The Laptev, East Siberian, and Chukchi sea shelves (Fig. 1) and their transition to deep water basins are still in a very early stage of geological exploration. All existing bathymetric, magnetic, gravimetric and geologic data, including shallow wells in the New Siberian Islands area and deep wells in the US sector of the Chukchi Sea shelf, were recently summarized in the process of compilation of the State Geological Map of the Russian Federation at a scale of 1:1,000,000. The work was carried out by the Geological Map Division of VNIIOkeangeologia during the last 7 years and includes a special geologic legend that unifies the entire offshore area. The level of earth science exploration varies drastically from one area to another, and even in the relatively better studied parts it remains insufficient for unambiguous characterization of geological structure and history, especially in places where subbottom geology is imaged exclusively on the basis of geophysical evidence and cannot be reliably correlated with island outcrops. Consequently, the views expressed by the authors remain to a large extent speculative and will have to be verified by future research activities.

In the present study attention is mainly concentrated on the specific structure of the sedimentary cover--basement age and structure are discussed briefly. The most relevant information was obtained from deep seismic profiling, which gives data on the structure of the basement surface, tectonic structures, and thickness and unconformities within the sedimentary cover. An elaboration of the tectonic model for all of the Russian eastern Arctic shelf is accompanied by critical discussions of the existing views on problems which have been hampered by absence of deep offshore wells. That is why the age and nature of the basement and sedimentary cover remain thus far, speculative.

Data sets

This paper is based mainly on the State Geological Map at a scale of 1:1,000,000, every sheet of which presents a separate geoinformational system (GIS) layer, including initial geological and geophysical databases along with geological maps. A set of maps consists of bedrock, Quaternary, bottom sediments, geomorphologic, tectonic, and deep-structural maps. The essential parts of the digitized database comes from geological observations during mapping of the mainland and islands, provided in the eastern Arctic region by the Russian Geological Survey from the middle of the last century. The geological investigations of the offshore area are rather poor. They include bottom sampling, piston cores and rare shallow seismic profiles.

Several shallow boreholes were collected around the New Siberian Islands, and deep ones on the Alaska Shelf (USA). The major source of information used in this presentation therefore come from various geophysical methods of investigations by the Marine Arctic Geological Enterprise (MAGE) of Murmansk (Sekretov, 1998; 1999a; 1999b; 1999c), Laboratory of Regional Geodynamics (LARGE) from Moscow (Drachev et al., 1998), and joint research by the oil company “Sevmorneftegeofizika” of Murmansk and the Geological Institute BGR of Hannover, Germany (Roeser et al., 1995; Hinz et al., 1998; Franke et a.l, 1999), and joint research by the oil company “Dalmorneftegeofizika” of Yuzhno-Sakhalinsk, Russia and Halliburton (USA).

Stratigraphy of the sedimentary cover

Two large provinces are distinguished on the Russian eastern Arctic Shelf that are defined by the age of their folded basement (late Mesozoic and Caledonian) and differ from each other by the age of their sedimentary cover. The boundary of the provinces can be traced to the western Chukchi and East-Siberian seas using  seismic reflection data and borehole sections from northern Alaska and the Chukchi Shelf, where two types of the sedimentary cover are established with good confidence. American geologists (Grantz et al., 1975; 1990 and Thurston and Theiss, 1987) divide the sedimentary cover into 2 sequences--Ellesmerian (Late Devonian to Early Cretaceous) and Brookian (Early Cretaceous to Cenozoic) with the regional Lower Cretaceous unconformity (LCU) at the base of the Barremian Stage separating them . The Ellesmerian Sequence is spread only in the province with Caledonian basement, and its source rocks were located to the north in the area of the present day Arctic Ocean. Brookian Sequence sediments are distributed in both provinces, covering the Ellesmerian Sequence in one, and in the other, late Mesozoic folded basement rocks where they constitute the whole volume of the sedimentary cover. The boundary between the late Mesozoic and Caledonian basements and consequently the southern boundary of the Ellesmerian Sequence distribution is traced from Cape Lisburne in western Alaska, west along the northern margin of the Herald Arch 100-150 km north of Wrangel Island, and on across the East-Siberian Sea to Vil`kistsky Island, in a southwest direction from the De-Long Archipelago.

The Ellesmerian Sequence can be distinguished on seismic records in the North Chukchi basin, where its thickness reaches 7-8 km, and the total thickness of the sedimentary cover is no less than 20 km. The assumption of Grantz et al. (1975) about the presence here of the Franklinian Sequence at the base of the cover is hardly probable, because his data show only Endicott Group strata in some grabens on the Chukchi Shelf that may be as thick as 7 km, which does not leave much room for Franklinian Sequence strata in the North Chukchi Basin.  After our interpretayion of a seismic profile SC-90-01 of “Dalmorneftegeofizika” Enterprise the sedimentary cover of the North Chukchi depression is divided into 7 seismic unites by reflectors (Ch-I)-(Ch-VII), and their correlation to the reflectors of Thurston and Theiss (Thurston & Theiss, 1987) is shown on Fig. 2. 

 Older, basal sedimentary cover exists north of the North Chukchi basin, as evidenced from Lower Paleozoic rocks dredged on the continental slope, on the Mendeleev Ridge, and on the steep eastern slope of the Northwind Escarpment, where a continuous stratigraphic section beginning with Cambrian rocks was reported (Grantz et al., 1998). Similar data were obtained by Russian investigators in the southern part of the Mendeleev Ridge, where paleontologically dated carbonate-terrigenous Upper Silurian to Lower Permian rocks were dredged (Kaban’kov et al., 2001). Low lithification and the occurrence of kaolinitic cement in sandstones, proves these dredged Paleozoic rocks are strata from sedimentary cover.

On the East-Siberian shelf, Ellesmerian Sequence strata were recognized on a LARGE seismic profile (Fig. 3) 170 km east-northeast of Novaya Sibir` Island (Drachev et al., 2001). Here, two reflectors “A” and “B” are distinguished beneath reflector “B-1” (LCU). The strata between these reflectors thin and are truncated by reflector B-1 to the north (toward the De-Long rise), but to the south, they increase to 7 km in thickness. Besides the increase in thickness, there is an increase in deformation of the strata, which gradually becomes more intense until it is manifested as a rather sharp transition to acoustic basement. We believe this transition of Ellesmerian Sequence to the folded state marks the boundary between late Mesozoic and Caledonian basement provinces.

On Novaya Sibir` Island, drilling revealed late Mesozoic basement (folded Jurassic terrigenous rocks) beneath Pliocene–Quaternary sediments. Magnetic anomalies over Novaya Sibir` Island are similar to the anomalies seen from the cassiterite-bearing granites of Bolshoy Lyahovsky Island . Placer cassiterite found on Novaya Sibir` Island confirms the presence of stanniferous granites in the basement. And, in the basal Pliocene layers overlying the deformed Jurassic rocks, grains of molybdenite and sphalerite are also found.

Caledonian basement is exposed on Henrietta Island where outcrops of folded volcanic and clastic rocks, bearing sills, dykes and sheets of basalts, andesite-basalts, and porphyritic diorite occur (Vinogradov et al., 1974). Basalts and porphyritic diorite dated bv the potassium–argon method are 310-450 ma and porphyritic diorite dated using the argon–argon method are in the 400-440 ma interval. Fragments of gneissic, granitic, and quartzitic rocks and schists in gritstone of Henrietta Island are evidence that the Caledonian fold basement zones in the De-Long rise province include blocks of older consolidation. More evidence of these is the presence of flat-lying Cambrian and Ordovician strata on Benetta Island, but these lower Paleozoic rocks may not be part of the sedimentary cover  because seismic reflection signatures typical of sedimentary cover are absent on the nearby profiles.

The Ellesmerian Sequence is divided by American authors (Grantz et al., 1975; 1990 and Thurston and Theiss, 1987) into two parts: Lower Ellesmerian and Upper Ellesmerian Sequences, separated by Permian unconformity (PU) at the base of Upper Permian strata. In the wells drilled on the Alaska coast, the sedimentary cover begins with the Upper Ellesmerian Sequence, and the “PU” reflector marks the acoustic basement. However, in the deep depressions where the “PU” horizon separates subparallel reflectors of the Lower and Upper Ellesmerian sequences, it may be lost. This is what we believe happened on the seismic profile in Thurston and Theiss (1987) Plate 5, published before the wells in the Chukchi Sea were drilled. Within the Ellesmerian Sequence below the Lisburne Group, there is a well expressed reflector  identified as an unconformity between Endicott Group (D₃-C₁) and Lisburne Group (C_2-3). On the southern slope of De-Long rise, the presence of Lisburne Group rocks is confirmed by dredge samples that contained  fragments of siliceous limestone with C_2-3 fauna in Neogene volcanics of Zhokhov Island.

The Brookian Sequence (Barremian Stage to Cenozoic) is divided by American scientists into Lower Brookian (K₁br-al) and Upper Brookian (K₂-KZ) sequences. In our interpretation, based on seismic data analysis across the whole eastern Arctic Shelf (from the Laptev to Chukchi seas), we propose dividing the stratigraphic section into Cretaceous (K₁br-K₂) and Cenozoic parts. The lower sequence is much thicker and is characterized by numerous plastic and disjunctive deformations as a result of fragmented topography of the basement surface. The upper sequence is a continuous but less thick mantle type deposit. It is seismically transparent and is not disturbed by syndepositional deformations.

Discussion

The uniform structure of the whole sedimentary cover suggests its uniform age. For a long time this postulate did not seem to apply for the Laptev shelf, where on the west it was thought to be late Proterozoic to Cenozoic in age. However, new multichannel seismic data, obtained by Marine Arctic Geological Expedition (MAGE, Murmansk, Russia) and by Regional Geodynamic Laboratory (LARGE, Moscow, Russia) do not support this conception (Drachev et al., 1998; Vonogradov & Drachev, 2000; Gusev et al., 2002). On the seismic profiles near the southern coast of the Laptev Sea (Oleneksky Bay, Buor-Khaya Bay) a floor of sedimentary cover lies on the Late Mesozoic folded basement, protruded on the coast. On the seismic profile along the Khatanga Bay one can see a sharp changing in the wave characters at the front of Late Mesozoic folding. This front is traced from the eastern coast of Bolshoi Begichev Island to Tsvetkov Cape on the southern-eastern coast of the Taimyr Peninsula (Vinogradov & Drachev, 2000). Horizontal seismic reflectors, typical for the thick sedimentary cover of the northern flank of the Siberian Platform are sharply terminated at this boundary. On the northern-eastern fragment of the seismic profile the wave record becomes chaotic similar to the acoustic basement along all the profile. It confirms that the cover of the Siberian Platform does not continue beyond the Khatanga Bay. 

There is still a difference between the west and central parts of the Laptev Sea and its eastern part. In the eastern part the main stage of folding is connected with Early Cretaceous time because coal-bearing molasse rocks of Aptian-Albian age on Kotel`ny Island overly folded Paleozoic and Mesozoic formations with a sharp unconformity. On the central and western part of the Laptev Shelf, riftogenous processes were superimposed at the final stage of the Early Cretaceous deformation resulting in rather weakly deformed basement rocks being buried under rift formations.  

The Late Cretaceous formation history of the sedimentary cover needs a special discussion. During Late Cretaceous time all of the area to the northeast of the Lena River mouth underwent an intense denudation. This conclusion is supported by the fact that the main stage of intrusive activity including large granite batholiths in northeast Russia took place at the end of Early Cretaceous to the beginning of Late Cretaceous, and erosion products are found in Paleocene sediments. Further evidence is deep denudational truncation and weathering crust reported in the Tiksi Bay area, where Paleocene sediments, including quartz siltstones, cover the greenshist Verkhoyan suite. If to adopt a depth of granitic massives formations equal no less than 3 km, a volume of denudated rocks on the North-East of Russia for the Late Cretaceous time (from granitic origin up to their denudation) may make up about 6,5 million km³.

On the adjacent shelf, continental slope and Eurasian deep basin, nearly 7.5 million km³ of sediments were deposited. A quiescent stage set in between the Mesozoic and Cenozoic manifested by Paleocene peneplain facies in the areas of Tiksi Bay, Yano-Indigirskaya lowland, and New Siberian Islands. On seismic records this boundary is expressed by a high contrast regional unconformity between the Upper Cretaceous strata exhibiting numerous reflectors with syndepositional deformation filling rift grabens, and overlying continuous seismically “transparent” Paleocene strata.

In the North Chukchi basin, Upper Cretaceous strata with widely spread clinoforms acquire the role of an independent sequence, separated from Barremian–Albian rocks by a sharp angular unconformity (Thurston and Theiss, 1987, Plate 7).

Structure Provinces

Structure of the sedimentary cover on the eastern Arctic shelf of Russia generally is an assemblage of large basins and rises separating zones of persistent depressions such as the Laptev basin, New Siberian System of horsts and grabens, Chukchi-East-Siberian basin, De-Long rise and series of perioceanic depressions along the Shelf margin (Fig. 4).

Laptev basin occupies the central and western parts of the Laptev Sea. It is about 400 km wide on the north near the continental slope, and does not exceed 100 km in width on south-southeast in the Buor-Khaya Bay area. On the south and west, the Laptev basin is bounded by mountainous–folded formations of Mesozoides, and from the east by Lazarev fault separating the basin from New Siberian System of horsts and grabens. The internal structure of the Laptev basin is rather complex due to numerous faults, uplifts and troughs. Sedimentary strata reach thicknesses of 10-12 km in troughs and thin to 5-6 km over horsts.

The New Siberian System of horsts and grabens, between two basins – Laptev and Chukchi-East Siberian, extends from the continent to the shelf margin, it is 600 km wide in the south and about 400 km in the north. On the whole, it is an uplifted province with reduced and discontinuous cover, except Novosibirsky and Anisinsky grabens, where the sedimentary cover is thickest. The structures of the western and the eastern slopes of the New Siberian System are not alike. The structure of the western slope is highly contrasting due to numerous horsts and grabens. The eastern slope is rather gentle with single  grabens. In the axial part of New Siberian System, basement is exposed. The axial zone is a direct continuation of the Lomonosov Ridge, which can be regarded as a submeridional horst. The western slope of the Lomonosov Ridge is also dissected by numerous horst and grabens, and the eastern slope is relatively gentle with rare grabens. This resemblance between New Siberian System and Lomonosov Ridge suggests that they are part of a transregional positive tectonic feature that trends from the continent, through the shelf, to the Arctic Ocean basin. The axial zone of this transregional feature is traced further on land in the Yano-Indigirskaya lowland as basement protrusions, including several granite massifs of Cretaceous age, collectively named the Chokhchuro-Chekurdakhsky row.

The Chukchi-East-Siberian Basin is the largest structural province of the eastern Arctic shelf. It extends in latitudinal direction over 1300 km, widening from 450 km in the west to 900 km in the east (in American part of Chukchi Sea). From the west, the basin is bounded by the New Siberian System of horsts and grabens, from the north by De-Long Rise, from the south by rangy Mesozoides of North-East Russia. The coastal lowlands are part of the southern flank of the basin. To the south of the Herald Arch, the basin is bounded by Alaska mountains, and in the north – by the Barrow Arch.

The basin can be divided into northern and southern parts by the different ages of basement. The northern province is underlain by Caledonian basement, the southern province by late Mesozoic basement. The provinces are separated by large high–angle faults. In the northern province, two deep depressions–Zhokhovsky and North-Chukchi--are distinguished, separated along 174 degree west longitude by the Jannetsky  transverse uplift.

The Zhokhovsky depression extends for 600 km from the east to where it is 200 km wide to the west where it gradually narrows and disappears in the boundary zone between the New Siberian System of horsts and grabens and De-Long Rise. The thickness of Upper Paleozoic–Cenozoic cover in the axial zone of the depression reaches 10-12 km.

The North-Chukchi Basin within the Russian shelf is also traced for 600 km and its width varies from 250 km on the east to 160 km on the northwest. This basin is notable for its great cover thickness. On seismic records acoustic basement is recognized at the depth of 18 km with a good confidence (Fig 3). The structure is asymmetrical: its southern flank dipping steeper than its northern flank. The axial zone and the northern flank is disturbed by the transverse Andrianovsky uplift along the 170º meridian. The uplift is contoured by Barremian-Albian (LCU) isolines (and in older strata), having an amplitude of up to 3000 m. But it is not obviously expressed at the Upper Cretaceous floor level (mBU). Twenty to twenty five kilometers north, the axial zone of the North Chukchi basin shifts to younger strata.

The North-Chukchi basin is dissected by sublatitudinal and younger submeridional faults. Sublatitudinal faults often bound half grabens on the southern flank of the basin. Submeridional faults truncate Upper Cretaceous rocks and bound grabens and horsts, which are expressed in sea floor topography. In the north, the basin is bounded by the arch-like North Chukchi rise, which extends west-northwest for 500 km and varies from 50 to 75 km in width. To the northwest, the North Chukchi rise conjugates with the southeastern flank of De-Long rise, and on east-southeast probably with the Barrow Arch. Sedimentary cover thickness within the North Chukchi rise is estimated at 6-7 km.

The southern part of the Chukchi-East-Siberian basin differs from its northern part by the presence of predominantly submeridional structural trends inherited from the late Mesozoic basement. Sublatitudinal strike features typical of the northern part are partly preserved only on the Chukchi Shelf, such as the Herald Arch with thin sedimentary cover and basement projections (Wrangel Island), and the South Chukchi depression with the Cretaceous-Cenozoic cover up to 4-6 km.

Over most of the East-Siberian shelf the structural trend is predominantly submeridional with symmetrical features. The axial zone of the structural assemblage is characterized by the Melvillian graben, where Aptian-Cenozoic cover reaches 10 km. Actually, it can be regarded as a zone of extension, on the both sides of which there are uplifts: Chukchi and East-Chersky, flanked again by subsidence features: the South-Denbarsky and Ambarchiksky garbens. The basement fault zones, responsible for origin of these structures extend to the north, dissecting Zhokhov depression and De-Long rise. It is very likely that submerged parts of the East-Siberian shelf have riftogenous nature, similar to the Laptev basin.

The De-Long rise has a block-like round-to-triangular form, elongated in a west-northwest direction for 800 km. It is 400 km wide in the west and narrows to the east to 150 km. For the most part it is covered by thin (less than 1 km) Cretaceous-Cenozoic mantle type deposits with several projections of Caledonian and probably older basement. On the slopes of De-Long rise the cover is 3-4 km thick, underlain by Mesozoic and Paleozoic strata. The rise is dissected by faults, bounded grabens and horsts.

Conclusions

Analysis of the obtained results permits us to make three main conclusions on the age and structure of the sedimentary cover of the eastern Arctic Shelf of Russia.

1.      The sedimentary cover structure was formed during two stages that resulted in two structural trends and related tectonic features. The time boundary between these stages lies in Barremian-Aptian. In the older stage sublatitudinal trends predominated, and in younger stage--submeridional.

2.      Older sublatitudinal zonation was caused by structural features of the fold basement, whereas younger submeridional zonation appeared as a result of a unique ocean and shelf riftogeneous processes. Structural elements of the Arctic Ocean, such as the Eurasian Basin, Lomonosov Ridge, Mendeleev Rise, and Chukchi Basin generally express continuations of the shelf structure in their sedimentary cover. Besides the mentioned structural relationships between the ocean and shelf, one more is important–the position of thick lens of the Lower Brookian Sequence in the North-Chukchi basin just opposite the Chukchi basin in the Arctic Ocean, between Mendeleev Rise and Chukchi Plateau..

3.      An active riftogenous process started simultaneously over all of the eastern Arctic Shelf at the end of  Early Cretaceous and finished by the beginning of the Cenozoic, and an active tectogenesis migrated in time toward the Arctic Ocean. In the Cenozoic on the shelf-ocean boundary, perioceanic depressions were formed, whereas on the shelf in a quiescent environment, a thin and continuous sedimentary mantle was deposited. 

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