Cambrian-Ordovician boundary beds in the U.S.S.R.

Cambrian-Ordovician boundary beds are widely distributed in many regions of the U.S.S.R. and are represented by sediments of various facies types. Correlations between facies belts are the subject of discussions in some cases. In the East European Platform, Urals, Pay-Khoy, Vaigach, South Novaya Zemlya, and in northeastern U.S.S.R., the base of the Ordovician coincides with the base of a transgressive complex. In Kazakhstan and the Siberian Platform, the change in facies is recorded close to the systemic boundary. Detailed conodont biostratigraphy of the boundary interval is established only in Kazakhstan, Estonia, and the Leningrad area. Graptolites are rare at the boundary interval; the exact level of the first appearance of nematophorous graptolites is open to dispute. The main group used for subdivision and correlation of the boundary interval is trilobites. The best horizon for the Cambrian-Ordovician boundary in the U.S.S.R. is the base of the conodont Cordylodus proavus Zone. The widespread transgression and drastic changes in faunal composition are connected with this horizon. Geochemical anomalies and reversal of the paleomagnetic polarity are known at this horizon in Kazakhstan. The main reasons for global changes at the Cambrian-Ordovician boundary are probably the strong activation of continental plate movements and related changes in the global system of oceanic circulation