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>Yes, there are those who question the exact date of the Mexican impact, but
>it is really pretty easy to ascertain -- find a location in the geologic
>record where a near extermination of all life on earth occurred falling at
>the rough age of the crater and there you have it -- the KT boundary is the
>only place it could have occurred.
You (and many others, of course) are guilty of conflating two arguments -
the date of the K-T impact, and the date of Chicxulub. The Late Cretaceous
was a LONG period of time, but many people not trained in stratigraphy have
automatically assumed any Late K impact is a TERMINAL K impact.
Now, I personally think that Chixculub is a, if not the, terminal K impact.
However, Meyerhoff et al. have presented data showing sedimentary rocks of
middle Campanian through Maastrichitan (and up into the Paleogene)
*overlying* the disturbed layer. If true, than Chixculub occured 8 or 10
million years prior to the K-T boundary, and so could not be the causal
Now, of course, if the dating is correct, that suggests there was a
seperate, terminal K impact elsewhere. Additionally, there is the
possiblity that the overlying rocks were slumped into the crater
post-impact. Nevertheless, these possibilities should be examined, not
assumed (just because they favor a cherished hypothesis).
To sum up - the impact theory looks good for the terminal K extinctions,
and (somewhat less so) the impact site appears to be Chixculub. However,
these are two seperate arguments, and each must be examined independantly.
Thomas R. Holtz, Jr.
Vertebrate Paleontologist in Exile Phone: 703-648-5280
U.S. Geological Survey FAX: 703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA 22092