New data on the Neogene volcano-sedimentary complex of the ICHA depression: the tyrkachyn section (Western Kamchatka)
In the 90-m river bank of the middle reaches of the Icha River, thick strata of volcano-sedimentary deposits represented by pyroclastic rocks and siliceous tuffites with layers of diatomites were studied. Representative diatom assemblages assigned to the Middle Miocene Denticulopsis hyalina Zone (14.9-13.1 Ma) indicate relatively deep-water conditions of the marine basin. Comparison with the data on diatom assemblages from reference sections of Western Kamchatka indicates that the deposits of the Tyrkachin section belong to the upper part of the Kakert Formation. The petrographic composition and lithological structure of volcanogenic formations (members 1, 2, and 4) suggest that they are the product of pyroclastic flows associated with volcanic activity in central Kamchatka. It is assumed that Members 1 and 2 were formed in subaerial conditions, as indicated by the complete absence of marine microfossils, signs of primary volcanic stratification, and the absence of layering. The sediments of Member 4 with eroded contacts and pronounced layering apparently accumulated in a subaqueous setting.