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All kinds of odd observations



NEMESIS

There are lots of reasons whjy Nemesis can't exist, never did
exist and has bearing on dino extinction.  Terry Colvin raises
this idea once again.  I thought it was dead.

IRIDIUM

Terry also queries the relevance of iridium.  This is now 
recorded from over a hundred sites in the K-T boundary rocks from
all continents and from deep-sea cores.  Even though Alvarez et
al. only had th ree sites for their original paper they guessed
right that it would turn out to be world-wide.

BITING BY SAUROPODS

Peter Buchholz has doubted that a bite by a sauropod could be
much of a deterent.  Paul Willis has pointed to the bite of a
wombat.  Have any of you ever seen a man bitten by a camel?  In
the Sudan a camel bite is more feared (and probably causes more
deaths) than bites by any of the great  cats.

MASS EXTINCTIONS

Paul Willis rightly points out that extinction of the dinosaurs
cannot be treated in isolation.  At th e K-T boundary    85 %
of all species k nown from the fossil record disappeared.  Most
of these species were micro- or nano-fossils, and give a better
overall picture of what h appened at that ti me than any study
of dinos.  Th e nearest any   dino fossil has been found to the
K-T boundary is 2 meters.  That's p retty close for something
as big as a  dino, but it is not exactly at the boundary.  The
microfossils give a much clearer picture of the mass ex tinction.

By the way, the terminal Permian extinction (P-T) involved about
96 % of all known fossil sp ecies.  Th anks to those who corrected
my dates.  I screwed up by a mixture of failure to check, writing
from memory and  sloppy ty ping.

CRETACEOUS SEDIMENTS INSIDE CHICXOLUB CRATER

Any crater        more than about 4 miles across is a complex
crater.   The Barringer Crater in Arizona is perhaps as large
as any simple crater we can expect to see.  An i mpact causing
a crater larger than this will produce a complex c rater with
a central uprising and slumped walls.  Chicoxolub (I never can
spell that) has two main rings, the more marked one 180 km.
across and a fainter one,  300 km.  Alan Hildebrand (Geol.
Survey of CAnada) does not believe th at the 300 km ring is of
any great significance and p uts the primary diameter at about
182 km.  In any case there are slumped rocks throughout, where 
the walls have collapsed.  Since these were late Creteceous,
then of course all blocks (many of which remain intact in such
slumps - see Richard Grieve's papers--one good general account
is in Scientific American about 1988) were cretaceous in age.
Any drilling i nto th ese blocks would show a late Cretaceous
age, espceially in the outer ring.

The only true criterion for the age of the crater is the
radiometric age of the MELT at the bo ttom of the crater.
Two teams have now dated this - the Berkeley group and the
Geol. Survey of Canada team.  Both agree that the melt
dates to between 64 and 65 Ma.  Both have also found late
Cretaceous dates from the breccia that overlies the melt.

SNOUTERS

I'm glad so meone else h as found the Snouters.  I remember
reading the book with delight when it was first published in
English translation.  I recommend it to anyone.

MISTAKES

I seem to be very good at making mistakes.  I apologise once
more for my slips

David

--
>From: David Brez Carlisle
bk090@Freenet Carleton.CA