Brazos River Iridium and Refs.

[Thor A Hansen <thorenet@henson.cc.wwu.edu>]

Hi Phil!

The Brazos River iridium "spike" has several peaks, the lowest 
(stratigraphically) usually occurs within what we call the tsunami bed, 
and the highest (again stratigraphically) at the top of, or just above, the 
tsunami bed.  Because there should be a zone of fine-grained 
"settle-out" mud above the coarse grained main body of the tsunami bed, 
it is a little difficult to be precise about where the top of the 
tsunami is.  I say "usually occurs" because there are multiple outcrops 
across the KT boundary at Brazos and we have done iridium analyses on four 
of them.  Of course interpretations of the paleontology of these sections 
vary, e.g., compare Keller and Smit.  But the molluscan record is clear; 
beneath the tsunami bed the molluscan assemblage is composed of a typical 
Cretaceous, suspension-feeding community. Above the tsunami bed it is 
composed of a very low diversity assemblage of a few species of 
Cretaceous origin (some survivors, some reworked) and a few new species, 
in a community dominated by deposit feeders.  

For references see:

Sheehan and Hansen, 1986, Geology 14:868-870

Bourgeois, J., Hansen, T.A., Wiberg, P.L., Kauffman, E.G. 1988. Science, 
241:567-570.

Hansen, T.A., et al. 1987. Cretaceous Research 8:229-252.

Hansen, T.A., et al. 1993. Cret. Res. 14: 685-706.

Hansen, T.A., et al. 1993. Paleobiology 19:251-265.