Palynostratigraphy and palaeoenvironments of the lower to middle jurassic bagå formation of bornholm, Denmark

The Bagå Formation includes up to approximately 400 m of paralic, lacustrine and alluvial deposits without marine macrofossils. Detailed palynological and sedimentological studies have been undertaken of all available cored boreholes and exposures of the formation. A total of 150 palynological samples were examined from partially overlapping sections amounting to a total of ca. 650 m. The palynomorphs recovered, some of which are stratigraphically important, comprise very diverse and well-preserved assemblages from which 130 species including 67 spore, 32 pollen and 31 microplankton species are identified. The assemblages are subdivided into the Chasmatosporites, Spheripollenites-Leptolepidites and Callialasporites-Perinopollenites miospore zones and the Mendicodinium reticulatum, Luehndea spinosa and Nannoceratopsis gracilis dinoflagellate cyst zones. Three of these, the Chasmatosporites, Callialasporites-Perinopollenites and Mendicodinium reticulatum zones, are newly proposed here. The Bagå Formation spans the late Pliensbachian, Toarcian, Aalenian, Bajocian and Bathonian time intervals. This has an important implication for the recognition of the regional NW European mid-Jurassic unconformity on Bornholm, which is shown to occur within the formation and not at the base as previously thought. The unconformity divides the formation into a lower part with brackish-marine influence and an upper, non-marine part. The recognition of both the unconformity and late Pliensbachian-Toarcian marine incursions facilitate a close correlation with other Jurassic sections in the region. The study illustrates the value of combining palynology and sedimentology in interpreting the depositional history of non-marine and marginal marine successions.