EXTRATERRESTRIAL IMPACT EVENTS - THE RECORD IN THE ROCKS AND THE STRATIGRAPHIC COLUMN

The known terrestrial impact record is a biased sample of a much large r population of impact events. The biases are due to the modifying eff ects of terrestrial geologic processes, coupled with incomplete search es for impact structures and impact-related materials, Terrestrial imp act structures have the same basic forms as impact craters on the othe r planets of the inner solar system but, because of post-impact modifi cation by terrestrial geologic process, are recognised by the occurren ce of shock metamorphic effects. In some cases; siderophile anomalies have been identified in impact lithologies and have been used to estim ate the composition of the impacting body, Similar shock metamorphic e ffects and a siderophile anomaly in K-T boundary materials are indicat ive of a major impact event, which has been correlated with the format ion of the Chicxulub structure: Mexico. Evidence of a small number of other impacts occur in the stratigraphic record, most commonly as tekt ite or microtektite horizons. In some cases they are known to be accom panied by geochemical anomalies, In other cases a number of Ir anomali es have been reported in the stratigraphic record but there is no conf irmatory evidence that they are due to impact, The majority of known i mpact events in the stratigraphic record are from relatively recent ge ologic time. Logic dictates, however, that many more impacts must be r ecorded ill terrestrial sediments and model calculations indicate that relatively small impacts (D greater than or equal to 20 km) have the potential to cause atmospheric blow-out and, thus, global dispersion o f some of the impact products. Geochemical detection, however, of such events may not be easy; in some cases because of relatively small abs olute signals against the background of the daily infall of cosmic mat erial. In addition, non-chondritic bodies may result in no appreciable geochemical anomaly. In view of this, any claim to a geochemical sign ature of impact in the stratigraphic record should be accompanied by a physical search for impact materials; although, in the case of impact s into oceanic crust, this too will be difficult. Given the K-T experi ence, however, and the fact that large-scale impact on Earth is a natu ral consequence of the character of the solar system, the potential of impacts to provide local and global marker horizons can not be ignore d. Similarly, the fact that impacts may have the potential to result i n shortterm biologic or climatic excursions can not be dismissed arbit rarily, when considering the causes of such phenomena as stable isotop e anomalies in the stratigraphic record. (C) 1997 Elsevier Science B.V .