extinctions & craters

[Thomas_R_HOLTZ@umail.umd.edu (th81)]

Just to add a little more contraversy to the discussion, there are recent
publications which have removed either Manson or Chixchilub from the  K-T
boundary.  Izett et al. (Science) have shown very convincing evidence that
the Manson crater in Idaho occurred in the middle of the Campanian stage, or
some 13 million years before the boundary.  What is more interesting is that
this impact occurred just prior to, and only a few hundred km from, the
highest diversity dinosaur fauna known (the Judith River fauna of Alberta
and Montana).

Somewhat more contraversial is Meyerhoff et al.'s paper in the March? issue
of Geology, where they present evidence that the Chixchilub crater is a)
volcanic rather than impact generated and b) mid-Campanian rather than
terminal Maastrichtian.  The volcanic evidence is that there are multiple
layers of andesites (volcanic rocks) of varying compositions.  Impact
generated melts are generally single massive units of unvarying
composition.  Furthermore, the Yucatan andesites are not simple melts of the
host rock (as expected in impacts).  The (to me) more significant aspect is
the fact that the disturbed layers are capped by rocks containing middle
Campanian microfossils. However, Meyerhoff et al. (or anyone else) have yet
to described other cores from the crater wiht these features, so these bits
of information may not pan out.

Although this does not mean that the last of the non-avian dinosaurs were
not wiped out by a bollide impact, it does suggest that they were resistant
to (and had been subjected to) impacts of small to enormous size of the
Cretaceous.

Thomas R. HOLTZ
Vertebrate Paleontologist, Dept. of Geology
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Phone:301-405-4084