Introduction to Landforms and Geology of Japan

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

Faults and folds

Small faults and folds are common in the Misaki Formation. Reverse faults are dominant but normal faults and strike-slip faults are also found. Some faults formed in the unconsolidated or partially consolidated sediments have a dragged structure (Photo 7). Also, folds were made in unconsolidated sediments that flowed and deformed.

Faults

Photo 6: Reverse faults (Hamamoroiso) [Click to enlarge]
These faults look like a conjugate fault set.
[Conjugate fault set: A cross-cutting set of fault planes which ideally intersect at angles of 60 degrees and 120 degrees, and have both left-handed and right-handed shear senses. The line of intersection is parallel to the direction of intermediate principal stress (σ2). The maximum principal stress bisects the acute angle and the minimum principal stress bisects the obtuse angle. (Cited from Dictionary of Earth Sciences 2nd Ed. Oxford University Press, 1999)]

Dragged structure

Photo 7Beds dragged by faulting (Arasaki) [Click to enlarge]

Photo 8 shows an outcrop of slump structure, which was designated as a natural monument by Kanagawa Prefecture. A slump structure is formed by which unconsolidated sediments slide down on a slope because of a landslide and so on. In the outcrop in the photo, it appears that the beds in the nearly center part of the cliff slid down on a slope dipping to the left and the beds in the left side folded. A guide plate placed by the Miura City board of education beside the cliff explains that the beds that slid down from the right side produced the slump structure. However, there is a different interpretation. According to Ogawa 2007a, the dipping beds are identical with the horizontal beds under those in the lower part of the right side. Therefore, the dipping beds went up from the left side onto the horizontal beds by reverse faulting.

Slump structure

Photo 8: Slump structure (Kaitocho near Hamamoroiso) [Click to enlarge]

Vein structure

Short vertical lines arrayed laterally under a pen in Photo 9 at Arasaki are referred to as a vein structure. The veins can be traced along the bed containing the structure. Vein structures are found in some silt beds, not in sand beds. The orientations of most veins are north-south in the Miura and Boso Peninsulas. The vein structures in the Miura and Boso Peninsulas were first reported in outcrops on land.

The structure consists of veins with constant heights and spacing. The formation of structure is thought to need shear waves with very short wavelength (millimeters to centimeters) generated by the density flow, debris flow, landslide, or faulting. (Ohsumi and Ogawa, 2008)

Vein structure

Photo 9: Vein structure (Arasaki) [Click to enlarge]

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