Record of Prehistoric Debris Avalanches on the North Flank of Mount St. Helens Volcano, Washington

摘要

In the crater walls and deep canyons on the north flank of Mount St. Helens, dacitic volcaniclastic rocks and domes of Pine Creek age (2.5-3.0 ka and possibly older) are pervasively deformed and contain deposits of possibly two debris avalanches. The base of the younger avalanche deposit contains numerous logs, one of which yielded an age of 2590±120 14C years B.P. The Pine Creek section is capped by slightly faulted andesite and basalt of Castle Creek age (1.7-2.2 ka). In the northeast and northwest walls of the crater, dacite domes of Pine Creek age and older are pervasively fractured. North-dipping normal faults and low-angle thrusts cut the domes. We postulate that forceful intrusion caused the deformation and slope failure in late Pine Creek time, in a manner similar to emplacement of the bulging cryptodome in 1980.