Home – Home – Jornals – Russian Geology and Geophysics 2020 number 4

The comprehensive chorological analyses of benthic and planktonic faunal groups, the reconstruction of paleogeography, and the delineation and correlation of the same-type biofacies and geographic ranges of the fauna suggest that the Paleozoic tectonic blocks of the Verkhoyansk-Chukotka foldbelt (Tas-Khayakhtakh, Selennyakh, Omulevka, Omolon, Okhotsk, and Chukchi inliers) and Kotel’nyi Island originated in the same epicontinental sea basin of the Siberian paleocontinent, being parts of its passive margin located at the place of the recent foldbelt. We provide a rationale for paleontological, sedimentary, and morphometric diagnostic characteristics of rift zones in complex thrust-folded structures. A rift development, with activation phases in the Ordovician and Devonian, is found to have preceded the breakup of the passive margin in the late Paleozoic.

We present new data on the geology, geochronology, and geochemistry of volcanic complexes of the Uda-Murgal and Okhotsk-Chukotka belts that expose on the left bank of the Anadyr’ River in its middle course. The structural relationships between the stratons, supplemented by the U-Pb and 40Ar/39Ar dates of volcanics, indicate at least three compression events at this segment of the Pacific margin during the Cretaceous: pre-Aptian, early Albian, and late Turonian. The complexes of the Uda-Murgal and Okhotsk-Chukotka belts are separated by an early Albian unconformity, but the other two unconformities are also well pronounced. The studied segment of the Uda-Murgal belt evolved in the ensialic island arc setting till the Barremian. In the Aptian, after the accretion of the island arc to the continent, volcanism reactivated on the Andean-type margin. The main geochemical difference between the Uda-Murgal and Okhotsk-Chukotka belts is the different volume portions of silicic rocks. The less significant difference in the contents of trace elements indicates a change in the composition of the mantle protolith. The complexes of the Okhotsk-Chukotka belt show signs of geochemical zoning, both longitudinal and transverse relative to the strike of the continent-ocean boundary.

The Yllymakh massif is one of Mesozoic ring intrusions widespread in Central Aldan. The alkaline rocks composing it are greatly diverse in composition. Based on the obtained petrological, geochemical, and geochronological data, we have recognized three groups of rocks, marking the different phases of the massif formation. The rocks of two groups resulted from the fractionation of rock-forming minerals (pyroxene and plagioclase) and accessory apatite, which is reflected in the composition trends in the variation diagrams and in the REE patterns. Assimilation of the crustal material also significantly contributed to the diversity of rock-forming melts, which is evidenced from the increase in 87Sr/86Sr from group to group. At the same time, the ε_Nd value is steadily extremely low throughout the study area (on average, -13.5). The obtained Ar-Ar geochronological data indicate three independent stages of the massif formation: 140.0 ± 1.9, 130.0 ±1.9-131.0 ± 2.4, and 125.0 ± 1.9 Ma, which are close in time to the evolution stages of other Mesozoic massifs in Aldan.

The uppermost Bathonian-lowermost Boreal Berriasian clay horizons (Gol’chikha Formation) of the Yenisei-Khatanga regional depression are regarded as probable oil source strata. Considerable core recovery in the Middle Jurassic to Lower Cretaceous sections from the boreholes drilled in the Paiyakhskaya well site and the presence of oils in the overlying strata of the Shuratovo Formation permit us to carry out integrated stratigraphic (bio-, litho-, chemo-, and seismostratigraphic) and geochemical (organic matter and oils) studies of the entire section of the Gol’chikha Formation and boundary beds, to reveal oil-producing horizons, and to compare the genotype and maturation level of their oils with those of the potentially oil source organic matter (OM) of the rocks. A detailed biostratigraphic zonation of the sections of the Gol’chikha Formation based on microfossils has been carried out. Comparison of δ¹³C_org variations in the Volgian and in the lower beds of the Boreal Berriasian with those in the Barents Sea shelf and in the northeast of East Siberia provided the basis for more accurate definition of the boundaries of stages and substages in the intervals free of fossils in the Paiyakhskaya area. The studied section of the Gol’chikha Formation has been divided into eight lithologic members calibrated with bio- and seismostratigraphic units. The distinctive features allowing the definition of the upper boundary of the Gol’chikha Formation are proposed using GIS data. Analysis of bio- and chemostratigraphic data allowed the correlation of the seismic reflecting horizons defined in the Gol’chikha Formation and its boundaries with the geologic section and relevant litho- and biostratigraphic units. It has been established that the strata with the highest content of organic matter consist of the upper part of the Gol’chikha Formation (the Upper Volgian and basal Boreal Berriasian). According to the vitrinite reflectance data, the OM catagenesis in the Upper Volgian interval corresponds to the oil window, which is confirmed by pyrolysis data. Thus, these strata can be considered oil-producing. The low δ¹³C_org values confirm the predominantly marine OM composition. Analysis of oils from the Cretaceous productive strata of the Paiyakhskaya area shows that they formed from the marine OM of the upper part of the Gol’chikha Formation at the same accumulation stage. The comprehensive studies of the Gol’chikha Formation in the Paiyakhskaya well site and complete stratigraphic coverage of the sections confirm that they can be considered a hypostratotype.

Several negative δ13C excursions in benthic foraminifera from gas-bearing core LV50-05 sampled offshore on the eastern slope of Sakhalin Island, Sea of Okhotsk, in an area of active methane seepage record the local history of methane events (ME). The core chronostratigraphy has been contsrained from AMS 14C ages and biostratigraphic data. Benthic foraminifera ( Nonionellina labradorica and Uvigerina parvocostata ) from some core intervals show normal marine δ13C values (about -1‰), but some intervals are marked by extremely depleted compositions as low as -34.5‰ δ13C (relative to VPDB). The negative δ13C excursions are interpreted as a record of seabed methane emanation during primary and secondary biomineralization of carbonate foraminifera. The results reveal four Holocene methane events (ME) in the area: two brief (ME-1 at 700-900 yr BP and ME-2 at 1200-1400 yr BP) and two long (ME-3 at 2500-4700 yr BP and ME-4 at 7400-10000 yr BP) events.

The main factors controlling the bulk sedimentation in the region of the Mendeleev Rise and the adjacent part of the Arctic Ocean during the late Cenozoic were studied using a complex of geomorphological, lithological, and organic geochemical data. Samples for the study were collected during the cruises of the R/V Akademik Fedorov in 2000, 2005, and 2007 and the icebreaker Kapitan Dranitsyn in 2012. Analysis of the group and molecular compositions of the dispersed organic matter (DOM) in bottom sediments has shown that the input of terrigenous sediments enriched with abrasion products of lithified rocks from the eastern source province determines the Holocene-Pleistocene sedimentation on the continental slope of the East Siberian Sea and the Podvodnikov Basin. The individual characteristics of DOM of the late Cenozoic deposits from the underwater mountains of the Mendeleev Rise reflect the wide diversity of sedimentary sources and depositional conditions. Subaqueous erosion and redeposition of denudation products of source rocks and pre-Holocene sediments play an important part in sedimentation together with a terrigenous flow and ice transport.

We present results of study of the molecular composition of organic matter (OM) in the bottom sediments of the Laptev Sea by gas chromatography-mass spectrometry, isotope gas chromatography-mass spectrometry, and Rock-Eval pyrolytic analysis. The OM of all collected sediment samples shows a significant terrigenous contribution. Compounds that are biomarkers of methanotrophic microorganisms are also found. A positive correlation between the contents of the studied biomarkers and the contents of pelite and total organic carbon is observed at the sites with documented intense methane bubbling. For example, the average content of C30 hopenes at the “methane” stations is twice higher than that at the “background” ones. The average content of C32 αβ-hopanes in sediment samples from the methane seepage area is 1.5 times higher than that at the background stations. We suggest that the increased C30 αβ-hopane content (~1.5 times higher within the methane seepage area) and the decreased moretane index relative to the C31 hopane index are due to the inflow of OM of petroleum origin. The presence of biphenyl in sediments indicates its petroleum origin, which supports our assumption of the migratory nature of petroleum hydrocarbons in the methane seepage area. Triterpenoids found in the sediment OM indicate diagenetic bacterial transformation of OM in the methane seepage areas, which shows that methane has been released for a long time. We assume the intense activity of the consortium of methanotrophs and sulfate reducers in the methane seepage areas.

According to drilling and seismic data, the Late Sinian-Early Cambrian intracratonic rift was found in the Deyang-Anyue area of the Upper Yangtze craton. This rift is controlled by a tensional fault and extends in the N-NW direction with a N-S length of 320 km and an E-W width of 50-300 km. After three stages of the rift evolution, i.e., the formation stage, development stage, and dying stage, a favorable near-source accumulation assemblage formed. The research results indicate that: (1) the sedimentation stage of the Late Sinian Dengying Formation is the rift formation stage, during which trough-basin facies sedimentation is developed in the rift, while platform marginal-facies mounds and shoals are developed on both sides, controlling the formation and distribution of high-quality reservoirs in the Dengying Formation; (2) the sedimentation stage of the Early Cambrian Meishucun-Qiongzhusi Formation is the rift development stage, during which deep-water continental-shelf facies argillaceous rocks accumulated in the rift with a thickness of 500-1000 m, indicating the sedimentation of high-quality source rocks; (3) the sedimentation stage of the Canglangpu Formation is the rift dying stage, terminating the evolution history of the intracratonic rift by gap filling. The intracratonic rift is a key factor for the formation of the Anyue giant gas field, where the high-quality source rocks provide abundant gases for the giant gas field. A regional lateral sealing occurred during the rapid rise of the western paleouplift in Central Sichuan during the Late Yanshan-Himalayan period, favorable for the preservation of the Gaoshiti-Moxi giant gas field on the eastern wing of the rift. The intracratonic rift and its role in giant gas accumulation provide a significant reference for deep oil and gas exploration in paleocratonic basins.