Vol 24, No 5 (2016)

The first U‒Pb dates are obtained for detrital zircons from Upper Precambrian deposits of the Sredni (Zemlepakhtinskaya and Kuyakan formations) and Rybachi (Lonskii Formation) peninsulas. The spectra of ages of detrital zircons in sandstone samples from the Zemlepakhtinskaya and Kuyakan formations are similar to a significant extent to each other, which implies the dominant role of the same provenances. Most zircon grains are the Paleoproterozoic and Mesoproterozoic in age; some of them are characterized by Mesoarchean and Neoarchean ages. Zircons dated back to 1.0‒2.0 Ga with maxima at approximately 1.8, 1.5, 1.3, and 1.1 Ga are the most abundant. The youngest zircon grains are the Mesoproterozoic in age: 1050 ± 21Ma (i.e., close to the Mesoproterozoic‒Neoproterozoic boundary) and 1028 ± 21 Ma from the Zemlepakhtinskaya and Kuyakan formations, respectively. The distribution spectrum of ages obtained for zircons from sandstones of the Lonskii Formation significantly differs from that characteristic of zircons from sandstones of the Zemlepakhtinskaya and Kuyakan formations. The zircon population from the Lonskii Formation is dominated by detrital zircons with Neoarchean and Paleoproterozoic ages (2.8‒1.6 Ga); Paleoarchean and Mesoarchean grains are scarce. Their age maxima are registered at levels of approximately 2.7 and 1.8 Ga. The minimum age obtained for zircons from sandstones of the Lonskii Formation (1349 ± 35 Ma) allows the Rybachi block to be considered as being older as compared with the Sredni bock. Crystalline complexes of the Baltic Shield served as a main provenance for the Upper Precambrian deposits of the peninsulas under consideration. The dates obtained for detrital zircons from the Upper Precambrian deposits of the Sredni and Rybachi peninsulas are compared with similar data on the Upper Precambrian sequences of the Timan and Varanger Peninsula areas to reveal differences and similarities in the distribution of ages.

On the basis of the lithological-facies analysis, it was established that deposits of the Upper Itat Subformation, comprising the Dubinino locality of the Middle Jurassic flora and insects (Sharypovo district, Krasnoyarsk krai), accumulated in alluvial and lacustrine and, to a lesser extent, floodplain environments (floodplain and alluvial fan facies). The occurrence of remains of insects, macroremains of flora, spores, and pollen allowed us to make a paleoreconstruction of an area with a strongly dissected relief: continental fresh-water reservoir (lake) with varying degree of overflow, surrounded by hills covered with gymnospermous and ginkgo forests.

The dynamics of radiolarian evolution in the late Albian–Campanian is analyzed, and several stages are recognized. The first stage (late Albian–middle Cenomanian), related to the MCE regional anoxic event, showed low evolutionary tempos and hence lacked structural change in radiolarian communities. The second stage (late Cenomanian–early Turonian), corresponding to ОАЕ 2, which was a global anoxic event, is characterized by a decrease in the number of genera, while many genera showed increased diversification of species composition. At this stage, a considerable number of genera became extinct and appeared, suggesting an increased rate of the radiolarian evolution. The third stage (middle Turonian–early Coniacian), including the beginning of ОАЕ 3, is characterized by a stabilized number of genera. The fourth stage (late Coniacian–Santonian) completely encompasses ОАЕ 3 except for its very beginning. At this stage, the radiolarian communities underwent a significant structural change, while their rate of evolution increased considerably. Nevertheless, during the ОАЕ 3 stage, a distinct trend toward a decrease in generic diversity continued from the late Cenomanian to the middle Turonian. The fifth stage (Campanian) is characterized by quite significant changes in the assemblage composition, while the trend toward a gradual decrease in the number of genera steadily continued. At this stage, which coincided with a considerable cooling, twice as many genera became extinct as during ОАЕ 2. The analysis of the dynamics of radiolarian evolution showed that the anoxic MCE, ОАЕ 2, and ОАЕ 3 events did not result in degradation of radiolarian assemblages. This suggests that this group has significant stratigraphic potential. In general, the evolution of radiolarians in the Late Cretaceous was gradual. By the end of the Campanian, nearly half of the generic diversity was composed of genera which appeared at the beginning of the Cretaceous and earlier.