Abstract
Conodonts from the type region of the Oandu Stage (Katian, Upper Ordovician) in NE Estonia were studied. Here, the lower boundary of the stage corresponds to a discontinuity surface at the base of the Hirmuse Formation and, in conodont succession, is marked by the disappearance of Semiacontiodus sp. and Besselodus ? sp. The most characteristic taxa of the Oandu Stage are Phragmodus undatus, Icriodella cf. superba and Plectodina sp., which are rare or missing below and above this stratigraphic interval in Estonia. The comparison of conodont faunas from North and South Estonia suggests that the strata in North Estonia correspond to the upper part of the Oandu Stage only as identified in sections in South Estonia. However, the position of the lower boundary of the stage in South Estonia is highly problematic. The boundary between the Amorphognathus tvaerensis and A. superbus conodont zones in Estonia lies within the (upper) Oandu Stage. and A. superbus conodont zones in Estonia lies within the (upper) Oandu Stage.
Highlights
The strata corresponding to the nowadays Oandu Stage were first described as Hemicosmites-limestone (‘Hemicosmitenkalk von Wassalem’) from the Vasalemma region by Eichwald (1854) and later referred to as the Vasalemma Bed (‘Wasalemm’sche Schicht’) by Schmidt (1881)
The comparison of conodont faunas from North and South Estonia suggests that the strata in North Estonia correspond to the upper part of the Oandu Stage only as identified in sections in South Estonia
The Oandu Stage in its type region is best characterized by the occurrence of Phragmodus undatus, Icriodella cf. superba and Plectodina sp., i.e. taxa that are rare or missing below and above this stratigraphic interval in Estonia
Summary
The strata corresponding to the nowadays Oandu Stage were first described as Hemicosmites-limestone (‘Hemicosmitenkalk von Wassalem’) from the Vasalemma region by Eichwald (1854) and later referred to as the Vasalemma Bed (‘Wasalemm’sche Schicht’) by Schmidt (1881). Öpik (1934) described an interval of claystone and marlstone from eastern North Estonia and called it the Oandu Beds He found that these strata were a timeequivalent of the Vasalemma and Saku facies occurring in western North Estonia and proved that a distinct unit, represented by a specific set of rocks (facies), existed between the Keila and Rakvere stages. Later, this unit was mainly treated as the Vasalemma Stage or as the upper part of the combined Keila–Vasalemma Stage. Based on a much wider geographical distribution of the Oandu marlstone than of the Vasalemma limestone, Männil (1958) suggested to rename the stage as Oandu Stage, with its stratotype on the Oandu River in eastern North Estonia
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