How deep is the sand in the Sahara?

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.