Re: Supernova causing P/T boundry mass extinctions?

[Peter Sheehan <sheehan@xxxxxxxxxxxxxxxx>]

On Thu, 5 Jan 1995, Peter Harries (GLY) wrote:

> 
> 
> On Thu, 5 Jan 1995, Tom Holtz wrote:
> 
> > 
> > Maybe we have to chalk up the missing meter to the Signor-Lipps effect.
> > 
> Given the number of man hours that have gone into trying to find 
> vertebrate fossils in that meter section, I have hard time just 
> explaining it away through the Signor-Lipps effect.  That tends to come 
> into play when things have been sampled in a relatively uniform fashion.  
> This interval, however, certainly doesn't suffer from being under 
> examined.  Therefore, the lack of fossils, be it caused by some primary 
> or some drastic post-depositional processes, would seem to be real.
> 
> Peter Harries
> 


There has been very little work done on the one meter below the iridium 
layer in MT and ND.  There are only two places, one in ND and one in MT 
where the iridium anomaly has been found.  There are a few other places 
where slight enrichment probably indicates the level.  The places where 
it has been found are associated with swampy deposits.  The layer is not 
persistent--for example in MT it is not found at about the same 
stratigraphic horizon about 1/2 km away.

In most of the region the fallout layer was probably eroded shortly after 
deposition.  It was, after all, a thin layer of loose ash, and rain fall 
probably eroded it (especially since the plant cover had been 
destroyed).  As a result, the approximate boundary has been selected as a 
coal bed.  This coal bed is almost certainly slightly diachronous over the 
region.  

In areas of stream deposition the iridium layer was probably eroded 
immediately--by the stream.  The most abundant fossil bearing sediments 
are the stream beds--so here the meter of section below the iridium can 
not be identified.  Most of the collecting effort mentioned above has 
been through screen washing of stream sediments.  

This also brings up the issue of what one meter of section means.  In 
some depositional settings of the Hell Creek a meter might represent only 
one or two depositional events of only a days duration (for example in 
point bar deposits).  In other parts of the Hell Creek (for example the 
floodplain deposits) deposition of 1 meter of section might have taken 
many thousands of years.   

Dinosaur bones in floodplain deposits are very rare.  Only a few thousand 
have ever been found in the Hell Creek, and few of these have been carefully 
positioned relative to the boundary.   

My point is that the 1 meter from the boundary "rule" is most likely a 
Signor-Lipps effect--especially since in most places the boundary is just 
a close guess as to the actual position.

Peter Sheehan