Paragenetic rock associations as a reflexion of carbonate platform margin lithogeodinamic evolution in the late Ordovician (Ilych river basin, Northern Urals)

Six paragenetic associations on the basis of spatial-temporal relationships of lithotypes were recognized among the Upper Ordovician carbonate and terrigenous-carbonate rocks in sections of the Ilych River area, the Northern Urals. Their distribution characterizes frequent changes of depositional environments and the paleolandscape transformation of the carbonate platform outer margin during the Late Ordovician. In the Sandbian lime-lerrigenous mud was accumulated with activities of infauna (PA-1) at the continental slope foot of the platform-ramp. In the mid-Katian facially differentiated and diverse reef ecosystem (PA-2, PA-3) formed on the platform-shelf margin in a shallow environment. In the late Katian, in the conditions of the open platform-ramp, transgressive facies of carbonate sediments, concluding not only various fauna, but also periodic supply of debris from raised areas of the upper ramp, were accumulated (PA-4, PA-5). The Hirnantian is initially characterized by periodic eroding of the platform-shelf margin, and later by the accumulation of shallow crinoidal sands. Change of the paragenetic associations in succession reflects the lithogeodynamic evolution of the carbonate platform from the continental slope deep-water batyal to shallow-water environments of the platform-shelf during the Late Ordovician history of the northeastern European Platform.