Red beds

Primary red beds may also form by in situ (early diagenetic) reddening of the sediment by the dehydration of brown or drab colored ferric hydroxides.

For example, the most easily altered material would be olivine: e.g. A 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.

Secondary reddening phases might be superimposed on earlier formed primary red beds in the Carboniferous of the southern North Sea.

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.