Approximately 65 million years ago an asteroid or comet slammed into the ocean near what is now the Yucatan Peninsula in the Gulf of Mexico. The impact generated massive earthquakes and tsunamis, and spread a layer of ejecta around the Earth. This ejecta layer is commonly recognized in the rock record by its high levels of iridium, shocked quartz, and glass spherules. Depending on who you believe, this impact may have wiped out the dinosaurs. During the summer of 1999, my friend and colleague Phil Stoffer showed me an areally extensive, but stratigraphically restricted, zone of soft sediment deformation and clastic dikes at the base of the White River Badlands that he had found while doing field work for his dissertation. Phil's dissertation was on Late Maastrichtian stratigraphy of the Western Interior Seaway and tectonic influences on stratal dimensions. Phil determined that this "Disturbed Zone" is within the very top of the Late Cretaceous Fox Hills Formation. Based on Sr isotope data from belemnites, preliminary magnetostratigraphy, and ammonite zonations, we think that the Disturbed Zone is a distal manifestation of the Chicxulub Impact at the K/T Boundary. The slumping and clastic dikes represent the effects of a passing seismic wave from the initial impact. The overlying sand sheet(s) may represent a tsunamite. So what? It's just another K/T Boundary, right? Wrong. If this Disturbed Zone is a manifestation of the Chicxulub Impact, then the overlying 16 meters of marine sediments below the base of the Eocene/Oligocene White River Group are Paleocene! This would totally rewrite the paleogeography of the Western Interior Seaway and the timing of its retreat, as well as the Paleogene geologic history of this region east of the Black Hills. It's possible that the Seaway never left until well into the Paleocene. Could the Western Interior Seaway actually have persisited to become the Eocene Cannonball Sea...without a retreat?! We're in the process of collecting sediments for geochemical analysis and searching for eject materials. Check out some of the pictures and figures below. Feel free to comment.
Impact location
Clastic dikes and slumps in Badlands NP
Possible tsunamite sand sheet in Badlands
NP
Exhumed roll structures in Badlands NP 1
Exhumed roll structures in Badlands NP 2
Slumps and roll structures along the
Cheyenne River
Stratigraphic revisions
Picture of Dillon Pass in Badlands NP,
and revised stratigraphy
Picture of Sage Creek area in Badlands NP, and
revised stratigraphy
Our 1999 GSA abstract
News
article in Academic Press online magazine