[meteorite-list] A Crater Wrong?

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sun, 24 Aug 2008 17:36:00 -0500
Message-ID: <030801c90639$c9721810$f34ce146_at_ATARIENGINE>

Hi, EP,

    If we were looking at The Moon instead of Australia,
it would be less confusing. We are so much less opinionated
about the Moon than the home planet. Not mentioned in this
article is that there is already a huge crater in Australia, the
Acraman Crater, with a ring at 90 km and another at 160-km,
which has been dated to 578mya, virtually the same age as
has been proposed for this "crater" (584mya). This is the
Late Vendian period, geologically, and the time of a major
extinction.

    Acraman required a 3-mile (5 km) diameter impactor, and
produced at least a 1,000,000 Megaton impact, and has a
300-mile ejecta blanket. The Acraman event has been suggested
as having a major effect on the evolution of life on Earth:
http://aca.mq.edu.au/files/ahill_SETINews2003.pdf
The upper 2500 meters of the Acraman crater has eroded
away in the half billion years it's been there!

    When I "stand back" from the planet with Google Earth,
I see a vaguely defined giant outer ring, but not a circular one
as mapped in the article. It is clearly an east-west elliptical
feature and the outline is not marked by any consistently
elevated features (mountains, hills, faults) as a rim would
be. It is largely a gigantic contrast feature.

    It may be the outline of that pre-Cambrian inland sea that
we know did exist there. It looks suspiciously like an ancient
ejecta blanket to me. And, it could be both: an ancient seabed
filled with a kilometer(s)-deep ejecta blanket.

    Or, it could be nothing but a coincidental contrast feature.

    The Acraman crater is right on the "rim" of the ejecta
blanket ellipse. About the right size for a secondary crater
from so huge an impact, if one happened. (Any secondaries
closer in would be buried by ejecta.)

    The guy's whole case is entirely "suggestive" rather than
evidential, at least up to the pseudotachylite.. It's no surprise
the government geologists in Australia were uninterested. You
have to remember that "impact" is not entirely accepted theory
in Australia.

    As late 1978, the man called the "dean" of Australian geology,
was writing textbooks that asserted that the craters on the Moon
were all volcanic, that tektites were produced in cryptovolcanic
events, not impact craters (because there are no impact craters
on Earth), and that the impact theory was "an American fantasy."

    I would imagine Australian geologists are just getting free of
that training. Connelly should send his data to the Australian
geologists who found (and recognized) the Acraman crater.
Any impact this huge should have left a massive amount of
evidence in subsurface strata.

    On the other hand, Connelly could be finding and conflating
evidence of Acraman into his picture of events. Or, there could
be another crater, even bigger than Acraman, probably very
deeply buried under that old seabed. Connelly's small gravity
map doesn't show very circular or radial features. It's doesn't
look like Chicxulub, for example.

    If anyone wants to look at the full-sized gravity map of
Australia, here it is (it's a JPEG of about 7 Megabytes!):
http://www.ga.gov.au/image_cache/GA11515.jpg
Acraman doesn't show up on it, either, at least not on this
scale or in any obvious (to me) way, but it was gravitic
anomalies that got it spotted in 1997.

    Look at the Ring-and-Rift patterns for Australia (p. 169)!
http://books.google.com/books?id=K4tv9_cRtvUC&pg=PA169&lpg=PA169&dq=acraman+gravity+map&source=web&ots=zgCvftVNMU&sig=PPxR790YeI4-SxuwDCZ7D735Lxk&hl=en&sa=X&oi=book_result&resnum=2&ct=result#PPA169,M1
You couldn't ask for a better map of the ancient basined
surface of an earlier Earth than that. Wow!

    Not convinced yet... but I would never tell anybody with
a whacky idea to quit, just keep digging. It is interesting that
a local came up with a similar idea from different (and "on the
ground" evidence). And there is that pseudotachylite evidence.

    Time will tell.


Sterling K. Webb
-----------------------------------------------------------------------------
----- Original Message -----
From: "E.P. Grondine" <epgrondine at yahoo.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Sunday, August 24, 2008 1:23 PM
Subject: [meteorite-list] A Crater Wrong?


Hello Everyone -

http://www.thedailyjournal.com/apps/pbcs.dll/article?AID=/20080819/NEWS01/808190301/1002

I can't remember if I saw any mention of this on the list, but then now days
I can't remember many things. My guess is that someone here is probably
among those who told this fellow he was mistaken, but then who knows?

(PS - That La Luz sounds beautiful)

good hunting all,
E.P. Grondine
Man and Impact in the Americas






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Received on Sun 24 Aug 2008 06:36:00 PM PDT