Ludmila Kopaevich

Cretaceous palaeogeography of the North-Eastern Peri-Tethys

Abstract The Cretaceous stratigraphy and main palaeogeographic features of the North-Eastern Peri-Tethys are briefly summarised on the base of new data. The study is mainly focused on the time-slices that were chosen for the recently published Peri-Tethys Atlas: Early Hauterivian, Early Aptian, Late Cenomanian, Early Campanian and Late Maastrichtian. Two main epochs in the development of this area are recognised. The Early Cretaceous is characterised by the prevalence of the terrigenous sedimentation and the existence of a large longitudinal strait through the Russian Platform, which controlled the Boreal/Tethyan connection. The southward Boreal water movement prevailed during the Neocomian. The strongest Boreal transgression took place in the Late Hauterivian, when cool water reached the Crimea basin. The Aptian–Albian time was characterised by a northward Tethyan water movement, interrupted by the Early Albian Boreal transgression. Tectonic rebuilding of the region took place in the Albian–Cenomanian. It resulted in the disappearance of the sea-strait through the Russian Platform and in the opening of another sea-strait in the Turgai area in the Turonian. This strait connected the Peri-Tethyan seas, the Western Siberia Boreal basin and joined with the Western Interior Seaway in the other side of the Hemisphere. The Northern Hemisphere Megastrait appeared. During this time mainly carbonate sedimentation prevailed, but the existence of a new longitudinal strait determined a two-way water and faunal exchange. Tethyan water moved up to the south and the south-east parts of the Western Siberian basin, while the cold Boreal water influence can be recognised along the northern margin of the Russian Platform basin.

Cretaceous transgressions and regressions on the Russian Platform, in Crimea and Central Asia

Abstract This paper is a brief explanation of the diagrams of the Cretaceous transgressions and regressions on the Russian Platform, in the Crimea (Figures 1–3) and some regions of central Asia—the western flanks of the Tien Shan mountains, the Fergana basin, the Zeravshan-Gissar and Alaj mountains, and the Northern Pamirs (Figures 5–7). Internationally recognized stages are employed. They are interpreted by Sasonova (1967) for the Lower Cretaceous (K 1 ) of the Russian Platform, by Naidin (1977) for the Upper Cretaceous (K 2 ) of the Platform and the Crimea, by Djalilov (1971) and Pojarkova (1976) for the Upper Cretaceous of central Asia. General data on the stratigraphy of the Cretaceous of central Asia may be found in Anon 1977.

Shell morphology of cretaceous planktonic foraminifers as a means for paleoenvironment reconstructions

During the Cretaceous, there were several relatively short intervals with a relatively low oxygen content in the water column. They were named “Oceanic anoxic events.” These events changed essentially the characteristics of the water masses, which had a significant effect on the morphology of planktonic foraminifers. The proportion of taxa with elongated chambers of the last whorl periodically increased. This structural feature probably provided shells with certain advantage under nonstandard oceanographic conditions. The elongated chambers were probably advantageous for dwelling in conditions of a low oxygen content in the water column. The small size, extended last whorl chambers, and thin and light wall facilitated floating in the upper water layers, which are particularly rich in oxygen. This hypothesis is corroborated by widespread periodic lateral distribution of the morphotypes with extended chambers of the last whorl during rather short intervals with the low oxygen content in sea water, making them an important marker of paleoenvironments. Returning to the normal environmental parameters promoted an increase in the taxonomic diversity of planktonic foraminifers and appearance of new morphological features. At the same time, each of these events had certain distinctive features.

The planktonic foraminifera of the Jurassic. Part II: Stratigraphy, palaeoecology and palaeobiogeography

The stratigraphy, palaeoecology and palaeobiogeography are outlined for two genera and ten species of Jurassic planktonic foraminifera described in Gradstein et al. (Swiss J Palaeontol 2017. doi:10.1007/s13358-017-0131-z). The two genera are Globuligerina and Conoglobigerina. Globuligerina probably appeared in late Toarcian (late Early Jurassic) and Conoglobigerina first occurred in Middle Oxfordian (early Late Jurassic). Within the two genera ten species are defined, including from older to younger: Globuligerina dagestanica (Morozova), G. avariformis (Kasimova), G. balakhmatovae (Morozova), G. oxfordiana (Grigelis), G. bathoniana (Pazdrowa), G. jurassica (Hofman), G. oxfordiana (Grigelis) calloviensis Kuznetsova emended, G. tojeiraensis Gradstein, Conoglobigerina helvetojurassica (Haeusler), C. grigelisi Gradstein and C. gulekhensis (Gorbachik and Poroshina). Globuligerina balakhmatovae, G. oxfordiana and G. bathoniana are longer ranging, although there are subtle evolutionary trends in these taxa that may refine stratigraphic usage. Other taxa have shorter stratigraphic ranges. Using stratigraphic trends, test morphology and wall texture changes, the species are tentatively assembled in four evolutionary groups, termed A–D, and in six zones from Late Toarcian through Tithonian. The greater species diversity and possibly also specimen abundance took place in the Kimmeridgian, a time of high global sea level and greenhouse palaeoclimatic conditions. Jurassic planktonic foraminifera preferred marine continental margin conditions instead of the distal open ocean and spread from offshore Eastern Canada to offshore Western Australia along the margins of the Tethys Ocean. The group did not venture into austral or boreal realms, likely reasons why migration probably failed to the west coasts of South and North America.

Preface: The planktonic foraminifera of the Jurassic

Planktonic foraminifera originated in late Early Jurassic and, for reasons poorly understood, only underwent proliferation of species and geographic spreading from mid-Cretaceous onwards. Their evolutionary development in the oceans and adjacent seaways since early mid-Cretaceous has spurned a large volume of increasingly sophisticated literature. Specimens of this diverse group are now routinely subject to a wide variety of taxonomic, biostratigraphic, geochemical and molecular research. This microfossil group now plays a major role in studies on Cretaceous and Cenozoic palaeoclimatology and palaeoceanography. This global attention is in striking contrast to the interest and research effort put in their Jurassic and earliest Cretaceous forerunners.