Initiation and Impact of Siberian Traps Volcanism: Onset of Volcanism and Intrusive Contact Metamorphism on Oil-bearing Carbonates

Dougal Jerram

The Siberian Traps Large Igneous Province was formed during the end-Permian, about 251 million years ago. Basaltic melt was injected into the organic and salt rich Tunguska sedimentary basin, forming interconnected sill complexes and associated hydrothermal vent complexes. We have conducted field work in Siberia during 2004 to 2010 to study the formation and implications of the SiberianTraps volcanism. Thick deposits of basaltic tuff and tephra have been reported as widespread in the lower succession of the Siberian Traps, commonly taken as direct evidence for the explosive nature of the initial phase of volcanism. The field work in this study revealed that tuffs are virtually absent along a 125 km long transect along the Dyupkun lake, even though tuff is shown on available geological maps. Towards the south and west, the transition between the end-Permian sediments and the flood basalts is either characterized by minor (<2 meters) to no tephra deposits (Khantaika area), hyaloclastites and associated lake-deposited tephra (Kureika area), or massive tephra deposits from local eruptive centers (Severnaya area). The new results questions the notion of province-scale explosive volcanism in Siberia during the onset of flood volcanism. Within the intrusive complexes beneath the volcanics, contact metamorphism of the sedimentary rocks around dolerite sills and dikes generated greenhouse gases and halocarbons to such an extent that the process could be responsible for both the end-Permian carbon isotope excursion and the mass extinction. The key processes include 1) metamorphism of oil-saturated rock salt sequences (halocarbon production), 2) methane generation from metamorphism of organic-rich shales (methane production), and 3) decarbonation of dolostones (carbon dioxide production). The study area is located along the uplifted western flanks (east of Turukhansk) of the vast Tunguska Basin, and is thus representative for the geology of the deep parts of the basin further east. New data from a ca. 100 meter thick dike emplaced into an exhumed oil reservoir within Cambrian dolostones sheds new light on the processes involved in sediment metamorphism and volatile generation. We have analysed the petrography and geochemistry (including carbon isotopes) of the contact metamorphic carbonates on both sides of the dike. Dolomite-consuming reactions were responsible for a notable reduction in dolomite content towards the contact. Abundant oil-filled and dike-parallel veins in the innermost contact aureole are associated with a notable bleaching of the host dolostone, suggesting that the petroleum system was active during to emplacement. Thus the emplacement of the dike resulted in carbon dioxide generation from the dolostone metamorphism, methane generation from petroleum heating in the aureole, and to seepage of basin brines and petroleum during system cooling.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013