STRATOTYPE OF THE UST'YAREGA Regional Substage OF THE LOWER FRANIAN OF THE UPPER DEVONIAN (NORTHEAST OF THE EAST EUROPEAN PLATFORM)
D.B. Sobolev1, M.A. Soboleva1
1N.P. Yushkin Institute of Geology, FRC Komi SC UB RAS, Syktyvkar, Russia
Keywords: Southern Timan, Upper Devonian, Frasnian Stage, Ust'yarega Formation/Regional Substage, lithology, biostratigraphy, chemostratigraphy, global eustatic events.
Abstract
The paper presents the results of a comprehensive lithological, bio-, and chemostratigraphic study of the Ust'yarega Formation stratotype (Lower Frasnian), located on the northeastern flank of the Ukhta Anticline (South Timan). Based on lithofacies analysis, depositional environments are reconstructed and a relative sea-level curve is established. The ~42 m thick formation exhibits a distinct two-part subdivision. The lower subformation (~19 m), composed of fine-grained siliciclastic sediments, accumulated in middle- to outer-ramp settings during the filling stage of the Timan Graben. The upper subformation (~23 m), represented by argillaceous-carbonate sediments, transgressively overlies the underlying deposits along a regional unconformity surface. Its depositional settings corresponded to a deep shelf and a gentle slope of a residual basin located over the northeastern flank of the paleograben. The signatures of global eustatic events identified in the studied section are characterized by carbon (δ13C) and oxygen (δ18O) isotope data. The Genundewa Event is distinguished by stable positive δ13C values against the background of negative shifts in the underlying and overlying deposits. The subsequent Early–Middle Frasnian Carbon Isotope Excursion (EMF CIE) is recorded by three shifts: the Pre-Timan Event is marked by the first (positive) excursion, the Timan Event proper by the second (negative) excursion, and the Middlesex Event by the third (positive) excursion. The Tr Isotope Event represents the positive δ13C anomaly preceding the EMF CIE. The upper subformation is biostratigraphically constrained by FZ 2 – FZ 4 conodont zones and correlated with the Sargaevian Regional Substage. The chronostratigraphic interval corresponding to the lower subformation is found to be missing from the current regional stratigraphic chart. To fill this gap, it is proposed to introduce a separate regional unit – the Ust'yarega Regional Substage, which will provide a continuous stratigraphic succession of regional horizon stratotypes in South Timan.
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