紀伊半島西部四万十地帯にみられる鉄・マンガンに富んだ堆積岩

概要

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The sediments, enriched strongly in iron and manganese, called ferromanganese sediments herein, are discovered in the Shimanto terrane (s), in western Kii Peninsula, Southwest Japan. They are underlain by the lower pillow lava member, 1 to 3 meters thick, with a relatively irregular surface, and intercalate the upper pillow lava member, 1 to 2 meters thick, and are intruded by a coarse-grained olivine basalt sill. The ferromanganese sediments are layered and vary in thickness from 0.5 to 6 meters or more. They are dark red to reddish brown, and delicately laminated and banded, and include abundant radioralian remains as well as sponge spicules aligned in parallel with the lamination or banding. The constituent minerals are hematite, rhodochrosite, calcite and quartz. The chemical analyses show that there were two cycles of precipitation ; the total content of iron and manganees decreases generally upward and then increases, at the horizon 2.5 meters above the upper pillow lava member. There was a time lag between the basaltic volcanism and the enrichment of iron and manganese in the sediments. The stratigraphic and paleogeographic features of the western Kii Peninsula (YANAI, 1983) indicate that the ferromanganese sediments formed in association with the basaltic volcanism on a relatively shallow-sea area, but not in an axial zone and flanks of active spreading oceanic ridge.
特定非営利活動法人日本火山学会の論文
1983-07-01