Volcanostratigraphical Study on the Miocene Volcanism in the Shimane Prefecture, Japan

There may be no doubt in that the volcanostratigraphical method, as well as any other means so far ordinarily applied, is also to play an important role in the research of sedimentary formations intercalated with more or less amounts of pyroclastic rocks. From this point of view, the author here intends to report an outline of the study in relation to the regions composed of so-called "Green Tuff" in the San-in province.
Basing on the activities and petrographical features of respective volcanics, inspection for several volcanisms appeared in the related basin has been carried out, illustrating that the volcanics concerned are, in general, represented either by the calcic series that might have been active through the periods from the middle to the upper Miocene or by the alkaline suite that might have begun to extrude in the lower Pliocene after a very long and silent interval nearly at the close of the former eruption.
Both mineralogical and petrochemical characteristics of the calcic series have been specifically pursued for scrutiny in comparison to other kinds of volcanics found scattered in the same province. In consequence, it has become clear that the activities of the series in question should be classified into three cycles, each of which might be represented distinctively by a formation composed of the pyro-clastics showing certain petrographical characteristics and reasonably correlative to the stratigraphical division determined from the biostratigraphical or other data.