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Still More Sihetun

This is a reply to a 9 October post concerning the BBC2 special, 
"Dinosaurs in Your Garden" and is directed at a comment John Jackson made 
concerning the show:

"The Chinese site surprised me. I had expected fossils to have been 
just lying around on the ground, but the site is now, at any rate, a 
vertical wall of strata, where small samples could simply be removed like 
books from a library.  I find it hard to believe this is difficult to 
date.  I thought volcanic ash was easy."

Believe it.  First of all, there isn't any true volcanic ash in the sense 
you're thinking of in the quarry horizon, at least not that we have found 
yet (tuffaceous sandstones are not the same thing as an actual ash layer).
Secondly, volcanic ash is not all that easy if you want to be sure the 
date is real.  Laser fusion Ar-Ar on zircon grains has been the method of 
choice, but since Ar gets liberated above about 250 degrees C and is driven 
off from the sample, the proximity of the tuffs that we HAVE found NEAR 
Sihetun to balsaltic units (which have a melting temperature of 1200 C+), 
we have alteration and Ar ratio reset problems to contend with, which 
make the error bars on our graphs rather large.  As for the igenous rocks 
that ARE found at Sihetun,  the problems concerning getting an accurate 
date from those rocks is another story entirely...and this is after you 
finally figure out how these rocks are related to each other and to the 
pretty little birds you want to age in the first place.

Josh Smith
University of Pennsylvania
Department of Earth and Environmental Science
471 Hayden Hall
240 South 33rd Street
Philadelphia, PA  19104-6316
(215) 898-5630 (Office)
(215) 898-0964 (FAX)