key feature of this process, and exemplified by the reaction, is the production of a suite of by-products which are precipitated as authigenic phases. These include mixed layer clays (illite – montmorillonite), quartz, potassium feldspar and carbonates as well as the pigmentary ferric oxides. Reddening progresses as the diagenetic alteration becomes more advanced, and is thus a time-dependent mechanism. The other implication is that reddening of this type is not specific to a particular depositional environment. However, the favourable conditions for diagenetic red bed formation i.e. positive Eh and neutral-alkaline pH are most commonly found in hot, semi-arid areas, and this is why red beds are traditionally associated with such climates. Secondary red beds Secondary red beds are characterized by irregular color zonation, often related to sub-unconformity weathering profiles. The color boundaries may cross-cut lithological contacts and show more intense reddening adjacent to unconformities. Secondary reddening phases might be superimposed on earlier formed primary red beds in the Carboniferous of the southern North Sea. Post-diagenetic alteration may take place through reactions such as pyrite oxidation: 3O2 + 4FeS2→ Fe2O3 (hematite) + 8S E = −789 kJ/mol and siderite oxidation: O2 + 4FeCO3 → 2Fe2O3 (hematite) + 4CO2 E = −346 kJ/mol Secondary red beds formed in this way are an excellent example of telodiagenesis. They are linked to the uplift, erosion and surface weathering of previously deposited sediments and require conditions similar to primary and diagenetic red beds for their formation. See also Red Beds of Texas and Oklahoma Chugwater Formation Red Hills, Kansas Old Red Sandstone New Red Sandstone References External links Pictures of Permo-Triassic red beds in Palo Duro Canyon Sedimentary rocks