Eustatically‐controlled sedimentation in the Hettangian‐Sinemurian (Early Jurassic) of Poland and Sweden

ABSTRACT In earliest Jurassic times, terrigenous, continental and marginal marine deposition occurred in a large epeiric basin along the Tornquist Line in Europe. Detailed sedimentological studies allow recognition of palaeoenvironmental fluctuations in space and time. The main earliest Jurassic transgressions occurred in the early Hettangian, early Sinemurian, mid‐Sinemurian and latest Sinemurian and formed bounding discontinuities (transgressive surfaces) of considerable correlative significance. There is a step‐wise trend of increasing marine extension and influence during the early Hettangian, early Sinemurian, mid‐Sinemurian and latest Sinemurian‐earliest Pliensbachian transgressions. Four sequences, four transgressive systems tracts, three highstand systems tracts and three levels regarded as equivalents of maximum flooding surfaces are distinguished. In the case of type 2 sequences, when incised valley‐fill deposits are not developed and regional erosion is less common, it may be rather difficult to define the sequence boundaries, which are often concealed within the amalgamated fluvial deposits occurring in the neighbouring parts of two adjacent sequences (fluvial/deltaic sediments terminate the highstand systems tracts and in this setting the transgressive systems tracts start with continental deposits prior to the transgressive surfaces). Generally, an exact correlation can be achieved between the sequence stratigraphy of the northeast and northwest European Lower Jurassic and the eustatic curve proposed by EPR (assuming some changes proposed by A. Hallam). The establishment of this correlation hopefully will stimulate future studies of the sequence stratigraphy of poorly dated siliciclastic deposits of marginal basins. In this setting even minor changes in sea‐level may cause major changes in facies development over large areas.