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Foraminiferal zonation and Upper Jurassic of Caucasus

Vuks V.

Volumina Jurassica

2006

Vol. 4, no. 1

270-271

In the last century there were only two regions in Caucasus, which possessed the schemes of the foraminiferal zonation for the Upper Jurassic - Kabarda-Dagestan paleotectonic zone (central and eastern part of the Northern Caucasus) of Russia and Tsesi-Korta paleotectonic zone of Georgia. Moreover, there were two local foraminiferal zones distinguished in the Upper Jurassic of the Western Caucasus: the Spirillina cf. kuebleri local zone (lower part of the Upper Tithonian) established in the western part of the Abino-Gunajskaya paleotectonic zone, and the Marssonella doneziana-Ophthalmidium strumosum local zone (Oxfordian-Kimmeridgian) - in the Laba paleotectonic zones. Several foraminiferal assemblages were described in the Novorossijsko-Lazarevskaya, Abino-Gunajskaya and western part of the Lagonaki barrier reef paleotectonic (Lagonaki zone) zones. In the last years the foraminiferal zonation for the Upper Jurassic of the eastern part of the Lagonaki zone (Western Caucasus) was proposed. In these deposits there are two local foraminiferal zones: the Alveosepta jaccardi (Oxfordian-Kimmeridgian) zone, and the Melathrokerion spirialis (Tithonian) zone. Besides, analysis of the foraminiferal distribution in the Upper Jurassic of the Western Caucasus makes possible recognition of the forminiferal associations and introduction the names (typical species) for each association. In general, the Alveosepta ukrainica assemblage is characteristic for Oxfordian-Kimmeridgian of the southern part of the Novorossijsko-Lazarevskaya zone, eastern part of the Abino-Gunajskaya zone and western part of the Lagonaki zone. The Anchispirocyclina jurassica - Lenticulina ponderosa assemblage is typical for Tithonian of the northern part of the Novorossijsko-Lazarevskaya zone, eastern part of the Abino-Gunajskaya zone and western part of the Lagonaki zone. Therefore these names were proposed for the local foraminiferal zones corresponding to stratigraphical interval, where these associations did occur. In the Upper Jurassic of the Western Caucasus there is a preliminary biostratigraphical scheme, where two successions of the local foraminiferal zones are distinguished: the Alveosepta ukrainica(?) zone (Oxfordian- Kimmeridgian), and Anchispirocyclina jurassica - Lenticulina ponderosa zone (Tithonian) for the most part of the Western Caucasus, as well as the Alveosepta jaccardi zone (Oxfordian-Kimmeridgian) and the Melathrokerion spirialis (Tithonian) zone for the eastern part of the Lagonaki palaeotectonic zone. The boundaries of the foraminiferal zones for the Upper Jurassic of the Western Caucasus and Kabarda-Dagestan zone are not strictly corresponding, but the only one boundary between the local foraminiferal zones (corresponding to the base of the Lower Oxfordian) in the biostratigraphical schemes of the latter paleotectonic zone and the Tsesi-Korta zone of Georgia. The Late Jurassic foraminiferal assemblages of the Kabarda-Dagestan and Tsesi-Korta zones are much closer each other, than associations from the Kabarda-Dagestan area and Western Caucasus. The peculiar evolution of the foraminifers in the different areas strictly corresponds to the development of the Caucasus palaeobasins. The Western Caucasus assemblages are generally very similar and easy for correlation. But in the Lagonaki barrier reef zone and Abino-Gunajskaya zone there occur very specific facies and foraminiferal communities. So, the successions of the foraminiferal assemblages in the particular regions of Caucasus are not the same, because the development of foraminifers was controlled by environmental conditions.