Diagenesis of a bioclastic oyster deposit from the Lower Cretaceous (Chachao Formation). Neuquen Basin. Mendoza Province, Argentina

The Lower Cretaceous Chachao Formation in the Malargue anticline area consists of wackestone, packstone, and minor grainstone and mudstone rich in benthonic fauna that were deposited in a carbonate ramp. The carbonate diagenesis in the Valanginian Chachao Formation contains many processes with conspicuous effects, including micritization, dissolution, neomorphism, and cementation. The early diagenetic process is characterized by micritization, dissolution and mineralogic stabilization of components, and earlier cement phase represented by micrite cement and isopachous calcite cement, which have petrographic characteristics consistent with precipitation in a marine-phreatic diagenetic environment. Later diagenetic phenomena include granular calcite and syntaxial cement. Both of cement types are interpreted as typical of a meteoric-phreatic environment. Concentric-zoning pattern of alternating dull, and blotchy- to bright luminescent zones is interpreted as being caused by a decrease in redox potential (Eh), under conditions of a progressive marine burial meteoric-phreatic diagenetic environment. Geochemical data (Sr++, Na+, Mg++, Fe++, Mn++) and SEM features of the micrite suggest that original calcareous mud could have been calcite dominated (CDP). The 6110 composition of the granular calcite cement ranging from -2.84 parts per thousand to -4.27 parts per thousand. PDB and the delta C-13 values of the cement between -2.46 parts per thousand and -3.50 parts per thousand PDB are compatible with precipitation from a fluid that evolved meteoric-phreatic composition. The high depleted delta O-18 values of the Gryphaea oyster shells can be related to the dilution of the marine water with a fresh water influx, whereas shells with the heaviest delta C-13 isotopic compositions are probably related to the original marine signal, which suggest a closed diagenctic system for carbon.