Palynological analysis of the organic debris in the Lower Cretaceous Jydegård Formation, Bornholm, Denmark

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Abstract
The Lower Cretaceous Jydegård Formation has been palynologically analysed. The finegrained Rødbjerg Member was examined in two boreholes in the Nyker fault-block and the coarse-grained Tornhøj Member was examined in an exposed section in the Robbedale region. Organic debris from fine-grained beds throughout the formation was evaluated for dinoflagellate cyst assemblages, for the proportion of dinoflagellate cysts to pollen and spores and for palynofacies. The three methods reflect different parameters of the depositional environment and 5 organic facies have been defined on the basis of variation in these parameters. The organic facies are interpreted as mainly reflecting fresh versus brackish water and oxygenated versus anoxic depositional environments. The organic facies are compared with the sedimentological interpretation of the formation and a depositional history is constructed. The Rødbjerg Member is a lagoonal mud deposited in brackish water under highly anoxic bottom conditions. The environment became slightly more marine and oxygenated in the middle of the member but returned to brackish and finally fresh water in the upper part. The Tornhøj Member was deposited in a back-barrier environment, dominated by alternating fresh and brackish water in the lower part. The upper part was deposited in a fluviatile fresh water environment.