1998 Volume 52 Issue 6 Pages 464-474
K-Ar ages of 47 samples of lavas from the Middle Pleistocene Tarusawa Formation of Ontake volcano were determined by isotope dilution and sensitivity methods. The age data are mostly consistent with the observed stratigraphic relations among the lavas and tephras, and the results indicate that the Middle Pleistocene volcanic activity of Ontake volcano started at 0.78±0.14 Ma and ceased at 0.39±0.006 Ma. Volcanic activity is divided into Stage 1 (0.78-0.59 Ma) and Stage 2 (0.53-0.39 Ma). And Stage 1 is subdivided into Substage 1a (0.78-0.71 Ma), Substage 1b (0.71-0.66 Ma) and Substage 1c (0.66-0.59 Ma). The strata of Substages 1a and 1b consist mainly of voluminous volcaniclastic sediments and lake deposits with thin intercalation of lava flows, whereas strata in and since Substage 1c are mostly lava flows. Lavas in Substages 1a and 1b are mainly basalt and andesite. And they become mostly andesite to dacite since Substage 1c. In Substages 1a and 1b, lava flows dammed up streams here and there to make lakes. The magma interacted with water and produced voluminous volcaniclastic fragments which were mobilized as lahar or fluvial flows sediments and deposited within the surveyed area. In Substage 1c, after lavas had buried the original valleys, the volcano had started to grow up. The effusion of more viscous andesite and dacite lavas accelerated to make a steeper conelike topography, and the production of hyaloclastic debris terminated. Once the steep topography was formed, volcaniclastic sediments, if any, were rarely deposited on it. As the edifice grew as a single conical volcano, the timing of above-mentioned lithofacies change is nearly synchronous in all directions. Ages of some Middle Pleistocene marker tephras are inferred from the stratigraphic relations with the dated lavas, these are; Kanbara Pumice (Kb.Pm.) I (0.70-0.65 Ma), Kb.Pm.II (0.67-0.65 Ma) and Hottaruzawa Pumice (Htz.Pm.) (0.61-0.58 Ma). They will provide with important datum planes in the central Japan area.