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

Abstract

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.

Controlling the Iron Migration Mechanism for the Cretaceous Sediment Color Variations in Sichuan Basin, China

“…Sediment color is considered one of the best indicators of climate. 6 − 10 It is used to identify rock types and divide and compare the strata, and it is an important index of climate and environmental change. 11 , 12 According to the genetic sediments, the color of the sediments can be categorized into inherited color, authigenic color, and secondary color.…”

“…They are usually used to reconstruct the paleoclimate and reveal the intensity of chemical weathering, which plays a vital role in indicating the controlling factors of rock weathering and the environment. − The iron element widely present in sedimentary rocks is susceptible to changes in redox conditions and is very useful for determining the redox conditions of sediments and rocks. , At the same time, iron-bearing minerals can make the rock show a specific color. Sediment color is considered one of the best indicators of climate. − It is used to identify rock types and divide and compare the strata, and it is an important index of climate and environmental change. , According to the genetic sediments, the color of the sediments can be categorized into inherited color, authigenic color, and secondary color. Both hereditary color and self-generated color are primary colors…”