Stratigraphy, geochemistry and origin of Silurian black shales of the Russian Arctic

As part of our on-going research on Silurian organic-rich source rocks, a database of Silurian black shales in the Russian Arctic is being compiled from the literature with a complementary data set provided by original fieldwork and associated samples.

Our review of the literature shows that Silurian black shales are well developed in the Russian Arctic. They are recorded from northern Novaya Zemlya, Vaigach and Pai-Khoi, the northern and Polar Urals, East Siberia, Taimyr, Severnaya Zemlya, the New Siberian Islands, Verkhoyansk-Kolyma and Chukotka. In some regions as much as 1000 m of black shales are reported. Sections provide excellent graptolite data allowing good biostratigraphic control and correlation. For these, eleven, areas, we present stratigraphic charts from Llandovery to Pridoli with information on sediment thickness, biostratigraphy and depositional settings. In almost all of the areas examined black shales occur at or near the base of the Silurian. Similar graptolitic shales are also know from other parts of the world (e.g. northern Africa). They represent a synchronous depositional unit, most probably associated with a large-scale anoxic event, related to the early Silurian rapid rise of sea level that followed rapid Late Ordovician deglaciation (Lüning et al. 2000).

Our field studies cover Severnaya Zemlya, Taimyr, northern East Siberia, and Kotel’ny Island of the New Siberian islands, where the Silurian sequences contain black graptolitic shale and bituminous limestone intervals. Field and analytical studies have been undertaken to map the rocks, document their stratigraphy, identify the fossils, establish and describe facies, reconstruct the palaeogeography, and determine palaeoenvironmental conditions. The later was achieved by performing major, trace and rare earth element analysis with a focus on specific elements, geochemical proxies and ratios. Some geochemical proxies, including Th/U, V/Cr, and V/(V+Ni), together with sedimentary facies and faunal data indicate anoxic conditions in Central Taimyr during the Telychian, Wenlock, early Ludlow and early Pridoli. It should be noted that no samples have been analysed from the Rhuddanian and Aeronian strata of Taimyr. An anoxic environment is indicated for the Rhuddanian of Kotel’ny Island, and for the Telychian of Severnaya Zemlya. In East Siberia, Th/U ratios indicate anoxic conditions during the Rhuddanian, Ludlow and Pridoli, but oxic conditions during the Aeronian. Other geochemical proxies (e.g. V/Cr, V/(V+Ni) and Ni/Co), suggest rather wide ranging environments through time. The overall geochemical trends are in good agreement with the results of previous palaeontological and sedimentological studies and confirm that the Rhuddanian anoxic environment was a result of a global sea-level rise.