HIGH-RESOLUTION SEQUENCE STRATIGRAPHY OF A HETTANGIAN-SINEMURIAN PARALIC SUCCESSION, BORNHOLM, DENMARK

Sequence stratigraphic interpretation of paralic successions is compli cated by the complex interfingering of marine and continental strata. The successions may also include terrestrial extensions of marine para sequences and completely independent lacustrine parasequence analogues . Failure in recognizing the possible interbedding of these two indepe ndent parasequence types may lead to construction of sequence stratigr aphic schemes based on incompatible data sets. We have studied a Lower Jurassic paralic section from the Baltic island of Bornholm, situated in the Tornquist Zone, which demarcates the transition from the stabl e Precambrian Baltic Shield to the subsiding Danish Basin and Danish-P olish Trough. The Hettangian-Sinemurian Sose Bugt Member (Ronne Format ion) of Bornholm includes lacustrine, fluvial and restricted marine, e stuarine deposits reflecting the basin-margin position. Biostatigraphi c resolution is poor and a sequence stratigraphic interpretation of th e paralic succession is far from straightforward. A multidisciplinary approach including facies analysis, recognition and lateral tracting o f key surfaces, palynostratigraphy, palynofacies, coal petrography, pa laeopedology, clay mineralogy and source rock geochemistry is applied in order to obtain a high degree of precision in the interpretation of the paralic facies. In this way four sequences are recognized in the overall backstepping lacustrine to estuarine succession. Marine and ma rginal marine parasequences are distinguished from their purely lacust rine analogues, and an internally consistent sequence stratigraphic sc heme is proposed. This is compared and tentatively correlated with fos siliferous marine sediments in the Danish Basin and with published eus tatic cycle charts.