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 environ ments in an overall transgressive setting in the Sogne Basin, Danish C entral Graben, in the North Sea. The coal seams occur in nonmarine to marginal marine deposits. Coal petrographic analyses of the seams in t he West Lulu-2 well supplemented with a sequence stratigraphic analysi s suggest that peat accumulation was controlled by relative sea-level changes. In West Lulu-2 the coal-bearing upper part of the Middle Jura ssic is divided into sequences A and B. The sediments of sequence A ar e part of a transgressive systems tract, whereas the overlying sequenc e B contains lowstand and transgressive systems tract deposits. The co als are situated in the transgressive systems tracts. The coal seams f ormed during a rapid relative sea-level rise are generally characteriz ed 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 m arine flooding surfaces or to transgressive surfaces if they are situa ted at the base of a transgressive systems tract. A thick coal seam zo ne is associated with the stratigraphic level of the maximum flooding surface, which correlates with the time of formation of the highest ac commodation space.