The K/T Boundary: The Geologic Record
1990. "The K/T Boundary: The Geologic Record", Global Catastrophes in Earth History; An Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality, Virgil L. Sharpton, Peter D. Ward
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Evidence favoring a major impact event at the Cretaceous-Tertiary (K/T) boundary includes enrichments of siderophile elements and diagnostic indicators of shock metamorphism. Recently, however, proponents of an endogenic cause for the K/T mass extinctions have challenged this evidence, suggesting instead that volcanic processes can account not only for the K/T boundary's chemical signature but for its shocked minerals as well. Support for the latter contention has been drawn from reports of shocked minerals in volcanic terrains, and charges that certain large structures, characterized by intense deformation, central uplift, and indications of shock metamorphism, are actually volcanic “cryptoexplosion” features that have been interpreted as impact structures only because they contain shocked minerals. We explore the validity of these charges and present a perspective to the evidence of shock metamorphism at the K/T boundary based on field and laboratory studies of terrestrial impact structures. Reports of volcanically induced shock deformation rest solely on occurrences of equivocal features (mosaicism, single lamellae sets) that bear little resemblance to the diagnostic shock features found at meteorite impact structures and within K/T boundary clastic grains. Furthermore, the impact interpretation of structures containing indications of shock metamorphism is based on a suite of geologic and geophysical evidence, such as the morphological and structural expression of an appropriate crater form (shallow, rootless structure comprising a central uplift, annular trough, and raised rim), downward and outward attenuation of shock pressures and attendant deformation, characteristic breccia formations, and impact melts composed of near-surface lithologies and containing evidence of superheating. Not only do these and other observations substantiate the impact interpretation but they are incompatible with any endogenic mechanism of formation. The link between shock metamorphism and meteorite impact has withstood countless challenges over the course of three decades and is now established beyond reasonable doubt. The occurrence and worldwide distribution of shocked minerals at the K/T boundary is conclusive evidence for a major impact event at that time.