Pedogenic continental red beds: How, why and when red? Redox geochemistry and quantitative colour analysis of the Old Red Sandstone, South Wales, UK

Abstract

Continental red beds are coloured by secondary Fe(III)‐oxy‐hydroxides (mainly hematite) which makes them excellent archives of ancient near‐surface redox conditions. However, colour is often treated as a subjective physical property, and opinions differ widely as to the environment and timing of colouration of the red beds. This study utilizes diffuse spectral reflectance to address the degree of reddening (hematitization) of the Siluro‐Devonian Old Red Sandstone representing a dryland fluvial system with abundant palaeo‐Vertisols, coinciding with the period of land colonization by vascular plants. The hematitization was studied along with the stratigraphy, facies, pedogenic features, petrology, element and Molybdenum isotope geochemistry. The mudstones and palaeosols are typically red while sandstones are often grey. The red colouration corresponds to approximately 30 to 36% of red band (625 to 700 nm) reflectance and high hematite to goethite ratios. The red sediments contain hematitized biotite, submicronic hematite in mudstone and sandstone matrix, and hematite cement in calcretes. The hematitization was synchronous with formation of pedogenic mudcracks, Vertic slickensides, calcretes, reduction spots, sometimes hydrodynamic reworking, and accompanied by local (grain‐scale) depletion in Fe(II), enrichment in Fe(III), and mobilization of U, V, As, Cu, Zn and Mo. It is estimated that the hematite growth was rapid (several hundred to thousand years) but the average sedimentation rates were low (approximately 3 to 10 cm/kyr). Erosion led to development of reactivation surfaces separating layers with contrasting hematite content and geochemistry. It is suggested that high rates of pedogenesis and low sedimentation rates favour the reddening and may represent an important control on the formation of continental red beds. Limited levels of soil waterlogging, and the published δ13C values (−9 to −12‰ Vienna Pee Dee Belemnite) of the pedogenic carbonate, might reflect the changes in the nature of the soil biomantle during the early Devonian land colonization by vascular plants.