Lower Viruan discontinuity surfaces in central Sweden

Near the base of the Viru Series (Middle Ordovician) in Västergötland, south-central Sweden, discontinuity surfaces in the limestones mark breaks in sedimentation. The morphology of these surfaces has been studied in Hällekis Quarry at Kinnekulle and Gullhögen Quarry at Billingen, with particular emphasis on lateral variations. Four main morphological types of discontinuity surface are distinguished. One has furrows arranged in a polygonal pattern which may represent mudcracks. The basal Viruan discontinuity surface in Gullhögen Quarry shows laminated structures resembling stromatolites. At Hällekis, the basal Viruan discontinuity surface has sac-shaped cavities up to 10 cm deep, produced by boring organisms. At Billingen, the amount of boring by hard-bottom endofauna was low and the discontinuity surfaces were mostly associated with high energy conditions, as indicated by conglomerates, blankets of oolites, and erosional phenomena. Stromatolites and mudcracks suggest shallow water conditions and even some periods of emersion.