[S-EM17] 地磁気・古地磁気・岩石磁気

[SEM17-10] 古地磁気学的手法による富士火山 滝沢1溶岩流の層序区分

*馬場 章¹、吉本 充宏¹、内山 高¹、渋谷 秀敏²、望月 伸竜² (1.山梨県富士山科学研究所、2.熊本大学)

キーワード：古地磁気永年変化、古地磁気方位データ、岩相層序ユニット、トレンチ調査、富士火山

Fuji volcano is one of the largest active volcano in Japan. Although stratigraphic studies accumulated, it is difficult to discriminate stratigraphic units of lava flows each other from field survey even with petrography, tephrochronology and ¹⁴C dating. Here, we present a paleomagnetic study of Takizawa 1 lava flow, which is estimated the eruption age (BCE 700-600) from tephrochronology (Uesugi, 1998). Takizawa 1 lava flow is characterized by high K₂O content larger than 0.9, and exposed scatteredly to the Takizawa riverbed. The direction of Takizawa 1 lava flow (D=-8.2°,I=44.0°,α95=1.98°) agree with that for the expected age referring to Holocene paleomagnetic secular variation at Lake Biwa(Ali et al.,1999). However, by our trench excavation, Takizawa 1 lava flow was found to underlie the Fuji black soil, which was formed BCE 6,000-3,600, so that it is a member of Fujinomiya stage (BCE 15,000-6,000). Furthermore, the petrological features and paleomagnetic direction are quite similar to the Saruhashi lava flow, whose age is estimated as BCE 10,000-8,000 by ¹⁴C dating. Our findings suggest that Takizawa 1 lava flow is the same stratigraphic unit as Saruhashi lava flow and thereby paleomagnetic method is effective for dating and identification of stratigraphic unit.