Tektites and Their Origin

Abstract

The known terrestrial impact record is a biased sample of a much larger population of impact events. The biases are due to the modifying effects of terrestrial geologic processes, coupled with incomplete searches for impact structures and impact-related materials. Terrestrial impact structures have the same basic forms as impact craters on the other planets of the inner solar system but, because of post-impact modification by terrestrial geologic process, are recognised by the occurrence of shock metamorphic effects. In some cases, siderophile anomalies have been identified in impact lithologies and have been used to estimate the composition of the impacting body. Similar shock metamorphic effects and a siderophile anomaly in K–T boundary materials are indicative of a major impact event, which has been correlated with the formation of the Chicxulub structure, Mexico. Evidence of a small number of other impacts occur in the stratigraphic record, most commonly as tektite or microtektite horizons. In some cases they are known to be accompanied by geochemical anomalies. In other cases a number of Ir anomalies have been reported in the stratigraphic record but there is no confirmatory evidence that they are due to impact. The majority of known impact events in the stratigraphic record are from relatively recent geologic time. Logic dictates, however, that many more impacts must be recorded in terrestrial sediments and model calculations indicate that relatively small impacts (D≥20 km) have the potential to cause atmospheric blow-out and, thus, global dispersion of some of the impact products. Geochemical detection, however, of such events may not be easy; in some cases because of relatively small absolute signals against the background of the daily infall of cosmic material. In addition, non-chondritic bodies may result in no appreciable geochemical anomaly. In view of this, any claim to a geochemical signature of impact in the stratigraphic record should be accompanied by a physical search for impact materials; although, in the case of impacts into oceanic crust, this too will be difficult. Given the K–T experience, however, and the fact that large-scale impact on Earth is a natural consequence of the character of the solar system, the potential of impacts to provide local and global marker horizons can not be ignored. Similarly, the fact that impacts may have the potential to result in short-term biologic or climatic excursions can not be dismissed arbitrarily, when considering the causes of such phenomena as stable isotope anomalies in the stratigraphic record.