Fenestral Limestones: Specific Features of Late Devonian Seas in the Timan-Northern Ural Region

摘要

The article is dedicated to the lithological-paleoecological analysis of Upper Devonian fenestral limestones from different areas of the Timan-northern Ural region, which reflect paleogeographic settings from the coastal zone to the marginal part of the shelf. It is established that their main structural elements are represented by fenestrae, peloids, calcispheres, and occasional microzoo- and phytobenthos remains. The presence of a dark micritic envelope on calcispheres is their specific textural feature. The envelope of calcispheres represents mineralized vegetable mucus, which reflects their planktonic type of dwelling. These structures include representatives of radiolarian skeletons, Charophyceae and Chlorophyceae algae, and foramin-iferal tests. The trophic system consists of five levels. Carbonate sediments accumulated in relatively shallow-water and lagoonal settings occupied mostly by tidal and shallow-water subtidal microbial mats. In these lagoons, intermittently limited circulation stimulated the formation of anoxic conditions in bottom waters. Development of anoxic conditions in the bottom water layer, sulfate reduction, and freshwater influence determined the formation of a specific paleocoenosis of fenestral limestones, which could not form biogenic frameworks.