[meteorite-list] Largest Crater in the Sahara Desert and LDG

From: Norm Lehrman <nlehrman_at_meteoritecentral.com>
Date: Fri Mar 3 17:33:51 2006
Message-ID: <20060303223349.33373.qmail_at_web81008.mail.mud.yahoo.com>

Bernd & list,

This is indeed exciting, and may finally justify LDG
being recognized as a true tektite rather than a
simple impactite.

Although the article doesn't give us much for location
beyond "at the northern tip of the Gilf Kebir region",
that's close enough, as the LDG strewn field is
immediately north of the Gilf Kebir.

The 28.5 ma date for LDG should be a good number
(fission track). The "100 million year sandstone"
mentioned as the crater target rock is perfect. For
years it has been argued that the Nubia group
sandstones are the geochemically perfect precursor for
LDG. Interestingly, this raised a problem for
researchers looking for a local LDG source crater as
there are good geological arguments that the Nubia
sandstones were covered by younger formations in the
LDG strewn field at 28.5 ma and would not have been
available as target rocks.

With the revelation that this newly recognized crater
did indeed impact the sandstones, we're almost there.
Now, all we have to do is eject the LDG a hundred km
or so northwards and the picture works fine. (The long
axis of the strewn field is roughly N-S).

Where is the dividing line between impactite and
tektite? I'd like to hear what others may understand,
but my impression is that it fundamentally hinges on
distance the glassy material is ejected from the
crater. Material found only in and immediately around
the source crater is impactite. Stuff blasted tens to
hundreds of km or more crosses the definitional
boundary into "tektites".

If this is the criterion, LDG was already home free in
my book insofar as the known strewn field has a long
axis of at least 150 km, so even if there was a
now-erosionally removed crater at one end of the
strewn field proper, some of the glass would've
already required over 100 km ejection distance.

Now, I'm guessing we may be talking a couple hundred
km, maybe more. Is that sufficiently far to
legitimize LDG as a true tektite? (From
Ries-Norlingen to the Czech moldavite fields is about
300 km).

Cheers,
Norm
http://tektitesource.com

--- bernd.pauli_at_paulinet.de wrote:

> Hi Ron and List,
>
> Like so many others, I was eagerly flying over the
> lines in search of
> a hint to LDG (Libyan Desert Glass),and, there it is
> (of course ;-):
>
> "since its shape points to an origin of
> extraterrestrial impact, it will likely prove to
> be the event responsible for the extensive field
> of 'Desert Glass'-yellow-green silica
> glass fragments found on the desert surface between
> the giant dunes of the Great Sand Sea
> in southwestern Egypt."
>
> But:
>
> "may have been formed by a meteorite impact tens of
> millions of years ago."
>
> How many *tens* of millions of years ago ???
>
> If current age estimates are correct, LDG has an age
> of ~28 Ma.
>
> Any thoughts out there, ... Norm?
>
> Cheers,
>
> Bernd
>
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Received on Fri 03 Mar 2006 05:33:49 PM PST