三瓶小豆原埋没林に刻まれた火砕流の記録

抄録・要旨

Large standing trees which were buried in situ by Holocene volcaniclastic and debris flow deposits are exposed in the river floor at Azukibara，Shimane Prefecture．Some parts of the buried trees are black．Carbonization temperatures of surface samples from the trees and of wood fragments from within the pyroclastic flow deposits were estimated using recently developed H/C themlometry．One tree about 10 m in height shows differing color and carbonization temperatures，in ascending order：（1）Lowest part：ocher coloration（not heated or carbonized）．（2）Lower part，buried in pyroclastic flow deposits：black，estimated temperature of 319-322℃．Carbonized wood from the pyroclastic flow deposits at this level is also black，and yields an emplacement temperature of 372℃．（3）Middle part：brown，262-275℃，below the lower limit for the thermometer．（4）Upper middle part：dark brown，334℃．Reworked carbonized wood in enclosing sandstone beds is black，and gives temperatures of 343-356℃．（5）uppermost part（top）：brown，285-305℃．Parts 2 and 4 record shows that at least two pyroclastic flow events took place．Most of the pyroclastic deposits along the river have been eroded by later stream action or debris flows．Carbonization temperatures of wood buried in situ provide a useful method for estimation of emplacement temperatures of past pyroclastic flows．