PETROGRAPHIC FACIES ANALYSIS OF LOWER AND MIDDLE JURASSIC COAL SEAMS ON THE ISLAND OF BORNHOLM, DENMARK

The Lower-Middle Jurassic succession on the island of Bornholm, Denmar k, comprises a wide range of clastic depositional environments: lacust rine, fluvial, coastal plain, tidal flat and shallow marine. The succe ssion contains a number of coal seams and carbonaceous seams, permitti ng: (1) evaluation of the relationship between the different clastic e nvironments and the associated environments of organic matter depositi on and (2) assessment of facies changes within coal/carbonaceous seams from different peat-forming environments. Coal petrographic facies an alysis was performed by maceral analysis of 61 coal samples from the L ower Jurassic Ronne Formation, comprising the Munkerup Member, Sose Bu gt Member and Galgelokke Member, and from the upper Lower-Middle Juras sic Baga Formation. The seams in the Munkerup Member and the Sose Bugt Member are mainly allochthonous, whereas those of the Galgelokke Memb er and the Baga Formation are mainly autochthonous. The seams represen t a broad spectrum of depositional environments; from raised bogs to f reshwater swamps and marine-influenced swamps. The facies are subaquat ic to telmatic and terrestrial. Compositional differences between the seams are attributed to vegetation characteristics, depositional facie s and differences in the degradation of the organic matter. The origin al peat-forming vegetation was composed of cellulose rich, shrub-like plants, tree ferns, herbaceous plant communities and a varying amount of trees. The peat-forming environments represented by the carbonaceou s seams in the Munkerup and Sose Bugt Members are compatible with the depositional environments indicated by the sediments. However, the coa l seam in the Galgelokke Member represents deposition in a raised bog, conflicting with sedimentary interpretation of a clastic tidal flat e nvironment for the member. The Baga Formation is considered to have fo rmed under a lower/upper delta plain or fluvial environment. The compo sition of the coal seams supports the interpretation of a system of me andering streams in a upper delta plain to fluvial environment.