An integrated biostratigraphical analysis of the Volkhov-Kunda (Lower Ordovician) succession at Fågelsång, Scania, Sweden

The Komstad Limestone is composed of dark grey to black micritic limestone deposited in an outer shelf environment on the margin of the East European Platform. It represents an interval of low sea level during the late Volkhov to early Kunda that led to the spread of limestones into the shale-dominated western lithofacies belt. The Baltoniodus norrlandicus, Lenodus antivariabilis, Lenodus variabilis, Yangtzeplacognathus crassus and Lenodus pseudoplanus conodont zones were identified in the upper Volkhov to lower Kunda interval. Based on the most recent taxonomic and biostratigraphical framework, the Megistaspis limbata (with two subzones), Asaphus expansus and Asaphus raniceps trilobite zones were identified. A shale intercalation in the lower part of the Komstad Limestone contains the graptolites Phyllograptus cor in association with Glyptograptus sp. and is referred to Darriwilian 1 (Upper Arenig). In the Baltoscandian zonation this matches the upper part of the Didymograptus hirundo graptolite Zone. The upper level of the Komstad Limestone and the basal part of the overlying Almelund Shale do not contain graptolites, whereas the succeeding black shales of the Almelund Shale belong to the Holmograptus lentus Zone (Llanvirn). The Arenig-Llanvirn boundary is situated at or very near the top of the Komstad Limestone. The conodont assemblage in the lower part of the Komstad Limestone at Fågelsång is associated with forms of Gondwanan affinity, which probably reflects the cool water environment of the outer shelf setting. The higher part of the limestone contains the Whiterockian conodont species Dzikodus sp. and Histiodella tableheadensis. These important Laurentian forms occur together with abundant Gothodus sp. 1 and Cyclopyge umbonata in the base of the Asaphus expansus Zone. The arrival of the Laurentian taxa - as well as Cyclopyge with an 'Gondwana' affinity - is related to a transient sea level rise at the base of the A. expansus trilobite Zone.