Silicification and dolomitization of anhydrite nodules in argillaceous terrestrial deposits: an example of meteoric-dominated diagenesis from the Triassic of central Spain

Abstract

Cauliflower-shaped nodules are widespread in a single red mudstone bed in
the Buntsandstein (Triassic) facies of the Iberian Range. They consist mostly of
quartz, dolomite and calcite, but other minerals, such as barite, kaolinite and
iron oxyhydroxides, are also present. The nodules are spherical, ovoid or
elongate in shape and range from 1 to 8 cm across. The surface of the nodules
is irregular, and some show a pedogenic coating of microspar. The
sedimentological and petrographic data suggest that the initial anhydrite
nodules formed through a progressive increase in the porewater concentration
of Ca2+ and SO4
2– in a vadose environment, occasionally under the influence of
pedogenic processes. Partial replacement of the anhydrite by megaquartz
occurred under more dilute conditions in the same type of setting, as indicated
by the presence of organic filaments on the quartz crystal surfaces. In type A
nodules, the dissolution of the innermost anhydrite was complete, and
different types of quartz cement filled the porosity. Fracturing and meteoric
cementation by calcite and minor amounts of kaolinite were the latest
processes affecting these nodules. In type B nodules, the dissolution of the
anhydrite was incomplete, inhibiting quartz cementation and enabling later
dolomitization of the anhydrite. Dolomitization appears to have been driven by
sulphate reduction, as indicated by the presence of bacterial bodies within the
dolomite crystals. Dedolomitization and precipitation of barite, kaolinite and
calcite spar cements occurred later under the influence of meteoric solutions.
The nodules may mark the former locations of the water table and provide
evidence for an episode of highly evaporitic conditions throughout wide areas
of the basin. Their occurrence reveals not only a complex diagenetic history but
is also indicative of palaeogeographic and palaeoclimatic conditions.