When the sky fell on our heads: Identification and interpretation of impact products in the sedimentary record

Abstract

Research on impact products, mechanisms and consequences has benefited greatly in the last ten years from the highly publicized Cretaceous‐Tertiary (KT) boundary debate. This debate has led to an increasing awareness among Earth scientists of the importance and potential effects of impact events in Earth history. With more and more researchers realizing the role played by impacts and becoming familiar with the characteristics of impact materials, it is likely that in the coming years, an increasing number of impact events will be identified, in agreement with the Earth cratering rate estimations (Grieve, 1984; Shoemaker et al., 1990).The increasing recognition of impact event is demonstrated by the discovery or identification in recent years of several impact craters, some of large diameter: Chicxulub, >180 km in Yucatan; Mjølnir, 40 km in the Barents Sea; Ames, 16 km in Oklahoma; Avak 12 km in Alaska; Pretoria Saltpan, 1.1 km and Kalkkop, 640 m both in South Africa. Sediments record impact events in the form of impact glass, shocked minerals, Ir anomalies, and perhaps also polymic‐chaotic breccias and/or tsunami deposits. Identification of impact products, particularly in ancient rocks, is hampered by poor preservation, diagenetic alteration, metamorphism, misinterpretation, or even lack of interest or awareness of their significance. Impact products represent very short term events, most likely geographically localized and stratigraphically concentrated over a thin interval of sediments and often diluted by normal background deposition. Finding these impact signatures requires very detailed petrographic and geochemical investigations with closely spaced sampling (a few cm). Such detail is not commonly used when studying ancient Mesozoic and Paleozoic sediments.