Taphonomy of modern traces in volcaniclastic deposits: the eruption of Chaiten volcano (2008-2010) as a natural laboratory

RICARDO NESTOR MELCHOR; JORGE FERNANDO GENISE; MARÍA VICTORIA SÁNCHEZ; LAURA SARZETTI; ALDO MARTÍN UMAZANO

Sao Leopoldo

Simposio; Simposio Latinoamericano de Icnología; 2010

UNISINOS

Neoichnological studies on rhyolithic pyroclastic deposits of the recent plinian eruptive cycle of the Chaiten volcano (42' 50' 10" S, 72' 39' 15" W) were conducted during December 2009 in Chile and western Chubut and Rio Negro provinces of Argentina. The recognized deposits include air-fall (in soils and shallow ponds) and water-lain tephra (stream and minor mass flow), and rare reworked windblown dunes. The ash plume was directed to the east and southeast of the volcano, reaching occasionally the Atlantic coast of Patagonia. After nineteen months of the start of the eruption (which still continues although with reduced energy), prese rvation of ash in soil profiles was patchy in the Argentine steppe. Ash thickness increased (up to 40 mm) towards the more humid western Nothophagus forest. The particular distribution of prese rved air-fall tephra would be related to mean annual rainfall and vegetation or litter cover, in addition to intenSity of prevalent winds, local relief and occurrence of rain or snow immediately after deposition. We found a large variety of traces in soils and ponds. The latter contained bird tracks, vertebrate swimming traces, thin invertebrate trails, and chironomid cases in water-lain ash. In soils covered by grass-dominated vegetation growing on ash, insect nest s and pupation chambers, and earthworm, slug and rodent burrows were identified. Burrowing by earthworms in grass-covered soi ls produced an ichnofabric that resembles the diffuse boxwork described for Jurassic-Cretaceous rocks from Patagonia. Nesting or pupating insects (potential trace-fossil equivalents are listed between brackets) in ash or crossing through the as h layer were identified as: Dichotomius torulosus (Coleoptera, Scarabaeinae) (Coprinisphaera), Frickius vario/osus (Coleoptera, Geotrupinae) (Scophichnium, Pollichnus), Diphaglosso goyi and Codeguala albopi/osa (Hymenoptera, Colletidae) (Celliforma), Corynura sp. (Hymenoptera, Halictid ae) (Cel/icolichnus), Augochlorini indet. (Hymenoptera, Halictidae) (Corimbotichnus), Cicadidae indet. (Auchenorrhyncha) (Feoichnus), Sphingidae indet. (Lepidoptera) (Teisseirei), Sphecidae indet. (Hymenoptera) (Rebuffoichnus) and Camponotus sp. (Hymenoptera, Formicidae) (Krausichnidae).