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. Johnson et al. (1997) have also showed how secondary reddening phases might be superimposed on earlier formed primary red beds in the Carboniferous of the southern North Sea. The general conditions leading to post-diagenetic alteration have been described by Mücke (1994). Important reactions include 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.
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