Abstract : The Oligocene Red Molasse (Barles) is a cylothemic formation: chenal (sandstone) and overbank (siltstone and mustone), its original colour is gray. The Eocene Sands (Royans) have a whiteoriginal colour.Red coloration in these two formations is authigenic and produced by post-depositional process . The analysis by X-Ray powder, by elecron microscopy and by geochemistry show that the nature of the iron bearing detrital grains is different between two formations but theirs process of redness is similar. 1) Nature of the iron bearing detrital grains: In the Oligocene Red Molasse, the iron bearing detrital grains are the glauconitic grains. In the Eocene Sands they are both ferromagnesians detrital grains and the weathering materials. 2) Process: After their deposition, the sediments of these two formations have the same process of the red coloration: a) The alteration process by hydrolysis of the iron bearing detrital grains frees the ferric oxides. This alteration, resulting from infiltrated water and to oxidizing environment of the surface, involves a drop of iron bearing grains content in the red facies which are always in the upper part of the both formations. Moreover, the alteration of glauconitic grains could produce the interstratified smectite. This alteration process takes place in the acid environment showing by the authigenic hexagonal kaolinite plates and by booklet kaolinite came from the reorganisation of detrital kaolinite. b) Deshydratation of goethite: In the boundary of red facies and original facies, there is uniquely the goethite and in the red facies we have both goethite and hematite. That means that the ferric oxides crystallize in goethite as the red precursor which deshydrates next into hematite according to thermodynamic study. This phenomena is supported by the authigenic acicular goethite and by the hexagonal hematite plates (electron microscopy) . c)According to the evolution of chemical species, this alteration process is simultaneously followed by the leaching from the surface to the lower part of some chemical species as Si02 , Na20 and by the accumulation at the upper part of the others (AI 20 3 , Fe203, Ti02 ). This leaching is emphasized by a penetrating form of red facies into original facies. The others observed process are the transformation of the clay mineraIs, the calcitic epigenesis and the quartz dissolution.