The Ozerninsky Sequence of the Uda–Vitim Zone (Western Transbaikal Region, Russia): Composition, Age, and Formation Environment

The paper reports the lithological and biostratigraphic study of the Ozerninsky Sequence, the accumulation of which is associated with the onset of the early Hercynian stage in the evolution of the Uda–Vitim structural–formational zone of the Baikal–Vitim fold system and the formation of gold–pyrite–polymetallic deposits of the Ozerninsky ore cluster. The sequence is isolated from the Lower Cambrian Oldynda Formation. It is composed predominantly of graywacke arkose and quartz graywacke and includes two subsequences. The lower subsequence is represented by the quartz–feldspar sandstone and siltstone with interlayers and lenses of polymictic conglomerate and gritstone, while the upper subsequence is composed of intercalated polymictic sandstone and siltstone, calcareous siltstone, and limestone with interlayers of clayey, clayey–siliceous, carbonaceous–clayey siltstone, and mudstone. The paleontological characteristics of the Ozerninsky Sequence determines its stratigraphic affiliation to the Lower (Emsian Stage)–Upper (Lower Frasnian Substage) Devonian. The composition, structure, and paleo-biota indicate that the sediments formed in the open shelf sea environment with a significant input of terrigenous component. The lower subsequence was accumulated in a shelf shoaling conditions at active hydrodynamic mode, while the upper subsequence was accumulated in the most submerged parts of the shelf, in areas with stagnant hydrogeological regime.