Japan Geoscience Union Meeting 2016

Presentation information


Symbol S (Solid Earth Sciences) » S-VC Volcanology

[S-VC48] Volcanic and igneous activities, and these long-term forecasting

Sun. May 22, 2016 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall HALL6)

Convener:*Teruki Oikawa(Institute of Earthquake and Volcano Geology, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Takeshi Hasegawa(Department of Earth Sciences, College of Science, Ibaraki University), Daisuke MIURA(Geosphere Sciences, Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry), Yoshihiro Ishizuka(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Nobuo Geshi(Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology)

5:15 PM - 6:30 PM

[SVC48-P02] The Ohachidaira caldera-forming eruption and associated deposits, Taisetsu volcano group, Japan

*Yuki Yasuda1, Keiko Suzuki-Kamata1 (1.Graduate School of Science, Kobe University)

Keywords:Ohachidaira caldera, Taisetsu volcano group, caldera-forming eruption, ignimbrite, lithic breccia

The Sounkyo member was produced during the Ohachidaira caldera-forming eruption that occurred in the central part of Taisetsu volcano group, central Hokkaido, Japan. At distal sites, the member comprises a pumice-fall deposit and the overlying Sounkyo ignimbrite (about 6.5 km3), previously named Px-type pyroclastic-flow deposit by Sato and Wada (2012). Proximal deposits, occurred around the Ohachidaira caldera, comprise the following pyroclastic sequence from base to top: pumice and scoria-fall deposit (SK-A); ignimbrite (SK-B); lithic breccia (SK-C); scoria-fall deposit (SK-D); and pyroclastic-surge deposit (SK-E). SK-A mantles the land surface, attains a maximum thickness of 60 m in the caldera rim exposures, and shows an outward decrease in thickness, grain-size, and the degree of welding. SK-B is a valley-filling ignimbrite as much as 45 m thick composed mainly of pumice and scoria clasts up to 70 cm in diameter and gray ash matrix with a small amount of lithic fragments, and varies vertically from massive facies (up to 15 m thick) to crudely parallel-stratified facies. SK-C (up to 27 m thick) is massive and poorly sorted, consisting predominantly of coarse lithic blocks, up to 2.6 m in diameter, and subordinately of pumice lapilli, with fines-depleted coarse ash matrix, and varies from clast-supported to matrix-supported. SK-C thickens into topographic depressions, contains abundant rounded pumice clasts, lacks impact structures even beneath meter-sized lithic blocks, and grades downward into SK-B ignimbrite and laterally into a fine-bearing, matrix-supported, lithic breccia, indicating a flow origin. SK-D is locally exposed and has an average thickness of 1 m. SK-E (up to 15 m thick) is a cross-stratified pyroclastic-surge deposit. The grain-size and component characteristics of SK-E are similar to those of SK-B. Field evidence shows that the distal pumice-fall deposit represents a lateral counterpart of SK-A. Hence the Sounkyo ignimbrite might be a distal equivalent of SK-B. The coarse lithic breccia (SK-C) overlies the voluminous ignimbrite (SK-B), implying that a vent widening occurred at the end of the climactic eruption.