Would this be the official explanation for the Richat structure (seems logical, imho).

[The so-called Richat Structure is a geological formation in the Maur Adrar Desert in the African country of Mauritania. Although it resembles an impact crater, the Richat Structure formed when a volcanic dome hardened and gradually eroded, exposing the onion-like layers of rock.]

http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=10789

---

[On Earth, circular structures are rare but, as mapping parts of the continents since the early 19th Century led to discoveries of such features, a number had been identified by the middle of the 20th Century. One of particular note is the series of concentric ridges in Mauritania (West Africa) known as the Richat structure (about 40 km outer diameter). The structure was first discovered from space by Gemini 4 astronauts McDivitt and White. This structure results from doming of sedimentary rocks by an igneous intrusion; erosion has left the quartzite ridges as hogbacks (inclined strata that are raised relative to softer rocks between circles) that outline the "bulls-eye" appearance of the structure. Here is a view taken by the ASTER sensor of the Richat structure (which briefly was thought to be of impact origin until a visit by R.S. Dietz found field evidence that precluded that possibility and confirmed a purely internal origin:]

http://rst.gsfc.nasa.gov/Sect18/Sect18_1.html

---

Still wondering if this is similar in origin (when will we know for sure!):

http://www.msss.com/mars_images/moc/8_2002_releases/bullseye/

Mauritania (NW Africa region) and Meridiani both have an abundance of Hematite.

Just interesting:

http://www.carleton.ca/~tpatters/teaching/intro/precambrian/precambrian2.html.

(... about to wonder out loud about BIFs, so I'll stop. Phil has more patience ;)