New data on Ordovician-Silurian conodonts and stratigraphy from the Severnaya Zemlya Archipelago, Russian Arctic

Thirty samples from 22 sections collected by the SWEDARCTIC international expedition to Severnaya Zemlya in 1999 contained Ordovician and Silurian conodont faunas. Several taxa, including Apsidognathus cf. milleri, Aulacognathus cf. kuehni, Nudibelodina sensitiva, Ozarkodina broenlundi and Pterospathodus eopennatus, allow precise dating of the strata in this region for the first time. The occurrence of Aphelognathus pyramidalis and Rhipidognathus aff. R. symmetricus in samples from the Strojnaya Formation fits well with the earlier dating of these strata as latest Ordovician. However, Aphelognathus sp. in sample BG-99/14-a, collected from the upper Ushakov Formation, indicates that at least in the lower reaches of the Ushakov River the top of this formation is considerably younger than considered earlier: the sampled strata are Late, not Early Ordovician in age. In the Ordovician and Silurian the present-day Severnaya Zemlya region was dominated by extensive shallow-water, mainly semi-restricted basin environments with habitat specific faunas. The occurrence of Riphidognathus aff. R. symmetricus at some levels in the Upper Ordovician suggests extreme shallowing episodes in the basin. On Severnaya Zemlya, normal-marine' faunas (including Pt. eopennatus) invaded the distal peripheral regions of the wide shallow-water platform at times of maximum sea-level rise only. The occurrence of Oz. broenlundi and N. sensitiva indicates that in the early Silurian the Severnaya Zemlya basin was quite well connected to the basins over modern North Greenland as well as to the Baltic Palaeobasin. The lower Silurian conodont assemblages in the Vodopad to Samojlovich formations are most similar to those described from the eastern Timan-northern Ural region