Tephrostratigraphy after marine oxygen isotope stage 11 in the western part of Yokohama City, Japan

Kasama, T.

Kanagawa Kenritsu Hakubutsukan Kenkyu Hokoku, Shizen Kagaku = Bulletin of the Kanagawa Prefectural Museum, Natural Science 35: 1-5

2006


Accession: 020211611

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Abstract
In the middle of 1980s to the first half of 1990s, many public works had been done in the western part of Yokohama City and resulted in giving us important geological knowledge. Almost marine deposits except the Kazusa Group are divided into the Konan (Kamikurata) Formation [marine oxygen isotope stage 11] and the Shimosueyoshi Formation [marine oxygen isotope stage 5e] in this area. Deposits between the Konan formation and the Shimosueyoshi formation mainly consist of subaerial tephras derived from the Hakone Volcano, in spite of including high sea level stages. Scoria layers adjacent to Kn-1 suggest frequent but small eruptions. Such tephras are not found other horizon. Some tephras contain accretionary lapilli, which are thought to have resulted from the phreatoplinian eruption, because Yokohama City is about 50 km distant from the Hakone Volcano. In the marine oxygen isotope stage 5 to 1, the most remarkable tephra is Hakone Younger pumice flow deposits (Hk-T(pfl)). The plinian deposit located just under the ignimbrite is called Tokyo Pumice (Hk-TP) [marine oxygen isotope stage 4]. Hk-T(pfl) deposits consist of two facies. Massive facies (flow1(1a+1b), flow2(2a+2b)) are distributed along the Akuwa River, the Nase River and the Kashio River, because these rivers had existed at the present place since marine isotope stage 4 and stratified facies (ST(a)-ST(f)) are distributed on the top of the Tama Hills. Hk-T(pfl) is thought to flow over the watershed of the Tama Hills.