Geologic stages of the Paleogene and Neogene evolution of the Arctic shelf in the Ob’ region (West Siberia)

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ScienceDirect Russian Geology and Geophysics 55 (2014) 483–494 www.elsevier.com/locate/rgg

Geologic stages of the Paleogene and Neogene evolution of the Arctic shelf in the Ob’ region (West Siberia) V.S. Volkova * A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia Received 26 November 2012; accepted 23 May 2013

Abstract The paper is concerned with the structure of the Arctic shelf sediments in the Ob’ region in the Paleogene and Neogene, sampled from boreholes drilled on the Yamal Peninsula, in the lower reaches of the Pur and Taz Rivers. The specifics of Paleogene marine sedimentation in the central and northern West Siberian Plain are studied. The effect of abiotic (tectonic) factors on the completeness of the geologic record is considered as well as the effect of recent (Oligocene–Neogene) tectonic processes on topography and sedimentation. The borehole sections are compared with the main seismic sections of the Kara Sea and Lomonosov Ridge. Keywords: Paleocene; Eocene; Oligocene; Talitsa, Serov, Nyurol’ka, and Tavda Formations; dinocysts; spores; pollen; stages; events; U-shaped valleys

Introduction The Paleogene and Neogene sediments in the Arctic part of the North Ob’ region (West Siberia) belong to a Meso-Cenozoic cover, and their thickness is more than 800 m. They are characterized by an incomplete geologic record. The stages of their geologic evolution are usually reconstructed from the sedimentation history of the central West Siberian Plate. There are no complete Paleogene or Neogene sections in the Arctic region. The most complete sections of these sediments are localized to the south of the Trans-Siberian Railroad, in the Om’ basin, in which they have a good paleontological substantiation and a good paleomagnetic description (Akhmet’ev et al., 2004a,b; Babushkin, 2001; Gnibidenko, 2006; Nikitin, 2006; Volkova, 2002; Volkova et al., 2002; Zal’tsman, 1968, 1981). On the sides of the Om’ basin, at the boundary with the North Kazakhstan monocline, a decrease in the thickness of the Paleogene and Neogene sediments caused the omission of magneto- and spore–pollen zones from the section (Gnibidenko et al., 2011). On the margins of the Arctic shelf, not only a decrease in the thickness of individual beds and formations is observed but also the omission of Upper Eocene, Oligocene, and Neogene sediments. These processes are due to Late Cenozoic tectonic events. Paleontological data (dinoflagellates, spores, and pollen) play an important role in the * Corresponding author. E-mail address: [email protected] (V.S. Volkova)

estimation of the sediment age and the completeness of the geologic record. In studies of the structure of sections of marine and continental sediments, it is very important to consider the sediment structure and succession, the character of stratigraphic boundaries with regard to lithologic composition, and paleontologic characteristics. Dinoflagellate study has been the preferred paleontological method for West Siberia in recent years. Dinoflagellates were studied by palynologists together with spores and pollen. The detected succession of dinoflagellate zones correlates with nanoplanktonic zones and spore– pollen data. Importantly, dinocysts—organic-walled planktonic algae— are observed both in carbonate and in carbonate-free facies, and their coexistence with spores and pollen in microscopic specimens permits a direct correlation of marine and continental sediments. Dinocysts play an important role in zonal stratigraphy owing to their rapid evolution, high morphologic diversity, and fast colonization of sea regions. In recent years, this group of phytoplankton has been used in Paleogene stratigraphy, especially in boreal provinces with a petroleum potential (Berggren et al., 1995; Costa et al., 1988). The dinocyst zoning of the Paleogene marine sediments of West Siberia was first developed by I.A. Kul’kova (1987, 1988, 1994). It was proposed in (Kul’kova and Shatsky, 1990) to determine the boundaries between units from index species. Paleogene sediments were studied from a series of boreholes in the Kulunda and Om’ basins, mainly south of Khanty-Man-

1068-7971/$ - see front matter D 201 4, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2014.03.006

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Fig. 1. The position of boreholes studied by the palynological method at the Trofimuk Institute of Petroleum Geology and Geophysics. The boundaries of the West Siberian Plain are shown by dashed lines.

siisk. Six successive dinoflagellate zones were detected by I.A. Kul’kova in the Paleogene sediments: DN2, DN3, DN7, DN10, DN12, and DN14 (Babushkin, 2001). The Cerodinium speciosum Zone was detected and traced (Babushkin, 2001; Kul’kova and Shatsky, 1990) in the upper Talitsa–lower Serov Formations. By nanoplankton, it corresponds to the Ellipsolithus macellus Subzone–Fasciculithus tympaniformis Zone and lower Heliolithus Subzone (Selandian). The Apectodinium homomorphum Zone was observed in the upper Serov Formation; by nanoplankton, this is the upper Heliolithus Subzone (upper Selandian) and the Discoaster multiradiatus Zone (Thanetian). The Wetzeliella meckelfeldensis, Dracodinium simile, D. varielongitudum Zone (collective occurrence) was recognized within the Irbit Formation. By nanoplankton, it corresponds to the Discoaster diastypus Zone (Ypresian). The Charlesdowniea coleothrypta s.l. Zone, traced in the Nyurol’ka Formation, is divided into two subzones. By nanoplankton, this zone corresponds to the Marthasterites tribrachiatus, Discoaster lodoensis, D. sublodoensis Zone and, partly, to the Nannotetrina fulgens Zone (upper Ypresian– Lutetian). The Kisselovia ornata, Wetzeliella irtyshensis, Areosphaeridium diktyoplokus Zone (joint occurrence), which is the most pronounced in the lower Tavda Formation, corre-

sponds to the Reticulofenestra umbilica Nanoplanktonic Zone (Barthonian). The Charlesdowniea clathrata angulosa Zone in the upper Tavda Formation corresponds to the Discoaster barbadiensis Nanoplanktonic Zone (Priabonian). The number of the dinoflagellate zones later increased to 15 after more detailed subdivision of the Paleogene marine sediments owing to studies carried out by N.I. Zaporozhets (Akhmet’ev et al., 2004a) and A.I. Yakovleva (2008).

Paleogene stage We will try to consider the structure of the subarctic areas of the North Ob’ region on the basis of the zoning of the Paleogene marine sediments of West Siberia. The same marine formations (Talitsa, Serov, Irbit, Nyurol’ka, and Tavda) as in the center of the plain (Babushkin, 2001; Chirva, 1960; Lyubomirova, 1960) are developed here. The sections differ in the degree of completeness of the geologic records. We used data from borehole sections on the northwestern Yamal Peninsula (Fig. 1, boreholes 1 and 3), along the Gulf of Ob’ in the southeast of this peninsula (boreholes 1K, 12RR, and 12RG), and in the lower reaches of the Pur (boreholes 11 and 32) and Taz Rivers (borehole 29). The sections of these boreholes reveal the structure of the Paleogene marine sedi-

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ments of the shelf zone of the Arctic epicontinental seas. The sediments are exposed here from Upper Cretaceous to Eocene. Lower Paleocene (Danian) The oldest sediments on the Yamal Peninsula are assigned to the Lower Paleocene. They were stripped in boreholes 12RR and 12RG (Fig. 1), below the Ob’ River level, at a depth of 160 m, and their thickness is more than 200 m. Silty, micaceous, and, sometimes, opoka-like clays in these boreholes belong to the Upper Tibeisale Subformation (Lower Paleocene) (Volkova et al., 2002). The Lower Paleocene sediments in the lower reaches of the Pur River (Fig. 2, borehole 11) (interval 281–300 m) overlie eroded Upper Cretaceous sediments, separated from them by a 5-m bed of sand with plant detritus. The Lower Paleocene is represented by compact dark gray, almost black clay with carbonaceous remnants. The sediments are similar in lithologic composition to those of the Talitsa Formation (southern West Siberia). In the Pur–Taz interfluve (Fig. 3, borehole 29), the sediments in the interval 260–280 m are replaced along strike by fine-grained micaceous sand with carbonaceous remnants and black clay interbeds. The sediments are tentatively assigned to the Danian Stage. Dinocysts are not found. We presume that they were deposited under continental conditions. Only at the boundary with the overlying Serov Formation, fragments of Cordosphaeridium dinoflagellates are observed. Within the interval 290–286 m, an assemblage with Wodehouseia fibriata Stanley is assigned to the Danian in the borehole. This part of the section contains a large amount of pollen of Triatriopollenites aroboratus Pfl., Tricolporopollenites spp., Extratriporopollenites clarus Pfl., and Trudopollis supplingensis (R. Rot Pfl.). The species Wodehouseia fibriata, typical of the Danian (?) sediments of Alaska and the upper Maastrichtian sediments of Canada (McIntyre, 1975), is widespread in the assemblage. Gymnospermous plants are represented by Taxodiaceae and Pinaceae. This assemblage is typical of the lower part of the Talitsa Subformation of Paleogene continental sediments. Upper Paleocene (Selandian and Thanetian) Upper Paleocene sediments are present in boreholes 29 (280–230 m), 12RR, 12RG, and 11 (Figs. 2, 3). The sediments on the left bank of the Taz River are assigned to the Upper Tibeisale Subformation, and those in the Pur basin, to the Upper Talitsa Subformation and Serov Formation. The sediments, which overlie eroded Paleocene clays, are slightly kaolinized fine- and medium-grained sands (Upper Talitsa Subformation) in boreholes 11 (interval 300–280 m) and 29 (280–260 m). The Serov Formation consists of diatomites and opoka-like clays with sand interbeds. The sediment thickness varies from 80 to 100 m. Dinocysts are observed in the sections of boreholes 12RR and 12RG. Species of the genus Apectodinium indicate late Late Paleocene–early Early Eocene sedimentation (Babushkin, 2001). The dinocysts coexist with

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abundant Myrtaceae pollen, diverse Triatriopollenites (including T. aroboratus Pfl. and T. excelsus Pfl.), and persistent pollen of Stemma Normapolles-Trudopollis menneri Zakl. and Nudopollis terminalis Pfl. Plant taxa similar to present-day ones (Platycaryapollenites, Ulmoideipites krempii And., Casuarinidites, Quercus sparsa (Mart.) Samoil., Juglans nigripites, Engelhardtia) yield a considerably large quantity of pollen. The assemblage contains Anacolosidites insignis Samoil. pollen and an insignificant but permanently present amount of Projectoporites spinulosus Samoil. and Regina excelsis Mtch. pollen. Orbiculapollis, Aquilapollenites, and Proteacidites adenanthoides Samoil. pollen occurs singly. Coniferous plants are represented by Taxodiaceae and Pinaceae pollen. The spores belong to sphagnum mosses. This composition is typical of the Upper Paleocene sediments of the Malyi Atlym area and the Middle Ob’ area of the Taz Peninsula (Tab-Yakha) (Samoilovich, 1961) as well as of the Trans-Urals facies region (Bakieva, 2003; Vasil’eva, 1990). For the Upper Paleocene sediments, Kul’kova detected dinocyst zones with the index species Cerodinium speciosum and C. markovae (zones NP4–NP6) (Babushkin, 2001). Dinoflagellates Ceratiopsis diebeli, Cordosphaeridium inodes (Klumpp) Eis., Areoligera, and Eocladopyxis are observed in borehole 11 (interval 250–190 m). This composition is similar to that of the Paleocene (Selandian) assemblage of Belgium. In the subarctic regions (boreholes 29, 11, and 32) (Figs. 2, 3, 4), the Paleocene sediments coexist with Lower, Middle, and Upper Eocene sediments, assigned to the Irbit, Nyurol’ka, and Tavda Formations. They are represented by marine facies up to 150 m thick. The sediments in all the boreholes are overlain by Pliocene–Quaternary rocks. They were studied in boreholes 32, 11, and 29 from intervals of 100–200 m. The Eocene sediments have an eroded surface. The sediments in all the boreholes are tabular dark gray diatomites with silt interbeds and rare interbeds of pebbles and glauconite sands. The Wetzeliella astra–Dracodinium simile Zone, detected in borehole 11 (interval 193–125 m), corresponds to zones NP10–NP11. Dinocysts of the Wetzeliella meckelfeldensis Zone in borehole 29 are detected at a depth of 180–150 m, and zones with Dracodinium varielongitudum—D. simile are observed in the interval 150–110 m. Charlesdowniea coleothrypta, which is typical of this zone, appears in the upper part of the interval. No pollen or spores are found in this zone. Microfossils are observed only in borehole 32 (interval 110–100 m), and they belong to the assemblage of Araliaceoipollenites euphorii and Sapotaceoidaepollenites manifestus. Also, Middle Eocene sediments are present in the boreholes. They are described as the Nyurol’ka Formation (upper Lyulinvor Horizon). These sediments are very widespread on the Yamal Peninsula, in the lower reaches of the Pur and Taz Rivers. Their thickness is more than 70 m, and they are usually localized at a depth of 100–65 m. The Middle Eocene is represented by greenish gray diatomaceous shales. The sediments in borehole 11 contain dinocysts of the Charlesdowniea coleothrypta Zone along with the species Rhom-

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Fig. 2. Borehole 11, lower reaches of the Pur River. 1, diatomites interbedded with diatomaceous shales; 2, clays; 3, diatomites; 4, sands; 5, clay loams; 6, silts; 7, gravel and pebbles; 8, carbonized remnants; 9, core sampling sites. The age of the formations in the figures is given in the text and on the columns.

bodinium glabrum and Charlesdowniea coleothrypta rotundata. The dinocysts coexist with a very typical assemblage of tricolporoidate pollen: Castanea crenataeformis, Araliaceoipollenites euphorii, Sapotaceoidaepollenites manifestus, Pompeckjoidaepollenites subhercynicus, and Castanopsis pseudocingulum. Also, Platycaryapollenites, Comptonia, and other specimens are present. This assemblage is similar to those detected by L.A. Panova (1968) for the Globorotalia aragonensis Zone and the lower Acarinina bullbrooki Zone. The Upper Eocene sediments in borehole 29 (interval 98–78 m) contain dinoflagellates of the Charlesdowniea coleothrypta Zone (DN10) (Babushkin, 2001). The species Ch. coleothrypta coexists with Rhombodinium cerciatum. The dinocysts are present together with pollen of the spore–pollen assemblage Araliaceoipollenites euphorii—Sapotaceoidaepollenites manifestus—Pompeckjoidaepollenites subhercynicus, Castanopsis

pseudocingulum (zone 7 (Kul’kova, 1994)). It correlates with the Charlesdowniea coleothrypta rotundata Zone (AndreevaGrigorovich, 1991) and corresponds to zones NP12–NP15 (upper Ypresian–Lutetian). The Upper Eocene in the subarctic region is represented by the Tavda Horizon. The last marine formation is exposed in all the boreholes, though its upper part is eroded. The sediments here are no more than 45 m thick. The Eocene marine sediments in the Arctic region are overlain by young Pliocene–Quaternary sediments. The Eocene marine sediments are bedded thin-tabular gray and dark gray clay. Clay in the upper part contains plant remnants, and the section ends with fine-grained micaceous sand, which indicates the shallowing of the basin and the proximity of the shoreline. Dinocysts Rhombodinium pentagonum Vozzh., Lentinia serrata Bujak, and Areosphaeridium diktyoplokus Klumpp are found in borehole 32. Also, the sediments of this

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Fig. 3. Borehole 29, Pur–Taz interfluve. See legend in Fig. 2.

zone contain pollen of the assemblage Castanopsis pseudocingulum—Rhoipites and Bolivina asiatica (Barthonian), Castanopsis pseudocingulum—Quercus gracilis—Tricolporopollenites liblarensis, which correlates with the Middle Eocene assemblage of Panova and corresponds to the Globigerina turcmenica and Bolivina asiatica Zone of planktonic foraminifers (Barthonian). The dinocysts in borehole 32 (interval 42–30 m), which belong to the assemblage Rhombodinium pentagonum—Charlesdowniea clathrata angulosa, characterize the late Middle–early Upper Eocene. This interval contains the pollen assemblage Quercus gracilis—Q. graciliformis, Castanea crenataeformis, and Rhoipites pseudocingulum. Note the appearance of Carya simplex, Corylopsis, and Hamamelis pollen in small amounts. According to Kul’kova, this assemblage correlates with that from the Barthonian sediments of Western Europe. We emphasize that the dinocyst composition of the sediments permits their comparison with analogs of the Paleocene and Eocene sediments of Western Europe (Selandian, Thanetian, Ypresian, Lutetian, and Bar-

thonian). The lower Tavda Formation is dated at the late Lutetian–Barthonian. Priabonian sediments are missing from this area in the Arctic region. Another distinctive feature of this area is the lack of Oligocene or Neogene continental sediments (Volkova, 1999). The main stages of the geologic evolution of Paleogene transgression (from late Paleocene to Oligocene) were considered in some publications (Chirva, 1960; Gnibidenko et al., 2011; Kulkova and Volkova, 1997; Lyubomirova, 1960; Volkova et al., 2005). The maximum transgression was observed in the Middle Eocene. The Eocene sea basin extended far northward. The climate at that time was near-subtropical. The average annual temperature varied from +20 to +25 °C, and precipitation reached 1000–1400 mm/yr (Kulkova and Volkova, 1997; Volkova, 2011). These conditions favored the formation of hydrocarbons. Note that the Siberian sea basin experienced multiple sea level variations. This is especially true of the Tavda basin. Apparently, transgression was followed by short-term regression, and more deep-water facies were replaced by shallow-water ones. This

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Fig. 4. Borehole 32, lower reaches of the Pur River. See legend in Fig. 2.

is evidenced by the repeated dispersal of Hydropteris indutus (Azolla). For example, there are two levels of dispersal on the edge of the Tavda basin (borehole 9, south of Lake Chany). One is localized at a depth of 405–350 m in the lower subformation of the Tavda basin, and the other is localized at a depth of 300–350 m (Priabonian), chrons R3tv–N2tv and N1tv (Gnibidenko, 2006; Volkova et al., 2005). According to M.A. Akhmet’ev et al. (2004a,b, 2010), the level of the World Ocean became lower in the Barthonian and Priabonian. The West Siberian Tavda basin lost connection with the Arctic basin. The termination of water exchange with the Arctic basin through the West Siberian inland basin caused the formation of a closed estuary-type system of currents therein (Akhmet’ev and Zaporozhets, 2011). All this favored the desalinization of the basin, as evidenced by the stunted development of the foraminifers. In the view of M.A. Akhmet’ev, the formation of Azolla beds in the “high” Arctic and Lomonosov Ridge is related to the cooling at that time. The temperature of seawater during the phase of cooling and desalinization was no more than 10 °C (Brinkhuis et al., 2006). According to (Akhmet’ev and Zaporozhets, 2010), signs of cooling are detected for the final phase of formation of the Azolla beds during the Late Tavda time. It caused an increase in the abundance of coniferous-tree pollen in the spectra. The composition of spores and pollen from the sediments of the Tavda basin indicates a decrease in the average annual temperatures in the

late Priabonian, which were 7–10 °C lower than those in the Barthonian (Volkova, 2011). In our view, the presence of Azolla beds and their age (in the Lomonosov Ridge) call for further study. The Azolla beds in the Lomonosov Ridge were detected by ACEX participants in the section of borehole MOOO4A. Two peaks of the distribution of these beds are observed at depths of 313.35 and 302.00–301.35 m. Actually, these peaks are similar in depth to the stages with Hydropteris indutus detected by us in the Tavda basin. The existence of one more level at the boundary between the Nyurol’ka and Tavda Horizons is hypothesized. During a visit to the Laboratory of Mesozoic and Cenozoic Paleontology and Stratigraphy (Trofimuk Institute of Petroleum Geology and Geophysics), the expedition members showed us argillaceous rock which was Azolla conglomerate. The Azolla species were not defined, and their exact age was hard to determine. According to (Akhmet’ev and Zaporozhets, 2010), their origin is the same in the Arctic and in the West Siberian sea basin (Hydropteris indutus). Their appearance is due to the lowering of the World Ocean level, which caused the desalinization of the Arctic basin waters and the transformation of a system of currents open toward the Tethys Ocean into a half-closed one. This resulted in the flourishing of Azolla beds in desalinized waters near the Lomonosov Ridge.

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Along with the hindered water exchange of the Polar basin with other oceanic regions during the period when the World Ocean level was low, another cause of the desalinization is hypothesized in (Akhmet’ev and Zaporozhets, 2010, 2011)—a dramatic increase in the precipitation–evaporation ratio in polar regions during a period of humid climate. According to the spore–pollen analysis, precipitation in West Siberia during the second half of the period of the Tavda basin evolution decreased to 1000–1200 mm/yr, but the percentage of their evaporation and their effect on the desalinization of the basin waters are hard to calculate. As was pointed out by V.M. Sinitsyn (1965), the general atmospheric circulation above Eurasia remained unchanged in the Paleogene. Only in the late Eocene, precipitation decreased to 900–800 mm/yr. The floristic composition of the phytogeographic zones in the Eocene was different from the present composition. Near-subtropical forests existed in the Arctic at that time (Kulkova and Volkova, 1997; Volkova et al., 2005). The geologic structure of the continental margins found its reflection at the bottom of the Kara Sea and then in the seismic complexes of the Lomonosov Ridge. The Barents–Kara shelf sea belongs to the eastern Pacific. Geologic structure in passing to the continental regions and the atmospheric dynamics are considered in (Gramberg and Pogrebitskii, 1984; Gramberg and Stroev, 1992). The authors showed that the structure of continental margins depends on many factors and processes determining the evolution of oceans and related sedimentation, including that of the sedimentary cover. Two peculiarities were pointed out by I.S. Gramberg in the evolution of the Atlantic: (1) the formation of oceanic lithosphere and (2) slow opening of the Atlantic and fast opening of the Pacific. The subsequent tectonic events affected the buildup of Meso-Cenozoic and Cenozoic sediments and the development of biota. The sediment thickness and the character of the structures were inherited from the structures of the Mesozoic cover (Alekseev et al., 1991; Gurevich and Musatov, 1989). In the Kara Sea (Shipilov and Tarasov, 1989), a section was compiled for the sedimentary cover (Jurassic, Cretaceous, and Paleogene sediments) as well as for a considerable part of the intermediate complex filling deep troughs, which includes Triassic sediments. It was found that the sedimentary cover is localized at an average depth of 4.4 km in the Kara Sea and at a depth of 16–18 km in the Barents Sea. It is more uniformly distributed in the Kara Sea than in the Barents Sea. The Kara basin is divided into the northern and southern parts based on geologic structure and the thickness of the sedimentary cover. The maximum thickness of the sedimentary cover is observed in the northern Kara basin (more than 10 km). Seismic studies show a change in the cover thickness from west to east. This indicates the reciprocal character of tectonic processes going on with retarded tectonic cycles (Shipilov and Tarasov, 1989). According to V.I. Gurevich and E.E. Musatov (1989), the thickness of the Paleogene–Neogene and Quaternary covers in the Kara basin correlates with the deep localization (inherited) of the key horizons of Lower Triassic and Middle–

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Upper Jurassic surfaces, which, in turn, depends on the top of the basement. The thickness of the Paleogene cover reaches 500 m in the basins and decreases to 10 m in the area of Novaya Zemlya. When the paleoecology and paleogeography of the Arctic Ocean were considered, it was pointed out in (Danilov and Shilo, 1998; Krapivner et al., 1989) that its Paleogene coincided with this period on the continent and had a marine character before the early Oligocene against the background of slow uplifting, which had ended by the early Oligocene. In the Oligocene, the formation of the deep basin of the Arctic Ocean was accompanied by the general regression of the Barents–Kara Sea. Seismic studies on the western Barents– Kara margin revealed pre-Paleogene fjords. They inherit pre-Cenozoic (Devonian–Jurassic–Cretaceous) paleograbens. A thick unit of Paleogene (?) and Neogene–Quaternary sediments is detected therein. However, it is presumed in (Musatov and Musatov, 1989) that grabens such as the Saint Anna or Voronin graben were initiated later, in the Quaternary. They lack Paleogene sediments but are filled with Quaternary deposits immediately overlying Cretaceous sediments. These landforms are produced by recent tectonics. A continuous section of Paleocene–Eocene and Lower Oligocene rocks is observed in the western part of the basin. The Paleogene cover in the Kara Sea region is up to 0.7 km thick. According to the seismic data, the Paleogene sediments are not ubiquitous. For example, there are no Paleogene sediments on Sverdluk Island. Upper Cretaceous and Paleogene sediments are exposed in the Nizhnyaya Zemlya trench. On the Yamal Peninsula, near Cape Kharasovei, a thick (up to 150 m) Quaternary unit in the geoacoustic section and borehole 1 overlies Paleogene rocks with angular unconformity. A similar structure of the sedimentary cover in seas was described by A.P. Lisitsyn (1974). During the post-Paleogene time, the subsequent sedimentation in the western and eastern Arctic shelf of Russia was closely related to tectonics. The latter caused a change of the transgressive and regressive phases of the Eocene sea, which affected the rhythm of sedimentation both on the continent and at the bottom of the Kara Sea. When considering the paleoecology and paleogeography of the Arctic seas, B.I. Kim and I.V. Reinen (1989) pointed out the synchronism of the tectonic processes determining the close relationship between the rhythm of sedimentation and topography in the Neogene. According to (Spiridonov et al., 1989; Suzdal’skii, 1972), the Pliocene–Quaternary cover in the Barents–Kara shelf was up to 200 m thick, and the younger Quaternary sediments were up to 100 m thick. Ridgelike glacial forms are observed in the eastern Kara Sea at a depth of 150–200 m. Valleys incised into the Paleogene rocks in the Barents–Kara shelf are of great interest. According to A.P. Lastochkin (1978), the presence of U-shaped valleys in all the platforms of Eurasia indicates that the processes were similar and, most likely, simultaneous. The incision depth reaches 270–300 m. Pliocene sediments more than 200 m thick are stripped on the eastern Yamal Peninsula, near Kamennyi Mys Village, borehole 1. According to our data (Volkova, 1999), the Pliocene sediments are here overlain by

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the Danian–Paleocene Talitsa Formation. Studies of buried valleys should take into account the time they were initiated (start of the incision) and filled with sediments. Their initiation might be coeval with the renewal of tectonic activity, fall of the World Ocean level, and regression of the Tavda basin. The valleys were filled with sediments during a later period, at a higher level of the World Ocean. Analysis shows variation in the elevations of the Paleocene–Eocene rocks in the Kara shelf. The sediment thickness is also variable. The thickness of the Talitsa Formation is up to 120–130 m in the Yamal–Taz region and no more than 60 m in the lower reaches of the Pur River (Volkova, 2002). It varies from 50 to 10 m in the upper reaches of the Vogulka River, near the Sartyn’ya River, and decreases to 10 m in the Urals area. Researchers presume that Paleocene sediments overlie eroded Jurassic sediments both in the South Kara and North Kara basins. In the shelf, near the North Siberian step, the Jurassic rocks wedge out; the thickness of the Cretaceous rocks decreases by two times; and, correspondingly, the thickness of the Paleogene sediments decreases to 50–10 m. The Paleogene sediments on the Arctic banks of the North Ob’ region and those at the bottom of the Kara Sea had identical paleontological characteristics. However, the Barents–Kara shelf had a complicated evolution, reflected in the sediment structure, biota changes, and the distribution of thickness of the Meso-Cenozoic cover. The situation was particularly complicated at the Eocene/Early Oligocene boundary. At that time, nonuniform tectonic movements in the shelf of the North Ob’ region destroyed most of the Eocene, Oligocene, and Neogene sediments. Large-scale studies of the sedimentary cover of the Laptev Sea part of the Lomonosov Ridge (Rekant and Gusev, 2012) show that the structural cover of the Lomonosov Ridge formed in the Lower Cretaceous. The initiation and evolution of the horst–graben structure of the ridge and the fault zone belong to this period. The formation of correlate sediments takes place simultaneously. Their study shows that intense formation of deep-water basins and separating rises of the Lomonosov Ridge began in the late Oligocene–Early Miocene. An important conclusion is made that the ridge structure cannot be considered in isolation from the structure of the part of the Laptev Sea–East Siberian Sea continental margin adjoining in the south. The structure of the sedimentary cover of the Lomonosov Ridge is a structural extension of the New Siberian system of horsts and grabens, recognized in the shelf. Also, digital models were constructed for the Moho topography as well as the distribution of, and variation in, the total thickness of the crust and its consolidated part within the deep-water zone of the Arctic Ocean. It was shown that the total crustal thickness varies in the deep-water basins from 10 to 16–30 km (Glebovsky et al., 2013). Qualitatively new geological data on the Arctic region have been obtained in recent years by ACEX-302. Mesozoic and Cenozoic sediments were stripped in five boreholes drilled in the Lomonosov Ridge between 87° and 88° N. Two boreholes (MOO2A and MOOO4A) are localized on the ridge slope. A 428-m section was studied in the boreholes (Kim and Glezer,

2007). The Upper Cretaceous sediments stripped at the bottom of borehole MOOO4A are unconformably overlain by Upper Paleocene sediments. Data on marine fauna and flora permitted J. Backman et al. (2006) to divide the section into four complexes. The first one (220.24 m below seafloor (m b.s.f.)) is assigned to the Holocene–Middle Eocene; the second (220.24–313.01 m b.s.f.), to the Middle Eocene; the third (313.01–404.79 m b.s.f.), to the Upper Paleocene–Lower Eocene; and the fourth (424.5–426.7 m b.s.f.), to the Upper Cretaceous. Diatoms, silicoflagellates, dinocysts, radiolarians, and foraminifers were studied from the sediments. The comprehensive biostratigraphic subdivision was compared with the biotratigraphic zones on silicoflagellates recognized in (Glezer, 1979a,b; Glezer and Stepanova, 1994). The silicoflagellate zones were compared with the dinocyst zones of the West Siberian Paleogene, recognized by Kul’kova (Kul’kova, 1987, 1994; Kul’kova and Shatsky, 1990; Volkova, 2002). As a result, the ages in the borehole sections were determined and compared based on microfossils with the data on the littoral sections of the Ob’ Arctic zone. Along with stages of geologic evolution, stages of transgression and regression—short moments of sea shallowing—were detected and correlated in time. The Late Cretaceous in the Lomonosov Ridge corresponds to an unconformity surface related to the fall of the sea level. The next stage belongs to the Paleocene. Early Paleocene reflects a phase of tectonic stabilization. No marine organisms are observed on the continent, in West Siberian sediments. A regime of peneplanation and crustal weathering was established at that stage. At that time, chemical weathering crusts were widespread on the Baltic Shield (Afanas’ev, 1977) and in the Polar Urals (Kaletskaya and Miklukho-Maklai, 1961) as well as on the New Siberian Islands and Laptev Sea shore (Kim and Slobodin, 1991). They consisted of variegated clays and clay loams. Their maximum thickness in the boreholes between the Lyakhovsky Islands was 23 m (Kim and Slobodin, 1991). A long epoch of peneplanation on the Lomonosov Ridge caused the erosion and leveling of topography. The erosion led to the omission of the Lower Paleocene sediments from the section of borehole MOOO4A. The Upper Paleocene sediments here overlie the eroded Upper Cretaceous sediments. A long transgression stage began later, in the late Paleocene. In northern West Siberia, the age of this transgression corresponds to that of the Tibeisale Formation (boreholes 12RR and 12RG on the Yamal Peninsula and borehole 1K on Cape Kharasovei). In the Kara shelf, it corresponds to the seismic unit stripped in boreholes in the Leningradskaya and Rusanovskaya areas. These sediments belong here to seismic complex LR3. Their accumulation indicates the subsidence of the Lomonosov Ridge below sea level (Kim and Glezer, 2007). A short-term regression took place at the Paleocene/Eocene boundary. Analysis of the time sections of seismic profiles on the Lomonosov Ridge shows the onlap of the Eocene seismic complex LR4 on the underlying Upper Paleocene complex LR3, suggesting the formation of an unconformity surface owing to the lack of sedimentation (Butsenko, 2001). Transgression is observed in the West Siberian Eocene. Synchro-

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nous marine sediments are detected in the Kara shelf (Rusanovskaya and Leningradskaya areas). The stripped sediments contain vast assemblages of diatoms, silicoflagellates, and dinocysts, which are compositionally identical to the microfossils from the Irbit and Lyulinvor Formations of the West Siberian Plain. The Lomonosov Ridge sediments correspond to the Eocene seismic complex LR4. According to Kim (2003), the maximum transgression took place in the Middle Eocene, as was the case with West Siberia. A regression is detected at the Oligocene/Eocene boundary. It is assumed in (Kim and Glezer, 2007) that the rise of the islands (Spitsbergen, Franz Josef Land, and Severnaya Zemlya) during that time with the second phase of regression caused the separation of the Arctic basin in the north and the West Siberian basin in the south, and a continental regime was established in the uplifted areas. The start of the West Siberian Oligocene corresponds to the Atlym Formation of continental sediments. It is not preserved everywhere in the Arctic sector. The Atlym Formation in West Siberia overlies eroded marine sediments of the Upper Eocene. However, according to Kim, the Early Oligocene marine sediments are detected in the basins of the Lomonosov Ridge. The presence of marine sediments in the ridge cover indicates its position below the sea level, but this question is disputable. Neogene stage A new stage of tectonic stabilization is observed at the Oligocene/Early Miocene boundary. New chemical weathering crust formed outside the area of lake basins during that time. The crust consists of kaolinite and kaolin-hydromica variegated clays, especially in southeastern West Siberia. The Oligocene–Miocene crust is no more than 8–20 m thick. The crust in the Mackenzie District (Cariboo Mountains), Canada, is composed of kaolin up to 40 m thick (Kim and Slobodin, 1991). Canadian geologists assign the crust to the Late Oligocene. This period in the Lomonosov Ridge corresponds to a washout. This time interval corresponds to the largest fall of the ocean level in the Earth’s history. The data on the Middle and Late Miocene paleogeography are not enough. Therefore, it is hard for us to acknowledge the existence of extensive transgression, at the onset of which there was fauna exchange between the Arctic (whose water was not cool at that time) and North Pacific Oceans (Alekseev et al., 1991). The absence of ice in the Arctic Ocean is evidenced by the average annual temperatures, which are quite high for this time interval (Volkova, 2011). In Kim’s view, the fauna exchange took place not through the Bering Strait but through the basin of the middle and upper reaches of the Kolyma River, in which marine sediments with a mixed assemblage of marine, brackish-water, and freshwater diatoms were detected (Kim and Slobodin, 1991). However, this issue calls for further study. The marine genesis of the sediments in the western sector of the Russian Arctic (Gladulina beds (Kolva Formation in the Pechora area) and the Ust’-Solenaya Formation (near the Yenisei River, West Siberia)) remains debatable (Gramberg

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and Kulakov, 1983) Members of the former Scientific Research Institute of Arctic Geology (NIIGA) recognize coeval sediments on the Taimyr Peninsula, Severnaya Zemlya, Begichev Island (Laptev Sea basin), New Siberian Islands, Novaya Zemlya, and Fadeevskii Island (Kim and Slobodin, 1991). A foraminiferal asssemblage is detected in the Middle and Upper Miocene sediments of the study areas. Middle–Upper Miocene sediments are stripped in boreholes in the lower reaches of the Kolyma River and within the Chukchi shelf (Alekseev et al., 1991). The Middle and Upper Miocene marine sediments in the Lomonosov Ridge are observed only in borehole MOO2A. The sediments correspond to seismic complex LR6. The presence of these sediments in the ridge confirms the formation of coeval deposits in the ocean basins, in which they correspond to seismic complexes. The global regression of the World Ocean in the late Late Miocene caused the uplifting of land and the drying of shelf. This stage on the shore framing was manifested everywhere in interregional erosion and the initiation of U-shaped valleys in Eurasian shelf. As was stated above, the buried valleys are more than 200 m deep in the Yamal shelf and more than 380 m deep in the shelf near Dudinka, by the Yenisei River (Volkova, 2010). The onlap of complex LR7 on the underlying complex LR6 (Middle–Upper Miocene) is observed in the Lomonosov Ridge. The formation of an unconformity surface takes place in the absence of sedimentation. We cannot agree with the NIIGA geologists in that the Pliocene was marked by a long epoch of continuous-discontinuous transgression, the most extensive in the Arctic. Marinism, which was particularly prosperous in the 1970s, was argued against by S.L. Troitskii (1975). He showed the complicated evolution of the Arctic shelf in the Neogene and Quaternary. As pointed out above, the Neogene is characterized by the widespread occurrence of lake basins and differentiated tectonic movements. The first half of the Neogene (Miocene) was a time of tectonic quiescence in northern Siberia (Strelkov, 1965; Vdovin, 1979). The recent tectonic movements had low amplitudes and, according to I.L. Kuzin (2002), were almost not reflected in the sediment structure. However, they caused the shrinkage of the Early Miocene Abrosimovka lake basin and its breakup into several small waterbodies with the accumulation of clays with interbeds of buried soils and brown coals. A latitudinal system of Ob’–Yenisei uplifts without sedimentation appeared in the northern part of the plain during the Neogene. Thin sediments were destroyed by denudation. Actually, denudation plains formed on the uplifts, and lacustrine sediments accumulated north and south of them. In the Middle and Late Miocene and Pliocene, renewed tectonic activity had a great influence on the formation of the sediments and topography. In the north of the plain (shelf zone), it resulted in the erosion of a considerable part of the Eocene, Oligocene, and Neogene rocks. Core studies (Volkova, 1999) showed the high elevation of the Upper Cretaceous sediments on the Yamal Peninsula. The Upper Cretaceous rocks are localized at a depth of only 350 m on the Yamal Peninsula and at a depth of 225 m on the Taz Peninsula. The eroded surface of the Cretaceous rocks

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is overlain by the compact clays and clay loams (up to 150 m thick) of the Lower Paleocene Tibeisale Formation, which, in turn, is overlain by Upper Pliocene–Quaternary sediments with a large sedimentation gap. It has been found that the northern paleoshelf lacks not only Neogene but also Eocene and Oligocene sediments. Tectonic activity at 7–6 Ma caused the subsequent incision and deepening of the buried valleys. They were filled with sediments in the Late Pliocene at a high sea level (Zarkhidze et al., 1991). The northern U-shaped valleys are incised into the surface of the Upper Cretaceous and Paleocene sediments. The authors of (Slobodin and Kim, 1990) are right in that the filling of the U-shaped valleys took place at a high level of the World Ocean (up to the maximum values). This is evidenced by the position of shorelines at elevations of 320–(255–270) m, according to V.Ya. Slobodin. Shorelines are observed in the Urals, on Paikhoi, and on Novaya Zemlya. Slobodin states that they are known in the Arctic, on the shores and islands of the Arctic Ocean. At the current stage of research, no marine sediments have been found in the U-shaped valleys. The valleys are filled with Upper Pliocene continental sediments; in the lower reaches of the Yenisei River, they are filled with Quaternary marine sediments. The position of the buried valleys in the Ob’ basin coincides with that of present-day rivers. The sea level during the incision was 299–300 m below the current one (Strelkov, 1965). The Neogene surface oscillated with an amplitude of up to 400–500 m. The amplitude might have been higher at the boundary of the Early Pliocene, particularly during the period of ascending tectonic movements. The present amplitude of elevations in northern West Siberia is no more than 220 m. The thickness of the Pliocene sediments in the buried valleys is more than 200 m. Undoubtedly, they were also renewed owing to other abiotic facotrs, including permafrost and solifluction (Zol’nikov et al., 2004). The U-shaped valleys on the central Taz Peninsula (Arkhipov et al., 1994; Volkova, 1999) lack Miocene and Pliocene sediments. Paleocene sediments overlain by Lower–Middle Quaternary moraine deposits are localized here at a depth of 200 m. The structure of the sections suggests that the Taz Peninsula in the Pliocene experienced stronger ascending movements, which determined the existence of the Taz system of uplifts; the latter inherit the general structures of the Meso-Cenozoic sedimentary cover. In the late Pliocene, the subsidence trend in the lower reaches of the Ob’ River was more stable, which led to the accumulation of lacustrine alluvium in the buried valleys. The discordant (unconformable with the structure) bedding of Tertiary sediments (Arkhipov, 1971; Arkhipov et al., 1994) is due to the young tectonic movements which favored the subsidence of the Ob’ Canal and the lower reaches of the Ob’ River. Owing to the discordant bedding of the Tertiary sediments, they all (Paleogene and Neogene) pinch out northward of the Malyi Atlym dislocations and dip southward, into the Khanty-Mansi depression. Only Paleocene sediments extend northward, to Peregrebnoe Village. The surface of the pre-Quaternary sediments in some boreholes is localized at a depth of 150–170 m, and the Pliocene sediments are overlain by boulder clay loams and clays with Paleogene megaclasts

and outliers. On the Taz and Yamal Peninsulas, these sediments are described as moraines and assigned to the upper Eopleistocene (Volkova and Babushkin, 2000). Along with the sedimentation, the ancient glacier showed intense exaration activity by renewing valleys and destroying and reworking the pre-Quaternary sediments. Later, in the Pleistocene, the north of the plain experienced multiple glaciations alternating with marine transgressions (Arkhipov and Volkova, 1994; Grosval’d, 1999; Grosval’d and Glazovskii, 1988; Lavrushin, 2010). All this took place during a new wave of tectonic activity, which also had a great influence on sedimentation, the evolution of topography (particularly the river network), and multiple increases in the area of the Arctic shelf. Conclusions Analysis of biotic and abiotic data suggests that the areas of the Barents and Kara Seas developed in the Paleogene together with the Eurasian ocean subbasin and the Barents– Kara continental-margin plate, making up the West Arctic zone of continent–ocean transition (Alekseev et al., 1991; Gramberg and Stroev, 1992; Zarkhidze et al., 1991). As a result of early Oligocene tectonic activity, the Lomonosov Ridge was only a fragment of continental crust detached by rifting from the Eurasian shelf and transported to high latitudes (Akhmet’ev et al., 2010; Poselov et al., 2010, 2012). Oligocene in the Arctic shelf was characterized by continental sedimentation in a warm climate (Kulkova and Volkova, 1997; Volkova, 2011). The Neogene stage was the most complicated for the Arctic zone. The Neogene tectonics gave rise to deep valleys and fjords filled with sediments in the Pliocene at a high sea level. In the late Pliocene and the first half of the Quaternary, the Arctic region experienced the first ancient glaciation with a dramatic decrease in the air temperature (Kulkova and Volkova, 1997; Volkova, 2011). The study was carried out under the research and development plan and project no. 23 of the Russian Academy of Sciences. References Afanas’ev, A.P., 1977. Phanerozoic Weathering Crusts of the Baltic Shield and Related Mineral Resources [in Russian]. Nauka, Leningrad. Akhmet’ev, M.A., Zaporozhets, N.I., 2010. Formation of Azolla beds at the Middle/Late Eocene boundary owing to the shallowing of surface waters in the sea basin of West Siberia, in: Paleostratigraphy-2010. PIN RAN, Moscow, pp. 12–13. Akhmet’ev, M.A., Zaporozhets, N.I., 2011. The mechanism of formation of Azolla beds in the Eocene sediments of the Lomonosov Ridge (high-latitude Arctic) and Tavda basin (West Siberian Plate), in: Shurygin, B.N., Lebedeva, N.K., Goryacheva, A.A. (Eds.), Mesozoic and Cenozoic Paleontology, Stratigraphy, and Paleogeography of Boreal Regions (Proc. Sci. Conf. in Honor of the 100th Birthday of V.N. Saks (Novosibirsk, 18–22 April 2011)) [in Russian], Vol. 2: Cenozoic. INGG SO RAN, Novosibirsk, pp. 17–22. Akhmet’ev, M.A., Aleksandrova, G.N., Beniamovskii, V.N., Vitukhin, D.I., Glezer, Z.I., Gnibidenko, Z.N., Dergachev, V.D., Dolya, Zh.A., Zaporozhets, N.I., Kozlova, G.E., Kul’kova, I.A., Nikolaeva, I.A., Ovech-

V.S. Volkova / Russian Geology and Geophysics 55 (2014) 483–494 kina, M.N., Radionova, E.P., Strel’nikova, N.I., 2004a. New data on the marine Paleogene of the southwestern Siberian Plate, Paper 1. Stratigrafiya. Geologicheskaya Korrelyatsiya 12 (1), 67–93. Akhmet’ev, M.A., Aleksandrova, G.N., Beniamovskii, V.N., Vitukhin, D.I., Glezer, Z.I., Gnibidenko, Z.N., Dergachev, V.D., Dolya, Zh.A., Zaporozhets, N.I., Kozlova, G.E., Kul’kova, I.A., Nikolaeva, I.A., Ovechkina, M.N., Radionova, E.P., Strel’nikova, N.I., 2004b. New data on the marine Paleogene of the southern West Siberian Plate, Paper 2. Stratigrafiya. Geologicheskaya Korrelyatsiya 12 (5), 65–86. Akhmet’ev, M.A., Zaporozhets, N.I., Iakovleva, A.I., Aleksandrova, G.N., Beniamovsky, V.N., Oreshkina, T.V., Gnibidenko, Z.N., Dolya, Zh.A., 2010. Comparative analysis of marine Paleogene sections and biota from West Siberia and the Arctic region. Stratigrafiya. Geologicheskaya Korrelyatsiya 18 (6), 78–103. Alekseev, M.N., Arkhangelov, A.A., Ivanova, I.M., Kim, B.I, Patyk-Kara, N.G., Reinen, I.V., Sekretov, S.B., Sharudo, O.I., 1991. The Laptev and East Siberian Seas: Cenozoic, in: Alekseev, M.N. (Ed.), Palaeogeographical Atlas of the Shelf Regions of Eurasia for the Mesozoic and Cenozoic. Robertson Group, Great Britain–GIN AN SSSR, Moscow, Vol. 1, pp. 114–120. Andreeva-Grigorovich, A.S., 1991. Zonal Stratigraphy of the Paleogene of the Soviet Union from Microphytoplankton Data (Dinocysts and Nanoplankton). Extended Abstract Doctoral (Geol.–Mineral. Dissertation). Inst. Geol. Sci., Kiev. Arkhipov, S.A., 1971. The Quaternary Period in West Siberia [in Russian]. Nauka, Novosibirsk. Arkhipov, S.A., Volkova, V.S., 1994. Geologic History, Landscapes, and Climates of the West Siberian Pleistocene (Trans. OIGGM) [in Russian]. OIGGM SO RAN, Novosibirsk, Issue 823. Arkhipov, S.A., Levchuk, L.K., Shelkoplyas, V.N., 1994. Stratigraphy and geological structure of the Quaternary in the Lower Ob–Yamal–Taz region of West Siberia. Geologiya i Geofizika (Russian Geology and Geophysics) 35 (6), 87–104. Babushkin, A.E. (Ed.), 2001. A Unified Regional Stratigraphic Chart of the Paleogene and Neogene Sediments of the West Siberian Plain [in Russian]. SNIIGGiMS, Novosibirsk. Backman, J., Moran, K., McInroy, D.B., Mayer, L., 2006. Arctic Coring Expedition (ACEX): Paleoceanographic and Tectonic Evolution of the Central Arctic Ocean. Edinburgh, UK, Integrated Ocean Drilling Program Management International, Inc. (Proceedings of the Integrated Ocean Drilling Program, Vol. 302). Bakieva, L.B., 2003. Palynological characteristics of Paleocene deposits in the northern West Siberian Plate. Stratigrafiya. Geologicheskaya Korrelyatsiya 11 (5), 58–71. Berggren, W.A., Kent, D.V, Swisher, C.C. III, Aubry, M.P., 1995. A revised Cenozoic geochronology and chronostratigraphy, in: Berggren, W.A., Kent, D.V., Aubry, M.-P., Hardenbol, J. (Eds.), Geochronology, Time Scales, and Global Stratigraphic Correlation. SEPM Spec. Publ. 54, 129–212. Brinkhuis, H., Schouten, S., Collinson, M.E., Sluijs, A., Sinninghe Damsté, J.S., Dickens, G.R., Huber, M., Cronin, T.M., Onodera, J., Takahashi, K., Bujak, J.P., Stein, R., van der Burgh, J., Eldrett, J.S., Harding, I.C., Lotter, A.F., Sangiorgi, F., van Konijnenburg-van Cittert, H., de Leeuw, J.W., Matthiessen, J., Backman, J., Moran, K., and the Expedition-302 Scientists, 2006. Episodic fresh surface waters in the Eocene Arctic Ocean. Nature 441 (7093), 606–609. Butsenko, V.V., 2001. Seismostratigraphic Analysis of the Sedimentary Cover of the Western Amerasian Basin. Extended Abstract Cand. Sci. (Geol.– Mineral.) Dissertation. St. Petersburg State Univ., St. Petersburg. Chirva, S.A., 1960. The Tertiary deposits of the southeastern Taz Peninsula, in: Essays on the Geology of the Northern West Siberian Plain (Trans. VNIGRI) [in Russian]. Gostoptekhizdat, Leningrad, Issue 158, pp. 23–39. Costa, L.I., Manum, S., Meyer, K.-J., 1988. The regional distribution of dinoflagellates; correlation of the interregional zonation with the local zones and with the regional lithostratigraphy. Great Britain/Norway, The Viking Graben, in: Venken, R. (Ed.), The Northwest European Tertiary Basin: Results of the International Geological Correlation Programme, Project 124. Geol. Jahrb., Reihe A, Heft 100, pp. 330–332.

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Danilov, I.D., Shilo, N.A., 1998. Transgressive–regressive cycles in evolution of the Arctic Ocean during the Late Cenozoic. Stratigrafiya. Geologicheskaya Korrelyatsiya 6 (6), 92–100. Glebovsky, V.Yu., Astafurova, E.G., Chernykh, A.A., Korneva, M.S., Kaminsky, V.D., Poselov, V.A., 2013. Thickness of the Earth’s crust in the deep Arctic Ocean: results of a 3D gravity modeling. Russian Geology and Geophysics (Geologiya i Geofizika) 54 (3), 247–262 (327–344). Glezer, Z.I., 1979a. Diatom zonation of Paleogene sediments. Sovetskaya Geologiya, No. 11, 19–30. Glezer, Z.I., 1979b. Silicoflagellate Zonation of the Paleogene Sediments of the Mediterranean Paleogeographic Region in the Territory of the Soviet Union (Trans. of the XXII Session of the All-Union Paleontol. Soc.) [in Russian]. Nauka, Leningrad, pp. 29–42. Glezer, Z.I., Stepanova, G.V., 1994. Diatom and silicoflagellate division and correlation of the Paleogene sediments of the Kara Sea. Regional’naya Geologiya i Metallogeniya (2), 148–153. Gnibidenko, Z.N., 2006. Cenozoic Paleomagnetism of the West Siberian Plate [in Russian]. Geo, Novosibirsk. Gnibidenko, Z.N., Volkova, V.S., Kuz’mina, O.B., Dolya, Zh.A., Khazina, I.V., Levicheva, A.V., 2011. Stratigraphic, paleomagnetic, and palynological data on the Paleogene–Neogene continental sediments of southwestern West Siberia. Russian Geology and Geophysics (Geologiya i Geofizika) 52 (4), 466–473 (596–605). Gramberg, I.S., Kulakov, Yu.N. (Eds.), 1983. Main Problems of the Late Cenozoic Paleogeography of the Arctic (Trans. Sevmorgeologiya) [in Russian]. Nedra, Leningrad, Vol. 190. Gramberg, I.S., Pogrebitskii, Yu.E. (Eds.), 1984. Seas of the Soviet Arctic, Vol. 9 of Geologic Structure of the Soviet Union and Distribution Patterns of Mineral Resources (Ed. by E.A. Kozlovskii) [in Russian]. NIIGA, Leningrad. Gramberg, I.S., Stroev, P.A., 1992. Deep Structure and Geodynamics of the Atlantic and Pacific Lithosphere [in Russian]. Nauka, Moscow. Grosval’d, M.G., 1999. Eurasian Hydrosphere Catastrophes and the Arctic Glaciation [in Russian]. Nauchnyi Mir, Moscow. Grosval’d, M.G., Glazovskii, A.F., 1988. Interaction of glaciations with the ocean: paleogeographic aspects. Scientific and Technical Advances, Ser. Paleogeography. VINITI, Moscow, Vol. 5. Gurevich, V.I., Musatov, E.E., 1989. Influence of the structure of the sedimentary cover on the Late Cenozoic sedimentation of the Barents– Kara shelf, in: Matishov, G.G. (Ed.), Problems of the Cenozoic Paleoecology and Paleogeography of Arctic Seas (Abstracts of III All-Union Conf.) [in Russian]. GI KNTs AN SSSR, Apatity, pp. 20–21. Kaletskaya, M.S., Miklukho-Maklai, A.D., 1961. Mesozoic weathering crust on the western slope of the Polar and Subpolar Urals. Dokl. AN SSSR 139 (6), 1425–1427. Kim, B.I., 2003. History of formation of the Eurasian basin (seismic complexes, structure, thickness of the cover, stages of evolution). Rossiiskii Geofizicheskii Zh., Nos. 31–32, 53–70. Kim, B.I., Glezer, Z.I., 2007. Sedimentary cover of the Lomonosov Ridge: Stratigraphy, structure, deposition history, and ages of seismic facies units. Stratigrafiya. Geologicheskaya Korrelyatsiya 15 (4), 63–83. Kim, B.I., Reinen, I.V., 1989. Evolution of the East Arctic shelf and paleogeography in the Pleistocene, in: Matishov, G.G. (Ed.), Problems of the Cenozoic Paleoecology and Paleogeography of Arctic Seas (Abstracts of III All-Union Conf.) [in Russian]. GI KNTs AN SSSR, Apatity, pp. 44–45. Kim, B.I., Slobodin, V.Ya., 1991. Main stages of development of the East Arctic shelf of Russia and the Canadian Arctic in the Paleogene and Neogene, in: Gramberg, I.S. (Ed.), Geology of the Folded Framing of the Amerasian Subbasin [in Russian]. VNIIOkeangeologiya, St. Petersburg, pp. 104–116. Krapivner, B.S., Skorobatenko, A.V., Tarasov, G.A., Shipilov, E.V., Gritsenko, I.I., 1989. Problems of the Cenozoic evolution of the Barents shelf, in: Matishov, G.G. (Ed.), Problems of the Cenozoic Paleoecology and Paleogeography of Arctic Seas (Abstracts of III All-Union Conf.) [in Russian]. GI KNTs AN SSSR, Apatity, pp. 49–50. Kulkova, I.A., Volkova, V.S., 1997. Landscapes and climate of West Siberia in the Paleogene and Neogene. Geologiya i Geofizika (Russian Geology and Geophysics) 38 (3), 581–595 (621–635).

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Editorial responsibility: B.N. Shurygin