1 Institute of Petroleum Geology, Russian Academy of Sciences, Siberian Branch, 630090, Novosibirsk, Ac. Koptyug av., 3, Russia. E–mail: SennikovNV@uiggm.nsc.ru

Key words: Events. Facial. Depositional environments. Biota. Ordovician.

The Altai-Sayan Folded Area is the large complicated geological structure situated in the south of Siberia. It includes several units: Gorny Altai, Salair, Kuznetsky Alatau, Kuzbass and Minusa depressions, West Sayan and Tuva. During Ordovician synchronic and heterochronic events happened with the mentioned structures. These events resulted in the change of the facial depositional environments, expansion range as well as in composition and structure of the biota developed.

In the Ordovician Altai–Salair–Kuznetsky basin (see Figure) at the earliest stage of its development was the same shape and possessed the same depositional environments as it was in Cambrian time. The main role in the establishment of the specific Altai–Salair–Kuznetsky basin belong to the event, connected with fade of the Salair Late Cambrian–Early Ordovician volcanic arc. The event began at the very end of Batyrbai – beginning of the Tremadoc stages and resulted in the formation of the subsea highs represented by small reefs (Tolstochikha Formation) on the remains of the volcanic arc. During Tremadocian irregular mosaic depositional environments were distributed in Altai–Salair–Kuznetsky basin. They could be regarded as a row: 1) nearshore– shallow water, including ones near large rivers deltas, coarse–grained; 2) closed to the shore part of the inner shelf, terrigenous–carbonate; 3) back–arc fine terrigenous; 4) volcanic islands, volcanic–terrigenous; 5) seamounts, carbonate–reefy; 6) open basins (oceanic), volcanic–siliceous–terrigenous. Taxonomically rich trilobites associations with populations of high density, rare brachiopods, graptolites and conodonts inhabited the aquatorium (Decisions..., 1983; Stratigraphy..., 1984; Petrunina, 1990; Sennikov, 1994; Petrunina et al., 2001).

Two orogenic events occurred in Late Tremadocian – Early Arenigian and Early Llanvirnian. They were associated with a joining of the volcanic arc with the Siberian continent (Gorny Altai, Salair, Kuznetsky Alatau). Basal conglomerates with unconformity overlaid different Cambrian horizons occurred in the basement of Arenigian in Gorny Altai (Voskresenka and Tuloy Formations), Salair (Karastun Formation) and Kuznetsky Alatau (Vasil’evka Formation). In the lower part of the Middle Ordovician almost in all Altai–Salair basin the single momentary orogenic stage is established. It is characterized by presence of the short–time coarse–grained sediments – conglomerates, gravelstones, coarse–grained sandstones from the basal beds of the Zaichikha, Bugryshikha, Sumulta, Karasa Formations (Decisions..., 1983, Sennikov et al., 2000c).

Deep and extent regression at the Tremadocian/Arenigian boundary occurred not only in the Altai–Salair–Kuznetsky basin as a marginal sea of the Siberian Continent. This regression is well fixed in many other paleobasins, for example in epicontinental basin of the Baltoscandia (Dronov et al., 2002).

In the Arenigian transition from the active to passive continental margin stage occurred. Only in the Kuznetsky Alatau conditions succeed from Tremadocian active margin preserved and volcanic activity prolonged (Alzass Formation). All other territory of the Altai–Salair–Kuznetsky basin (Gorny Altai, Salair) become the united amagmatic shelf paleobasin (Voskresenka, Tuloi, Ilovaty, Izyrak Formations) (Decisions..., 1983). In the Arenigian Altai–Salair–Kuznetsky basin facial depositional environments were not mosaic as at previous stages, but they were grouped into linear zones as regular sub–parallel belts: 1) near–shore coarse terrigenous, 2) inner shelf terrigenous, 3) seamounts thin fine terrigenous, 4) slope fine terrigenous olistostrome, 5) open–basin (oceanic) volcanic–siliceous–terrigenous.

Arenigian was transitional for the faunal association as well (Stratigraphy..., 1984). Mainly pelagic graptolites assemblages inhabited shelf of the paleobasin, represented by taxonomic reach populations with huge number of specimens. Isolated chitinozoan taxa occurred sporadically. Arenigian benthic associations were comparatively scanty and contained trilobite assemblages (usually 3–5 taxa) and brachiopods (2–3 taxa). Ocean was populated by pelagic conodonts, radiolarians, and isolated benthic siliceous sponges with medium density and low taxonomic diversity (Sennikov et al., 2002b).

1-4 – deep-seated faults: 1. suture zones, 2. zones of the main strike-slip faults, 3. fault-thrust zones, 4. large tectonic zones.

Change in the complexes of benthic groups (brachiopods and trilobites) is noted after the event on the Early and Middle Ordovician boundary. Mentioned complexes are characterized by wide taxonomic diversity and increase of the presence endemics together with cosmopolitan taxa. Benthic populations reach their highest density. Pelagic groups (graptolites and chitinozoans) are represented by 2–3 taxa, with extremely low population density.

In the Llanvirnian, Caradocian and Early Ashgillian time the depositional environments facies have been changed. Among them found: 1) near–shore coarse terrigenous, 2) inner shelf terrigenous, rarely carbonate–terrigenous, 3) seamounts thin fine carbonate–bioherm, 4) slope fine terrigenous flysch.

Beginning with the Llanvirnian Gorny Altai and Salair paleobasins were inhabited by the all–possible varieties of marine organisms – from nektonic and planktonic to benthic. The spatial regularities in the distribution of the faunal associations in the extremely shallow–water and comparatively deep–water conditions become noticed.

In the Middle Ordovician Gorny Altai basin presence of the oolitic limestones should be noted in the basement of the Khankhara Formation. This is the evidence of the large–scale straightening of paleobasin bottom relief with lack of the vertical differentiated movements of the single parts and its general shallow–water conditions with depths up to base of the regular waves (to 10 m). In the composition of the benthic associations this event resulted in comparatively sharp renovation of the composition within trilobites, brachiopods and reef–dwellers and reef–builders appeared (algae, bryozoans, tabulates). Benthos population density was high, and taxonomic diversity considerably high. Pelagic associations (graptolite, chitinozoans) were extremely rare at that time, with low population density and taxonomic diversity no more than 2–5 taxa.

At the end of Ordovician (Late Ashgillian) the huge carbonate platform (Orlov and Veber Formations) occurred at the inner shelf margin, Gorny Altai and Salair. It was formed on the seamounts and become the last structure of the shelf paleobasin. Reef systems started forming systematically on the carbonate platform. Those stable structures reached the sea level, withstood currents and storms and had influence on the character of sedimentation as well as on composition and structure of the faunal associations. Constant in time broad facial zones (belts) with distinct margins: 1) near–shore– shallow water coarse–grained terrigenous, 2) back–reef fine terrigenous, 3) inner slope of carbonate platform, terrigenous–reefy, 4) central part of the carbonate platform, carbonate–reefy, 5) outer slope of carbonate platform terrigenous–carbonate, 6) slope terrigenous–carbonate, olistostrome; and characterized by specific sedimentation started forming within the paleobasin.

Taxonomically diverse benthic, plankton and nekton fauna inhabited the Altai–Salair Late Ordovician basin. Population density is also high, with complicated symbiotic relations.

The following characteristics have been noted for the Middle Ordovician Altai–Salair basin: a) high density of the populations and taxonomic diversity, b) wide range of different organisms, c) presence of the reef–dwellers and reef–builders, d) reach plankton (conodonts, graptolites, radiolarians, chitinozoans), e) few endemics, f) polytaxa associations prevail. These are typical characteristics for the tropical paleobasins.

In the Middle Ordovician, volcanic activity and several orogenic phases took place in Tuva. Only local (Mugur–Aksy Formation) populations of the pelagic radiolarians were developed during the active volcanic activity.

During Ordovician four well–distinct regionally coarse–grained (varicolored and grey–colored conglomerates) strata that are the evidence of the orogeny were found in the Tuva basin (see Figure). They are: 1) from the lower part of Arenigian, 2) boundary Arenigian–Llanvirnian, 3) boundary Llanvirnian–Caradocian, 4) from the lowermost Ashgillian (Decisions..., 1983; Sennikov et al., 2000a). The shape and area of the Tuva Ordovician basin were considerably changed by every orogeny event. Facial environments possessed mosaic and irregular structure. The following conditional row is proposed: 1) near–shore coarse–grained terrigenous, 2) back–arc terrigenous, rare carbonate–terrigenous, 3) volcanic islands, volcanic–siliceous–terrigenous. Only the spot–like populations represented by the monoplacophors, lingulids – marine inarticulate brachiopods, gastropods and organisms with unknown systematic position inhabited those unstable waters with deviate from the normal marine conditions. Other fauna like trilobites, brachiopods, crinoids, bryozoans, gastropods, also single pelagic graptolites and chitinozoans and reach conodont associations were found only from the carbonate–terrigenous type of the back–arc sections (Tarlyk Formation) (Sennikov et al., 2000b; Sennikov et al., 2002). The Ordovician endemics associations of the Tuva basin reach up to 50–90%.

The following characteristics were established in the composition and structure of the Ordovician and Silurian of the Tuva faunal associations: a) low number and scanty of the taxonomic diversity, b) generally monotonous fauna, c) lack of the reef–builders and reef–dwellers, d) scanty of the plankton, e) prevalence of the endemic associations of low rank organisms, f) mainly monotaxa faunal associations. By mentioned characteristics Tuva paleobasin is assigned to the moderate climatic belts basins of the middle latitudes.

In the Middle Llanvirnian local orogeny related to the collision of the Tuva–Mongolian microcontinent and the West Sayan volcanic arc took place in West Sayan. Specific sedimentary palebasin was formed simultaneously to this stage of orogeny in the West Sayan. The marine olistostromes were found from there (Manchurek Formation) (Sennikov et al., 2000d). Faunal associations of the benthic organisms possess very scanty taxanomic diversity, represented only by two or three groups of organisms (brachiopods, crinoids, bryozoans). Very few pelagic organisms (conodonts) were found.

Studies were supported by the RFBR Grant Nº 02–05–64789.