11:00 〜 13:00
[SVC29-P01] 阿寒カルデラ南東部で掘削した定方位ボーリングコアの火山層序学的検討
キーワード：阿寒カルデラ、定方位ボーリングコア、降下火砕物、火砕流堆積物
Akan caldera is a large caldera formed at the Akan volcano located in the eastern part of Hokkaido, Japan. Outside the rim of the caldera there are thick deposits of pyroclastic materials that were ejected during its formation. The stratigraphy, distribution and petrological characteristics of these deposits were clarified by Hasegawa and Nakagawa (2007), and the deposits erupted from Akan caldera were roughly divided into 17 eruption groups from Ak1 to Ak17 (in descending stratigraphic order). In addition, pyroclastic deposits from the adjacent Kutcharo caldera and distal air-fall ash layers from central Hokkaido were found intercalated with the Akan pyroclastic deposits. From these findings, it was clarified that the caldera-forming activity of the Akan volcano began at least in the Early Pleistocene and continued over a period of more than one million years.
Our study aims to clarify the time scale of each eruption group recognized in the pyroclastic deposits around Akan caldera. In order to examine the methods for estimating the durations of super large eruptions based on pyroclastic deposits using paleomagnetic methods (Hasegawa et al., 2018), oriented drilling was carried out in Tsurui Village, southeast of Akan caldera, and core samples from various depths down to an excavation depth of 100 m were obtained.
The core samples are mainly composed of pyroclastic deposits, reworked pyroclastic deposits and fluvial deposits. Pyroclastic fall deposits are predominant at depths of <40 m, and pyroclastic flow deposits are predominant at depths of >40 m. In addition, fluvial deposits are mainly located near depths of 80 to 90 m. As characteristic pyroclastic deposits, welded pyroclastic flow deposits were found near a depth of 96 m, and fine-grained pumice beds possibly originating from a source other than the Akan caldera were found near the depths of 6.4 m and 15.7 m.
In this presentation, prior to paleomagnetic investigation, we will discuss the eruptive history of Akan caldera using stratigraphic position and facies based on the detailed description of the core samples.
Our study aims to clarify the time scale of each eruption group recognized in the pyroclastic deposits around Akan caldera. In order to examine the methods for estimating the durations of super large eruptions based on pyroclastic deposits using paleomagnetic methods (Hasegawa et al., 2018), oriented drilling was carried out in Tsurui Village, southeast of Akan caldera, and core samples from various depths down to an excavation depth of 100 m were obtained.
The core samples are mainly composed of pyroclastic deposits, reworked pyroclastic deposits and fluvial deposits. Pyroclastic fall deposits are predominant at depths of <40 m, and pyroclastic flow deposits are predominant at depths of >40 m. In addition, fluvial deposits are mainly located near depths of 80 to 90 m. As characteristic pyroclastic deposits, welded pyroclastic flow deposits were found near a depth of 96 m, and fine-grained pumice beds possibly originating from a source other than the Akan caldera were found near the depths of 6.4 m and 15.7 m.
In this presentation, prior to paleomagnetic investigation, we will discuss the eruptive history of Akan caldera using stratigraphic position and facies based on the detailed description of the core samples.