Petrology, sedimentology, geochemistry, and absolute age of Triassic volcanosedimentary rocks from the southwest of the West Siberian geosyneclise (Kurgan Region)

Abstract

In this study we report new Ar-Ar ages for basalt (261.0 ± 15, 249.4 ± 3.0, 247.0 ± 3.1 Ma) and rhyolite (246.9 ± 2.6 Ma) samples collected from the Triassic volcanosedimentary sequence, which is one of the remarkably complete and structurally complex sections in the southwest of the West Siberian geosyneclise (WSG) (East Kurganskaya-42 (EK-42) and East Kurganskaya-43 (EK-43) wells), largely comprising basalts and conformable layers of rhyolites, acid tuffs, limestones, and silty mudstones. It was shown that basalts, which constitute most of the section, are geochemically similar to medium-K tholeiitic and calc-alkaline basalts in the southern extension of the Koltogory–Urengoi rift (Nikol’skaya-1 well). Rhyodacites and rhyolites from the section of interest belong to the potassic to potassic-sodic series of normal alkalinity. The trace element compositions of the rhyolites and host basalts suggest fractionation from primary magmas during formation of felsic rocks. No low-K sheet-flow basalts have been encountered in the sections penetrated by the three wells (EK-42, EK-43, and Nikol’skaya-1), as opposed to sections in the northern part of the Koltogory–Urengoi rift (TSD-6 well) and the Siberian flood basalt province. Despite their small thickness, the sedimentary units provide evidence for different depositional settings during the evolution of a volcanosedimentary basin. Similar to the Koltogory–Urengoi rift, deposition of the Presnogor’kovsk graben fill indicates intermittent marine conditions with accumulation of hyaloclastites, and fine, calcareous, silt- and clay-rich carbonaceous oozes associated with a background sedimentation, which contain abundant marine fauna, pyrite and rare distal turbidites with glauconites. The geochemical features of these sedimentary rocks were largely defined by a bimodal syndepositional volcanism. During the Early Triassic, a system of rifts and grabens that was developed within the continental plateau over much of the WSG allowed the intermittent ingress of sea-water from the north (in present coordinates) far southwards and southwestwards up to the latitude of Omsk and Kurgan. As a result of extensive erosion of the continental plateau, the packages of low-K sheet-flow basalts were removed from the upper part of the sections of deep wells, which became dominated by the eroded roots of the West Siberian traps (subalkaline and alkaline rift basalts).

Morphotectonic and Petrological Characteristics of Permo-Triassic Traps of Siberia

“…In [20] considered all post-Paleozoic igneous events of the Russian sector of Asia were considered as rifting processes, although there are no signs of classical rifts of the PTTS igneous systems in the described structural forms of the magma development in the sedimentary cover and subaerial mantle flows for SC and Taimyr [21][22][23][24][25][26][27][28][29]. The described manifestations of subaerial volcanism beneath the Mesozoic cover on the WSP territory [30][31][32][33] also do not contain geological and structural information about morphological structures common for classical rifting, therefore morphological structures characteristics of the PTTS manifestation of terrestrial magmatism should be studied by instrumental techniques.…”

Linking sediment supply variations and tectonic evolution in deep time, source-to-sink systems—The Triassic Greater Barents Sea Basin

“…The maximum extent of the Lower-Middle Triassic (Anisian) West Siberian Catchment includes three areas: West Siberian Basin with the western part of the Siberian Platform, the northern Central Asian Orogenic Belt to the south, and Taimyr to the north. The West Siberian Basin was composed of a rift valley system where rift shoulders were a local sediment source and grabens acted as transportation pathways (Saraev et al, 2011). By the end of the Anisian, magmatic activity related to the Siberian Traps ceased in the West Siberian Basin, and the northern part of the basin started to subside (Meshcheryakov and Karaseva, 2010; Sømme et al, 2018); the West Siberian Catchment decreased in size during Ladinian-Norian time (Fig.…”

Ooidal ironstones in the Meso-Cenozoic sequences in western Siberia: assessment of formation processes and relationship with regional and global earth processes

“…While the exhalation of fluids in oxic conditions leads to the rapid precipitation of iron as oxyhydroxides (goethite, lepidocrocite) as in the Bakchar ironstone deposit (Kimberley 1989;Sturesson et al 2000;Rudmin et al 2019). Global volcanic events possibly triggered the hydrothermal convection in continental rift systems of the West Siberian Plate (Surkov 2002;Saraev et al 2011;Vibe et al 2018). Metal-saturated aqueous fluids moved into the near-bottom waters through convection, similar to the continental rift system of the Dead Sea (Dill et al 2010), or through underwater exhalation similar to the system of Mahengetang (Heikoop et al 1996), the Hawaiian Islands (Hearty et al 2010) and Trinidad Island (Kimberley 1994).…”