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Nikolay Sennikov , Trofimuk Institute of Petroleum Geology and Geophysics SB RAS (Russian Federation)
Olga Obut, Trofimuk Institute of Petroleum Geology and Geophysics SB RAS (Russian Federation)
Elena Bukolova, Trofimuk Institute of Petroleum Geology and Geophysics SB RAS (Russian Federation)
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Identification of the boundaries of "new" Ordovician stages such as Floian, Dapingian, Darriwilian, Sandbian and Katian in different countries and geological regions is one of the important topics occurred after the new global standard set of chronostratigraphic subdivisions has been proposed for the Ordovician System. Allocation of such boundaries in every geological region would allow to determine chronostratigraphic position of the regional stages and designate the precise age of the local stratigraphic units - formations. Most preferable method of determination of such boundaries in the sections of the certain region is recognition of graptolite (or conodont) zones cognominal with zones indicated GSSP's for the stage boundaries.
Darriwilian is the first approved stage in the Ordovician standard of ISS. It was the base of the Darriwilian where the first GSSP for the Ordovician ISS was established (Chen, Bergstrom, 1995, Mitchell et al., 1997; Ordovician News, 1997; Chen Xu et all., 2006).
GSSP of the Darriwilian Stage lower boundary is proposed in the Huangnitang section, China (Zhang Yuandong et all., 2007). It coincides with the appearance of the graptolite zonal index-species Undulograptus austrodentatus (Harris et Keble). The austrodentatus zone in Huangnitang section is underlined by Exigraptus clavus zone, characterized by Expansograptus hirundo (Salter) and Undulograptus sinodentatus (Mu et Lee). The I. caduceus imitatus zone is conditionally recognized under Exigraptus clavus zone, though this level is represented by limestones with conodonts, and graptolites are absent. At the lower Darriwilian boundary in the Global Boundary Stratotype Section graptolites appeared from the underlined zones including different species of genera Cardiograptus Harris et Keble, Areniograptus Yu et Fang, Undulograptus sinodentatus (Mu et Lee) and some others are common along with the index-species Undulograptus austrodentatus (Harris et Keble) (Zhang Yuandong et all., 2007).
Key section "Maralikha" is situated in NW Gorny Altai, near Maralikha village. Voskresenka Formation is cropped out there. It is represented by sandstones, mudstones and siltstones with underwater "twisting" structures. The Maralikha section is subdivided into several beds with diverse graptolite fauna. The lower 1-3 beds are characterized by the angustifolius elongatus and gibberulus graptolite zones. Middle part of the section (4, 5 beds) coincides with I. caduceus imitatus zone, following by hirundo, Cardiograptus, sinodentatus zone. Base of the 6 bed is aligned with lower Darriwilian boundary, overlain by austrodentatus zone. The "Maralikha" section is proposed as a reference section for designation of the lower Darriwilian Stage boundary in Siberia.
This study has been supported by Russian Science Support Foundation grant 1 08-05-00729. Contribution to IGCP 503 Project.
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