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

Abstract. 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.