The Sedimentary Record of Impact Events

Abstract

Some impact events in the geological record have been recognized because of the discovery of ejecta layers. Ejecta are classified into proximal ejecta, which are found in the immediate vicinity of an impact crater, within 5 crater radii from the crater rim). Distal ejecta consist of (usually fine-grained) rock and mineral fragments, and/or contain impact glasses. The study of distal ejecta, e.g., at the Cretaceous-Tertiary boundary, has led to the discovery of large impact structures (in this case, the Chicxulub structure in Mexico). Tektites are also distal ejecta. They occur in geographically extended strewn fields, of which four are currently known: the Australasian (0.78 Ma), the Ivory Coast (1.07 Ma), the Central European (15 Ma), and the North American (35 Ma) strewn fields. The extent of three of the four fields is defined by the occurrence of microtektites in deep-sea sediments, where they occur in a stratigraphie position that is identical to the radiometric age of the tektites. Source craters have been identified for three of the four strewn fields; the source crater remains elusive for the Australasian field. Distal ejecta (also called impactoclastic layers) can serve as event markers in the geological record, because they allow immediate determination if there is a correlation between an impact event and a biological response.