Dinocyst stratigraphy and palynofacies studies used for refining a sequence stratigraphic model—uppermost Oligocene to lower Miocene, Jylland, Denmark

Abstract

The uppermost Oligocene– Miocene deposits in the central and southern parts of Jylland, Denmark, comprise a complex succession of siliciclastic, paralic deposits. The coarse-grained parts of the succession comprise important groundwater reservoirs. A sequence stratigraphic model has gradually been developed during the last 7 years and the succession is now subdivided into six sequences (sequence A–F). The model is based on a combination of seismic data, geophysical logs and lithological data from boreholes and sedimentological measurements of outcrops. The present study documents the composition of assemblages of organic-walled dinoflagellates (dinocysts) in the lower part of the succession (sequences A–C). The data come from seven water supply boreholes and one outcrop section. The dinocyst data are an important tool for pointing out the sequence stratigraphic surfaces in the individual boreholes, showing distinct changes at the sequence boundaries and increased relative abundance and diversity of dinocysts at marine flooding surfaces. The dinocysts are also used for correlating between the studied boreholes and outcrops, for interpreting changes in the depositional environment and for dating the sequences in more detail than earlier datings based on molluscs and foraminifers. The datings allow correlation to eustatic sea-level curves. The good match between the sequence development and climatic changes suggests that eustatic sea-level changes were the main factor in sequence formation and changes in the depositional environment.