Spectacular, totally exhumed, carbonate mud buildups of Givetian age can be found in the southern Ahnet basin of the central Sahara. Their geometries range from isolated mud mounds to mud ridges up to 8 km long. The latter are the result of lateral coalescence of closely spaced, individual mounds. Mounds up to 40 m high and ridges up to 85 m high show original slopes of 25° to 65°. Frame-builders in the massive core of the buildups are restricted to scattered small tabulate and solitary rugose corals. The total absence of stromatoporoids, colonial rugose corals, and algae, as well as the lack of debris at the toes of the buildups, shows that they were constructed in deep water below the photic zone and storm wave base. The arrangement of the buildups in north- to northwest-trending clusters and ridges suggests that their distribution was controlled by early Variscan extensional movements inherited from Precambrian patterns.

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