| Research Abstract |
The main results of this study can be divided into the following three parts.
(1) A boring investigation, totally 100m in core length, for a cold-seep chemosynthetic community in the lower Plesitocene Koshiba Formation at Segami, Yokohama City indicates that it has about 40 m in height, normal for the bedding plane, and about 20 m in width, parallel for the bedding plane. The litho-facies of the core are of upward fining from mudstone, the upper part of the Ofuna Formation, to muddy sandstone, the lower part of the Koshiba Formation. This result evidently suggests that the seepage at Segami continued for a long period of time, and the community occurred in a regressive phase in the depositional cycle. The middle part of the core has a representative litho-facies, such as an oblique litho-boundary for the bedding plane, crashed shell fragments of large bivalves, probably Lucinoma sp., in several horizons, and a few scoria beds entirely replaced their matrix with white carbonates. These litho-facies suggest an explosive eruption of methane that has been considered to be a cause of pock marks observed on the present oceanic floor.
(2) A Pleistocene, cold-seep chemosynthetic community is discovered from the Shintomi Town, Miyazaki Prefecture. This community is exposed in the range of 100m in N-S direction, 60m in E-W, and 10m in thickness, and occurs in the upward fining, regressive sequence.
(3) Holocene concretions including Conchocele sp. and/or Calyptogena sp. are discovered from off Wakkanai City, Hokkaido. The depleted 13C of the carbonate cements of the concretions suggests that these communities were depended upon the methane seepage.
|
