Age of Estonian kukersite oil shale - Middle or Late Ordovician?

 Modern principles of stratigraphy and introduction of a new international time scale for the Ordovician Period have changed the view on regional stratigraphy. Seven global stages and three series allow for a more universal application of the Ordovician time scale, but in some cases the newly defined units differ from their former usage. The regional successions can only be dated by comparing them with global type sections, which for the Upper Ordovician is located in Scania, southern Sweden. Using biostratigraphical methods, we provide some new insights into correlation between the inter­national standard and the Uhaku and Kukruse regional stages in Estonia. The Kukruse Stage and the deposits of kukersite oil shale, which have commonly been treated as middle part of Middle Ordovician, must, in fact, be considered as Upper Ordovician.