Paleozoic Reef Formation in the Pechora Syneclise and the Caspian Basin: A Comparative Analysis

In two marginal depressions of the East European Platform (Pechora Syneclise and Caspian Basin), reef structures are widespread, but their stratigraphic intervals are far from similar. The reef formation began in the Caradocian in the Pechorian Ural, in the second half of the Llandoverian in the Pechora Syneclise, reached the maximum in the Frasnian, and gave way to the development of reef mounds in the Famennian. The structures are represented both as asymmetric reefs framing shallow zones at their boundary with the relatively deep paleobasins and as solitary structures within the latter. Reefs of the next global maximums (late Visean–Serpukhovian and Lower Permian) are confined to some places at the boundary with the Ural paleocean and its relict (Cis-Ural foredeep). The Caspian Basin accommodates reefs of all three global maximums of development—both asymmetric reef systems framing the shelf edges and symmetric intra-basin isolated structures. Such difference is due to the different paleogeomorphological type of basins. The Caspian Basin was sharply differentiated in depth throughout the Middle and Late Paleozoic, resulting in the formation of reefs rising above the bottom of these basins. In the Pechora Syneclise, depth differentiation of the basins occurred only in the Late Devonian. The Visean–Serpukhovian and early Permian shallow seas were generally weakly dissected and did not provide conditions for the formation of thick reefs protruding above the seafloor. Such reefs were formed only on the Ural paleocean edge in the Visean–Serpukhovian and on the Cis-Ural foredeep edge in the Permian.