Silicificación y dolomitización de nódulos de anhidrita en sedimentos litíticos continentales. Un caso particular de diagénesis meteórica en el Triásico de la Cordillera Ibérica

Abstract

Cauliflower nodules occur in a red mudstone bed of the Buntsandstein facies of the Iberian Ranges. They consist mostly of quartz, dolomite and calcite, but barite, kaolinite and iron oxides-hydroxides are also present. The nodules are spherical, ovoid or elongated in shape and range from I to 8 cm across. Their surface is irregular and some show a pedogenic carbonate coating. The initial anhydrite nodules formed through a progressive increase in the concentration of Ca+2 and S 0 42 in the meteoric porewaters in a superficial vadose environment. Partial replacement of the anhydrite by megaquartz occurred under more dilute conditions. From this point forward, the evolution of nodules followed two paths. In type A nodules, the dissolution of the anhydrite was complete, and quartz cements filled the porosity. In type B nodules, the dissolution of the anhydrite was incomplete, inhibiting quartz cementation and allowing dolomitization, which was mainly driven by sulphate reduction, indicated by the presence o f bacteria within the dolomite crystals. Dedolomitization, fracturation and precipitation of barite, kaolinite and calcite spar cements occurred later under the influence of meteoric solutions. The presence of these nodules not only reveals a complex meteoric diagenetic history but is also indicative of palaeogeographic and palaeoclimatic conditions.