The Richat Structure, also known as the Eye of the Sahara and Guelb er
Richat, is a huge circular formation in the Sahara desert of
west–central Mauritania near Ouadane. This structure is a deeply eroded,
slightly elliptical, 40-km in diameter, dome. This prominent circular
feature in the Sahara desert has attracted attention since the earliest
space missions because it looks like a gigantic bull’s-eye. The
sedimentary rock exposed in this dome range in age from Late Proterozoic
(2.5 billion years) within the center of the dome to Ordovician (480
million years) sandstone around its edges.
It was originally thought to have been an impact crater when it was
first seen on a large scale by scientists. More research eventually
revealed that it was not actually an impact crater, but instead a a
circular anticline whose crest has been eroded away.
Erosion has created circular cuestas represented by three nested rings
dipping outward from the structure. The center of the structure consists
of a limestone-dolomite shelf that encloses a kilometer-scale siliceous
breccia and is intruded by basaltic ring dikes, kimberlitic intrusions,
and alkaline volcanic rocks. Several hypotheses have been presented to
explain the spectacular Richat structure and breccia, but their origin
remains enigmatic. The breccia body is lenticular in shape and
irregularly thins at its extremities to only a few meters. The breccia
was created during karst dissolution and collapse. Internal sediments
fill the centimeter- to meter-scale cavities. Alkaline enrichment and
the presence of Cretaceous automorphous neoformed K-feldspar demonstrate
the hydrothermal origin of these internal sediments and their
contemporaneity with magmatism. A model is proposed in which doming and
the production of hydrothermal fluids were instrumental in creating a
favorable setting for dissolution. The circular Richat structure and its
breccia core thus represent the superficial expression of a Cretaceous
alkaline complex with an exceptionally well preserved hydrothermal karst
infilling at its summit.
There are a few other similar looking features that can be found in
Africa: The Jebel Uwaynat that borders Sudan, Egypt and Libya and The
Brandberg Intrusion in Namibia.
Source :- http://www.unbelievableinfo.com/2013/12/the-richat-structure.html
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