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During the Permian, large fluvio-lacustrine basins occurred within the West European Hercynian chain. The deposits were mainly detrital (conglomerates, sandstones and pelites). The two studied sites, Couy borehole and Aumance basin contain several dolomitic beds alternating with pelites containing Ostracods. The carbonate layers constitute various dolomitized facies with dolosparite, laminated and bituminous dolomites, oolites, oncolites and stromatolites, and pedological nodules. Microfacies are varied. The dolomite crystals can be either isolated rhombs scattered in pelites or rhombs and xenomorphic crystals cementing sandstones; in dolomicrites, dolomicrosparites and dolosparites, crystals are xenomorphous and very irregular in size.All the layers are interpreted as the result of the dolomitization of an initial calcitic mud. Some columnar stromatolites had their thin primary fabric well preserved by siliceous early replacement, while dolomitization poorly preserved laminations. The fluvio-lacustrine sediments associated with carbonates are red, mottled and grey. The carbonates were deposited in small lacustrine basins and in the flood plain pelites. Climate conditions were not desertic, but dry tropical with alternating wet and dry seasons allowing for a periodic hydromorphy responsible for oxido-reduction of iron, and carbonate nodulization.