Impact stratigraphy: Old principle, new reality

TitleImpact stratigraphy: Old principle, new reality
Publication TypeJournal Article
Year of Publication2008
AuthorsKeller, G
JournalGeological Society of America Special Papers

Impact stratigraphy is an extremely useful correlation tool that makes use of unique events in Earth's history and places them within spatial and temporal contexts. The K-T boundary is a particularly apt example to test the limits of this method to resolve ongoing controversies over the age of the Chicxulub impact and whether this impact is indeed responsible for the K-T boundary mass extinction. Two impact markers, the Ir anomaly and the Chicxulub impact spherule deposits, are ideal because of their widespread presence. Evaluation of their stratigraphic occurrences reveals the potential and the complexities inherent in using these impact signals. For example, in the most expanded sedimentary sequences: (1) The K-T Ir anomaly never contains Chicxulub impact spherules, whereas the Chicxulub impact spherule layer never contains an Ir anomaly. (2) The separation of up to 9 m between the Ir anomaly and spherule layer cannot be explained by differential settling, tsunamis, or slumps. (3) The presence of multiple spherule layers with the same glass geochemistry as melt rock in the impact breccia of the Chicxulub crater indicates erosion and redeposition of the original spherule ejecta layer. (4) The stratigraphically oldest spherule layer is in undisturbed upper Maastrichtian sediments (zone CF1) in NE Mexico and Texas. (5) From central Mexico to Guatemala, Belize, Haiti, and Cuba, a major K-T hiatus is present and spherule deposits are reworked and redeposited in early Danian (zone P1a) sediments. (6) A second Ir anomaly of cosmic origin is present in the early Danian. This shows that although impact markers represent an instant in time, they are subject to the same geological forces as any other marker horizons—erosion, reworking, and redeposition—and must be used with caution and applied on a regional scale to avoid artifacts of redeposition. For the K-T transition, impact stratigraphy unequivocally indicates that the Chicxulub impact predates the K-T boundary, that the Ir anomaly at the K-T boundary is not related to the Chicxulub impact, and that environmental upheaval continued during the early Danian with possibly another smaller impact and volcanism.  PDF