ICE-MELT COLLAPSE PITS AND ASSOCIATED FEATURES IN THE 1991 LAHAR DEPOSITS OF VOLCAN-HUDSON, CHILE - CRITERIA TO DISTINGUISH ERUPTION-INDUCED GLACIER MELT

In subaerial volcaniclastic sequences, structures formed by ice blocks can provide information about a volcano's history of lahar generation by glacier melt. At Volcan Hudson in Chile, catastrophic lahars were initiated by eruption-induced melting of glacier ice in August and Oct ober 1991. They transported large ice blocks 50 km down the Rio de los Huemules valley to the sea. Large current crescents with lee-side len ses were formed where ice blocks were deposited during waning stages o f the flood. When stranded blocks of ice melted, they left cone-shaped and ring-shaped heaps of ice-rafted debris on the sediment surface. S everal hundred ice blocks were completely buried within the aggrading lahar sediment, and when these melted circular collapse pits formed in the sediment. Collapse types included subsided coherent blocks of sed iment bounded by an outward-dipping ring-fracture, trapdoor structures with horseshoe-shaped fractures, downsag pits with centroclinal dips locally up to 60 degrees, pits with peripheral graben and crevasses, p iecemeal (highly fragmented) collapse structures and funnel-shaped pit s containing disaggregated sediment. A sequence of progressive collaps e is inferred in which initial downsag and subsidence on an outward-di pping ring fracture produces a small diameter pit. This is followed by widening of the pit by progressive development of concentric ring fra ctures and downsag outside the early formed pit, and by collapse of ov erhanging pit walls to produce vertical to inward-dipping walls and ap rons of collapse debris on the pit floor. The various structures have potential for preservation even in regions prone to high rainfall and flooding, and they can be used to indicate that former lahars containe d abundant blocks of ice.