First radiometric dating of tonsteins from coal-bearing succession of the Kuznetsk Basin: U-Pb geochronology of the Tailugan Formation

Abstract

Tonsteins, predominantly solid kaolinite clay interbeds, are widespread in the coals of the Kuznetsk Basin and usually contain idiomorphic zircon grains of magmatic origin in quantities suitable for uranium-lead (U-Pb) radiometric dating. For the first time, tonstein zircons from coal seam 78 of the Tailugan Formation (Fm) are dated by two methods: Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Chemical Abrasion Isotope-Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS). The CA-ID-TIMS datings of 257.0 ± 1.3 Ma and 256.6 ± 0.4 Ma determine the age of the lower boundary of the Tailugan Fm and the Tailuganian Regional Stage at 257.0 Ma. We estimate the duration of the Tailuganian to be around 4.22 million years. The dating results make it possible to directly correlate the Tailuganian of the Kuzbass with the upper half of the Wuchiapingian and with most of the Changhsingian of the International Chronostratigraphic Scale. The accumulation rate of total sediments of the Tailugan Fm, calculated without considering the compaction index, is approximately 0.13–0.18 mm/year, and the rate of coal accumulation is 0.024 mm/year. Such values are comparable to the accumulation rates of the Late Paleozoic coal-bearing strata of the Donetsk Basin and Western Australia. The rate of peat accumulation during the Tailuganian, calculated at a compaction index of 10:1, has been estimated at 0.24 mm/year. This value is comparable to the rates of peat accumulation in the Holocene bogs of the Kuznetsk Alatau in Siberia and the European part of Russia. The presence of tonsteins in the coal seams of the Tailugan Fm suggests that the upper surface of the peat at the time of its accumulation was below the water level, serving as a protective screen for the thin volcanic deposits and preventing its erosion. The assemblages of macroflora, ostracods, conchostracans and bivalves considerably vary through the section of the Tailugan Fm. This variation makes it possible to identify in Kuzbass the stratigraphic level that corresponds to the boundary between the Wuchiapingian and Changhsingian stages in the future.