Syndepositional processes in the pigmentation of oceanic red beds: evidence from the Basque-Cantabrian Basin (northern Spain)

Oceanic red beds (ORBs) are present in Upper Cretaceous and Danian deep-marine deposits in the Basque-Cantabrian Basin of northern Spain. The presence and regularity of the succession of marl-limestone couplets is exceptional based on the macroscopic, microscopic and geochemical evidence collected. Five types of marl-limestone couplets are identified based on the colour, and a high maximum sedimentation rate (3.6 cm ka(-1) ) is noted. The oxidizing activity of deep, cold-water masses is indicated by the oxygen isotope signal in the lower-upper Maastrichtian and Danian sections and the presence of the boreal inoceramid Spyridoceramus tegulatus. In theory, the variation in colour from grey to greenish-yellow, purple and pink up to red tones correlates with the Fe2+/(Fe2++Fe3+) ratio. It is interpreted as the possible palaeoenvironmental transit of particles that sediment out slowly in oxic environments when they circulate through cooler, oxidizing water masses. The colour is considered to be a depositional feature, and hematite, detected by X-ray diffraction, is the main staining agent, without discarding the possible redistribution of previous oxyhydroxides passing to hematite as a final product. The cell filling of the foraminifer shells does not incorporate appreciable amounts of Fe and Mg during diagenesis. Bacterial activity is detected using scanning electron microscopy images, both in the coccolith debris and in the detrital micas, although there is uncertainty as to its importance in the staining process.

Automated summary

Oceanic red beds (ORBs) in the Basque-Cantabrian Basin of northern Spain show exceptional features in Upper Cretaceous and Danian deep-marine deposits. The color variations and sedimentation rates indicate the oxidizing activity of deep, cold-water masses. The presence of hematite as the main staining agent in the red beds suggests a slow sedimentation process in oxic environments.