ABSTRACT: The End--Cretaceous Extinction Need Not Have Been Impact-Related

HALLAM, A., University of Birmingham, Birmingham, England

Mass extinctions are considered to result from the disruption of community structure by a catastrophic event. Catastrophe in this context has been defined as "biospheric perturbations that appear instantaneous when viewed at the level of resolution provided by the geological record." This resolution is only rarely likely to be as little as a few thousand years. For many characteristic Mesozoic groups of marine invertebrates that disappeared at the end of the Cretaceous, such as rudists, inoceramids, ammonites, and belemnites, their extinction was much more prolonged than this. The evidence for terrestrial vertebrates is much more equivocal, the only really good data coming from Montana, where controversy persists about how catastrophic the extinction was. From where there is good evid nce for a catastrophe, among the calcareous plankton, there is no support for the kind of spectacular environmental disruptions on a timescale of less than a year proposed by some impact supporters. Furthermore, there are clear indications from other mass extinction horizons that dramatic changes on a short timescale can result from events confined to our planet. Many such changes are associated with episodes of oceanic anoxia, often related to times of rapid sea-level rise. This may be a more effective promoter of marine extinctions than the more usually discussed global sea-level falls. The combination of both, in a so-called regression-transgression couplet, provides an extremely powerful means of severely reducing habitat area. It seems an extraordinary coincidence that a K/T bolide mpact should have occurred just at the time of one of the most notable regression-transgression couplets in the Phanerozoic record.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)