Origin of red beds in the Ringerike Group (Silurian) of Norway

Abstract

The Ringerike Group is a meandering fluviatile succession which is about 60% red. Most of the red zones are formed of mudrocks and siltstones and correspond to the fine members of fining-upwards cyclothems. The majority of coarse members are drab coloured. Textural studies of thin and polished sections show that the red colour is caused by finely crystalline hematite as matrix and grain-coatings. This hematite apparently crystallized post-depositionally. Hematite also occurs in other textural sites: within altered phyllosilicates, as detrital grains and as totally pseudomorphed phyllosilicates. This, and the lack of consistency between colour and clay mineralogy, suggests that the red beds have had a long and complex diagenetic history. Iron analyses indicate that the red beds are enriched in Fe 3+ and total iron (FeO) by about 1%. This is thought to have been derived from the pre-depositional weathering of iron minerals and introduced into the sediments as amorphous iron hydroxide or iron-bearing clays. Crystallization of iron hydroxide under oxidizing conditions and the post-depositional alteration of iron-silicates and oxides is thought to be responsible for the formation of the red beds.