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Sep 25 / Dave Schumaker

Richat Structure in Mauritania

I stumbled across this on Universe Today’s “Where in The Universe Challenge” and thought it made a very interesting geologic phenomenon to write about. (Apologies to Ron and the gang for potentially stealing an idea from the Where on Google Earth challenge.)

Source: Google Earth

What does the above photograph look like on your first impression? I immediately said an impact crater and was amazed at how well defined this was. Especially when you find out this feature has a diameter of 30 kilometers!

This interesting feature is known as the Richat Structure and is located in the African country of Mauritania. Below is an embedded Google Maps frame showing the location.

View Larger Map

This structure is so big, that while zooming in to Mauritania on Google Maps, I was immediately able to see it on a rather zoomed out view.

Anyway, what exactly is this structure? 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.

Kim from All of My Faults Are Stress Related adds this comment after reading a paper linked in the Wikipedia article on the structure:

It’s not just a circular anticline – it’s a dome above a Cretaceous pluton. (The article describes the intrusion as a plug, but I wonder if we’re not looking at the top of a laccolith – like the Henry Mountains in Utah, but not eroded through to the pluton itself.)

The paper is titled “Resolving the Richat enigma: Doming and hydrothermal karstification above an alkaline complex,” by Matton G, Jébrak M, Lee JK:

The Richat structure (Sahara, Mauritania) appears as a large dome at least 40 km in diameter within a Late Proterozoic to Ordovician sequence. 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.

For readers not familiar with anticlines, below is a cross section of an anticline to help visualize what might be going on.

Source: Wikipedia – Anticline

According to the website Google Sightseeing, 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.

[Via Universe Today]

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