New paleoecological and biostratigraphical data on the "Calcaires delemontiens" formation ("Delsberger Kalke", Late Oligocene, Swiss Jura Mountains)

The "Calcaires delemontiens" were first described by GREPPIN (1870) This Fo rmation is located in the Jura synclines and corresponds to the distal part of the Swiss Molasse basin. Outcrops and boreholes (Saicourt, Reconvilier, SE Pre Godat, SW Devant la Melt, Courfaivre and Brochene Fluh) have been s tudied in detail. New paleoecological and biostratigraphical data were obta ined through sedimentological and paleontological analyses. The formation r epresents shallow lacustrine environments (0-4 meters), as indicated by the occurrence of charophyte meadows, ostracod associations and roots and trav ertine levels. The freshwaters lakes were strongly supersaturated with resp ect to carbonates, allowing the dominance of the ostracod Metacypris danubi alis punctata. However, the presence of the halophilic ostracod Hemicypride is dacica grekoffi, at the top of the Courfaivre section and the Reconvilie r borehole Re2, points to locally brackish environments. The "Calcaires delemontiens" have been dated as Late Chattian in the Broche ne Fluh section by ENGESSER (1990, mammals) and SCHLUNEGGER et al. (1996, m agnetostratigraphy). However, according to the charophytes discussed in thi s study, the top of the Brochene Fluh section reaches the Lower Aquitanian (Nitida zone). Moreover, a clear Early Aquitanian age (MN1, Boudry 2) has b een found in the SE Pre Godat section by THEILER (1998). Consequently, the Formation of the "Calcaires delemontiens" has to cover the Upper Chattian t o Lower Aquitanian (MP29 to MN1).