How deep is the sand in the Sahara?

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I asked a similar question of Dr. Christopher S. Swezey in 2001:

Hi - I recently came across your letter to Science in the 10/8/99
issue and thought you might be able to answer a question I have had
since visiting the Sahara in 1985: how deep is the sand at its
deepest? Or how deep at the greatest depth that has been sampled or
imaged? Are your thermoluminscence data from drilling core samples?

Thanks in advance for satisfying my idle curiosity.

Gantt Galloway

Here's the answer he gave me:

Much of the Sahara is characterized by bare rock, and the sand in the
Sahara tends to occur in discrete groups (usually topographic basins).
Large areas covered by sand are called sand seas or "ergs." Within ergs,
however, the amount of sand cover is variable. Dunes take on different
shapes, as a function of wind characteristics and the amount of sand
available. Barchan dunes (small crescentic dunes) form where the wind is
unidirectional and where there is not much sand available. The interdune
areas associated with barchan dunes consist of bare rock or some other
non-sandy substrate. In contrast, star dunes form where the winds are
multidirectional and where there is a lot of sand available.

I am most familiar with the Grand Erg Oriental (Great Eastern Sand Sea) of
Tunisia and Algeria. This erg is characterized by barchan dunes and other
small linear dunes in the north, larger dunes with various linear and
grid-like shapes further south, and star dunes at the extreme southeastern
part of the erg. The tallest star dunes of this erg are 320 m high, and
the interdune areas around the star dunes are filled with sand (Star dunes
in most other regions of the Sahara are not as tall as the star dunes of
the Grand Erg Oriental, so I assume that the sand cover is less thick in
these other regions). I am not certain how thick the sand would be if you
drilled a well in the middle of an interdune area among the Erg Oriental
star dunes (maybe 150 m, maximum?). I do know that the dunes of the Erg
Oriental rest on top of sandstones of Miocene age (~5-23 million years
old), that these sandstones are exposed in the interdune areas to the
north. Some oil companies have drilled wells in the southern part of the
sand sea, but their targets are very deep and they rarely report the
thicknesses of the younger sediments near the surface.

My thermoluminescence dates from the Grand Erg Oriental were from small
outcrops on the northern margin of the erg, where wind-blown sand deposits
interfinger with lake and river deposits. I chose this area because it is
a basin that lies below sea level, and is still subsiding. I thought that
this might have the oldest record of wind-blown sediments in the Sahara. I
found out, however, that most of these sediments are relatively young
(<18,000 years old), and that these relatively young sediments rest
directly on top of much older Miocene sandstones. This pattern of
relatively young sediments (thousands of years old) resting on top of much
older sandstones (millions of years old) is consistent with what is
observed in most of the rest of the Sahara. If I were to go back to the
Sahara today and look for the oldest record of wind-blown sand, I would
probably look in the area of Lake Chad (another basin that is subsiding,
and filling up with sediments).

Anyway, that may be more information that you wanted. I did most of my
work in the Sahara when I was in graduate school (Univ. of Texas at Austin,
and Univ. Louis Pasteur in Strasbourg - France). When I wrote the letter
to Science, I was employed by British Petroleum in Texas, but about a year
and a half ago I quit BP and took a job with the U. S. Geological Survey.
I am currently working on putting together a story on the origin of the
Sahara Desert, and I am also working on various projects associated with
the Appalachians.

Thanks for contacting me, and let me know if you have any other questions.

- Chris.
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