Tabulate corals from Lower Silurian of the north-western Gorny Altai

Khabibulina R. A.

A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia,

khabibullinaRA@ipgg.nsc.ru

Collections of tabulate corals from three Early Silurian reef-complex sections in Gorny Altai were investigated for facies confinement. Silurian reefs are characterized by significant size (50-1000 m in diameter) and thickness (up to hundreds m). The largest among reef organogenic structures are elevated reefs consist of medium and large size reefs, with specific terrigenous-carbonate and carbonate-terrigenous facies of backreef basins and extended on tens of kilometers. Having linear length up to hundreds of kilometers such reef massifs could be 10-15 km in cross-section. Early Silurian carbonates in Gorny Altai are typical examples of reef massifs. Reefs and the adjacent backreefs are situated in 3-10 km from each other in the north-western part of Gorny Altai, near the Cherny Anui village.

Studied sections represent three Lower Silurian stratigraphic units: lower Telychian, upper Telychian and lower Sheinwoodian. Terrigenous Syrovaty Formation, reef carbonate and terrigenous-carbonate Polaty Formation, terrigenous with rare carbonate intercalates Chesnokovka Formation and reef carbonate Chagyrka Formation were investigated. Central part of reef is represented by massive bioherm limestones, marginal parts of reef – intercalation of clayey limestones and terrigenous rocks. In the backreef basin distant from slope base intercalation of limestones, sandstones and mudstones was observed.

In the central part of the reefal carbonate platform (Section Chichka) and in backreef facies (Section Mayak) the following tabulate corals assemblages were recovered: Mesofavosites dualis Sokolov, Favosites gotlandicus Lamark, Mesosolenia festivus Tchernychev, Halisites catenularius Linne, Cladopora seriata Hall, Striatopora flexuosa Hall, Catenipora escharoides Lamark and Subalveolitella repentina Sokolov. Population analysis for species Mesofavosites dualis Sokolov, Favosites gotlandicus Lamark and Cladopora seriata Hall was done. Therefore variations of quantitative characteristics including corallite and pores diameter as well as wall thickness were studied. According to typological species concept very often pores diameter is regarded as species or subspecies criteria. However the typological approach can not reveal all variability of tabulates. According to population analysis species are characterized by a direct correlation between the diameter of corallites and the pore diameter, diameter of corallites and wall thickness. These were proved by data on altaian tabulates Mesofavosites dualis Sokolov and Favosites gotlandicus Lamark.

Inner part of the reef (its base) facies could be observed in the Section Turata, near Cherny Anui village. The longest stratigraphic distribution was noted for populations of Mesofavosites dualis Sokolov, Favosites gotlandicus amark and Mesosolenia festivus Tchernychev. Populations of Cladopora seriata Hall and Parastriatopora rhizoides Sokolov have short stratigraphic distribution. Species Favosites gotlandicus Lamark was found from fine and coarse-grained, fine-clayey, pelitic, massive limestones and fine-grained carbonate sandstones intercalated with clayey limestones. Species Mesofavosites dualis Sokolov often occur in clayey and massive limestones, siltstones intercalated with carbonates, and in calcareous sandstones. Species Mesosolenia festivus Tchernychev most often present in clayey or massive limestones and fine-grained calcareous sandstones. Species Halisites catenularius Linne was obtained from clayey limestones and fine-crystalline detrital limestones, also occur in fine-grained calcareous sandstones.

In the central part of the reef carbonate platform tabulate corals were recovered as from limestones as from sandstones. In backreef facies tabulates were mainly found from limestones and rarely from mudstones. In the marginal part of the reef tabulates are mainly distributed in limestones, rarely in sandstones, mudstones and calcareous siltstones.

In the Section Chichka all tabulate assemblage ranges from underlying Polaty Formation to overlying Chesnokovka Formation. In the Section Mayak 8 species were defined in Polaty Formation and 4 of them range to Chesnokovka Formation. In the Section Turata 7 species were defined in Polaty Formation and 5 of them range to Chesnokovka Formation. And only 2 species Favosites gotlandicus Lamark and Cladopora seriata Hall range up to Chagyrka formation in the Section Turata.

Studied tabulate corals from reef complex achieve the highest diversity peak in the late Telychian, and only two species are known from Sheinwoodian. For tabulate corals central part of the reef facies are more favorable. In the marginal part of the reef they are found in terrigenous and carbonate rocks, but rarely. The greatest taxonomic diversity and rise achieved massive corals of the family Favositidae Dana, less diverse and rare are massive-branched representatives of family Pachyporidae Gerth.