[meteorite-list] Secrets of the Wabar Craters - Part 1 of 2

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:08:32 2004
Message-ID: <3D94DE3E.DE3258E2_at_lehrer.uni-karlsruhe.de>

WYNN J.C. and SHOEMAKER E.M. (1997) Secrets of the Wabar Craters
– A team of geologists explores this enigmatic meteorite-impact site
in southern Saudi Arabia (Sky and Telescope, 1997, pp. 44-48):

In their explorations of the red planet, Mars Pathfinder and Mars Global
Surveyor have encountered a well-known class of surface features: impact
craters. Although common on the Moon and Mars, craters are rare on Earth
- so far only about 156 have been confirmed worldwide. Astronomers and
geologists now realize that the ancient, inactive surfaces of these
neighboring worlds have preserved their long history of cosmic
bombardment and cratering. In contrast, the Earth's crust is very
dynamic. Tectonic plate movement, wind and water erosion, and
sedimentation have conspired to obliterate most evidence of impacts,
especially smaller and older ones. These crust-changing processes have
had much less effect on Mars's desertlike surface, however, and so the
red planet has retained a great many impact features. Several possible
analogues for Martian craters exist here on Earth. One of the youngest
and best-preserved is in the Rub' al-Khali, the Empty Quarter of Saudi
Arabia, which is one of the most inaccessible and formidable deserts on
our lanet.

The Ancient City of Wabar

In 1932 the British explorer Harry St. John Philby became only the
second westerner to cross the Empty Quarter. His arduous trip took more
than two months, with many of his camels dying during the journey.
Philby's guides had told him of an ancient city destroyed by God because
of the wickedness and impiety of its king. (The city, called 'Ubar in
the Qu’ran, has since been found in northern Oman along the ancient
Frankincense Trail.)
When Philby finally reached the site, which he dubbed Wabar, he was
disappointed: he didn't see any ruins or evidence of human handiwork,
only two shallow depressions in the buff-colored sand surrounded by lots
of dark debris that he initially mistook for lava. His guides brought
him handfuls of black, glassy beads - the pearls adorning the women of
the destroyed city, they said. (The guides were certain they could sell
them for profit in Makkah when they returned.) They also brought to
Philby a chunk of rusted iron, suggesting that this was the evidence of
human handiwork he was searching for.
As Philby examined the rabbit-size object, it slowly dawned on him that
the craters were not of volcanic origin. With growing excitement he
began to realize that he was holding the evidence of a visitor from
beyond Earth's atmosphere. Laboratory examination later in Britain
confirmed his suspicion: the Wabar craters were, in fact, the result of
a meteorite impact.

The Expedition

Because the Earth has so few recognizable impact sites, each one demands
careful scientific inspection. So in March 1995 we undertook an
exhausting, 10-day expedition deep into the Empty Quarter to map and
analyze the Wabar impact-crater complex. Our transportation consisted of
Hummer vehicles, civilian versions of the military Humvees. It would
normally take at least three days for a caravan of four-wheel-drive
vehicles to reach the site, because they bog down constantly in the
fine, talc-like sand. But the Hummer's variable-inflation tires can roar
across dune tops at 40 kilometers per hour.
We completed the 750-km drive from Riyadh to Wabar in just under 17
hours, leaving in the early morning and camping at night in dune fields
that stretch uninterrupted for hundreds of kilometers in all directions.
The ride itself is endlessly fascinating, as the dunes constantly change
appearance across the Empty Quarter. At Wabar they look like huge ocean
waves rolling randomly in all directions.
Wabar's craters and their associated debris fields are situated at
latitude 21° 30.2' north and longitude 050° 28.4' east. The cluster is
oriented northwest-southeast and measures roughly 1,000 by 500 meters.
Although numerous visitors have examined the site since Philby's first
report 63 years ago, no detailed investigation had been made prior to
our expedition. We spent five days conducting systematic geological and
geophysical surveys of the craters and their surroundings.
At least three craters were exposed at the time of our visit (see the
map on the facing page). We dubbed them the "11-meter," "Philby A" (64
meters across), and "Philby B" (116 meters). Contrary to previous
reports, these impact features have been formed entirely in the loose
sands of the active Rub' al-Khali. No outcrops were exposed near the
craters, nor were any fragments of bedrock found in the strewnfields.
The crater walls are lined with breccia, fragments of shock-compressed
sand that had been metamorphosed into a firm aggregate we called
"instant rock." The exposed rims of the two largest craters are mantled
with fragments of black, slaggy glass called impactite (consisting of
about 10 percent iron-nickel from the shock-melted meteorite and 90
percent local sand) and chunks of instant rock that look like bleached
sandstone.
At the 11-meter crater, the ejecta rim has been entirely eroded away and
only the fused sand lining the lower crater walls remains visible. The
fact that the craters have survived the shifting sands of the Rub'
al-Khali is due solely to impactite glass and instant rock that mantle
the crater rims and walls and resist scouring by the wind. The 11-meter
crater was discovered by one of the authors (Wynn) during a visit in
December 1994. Nobody had reported this feature before; apparently it
was exposed only recently by prevailing winds.
When first seen in 1932, Philby A’s rim was stfll exposed entirely.
Sixty-three years later only its southeastern section remains visible,
as the northwestern half is now covered by a prominent seif
(sword-shaped) dune. Beyond the rim lies a fallout deposit of glass
bombs, chunks of instant rocks, and oxidized meteorite fragments
scattered across a meter-thick ejecta blanket.
We excavated a trench in the southern rim to study the layering history
and obtain samples for dating the precrater sand deposit. Normally it
doesn't rain for years at a time in the Rub' al-Khali, but by an amazing
coincidence it rained the night before our arrival. The damp sand held
the walls of the trench long enough for us to obtain a cross-section.
The structure of this crater in the sand resembles that of Meteor Crater
near Winslow, Arizona, with the topmost layers clearly deformed and
folded back onto themselves (see page 49).
Reaching Philby B, the largest crater, we found only the rim crest and
some of its breccia-lined wall remaining. The crater was well exposed in
1932 and may have been about 12 meters deep at that time. The depth was
just 2 meters when Wynn visited the crater in 1994, but wind scouring
increased this to about 3 ½ meters by the foflowing year. A fallout
deposit of glassy black slag and instant rock capped the southern
exposed rim, but the badly eroded eastern rim had been reduced to glassy
rubble.
In the most distant, northwestern patches of the complex we spotted a
smattering of glass spheres and teardrops a few millimeters in diameter
- the "pearls" that Philby's guides had excitedly gathered. Most of
these were likely entrained in a very hot, turbulent explosion plume
that carried them downwind (roughly northwest). In effect, it rained
molten glass for more than a half kilometer from the impact. Black iron
meteorite fragments and crusty balls of oxidized metal are also
dispersed in the area and are most abundant near the craters.
Received on Fri 27 Sep 2002 06:39:58 PM PDT