四万十地向斜南縁部における後期白亜紀酸性火山活動

概要

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Thick acid tuff layers are widely distributed in the Southern Shimanto terrane of the western Kii Peninsula, Southwest Japan (YANAI, 1980). They are interstratified within the Late Cretaceous strata (Coniacian-Santonian∿Maastrichtian?) of sandy mudstone, lithic sandstone rich in volcanic rock fragments, and submarine slump and sliding breccia. Those strata were accumulated in a shallow-sea environment (YANAI, 1982). The acid tuff layers tend to decrease in thickness from south to north. The grain size also decreases in the same direction. The acid tuff layers gradually change their lithofacies toward north from coarse-grained pumice tuff and volcaniclastic sandstone rich in pumice fragments to finegrained vitric tuff with sedimentary structures characteristic of turbidites. These lines of evidence suggest that the source area of these pyroclastic materials presumably lay to and/or on the south of the depositional area, the Shimanto geosynclinal basin sensu stricto. The Late Cretaceous volcanic activities are also known in the Eo-Nippon Cordillera, to the north of the geosynclinal basin (eg. KOITO, 1975). Therefore, the violent acid volcanic activities were found at least in two rows on both north and south sides of the geosynclinal basin. The ancient subduction axis was situated farther to the south of the geosynclinal basin during Late Cretaceous time.
特定非営利活動法人日本火山学会の論文
1983-04-01