6:15 PM - 7:30 PM
[SVC47-P03] The occurrence and grain-size characteristics of distal facies of Aso-4 ignimbrite in Yamaguchi Prefecture
Keywords:Aso-4 ignimbrite, Yamaguchi Prefecture, grain-size characteristics
Aso-4T ignimbrite distributes widespread in central to western region of Yamaguchi Prefecture with relatively thin (2-3 m) thickness in contrast with Aso-4A ignimbrite which fills valley to a thickness of more than 40 m in Oita Prefecture. Aso-4 ash-fall deposits as 15 cm-thick and partially modified by water current and resedimented at Miyoshi city in Tokushima Prefecture.
Aso-4T ignimbrite is significantly altered and rich in clay. Oppositely, Aso-4A ignimbrite is coarse, poorly sorted and includes small amount of clay. Aso-4 ash is fine , relatively sorted and rich in clay. The maximum size of pumice in Aso-4T at 130-160 km from the source caldera is 1.0 to 1.2 cm, that in Aso-4A at 50-70 km is 20-30 cm and that in Aso-4 ash-fall at 314 km is 5 mm. The maximum length of hornblende in Aso-4T at 50 km and 130-160 km is 3.8 mm and 2.8-3.1 mm. That in Aso-4A at 50 km is 3.4 mm and in Aso-4 ash-fall at 314 km and 680 km is 2.5 mm and 0.9 mm.
The aspect ratio of hornblende in Aso-4T is 1.6-8.0, that in Aso-4A at 50-70 km is 1.5-8.0 and that in Aso-4 ash-fall at 314-682 km is 1.0-6.0.
Hornblende is resistant to alteration so that it suites to the study of grain-size characteristics of significantly altered ignimbrite such as Aso-4T. We recognized the tendency of the grain-size characteristics as follows:
1) the grain-size of hornblende in Aso-4A ignimbrite is slightly larger than that in Aso-4T ignimbrite
2) longitudinal variations of maximum size of hornblende in Aso-4A and 4T ignimbrites are relatively homogeneous than that in Aso-4 ash-fall
3) aspect ratios of hornblende in both ignimbrites are high than that in Aso-4 ash-fall.