山形県白鷹火山カルデラの構造と岩屑なだれ堆積物

Structures of the Quaternary Shirataka volcanic calderas and the occurrences of debris avalanches have been studied on the basis of field mapping data，borehole core，and paleomagnetism measurements．The debris avalanche deposits distributed in and adjacent to the calderas are determined to be 50 to 60 meters in thickness，and it extensively cover rhyolitic pyroclastic flow deposits and thick andesite lava．Although the northern and eastern walls of the calderas are not recognized，borehole data indicate the existence of low calderas wall beneath the debris avalanche deposits in the eastern portion．Therefore，the calderas shape a general horseshoe structure opening to the north．
Thin paleosols layers interbedded in the debris avalanche deposits mean that several deposition events occurred．The deposition age is estimated to be from early to middle Pleistocene．Centralization of paleomagnetism direction of the deposits means that they may have been deposited under high temperature condition influenced by the volcanic activity．Consequently，even through in several episodes，occunrences of sector collapse，formations of calderas and depositions of debris avalanches may have finished at least before several ten thousands years ago．