Palaeoenvironmental analysis of mid-Cretaceous coastal lagoonal deposits (Lusitanian Basin, W Portugal)

The sedimentary record of fossil coastal lagoons is an under-explored archive of environmental change in ancient coastal domains. The significant continentality of coastal lagoons, the small volumes of their water bodies, very shallow depths and intermittent connectivity to oceanic aquafacies all result in a highly complex depositional system. The matrix micrite chemostratigraphy (carbon and oxygen isotope ratios) of two mid-Cenomanian shallow coastal lagoon sections in the western Lusitanian Basin, Portugal, are here presented in comparison to the geochemistry of different diagenetic carbonate materials. Geochemical evidence is placed against independent data from field observations, sedimentology, microfacies analysis, and ostracod palaeoecology. Marine ingressions and coastal lagoonal environments are recorded both in sedimentological, micropalaeontological and geochemical data sets. Intervals of more open, marine conditions are evidenced by overall higher (0 to 2‰) carbon and less 18O-depleted (− 3‰) isotope ratios. The opposing pattern is observed for periods of more restricted lagoonal conditions. The combination of a detailed ostracod palaeoecology with geochemical and sedimentological findings is considered a strength of this paper as it allowed to disentangle the complex geochemical signals of ancient coastal lagoonal deposits. The comparison with Recent coastal lagoons provided new arguments to clarify interpretations based on ancient records.