Co-occurrence of the foraminifer <i>Mohlerina basiliensis</i> with <i>Bacinella–Lithocodium</i> oncoids: palaeoenvironmental and palaeoecological implications (Late Oxfordian, Swiss Jura)

The stratigraphical and spatial distribution of the foraminifer Mohlerina basiliensis and Bacinella–Lithocodium oncoids was studied in the lagoonal deposits of the oncoid-rich Hauptmumienbank Member from the Swiss Jura Mountains (Late Oxfordian, NW Switzerland). Mohlerina basiliensis has a double-layered wall structure (thick outer hyaline-radial calcite layer with a lamellar structure and thin inner microgranular calcite layer) and is preferentially found in low-energy facies and associated with normal-marine bioclasts and Bacinella–Lithocodium oncoids. The distribution of Mohlerina basiliensis is relatively patchy. The ‘distal’ platform facies present a higher abundance and a more extensive stratigraphic occurrence of this foraminifer. The timing of the ecological evolution is given by the sequence- and cyclostratigraphic framework and illustrates the stepwise flooding of the platform. Bacinella–Lithocodium oncoids have diameters of a few millimetres to several centimetres and are composed of an alternation of Bacinella–Lithocodium meshwork and thin micritic laminae. They are found preferentially in low-energy facies (bioclastic wackestones) of open lagoons. The distribution of Bacinella–Lithocodium oncoids through time and space is also patchy; however, they appear more abundantly in ‘distal’ platform facies. This study reveals the co-occurrence of Mohlerina basiliensis and Bacinella–Lithocodium oncoids and thus suggests similar environmental conditions.