Abstract

Recent debates concerning the origin of Emsian carbonate mud buildups exposed at Hamar Laghdad in the eastern Anti-Atlas of Morocco centre around geochemical studies that suggest authigenic or chemosynthetic processes induced precipitation of mud from hydrothermal fluids. Geochemical data alone cannot be used to demonstrate conclusively the origin of the mud comprising the buildups, but an in situ source and early cementation can be inferred from morphology, internal architecture, and sedimentary structures.

Morphological types vary from symmetrical mounds up to 12 m high to asymmetrical pinnacles that locally coalesced, forming elongate multicored complexes up to 56 m high. Their steep-sided nature (margins dipping up to 54°) and sporadic occurrence of erect invertebrates suggest localized production of mud rather than hydrodynamic piling or baffling from suspension. Irregular nodular beds that thicken internally imply centralized production and a local source for the mud. Displaced nodules and slump folds indicate early cementation of mud to at least a firm consistency, followed by downslope movement.

Abrupt termination of carbonate production during late transgression resulted in drowning of the mud buildups and adjacent platform. Except for a thin condensed bed deposited during maximum flooding, the buildups and platform (transgressive systems tract deposits) remained uncovered for an extended period (upper inversus-laticostatus to lower serotinus zones) before being buried beneath progradational clinoforms of nodular limestone and marlstone (regressive systems tract deposits).

Overlying Middle Devonian carbonates contain evidence of hydrothermal seepage, based on the occurrence of solemyid and anomalodesmatan bivalves, which are thought to be chemosymbiotic. These bivalves occur in a mud buildup rooted on an Emsian counterpart, suggesting periodic hydrothermal activity beneath the buildups. A hydrothermal origin for the Hamar Laghdad mud buildups appears to be consistent with other mud buildups of similar age in other parts of northwest Africa. Vast deposits of coeval Minette-type ooidal ironstones, which also have a seep origin in the Tindouf Basin, south of the Anti-Atlas, testify to occasional widespread hydrothermal activity along the northwest margin of Gondwana during much of the Devonian.

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