Home – Home – Jornals – Russian Geology and Geophysics 2019 number 12

The history of Quaternary glaciation and postglacial uplift in Fennoscandia is considered in relation to the surface topography, gravity, and number of glacial deposits and is compared to the respective processes in North America. The surface topography and the gravity field are correlated over the Fennoscandian region as a whole and for two reference areas of South Norway and the Kola Peninsula. The gravity field is mapped as free-air and Bouguer gravity anomalies using composed global models based on satellite data. The impact of glaciation may be responsible for zoned patterns of both topography and gravity. The glaciation centers of Norway are marked by uplifts reaching 2470 and 1500 m and by circular Bouguer gravity lows of d-200 mGal, which correspond to residual crust thickening. The gravity patterns of the Kola Peninsula and Norway consist of circular and polygonal anomalies caused by both postglacial rebound and rock density variations. The general uplift and related extension of the crust led to the formation of fractures of different sizes, fr om 100-200 km long and 1-2 km deep fjords on Norway to 1-2 km long and 30-20 m deep local fractures in the Kola Peninsula. The gravity field of central North America is characterized by a generalized map of free-air anomalies within the limits of glaciation and a map of free-air anomalies compiled from the global database. Glaciation and its records in the surface topography and gravity patterns are described in more detail for Wisconsin state (USA), wh ere extension fractures are similar to those on the Kola Peninsula. The models explaining the correlation between postglacial uplift and gravity variations can be further updated using seismic and tectonic data, as it was shown previously for the case of Finland.

This is a review of previous works on glaciotectonics and recent epeirogenesis of the northern West Siberian sedimentary basin and adjacent parts of the Urals and the Siberian Craton. It is supported by the addition of detailed structural profiles of the disharmonic disturbances around Maly Atlym settlement on the Lower Ob’ never published before. The collected data highlight sources and results of neotectonic phenomena in the north of West Siberia and their impact on distribution of hydrocarbon deposits. The rugged topography of the northern Urals and Mid-Siberian Plateau, discordant with the regional tectonic structure, is generated by compensation uplifts along the margins of thick ice sheets in the West Siberian North. Ice load vacillations were an important factor of geographical separation of hydrocarbons liquid and gaseous phases. This is evident fr om the west-east zonation of petroleum deposits, discordant with the south-north strike of ancient structures but concordant with thickness zonation of ice sheets. The structure of the alpine-type disturbances penetrating up to 400 m into the sedimentary basin on the Lower Ob’ reveals that the variations of their tectonic style do not fit the mechanical properties of thawed Palaeogene rocks. However, they are more understandable assuming their origin from deviatoric stress in pressurized perennially frozen Palaeogene rocks. Such conditions are feasible at the base of a growing thick ice sheet. Ice sheets did significant work of glaciotectonic erosion of soft Tertiary rocks of perennially frozen substrate soldered with glacial ice. This type of erosion is evident in the Ob’ catchment area wh ere whole blocks of intact sand and clay were transported by glaciers over hundreds of kilometres. Glaciotectonic erosion of glacier substrate is sufficient for explaining the well-known stratigraphic hiatus between Quaternary and Upper Cretaceous formations of the Siberian Arctic instead of the popular but illogical tectonic inversion of the sedimentary basin.

This research attempted to reconstruct the sedimentary environment and depositional sequences of the Qom Formation in Central Iran, using biofacies and taphofacies analyses. The Qom Formation in the Andabad area (3º48ʹ12.6″N, 47º59ʹ28″E) is 220 m in thickness. The thickness of the Qom Formation in the Nowbaran area (35º05ʹ22.5″N, 49º41ʹ00″E) was found to be 458 m. In both areas, the formation consists mainly of shale and limestone. The lower boundary between the Qom and Lower Red Formations is unconformable in both areas. In the Nowbaran area, the Qom Formation is covered by recent alluvial sediments. In the Andabad area, the Qom Formation is unconformably overlain by the Upper Red Formation. A total of 122 limestone and 15 shale rock samples were collected from the Andabad area, and 94 limestone and 24 shale rock samples were collected from the Nowbaran area. Analysis of the collected samples resulted in the recognition of nine biofacies, one terrigenous facies, and five taphofacies within the Qom Formation in both areas. Based on the vertical distributions of biofacies, the Qom Formation is deposited on an open shelf carbonate platform. This carbonate platform can be divided into three subenvironments: inner shelf (restricted and semirestricted lagoon), middle shelf, and outer shelf. Two third-order and one incomplete depositional sequences were identified in the Nowbaran area, but in the Andabad area, three third-order and one incomplete depositional sequences were distinguished.

We present results of research into the tectonic, lithofacies, and geochemical formation conditions of the Erema-Chona oil and gas pool. We characterize present-day structures and their formation history, consider the composition, structure, formation conditions, postdepositional alterations, and porosity-permeability properties of the Osa, Ust’-Kut, Preobrazhenka, Erbogachen, and Upper Chona Horizons, assess the quality of the overlying seals, and describe the technique and results of quantitative estimation of the petroleum potential of seals. Modern technologies for the development of oil reserves are also presented, along with geological and economic assessment of hydrocarbon resources of the study object.

Within the city of Ulan-Ude, several sites of bastnaesite-fluorite rocks and calcite-containing rocks were found. They are confined to the exposures of Paleozoic schists and quartzites. The rocks have an age of 134.2 ± 2.6 Ma. They are brecciated lenticular and vein-like bodies cemented mainly with bastnaesite-fluorite aggregate. The content of fluorite in the rocks is several tens of percent, and the content of bastnaesite-(Ce) is 20-30%, often reaching 50%. Among the secondary minerals, there are monazite-(Ce), albite, and K-feldspar, and the accessory minerals are zircon, Nb-containing rutile, and manganilmenite. Light lanthanides are predominant among REE in the rocks. Bastnaesite and fluorite contain brine-melt fluid inclusions with homogenization temperatures of 490-520 ºC. The salts of these inclusions are composed of predominant Na and Ca sulphates and subordinate Ca and REE carbonates, and the gas phase contains CO₂. Gas inclusions and part of water-salt inclusions homogenized at 150-200, 290-350, and 430-450 ºC. The salts of late fluids are composed of Ca and REE carbonates, K and/or Na chlorides, Ca, Mg, and Fe hydrosulphates, and Ca and Na hydrocarbonates, and the gas phase contains CO₂ ± H₂. The isotopic compositions of carbon (-5.9 to -8.3‰ d¹³C_V-PDB) and oxygen (4.3 to 8.3‰ d¹⁸O_V-SMOW) in bastnaesite and calcite fall in the PIC square specific to unaltered intrusive carbonatites. The primary strontium isotope ratios in fluorite and bastnaesite are equal to 0.70559-0.70568. The proximal location, close ages, and mineral and geochemical features indicate a genetic relationship of the studied rocks with the late Mesozoic carbonatites of southwestern Transbaikalia. The finding of this rock occurrence indicates the existence of one more carbonatite-bearing area and expands the distribution area of such rocks, which makes southwestern Transbaikalia promising for REE mineralization.

A model of the formation of the Teletskoe-Chulyshman metamorphic belt (TCMB) in Gorny Altai has been elaborated. The estimated pressure (not exceeding 3-4 kbar) and temperature (about 740 ºС) indicate an increased regional crustal thermal gradient equal to 60-90 ºС/km during the formation of the metamorphic belt. The age of migmatites of the Chulyshman complex has been evaluated at 483.9 ± 5.7 Ma (Early Ordovician) by U/Pb (SHRIMP) zircon dating. The paleogeodynamic setting of the TCMB formation and the protolith nature are identified based on the geochemical and petrochemical parameters of the metamorphic rocks. Structural parameters and numerical modeling show that the Chulyshman migmatite-gneiss complex is an apical part of the thermal-dome structure formed under the thermal impact of a magmatic basic heat source in the base of the crust and displaced to the relevant depths via thrusts and crustal extrusion during the Early Ordovician accretion-collision event. Matching the metamorphism parameters and the numerical-modeling results for the crustal thermal regime, we determined the rate of the anatectic front displacement along the thrust to be at least 6 cm/yr.