Well, back to the desert for me...Ouadane, Mauritania in this case. The Richat structure is a spectacular expression of an annular drainage pattern in the Sahara. I came across this while straying from a search in the fold complexes of the Atlas Mountains in Morocco. I also found a nicely enhanced image of the structure posted on a Google Earth bulletin board. I agree that it appears to be a very well defined domal uplift of a sequence of sedimentary rocks, but I don't know details.

Flash Earth Link

http://www.flashearth.com/?lat=21.110995&lon=-11.357303&z=11.5&r=0&src=msl

Link to Google Earth community discussion of feature:

http://bbs.keyhole.com/ubb/showflat.php?Number=11619

Link to images of this feature found in Google Search:

http://www.google.com/images?sa=3&q=richat+structure&btnG=Search+images

More evidence of this feature as a geoblogger favorite:

http://geology.rockbandit.net/2008/09/25/richat-structure-in-mauritania/