Lithology and geochemistry of Triassic deposits in the southwest of the West Siberian geosyneclise

Abstract

Sedimentological and geochemical studies of a Triassic volcanosedimentary section of the Tura Group on the southern margin of the Koltogory-Urengoi Rift have shown a wide occurrence of fine- and very fine-clastic turbidites with admixtures of glauconite and basic and acid pyroclastic material. According to petrographic and geochemical data, the section sandstones are mainly graywackes and partly litharenites, and the mudstones are montmorillonite clayey rocks with admixtures of chlorite, hydromica, and kaolinite. The sourcelands of the peneplaned plateau abound in basic volcanics, which suggests (by analogy with the Triassic strata of the Siberian Platform) a wider occurrence of flood basalts in the past. Most of these basalts were, most likely, eroded, which resulted in a predominance of rift basalts on the pre-Jurassic weathering surface. Along with tephra and volcanoclastics, the clastic rocks are dominated by products of chemical weathering developed after basic volcanics. Marine deposits revealed in the Koltogory-Urengoi Rift section suggest the inflow of sea waters from the northern (in the modern frame of reference) ocean basin localized north of the modern Severnaya Zemlya Archipelago. The sedimentation basin of the rift was a narrow but extended bay of the Red Sea type, where the formation of marine deposits was accompanied by the eruption of basalts in the area of Nikol’skaya BH-1 lasting up to the late Middle Triassic.