6

Ordovician has been explained by glaciation on Gonwana, which caused an eustatic fall of sea level. Global regression resulted in considerable reduction of areas of shallow-water environments, and most of the seas on continental shelves disappeared. Reduction of the areas of suitable environments caused changes in the faunas; many taxa disappeared. In the stratigraphic sequence this event is marked by the appearance of extremely shallow- water sediments, by extensive gaps, and by denudation ofolder sediments.
Such rapid sea-level fall and, probably, also denudation of older sediments (Lolashor Stage) can be obsereved in the sequence in the Subpolar Urals. As the Timan-Urals palaeobasin was located at low latitudes during the late Ordovician-Silurian period, the direct climatic effect of glaciation on the environments in this basin was relatively unimportant. It is possible that a general shallowing of the basin, and a decrease in the supply of oceanic water, resulted in an increase in temperature in the basin. This possibility is indicated by wide distribution of coral- stromatoporoidal faunas in Yaptikshor time, and appearance of gigantic forms of fauna (Holorchynchus giganteus Kiaer, rugose corals with length up to 15 cm and with diameter up to 7 cm) in section 108. Climatic changes at the end of the Ordovician led to increased aridity and deposition ofsulphates in the pre-Urals Depression. In the region of the mouth of the Durnayu River (sections 211, 212, 217), strata of Llandovery, Wenlock and Ludlow age were examined, and in the region of the mouth of the Syv'yu River (section 236), Silurian and Devonian boundary beds were studied. Sections 211, 212 and 217 were sampled in detail for carbon isotopes (d¹³C). Isotopes will be analyzed in the Institute of Geology at Tallinn Technical University. Hopefully, the results of the isotopic studies will help specify the interval of the Ireviken Event recognized at and just above the Llandovery-Wenlock boundary all over the world (Jeppsson, 1997). During this event, most of the taxa of the rich Telychian faunas became extinct. Particularly distinct (and well studied) are changes in the conodont succession (Jeppsson, 1997; Jeppsson & Männik, 1993).
Detailedand complex studies of the nature of events, including the Ireviken Event, allow us to estimate the general causes and mechanisms of the faunal changes/extinctions in the Earth's history. Also, the spcification of the the level of the Ireviken Event in the Silurian sequence in the Subpolar Urals allows a solution to the problem of the Llandovery-Wenlock boundary in this region - does it correspond to the level close to the contact between the Filipp''el' and Marshrutnyj stages (based on macrofaunas) or does it occur in the (middle or upper) Ust'Durnayu Stage (based on conodonts - Männik, pers. obs.).
Results of our studies allow inclusion of the Timan-Urals region in the system of global stratigraphical correlations. Also, the data available now permit us:
1) to correct and complete the regional stratigraphic scheme;
2) to correlate the core sections from the Timan-Urals region;
3) to correlate the sections in the Timan-Urals region with those from other parts of the world.
During the expedition possible themes and routes of geological excursions to the national park Yugyd Va were worked out. Directly in the sections, the possibilities of their demonstration were discussed, main stratigraphical boundaries and characteristic beds were marked by different colours, and the program and route of the planned international excursion to this region during CAPV-2000 were worked out.
According to the agreed schedule the excursion to the Subpolar Urals will last 7 days. The sequence will be demonstrated from three camps:
1. Camp close to the section 88 - sections 73, 74, 75, 76, 88, 89 and 108 will be demonstrated.
2. Camp close to the section 211 - section 211, 212 and 217 will be demonstrated.
3. Camp in the mouth of the Syv'yu River - section 105, 231, 236 will be demostrated. On the 7th day all participants will return to Ukhta where for two days they will be able to study some core sections from the Timan-Urals region and participate in a one-day excursion to Devonian sections around Ukhta. Workshop in Syktyvkar
Time:August 3-7, 1999.
Participants:
A. I. Antoshkina, Institute of Geology of the Komi Science Centre, Ural Division of Russian Academy of Sciences
(Syktyvkar, Russia);
N. V. Belyaeva, Institute of Geology of the Komi Science Centre, Ural Division of Russian Academy of Sciences
(Syktyvkar, Russia);