Facies, stratigraphy and diagenesis of Middle Devonian reef- and mud-mounds in the Mader (eastern Anti-Atlas, Morocco)

Abstract

During the Devonian, the eastern Anti-Atlas formed a part of the northwestern continental margin of Gondwana which was a mid-latitudinal (30-40⁰ S), temperate-water carbonate province. In the Mader region, ten carbonate mounds (one reef-mound and nine mud-mounds), distributed over five discrete localities, are intercalated within a 200-400 m thick Middle Devonian succession. The arid climate of the northwestern margin of the Sahara has exhumed these mounds which display perfectly their original morphologies and relations to off-mound lithologies.

The carbonate mounds of the Mader area consist of massive, stromatactis-bearing boundstones (wackestones and floatstones in a purely descriptive manner) with the bulk of the mound volume consisting of fine-grained carbonate (microspar). High accumulation rates (0.2-0.8 m/1000 a), purity of mound carbonates (>95% CaCo₃) and homogeneous Mg-calcite mineralogy strongly argue for in situ carbonate production by microbial (cyanobacterial/bacterial) communities. In addition, other indications (calcified cyanobacteria in the immediate neighbourhood of stromatactis fabrics, dark crusts surrounding stromatactis fabrics and alignment of stromatactis fabrics parallel to the accretionary mound surfaces) suggest a close relationship between stromatactis formation and carbonate production. Microbial communities probably flourished on the mound surfaces, precipitating fine-grained carbonates and consolidating the steepmound flanks by their mucilages. Once embedded, these communities decayed and were successively replaced by calcite cements, finally resulting in stromatactis fabrics.

The facies model proposed for the three most conspicuous mound occurrences (Aferdou el Mrakib, Guelb el Maharch, Jebel el Otfal) is 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 Middle Devonian facies pattern varying from shallow to deeper water environments and by different faunal associations of the carbonate mounds. The Aferdou el Mrakib reef-mound was established at moderate water depth (midramp setting), because it contains abundant frame-builders (stromatoporoids, colonial rugose corals) but lacks indications for euphotic conditions, like calcareous algae and micritic envelopes. The Guelb el Maharch and Jebel el Otfal mud-mounds contain a much more impoverished fauna, dominated by crinoids and tabulate corals (auloporids), indicating a deeper bathymetric position (outer ramp setting) on the ramp. Further, but rather unspectacular mud-mounds (SE' Zireg, Jebel Ou Driss) are situatedapart from the ramp at localities in the southern and the southwestern Mader area respectively.

Mound growth was possibly initiated by hydrothermalseepage at the seafloor though no evidences for hydrothermal activity, like mineralizations or depleted δ¹³C values, have been found to date. Slightly elevated temperatures may have stimulated the benthic fauna, especially crinoids, forming flat in situ lenses, which in turn served as substrates for microbial colonization.

Termination of mound growth in the Mader Basin is connected with the subsidence-caused drowning of the carbonate ramp. Poorly-fossiliferous, laminated mudstones overlie the mounds and suggest a southward-directed extension of basinal facies onlapping the ramp and its mounds and resulting in poorly oxygenated seafloor conditions.

Diagenesis of the Mader Basin carbonate mounds includes early marine, shallow marine burial and deeper burial cementation, recrystallization of the fine-grained mound carbonates, stylolitization and dolomitization. Radiaxial calcites (RC) precipitated in the marine environment and are believed to have preserved a nearly primary marine stable isotopic composition of the Mader Basin seawater with mean values of δ¹⁸ O=-2.6(+-0.2)promili PDB and δ¹³C=+2.7(+-0.5) ‰PDB. The exceptional high δ¹⁸O values,compared with other Middle Devonian data derived from North American studies, are interpreted as resulting from the mid-latitudinal, temperate-water settings of the Mader Basin carbonate mounds. The diagenetic history is characterized by progressive burial conditions. Meteoric influences can be ruled out because the progressively deepening bathymetric evolution of the Mader Basin excludes subaerial exposure. All diagenetic events, especially cement zones, are probably diachronous and therefore cannot be correlated within in the Mader Basin and not even within individual mounds. Fault-related dolomitization, postdating Variscan compression was the last diagenetic event which affected the carbonate mounds of the Mader area.