An exhumed Palaeozoic underwater scenery: the Visean mud mounds of the eastern Anti-Atlas (Morocco)

Abstract

About one hundred carbonate mud mounds, covering an area of 440 km 2 in the eastern Anti-Atlas of Morocco, constitute one of the largest mound agglomerations known so far from Lower Carboniferous settings. They occur within a 4000-m-thick succession of shales with intercalated bedded limestones, sandstones, and siltstones. According to conodont and goniatite biostratigraphy, mound formation started in the early Gnathodus texanus Zone and terminated during the G. bilineatus Zone of the Visean stage. Individual mounds are a few metres to 30 m high, have base diameters of up to 300 m and are concentrated in several parallel, WNW–ESE running belts. From their lithology and facies relationships, four types of mounds can be distinguished: (1) massive crinoidal wacke- or packstones without stromatactis; (2) massive crinoidal wacke- or packstones with rare stromatactis; (3) similar to (2), but allochthonous; and (4) biodetrital (skeletal) grainstone mounds. While carbonate deposition in types (1) to (3) was probably triggered by microbial precipitation, type (4) is the result of a predominantly mechanical accumulation of skeletal debris. Biota in the four types comprise a great variety of invertebrates, among which crinoids, sponges, and bryozoans are most common. Diagenesis of the mound carbonates was dominated by recrystallization of micritic matrix and organic remains and late burial cementation. Oxygen and carbon isotope data of brachiopod and crinoid ossicles, matrix, and early marine cements plot in a large field and do not allow definite conclusions about the composition of the ambient seawater. Microbial activity and the absence or scarcity of green algae, colonial corals and coralline sponges suggest deposition of the mounds in moderate water depth close to the lower limit of the photic zone. D 2001 Elsevier Science B.V. All rights reserved.