1 Institute of Geology, Komi Scientific Centre, Russian Academy of Sciences, Uralian Branch, Russia. E–mail: antoshkina@geo.komisc.ru

Key words: Carbonates. Stratigraphy. Upper Ordovician. Subpolars Urals. Baltica.

The Upper Ordovician carbonate succession of the Subpolar Urals formed on the northeastern margin of the Baltica palaeocontinent (= Pechora Plate or Timan–Northern Urals region). From the second half of the Middle Ordovician, carbonate sedimentation occurred on the passive margin. From the west to the east of the basin inner to outer carbonate ramp is recognized (Antoshkina et al., 2000). Among the major factors controlling formation and accumulation of carbonate sediments are tectonic situation, relative sea level fluctuation, climatic stability and biotic balance. It is possible that in the end of the Early Ashgillian differentiation between stable and tectonically active structural zones along large regional faults began and the Kos’yu–Rogovskaya pericratonic depression was formed.

The Upper Ordovician (= Ashgillian) of the West Urals includes the Rassokha, Polydov, Syr’ya, and Kyr’ya horizons (regional stages). In the Subpolar Urals region Upper Ordovician successions (600–870 m thickness) have been subdivided into three formations: Zyb (or Ust’Zyb), Malaya Tavrota, and Yaptikshor (Figure 1). These formations have the stratotypes in the Kozhym River area (Antoshkina et al., 1989; 2000).

The Zyb Formation (up to 78 m thickness) is dominated by dolomitized sorted bioclastic packstones/wackestones with cross–bedded in many layers, lens–like beds of carbonate gravelstones and conglomerates were also found. Rare pelagic (conodonts) and benthic (brachiopods, gastropods) faunal groups have been found in this formation. The Malaya Tavrota Formation includes two members – Lower and Upper Malaya Tavrota subformations. The former consist of grey mostly dolomitized and poorly fossiliferous limestones, 372 m thickness. Small brachiopods, bryozoans, tabulate corals, stromatoporoids, rare orthoceratids, and trilobites occur in more transgressive units of sedimentary cycles of this subformation. Upper Malaya Tavrota Subformation consists of two lithological patterns: laminar beds with stromatolites and reef massive dolomite limestones, 100 to 260 m thickness. The Yaptikshor Formation (up to 100 m thickness) consists mostly of thick dark–grey, partly argillaceous with rich fossils and strongly dolomitized limestones. Brachiopods Holorhynchus giganteus and Proconchidium muensteri are common (Antoshkina et al., 1989). In the West Urals stratigraphical chart the position of the Ordovician–Silurian boundary was shown between Kyr’ya and Yarenev horizons (Antsygin et al., 1993). The isotope data of the problematic in age units (up to 60 m thickness) from the lower part of the Yareney Formation in the Kozhym River section have shown a great similarity with that of uppermost Estonian sequences of Hirnantian in age (Beznosova et al., 2002). The Yareney Formation with Holorhynchus giganteus is regarded as the uppermost Ordovician units aligned with Beds (Stage) 5b in the Oslo region. Isotopic dating of the sediments with Hirnantia fauna is still debatable (Brenchley et al., 1997).

The base of the Hirnantia Stage is recognised by changes in lithology that generally marks a change to shallower marine facies. The uppermost Ordovician sequence in the Subpolar Urals is characterized by monotonous white–grey massive–bedded strongly dolomitized bioclastic limestones containing abundant crinoidal debris and leached large stromatoporoids and tabulate corals. Traces of burrows are recognized as dark–grey clotted bands into massive structures. It is shown that deposits of the uppermost Ordovician sequence formed in clearly shallower environments than deposits of the underlying dark–grey argillaceous and richly fossiliferous Yaptikshor Formation. This fact coincides with already proposed stratigraphically higher, than it was earlier regarded, position of the Ordovician/Silurian boundary on Subpolar Urals (Beznosova, 2000).

The Late Ordovician epicontinental basin was characterized by a low average rate of sedimentation in comparison to the more ancient Middle Ordovician and more young Llandoverian sedimentary basins. The composition of the Ashgillian deposits shows evidence of a crisis in sedimentary situation on Early and Late Ashgillian boundary and gradual degradation of carbonate sedimentation in westward sea basin. In the end of the Polydov age the Early Ashgillian ramp transformed in the epicontinental platform as a result of marginal uplift. A global eustatic fall of sea level and generation of barrier at the shelf margin resulted in formation of an extensive intrashelf lagoon. Carbonate–sulfate strata accumulated in the pericratonic area. At the beginning of Late Ashgillian during the first Paleozoic reef building evaporate shelf margin grew in the pre–Urals area.

Figure 1. The Upper Ordovician stratigraphic subdivisions of the Subpolar Urals, their depositional environments and sea level changes.

The growth of the Upper Ashgilian reef (Syr’ya age) was first time stopped by a sea level fall. The top of the Bad’ya reef is a subaerial exposure surface with karst erosion. It possesses erosion fissures are up to 3 m deep extend from the top of the reef downwards (Antoshkina, 1996). A thin layer of black shale overlying the reef top marked an abrupt sea level rise in the earliest Kyr’ya age, causing the termination of reef growth (Figure 1). The next sea level fall is recognized at the base of the uppermost Ashgillian sequence. Near the end of the Ordovician the carbonate platform was exposed during the glacio–eustatic sea level lowstand. Accordingly, it is most probable that the uppermost Hirnantian Stage and the whole Ruddanian Stage are missing in the West–Uralian successions. The first Silurian brachiopod species from this region identified as Pentamerus sp. (aff. oblongus) occurring in Aeronian (Beznosova et al., 2002).

Comparative analysis of the parities among intensive reef building, growth stops and erosion of upper part of reefs allow to make reconstructions of sea level curve for the Upper Ordovician succession of the Subpolar Urals (Figure 1). Some sequence boundaries in subtidal settings are expressed as rapid deepening due to accommodation increases. No attempts were made to quantify the curve, but relied upon the facies successions and sequence boundary development to indicate relative water depth. The curve requires further testing but may be compared to that for the other basins on the margin of the Baltica paleocontinent.

Accepted: June 15, 2003