Abstract

The Devonian Coumiac carbonate series is well known since it is the point reference in time for the Frasnian-Famennian boundary. Its fossil content has been extensively studied, in particular conodonts, ammonoids and trilobites. However, little attention has been paid to its sedimentology and sequence interpretation. The succession is condensed, as the Frasnian is reduced to 20-30 metres of red and grey argillaceous and micritic carbonates. Four microfacies are recognized. MF1 is a shale and argillaceous mudstone rich in radiolarians. MF2 is a sponge wackestone/bafflestone with variable amounts of tentaculites. MF3 is a laminated wackestone/packstone with tentaculites, ammonoids and bivalves. MF4 is a crinoidal packstone. Numerous (more than twenty) hardgrounds are underlined by the presence of hematite concentrations, ferruginous microstromatolites, bioerosions and microbreccias. Their number increases from bottom to top and reaches a maximum at the F/F boundary. Some shallow water resedimented fauna and flora (stromatoporoids, calcispheres, cyanobacteria, blue green algae) are present. Some are reworked microbreccias derived from a distant ramp, some are rafted elements. Despite the difficulty to establish a bathymetric sequence, the ordering of the sequence is established using mostly sedimentological criteria e.g. tempestite laminations, graded-bedding, granulometry. They allow to recognize a distal-proximal gradient. MF1 is the deepest facies, just below the storm wave base, without evidence of turbidites or mudflows. MF2 and MF3 are still below fair-weather wave base, most of the sponge bafflestones of MF2 being in situ. MF3 is composed of reworked hemipelagic fauna (tentaculites/ammonoids). Both are still below the photic zone. MF4 is near fair-weather wave base and reaches the photic zone. Fossils are derived from the destruction of nearby crinoidal meadows. The Coumiac succession is a distal carbonate ramp, its lower part being characterized by hemipelagic sedimentation. The remainder of the series is a progressive regression punctuated by six important eustatic fluctuations. The red pigmentation is caused by ferro-oxidizing bacterial activity. Hematite is present in hardgrounds, sponge perforations, oncoids, simple and complex microstromatolites, dichotomic tufts and mud cracks. These bacterial bioconstructions seem to develop in very quiet dysaerobic waters associated to important sequential fluctuations. The bioconstructions indicate mostly calm sedimentation, well below the photic zone and in poorly oxygenated waters. The work shows also the importance of hiatuses associated to the F/F boundary.

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