Aspects of carbonate sedimentation in the Ordovician of Baltoscandia

In the Baltoscandian epicontinental sea the normal type of Ordovician carbonate sediments was a mixture, in various proportions, of skeletal sand and carbonate mud. In some marginal areas sediments of Bahaman type were deposited during the latest part of the Viruan Epoch, and such sediments were widely distributed from the very latest Ordovician onwards. It is suggested that deposition mostly took place in a temperate climatic zone and that the bahamitic deposits indicate an increase of temperature to subtropical or tropical level.The succession of the Ordovician lithofacies belts in Baltoscandia is mostly roughly symmetrical with regard to grain size, but asymmetrical as to the distribution of terrigenous mud. The position of the boundary between the terrigenous and carbonate mud belts may have been a result of competition in the material transport from the west and from the east. The direction of the lithofacies belts is not parallel to the axis of the Caledonian geosynclinal zone. The predominance of carbonate sediments in the Baltoscandian Ordovician epicontinental sea probably does not reflect particularly good conditions for production of carbonate material, but rather, a very low supply of terrigenous matter. In the Central Baltic area changes of sea level were probably largely opposite to those in Västergötland. This indicates differential isostatic movements in the Baltoscandian cratonic shield during parts of the Ordovician Period.