Abstract

Reports of shock-deformed phenocrysts from the Toba ignimbrite deposits, Sumatra, have prompted considerable debate over whether shock-deformation products are clear evidence of a meteorite impact origin for the Cretaceous/Tertiary (K/T) boundary deposits as well as terrestrial "cryptoexplosion" structures. Evidence presented in favor of volcanically induced shock at Toba includes kinked biotites and rare occurrences of single sets of lamellae in quartz grains but rests most heavily upon occurrences of mosaic extinction patterns in plagioclase phenocrysts. Our analysis of several of the same Toba samples reveals that these mosaic patterns are related to distinct compositional zoning and cannot be attributed to deformation of the crystal lattice that shock would produce. Additionally, in more than 200 quartz grains examined, we detected no occurrences of microdeformation features or mosaic textures similar to those associated with known impact structures and the K/T boundary. We conclude that evidence of shock deformation in the Toba deposits has not been demonstrated.

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