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.

中文翻译：

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日本北海道中部 Ohachidaira 火山和山顶火山口演化的时间-地层框架的古地磁约束

我们提出了 Ohachidaira 火山附近近端沉积物的修订地层及其从 21 个地点（193 个样本）获得的古地磁方向。我们确定了来自 Ohachidaira 火山的四个近端火山碎屑岩成员，它们是在早期建筑物喷发火山作用之后产生的，从老到年轻：（1）开口基性火山碎屑密度流（PDC）沉积物（Kobachidaira ignimbrite）；(2)形成maar的基性火山灰环矿床(Mamiyadake tephra ring)；(3) 镁铁质至硅质火山口形成矿床（层云峡成员）；(4) 镁铁质 PDC 矿床（Kumonotaira ignimbrite）。从Kobachidaira 火山岩和层云峡段获得的良好分组的古地磁方向表明，与地磁长期变化相比，每个矿床的形成时间间隔都很短——即不到一个世纪。反过来，Mamiyadake tephra 环的分散古地磁方向记录了地磁场在更长的时间间隔内发生的长期变化——即至少750年，可能更长。与底层层云峡段具有相似古地磁方向的 Kumonotaira 熔凝岩可能是在层云峡喷发期间产生的。通过将我们的数据与先前报道的两个岩石学上不同的远侧熔凝灰岩的古地磁方向进行比较，我们得出结论，层云峡段是远侧富含辉石的凝灰岩的近端相关物，而远侧富含角闪石的凝灰岩可能不是 Ohachidaira起源。修正后的地层表明 Ohachidaira 火山是一座马尔-破火山口复合火山，并且山顶的火山口是一个由爆炸性侵蚀和通风口壁的同步崩塌逐渐形成的喇叭形漏斗。体积估计表明层云峡喷发从根本上扩大了喷口并形成了火山口。我们的数据证实，古地磁方向的比较可以为 Ohachidaira 火山碎屑岩序列提供有用的识别和关联工具。