The graptolite Expansograptus hirundo zone in the Gorny Altai sections

Bukolova E.V.

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

Bukolovaev@ipgg.nsc.ru

During the 70’s of the last century important collections of Ordovician graptolites were gathered in the north-eastern part of the Gorny Altay region. In recent years re-investigation of Ordovician sequences add new data on graptolites that led to ratification of the complete graptolite zonation for Gorny Altai, including the Expansograptus hirundo Zone first defined in 1978 (Obut, Sennikov, 1978). Graptolites and their distribution in the E. hirundo zone were investigated during 2007-2011 in three sections geographically distant from each other: “Maralikha”, “Tuloy” and “Pridorozhnyi”.

In the “Maralikha” section 6 levels characterized by graptolites were established for the E. hirundo Zone. Among them 9 genera and 10 species - Isograptus caduceus imitatus Harris, Acrograptus cognatus (Harris et Thomas), Glossograptus ex. gr. acanthus Elles et Wood, Loganograptus logani (Hall), Expansograptus extensus (Hall), Expansograptus hirundo (salter), Expansograptus suecicus suecicus (Tullberg), Expansograptustaimyrensis Obut et Sobolevskaya, Pseudotrigonograptus angustus Mu et Lee, Undulograptus sinodentatus (Mu et Lee), Cardiograptus sp., Glossograptus sp., Pseudoclimacograptus sp. Absent in other sections Undulograptus sinodentatus (Mu et Lee) is rather abundant (12 specimens) in this section. All other taxa are represented by 1-2 specimens. Comparison of taxanomic composition of graptolites from three mentioned above sections revealed that those recovered from the “Maralikha” section differ from graptolite fauna of two other sections.

In the “Tuloy” section 19 levels characterized by graptolites were found. Among them 16 genera and 29 species - Acrograptus nicholsoni (Lapworth), Corimbograptus deflexus (Elles et Wood), Isograptus paraboloides Tzaj and others. Most abundant in the E. hirundo Zone are species belong to Expansograptus Boucek et Pribyl – more than 50 specimens and Isograptus Moberg – more than 20 specimens.

In the “Pridorozhnyi” section 5 levels with graptolites were found in the E. hirundo Zone. Among identified 5 genera and 10 species are Expansograptus suecicus suecicus (Tullberg), Isograptus gibberulus (Nocholson), Corymbograptus sp. and others. All taxa recovered from this section were found in the “Tuloy” section (abundant) and the “Maralikha” section (single). In this section most representative for the E. hirundo Zone are species of Isograptus Moberg – more than 50 % of total number.

Analysis of taxanomic composition of graptolites from all three sections revealed that association of the “Tuloy” section includes taxa from both other sections. On the other hand in the “Pridorozhnyi” section was identified Isograptus primulus Harris absent in the other two sections. More than half of taxa found in the E. hirundo zone in the "Maralikha" section do not match those found in the other two sections.

It could be supposed that the "Maralikha" section (located in the western part of Gorny Altai) contains almost completely different graptolite association due to the geographic distance separated it from the other two sections, located in the northeastern part of Gorny Altai. This fact suggests that the Eastern and Western parts of the Northern Altay Mountains have been different sedimentary basins during the Middle Ordovician.

The study was supported by RFBR grant 11-05-0055; Contribution to IGCP 591.