RE: Extinction

[Phillip Bigelow <bh162@scn.org>]

On Sat, 25 Mar 2000, Steve  Brusatte wrote:

> On Fri, 24 Mar 2000 17:38:58   Phillip Bigelow wrote:
> >
> >Uhhh...  Not to throw a wrench into the smoothly running machinery of this
> >discussion, but, question:  Was the trackway located below a known K-T
> >boundary layer (~2 cm-thick layer containing iridium spike, shocked
> >quartz, spherules, soot, etc.), OR was the trackway found in a zone that
> >is *inferred* to be at K-T boundary because it appears to be at the same
> >stratigraphic level as other known boundary layers in the formation?


> Well, good question.  I do not know the exact answer, but I believe that
> there was an iridium spike.  Lockley quotes, "To their surprise, as well
> as my own, it took less than a minute for me to find tracks only 15
> inches below the iridium layer."  This is quoted from page 224 in The
> Eternal Trail. 



If Lockley states it *that* way, then it probably is true (i.e., that
there is indeed an iridium layer in the *same* section as the footprints; 
in other words, he can place one hand on the track layer and his other
hand on the boundary bed). That's all I needed to know! :-)

BTW:  15 inches of stratigraphic thickness can represent one year's
accumulation of sediment OR as much as 10,000 years accumulation of
sediment.  It all depends on the rate of deposition in that little area
(which is highly variable even within one formation). And if a flood came
through during the intervening time and stripped away 1-5 inches of
sediment before deposition resumed, then we've got even more problems!

As George O. noted:  In order to say anything *definative* regarding the
relationship between airfall impact deposition and faunal survival, we
need to find dino footprints at least *within* the boundary clay layer, OR
above it!* All other relationships yield interpretive conclusions
(although if that is all a scientist has to work with,
then...well...interpretive conclusions are better than nothin'!). :-) 



> Lockley also gives two other references:
> Lockley, Martin.  "Tracking Dinosaurs."
> Lockley, Martin and A. Hunt, "Dinosaur Tracks and other fossil footprints of 
> the Western United States."


Got 'em.  Love 'em.


* = I'm not going to touch this subject with a 15-inch pole, but: K. 
Rigby, Jr. and Chinese co-workers claimed (a while ago) to have found dino
tracks above an iridium-rich layer in U. Cretaceous sediments somewhere in
Asia. Since then I have heard nothing more on this claim.  Has that
section of rock been re-interpreted?  Could the iridium spike be instead
from the Manson impact?


              <pb>
--