Sandbian (Late Ordovician) conodonts in Estonia: distribution and biostratigraphy

The boundaries of the Sandbian Stage are defined by graptolites. Because of their rare occurrence in the Estonian part of the Baltoscandian palaeobasin, other fossils, including conodonts, are used for biostratigraphy as good alternatives. Since the definition of Sandbian Stage/Age in 2002, information about conodonts from this time interval in Baltsocandia has improved considerably. In this paper, existing data on Sandbian conodont faunas from the Estonian part of the Baltoscandian palaeobasin, including the most recent information from the Velise V-97 core section (western part of mainland Estonia), is evaluated. Dating of these strata in the region is addressed and partly updated. Comparison of conodont successions from other regions of Baltoscandia with those from the Estonian sections is used to check consistency of known conodont zones. The rich conodont succession from the Velise V-97 drill core is considered a representative one for the Sandbian-age carbonate ramp environment in Baltoscandia. Most of the principal conodont zones within the Sandbian Stage were recognised whilst the lower and upper boundary intervals of the stage are poorly resolved. Despite the rich conodont faunas available, the boundaries of the Sandbian Stage are currently not clearly definable in the conodont succession. However, further detailed studies on the morphology and evolution of the earliest Amorphognathus (A. inaequalis and A. tvaerensis in particular), based on rich and well preserved material from different parts of Baltoscandia, might provide useful criteria for location of the Sandbian and Katian boundaries in non-graptolitiferous carbonate sections