Sohei Kaizuka

Recent formations and their basal topography in and around Tokyo Bay, central Japan

Abstract This paper summarizes the subsurface geology of the recent (both Holocene and latest Pleistocene formations and the buried topography beneath them in and around Tokyo Bay, the type area of the late Quaternary in Japan. Buried abrasion platforms in the buried topography are classified into upper (ca. 0 to −10 m high) and lower (ca. −20 to −40 m) platforms; upper and lower buried river terraces are also distinguished, and are correlated to the subaerial late Pleistocene terraces of Tc1 and Tc2, respectively. A buried valley system is elucidated, of which the trunk valley floor reaches −70 m in Tokyo and emerges into a flat surface at the shelf edge in the entrance to Tokyo Bay. Approximate dates for these geomorphic surfaces are given. The height of sea level contemporaneous with the buried valley floor (ca. 20,000–15,000 yr BP) is estimated at about −135 m. The recent formations are divided into two members, upper and lower, by a middle sand bed, in addition to the lowest buried valley floor gravel. The lower member, which is composed of brackish to marine deposits of complicated lithofacies, was accumulated in narrow drowned valleys during the early stage of the Yurakucho (Flandrian) transgression. The middle sand bed is the foreset bed of deltas, which was formed during a slight regression between ca. 11,000 and 10,000 yr BP. The upper member, which consists mainly of widespread homogeneous marine clay and deltaic sand, was accumulated in a wide bay and its embayments during the late stage of the Yurakucho transgression and the following stage of a relatively stable sea level.

Coastal evolution at a rapidly uplifting volcanic island: Iwo-jima, western pacific ocean

Abstract Iwo-jima is the emergent part of a large submarine volcano on the Izu-Ogasawara island arc. Approximately 25 marine terraces form the major geomorphic components of the island. These have been formed in response to rapid domal uplift and marine abrasion, both of which may have been irregular in time and in space. The oldest marine terrace at about 110 m above sea-level has an age of 500–800 14 C BP, indicating an average uplift rate of 150–200 mm/year. Surrounding the island is a ca. 1 km wide wave-cut and wave-built platform less than 15 m deep. Marine planation has maintained the platform at nearly the same level despite rapid uplift.