Resolving the Richat enigma; doming and hydrothermal karstification above an alkaline complex

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 limeston...

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Bibliographic Details
Published in:Geology (Boulder) 2005-08, Vol.33 (8), p.665-668
Main Authors: Matton, Guillaume, Jébrak, Michel, Lee, James K. W
Format: Article
Language:English
Subjects:
absolute age
Africa
alkali feldspar
alkalic composition
Ar/Ar
breccia
buried features
collapse structures
complexes
Cretaceous
cuestas
dates
domes
erosion features
feldspar group
framework silicates
geochemistry
Geology
geomorphology
hydrothermal conditions
K-feldspar
karst
karstification
magmatism
Mauritania
Mesozoic
paleokarst
reaction rims
remote sensing
Richat Mountain
ring structures
Sahara
sedimentary rocks
Sediments
silicates
silicification
solution
structural analysis
Structural geology
Taoudenni
Volcanoes
West Africa
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Summary: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.
ISSN:0091-7613
1943-2682
DOI:10.1130/G21542AR.1