Lithologic, paleontologic and geochemical foundation of the Early Wenlockian (Early Silurian) sedimentation break in the western slope of the Northern Urals section (Verkhnaya Kos’yu River)

Research subject. Lower Silurian carbonate deposits in the P-VK6 section (basin of the upper reaches of the Ilych River, Northern Urals).

Materials and methods. The material for the article comprises a field description of a section and 24 rock samples studied by classical optical microscopic and isotopic methods. The extraction of conodont elements from carbonate rocks was carried out according to a standard procedure in 8% acetic acid.

Results. The surface of the sedimentation break is represented by an erosional surface and voids in the microbial laminites. Two types are distinguished among the underground palaeokarst elements: (i) vertical cavities (up to 2.0–3.0 cm) filled with gravitational calcite with fibrous extinction and blocky sparite; and (ii) palaeovoids (5.0–20 cm) with irregular to tube-shaped, filled with laminar clay-carbonate sediments. The first type is located at a depth of up to 2.5 m directly below the surface of the sedimentation break. This interval is also highlighted by the Meteoric Calcite Line isotopic trend (variable δ13C values and invariable δ18O values). The second type is filled mainly with clayey-dolomite material, probably the insoluble sediments derived from rock dissolution. The maximum recording depth for these elements is 5.0 m below the surface of the sedimentation break. Biostratigraphic analysis of conodonts demonstrated that the rocks of the Marshrut regional stage of the Telychian underwent karsting. From the deposits overlying the paleokarst, species Ozarkodina bohemica (Walliser) was determined, which made it possible for the first time to paleontologically confirm the regional gap throughout the Marshrut regional stage of the Llandovery age and Vo`jvyv regional stage of the Wenlock age.

Conclusions. The sedimentation break age, based on biostratigraphic data and regional background, is determined to be Early Wenlockian.

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