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1.
Based on the LA-ICP-MS data, detrital zircons from the tillite-type conglomerates of the Tanin Formation (Serebryanka Group) on the western slope of the Central Urals include approximately equal proportions of crystals with Neoarchean and Paleoproterozoic U-Pb ages. Therefore, we can assume that crystalline rocks of the basement beneath the eastern part of the East European Craton served as a provenance for aluminosilicate clastics in the initial Serebryanka period. Detrital zircons from sandstones of the Kernos Formation have the Meso-Neoarchean (∼15%), Paleoproterozoic (∼60%), and Mesoproterozoic (∼26%) age. Comparison of the obtained data with the results of the study of detrital zircons from Riphean and Vendian sandstones of the Southern Urals shows that the Riphean and Lower Vendian rocks are mainly represented by erosional products of Middle and Upper Paleoproterozoic crystalline rocks that constitute the basement of the East European Craton. In addition, a notable role belonged to older (Lower Proterozoic, Neoarchean and Mesoarchean) rock associations during the formation of the Serebryanka Group. The terminal Serebryanka time (Kernos Age) differed from its initial stage (Tanin Age) by the appearance of Mesoproterozoic complexes in provenances. According to available data, these complexes played an insignificant role in the formation of Riphean-Vendian rocks in the neighboring South Uralian segment. This implies a spatiotemporal diversity of clastic material sources for Upper Precambrian rocks in the western megazone of the Southern and Central Urals.  相似文献   

2.
The rift-related geodynamic setting of the Late Precambrian geological evolution on the western slope of the South Urals is reconstructed on the basis of localization of lithotectonic complexes of this age, their formation conditions, and the geochemistry of rocks. The Early Riphean stage comprises accumulation of coarse-clastic rocks intercalating with alkaline volcanic rocks of the Navysh Complex, which is a constituent of the Ai Formation, and emplacement of doleritic and picritic intrusions of the Shuida Complex and melanocratic dolerite and gabbrodolerite of the Yusha Complex. The Middle Riphean stage is characterized by wide-spread coarse-clastic terrigenous rocks of the Mashak Formation that intercalate with volcanic rocks of the bimodal basalt-rhyolite association, the Berdyaush pluton of rapakivi granite, the Kusa-Kopan layered intrusive complex, the Lapyshta Complex of dolerites and picrites, and numerous occurrences of gabbrodolerites. The terrigenous rocks of the Vendian stage include conglomerate, gravelstone, and sandstone of the Asha Group, while igneous rocks comprise alkaline volcanics of the Arsha Complex, alkali gabbroids of the Miseli Complex, and melanocratic syenite of the Avashla Complex. The geological evolution of the region is distinguished by local (failed or aborted) rifting. The occurrence of lithotectonic complexes is controlled by dynamic conditions of rifting. A certain inheritance in the evolution may be traced for the Early and Middle Riphean and partly for the Late Riphean and Vendian.  相似文献   

3.
The geochemical features of basal fine-grained terrigenous rocks from the Riphean sedimentary megasequences of the Southern Urals, Uchur-Maya region, and Yenisei Range were compared in order to estimate the maturity of the continental crust that was formed by the beginning of the Riphean. It was shown that initial shales from the base of the Riphean sequence of the Yenisei Range and fine-grained aluminosiliciclastic rocks from the base of the Riphean sections of the Southern Urals were formed by the erosion of a rather mature continental crust. In contrast, fine-grained terrigenous rocks from the base of the Riphean of the Uchur-Maya region were derived from immature Late Archean protoliths or their Early Proterozoic analogs. The fine-grained terrigenous rocks of the three sedimentary megasequences show different variations in the (La/Yb)N ratio. In the Southern Urals, this ratio is high (12–15) in the Burzyan Group and decreases upsection to 6–10. In the shales of the Uchur-Maya region, the (La/Yb)N ratio decreases upsection, and the La/Sc ratio shows a sympathetic behavior. This is due to a decrease in the proportion of “primitive” tonalite-trondhjemite associations of the Archean granite-greenstone terranes in the provenance area with time and the appearance of intra-plate (riftogenic?) granitoids and significant amounts of basic and ultrabasic rocks. The latter marks the onset of large rift-forming events in the Uchur-Maya region at the beginning of the Late Riphean. The (La/Yb)N of the studied rocks from the Yenisei Range are mostly similar to the PAAS ratio, but higher values were found in the Upper Vorogovka and Chingasan groups, which was related to the contribution of strongly LREE-enriched granitoids and rift felsic and alkali basaltic volcanic associations to the formation of the terrigenous material. A comparison of Rb, Sr, Y, Zr, Ba, Hf, Th, U, Cr, and Ni contents and Zr/Y, (La/Yb)N, Ni/Co, Cr/Th, Cr/Sc, and La/Th ratios in the fine-grained terrigenous rocks of the Riphean megasequences of the Southern Urals, Uchur-Maya region, and the Yenisei Range with those in the model geochemical objects (PAAS, UCPR1, UCAR2, and others) showed that, in terms of most of the parameters, the Riphean fine-grained terrigenous rocks from the three regions are similar to each other, PAAS, and Proterozoic cratonic shales. This indicates a fairly high general maturity of the protoliths that were eroded during the Riphean in the eastern East European craton and in the southeastern and southwestern parts of the Siberian craton.  相似文献   

4.
The Grenvillian orogeny (～1250 to 980 Ma) was one of the most significant Riphean events. It determined the formation of many structures observable now in North and South America, northwestern Europe, South Africa, Western Australia, Antarctica, and other regions. Nevertheless, its reflections in the most complete and relatively well investigated Upper Precambrian sedimentary sections of northern Eurasia such as the Riphean stratotype (Bashkir meganticlinorium) and hypostratotype (Uchur-Maya region) still remain unknown. This is primarily true of the petrographic and chemical compositions of terrigenous rocks. This work is dedicated to the analysis of peculiar features in variations of the whole-rock chemical composition of sandstones and fine-grained clastic rocks (shales, mudstones, fine-grained clayey siltstones) that constitute Middle-Upper Riphean boundary layers of the Bashkir meganticlinorium, Kama-Belaya aulacogen, and Uchur-Maya region. The analysis reveals no tendency for the decrease in the degree of the chemical and, consequently, mineralogical maturity in the upward direction through the Middle-Upper Riphean sections in the above-mentioned regions. The whole-rock compositions of fine-grained clastic rocks associated with sandstones correspond mostly to that of “common” Upper Precambrian clayey rocks. The formation of practically the entire Yurmatinian-Karatavian succession in the Bashkir meganticlinorium proceeded under relatively stable TDM and ?Nd(T) values. The period of 1250 to 980 Ma in the central and eastern parts of the Siberian Platform was marked by repeated rifting episodes alternating with accumulation of mature platformal sediments, although repercussions of Grenvillian collisional processes are missing from this region as well. The performed analysis provides grounds for the conclusion that contribution of the Grenvillian events to the formation of most complete Riphean successions in northern Eurasia was insignificant.  相似文献   

5.
Analysis of the litho-geochemistry of fine-grained terrigenous rocks (metapelites, shales, and mudstones) of sedimentary megasequences in the Southern Urals, Uchur-Maya area, and the Yenisei Kryazh indicates that Riphean sequences in these regions are dominated by chlorite-hydromica rocks, with montmorillonite and potassic feldspar possibly occurring only in some of the lithostratigraphic units. According to the values of their hydrolysate modulus, most clay rocks from the three Riphean metamorphosed sedimentary sequences are normal or supersialites, with hydrosialites and hydrolysates playing subordinate roles. The most lithochemicaly mature rocks are Riphean clays in the Yenisei Kryazh (Yenisei Range). The median value of their CIA is 72, whereas this index is 70 for fine-grained aluminosilicate rocks from the Uchur-Maya area and 66 for fine-grained terrigenous rocks of the Riphean stratotype. Hence, at ancient water provenance areas from which aluminosilicate clastic material was transported in sedimentation basins in the southwestern (in modern coordinates) periphery of the Siberian Platform, the climate throughout the whole Riphean was predominantly humid. At the same time, the climate at the eastern part of the East European Platform was semiarid-semihumid. The K2O/Al2O3 ratio, which is employed as an indicator of the presence of petro-and lithogenic aluminosilicate clastic component in Riphean sedimentary megasequences, shows various tendencies. According to their Sc, Cr, Ni, Th, and La concentrations and the Th/Sc ratio, the overwhelming majority of Riphean shales and mudstones notably differ from the average Archean mudstone and approach the average values for post-Archean shales. This suggests that mafic Archean rock in the provenance areas did not play any significant role in the origin of Riphean sedimentary megasequences. The Co/Hf and Ce/Cr ratios of the terrigenous rocks of the three Riphean megaseqeunces and their (Gd/Yb) N and Eu/Eu* ratios place these rocks among those containing little (if any) erosion products of primitive Archean rocks. According to various geochemical data, the source of the great majority of fine-grained aluminosilicate clastic rocks in Riphean sediment megasequences in our study areas should have been mature sialic (felsic), with much lower contents of mafic and intermediate rocks as a source of the clastic material. The REE patterns of the Riphean shales and metapelites in the Bashkir Meganticlinorium, Uchur-Maya area, and Yenisei Kryazh show some features that can be regarded as resulting from the presence of mafic material in the ancient provenance areas. This is most clearly seen in the sedimentary sequences of the Uchur-Maya area, where the decrease in the (La/Yb) N ratio up the sequence of the fine-grained terrigenous rocks from 15–16.5 to 5.8–7.1 suggests that mantle mafic volcanics were brought to the upper crust in the earliest Late Riphean in relation to rifting. Analysis of the Sm-Nd systematics of the Riphean fine-grained rocks reveals the predominance of model age values in the range of 2.5–1.7 Ga, which can be interpreted as evidence that the rocks were formed of predominantly Early Proterozoic source material. At the same time, with regard for the significant role of recycling in the genesis of the upper continental crust, it seems to be quite possible that the ancient provenance areas contained Archean complexes strongly recycled in the Early Proterozoic and sediments formed of their material. An additional likely source of material in the Riphean was mafic rocks, whose variable contribution is reflected in a decrease in the model age values. Higher Th and U concentrations in the Riphean rocks of the Yenisei Kryazh compared to those in PAAS indicate that the sources of their material were notably more mature than the sources of fine-grained aluminosilicate clastic material for the sedimentary megaseqeunces in the Southern Urals and Uchur-Maya area.  相似文献   

6.
In the mid-1980s, it was concluded based on geochemical study that Th, Sc, La concentrations and ratios Th/Sc, La/Sc and Eu/Eu* did not wary significantly in the post-Archean time. It was impossible to judge about compositional variations of upper crust during the Riphean and Vendian, because data of that time characterized a limited number of samples from the post-Archean basins of Australia, New Zealand, and Antarctic. Considered in this work are variations of Eu/Eu*, LREE/HREE, Th/Sc, and La/Sc ratios in Upper Precambrian fine-grained siliciclastic rock of the Southern Urals western flank (Bashkirian meganticlinorium) and Uchur-Maya region (Uchur-Maya plate and Yudoma-Maya belt). As is established, only the Eu anomaly in the studied siliciclastic rocks is practically identical to this parameter of the average post-Archean shale. Three other parameters plot on the Riphean-Vendian variation curves with positive and negative excursions of diverse magnitude, which do not coincide always in time. It is assumed that these excursions likely mark stages of local geodynamic activity, destruction of pre-Riphean cratons, and progressing recycling of sedimentary material during the Riphean.  相似文献   

7.
Sedimentation environments were reconstructed for the Early Vendian successions of the western slope of the Central Urals, which comprises one of the most complete sections of the terminal Precambrian system in northern Eurasia. It was shown that, despite the presence of several diamictite levels in the sections of the Serebryanka Group, mature and multiply recycled fine-grained siliciclastic materials (CIA = 65–77) were delivered into the sedimentation basin over the whole Early Vendian. Based on the lithochemical characteristics of shales, the climate of Serebryanka time can be estimated as semiarid-semihumid, similar to that dominating in Late Vendian paleocatchments. Based on relatively high Mo/Mn values (0.011–0.024), it was suggested that anoxic or similar conditions existed in the basin of Buton time, whereas other sedimentary complexes of the Serebryanka Group were formed in well aerated environments. The systematics of Sr, Ba, Zr, Cu, and V in fine-grained siliciclastic rocks and Sr isotopic data for carbonate rocks indicate that the sediments were accumulated in a fresh-water basin. The values of trace-element indicator ratios, e.g., Th/Sc, La/Sc, Th/Cr and others, in the shales of the Serebryanka Group and Nd model age estimates indicate that a variety of mainly Early Proterozoic complexes, ranging from granitoids to basic rocks, occurred in the Early Vendian paleocatchments. The basic rocks were eroded most extensively probably in the end of Serebryanka time. Based on the Ce/Ce* values of shales, it was concluded that submarine volcanism had no significant influence on sedimentation processes in the Early Vendian. An exception is Koiva and Kernos time, when hematite-bearing shales were accumulated in association with pillow basalts in some zones of the basin. The distribution of the compositions of shales from various formations of the Serebryanka Group in discrimination diagrams suggests that the Early Vendian sedimentary sequences were formed in passive geodynamic settings.  相似文献   

8.
Relationship of large magmatic/plume events, warm periods, and epochs of accumulation of variably Сorg–rich sediments is considered by the example of Riphean low-carbonaceous shales of the western slope of the South Urals. It is shown that this relationship presumably exists for some of the indicated events (as for Early Precambrian superplume events), and does not exist for others. The formation of organic carbon-rich sediments in the Late Precambrian was likely controlled by other external and internal factors than in the Early Precambrian, relationships between which are so far poorly understood. It is also possible that the lesser scale of plume events in the Late Precambrian as compared to previous epochs could lead to the obliteration of their influence on the formation of Сorg–rich sedimentary rocks, as well as to the local accumulation of the latters.  相似文献   

9.
The provenance composition and geological evolution of different segments in the distal zone of the Late Vendian foreland basin of the Timan orogen were deciphered on the basis of sequence stratigraphic reconstructions and precision geochemical data on the Upper Vendian fine-grained terrigenous rocks from the Southeast White Sea region, Vychegda, Verkhnekama, and Shkapovo-Shikhan basins, and the Kvarkush-Kamennogorsk and Bashkirian meganticlinoriums. The Upper Vendian of the Southeast White Sea region is subdivided into four sequences: Agma, Solza, Zimnie Gory, and Erga. The tracing of sequence boundaries and lateral facies associations from the Southeast White Sea Region to the South Urals made it possible to identify the main stages of the evolution of sedimentary filling of the foreland basin: (I) shallow water epiplatformal basin, (II) isolated basin, (III) lowstand system tract, and (IV) progradation of delta platforms. Position of data points of the Upper Vendian shales, silty mudstones, and mudstones plotted in the diagrams Th-La, Ni-Cr, GdN/YbN-Eu/Eu*, Sc-Th/Sc, La/Sm-Sc/Th, and Co/Hf-Ce/Cr, together with their REE systematics and data on Nd model ages indicates that the Proterozoic, including Neoproterozoic rocks of the Timan thrust-folded belt served as the main source for fine aluminosiliciclastic material in the distal zones of the Timan foreland basin. Obtained results are consistent with the concept that the Vendian stage of the evolution of the Eastern European platform is related to the intense input of allochthonous material in its peripheral area owing to the activation of orogenic processes in the adjacent mobile belts and allow us to significantly specify the paleogeographical reconstructions.  相似文献   

10.
Rb-Sr and K-Ar characteristics of Vendian and Upper Riphean sections in various structural units of the East European Platform are studied. It is shown that Neoproterozoic clayey rocks of the platform underwent postsedimentary transformations, primarily owing to processes of K accumulation (illitization). Their intensity decreases with depth and isotopic signatures of provenance rocks are partly retained in Riphean rocks. Stages of the most active transformations approximately 400 and 600 Ma ago are manifested in the Vendian clayey rocks. Events of approximately 1000 Ma ago are recorded in Riphean rocks of the East European Platform. The least altered rocks of the Pachelma aulacogen can retain ancient hydrocarbon pools.  相似文献   

11.
The first results of U–Pb dating of detrital zircons from Upper Ordovician sandstones of the Bashkir uplift in the Southern Urals and U–Pb isotopic ages available for detrital zircons from six stratigraphic levels of the Riphean–Paleozoic section of this region are discussed. It is established that the long (approximately 1.5 Ga) depositional history of sedimentary sequences of the Bashkir uplift includes a peculiar period lasting from the Late Vendian to the Emsian Age of the Early Devonian (0.55–0.41 Ga). This period is characterized by the following features: (1) prevalence of material from eroded Mesoproterozoic and Early Neoproterozoic crystalline complexes among clastics with ages atypical of the Volga–Urals segment of the East European Platform basement; (2) similarity of age spectra obtained for detrital zircons from different rocks of the period: Upper Vendian–Lower Cambrian lithic sandstones and Middle Ordovician substantially quartzose sandstones.  相似文献   

12.
In this study we discuss the problem of dating the Kiryabinka complex. The data collected on zircons from pyroxenites of the Kiryabinka polyphase pyroxenite-gabbro complex can help address a number of controversial issues regarding the Precambrian geology of the Southern Urals. First, the age of the complex (T = 680 ± 3.4 Ma) can be assigned within the late Riphean (RF4, Arshinian) or the middle Neoproterozoic (Cryogenian). The available zircon dates from gabbroic and granitoid rocks in the western flank of the Southern Urals (Berdyaush, Akhmer, and Barangul massifs) are supplemented with a new age of ultramafic rocks, the differentiates of a basaltic magma, which further corroborate the conclusion about the Upper Riphean age of the country rocks.  相似文献   

13.
We consider the general and specific features of the evolution of the composition of fine-grained terrigenous rocks in the Riphean sedimentary megasequences of the Southern Urals, Uchur-Maya region, and Yenisei Ridge. It has been established that the crust on the southwestern (in the modern frame of references) periphery of the Siberian craton was geochemically the most mature segment of the Riphean continental crust. For example, the fine-grained clastic rocks and metapelites of all Riphean lithostratigraphic units of the Yenisei Ridge have higher median contents of Th than the most mature Paleoproterozoic crust, and in median contents of Y and Cr/Th values they are the most similar to it. In the Southern Urals and Uchur-Maya region, some units of the Riphean sedimentary sequences show median contents of Y and Th and Cr/Th values close to those of primitive Archean crust. Analysis of Cr/Th variations in the fine-grained terrigenous rocks of all three megasequences shows that the minimum Cr/Th values, evidencing a predominance or the abundance of felsic rocks in provenances, are typical of the Riphean argillaceous shales and metapelites of the Yenisei Ridge. The distinct Cr/Th and Cr/Sc increase in the fine-grained clastic rocks of the Chingasan Group of the ridge reflects the large-scale destruction of continental crust during the formation of rift troughs as a result of the Rodinia breakup in the second half of the Late Riphean. The Cr/Th variations in the Lower and Middle Riphean argillaceous shales and mudstones of the Bashkirian mega-anticlinorium and Uchur-Maya region are in agreement, which evidences the subglobal occurrence of rifting in the early Middle Riphean (so-called “Mashak rifting”).  相似文献   

14.
The analysis of lithogeochemical data on the Upper Riphean and Vendian sandstones from the Bashkirian anticlinorium showed that sandstone associations formed in a passive sluggish tectonic regime in the middle Late Vendian were replaced by associations accumulated in the more active tectonic settings. This is well seen in the SiO2-K2O/Na2O and (Fe2O3* + MgO)-TiO2 diagrams reflecting the particular and median compositions of psammites. The lithochemical characteristics of sandstones were examined to determine the compositional variation of rock complexes eroded on paleodrainage areas. Quartz-rich sedimentary, metasedimentary, and metamorphic rocks, as well as felsic igneous rocks prevailed in the paleodrainage areas throughout the entire Late Riphean and Early Vendian, while the main sources of clastic material in the Late Vendian were igneous intermediate and basic rocks. With allowance made for the previous comparative-lithological data and some other materials, significant similarity in the position and orientation of compositional fields of psammites from the middle and upper levels of the Asha Group (Bashkirian anticlinorium) with fields of psammites from different syncollisional (flysch and molasse) basins in the SiO2-K2O/Na2O, K2O/Na2O-SiO2/Al2O3, F1–F2 and other diagrams suggests that the middle Late Vendian (beginning from the Basa level) was marked by a variation in tectonic/geodynamic settings of sandstone accumulation and in composition of the eroded paleodrainage systems. The revealed trend agrees well with concept of the existence of the Late Riphean-Vendian Pechora paleocean.  相似文献   

15.
Timan comprises the southwest edge of the Pechora Plate. The plate basement is composed of variably metamorphosed sedimentary, mainly terrigenous, and igneous rocks of the Late Precambrian age that are generally overlain by Ordovician-Cenozoic platform cover. Poor exposition and discontinuous distribution of the Upper Precambrian outcrops of dominantly fossil-free sedimentary rocks cause considerable disagreements in stratigraphic correlation. This applies equally to North Timan, which represents an uplifted block of basement, in which sedimentary-metamorphic rocks form the Barminskaya Group (～5000 m thick), previously dated as Early Riphean to Vendian. Earlier Rb-Sr and Sm-Nd isotope dating of schist and cross-cutting gabbro-dolerite and dolerite established the timing of greenschist facies metamorphism at 700 Ma. Thus, Late Riphean age of the Barminskaya Group has been suggested. Results of local U-Pb dating of detrital zircon from silty sandstones of the Malochernoretskaya Formation, which constitutes the middle part of the outcropping section of the Barminskaya Group, confirm this conclusion. Age data for 95 zircon grains cover the range of 1035–2883 Ma with age peaks at 1150, 1350, 1550, 1780, and 1885 Ma. The minimum age of zircons, considered as the lower age constraint on sediment deposition, provides grounds to date the Barminskaya Group as Late Riphean and indicates eroded rock complexes of the Fennoscandian Shield as the possible provenance areas.  相似文献   

16.
New structural, petrological, chemical, isotope, and paleomagnetic data have provided clues to the Late Riphean–Paleozoic history of the Uda–Vitim island arc system (UVIAS) in the Transbaikalian sector of the Paleoasian ocean, as part of the Transbaikalian zone of Paleozoids. The island arc system consists of three units corresponding to main evolution stages: (i) Upper Riphean (Late Baikalian), (ii) Vendian–Lower Paleozoic (Caledonian), and (iii) Middle–Upper Paleozoic (Hercynian). The earliest stage produced the base of the system composed of Late Riphean ophiolite (971–892 Ma, U-Pb) and volcanic (837–789 Ma, U-Pb) and sedimentary rocks (hemipelagic siliceous sediments and dolerite sills) which represent the Barguzin–Vitim oceanic basin and the Kelyana island arc. The main event of the second stage was the formation of the large UVIAS structure (over 150,000 km2) which comprised the Transbaikalian oceanic basin, the forearc and backarc basins, and the volcanic arc itself, and consisted of many volcanic-tectonic units exceeding 100 km2 in area (Eravna, Oldynda, Abaga, etc.). Lithology, stratigraphy, major–element compositions, and isotope ages of Vendian–Cambrian volcanic rocks and associated sediments indicate strong differentiation of calc-alkaline series and the origin of the island arc system upon oceanic crust, in a setting similar to that of the today’s Kuriles–Kamchatka island arc system. The Middle–Upper Paleozoic stage completed the long UVIAS history and left its imprint in sedimentary and volcanic rocks in superposed trough basins. The rocks were studied in terms of their biostratigraphic and isotope age constraints, as well as major- and trace-element compositions, and were interpreted as products of weathering and tectonic-magmatic rework of the UVIAS units.  相似文献   

17.
Lithogeochemical features of Riphean fine-grained terrigenous rocks of the Kama-Belaya aulacogen are discussed. It is shown that aluminosiliciclastic material delivered to the aulacogen during the Riphean was characterized by a low maturity degree. The successively increasing K2O/Al2O3 values in the Riphean summary section correlate negatively with the CIA index values, indicating a gradually strengthening tendency for climate aridization in erosion zones. Data on some indicator ratios of trace elements and REE systematics in Riphean silty mudstones and shales of the Kama-Belaya aulacogen imply the involvement of mafic and ultramafic rocks, in addition to acid igneous and metamorphic varieties, in erosion during accumulation of the Nadezhdino, Tukaevo, Ol’khovka, Usinsk, and Priyutovo formations. Comparison of data on the composition of rocks in provenances based on the mineralogical-petrographic study of sandstones and investigation of geochemical features of silty mudstones and shales revealed their sufficiently high similarity. The geochemical data made it possible to specify the composition of rocks in provenances. Low Ce/Cr values in the fine-grained terrigenous rocks of the Lower Riphean Kyrpy Group suggest their formation with a significant contribution of erosion products of the Archean substrate, which is atypical for higher levels of the section. Thus, the Early-Middle Riphean transition period was likely marked by substantial changes in the mineral composition of material delivered to the Kama-Belaya aulacogen. The lack of exhalative components in the examined specimens of silty mudstones and shales points to a relatively low permeability of the Earth’s crust in the eastern East European Platform through the entire Riphean.  相似文献   

18.
Diverse assemblages of cellularly preserved Precambrian microorganisms have been discovered in cherty stromatolitic sediments from six formations in the Soviet Union: Sukhotungusin Fm. (Middle Riphean, Siberia); Valukhtin Fm. (Middle Riphean, Siberia); Shorikha Fm. (Upper Riphean, Siberia); Minyar Fm. (Upper Riphean, Bashkiria); Olkhin Fm. (Upper Riphean, Siberia); and Chichkan Fm. (Vendian, Kazakstan). These cyanophyte-dominated microbial communities, occurring in both stratiform (cf. Stratifera) and columnar stromatolitic deposits (Baicalia hirta and Conophyton gaubitza), are the first stromatolite-building microbiotas to be reported from the Soviet Union; collectively they comprise more than one-fifth of all such Precambrian assemblages now known.  相似文献   

19.
Detrital zircons (DZs) from arkose sandstones of the Upper Riphean Zilmerdak Formation (Southern Urals) yielded ages in the range of 3039–964 Ma. Grains with Late Karelian and Early and Middle Riphean ages compose 35, 34, and 26% of the total number of the analyzed zircons, respectively. This is similar to the age spectra of the Vendian sandstones (Asha Group), but it differs significantly from the age distribution typical of the Riphean stratotype sandstones.  相似文献   

20.
Lithogeochemical features of the Vendian mudstones and silty mudstones taken from Borehole Keltma 1 in the southern part of the Vychegda trough of the Mezen syneclise are discussed. It is shown that fine-grained clastic rocks of the Ust-Pinega, Krasavino, and Mezen formations have similar chemical compositions, suggesting their accumulation in sufficiently similar settings. The main part of the studied samples has K2O/Al2O3 < 0.4. This fact, in combination with the absence of TM-FM and NPM-HM correlations, indicates a significant contribution of recycled aluminosiliciclastics in their composition. At the same time, the absence of correlation between CIA and indicator ratios of rock composition in the paleodrainage basins, such as Th/Cr and Th/Sc, indicates that CIA and some other lithochemical indicators appropriately reflect the paleoclimatic conditions in source areas surrounding a basin. The CIA value in most of the analyzed samples is no more than 70. Thus, the Keltma section is similar to Upper Vendian sequences of the Kvarkush-Kamennogorsk anticlinorium and the Shkapovo-Shikhany depression. It has been established that felsic and intermediate magmatic rocks coupled with a significant contribution of quartz-rich sediments served as the source of fine aluminosiliciclastics for the southern Vychegda trough during the Vendian. High Ce/Cr values in the mudstones and silty mudstones suggest that the geochemically primitive Archean protoliths were not involved in the washout. In the SiO2-K2O/Na2O diagram, the Vendian mudstones and silty mudstones are plotted in the field of sediments of active continental margins. Typical low values of Mo/Mn and some other redox indices in these rocks indicate that oxidizing environment predominated in bottom waters of the sedimentation basin during the entire Vendian. Analysis of variations of the lithochemical indicators upward the Vendian sedimentary successions in borehole Keltma 1 made it possible to divide the section into three sequences of different lithofacies and paleontological compositions.  相似文献   

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