PARSING THE TIMING OF EVENTS AT THE CRETACEOUS-PALEOGENE BOUNDARY (Invited Presentation)

The synchrony of timing is a critical test for possible causes of the Cretaceous-Paleogene boundary (KPB) ecological crisis. Recent geochronological work has significantly improved our ability to align the sequence of events for the KPB with the Chicxulub impact and volcanism in the Deccan Traps. Chicxulub unequivocally produced the iridium anomaly widely used to define the KPB, as is clear from stratigraphy and recently confirmed by ⁴⁰Ar/³⁹Ar geochronology. A minority view that Chicxulub pre-dated the KPB by as much as 300 ka is clearly untenable. A more challenging question concerns the relationship between the Deccan Traps and the KPB/Chicxulub impact. It is now incontrovertibly clear that these events occurred during the interval of Deccan volcanism, and most probably at the time when the volcanism underwent a state shift that appears to reflect a fundamental change in the magmatic plumbing system. The transition is manifest in changes in eruption tempo, flow volume, and geochemistry beginning at the contact between the Bushe and Poladpur Formations of the Western Ghats and persisting throughout the Wai Subgroup. Current data indicate that the 66.04 Ma age of the KPB/Chicxulub impact occurred within 50 ka of this transition. The transition does not coincide with a significant hiatus in volcanism, nor is there evidence for any hiatus longer than ~50 ka within the Western Ghats stratigraphy, hence the notion of three discrete pulses of volcanism should be abandoned. Ongoing work is aimed at refining the age of the Bushe/Poladpur transition, and testing for regional synchrony. The results will provide a rigorous test of the hypothesis (Richards et al., 2015) implicating the Chicxulub impact as the cause of the Deccan state shift. In any case, the onset of the most massive Deccan eruptions at the KPB poses challenges to deconvolution of the relative contribution of impact and volcanism to the ecological crisis.