The Upper Ordovician reef Bol’shaya Kos’yu, Ilych River, the Northern Urals (structure, paleobiocenosis, microfacies, model of formation)

Abstract

Subject. The Upper Ordovician reef Bol’shaya Kos’yu, located on the western slope of the Northern Urals in the basin of the Ilych River is a unique object among coeval reefs due to the weak dolomitization of rocks and the ability to trace the phases of its development. Materials and methods. The article presents the results of paleontological, paleoecological and microfacial analyses of reef limestones selected by the author from 12 natural outcrops with a total capacity of about 150 m located along both banks of the Bol’shaya Kos’yu River. Results. The structure of the reef, morphology and taxonomic composition of the frame-forming biota were established, and the important role of sphinctozoal sponges was first identified. Among the reef limestones in the main genetic characteristics that reflect specific depositional environments, was identified three types of microfacies. Type 1 – reef microfacies – framestones and buflestones. Type 2 – microfacies of interbiohermic space and inter-carcass cavities exposed to wave and tidal action – bioclastic and peloid pakstones, biolithoclastic grainstones and lithoclastic rudstones. Type 3 – microfacies of the hidden microcosmic cavities, protected from the active wave impact – wackstone. Сonclusion. The composition of the microfacies and frame-forming biota characterize difficult differentiated, both on a section, and structure the reef, which was formed in the conditions of the carbonate platform-shelf margin with active hydrodynamics. Its growth occurred during transgression, which ended by the end of the middle katian a major regression that stopped its development. The revealed paleontological, paleoecological and microfacial data indicate that starting from the middle katian the first identified sphinctozoal sponges could settle in sublittoral environments with active hydrodynamics at depths up to 20 m.

Keywords

• sphinctozoa
• paleobiocenosis
• microfacies
• reef
• upper ordovician
• ilych river
• northern urals