Geological Implications of Impacts of Large Asteroids and Comets on the Earth
Richard A. F. Grieve, 1982. "The record of impact on Earth: Implications for a major Cretaceous/Tertiary impact event", Geological Implications of Impacts of Large Asteroids and Comets on the Earth, Leon T. Silver, Peter H. Schultz
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Approximately one hundred terrestrial hypervelocity impact structures, ranging in diameter from a few tens of meters to ∼ 140 km, are currently known. All are on land, with major concentrations occurring on the North American and European cratons. With the exception of three, all recognized structures are Phanerozoic in age, with ∼35% being <100 my old. There is no known structure commensurate in age with an impact event of the magnitude proposed in the hypothesis of Alvarez and others (1980) for an impact-induced biological crisis at the Cretaceous-Tertiary boundary. A size-frequency distribution of N∝D−2 for large terrestrial impact structures and a calculated cratering rate of 0.35 × 10−14 km−2 y−1 for structures with D >20 km, indicates, however, that an impact event capable of producing a ≲ 200 km-sized structure may be expected somewhere on the earth’s land surface every 65 m.y. There are three or four known structures with D >25 km and ages of 65 ± 5 m.y. This, however, may not represent a statistically significant increase in the cratering rate at the end of the Cretaceous.
Consideration of cratering mechanics indicates that if the proposed Cretaceous-Tertiary impact event occurred in the ocean it had the potential to locally excavate the oceanic crust and bring upper mantle material to the surface, thus producing an as yet to be detected geophysical anomaly. Although cratering efficiency calculations suggest that ~103 projectile masses of material may be excavated in a major impact event, the bulk of this ejecta is locally confined. If, as has been suggested, the siderophile-enrichments in the Cretaceous-Tertiary boundary layer indicate the presence of projectile-contaminated ejecta from a major impact, then the source of this material is most likely early-time ejecta accelerated upwards as the projectile is penetrating the target rocks. This is supported by the relative abundance and distribution of meteoritic siderophile elements in impact melt rocks, which indicate that the bulk (~95%) of the projectile mass may be lost from the immediate area of the crater. The difficulties in defining projectile types from siderophile anomalies in the relatively well-known environment of impact melt rocks suggest that more detailed geochemistry and mineralogy will be necessary before the siderophile enrichments at the Cretaceous-Tertiary boundary can be linked to a specific meteoritic compositional class. In general, the record of terrestrial cratering is not inconsistent with the proposal that a major impact event occurred at the Cretaceous-Tertiary boundary. It does not, however, supply any direct confirmation that such an event produced a biological crisis.
- impact craters
- impact features
- lower Tertiary
- meteor craters
- siderophile elements
- stratigraphic boundary
- Upper Cretaceous