The Viruan (Middle Ordovician) of Kinnekulle and Northern Billingen, Västergötland

The stratigraphy of the Viruan (Middle Ordovician) sequence of Kinnekulle and northern Billingen in Västergötland, south-central Sweden, is described from the information obtained from cores of three borings and from examinations of some selected outcrops. The Aserian beds on the south-eastern slope of northern Billingen are very thin. They are separated from the under- and overlying beds by a considerable break and belong exclusively to the upper part of the stage (Vikarby Limestone). On Kinnekulle the Aserian deposits are entirely missing. On the south-eastern slope of northern Billingen the Vikarby Limestone is overlain by a few variegated, preponderantly calcarenitic beds of Lasnamägian age for which the new term Skövde Limestone is proposed. Their earrelation with Lasnamägian divisions in the other districts is uncertain. Overlying the Skövde Limestone is a thick sequence consisting mostly of mudstones with limestone nodules here referred to the new division Gullhögen Formation. On eastern Billingen the lowermost part of the Gullhögen Formation is probably of late Lasnamägian age, whereas the age of the main part of the formation is early Uhakuan. At present i t is impossible by either fauna! or lithological criteria to determine within the Gullhögen Formation the leve! of the boundary between the Lasnamägi and Uhaku Stages. On Kinnekulle the Skövde Limestone is missing, and the Gullhögen Formation rests directly upon the Ontikan Kunda Stage. Because the lowermost part of the Gullhögen Formation, as developed on Billingen, is obviously also missing on Kinnekulle, the possibility is not excluded that there the basal Viruan beds are of early Uhakuan age. The late Uhakuan beds of Västergötland are developed as a tongue of the Furudal Limestone and are designated here with the new name Ryd Limestone. The Ludibundus beds are subdivided into two formations, viz. the Dalby Formation and the Skagen Formation (new name, corresponding to the suprabentonitic Ludibundus beds in Västergötland). On Kinnekulle and Mösseberg the Dalby Limestone can be subdivided into two members. The Lower Member is relatively thin and consists of calcilutites in the lower part and calcarenitic calcilutites to calcarenites in the upper part. The Upper Member has a thin, oölitic calcareous mudstone with chamosite oöids at the base and consists preponderantly of mudstones with occasional beds of calcilutite. On Billingen and Varvsberget the whole formation is composed of Iimestones which are calcilutitic in the basal part and preponderantly calcarenitic in the main upper part. On eastern Billingen the formation has its minimum known thickness in Sweden. The Skagen Limestone consists of calcilutites and calcareous mudstones and contains a distinctive fauna. It is suggested that the Skagen Limestone of Västergötland comprises the lower part of the suprabentonitic Ludibundus beds only, and that beds corresponding to the upper part of the suprabentonitic Ludibundus beds of Östergötland and the Siljan district are missing in Västergötland. If this is correct, the Skagen and Mossen Formations on Kinnekulle are separated by a considerable break which probably also involves beds comparable to the lower part of the zone of Dicranograptus clingani. On the south-eastern slope of northern Billingen beds comparable to this graptolite zone are completely missing, and the Skagen Limestone is directly overlain by Harjuan (Upper Ordovician) strata. Discontinuity surfaces at the base of the Viru Series are described in some detail.