The Chicxulub structure on Yucatan is now commonly believed to have been formed by the Cretaceous/Tertiary (K/T) boundary bolide impact that caused the catastrophic extinction of organisms from dinosaurs to microplankton. However, the mass extinction began well before the K/T boundary and the kill-effect that may be directly attributed to a K/T impact is relatively small (only planktonic foraminifera and nannoplankton affected), highly selective (only tropical-subtropical species extinct) and restricted to low latitudes. Moreover, key evidence cited in support of Chicxulub as K/T impact crater is still controversial (e.g., impact origin of glass), or contradictory : the so-called "impact-generated megatsunami deposits" in northeastern Mexico contain burrowing horizons that indicate deposition occurred over an extended period of time. This database suggests a multi-event scenario that includes a pre-K/T event (impact or volcanism) that formed the spherule deposits in northeastern Mexico and a K/T event (Ir anomaly, mass extinction) with both events coinciding with climatic and sea level fluctuations during the last 200-300 kyr of the Maastrichtian.

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