FRACTAL ANALYSIS AND ORIGIN OF A CHERT MEGABRECCIA IN THE RICHAT DOME, MAURITANIA, AFRICA

The Richat complex is one of the most spectacular terrestrial circular structures visible from space. Located in Proterozoic-Cambrian sediments of the Mauritanian part of the Sahara desert, the Richat complex form a domal structure of 40 km in diameter. The center of the structure consists of a limestone-dolomite shelf intruded by a kilometric scale chert megabreccia. Numerous theories have been proposed for the origin of this mysterious breccia including meteoritic impact, plutonism and intraformational processes.

The stratigraphy of the Richat dome includes limestone, dolomite, sandstone, mudstone and chert. Field and petrologic studies indicate that the breccia is composed of angular and rounded heterolithic chert fragments in a silica cement. Silica dissolution and replacement were observed in the breccia whereas dolomitization and sulphidation were observed in the limestones. Early ductile deformation in the chert indicates an early development of silicification. The chert fragments display a bimodality in the boundary fractal dimension (Euclidian distance mapping) that implies two phases of breccia formation. Field observations suggest that the breccia was form by a process of syngenetic karst-dissolution collapse. The most likely interpretation is that the Richat chert was formed during a shelf high-stand and persisted in a sub-emergent setting from the Proterozoic up to the Cretaceous. Thus, the Richat chert appears to be an indication of a long term high-standing structural position.