Middle Devonian reef and mud mounds on a carbonate ramp: Mader Basin (eastern Anti-Atlas, Morocco)

Abstract

Abstract The Mader Basin is a palaeogeographical unit of the eastern Anti-Atlas, once situated on the northwestern margin of Gondwana, which was a mid-latitudinal (30–45°S) carbonate province during Devonian time. In this basin, isolated reef and mud mounds are intercalated within a 200–300 m thick Middle Devonian succession. The mounds were established on a 40 km wide, tectonically controlled homoclinal ramp, which developed between an area of uplift (Mader Platform) and another area of strong subsidence (depocentre of the Mader Basin). The bathymetric gradient of this ramp is reflected by a facies pattern varying from shallow ramp to basinal environments and by different faunal associations of the carbonate mounds. The lithology of the Mader Basin carbonate mounds consists of massive, stromatactis-bearing, bioclastic wackestones, with the bulk of the mound volume consisting of microspar. High accumulation rates (0.2–0.8 m/ka) and the purity of mound carbonates (>95% CaCO 3 ) compared with the marly inter- and off-mound facies strongly favour autochthonous carbonate production. In addition, several indications (calcimicrobes in the immediate neighbourhood of stromatactis, dark crusts surrounding stromatactis and alignment of stromatactis parallel to the accretionary mound surfaces) suggest a close relationship between microbial carbonate production and stromatactis formation. Microbial communities probably flourished on the mound surfaces, precipitating fine-grained carbonates and consolidating the steep mound flanks by their mucilagenic secretions. Once embedded, these communities decayed and left behind stromatactis cavities, which were subsequently filled by calcite cements. Drowning of the Mader Basin carbonate ramp was caused by progressive deepening related to basinal subsidence, which was superimposed by rapid eustatic sea-level rises. Onlapping of poorly oxygenated basinal waters was the direct cause for the termination of mound growth.