Organic facies development within Middle Jurassic coal seams, Danish Central Graben, and evidence for relative sea-level control on peat accumulation in a coastal plain environment

During Middle Jurassic times peat accumulated in coastal plain environments in an overall transgressive setting in the Sogne Basin, Danish Central Graben, in the North Sea. The coal seams occur in nonmarine to marginal marine deposits. Coal petrographic analyses of the seams in the West Lulu-2 well supplemented with a sequence stratigraphic analysis suggest that peat accumulation was controlled by relative sea-level changes. In West Lulu-2 the coal-bearing upper part of the Middle Jurassic is divided into sequences A and B. The sediments of sequence A are part of a transgressive systems tract, whereas the overlying sequence B contains lowstand and transgressive systems tract deposits. The coals are situated in the transgressive systems tracts. The coal seams formed during a rapid relative sea-level rise are generally characterized by petrographic features related to a permanently high groundwater table. Coals formed during a slow rise typically are 'drier' in their petrographic composition. The highest pyrite contents are observed in the 'wet' coals. The coal seams represent terrestrial equivalents to marine flooding surfaces or to transgressive surfaces if they are situated at the base of a transgressive systems tract. A thick coal seam zone is associated with the stratigraphic level of the maximum flooding surface, which correlates with the time of formation of the highest accommodation space.