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More Extinction (with refs!!!)



Peter Sheehan <sheehan@csd4.csd.uwm.edu> wrote: 
Peter said:
>Williams' Fig. 1c (fig 1, part 3).  He shows many dots randomly 
>distributed, then 2-3 meters below the top of the Hell Creek the dots 
>are scarce.  I see no evidence for this.  
>To begin with the iridium layer is not spread all over N.D. and Montana.  
>It is well developed in two places--one in MT and one in ND.  Over most 
       ^^^^ ^^^^^^^^^?
>of the region the iridium layer is not present or at least has not been 
>recognized!  So how can you tell when you are 2-3 meters below the iridium 
>horizon?

   Peter is correct about the sparce distribution of the iridium "spike" in
the boundary zone (Peter should know; he is one of the experts on this!).
However, I prefer to use the more appropriate term "iridium spike" rather
using "iridium layer", because there really is no "iridium layer" anywhere
(not even at Gubbio, Italy!).  Rather, the iridium is an *enrichment* from a
rather uniform background concentration in the surrounding rock of 
1 to 2 nannograms/gram of rock (Smit and Van Der Kaars, 1984).  The iridium
occurs in a clay layer at Gubbio, Italy (the Alvarez et.al site), at the
Fish Clay in Denmark, and I think in Spain (no ref. on this, sorry).  In
addition, the spike is present in boundary zones on the deep sea sediments.
Well, back to the American West...
In the case of the Herpijunk Promontory site, the iridium spike is, indeed,
a SPIKE:  11.7 nannograms/gram of rock. (Smit and Van Der Kaars, 1984). 
Iridium spikes are by nature well developed...otherwise they would not appear
as spikes!  In the American West, Montana actually has more than one site,
although Peter may take issue with me as to what actually constitues a
"site":  In Montana, there is apparently a suite of "sub-sites" in the Fort
Peck area. 1) Locality 2 of Fastovsky (1987), aka, the "Brownie Butte site";
           2) The "Herpijunk Promontory" site of Smit and Van Der Kaars
             (1984). Snow Creek area.
           3) A purported site, discovered by Alvarez (Wally, not Louis),
              Clemens and other co-workers, also found in the Snow Creek
              area.  This site is informally and non-biasly <sicker> dubbed
              "Iridium Hill" by Alvarez and his crew. 
              Alvarez, L.W. (1983), Proc. Natl. Acad. Sci. U.S.A., 
                  vol. 80, p. 627.

In all three of these sites, a clay layer was either containing the spike,
OR, the clay layer was adjacent to the spike.  This implies (at least to me) a 
typical sampling bias that unfortunately is unavoidable in such "in search
of" expeditions.  The clay looked unusual, so the researchers looked at it
more closely.  The only other alternative would be to do what foram and
nannofossil experts call "continuous sampling"...and these guys will
probably tell you that you don't want to do that.  It is hard, slow, and
expensive, because you are looking at an entire core of rock, rather than a
unusual clay layer.  As far as I know, no one has attempted continuous
sampling for iridium in these continental sediments.
  There are of course other continental iridium spikes at or near the K/T
boundary in the American West:
    a) the North Dakota site that Peter Sheehan mentioned.
    b) The Raton basin of New Mexico has an iridium "anomaly" that is
associated with a sharp pollen change (C. J. Orth, J.S. Gillmore, C.L.
Pillmore, R.H. Tschudy, J.E. Fassett, p. 423, _in_ L.T. Silver and P.H.
Schultz, Eds., Geol. Soc. Am. Special Paper 190, year 1982).
    c) And I recall that someone found a spike in the continental deposits of
the San Juan Basin (sorry, no refs. on this...does anybody know?). 
  
 In Texas, the Brazos River section has an iridium spike directly over those big
tidal wave deposits...but of course, that is a marine site, not a
continental one. Dr. Tor Hanson lurks on this listserver...perhaps he has
something to contribute to this discussion...Tor?? 

Peter also mentioned how hard it is to estimate where the boundary zone is.
He is making an *understatement* when he says that!  The boundary
between the Hell Creek and the overlying Paleocene Tullock Formation has
been defined as the lowest continuous coal layer. As part of this
definition, the Hell creek Formation is everything below that layer that
still contains dinosaur bones. (Brown, 1907).  In all places where these
guys have found that iridium spike, they were looking in areas where that
lower coal layer is well-exposed (the lower coal is called "lower Z coal").
The problem is that the lower Z coal is not found everywhere; in fact, it is
frustrating to approximate the boundary if that coal layer isn't present!
(sort of an oxymoron isn't it? "lowest continuous coal layer that isn't
always present").  Therefore, in most places, the K/T boundary probably
occurs in a drab, unremarkable section of rock, and would be extremely
difficult (and costly in terms of time and money) to pinpoint. 

Peter continues:
>Dinosaur bones are rare in the Hell Creek.  If I remember correctly in the 
>two places where the iridium layer has been found the nearest dinosaur 
>bones are many meters below the horizon--don't quote me on this.  
 
I'm not certain about this myself, (in other words I don't have a ref. on
this!) but I believe that in the Brownie Butte area, the strat. 
distance is less than a meter from the highest dino bone to the iridium spike.  
No extrapolation.  As I recall, there was some discussion about whether the
dino material was re-worked....nothing published though. 

Peter continues:
>The other facies in Fig. 4--flood plain muds and silts-- show dinosaur bones 
>decreasing in abundance near the top of the Hell Creek.   Again, 
>the exact position of the iridium layer is not known--even though 
>its' position can be guessed within a few meters.  How can you tell what 
>happens to abundance just below a layer which you can not identify in the 
>outcrop?

I'll buy this.   However, this doesn't bode well for your catastrophic
hypothesis, just as it doesn't bode well for the gradualists.  Frankly, I am
still unconvinced, but remain open-minded. 


In closing (finally), I have visited the Brownie Butte site on a couple
occasions.  As you push away the weathered sediment of the hill to look at
the coal/clay layer, you really get a shiver up your spine, knowing that an
event so powerful occurred 65 million years ago that it left a mark ("scar")
in the earth: a strip of dust that marks "one really bad day" in earth's
history.
 (sorry about the length)