The geology of the upper Middle - Upper Miocene Gram Formation in the Danish area

Sediments of the upper Middle - Upper Miocene Gram Formation are distributed in central, west and south Jylland, Denmark. The Gram Formation is subdivided into three units: bottom to top the glaucony-rich, lowermost part, the Gram Clay, and the Gram sand. The sediments of the Gram Formation were deposited in an open marine depositional environment. The sediments of the Gram Formation were deposited in an open-marine environment. The glaucony-rich part was laid down under maximum transgression of the sea during the late Middle Miocene, when the north-eastern boundary of the North Sea was possibly located near the Sorgenfrei-Tornquist Zone in northern Jylland and Skåne. Water depths were up to 100 m in the area covered by the present-day west Jylland, but exceeded 500 m in the central part of the North Sea. In the Upper Miocene, outbuilding of clastic sedimentary wedges from the Fennoscandian Shield occurred from both the north and east. This resulted in a high sedimentation rate during the deposition of the Gram Clay. Locally, especially around salt structures, deeper, partly anoxic, restricted depositional environments prevailed. Expansion of these anoxic environments may have resulted in extreme environmental conditions and resulted in mass mortality shown by well-defined concretion horizons containing impressions of crabs. The uppermost part of the Gram Clay, with fine-grained storm-sand layers, and the Gram sand indicate periodical proximity of the shoreline. Possibly as a result of increased subsidence of the North Sea Basin in the Upper Miocene, up to 400 m of mainly shelfal deposits were laid down in most parts of the eastern North Sea Basin.