by A. Préat et  al., December 2018 in GeologicaBelgica (with .pdf)

Abstract

Explaining the color of rocks is still a complex problem. This question was raised long ago in the community of geologists, particularly for the pigmentation of the ‘red marbles’ of the Frasnian of Belgium at the beginning of the last century, with many unsatisfactory hypotheses. Our recent analysis of different red carbonate rocks in Europe and North Africa (Morocco) may provide an alternative explanation for the color of these rocks. For this it was necessary to bring together diverse and complementary skills involving geologists, microbiologists and chemists. We present here a synthesis of these works. It is suggested that the red pigmentation of our studied Phanerozoic carbonate rocks, encompassing a time range from Pragian to Oxfordian, may be related to the activity of iron bacteria living in microaerophilic environments. A major conclusion is that this red color is only related to particular microenvironments and has no paleogeographic or climatic significance. All red carbonates have not necessarily acquired their pigmentation through the process established in this review. Each geological series must be analyzed in the light of a possible contribution of iron bacteria and Fungi.

…