The high Arctic (figure 1) is dominated by the least known ocean on Earth. It is composed of two basins, the Amerasian and the Eurasian basins, separated by the Lomonosov Ridge, a sliver of continental lithosphere that apparently detached from the Eurasian shelf by the opening of the Eurasian Basin in early Cenozoic time (Lawver et al., 1988). While the origin of the Eurasian Basin is relatively well established, the evolution of the Amerasian Basin is still controversial. A good knowledge of the bedrock geology and the tectonic history of the Arctic shelves is crucial both for reconstruction of the Arctic before the opening of the ocean basins and for an understanding of the tectonic evolution of the basins themselves.

Our SWEDARCTIC studies during the last decade have concerned Eurasia’s northern margin, from Svalbard via the Timan Ridge and the Polar Urals, to Taymyr and Severnaya Zemlya. Since the late 1990s the focus has been on October Revolution Island in the Severnaya Zemlya archipelago. The latter is located on the edge of the Kara Shelf in northernmost Russia (figure 1) between 78° and 81°N. Its topography, in general, is very gentle, with a mountainous terrain only in northern Bolshevik Island and easternmost October Revolution Island. The islands of the archipelago comprise 36,000 km² of tundra and ice-caps (figure 2), the latter covering large areas of the islands.

Severnaya Zemlya, northernmost Taymyr and large parts of the eastern Kara Shelf comprise a terrane (the North Kara Terrane) that was accreted to Siberia in late Palaeozoic time to form the Taymyr-Severnaya Zemlya orogenic belt as we know it today (Zonenshain et al., 1990). Although this period of deformation dominates the structure of the Neoproterozoic to Devonian bedrock, an early Palaeozoic deformational episode has been recognized; this has been the focus of our research.

Major structures and rock units on the Severnaya Zemlya islands and on Taymyr strike towards the continental slope, and are therefore likely to be found beneath the Lomonosov Ridge, and perhaps beneath the margins of the Amerasian Basin. The subject of this research programme is to define the structure and tectonic history of the North Kara Terrane. Its location on the edge of the Eurasian shelf makes it an important piece in the Arctic puzzle – how the different fragments of continental lithosphere were arranged before the opening of the Arctic basins – a problem to be approached by trans-Arctic correlation of geological evidence.

Geology

The Neoproterozoic strata of the Severnaya Zemlya archipelago (figure 2), reaching up into the Vendian, consist mainly of turbidites; they dominate the bedrock of Bolshevik Island and the northernmost parts of the Taymyr Peninsula. The relationship between the Neoproterozoic succession and overlying Cambrian strata (well exposed on October Revolution Island) is disputed and has not been proven to be exposed on either of the islands. Perhaps after a significant hiatus, sedimentation continued in the Early to Middle Cambrian in a shallow marine environment and later in a basin with an increasing amount of black shale and subordinate limestone. Late Cambrian shale deposition was influenced by the influx of turbidites. Folding, uplift and erosion interrupted deposition in the latest Cambrian and/or earliest Ordovician. In the Early to Middle Ordovician, sandstones (locally conglomerates), limestones/dolomites, marls, evaporites and gypsiferous shales were deposited – a volcanic component is prominent. Limestones dominate the Upper Ordovician and Silurian, and red shales appear in the uppermost Silurian. The Devonian is developed in Old Red Sandstone facies.

In the northwestern part of October Revolution Island, Ordovician to Devonian strata occur in major, gently northwest-plunging, northeast-vergent folds. The fold-axes swing southwards into a northnortheast-trending, dextral strike-slip fault zone (the Fjordove Lake Fault-zone, Figure 2), dividing the October Revolution Island bedrock geology into two parts. East of the fault zone, Cambrian and Ordovician strata occur in open (in the south) to close (in the north) upright folds with southsouthwest-plunging fold-axes. The deformed Palaeozoic succession is unconformably overlain by Upper Carboniferous, Permian and Mesozoic strata.

Fieldwork

In 1998, 1999, 2002 and 2003 international geoscientific expeditions visited Taymyr and the Severnaya Zemlya archipelago to work on the tectonic evolution, studying stratigraphy, palaeontology, structural geology and palaeomagnetism.

In the summer 2003, a field party of four geoscientists visited October Revolution Island with plans to work on the little known and controversial Precambrian/Cambrian boundary and continue research on the Middle Cambrian to Middle Ordovician units of southeastern October Revolution Island with emphasis on the Lower Ordovician regional unconformity. Only the latter could be accomplished due to helicopter problems. The summer season is usually short – from early July to late August – but favourable for fieldwork; 2003 was exceptionally cold and in late July and the first half of August the winter snowcover had not left the islands except locally in a small area around our campsite. These unfavourable conditions complicated fieldwork in a terrane where the outcrops are few even under the best conditions.

During the field season 2003, our studies included ground control for remote sensing, detailed studies of stratigraphy, structure and palaeontology and the collection of material for analyses of isotope age and provenance. Work on the Lower Ordovician angular unconformity was expanded to include two additional sections across this previously controversial contact between Upper Cambrian shales/sandstones and Ordovician conglomerates, sandstones and various other strata. Palaeontological and geochronological samples have been collected for dating the rocks above and below the unconformity and therewith the period of deformation and erosion. Another study, integrating remote sensing and image interpretation of Landsat 5 TM and Corona KH-4A satellite imagery with field-investigations in key-areas, has been finished with ground control of geology and the spatial accuracy of the imagery; corrections of the geological interpretation of southeastern October Revolution Island were possible (Lorenz, submitted).

The results of our research have been presented at the ICAM IV (International Conference on Arctic Margins) in Halifax (Lorenz et al., 2003; Lorenz, 2003; Gee, 2003) and manuscripts are being prepared for publication in the conference proceedings.