Geological structure features of Menzelinsky, Timerovsky and Olginsky fields of the Republic of Tatarstan as a result of their genetic nature

The Menzelinsky, Timerovsky and Olginsky fields are located in the northeast of the Republic of Tatarstan and are associated with uplifts, sharply pronounced along the Tulskian surface of the Lower Carboniferous. With a small area of uplifts (1.5-2.2 km across), they are distinguished by a considerable (220-380 m) height and steep wings (up to 40°) of the carbonate rock array overlapped by the RadaevskianTulskian terrigenous strata. The carbonate array is represented by deposits of the Tournaisian-Famennian age on the arches of uplifts and by Famennian deposits on their wings. It is accepted to classify such carbonate structures as frameless reefs formed on local seabed areas with a combination of specific conditions for their continuous growth. In the opinion of the authors of the present article, characterized and similar uplifts were formed due to erosion-karst processes that took place on the Tournaisian continental land after the regression of the Tournaisian Sea in the east of the Russian Platform. Erosion and karst processed the limestone paleosurface of the continent during the entire Kosvinian time. As a result of the Late Radaevskian-Bobrikovian cycle of marine transgression, the entire Tournaisian-Famennian surface was covered by the terrigenous rock strata of Radaevskian-Tulskian on the wings of the uplifts and by Bobrikovian-Tulskian strata – on the arches.