Lower Viruan (Middle Ordovician) microplankton from the Ekön borehole No 1 in Östergötland, Sweden

The present study deals with organic-walled microplankton having probable phytoplanktic affinities and belonging to the Group Acritarcha Evitt, 1963. A continuously cored well through the Lower Viruan (Middle Ordovician) at Ekön, in the county of Östergötland in Sweden, has revealed the presence of twenty-nine species of the acritarch genera Aremoricanium, Baltisphaeridium, Goniosphaeridium, Multiplicisphaeridium, Peteinosphaeridium and Tylotopalla. Five new species are proposed, i.e.: Aremoricanium carolineae, Baltisphaeridium annelieae, Baltisphaeridium christoferii, Baltisphaeridium ingerae and Goniosphaeridium christianii. A biostratigraphic account is given and a quantitative analysis is carried out with regards to the phytoplanktic density. The acritarch assemblage of the Lower Viruan of Ekön is compared with assemblage material from equivalent strata of the Gammalsby boring from Öland and the Grötlingbo boring from Gotland.