STRUCTURE OF THE RUSSIAN EASTERN ARCTIC SHELF V.A. Vinogradov, E.A. Gusev, and B.G. Lopatin, All-Russia Geological Institute for Geology and Mineral Resources of World Ocean (VNIIO), St. Petersburg, Russia, e-mail: [email protected] ABSTRACT sheets. A set of maps consists of tectonic, deep- This study of the Laptevs, East Siberian, and structural, bedrock, Quaternary and bottom sediment, Chukchi sea shelves concentrates mainly on the and geomorphologic maps. Onshore portions of the specific structure of the sedimentary cover and briefly digitized database come from geological mapping of discusses age and structure of the basement. An the eastern Arctic mainland and islands by the Russian elaboration of the tectonic model for the entire Russian Geological Survey in the middle of the last century. eastern Arctic shelf is accompanied by discussions of Geological investigations of the offshore area are rather the existing views on problems which have been poor. They include bottom sampling, piston cores and hampered by absence of deep offshore wells. The most rare shallow seismic profiles. Although several shallow relevant information was obtained from deep seismic boreholes were collected around the New Siberian profiling, which provides data on thicknesses and Islands and exploration wells were drilled on the U.S. unconformities within the sedimentary cover, tectonic Alaska Shelf, the major source of offshore information structures, and structure of the basement surface. But used in this study is from various geophysical methods without well control the age and nature of the basement of investigations by the Marine Arctic Geological and sedimentary cover remain thus far, speculative and Enterprise (MAGE) of Murmansk (Sekretov, 1998; will have to be verified by future research activities. 1999a; 1999b; 1999c), Laboratory of Regional Geodynamics (LARGE) from Moscow (Drachev et al., INTRODUCTION 1998), and joint research efforts by the oil company The Laptevs, East Siberian, and Chukchi sea “Sevmorneftegeofizika” of Murmansk and the shelves (Fig. 1) and their transition to deep water basins Geological Institute BGR of Hannover, Germany are still in a very early stage of geological exploration. (Roeser et al., 1995; Hinz et al., 1998; Franke et a.l, All existing bathymetric, magnetic, gravimetric and 1999), and by the oil company “Dalmorneftegeofizika” geologic data, including shallow wells in the New of Yuzhno-Sakhalinsk, Russia and Halliburton (USA). Siberian Islands area and deep wells in the U.S. sector of the Chukchi Sea shelf, were recently summarized in STRATIGRAPHY OF THE SEDIMENTARY the process of compiling the State Geological Map of COVER the Russian Federation at a scale of 1:1,000,000. The Two large provinces are distinguished on the work was carried out by the Geological Map Division Russian eastern Arctic Shelf. These two provinces in of the All Russian Scientific Research Institute for the western Chukchi and East-Siberian seas are Geology and Mineral Resources of the Worlds Oceans generally underlain by basement of either late (VNIIOkeangeologia) during the last 7 years and Mesozoic or Caledonian age. The boundary between includes a special geologic map index that unifies the these is complex in some areas, with the former entire offshore area. The level of earth science overprinting the latter, but can generally be traced from exploration varies drastically from one area to another, Cape Lisburne in western Alaska, northwest along the and even in the relatively better studied parts it remains northern margin of the Herald Arch to100-150 km insufficient for unambiguous characterization of north of Wrangel Island, and on across the East- geological structure and history, especially in places Siberian Sea to Vil`kistsky Island, southwest of the De- where subbottom geology is imaged exclusively on the Long Archipelago. basis of geophysical evidence and cannot be reliably These provinces are generally characterized by correlated with island outcrops. unique sedimentary cover with distinct stratigraphic ages and are traced using seismic reflection data and DATA SETS borehole sections from northern Alaska and the This paper is based mainly on the State Geological Chukchi Shelf where American investigators (Grantz et Map at a scale of 1:1,000,000, every sheet of which al., 1975; 1982; and 1990 and Thurston and Theiss, presents a separate geoinformational system (GIS) 1987) identified two major types of sedimentary cover layer. All existing bathymetric, magnetic, gravimetric with good confidence. They divide sedimentary cover and geologic data, including shallow wells in the New into 2 main sequences, the Ellesmerian and Brookian, Siberian Islands area and deep wells in the U.S. sector separated by the regional Lower Cretaceous of the Chukchi Sea shelf, were used to construct maps unconformity (LCU) at the base of the Barremian 90 ICAM IV Proceedings Figure 1. Scheme of geographical objects. Legend: 1 – shelf break; 2 – boundaries of geographical objects; 3 – small islands. Map: Objects: 1 – Khatanga bay, 2 – Bolshoy Begichev island, 3 – Tsvetkov cape, 4 – Olenek bay, 5 – Tiksi bay, 6 – Buor-Khaya bay, 7 – Yano-Indigirskaya lowland, 8 – Bol`shoy Lyakhovsky island, 9 – Kotel`ny island, 10 – Zemlya Bunge, 11 – Faddeevsky island, 12 – New Siberia island, 13 – Vil`kistsky island, 14 – Zhokhov island, 15 – Benetta island, 16 – Henrietta island, 17 – Lomonosov Ridge, 18 – Mendeleev Rise, 19 – Chukchi Basin, 20 – Wrangel island. Stage. Later researchers have distinguished a third “Eoellesmerian” Sequence in the North Chukchi Basin sequence in the U.S. Chukchi Sea—the Rift Sequence at the base of the Ellesmerian cover but it is not clear (Sherwood et al., 2004) which we have not mapped but what age they are. We believe his data show lower may be equivalent in part to our J-K1h sequence in Ellesmerian Endicott Group strata in some grabens that Figure 3. This sequence underlies the Brookian and may be as thick as 7 km, in the North Chukchi Basin. represents the initial stage of opening of the Canada Figure 2 shows our interpretation of a seismic profile Basin. The second major sequence is the Late Devonian collected by “Dalmorneftegeofizika” Enterprise which to Early Cretaceous Ellesmerian Sequence, which is divides the sedimentary cover of the North Chukchi generally found in the province with Caledonian depression into 7 seismic units separated by reflectors basement and had source rocks located to the north in Ch-I-VII, and a correlation to the reflectors mapped by the area of the present day Arctic Ocean. The Early Thurston and Theiss (1987). Older, basal sedimentary Cretaceous to Cenozoic Brookian Sequence sediments cover exists north of the North Chukchi Basin, as are distributed in both provinces, covering the evidenced from lower Paleozoic rocks dredged on the Ellesmerian Sequence in one, and in the other, covering continental slope, on the Mendeleev Ridge, and on the late Mesozoic folded basement rocks and comprising steep eastern slope of the Northwind Escarpment, the whole volume of the sedimentary cover. where a continuous stratigraphic section beginning with Ellesmerian Sequence strata can be distinguished Cambrian rocks was reported by Grantz et al.(1998). on seismic records in the North Chukchi basin, where Similar data were obtained by Russian investigators their thickness reaches 7-8 km, and where the total from the southern part of the Mendeleev Ridge, where thickness of the sedimentary cover is no less than 20 they dredged moderately lithified carbonate- km. Grantz et al. (1975 and Grantz et al., 1982) had terrigenous rocks exhibiting the occurrence of reported the presence of older, possibly Franklinian or kaolinitic cement in sandstones, and were Vinogradov et al. 91 ICAM IV Proceedings Figure 2. Structure of the sedimentary cover of the Southern side of the North-Chukchi depression with interpreted reflectors (Ch-I) – (Ch- VII). The age and correlation with the seismic horizons of the U.S. Chukchi Sea are shown at right: uBU (Upper Brookian Unconformity), mBU (Mid-Brookian Unconformity), LCU (Lower Cretaceous Unconformity), PU (Permian Unconformity), and EU (Ellesmerian Unconformity). Seismic profile fragment SC-90-01 of “Dalmorneftegeofizika” Enterprise. 92 Vinogradov et al. Vinogradov Figure 3. Fragment of seismic profile (89001, LARGE) approximately 170 km east-southeast of New Siberian Island. The boundary between Epi Late Mesozoic (to the left of the fault) and Epi-Caledonian (to the right of the fault) regions of the East Siberian Sea shelf. B-I reflector is equivalent to the Lower Cretaceous Unconformity (LCU) and PZ3-K1 is equivalent to the Ellesmerian Sequence. 93 paleontologically dated as Upper Silurian to Lower the acoustic basement. However, in deep depressions Permian in age (Kaban’kov et al., 2001). where the “PU” horizon separates subparallel reflectors On the East-Siberian shelf, Ellesmerian Sequence of the Lower and Upper Ellesmerian sequences, it may strata were recognized on a LARGE seismic profile be lost. This is what we believe happened on the (Fig. 3) 170 km east-northeast of New Siberian Island seismic profile in Thurston and Theiss (1987) Plate 5, (Drachev et al., 2001). Here, two reflectors “A” and published before the wells in the Chukchi Sea were “B” are distinguished beneath reflector “B-1” (LCU). drilled. Within the Ellesmerian Sequence below the The strata between these reflectors thin and are Lisburne Group, there is a well expressed reflector truncated by reflector B-1 to the north (toward the De- identified as an unconformity between Endicott Group Long rise), but to the south, they increase to 7 km in (D3-C1) and Lisburne Group (C2-3). On the southern thickness. Besides the increase in thickness, there is an slope of De-Long rise, the presence of Lisburne Group increase in deformation of the strata, which gradually rocks is confirmed by dredge samples that contained becomes more intense until it is manifested as a rather fragments of siliceous limestone with C2-3 fauna in sharp transition to acoustic basement. We believe this Neogene volcanics of Zhokhov Island.