Svetlana O. Zorina

Euxinia as a dominant process during OAE1a (Early Aptian) on the Eastern Russian Platform and during OAE1b (Early Albian) in the Middle Caspian

A detailed study of the mineral composition and microstructure of the black shales associated with OAE1a (Eastern Russian Platform), OAE1b (Middle Caspian), and the host rocks has been carried out using X-ray diffraction, scanning electron microscopic, and microprobe analyses. The results provide important constraints for depositional environments in the sedimentary basins. Black shales with pyrite framboids imply euxinic (sulfidic) conditions with increased organic matter preservation. Disintegrating framboids suggest partial or complete dissolution of the organic matter inside the framboids due to increasing water oxygenation. OAE1a on the Eastern Russian Platform is heterogeneous as it includes thin interbeds of concretionary coccolith limestones within the interval of bituminous shales, and correlates with the Lower Aptian Rhagodiscus angustus nannofossil zone. The coccolith limestones indicate short intermittent episodes of interrupted stagnation, rapid oxygenation, and restoration of normal marine conditions. The presence of montmorillonite, albite, microcline, and diopside in the bituminous siltstones and in the host siltstones of OAE1a on the Eastern Russian Platform, as well as a high content of titanium in the black siltstones correlated with OAE1b in the Middle Caspian allow proposing significant input of pyroclastic material into the extant sea.

New palynological data for Toarcian (Lower Jurassic) deep-marine sandstones of the Western Caucasus, southwestern Russia

Abstract Information on Jurassic palynomorphs from the Greater Caucasus is potentially of great importance, but its availability to the international research community is severely limited. New palynological data for Toarcian deposits of the Western Caucasus are recorded in the present paper. Particularly, dinoflagellate cysts are described for the first time from the Bagovskaja Formation; palynomorphs are found in sandstone levels within this unit. The most representative assemblage includes pollen (with predominant bisaccate pollen), spores (Cyathidites being commonest), and dinoflagellate cysts amongst which the predominant taxon is Nannoceratopsis spiculata. The dinocyst assemblage implies a late Toarcian age for the upper part of the Bagovskaja Formation. On the basis of these new palynostratigraphical results, the range of the formation is extended; previously, only the lower part had been dated on ammonite evidence.

An Event-Based Depositional Model for the Paleocene–Eocene Sandy–Clayey–Siliceous Sequence of the Russian Platform

An event-based depositional model for the Paleocene—Eocene sandy–clayey–siliceous deposits of the Russian Platform was proposed. The model was based on pulsational input of pyroclastic material and intrusion of sandy injectites. These processes should be taken into account to identify the stratigraphic position of the Paleocene–Eocene lithostratigraphic units in the eastern, southeastern, and southern parts of the Russian Platform.

Mineralogical composition of the Lower and Upper Kazanian (Mid-Permian) rocks and facies distribution at the Petchischi region (Eastern Russian Platform)

The mineral composition proportions of carbonate rocks of Kazanian (Mid-Permian) age in the Petchischi region (eastern part of the Russian Platform) was identified by X-ray powder diffraction, ICP-MS and optical microscopy. The Lower Kazanian deposits are presented predominantly by bio-dolomicrites with changing terrigenous component and the lack of gypsum-bearing layers in the succession. Dolomicrites are prevalent in the Upper Kazanian succession, which is composed of alternation of gypsum-bearing dolomites, clayey dolomites and pure dolomites. The discovered bentonite-bearing component in marls and bentonite clays are proposed as evidence of volcanic activity in the Urals in the Kazanian stage. Two marine facies on the Eastern Russian Platform in the Kazanian: peritidal shallow flat and coastal sabkha agree well with the trends of δ18O and δ13C ratios.

Mesozoic Platform Megasequences in the Eastern Part of the Russian Plate

On the basis of investigations of the sequences and chronostratigraphy of the Middle Jurassic–Lower Cretaceous deposits in the eastern part of the Russian Plate, five megasequences are identified: the upper Bajocian–Callovian, Oxfordian–Berriasian, Valanginian, upper Hauterivian–Aptian, and Albian megasequences. These were formed under stable tectonic–eustatic conditions (without long breaks in sedimentation). The megasequences are characterized by peculiar sediment compositions: mineral assemblages are specific to individual horizons, units, or the entire megasequence, and they include a specific complex of solid mineral resources.