Arenig (Lower Ordovician) conodonts and biozonation in the eastern Siljan district, central Sweden

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Abstract
Based on biostratigraphic investigations in the eastern Siljan district, central Sweden, and combined with earlier studies at Hunneberg, south-central Sweden, it is shown that the early post-Tremadoc-ageParoistodus proteusconodont Zone can be divided into four successive subzones. These are, in ascending order: theDrepanoistodusaff.D. amoenusSubzone; theTripodusSubzone; theParacordylodus gracilisSubzone; and theOelandodus elongatus-Acodus deltatus deltatusSubzone. The lowermost of the subzones is a concurrent range zone, with its reference section at Storeklev, Hunneberg, and the other three interval zones with the reference section at Sjurberg in the eastern Siljan district. Correlations between these two areas, as well as with the Flåsjö area, Jämtland, the Finngrundet core, and some other areas, are discussed, and it is concluded that theDrepanoistodusaff.D. amoenusSubzone and theTripodusSubzone together correspond to theMegistaspis(Ekeraspis)armatatrilobite Zone, and occur below theHunnegraptus copiosusgraptolite Zone. TheParacordylodus gracilisSubzone is equivalent with the lower part of theMegistaspis(Varvaspis)planilimbatatrilobite Zone, and may correspond to theHunnegraptus copiosusZone. The uppermost subzone, theOelandodus elongatus-Acodus deltatus deltatusSubzone, equals the upper part of theM.(V.)planilimbataZone, and the local base of theTetragraptus phyllograptoidesgraptolite Zone is close to the base of this conodont subzone. The last occurrences ofT. phyllograptoidesare close to the top of this conodont subzone. Also included in the investigation are the uppermost part of the uppermost TremadocPaltodus deltiferZone and the conodont zones overlying theP. proteusZone in the eastern Siljan district, thePrioniodus elegansZone and theOepikodus evaeZone; the latter zone corresponds to the trilobite zones ofMegalaspides(M.)dalecarlicusandMegistaspis(Varvaspis)estonica.A few samples from the superimposedBaltoniodus triangularis, B. navis, andParoistodus originalisZones have been investigated and are described as well. The correlation of these conodont zones and subzones with those of areas outside Baltoscandia, particularly North America, is also discussed.