New evidence concerning the age and biotic effects of the Chicxulub impact in NE Mexico

Abstract

In the 1990s the Chicxulub impact was linked to the K-T boundary by impact spherules at the base of a sandstone complex that was interpreted as an impact-generated tsunami deposit. Since that time a preponderance of evidence has failed to support this interpretation, revealing long-term deposition of the sandstone complex, the K-T boundary above it and the primary impact spherule ejecta interbedded in Late Maastrichtian marls below. Based on evidence from Mexico and Texas we suggested that the Chicxulub impact predates the K-T boundary. Impact-tsunami proponents have challenged this evidence largely on the basis that the stratigraphically lower spherule layer in Mexico represents slumps and widespread tectonic disturbance, although no such evidence has been presented. The decades-old controversy over the cause of the K-T mass extinction will never achieve consensus, but careful documentation of results that are reproducible and verifiable will uncover what really happened at the end of the Crectaceous. This study takes an important step in that direction by showing (1) that the stratigraphically older spherule layer from El Peñon, NE Mexico, represents the primary Chicxulub impact spherule ejecta in tectonically undisturbed sediments and (2) that this impact caused no species extinctions.

Deccan volcanism, the KT mass extinction and dinosaurs

“…A regional study of late Maastrichtian sediments discovered the primary impact spherule layer interbedded in marls and claystones 4 m below the sandstone complex at El Penon (fi gure 4) (Keller et al 2003a(Keller et al , 2009b. This spherule layer contains abundant amalgamated impact melt rock and spherules with convex/ concave contacts and calcite cement in the lower part, decreasing abundance of spherules and increasing marl matrix in the upper part followed by normal marl sedimentation with common planktic foraminifera (fi gure 4I-Q).…”

Atmospheric halogen and acid rains during the main phase of Deccan eruptions: Magnetic and mineral evidence

“…The most famous case is the Cretaceous-Paleogene boundary (KPB or Cretaceous-Tertiary boundary [KTB]) mass extinction, which is still the center of acrimonious debates between partisans of the bolide impact theory (Schulte et al, 2010) and those who favor a terrestrial origin linked to Deccan Traps volcanism (Courtillot, 2012;Keller et al, 2012). Others argue that each process, individually, is unlikely to have caused a global biological collapse, and a scenario of multiple causes is preferred (White and Saunders, 2005;Keller et al, 2009;Renne et al, 2013).…”

Grain size of Cretaceous-Paleogene boundary sediments from Chicxulub to the open ocean: Implications for interpretation of the mass extinction event

“…The typical plankton extinction record and the associated diagnostic grain size change across the K-Pg boundary at Site 1212 provides a basis for interpreting the extinction record and stratigraphy of more proximal and higher energy locations. Keller et al (1997Keller et al ( , 2004Keller et al ( , 2007Keller et al ( , 2009) and others proposed that impact-related units in shelf sections in northeastern Mexico and Texas preceded the paleontological K-Pg boundary by as much as 300 k.y. This conclusion was based on: (1) the correlation of spherule-bearing cross-bedded sand lenses and layers to the Plummerita hantkeninoides (CF1) planktonic foraminiferal zone, which represents the last 300 k.y.…”

“…This issue is especially signifi cant at the K-Pg boundary in the Gulf of Mexico, where sediments were deposited by a range of high-energy media including gravity fl ows, high-energy water disturbance, and tsunami-generated currents. These processes com-plicate biostratigraphic interpretations and have ignited the debate as to whether the impactrelated deposits correlate to the mass extinction at the K-Pg boundary (e.g., Keller et al, 1997Keller et al, , 2004Keller et al, , 2007Keller et al, , 2009.…”

“…These authors interpreted as much as 300 k.y. between the Chicxulub impact and K-Pg boundary, and have proposed that Deccan (India) volcanism, not the impact, was responsible for the mass extinction event (e.g., Keller et al, 2009). Eruption of the Deccan and Rajahmundy Traps of India overlaps the K-Pg boundary, and the role of this volcanism in the extinctions has long been postulated as an alternative to the Chicxulub impact (e.g., Offi cer and Drake, 1983).…”