On Soundings at Tosabae

Abstract

Tosabae is a marine bank situated at the entrance of Kii Strait 50km off the coast of Cape Muroto, Tosa Province. Its shallowest part is 154m in the western corner of the bank. The bank, which is outlined by a 400m bathymetric line, is oval, 31km long and 15km broad. In 1927 the surveying ship, “Sôyô-maru” of the Imperial Fisheries Experimental Station dredged many bottom samples from the bank, the predominant materials being rock fragments, followed by gravels, sands, and organic remains. 1. Rock fragments. These are calcareous and marly shale, fine black sandstone, and gabbro. The calcareous shale under the microscope shows tests of fossil foraminifera, such as Rotalia sp., Globigerina sp., and Lenticulina rotulata Lamark. Only one, piece of gabbro was found among the rock fragments. It is greenish gray and shows fine schistose structure. Under the microscope, most of the crystals of monoclinic pyroxene are crushed and in parts have the parallel structure of mylonite. 2. Gravels. Two gravels of marly shale were found in the bottom samples; of angular shape but very smooth surface. 3. Sands. The sand grains consist of organic remains, quartz, feldspar, hornblende, etc. The abrade percentage of the quartz grains in the sand is far greater than that of the quartz grains in the rock. 5. Organisms. The organic remains are foraminifera, coral, bryozoa, and mollusca. The Tosabae bank, which rises from the deep sea floor outside the continental shelf around Cape Muroto, is separated from the cape by a trench more than 700m deep, across which it is most natural to conclude that transportation of rock from the terrestrial region of Sikoku is impossible for which reason the dredgings are of geological importance, being probably the components forming the main part of the bank. The lithological characters of the sedimentary rocks just mentioned are found in the geological elements making up the Neogene Tertiary developed in Aki Province near Cape Muroto. Gabbro is also found in the Akigawa series, the Mesozoic formation developed at Cape Muroto. The bank of Tosabae therefore is closely related both geologically and topographically to the terrestrial part of the Muroto region.