The Lower Viruan in the autochthonous Ordovician sequence of Jämtland

The stratigraphy and lithology of the lower Viruan Aseri and Lasnamägi Stages of the autochthonous Ordovician of Jämtland, south-western part of northern Sweden, are described in some detail. lt is shown that the same topo- and lithostratigraphic units previously distinguished on Öland and in the Siljan district can be applied. The Kårgärde and Vikarby Members of the Aserian Segerstad Limestone are recognized in all areas investigated, whereas the Lasnamägi Stage shows a more incomplete development, with the Skärlöv Limestone missing in the Brunflo area and the Folkeslunda Limestone missing in the Lockne area. Stromatolites in the form of mats and domes are an importallt constituent in all pre-Folkeslunda rocks studied and characterize especially the Vikarby and Seby Limestones, for which a new term, the Lunne facies, is introduced. Certain palaeoenvironmental conclusions are drawn from the varying development of the stromatolites.