Re: Extinction

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>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
agent.

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.                                   
tholtz@geochange.er.usgs.gov
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
U.S.A.