Organic buildups and reefs on the palaeozoic carbonate platform margin, Pechora Urals, Russia

In the Pechora Urals, Palaeozoic organic structures are widespread and are represented by various types, from small bioherms to large barrier reefs which can be observed throughout Upper Ordovician-Lower Permian strata. In the pre-orogenic history of Palaeozoic carbonate sedimentation and reef growth three stages are distinguished. Each stage is characterized by a distinct reef geometry, growth rate and depositional setting. (1) The reefs of the lower Palaeozoic (Ashgillian-lower Emsian) stage were restricted to the outer shelf margin, and they were wave-resistant stromatolite-rich frameworks, except for upper Llandoverian knell and patch reefs. The total thickness of the lower Palaeozoic reefs is 60-1200 m. (2) The middle Palaeozoic (middle Frasnian-lower Tournaisian) organic buildups are quiet-water carbonate bank-type structures generally between 300 and 700 m thick. They were formed on both the outer and the inner shelf margin; however, they are better developed and more characteristic of the latter setting. Stromatoporoid-microbial and microbial associations form the main buildup facies separated by bedded interbuildup sediments. The upper Palaeozoic (upper Visean-Sakmarian) organic structures are represented by both isolated wave-resistant shelf margin reefs, up to 200 m thick, and quiet-water carbonate banks, up to 330 m thick. These buildups were located in the outer and inner shelf margin. Framework-builders were mainly branching and platy stromatoporoids associated with stromatolites in the reefs, and calcified microbes and phylloid algae in the carbonate banks. The distribution and geometry of the buildups was mainly determined by regional tectonic events caused by the passive-to-active margin transition of the northeastern European Platform. The size of the reefs: depended on the magnitude and duration of sea-level fluctuations. (C) 1998 Elsevier Science B.V. All rights reserved.