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Main criteria for predicting the presence of ancient reefs are paleoclimatic, paleotectonic, and lithologie. The paleoclimatic criterion is used to identify warm climatic zones, restricted to the equatorial belt of the globe, where reef formation was possible. Regions of differential tectonic movements, which are the essential factors for reef formation, are located by use of the paleotectonic criterion. The lithologie criterion distinguishes marine sedimentary complexes associated with reefs. By use of these criteria, three middle Paleozoic reef provinces have been distinguished in the Soviet Arctic—the Pechora-Novaya Zemlya, Taymyr-Tungusska, and East Siberia-Chukchi regions.

The Pechora-Novaya Zemlya province contains Silurian-Devonian reefs. In the Novaya Zemlya-Urals area, reef complexes are confined to edges of géosynclinal downwarps and tectonic scarps in the outer part of a miogeosynclinal area. In the Pechora depression. Upper Devonian reef masses formed on tectonic scarps bordering trenchlike troughs of the platform. In Vaygach, Silurian reefs belong to the sedimentary complex of a shallow carbonate platform. A Devonian reef massif up to 2,000 m thick formed along a trough margin including part of northeastern Pay-Khoy and reaching southeastern Vaygach. Facies changes from reef to nonreef deposits may be decisive in formation of stratigraphie oil and gas traps. The aim of prospecting in the Pay-Khoy-Novaya Zemlya region is to search for high-porosity zones in reef massifs among petroliferous black shales and limestones.

Almost the whole spectrum of Silurian-Devonian deposits, especially rich in clay, clayey carbonate, and sulfate rocks, is represented on the sides of the Tun-gusska syneclise and in the northern Taymyr margin of the Siberian platform. Bioclastic skeletal and biogenic limestones also are present. Carbonate sedimentation in the Tungusska basin was closely related to transgressions and regressions of the sea. Association of coral-stromatoporoid limestones of varied thickness with shales suggests the possibility of oil traps in Wen-lockian deposits. Devonian reefs are probably present at the juncture of the Tungusska and Taymyr basins and in the Yenisey-Khafanga trough.

In the East Siberia-Chukchi reef province, Silurian and Devonian carbonate deposits with indications of reef formation are located north of the continental margin. Most paleotectonic reconstructions for the northeast margin of the USSR indicate the transition from a eugeosyncline at the south to a miogeosyncline and, at the north, the Hyperborean platform. The outer part of the miogeosyncline underwent differential tectonic movements favorable for reef formation. Reefs might also have been formed at the edges of troughs within the Hyperborean platform. Localized occurrences of reeflike limestones in volcanic sequences of the eugeo-synclinal zone of the Koryak upland could have formed as atolls on submarine volcanic cones.

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