A Tonian age for the Visingsö Group in Sweden constrained by detrital zircon dating and biochronology: implications for evolutionary events

Detrital zircon U–Pb ages from samples of the Neoproterozoic Visingsö Group, Sweden, yield a maximum depositional age of ≤ 886±9 Ma (2σ). A minimum depositional age is established biochronologically using organic-walled and vase-shaped microfossils present in the upper formation of the Visingsö Group; the upper formation correlates with the Kwagunt Formation of the 780–740 Ma Chuar Group in Arizona, USA, and the lower Mount Harper Group, Yukon, Canada, that is older than 740 Ma. Mineralized scale microfossils of the type recorded from the upper Fifteenmile Group, Yukon, Canada, where they occur in a narrow stratigraphic range and are younger than 788 Ma, are recognized for the first time outside Laurentia. The mineralized scale microfossils in the upper formation of the Visingsö Group seem to have a wider stratigraphic range, and are older than c. 740 Ma. The inferred age range of mineralized scale microfossils is 788–740 Ma. This time interval coincides with the vase-shaped microfossil range because both microfossil groups co-occur. The combined isotopic and biochronologic ages constrain the Visingsö Group to between ≤ 886 and 740 Ma, thus Tonian in age. This is the first robust age determination for the Visingsö Group, which preserves a rich microfossil assemblage of worldwide distribution. The organic and mineralized microorganisms preserved in the Visingsö Group and coeval successions elsewhere document global evolutionary events of auto- and heterotrophic protist radiations that are crucial to the reconstruction of eukaryotic phylogeny based on the fossil record and are useful for the Neoproterozoic chronostratigraphic subdivision.