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Geology; August 2005; v. 33; no. 8; p. 665-668; DOI: 10.1130/G21542AR.1
© 2005 Geological Society of America
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Resolving the Richat enigma: Doming and hydrothermal karstification above an alkaline complex

Guillaume Matton1, Michel Jébrak1 and James K.W. Lee2

1 Department of Earth and Atmospheric Sciences, University of Quebec at Montreal, 201 President Kennedy Avenue, Montreal, Quebec H3C 3P8, Canada
2 Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada

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.

Key Words: karst • hydrothermal • breccia • silicification • Africa






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