Integrated palaeoenvironmental characterization of ancient coastal lagoons (Cenomanian, Lusitanian Basin, Portugal)

Coastal lagoons are a highly complex depositional system, connected at least intermittently to the ocean, characterized by strong continentality, small volume of their water bodies and very shallow depths. Two mid-Cenomanian... [ view full abstract ]

Coastal lagoons are a highly complex depositional system, connected at least intermittently to the ocean, characterized by strong continentality, small volume of their water bodies and very shallow depths. Two mid-Cenomanian sections representing deposits of neighbouring coastal lagoons were tested in order to evaluate environmental and diagenetic controls in these very dynamic and highly sensitive environments. Palaeoecological data (ostracods, foraminifers, algae) was combined with detailed sedimentology (field and microfacies analysis) and placed against evidence from cathodoluminescence, carbon and oxygen isotope and elemental geochemistry of different carbonate materials. A comparison with data collected from waters and sediments along modern analogous lagoons provided valuable information.
Despite a mild meteoric influence, obtained matrix micrite fluctuations could be traced back to palaeoenvironmental changes at both studied sites. The geochemical stratigraphic record proved to be coherent with palaeoenvironmental conditions indicated by the mentioned independent proxies. Intervals of more open, marine conditions are evidenced by overall higher (0 to 2‰) carbon isotope values and less 18O-depleted (-3‰) isotope ratios. Interestingly, the opposing pattern is observed for periods of more restricted lagoonal conditions. Higher values carbon isotope values of marine character prevail during open lagoonal conditions and lower values are present during restricted periods and point to changes in shallow-water carbonate production and/or higher detrital input. Matrix micrite oxygen isotope ratios reveal a combined temperature and salinity control that was tentatively resolved by comparison with modern settings. Accordingly, cooler, normal marine salinity water masses prevailed during periods of higher connectivity with open-waters. Slightly warmer waters could be present during restricted conditions, as commonly verified along a variety of present day cases. But the strongest response of O-isotope record related to sharp salinity decrease due to prevailing humid climatic conditions, further supported by sedimentologic interpretations and ostracod palaeocology.