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Re: SVP press coverage



Here is the website of Keller and team's findings:

http://geoweb.princeton.edu/people/faculty/keller/chicxulub.html

Hm...

Obviously, the following two points are not enough to dismiss Keller's results from all the sites outside the crater. But I think they should be mentioned nevertheless.

- There is a U/Pb-date from zircons from the Chicxulub crater, namely 64.98 +- 0.05 Ma. On the other hand, it has now again become fashionable to put the K-Pg boundary at 65.5 Ma (e. g. http://www.stratigraphy.org/gssp.htm). Keller doesn't mention _either_ of these. I'm confused.
- At Yaxcopoil-1, a bore hole inside the crater, the suevite breccia is overlain by 50 cm of what does seem to be Cretaceous sediment. Keller knows the argument that those 50 cm are backwash/crater infill; and she goes on to argue against this view _by describing the lithology of those 50 cm!_ I wonder if her views about the infilling of such a giant crater are perhaps far too benign. What if these 50 cm belong to one boulder of a big breccia? I mean a boulder that _was already rock_ when the impact happened, broke off of the crater wall (which was originally vertical at the top, and tens of km high!), and came to lie on top of the suevite breccia. The drill core is just a few cm across, so it's easily capable of missing the boundaries of a boulder of, say, 1/4 m³. If I'm right, this makes the entire following quote irrelevant:


From the conclusions section:

> a) Yaxcopoil-1 contains a 50 cm thick laminated micritic
> limestone between the impact breccia and the KT boundary. This
> interval contains five thin green layers of glauconite formation with
> each interval bioturbated. This indicates deposition occurred in
> variable, but generally low energy environments interrupted by long
> pauses of little or no deposition and glauconite formation.
>
> b) There are no grain-size grading (except for dolomite rhombs that
> may give that impression), no cross-bedding, no flaser bedding, no
> sand grains in insoluble residues, that would indicate high energy
> deposition due to backwash and crater infill. Though such evidence is
> present in the top 15 m of the impact breccia.

-Heather Yager

There's a test for this: there must be an unconformity exactly under the K-Pg boundary glauconite. Judging from the irregular shape of the glauconite band in
http://geoweb.princeton.edu/people/faculty/keller/fig21.jpg, I think this is possible. But then I don't know how this particular glauconite was determined to constitute the boundary. The text at http://geoweb.princeton.edu/people/faculty/keller/chicxpage2.html doesn't tell.