Lower Ludfordian (Upper Silurian) slope deposits in the Lyagadin section (Ilych River, the Northern Urals)

Abstract

Research subject. Upper Silurian (Lower Ludfordian) slope deposits of the Lyagadin section (Ilych River, Northern Urals).Materials and methods.The composition and structure of 40 rock samples collected across the section during fieldworks in 2019 were studied. Thin sections were examined under a polarising microscope. Eight samples were investigated by silicate analysis, and clay minerals from 2 samples were determined by X-ray diffraction analysis of oriented samples.Results. The Lyagadin section with a visible thickness of 70 m is represented by an uneven alteration of clayey-siliceouscalcareous schists, micro-grained limestones with burrows, micro-fine-grained layered limestones, fine lime sandstones, coarse- and medium-grained lime sandstones, lime conglogravelites and lime conglobreccias. Clayey-siliceous-calcareous schists and micro-grained limestones with burrows characterise the background periplatform clayey-carbonate deposits. The part of micro-fine-grained layered limestones (their horizontally layered varieties) are bottom current sediments. Gravity flow deposits are represented by clastic rock assemblage. The most characteristic of them is the assemblage composed of lime conglogravelites with an erosional bottom, which pass into gradational-layered lime sandstones, first coarse- and medium-grained, and further the into their fine-grained varieties. They are covered, in turn, with micro- fine-grained limestones with cross or convolute stratifications. That association is interpreted as calciturbidites. Calcidebrites are characterised by lime conglobreccias. The deposits of grainy flows (?) are represented by the layers of lime sandstones not revealing the characteristic features of the Bouma sequence.Conclusion. The reconstructed background sedimentation on the continental slope during Early Ludfordian was characterised by the accumulation of hemi-pelagic clay deposits together with a certain amount of microscopic quartz, orthoclase and organic matter. Periodically appearing thin layers of micro-grained limestones with burrows correspond to the periods of increased supply of carbonate material, which could be attributed to “marine snow”. The irregular nature of the distribution of gravity flow deposits in the Lyagadin section suggests that the studied slope deposits are, most likely, the formation of the outer apron of slope bottom, rather than calciclastic fans.