EJECTA VERSUS CRATERING RECORD ON EARTH

Asteroid or comet impact can now be considered as an integral part of the uniformitarian concept as it is accepted that some of these events have influenced the course of Earth evolution. Unfortunately most of the Earth cratering and ejecta record is unavailable, erased or hidden by tectonic activity, erosion or sedimentation. Compare to the more than 160 impact structures known today only a dozen (or less) ejecta layers have been found in the sedimentary record. And some are controversial. When discovered ejecta layers can often be difficult to correlate with a known impact structure. The history of biological and climatic changes is recorded exclusively in sediments. Finding and studying impact ejecta material in a sedimentary sequence is thus of paramount importance to precisely pinpoint temporal correlation and consequence of an impact on Earth?evolution. In the Cenozoic, several impact layers are known. The KT boundary case, with its bountiful proximal and distal layers, can be considered a case study to learn about the production, transport, distribution sedimentation of ejecta. The variety of the paleoenvironments hosting KT ejecta also provides key data on the effect of diagenesis on the alteration and preservation of ejecta fragments. Few ejecta beds are recognized in the Mesozoic and in the Paleozoic. For the Paleozoic, this can perhaps be explained by the fact that deep water settings - ideal for the preservation of ejecta material - are rarely preserved. Ejecta layers are known in the Precambrian of Australia and South-Africa, and their tickles and extension illustrate the magnitude of impact events in this period. This paper will further review the known ejecta layers, discuss possible causes for their scarcity and poor preservation and propose ways to improve the search for ejecta beds in sediments.