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ABSTRACT

Rocks exposed at Punta Cono include very fine-grained to coarse-grained tuffs, lapilli tuffs, and tuff breccias deposited in a deep marine environment. Syndepositional basaltic intrusive activity was common. In one locality a hyaloclastite-peperite complex formed. Slumped sections with fluidal basalt ‘clasts’, derived from intrusions that entered the sediment pile from below, are present elsewhere. Abundant soft-sediment folds in fine-grained laminated subaqueous fall-out tuff suggest steep gradients; these are cut by shallow channels filled with coarse-grained tuff, lapilli tuff, and rare tuff breccia. The combination of marine fossils, extreme textural immaturity, abundant slump features, and syndepositional magmatism indicates deposition upon the submarine flanks of an active volcano.

Recognition of magma-wet sediment interaction is hampered in volcaniclastic rocks because of the similarity between host and intrusive fragments. Products of magma-water-sediment interactions at Punta Maria include: (1) jigsaw-puzzle hyaloclastite, formed by non-explosive hydroclastic fragmentation of magma upon contact with water and water-bearing sediment; (2) peperites, produced by mixing of magma with sediment; and (3) an unusual tuff breccia unit, the result of non-explosive mixing of ‘wisps’of lava with sediment during remobilization of an unconsolidated section. Low-explosivity magma-water-sediment interactions are favoured by relatively high hydrostatic pressures in sub-wave base settings.