This study reexamined molluscan fossils preserved in the calcareous sandstone collected from the eastern cliff of the Senoumi Gorge, Suruga Trough, through the submersible “Shinkai 2000” Dive 579. As a result, we identified a total of six species including an extinct bivalve Mimachlamys satoi, which is a characteristic of the Pliocene to Early Pleistocene Kakegawa Fauna. The fossil assemblage dominated by sessile epifauna is comparable to the recent molluscan dead assemblages commonly found on the crests of the Omaezaki and the Izu Spurs, 50–200 m in bathymetric depth. The present discovery is consistent with the previous argument that a shallow-marine ridge with an NE–SW strike, an early precursor of the Izu Peninsula, were formed by the tectonic movements that are assumed to have occurred around 3–1 Ma. The new evidence also suggests that the calcareous sandstone was deposited on the crest of the ridge just after the tectonic movements. Our result provides a new insight into the tectonic history of the Suruga Trough where the Izu-Bonin volcanic arc is subducting beneath the Southwest Japan arc.

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The elemental X-ray maps of a layered gabbro sample obtained by a scanning X-ray analytical microscope have been used to produce mineral-distribution maps of the layered gabbro. These element-distribution maps have been transformed into mineral-distribution maps by Michibayashi et al. (2002)ʼs method. To examine the mineral-distribution maps, we tested two texture analyses: spatial distribution method and Hough transform method. As a result, the spatial distribution method could have a possibility to reveal anisotropic characters of the layered gabbro, whereas the Hough transform method would be partially able to show its anisotropic nature. Further study will be required to develop a realistic texture analysis for the mineral-distribution maps obtained by this procedure.

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