%0 Journal Article
%A Alonso-Zarza, Ana María
%A Sánchez Moya, Yolanda
%A Bustillo Revuelta, María Ángeles
%A Sopeña, Alfonso
%A Delgado Huertas, Antonio
%T Silicification and dolomitization of anhydrite nodulesin argillaceous terrestrial deposits: an exampleof meteoric-dominated diagenesis from the Triassicof central Spain
%D 2002
%@ 0037-0746
%U https://hdl.handle.net/20.500.14352/59045
%X Cauliflower-shaped nodules are widespread in a single red mudstone bed inthe Buntsandstein (Triassic) facies of the Iberian Range. They consist mostly ofquartz, dolomite and calcite, but other minerals, such as barite, kaolinite andiron oxyhydroxides, are also present. The nodules are spherical, ovoid orelongate in shape and range from 1 to 8 cm across. The surface of the nodulesis irregular, and some show a pedogenic coating of microspar. Thesedimentological and petrographic data suggest that the initial anhydritenodules formed through a progressive increase in the porewater concentrationof Ca2+ and SO42– in a vadose environment, occasionally under the influence ofpedogenic processes. Partial replacement of the anhydrite by megaquartzoccurred under more dilute conditions in the same type of setting, as indicatedby the presence of organic filaments on the quartz crystal surfaces. In type Anodules, the dissolution of the innermost anhydrite was complete, anddifferent types of quartz cement filled the porosity. Fracturing and meteoriccementation by calcite and minor amounts of kaolinite were the latestprocesses affecting these nodules. In type B nodules, the dissolution of theanhydrite was incomplete, inhibiting quartz cementation and enabling laterdolomitization of the anhydrite. Dolomitization appears to have been driven bysulphate reduction, as indicated by the presence of bacterial bodies within thedolomite crystals. Dedolomitization and precipitation of barite, kaolinite andcalcite spar cements occurred later under the influence of meteoric solutions.The nodules may mark the former locations of the water table and provideevidence for an episode of highly evaporitic conditions throughout wide areasof the basin. Their occurrence reveals not only a complex diagenetic history butis also indicative of palaeogeographic and palaeoclimatic conditions.
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