Visean carbonate deposits in the Ilych river basin (Northern Urals): lithology, isotopes, evolution of sedimentation

The paper presents the results of lithological studies and the first data on carbon and oxygen isotope composition in limestones of Visean deposits at the eastern outcrops (Ilych River, Northern Urals). The main part of studied rocks are represented by different types of limestone: bioclastic and oolitic wackestones and grainstones, peloids wackestones and calcareous rudstones. Sometimes limestones are variosly dolomitized and silicified. Fine-grained quartz sandstones and claystone occur in the lower part of the studied section. A total of 144 samples of limestones have been analysed for their carbon and oxygen isotopic values: d13СDPB = -2.9...+4.2 ‰ and d18ОSMOW = +23...+28.5 ‰. Most of these values are located within Visean marine carbonates of the East European platform and the Urals. Variations in the carbon and oxygen isotopic composition of limestones are in some extent consistent with the revealed trend of changes in the conditions of sedimentation in the Eastern Upper Pechora paleobasin in Visean. In Visean at the observed area the sedimentation occurred within shallow-water carbonate shelf. The transition from terrigenous to calcareous sedimentation occurred in the period of maximum transgression within Radaevkian-Tulian time. It took place gradually with a few short-lived inversions. Similarly this transition was reflected by the isotopes composition. Change curves of average values d13С have the form of a sine wave with an amplitude within +2.4...+3.9 ‰. Average values of d18О vary linearly from +23 to +27 ‰. Almost until the end of the Mikhailovian time the conditions of sublittoral sedimentation dominated. At this time there was a gradual shallowing of the sea. Сoncurrently, the value of carbon isotopes become heavier from +0.9 to +3.6 ‰ to the mid-Aleksinian, and later d13C = +3 ± 0.25 ‰. At the end of Mikhailovian time the Islands area was formed nearby the Ilych River basin. This change of condition of sedimentation is well reflected in the isotope system of the carbonate rocks. The isotopic composition of carbon have a sharp relief from +3 to -2.9 ‰ and d18О has a small relief from +26.5 to +25 ‰.