Taymyr ­­– enigma of the Siberian Arctic

 Project leader: David G. Gee, Department of Earth Sciences, Uppsala University

 The world, as we know it today, is largely shaped by the last hundred million years of plate movements. Our present environments, from topography to climate, are dictated by the collision and on-going convergence of Eurasia with the southern continents Africa and India-with the rise of the Alpine mountain belts (orogens) that reach from southern Europe, through the Middle East to the Himalayas, Tibet and southern China. About two hundred million years prior to these Alpine orogenies, at a time when Africa and India were located far to the south and separated from Europe by wide oceans, Eurasia grew into one vast continent during a long period of collisional interaction between ancient continental massifs (cratons). Two of these massifs contained the old cores of Asia and Europe, the Siberian and East European cratons respectively. Our work in Taymyr has mainly concerned this older (Palaeozoic and latest Precambrian) history of continental collision, which established the framework for deposition of the Mesozoic and younger basins of the Kara and Barents Seas with their vast hydrocarbon resources. Collision of the Siberian and the East European cratons in the late Palaeozoic resulted in the development of the Uralides, a mountain belt that defines the boundary between Asia and Europe today. One of the most prominent features of Eurasia, the deeply eroded Ural mountains, reaches 3000 km from near the Arai Sea northwards to the Arctic shelf. The Uralian structures can be followed into Novaya Zemlya (dividing the Barents from the Kara Seas) from where they apparently curve back across the Kara Sea to Taymyr. Yet in Taymyr many of the characteristic features of the Uralian orogen are lacking the evidence of Palaeozoic oceans, subduction complexes and volcanic arc magmatism that dominate the orogeny further west. The old mountain belt of Taymyr, now reduced by weathering and erosion to a vast tundra wilderness, extends for nearly 1000 km from the southern Kara Sea in the west to the Laptev Sea in the east. A large range of hills (locally mountains, up to nearly 1150 m a.s.l.) defines the eastern and southern margins of this fold and thrust belt along the northern margin of the Siberian craton. Cape Chelyuskin (77°37’N) is Taymyr’s (and Russia’s) northernmost point on land, but the orogen extends off-shore northeastwards and northwards via the Severnaya Zemlya islands to the edge of the High Arctic continental shelf.

Taymyr’s bedrock

Taymyr’s bedrock geology is dominated by Palaeozoic and early Mesozoic rocks that were deposited on an older orogenic complex of late Precambrian (Neoproterozoic) age. During the latest Palaeozoic to earliest Mesozoic, all these rocks were transported southwards towards the Siberian craton. The movement started in the north and migrated southwards; thus, Jurassic strata unconformably overlie the folded complexes of central and northern Taymyr, whereas in the front of the orogeny furthest to the southeast, they are involved in the folding and S-vergent transport. Taymyr’s bedrock divides into three E-W trending belts – the Northern, Central and Southern Belts. The Northern Belt (Kara Sea Terrane) is dominated by unfossiliferous (probably Neoproterozoic) turbidities (deep water sandstones, siltstones and shales) intruded extensively by granites, a few of which yield Carboniferous ages.

The Central Belt is largely composed of Neoproterozoic sedimentary and volcanic rocks, including fragmented ophiolites and island arc magmatic suites; continental crust of Grenvillian and possibly older age occurs as subordinate units, intercalated within the Central Belt. These Precambrian rocks were folded and thrust together in the latest Neoproterozoic (Vendian), prior to the deposition of overlying late Vendian, Palaeozoic and early Mesozoic successions. Lower and Middle Palaeozoic strata overlie the older rocks with major unconformity and were folded together with the latter in the latest Palaeozoic as part of the S-vergent fold belt.

Taymyr’s Southern Belt is composed of Palaeozoic and early Mesozoic strata (including prominent Triassic volcanics and mafic sills). The older strata occur along the northern margin of the Southern Belt where they are folded and thrust southwards onto the late Palaeozoic and Mesozoic rocks. Prominent facies changes occur in the lower Palaeozoic strata, from platform successions dominated by carbonates in the south into basinal shales in the north.

The Precambrian rocks of Taymyr, forming the unconformably underlying basement to the Palaeozoic successions, appear to compose the northern part of an extensive orogen. This so-called Baikalian orogenic belt encircled the Siberian craton in the late Precambrian (Neoproterozoic). These Precambrian rocks, with their evidence of Neoproterozoic oceans and subduction-related volcanic-arc complexes, contrast markedly with the overlying continental shelf and basinal successions of the Palaeozoic. The possibility arises that the Neoproterozoic and Palaeozoic orogenic histories were linked together via a long-lived subduction and accretionary process similar to that going on today along the western margin of the Pacific Ocean (Sengor et al., 1993). Whether or not this was the case, there can be little doubt that the Neoproterozoic tectonic evolution of Taymyr had a profound influence on the younger history, controlling the development of sedimentary basins and the character of subsequent deformation.

The aim of the project

The geological goals of the Taymyr project have been outlined by Gee and Ziegler in EUROPROBE’s science presentation (Gee and Zeyen 1996) and in a successful project proposal to the EU program for cooperation with scientists of the former Soviet Union (INTAS) – a two year contract providingsupport for Russian partners beginning in 1999. We are seeking a new understanding of the geodynamic evolution of the Eurasian High Arctic and, in this context, Taymyr plays a central, enigmatic role. Analysis of the Neoproterozoic to Phanerozoic dynamic development of the High Arctic involves a wide range of geological disciplines and access to a voluminous, mainly unpublished Russian data-base. Thus it requires a large contingent of scientists, each discipline anchored in partnership with Russian colleagues.

Principle methods and techniques

The existing Taymyr data-base, summarized in a 1:500 000 geological map (Bezzubtsev et al., 1983) and various recent presentations of the geology (Simonov and Malitch 1995, 1997; Vernikovsky 1997 and references therein), provide a foundation that can be interpreted in various ways. Bedrock structures (e.g. folds and faults) need the support of kinematic analysis and good control of timing. Likewise age control is essential both for stratigraphy (where paleontology plays a key role in the Phanerozoic sequences) and for magmatic suites (where isotope-age and provenance studies are vital). Isotopic studies are also proving important for constraining the age and provenance of unfossiliferous sequences. Thus, much of our fieldwork in 1998 concerned acquiring the necessary rock samples for analysis at the isotope laboratory of the Swedish Museum of Natural History in Stockholm. The establishment of a new Ar/Ar laboratory in Trondheim during 1999 will also be important for this work. In older, deformed rock complexes where the primary relationships have been disturbed by later movements and metamorphism, petrologic studies of the magmatic suites is essential for providing evidence of the character of the ancient environments, e.g. the oceanic and island-arc terranes. The geodynamic evolution during the Neoproterozoic and Palaeozoic appears to have been controlled by plate interaction, driven by mantle convection; essential for reconstructing the former is a knowledge of the movement of the different High Arctic terranes. Latitudinal movements and rotations of terranes can be defined by palaeomagnetic methods and here the laboratories in Trondheim, Bergen and Novosibirsk will play important roles. Within the Phanerozoic successions, where palaeontology provides the essential control of age, regional fauna affinities are of particular importance. For example, the Upper Ordovician shelly faunas of Taymyr have yielded brachiopods closely similar to those found in the Boda limestone (near Siljan) in Sweden, suggesting that these strata now located close to the Siberian platform, were originally deposited on the continental shelf of Baltica (Cocks and Modzalevskaya 1997). This possibility calls into question the Siberian affinities of the Palaeozoic successions and introduces the speculative hypothesis of a Caledonian-age suture located beneath the Khatanga trough, to the south of the Taymyr orogen. 

1998 fieldwork

Seventeen geoscientists participated in the 1998 fieldwork on the Taymyr bedrock. The work concentrated on Taymyr’s Southern and Central Belts, with two parties of 5-7 people each. The field season was divided into two parts (15th July-6th August, 6th August-26th), with some participants staying for both periods. The southern party concentrated on a section through the southern Taymyr fold and thrust belt, starting on the northern shore of lake Taymyr and moving northwards with the help of zodiacs to the Taymyr River and downstream to the edge of the Central Belt. During the first period, the work was led by Robert Scott and concentrated on the Early Mesozoic and Late Palaeozoic successions. During the second period, Simon Inger took over the leadership of the traverse through the Early-Mid Palaeozoic section. The party concerned largely with the Central Belt started with a helicopter drop-off in the upper reaches of the Shrenk River, a few kilometres north of the contact with the Kara Sea Terrane. Fieldwork southwards along the Shrenk River was to be facilitated by tracked vehicles, but these failed and the party was moved downstream by helicopter. Towards the end of the first period the camp was moved by a Khatanga expedition vehicle. During the second period, after rotation of personnel and Vicky Pease taking over the leadership from David Gee, the party was moved to Cape Chelyuskin. A Norilsk tracked vehicle then transported the group eastwards and southwards, allowing work on many of the rock units of Taymyr’s Central Belt.

Preliminary results

For most of the science, laboratory work during the coming months will be essential before we can report new results. Nevertheless, some studies have already been reported at the Third International Conference on Arctic Margins (ICAM III) held in Celle (near Hannover) in mid-October 1998. The fieldwork on structure and stratigraphy provided clear evidence that Taymyr’s Central and Southern Belts are essential parts of the same fold and thrust system. The prominent boundary between these two ”terranes”, highlighted by some authors, is essentially controlled by the Early

Palaeozoic sedimentary facies changes, from shelf environments in the south into basinal in the north. Basement rheology of the underlying Baikalian complex probably controlled these facies changes in the younger cover. By contrast, the fault contact between the Central Belt and the Northern Belt appears to be a major tectonic boundary of profound importance for High Arctic geology. The Kara Sea Terrane, on present evidence, is indeed an exotic complex – an independent microcontinent, unrelated to Siberia and maybe also Baltica.