Abstract
Darriwilian conodonts have been studied in numerous sections of Baltoscandia, but few data are available from the classical outcrop area in northern Estonia. In this paper we report the succession of Darriwilian conodonts and chitinozoans from the bed-by-bed sampled Uuga and Osmussaar cliffs, NW Estonia. Standard Baltoscandian conodont and chitinozoan zones and subzones were identified from the topmost Volkhov to Uhaku regional stages. The global Dapingian–Darriwilian boundary coincides with the Volkhov–Kunda boundary in NW Estonia. The Kunda Stage is represented by its middle or upper part only, and the Aseri Stage is probably missing in both sections studied. The lowermost part of the Lasnamagi Stage with the conodont Yangtzeplacognathus foliaceus is also very condensed. The rest of the Lasnamagi and Uhaku stages, corresponding to the main part of the Pygodus serra conodont Zone, is well represented and can be correlated across Estonia and Sweden. Subtle regional variations in this interval indicate very uniform depositional conditions over wide areas of the Baltoscandian palaeobasin. Our results suggest that the Y. foliaceus Subzone in Estonia needs further assessment. Stratigraphically well-constrained Y. protoramosus appears to be more common than previously thought, and is thus a valuable regional subzonal index. The integrated conodont and chitinozoan scale provides more than 20 biostratigraphically important levels for local and regional correlations, probably approaching temporal resolution in the order of 0.1 Ma for the late Darriwilian.
Highlights
Well-preserved and fossiliferous carbonate rocks of Darriwilian age, Middle Ordovician, are widespread in Baltoscandia and widely utilized locally as building material
Darriwilian conodonts have been studied in numerous sections of Baltoscandia, but few data are available from the classical outcrop area in northern Estonia
In this paper we report the succession of Darriwilian conodonts and chitinozoans from the bed-by-bed sampled Uuga and Osmussaar cliffs, NW Estonia
Summary
Well-preserved and fossiliferous carbonate rocks of Darriwilian age, Middle Ordovician, are widespread in Baltoscandia and widely utilized locally as building material. Divided into the uppermost Volkhov, Kunda, Aseri, Lasnamägi and Uhaku regional stages, they bear evidence of rapid diversification of many groups of organisms (Hints et al 2010 and references therein). The standard North Atlantic conodont biozonation (Bergström 1971), widely used in Baltoscandia and elsewhere, is based on several successive zones and subzones, some of which have clearly evolutionary significance (Bergström 1983; Zhang 1998). Darriwilian conodont zones and subzones have been identified in numerous sections in Scandinavia and Estonia. Very few data are still available from the classical outcrop area of northern Estonia, which serves as the standard for many Baltic regional stages. In drill cores the sampling density and sample size often limit the temporal resolution and complicate detailed comparison of biozonations based on different fossil groups
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