result showed that the compressive strength, abrasive hardness, and impact strength of the siltstone are higher than the tuffaceous rock. This indicates that the siltstone is more resistant to external destructive forces such as compression, impact, and abrasion given by wave than the tuffaceous rock, which conflicts with the actual form mentioned above. Because wave-cut benches in an intertidal zone have times when they are below sea level and above sea level, the rocks are repeatedly subjected to drying and wetting conditions. They, therefore, investigated the water absorption characteristic of the rocks. The result demonstrated that, for the siltstone, the expansion on water absorption and shrinkage following drainage are greater than those of the tuffaceous rock. Consequently, it was presumed that the repetition of expansion and shrinkage of the siltstone under the drying and wetting conditions caused internal stress and produced countless minor joints, resulting in which the rock was easily broken by wave erosion. The expansion and shrinkage of the tuffaceous rock is much less than the siltstone. Thus, the tuffaceous beds were to be ridges and the silt beds to be grooves.

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