Petrographic facies analysis of Lower and Middle Jurassic coal seams on the island of Bornholm, Denmark

Abstract

The Lower-Middle Jurassic succession on the island of Bornholm, Denmark, comprises a wide range of clastic depositional environments: lacustrine, fluvial, coastal plain, tidal flat and shallow marine. The succession contains a number of coal seams and carbonaceous seams, permitting: (1) evaluation of the relationship between the different clastic environments and the associated environments of organic matter deposition and (2) assessment of facies changes within coal/carbonaceous seams from different peat-forming environments. Coal petrographic facies analysis was performed by maceral analysis of 61 coal samples from the Lower Jurassic Rønne Formation, comprising the Munkerup Member, Sose Bugt Member and Galgeløkke Member, and from the upper Lower-Middle Jurassic Bagå Formation. The seams in the Munkerup Member and the Sose Bugt Member are mainly allochthonous, whereas those of the Galgeløkke Member and the Bagå Formation are mainly autochthonous. The seams represent a broad spectrum of depositional environments; from raised bogs to freshwater swamps and marine-influenced swamps. The facies are subaquatic to telmatic and terrestrial. Compositional differences between the seams are attributed to vegetation characteristics, depositional facies and differences in the degradation of the organic matter. The original 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 carbonaceous seams in the Munkerup and Sose Bugt Members are compatible with the depositional environments indicated by the sediments. However, the coal seam in the Galgeløkke Member represents deposition in a raised bog, conflicting with sedimentary interpretation of a clastic tidal flat environment for the member. The Bagå Formation is considered to have formed under a lower/upper delta plain or fluvial environment. The composition of the coal seams supports the interpretation of a system of meandering streams in a upper delta plain to fluvial environment.