紀伊半島東部, 海山地域の四万十帯上部白亜系

概要

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The Geology of the Shimanto Belt has been examined in the Miyama area, north of Owase, eastern Kii Peninsula, Southwest Japan. The strata in this area can be divided lithologically into three thrust-bounded formations or tectonostratigraphic units : the Mizunomitoge, Ookogawa and Shimakatsuura Formations from north to south. The Mizunomitoge Formation consists mainly of melange rocks with block-in-matrix fabrics. The Ookogawa Formation is a unit of sandstone-dominated coherent beds, and the Shimakatsuura Formation is also a unit of coherent beds mainly composed of alternating beds of sandstone and mudstone. Radiolarian fossils from these formations indicate that they are of Upper Cretaceous age. The mudstones from these formations show nearly the same illite crystallinity value belonging to the level of diagenetic and lower anchimetamorphic zones. The diagenesis and metamorphism occurred probably by the rise of geothermal gradient in a short period during the Early and Middle Miocene. The thermal maturity also suggests that these strata were accreted in a shallower tectonic level of a convergent margin.
静岡大学の論文