Pelagic gastropod assemblages from the kazusa group of the boso peninsula japan and pliocene pleistocene climatic changes

Ujihara, A.

Journal of the Geological Society of Japan 92(9): 639-652


ISSN/ISBN: 0016-7630
Accession: 006077555

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Pelagic gastropods have been collected from 210 localities in the Kazusa Group (late Pliocene to middle Pleistocene) of the middle part of the Boso Peninsula, Chiba Prefecture, Japan. They consist of two heteropod species and nineteen pteropod species. Nineteen of them are warm water living forms. Present are four pelagic gastropod assemblages, I, II, III, and IV assemblages, which represent past pelagic communities with different geographical distribution patterns. The I assemblage is of the Transition Zone, the II assemblage of the Transition Zone and higher latitudes of the Circumglobal Warm-water Region, the III assemblage of the Circumglobal Warm-water Region and lower latitudes of the Transition Zone and the IV assemblage of lower latitudes of the Circumglobal Warm-water Region. These assemblages alternate vertically with one another in the Group. It seems to be due to changes in hydrographic condition in this area, principally of water temperature. The paleotemperature is discussed in detail on the basis of pelagic gastropod assemblages. Four notable peaks of water temperature which were as high as that of the present tropical water are recognized. They are stratigraphically situated in the lowermost part and the middle part of the Kiwada Formation, the middle part of the Otadai Formation and the middle part of the Kokumoto Formation.