Re: Suspicious impact craters and iridium layers

[Phil Bigelow <bigelowp@juno.com>]

On Mon, 13 Jun 2005 09:07:08 +0200 (CEST)
=?iso-8859-2?Q?Vladim=EDr=20=20Socha?= <Seismosaurus@seznam.cz> writes:
> Good day to all listmembers,
> I finally finished reading David M. Raup's book "Extinction: Bad 
> Genes or Bad Luck?" and I'd like to compare new data with those from 
> the book (published in 1990). Firstly, in the book there's a short 
> list of all strata with (as for 1990)discovered iridium anomalies:
> Frasnian (Devonian) 367 mya
> Callowian (Jurassic) 157 mya
> Cenomanian (Cretaceous) 90 mya
> KT boundary 65 mya
> Eocene (Tertiary) 35 mya
> Miocene (-"-) 12 mya
> Pliocene (-"-) 3 mya


If I had to guess, I'll bet that with future chemostratigraphic studies,
there will be vastly more Ir anomalies discovered.  Hundreds, perhaps
thousands of them.  Raup only listed the tip of the Ir iceberg.  It all
depends on how one defines a "significant Ir anomaly".  For instance, is
50% over background "significant"?

A word about Raup's 1990 book:  When he wrote it, many radiometric dates
for both impact craters and stratabound Ir were poorly known.  Sadly,
many still remain poorly known.


> My question is: were any other layers found from 1990 until today? 

A buttload of Precambrian spherule layers (well, maybe not a buttload,
but a few).
At least one Oligocene Ir/spherule layer has been found.  One of my
current summer vacation activities is searching for spherule layers in
Belt Supergroup rocks.  One of these days, folks.  One of these
days......


> Were some of the listed revisited and/or rejected?

Most have at least been revised.  Some others were later found to only be
close to extinction horizons, but as we know, "close" don't earn ya a
seegar.

Even the well-studied Chicxulub crater has had its radiometric date
changed more than once.

<pb>
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