FOUR ARGUMENTS WHY THE CHICXULUB IMPACT HAS CAUSED THE CRETACEOUS-PALEOGENE MASS-EXTINCTIONS

Some geologists have uncoupled the Chicxulub impact from the K/Pb boundary (KPb) mass-extinctions by interpreting observations in conflict with the data. Detailed investigations by many scientists (e.g.Schulte et al 2009 GCA) has confirmed that the Chicxulub impact coincides with the mass-extinctions at the KPb . We mention four contentious points. Details are on: http://www.geolsoc.org.uk/chicxulub

1) The KPb is defined at a mass-mortality level, below an iridium anomaly, and a clay layer without Paleocene foraminifers at its base. Placing the KPb at the first occurrence of Paleocene foraminifers gives an erroneous position above the true KPb. Dolomite crystals overlying the impact ejecta in the Yax-1 drillcore were misinterpreted as foraminifers. There is no other convincing evidence of thriving Maastrichtian biota above the Chicxulub impact ejecta.

2) The iridium anomaly and coarse impact ejecta in the Gulf occur at diffent stratigraphic horizons, and were interpreted as evidence for two impacts. However, the same impact evidence is within one dual layer, composed of an upper layer with iridium amalgamated on a layer with coarse impact ejecta at ODP leg 207 and the US western interior. Continuous sequences even further from the Gulf display only a single impact layer. Therefore, both the iridium and coarse ejecta in the Gulf are simplest explained by one, -Chicxulub- impact.

3) The complex structure of -and burrowing in- the clastic beds in the Gulf area between the iridium and coarse ejecta lead to impression of long time intervals between beds. However, all beds are eroded into each other. The burrowed levels can better be explained by quick back-burrowing by tsunami disturbed arthropods, and as a burrow-fabric penetrated from above the clastic beds. Climbing ripples, mis-interpreted as HCS wave-ripples, give the impression of infrequent storms, but are more consistent with tsunami currents.

4) In Mexico, multiple layers of coarse ejecta below the clastic beds are interpreted as evidence for a pre KPb age of the Chicxulub impact. But the layers are discontinuous and display extreme ball-and-pillowing. Similarly, a clay-layer in Brazos, Texas has been interpreted as coarse ejecta. However, we have found no coarse ejecta of Chicxulub in there, but we did find small volcanic phenocrysts, evidence for a bentonitic ash layer.