On Soundings at Musasitai

Abstract

The existence of a conspicuous zone of islands and banks fringing the northern coast of the Japan Sea is well known. In 1922, H. I. M. S. “Musasi” discovered a large bank at the northern end of that zone, Iookm west off the coast of Tesio Province, Hokkaidô. It is outlined by 200m bathymetric line, gourd-shaped, and its shallowest part, which is in the center of the northern part, 31m. The bank has a significant flat, broad plain at a depth of 140-160m., the margin of the flat plain descending abruptly to the deep sea-floor. In the course of the soundings, thirty bottom samples were obtained from the bank and these samples were placed at my disposal through the courtesy of Commander Asahina, Hydrographic Department, Imperial Japanese Navy. An examination of the bottom samples, showed a predominance of gravels, followed by sand, mud, and rock fragments. I Rock fragments. The rock fragments, which were obtained from three stations, are plagio-liparite, glassy basalt, siliceous shale, and diatom-earth. The diatom-earth and siliceous shale show, under the microscope, some organic remains, such as fossil coscinodiscus sp., sponge spicules, etc. Although the fossil diatom and other organisms in the sedimentary rock fragments are of little value for accurate correlation of the rock strata, the fact remains that the general character of the rock fragments in question closely resembles those of the Tertiary rocks developed in the adjoining ter-restrial region, where, it may be added, liparite and basalt are also common in the Tertiary and Quaternary formations. 2 Gravels. Gravels were obtained from II stations on flat plain of the bank and the deep sea floor around it. They are quite water-worn, with neither striae nor pits on the surface, although rarely encrusted with calca-reous algae, bryozoa, etc. The lithological characters of the gravels micro-scopically determined may be classified into igneous and sedimentary rocks. The gravels of igneous rocks are liparite and basalt, probably derived from the bed rocks. The gravels of sedimentary rocks are diatomaceous sandy shale, siliceous shale containing fossil sponge spicules, their lithological charac-ters being closely related to the Tertiary rocks developed in the adjacent terrestrial regions. Gravels of hornstone and siliceous shale are rarely found among them, suggesting that they were derived from the Tertiary conglome-rate bed containing abundant pebbles of Palaeozoic. rocks. 3 Sand. Sand was obtained from many stations on the bank and the deep sea floor around it. The sand grains consist of angular quartz, feldspar, coloured minerals, glauconite, and organic remains. The most noticeable fact is that tests of foraminifera are abundant in the coarse sand obtained from sea floor shallower than 200m., decreasing abruptly in number. with increase of depth. 4 Mud. Mud is found below 400m. That obtained from the deep sea floor contained abundant diatom remains, sometimes looking like diatom ooze. Since the Musasi-tai, which rises from the deep sea floor outside the con-tinental shelf along Hokkaido, is separated from the mainland by a trench more than 300m. deep, it is natural to conclude that transportation of rocks and gravels from the mainland of Hokkaido across this trench is impossible, for which reason the soundings are of geological impotance, due to their yielding rock components that probably from the main part of the bank. The lithological charactesr of the rock fragments and gravels just mentioned are found to be the same as the geological elements making up the Neogene Tertiary developed in the adjacent mainland. From the foregoing facts, the bank as a whole represents a recently sub-merged land block, closely related, geologically as well as topographically, to the mainland of Hokkaidô.