Epifaunal worm tubes on Lower Jurassic (Lower Lias) ammonites from Dorset

Over 40 ammonites, mostly Promicroceras, with epifaunal worm tubes are described from the Lower Jurassic, Charmouth Mudstone Formation (Lias Group) of Dorset. Serpulids that were overgrown by the ammonites or responded to the ammonites’ growth attached to juvenile, living ammonites. Some epifaunal serpulids attached post-mortem, indicating oxygenated bottom water, which was rare in the lower Charmouth Mudstone Formation. Other serpulids do not conform to either pre- or post-mortem growth predictions and require individual assessment. The commonest pattern of growth for serpulids on live juvenile ammonites was attachment in the umbilical seam, with later growth onto, and finally around, the venter. Reconstructing this pattern shows that serpulids kept their aperture at 6 o’clock with respect to the orientation of the living ammonite (105-115° behind the ammonite aperture) throughout life. Reorientation of growth lines in serpulid worms just before the aperture suggests some worm tubes were fully grown. The 6 o’clock position of the aperture enabled feeding currents generated by the worms to parallel currents generated by swimming ammonites, thus maximizing food gathering and confirming that ammonites swam backwards. The mid-ventral position enabled the worm to deploy its branchia on both sides of the ammonite. Growth on ammonites was beneficial to the worms, but parasitic to the ammonites. Promicroceras with epifaunal worm tubes died at smaller sizes than unencumbered examples and size at death correlates inversely to extra weight of worm tubes. Uniformitarian comparisons suggest fossil serpulid worms grew in one season and that Promicroceras reached full size in two or three years.     

Biofacies of Upper Jurassic and Lower Cretaceous sediments of central West Siberia

Paleontological study of Upper Jurassic and Lower Cretaceous sediments recovered by boreholes in the Agan-Vakh and Nadym-Vengapur interfluves clarified environments of their deposition. As is shown, influx of siliciclastic material to central areas of the West Siberian sea basin varied through time. Taxonomic composition and ecological structure of nektonic and benthic fossil assemblages are analyzed and considered in terms of environmental factors such as hydrodynamics, aeration, temperature, and salinity of seawater.     

Lower Jurassic palynostratigraphy of Eastern Siberia

This paper presents the results of a palynological study of the natural outcrops of the Lower and Middle Jurassic on the Kelimyar River (Outcrops 5, 6, 7, 14, and 16) and two boreholes, Middle-Nakynskaya and Ygyata-Tyungskaya 1, in Eastern Siberia. The Ukugut, Tyung, Motorchuna, Suntar, Kyrin, and Kelimyar formations were studied. Six biostrata with dinocysts were established for the Upper Sinemurian–Toarcian. The analysis of the stratigraphic ranges of dinocysts in the sections allowed the identification of the stratigraphic ranges for important genera of dinocysts, which enabled the updating of the geochronology of some biostrata. Three main stages of the evolution of dinocysts in the Early Jurassic in the Siberian Paleobasin were characterized by the appearance, diversification, and disappearance of some orders of dinocysts at certain times. Seven biostrata (beds with characteristic palynoassemblages) were established for the upper Sinemurian–Toarcian on the basis of the study of the taxonomic composition of spores and pollen of land plants and successive changes in the composition of the palynospectra.     

Palynostratigraphy of the Lower-Middle Jurassic deposits,borehole section Vostok 4 (Southeast of West Siberia)

This work presents the results of palynological investigation of the Vostok 4 borehole section situated in the Azharma region of the Ob-Irtysh facies area, where drilling has penetrated through the Urmanskaya, Ilansk, Peshkovskaya, and Tyumen formations. Six palynoassemblages are distinguished in the section based on the analyzed distribution of spores, pollen of terrestrial plants, and microphytoplankton. According to their succession, the section is subdivided into biostratigraphic units ranked as beds with palynomorphs. Comparative analysis and correlation with the Siberian palynostratigraphic standard scale of the Lower-Middle Jurassic, where succession of palynoassemblages is coordinated with ammonite zones, substantiate the Pliensbachian-Bajocian age of the deposits studied. Diverse microphytoplankton (dinoflagellate cysts, acritarchs, prasinophytes, zignematacean and colonial algae) occurs throughout the section. The facies environment of sedimentation is reconstructed based on quantitative proportions of microphytoplankton taxa.     

Callovian and Upper Jurassic foraminiferal and ammonite biostratigraphy of the Shaim petroleum region (West Siberia)

The results of biostratigraphic analysis of foraminiferal assemblages were used to identify a nearly complete succesion of biostratigraphic units in the Callovian-Upper Jurassic section of the Shaim region. The finds of the ammonites from different parts of the Callovian-Volgian sections helped to correlate the identified foraminiferal biostratigraphic units to the General Stratigraphic Scale. Analysis of variations in the taxonomic composition and structure of foraminiferal assemblages allowed refinement of paleontological characteristics of the foraminiferal biostratigraphic units for different facies conditions in the southwest of the West Siberian Basin.     

Problems of phytostratigraphy and the correlation of the Lower Jurassic continental sediments in West Siberia and Kuznetsk and Kansk-Achinsk basins

Paleofloral and palynological records of Lower Jurassic sediments in West Siberia, Kuznetsk (Kuzbass), and Kansk-Achinsk basins and their correlation are discussed. In a number of recent papers dedicated to the Jurassic stratigraphy of Siberia this problem is ambiguously treated. The reference palynological scale has been developed for the Jurassic West Siberian sediments and an uninterrupted succession of floral assemblages associated with it and with regional stratigraphic units has been recognized. On this basis the scheme of the correlation between the Lower Jurassic sediments of the Kansk-Achinsk and Kuznetsk basins and West Siberia permitting a better age estimate of coal-bearing deposits, is proposed.     

Paleomagnetic investigations of Jurassic and Lower Cretaceous sedimentary rocks of Siberia

The paper contains recent information on the history of the earth's magnetic field in the Jurassic and Lower Cretaceous, obtained in paleomagnetic exploration of the key paleontological sections of sea deposits in the Bay of Anabar and Khatanga basin. Six paleomagnetic zones in the Valanginian were found for the first time, as well as alternation of paleomagnetic zones in Middle and Lower Jurassic rocks. Comparison of paleomagnetic sections of different regions indicates inversions of the magnetic field of the earth. The pole position was calculated for the Lower Valanginian: λ = = 174° E; Φ = 63.2° N. Possibility is seen for the use of rocks possessing high magnetic viscosity and low values of Q and of the collapse field, for paleomagnetic purposes. – Authors.     

Temperature monitoring in Bakchar bog (West Siberia)

We report the results of continuous temperature monitoring (812 days, from 28 June 2005 to 26 September 2007) in a 80 cm layer of peat soil in Bakchar bog (West Siberia), at sampling rates of 60 min in wintertime and 15 min in summertime. Both annual and daily temperature patterns are controlled by water table position and weather conditions. Wintertime soil temperature patterns are disturbed by the formation of a seasonal frozen layer with its thickness (freezing depth) depending on the time when steady snow cover sets up and on soil moisture. During the period of frozen layer thawing, the temperature of peat becomes sensitive to peat moisture and water table position as well as to the air and peat surface temperature. The warm-season soil temperature patterns bear effects of peat warming by rainwater percolation, both in nightand daytime. The patterns with soil warming during rainfall and phase change during seasonal freezing-thawing cycles record disturbances to conductive heat transfer.     

First findings of dinocysts in the upper Oligocene Turtas formation in Southern Tyumen oblast (West Siberia)

A new occurrence of dinocysts Pseudokomewuia in continental Cenosoic (Oligocene–Miocene) rocks of the West Siberian plain is identified. A complex of palynomorphs with dominant microphytoplankton (dinocysts) was found for the first time in the lacustrine-marsh sediments of the south of Tyumen oblast. The variable composition of the complex with Pseudokomewuia aff. laevigata He and Pseudokomewuia aff. granulata He species is traced upward through the section. The layers that correspond to the maximum abundance of Pseudokomewuia (akme) are distinguished and correlated with sections studied previously, which are located in the Barabinsk lithofacial region. Sediments with dinocysts are compared with sediments of the same age from North America, Northern Europe, and Southern China.     

The upper Volgian Substage in Northeast Siberia (Nordvik Peninsula) and its Panboreal correlation based on ammonites

Section of the middle and upper Volgian substages and basal Boreal Berriasian in the Cape Urdyuk-Khaya (Nordvik Peninsula) is largely composed of dark argillites substantially enriched in C_org. Characteristic of the section is a continuous succession of ammonite, foraminiferal, ostracode, and dinocyst zones known also in the other Arctic areas. Boundaries of the upper Volgian Substage are recognizable only based on biostratigraphic criteria. The succession of the middle Volgian Taimyrosphinctes excentricus to basal Ryazanian Hectoroceras kochi zones is characterized. The range of the substage is revised. The lower Exoticus Zone, where ammonites characteristic of the Nikitini Zone upper part in the East European platform have been found, is referred to the middle Volgian Substage. Newly found ammonites are figured. Two possible positions of the Jurassic-Cretaceous boundary in the Arctic region, i.e., at the lower and upper boundaries of the Chetae Zone at the top of the upper Volgian Substage, are discussed.     

Sedimentation models for the Jurassic stratum in the Potanai oilfield (Western Siberia)

The sedimentation models for the upper part of the Tyumen Formation (the J_2?C3 productive layers) and the Abalak Formation (J ₁ ² and J ₁ ¹ ) were constructed on the basis of data from lithological studies of Jurassic stratum cores (the Potanai oilfield in the northeast Shaim megaswell). The models show that the sedimentation environment sequentially changed from the continental to the deltaic, lagoonal, and shelf types through the Middle and Late Jurassic.     

Vendian of the Fore-Yenisei sedimentary basin (southeastern West Siberia)

Fossiliferous Upper Vendian strata are discovered in the Upper Proterozoic to Lower Paleozoic Fore-Yenisei sedimentary basin under a thick Mesozoic-Cenozoic cover in southeastern West Siberia. Two depositional systems are recognized based on sedimentological features: (1) wave- and current-agitated shoreface-forereef-biohermal reef system (Vostok-3 Borehole section) and (2) tidal flat-evaporite basin (Averinskaya-150 Borehole section). The forereef facies yielded fossilized tubular calcareous skeletons of reef-building metazoans Cloudina riemkeae, Cloudina hartmannae and Cloudina carinata, phosphatized Namacalathus-like fossils, and a diversity of tubular phosphatized and agglutinated tubular fossils. The fossil assemblage can be interpreted as the evidence of ecological complexity of the reef system. Paleontological characteristics suggest correlation of the Vendian strata with the lowermost Purella antiqua Assemblage Zone and the boundary interval with the underlying Anabarites trisulcatus Assemblage Zone of the Siberian Platform. Therefore, at least in the late Proterozoic, the Fore-Yenisei sedimentary basin was part of a larger pericratonic depositional system on the western margin of the Siberian Craton.     

A lithofacies description of Jurassic sediments in the south of the Predyenisei petroleum subprovince,West Siberia

We have studied Jurassic sections in the Predyenisei subprovince of the West Siberian petroleum basin, which were penetrated in the Vostok-1, Vostok-3, and Vostok-4 stratigraphic wells. The Urman, Togur, Ilan, Peshkovo, Tyumen’, Naunak, and Mar’yanovka Formations are described from a detailed comprehensive core analysis and log data. The depositional environment for these sediments was predominantly continental. There is evidence for short transgressions in the Ilan (Lower Toarcian) and Peshkovo (Upper Toarcian) Formations, as well as the Upper Urman (Upper Pliensbachian) and the Upper Tyumen’ (Bajocian) Subformations. In the Upper Naunak Subformation (Oxfordian), there was a change of facies from continental to littoral continental and littoral marine. The Mar’yanovka Formation developed in normal marine shallow- or moderately deep-water environments. Although good reservoirs are common throughout the Jurassic section in the southeast of West Siberia, only small, lithologically screened deposits are predicted here.     

A revision of some British Lower Bajocian stephanoceratid ammonites

Members of the ammonite family Stephanoceratidae from the Middle Jurassic, Lower Bajocian, Laeviuscula to Humphriesianum zones of South West England are revised. These comprise faunas resulting from periodic migrations and possibly hybridization. The taxonomic positions of Teloceras Mascke, 1907, Kumatostephanus Buckman, 1922, Gibbistephanus Buckman, 1928 and Pseudoteloceras Pavia and Fernández-López, 2016 are considered in the light of new research. A new subfamily Kumatostephaninae is erected and hypotheses are presented regarding the evolution of some Early Bajocian stephanoceratids. Pseudoteloceras digbyi sp. nov. represents the earliest ‘Teloceras-like’ morphology from the late Sauzei Zone. Refinement is made of the scheme of faunal horizons for the Lower Bajocian.     

Platinum group elements in Permo-Triassic volcanics in West Siberia (the first data)

The first data on PGE contents in the volcanic rocks of the West Siberian Plate are presented. Analysis has shown that most of the studied rocks have clarke contents of these elements. Rocks from the central areas of paleorift valleys are enriched in ΣPGE (2.0–32.0 ppb), particularly in Pt (0.1–24.2 ppb) and Pd (0.3–8.0 ppb), which might be related to the action of plume. The magmatic PGE pattern confirms the earlier conclusions about the mantle genesis of the studied rocks.