Paleomagnetic constraints on a time-stratigraphic framework for the evolution of Ohachidaira volcano and the summit caldera, central Hokkaido, Japan

Abstract

We present a revised stratigraphy of the proximal deposits around Ohachidaira volcano and their paleomagnetic directions obtained from 21 sites (193 samples). We identify four proximal pyroclastic members from Ohachidaira volcano that were produced after the early edifice-building effusive volcanism, from older to younger: (1) vent-opening mafic pyroclastic density current (PDC) deposits (Kobachidaira ignimbrite); (2) maar-forming mafic tephra-ring deposits (Mamiyadake tephra ring); (3) mafic to silicic caldera-forming deposits (Sounkyo Member); and (4) mafic PDC deposits (Kumonotaira ignimbrite). Well-grouped paleomagnetic directions obtained from the Kobachidaira ignimbrite and the Sounkyo Member indicate that each deposit was formed over a short time interval compared to geomagnetic secular variation―that is, less than a century. Conversely, scattered paleomagnetic directions of the Mamiyadake tephra ring record the secular variation of the geomagnetic field that took place over a longer time interval―that is, at least 750 yr, probably more. The Kumonotaira ignimbrite, which has a similar paleomagnetic direction to the underlying Sounkyo Member, may have been produced during the Sounkyo eruption. From comparison between our data and the previously reported paleomagnetic directions of two petrologically distinct distal ignimbrites, we conclude that the Sounkyo Member is a proximal correlative of the distal pyroxene-rich ignimbrite, and that the distal hornblende-rich ignimbrite may not be of an Ohachidaira origin. The revised stratigraphy suggests that Ohachidaira volcano is a maar-caldera complex volcano, and that the summit caldera is a flaring funnel formed incrementally by explosive erosion and syn-eruptive collapse of the vent walls. Volume estimates imply that the Sounkyo eruption fundamentally enlarged the vent and formed the caldera. Our data confirm that the comparison of paleomagnetic directions can offer a useful identification and correlation tool for the Ohachidaira pyroclastic sequences.