Palaeoclimate indicators (clay minerals,calcareous nannofossils,stable isotopes) compared from two successions in the late Jurassic of the Volga Basin (SE Russia)

A study of clay mineral and calcareous nannofossil abundances in late Jurassic–early Cretaceous sediments from the Volga Basin, SE Russia, is presented. From these results, we are able to compare some general patterns of mineralogical and palaeontological change for the Volga Basin to the palaeoclimate models developed for northern Europe and beyond. The two successions examined comprise calcareous mudstones with black organic‐rich shale horizons, overlain by a series of phosphatic silty sands. Clay mineralogical results show a progressive decrease in kaolinite and the concomitant increase of smectite and illite through the middle Volgian, followed by an abrupt increase in kaolinite in the late Volgian. The clay mineral evidence suggests increasing aridity at the end of the Jurassic, similar, in part, to many western European successions. Because of differential settling of clay minerals, superimposed upon this possible climatic signature is likely to be the effect of relative sea‐level change. Calcareous nannofossil analysis from a single section reveals a shift through the middle Volgian from low nutrient, warm water assemblages dominated by Watznaueria to cooler surface water and high nutrient assemblages dominated by Biscutum constans. These observations suggest that increased aridity is also associated with climatic cooling. Black shales are associated with increased productivity, higher sea levels and increases in smectite content. Hence, periods of low (chemical) hinterland weathering during semi‐arid conditions are paradoxically associated with relatively nutrient‐rich waters, and organic‐rich shales. Comparison of published carbon and oxygen stable isotope results from this and other sections to the clay mineral and nannofossil data confirms the palaeoclimatic interpretation. This study significantly improves the published biostratigraphically constrained clay mineral database for this time period, because other European and North American successions are either non‐marine (and thus poorly dated), absent (through penecontemporaneous erosion) or condensed. Copyright © 2002 John Wiley & Sons, Ltd.     

A unique find of Hercoglossa pavlovi (Arkh.) (Nautiloidea) in paleocene sediments of the Volga region

A unique find of Hercoglossa pavlowi (Arkhangelsky) from Paleocene sediments of the Saratov Volga region is described. It is assumed that the shell was transported in the post-mortem condition from lower latitudes to the sea basin that covered the East European Platform at that time.     

The Early Cretaceous Anoxic Basin of the Russian Plate: Sedimentology and Geochemistry

The profile comprising a series of lower Aptian sections from Ul'yanovsk to Saratov in the Russian Plate has been studied. It is shown that the unit of organic-rich rocks is characterized by the lack of bioturbation and elevated concentration of many chemical elements. The petrography and geochemistry of organic matter (OM) indicate the prevalence of basinal OM in carbonaceous sediments, while continental OM dominates in host rocks. Sedimentological, biotic, and geochemical data testify to the deposition of organic-rich sediments under anoxic conditions. The anoxic environment in the Aptian basin of the Russian Plate correlates with the global OASE-1a anoxic event. The mechanism of Aptian carbonaceous sedimentation is discussed.     

Report on the 4th International Meeting of the IUGS Lower Cretaceous Ammonite Working Group, the “Kilian Group” (Dijon, France, 30th August 2010)

The 4th Kilian Group meeting (Dijon, France, 30th August 2010) focused on the Aptian and Albian Stages. For the Aptian, a two-fold division of the stage was adopted for the Mediterranean area with a boundary between the Dufrenoyia furcata and Epicheloniceras martini Zones. The main changes to the zonal scheme concern the Lower Aptian with: the introduction of a Deshayesites luppovi Subzone in the upper part of the Deshayesites oglanlensis Zone; the replacement of Deshayesites weissi by Deshayesites forbesi as new index-species of the second interval zone; the introduction of a Roloboceras hambrovi Subzone in the upper part of the D. forbesi Zone; and the subdivision of the D. furcata Zone into the D. furcata and Dufrenoyia dufrenoyi Subzones. For the Albian, the upper part of the Douvilleiceras mammillatum Zone (Lower Albian) is now characterized by a Lyelliceras pseudolyelli Subzone. The main amendments concern the Upper Albian. The base of this substage is defined by the base of the Dipoloceras cristatum Zone. Above it, the Upper Albian zonal scheme comprises in stratigraphic order the Mortoniceras pricei, Mortoniceras inflatum, Mortoniceras fallax, Mortoniceras rostratum, Mortoniceras perinflatum and Arrhaphoceras briacensis Zones.     

Thalassinoides burrows (decapoda dwelling structures) in lower cretaceous sections of southwestern and central Crimea

Burrows of Thalassinoides, which are attributed to the group of dwelling structures, occur in all the marine and coastal facies. The Lower Cretaceous sections of southwestern and central Crimea yielded the representative collection of Thalassinoides burrows belonging to the ichnospecies Th. suevicus (Rieth, 1932), which served as an object for this investigation. The burrows are confined to coarse-grained terrigenous, carbonate, and mixed sediments and contain assemblages of ichnofossils indicating coastal and shallow-water marine Skolithos and Cruziana ichnofacies. In the Mesozoic-Cenozoic, the producers of the Thalassinoides burrows were decapods, confirmed by finds of crayfish Hoploparia in them.     

Ammonites from the <Emphasis Type="Italic">Deshayesites</Emphasis> genus from Aptian (Lower Cretaceous) sediments in the Mountainous Crimea

Stratigraphically important ammonites Deshayesites ex gr. deshayesi (d’Orbigny, 1840), Deshayesites sp. juv., and Paradeshayesites aff. callidiscus [Casey, 1961] from lower Aptian pelagic sediments at the Verkhorech’e Village have been described and depicted. The new finds and revision of previously found ammonites allow the recognition of ammonite zones Deshayesites volgensis and Deshayesites deshayesi in the Southwestern Crimea.