Diagenetic evolution of opaque and transparent heavy minerals reflecting colour genesis in continental fluvial buntsandstein red beds of the eifel (F.R. Germany)

Abstract

The postsedimentary evolution of the mainly fluvial Eifel Buntsandstein is characterized by an isochemical diagenesis in entirely fresh-water environment under conditions of slow pore water flow. The closed system favoured the internal coexistence of corrosion and authigenesis of various minerals and results in almost absence of net dissolution and rarity of net cementation in the sequence. The succession of events in time and space requires a mainly acidic initial composition of the pore solutions to enable leaching of detrital minerals, and a subsequent change to basic conditions allowing reprecipitation. Apart from light minerals, the occurrence of both dissolution and neoformation of various transparent and opaque accessories has been observed in both thin and polished sections. The authigenic appearance of stable heavy minerals underlines the compositional range of the postsedimentary processes, whereas the diagenetic evolution of the opaque accessories reflects the temporal range of the diagenetic alterations and especially provides important information on the origin of the red colour of the clastic sequence. The features of corrosion and authigenesis are documented for tourmaline, zircon, apatite and garnet from the transparent assemblage and for rutile, anatase, ilmeno-hematite, titaniferous magnetite, hematite pseudomorph after framboidal pyrite and rutilo-ilmenite from the opaque community. The authigenic neoformation of several transparent accessories and the diagenetic evolution of the opaque heavy minerals provide the most decisive contributions to the origin of the red colour which is enhanced by further sedimentological and petrographical criteria. The hematite pellicles surrounding sedimentary grains are predominantly of detrital provenance. After deposition, the primary grain envelopes as well as the heavy minerals are subjected to diagnetic alterations, beginning already syngenetically-eodiagenetically and persisting via phases of syndiagenesis, anadiagenesis and epidiagenesis (sensu Fairbridge 1967) up to recent time. Neoformed fine-dispersed iron oxides only subordinately originate by mobilization of iron oxides from micas and opaque heavy minerals. In micas, liberated iron oxides are predominantly reprecipitated between cleavage cracks forming pseudomorphs of hematite after biotite. Opaque heavy minerals are mainly transformed in situ into other iron and titanium oxides. Authigenic origin of fine-crystalline pigment in the pore space during course of the diagenetic evolution is only possible from iron oxides having been mobilized by skeletization of intergrowth and exsclution fabrics and by complete replacement of ilmeno-hematite and titaniferous magnetite by titanium oxides. The most important sedimentological criteria for the provenance of the red colour are the consistent differences in chroma hue and saturation between aeolian and fluvial deposits, the red colour of both channel and floodplain sediments, the absence of diachronous colour boundaries and the preservation of coexistent bleaching schlieren and patches of intensified chroma hue as well as the colour zonation within the Violette Horizonte calcrete palaeosols. The diagenetic evolution of opaque and transparent heavy minerals and the sedimentological relationships result in conclusion that the predominant amount of diagenetic iron oxides has been mobilized from primery grain pellicles. Dispersed or concentrated reprecipitation of liberated iron oxides takes place throughout all stages of diagenesis. The terrestrial deposits are mainly primary red beds, and the pigment distribution has been secondarily overprinted during diagenesis.