Diagenesis of radiaxial fibrous calcites in a subunconformity, shallow‐burial setting: Upper Triassic and Liassic, Northern Calcareous Alps, Austria

Abstract

ABSTRACTMultiple generations (I‐IV) of radiaxial fibrous calcite (RFC) cement crusts are intercalated with meteoric internal sediments and microsparitic calcite cements within karst dissolution cavities in Upper Triassic Steinplatte and basal Liassic Hirlatz carbonates in the Northern Calcareous Alps of Austria. Stratigraphic, petrographic and geochemical evidence suggest that RFC precipitated as high‐Mg calcite from marine fluids in the shallow‐burial (<180m) environment. Estimated original δ18O and δ13C isotopic compositions of Late Triassic and Early Jurassic marine waters are −1.88 to −2.21%‰ and +2.37 to +2.89%‰ (PDB), respectively. The RFC subsequently was altered to low‐Mg calcite by meteoric or mixed meteoric‐marine fluids introduced during periods of subaerial exposure.RFC cement generations I‐IV in Steinplatte reef‐slope beds, and I‐II in Steinplatte reef and lagoon and Hirlatz samples retained near‐original marine oxygen and carbon isotopic compositions (‐1.88 and +2.89%‰ PDB, −2.11 and +2.37%‰ PDB, respectively) and petrographic identity during the conversion to low‐Mg calcite. In contrast, RFC generations III and IV in the Hirlatz and upper Steinplatte were intensely leached and partly replaced by coarse, blocky calcites of meteoric origin, with attendant modification of original crystal fabrics and isotopic compositions (−4.65 to −5.22%‰ PDB δ18O, +2.54 to +3.14%‰ PDB δ13C). These differences in diagenetic behaviour are related to contrasting processes of fluid‐rock systems and original microfabric variations in the RFC cements. The karst‐affected, phreatic‐marine burial environment of RFC formation and subsequent diagenetic alteration inferred for these rocks have not been previously documented in other ancient rocks.